B-5_Materials_-_Welding_-_NDE_-_Part_1[1].pdf

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CONFIDENTIAL: for internal use only

BASIC COURSE FOR SURVEYORS - Module 5

METALLURGY, WELDING and NDE

Rev. 2


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Section 1:

GENERAL METALLURGY

INDEX

METALLURGY

1. General Metallurgy

- Crystalline structure of metals and the allotropic states of

iron

- Metal Alloys

- Solidification Structures

- Strain Hardening and Recrystallisation

2. Mechanical Properties of Metals- Strength

- Ductility

- Hardness

- Toughness

3. Steel Metallurgy

- Ferrous-Carbon State Diagram- Structural Transformations in Steel

4. Manufacturing and Classification of Steels- Methods of Steel Production, Refining, Casting and

Rolling

- Classification of Steels

5. Heat Treatment for Steel- Annealing- Normalising

- Quenching

- Tempering- Stress Relieving

WELDING

1. Metallurgy of Welds in Steels- The Weld Zone- The Heat Affected Zone

2. Welding Defects

- Hydrogen in welding and cold cracking

- Hydrogen absorption in welding- Cracks in the HAZ (Cold Cracks)

- Hot Cracks

- Remedies for avoiding hot cracks- Lamellar Tearings

3. Heat Phenomena in Welds- Shrinkage and Residual Stress in Welds


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- Effect of shrinkage and residual stress

- Practical methods for attenuating residual strength

4. Welding Processes- Manual metal arc welding (shielded metal arc welding)- Submerged arc welding- MIG and MAG welding processes

- TIG welding process

5. Weldability of metals- Mild steels- Low manganese alloy steels

- High resistance quenched and tempered steel

- Molybdenum-chromium steels- Nickel steels- Austenitic chromium-nickel stainless steels

- Austeno-ferritic stainless steels (DUPLEX)

- Aluminium and aluminium alloys- Copper and copper alloys

NON-DESTRUCTIVE TESTING

1. Methods and techniques- Visual examination- Die penetrant testing

- Magnetic particles testing

- Radiografic testing- Ultrasonic testing

2. Extent

- Quality levels Limit for imperfections

Personnel qualification


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METALLURGY, WELDING AND NDT

1 GENERAL METALLURGY

1.1 Crystalline structure of metals and the allotropic states of iron

Crystalline structure

In their solid state, pure metals are made up of an aggregate of atoms that are alike, arranged in a veryspecific order that is constant for each type of metal.

Unlike amorphous substances in which the elementary particles (molecules or atoms) are randomly arrangedin space, the spatial distribution of atoms in metals is governed by well-defined laws of crystallographic

symmetry therefore metals have a crystalline structure.The atoms that provide this specific structure are located at individual points in a fundamental setting that ischaracteristic of a crystalline structure the elementary cell.

In general, each cell can be considered as a polyhedron, the shape of which is determined by the metalatoms positioned at its vertices.

Each elementary cell is surrounded by a certain number of identical, iso-oriented cells, with which it shares acertain number of atoms that make up the links in a three-dimensional crystalline matrix.

Apart from a few exceptions, the matrixs in pure industrial metals belong to the following threecrystallographic systems:

centred body cubic system (Fig. 1.1)

centred faces cubic system (Fig. 1.2)compact hexagonal system (Fig. 1.3)

The centred body cubic system has an atom at each of the 8 corners of the cube and an atom in the centreof the cube.

The centred faces cubic system has cubic cells with an atom at each of the 8 corners of the cube and anatom in the centre of each of the 6 faces.

The compact hexagonal matrix has cells that have a hexagonal prism shape, with an atom at each of the 6corners at the base of the prism, plus an atom in the centre of each base and three atoms midway up theheight of the prism at the corners of an equilateral triangle inside the hexagonal prism.

Figure 1.1


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Figure 1.2

Figure 1.3

Allotropic states of iron

Allotropy is the result of the existence of different crystalline structure for the same chemical species. Each ofthese exists in a stable state under specific pressure and temperature conditions.

Iron is a chemical element that can present itself in its solid state, in three stable allotropic forms at differenttemperatures (Fig. 1.4).

Alpha iron, which is stable from ordinary temperatures all the way up to 910C, crystall ises with a ce ntredbody cubic system.

