SCHOOL OF MECHANICAL

DEPARTMENT OF MECHANICAL ENGINEERING

LESSON NOTES

U4MEA04 MATERIAL SCIENCE AND ENGINEERING

METALLURGY

VELTECH Dr.RR & Dr.SR TECHNICAL UNIVERSITY

SYLLABUS

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U4MEA04 MATERIAL SCIENCE AND ENGINEERING METALLURGY L T P C(common for MECH, AUTO) 3 0 0 3

OBJECTIVES:To impart knowledge on the structure, properties, treatment, testing and applications of metals and non-metallic materials so as to identify and select suitable materials for various engineering applications.

UNIT I: Crystallography 9

Classification of Materials- Engineering properties of materials( band energy and activation energy)- Structure of Solid materials- BCC- FCC & HCP Structures- Atomic packing factor-Polymorphism and Allotropy- Miller Indices- Crystal imperfection, point, line, surface and volume defects- Metallographic Analysis- Specimen preparation, metallurgical and scanning electron microscopes

UNIT II: Mechanical Properties of materials & Fracture 9

Mechanisms of  Plastic and Elastic deformations, Slip and Twinning- Strengthening mechanisms, recover recrystallization and Grain growth- Strain hardening- Work hardening, Precipitation hardening. Types of   Fracture- Ideal fracture stress, ductile and brittle fracture- Griffith’s theory, creep mechanisms of Creep- Creep resistant materials- Fracture failure SN curve- prevention of fatigue failure- Ductile and Brittle transition, Cup and Cone type fracture.

UNIT III: Ferrous and Non ferrous Metals and Phase Diagram 9

Ferrous and Non ferrous Metals- Effect of alloying additions on steel (Mn, Si, Cr, Mo, V Ti & W) - stainless steels – HSLA - maraging steels – Gray, White malleable, spheroidal - Graphite - alloy cast irons -Copper and Copper alloys – Brass, Bronze and Cupronickel – Aluminum and Al-Cu – precipitation strengthening treatment – Bearing alloys.

Solid Solution, Inter metallic Compound cooling curves, types of Equilibrium diagrams, Lever rules- Phase diagrams- Gibbs phase rule- Iron carbide diagram-TTT diagram

UNIT IV: Mechanical testing 9

Tensile test- Stress Strain curves for Ductile and Brittle materials- Mild steel, Copper, Concrete, and Cast iron Proof Stress, Yield point phenomenon, Luder’s bands- compression and shear loads, Hardness tests (Brinnel, Vicrex and Rockwell) - Impact test- Izod and Chorpy, Fatigue and creep test, fracture toughness tests

UNIT V: Heat Treatment 9

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Definition – Full annealing, stress relief, recrystallisation and spheroidizing –normalising, hardening and Tempering of steel. Isothermal transformation diagrams – cooling curves superimposed on I.T. diagram CCR - Hardenability, Jominy end quench test – Austempering, martempering Case hardening, carburising, nitriding, cyaniding, carbonitriding – Flame and Induction hardening.

TOTAL : 45 periodsTEXT BOOKS1. Dieter, G. E., Mechanical Metallurgy, McGraw Hill, Singapore, 2001

2. Thomas H. Courtney, Mechanical Behaviour of Engineering materials, McGraw Hill,

Singapore, 2000

REFERENCE BOOKS

1. Kenneth G.Budinski and Michael K.Budinski “Engineering Materials” Prentice-Hall of India Private Limited, 4th Indian Reprint 2002.

2. William D Callsber “Material Science and Engineering”, John Wiley and Sons 1997.

3. Raghavan.V “Materials Science and Engineering”, Prentice Hall of India Pvt., Ltd., 1999.4. Sydney H.Avner “Introduction to Physical Metallurgy” McGraw Hill Book Company,

1994.

UNIT – I

CRYSTALLOGRAPHY

Classification of Materials:

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Common engineering materials that come within the scope of Material Science may be classified broadly into the following three types:-

1. Metals (Ferrous and Non-Ferrous) 2. Ceramics 3. Organic Polymers

Band energy:

Energy bands consisting of a large number of closely spaced energy levels exist in crystalline materials. The bands can be thought of as the collection of the individual energy levels of electrons surrounding each atom. The wavefunctions of the individual electrons, however, overlap with those of electrons confined to neighboring atoms. The Pauli exclusion principle does not allow the electron energy levels to be the same so that one obtains a set of closely spaced energy levels, forming an energy band. The energy band model is crucial to any detailed treatment of semiconductor devices. It provides the framework needed to understand the concept of an energy bandgap and that of conduction in an almost filled band as described by the empty states.In this section, we present the free electron model and the Kronig-Penney model. Then we discuss the energy bands of semiconductors and present a simplified band diagram. We also introduce the concept of holes and the effective mass.

Activation Energy

The energy that an atomic system must acquire before a process (such as an emission or reaction) can occur "catalysts are said to reduce the energy of activation during the transition phase of a reaction"

What is Activation Energy?

... life to take place, cells must use enzymes to selectively lower the activation energy of reactions. Enzymes are protein molecules that act as biological

Structure of Solid materials

In the context of correspondence, blind carbon copy (abbreviated Bcc:) refers to the practice of sending a message to multiple recipients in such a way that conceals individual email addresses (mentioned in "to"

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field of the mail) from the complete list of recipients. [1] In some circumstances, the typist must ensure that multiple recipients of such a document not see the names of other recipients. To achieve this, the typist can:

Add the names in a second step to each copy, without carbon paper Set the ribbon not to strike the paper, which leaves names off the

top copy (but may leave letter impressions on the paper)

To specify recipients, an e-mail message may contain addresses in any of these three fields:

To: Primary recipients Cc: Carbon copy to secondary recipients—other interested parties Bcc: Blind carbon copy to tertiary recipients who receive the mes-

sage without anyone else (including the To, Cc, and Bcc recipients) seeing who the tertiary recipients are.

It is common practice to use the Bcc: field when addressing a very long list of recipients, or a list of recipients that should not (necessarily) know each other, e.g. in mailing lists. [2]

FCC

The purpose of this Reference Guide is to familiarize you with the Federal Communications Commission’s (FCC’s) procedures and regulations concerning the processing of requests for documents made under the Freedom of Information Act (FOIA). Following the guidance below will make it more likely that you will receive the information that you are seeking in the shortest amount of time, and without unnecessary expense. This Reference Guide also provides information on how to obtain routinely available FCC documents through means other than making a FOIA request

Similarities and Difference Between the FCC and HCP Structure

The face centered cubic and hexagonal close packed structures both have a packing factor of 0.74, consist of closely packed planes of atoms, and have a coordination number of 12. The difference between the fcc and hcp is the stacking sequence. The hcp layers cycle among the two equivalent shifted positions whereas the fcc layers cycle between three positions. As can be seen in the image, the hcp structure contains only two types of planes with an alternating ABAB arrangement. Notice how the atoms of the third plane are in exactly the same position as the atoms in the first plane. However, the fcc structure contains three types of planes with a ABCABC arrangement. Notice how the atoms in rows A

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and C are no longer aligned. Remember that cubic lattice structures allow slippage to occur more easily than non-cubic lattices, so hcp metals are not as ductile as the fcc metals.

The table below shows the stable room temperature crystal structures for several elemental metals.

MetalCrystal Structure

Atomic Radius (nm)

Aluminum FCC 0.1431Cadmium HCP 0.1490Chromium BCC 0.1249Cobalt HCP 0.1253Copper FCC 0.1278Gold FCC 0.1442Iron (Alpha) BCC 0.1241Lead FCC 0.1750Magnesium HCP 0.1599Molybdenum BCC 0.1363Nickel FCC 0.1246Platinum FCC 0.1387Silver FCC 0.1445Tantalum BCC 0.1430Titanium (Alpha) HCP 0.1445Tungsten BCC 0.1371Zinc HCP 0.1332

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A nanometer (nm) equals 10-9 meter or 10 Angstrom units.

HCP Structures

Primary Metallic Crystalline Structures (BCC, FCC, HCP)

As pointed out on the previous page, there are 14 different types of crystal unit cell structures or lattices are found in nature. However most metals and many other solids have unit cell structures described as body center cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp). Since these structures are most common, they will be discussed in more detail.

Body-Centered Cubic (BCC) Structure

The body-centered cubic unit cell has atoms at each of the eight corners of a cube (like the cubic unit cell) plus one atom in the center of the cube (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. It is said to have a coordination number of 8. The bcc unit cell consists of a net total of two atoms; one in the center and eight eighths from corners atoms as shown in the middle image below (middle image below). The image below highlights a unit cell in a larger section of the lattice.

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The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. The bcc structure is often the high temperature form of metals that are close-packed at lower temperatures. The volume of atoms in a cell per the total volume of a cell is called the packing factor. The bcc unit cell has a packing factor of 0.68.

Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium, barium, vanadium, alpha-iron and tungsten. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure. It should be noted that there are other important mechanisms for hardening materials, such as introducing impurities or defects which make slipping more difficult. These hardening mechanisms will be discussed latter.

Face Centered Cubic (FCC) Structure

The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination number of 12. The fcc unit cell consists of a net total of four atoms; eight eighths from corners atoms and six halves of the face atoms as shown in the middle image above. The image below highlights a unit cell in a larger section of the lattice.

In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the bcc structure. The atoms from one layer

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nest themselves in the empty space between the atoms of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls that are aligned in columns and rows. When a few additional balls are tossed in the box, they will not balance directly on top of the balls in the first layer but instead will come to rest in the pocket created between four balls of the bottom layer. As more balls are added they will pack together to fill up all the pockets. The packing factor (the volume of atoms in a cell per the total volume of a cell) is 0.74 for fcc crystals. Some of the metals that have the fcc structure include aluminum, copper, gold, iridium, lead, nickel, platinum and silver.

Hexagonal Close Packed (HPC) Structure

Another common close packed structure is the hexagonal close pack. The hexagonal structure of alternating layers is shifted so its atoms are aligned to the gaps of the preceding layer. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer just like in the fcc structure. However, instead of being a cubic structure, the pattern is hexagonal. (See image below.) The difference between the HPC and FCC structure is discussed later in this section.

The hcp structure has three layers of atoms. In each the top and bottom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon. The middle layer has three atoms nestle in the triangular "grooves" of the top and bottom plane. Note that there are six of these "grooves" surrounding each atom in the hexagonal plane, but only three of them can be filled by atoms.

As shown in the middle image above, there are six atoms in the hcp unit cell. Each of the 12 atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and each of the three atom in the middle layer contribute 1 atom. The image

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on the right above attempts to show several hcp unit cells in a larger lattice.

The coordination number of the atoms in this structure is 12. There are six nearest neighbors in the same close packed layer, three in the layer above and three in the layer below. The packing factor is 0.74, which is the same as the fcc unit cell. The hcp structure is very common for elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc and zirconium.

Atomic packing factor

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In crystallography, atomic packing factor (APF) or packing fraction is the fraction of volume in a crystal structure that is occupied by atoms. It is dimensionless and always less than unity. For practical purposes, the APF of a crystal structure is determined by assuming that atoms are rigid spheres. The radius of the spheres is taken to be the maximal value such that the atoms do not overlap. For one-component crystals (those that contain only one type of atom), the APF is represented mathematically by

where Natoms is the number of atoms in the unit cell, Vatom is the volume of an atom, and Vunit cell is the volume occupied by the unit cell. It can be proven mathematically that for one-component structures, the most dense arrangement of atoms has an APF of about 0.74. In reality, this number can be higher due to specific intermolecular factors. For multiple-component structures, the APF can exceed 0.74.

Content 1 Worked out example

o 1.1 Body-centered cubic crystal structure o 1.2 Hexagonal close-packed crystal structure

2 APF of common structures 3 See also

4 References

[edit] Worked out example[edit] Body-centered cubic crystal structure

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BCC structure

The primitive unit cell for the body-centered cubic (BCC) crystal structure contains nine atoms: one on each corner of the cube and one atom in the center. Because the volume of each corner atom is shared between adjacent cells, each BCC cell contains two atoms.

Each corner atom touches the center atom. A line that is drawn from one corner of the cube through the center and to the other corner passes through 4r, where r is the radius of an atom. By geometry, the length of the diagonal is a√3. Therefore, the length of each side of the BCC structure can be related to the radius of the atom by

Knowing this and the formula for the volume of a sphere((4 / 3)pi r3), it becomes possible to calculate the APF as follows:

[edit] Hexagonal close-packed crystal structure

HCP structure

For the hexagonal close-packed (HCP) structure the derivation is similar. The side length of the hexagon will be denoted as a while the height of the hexagon will be denoted as c. Then:

a = 2r

It is then possible to calculate the APF as follows:

APF of common structures

By similar procedures, the ideal atomic packing factors of all crystal structures can be found. The common ones are collected here as reference, rounded to the nearest hundredth.

Simple cubic : 0.52 Body-centered cubic : 0.68 Hexagonal close-packed : 0.74 Face-centered cubic : 0.74 Diamond cubic : 0.34

Miller index

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Planes with different Miller indices in cubic crystals

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Examples of directions

Miller indices are a notation system in crystallography for planes and directions in crystal (Bravais) lattices.

In particular, a family of lattice planes is determined by three integers ℓ, m, and n, the Miller indices. They are written (hkl), and each index denotes a plane orthogonal to a direction (h, k, l) in the basis of the reciprocal lattice vectors. By convention, negative integers are written with a bar, as in 3 for −3. The integers are usually written in lowest terms, i.e. their greatest common divisor should be 1. Miller index 100 represents a plane orthogonal to direction ; index 010 represents aℓ plane orthogonal to direction m, and index 001 represents a plane orthogonal to n.

There are also several related notations[1]:

the notation { mn} denotes the set of all planes that are equivalentℓ to ( mn) by the symmetry of the lattice. ℓ

In the context of crystal directions (not planes), the corresponding notations are:

[ mn], with square instead of round brackets, denotes a direction inℓ the basis of the direct lattice vectors instead of the reciprocal lat-tice; and

similarly, the notation 〈hkl〉 denotes the set of all directions that are equivalent to [ mn] by symmetry. ℓ

Miller indices were introduced in 1839 by the British mineralogist William Hallowes Miller. The method was also historically known as the Millerian system, and the indices as Millerian,[2] although this is now rare.

The precise meaning of this notation depends upon a choice of lattice vectors for the crystal, as described below. Usually, three primitive lattice vectors are used. However, for cubic crystal systems, the cubic lattice vectors are used even when they are not primitive (e.g., as in body-centered and face-centered crystals).

Contents 1 Definition 2 Case of cubic structures 3 Case of hexagonal and rhombohedral structures 4 The crystallographic planes and directions 5 Integer vs. irrational Miller indices: Lattice planes and qua -

sicrystals

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6 See also 7 References

8 External links

Definition

Examples of determining indices for a plane using intercepts with axes; left (111), right (221).

There are two equivalent ways to define the meaning of the Miller indices:[1] via a point in the reciprocal lattice, or as the inverse intercepts along the lattice vectors. Both definitions are given below. In either case, one needs to choose the three lattice vectors a1, a2, and a3 as described above. Given these, the three primitive reciprocal lattice vectors are also determined (denoted b1, b2, and b3).

Then, given the three Miller indices , m, n ( mn) denotes planesℓ ℓ orthogonal to the reciprocal lattice vector:

That is, ( mn) simply indicates a normal to the planes in the ℓ basis of the primitive reciprocal lattice vectors. Because the coordinates are integers, this normal is itself always a reciprocal lattice vector. The requirement of lowest terms means that it is the shortest reciprocal lattice vector in the given direction.

Equivalently, ( mn) denotes a plane that intercepts the three pointsℓ a1/ , ℓ a2/m, and a3/n, or some multiple thereof. That is, the Miller indices are proportional to the inverses of the intercepts of the plane, in the basis of the lattice vectors. If one of the indices is zero, it means that the planes do not intersect that axis (the intercept is "at infinity").

Considering only ( mn) planes intersecting one or more latticeℓ points (the lattice planes), the perpendicular distance d between adjacent lattice planes is related to the (shortest) reciprocal lattice vector orthogonal to the planes by the formula: .[1]

The related notation [ mn] denotes the ℓ direction:

That is, it uses the direct lattice basis instead of the reciprocal lattice. Note that [ mn] is ℓ not generally normal to the ( mn) planes,ℓ except in a cubic lattice as described below.

Case of cubic structures

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For the special case of simple cubic crystals, the lattice vectors are orthogonal and of equal length (usually denoted a); similar to the reciprocal lattice. Thus, in this common case, the Miller indices ( mn)ℓ and [ mn] both simply denote normals/directions in ℓ Cartesian coordinates.

For cubic crystals with lattice constant a, the spacing d between adjacent ( mn) lattice planes is (from above):ℓ

Because of the symmetry of cubic crystals, it is possible to change the place and sign of the integers and have equivalent directions and planes:

Coordinates in angle brackets such as 〈100〉 denote a family of di-rections which are equivalent due to symmetry operations, such as [100], [010], [001] or the negative of any of those directions.

Coordinates in curly brackets or braces such as {100} denote a family of plane normals which are equivalent due to symmetry op-erations, much the way angle brackets denote a family of direc-tions.

For face-centered cubic and body-centered cubic lattices, the primitive lattice vectors are not orthogonal. However, in these cases the Miller indices are conventionally defined relative to the lattice vectors of the cubic supercell and hence are again simply the Cartesian directions.

Case of hexagonal and rhombohedral structures

Miller-Bravais indices

With hexagonal and rhombohedral lattice systems, it is possible to use the Bravais-Miller index which has 4 numbers (h k i l)

i = −h − k.

Here h, k and l are identical to the Miller index, and i is a redundant index.

This four-index scheme for labeling planes in a hexagonal lattice makes permutation symmetries apparent. For example, the similarity between (110) ≡ (1120) and (120) ≡ (1210) is more obvious when the redundant index is shown.

In the figure at right, the (001) plane has a 3-fold symmetry: it remains unchanged by a rotation of 1/3 (2π/3 rad, 120°). The [100], [010] and the [110] directions are really similar. If S is the intercept of the

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plane with the [110] axis, then

i = 1/S.

There are also ad hoc schemes (e.g. in the transmission electron microscopy literature) for indexing hexagonal lattice vectors (rather than reciprocal lattice vectors or planes) with four indices. However they don't operate by similarly adding a redundant index to the regular three-index set.

For example, the reciprocal lattice vector (hkl) as suggested above can be written as ha*+kb*+lc*if the reciprocal-lattice basis-vectors are a*, b*, and c*. For hexagonal crystals this may be expressed in terms of direct-lattice basis-vectors a, b and c as

Hence zone indices of the direction perpendicular to plane (hkl) are, in suitably-normalized triplet form, simply [2h+k,h+2k,l(3/2)(a/c)2]. When four indices are used for the zone normal to plane (hkl), however, the literature often uses [h,k,-h-k,l(3/2)(a/c)2] instead[3]. Thus as you can see, four-index zone indices in square or angle brackets sometimes mix a single direct-lattice index on the right with reciprocal-lattice indices (normally in round or curly brackets) on the left.

The crystallographic planes and directions

Dense crystallographic planes

The crystallographic directions are fictitious lines linking nodes (atoms, ions or molecules) of a crystal. Similarly, the crystallographic planes are fictitious planes linking nodes. Some directions and planes have a higher density of nodes; these dense planes have an influence on the behaviour of the crystal:

optical properties : in condensed matter, the light "jumps" from one atom to the other with the Rayleigh scattering; the velocity of light thus varies according to the directions, whether the atoms are close or far; this gives the birefringence

adsorption and reactivity: the adsorption and the chemical reac-tions occur on atoms or molecules, these phenomena are thus sen-sitive to the density of nodes;

surface tension : the condensation of a material means that the atoms, ions or molecules are more stable if they are surrounded by other similar species; the surface tension of an interface thus varies according to the density on the surface

o the pores and crystallites tend to have straight grain bound-aries following dense planes

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o cleavage dislocations (plastic deformation)

o the dislocation core tends to spread on dense planes (the elastic perturbation is "diluted"); this reduces the friction (Peierls-Nabarro force), the sliding occurs more frequently on dense planes;

o the perturbation carried by the dislocation (Burgers vector) is along a dense direction: the shift of one node in a dense di-rection is a lesser distortion;

o the dislocation line tends to follow a dense direction, the dis-location line is often a straight line, a dislocation loop is often a polygon.

For all these reasons, it is important to determine the planes and thus to have a notation system.

Integer vs. irrational Miller indices: Lattice planes and quasicrystals

Ordinarily, Miller indices are always integers by definition, and this constraint is physically significant. To understand this, suppose that we allow a plane (abc) where the Miller "indices" a, b, and c (defined as above) are not necessarily integers.

If a, b, and c have rational ratios, then the same family of planes can be written in terms of integer indices ( mn) by scaling ℓ a, b, and c appropriately: divide by the largest of the three numbers, and then multiply by the least common denominator. Thus, integer Miller indices implicitly include indices with all rational ratios. The reason why planes where the components (in the reciprocal-lattice basis) have rational ratios are of special interest is that these are the lattice planes: they are the only planes whose intersections with the crystal are 2d-periodic.

For a plane (abc) where a, b, and c have irrational ratios, on the other hand, the intersection of the plane with the crystal is not periodic. It forms an aperiodic pattern known as a quasicrystal. In fact, this construction corresponds precisely to the standard "cut-and-project" method of defining a quasicrystal, using a plane with irrational-ratio Miller indices. (Although many quasicrystals, such as the Penrose tiling, are formed by "cuts" of periodic lattices in more than three dimensions, involving the intersection of more than one such hyperplane.)

Imperfections of crystal structure

Dr. Dmitri Kopeliovich

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There are three conventional types of crystal imperfections:

Point defects

The simplest point defects are as follows:

Vacancy – missing atom at a certain crystal lattice position; Interstitial impurity atom – extra impurity atom in an interstitial

position; Self-interstitial atom – extra atom in an interstitial position; Substitution impurity atom – impurity atom, substituting an

atom in crystal lattice; Frenkel defect – extra self-interstitial atom, responsible for the va-

cancy nearby.

Line defects

Linear crystal defects are edge and screw dislocations.

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Edge dislocation is an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics: ductility and malleability.

Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part. It is called screw as atomic planes form a spiral surface around the dislocation line.

For quantitative characterization of a difference between a crystal distorted by a dislocation and the perfect crystal the Burgers vector is used.

The dislocation density is a total length of dislocations in a unit crystal volume. The dislocation density of annealed metals is about 1010 - 1012 m−². After work hardening the dislocation density increases up to 1015 - 1016 m-². Further increase of dislocation density causes crackes formation and fracture.

Planar defects

Planar defect is an imperfection in form of a plane between uniform parts of the material. The most important planar defect is a grain boundary. Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis. Depending on the rotation axis direction, two ideal types of a grain boundary are possible:

Tilt boundary – rotation axis is parallel to the boundary plane; Twist boundary - rotation axis is perpendicular to the boundary

plane:

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An actual boundary is a “mixture” of these two ideal types.

Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.

Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.

Grains, divided by small-angle boundaries are also called subgrains.

Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.

Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.

Diffusion along grain boundaries is much faster, than throughout the grains.

Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.

Discuss the article and ask questions in our Materials Forum

Related internal links Metals crystal structure Solid solutions Diffusion in alloys Grain structure Crystal lization

SPECIMEN PREPARATION for SCANNING ELECTRON MICROSCOPY

 Materials for Sample Preparation

A list of equipment and materials necessary for preparation of polished specimens is given in Table 1. For some items, substitution may be possible if comparable supplies are available in the laboratory. The list

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is presented in order of use of the equipment or supplies.

 Table 1. Equipment and Supplies for Preparation of Polished Sections

Item Purpose

Diamond blade slab saw large-sized sample slabbingDiamond blade wafering saw

cutting of thin (mm-sized) sections

Propylene glycol diamond saw cutting lubricantAlcohol: 200 proof ethanol

cutting lubricant, cleaning aid

Ultrasonic bath specimen cleaningSpecimen jars and lids for replacement stepsPotting epoxies (medium and low viscosity)

for powders and hardened pastes

Dye, blue or red, alcohol miscible

to estimate alcohol replacement depth

Refrigerator epoxy storageVacuum chamber and pump

vacuum impregnation

Drying / curing oven capable of at least 65 ºCGlass plate (400 x 400 mm)

smooth surface for grinding

Lapidary wheel (minimum 200 mm)

grinding and polishing

Mold cups potting specimensAluminum foil (extra heavy duty)

for forming odd-sized specimen molds</TD< tr>

Mold releasefacilitates removal of specimen / epoxy

Metal trays to hold specimens

contains any leaking epoxy

Diamond pen label engravingAbrasive papers (silicon carbide)

coarse to fine grinding, 100 to 600 grit

Polishing cloths (low-relief)

µm and finer polishing

Diamond paste for 6, 3, 1, 0.25 µm in non-aqueous

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polishing suspensionLint-free cloths specimen handling and cleaningCompressed air specimen cleaning and dryingVacuum dessiccators specimen storage

UNIT – II

MECHANICAL PROPERTIES OF MATERIALS &

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FRACTURE

Mechanism of slip and twinning

The objectives are to: (1) demonstrate the mechanisms of deformation in body centered cubic (BCC), face centered cubic (FCC), and hexagonal close-packed (HCP)-structure metals and alloys and in some ceramics as

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well; (2) examine the deformed microstructures (slip lines and twin boundaries) in different grains of metallic and ceramic specimens; and (3) study visually the deformed macrostructure (slip and twin bands) of metals and alloys. Some of the topics covered include: deformation behavior of materials, mechanisms of plastic deformation, slip bands, twin bands, ductile failure, intergranular fracture, shear failure, slip planes, crystal deformation, and dislocations in ceramics.

KEYWORDS: Alloys, Body Centered Cubic Lattices, Ceramics, Crystal Defects, Crystal Structure, Edge Dislocations, Face Centered Cubic Lattices, Mechanical Twinning, Metals, Microstructure, Ductility, Failure Analysis, Fracturing, Packing Density, Plastic Deformation

Strengthening mechanisms

Work hardening

Main article: Work hardening

The primary species responsible for work hardening are dislocations. Dislocations interact with each other by generating stress fields in the material. The interaction between the stress fields of dislocations can impede dislocation motion by repulsive or attractive interactions. Additionally, if two dislocations cross, dislocation line entanglement occurs, causing the formation of a jog which opposes dislocation motion. These entanglements and jogs act as pinning points, which oppose dislocation motion. As both of these processes are more likely to occur when more dislocations are present, there is a correlation between dislocation density and yield strength, where G is the shear modulus, b is the Burgers vector, and is the dislocation density.

Increasing the dislocation density increases the yield strength which results in a higher shear stress required to move the dislocations. This process is easily observed while working a material. Theoretically, the strength of a material with no dislocations will be extremely high (τ=G/2) because plastic deformation would require the breaking of many bonds simultaneously. , at moderate dislocation density values of around 107-109 dislocations/m2, the material will exhibit a significantly lower mechanical strength. Analogously, it is easier to move a rubber rug across a surface by propagating a small ripple through it than by dragging the whole rug. At dislocation densities of 1014 dislocations/m2 or higher, the strength of the material becomes high once again. It should be noted that the dislocation density can't be infinitely high because then the material would lose its crystalline structure.

Figure 1: This is a schematic illustrating how the lattice is strained by

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the addition of substitutional and interstitial solute. Notice the strain in the lattice that the solute atoms cause. The interstitial solute could be carbon in iron for example. The carbon atoms in the interstitial sites of the lattice creates a stress field that impedes dislocation movement.

[edit] Solid Solution Strengthening/Alloying

Main article: Solid solution strengthening

For this strengthening mechanism, solute atoms of one element are added to another, resulting in either substitutional or interstitial point defects in the crystal (see Figure 1). The solute atoms cause lattice distortions that impede dislocation motion, increasing the yield stress of the material. Solute atoms have stress fields around them which can interact with those of dislocations. The presence of solute atoms impart compressive or tensile stresses to the lattice, depending on solute size, which interfere with nearby dislocations, causing the solute atoms to act as potential barriers to dislocation propagation and/or multiplication.

The shear stress required to move dislocations in a material is:

Where c is the solute concentration and ε is the strain on the material caused by the solute.

Increasing the concentration of the solute atoms will increase the yield strength of a material, but there is a limit to the amount of solute that can be added, and one should look at the phase diagram for the material and the alloy to make sure that a second phase is not created.

In general, the solid solution strengthening depends on the concentration of the solute atoms, shear modulus of the solute atoms, size of solute atoms, valency of solute atoms (for ionic materials), and the symmetry of the solute stress field. Note that the magnitude of strengthening is higher for non-symmetric stress fields because these solutes can interact with both edge and screw dislocations whereas symmetric stress fields, which cause only volume change and not shape change, can only interact with edge dislocations.

Figure 2: This is a schematic illustrating how the dislocations can interact with a particle. It can either cut through the particle or bow around the particle and create a dislocation loop as it moves over the particle.

[edit] Precipitation Hardening

Main article: Precipitation strengthening

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In most binary systems, alloying above a concentration given by the phase diagram will cause the formation of a second phase. A second phase can also be created by mechanical or thermal treatments. The particles that compose the second phase precipitates act as pinning points in a similar manner to solutes, though the particles are not necessarily single atoms.

The dislocations in a material can interact with the precipitate atoms in one of two ways (see Figure 2). If the precipitate atoms are small, the dislocations would cut through them. As a result, new surfaces (b in Figure 2) of the particle would get exposed to the matrix and the particle/matrix interfacial energy would increase. For larger precipitate particles, looping or bowing of the dislocations would occur which results in dislocations getting longer. Hence, at a critical radius of about 5 nm, dislocations will preferably cut across the obstacle while for a radius of 30 nm, the dislocations will readily bow or loop to overcome the obstacle.

The mathematical descriptions are as follows:

For Particle Bowing-

For Particle Cutting-

Figure 3: This is a schematic roughly illustrating the concept of dislocation pile up and how it effects the strength of the material. A material with larger grain size is able to have more dislocation to pile up leading to a bigger driving force for dislocations to move from one grain to another. Thus you will have to apply less force to move a dislocation from a larger than from a smaller grain, leading materials with smaller grains to exhibit higher yield stress.

[edit] Grain Boundary Strengthening

Main article: Grain boundary strengthening

In a polycrystalline metal, grain size has a tremendous influence on the mechanical properties. Because grains usually have varying crystallographic orientations, grain boundaries arise. While an undergoing deformation, slip motion will take place. Grain boundaries act as an impediment to dislocation motion for the following two reasons:1. Dislocation must change its direction of motion due to the differing orientation of grains.[4]

2. Discontinuity of slip planes from grain 1 to grain 2.[4]

The stress required to move a dislocation from one grain to another in order to plastically deform a material depends on the grain size. The average number of dislocations per grain decreases with average grain

25

size (see Figure 3). A lower number of dislocations per grain results in a lower dislocation 'pressure' building up at grain boundaries. This makes it more difficult for dislocations to move into adjacent grains. This relationship is the Hall-Petch Relationship and can be mathematically described as follows:

Where k is a constant, d is the average grain diameter and σy,0 is the original yield stress.

The fact that the yield strength increases with decreasing grain size is accompanied by the caveat that the grain size cannot be decreased infinitely. As the grain size decreases, more free volume is generated resulting in lattice mismatch. Below approximately 10 nm, the grain boundaries will tend to slide instead; a phenomenon known as grain-boundary sliding. If the grain size gets too small, it becomes more difficult to fit the dislocations in the grain and the stress required to move them is less. It was not possible to produce materials with grain sizes below 10 nm until recently, so the discovery that strength decreases below a critical grain size is still exciting.

[edit] Strengthening Mechanisms in Amorphous Materials

[edit] Polymer

Polymers fracture via breaking of inter- and intra molecular bonds; hence, the chemical structure of these materials plays a huge role in increasing strength. For polymers consisting of chains which easily slide past each other, chemical and physical cross linking can be used to increase rigidity and yield strength. In thermoset polymers (thermosetting plastic), disulfide bridges and other covalent cross links give rise to a hard structure which can withstand very high temperatures. These cross-links are particularly helpful in improving tensile strength of materials which contain lots of free volume prone to crazing, typically glassy brittle polymers [6]. In thermoplastic elastomer, phase separation of dissimilar monomer components leads to association of hard domains within a sea of soft phase, yielding a physical structure with increased strength and rigidity. If yielding occurs by chains sliding past each other (shear bands), the strength can also be increased by introducing kinks into the polymer chains via unsaturated carbon-carbon bonds [6].

Increasing the bulkiness of the monomer unit via incorporation of aryl rings is another strengthening mechanism. The anisotropy of the molecular structure means that these mechanisms are heavily dependent

26

on the direction of applied stress. While aryl rings drastically increase rigidity along the direction of the chain, these materials may still be brittle in perpendicular directions. Macroscopic structure can be adjusted to compensate for this anisotropy. For example, the high strength of Kevlar arises from a stacked multilayer macrostructure where aromatic polymer layers are rotated with respect to their neighbors. When loaded oblique to the chain direction, ductile polymers with flexible linkages, such as oriented polyethylene, are highly prone to shear band formation, so macroscopic structures which place the load parallel to the draw direction would increase strength[6].

Mixing polymers is another method of increasing strength, particularly with materials that show crazing preceding brittle fracture such as atactic polystyrene (APS). For example, by forming a 50/50 mixture of APS with polyphenylene oxide (PPO), this embrittling tendency can be almost completely suppressed, substantially increasing the fracture strength [6].

[edit] Glass

Many silicate glasses are strong in compression but weak in tension. By introducing compression stress into the structure, the tensile strength of the material can be increased. This is typically done via two mechanisms: thermal treatment (tempering) or chemical bath (via ion exchange).

In tempered glasses, air jets are used to rapidly cool the top and bottom surfaces of a softened (hot) slab of glass. Since the surface cools quicker, there is more free volume at the surface than in the bulk melt. The core of the slap then pulls the surface inward, resulting in an internal compressive stress at the surface. This substantially increases the tensile strength of the material as tensile stresses exterted on the glass must now resolve the compressive stresses before yielding.

σy = modified = σy,0 + σcompressive

Alternately, in chemical treatment, a glass slab treated containing network formers and modifiers is submerged into a molten salt bath containing ions larger than those present in the modifier. Due to a concentration gradient of the ions, mass transport must take place. As the larger cation diffuses from the molten salt into the surface, it replaces the smaller ion from the modifier. The larger ion squeezing into surface introduces compressive stress in the glass's surface. A common example is treatment of sodium oxide modified silicate glass in molten

27

potassium chloride.

[edit] Applications & Current Research

Strengthening of materials is useful in many applications. One main application of strengthened materials is for construction. In order to have stronger buildings and bridges, one must have a strong frame that can support high tensile or compressive load and resist plastic deformation. The steel frame used to make the building should be as strong as possible so that it does not bend under the entire weight of the building. Polymeric roofing materials would also need to be strong so that the roof does not cave in when there is build-up of snow on the rooftop.

Research is also currently being done to increase the strength of metallic materials through the addition of polymer materials such as bonded carbon fiber reinforced polymer to (CFRP)[1].

[edit] Molecular Dynamics Simulations

The use of computation simulations to model work hardening in materials allows for the direct observation of critical elements that rule the process of strengthening materials. The basic reasoning derives from the fact that, when examining plasticity and the movement of dislocations in materials, a focus on the atomistic level is many times not accounted for and the focus rests on the contiuum description of materials. Since the practice of tracking these atomistic effects in experiments and theorizing about them in textbooks cannot provide a full understanding of these interactions, many turn to molecular dynamics simulations to develop this understanding.[7]

The simulations work by utilizing the known atomic interactions between any two atoms and the relationship F = ma, so that the dislocations moving through the material are ruled by simple mechanical actions and reactions of the atoms. The interatomic potential usually utilized to estimate these interactions is the Lennard – Jones 12:6 potential. Lennard – Jones is widely accepted because its experimental shortcomings are well-known.[7][8] These interactions are simply scaled up to millions or billions of atoms in some cases to simulate materials more accurately.

Molecular dynamic simulations display the interactions based upon the governing equations provided above for the strengthening mechanisms. They provide an effective way to see these mechanisms in action outside the painstaking realm of direct observation during experiments.

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recover recrystallization and Grain growth

RECOVERY, RECRYSTALLIZATION, AND GRAIN GROWTH are microstructural changes that occur during annealing after cold plastic deformation and/or during hot working. These three mechanisms are sometimes referred to as restoration processes, because they restore the microstructural configuration to a lower energy level. All three processes involve diffusion and thus depend on thermal activation to cause rearrangement of dislocations and grain boundaries. The mechanisms of recovery and recrystallization also depend on the extent of plastic deformation (either during hot working or by cold work prior to annealing). In contrast, grain growth is not in direct response to deformation, but it is a thermally driven restoration process that results in lower surface energy of individual grains. Recovery and recrystallization can occur during hot working or during annealing after cold plastic deformation. When a metal is cold worked by plastic deformation, a small portion of the mechanical energy expended in deforming the metal is stored in the specimen. This stored energy resides in the crystals as point defects (vacancies and interstitials), dislocations, and stacking faults in various forms and combinations, depending on the metal (see the article “Plastic Deformation Structures” in this Volume). Therefore, a cold-worked specimen, being in a state of higher energy, is thermodynamically unstable. With thermal activation, such as provided by annealing, the cold-worked specimen tends to transform to states of lower energies through a sequence of processes with microstructural changes, as shown schematically in Fig.. Such classification is approximate; some overlapping between the stages usually occurs because of microstructural nonhomogeneity of the specimen. To some extent, the annealing behavior of a metal may be different from metal to metal and for the same metal of different purity, but the basic phenomena involved in the various annealing stages are similar. During recovery, accumulated strain is relieved to some extent by microstructural and submicroscopic rearrangements, but the grains are not entirely strain-free. At higher temperatures, strain-free grains are created during the restoration process of recrystallization. Along with the microstructural changes, the properties of the specimen also change correspondingly (Fig.). Thus, deformation and annealing are important processing methods for producing desired properties of the material by controlling its microstructures. Similar restoration process can also occur during hot working. This is shown in Fig. for hot working with moderate amount of reduction (strain) during working (Fig.a) and high strain (Fig.b). The regions of static recovery and recrystallization, which occur after deformation, are analogous to restoration of worked structure by annealing. In addition, dynamic recovery can occur during deformation at high temperature. Figure also illustrates the occurrence of either static or dynamic recrystallization at moderate or high strains,

29

respectively, depending on the stacking-fault energy of a metal. Stacking faults in crystalline structures are planar-type defects that influence hardening and recrystallization (see the article “Plastic Deformation Structures” in this Volume).

From: ASM Handbook Volume 14A, Metalworking: Bulk Forming (ASM International)Published: 2005Pages: 552-562

Strain hardening

The strain hardening exponent (also called strain hardening index), noted as n, is a materials constant which is used in calculations for stress-strain behaviour in work hardening.

In the formula σ = K ε n, σ represents the applied stress on the material, ε is the strain and K is the strength coefficient. The value of the strain hardening exponent lies between 0 and 1. A value of 0 means that a material is a perfectly plastic solid, while a value of 1 represents a 100% elastic solid. Most metals have an n value between 0.10 and 0.50.

Stress–strain curve

From Wikipedia, the free encyclopedia  (Redirected from Stress-strain curve)Jump to: navigation, search

Fig.1 Stress–strain curve showing typical yield behavior for nonferrous alloys. Stress (σ) is shown as a function of strain (ε)1: True elastic limit2: Proportionality limit3: Elastic limit4: Offset yield strength

This article does not cite any references or sources.

Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

During tensile testing of a material sample, the stress–strain curve is a graphical representation of the relationship between stress, derived from measuring the load applied on the sample, and strain,

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derived from measuring the deformation of the sample, i.e. elongation, compression, or distortion. The area under the elastic portion of the curve is known as the modulus of resilience.

The nature of the curve varies from material to material. The following diagrams illustrate the stress–strain behaviour of typical materials in terms of the engineering stress and engineering strain where the stress and strain are calculated based on the original dimensions of the sample and not the instantaneous values.

Contents[hide]

1 Ductile materials 2 Brittle materials 3 See also

4 External links [edit] Ductile materials

Fig 2. A stress–strain curve typical of structural steel1. Ultimate Strength2. Yield Strength3. Rupture4. Strain hardening region5. Necking region.A: Apparent stress (F/A0)B: Actual stress (F/A)

Steel generally exhibits a very linear stress–strain relationship up to a well defined yield point (Fig.2). The linear portion of the curve is the elastic region and the slope is the modulus of elasticity or Young's Modulus. After the yield point, the curve typically decreases slightly because of dislocations escaping from Cottrell atmospheres. As deformation continues, the stress increases on account of strain hardening until it reaches the ultimate strength. Until this point, the cross-sectional area decreases uniformly because of Poisson contractions. The actual rupture point is in the same vertical line as the visual rupture point.

However, beyond this point a neck forms where the local cross-sectional area decreases more quickly than the rest of the sample resulting in an increase in the true stress. As shown in Fig.2, On an engineering stress–strain curve this is seen as a decrease in the apparent stress. However if the curve is plotted in terms of true stress and true strain the stress will continue to rise until failure. Eventually the neck

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becomes unstable and the specimen ruptures (fractures).

Less ductile materials such as aluminum and medium to high carbon steels do not have a well-defined yield point. For these materials the yield strength is typically determined by the "offset yield method", by which a line is drawn parallel to the linear elastic portion of the curve and intersecting the abscissa at some arbitrary value (most commonly 0.2%). The intersection of this line and the stress–strain curve is reported as the yield point. Plastic region is the point where the material will stay deformed, The elastic region is the point where the material can stretch no further. Failure point is when the object breaks.

[edit] Brittle materials

Fig.3 Stress Strain Curve for Brittle materials

Brittle materials such as concrete and carbon fiber do not have a yield point, and do not strain-harden. Therefore the ultimate strength and breaking strength are the same. A most unusual stress-strain curve is shown in Fig.3. Typical brittle materials like glass do not show any plastic deformation but fail while the deformation is elastic. One of the characteristics of a brittle failure is that the two broken parts can be reassembled to produce the same shape as the original component as there will not be a neck formation like in the case of ductile materials. A typical stress strain curve for a brittle material will be linear. Testing of several identical specimen, cast iron, or soil, tensile strength is negligible compared to the compressive strength and it is assumed zero for many engineering applications. Glass fibers have a tensile strength stronger than steel, but bulk glass usually does not. This is because of the stress intensity factor associated with defects in the material. As the size of the sample gets larger, the size of defects also grows. In general, the tensile strength of a rope is always less than sum of the tensile strength of its individual fibers.

Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving required properties.

There are two general objectives of heat treatment: hardening and annealing.

Hardening

Hardening is a process of increasing the metal hardness, strength,

32

toughness, fatigue resistance.

Strain hardening (work hardening) – strengthening by cold-work (cold plastic deformation).

Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.

Grain size strengthening (hardening) – strengthening by grain refining.

Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.

Solid solution hardening – strengthening by dissolving an alloy-ing element.

Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements are more effective in solid solution hardening, than substitution elements.

Dispersion strengthening – strengthening by addition of second phase into metal matrix.

The second phase boundaries resist the dislocations motions, increasing the material strength. The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix (composite materials).

Hardening as a result of Spinodal decomposition. Spinodal structure is characterized by strains on the coherent boundaries between the spinodal phases causing hardening of the alloy.

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Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersatu-rated solid solution.

The second phase boundaries resist the dislocations motions, increasing the material strength.

The age hardening mechanism in Al-Cu alloys may be illustrated by the phase diagram of Al-Cu system (see figure below)

When an alloy Al-3%Cu is heated up to the temperature TM, all CuAl2 particles are dissolved and the alloy exists in form of single phase solid solution (α-phase). This operation is called solution treatment.

Slow cooling of the alloy will cause formation of relatively coarse particles of CuAl2 intermetallic phase, starting from the temperature TN.

However if the the cooling rate is high (quenching), solid solution will retain even at room temperature TF. Solid solution in this non-equilibrium state is called supersaturated solid

Solution. Obtaining of supersaturated solid solution is possible when cooling

is considerably faster, than diffusion processes.

As the diffusion coefficient is strongly dependent on the temperature, the precipitation of CuAl2 from supersaturated solution is much faster at elevated temperatures (lower than TN).This process is called artificial aging. It takes usually a time from several hours to one day.

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When the aging is conducted at the room temperature, it is called natural aging. Natural aging takes several days or more.

Precipitation from supersaturated solid solution occurred in several steps:

Segregation of Cu atoms into plane clusters. These clusters are called Guinier-Preston1 zones (G-P1 zones).

Diffusion of Cu atoms to the G-P1 zones and formation larger clus-ters, called GP2 zones or θ” phase. This phase is coherent with the matrix .

Formation of θ’ phase which is partially coherent with the matrix. This phase provides maximum hardening.

Annealing

Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealing temperature), followed by controlled cooling.

Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the grain structure into more stable state.

Annealing stages:

Stress relief (recovery) – a relatively low temperature process of reducing internal mechanical stresses, caused by cold-work, cast-ing or welding.

During this process atoms move to more stable positions in the crystal lattice. Vacancies and interstitial defects are eliminated and some dislocations are annihilated.

Recovery heat treatment is used mainly for preventing stress-corrosion cracking and decreasing distortions, caused by internal stresses.

Recrystallization – alteration of the grain structure of the metal.

If the alloy reaches a particular temperature (recrystallization or annealing temperature) new grains start to grow from the nuclei formed in the cold worked metal. The new grains absorb imperfections and distortions caused by cold deformation. The grains are equi-axed and independent to the old grain structure.

As a result of recrystallization mechanical properties (strength,

35

ductility) of the alloy return to the pre-cold-work level.

The annealing temperature and the new grains size are dependent on the degree of cold-work which has been conducted. The more the cold-work degree, the lower the annealing temperature and the fine recrystallization grain structure. Low degrees of cold-work (less than 5%) may cause formation of large grains.

Usually the annealing temperature of metals is between one-third to one-half of the freezing point measured in Kelvin (absolute) temperature scale.

Grain growth (over-annealing, secondary recrystallization) – growth of the new grains at the expense of their neighbors, occur-ring at temperature, above the recrystallization temperature.

This process results in coarsening grain structure and is undesirable.

Seminar on Brittle and Ductile Fracture Manipal Institute of Technology Department of Mechanical Engineering Effect of Specimen Orientation

The notched impact properties of rolled or forged products vary

with the orientation in the plate or bar. The figure shows typical form of

energy-temperature curves for specimen cit in longitudinal and

transverse directions of the rolled plate. Specimen A and B are oriented

in longitudinal directions. The graphs shows that considerably large

differences are expected for different specimen orientations at high

energy levels, but difference becomes much less at energy levels below

30J. Since the ductility transition temperatures are evaluated in this

region of energy, it seems that specimen and notch orientation are not

36

very important. If, however materials are compared on the basis of room

temperature impact properties, orientation can greatly affect the results.

Effect of orientation of specimen on transition temperature

Seminar on Brittle and Ductile Fracture Manipal Institute of Technology Department of Mechanical Engineering Effect of specimen thickness

Probably the chief deficiency of Charpy impact test is that the small specimen is not always a realistic model of the actual situation. Not only does the small specimen lead to considerable scatter, but a specimen with a thickness of 10mm cannot provide the same constraint as would be found in a structure with a much greater thickness, at a particular service temperature the standard Charpy specimen shows a high shelf energy, while actually the same material in a thick section structure has low toughness at the same temperature. Effect of section thickness on transition-temperature curves

Mechanism of Creep in Brittle Rock

A review of the experimental evidence suggests that creep in brittle rock at low temperature is due to time-dependent cracking. A transient creep law is derived from the mechanism of time-dependent cracking in an inhomogeneous brittle material. The behavior is described as a Markov process with a stationary transition probability that is obtained from experimental observations of static fatigue in glasses. The model is compared with experimental observations and found to predict the observed stress dependence of creep.

37

Abstract

Creep-resistant materials are used in machines and facilities operated at high temperatures e.g. power engineering equipment. They must be able to withstand the highest possible operating loads at elevated temperatures and also be sufficiently resistant to high-temperature corrosion. In contrast to heatresistant materials, the mechanical properties of creep-resistant materials are of prime importance.

Fulltext Preview

S–N curve

The S–N curve obtained from cantilever-type rotary bending fatigue tests

using hour-glass-shaped specimens of high carbon-chromium bearing

steel clearly distinguished the fracture modes into two groups each

having a different crack origin. One was governed by crystal slip on the

specimen surface, which occurred in the region of short fatigue life and a

high stress amplitude level. The other was governed by a non-metallic

inclusion at a subsurface level which occurred in the region of long

fatigue life and low stress amplitude. The inclusion developed a fish-eye

fracture mode that was distributed over a wide range of stress amplitude

not only below the fatigue limit defined as the threshold for fracture due

to the surface slip mode but also above the fatigue limit. This remarkable

shape of the S–N curve was different from the step-wise one reported in

previous literature and is characterized as a duplex S–N curve composed

of two different S–N curves corresponding to the respective fracture

modes. From detailed observations of the fracture surface and the

fatigue crack origin, the mechanisms for the internal fracture mode and

the characteristics of the S–N curve are discussed.

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UNIT – III

FERROUS AND NON FERROUS METALS AND PHASE

DIAGRAM

KEY to METALS Search by Metal Properties

You can quickly and easily search over 4 million metal properties records by designation, countries/standards, type, standard number, chemical composition, mechanical properties, other properties or any combination of these criteria. For example, let’s look for a Chinese stainless steel, which needs to have C < 0.05, Cr > 16%, Ni > 8%, tensile stress over 750 MPa.

Click Advanced Search from the main window. Next, click STEEL radio

39

button, choose China/GB in the Country/Standard list. Check boxes Stainless Steel, and enter requested steel properties and alloying elements.

The search results screen appears. Click on a steel from the list to review its properties, in this example the first will be selected – S31723.

After clicking on the material, a list of subgroups appears. In KEY to METALS, the term “subgroups” refers to standard specifications that define steel properties; in this case the specification GB/T 4234 is selected. Note that properties defined according to different specifications may differ significantly.

Steel properties within the KEY to METALS Database include composition, cross-reference tables, mechanical properties, physical properties, and even creep and fatigue properties. Click on the examples below to enlarge them.

There is no simple definition of metal; however, any chemical element having "metallic properties" or "metal properties" is classed as a metal. Typical metal properties are luster, good thermal and electrical conductivity, and the capability of being permanently shaped or deformed at room temperature.

The properties of metals make them suitable for different uses in daily life.

Chemical elements lacking the typical metal properties are classed as nonmetals. A few elements, known as metalloids, sometimes behave like a metal and at other times like a nonmetal. Some examples of metalloids are as follows: carbon, phosphorus, silicon, and sulfur.

The properties of different metals can be combined by mixing two or more of them together. The resulting substance is called an alloy. Pure

40

elemental metals are often too soft to be of practical use which is why much of metallurgy focuses on formulating useful alloys.

Steel, for example, is a mixture of iron and small amounts of carbon and other elements. Other alloys like brass (copper and zinc) and bronze (copper and tin) are easy to shape and beautiful to look at. Bronze is also used frequently in ship-building because it is resistant to corrosion from sea water.

Titanium is much lighter and less dense than steel, but as strong; and although heavier than aluminum, it's also twice as strong. It's also very resistant to corrosion. All these factors make it an excellent alloy material. Titanium alloys are used in aircraft, ships, and spacecraft, as well as paints, bicycles, and even laptop computers!

Copper is a good conductor of electricity and is ductile. Therefore copper is used for electrical cables.

Gold and silver are very malleable, ductile and very non-reactive. Gold and silver are used to make intricate jewelry. Gold is especially suitable for this purpose since it does not tarnish. Gold can also be used for oxidation-free electrical connections.

Iron and steel are both hard and strong. Therefore they are used to construct bridges and buildings. A disadvantage of using iron is that it tends to rust, whereas most steels rust, but they can be formulated to be rust free.Aluminum is a good conductor of heat and is malleable. It is used to make saucepans and foil, and also airplane bodies due to the fact that is very light.

Stainless steel or galvanized steel are used where resistance to corrosion is important. Aluminum alloys and magnesium alloys are used for applications where strength and lightness are required.

Copper-nickel alloys such as Monel are used in highly corrosive environments and for non-magnetic applications. Nickel-based superalloys like Inconel are used in high temperature applications such as turbochargers, pressure vessels, and heat exchangers. For extremely high temperatures, single crystal alloys are used to minimize creep.

Maraging Steels:

Maraging steels (a portmanteau of martensitic and aging) are iron alloys which are known for possessing superior strength and toughness without losing malleability, although they can not hold a good cutting

41

edge. 'Aging' refers to the extended heat-treatment process. These steels are a special class of low-carbon ultra-high-strength steels which derive their strength not from carbon, but from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel.[1]

Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt, molybdenum, and titanium.[1] Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made.

The common, non-stainless grades contain 17–19% nickel, 8–12% cobalt, 3–5% molybdenum, and 0.2–1.6% titanium. Addition of chromium produces stainless grades resistant to corrosion. This also indirectly increases hardenability as they require less nickel: high-chromium, high-nickel steels are generally austenitic and unable to transform to martensite when heat treated, while lower-nickel steels can transform to martensite.

Properties

Due to the low carbon content maraging steels have good machinability. Prior to aging, they may also be cold rolled to as much as 80–90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the properties of heat affected zone.[1]

When heat-treated the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content the alloys have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. They can also be nitrided to increase case hardness. They can be polished to a fine surface finish.Non-stainless varieties of maraging steels are moderately corrosion-resistant, and resist stress corrosion and hydrogen embrittlement. More corrosion protection can be gained by cadmium plating or phosphating.

[edit] Heat treatment cycle

The steel is first annealed at approximately 820 °C (1,510 °F) for 15–30 minutes for thin sections and for 1 hour per 25 mm thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling to room temperature to form a soft, heavily-dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (precipitation hardening) of the more common alloys for approximately 3 hours at a temperature of 480 to 500 °C produces a fine dispersion of Ni3(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute elements added for such precipitation. Overaging leads to a reduction in stability of the primary,

42

metastable, coherent precipitates, leading to their dissolution and replacement with semi-coherent Laves phases such as Fe2Ni/Fe2Mo. Further excessive heat-treatment brings about the decomposition of the martensite and reversion to austenite.

Newer compositions of maraging steels have revealed other intermetallic stoichiometries and crystallographic relationships with the parent martensite, including rhombohedral and massive complex Ni50(X,Y,Z)50, or Ni50M50 in simplified notation.

[edit] Uses

Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into thinner rocket and missile skins than other steels, reducing weight for a given strength. Maraging steels have very stable properties, and, even after overaging due to excessive temperature, only soften slightly. These alloys retain their properties at mildly elevated operating temperatures and have maximum service temperatures of over 400 °C (752 °F).[citation needed] They are suitable for engine components, such as crankshafts and gears, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial load. Their uniform expansion and easy machinability before aging make maraging steel useful in high-wear components of assembly lines and dies. Other ultra-high-strength steels, such as Aermet alloys, are not as machinable because of their carbide content.

In the sport of fencing, blades used in competitions run under the auspices of the Fédération Internationale d'Escrime are often made with maraging steel. Maraging blades are required in foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less blade breakage and fewer injuries. The notion that such blades break flat is a fencing urban legend: testing has shown that the blade-breakage patterns in carbon steel and maraging steel blades are identical[citation needed]. Stainless maraging steel is used in bicycle frames and golf club heads. It is also used in surgical components and hypodermic syringes, but is not suitable for scalpel blades because the lack of carbon prevents it from holding a good cutting edge.

Maraging steel production, import, and export by certain states, such as the United States,[2] is closely monitored by international authorities because it is particularly suited for use in gas centrifuges for uranium enrichment; lack of maraging steel significantly hampers this process. Older centrifuges used aluminum tubes; modern ones, carbon fiber composite.

Physical properties

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Density : 8.1 g/cm³ (0.29 lb/in³) Specific heat , mean for 0–100 °C (32–212 °F): 813 J/(kg·K) (0.108

Btu/(lb·°F)) Melting point : 2575 °F, 1413 °C Thermal conductivity : 25.5 W·m/(m²·K) Mean coefficient of thermal expansion: 11.3×10−6 Yield tensile strength : typically 1030–2420 MPa (150,000–350,000

psi)[3] Ultimate tensile strength: typically 1600–2500 MPa (230,000–

360,000 psi). Grades exist up to 3.5 GPa (500,000 psi) Elongation at break: up to 15% KIC fracture toughness: up to 175 MPa-m½ Young's modulus : 210 GPa[4] Shear modulus : 77 GPa Bulk modulus : 140 GPa Hardness (aged): 50 HRC (grade 250); 54 HRC (grade 300); 58

HRC (grade 350)

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γράφειν (graphein): "to draw/write", for its use in pencils, where it is commonly called lead (not to be confused with the metallic element lead). Unlike diamond (another carbon allotrope), graphite is an electrical conductor, a semimetal. Thus, e.g., it is useful in arc lamp electrodes. Graphite is the most stable form of carbon under standard conditions. Therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds. Graphite may be considered the highest grade of coal, just above anthracite and alternatively called meta-anthracite, although it is not normally used as fuel because it is hard to ignite.

There are three principal types of natural graphite, each occurring in different types of ore deposit:

1. Crystalline flake graphite (or flake graphite for short) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken and when broken the edges can be irregular or angular;

2. Amorphous graphite occurs as fine particles and is the result of thermal metamorphism of coal, the last stage of coalification, and is sometimes called meta-anthracite. Very fine flake graphite is sometimes called amorphous in the trade;

3. Lump graphite (also called vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin.

Highly ordered pyrolytic graphite or highly oriented pyrolytic graphite

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(HOPG) refers to graphite with an angular spread between the graphite sheets of less than 1°. This highest-quality synthetic form is used in scientific research.[3] The name "graphite fiber" is also sometimes used to refer to carbon fiber or carbon fiber-reinforced polymer.

Occurrence

Graphite output in 2005

Minerals associated with graphite include quartz, calcite, micas, iron meteorites, and tourmalines. Graphite has various other characteristics. Thin flakes are flexible but inelastic, the mineral can leave black marks on hands and paper, it conducts electricity, and displays superlubricity. Its best field indicators are softness, luster, density and streak.

According to the United States Geological Survey (USGS), world production of natural graphite in 2008 was 1,110 thousand tonnes (kt), of which the following major exporters are: China (800 kt), India (130 kt), Brazil (76 kt), North Korea (30 kt) and Canada (28 kt). Graphite is not mined in US, but US production of synthetic graphite in 2007 was 198 kt valued at $1.18 billion. US graphite consumption was 42 kt and 200 kt for natural and synthetic graphite, respectively.[4]

Scanning tunneling microscope image of graphite surface atoms

graphite's unit cell

ball-and-stick model of graphite (2 graphene layers)

side view of layer stacking

[edit] Properties

Graphite has a layered, planar structure. In each layer, the carbon atoms are arranged in a hexagonal lattice with separation of 0.142 nm, and the distance between planes is 0.335 nm.[5] The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral), have very similar physical properties (except that the graphene layers stack slightly differently).[6] The hexagonal graphite may be either flat or buckled.[7]

The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated

45

above 1300 °C.[8] The layering contributes to its lower density.[clarification

needed]

The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another.Graphite can conduct electricity due to the vast electron delocalization within the carbon layers (a phenomenon called aromaticity). These valence electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.

Graphite and graphite powder are valued in industrial applications for its self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are solely due to the loose interlamellar coupling between sheets in the structure.[9] However, it has been shown that in a vacuum environment (such as in technologies for use in space), graphite is a very poor lubricant. This observation led to the discovery that the lubrication is due to the presence of fluids between the layers, such as air and water, which are naturally adsorbed from the environment. This molecular property is unlike other layered, dry lubricants such as molybdenum disulfide. Recent studies suggest that an effect called superlubricity can also account for graphite's lubricating properties. The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steel,[10][11] and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity). It is also corrosive to aluminium in the presence of moisture. For this reason, the US Air Force banned its use as a lubricant in aluminium aircraft,[12] and discouraged its use in aluminium-containing automatic weapons.[13] Even graphite pencil marks on aluminium parts may facilitate corrosion.[14] Another high-temperature lubricant, hexagonal boron nitride, has the same molecular structure as graphite. It is sometimes called white graphite, due to its similar properties.

When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon. This material is useful for blood-contacting implants such as artificial heart valves. It is also highly diamagnetic, thus it will float in mid-air above a strong magnet.

Natural and crystalline graphites are not often used in pure form as structural materials, due to their shear-planes, brittleness and inconsistent mechanical properties.

[edit] History

46

Graphite was used by the 4th millennium B.C. Marica culture to create a ceramic paint to decorate pottery during the Neolithic Age in southeastern Europe.[15]

Some time before 1565 (some sources say as early as 1500), an enormous deposit of graphite was discovered on the approach to Grey Knotts from the hamlet of Seathwaite in Borrowdale parish, Cumbria, England, which the locals found very useful for marking sheep.[16][17] This particular deposit of graphite was extremely pure and soft, and could easily be broken into sticks. This remains the only deposit of graphite found in this packed form.[18]

[edit] Other names

Graphite has also been known historically under other names: blacklead and plumbago.[19]

Plumbago was commonly used for its massive mineral form. Both of these names arise from confusion with the similar-appearing lead ores, particularly galena. The Latin word for lead is plumbum, which gave its name to both the English term for this grey metallic-sheened mineral and even the leadworts or plumbagos, plants with flowers that resemble this colour.

Blacklead has usually been applied to a powdered or processed form, where this fine powder then appears as a matt non-metallic black.

[edit] Uses of natural graphite

Natural graphite is mostly consumed for refractories, steelmaking, expanded graphite, brake linings, foundry facings and lubricants.[4]

Graphene, which occurs naturally in graphite, has unique physical properties and might be one of the strongest substances known; however, the process of separating it from graphite will require some technological development before it is economically feasible to use it in industrial processes.

[edit] Refractories

This end-use begins before 1900 with the graphite crucible used to hold molten metal; this is now a minor part of refractories. In the mid 1980s, the carbon-magnesite brick became important, and a bit later the alumina-graphite shape. Currently the order of importance is alumina-graphite shapes, carbon-magnesite brick, monolithics (gunning and ramming mixes), and then crucibles.

Crucibles began using very large flake graphite, and carbon-magnesite

47

brick requiring not quite so large flake graphite; for these and others there is now much more flexibility in size of flake required, and amorphous graphite is no longer restricted to low-end refractories. Alumina-graphite shapes are used as continuous casting ware, such as nozzles and troughs, to convey the molten steel from ladle to mold, and carbon magnesite bricks line steel converters and electric arc furnaces to withstand extreme temperatures. Graphite Blocks are also used in parts of blast furnace linings where the high thermal conductivity of the graphite is critical. High-purity monolithics are often used as a continuous furnace lining instead of the carbon-magnesite bricks.

The US and European refractories industry had a crisis in 2000–2003, with an indifferent market for steel and a declining refractory consumption per tonne of steel underlying firm buyouts and many plant closings. Many of the plant closings resulted from the acquisition of Harbison-Walker Refractories by Radex-Heraklith, Inc. (RHI); some plants had their equipment auctioned off. Since much of the lost capacity was for carbon-magnesite brick, graphite consumption within refractories area moved towards alumina-graphite shapes and monolithics, and away from the brick.The major source of carbon-magnesite brick is now imports from China. Almost all of the above refractories are used to make steel and account for 75% of refractory consumption; the rest is used by a variety of industries, such as cement.

According to the USGS, US natural graphite consumption in refractories was 11,000 tonnes in 2006.[4]

[edit] Steelmaking

Natural graphite in this end use mostly goes into carbon raising in molten steel, although it can be used to lubricate the dies used to extrude hot steel. Supplying carbon raisers is very competitive, therefore subject to cut-throat pricing from alternatives such as synthetic graphite powder, petroleum coke, and other forms of carbon. A carbon raiser is added to increase the carbon content of the steel to the specified level. An estimate based on USGS US graphite consumption statistics indicates that 10,500 tonnes were used in this fashion in 2005.[4]

[edit] Expanded graphite

Expanded graphite is made by immersing natural flake graphite in a bath of chromic acid, then concentrated sulfuric acid, which forces the crystal lattice planes apart, thus expanding the graphite. The expanded graphite can be used to make graphite foil or used directly as "hot top" compound to insulate molten metal in a ladle or red-hot steel ingots and decrease heat loss, or as firestops fitted around a fire door or in sheet metal collars surrounding plastic pipe (during a fire, the graphite expands and chars to resist fire penetration and spread), or to make high-performance

48

gasket material for high-temperature use. After being made into graphite foil, the foil is machined and assembled into the bipolar plates in fuel cells. The foil is made into heat sinks for laptop computers which keeps them cool while saving weight, and is made into a foil laminate that can be used in valve packings or made into gaskets. Old-style packings are now a minor member of this grouping: fine flake graphite in oils or greases for uses requiring heat resistance. A GAN estimate of current US natural graphite consumption in this end use is 7,500 tonnes.[4]

[edit] Intercalated graphite

Main article: Graphite intercalation compound

Structure of CaC6

Graphite forms intercalation compounds with some metals and small molecules. In these compounds, the host molecule or atom gets "sandwiched" between the graphite layers, resulting in compounds with variable stoichiometry. A prominent example of an intercalation compound is potassium graphite, denoted by the formula KC8. Graphite intercalation compounds are superconductors. The highest transition temperature (by June 2009) Tc = 11.5 K is achieved in CaC6 and it further increases under applied pressure (15.1 K at 8 GPa).[20]

[edit] Brake linings

Natural amorphous and fine flake graphite are used in brake linings or brake shoes for heavier (nonautomotive) vehicles, and became important with the need to substitute for asbestos. This use has been important for quite some time, but nonasbestos organic (NAO) compositions are beginning to cost graphite market share. A brake-lining industry shake-out with some plant closings has not helped either, nor has an indifferent automotive market. According to the USGS, US natural graphite consumption in brake linings was 6,510 tonnes in 2005.[4]

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[edit] Foundry facings and lubricants

A foundry facing mold wash is a water-based paint of amorphous or fine flake graphite. Painting the inside of a mold with it and letting it dry leaves a fine graphite coat that will ease separation of the object cast after the hot metal has cooled. Graphite lubricants are specialty items for use at very high or very low temperatures, as forging die lubricant, an antiseize agent, a gear lubricant for mining machinery, and to lubricate locks. Having low-grit graphite, or even better no-grit graphite (ultra high purity), is highly desirable. It can be used as a dry powder, in water or oil, or as colloidal graphite (a permanent suspension in a liquid). An estimate based on USGS graphite consumption statistics indicates that 2,200 tonnes was used in this fashion in 2005.[4]

[edit] Other uses

Natural graphite has found uses as the marking material ("lead") in common pencils, in zinc-carbon batteries, in electric motor brushes, and various specialized applications.

[edit] Uses of synthetic graphite

[edit] Electrodes

These electrodes carry the electricity that heats electric arc furnaces, the vast majority steel furnaces. They are made from petroleum coke after it is mixed with petroleum pitch, extruded and shaped, then baked to sinter it, and then graphitized by heating it above the temperature (3000 °C) that converts carbon to graphite. They can vary in size up to 11 ft. long and 30 in. in diameter. An increasing proportion of global steel is made using electric arc furnaces, and the electric arc furnace itself is getting more efficient and making more steel per tonne of electrode. An estimate based on USGS data indicates that graphite electrode consumption was 197,000 tonnes in 2005.[4]

[edit] Powder and scrap

The powder is made by heating powdered petroleum coke above the temperature of graphitization, sometimes with minor modifications. The graphite scrap comes from pieces of unusable electrode material (in the manufacturing stage or after use) and lathe turnings, usually after crushing and sizing. Most synthetic graphite powder goes to carbon raising in steel (competing with natural graphite), with some used in batteries and brake linings. According to the USGS, US synthetic graphite powder and scrap production was 95,000 tonnes in 2001 (latest data).[4]

[edit] Neutron moderator

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Main article: Nuclear graphite

Special grades of synthetic graphite also find use as a matrix and neutron moderator within nuclear reactors. Its low neutron cross-section also recommends it for use in proposed fusion reactors. Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron, widely used as the seed electrode in commercial graphite deposition systems—this caused the failure of the Germans' World War II graphite-based nuclear reactors. Since they could not isolate the difficulty they were forced to use far more expensive heavy water moderators. Graphite used for nuclear reactors is often referred to as nuclear graphite.

[edit] Other uses

Graphite (carbon) fiber and carbon nanotubes are also used in carbon fiber reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC). Products made from carbon fiber graphite composites include fishing rods, golf clubs, bicycle frames, and pool cue sticks and have been successfully employed in reinforced concrete. The mechanical properties of carbon fiber graphite-reinforced plastic composites and grey cast iron are strongly influenced by the role of graphite in these materials. In this context, the term "(100%) graphite" is often loosely used to refer to a pure mixture of carbon reinforcement and resin, while the term "composite" is used for composite materials with additional ingredients.[21]

Graphite has been used in at least three radar absorbent materials. It was mixed with rubber in Sumpf and Schornsteinfeger, which were used on U-boat snorkels to reduce their radar cross section. It was also used in tiles on early F-117 Nighthawks. Modern smokeless powder is coated in graphite to prevent the buildup of static charge.

[edit] Graphite mining, beneficiation, and milling

Graphite is mined around the world by both open pit and underground methods. While flake graphite and amorphous graphite are both mined open pit and underground, lump (vein) graphite is only mined underground in Sri Lanka. The open pit mines usually employ equipment (i.e. bulldozers) to scoop up the ore, which is usually put in trucks and moved to the plant. Since the original rock is usually lateritized or weathered, this amounts to moving dirt with flecks or pieces of graphite in it from the pit (blasting is seldom required). The underground graphite mines employ drilling and blasting to break up the hard rock (ore), which is then moved by mine cars pulled by a locomotive, or moved by automotive vehicles, to the surface and then to the plant. In less-developed areas of the world, the ore can be mined by pick and

51

shovel and transported by mine cars pushed by a laborer or by women carrying baskets of ore on their heads.

Graphite usually needs beneficiation, although thick-bedded amorphous graphite and vein graphite is almost always beneficiated, if beneficiated at all, by laborers hand-picking out the pieces of gangue (rock) and hand-screening the product. The great majority of world flake graphite production is crushed and ground if necessary and beneficiated by flotation. Treating graphite by flotation encounters one big difficulty: graphite is very soft and "marks" (coats) the particles of gangue. This makes the "marked" gangue particles float off with the graphite to yield a very impure concentrate. There are two ways of obtaining a saleable concentrate or product: regrinding and floating it again and again (up to seven times) to obtain a purer and purer concentrate, or by leaching (dissolving) the gangue with hydrofluoric acid (for a silicate gangue) or hydrochloric acid (for a carbonate gangue).

In the milling process, the incoming graphite products and concentrates can be ground before being classified (sized or screened), with the coarser flake size fractions (below 8 mesh, 8–20 mesh, 20–50 mesh) carefully preserved, and then the carbon contents are determined. Then some standard blends can be prepared from the different fractions, each with a certain flake size distribution and carbon content. Custom blends can also be made for individual customers who want a certain flake size distribution and carbon content. If flake size is unimportant, the concentrate can be ground more freely. Typical final products include a fine powder for use as a slurry in oil drilling; in zirconium silicate, sodium silicate and isopropyl alcohol coatings for foundry molds; and a carbon raiser in the steel industry (Synthetic graphite powder and powdered petroleum coke can also be used as carbon raiser). Rough graphite is typically classified, ground, and packaged at a graphite mill; often the more complex formulations are also mixed and packaged at the mill facility. Environmental impacts from graphite mills consist of air pollution including fine particulate exposure of workers and also soil contamination from powder spillages leading to heavy metals contaminations of soil. Dust masks are normally worn by workers during the production process to avoid worker exposure to the fine airborne graphite and zircon silicate.

[edit] Graphite recycling

The most common way graphite is recycled occurs when synthetic graphite electrodes are either manufactured and pieces are cut off or lathe turnings are discarded, or the electrode (or other) are used all the way down to the electrode holder. A new electrode replaces the old one, but a sizeable piece of the old electrode remains. This is crushed and

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sized, and the resulting graphite powder is mostly used to raise the carbon content of molten steel. Graphite-containing refractories are sometimes also recycled, but often not because of their graphite: the largest-volume items, such as carbon-magnesite bricks that contain only 15–25% graphite, usually contain too little graphite. However, some recycled carbon-magnesite brick is used as the basis for furnace repair materials, and also crushed carbon-magnesite brick is used in slag conditioners. While crucibles have a high graphite content, the volume of crucibles used and then recycled is very small.