Alpha iron is magnetic at ordinary temperatures, but at 770C it loses its magnetic properties. It is thenreferred to as beta iron, but this should not be considered as a new allotropic phase as it retains the samecentred body cubic crystalline matrix as alpha Fe.

Gamma iron is stable between 910C and 1440C and i s a real allotropic form of Fe as it has a centred facecubic matrix. It is not magnetic and has a solvent power for C that is much higher than alpha iron.

Delta iron is obtained due to allotropic transformation of gamma Fe when it is heated beyond 1440C an d isstable up to 1539C, which is the melting point for pure iron. It has a centred body cubic matrix, like that ofalpha Fe, but it is not magnetic.

Allotropic transformations, which, as we will see later, form the basis of the study of metallurgy, come aboutdue to the diffusion of Fe atoms and C atoms melted in the Fe. These atoms tend to migrate in the materialand therefore to homogenise its final structure, and this process is enhanced by temperature.

We will also see what dangerous effects failure of these C and Fe atoms to diffuse during solidification canhave.


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Figure 1.4

Metal Alloys

In practice, it is unlikely that pure metals will be used in steel construction. In order to obtain specificmechanical, physical, and chemical properties, mixtures of two or metals are normally used, and these areknown as metal alloys.

Alloys can also include elements that are not normally considered as metals, such as carbon, phosphorous,silicon, etc.

There are two basic types of alloys:

solid solutions

juxtaposition alloys

Solid Solutions

Solid solutions can be obtained in two ways:

- by substitution

- by insertion

Substitution Solid Solutions (Fig. 1.5)

Substitution solid solutions are the solid solutions most commonly

found in metals, and occur if some of the atoms in the solvent


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metals crystalline matrix are replaced by the same number of

atoms of the melted metal.

In general the solubility range is greater for metals

with atomic volumes that are very nearly the same,

these tend to crystallise into identical

crystallographic systems, and are reasonably close

on the electro-chemical table of elements.

It has been shown experimentally that for the same crystalline

structure and vicinity on the electro-chemical table of

elements, substitution alloys are formed with wide ranges of

solubility where the metals have atomic diameters that do notdiffer by more than 15% from one another.

Insertion Solid Solutions (Fig. 1.6)

Insertion solid solutions are formed by inserting one or more

atoms of the dissolved element into the links in the crystalline

matrix of the solvent metal.

In order for this to take place, the dissolved element must have

an atomic diameter that is significantly smaller (60% smaller)

than that of the solvent metal. The smaller the atom size of the

element to be dissolved (the solute) is in relation to the inter-

atomic spaces in the solvent, the easier it is for the solute to

insert itself into the crystalline matrix of the metal that

constitutes the basic matrix.

Only a few elements are able to satisfy this requirement when

dealing with iron the most important are hydrogen, carbon,

nitrogen, and boron.

Experiments have also shown that central face cubic and

compact hexagonal crystalline structures are the structures

that are most favourable for forming insertion solutions.

Juxtaposition Alloys

Juxtaposition alloys are formed when the solubilitybetween the metals in their solid state is very limited or

non-existent.

In such cases alloys are made up of a number of

grains of the various metals or the possible partially

solid solutions in juxtaposition to one another.

These are known as juxtaposition alloys and include

the vast majority of industrial alloys.

One typical example is pearlite (Fe-C alloy, with 0,8%

C by weight). This is a juxtaposition alloy made up offerrite and cementite (Fig. 1.7).

Fig. 1.7

Fig. 1.5

Fig. 1.6

Solvent element

Soluted element

Solvent element

Soluted element


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Finally, it is worth noting that certain metals may react with one another chemically to form

intermetallic compounds. This is the case with Fe-C alloys, for example, that contain the Fe3C

(cementite) intermetallic compound, with 6,67% of C by weight.

Solidification Structures

In their liquid state, metal atoms have a high energy level given to them by the heat absorbed

during heating up from the solid state until the metal melts.They are fairly free to move around in the heart of the liquid mass as they are obliged to maintain a

reciprocal distance but not their position, and they move at a speed that increases as the

temperature of the liquid increases.