A high-quality flake graphite product that closely resembles natural flake graphite can be made from steelmaking kish. Kish is a large-volume near-molten waste skimmed from the molten iron feed to a basic oxygen furnace, and is a mix of graphite (precipitated out of the supersaturated iron), lime-rich slag, and some iron. The iron is recycled on site, so what is left is a mixture of graphite and slag. The best recovery process uses hydraulic classification (Which utilizes a flow of water to separate minerals by specific gravity: graphite is light and settles nearly last.) to get a 70% graphite rough concentrate. Leaching this concentrate with hydrochloric acid gives a 95% graphite product with a flake size ranging from 10 mesh down.

Alloy Cast Iron 

Cast iron containing the usual components, as well as specially introduced alloying elements, which impart to it specific properties, such as increased strength, durability, and refractoriness. Alloy cast irons are usually classified chemically (for example, nickel or chromium cast iron). In naturally alloyed cast iron the alloying additives come from the iron ore.

Copper and Copper Alloys (Brass, Bronze)

MatWeb has data sheets for 2200+ coppers and copper alloys such as brass and bronze, including unique data for different product forms and heat treatments.  Once you navigate to specific data sheets, you will find the properties you need for the copper alloys of your choice - such as the tensile strength and melting point of SAE 40 Bronze or the density of Leaded Muntz Metal, UNS C36500.

The fastest and surest way to find the database entries for all copper alloys is to follow the link to our Search By Material Category and then select Metal --> Non-Ferrous --> Copper Alloy.

You can also choose "Copper Alloy" in conjunction with property values when you Search By Property.  This will help you to naviagate through our large collection to find the exact copper, brass, or bronze alloys that meet your property specifications.

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Copper alloys can be difficult to find by a text-based search due to ambiguous common names and misnomers. For example, the term "Silver-bearing tough pitch copper" is used for alloys with at least four different composition ranges with different UNS numbers.  When choosing text for our Quick Search, please keep in mind that the MatWeb text-based search is sensitive to punctuation, spacing, and abbreviations; it will treat Copper-tin differently than Copper tin.  The UNS number is the most widely available search term but coppers can sometimes be found in MatWeb by entering ASTM, CDA, CEN, and ISO designations. The UNS Number is often created from the Copper Development Agency (CDA) number by adding a 'C' at the begionning and two zeroes at the end (ex: CDA 360 = UNS C36000).No matter how you reach MatWeb's entries for specific copper, brass, or bronze entries, you will find complete physical property data.  The MatWeb staff has collected extensive data to bring you values for properties such as composition, density, hardness, ultimate tensile strength, electrical resisitivity or conductivity, elastic modulus, etc.

For more information about copper, choose one of the following links:

How Alloying Elements Affect the Properties of Copper ASTM B601 Heat Treating Temper Codes for Coppers and

Copper Alloys ASTM Specification Codes for Coppers and Copper Alloys Federal Specification Codes for Coppers and Copper Alloys

Small amounts of alloying elements are often added to metals to improve certain characteristics of the metal.  Alloying can increase or reduce the strength, hardness, electrical and thermal conductivity, corrosion resistance, or change the color of a metal.  The addition of a substance to improve one property may have unintended effects on other properties.  This page describes the effects of various alloying elements on copper and copper alloys such as brass and bronze.

Strength

Solid solution strengthening of copper is a common strengthening method.  Small amounts of an alloying element added to molten copper will completely dissolve and form a homogeneous microstructure (a sin-gle phase).  At some point, additional amounts of the alloying element will not dissolve; the exact amount is dependent on the solid solubility of the particular element in copper.  When that solid solubility limit is ex-ceeded, two distinct microstructures form with different compositions and hardnesses.  Copper by itself is relatively soft compared with com-mon structural metals.  An alloy with tin added to copper is known as bronze; the resulting alloy is stronger and harder than either of the pure

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metals.  The same is true when zinc is added to copper to form alloys known as brass.  Tin is more effective in strengthening copper than zinc, but is also more expensive and has a greater detrimental effect on the electrical and thermal conductivities than zinc.  Aluminum (forming al-loys known as aluminum bronzes), Manganese, Nickel, and Silicon can also be added to strengthen copper.

Another copper strengthening method is precipitation hardening.  The process involves quenching a supersaturated solid solution from an elevated temperature, then reheating to a lower temperature (aging) to allow the excess solute to precipitate out and form a second phase.  This process is often used for copper alloys containing beryllium, chromium, nickel, or zirconium.  Precipitation hardening offers distinct advantages.  Fabrication is relatively easy using the soft solution-annealed form of the quenched metal.  The subsequent aging process of the fabricated part can be performed using relatively inexpensive and unsophisticated furnaces.  Often the heat treatment can be performed in air, at moderate furnace temperatures, and with little or no controlled cooling.  Many combinations of ductility, impact resistance, hardness, conductivity, and strength can be obtained by varying the heat treatment times and temperatures.

Electrical and Thermal Conductivity.

Pure copper is a very good conductor of both electricity and heat. The In-ternational Annealed Copper Standard (IACS; a high purity copper with a resistivity of 0.0000017 Ohm-cm) is still sometimes used as an electri-cal conductivity standard for metals. The best way to increase the elec-trical and thermal conductivity of copper is to decrease the impurity lev-els.  The existence of impurities and all common alloying elements, ex-cept for silver, will decrease the electrical and thermal conductivity of copper.  As the amount of the second element increases, the electrical conductivity of the alloy decreases.  Cadmium has the smallest effect on resulting alloy's electrical conductivity, followed by increasing effects from zinc, tin, nickel, aluminum, manganese, silicon, then phosphorus.  Although different mechanisms are involved in thermal conductivity, the addition of increasing amounts of elements or impurities also produces a drop in thermal conductivity.  Zinc has very minor effect on the thermal conductivity of copper, followed by increasing effects from nickel, tin, manganese, silicon, and serious effects from phosphorus.  Phosphorus is often used to deoxidize copper, which can increase the hardness and strength, but severely affect the conductivity.  Silicon can be used in-stead of phosphorus to deoxidize copper when conductivity is important.

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Color

Pure copper has a reddish gold color which quickly oxides to a dull green.  Since copper often contains natural impurities or is alloyed with more than one element, it is difficult to state the specific effect each al-loying element has on the resulting alloy's color.  Electrolytic tough pitch copper contains silver and often trace amount of iron and sulfur and has a soft pink color.  Gilding copper is a reddish brown color and contains zinc, iron, and lead.  Brass is often used as an ornamental metal, since it has an appearance very similar to that of gold and is much less expen-sive.  Brasses contain varying amounts of zinc, iron, and lead and can vary from reddish to greenish to brownish gold.  Nickel silver, which contains nickel, zinc, iron, lead, and manganese, can have a grayish-white to silver appearance.

ASTM B601 Heat Treating Temper Codes for Copper Metal and Copper Alloys

Because of the wide variation in important physical properties as a function of an alloy's treatment history, it is imperitive to know and understand the temper in which a copper alloy is to be treated prior to its application.

Copper Temper Name

ASTM Code

1/8 Hard H001/4 Hard H011/2 Hard H023/4 Hard H03Hard H04Extra Hard H06Spring H08Extra Spring H10Special Spring H12Ultra Spring H13Super Spring H14Extruded and Drawn H50Pierced and Drawn H52Light Drawn, Light Cold Rolled

H55

Drawn General Purpose

H58

Cold Heading and Forming

H60

Rivet H63Screw H64

As Manufactured Tempers

ASTM Code

As Sand Cast M01As Centrifugal Cast M02As Plaster Cast M03As Pressure Die Cast M04As Permanent Mold Cast M05As Investment Cast M06As Continuous Cast M07As Hot Forged and Air Cooled

M10

As Forged and Quenched M11As Hot Rolled M20As Hot Extruded M30As Hot Pierced M40As Hot Pierced and Rerolled

M45

Cold Worked and Stress Relieved TempersASTM CodeH01 Temper and Stress Relieved

HR01

H02 Temper and Stress HR02

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Bolt H66Bending H70Hard Drawn H80Medium Hard Drawn Electrical Wire

H85

Hard Drawn Electrical Wire

H86

As Finned H90

RelievedH04 Temper and Stress Relieved

HR04

H08 Temper and Stress Relieved

HR08

H10 Temper and Stress Relieved

HR10

As Finned HR20Drawn and Stress Relieved

HR50

Cold Rolled and Order Strengthened Tempers

ASTM Code

H04 Temper and Order Heat Treated

HT04

H08 Temper and Order Heat Treated

HT08

ASTM Specification Codes for Copper and Copper Alloys - Brass and Bronze

Copper or Copper Alloy Product

ASTM Spec

Hard drawn copper wire

B1

Medium hard drawn copper wire

B2

Annealed copper wire B3Bronze trolley wire B9Free cutting brass rod, bar, and shapes for use in screw machines

B16

Cartridge brass sheet, strip, plate, bar, and disks

B19

Naval brass rod, bar, and shapes

B22

Bronze castings for bridges and turntables

B22

Copper alloy ingots B30Tinned soft or annealed copper electrical wire

B33

Standard sized seamless copper pipe

B43

Copper trolley wire B47Annealed rectangular B48

Copper or Copper Alloy Product

ASTM Spec

Copper conductors for electronic equipment wire hookups

B286

Manganese brass (Cu-Zn-Mn) sheet and strip

B291

Silver coated annealed copper wire

B298

Free cutting copper rod and bar

B301

Threadless copper pipe B302Copper drainage tube (DWV)

B306

Seamless copper alloy pipe and tube

B315

Nickel coated annealed copper wire

B355

Copper and copper alloy seamless condenser and heat exchanger tubes with fins

B359

Hard drawn copper capillary tube for

B360

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and square bare copper electrical wireHot rolled copper rod redraw for electrical use

B49

Steam or valve bronze castings

B61

Bronze rough castings for locomotive wear parts

B66

Car and tender lined journal bearings

B67

Seamless bright annealed copper tube

B68

Seamless copper tube B75Seamless copper water tube

B88

General purpose and pressure vessel Cu-Si alloy plate, sheet, strip, and rolled bar

B96

Cu-Si alloy rod, bar, and shapes

B98

General purpose Cu-Si alloy wire

B99

Rolled copper alloy bearing and expansion plates and sheets for bridges and structural use

B100

Lead coated copper sheet

B101

Phosphor bronze plate, sheet, strip, and rolled bar

B103

Hard drawn copper alloy wires for electrical conductors

B105

Copper and copper alloy seamless condenser tubes and ferrule stock

B111

Fig. 9 deep section grooved and fig. 8 Cur trolley wire for

B116

restrictor applicationsCu-Ni alloy castings B369Copper sheet and strip for building construction

B370

Cu-Zinc-Si alloy rod B371Seamless copper and copper alloy rectangular wave guide tube

B372

U-bend seamless copper and copper alloy heat exchanger and condenser tubes

B395

Cu-Ni-Si alloy rod and bar

B411

Cu-Ni-Si alloy wire B412Cu-Al-Si-Co alloy and Cu-Ni-Al-Si alloy sheet and strip

B422

Gear bronze castings B427Copper and copper alloy clad steel plate

B432

Cu-Co-Be (UNS C17500) and Cu-Ni-Be (UNS C17510) rod and bar

B441

Welded copper tube B447Copper foil, strip and sheet for printed circuits and carrier tapes

B451

Leaded brass (Cu-Zn-Pb) rod

B453

Leaded brass (Cu-Zn-Pb) extruded shapes

B455

Cu-Fe alloy plate, sheet, strip, and rolled bar

B465

Seamless Cu-Ni pipe and tube

B466

Welded Cu-Ni pipe B467Seamless Cu-Ni pipe and tube

B469

Bonded copper B470

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industrial haulageLeaded brass plate, sheet, strip, and rolled bar

B121

Copper and copper alloy forging rod, bar, and shapes

B124

Cartridge brass cartridge case cups

B129

Commercial bronze strip for bullet jackets

B130

Copper alloy bullet jacket cups

B131

Copper rod, bar, and shapes

B133

Brass wire B134Seamless brass tube B135Manganese bronze rod, bar, and shapes

B138

Phosphor bronze rod, bar, and shapes

B139

Leaded red brass hardware bronze rod, bar, and shapes

B140

Aluminum bronze sand castings

B148

Aluminum bronze rod, bar, and shapes

B150

Nickel Silver (Cu-Ni-Zn) and Cu-Ni rod and bar

B151

Copper sheet, strip, plate, and rolled bar

B152

Phosphor bronze wire B159Aluminum bronze, plate, sheet, strip, and rolled bar

B169

Copper alloy condenser tube plates

B171

Rope lay standard conductors w/ bunch stranded members

B172

Rope lay standard conductors w/ concentric stranded members

B173

conductors for use in hookup wire for electronic equipmentCast Cu-Ni ship tailshaft sleeves

B492

Compact round concentric-lay-stranded copper conductors

B496

Copper alloy continuous castings

B505

Copper clad stainless steel sheet and strip for building construction

B506

Copper alloy strip for flexible metal hose

B508

Cu-Co-Be alloy, plate, sheet, strip, and rolled bar

B534

Welded copper and copper alloy heat exchanger tube

B543

Seamless and welded Cu-Ni tubes for water desalination plants

B552

Light gauge UNS C26000 brass strip in narrow widths for heat exchanger tubing

B569

Beryllium Copper (Cu-Be) alloy forgings and extrusions

B570

General purpose copper alloy sand castings

B584

Welded brass tube B587Cu-Zn-Sn alloy plate, sheet, strip, and rolled bar

B591

Cu-Zn-Al-Co alloy plate, sheet, strip, and rolled bar

B592

Welded copper alloy pipe

B608

High-strength, high-B624

59

Bunch stranded conductors

B174

Brass die castings B176Copper bus bar, rod, and shapes

B187

Seamless copper bus pipe and tube

B188

Lead copper and Lead copper alloy coated soft copper electrical wire

B189

Beryllium Copper (Cu-Be) alloy plate, sheet, strip, and rolled bar

B194

Beryllium Copper (Cu-Be) alloy rod and bar

B196

Beryllium Copper (Cu-Be) alloy wire

B197

Nickel Silver (Cu-Ni-Zn) and Cu-Ni alloy wire

B206

Cored, annular, concentric-lay stranded copper conductors

B226

Concentric-lay stranded copper and copper clad steel conductors

B229

Tinned hard drawn and medium hard drawn copper electrical wire

B246

Copper base alloy centrifugal castings

B271

Copper flat wire and strip w/rolled or drawn edges

B272

Seamless copper tube for air conditioning and refrigerator use

B280

Copper and copper alloy hot pressed die forgings

B283

conductivity copper alloy electronic wireCopper and copper alloy solar heat absorber panels

B638

Welded copper and copper alloy tube for AC or refrigeration use

B640

Seamless and type D welded copper distribution tube

B641

Beryllium Copper (Cu-Be) alloy seamless tube

B643

Brass, Cu, and Cr plated pipe nipples

B687

Cu, copper alloy, and Cu-clad stainless steel sheet and strip for electrical cable shielding

B694

UNS C69100 seamless copper alloy pipe and tube

B706

Welded copper water tube

B716

Fine wire and rope-lay bunch stranded copper electrical conductors

B738

Cu-Ni-Sn spinodal alloy strip

B740

Seamless copper tube coils

B743

Cu-Zn alloy sheet and strip

B747

Sand cast copper alloy for valve applications

B743

Cu-Co-Be alloy sheet and strip

B768

Round wire for use as grid side rods in electron tubes

F9

Wrought oxygen-free copper for electronic devices

F68

Wrought electronic F96

60

grade copper alloysGeneral use nonferrous nuts

F467

General use nonferrous bolts, hex cap screws, and studs

F468

Miscellaneous

Federal Specification Codes for Coppers and Copper Alloys

Copper or Copper Alloy Product Federal Spec

Flat or Tubular, tin-coated or silver-coated, copper braid or wire

QQ-B-575

Leaded and non-leaded brass plate, bar, sheet, and strip QQ-B-613

Leaded and non-leaded brass forging and flat products w/ finished edges (i.e. bar and strip)

QQ-B-626

Naval brass forgings, rod, shapes, wire and flat products w/ finished edges (i.e. bar and flat wire)

QQ-B-637

Naval brass plate, bar, sheet, and strip QQ-B-639

Copper, Cu-Zn, and Cu-P brazing alloy QQ-B-650

Aluminum bronze ingots QQ-B-675

Manganese bronze bar, forgings, flat wire, plate, rod, shapes, sheet, strip

QQ-B-728

Phosphor bronze bar, flat wire, plate, sheet, strip, and structural shapes

QQ-B-750

Copper alloy castings QQ-C-390

Aluminum bronze bar, plate, sheet, and strip QQ-C-450

Aluminum bronze flat products with finished edges QQ-C-465

Copper flat products, rods, and shapes with finished edges

QQ-C-502

Copper ingot QQ-C-521

Yellow brass, manganese brass, aluminum- manganese bronze, and manganese bronze ingots

QQ-C-523

Leaded and non-leaded red brass, semi-red brass, and QQ-C-

61

tin bronze ingots 525Beryllium-copper (BeCu) bar, rod, and wire QQ-C-

530Beryllium-copper (BeCu) strip QQ-C-

533Copper flat products with slit, slit and edge-rolled, sheared, sawed, or machined edges

QQ-C-576

Nickel silver (Cu-Ni-Zn) sheet, strip and bar QQ-C-585

Nickel silver (Cu-Ni-Zn) flat products with finished edges, rod, and shapes

QQ-C-586

Cu-Si, Cu-Zn-Si, and Cu-Ni-Si alloy flat products, forgings, rod, shapes, and wire

QQ-C-591

Copper-Nickel (Cu-Ni) alloy welding rod QQ-R-571

Copper alloy wire QQ-W-321

Uninsulated electrical wire QQ-W-343

Seamless standard size red brass pipe, regular and extra strong

WW-P-351

Seamless standard size copper pipe WW-P-377

Brass or bronze pipe fittings WW-P-460

Copper and copper alloy tube fittings WW-T-725

Seamless brass tubing WW-T-791

Seamless copper tubing for use with solder-type or flared-tube fittings

WW-T-799

Brass or bronze unions WW-T-516

Brass or bronze butterfly valves WW-T-967

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.[1] In comparison, bronze is principally an alloy of copper tin.[2] Bronze does not necessarily contain tin, and a variety of alloys of copper, including alloys with arsenic, phosphorus, aluminum, manganese, and silicon, are commonly termed "bronze". The term is applied to a variety of brasses and the distinction is largely historical.[3] Brass is a substitutional alloy. It is used for decoration for its bright gold-like appearance; for applications

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where low friction is required such as locks, gears, bearings, doorknobs, ammunition, and valves; for plumbing and electrical applications; and extensively in musical instruments such as horns and bells for its acoustic properties. It is also used in zippers. Because it is softer than most other metals in general use, brass is often used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases.[4]

Brass has a muted yellow color, which is somewhat similar to gold. It is relatively resistant to tarnishing, and is often used as decoration and for coins. In antiquity, polished brass was often used as a mirror.

Although forms of brass have been in use since prehistory,[5] its true nature as a copper-zinc alloy was not understood until the post medieval period because the zinc vapour which reacted with copper to make brass was not recognised as a metal.[6] The King James Bible makes many references to "brass".[7] The Shakespearean English form of the word 'brass' can mean any bronze alloy, or copper, rather than the strict modern definition of brass.[citation needed] The earliest brasses may have been natural alloys made by smelting zinc-rich copper ores.[8] By the Roman period brass was being deliberately produced from metallic copper and zinc minerals using the cementation process and variations on this method continued until the mid 19th century.[9] It was eventually replaced by speltering, the direct alloying of copper and zinc metal which was introduced to Europe in the 16th century.[8]

Contents

[hide] 1 Properties 2 Lead content 3 Applications 4 Season cracking 5 Brass types 6 History

o 6.1 Early copper zinc alloys o 6.2 Brass making in the Roman World o 6.3 Brass making in the Medieval Period o 6.4 Brass making in Renaissance and Post Medieval Europe

7 See also 8 References 9 External links

[edit] Properties

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Microstructure of rolled and annealed brass (400X magnification)

The malleability and acoustic properties of brass have made it the metal of choice for brass musical instruments such as the trombone, tuba, trumpet, cornet, euphonium, tenor horn, and the French horn. Even though the saxophone is classified as a woodwind instrument and the harmonica is a free reed aerophone, both are also often made from brass. In organ pipes of the reed family, brass strips (called tongues) are used as the reeds, which beat against the shallot (or beat "through" the shallot in the case of a "free" reed).

Brass has higher malleability than bronze or zinc. The relatively low melting point of brass (900 to 940°C, depending on composition) and its flow characteristics make it a relatively easy material to cast. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses. The density of brass is approximately 8400 to 8730 kilograms per cubic metre[10] (equivalent to 8.4 to 8.73 grams per cubic centimetre).

Today almost 90% of all brass alloys are recycled.[11] Because brass is not ferromagnetic, it can be separated from ferrous scrap by passing the scrap near a powerful magnet. Brass scrap is collected and transported to the foundry where it is melted and recast into billets. Billets are heated and extruded into the desired form and size.

Aluminium makes brass stronger and more corrosion resistant. Aluminium also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self healing. Tin has a similar effect and finds its use especially in sea water applications (naval brasses). Combinations of iron, aluminium, silicon and manganese make brass wear and tear resistant.

[edit] Lead content

To enhance the machinability of brass, lead is often added in concentrations of around 2%. Since lead has a lower melting point than

64

the other constituents of the brass, it tends to migrate towards the grain boundaries in the form of globules as it cools from casting. The pattern the globules form on the surface of the brass increases the available lead surface area which in turn affects the degree of leaching. In addition, cutting operations can smear the lead globules over the surface. These effects can lead to significant lead leaching from brasses of comparatively low lead content.[12]

Silicon is an alternative to lead; however, when silicon is used in a brass alloy, the scrap must never be mixed with leaded brass scrap because of contamination and safety problems.[13]

In October 1999 the California State Attorney General sued 13 key manufacturers and distributors over lead content. In laboratory tests, state researchers found the average brass key, new or old, exceeded the California Proposition 65 limits by an average factor of 19, assuming handling twice a day.[14] In April 2001 manufacturers agreed to reduce lead content to 1.5%, or face a requirement to warn consumers about lead content. Keys plated with other metals are not affected by the settlement, and may continue to use brass alloys with higher percentage of lead content.[15][16]

Also in California, lead-free materials must be used for "each component that comes into contact with the wetted surface of pipes and pipe fittings, plumbing fittings and fixtures." On January 1, 2010, the maximum amount of lead in "lead-free brass" in California was reduced by more than an order of magnitude from 4% to 0.25% lead. The common practice of using pipes for electrical grounding is discouraged, as it accelerates lead corrosion.[17][18]

[edit] Applications

This section is in a list format that may be better presented using prose. You can help by converting this section to prose, if appropriate. Editing help is available. (November 2009)

Harsh environments: The so called dezincification resistant (DZR) brasses are used where there is a large corrosion risk and where normal brasses do not meet the standards. Applications with high water temperatures, chlorides present or deviating water qualities (soft water) play a role. DZR-brass is excellent in water boiler systems. This brass alloy must be produced with great care, with special attention placed on a balanced composition and proper production temperatures and parameters to avoid long-term failures.

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Germicidal properties: The copper in brass makes brass germicidal, via the oligodynamic effect. For example, brass doorknobs disinfect themselves of many bacteria within eight hours.[19] This effect is important in hospitals, and useful in many contexts.

Brass door hardware: Brass hardware is generally lacquered when new, which prevents tarnishing of the metal. Freshly polished brass is similar to gold in appearance, but becomes more reddish within days of exposure to the elements. A traditional polish is Brasso.

Other: Brass was used to make fan blades, fan cages and motor bearings in many antique fans that date before the 1930s. Brass can also be used for fixings for use in cryogenic systems.[20] Brass has also been used to make cymbals for the modern drum kit.

[edit] Season cracking

Cracking in brass caused by ammonia attack

Brass is susceptible to stress corrosion cracking, especially from ammonia or substances containing or releasing ammonia. The problem is sometimes known as season cracking after it was first discovered in brass cartridge cases used for rifle ammunition during the 1920s in the Indian Army. The problem was caused by high residual stresses from cold forming of the cases during manufacture, together with chemical attack from traces of ammonia in the atmosphere. The cartridges were stored in stables and the ammonia concentration rose during the hot summer months, so initiating brittle cracks. The problem was resolved by annealing the cases, and storing the cartridges elsewhere.

[edit] Brass types

Admiralty brass contains 30% zinc, and 1% tin which inhibits dez-incification in many environments.

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Aich's alloy typically contains 60.66% copper, 36.58% zinc, 1.02% tin, and 1.74% iron. Designed for use in marine service owing to its corrosion resistance, hardness and toughness. A characteristic ap-plication is to the protection of ships' bottoms, but more modern methods of cathodic protection have rendered its use less common. Its appearance resembles that of gold.[21]

Alpha brasses with less than 35% zinc, are malleable, can be worked cold, and are used in pressing, forging, or similar applica-tions. They contain only one phase, with face-centered cubic crys-tal structure. Prince's metal or Prince Rupert's metal is a type of alpha brass containing 75% copper and 25% zinc. Due to its beautiful yellow color, it is used as an imitation of gold. [22] The alloy was named after Prince Rupert of the Rhine.

Alpha-beta brass (Muntz metal), also called duplex brass, is 35–45% zinc and is suited for hot working. It contains both α and β' phase; the β'-phase is body-centered cubic and is harder and stronger than α. Alpha-beta brasses are usually worked hot.

Aluminium brass contains aluminium, which improves its corro-sion resistance. It is used for seawater service[23] and also in Euro coins (Nordic gold).

Arsenical brass contains an addition of arsenic and frequently aluminium and is used for boiler fireboxes.

Beta brasses, with 45–50% zinc content, can only be worked hot, and are harder, stronger, and suitable for casting.

Cartridge brass is a 30% zinc brass with good cold working prop-erties.

Common brass, or rivet brass, is a 37% zinc brass, cheap and standard for cold working.

DZR brass is dezincification resistant brass with a small percent-age of arsenic.

Gilding metal is the softest type of brass commonly available. An alloy of 95% copper and 5% zinc, gilding metal is typically used for ammunition components.

High brass contains 65% copper and 35% zinc, has a high tensile strength and is used for springs, screws, and rivets.

Leaded brass is an alpha-beta brass with an addition of lead. It has excellent machinability.

Lead-free brass as defined by California Assembly Bill AB 1953 contains "not more than 0.25 percent lead content".[17]

Low brass is a copper-zinc alloy containing 20% zinc with a light golden color and excellent ductility; it is used for flexible metal hoses and metal bellows.

Manganese brass is a brass most notably used in making golden dollar coins in the United States. It contains roughly 70% copper, 29% zinc, and 1.3% manganese.[24]

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Muntz metal is about 60% copper, 40% zinc and a trace of iron, used as a lining on boats.

Nickel brass is composed of 70% copper, 24.5% zinc and 5.5% nickel used to make pound coins in the pound sterling currency.

Naval brass, similar to admiralty brass, is 40% zinc and 1% tin.

Nordic gold , used in 10, 20 and 50 cts euro coins, contains 89% copper, 5% aluminium, 5% zinc, and 1% tin.

Red brass is both an American term for the copper-zinc-tin alloy known as gunmetal, and an alloy which is considered both a brass and a bronze. It typically contains 85% copper, 5% tin, 5% lead, and 5% zinc.[25] Red brass is also an alternative name for copper al-loy C23000, which is composed of 14–16% zinc, 0.05% iron and lead, and the remainder copper.[26] It may also refer to ounce metal, another copper-zinc-tin alloy.

Rich low brass (Tombac) is 15% zinc. It is often used in jewelry applications.

Tonval brass (also called CW617N or CZ122 or OT58) is a copper-lead-zinc alloy. It is not recommended for seawater use, being sus-ceptible to dezincification.[27][28]

White brass contains more than 50% zinc and is too brittle for general use. The term may also refer to certain types of nickel sil-ver alloys as well as Cu-Zn-Sn alloys with high proportions (typi-cally 40%+) of tin and/or zinc, as well as predominantly zinc cast-ing alloys with copper additive.

Yellow brass is an American term for 33% zinc brass.

[edit] History

[edit] Early copper zinc alloys

In West Asia and the Eastern Mediterranean early copper zinc alloys are now known in small numbers from a number of third Millennium BC sites in the Aegean, Iraq, the United Arab Emirates, Kalmikia, Turkmenistan and Georgia and from 2nd Millennium BC sites in West India, Uzbekistan, Iran, Syria, Iraq and Palestine.[29] However, isolated examples of copper-zinc alloys are known in China from as early as the 5th Millennium BC.[30]

The compositions of these early "brass" objects are very variable and most have zinc contents of between 5% and 15% wt which is lower than in brass produced by cementation.[31] These may be "natural alloys" manufactured by smelting zinc rich copper ores in reducing conditions. Many have similar tin contents to contemporary bronze artefacts and it is possible that some copper-zinc alloys were accidental and perhaps not even distinguished from copper.[31] However the large number of copper-zinc alloys now known suggests that at least some were deliberately

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manufactured and many have zinc contents of more than 12% wt which would have resulted in a distinctive golden color.[32]

By the 8th-7th century BC Assyrian cuneiform tablets mention the exploitation of the "copper of the mountains" and this may refer to "natural" brass.[33] Oreichalkos, the Ancient Greek translation of this term, was later adapted to the Latin aurichalcum meaning "golden copper" which became the standard term for brass.[34] In the 4th century BC Plato knew oreichalkos as rare and nearly as valuable as gold[35] and Pliny describes how aurichalcum had come from Cypriot ore deposits which had been exhausted by the 1st century AD.[36] '

[edit] Brass making in the Roman World

During the later part of first Millennium BC the use of brass spread across a wide geographical area from Britain [37] and Spain [38] in the west to Iran, and India in the east.[39] This seems to have been encouraged by exports and influence from the Middle-East and eastern Mediterranean where deliberate production of brass from metallic copper and zinc ores had been introduced.[40] The 4th century BC writer Theopompus, quoted by Strabo, describes how heating earth from Andeira in Turkey produced "droplets of false silver", probably metallic zinc, which could be used to turn copper into oreichalkos.[41] In the 1st century BC the Greek Dioscorides seems to have recognised a link between zinc minerals and brass describing how Cadmia (zinc oxide) was found on the walls of furnaces used to heat either zinc ore or brass and explaining that it can then be used to make brass.[42]

By the first century BC brass was available in sufficient supply to use as coinage in Phrygia and Bithynia,[43] and after the Augustan currency reform of 23 BC it was also used to make Roman dupondii and sestertii.[44] The uniform use of brass for coinage and military equipment across the Roman world may indicate a degree of state involvement in the industry,[45] and brass even seems to have been deliberately boycotted by Jewish communities in Palestine because of its association with Roman authority.[46]

Brass was produced by the cementation process where copper and zinc ore are heated together until zinc vapour is produced which reacts with the copper. There is good archaeological evidence for this process and crucibles used to produce brass by cementation have been found on Roman period sites including Xanten [47] and Nidda [48] in Germany, Lyon in France [49] and at a number of sites in Britain.[50] They vary in size from tiny acorn sized to large amphorae like vessels but all have elevated levels of zinc on the interior and are lidded.[49] They show no signs of slag or metal prills suggesting that zinc minerals were heated to produce zinc vapour which reacted with metallic copper in a solid state reaction. The

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fabric of these crucibles is porous, probably designed to prevent a build up of pressure, and many have small holes in the lids which may be designed to release pressure[49] or to add additional zinc minerals near the end of the process. Dioscorides mentioned that zinc minerals were used for both the working and finishing of brass, perhaps suggesting secondary additions.[51]

Brass made during the early Roman period seems to have varied between 20% to 28% wt zinc.[52] The high content of zinc in coinage and brass objects declined after the first century AD and it has been suggested that this reflects zinc loss during recycling and thus an interruption in the production of new brass.[53] However it is now thought this was probably a deliberate change in composition[54] and overall the use of brass increases over this period making up around 40% of all copper alloys used in the Roman world by the 4th century AD.[55]

[edit] Brass making in the Medieval Period

Baptism of Christ on the 12th century Baptismal font at St Bartholomew's Church, Liège.

Little is known about the production of brass during the centuries immediately after the collapse of the Roman Empire. Disruption in the trade of tin for bronze from Western Europe may have contributed to the increasing popularity of brass in the east and by the 6th-7th centuries AD over 90% of copper alloy artefacts from Egypt were made of brass.[56]

However other alloys such as low tin bronze were also used and they vary depending on local cultural attitudes, the purpose of the metal and access to zinc, especially between the Islamic and Byzantine world.[57]

Conversely the use of true brass seems to have declined in Western Europe during this period in favour of gunmetals and other mixed alloys[58] but by the end of the first Millennium AD brass artefacts are found in Scandinavian graves in Scotland,[59] brass was being used in the manufacture of coins in Northumbria [60] and there is archaeological and

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historical evidence for the production of brass in Germany[61] and The Low Countries [62] areas rich in calamine ore which would remain important centres of brass making throughout the medieval period,[63]

especially Dinant - brass objects are still collectively known as dinanterie in French. The Baptismal font at St Bartholomew's Church, Liège in modern Belgium (before 1117) is an outstanding masterpiece of Romanesque brass casting.

The cementation process continued to be used but literary sources from both Europe and the Islamic world seem to describe variants of a higher temperature liquid process which took places in open topped crucibles.[64] Islamic cementation seems to have used zinc oxide known as tutiya or tutty rather than zinc ores for brass making resulting in a metal with lower iron impurities.[65] A number of Islamic writers and the 13th century Italian Marco Polo describe how this was obtained by sublimation from zinc ores and condensed onto clay or iron bars, archaeological examples of which have been identified at Kush in Iran.[66]

It could then be used for brass making or medicinal purposes. In 10th century Yemen al-Hamdani described how spreading al-iglimiya, probably zinc oxide, onto the surface of molten copper produced tutiya vapour which then reacted with the metal.[67] The 13th century Iranian writer al-Kashani describes a more complex process whereby tutiya was mixed with raisins and gently roasted before being added to the surface of the molten metal. A temporary lid was added at this point presumably to minimise the escape of zinc vapour.[68]

In Europe a similar liquid process in open topped crucibles took place which was probably less efficient than the Roman process and the use of the term tutty by Albertus Magnus in the 13th century suggests influence from Islamic technology.[69] The 12th century German monk Theophilus described how preheated crucibles were one sixth filled with powdered calamine and charcoal then topped up with copper and charcoal before being melted, stirred then filled again. The final product was cast, then again melted with calamine. It has been suggested that this second melting may have taken place at a lower temperature to allow more zinc to be absorbed.[70] Albertus Magnus noted that the "power" of both calamine and tutty could evaporate and described how the addition of powdered glass could create a film to bind it to the metal.[71] German brass making crucibles are known from Dortmund dating to the 10th century AD and from Soest and Schwerte in Westphalia dating to around the 13th century confirm Theophilus' account as they are open topped, although ceramic discs from Soest may have served as loose lids which may have been used to reduce zinc evaporation, and have slag on the interior resulting from a liquid process.[72]

[edit] Brass making in Renaissance and Post Medieval Europe

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The Renaissance saw important changes to both the theory and practice of brassmaking in Europe. By the 15th century there is evidence for the renewed use of lidded cementation crucibles at Zwickau in Germany.[73]

These large crucibles were capable of producing c.20 kg of brass.[74]

There are traces of slag and pieces of metal on the interior. Their irregular composition suggesting that this was a lower temperature not entirely liquid process.[75] The crucible lids had small holes which were blocked with clay plugs near the end of the process presumably to maximise zinc absorption in the final stages.[76] Triangular crucibles were then used to melt the brass for casting.[77]

16th century technical writers such as Biringuccio, Ercker and Agricola described a variety of cementation brass making techniques and came closer to understanding the true nature of the process noting that copper became heavier as it changed to brass and that it became more golden as additional calamine was added.[78] Zinc metal was also becoming more commonplace By 1513 metallic zinc ingots from India and China were arriving in London and pellets of zinc condensed in furnace flues at the Rammelsberg in Germany were exploited for cementation brass making from around 1550.[79]

Eventually it was discovered that metallic zinc could be alloyed with copper to make brass; a process known as speltering[80] and by 1657 the German chemist Johann Glauber had recognised that calamine was "nothing else but unmeltable zinc" and that zinc was a "half ripe metal."[81] However some earlier high zinc, low iron brasses such as the 1530 Wightman brass memorial plaque from England may have been made by alloying copper with zinc and include traces of cadmium similar those found in some zinc ingots from China.[80]

However the cementation process was not abandoned and as late as the early 19th century there are descriptions of solid state cementation in a domed furnace at around 900-950 degrees Celsius and lasting up to 10 hours.[82] The European brass industry continued to flourish into the post medieval period buoyed by innovations such as the 16th century introduction of water powered hammers for the production of battery wares.[83] By 1559 the Germany city of Aachen alone was capable of producing 300,000 cwt of brass per year.[83] After several false starts during the 16th and 17th centuries the brass industry was also established in England taking advantage of abundant supplies of cheap copper smelted in the new coal fired reverberatory furnace.[84] In 1723 Bristol brass maker Nehemiah Champion patented the use of granulated copper, produced by pouring molten metal into cold water.[85] This increased the surface area of the copper helping it react and zinc contents of up to 33% wt were reported using this new technique.[86]

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In 1738 Nehemiah's son William Champion patented a technique for the first industrial scale distillation of metallic zinc known as distillation per descencum or "the English process."[87] This local zinc was used in speltering and allowed greater control over the zinc content of brass and the production of high zinc copper alloys which would have been difficult or impossible to produce using cementation, for use in expensive objects such as scientific instruments, clocks, brass buttons and costume jewellery.[88] However Champion continued to use the cheaper calamine cementation method to produce lower zinc brass [88] and the archaeological remains of bee-hive shaped cementation furnaces have been identified at his works at Warmley.[89] By the mid late 18th century developments in cheaper zinc distillation such as John-Jaques Dony's horizontal furnaces in Belgium and the reduction of tariffs on zinc [90] as well as demand for corrosion resistant high zinc alloys increased the popularity of speltering and as a result cementation was largely abandoned by the mid 19th century.[91]

Bronze

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminum, or silicon. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal. However, since "bronze" is a somewhat imprecise term, and historical pieces have variable compositions, in particular with an unclear boundary with brass, modern museum and scholarly descriptions of older objects increasingly use the more cautious term "copper alloy" instead.[1]

The word Bronze is believed to be cognate with the Italian: bronzo and German: brunst, perhaps ultimately taken from the Persian word birinj ("bronze") or possibly from the Latin name of the city of Brindisi

HISTORY

The discovery of bronze enabled people to create metal objects which were better than was previously possible. Tools, weapons, armor, and various building materials, like decorative tiles, made of bronze were harder and more durable than their stone and copper ("Chalcolithic") predecessors. Initially bronze was made out of copper and arsenic to form arsenic bronze. It was only later that tin was used, becoming the sole type of bronze in the late 3rd millennium BC. Tin bronze was superior over arsenic bronze in that the alloying process itself could more easily be controlled (as tin was available as a metal) and the alloy was stronger and easier to cast. Also, unlike arsenic, tin is not toxic.