When the liquid cools, the movement of the atoms decreases until, at a certain temperature and in

a generic point in the liquid mass, the inter-atomic forces of attraction gain the upper hand and

limit some atoms to a set position in relation to one another, thereby forming the first cell of the

crystalline matrix.

This is known as the first germ of solidification, and other cells form alongside it as the temperature

drops.

The growth of the crystalline structure does not take place casually but, as we have already seen,

according to well-defined crystallographic laws. In fact, the cells share the atoms in adjoining faces

and development takes place by branching out in the directions of the crystallographic axes

(solidification process said to be arborescent or dendritic - Fig. 1.8).

The result of the development of each germ of solidification is said to be crystal, grain, or

dendrite. Each grain contains a large number of elementary cells that are all oriented in the

same way in relation to the initial cell. This orientation varies from grain to grain.

Oriented dendritic structures

Fig. 1.8

When the temperature drops in a liquid masson the verge of solidifying, this does not take

place at an equal rate in all directions as the

specific external conditions result in directions

with a greater rate of change in temperature.

The dendrites that form starting from the

points that cool first develop in the direction

that has the steepest temperature gradient.

The grains therefore take on an elongated

shape in a direction perpendicular to the

isothermal surfaces in the liquid in the process

of solidifying.

In this case, dendritic crystallisation is referred to as being columnar, whereas when there are nopreferential directions of solidification it is referred to as being equiaxial.


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In an ingot in its molten state for example,

solidification starts in the area in contact with

the cold sides of the ingot mould. The

perimeter grains are therefore elongated

towards the centre, providing a decidedly

columnar structure.

In the centre, where cooling is practically

unaffected by the cold sides, crystallisation is

equiaxial.

In welding, due to the limited dimensions

involved, solidification is completelydominated by a columnar dendritic

phenomenon and no space is available for

equiaxial structures (Fig. 1.9).Fig. 1.9

Strain Hardening and Recrystallisation

Strain Hardening

Strain hardening is a fundamental characteristic in the plastic deformation of metal materials and

involves the continual increase in the force required to cause the crystalline matrix deformation, asdeformation itself increases.

This phenomenon can be seen in the load

elongation graph that is obtained in a

mono-axial tensile test and shows an ever

upward trend on the curve over the plastic

deformation stage.

Strain hardening is explained as the

interaction between atomic planes (the

matrix deformation occurs through sliding of

atomic planes) and between solution alloys

and intermetallic compounds (i.e. carbides)inside the matrix and at the grain boundaries

that increases the load required to deform

the matrix itself as deformation proceeds.

Strain hardening that is obtained by any cold

plastic operation could be dangerous as, in

addition to resulting in an increase in the

ultimate tensile strength and yield point, it also

causes hardness to increase and ductility and

toughness to decrease (Fig. 1.10).

Fig. 1.10

Recrystallisation

This is the process that sees the crystalline matrix of material that has undergone plasticdeformation rebalanced as a function of time or temperature.


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The speed with which recrystallisation occurs is very slow at low temperatures. The minimum

temperature at which this takes place within a sufficiently short period of time is called the

recrystallisation temperature. Crystallisation tends to take place more quickly at higher

temperatures.

When plastic deformation of a metal piece is carried out at a temperature exceeding this

recrystallisation temperature, recrystallisation takes place immediately after plastic working (hot

working), whereas if the temperature is lower, plastic working causes strain hardening (cold

working) and recrystallisation does not occur other than after subsequent heat treatment.

Recrystallisation after cold plastic deformation therefore removes the effects of strain hardening,

lowering the strength and hardness of the material and increasing its ductility and toughness. As a

result of the new crystalline structure formed, this process also allows the grain of the metal to be

made finer.


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2. Mechanical Properties of Metals

The mechanical properties of metals are taken as indicating all the properties that solid substances(in this case metals) are found to have when a force is applied to them and that therefore indicate

the possibility of using a metal for structural purposes.

Metals are highly regarded in the construction industry as they are strong, hard, ductile and tough.

The combination of these properties can also be varied within extensive limits by adding alloying

elements, heat treatment, or mechanical action.

Strength

In the case of metal materials the term strength is used to mean a wide range of things, as it is usedin relation to the other characteristics of the materials (plasticity, hardness, toughness) and

therefore indicates the combination of all that is known about the relationship between applied

loads, internal forces (tension) and deformation.

Of the mechanical tests used, the single-axis tensile test is one of the best defined, in terms of

concept, to determine this relationship, as at least in theory it provides complete uniformity in the

application of the load over a rather well-defined area of material, known as the gauge length Lo

of the test piece (Fig. 1.11).

Fig. 1.11

In order for the load to be applied uniformly over the entire gauge length, this must have a

constant section and the load must be applied at its ends in two areas known as heads,

connected to the gauge length very gradually.

The test piece subjected to a mono-axial load can have a circular, square or rectangular section,

or it may be in the form of a length of pipe when carrying out tensile testing of tubular products.

In general, tensile tests on a metal alloy include four stages or periods (Fig. 1.12):- Ultimate tensile strength (1)

- Yield strength (2)


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- Ultimate load (3)

- Period of elastic elongation

- Period of yielding

- Period of uniform plastic elongation (4)

- Period of non-uniform plastic elongation or necking down (5)

If the load on the test piece is removed during elastic elongation, no permanent deformation

occurs.

The yield period marks the transition from elastic to plastic behaviour. The yield load (indicated as

Rs) is the load at which, for the first time, an increase in elongation occurs without a simultaneous

increase in loading, or with a decrease in loading. The period of plastic deformation occurs

immediately after yielding and ends when the maximum load or ultimate tensile strength is

reached.

The elongation of the test piece is localised over a short length of the test piece, where transverse

deformation (necking down) occurs and progresses until the test piece breaks (Fig. 1.13).

Fig. 1.12

Fig. 1.12: necking down in the test piece


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Ductility

Ductility is normally defined as a materials capacity to deform plastically without breaking, and isindicated by the extent to which a test piece subjected to a tensile test deforms plastically until it

breaks.

Good ductility is very important in a metallic material as:- it is an indication of the materials capacity to withstand cold deformation

- it gives an indication of the plasticity reserve that is available to withstand breaking due to

sudden overloading

- it means that you do not have to worry too much about the residual tension that is an

inevitable consequence of stresses induced by welding

- it gives an indication of the materials strength in relation to lamellar tearing.

In order to test the ductility of the material, a test piece with a rectangular section is submitted to a

bending test (Fig. 1.13). Good results from this test indicate that the material is sufficiently plastic, as

the outer layers of material have been elongated.

The size of the grain is very important in terms of ductility. As the grain size decreases, the materials

ductility increases.

Fig. 1.13

Hardness


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Hardness can be defined as a metalmaterials capacity to withstand a body

(indenter) seeking to penetrate the metal

material by deforming it plastically (by

compression).

The hardness of a test piece is determined on

the basis of the size of an indent left by the

indenter to which a specific force is applied.

There are various methods for measuring

hardness that differ in terms of:

- the shape of the indenter;

- the extent of the force applied;

- the method used to evaluate thehardness.

Fig. 1.14 - durometer

Brinell Hardness

A load F is applied to a ball made ofhardened steel, and is kept in place for a

certain time. Once this time has expired, the

load is removed and the maximum diameter

of the circular imprint the sphere has left in

the material is measured.The Brinell number is given by the ratio F/S

between the load and the area of the

imprint, and is indicated by the symbol HB

(Fig. 1.15). The Brinell Test is not reliable for

materials with an HB value that exceeds 450.

Fig. 1.15

Vickers Hardness


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A load F is applied to a diamond indenterhaving a pyramidal shape with a square base

and angle at the vertex of 136, with the load

being applied for a certain amount of time.

The load F is removed and an indentation is

found that is considered as being a square-

based pyramid with the same vertex angle as

the indenter.

The area of this indentation is then calculated

after having measured the diagonal of thebase of the pyramid. The Vickers Hardness is

the ratio F/S between the load and the area

of the imprint, and is indicated by the symbol

HV (Fig. 1.16).

Fig. 1.16

Rockwell B and C Hardness

An initial load F0is applied to an indenter and kept in place for a certain time t0causing an imprintwith a depth of e0. A load F1is then added to F0for a time t1and the imprint reaches a depth of e1.

Load F1is then removed, leaving load F0. The imprint then has a depth e2, which is less than e1but

greater than e0due to plastic deformation.

The difference (e2 e0) indicates the hardness. This difference is lower in harder materials.