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The earliest tin-alloy bronzes date to the late 4th millennium BC in Susa (Iran) and some ancient sites in Luristan (Iran) and Mesopotamia (Iraq).

Copper and tin ores are rarely found together (exceptions include one ancient site in Thailand and one in Iran), so serious bronze work has always involved trade. In Europe, the major source for tin was Great Britain's deposits of ore in Cornwall, which were traded as far as Phoenicia in the Eastern Mediterranean.

Though bronze is generally harder than wrought iron, with Vickers hardness of 60–258[3] vs. 30–80,[4] the Bronze Age gave way to the Iron Age; this happened because iron was easier to find. Bronze was still used during the Iron Age, but, for many purposes, the weaker wrought iron was found to be sufficiently strong. Archaeologists suspect that a serious disruption of the tin trade precipitated the transition. The population migrations around 1200–1100 BC reduced the shipping of tin around the Mediterranean (and from Great Britain), limiting supplies and raising prices.[5] As ironworking improved, iron became cheaper; and as cultures advanced from wrought iron to forged iron, they learned how to make steel, which is stronger than bronze and holds a sharper edge longer.[6]

Composition

There are many different bronze alloys but modern bronze is typically 88% copper and 12% tin.[7] Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4–5% tin are used to make coins, springs, turbines and blades. Historical "bronzes" are highly variable in composition, as most metalworkers probably used whatever scrap was to hand; the metal of the 12th century English Gloucester Candlestick is bronze containing a mixture of copper, zinc, tin, lead, nickel, iron, antimony, arsenic with an unusually large amount of silver - between 22.5% in the base and 5.76% in the pan below the candle. The proportions of this mixture may suggest that the candlestick was made from a hoard of old coins. The Benin Bronzes are really brass, and the Romanesque Baptismal font at St Bartholomew's Church, Liège is described as both bronze and brass.

Commercial bronze (90% copper and 10% zinc) and Architectural bronze (57% Copper, 3% Lead, 40% Zinc) are more properly regarded as brass alloys because they contain zinc as the main alloying ingredient. They are commonly used in architectural applications.[8][9]

Bismuth bronze is a bronze alloy with a composition of 52% copper, 30% nickel, 12% zinc, 5% lead, 1% bismuth. It is able to hold a good polish and so is sometimes used in light reflectors and mirrors.[10]

Other bronze alloys include aluminum bronze, phosphor bronze,

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manganese bronze, bell metal, speculum metal and cymbal alloys.

[edit] Properties

Assorted ancient bronze castings

Bronze is considerably less brittle than iron. Typically bronze only oxidizes superficially; once a copper oxide (eventually becoming copper carbonate) layer is formed, the underlying metal is protected from further corrosion. However, if copper chlorides are formed, a corrosion-mode called "bronze disease" will eventually completely destroy it.[11]

Copper-based alloys have lower melting points than steel or iron, and are more readily produced from their constituent metals. They are generally about 10 percent heavier than steel, although alloys using aluminum or silicon may be slightly less dense. Bronzes are softer and weaker than steel—bronze springs, for example, are less stiff (and so store less energy) for the same bulk. Bronze resists corrosion (especially seawater corrosion) and metal fatigue more than steel and is also a better conductor of heat and electricity than most steels. The cost of copper-base alloys is generally higher than that of steels but lower than that of nickel-base alloys.

Copper and its alloys have a huge variety of uses that reflect their versatile physical, mechanical, and chemical properties. Some common examples are the high electrical conductivity of pure copper, the excellent deep drawing qualities of cartridge case brass, the low-friction properties of bearing bronze, the resonant qualities of bell bronze, and the resistance to corrosion by sea water of several bronze alloys.

The melting point of Bronze varies depending on the actual ratio of the alloy components and is about 950 °C.

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[edit] Uses

Ewer from 7th century Iran. Cast, chased, and inlaid bronze. New York Metropolitan Museum of Art

Bronze was especially suitable for use in boat and ship fittings prior to the wide employment of stainless steel owing to its combination of toughness and resistance to salt water corrosion. Bronze is still commonly used in ship propellers and submerged bearings.

In the twentieth century, silicon was introduced as the primary alloying element, creating an alloy with wide application in industry and the major form used in contemporary statuary. Aluminum is also used for the structural metal aluminum bronze.

It is also widely used for cast bronze sculpture. Many common bronze

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alloys have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mold. Bronze parts are tough and typically used for bearings, clips, electrical connectors and springs.Spring bronze weatherstripping comes in rolls of thin sheets and is nailed or stapled to wood windows and doors. There are two types, flat and v-strip. It has been used for hundreds of years because it has low friction, seals well and is long lasting. It is used in building restoration and custom construction.

Bronze also has very little metal-on-metal friction, which made it invaluable for the building of cannon where iron cannonballs would otherwise stick in the barrel.[citation needed] It is still widely used today for springs, bearings, bushings, automobile transmission pilot bearings, and similar fittings, and is particularly common in the bearings of small electric motors. Phosphor bronze is particularly suited to precision-grade bearings and springs. It is also used in guitar and piano strings.

Unlike steel, bronze struck against a hard surface will not generate sparks, so it (along with beryllium copper) is used to make hammers, mallets, wrenches and other durable tools to be used in explosive atmospheres or in the presence of flammable vapors.

[edit] Bronze statues

See also: Bronze sculpture

Indian Hindu artisans from the period of the Chola empire in Tamil Nadu, used bronze to create intricate statues via the lost wax casting method with ornate detailing depicting the Gods of Hinduism mostly, but also the lifestyle of the period. The art form survives to this day, with many silpis, craftsmen, working in the areas of Swamimalai and Chennai.

In antiquity other cultures also produced works of high art using bronze. For example: in Africa, the bronze heads of the Kingdom of Benin; in Europe, Grecian bronzes typically of figures from Greek mythology; in east Asia, Chinese bronzes of the Shang and Zhou dynasty — more often ceremonial vessels but including some figurine examples.

Bronze continues into modern times as one of the materials of choice for monumental statuary.

[edit] Musical instruments

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Bronze is the most popular metal for top-quality bells, particularly bell metal, which is about 23% tin.

Nearly all professional cymbals are made from a bronze alloy. The alloy used in drum kit cymbal bronze is unique in the desired balance of durability and timbre.

Modern cymbals consist of several types of bronze, the most common being B20 bronze, which is roughly 20% tin, 80% copper, with traces of silver. Zildjian and Sabian use this alloy for their professional lines. A Swiss company, Paiste, uses a softer B8 bronze which is made from 8% tin and 92% copper in nearly all of their cymbals. Zildjian and Sabian use this metal too, in their budget priced cymbals.

As the tin content in a bell or cymbal rises, the timbre drops. [12] As well as B8 and B20, Meinl Percussion uses B10 (10% tin) and B12 (12% tin) alloys for cymbals, which have timbres roughly between B8 and B20.[13]

Bronze is also used for the windings of steel strings of various stringed instruments such as the double bass, piano, harpsichord, and the guitar, replacing former gut and nylon strings. Bronze strings are commonly reserved on pianoforte for the lower pitch tones, as they possess a superior sustain quality to that of high-tensile steel.[14]

Bronzes of various metallurgical properties are widely used in struck idiophones around the world, notably in South East Asia, and most famously for the Javanese gamelan and other glockenspiel-like musical instruments. The earliest bronze archeological finds in Indonesia date from 1–2 BCE, including flat plates probably suspended and struck by a wooden or bone mallet.[14][15]

Some companies are now making saxophones from phosphor bronze (3.5 to 10% tin and up to 1% phosphorus content).

[edit] Medals

A large bronze cast medallion, some 9.5 by 8.7 centimetres in measurement, created by the celebrated medalist Valerio Belli in the sixteenth century.Bronze has been used in the manufacture of various types of medals for centuries, and are known in contemporary times for being awarded to the second-runner up in sporting competitions and other events. The later usage was in part attributed to the choices of Gold, Silver and Bronze to represent the first three Ages of Man in Greek mythology: the Golden Age, when men lived among the gods; the Silver age, where youth lasted a hundred years; and the Bronze Age, the era of heroes, and

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was first adopted at the 1904 Summer Olympics. At the 1896 event, silver was awarded to winners and bronze to runners-up, while at 1900 other prizes were given, not medals.

[edit] See also

Fragment of the grave of Cyprian Kamil Norwid in the Bards' crypt in Wawel Cathedral, Kraków, Poland by sculptor Czesław Dźwigaj

List of copper alloys Art object Bronze medal Bronze sculpture Bronzing Chinese bronze inscriptions French Empire mantel clock Seagram Building DZR Dezincification Resistant Brass UNS C69100

Cupronickel

Cupronickel or copper-nickel (sometimes incorrectly referred to as "cupernickel") is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese. Cupronickel is highly resistant to corrosion in seawater, because its electrode potential is adjusted to be neutral with regard to seawater. Because of this, it is used for piping, heat exchangers and condensers in seawater systems as well as marine hardware, and sometimes for the propellers, crankshafts and hulls of premium tugboats, fishing boats and other working boats.

A more familiar common use is in silver-coloured modern circulation coins. A typical mix is 75% copper, 25% nickel, and a trace amount of

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manganese. In the past true silver coins were debased with cupronickel. Despite high copper content, the colour of cupro-nickel remarkably is silver.It is used in thermocouples, and the 55% copper/45% nickel alloy constantan is used to make resistors whose resistance is stable across changes in temperature.

Monel metal is a nickel-copper alloy, containing minimum 63% nickel.

Other names

Aside from cupro-nickel, many other terms exist which describe the same material. Still registered as tradenames are Alpaka or Alpacca (registered trademark), Argentan Minargent, the French term name Cuivre blanc Occasionally cupro-nickel is also referred to as "hotel silver", plata alemana (Spanish for "German Silver"), "German silver" and "Chinese silver"[1].

[edit] History

Cupronickel coin of king Pantaleon c. 170 BCE.Obv: Bust of Dionysos with a wreath of leaves.Rev: Panther with a small bell around the neck, touching a vine with the left leg. Greek legend: BASILEOS PANTALEONTOS "King Pantaleon".Cupro-nickel was known to the Romans as an artificial "white" gold or silver termed "claudianum" and very possibly the"molybdochalcum" of the Alexandrians.

The cupro-nickel alloy was known by Chinese since circa 3rd century BCE as "white copper" (some weapons from the Warring States Period were in copper-nickel alloy).[2]

The ancient Greeks were producing it and a lower quality imitation of it in the Aegean Bronze Age and known as "orichalum". The Greco-Bactrian kings Euthydenus II dating from 180 to 170 BCE and his younger brothers Pantaleon and Agathocles around 170 BC.[3]

The theory of Chinese origins of Bactrian cupro-nickel was suggested in 1868 by Flight, who found the coins and considered the oldest cupro-nickel coins yet discovered were of a very similar alloy to Chinese

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paktong.[4] Cunningham in 1873 argued the coins must have been the result of overland trade from China, through India to Greece — highly controversial at the time and much derided. In 1973, Cheng and Schwitter in their new analyses argued the Bactrian alloys (copper, lead, iron, nickel and cobalt) were closely similar to Chinese paktong, and that out of nine known Asian nickel deposits, only those in China could provide same identical chemical content ratios.[4] However this hypothesis, although widely publicised, was later disproven by a perhaps over-enthusiastic oversight of the well-known Persian arsenic-nickel mines much closer to Bactria and known to be exploited by the Greeks and Persians.[4]

[edit] Chinese history of cupronickel

The author-scholar Ho Wei describes most exactly the process in circa 1095 CE, which suggest the Chinese were not aware that nickel was a metal in its own right. The paktong alloy was described as being made from adding small pills of naturally-occurring "Yunnan" ore to a bath of molten copper. When a crust of slag formed, saltpeter was added, the alloy stirred and the ingot immediately cast. Zinc is mentioned as an ingredient — but not detailed when exactly it was added. The ore used is noted as solely available from Yunnan, related from the story:

San Mao Chun were at Tanyang during a famine year when many people died, so taking certain chemicals, Ying projected them onto silver, turning it into gold, and he also transmuted iron into silver — thus enabling the lives of many to be saved [through purchasing grain through this fake silver and gold] Thereafter all those who prepared chemical powders by heating and transmuting copper by projection called their methods "Tanyang techniques".[5]

The late Ming and Ching literature have very little information about paktong. However, it is first mentioned specifically by name in the Thien Kung Khai Wu of circa 1637:

When lu kan shih (zinc carbonate, calamine) or wo chhein (zinc metal) is mixed and combined with chih thung (copper), one gets 'yellow bronze' (ordinary brass). When phi shang and other arsenic substances are heated with it, one gets 'white bronze' or white copper: pai thong. When alum and niter and other chemicals are mixed together one gets ching thung: green bronze.[5]

Ko Hung of the 300 CE stated:" The Tanyang copper was created by throwing a mercuric elixir into Tanyang copper and heated- gold will be formed." However, the Pha Phu Tsu and the Shen I Ching describing a statue in the Western provinces as being of silver, tin, lead and Tanyang

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copper — which looked like gold, and could be forged for plating and inlaying vessels and swords.[4]

Needham et al. argue that cupro-nickel was at least known as a unique alloy by the Chinese during the reign of Liu An in 120 BCE in Yunnan. Moreover the Yunnanese State of Tien was founded in 334 BCE as a colony of the Chu. Most likely modern paktong was unknown to Chinese of the day — but the naturally occurring Yunnan ore upro-nickek alloy was likely a valuable internal trade commodity.[4]

[edit] Western re-discovery

The alloy seems to have been re-discovered by the West during alchemy experiments, notably Andreas Libavius, in his Alchemia of 1597 where he mentions a surface-whitened copper aes album by mercury or silver, but in De Natura Metallorum in Singalarum Part 1, of 1599 the same term was applied to '"tin" from the East Indies (modern-day Indonesia and the Philippines) and given the Spanish name: tintinaso.[5]

Bishop Watson of Cambridge appears to be the first to discover cupro-nickel was an alloy of three metals. In attempting to re-discover the secret of white-copper critiqued du Halde's History of China (1688) confusing the term paktong', Bishop Watson noted the Chinese of his day did not form it as an alloy, but smelt readily available unprocessed ore:

appeared from a vast series of experiments made at Peking- that it occurred naturally as an ore mined at the region, the most extraordinary copper is pe-tong or white copper: it is white when dug out of the mine and even more white within than without. It appears , by a vast number of experiments made at Peking, that its colour is owing to no mixture; on the contrary, all mixtures diminish its beauty, for, when it is rightly managed it looks exactly like silver and were there not a necessity of mixing a little tutenag or such metal to soften it, it would be so much more the extraordinary as this sort of copper is found no where but in China and that only in the Province of Yunnan". Notwithstanding what is here said, of the colour of the copper being owing to no mixture, it is certain the Chinese white copper as brought to us, is a mixt [sic: mixed] metal; so that the ore from which it was extracted must consist of various metallic substances; and from such ore that the natural orichalum if it ever existed, was made".[4]

During the peak European importation of Chinese white-copper during 1750 to 1800, increased attention was made to its discovering its constituents—Peat and Cookson found that: "the darkest proved to contain 7.7% nickel and the lightest said to be indistinguishable from

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silver with a characteristic bell-like resonance when struck and considerable resistance to corrosion, 11.1%".

Another trial by Fyfe estimated the nickel content at 31.6%. Guesswork ended when a Dr Dinwiddie of the Macartney Embassy of 1793 brought back, at considerable personal risk (smuggling of paktong ore was a capital crime by the Chinese Emperor) some of the ore from which paktong was made.[6] Cupro-nickel was now widely understood and published by E. Thomason, in 1823, in his submission later rejected for not being new knowledge to the Royal Society of Arts.

Efforts to duplicate exactly the Chinese paktong failed in Europe due to a general lack of requisite complex cobalt-nickel-arsenic naturally occurring ore. However, the Schneeburg district of Germany, where the famous Blaufarbenwerke made cobalt blue and other pigments solely held the requisite complex cobalt-nickel-arsenic ores in Europe.At the same time the Prussian Verein zur Beförderung des Gewerbefleißes ("Society for the Improvement of Business Diligence/Industriousness") offered a prize for the mastery of the process and unsurprisingly, Dr E.A. Geitner and J.R. von Gersdoff of Schneeburg duly won the prize and launched their German silver under the trade name Argentan and Neusilber ("new Silver")[6]

In 1829, Percival Norton Johnston persuaded Dr Geitner to establish a foundry in Bow Common behind Regents' Park Canal in London and obtained ingots of nickel-silver of 18% Ni, 55% Cu and 27% Zn.[6]

Between 1829 and 1833—Percival Norton Johnson was the first man to refine cupro-nickel on the British Isles: and became a wealthy man producing in excess of 16.5 tonnes per year, mainly made into cutlery by the Birmingham firm William Hutton and sold under the trade-name "Argentine".Johnsons' most serious competitor, Charles Askin and Brok Evans, under the brilliant chemist Dr. EW Benson devised greatly improved methods of cobalt and nickel suspension and marketed their own brand of nickel-silver: British Plate.[6]

[edit] Coinage

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Five Swiss franks

In Europe, Switzerland pioneered the nickel billion coinage in 1850, with the addition of silver. In 1879, Switzerland adopted the far cheaper 75:25 copper to nickel ratio then being used by the Belgians, the United States, and Germany.

Cupro-nickel was not used again in coinage until the 20th century. Cupro-nickel is the cladding on either side of United States Half Dollars (50¢) since 1971, and all quarters (25¢) and dimes (10¢) made after 1964. Currently some circulating coins like the United States Jefferson Nickel (5¢),[7] the Swiss franc, and the South Korean 500 and 100 won are made of solid cupro-nickel (75/25 ratio).[8]

[edit] Marine engineering grades

By the 1920s, a 70-30 copper-nickel grade was developed for naval condensers. Soon afterwards, a 2% manganese and 2% iron alloy was also developed for a UK power station which needed better erosion resistance because the levels of entrained sand in the seawater. A 90-10 alloy first became available in the 1950s, initially for seawater piping, and is now the more widely used alloy.

The alloys are:

UNS Standard Compositions* of wrought alloys. Maximum or Range

Alloy UNS No.Common name

Nickel IronManganese

Copper

C70600 90-10 9-11 1-1.8 1 RemainderC71500 70-30 29-33 0.4-1.0 1 RemainderC71640 66-30-2-2 29-32 1.7-2.3 1.5-2.5 Remainder

These values may vary in other standards

There are subtle differences in corrosion resistance and strength which determine which alloy is selected. Descending the table, the maximum allowable flow rate in piping increases as does the strength.

Typical minimum mechanical properties and maximum velocities

Copper Nickel Alloy

Yield strength (MPa)

Tensile strength (MPa)

Typical max. velocity at 100 mm piping bore diameter (m/s)

90-10 105 125 3.5

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70-30 275 358 4.066-30-2-2 170 435 6.0

Panels after 12 months exposure at Langstone Harbour, UK. Left to right: steel, 90-10 copper-nickel sheathed steel, copper-nickel; all three with aluminium sacrificial anodes for corrosion protection. Far right: unprotected copper nickel showing no macrofouling

In seawater, the alloys have excellent corrosion rates which remain low as long as the maximum design flow velocity is not exceeded. This velocity depends on geometry and pipe diameter. They have high resistance to crevice corrosion, stress corrosion cracking and hydrogen embrittlement that can be troublesome to other alloy systems. Copper-nickels naturally form a thin protective surface layer over the first several weeks of exposure to seawater and this provides its on-going resistance. Additionally, they have a high inherent biofouling resistance to attachment by macrofoulers ( e.g. seagrasses and molluscs) living in the seawater. To use this property to its full potential, the alloy needs to be free of the effects or insulated from any form of cathodic protection.

However, copper-nickels can show high corrosion rates in polluted or stagnant seawater when sulfides or ammonia are present. It is important therefore to avoid exposure to such conditions particularly during commissioning and refit while the surface films are maturing. Ferrous sulfate dosing to sea water systems can provide improved resistance.

As copper and nickel alloy with each other easily and have simple structures, the alloys are ductile and readily fabricated. Strength and hardness for each individual alloy is increased by cold working; they are

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not hardened by heat treatment. Joining of 90-10 (C70600) and 70-30 (C71500) is possible by both welding or brazing. They are both weldable by the majority of techniques although autogenous (welding without weld consumables) or oxy-acetylene methods are not recommended. 70-30 rather than 90-10 weld consumables are normally preferred for both alloys and no post-weld heat treatment is required. They can also be welded directly to steel providing a 65% nickel-copper weld consumable is used to avoid iron dilution effects. Brazing requires appropriate silver-base brazing alloys. The C71640 alloy tends to be used as seamless tubing and expanded rather than welded into the tube plate.

The Asperida nearing 40 years old; its 70-30 copper-nickel hull is being inspected before being refitted.

Applications for copper-nickels have withstood the test of time as they are still widely used and range from seawater system piping, condensers and heat exchangers in naval vessels, commercial shipping, multi-stage flash desalination and power stations. They have also been used as splash zone cladding on offshore structures and protective cladding on boat hulls as well as for solid hulls themselves.

For a discussion about copper nickel alloys in aquaculture, go to: [[1]]

[edit] Other usage

Single-core thermocouple cables use a single conductor pair of thermocouple conductors such as iron-constantan, copper constantan or nickel-chromium/nickel-aluminium. These have the heating element of constantan or nickel-chromium alloy within a sheath of copper, cupro-

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nickel or stainless steel.[9]

Beginning around the turn of the 20th century, bullet jackets were commonly made from this material. It was soon replaced with gilding metal to reduce metal fouling in the bore.

Currently, cupronickel remains the basic material for silver-plated cutlery. It is commonly used for mechanical and electrical equipment, medical equipment, zippers, jewelry items, and as material for strings for string instruments.

For side-arms (pistols), nickel is the favoured metal for the trigger. For high-quality cylinder lock and locking systems, the cylinder core is made from wear-resistant cupronickel.

[edit] Physical properties

Cupro-nickel (70/30 ratio) melts at 1170 °C and has a density of 8,910 kg/m3 (0.322 lb/in3).[10]

Broad properties overview for the whole family of Cupro-nickel alloys:

Melting point : from about 900 °C, depending on alloy content Density : 8.1 to 8.7 g·cm−3 (depending on the mix) Electrical conductivity : approx (3–5)×106 (Ω·m)−1 Thermal conductivity : about 25-35 W·m−1·K−1 Thermal expansion coefficient : 16 to 17 µm·m−1·K−1

Aluminium (UK i / ̩ æ l j ʉ ̍ m ɪ n i ə m / AL-ew-MIN-ee- ə m )[4] or aluminum (US i / ə ̍ l u ː m ɪ n ə m / ə -LOO-mi-n ə m ) is a silvery white member of the boron group of chemical elements. It has the symbol Al and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element, after oxygen and silicon. It makes up about 8% by weight of the Earth's solid surface. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals.[5] The chief source of aluminium is bauxite ore.

Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are very important in other areas of transportation and building. Its reactive nature makes it useful as a catalyst or additive in chemical mixtures, including ammonium nitrate explosives, to enhance blast power.

Despite its prevalence in the environment, aluminum salts are not known

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to be used by any form of life. Also in keeping with the element's abundance, it is well tolerated by plants in soils (in which it is a major component), and to a lesser extent, by animals as a component of plant materials in the diet (which often contain traces of dust and soil). Soluble aluminum salts have some demonstrated toxicity to animals if delivered in quantity by unnatural routes, such as injection. Controversy still exists about aluminum's possible long-term toxicity to humans from larger ingested amounts.

Characteristics

Etched surface from a high purity (99.9998%) aluminium bar, size 55×37 mm

Aluminium is a soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. Aluminium is nonmagnetic and nonsparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.[6]

Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.

Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.[6] This corrosion resistance is also often greatly reduced when many aqueous salts are present, particularly in the presence of dissimilar metals.

Aluminium atoms are arranged in a face-centered cubic (fcc) structure. Aluminium has a stacking-fault energy of approximately 200 mJ/m2.[7]

Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints.

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Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.[8]

Aluminium is a good thermal and electrical conductor, having 62% the conductivity of copper. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 kelvins and a critical magnetic field of about 100 gauss (10 milliteslas).[9]

Creation

Stable aluminium is created when hydrogen fuses with magnesium in either large stars or in supernovae.[10]

Isotopes

Main article: Isotopes of aluminium

Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2×105 y) occur naturally; 27Al has a natural abundance above 99.9%. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales.[11] Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding drastically reduces 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[12]

Natural occurrence

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See also: Aluminium in Africa

In the Earth's crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon).[13] Because of its strong affinity to oxygen, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Native aluminium metal can be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[14] It also occurs in the minerals beryl, cryolite, garnet, spinel and turquoise.[15] Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and sapphire, respectively.[13] Pure Al2O3, known as corundum, is one of the hardest materials known.[15]

Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3-2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.[16] Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica but the primary mining areas for the ore are in Ghana, Indonesia, Russia and Surinam.[17]

Smelting of the ore mainly occurs in Australia, Brazil, Canada, Norway, Russia and the United States.[17] Because smelting is an energy-intensive process, regions with excess natural gas supplies (such as the United Arab Emirates) are becoming aluminium refiners.

Production and refinement

Although aluminium is the most abundant metallic element in the Earth's crust, it is never found in free, metallic form, and it was once considered a precious metal more valuable than gold. Napoleon III, Emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the others made do with gold.[18][19] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time, when aluminium was as expensive as silver.[20] Aluminium has been produced in commercial quantities for just over 100 years.

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Bauxite

Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. There is an indirect carbothermic reduction possible by using carbon and Al2O3, which forms an intermediate Al4C3 and this can further yield aluminium metal at a temperature of 1900–2000°C. This process is still under development. This process costs less energy and yields less CO2 than the Hall-Héroult process, the major industrial process for aluminium extraction.[21]

Aluminium oxide has a melting point of about 2,000 °C (3,600 °F). Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite with calcium fluoride and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C (1,740 to 1,800 °F). Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium and sodium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. The reaction at the cathode is:

Al3+ + 3 e− → Al

Here the aluminium ion is being reduced. The aluminium metal then sinks to the bottom and is tapped off, usually cast into large blocks called aluminium billets for further processing.

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At the anode, oxygen is formed:2 O2− → O2 + 4 e−

This carbon anode is then oxidized by the oxygen, releasing carbon dioxide:

O2 + C → CO2

The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process.

Unlike the anodes, the cathodes are not oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

World production trend of aluminium

Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kiloamperes; state-of-the-art smelters[22] operate at about 350 kA. Trials have been reported with 500 kA cells.

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, Ghana, the South Island of New Zealand, Australia, the People's Republic of China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.[23]

Aluminium output in 2005

In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share, followed by Russia, Canada, and the USA, reports the British Geological Survey.

Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina.[24] Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some

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refining problems, some being high in silica, but have the advantage of being shallow and relatively easy to mine.[25]

See also: Category:Aluminium minerals

Recycling

Aluminium recycling code

Main article: Aluminium recycling

Aluminium is 100% recyclable without any loss of its natural qualities. Recovery of the metal via recycling has become an important facet of the aluminium industry.

Recycling involves melting the scrap, a process that requires only 5% of the energy used to produce aluminium from ore, though a significant part (up to 15% of the input material) is lost as dross (ash-like oxide).[26]

The dross can undergo a further process to extract aluminium.

Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public awareness.

In Europe aluminium experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles.[27]

Recycled aluminium is known as secondary aluminium, but maintains the same physical properties as primary aluminium. Secondary aluminium is produced in a wide range of formats and is employed in 80% of the alloy injections. Another important use is for extrusion.

White dross from primary aluminium production and from secondary recycling operations still contains useful quantities of aluminium that can be extracted industrially.[28] The process produces aluminium billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia), which spontaneously ignites on contact with air;[29] contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, the waste has found use as a filler in asphalt and concrete.[30]

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Chemistry

Oxidation state +1

AlH is produced when aluminium is heated in an atmosphere of hydrogen. Al2O is made by heating the normal oxide, Al2O3, with silicon at 1,800 °C (3,272 °F) in a vacuum.[31]

Al2S can be made by heating Al2S3 with aluminium shavings at 1,300 °C (2,372 °F) in a vacuum.[31] It quickly disproportionates to the starting materials. The selenide is made in a parallel manner.

AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium. Aluminium halides usually exist in the form AlX3, where X is F, Cl, Br, or I.[31]

Oxidation state +2

Aluminium monoxide, AlO, has been detected in the gas phase after explosion[32] and in stellar absorption spectra.[33]

Oxidation state +3

Fajans' rules show that the simple trivalent cation Al3+ is not expected to be found in anhydrous salts or binary compounds such as Al2O3. The hydroxide is a weak base and aluminium salts of weak acids, such as carbonate, cannot be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.

Aluminium hydride, (AlH3)n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent. Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li[AlH4]. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry, particularly as a reducing agent. The aluminohalides have a similar structure.Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and forming aluminates with alkalis. It exists in various crystalline forms.

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Aluminium carbide, Al4C3 is made by heating a mixture of the elements above 1,000 °C (1,832 °F). The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al2(C2)3, is made by passing acetylene over heated aluminium.

Aluminium nitride, AlN, can be made from the elements at 800 °C (1,472 °F). It is hydrolysed by water to form ammonia and aluminium hydroxide. Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine.

Aluminium oxide, Al2O3, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride, and carborundum. It is almost insoluble in water. Aluminium sulfide, Al2S3, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic.

Aluminium iodide, AlI3, is a dimer with applications in organic synthesis. Aluminium fluoride, AlF3, is made by treating the hydroxide with HF, or can be made from the elements. It is a macromolecule, which sublimes without melting at 1,291 °C (2,356 °F). It is very inert. The other trihalides are dimeric, having a bridge-like structure.

When aluminium and fluoride are together in aqueous solution, they readily form complex ions such as [AlF(H2O)5]2+, AlF3(H2O)3, and [AlF6]3−. Of these, [AlF6]3− is the most stable. This is explained by the fact that aluminium and fluoride, which are both very compact ions, fit together just right to form the octahedral aluminium hexafluoride complex. When aluminium and fluoride are together in water in a 1:6 molar ratio, [AlF6]3− is the most common form, even in rather low concentrations.Organometallic compounds of empirical formula AlR3 exist and, if not also polymers, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium.

Analysis

The presence of aluminium can be detected in qualitative analysis using aluminon.

Applications

General use

Aluminium is the most widely used non-ferrous metal.[34] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded

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that of any other metal except iron (837.5 million tonnes).[35] Forecast for 2012 is 42–45 million tons, driven by rising Chinese output.[36] Relatively pure aluminium is encountered only when corrosion resistance and/or workability is more important than strength or hardness. A thin layer of aluminium can be deposited onto a flat surface by physical vapour deposition or (very infrequently) chemical vapour deposition or other chemical means to form optical coatings and mirrors. When so deposited, a fresh, pure aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation.

Pure aluminium has a low tensile strength, but when combined with thermo-mechanical processing, aluminium alloys display a marked improvement in mechanical properties, especially when tempered. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength-to-weight ratio. Aluminium readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon (e.g., duralumin). Today, almost all bulk metal materials that are referred to loosely as "aluminium", are actually alloys. For example, the common aluminium foils and beverage cans are alloys of 92% to 99% aluminium.[37]

Household aluminium foil

Aluminium-bodied Austin "A40 Sports" (circa 1951)

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Aluminium slabs being transported from a smelter.

Some of the many uses for aluminium metal are in:

Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles etc.) as sheet, tube, castings etc.

Packaging (cans, foil, etc.) Construction (windows, doors, siding, building wire, etc.) A wide range of household items, from cooking utensils to baseball

bats, watches.[38] Street lighting poles, sailing ship masts, walking poles etc. Outer shells of consumer electronics, also cases for equipment e.g.

photographic equipment. Electrical transmission lines for power distribution MKM steel and Alnico magnets Super purity aluminium (SPA, 99.980% to 99.999% Al), used in

electronics and CDs. Heat sinks for electronic appliances such as transistors and CPUs. Substrate material of metal-core copper clad laminates used in

high brightness LED lighting. Powdered aluminium is used in paint, and in pyrotechnics such as

solid rocket fuels and thermite. Aluminium can be reacted with hydrochloric acid to form hydrogen

gas. A variety of countries, including France, Italy, Poland, Finland, Ro-

mania, Israel, and the former Yugoslavia, have issued coins struck in aluminium or aluminium-copper alloys.[39]

Aluminium compounds

Aluminium ammonium sulfate ([Al(NH4)](SO4)2), ammonium alum is used as a mordant, in water purification and sewage treatment, in paper production, as a food additive, and in leather tanning.

Aluminium acetate is a salt used in solution as an astringent. Aluminium borate (Al2O3 B2O3) is used in the production of glass

and ceramic. Aluminium borohydride (Al(BH4)3) is used as an additive to jet fuel. Aluminium bronze (CuAl5)

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Aluminium chloride (AlCl3) is used in paint manufacturing, in an-tiperspirants, in petroleum refining and in the production of syn-thetic rubber.

Aluminium chlorohydrate is used as an antiperspirant and in the treatment of hyperhidrosis.

Aluminium fluorosilicate (Al2(SiF6)3) is used in the production of synthetic gemstones, glass and ceramic.

Aluminium hydroxide (Al(OH)3) is used: as an antacid, as a mor-dant, in water purification, in the manufacture of glass and ce-ramic and in the waterproofing of fabrics.

Aluminium oxide (Al2O3), alumina, is found naturally as corundum (rubies and sapphires), emery, and is used in glass making. Syn-thetic ruby and sapphire are used in lasers for the production of coherent light. Used as a refractory, essential for the production of high pressure sodium lamps.

Aluminium phosphate (AlPO4) is used in the manufacture: of glass and ceramic, pulp and paper products, cosmetics, paints and var-nishes and in making dental cement.

Aluminium sulfate (Al2(SO4)3) is used in the manufacture of paper, as a mordant, in a fire extinguisher, in water purification and sewage treatment, as a food additive, in fireproofing, and in leather tanning.

Aqueous aluminium ions (such as found in aqueous aluminium sul-fate) are used to treat against fish parasites such as Gyrodactylus salaris.

In many vaccines, certain aluminium salts serve as an immune ad-juvant (immune response booster) to allow the protein in the vac-cine to achieve sufficient potency as an immune stimulant.

Aluminium alloys in structural applications

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Aluminium foam

Main article: Aluminium alloy

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).

The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation.

One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life.Another important property of aluminium alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminium, unlike steel, melts without first glowing red. Forming operations where a blow torch is used therefore require some expertise, since no visual signs reveal how close the material is to melting. Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing

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by heat-treating the parts in an oven, followed by gradual cooling—in effect annealing the stresses.

The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region.

Household wiring

See also: Aluminium wire

Compared to copper, aluminium has about 65% of the electrical conductivity by volume, although 200% by weight. Traditionally copper is used as household wiring material. In the 1960s aluminium was considerably cheaper than copper, and so was introduced for household electrical wiring in the United States, even though many fixtures were not designed to accept aluminium wire. In some cases the greater coefficient of thermal expansion of aluminium causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection. Also, pure aluminium has a tendency to creep under steady sustained pressure (to a greater degree as the temperature rises), again loosening the connection. Finally, galvanic corrosion from the dissimilar metals increased the electrical resistance of the connection.

All of this resulted in overheated and loose connections, and this in turn resulted in fires. Builders then became wary of using the wire, and many jurisdictions outlawed its use in very small sizes in new construction. Eventually, newer fixtures were introduced with connections designed to avoid loosening and overheating. The first generation fixtures were marked "Al/Cu" and were ultimately found suitable only for copper-clad aluminium wire, but the second generation fixtures, which bear a "CO/ALR" coding, are rated for unclad aluminium wire. To adapt older assemblies, workers forestall the heating problem using a properly done crimp of the aluminium wire to a short "pigtail" of copper wire. Today, new alloys, designs, and methods are used for aluminium wiring in combination with aluminium termination.

History

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The statue of the Anteros (commonly mistaken for either The Angel of Christian Charity or Eros) in Piccadilly Circus London, was made in 1893 and is one of the first statues to be cast in aluminium.

Ancient Greeks and Romans used aluminium salts as dyeing mordants and as astringents for dressing wounds; alum is still used as a styptic. In 1761 Guyton de Morveau suggested calling the base alum alumine. In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first termed alumium and later aluminum (see Etymology section, below).

The metal was first produced in 1825 (in an impure form) by Danish physicist and chemist Hans Christian Ørsted. He reacted anhydrous aluminium chloride with potassium amalgam and yielded a lump of metal looking similar to tin.[40] Friedrich Wöhler was aware of these experiments and cited them, but after redoing the experiments of Ørsted he concluded that this metal was pure potassium. He conducted a similar experiment in 1827 by mixing anhydrous aluminium chloride with potassium and yielded aluminium.[40] Wöhler is generally credited with isolating aluminium (Latin alumen, alum), but also Ørsted can be listed as its discoverer.[41] Further, Pierre Berthier discovered aluminium in bauxite ore and successfully extracted it.[42] Frenchman Henri Etienne Sainte-Claire Deville improved Wöhler's method in 1846, and described his improvements in a book in 1859, chief among these being the substitution of sodium for the considerably more expensive potassium.

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(Deville's book is De l'aluminium, ses propriétés, sa fabrication (Paris, 1859). Deville likely also conceived the idea of the electrolysis of aluminium oxide dissolved in cryolite; Charles Martin Hall and Paul Héroult might have developed the more practical process after Deville.)

Before the Hall-Héroult process was developed, aluminium was exceedingly difficult to extract from its various ores. This made pure aluminium more valuable than gold.[43] Bars of aluminium were exhibited at the Exposition Universelle of 1855,[44] and Napoleon III was said[citation

needed] to have reserved a set of aluminium dinner plates for his most honoured guests.

Aluminium was selected as the material to be used for the apex of the Washington Monument in 1884, a time when one ounce (30 grams) cost the daily wage of a common worker on the project;[45] aluminium was about the same value as silver.

The Cowles companies supplied aluminium alloy in quantity in the United States and England using smelters like the furnace of Carl Wilhelm Siemens by 1886.[46] Charles Martin Hall of Ohio in the U.S. and Paul Héroult of France independently developed the Hall-Héroult electrolytic process that made extracting aluminium from minerals cheaper and is now the principal method used worldwide. The Hall-Heroult process cannot produce Super Purity Aluminium directly. Hall's process,[47] in 1888 with the financial backing of Alfred E. Hunt, started the Pittsburgh Reduction Company today known as Alcoa. Héroult's process was in production by 1889 in Switzerland at Aluminium Industrie, now Alcan, and at British Aluminium, now Luxfer Group and Alcoa, by 1896 in Scotland.[48]

By 1895 the metal was being used as a building material as far away as Sydney, Australia in the dome of the Chief Secretary's Building.

Many navies have used an aluminium superstructure for their vessels; the 1975 fire aboard USS Belknap that gutted her aluminium superstructure, as well as observation of battle damage to British ships during the Falklands War, led to many navies switching to all steel superstructures. The Arleigh Burke class was the first such U.S. ship, being constructed entirely of steel.

In 2008 the price of aluminium peaked at $1.45/lb in July but dropped to $0.70/lb by December.[49]

Etymology

Nomenclature history

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The earliest citation given in the Oxford English Dictionary for any word used as a name for this element is alumium, which British chemist and inventor Humphry Davy employed in 1808 for the metal he was trying to isolate electrolytically from the mineral alumina. The citation is from the journal Philosophical Transactions of the Royal Society of London: "Had I been so fortunate as to have obtained more certain evidences on this subject, and to have procured the metallic substances I was in search of, I should have proposed for them the names of silicium, alumium, zirconium, and glucium."[50][51]

Davy settled on aluminum by the time he published his 1812 book Chemical Philosophy: "This substance appears to contain a peculiar metal, but as yet Aluminum has not been obtained in a perfectly free state, though alloys of it with other metalline substances have been procured sufficiently distinct to indicate the probable nature of alumina."[52] But the same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, in a review of Davy's book, objected to aluminum and proposed the name aluminium, "for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound."[53]

The -ium suffix conformed to the precedent set in other newly discovered elements of the time: potassium, sodium, magnesium, calcium, and strontium (all of which Davy isolated himself). Nevertheless, -um spellings for elements were not unknown at the time, as for example platinum, known to Europeans since the sixteenth century, molybdenum, discovered in 1778, and tantalum, discovered in 1802. The -um suffix is consistent with the universal spelling alumina for the oxide, as lanthana is the oxide of lanthanum, and magnesia, ceria, and thoria are the oxides of magnesium, cerium, and thorium respectively.

The spelling used throughout the 19th century by most U.S. chemists ended in -ium, but common usage is less clear.[54] The -um spelling is used in the Webster's Dictionary of 1828. In his advertising handbill for his new electrolytic method of producing the metal 1892, Charles Martin Hall used the -um spelling, despite his constant use of the -ium spelling in all the patents[47] he filed between 1886 and 1903.[55] It has consequently been suggested that the spelling reflects an easier to pronounce word with one fewer syllable, or that the spelling on the flier was a mistake. Hall's domination of production of the metal ensured that the spelling aluminum became the standard in North America; the Webster Unabridged Dictionary of 1913, though, continued to use the -ium version.

In 1926, the American Chemical Society officially decided to use

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aluminum in its publications; American dictionaries typically label the spelling aluminium as a British variant.

The name "aluminium" derives from its status as a base of alum. "Alum" in turn is a Latin word that literally means "bitter salt".[56]

Present-day spelling

Most countries use the spelling aluminium (with an i before -um). In the United States, this spelling is largely unknown, and the spelling aluminum predominates.[57][58] The Canadian Oxford Dictionary prefers aluminum, whereas the Australian Macquarie Dictionary prefers aluminium.

The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminium as the standard international name for the element in 1990, but three years later recognized aluminum as an acceptable variant. Hence their periodic table includes both.[59] IUPAC prefers the use of aluminium in its internal publications, although nearly as many IUPAC publications use the spelling aluminum.[60]

Health concerns

NFPA 704000Fire diamond for aluminium shot

Despite its natural abundance, aluminium has no known function in living cells and presents some toxic effects in elevated concentrations. Its toxicity can be traced to deposition in bone and the central nervous system, which is particularly increased in patients with reduced renal function. Because aluminium competes with calcium for absorption, increased amounts of dietary aluminium may contribute to the reduced skeletal mineralization (osteopenia) observed in preterm infants and infants with growth retardation. In very high doses, aluminium can cause neurotoxicity, and is associated with altered function of the blood-brain barrier.[61] A small percentage of people are allergic to aluminium and experience contact dermatitis, digestive disorders, vomiting or other

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symptoms upon contact or ingestion of products containing aluminium, such as deodorants or antacids. In those without allergies, aluminium is not as toxic as heavy metals, but there is evidence of some toxicity if it is consumed in excessive amounts.[62] Although the use of aluminium cookware has not been shown to lead to aluminium toxicity in general, excessive consumption of antacids containing aluminium compounds and excessive use of aluminium-containing antiperspirants provide more significant exposure levels. Studies have shown that consumption of acidic foods or liquids with aluminium significantly increases aluminium absorption,[63] and maltol has been shown to increase the accumulation of aluminium in nervous and osseus tissue.[64] Furthermore, aluminium increases estrogen-related gene expression in human breast cancer cells cultured in the laboratory.[65] The estrogen-like effects of these salts have led to their classification as a metalloestrogen.Because of its potentially toxic effects, aluminium's use in some antiperspirants, dyes (such as aluminium lake), and food additives is controversial. Although there is little evidence that normal exposure to aluminium presents a risk to healthy adults,[66] several studies point to risks associated with increased exposure to the metal.[67] Aluminium in food may be absorbed more than aluminium from water.[68] Some researchers have expressed concerns that the aluminium in antiperspirants may increase the risk of breast cancer,[69] and aluminium has controversially been implicated as a factor in Alzheimer's disease.[70]

The Camelford water pollution incident involved a number of people consuming aluminium sulfate. Investigations of the long-term health effects are still ongoing, but elevated brain aluminium concentrations have been found in post-mortem examinations of victims who have later died, and further research to determine if there is a link with cerebral amyloid angiopathy has been commissioned.[71]

According to The Alzheimer's Society, the overwhelming medical and scientific opinion is that studies have not convincingly demonstrated a causal relationship between aluminium and Alzheimer's disease.[72]

Nevertheless, some studies, such as those on the PAQUID cohort,[73] cite aluminium exposure as a risk factor for Alzheimer's disease. Some brain plaques have been found to contain increased levels of the metal.[74]

Research in this area has been inconclusive; aluminium accumulation may be a consequence of the disease rather than a causal agent. In any event, if there is any toxicity of aluminium, it must be via a very specific mechanism, since total human exposure to the element in the form of naturally occurring clay in soil and dust is enormously large over a lifetime.[75][76] Scientific consensus does not yet exist about whether aluminium exposure could directly increase the risk of Alzheimer's disease.[72]

Effect on plants

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Aluminium is primary among the factors that reduce plant growth on acid soils. Although it is generally harmless to plant growth in pH-neutral soils, the concentration in acid soils of toxic Al3+ cations increases and disturbs root growth and function.[77][78][79]

Most acid soils are saturated with aluminium rather than hydrogen ions. The acidity of the soil is therefore a result of hydrolysis of aluminium compounds.[80] This concept of "corrected lime potential"[81] to define the degree of base saturation in soils became the basis for procedures now used in soil testing laboratories to determine the "lime requirement"[82] of soils.[83]

Wheat's adaptation to allow aluminium tolerance is such that the aluminium induces a release of organic compounds that bind to the harmful aluminium cations. Sorghum is believed to have the same tolerance mechanism. The first gene for aluminium tolerance has been identified in wheat. It was shown that sorghum's aluminium tolerance is controlled by a single gene, as for wheat.[84] This is not the case in all plants.

See also

Book:Aluminium

Books are collections of articles that can be downloaded or ordered in print. Aluminium: The Thirteenth Element Aluminium alloy The Aluminum Association Aluminium battery Aluminium foil Beverage can Institute for the History of Aluminium (IHA) List of countries by aluminium production Aluminium industry in Russia

Al-Cu

Copper has been the most common alloying element almost since the beginning of the aluminum industry, and a variety of alloys in which copper is the major addition were developed.

In the cast alloys the basic structure consists of cored dendrites of aluminum solid solution, with a variety of constituents at the grain boundaries or interdendritic spaces, forming a brittle, more or less

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continuous network of eutectics. Wrought products consist of a matrix of aluminum solid solution with the other solible and insoluble constituents dispersed within it.

Copper has been the most common alloying element almost since the beginning of the aluminum industry, and a variety of alloys in which copper is the major addition were developed. Most of these alloys fall within one of the following groups:

Cast alloys with 5% Cu, often with small amounts of silicon and

magnesium.

Cast alloys with 7-8% Cu, which often contain large amounts of iron and silicon and appreciable amounts of manganese, chromium, zinc, tin, etc.

Cast alloys with 10-14% Cu. These alloys may contain small amounts of magnesium (0.10-0.30% Mg), iron up to 1.5%, up to 5% Si and smaller amounts of nickel, manganese, chromium.

Wrought alloys with 5-6% Cu and often small amounts of man-ganese, silicon, cadmium, bismuth, tin, lithium, vanadium and zirconium. Alloys of this type containing lead, bismuth, and cadmium have superior machinability.

Durals, whose basic composition is 4-4.5% Cu, 0.5-1.5% Mg, 0.5-1.0% Mn, sometimes with silicon additions.

Copper alloys containing nickel, which can be subdivided in two groups: the Y alloy type, whose basic composition is 4% Cu, 2% Ni, 1.5% Mg; and the Hyduminiums, which usually have lower copper contents and in which iron replaces 30me of the nickel.

In most of the alloys in this group aluminum is the primary constituent and in the cast alloys the basic structure consists of cored dendrites of aluminum solid solution, with a variety of constituents at the grain boundaries or interdendritic spaces, forming a brittle, more or less continuous network of eutectics.

Wrought products consist of a matrix of aluminum solid solution with the other constituents dispersed within it. Constituents formed in the alloys can be divided in two groups: in the soluble ones are the constituents containing only one or more of copper, lithium, magnesium, silicon, zinc; in the insoluble ones are the constituents containing at least one of the more or less insoluble iron, manganese, nickel, etc.

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The type of soluble constituents formed depends not only on the amount of soluble elements available but also on their ratio. Available copper depends on the iron, manganese and nickel contents; the copper combined with them is not available.

Copper forms (CuFe)Al6 and Cu2FeAl7, with iron, (CuFeMn)Al6 and Cu2Mn3Al20 with manganese, Cu4NiAl, and several not too well known compounds with nickel and iron. The amount of silicon available to some extent controls the copper compounds formed. Silicon above 1% favors the FeSiAl5, over the iron-copper compounds and (CuFeMn)3Si2Al15, over the (CuFeMn)Al6 and Cu2Mn3Al20 compounds.

Similarly, but to a lesser extent, available silicon is affected by iron and manganese contents. With the Cu:Mg ratio below 2 and the Mg:Si ratio well above 1.7 the CuMg4Al6 compound is formed, especially if appreciable zinc is present. When Cu:Mg > 2 and Mg:Si > 1.7, CuMgAl2

is formed. If the Mg:Si ratio is approximately 1.7, Mg2Si and CuAl2 are in equilibrium. With the Mg:Si ratio 1 or less, Cu2Mg8Si6Al5, is formed, usually together with CuAl2. When the copper exceeds 5%, commercial heat treatment cannot dissolve it and the network of eutectics does not break up. Thus, in the 10-15% Cu alloys there is little difference in structure between the as-cast and heat treated alloys.

Magnesium is usually combined with silicon and copper. Only if appreciable amounts of lead, bismuth or tin are present, Mg2Sn, Mg2Pb, Mg2Bi3 can be formed.

The effect of alloying elements on density and thermal expansion is additive; thus, densities range from 2 700 to 2 850 kg/m3, with the lower values for the high-magnesium, high-silicon and low-copper alloys, the higher for the high-copper, high-nickel, high-manganese and high-iron contents.

Expansion coefficients are of the order of 21-24 x 10-6 1/K for the 300-4000 K range and 23-26 x 10-6 1/K for the 300-700 K range, with the higher values for the high-magnesium, low-copper and low-silicon alloys, the lower ones for the higher silicon and higher copper contents. At subzero temperatures the coefficient decreases practically in the same way as that of pure aluminum. However, release of casting stresses or precipitation and solution of copper and magnesium produce changes in length of up to 0.2%, which may affect the dimensional accuracy of parts exposed to high temperature. Subzero treatment of castings to reduce warpage has been recommended. Specific heat of the commercial alloys is practically the same as for the binary aluminum-copper. Thermal conductivity is little affected by alloying elements other than copper: for the commercial alloys with 4-

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12% Cu, < 4% other elements, it is approximately 70% of that of pure aluminum at room temperature, some 75-80% at 600 K and 30-35% at 200 K.

Electric conductivity is very sensitive to copper in solution, and to a much lesser extent to magnesium and zinc, but is little affected by alloying elements out of solution. In an alloy with 5% Cu in solution the conductivity is approximately half that of pure aluminum (30-33% IACS), but in the annealed state an alloy with 12% Cu and up to 5% other elements has a conductivity of 37-42% IACS, only 25-30% lower than that of pure aluminum.

The mechanical properties of the alloys vary over an extremely wide range, from those of the sand cast 8% Cu alloys, which are among the lowest in aluminum alloys, to those of durals or wrought 5% Cu alloys, which may reach values of up to 650 MPa.

Higher purity, special compositions, fabricating techniques or heat treatments may produce higher properties. Porosity, poor feeding of castings, excessive amounts of impurities, segregation and poor quality control in fabrication may reduce the properties well below the determined limits. Surface defects reduce the properties of castings more than internal ones. Prestrain or elastic strain during testing have no effect on properties. Ultrasonic vibration may reduce or increase them; and irradiation at cryogenic temperatures may slightly increase strength. Dynamic loading may produce strength and ductility values higher or lower, depending on the speed, but not at high temperature. Temperatures below room temperature increase strength and hardness, with some loss of ductility and a decrease in anisotropy.

Correspondingly, exposure to temperatures above room temperature eventually results in a decrease in strength and hardness with a decided increase in elongation. Heat treatment has a substantial effect: if the alloys are quenched from high temperature and only naturally aged, exposure to temperatures in the range up to 500-600 K may produce a temporary increase in hardness and strength due to artificial aging. Eventually this increase disappears, the faster the higher the temperature, and the normal decline sets in, as in alloys already aged to peak hardness. Prolonged heating (for up to 2 years) results in appreciable softening at all temperatures. For intermediate exposure times this softening is less if the materials are thermo-mechanically treated. In short-time tests fast heating to test temperature increases the strength.

Impact resistance is low, as for all aluminum alloys: in the Charpy test values range from a minimum of 2-3 x 104N/m for cast alloys with 7% Cu

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to a maximum of 30-40 x 104N/m for wrought products in the naturally aged temper. Notch sensitivity is usually low, especially in the wrought alloys, or in the cast alloys heat treated to maximum ductility. The plane strain fracture toughness ranges from 85 to 100% of the yield strength, depending on a variety of factors. Both impact resistance and notch toughness increase with increasing temperature, but the decrease with subzero temperatures is limited. In the softer alloys at 70 K the difference is within error of testing; only for the higher-strength alloys is the decrease appreciable.

Shear strength is of the order of 70-75% of tensile strength, even at high temperature; bearing strength is approximately 1.5 of tensile; compressive yield strength is 10-15% higher or lower than ultimate tensile strength.

Most alloying elements raise the modulus of elasticity of aluminum, but the increase is not substantial: for the aluminum-copper alloys the modulus of elasticity at room temperature is of the order of 70-75 GPa and practically the same in tension and in compression. It changes regularly with temperature from a value of 76-78 GPa at 70 K to a value of the order of 60 GPa at 500 K. The change during aging is negligible for practical purposes. The Poisson ratio is slightly lower and of the order of 0.32-0.34, and so is the compressibility. The Poisson ratio increases with increasing temperature.

Many of the cast alloys and of the aluminum-copper-nickel alloys are used for high-temperature applications, where creep resistance is important. Resistance is the same whether the load is tensile or compressive.

Wear resistance is favored by high hardness and the presence of hard constituents. Alloys with 10-15% Cu or treated to maximum hardness have very high wear resistance.

Silicon increases the strength in cast alloys, mainly by increasing the castability and thus the soundness of the castings, but with some loss of ductility and fatigue resistance, especially when it changes the iron-bearing compounds from FeM2SiAl8 or Cu2FeAl7, to FeSiAl5.

Magnesium increases the strength and hardness of the alloys, but, especially in castings, with a decided decrease in ductility and impact resistance.

Iron has some beneficial strengthening effect, especially at high temperature and at the lower contents (< 0.7% Fe).

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Nickel has a strengthening effect, similar to that of manganese, although more limited because it only acts to reduce the embrittling effect of iron. Manganese and nickel together decrease the room-temperature properties because they combine in aluminum-manganese-nickel compounds and reduce the beneficial effects of each other. The main effect of-nickel is the increase in high-temperature strength, fatigue and creep resistance.

Titanium is added as grain refiner and it is very effective in reducing the grain size. If this results in a better dispersion of insoluble constituents, porosity and nonmetallic inclusions, a decided improvement in mechanical properties results.

Lithium has an effect very similar to that of magnesium: it increases strength, especially after heat treatment and at high temperatures, and there is a corresponding decrease in ductility. Zinc increases the strength but reduces ductility.

Precipitation hardening

Precipitation hardening, also called age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel and titanium, and some stainless steels. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material. The impurities play the same role as the particle substances in particle-reinforced composite materials. Just as the formation of ice in air can produce clouds, snow, or hail, depending upon the thermal history of a given portion of the atmosphere, precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging.

Note that two different heat treatments involving precipitates can alter the strength of a material: solution heat treating and precipitation heat treating. Solid solution strengthening involves formation of a single-phase solid solution via quenching and leaves a material softer. Precipitation heat treating involves the addition of impurity particles to increase a material's strength.[1] Precipitation hardening via precipitation heat treatment is the main topic of discussion in this article.

Kinetics versus thermodynamics

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This technique exploits the phenomenon of supersaturation, and involves careful balancing of the driving force for precipitation and the thermal activation energy available for both desirable and undesirable processes.

Nucleation occurs at a relatively high temperature (often just below the solubility limit) so that the kinetic barrier of surface energy can be more easily overcome and the maximum number of precipitate particles can form. These particles are then allowed to grow at lower temperature in a process called aging. This is carried out under conditions of low solubility so that thermodynamics drive a greater total volume of precipitate formation.

Diffusion's exponential dependence upon temperature makes precipitation strengthening, like all heat treatments, a fairly delicate process. Too little diffusion (under aging), and the particles will be too small to impede dislocations effectively; too much (over aging), and they will be too large and dispersed to interact with the majority of dislocations.

[edit] Alloy design

Precipitation strengthening is possible if the line of solid solubility slopes strongly toward the center of a phase diagram. While a large volume of precipitate particles is desirable, a small enough amount of the alloying element should be added that it remains easily soluble at some reasonable annealing temperature.Elements used for precipitation strengthening in typical aluminum and titanium alloys, make up about 10% of their composition. While binary alloys are more easily understood as an academic exercise, commercial alloys often use three components for precipitation strengthening, in compositions such as Al(Mg, Cu) and Ti(Al, V). A large number of other constituents may be unintentional, but benign, or may be added for other purposes such as grain refinement or corrosion resistance. In some cases, such as many aluminum alloys, an increase in strength is achieved at the expense of corrosion resistance.

The addition of large amounts of nickel and chromium needed for corrosion resistance in stainless steels means that traditional hardening and tempering methods are not effective. However, precipitates of chromium, copper or other elements can strengthen the steel by similar amounts in comparison to hardening and tempering. The strength can be tailored by adjusting the annealing process, with lower initial temperatures resulting in higher strengths. The lower initial temperature increase driving force of nucleation. More driving force means more nucleation sites, and more sites, means more places for dislocations to be disrupted while the finished part is in use.

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Many alloy systems allow the aging temperature to be adjusted. For instance, some aluminium alloys used to make rivets for aircraft construction are kept in dry ice from their initial heat treatment until they are installed in the structure. After this type of rivet is deformed into its final shape, aging occurs at room temperature and increases its strength, locking the structure together. Higher aging temperatures would risk over-aging other parts of the structure, and require expensive post-assembly heat treatment. Too high of an aging temperature promotes the precipitate to grow too readily.

[edit] Theory

The primary species of precipitation strengthening are second phase particles. These particles impede the movement of dislocations throughout the lattice. You can determine whether or not second phase particles will precipitate into solution from the solidus line on the phase diagram for the particles. Physically, this strengthening effect can be attributed both to size and modulus effects, and to interfacial or surface energy.

The presence of second phase particles often causes lattice distortions. These lattice distortions result when the precipitate particles differ in size from the host atoms. Smaller precipitate particles in a host lattice leads to a tensile stress, whereas larger precipitate particles leads to a compressive stress. Dislocation defects also create a stress field. Above the dislocation there is a compressive stress and below there is a tensile stress. Consequently, there is a negative interaction energy between a dislocation and a precipitate that each respectively cause a compressive and a tensile stress or vice versa. In other words, the dislocation will be attracted to the precipitate. In addition, there is a positive interaction energy between a dislocation and a precipitate that have the same type of stress field. This means that the dislocation will be repulsed by the precipitate.

Precipitate particles also serve by locally changing the stiffness of a material. Dislocations are repulsed by regions of higher stiffness. Conversely, if the precipitate causes the material to be locally more compliant, then the dislocation will be attracted to that region.

Furthermore, a dislocation may cut through a precipitate particle. This interaction causes an increase in the surface area of the particle. The area created is where, r is the radius of the particle and b is the magnitude of the burgers vector. The resulting increase in surface energy is where is the surface energy. The dislocation can also bow around a precipitate particle.

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[edit] Governing Equations

There are two equations to describe the two mechanisms for precipitation hardening:

Dislocations cutting through particles:

where τ is material strength, r is the second phase particle radius, γ is the surface energy, b is the magnitude of the Burgers vector, and L is the spacing between pinning points. This governing equation shows that the strength is proportional to r, the radius of the precipitate particles. This means that it is easier for dislocations to cut through a material with smaller second phase particles (small r). As the size of the second phase particles increases, the particles impede dislocation movement and it becomes increasingly difficult for the particles to cut through the material. In other words, the strength of a material increases with increasing r.Dislocations bowing around particle:

Where τ is the material strength, G is the shear modulus, b is the magnitude of the Burgers vector, L is the distance between pinning points, and r is the second phase particle radius. This governing equation shows that for dislocation bowing the strength is inversely proportional to the second phase particle radius r. Dislocation bowing is more likely to occur when there are large particles present in the material.

These governing equations show that the precipitation hardening mechanism depends on the size of the precipitate particles. At small r, cutting will dominate, while at large r, bowing will dominate.

Looking at the plot of both equations, it is clear that there is a critical radius at which max strengthening occurs. This critical radius is typically 5-30 nm.

[edit] Some precipitation hardening materials

2000-series aluminum alloys (important examples: 2024 and 2019, also Y alloy and Hiduminium)

6000-series aluminum alloys (important example: 6061 for bicycle frames and aeronautical structures)

7000-series aluminum alloys (important examples: 7075 and 7475) 17-4PH stainless steel (UNS S17400) Maraging steel Inconel 718

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Alloy X-750 René 41 Waspaloy

See also Alfred Wilm Strength of Materials Strengthening mechanisms of materials Metallurgy Superalloy

Bearing alloys.

Properties of the Alloys: Tinbase babbitts commonly containcopper and antimony following the pattern, though not necessarily the proportions, of Isaac Babbitt’s original alloy. They have hardness up to 32BHN which gives them excellent load-carrying characteristics. They show low friction resistance, low wear, good run-in properties and good emergency behavior in the absence of adequate lubrication. They “wet” easily and maintain an oil film, resist corrosion, are easily cast and bonded and retain good mechanical properties at elevated temperatures. Conventional leadbase babbitts contain antimony and tin, which greatly increase the strength and hardness of lead. Properties of the lead-base alloy simprove with the addition of antimony up to a maximum of 18%, above which the alloy becomes excessively brittle. The addition of tin to the lead and antimony improves mechanical and casting properties. At 10% tin, room-temperature strength and hardness reach a maximum. The lead-antimony-tin alloys are not the equal of tin-base alloys but are fully adequate for lower loads and moderate temperatures. Though alloys with lower tin content are easier to handle in the kettle, they are more difficult to bond. The very good frictional properties, reasonably good corrosion resistance and low cost of the lead-antimony-tin alloys makes them ideal for a wide range of applications.The lead-antimony-arsenic alloys are the equal of tin-base alloys in their ability to retain hardness and strength at elevated temperatures. In this respect they are superior to conventional lead-base alloys.

Bearing Operating Conditions:

The method or efficiency of lubrication is one of the factors affecting the choice of an alloy. Under poor lubricating conditions, an alloy of good conformity and run-in behavior is required. Temperature, rotating speed, pressure per unit area and even the procedure for fabricating the bearing have an influence on alloy selection; the design of the bearing and its bonding are also significant. For example, a thick lining,

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mechanically anchored, requires a babbitt of good ductility at room temperature that will seat itself in the anchors under load. The tin-base alloys, which have good plasticity at room temperature, adjust well to these conditions under moderate to severe loads. For bearings which are difficult to seal and align, and where line contact occurs in the early moments of operation before full lubrication is established, the conventional lead-base alloys have the required ductility and conformability. Their use is limited, however, to moderate speeds and loads.Where thin linings and precision castings are used, certain leadbase alloys containing only a nominal 1% tin should be considered; in properly designed and properly cast bearings they perform as well as tin-base babbitts and are much less expensive. They have excellent fatigue resistance, which is important to bearings of this type. Naturally, they do not have the ductility of lead-base antimony-tin bearings but this is a minor factor with thin liners. Most important of all: In selecting a bearing alloy, seek the advice of your Fry Technology representative. Through Fry Technology you can draw on our group’s Central Research Department and their many years of experience in the theory and application of bearing alloys.

Melting of Bearing Alloys

Because of the relatively low melting point of bearing metal alloys, it is easy to convert in gots to liquid alloy. To make molten metal suitable for casting, however, requires careful control. The melting pot can be of any size suitable for the amount of metal needed. Heatresistant iron containing nickel, chromium or molybdenum is the preferred material for its long service life. Clay graphite crucibles are sometimes used where contamination from iron is a serious problem.

The melting pot must be clean. After melting, the pot should be scraped to remove accumulations of metal and dross. If not, subsequent casting may show hard spots on the machined surface. A semi-spherical melting pot with a flange supported by a refractory shell is recommended, and heating should be arranged so that a uniform temperature prevails throughout the melt. Uneven heating may cause segregation or allow partial solidification. Segregation may occur in tin-base alloys of high copper content and may result in a deposit in the kettle after pouring which is much higher in copper than the desired alloy. This obviously deprives the cast metal of some of its specified alloy content of copper. After complete melting, the metal should be stirred, manually or mechanically, to insure uniformity of the melt but carefully to avoid producing too much dross. Manual stirring is best done with a circular perforated plate on a long-handled steel rod. Stirring is from the bottom upward using a figure “8” motion. After stirring, the metal should remain at rest for a few minutes, then be skimmed. The temperature of

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the melt should be controlled-or checked-by pyrometer. Constant thermal control is required for efficient and uniform results. High temperatures lead to excessive drossing, which is wasteful. Further, dross may be carried over into the casting and causefailure of the bearing. Also, fuel costs are higher and pot life shortened. On the other hand, a melt temperature which is too low can cause segregation in the pot as well as premature solidification before bonding on the shell takes place. The most suitable pyrometer is the shielded type which is submerged in the melt and records on a wall-mounted instrument. No portion of the melt should ever be allowed to remain between the solidus and liquidus temperatures for any length of time. This often happens when the casting set-up is not quite ready or when metal is left overnight for use the next day. Under these circumstances, crystal aggregates are precipitated. The heavier crystals sink, the lighter float. Stirring after reheating does now always dissolve all the crystals and the result may be hard spots in the bearings. Above all, metal should not be allowed to solidify in the melting pot overnight. Unused metal at day’s end should be poured into pigs, and the pot thoroughly cleaned before re-use.

Accumulations of dross, sweepings, skimmings and machine-shop borings should be sold to a smelter or collector, not used in the pot. Clean borings of known constituents may be used in the melt if magnetically screened to remove ferrous chips and particles. This will not remove brass and bronze chips which may also be harmful. Of course, alloys must not be mixed. It cannot be emphasized too strongly that metal waste should be sold rather than re-used. If used, it must be clean and sorted with scrupulous care. Good housekeeping is imperative to thecasting of dependable bearings.

Additions to the melt should be made in such a way as to assure rapid coalescence with the bath to prevent oxidation of the metals being added. When melting, pouring, machining or otherwise working with these alloys, care should be taken to comply with health standards promulgated by the Federal Occupational Health and Safety Administration or the state equivalent relating to the concentrations of airborne metal fumes and dust and work practices. Upon request, Fry Technology will supply to customers copies of Material Safety Data Sheets for the major constituents of these alloys. Employees should be fully informed of any hazards that may exist and the necessary steps to be taken to eliminate or minimize them.

Bonding the Bearing

There are two basic methods – chemical or metallurgical, and mechanical – by which babbitt metals are bonded to the supporting shell. Chemical bonding is the preferred modern practice and is used almost exclusively.

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Mechanical bonding is sometimes used for bearings of an inch or more in thickness which are secured to the shell with the help of grooves, dovetails, anchors, undercuts or holes to keep the bearing metal in place. The metallurgical bond is a thin layer of alloy between the bearing metal and the shell or support; the bond alloys with both, and secures them firmly in relation to one another. The bonding layer, though strong, is brittle and must be as thin as is practicable to minimize stress concentration in the area. Tin-base bearing alloys are commonly bonded to steel and bronze shells. In the former case, tin-iron compounds are formed at the bond and in the latter, tincopper. The tin-copper compound is weaker than the tin-iron compound and dictates the preference for steel shells, though bronze shells are serviceable if bonding is properly done. Lead-base alloys give equally good results with either type of shell. For arsenic-hardened alloys, the steel shell is preferred. Bronze shells invite the possibility of forming a weak and brittle copper-arsenic bonding layer. This can be avoided, however, by careful control of shell and bearing metal temperature and by rapid solidification of the bearing metal into the copper constituent of the bronze.

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase.This often happens when the two elements (generally metals) involved are close together on the periodic table; conversely, a chemical compound is generally a result of the non proximity of the two metals involved on the periodic table.[1]

Details

The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. Both of these types of solid solution affect the properties of the material by distorting the crystal lattice and disrupting the physical and electrical homogeneity of the solvent material.[2]

Some mixtures will readily form solid solutions over a range of concentrations, while other mixtures will not form solid solutions at all. The propensity for any two substances to form a solid solution is a complicated matter involving the chemical, crystallographic, and quantum properties of the substances in question. Solid solutions, in accordance with the Hume-Rothery rules, may form if the solute and solvent have:

Similar atomic radii (15% or less difference) Same crystal structure

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Similar electronegativities Similar valency

The phase diagram in Fig. 1 displays an alloy of two metals which forms a solid solution at all relative concentrations of the two species. In this case, the pure phase of each element is of the same crystal structure, and the similar properties of the two elements allow for unbiased substitution through the full range of relative concentrations.

Solid solutions have important commercial and industrial applications, as such mixtures often have superior properties to pure materials. Many metal alloys are solid solutions. Even small amounts of solute can affect the electrical and physical properties of the solvent.

Fig. 2 This binary phase diagram shows two solid solutions: α and β.

The binary phase diagram in Fig. 2 at right shows the phases of a mixture of two substances in varying concentrations, α and β. The region labeled "α" is a solid solution, with β acting as the solute in a matrix of α. On the other end of the concentration scale, the region labeled "β" is also a solid solution, with α acting as the solute in a matrix of β. The large solid region in between the α and β solid solutions, labeled "α and β", is not a solid solution. Instead, an examination of the microstructure of a mixture in this range would reveal two phases — solid solution α-in-β and solid solution β-in-α would form separate phases, perhaps lamella or grains.