The most important scales are the B and C scales, obtained by using spherical and conical

indenters, respectively.

Fig. 1.17

Microhardness Portable Durometers

Microhardness tests can be used to determine the hardness even of individual grains, and arecarried out on smooth polished surfaces. Either a metallography microscope with very small

indenters or special ultrasound instruments are used.

Portable durometers can be of the spring-loaded type (although these are seldom used as the

spring loses its calibration) or of a POLDI type.

With this latter type the load is applied by striking a punch with a spherical end with a hammer.

Between the sphere and the part that is struck there is a metal bar of known hardness. The spheretherefore causes an indent in both the bar and the test piece and these are used to determine the


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hardness of the test piece, with the help of hardness conversion tables, without the intensity of the

blow playing any part, as this may be arbitrary.

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Toughness

Toughness can be defined as a metals capacity to prevent breaking when it is subjected todynamic loads and therefore in an unfavourable state for absorbing the energy of plastic

deformation.

In fact, experience has shown that a material considered as being ductile after normal tensile or

bending tests may act in a fragile manner (that is, without toughness, breaking with very little

deformation) when it is subjected to dynamic forces other than those used for the two tests referred

to above.

It has been found that under certain

conditions a ductile material may not be

tough, while generally material that is not very

ductile is not very tough either.

The toughness of a metal is influenced by

three factors:

- the speed with which the load isapplied;

- the type of force applied (mono-axial or multi-axial);

- the temperature of the metal (Fig.1.18).

Fig. 1.18

If a ductile metal is loaded quickly, applying a multi-axial force and at low temperature, it mayreact with limited toughness.

Where possible, these conditions are simulated in a impact test, which, in terms of simplicity and

ease of execution is the test most commonly used for determining the toughness of materials.

This test is based on the impact of a weight against a test piece with a notch acting as a stress

raiser (the notch causes a localised increase in the stress and transforms mono-axial stress into multi-

axial stress). This is done at a certain temperature that relates to the working temperature for the

material being tested.

During the test the weight made up of a pendulum hammer strikes a test piece (55 x 10 x 10 mm)

resting on two supports 40 mm apart. The energy (in Joules) absorbed by the test piece to break it is

measured (Fig. 1.19 & 1.20).

The test piece most commonly used for impact tests is known as a Charpy V test piece, where the

notch is V-shaped with an opening angle of 45, depth of 2 mm, and radius at the bottom of the

notch of 0,25 mm.


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The size of the grain is very important for good impact properties. The smaller the grain, the greater

the toughness.

Generally, it is not possible to predict how a material will behave at a temperature even slightly

lower than the test temperature, due to the tenacity transition phenomenon that we will look at

when dealing with brittle fracture in welded structures (Fig. 1.21a/b).

Fig. 1.19 Fig. 1.20



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Fig. 1.21a - Carbon steel

Fig. 1.21b austenitic stainless steel


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3. Steel Metallurgy

Ferrous-Carbon State Diagram

The Fe-C diagram is a graphic representation of the allotropic transformations that take place inrelation to temperature (Fig. 1.22).

Fig. 1.22

It can therefore be used to identify the range over which the various steel structures that can befound in steel exist, bearing in mind the C content and the steel temperature.

The indications that can be taken from this diagram only apply for very slow heating and cooling.

The % of C content is shown on the abscissa (x-axis) and the temperature on the ordinate (y-axis).

The diagram clearly shows that when the C content is below 2% we are dealing with steel, while

above this percentage we are dealing with cast iron.

Steels with a C content below 0,8% are said to be hypo-eutectoid and those between 0,8 and 2%

are referred to as hyper-eutectoid.


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While pure Fe solidifies at a temperature of 1539C, Fe-C alloys solidify over a range that goes

upwards as the C content increases. The liquid and solid lines indicate the start and finish

temperatures for solidification for each alloy.

We also find that, while allotropic transformation of pure Fe takes place at a constant temperature

of 910C, in the Fe-C alloy this transformation takes place gradually over the interval between lines

A3and A1.

A3 indicates the boundary between the austenitic and austenitic-ferritic fields and shows the

temperature at which austenite and ferrite are balanced in the case of hypoeutectic steels, above

which only austenite is stable and below which ferrite is found.

A1 indicates the temperature at which pearlite forms during cooling for steel with a C content

greater than or equal to 0,025% and this is constant where the C content varies.

To explain the meaning of the various lines and to comment on the phenomena they involve, lets

look at what happens when an Fe-C alloy with a C content of < 0,80% (hypo-eutectoid) cools,

starting from a temperature above its melting temperature (Fig. 1.23).

Fig. 1.23

Starting from the top we come across the liquid line where solidification starts with the formation of

the first austenite cells. Over the interval between the liquid curve and the solid curve, solidification

continues with the gradual disappearance of the liquid. When curve I-E is reached, solidification is

complete and nothing else occurs until we come to line A3(curve G-S).

At this point the transformation from austenite to ferrite in the solid state begins (gamma alpha

transformation).

Since ferrite dissolves only a limited amount of C, the residual austenite gradually becomes richerand richer in C, following line A3until it reaches line A1where ferrite grains (with C = 0,025%) and

austenite grains (with C = 0,80%) are found together.


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On line A1at a temperature of 723C (length P-S), the residual austenite is transformed into pearlite,

which is a eutectoid compound made up of very thin alternate parallel laminations of ferrite and

Fe3C iron carbide.

Upon further cooling and until ambient temperature is reached, nothing further occurs if we

overlook the very small amounts of tertiary Fe3C iron carbide deposited as a result of the

decreasing solubility of C in alpha Fe.

At ambient temperature the micrographic structure is made up of grains of primary ferrite, derived

from the austenite over the A3 - A1 interval, and grains of pearlite that formed at the eutectic

temperature, each made up of thin laminations of juxtaposed ferrite and Fe3C iron carbide.

Fig. 1.24 shows the different structures of

eutectoid steel: starting from the top, the

alloy follows the same structural

transformations as the previous case until it

reaches the eutectoid at 723C (point S).

Here, the austenite will become unstable and

will be transformed completely into grains of

pearlite formed by alternate parallel

laminations of ferrite and Fe3C iron carbide.

This structure remains stable until ambient

temperature.

Fig. 1.24

Fig. 1.25 shows the different structures of a

hypereutectoid steel: in this third case as well,

the alloy follows the same structural

transformations as the previous two until it

meets curve ACM(curve S-E).

Under this curve, the austenite becomes over-

saturated with carbon, which leaks out of the

matrix in the form of Fe3C iron carbide. As the

temperature drops, the percentage of Fe3C

iron carbide increases until it meets the

straight line at 723C. At this point the residual

austenite (with C=0,80) is transformed

completely into pearlite.

After cooling, the steel will be characterised

by a mixed Fe3C iron carbide-pearlite

structure.

Fig. 1.25


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Structural Transformations in Steel

A study of the transformations in steel in a state of equilibrium represented by the ferrous-carbon

diagram does not provide any information on the actual transformations that take place upon

cooling, which are closely linked to the rate of cooling and thus the time over which this takes

place.

During cooling below A3, austenite (Fig. 1.26) decomposes, giving rise to two processes:

- transformation of the centred face matrix of austenite into the centred body cubic matrix of

ferrite (Fig. 1.27)

- separation of the carbon from the austenite, in the form of cementite (Fig. 1.28).

This transformation of the matrix takes place easily as the two matrixs relate to one another in

simple geometric terms.

Fig. 1.26 fig. 1.27 Fig. 1.28

As far as the separation of carbon is concerned, as the rate of cooling increases, an unstable

equilibrium is produced in the material with a phenomenon of under-cooling. As this increases, the

forces that tend to bring about a change in the matrix grow. The austenite matrix becomes more

and more unstable and the carbon it contains is in an ever-increasing state of over-saturation.

The delay in the transformation due to the fast cooling speed resulting in over-saturation of

austenite leads to the formation of structures that are more and more fine.

If this speed increases acicular ferrite structure with carbides finely dispersed appears. Such a fine

structure is, generally, tough (Fig. 1.29 and 1.30).


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Fig. 1.29

Fig. 1.30Fig. 1.31

If the rate of cooling is increased further, the

carbon that does not find a place for itself in

the centred body matrix is unable to organise

itself in order to bring about a second phase.