[edit] Application

In the phase diagram, the unalloyed extreme left and right concentrations, and the dip in the center, the material will be solid and become liquid as heat is added, where at other proportions the material will enter a mushy or pasty phase. The mixture at dip point of the diagram is called a eutectic alloy. Lead-tin mixtures formulated at that point (37/63 mixture) are useful when soldering electronic components, particularly if done manually, since the solid phase is quickly entered as the solder cools. In contrast, when lead-tin mixtures were used to solder seams in automobile bodies a pasty state enabled a shape to be formed with a wooden paddle or tool, so a 70-30 lead to tin ratio was used. (Lead is being removed from such applications owing to its toxicity and consequent difficulty in recycling devices and components that include lead.)

[edit] Exsolution

When a solid solution becomes unstable — due to a lower temperature,

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for example — exsolution occurs and the two phases separate into distinct microscopic to megascopic lamellae. An example of this is the alkali feldspar mineral variety perthite in which thin white albite layers alternate between typically pink microcline.

[edit] Examples

Chen, Jing; Xu, Zhi-qin; Chen, Z-Z.; Li, T-F.; Chen, F-Y. (December 2005). "Pargasite and ilmenite exsolution texture in clinopyroxene from the Hujialing Garnet-Pyroxenite, Su-lu U.H.P. Terrane, Cen-tral China: A geodynamic Implication". European Journal of Miner-alogy 17 (6): 895–903. doi:10.1127/0935-1221/2005/0017-0895. http://www.uni-graz.at/IEC-7/PDF-files/Chen.pdf.

Petersen, U.. "Introduction to Ore Microscopy II; Mineral Paragen-esis". http://www.mines.utah.edu/~wmep/59298/592PDF/rlm2.pdf.

[edit] See also

Phase diagram Eutectic point

Lever rule

From Wikipedia, the free encyclopediaJump to: navigation, search

A phase diagram with tie line (LS). The x dimension defines the percent weight of elements A and B.

The lever rule is a tool used to determine weight percentages of each phase of a binary equilibrium phase diagram. It is used to determine the percent weight of liquid and solid phases for a given binary composition and temperature that is between the liquidus and solidus.[1]

[edit] Calculations

Before any calculations can be made a tie line is drawn on the phase diagram to determine the percent weight of each element; on the phase diagram to the right it is line segment LS. This tie line is drawn horizontally at the compositions temperature from the liquid to the solidus. The percent weight of element B at the liquidus is given by wl

and the percent weight of element B at the solidus is given by ws. The percent weight of solid and liquid can then be calculated using the following lever rule equations:[1]

where wo is the percent weight of element B for the given composition.

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Iron-Iron Carbide Diagram

Figure 1. Iron-ironcarbide diagram

Cementite is also known as iron carbide which has a chemical formula, Fe3C. It contains 6.67 % Carbon by weight. It is a typical hard and brittle interstitial compound of low tensile strength (approximately 5,000 psi) but high compressive strength. Its crystal structure is orthorhombic.

  AUSTENITE ( iron):

It is also known as ( ) gamma-iron, which is an interstitial solid solution of carbon dissolved in iron with a face centered cubic crystal (F.C.C) structure. Average properties of austenite are:

 Tensile strength 150,000 psi.

Elongation 10 % in 2 in gage length.

Hardness Rockwell C 40

Toughness High

Table 1. Properties of Austenite

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Figure 2. Austenite (gamma iron) crystal structure

Austenite is normally unstable at room temperature. Under certain conditions it is possible to obtain austenite at room temperature.

  FERRITE ( iron):

It is also known as ( ) alpha -iron, which is an interstitial solid solution of a small amount of carbon dissolved in iron with a Body Centered Cubic (B.C.C.) crystal structure. It is the softest structure on the iron-iron carbide diagram. Average properties are:

 Tensile Strength 40,000 psi

Elongation 40 % in 2 in gage length

Hardness Less than Rockwell C 0 or less than Rockwell B 90.

Toughness Low

Table 2. Properties of Ferrite.

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Figure 2. Ferrite (alpha iron) crystal structure

PEARLITE ( + Fe3C)

It is the eutectoid mixture containing 0.83 % Carbon and is formed at 1333oF on very slow cooling. It is very fine platelike or lamellar mixture of ferrite and cementite. The structure of pearlite includes a white matrix (ferritic background) which includes thin plates of cementite. Average properties are:

 Tensile Strength 120,000 psi

Elongation 20 % in 2 in gage length

Hardness Rockwell C 20 or BHN 250-300

Table 3. Properties of pearlite.

Figure 3. Pearlite microstructure (Light background is the ferrite matrix, dark lines are the cementite network)

A fixed amount of carbon and a fixed amount of iron are needed to form cementite (Fe3C). Also, pearlite needs fixed amounts of cementite and ferrite.

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If there is not enough carbon, that is less than 0.83 %, the carbon and the iron will combine to form Fe3C until all the carbon is consumed. This cementite will combine with the required amount of ferrite to form pearlite. The remaining amount of ferrite will stay in the structure as free ferrite. Free ferrite is also known as proeutectoid ferrite. The steel that contains proeutectoid ferrite is referred to as hypoeutectoid steel.

If, however, there is an excess of carbon above 0.83 % in the austenite, pearlite will form, and the excess carbon above 0.83 % will form cementite. The excess cementite deposits in the grain boundaries. This excess cementite is also known as proeutectoid cementite.

LEDEBURITE ( + Fe3C)

It is the eutectic mixture of austenite and cementite. It contains 4.3 % Carbon and represents the eutectic of cast iron. Ledeburite exists when the carbon content is greater than 2 %, which represents the dividing line on the equilibrium diagram between steel and cast iron.

 ( ) DELTA IRON:

Delta iron exists between 2552 and 2802 oF. It may exist in combination with the melt to about 0.50 % Carbon, in combination with austenite to about 0.18 % Carbon and in a single phase state out to about 0.10 % carbon. Delta iron has the Body Centered Cubic (B.C.C) crystal structure and is magnetic.

UNIT – IV

MECHANICAL TESTING

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Ultimate tensile strength

From Wikipedia, the free encyclopediaJump to: navigation, search

Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength,[1][2] is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. Mechanically, tensile strength is is opposite of compressive strength, although in many materials the magnitudes of these two strengths are quite different.

The UTS is usually found by performing a tensile test and recording the stress versus strain; the highest point of the stress-strain curve is the UTS. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.

Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood.

Tensile strength is defined as a stress, which is measured as force per unit area. In the SI system, the unit is pascal (Pa) or, equivalently, newtons per square metre (N/m²). The customary unit is pounds-force per square inch (lbf/in² or PSI), or kilo-pounds per square inch (KSI), which is equal to 1000 PSI; kilo-pounds per square inch are commonly used for convenience when measuring tensile strengths.

Contents

[hide] 1 Concept

o 1.1 Ductile materials o 1.2 Brittle materials o 1.3 Liqvors

2 Testing 3 Typical tensile strengths

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4 See also 5 References

6 Further reading

[edit] Concept

[edit] Ductile materials

Stress vs. Strain curve typical of aluminum #Ultimate strength #Yield strength #Proportional limit stress #Fracture #Offset strain (typically 0.2%)

Stress vs. strain curve typical of structural steel#Ultimate strength #Yield strength #Fracture #Strain hardening region #Necking region A: Engineering stress

B: True stress

Many materials display linear elastic behavior, defined by a linear stress-strain relationship, as shown in the figure up to point 2, in which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will widen, but will return to its original shape and size when unloaded. Beyond this linear region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen will not return to its original size and shape when unloaded. Note that there will be elastic recovery of a portion of the deformation. For many applications, plastic deformation is unacceptable, and is used as the design limitation.

After the yield point, ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress-strain curve (curve A); this is because the engineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress-strain curve, and the engineering stress coordinate of this point is the tensile ultimate strength, given by point 1.

The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used to for quality control, because of the ease of testing. It is also used to rough determine material types for unknown samples.[3]

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[edit] Brittle materials

Brittle materials, such as concrete and carbon fiber, are characterized by failure at small strains. They often fail while still behaving in a linear elastic manner, and thus do not have a defined yield point. Because strains are low, there is negligible difference between the engineering stress and the true stress. Testing of several identical specimens will result in different failure stresses, this is due to the Weibull Modulus of the brittle material.

The UTS is a common engineering parameter when design brittle members, because there is no yield point.[3]

[edit] Liqvors

Tensile strength can be defined for liqvors as well as solors. For example, when a tree draws water from its roots to its upper leaves by transpiration, the column of water is pulled upwards from the top by capillary action, and this force is transmitted down the column by its tensile strength.

[edit] Testing

Round bar tensile specimen after testing

Main article: Tensile testing

Typically, the testing involves taking a small sample with a fixed cross-section area, and then pulling it with a controlled, gradually increasing

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force until the sample changes shape or breaks.

When testing metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.[4]

[edit] Typical tensile strengths

MaterialYieldstrength(MPa)

Ultimatestrengtha

(MPa)

Density(g/cm³)

carbon nanotube b 011,000 (SW)–150,000 (MW)

00.037–01.34

graphene 130,000[5] 01.0silicon, monocrystalline (m-Si) 007,000 02.33colossal carbon tube 007,000 01.0Toray carbon fiber (T1000G)[6] 006,370 01.80polybenzoxazole (Zylon)[7] 005,800 01.56carbon fiber 005,650 01.75basalt fiber [8] 004,840 02.7S-Glass 004,710 02.48silica glass strands, ultra-pure[9] 004,100 02.2aramid, polyparaphenylene terephalamide (Kevlar-49)

003,620 01.45

carbon nanotube, first ? 003,600 01.3UHMWPE strands [10] [11]

(Dyneema or Spectra)002,300–003,500 00.97

E-Glass 003,450 02.57silicon carbide (SiC) 003,440 03.21Vectran 002,850–003,340 01.41boron 003,100 02.46aramid, polyparaphenylene terephalamide (Twaron)

002,920 01.44

steel, AISI 1060, 0.6% carbon (piano wire)[12] 002,200–002,482 07.8

sapphire (Al2O3) 001,90003.9–04.1

steel, prestressing strands 001,650 001,860 07.8Darwin's bark spider silk 000,970 001,850[13]

Liquidmetal alloy 001,723 000,550–001,600 06.1tungsten 001,510 19.25carbon nanotube-polyvinyl 001,200

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alcohol composite[14]

spider silk c 001,000 01.3titanium alloy, 6% Al, 4% V 000,830 000,900 04.51stainless steel, AISI 302, cold-rolled

000,520 000,860 08.19

carbon steel, AISI 1090 000,640 000,841 07.58steel, high-strength alloy, ASTM A514

000,690 000,760 07.8

steel, API 5L X65[15] 000,448 000,531 07.8steel, AISI 4130, quench & temper at 1200°F[16] 000,703 000,814 07.75

silk, silkworm 000,500 01.3brass 000,200+ 000,550 05.3bamboo 000,350–000,500 00.4beryllium, 99.9%[17] 000,345 000,448 01.84steel, structural ASTM A36 000,250 000,400 07.8aluminium alloy, 2014-T6[18] 000,414 000,483 02.8human hair d 000,380 01.35cupronickel, 10% Ni, 1.6% Fe, 1% Mn

000,130 000,350 08.94

aluminium alloy, 6063-T6 000,248 02.63copper, 99.9% 000,070 000,220 08.92cast iron, ASTM A48, 4.5% C 000,130 000,200 07.15

bone (limb)000,104–000,121

000,130 01.6

polypropylene000,012–000,043

000,019.7–000,080 00.91

Nylon, 6/6 000,045 000,075 01.15polyethylene, ultra-high molecular weight (UHMWPE)

000,023 000,046 00.97

pinewood 000,040 00.5polyethylene, high-density (HDPE)

000,026–000,033

000,037 00.95

glass [19] 000,033 02.53

epoxy adhesive [20] 000,012–000,03000.75–02.

concrete000,003–000,030 (compressed)

02.4

marble 000,015 02.56rubber 000,015 01.1

^a Many of the values depend on manufacturing process and purity/composition.

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^b Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with labs producing them at a tensile strength of 63 GPa[21], still well below their theoretical limit of 300 GPa[citation needed]. The first nanotube ropes (20mm) whose tensile strength was published (in 2000) had a strength of 3.6 GPa.[22] The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid)[23].

^c The strength of spider silk is highly variable. It depends on many factors including kind of silk (Every spider can produce several for sundry purposes.), species, age of silk, temperature, humidity, swiftness at which stress is applied during testing, length stress is applied, and way the silk is gathered (forced silking or natural spinning)[24]. The value shown in the table, 1000 MPa, is roughly representative of the results from a few studies involving several different species of spider however specific results varied greatly.[25]

^d Human hair strength varies by ethnicity and chemical treatments.

Typical properties for annealed elements[26]

Element

Young'smodulus(GPa)

Offset oryield strength(MPa)

Ultimatestrength(MPa)

silicon 1075000–9000

tungsten 411 5500550–0620

iron 211 080–100 0350

titanium 120 100–2250240–0370

copper 130 033 0210tantalum 186 180 0200

tin 047 009–0140015–0200

zinc (wrought)

1050110–0200

nickel 170 014–0350140–0195

silver 083 0170gold 079 0100

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aluminium 070 015–0200040-0050

lead 016 0012

[edit] See also

Flexural strength Specific strength Strength of materials Tensile structure Toughness Ultimate failure Stress-Strain Curves

The relationship between the stress and strain that a material displays is known as a Stress-Strain curve. It is unique for each material and is found by recording the amount of de-formation (strain) at distinct inter-vals of tensile or compressive load-ing. These curves reveal many of the properties of a material (including data to establish the Modulus of Elasticity, E). What does a compari-son of the curves for mild steel, cast iron and concrete illustrate about their respective properties?

It can be seen that the concrete curve is almost a straight line. There is an abrupt end to the curve. This, and the fact that it is a very steep line, indicate that it is a brittle material. The curve for cast iron has a slight curve to it. It is also a brittle material. Both of these materials will fail with little warning once their limits are sur-passed. Notice that the curve for mild steel seems to have a long gently curving "tail". This indicates a behavior that is distinctly dif-ferent than either concrete or cast iron. The graph shows that after a certain point mild steel will continue to strain (in the case of ten-sion, to stretch) as the stress (the loading) remains more or less constant. The steel will actually stretch like taffy. This is a material property which indicates a high ductility. There are a number of significant points on a stress-strain curve that help one understand and predict the way every building material will behave.

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An example plot of a test on two grades of steel is illustrated above. If one begins at the origin and follows the graph a number of points are indicated. Point A is known as the proportional limit. Up to this point the relationship between stress and strain is exactly pro-portional. The number which describes the relationship between the two is the Modulus of Elasticity. This is discussed in more detail in the next lecture.

Strain increases faster than stress at all points on the curve beyond point A. Up to this point, any steel speciment that is loaded and un-loaded would return to its original length. This is known as elastic behavior. Point B is the point after which any continued stress re-sults in permanent, or inelastic, deformation. Thus, point B is known as the elastic limit. Since the stress resistence of the mate-rial decreases after the peak of the curve, this is also known as the yield point.

The line between points C and D indicates the behavior of the steel specimen if it experienced continued loading to stress indicated as point C. Notice that the dashed line is parallel to the elastic zone of the curve (between the origin and point A). When the specimen is unloaded the magnitude of the inelastic deformation would be de-termined (in this case 0.0725 inches /inch). If the same specimen was to be loaded again, the stress-strain plot would climb back up the line from D to C and continue along the initial curve. Point E in-dicates the location of the value of the ultimate stress. Note that this is quite different from the yield stress. The yield stress and ulti-mate stress are the two values that are most often used to deter-mine the allowable loads for building materials and should never be confused.

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A material is considered to have completely failed once it reaches the ultimate stress. The point of rupture, or the actual tearing of the material, does not occur until point F. It is interesting to note the curve that indicates the actual stress experienced by the speci-men. This curve is different from the apparent stress since the cross sectional area is actually decreasing. There is quite a bit to be learned from both the study of the ideal and actual behavior of all building materials. Changes in that body of knowledge have had large impacts on the way in which building structures are de-signed.

The earliest methods of design limited the stresses that a structure would be "allowed" to experience. Thus, the method of design was known as the Allowable Stress Method. Recognition of the addi-tional strength potential of most materials resulted in the Ultimate Stress Method of design. Contemporary thought centers on the lim-itation of the various service conditions of the structure at hand. This is known as the Limit States Design method. In the end, it is the authors opinion that the actual method of design is less impor-tant than the legal bodies would like us to believe. Human factors in the construciton process SHOULD prevent a good designer from pushing too hard against the envelope of safety.

STRESS-STRAIN CURVES David Roylance Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139 August 23, 2001 Introduction Stress-strain curves are an extremely important graphical measure

of a material’s mechanical properties, and all students of Mechan-ics of Materials will encounter them often. However, they are not without some subtlety, especially in the case of ductile materials that can undergo substantial geometrical change during testing. This module will provide an introductory discussion of several points needed to interpret these curves, and in doing so will also provide a preliminary overview of several aspects of a material’s mechanical properties. However, this module will not attempt to survey the broad range of stress-strain curves exhibited by modern engineering materials (the atlas by Boyer cited in the References section can be consulted for this). Several of the topics mentioned here — especially yield and fracture — will appear with more detail in later modules.

“Engineering” Stress-Strain Curves Perhaps the most important test of a material’s mechanical response is the tensile test1, in

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which one end of a rod or wire specimen is clamped in a loading frame and the other subjected to a controlled displacement δ (see Fig. 1). A transducer connected in series with the specimen pro-vides an electronic reading of the load P(δ) corresponding to the displacement. Alternatively, modern servo-controlled testing ma-chines permit using load rather than displacement as the controlled variable, in which case the displacement δ(P) would be monitored as a function of load.

The engineering measures of stress and strain, denoted in this module as σe and _e respectively, are determined from the mea-sured the load and deflection using the original specimen cross-sec-tional area A0 and length L0 as σe = P

A0 , _e = δ L0 (1)

When the stress σe is plotted against the strain _e, an engineering stress-strain curve such as that shown in Fig. 2 is obtained. 1Stress-strain testing, as well as almost all experimental proce-dures in mechanics of materials, is detailed by standards-setting or-ganizations, notably the American Society for Testing and Materi-als (ASTM). Tensile testing of metals is prescribed by ASTM Test E8, plastics by ASTM D638, and composite materials by ASTM D3039.

Figure 1: The tension test. Figure 2: Low-strain region of the engineering stress-

strain curve for annealed polycrystalline copper; this curve is typi-cal of that of many ductile metals.

In the early (low strain) portion of the curve, many materials obey Hooke’s law to a reasonable approximation, so that stress is pro-portional to strain with the constant of proportionality being the modulus of elasticity or Young’s modulus, denoted E:

σe = E_e (2) As strain is increased, many materials eventually deviate from this

linear proportionality, the point of departure being termed the pro-portional limit. This nonlinearity is usually associated with stress-induced “plastic” flow in the specimen. Here the material is under-going a rearrangement of its internal molecular or microscopic structure, in which atoms are being moved to new equilibrium posi-tions. This plasticity requires a mechanism for molecular mobility, which in crystalline materials can arise from dislocation motion (discussed further in a later module.) Materials lacking this mobil-ity, for instance by having internal microstructures that block dislo-cation motion, are usually brittle rather than ductile. The stress-strain curve for brittle materials are typically linear over their full range of strain, eventually terminating in fracture without appre-ciable plastic flow.

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Note in Fig. 2 that the stress needed to increase the strain beyond the proportional limit in a ductile material continues to rise beyond the proportional limit; the material requires an ever-increasing stress to continue straining, a mechanism termed strain hardening. These microstructural rearrangements associated with plastic flow are usually not reversed when the load is removed, so the propor-tional limit is often the same as or at least close to the materials’s elastic limit. Elasticity is the property of complete and immediate recovery from an imposed displacement on release of the load, and the elastic limit is the value of stress at which the material experi-ences a permanent residual strain that is not lost on unloading. The residual strain induced by a given stress can be determined by drawing an unloading line from the highest point reached on the se - ee curve at that stress back to the strain axis, drawn with a slope equal to that of the initial elastic loading line. This is done because the material unloads elastically, there being no force driving the molecular structure back to its original position. A closely related term is the yield stress, denoted σY in these modules; this is the stress needed to induce plastic deformation in the specimen. Since it is often difficult to pinpoint the exact stress at which plastic de-formation begins, the yield stress is often taken to be the stress needed to induce a specified amount of permanent strain, typically 0.2%. The construction used to find this “offset yield stress” is shown in Fig. 2, in which a line of slope E is drawn from the strain axis at _e = 0.2%; this is the unloading line that would result in the specified permanent strain. The stress at the point of intersection with the σe − _e curve is the offset yield stress.

Figure 3 shows the engineering stress-strain curve for copper with an enlarged scale, now showing strains from zero up to specimen fracture. Here it appears that the rate of strain hardening 2 dimin-ishes up to a point labeled UTS, for Ultimate Tensile Strength (de-noted σf in these modules). Beyond that point, the material appears to strain soften, so that each increment of additional strain requires a smaller stress.

Figure 3: Full engineering stress-strain curve for annealed poly-crystalline copper. The apparent change from strain hardening to strain softening is an artifact of the plotting procedure, however, as is the maximum observed in the curve at the UTS. Beyond the yield point, molecular flow causes a substantial reduction in the speci-men cross-sectional area A, so the true stress σt = P/A actually borne by the material is larger than the engineering stress com-puted from the original cross-sectional area (σe = P/A0). The load must equal the true stress times the actual area (P = σtA), and as long as strain hardening can increase σt enough to compensate for the reduced area A, the load and therefore the engineering stress will continue to rise as the strain increases. Eventually, however,

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the decrease in area due to flow becomes larger than the increase in true stress due to strain hardening, and the load begins to fall. The strain hardening rate is the slope of the stress-strain curve, also called the tangent modulus, is a geometrical effect, and if the true stress rather than the engineering stress were plotted no max-imum would be observed in the curve.

At the UTS the differential of the load P is zero, giving an analytical relation between the true stress and the area at necking:

P = σ t A → dP = 0 = σ t dA + Adσ t dA

A = dσt σt (3)

The last expression states that the load and therefore the engineer-ing stress will reach a maximum as a function of strain when the fractional decrease in area becomes equal to the fractional increase in true stress. Even though the UTS is perhaps the materials prop-erty most commonly reported in tensile tests, it is not a direct mea-sure of the material due to the influence of geometry as discussed above, and should be used with caution. The yield stress σY is usu-ally preferred to the UTS in designing with ductile metals, although the UTS is a valid design criterion for brittle materials that do not exhibit these flow-induced reductions in cross-sectional area. The true stress is not quite uniform throughout the specimen, and there will always be some location - perhaps a nick or some other defect at the surface - where the local stress is maximum. Once the maxi-mum in the engineering curve has been reached, the localized flow at this site cannot be compensated by further strain hardening, so the area there is reduced further.

This increases the local stress even more, which accelerates the flow further. This localized and increasing flow soon leads to a “neck” in the gage length of the specimen such as that seen in Fig. 4.

Figure 4: Necking in a tensile specimen. Until the neck forms, the deformation is essentially uniform

throughout the specimen, but after necking all subsequent defor-mation takes place in the neck. The neck becomes smaller and smaller, local true stress increasing all the time, until the specimen fails. This will be the failure mode for most ductile metals. As the neck shrinks, the nonuniform geometry there alters the uniaxial stress state to a complex one involving shear components as well as normal stresses.

The specimen often fails finally with a “cup and cone” geometry as seen in Fig. 5, in which the outer regions fail in shear and the inte-rior in tension. When the specimen fractures, the engineering strain at break — denoted _f — will include the deformation in the

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necked region and the unnecked region together. Since the true strain in the neck is larger than that in the unnecked material, the value of _f will depend on the fraction of the gage length that has necked. Therefore, _f is a function of the specimen geometry as well as the material, and thus is only a crude measure of material ductility. Figure 5: Cup-and-cone fracture in a ductile metal.

Figure 6 shows the engineering stress-strain curve for a semicrys-talline thermoplastic. The response of this material is similar to that of copper seen in Fig. 3, in that it shows a proportional limit followed by a maximum in the curve at which necking takes place. (It is common to term this maximum as the yield stress in plastics, although plastic flow has actually begun at earlier strains.)

Figure 6: Stress-strain curve for polyamide (nylon) thermoplastic. The polymer, however, differs dramatically from copper in that the neck does not continue shrinking until the specimen fails. Rather, the material in the neck stretches only to a “natural draw ratio” which is a function of temperature and specimen processing, be-yond which the material in the neck stops stretching and new mate-rial at the neck shoulders necks down. The neck then propagates until it spans the full gage length of the specimen, a process called drawing.

This process can be observed without the need for a testing ma-chine, by stretching a polyethylene “six-pack holder,” as seen in Fig. 7. Not all polymers are able to sustain this drawing process. As will be discussed in the next section, it occurs when the necking process produces a strengthened microstructure whose breaking load is greater than that needed to induce necking in the untrans-formed material just outside the neck.

Figure 7: Necking and drawing in a 6-pack holder. “True” Stress-Strain Curves As discussed in the previous section, the engineering stress-strain curve must be interpreted with caution beyond the elastic limit, since the specimen dimensions experience substantial change from their original values. Using the true stress σt = P/A rather than the engineering stress σe = P/A0 can give a more direct measure of the material’s response in the plastic flow range. A measure of strain often used in conjunction with the true stress takes the increment of strain to be the incremental in-crease in displacement dL divided by the current length L : d_ t = dL l → _ t = _ L

l0 1 L dL = ln

This is called the “true” or “logarithmic” strain. During yield and the plastic-flow regime following yield, the material flows with negligible change in volume; increases in length are offset by decreases in cross-sectional area. Prior to necking, when the strain is still uniform along the specimen length, this volume constraint can be written:

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dV = 0 → AL = A 0 L 0 → L L0 A A0 (5)

The ratio L/L0 is the extension ratio, denoted as λ. Using these re-lations, it is easy to develop relations between true and engineering measures of tensile stress and strain (see Prob. 2):

σt = σe (1 + _e) = σeλ, _t = ln(1+_e) = ln λ (6)

These equations can be used to derive the true stress-strain curve from the engineering curve, up to the strain at which necking be-gins. Figure 8 is a replot of Fig. 3, with the true stress-strain curve computed by this procedure added for comparison. Beyond neck-ing, the strain is nonuniform in the gage length and to compute the true stressstrain curve for greater engineering strains would not be meaningful. However, a complete true stress-strain curve could be drawn if the neck area were monitored throughout the tensile test, since for logarithmic strain we have

L L 0 = A A 0 → _ t = ln = ln A L L0 A0 (7)

Ductile metals often have true stress-strain relations that can be described by a simple power-law relation of the form: 6 Figure 8: Comparison of engineering and true stress-strain curves for cop-per. An arrow indicates the position on the “true” curve of the UTS on the engineering curve.

σ t = A nt _ → log σt = logA + n log _t (8)

Figure 9 is a log-log plot of the true stress-strain data3 for copper from Fig. 8 that demonstrates this relation. Here the parameter n = 0.474 is called the strain hardening parameter, useful as a measure of the resis-tance to necking. Ductile metals at room temperature usually exhibit val-ues of n from 0.02 to 0.5. Figure 9: Power-law representation of the plas-tic stress-strain relation for copper. A graphical method known as the “Consid`ere construction” uses a form of the true stressstrain curve to quantify the differences in necking and drawing from material to mate-rial. This method replots the tensile stress-strain curve with true stress σt as the ordinate and extension ratio λ = L/L0 as the abscissa. From Eqn. 6, the engineering stress σe corresponding to any. Here percent strain was used for _t; this produces the same value for n but a different A than if full rather than percentage values were used. value of true stress σt is slope of a secant line drawn from origin (λ = 0, not λ = 1) to intersect the σt − λ curve at σt. Figure 10: Consid`ere construction. (a) True stress-strain curve with no tangents - no necking or drawing. (b)

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One tangent - necking but not drawing. (c) Two tangents - necking and drawing. Among the many possible shapes the true stress-strain curves could assume, let us consider the concave up, concave down, and sig-moidal shapes shown in Fig. 10. These differ in the number of tangent points that can be found for the secant line, and produce the following yield characteristics:

(a) No tangents: Here the curve is always concave upward as in part (a) of Fig. 10, so the slope of the secant line rises continuously. Therefore the engineering stress rises as well, without showing a yield drop. Eventually fracture intercedes, so a true stress-strain curve of this shape identifies a material that fractures before it yields. (b) One tangent: The curve is concave downward as in part (b) of Fig. 10, so a secant line reaches a tangent point at λ = λY . The slope of the secant line, and therefore the engineering stress as well, begins to fall at this point. This is then the yield stress σY seen as a maximum in stress on a conventional stress-strain curve, and λY is the extension ratio at yield. The yielding process begins at some adventitious location in the gage length of the specimen, and continues at that location rather than being initiated elsewhere because the secant modulus has been reduced at the first location. The specimen is now flowing at a single location with decreasing resistance, leading eventually to failure. Ductile metals such as alu-minum fail in this way, showing a marked reduction in cross sec-tional area at the position of yield and eventual fracture.(c) Two tangents: For sigmoidal stress-strain curves as in part (c) of Fig. 10, the engineering stress begins to fall at an extension ration λY , but then rises again at λd. As in the previous one-tangent case, material begins to yield at a single position when λ = λY , produc-ing a neck that in turn implies a nonuniform distribution of strain along the gage length. Material at the neck location then stretches to λd, after which the engineering stress there would have to rise to stretch it further. But this stress is greater than that needed to stretch material at the edge of the neck from λY to λd, so material already in the neck stops stretching and the neck propagates out-ward from the initial yield location. Only material within the neck shoulders is being stretched during propagation, with material in-side the necked-down region holding constant at λd, the material’s “natural draw ratio,” and material outside holding at λY . When all the material has been drawn into the necked region, the stress be-gins to rise uniformly in the specimen until eventually fracture oc-curs.

The increase in strain hardening rate needed to sustain the draw-ing process in semicrystalline polymers arises from a dramatic transformation in the material’s microstructure. These materials are initially “spherulitic,” containing flat lamellar crystalline plates,

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perhaps 10 nm 8 thick, arranged radially outward in a spherical do-main. As the induced strain increases, these spherulites are first deformed in the straining direction. As the strain increases further, the spherulites are broken apart and the lamellar fragments rear-ranged with a dominantly axial molecular orientation to become what is known as the fibrillar microstructure. With the strong cova-lent bonds now dominantly lined up in the load-bearing direction, the material exhibits markedly greater strengths and stiffnesses — by perhaps an order of magnitude — than in the original material. This structure requires a much higher strain hardening rate for in-creased strain, causing the upturn and second tangent in the true stress-strain curve.

Strain energyThe area under the σe − _e curve up to a given value of strain is the total mechanical energy per unit volume consumed by the ma-terial in straining it to that value. This is easily shown as follows:

In the absence of molecular slip and other mechanisms for energy dissipation, this mechanical energy is stored reversibly within the material as strain energy. When the stresses are low enough that the material remains in the elastic range, the strain energy is just the triangular area in Fig. 11:

Figure 11: Strain energy = area under stress-strain curve. Note that the strain energy increases quadratically with the stress or strain; i.e. that as the strain increases the energy stored by a given increment of additional strain grows as the square of the strain. This has important consequences, one example being that an archery bow cannot be simply a curved piece of wood to work well. A real bow is initially straight, then bent when it is strung; this stores substantial strain energy in it. When it is bent further on drawing the arrow back, the energy available to throw the arrow is very much greater than if the bow were simply carved in a curved shape without actually bending it.

Figure 12 shows schematically the amount of strain energy avail-able for two equal increments of strain Δ_, applied at different lev-els of existing strain. The area up to the yield point is termed the modulus of resilience, and the total area up to fracture is termed the modulus of toughness; these are shown in Fig. 13. The term “modulus” is used because the units of strain energy per unit vol-ume are N-m/m3 or N/m2, which are the same as stress or modulus of elasticity. The term “resilience” alludes to the concept that up to the point of yielding, the material is unaffected by the applied stress and upon unloading.

Figure 12: Energy associated with increments of strain

Table 1: Energy absorption of various materials.

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Material Maximum Maximum Modulus of Density Max. Energy Strain, % Stress, MPa Toughness, MJ/m3 kg/m3 J/kg Ancient Iron 0.03 70 0.01 7,800 1.3 Modern spring steel 0.3 700 1.0 7,800 130 Yew wood 0.3 120 0.5 600 900 Tendon 8.0 70 2.8 1,100 2,500 Rubber 300 7 10.0 1,200 8,000

Will return to its original shape. But when the strain exceeds the yield point, the material is deformed irreversibly, so that some residual strain will persist even after unloading. The modulus of resilience is then the quantity of energy the material can absorb without suffering damage. Similarly, the modulus of toughness is the energy needed to completely fracture the material. Materials showing good impact resistance are gen-erally those with high moduli of toughness.

Figure 13: Moduli of resilience and toughness.

Table 14 lists energy absorption values for a number of common materials. Note that natural and polymeric materials can provide extremely high energy absorption per unit weight. During loading, the area under the stress-strain curve is the strain energy per unit volume absorbed by the material. Conversely, the area under the unloading curve is the energy released by the material. In the elas-tic range, these areas are equal and no net energy is absorbed. But 4J.E. Gordon, Structures, or Why Things Don’t Fall Down, Plenum Press, New York, 1978. if the material is loaded into the plastic range as shown in Fig. 14, the energy absorbed exceeds the energy released and the difference is dissipated as heat.

Figure 14: Energy loss = area under stress-strain loop.

Compression The above discussion is concerned primarily with simple tension,

i.e. uniaxial loading that increases the interatomic spacing. How-ever, as long as the loads are sufficiently small (stresses less than the proportional limit), in many materials the relations outlined above apply equally well if loads are placed so as to put the speci-men in compression rather than tension. The expression for defor-mation and a given load δ = PL/AE applies just as in tension, with negative values for δ and P indicating compression. Further, the modulus E is the same in tension and compression to a good ap-proximation, and the stress-strain curve simply extends as a straight line into the third quadrant as shown in Fig. 15.

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Figure 15: Stress-strain curve in tension and compression. There are some practical difficulties in performing stress-strain

tests in compression. If excessively large loads are mistakenly ap-plied in a tensile test, perhaps by wrong settings on the testing ma-chine, the specimen simply breaks and the test must be repeated with a new specimen. But in compression, a mistake can easily damage the load cell or other sensitive components, since even af-ter specimen failure the loads are not necessarily relieved. Speci-mens loaded cyclically so as to alternate between tension and com-pression can exhibit hysteresis loops if the loads are high enough to induce plastic flow (stresses above the yield stress). The enclosed area in the loop seen in Fig. 16 is the strain energy per unit volume released as heat in each loading cycle. This is the well-known ten-dency of a wire that is being bent back and forth to become quite hot at the region of plastic bending. The temperature of the speci-men will rise according to the magnitude of this internal heat gen-eration and the rate at which the heat can be removed by conduc-tion within the material and convection from the specimen surface.

Figure 16: Hysteresis loop. Specimen failure by cracking is inhibited in compression, since

cracks will be closed up rather than opened by the stress state. A number of important materials are much stronger in compression than in tension for this reason. Concrete, for example, has good compressive strength and so finds extensive use in construction in which the dominant stresses are compressive. But it has essentially no strength in tension, as cracks in sidewalks and building founda-tions attest: tensile stresses appear as these structures settle, and cracks begin at very low tensile strain in unreinforced concrete.

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon Indian Institute of Technology Madras 1.6 Concrete (Part II)

This section covers the following topics. Properties of Hardened Concrete (Part II) Properties of Grout Codal Provisions of Concrete

1.6.1 Properties of Hardened Concrete (Part II)

The properties that are discussed are as follows.1) Stress-strain curves for concrete2) Creep of concrete3) Shrinkage of concrete

Stress-strain Curves for Concrete

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Curve under uniaxial compression

The stress versus strain behaviour of concrete under uniaxial com-pression is initially linear (stress is proportional to strain) and elastic (strain is recovered at unloading). With the generation of micro-cracks, the behaviour becomes nonlinear and inelastic. After the specimen reaches the peak stress, the resisting stress decreases with increase in strain.

IS:1343 - 1980 recommends a parabolic characteristic stress-strain curve, proposed by Hognestad, for concrete under uniaxial compression (Figure 3 in the Code). Figure 1-6.1 a) Concrete cube under compression, b) Design stress-strain curve for concrete under compression due to flexure Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon In-dian Institute of Technology Madras

The equation for the design curve under compression due to flexure is as follows.