It therefore remains in an over-saturated

solution causing a distortion of the

transformed cubic matrix, which becomes

tetragonal (Fig. 1.32).

This metastable phase is known as martensite(Fig. 1.31). It is extremely hard for two reasons:

the tetragonal matrix does not have all the

combinations of sliding planes (and therefore

the relative ductility) that the cubic matrix

has, and then the carbon that remains

trapped in the interstices impedes sliding. The

hardness and fragility of the martensite

increase as the carbon content increases.

Fig. 1.32


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4. Manufacturing and Classification of Steels

Methods of Steel Production, Refining, Casting and Lamination

Production of Pig Iron:

Pig iron is made by smelting iron ore with coke in a blast furnace.

Together with the coke and iron ore, limestone is put in the furnace (Fig. 1.33 & 1.34).

The blast furnace is used to produce pig iron (iron-carbon alloy) by reducing the metal oxides (for

example Fe2O3) naturally present due to the oxidising atmosphere. The oxygen bonds with the

carbon added to the material to be produced, forming carbon dioxide and leaving the pig iron.

The limestone purifies the pig iron in the bath, bonding with the impurities present and forming a

slag that floats on the surface and out of the iron.

The pig iron that is extracted from the blast furnace can be further refined for steel-making.

The heat for production of pig iron is generated by combustion of the coke mixed with the iron ore.

The combustion air is preheated inside special towers which are built into the plant. The blast

furnace is used for continuous production throughout its service life (several years), which normally

depends on the durability of its inner lining.

Fig. 1.33 Fig. 1.34


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Steel Production:

It is possible to distinguish three manufacturing phases in steel production:

- the first phase covers the actual production of molten steel (refining and processing)

- the second phase covers the casting of the steel from the ladle in ingots or continuous

castings

- the third phase covers the lamination of the semi-finished goods (ingots, slabs, billets).

Refining:

In brief, refining is the oxidisation process used to eliminate excess components (carbon and siliconin the cast iron) and dangerous impurities from the initial bath to produce a product that is as pure

as possible and that contains only the required quantity of carbon, manganese and silicon.

This process involves an initial load of molten, solid or mixed items in converters (Fig. 46 & 47) or

electric furnaces (Fig. 48).

Converters use a load of molten cast iron produced in blast furnaces as their main constituent,

whereas electric furnaces use a solid load of scrap recycled steel with cast iron and/or graphite

added.

The chemical reactions that occur during the refining process can be summarised as:

Decarburisation: the injection of oxygen causes an initial oxidation reaction in the iron and

silicon. The oxidation reaction in silicon is strongly exothermic, raises the temperature of the

bath and initiates the conversion of the carbon to carbon monoxide (CO) and carbon dioxide

(CO2), which are released as gases.

Deoxidation: during the decarburisation reactions, manganese is also oxidised and reacts with

the silicon and iron oxides to form silicates that pass into the slag and deoxidise the molten

bath.

Purification: the manganese and calcium and magnesium oxides or carbonates introduced

into the bath react with sulphur and phosphorus compounds to form manganese sulphides and

calcium phosphates that pass into the slag with consequent purification of the bath.The steel produced in this way in the converter or electric furnace still contains notable quantities of

dissolved gases, particularly oxygen, which must be eliminated for deoxidation to be completed in

the ladle with the addition of elements such as manganese, silicon and aluminium that react with

the oxygen to form oxides that precipitate in the metallic matrix.

Steels are divided into three categories on the basis of the degree of deoxidation, i.e. the level of

oxygen remaining in solution in the molten product:

Effervescent steels: these have a high oxygen content that causes notable development of

gaseous carbon monoxide during casting and solidification due to the FeO + C = Fe + CO

reaction which, in turn, causes the development of a continuous layer of blowholes in the

peripheral zones, particularly the upper peripheral zones. The outer surface layer of the ingot is

made up of purer, more compact metal with a low carbon content.


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Semi-killed steels: these do not undergo deoxidation to the extent of blocking the reaction

between the carbon and oxygen. They therefore have a structure with less porosity and fewer

blowholes than effervescent steels and a smaller central segregation.