For εc ≤ ε0

⎡ ⎛ ⎞ ⎛ ⎞ ⎤⎢ ⎜ ⎟ ⎜ ⎟ ⎥⎢⎣ ⎝ ⎠ ⎝ ⎠ ⎥⎦c cck ckf = f ε - εε ε20 0

For εc < εc ≤ εcu

fc = fck (1-6.2)

Here, fc = compressive stress fck = characteristic compressive strength of cubes εc = compressive strain ε0 = strain corresponding to fck = 0.002 εcu = ultimate compressive strain = 0.0035

For concrete under compression due to axial load, the ultimate strain is restricted to 0.002. From the characteristic curve, the design curve is defined by multiplying the stress with a size factor of 0.67 and dividing the stress by a material safety factor of γm = 1.5. The design

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curve is used in the calculation of ultimate strength. The following sketch shows the two curves.

ε0 εcu εc fc fck 0.447 fck Characteristic curve Design curve

Figure 1-6.2 Stress-strain curves for concrete under compression due to flexure

In the calculation of deflection at service loads, a linear stress-strain curve is assumed

up to the allowable stress. This curve is given by the following equation.

fc = Ecεc (1-6.3) Note that, the size factor and the material safety factor are not

used in the elastic modulus Ec. Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof.

Devdas Menon Indian Institute of Technology Madras For high strength concrete (say M100 grade of concrete and above)

under uniaxial compression, the ascending and descending branches are steep.

ε0 εc fc fck Es Eci

Figure 1-6.3 Stress-strain curves for high strength concrete under com-pression

The equation proposed by Thorenfeldt, Tomaxzewicz and Jensen is appropriate for high strength concrete.

⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ c c ck nk cn ε εf =fn - + εε 001 (1-6.4)

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The variables in the previous equation are as follows. fc = compressive stress fck = characteristic compressive strength of cubes in N/mm2 εc = compressive strain ε0 = strain corresponding to fck k = 1 for εc ≤ ε0 = 0.67 + (fck / 77.5) for εc > ε0. The value of k should be greater

than 1. n = Eci / (Eci – Es) Eci = initial modulus Es = secant modulus at fck = fck / ε0. The previous equation is applicable for both the ascending and de-

scending branches of the curve. Also, the parameter k models the slope of the descending branch, which increases with the charac-teristic strength fck. To be precise, the value of ε0 can be consid-ered to vary with the compressive strength of concrete.

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon

Indian Institute of Technology Madras Curve under uniaxial tension The stress versus strain behaviour of concrete under uniaxial ten-

sion is linear elastic initially. Close to cracking nonlinear behaviour is observed.

fc εc fc (a) (b)

Figure 1-6.4 a) Concrete panel under tension, b) Stress-strain curve for concrete under tension

In calculation of deflections of flexural members at service loads, the nonlinearity is neglected and a linear elastic behaviour fc = Ecεc is assumed. In the analysis of ultimate strength, the tensile strength of concrete is usually neglected.

Creep of Concrete Creep of concrete is defined as the increase in deformation with

time under constant load. Due to the creep of concrete, the pre-stress in the tendon is reduced with time.

Hence, the study of creep is important in prestressed concrete to calculate the loss in prestress.

The creep occurs due to two causes.1. Rearrangement of hydrated cement paste (especially the layered products)2. Expulsion of water from voids under load

If a concrete specimen is subjected to slow compressive loading, the stress versus strain curve is elongated along the strain axis as compared

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to the curve for fast loading. This can be explained in terms of creep. If the load is sustained at a level, the increase in strain due to creep will lead to a shift from the fast loading curve to the slow loading curve (Fig-ure 1-6.5).

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon

Indian Institute of Technology Madras fc Fast loading Slow loading Effect of creep εc

Figure 1-6.5 Stress-strain curves for concrete under compression

Creep is quantified in terms of the strain that occurs in addition to the elastic strain due to the applied loads. If the applied loads are close to the service loads, the creep strain increases at a decreas-ing rate with time. The ultimate creep strain is found to be propor-tional to the elastic strain. The ratio of the ultimate creep strain to the elastic strain is called the creep coefficient θ.

For stress in concrete less than about one-third of the characteris-tic strength, the ultimate creep strain is given as follows.εcr,ult = θεel (1-6.5)

The variation of strain with time, under constant axial compressive stress, is represented in the following figure.

strain Time (linear scale) εcr, ult = ultimate creep strain εel = elastic strain

Figure 1-6.6 Variation of strain with time for concrete under compres-sion

If the load is removed, the elastic strain is immediately recovered. However the recovered elastic strain is less than the initial elastic strain, as the elastic modulus increases with age.

There is reduction of strain due to creep recovery which is less than the creep strain.

There is some residual strain which cannot be recovered (Figure 1-6.7).

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon

Indian Institute of Technology Madras strain Time (linear scale) Residual strain Creep recovery Elastic recovery Unloading.

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Figure 1-6.7 Variation of strain with time showing the effect of unload-ing. The creep strain depends on several factors. It increases with the in-crease in the following variables.

1) Cement content (cement paste to aggregate ratio)2) Water-to-cement ratio3) Air entrainment4) Ambient temperature.

The creep strain decreases with the increase in the following variables.1) Age of concrete at the time of loading.2) Relative humidity3) Volume to surface area ratio.

The creep strain also depends on the type of aggregate. IS:1343 - 1980 gives guidelines to estimate the ultimate creep

strain in Section 5.2.5. It is a simplified estimate where only one factor has been considered. The factor is age of loading of the pre-stressed concrete structure. The creep coefficient θ is provided for three values of age of loading.

Table 1-6.1 Creep coefficient θ for three values of age of loading Age of Loading Creep Coefficient

7 days 2.2 28 days 1.6 1 year 1.1

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon Indian Institute of Technology Madras. It can be observed that if the structure is loaded at 7 days, the creep coefficient is 2.2.

This means that the creep strain is 2.2 times the elastic strain. Thus, the total strain is more than thrice the elastic strain. Hence, it is necessary to study the effect of creep in the loss of prestress and deflection of prestressed flexural members. Even if the struc-ture is loaded at 28 days, the creep strain is substantial. This im-plies higher loss of prestress and higher deflection.

Curing the concrete adequately and delaying the application of load provide long term benefits with regards to durability, loss of prestress and deflection.

In special situations detailed calculations may be necessary to mon-itor creep strain with time. Specialised literature or international codes can provide guidelines for such calculations.

Shrinkage of Concrete

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Shrinkage of concrete is defined as the contraction due to loss of moisture. The study of shrinkage is also important in prestressed concrete to calculate the loss in prestress.

The shrinkage occurs due to two causes.1. Loss of water from voids2. Reduction of volume during carbonation

The following figure shows the variation of shrinkage strain with time. Here, t0 is the time at commencement of drying. The shrinkage strain increases at a decreasing rate with time. The ultimate shrinkage strain (εsh) is estimated to calculate the loss in prestress. Shrinkage strain t0 Time (linear scale) εsh

Figure 1-6.8 Variation of shrinkage strain with time Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof.

Devdas Menon Indian Institute of Technology Madras Like creep, shrinkage also depends on several factors. The shrink-

age strain increases with the increase in the following variables.1) Ambient temperature2) Temperature gradient in the members3) Water-to-cement ratio4) Cement content.

The shrinkage strain decreases with the increase in the following vari-ables.

1) Age of concrete at commencement of drying2) Relative humidity3) Volume to surface area ratio.

The shrinkage strain also depends on the type of aggregate. IS:1343 - 1980 gives guidelines to estimate the shrinkage strain

in Section 5.2.4. It is a simplified estimate of the ultimate shrink-age strain (εsh).

For pre-tension εsh = 0.0003 (1-6.6) For post-tension (1-6.7) ( ) εsh = log10 t + 0.0002 2

Here, t is the age at transfer in days. Note that for post-tension, t is the age at transfer in days which approximates the curing time. It can be ob-

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served that with increasing age at transfer, the shrinkage strain reduces. As mentioned before, curing the concrete adequately and delaying the application of load provide long term benefits with regards to durability and loss of prestress. In special situations detailed calculations may be necessary to monitor shrinkage strain with time. Specialised literature or international codes can provide guidelines for such calculations.

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Devdas Menon

Indian Institute of Technology Madras1.6.2 Properties of Grout

Grout is a mixture of water, cement and optional materials like sand, water-reducing admixtures, expansion agent and pozzolans. The water-to-cement ratio is around 0.5.

Fine sand is used to avoid segregation.The desirable properties of grout are as follows.1) Fluidity2) Minimum bleeding and segregation3) Low shrinkage4) Adequate strength after hardening5) No detrimental compounds6) Durable.

IS:1343 - 1980 specifies the properties of grout in Sections 12.3.1 and Section 12.3.2.

The following specifications are important.1) The sand should pass 150 μm Indian Standard sieve.2) The compressive strength of 100 mm cubes of the grout shall not be less than 17 N/mm2 at 7 days.

Prestressed Concrete Structures Dr. Amlan K Sengupta and Prof. Dev-das Menon Indian Institute of Technology Madras

1.6.5 Codal Provisions of Concrete

The following topics are covered in IS:1343 - 1980 under the re-spective sections. These provisions are not duplicated here.

Table 1-6.2 Topics and sections

Workability of concrete Section 6 Concrete mix proportioning Section 8 Production and control of concrete Section 9 Formwork Section 10 Transporting, placing, compacting Section 13 Concrete under special conditions Section 14

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Sampling and strength test of concrete Section 15 Acceptance criteria Section 16 Inspection and testing of structures Section 17

Lüders band

From Wikipedia, the free encyclopediaJump to: navigation, search

A Lüders band is a localized band of plastic deformation which occurs in certain materials before fracture [1] . Those are well-known in low carbon steels and some Al-Mg Alloys. The reason for their apparition is the pinning of dislocations by interstitial atoms (in steel, those are typically carbon and nitrogen), which naturally tend to form ‘‘atmospheres’’ around them.

They were first reported by Guillaume Piobert and W. Lüders. The bands tend to form at the shoulders of tensile test pieces as the stress can be highest in these regions. The formation of luders bands depends on the grain size of the metal being stressed and the geometry of the material, with flatter specimens showing more bands than a round bar.[2]

[edit] See also

Portevin–Le Chatelier effect

[edit] References

Richard W. Hertzberg , Deformation and Fracture Mechanics of En-gineering Materials, 4th Edition, pp. 29–30 ISBN 0471012149

http://iopscience.iop.org/1478-7814/23/1/331

1. ̂ Intergranular strain and texture in steel Luders bands Roxana Hutanua, Lynann Claphama, , and R.B. Rogge; Acta Materialia, Vol-ume 53, Issue 12, July 2005, Pages 3517-3524

2. ̂ Macroscopic aspects of Lüders band deformation in mild steel V.S. Ananthan†, and E.O. Hall; Acta Metallurgica et Materialia, Vol-ume 39, Issue 12, December 1991, Pages 3153-3160

3. Compressive, Bearing, & Shear Properties

4. Compressive Properties

In theory, the compression test is simply the oppo-site of the tension test with respect to the direction

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of loading.  In compression testing the sample is squeezed while the load and the displacement are recorded.  Compression tests re-sult in mechanical properties that include the compressive yield stress, compressive ultimate stress, and compressive modulus of elasticity.

5. Compressive yield stress is measured in a manner identical to that done for tensile yield strength.  When testing metals, it is defined as the stress corresponding to 0.002 in./in. plastic strain.  For plas-tics, the compressive yield stress is measured at the point of per-manent yield on the stress-strain curve. Moduli are generally greater in compression for most of the commonly used structural materials.

6. Ultimate compressive strength is the stress required to rupture a specimen. This value is much harder to determine for a compres-sion test than it is for a tensile test since many material do not ex-hibit rapid fracture in compression.  Materials such as most plas-tics that do not rupture can have their results reported as the com-pressive strength at a specific deformation such as 1%, 5%, or 10% of the sample's original height.

7. For some materials, such as concrete, the compressive strength is the most important material property that engineers use when de-signing and building a structure.  Compressive strength is also commonly used to determine whether a concrete mixture meets the requirements of the job specifications. 

8. Bearing Properties

Bearing properties are used when designing mechanically fastened joints. The purpose of a bearing test is to determine the the defor-mation of a hole as a function of the applied bearing stress.  The test specimen is basically a piece of sheet or plate with a carefully prepared hole some standard distance from the edge. Edge-to-hole diameter ratios of 1.5 and 2.0 are common. A hardened pin is in-serted through the hole and an axial load applied to the specimen and the pin. The bearing stress is computed by dividing the load applied to the pin, which bears against the edge of the hole, by the bearing area (the product of the pin diameter and the sheet or plate thickness). Bearing yield and ultimate stresses are obtained from bearing tests. BYS is computed from a bearing stress defor-mation curve by drawing a line parallel to the initial slope at an off-set of 0.02 times the pin diameter. BUS is the maximum stress withstood by a bearing specimen.

9. Shear Properties

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A shearing stress acts parallel to the stress plane, whereas a tensile or compressive stress acts normal to the stress plane. Shear prop-erties are primarily used in the design of mechanically fastened components, webs, and torsion members, and other components subject to parallel, opposing loads. Shear properties are dependant on the type of shear test and their is a variety of different standard shear tests that can be performed including the single-shear test, double-shear test, blanking-shear test, torsion-shear test and oth-ers. The shear modulus of elasticity is considered a basic shear property. Other properties, such as the proportional limit stress and shear ultimate stress, cannot be treated as basic shear proper-ties because of “form factor” effects.

10. Permanent elimination of the yield-point

-p Creep and fatigue tests for solder alloys

The creep and cyclic fatigue tests of the lead-free solder, two-ball solder alloy and the solder joint were conducted by using the 6-axis thermal-mechanical micro fatigue tester. The specimens and the testing results are shown as follows.

Creep curve of a lead free solder alloy 

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Typical creep curves for two solder alloys 

153

A typical stress-controlled fatigue test of the lead-free solder

154

 

Typical displacement-controlled fatigue test:cyclic softening of two-ball single-lap shearing

 

155

Fatigue test of the solder joint

In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. It is denoted KIc and has the units of .

The subscript 'Ic' denotes mode I crack opening under a normal tensile stress perpendicular to the crack, since the material can be made thick enough to resist shear (mode II) or tear (mode III).

Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. If a material has a large value of fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with a low fracture toughness value.[1]

Fracture mechanics, which leads to the concept of fracture toughness, was largely based on the work of A. A. Griffith who, among other things, studied the behavior of cracks in brittle materials.

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A related concept is the work of fracture (γwof) which is directly proportional to , where E is the Young's modulus of the material.[2] Note that, in SI units, γwof is given in J/m2.

Contents

[hide] 1 Table of values 2 Crack growth as a stability prob -

lem 3 Transformation toughening 4 Conjoint action 5 Stress-corrosion cracking (SCC) 6 See also 7 References

8 Other references

[edit] Table of values

Here are some typical values of fracture toughness for various materials:

MaterialKIc (MPa-m1

/ 2)

Metals

Aluminum alloy (7075) 24

Steel alloy (4340) 50

Titanium alloy 44-66

Aluminum 14-28

Ceramics

Aluminum oxide 3-5

Silicon carbide 3-5

Soda-lime-glass 0.7-0.8

Concrete 0.2-1.4

Polymers

Polymethyl methacrylate 0.7-1.6

Polystyrene 0.7-1.1

Composites

Mullite fiber reinforced-mullite composite

1.8-3.3[3]

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Silica aerogels0.0008-0.0048[4]

[edit] Crack growth as a stability problem

Consider a body with flaws (cracks) that is subject to some loading; the stability of the crack can be assessed as follows. We can assume for simplicity that the loading is of constant displacement or displacement controlled type (such as loading with a screw jack); we can also simplify the discussion by characterizing the crack by its area, A. If we consider an adjacent state of the body as being one with a larger crack (area A+dA), we can then assess strain energy in the two states and evaluate strain energy release rate.

The rate is reckoned with respect to the change in crack area, so if we use U for strain energy, the strain energy release rate is numerically dU/dA. It may be noted that for a body loaded in constant displacement mode, the displacement is applied and the force level is dictated by stiffness (or compliance) of the body. If the crack grows in size, the stiffness decreases, so the force level will decrease. This decrease in force level under the same displacement (strain) level indicates that the elastic strain energy stored in the body is decreasing - is being released. Hence the term strain energy release rate which is usually denoted with symbol G.

The strain energy release rate is higher for higher loads and larger cracks. If the strain energy so released exceeds a critical value Gc, then the crack will grow spontaneously. For brittle materials, Gc can be equated to the surface energy of the (two) new crack surfaces; in other words, in brittle materials, a crack will grow spontaneously if the strain energy released is equal to or greater than the energy required to grow the crack surface(s). The stability condition can be written as

elastic energy released = surface energy created

If the elastic energy releases is less than the critical value, then the crack will not grow; equality signifies neutral stability and if the strain energy release rate exceeds the critical value, the crack will start growing in an unstable manner. For ductile materials, energy associated with plastic deformation has to be taken into account. When there is plastic deformation at the crack tip (as occurs most often in metals) the energy to propagate the crack may increase by several orders of magnitude as the work related to plastic deformation may be much larger than the surface energy. In such cases, the stability criterion has to restated as

158

elastic energy released = surface energy + plastic deformation energy

Practically, this means a higher value for the critical value Gc. From the definition of G, we can deduce that it has dimensions of work (or energy) /area or force/length. For ductile metals GIc is around 50 to 200 kJ/m2, for brittle metals it is usually 1-5 and for glasses and brittle polymers it is almost always less than 0.5. The I subscript here refers to mode I or crack opening mode as described in the section on fracture mechanics.

The problem can also be formulated in terms of stress instead of energy, leading to the terms stress intensity factor K (or KI for mode I) and critical stress intensity factor Kc (and KIc). These Kc and KIc (etc.) quantities are commonly referred to as fracture toughness, though it is equivalent to use Gc. Typical values for KIcare 150 MN/m3/2 for ductile (very tough) metals, 25 for brittle ones and 1-10 for glasses and brittle polymers. Notice the different units used by GIc and KIc. Engineers tend to use the latter as an indication of toughness.

[edit] Transformation toughening

Composites exhibiting the highest level of fracture toughness are typically made of a pure alumina or some silica-alumina (SiO2 /Al2O3) matrix with tiny inclusions of zirconia (ZrO2) dispersed as uniformly as possible within the solid matrix. (*Note: a wet chemical approach is typically necessary in order to establish the compositional uniformity of the ceramic body before firing).

The process of "transformation toughening" is based on the assumption that zirconia undergoes several martensitic (displacive, diffusionless) phase transformations (cubic → tetragonal → monoclinic) between room temperature and practical sintering (or firing) temperatures. Thus, due to the volume restrictions induced by the solid matrix, metastable crystalline structures can become frozen in which impart an internal strain field surrounding each zirconia inclusion upon cooling. This enables a zirconia particle (or inclusion) to absorb the energy of an approaching crack tip front in its nearby vicinity.

Thus, the application of large shear stresses during fracture nucleates the transformation of a zirconia inclusion from the metastable phase. The subsequent volume expansion from the inclusion (via an increase in the height of the unit cell) introduces compressive stresses which therefore strengthen the matrix near the approaching crack tip front. Zirconia "whiskers" may be used expressly for this purpose.

159

Appropriately referred to by its first discoverers as "ceramic steel", the stress intensity factor values for window glass (silica), transformation toughened alumina, and a typical iron/carbon steel range from 1 to 20 to 50 respectively.

[edit] Conjoint action

There are number of instances where this picture of a critical crack is modified by corrosion. Thus, fretting corrosion occurs when a corrosive medium is present at the interface between two rubbing surfaces. Fretting (in the absence of corrosion) results from the disruption of very small areas that bond and break as the surfaces undergo friction, often under vibrating conditions. The bonding contact areas deform under the localised pressure and the two surfaces gradually wear away. Fracture mechanics dictates that each minute localised fracture has to satisfy the general rule that the elastic energy released as the bond fractures has to exceed the work done in plastically deforming it and in creating the (very tiny) fracture surfaces. This process is enhanced when corrosion is present, not least because the corrosion products act as an abrasive between the rubbing surfaces.

Fatigue is another instance where cyclical stressing, this time of a bulk lump of metal, causes small flaws to develop. Ultimately one such flaw exceeds the critical condition and fracture propagates across the whole structure. The 'fatigue life' of a component is the time it takes for criticality to be reached, for a given regime of cyclical stress. Corrosion fatigue is what happens when a cyclically stressed structure is subjected to a corrosive environment at the same time. This not only serves to initiate surface cracks but (see below) actually modifies the crack growth process. As a result the fatigue life is shortened, often considerably.

[edit] Stress-corrosion cracking (SCC)

Main article: Stress corrosion cracking

This phenomenon is the unexpected sudden failure of normally ductile metals subjected to a constant tensile stress in a corrosive environment. Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides, mild steel cracks in the presence of alkali (boiler cracking) and copper alloys crack in ammoniacal solutions (season cracking). Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially chloride. With the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics

160

predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below KIc. In fact, the subcritical value of the stress intensity, designated as KIscc, may be less than 1% of KIc, as the following table shows:

Alloy KIc (MN / m3 / 2) SCC environment KIscc (MN / m3 / 2)

13Cr steel 60 3% NaCl 12

18Cr-8Ni 200 42% MgCl2 10

Cu-30Zn 200 NH4OH, pH7 1

Al-3Mg-7Zn 25 Aqueous halides 5

Ti-6Al-1V 60 0.6M KCl 20

The subcritical nature of propagation may be attributed to the chemical energy released as the crack propagates. That is,

elastic energy released + chemical energy = surface energy + deformation energy

The crack initiates at KIscc and thereafter propagates at a rate governed by the slowest process, which most of the time is the rate at which corrosive ions can diffuse to the crack tip. As the crack advances so K rises (because crack length appears in the calculation of stress intensity). Finally it reaches KIc , whereupon fast fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature. Stainless steels, for example, are employed because under most conditions they are 'passive', i.e. effectively inert. Very often one finds a single crack has propagated while the rest of the metal surface stays apparently unaffected.

[edit] See also

Puncture resistance Fracture mechanics Brittle-ductile transition zone Charpy impact test Impact (mechanics) Izod impact strength test Toughness of ceramics by indentation Shock (mechanics) Stress corrosion cracking Fracture toughening mechanisms

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UNIT – V

HEAT TREATMENT

Full annealing

Annealing (metallurgy)

From Wikipedia, the free encyclopediaJump to: navigation, search

It has been suggested that Annealing by short circuit be merged into this article or section. (Discuss)

This article needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (October 2010)

162

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature and maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

In the cases of copper, steel, silver, and brass, this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool. Unlike ferrous metals—which must be cooled slowly to anneal—copper, silver[1] and brass can be cooled slowly in air or quickly by quenching in water. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming.

Contents

[hide] 1 Thermodynamics of annealing

o 1.1 Stages of annealing 2 Annealing in a controlled atmos -

phere 3 Setup and equipment 4 Diffusion annealing of semiconduc -

tors 5 Specialized annealing cycles

o 5.1 Normalization o 5.2 Process annealing o 5.3 Full anneal o 5.4 Short cycle anneal

6 Resistive heating 7 See also 8 References 9 Further reading

10 External links

[edit] Thermodynamics of annealing

Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility.

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The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed "stress relief".

The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process.

The reaction facilitating the return of the cold-worked metal to its stress-free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradients within the body of the metal. The creation of lattice vacancies is governed by the Arrhenius equation, and the migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion.[2]

Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered. On heating at specific temperature and cooling it is possible to bring the atom at the right lattice site and new grain growth can improve the mechanical properties.

[edit] Stages of annealing

There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stresses which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.[3] The second phase is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses.[3] If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.

[edit] Annealing in a controlled atmosphere

The high temperature of annealing may result in oxidation of the metal’s surface, resulting in scale. If scale is to be avoided, annealing is carried out in an oxygen-, carbon-, and nitrogen-free atmosphere (to avoid oxidation, carburization, and nitriding respectively) such as endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen[clarification

needed]).

The magnetic properties of mu-metal (Espey cores) are introduced by

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annealing the alloy in a hydrogen atmosphere.

[edit] Setup and equipment

Typically, large ovens are used for the annealing process. The inside of the oven is large enough to place the workpiece in a position to receive maximum exposure to the circulating heated air. For high volume process annealing, gas fired conveyor furnaces are often used. For large workpieces or high quantity parts Car-bottom furnaces will be used in order to move the parts in and out with ease. Once the annealing process has been successfully completed, the workpieces are sometimes left in the oven in order for the parts to have a controlled cooling process. While some workpieces are left in the oven to cool in a controlled fashion, other materials and alloys are removed from the oven. After being removed from the oven, the workpieces are often quickly cooled off in a process known as quench hardening. Some typical methods of quench hardening materials involve the use of media such as air, water, oil, or salt.

[edit] Diffusion annealing of semiconductors

In the semiconductor industry, silicon wafers are annealed, so that dopant atoms, usually boron, phosphorus or arsenic, can diffuse into substitutional positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material.

[edit] Specialized annealing cycles

[edit] Normalization

Normalization is an annealing process in which a metal is cooled in air after heating in order to relieve stress.

It can also be referred to as: Heating a ferrous alloy to a suitable temperature above the transformation temperature range and cooling in air to a temperature substantially below the transformation range.

This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air. Small grains are formed which give a much harder and

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tougher metal with normal tensile strength and not the maximum ductility achieved by annealing. It eliminates columnar grains and dendritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.

[edit] Process annealing

Process annealing, also called "intermediate annealing", "subcritical annealing", or "in-process annealing", is a heat treatment cycle that restores some of the ductility to a work piece allowing it be worked further without breaking. Ductility is important in shaping and creating a more refined piece of work through processes such as rolling, drawing, forging, spinning, extruding and heading. The piece is heated to a temperature typically below the austenizing temperature, and held there long enough to relieve stresses in the metal. The piece is finally cooled slowly to room temperature. It is then ready again for additional cold working. This can also be used to ensure there is reduced risk of distortion of the work piece during machining, welding, or further heat treatment cycles.

The temperature range for process annealing ranges from 500 °F to 1400 °F, depending on the alloy in question.

[edit] Full annealFull annealing temperature ranges

A full anneal typically results in the second most ductile state a metal can assume for metal alloy. It creates an entirely new homogeneous and uniform structure with good dynamic properties. To perform a full anneal, a metal is heated to its annealing point (about 50°C above the austenic temperature as graph shows) and held for sufficient time to allow the material to fully austenitize, to form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the equilibrium microstructure is obtained. In some cases this means the material is allowed to air cool. In other cases the material is allowed to furnace cool. The details of the process depend on the type of metal and the precise alloy involved. In any case the result is a more ductile material that has greater stretch ratio and reduction of area properties but a lower yield strength and a lower tensile strength. This process is also called LP annealing for lamellar pearlite in the steel industry as opposed to a process anneal which does not specify a microstructure and only has the goal of softening the material. Often material that is to be machined, will be annealed, then be followed by further heat treatment to obtain the final desired properties.

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[edit] Short cycle anneal

Short cycle annealing is used for turning normal ferrite into malleable ferrite. It consists of heating, cooling, and then heating again from 4 to 8 hours.

[edit] Resistive heating

Resistive heating can be used to efficiently anneal copper wire; the heating system employs a controlled electrical short circuit. It can be advantageous because it does not require a temperature-regulated furnace like other methods of annealing.

The process consists of two conductive pulleys (step pulleys) which the wire passes across after it is drawn. The two pulleys have an electrical potential across them, which causes the wire to form a short circuit. The Joule effect causes the temperature of the wire to rise to approximately 400 °C. This temperature is affected by the rotational speed of the pulleys, the ambient temperature, and the voltage applied. Where t is the temperature of the wire, K is a constant, V is the voltage applied, r is the number of rotations of the pulleys per minute, and ta is the ambient temperature:

t = ((KV ²)/(r))+ta

The constant K depends on the diameter of the pulleys and the resistivity of the copper.

Purely in terms of the temperature of the copper wire, an increase in the speed with which the wire passes through the pulley system has the same effect as an increase in resistance. Therefore, the speed with which the wire can be drawn through varies quadratically as the voltage applied.

[edit] See also

Annealing (glass) Hollomon-Jaffe parameter Low hydrogen annealing Tempering

[edit] References

1. ̂ http://www.handyharmancanada.com/hbpm/silver/silver.htm 2. ̂ Van Vlack, L.H. Elements of Materials Science and Engineering,

Addison-Wesley, 1985, p 134

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3. ^ a b Verhoeven, J.D. Fundamentals of Physical Metallurgy, Wiley, New York, 1975, p. 326

[edit] Further reading

Thesis of Degree, Cable Manufacture and Tests of General Use and Energy. - Jorge Luis Pedraz (1994), UNI, Files, Peru.

Dynamic annealing of the Copper wire by using a Controlled Short circuit. = Jorge Luis Pedraz (1999), Peru: Lima , CONIMERA 1999, INTERCON 99,

[edit] External links

Annealing with induction : Ameritherm offers annealing overview and Application Notes

Annealing :efunda - engineering fundamentals Full Annealing :Material Science Annealing : Aluminum and Aircraft Metal Alloys

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Definition:  A heat treatment process that has the object of relieving internal stresses, refining the grain size and improving the mechanical properties. The steel is heated to 800-900oC according to analysis, held at temperature to allow a full soak and cooled in still air.

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HARDENING AND TEMPERING

V. Ryan © 2005 - 2009

PDF FILE - CLICK HERE FOR PRINTABLE WORKSHEET

Steel can be treated by intense heat to give it different properties of hardness and softness. This depends on the amount of carbon in the steel (only high carbon steel can be hardened and tempered).

CARBON CONTENT OF COMMON STEELS: Mild steel: 0.4% carbon, Medium carbon steel approximately 0.8% carbon, High Carbon Steel approximately  1.2% carbon (this steel is also known as Tool Steel and includes Silver Steel and Gauge Plate).

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Mild steel and medium carbon steel do not have enough carbon to change their crystalline structure and consequently cannot be hardened and tempered. Medium carbon steel may become slightly tougher although it cannot be harden to the point where it cannot be filed or cut with a hacksaw (the classic test of whether steel has been hardened). 

If steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under great pressure. On the other hand, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft. However, the industrial heat treatment of steel is a very complex and precise science.

In a school workshop most heat treatment of metals takes place on a brazing hearth. A rotating table and fire bricks are essential. The fire bricks reflect the intense heat back on to the metal being heated. This is achieved by arranging the bricks in a semi-circle behind the metal being heated. Without the bricks, heat would escape and this would limit the temperature that could be reached.

HARDENING AND TEMPERING

Heat treatment of steel in a school workshop is normally a two stage process. For example, if a high carbon steel or silver steel screw driver blade has been manufactured, at some point it will have to be ‘’hardened’ to prevent it wearing down when used. On the other hand it will have to be ‘tempered’. This second heating process reduces the hardness a little but toughens the

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steel. It also significantly reduces the brittleness of the steel so that it does not break easily. The whole process is called ‘hardening and tempering’.

The screw driver blade is heated, slowly at first, warming up the whole blade. Then the heat is concentrated on the area at the end of the blade. This gradually becomes ‘red’ hot.

The screw driver blade is removed quickly from the brazing heart, with blacksmiths tongs and plunged into clean, cold water. Steam boils off from the water as the steel cools rapidly. At this stage the blade is very hard but brittle and will break easily.

STAGE THREE:

The screw driver blade is cleaned with emery cloth and heated again on the brazing hearth. Heat is concentrated at the end of the steel blade. The steel must be watched very carefully as it changes colour quite quickly. A blue line of heat will appear near the end of the blade and it travels towards the tip as the temperature rises along the blade. When the line of blue reaches the tip the brazing torch is turned off.

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The blue indicates the correct temperature of ‘tempering’.

STAGE FOUR:The screw driver blade is placed on a steel surface, such as an anvil face. This conducts the heat away and allows slow cooling of the screw driver blade. When cold, the blade should be tough and hard wearing and unlikely to break or snap. This is due to the tempering process.

USEFUL COLOUR INDICATORS OF TEMPERATURE

When heating steel on the brazing hearth, colour changes take place. These can be used to indicate the temperature of the metal. The table opposite is a rough guide.

The table opposite shows the temperatures and the associated colours required when tempering steel for particular uses. For instance, when making wood turning tools, they must be heated to a brown colour, whilst tempering.

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Hardenability

From Wikipedia, the free encyclopediaJump to: navigation, search

Cross-section of Jominy test apparatus. Water from a pipe cools the bottom of the specimen and then hardness measurements are taken across the length of the specimen.

Jominy test apparatus

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (April 2009)The hardenability of a metal alloy is its capability to be hardened by heat treatment. It should not be confused with hardness, which is a measure of a sample's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material.

The hardenability of ferrous alloys, i.e. steels, is a function of the carbon content and other alloying elements and the grain size of the austenite. The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats. Substances like brine and water cool much more quickly than oil or air. Additionally, if the fluid is agitated cooling occurs even more quickly. The geometry of the part also affects the cooling rate: of two samples of equal volume, the one with higher surface area will cool faster.

The hardenability of a ferrous alloy is measured by a Jominy test: a round metal bar of standard size is transformed to 100% austenite through heat treatment, and is then quenched on one end with room-temperature water. The cooling rate will be highest at the end being quenched, and will decrease as distance from the end increases. The hardenability is then found by measuring the hardness along the bar: the farther away from the quenched end that the hardness extends, the higher the hardenability.

Jominy hardenability (quench) test     The Jominy Test involves heating a test piece from the steel (25mm diameter and 100mm long) to an austenitising temperature and quenching from one end with a controlled and standardised jet of water. Take a sample from the furnace and place it on the Jominy test fixtures

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and observe the cooling pattern.After quenching the hardness profile is measured at intervals from the quenched end after the surface has been ground back to remove any

effects of decarburisation (0.38mm is removed from the surface). The hardness variation along the test surface is a result of microstructural variation which arises since the cooling rate decreases with distance from the quenched end. The cooling rate along the Jominy test specimen varies from about 225 ?C s-1 to 2 ?C s-1. 

Austempering

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This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. The talk page may contain suggestions. (September 2009)

Austempering is an isothermal heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a lower Bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized Austenite known as ausferrite. It is primarily used to improve mechanical properties.

Austempering is defined by both the process and the resultant microstructure . Typical austempering process parameters applied to an unsuitable material will not result in the formation of Bainite or ausferrite and thusly the final product will not be called austempered. both microstructures may also be produced via other methods. for example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not refered to as austempered.

Contents

[hide] 1 History 2 Process

o 2.1 Austenitizing o 2.2 Quenching o 2.3 Cooling o 2.4 Tempering

3 Advantages of Austemper - ing

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4 References

[edit] History

The austempering of steel was first pioneered by Edgar C. Bain and Edmund S. Davenport in the 1930’s who were working for the United States Steel Corporation at that time. Bainite must have been present in steels long before its acknowledged

discovery date, but was not identified because of the limited metalographic techniques available and the mixed microstructures formed by the heat treatment practices of the time. Coincidental circumstances led inspired Bain to study isothermal phase transformations. Austneite and the higher temperature phases of steel were becoming more and more understood and it was already known that austenite could be retained at room temperature. through his contacts at the American Steel and Wire Company Bain was aware of isothermal transformations being used in industry and he began to concieve of new experiments [1]

Further research into the isothermal transformation of steels was a result of Bain and Davenport's discovery of a new microstructure consisting of an "acicular, dark etching aggregate." This microstructure was found to be "tougher for the same hardness than tempered Martensite" [2].

Commercial exploitation of bainitic steel did not become common overnight. Common heat treating practices at the time featured continuous cooling methods and were not capable, in practice, of producing fully Bainitic microstructures. The range of alloys available produced either mixed microstructures or excessive amounts of Martensite. The advent of low-carbon steels containing boron and molybdenum in 1958 allowed fully Bainitic steel to be produced by continuous cooling [1] [3]. Commercial use of bainitic steel thus came about as a result of the development of new heat treating methods, those that involve a step holding the work piece at a fixed temperature for a period of time sufficient to allow transformation became colelctively know as austempering.

One of the first uses of austempered steel was in rifle bolts during World War II[4]. The high impact strength possible at high hardnesses, and the relatively small section size of the components made austempered steel ideal for this application. Over subsequent decades austempering revolutionized the spring industry followed by clips and clamps. These components, which are usually thin, formed parts do not require expensive alloys and generally possess better elastic properties than their tempered Martensite counterparts. eventually austempered steel

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made its way into the automotive industry where one of its first uses was in safety critical components. The majority of car seat brackets and seat belt components are made of austempered steel because of its high strength and ductility[4].

These properties allow it to absorb significantly more energy during a crash without the risk of brittle failure. Currently, austempered steel is also used in bearings, mower blades, transmission gear, wave plate, and turf aeration tines [4]. in the second half of the twentieth century the austempering process began to be commercially applied to cast irons. Austempered

Ductile Iron (ADI) was first commercialized in the early 1970’s and has since become a major industry.

[edit] Process

The most notable difference between austempering and conventional quench and tempering is that it involves holding the workpiece at the quenching temperature for an extended period of time. The basic steps are the same whether applied to cast iron or steel and are as follows:

[edit] Austenitizing

In order for any transformation to take place the microstructure of the metal must be austenite. The exact boundaries of the austenite phase region depend on the chemistry of the alloy being heat treated. however, austenitizing temperatures are typically between 790 and 915°C (1455 to 1680°F) [5]. The amount of time spent at this temperature will vary with the alloy and process specifics for a through-hardened part the best results are acheived when austenitization is long enough to produce a fully austenitic metal microstructure (there will still be graphite present in cast irons) with a consistent carbon content. In steels this may only take a few minutes after the austenitizing temperature has been reached throughout the part section, but in cast irons it takes longer. This is because carbon must diffuse out of the graphite until it has reached the equilibrium concentration dictated by the temperature and the phase diagram.

This step may be done in many types of furnaces, in a high temperature salt bath, via direct flame or induction heating numerous patents exist for specific methods and variations.

[edit] Quenching

As with conventional quench and tempering the material being heat

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treated must be cooled from the austenitizing temperature quickly enough to avoid the formation of pearlite. The specific cooling rate that is necessary to avoid the formation of pearlite is a product of the chemistry of the austenite phase and thus the alloy being processed. the actually cooling rate is a product of both the quench severity, which is influenced by quench media, agitation, load:quenchant ratio, etc, and the thickness and geometry of the part. As a result, heavier section components required greater hardenability. in austempering the heat treat load is quenched to a temperature which is typically above the Martensite start of the austenite and held. In some patented processes the parts are quench just below the Martensite start so that the resulting microstructure is a controlled mixture of Martensite and Bainite.

The two important aspects of quenching are the cooling rate and the holding time. The most common practice is to quench into a bath of liquid nitrie-nitrate salt and hold in the bath. Because of the restricted temperature range for processing it is not usually possible to quench in water or brine, but high temperature oils are used for a narrow temperature range. some processes feature quenching and then removal from the quench media for and then holding in a furnace.

The quench and holding temperature are primary processing paramters that control the final hardness, and thus properties of the material.

[edit] Cooling

After quenching and holding there is no danger of cracking; parts are typically air cooled or put directly into a room temperature wash system.

[edit] Tempering

No tempering is required after austempering if the part is through hardened and fully transformed to either Bainite or ausferrite [5]. tempering adds another stage and thus cost to the process; it does not provide the same property modification and stress relief in Bainite or ausferrite that it does for virgin Martensite.

[edit] Advantages of Austempering

Austempering offers many manufacturing and performance advantages over traditional material/process combinations. It may be applied to numerous materials, and each combination has its own advantages, which are listed below. One of the advantages that is common to all austempered materials is a lower rate of distortion than for quench and tempering. This can be translated into significant cost savings by adjusting the entire manufacturing process.

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The most immediate cost savings are realized by machining before heat treatment. There are many such savings possible in the specific case of converting a quench and tempered steel component to austempered ductile iron (ADI). In North America and many other parts of the world the cost of ductile iron per kilogram is less than that for steel [citation needed]. Ductile iron is 10% less dense than steel and can be cast near to net shape, both characteristics that reduce the casting weight. Near net shap casting also reduces the machining cost further, which is already reduced by machining soft ductile iron instead of hardened steel. A lighter finished part reduces freight charges and the streamlined production flow often reduces lead time. In many cases strength and wear resistance can also be improved [4].

Process/Material combinations include:

austempered steel Carbo-Austempered steel Marbain steel Austempered Ductile Iron (ADI) Locally Austempered Ductil Iron (LADI) Austempered Gray Iron (AGI) Carbidic Austempered Ductile Iron (CADI)

When speaking of performance improvements, austempered materials are typically compared to conventionally quench and tempered materials with a tempered Martensite microstructure.

In steels above 40 Rc these improvements include:

Higher ductility, impact strength and wear resistance for a given hardness,

A low distortion, repeatable dimensional response, Increased fatigue strength, Resistance to hydrogen and environmental embrittlement.

In cast irons (from 250-550 HBW) these improvements include:

Higher ductility and impact resistance for a given hardness, A low distortion, repeatable dimensional response, Increased fatigue strength, Increased wear resistance for a given hardness.

[edit] References

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1. ^ a b Bhadeshis, H. K. D. H., "Bainite in Steels: Transformations, Microstructure, and properties" second edition, IOM Communica-tions, London, England, 2001

2. ̂ Bain, Edgar C., "Functions of the Alloying Elements in Steel" American Society for Metals, Cleveland, Ohio, 1939

3. ̂ Irvine, K.J. and Pickering, F.B JISI 188, 1958. 4. ^ a b c d http://www.appliedprocess.com 5. ^ a b "Heat Treater's Guide: Practices and procedures for Irons and

Steels" ASM International, Materials Park, Ohio, Second Edition,1995

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Carbon steel

From Wikipedia, the free encyclopediaJump to: navigation, search

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Iron alloy phases

v • d • e

Ferrite (α-iron, δ-iron)Austenite (γ-iron)Pearlite (88% ferrite, 12% cementite)MartensiteBainiteLedeburite (ferrite-cementite eutectic, 4.3% carbon)Cementite (iron carbide, Fe3C)

Steel classes

Crucible steelCarbon steel (≤2.1% carbon; low alloy)

Spring steel (low or no alloy)

Alloy steel (contains non-carbon elements)

Maraging steel (contains nickel)Stainless steel (contains ≥10.5% chromium)Weathering steelTool steel (alloy steel for tools)

Other iron-based materials

Cast iron (>2.1% carbon)

Ductile ironGray ironMalleable ironWhite iron

Wrought iron (contains slag)

Carbon steel, also called plain-carbon steel, is steel where the main alloying constituent is carbon. The American Iron and Steel Institute (AISI) defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following

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elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60."[1]

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

As the carbon content rises, steel has the ability to become harder and stronger through heat treating, but this also makes it less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.[2]

Eighty-five percent of all steel used in the United States is carbon steel.[1]

Contents

[hide] 1 Types

o 1.1 Mild and low carbon steel o 1.2 Higher carbon steels

2 Heat treatment 3 Case hardening 4 See also 5 References

6 Bibliography

[edit] Types

See also: SAE steel grades

Carbon steel is broken down in to four classes based on carbon content:

[edit] Mild and low carbon steel

Mild steel is the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low carbon steel contains approximately 0.05–0.15% carbon[1] and mild steel contains 0.16–0.29%[1] carbon, therefore it is neither brittle nor ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing.[3]

It is often used when large quantities of steel are needed, for example as structural steel. The density of mild steel is approximately 7.85 g/cm3

(0.284 lb/in3)[4] and the Young's modulus is 210,000 MPa (30,000,000

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psi).[5]

Low carbon steels suffer from yield-point runout where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop Lüder bands.[6]

[edit] Higher carbon steels

Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red-short. Low alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1,426–1,538 °C (2,599–2,800 °F).[7] Manganese is often added to improve the hardenability of low carbon steels. These additions turn the material into a low alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.65% manganese by weight.

Medium carbon steel

Approximately 0.30–0.59% carbon content.[1] Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.[8]

High carbon steel

Approximately 0.6–0.99% carbon content.[1] Very strong, used for springs and high-strength wires.[9]

Ultra-high carbon steel

Approximately 1.0–2.0% carbon content.[1] Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy. Note that steel with a carbon content above 2.0% is considered cast iron.

Steel can be heat treated which allows parts to be fabricated in an easily-formable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance. Steels are often wrought by cold working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature.

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[edit] Heat treatment

Iron-carbon phase diagram, showing the temperature and carbon ranges for certain types of heat treatments.Main article: Heat treatment

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are slightly altered. As with most strengthening techniques for steel, Young's modulus is unaffected. Steel has a higher solid solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through the eutectoid reaction affects the rate at which carbon diffuses out of austenite. Generally speaking, cooling swiftly will give a finer pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C) steel then the structure is full pearlite with small grains of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. Here is a list of the types of heat treatments possible:

Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formabil-ity. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.[10]

Full annealing : Carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more duc-tile.[11]

Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually

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heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward shows the area where process annealing occurs.

Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this tem-perature is maintained for a time and then the temperature is brought down below lower critical temperature and is again main-tained. Then finally it is cooled at room temperature. This method rids any temperature gradient.

Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (68 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.[12]

Quenching : Carbon steel with at least 0.4 wt% C is heated to nor-malizing temperatures and then rapidly cooled (quenched) in wa-ter, brine, or oil to the critical temperature. The critical tempera-ture is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline struc-ture, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approxi-mately three to four (with more carbon) fold harder than normal-ized steel.[13]

Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typi-cally in a molten salt bath at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained austenite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer mar-quenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer tempera-tures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process re-duce internal stresses and stress cracks, but it also increases the impact resistance.[14]

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Quench and tempering : This is the most common heat treatment encountered, because the final properties can be precisely deter-mined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eu-tectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restores ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.[15]

Austempering : The austempering process is the same as martem-pering, except the steel is held in the molten salt bath through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact re-sistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath.[16]

[edit] Case hardening

Main article: Case hardening

Case hardening processes harden only the exterior of the steel part, creating a hard, wear resistant skin (the "case") but preserving a tough and ductile interior. Carbon steels are not very hardenable; therefore wide pieces cannot be thru-hardened. Alloy steels have a better hardenability, so they can through-harden and do not require case hardening. This property of carbon steel can be beneficial, because it gives the surface good wear characteristics but leaves the core tough.

[edit] See also

Cold working Hot working

[edit] References

1. ^ a b c d e f g Classification of Carbon and Low-Alloy Steel, archived from the original on 2010-03-11, http://www.webcitation.org/5o9SDyEAb, retrieved 2010-03-11.

2. ̂ Knowles, Peter Reginald (1987), Design of structural steelwork (2nd ed.), Taylor & Francis, p. 1, ISBN 9780903384599, http://books.google.com/books?id=U6wX-3C8ygcC&pg=PA1.

3. ̂ Engineering fundamentals page on low-carbon steel 4. ̂ Elert, Glenn, Density of Steel,

http://hypertextbook.com/facts/2004/KarenSutherland.shtml, re-trieved 2009-04-23.

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5. ̂ Modulus of Elasticity, Strength Properties of Metals - Iron and Steel, http://www.engineersedge.com/manufacturing_spec/properties_of_metals_strength.htm, retrieved 2009-04-23.

6. ̂ Degarmo, p. 377. 7. ̂ Ameristeel article on carbon steel 8. ̂ Engineering fundamentals page on medium-carbon steel 9. ̂ Engineering fundamentals page on high-carbon steel 10. ̂ Smith, p. 388. 11. ̂ Smith, p. 386. 12. ̂ Smith, pp. 386–387. 13. ̂ Smith, pp. 373–377. 14. ̂ Smith, pp. 389–390. 15. ̂ Smith, pp. 387-388. 16. ̂ Smith, p. 391.

[edit] Bibliography

Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.

Oberg, E.; et al. (1996), Machinery's Handbook (25th ed.), Indus-trial Press Inc, ISBN 0831125993.

Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill, ISBN 0-07-295358-6.

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Carburized steel parts are rarely used without subsequent heat treatment, which consists of several steps to obtain optimum hardness in the case, and optimum strength and ductility in the core. Grain size of the core and case is refined. a. Refining  the  core  is accomplished  by  reheating  the  parts  to  a  point  just  above  the critical  temperature  of  the steel.  After soaking for a sufficient time to insure uniform heating, the parts are quenched in oil. b. The hardening temperature for the high carbon case is well below that of the core.  It is, therefore, necessary to heat the parts again to the critical temperature of the case and quench them in oil to produce the required hardness.  A soaking period of 10 minutes is generally sufficient. c. A  final  stress relieving  operation  is  necessary  to  minimize  the  hardening  stresses produced  by  the  previous treatment.  The stress relieving temperature is generally around 350 F.  This is accomplished by heating, soaking until uniformly heated, and cooling in still air.  When extreme hardness is desired, the temperature should be carefully held to the lower limit of the range. 2-41.    CYANIDING.    Steel  parts  may  be  surface-hardened  by  heating  while  in  contact  with  a  cyanid  salt, followed  by quenching.    Only  a  thin  case  is  obtained  by  this method  and  it  is,  therefore,  seldom  used  in  connection  with  aircraft construction or repair.  Cyaniding is, however, a rapid and economical method of case hardening, and may be used in some instances for relatively unimportant parts.  The work to be hardened is immersed in a bath of molten sodium or potassium cyanide from 30 to 60 minutes.  The

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cyanide bath should be mainlined at temperature to 760 C to 899 C (1,400 F to 1,650 F).  Immediately after removal from the bath, the parts are quenched in water.  The case obtained in this manner is due principally to the formation of carbides and nitrides on the surface of the steel.  The use of a closed pot and ventilating hood are required for cyaniding, as cyanide vapors are extremely poisonous. 2-42. NITRIDING.  This method of case hardening is advantageous due to the fact that a harder case is obtained than by carburizing.  Many engine parts such as cylinder barrels and gears may be treated in this way.  Nitriding is generally applied to certain special steel alloys, one of the essential constituents of which is aluminum.  The process involves the exposing  of  the  parts  to  ammonia  gas  or  other  nitrogenous materials  for  20  to  100  hours  at  950F.    The container in which the work and Ammonia gas are brought in contact must be airtight and capable of maintaining good circulation and even temperature throughout.  The depth of case obtained by nitriding is about 0.015 inch if heated for 50 hours.  The nitriding  process  does  not  affect  the physical  state  of  the  core  if  the  preceding  tempering  temperature was  950  F  or over. When a part is to be only partially treated, tinning of any surface will prevent it from being nitrides.  Nitrided surfaces can be reheated to 950 F with out losing any of their hardness.  However, if heated above that temperature, the hardness is rapidly  lost  and  cannot be  regained  by  retreatment.    Prior  to  any  nitriding  treatment,  all decarburized  metal  must  be removed to prevent flaking of the nitrided case.  When no distortion is permissible in the nitrided part, it is necessary to normalize the steel prior to nitriding to remove all strains resulting from the forging, quenching, or machining. 2-43.  HEAT TREATING EQUIPMENT. Equipment necessary for heat treating consists of a suitable means for bringing the metal to the required temperature measuring  and  controlling  device,  and  quenching medium.    Heat  may,  in  some  instances,  be  supplied  by  means  of  a forge  or  welding  torch;  however,  for  the  treatment  required  in aircraft  work,  a  furnace  is  necessary.    Various  jigs  and fixtures are sometimes needed for controlling quenching and preventing warping. 2-44.  FURNACES.  Heat treating furnaces are of many designs and no one size or type perfectly fills every heat treating requirement. The size and quantity of metal to be treated and the various treatments required determine the size and type  of  furnace  most  suitable  for each  individual  case.    The  furnace  should  be  of  a  suitable  type and  design  for  the purpose intended and should be capable of maintaining within the working zone a temperature varying not more than + or 14 C ( 25F) for the desired value

Nitriding

From Wikipedia, the free encyclopedia

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Nitriding is a heat treating process that alloys nitrogen onto the surface of a metal to create a case hardened surface. It is predominantly used on steel, but also titanium, aluminum and molybdenum.

Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, injectors and plastic-mold tools.

Contents

[hide] 1 Processes

o 1.1 Gas nitriding o 1.2 Salt bath nitriding o 1.3 Plasma nitriding

2 Materials for nitriding 3 History 4 See also 5 References 6 Further reading

7 External links

[edit] Processes

The processes are named after the medium used to donate nitrogen. The three main methods used are: gas nitriding, salt bath nitriding, and plasma nitriding.

[edit] Gas nitriding

In gas nitriding the donor is a nitrogen rich gas usually ammonia (NH3), which is why it is sometimes known as ammonia nitriding.[1] When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material. This process has been around for nearly a century though only in the last few decades has there been a

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concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over the other variants are:

All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding)

Large batch sizes possible - the limiting factor being furnace size and gas flow

With modern computer control of the atmosphere the nitriding re-sults can be tightly controlled

Relatively cheap equipment cost - especially compared with plasma

The disadvantages of gas nitriding are:

Reaction kinetics heavily influenced by surface condition - an oily surface or one contaminated with cutting fluids for example will de-liver poor results

Surface activation is sometimes required to successfully treat steels with a high chromium content - compare sputtering during plasma nitriding

Ammonia as nitriding medium - though not especially toxic it can be harmful when inhaled in large quantities. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion

[edit] Salt bath nitriding

In salt bath nitriding the nitrogen donating medium is a nitrogen containing salt such as cyanide salt. The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550–590 °C (1,022–1,094 °F). The advantages of salt nitriding are:

Quick processing time - usually in the order of 4 hours or so to achieve

Simple operation - heat the salt and workpieces to temperature and submerge until the duration has expired

The disadvantages are:

The salts used are highly toxic - Disposal of salts are controlled by stringent environmental laws in western countries and has in-creased the costs involved in using salt baths. This is one of the

195

most significant reasons the process has fallen out of favor in the last decade or so.

Only one process possible with a particular salt type - since the ni-trogen potential is set by the salt, only one type of process is possi-ble

[edit] Plasma nitriding

Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials.

In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with amonia). There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes.

Usually steels are very beneficially treated with plasma nitriding. Plasma nitriding advantage is related to the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only the performance of metal parts gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatigue strength of the metals being treated.

A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion.

This process was invented by Dr. Bernhardt Berghaus of Germany who later settled in Zurich to escape persecution of his community by the Nazis in 1939. It was only after his death in late 1960s that the process was acquired by Klockner group and popularized world over.

Plasma nitriding is often coupled with physical vapor deposition (PVD) process and labeled Duplex Treatment, to avail of immensely enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to generate a smooth jetblack layer of oxides which is very resistant to not only wear but corrosion.

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Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260°C to more than 600°C.[2] For instance, at moderate temperatures (like 420°C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties.

In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvants that could remain. This also helps the thermal stability of the plasma plant since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added in order keep the surface clear of oxides.

[edit] Materials for nitriding

Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best. Minimal amounts of material should be removed post nitriding to preserve the surface hardness.

Nitriding alloys are alloy steels with nitride forming elements such as aluminum, chromium, molybdenum and titanium.

[edit] History

Systematic investigation into the effect of nitrogen on the surface properties of steel only started in the 1920s. Investigation into gas nitriding began independently in both Germany and America. The process was greeted with enthusiasm in Germany and several steel grades were developed with nitriding especially in mind, these are the so called nitriding steels. The reception in America, on the other hand, was less impressive. With so little demand the process was more or less forgotten in the US. It was only after World War II that the process was reintroduced from Europe. A great deal of research has taken place in

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the last few decades to understand the thermodynamics and kinetics of the reactions taking place.

[edit] See also

Carburization Carbonitriding Surface finishing

[edit] References

1. ̂ Ion Nitriding and Nitrocarburizing of Sintered PM Parts, October 7, 2004

2. ̂ Zagonel, L; Figueroa, C; Droppajr, R; Alvarez, F (2006). "Influ-ence of the process temperature on the steel microstructure and hardening in pulsed plasma nitriding". Surface and Coatings Tech-nology 201: 452. doi:10.1016/j.surfcoat.2005.11.137.

[edit] Further reading

Ruth Chatterjee-Fischer - Wärmebehandlung von Eisenwerkstoffen: Nitrieren und Nitrocarburieren [Heat treatment of ferrous materi-als: nitriding and nitrocarburising] 1995 2nd Edition Expert Verlag ISBN 3-8159-1092

Chattopadhyay, Ramnarayan (2004). "Plasma Nitriding". Advanced Thermally Assisted Surface Engineering Processes. Berlin: Springer. pp. 90–94. ISBN 1-4020-7696-7.

[edit] External links

Key to Steel - Nitriding "MIL-S-6090A, Military Specification: Process for Steels Used In

Aircraft Carburizing and Nitriding". United States Department of Defense. 07 JUN 1971. http://www.everyspec.com/MIL-SPECS/MIL+SPECS+(MIL-S)/MIL-S-6090A_8810/.

Carbonitriding

From Wikipedia, the free encyclopediaJump to: navigation, search Not to be confused with Nitrocarburizing.

This article needs additional citations for verification.Please help improve this article by adding reliable references.

198

Unsourced material may be challenged and removed. (August 2009)

Carbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip, increasing the hardness and modulus near the surface. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult to work grades of steel.[1] Surface hardness of carbonitrided parts ranges from 55 to 62 HRC.

Certain pre-industrial case hardening processes include not only carbon-rich materials such as charcoal, but nitrogen-rich materials such as urea, which implies that traditional surface hardening techniques were a form of carbonitriding.

Contents

[hide] 1 Process 2 Characteristics of carbonitrided

parts 3 Advantages 4 Applications 5 See also

6 References

[edit] Process

Carbonitriding is similar to gas carburization with the addition of ammonia to the carburizing atmosphere. which provides a source of nitrogen. Nitrogen is adsorbed at the surface and diffuses into the workpiece along with carbon. Carbonitriding (around 850 °C / 1550 °F) is carried out at temperatures substantially higher than plain nitriding (around 530 °C / 990 °F) but slightly lower than those used for carburizing (around 950 °C / 1700 °F) and for shorter times. Carbonitriding tends to be more economical than carburizing, and also reduces distortion during quenching. The lower temperature allows oil quenching, or even gas quenching with a protective atmosphere.

[edit] Characteristics of carbonitrided parts

Carbonitriding forms a hard, wear resistant case, typically .07mm -.5mm thick, and generally has higher hardness than a carburized case. Case depth is tailored to the application, and a thicker case, increases wear

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life of the part. Carbonitriding alters only the top layers of the workpiece; and does not deposit an additional layer, so the process does not significantly alter the dimensions of the part.

Maximum case depth is typically restricted to 0.75mm; case depths greater than this take too long to diffuse to be economical. Shorter processing times are preferred to restrict the concentration of nitrogen in the case, as nitrogen addition is more difficult to control than carbon. An excess of nitrogen in the work piece can cause high levels of retained austenite and porosity, which are undesirable in producing a part of high hardness.

[edit] Advantages

Carbonitriding also has other advantages over carburizing; it has a greater resistance to softening during tempering and increased fatigue and impact strength. It is possible to use both carbonitriding and carburizing together to form optimum conditions of deeper case depths and therefore performance of the part in industry. This method is applied particularly to steels with low case hardenability, such as the seat of the valve. The process applied is initially carburizing to the required case depth (up to 2.5mm) at around 900-955°C, and then carbonitriding to achieve required carbonitrided case depth. The parts are then oil quenched, and the resulting part has a harder case than possibly achieved for carburization, and the addition of the carbonitrided layer increases the residual compressive stresses in the case such that the contact fatigue resistance and strength gradient are both increased.

[edit] Applications

Typical applications for case hardening are gear teeth, cams, shafts, bearings, fasteners, pins, automotive clutch plates, tools, and dies.

[edit] See also

Nitridization Carburization Differential hardening Quench polish quench Surface engineering Flame hardening VS induction hardening VS laser transfor-

mation hardening Hi Melrose

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1. Flame Hardening

Basic, cheap, relatively easy set-up. Process not very accurate and gives relatively thick hardened cases. Process relies on heat conduction from component surface, so relatively slow.

2. Induction Hardening

Relatively expensive and complex set-up. Process can be very accurate and repeatable and good for mass production. Process does not solely rely on heat conduction, but from internally generated heat from induced electrical currents. Can be extremely fast heating even for thick case hardening (frequency dependent).

3. Laser Transformation Hardening

Relatively expensive and complex set-up. Process can be very accurate and repeatable and good for mass production. Process relies on heat con-duction. Fast for thin transformation layers or lazer glazing, but time re-quired for deeper hardening.

It is best to look at this from the point of view of the component and its intended application. You should be able find detailed information on this subject by searching Google or the like or reading a good book on heat treatment.

Brinell scale

From Wikipedia, the free encyclopediaJump to: navigation, search Force diagram

The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness.

The typical test uses a 10 millimetres (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29,000 N; 6,600 lbf) force. For softer

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materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

where:

P = applied force (kgf) D = diameter of indenter (mm) d = diameter of indentation (mm)

The BHN can be converted into the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer's index (n) from Meyer's law. If Meyer's index is less than 2.2 then the ratio of UTS to BHN is 0.36. If Meyer's index is greater than 2.2, then the ratio increases.[1]

BHN is designated by the most commonly used test standards (ASTM E10-08[2] and ISO 6506-1:2005[3]) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.

HBW is calculated in both standards using the SI units as

where:

F = applied force (N) D = diameter of indenter (mm) d = diameter of indentation (mm)

Contents

[hide] 1 Common values 2 Standards 3 See also 4 References

o 4.1 Notes o 4.2 Bibliography

5 External links

[edit] Common values

When quoting a Brinell hardness number (BHN or more commonly HB), the conditions of the test used to obtain the number must be specified.

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The standard format for specifying tests can be seen in the example "HBW 10/3000". "HBW" means that a tungsten carbide (from the chemical symbol for tungsten) ball indenter was used, as opposed to "HBS", which means a hardened steel ball. The "10" is the ball diameter in millimeters. The "3000" is the force in kilograms force.

Brinell hardness numbersMaterial HardnessSoftwood (e.g., pine) 1.6 HBS 10/100

Hardwood2.6–7.0 HBS 1.6 10/100

Aluminium 15 HBCopper 35 HBMild steel 120 HB18-8 (304) stainless steel annealed

200 HB[4]

Glass 1550 HBHardened tool steel 1500–1900 HBRhenium diboride 4600 HBNote: Standard test conditions unless otherwise stated

[edit] Standards

International (ISO) and European (CEN) Standard o EN ISO 6506-1:2005: Metallic materials - Brinell hardness

test - Part 1: test method o EN ISO 6506-2:2005: Metallic materials - Brinell hardness

test - Part 2: verification and calibration of testing machine o EN ISO 6506-3:2005: Metallic materials - Brinell hardness

test - Part 3: calibration of reference blocks o EN ISO 6506-4:2005: Metallic materials - Brinell hardness

test - Part 4: Table of hardness values US standard (ASTM International)

o ASTM E10-08: Standard method for Brinell hardness of metallic materials.

[edit] See also

Brinelling Hardness comparison Knoop hardness test Leeb Rebound Hardness Test Rockwell scale

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Vickers hardness test

[edit] References

[edit] Notes1. ̂ Tabor, p. 17. 2. ̂ ASTM E10 - 08 Standard Test Method for Brinell Hardness of

Metallic Materials 3. ̂ ISO 6506-1:2005 Metallic materials - Brinell hardness test - Part

1: Test method 4. ̂ 304: the place to start, http://www.assda.asn.au/index.php?

option=com_content&task=view&id=65&Itemid=91, retrieved 2009-03-31.

[edit] Bibliography Tabor, David (2000), The Hardness of Metals, Oxford University

Press, ISBN 0198507763, http://books.google.com/?id=b-9LdJ5FHXYC.

[edit] External links

Rockwell to Brinell conversion chart Struers hardness conversion table Brinell Hardness HB conversion chart

Retrieved from "http://en.wikipedia.org/wiki/Brinell_scale"Categories: Hardness tests | Dimensionless numbers

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Charpy impact test

From Wikipedia, the free encyclopediaJump to: navigation, search

Materials failure

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modes

Buckling

Corrosion

Creep

Fatigue

Fracture

Impact

Mechanical overload

Thermal shock

Wear

Yielding

This box:

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The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. But a major disadvantage is that all results are only comparative.[1]

The test was developed in 1905 by the French scientist Georges Charpy. It was pivotal in understanding the fracture problems of ships during the second World War. Today it is used in many industries for testing building and construction materials used in the construction of pressure vessels, bridges and to see how storms will affect materials used in

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building.[2]

Contents

[hide] 1 Definition 2 Quantitative results 3 Qualitative results 4 Sample sizes 5 See also 6 Notes

7 External links

[edit] Definition

A vintage impact test machine.

The apparatus consists of a pendulum axe swinging at a notched sample of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after a big fracture.

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The notch in the sample affects the results of the impact test,[3] thus it is necessary for the notch to be of a regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.[4]

The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be found in ASTM E23[5], ISO 148-1[6] or EN 10045-1[7], where all the aspects of the test and equipment used are described in detail.

[edit] Quantitative results

The quantitative result of the impact test—the energy needed to fracture a material—can be used to measure the toughness of the material and the yield strength. Also, the strain rate may be studied and analyzed for its effect on fracture.

The ductile-brittle transition temperature (DBTT) may be derived from the temperature where the energy needed to fracture the material drastically changes. However, in practice there is no sharp transition and so it is difficult to obtain a precise transition temperature. An exact DBTT may be empirically derived in many ways: a specific absorbed energy, change in aspect of fracture (such as 50% of the area is cleavage), etc.[1]

[edit] Qualitative results

The qualitative results of the impact test can be used to determine the ductility of a material.[8] If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture.[1]

[edit] Sample sizes

According to ASTM A370,[9] the standard specimen size for Charpy impact testing is 10mm×10mm×55mm. Subsize specimen sizes are: 10mm×7.5mm×55mm, 10mm×6.7mm×55mm, 10mm×5mm×55mm, 10mm×3.3mm×55mm, 10mm×2.5mm×55mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products).

According to EN 10045-1,[7] standard specimen sizes are 10mmx10mmx55mm. Subsize specimens are: 10mmx7.5mmx55mm and

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10mmx5mmx55mm.

[edit] See also

Izod impact strength test Brittle Impact force

[edit] Notes

1. ^ a b c Meyers Marc A, Chawla Krishan Kumar (1998). Mechanical Behaviors of Materials. Prentice Hall. ISBN 9780132628174.

2. ̂ Jacobs James A, Kilduff Thomas F (2005). Engineering Materials Technology (5th ed.). Pearson Prentice Hall. pp. 153–155. ISBN 9780130481856.

3. ̂ Kurishita H, Kayano H, Narui M, Yamazaki M, Kano Y, Shibahara I (1993). "Effects of V-notch dimensions on Charpy impact test re-sults for differently sized miniature specimens of ferritic steel". Ma-terials Transactions - JIM (Japan Institute of Metals) 34 (11): 1042–52. ISSN 0916-1821.

4. ̂ Mills NJ (February 1976). "The mechanism of brittle fracture in notched impact tests on polycarbonate". Journal of Materials Sci-ence 11 (2): 363–75. doi:10.1007/BF00551448.

5. ̂ ASTM E23 Standard Test Methods for Notched Bar Impact Test-ing of Metallic Materials

6. ̂ ISO 148-1 Metallic materials - Charpy pendulum impact test - Part 1: Test method

7. ^ a b EN 10045-1 Charpy impact test on metallic materials. Test method (V- and U-notches)

8. ̂ Mathurt KK, Needleman A, Tvergaard V (May 1994). "3D analy-sis of failure modes in the Charpy impact test". Modeling and Simu-lation in Materials Science Engineering 2: 617–35. doi:10.1088/0965-0393/2/3A/014.

9. ̂ ASTM A370 Standard Test Methods and Definitions for Mechani-cal Testing of Steel Products

[edit] External links

Charpy Impact Testing module at steeluniversity.org , including a fully interactive simulation

Retrieved from "http://en.wikipedia.org/wiki/Charpy_impact_test"Categories: Fracture mechanics | Materials testing

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Log in / create account

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Interaction Help About Wikipedia Community portal Recent changes Contact Wikipedia

Toolbox What links here Related changes Upload file Special pages Permanent link Cite this page

Print/export Create a book Download as PDF

211

Printable version

Languages Deutsch Español فارسی Français Italiano Magyar Nederlands 日本語 Polski Slovenščina Ti ế ng Vi ệ t

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Alike License; additional terms may apply. See Terms of Use for de-tails.Wikipedia® is a registered trademark of the Wikimedia Founda-tion, Inc., a non-profit organization.

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Rockwell scale

From Wikipedia, the free encyclopediaJump to: navigation, search

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A hardness tester of the Rockwell type.

A closeup of the business end of a Rockwell-type hardness tester.

The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.[1] There are different

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scales, which are denoted by a single letter, that use different loads or indenters. The result, which is a dimensionless number, is noted by HRX where X is the scale letter.

When testing metals, indentation hardness correlates linearly with tensile strength.[2] This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.[citation needed]

Contents

[hide] 1 History 2 Operation 3 Scales and values

o 3.1 Typical values 4 Standards 5 See also 6 References

7 External links

[edit] History

The differential depth hardness measurement was conceived in 1908 by a Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, "the cone trial").[3] The differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections. The Brinell hardness test, invented in Sweden, was developed earlier—in 1900—but it was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive.

The Rockwell hardness tester, a differential-depth machine, was co-invented by Connecticut natives Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940). A patent was applied for on July 15, 1914.[4] The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The application was subsequently approved on February 11, 1919, and holds patent number #1,294,171. At the time of invention, both Hugh and Stanley Rockwell (not direct relations) worked for the New Departure Manufacturing Co. of Bristol, CT. New Departure was a major ball bearing manufacturer that, in 1916, became part of United Motors and, shortly thereafter, General Motors Corp. After leaving the Connecticut company, Stanley Rockwell, then in Syracuse, NY, applied for an improvement to the

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original invention on September 11, 1919, which was approved on November 18, 1924. The new tester holds patent #1,516,207.[5][6]

Rockwell moved to West Hartford, CT, and made an additional improvement in 1921.[6] Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.[7] Stanley started a heat-treating firm circa 1923, the Stanley P. Rockwell Company, which still exists in Hartford, CT. The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was most recently acquired by Instron Corp. in 1993.[8]

[edit] Operation

The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques.

It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation.[9] Also, readings should be taken from a flat perpendicular surface, because round surfaces give lower readings. A correction factor can be used if the hardness must be measured on a round surface.[10]

[edit] Scales and values

There are several alternative scales, the most commonly used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number.

Various Rockwell scales[11]

Scale

Abbreviation

Load Indenter Use

A HRA 60 120° diamond cone† Tungsten carbide

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kgf

B HRB100 kgf

1⁄16-inch-diameter (1.6 mm) steel sphere

Aluminium, brass, and soft steels

C HRC150 kgf

120° diamond cone Harder steels

D HRD100 kgf

120° diamond cone

E HRE100 kgf

1⁄8-inch-diameter (3.2 mm) steel sphere

F HRF60 kgf

1⁄16-inch-diameter (1.6 mm) steel sphere

G HRG150 kgf

1⁄16-inch-diameter (1.6 mm) steel sphere

†Also called a brale indenter

Except for one very limited exception,[clarification needed] the steel inden-ter balls have been replaced by tungsten carbide balls of the vary-ing diameters. Scales using the ball indenter have a "W" suffix added to the scale name to indicate usage of the carbide ball, for example "HR30T" is now "HR30TW".[citation needed]

The superficial Rockwell scales use lower loads and shallower impressions on brittle and very thin materials. The 45N scale employs a 45-kgf load on a diamond cone-shaped Brale indenter, and can be used on dense ceramics. The 15T scale employs a 15-kgf load on a 1⁄16-inch-diameter (1.6 mm) hardened steel ball, and can be used on sheet metal.

Readings below HRC 20 are generally considered unreliable, as are readings much above HRB 100.

[edit] Typical values Very hard steel (e.g. a good knife blade): HRC 55–62 (Hardened

tool steels such as D2)[12] Axes, chisels, etc.: HRC 40–45 (about 1045 carbon steel)[citation needed] Brass: HRB 55 (Low brass, UNS C24000, H01 Temper) to HRB 93

(Cartridge Brass, UNS C26000 (260 Brass), H10 Temper)[13]

Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring case-hardened specimens.

[edit] Standards

International (ISO)

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ISO 6508-1: Metallic materials -- Rockwell hardness test -- Part 1: Test method (scales A, B, C, D, E, F, G, H, K, N, T)

ISO 2039-2: Plastics -- Determination of hardness -- Part 2: Rockwell hardness

US standard (ASTM International)

ASTM E18 : Standard methods for Rockwell hardness and Rockwell superficial hardness of metallic materials

[edit] See also

Vickers hardness test Brinell hardness test Leeb Rebound Hardness Test Knoop hardness test Shore durometer Hardness comparison Holger F. Struer

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