Killed steels: these undergo forced deoxidation by the injection of deoxidising elements such as

silicon and aluminium to block the development of other decarbonising reactions and

consequent effervescence during solidification. After solidification these steels have a

compact structure without blowholes but with a shrinkage cone at the top of the ingot and

that must be removed before lamination. The addition of aluminium or other elements such as

vanadium, niobium or titanium causes further refining of the grain, producing fine-grained killed

steels.

The LD Converter

The LD process is the most commonly used in steel production and uses molten cast iron and a coldload of scrap and solid cast iron.

The oxidising agent used in the process is pure oxygen.

The LD process was introduced in the 1950s and takes its name from the Linz and Donawitz

Steelworks in Austria, which first used it.

The converter consists of a cylindrical steel container with a convex base, over which a truncated

cone-shaped part is fitted.

The converter is lined with dolomite or magnesite fettling and a basic slag is formed by theintroduction of fluxes such as lime and fluorine.

The converter is loaded in an inclined position and then the oxygen injection nozzle is

concentrically positioned above the opening for the injection of the oxygen (Fig. 1.35 & 1.36).

Fig. 1.35 Fig. 1.36

During the injection, which lasts about 20 min and is calculated on the basis of the oxygen volumenecessary for the desired final carbon percentage, silicon is the first element to be oxidised,

followed by manganese. These begin to form a slag with a certain basicity and decarburisation


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begins. When this phase is concluded, desulphurisation begins and the slag reaches its maximum

basicity.

When the required quantity of oxygen has been injected, the converter is inclined and a sample

taken for analysis. If this shows the required analytical values for the principal chemical elements

(carbon and manganese) and the impurities (sulphur and phosphorus), iron alloys are added for

deoxidation and correction of the casting, which is finished in the ladle.

The Electric Furnace

Electric furnaces are either indirect arc(the bath is heated by radiation from an arc that developsbetween electrodes above it) or direct arc(the arc is formed by electrodes and the bath and the

electric circuit is enclosed in the bath).

Depending on the type of steel required, the furnace (Fig. 1.37 & 1.38) is loaded with scrap as well

as with carbon-bearing material (coke, graphite powder, cast iron) and lime, feldspar and iron

mineral to form the slag.

The electric current from the electrodes melts the load with the aid of the injected oxygen. The

temperature of the bath and its surroundings is increased until the scrap is completely melted at

temperatures strictly correlated with the carbon content of the molten bath.

The refining of the bath begins with an oxidation phase, during which the injection of oxygen helps

decarburisation and the metal-slag interchange, above all favouring dephosphorisation of the

bath: the phosphorus that has accumulated in the slag is eliminated during the first scorification.

The slag is renewed with the addition of lime, feldspar and reduction materials such as ferrosilicon,

calcium silicide and aluminium. These deoxidisers reduce the iron oxide and the other metal oxides

in the slag, and during this reduction phase desulphurisation of the bath takes place.

Fig. 41.37 Fig. 1.38

Iron alloys are added to obtain the required chemical balance and the casting is then poured into

the ladle.


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By means of the electric furnace it is possible to produce a wider range of special steel products

than by the LD process: from unalloyed steels to high alloy steels. The electric furnace is particularly

irreplaceable in the production of alloyed and special steels as it is possible to obtain the necessary

content of the alloying elements by controlling their oxidation process.

Steel Casting

The casting systems used in steel manufacturing are:- casting in ingots that can be transformed into slabs and billets by cutting and laminating- continuous casting in slabs and billets.

Casting in lngots

This is the oldest casting process and now almost completely superseded by continuous casting.However, it remains the only technique for casting non-deoxidated steels (rimmed and semi-killed

steels).

Furthermore, ingots are still used for the production of forged steels of particularly large dimensions.

The steel is cast in cast iron moulds with walls of great thickness to enable a rapid solidification of

the molten material.

The moulds are generally in the form of truncated pyramids, with square or rectangular sections

and with the largest side uppermost to make it possible to easily release the ingot by pulling it

upwards.

The most commonly used method is direct casting: the steel mould that emerges from the bottom

of the ladle falls freely into the ingot (Fig. 1.39).

Another casting method used when a large number of ingots of small dimensions is required is

bottom casting: the steel is forced upwards into the mould from below, using a system of interlinked

canals made of refractory brick (Fig. 1.40).

Fig. 1.39 Fig. 1.40


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The solidification of the cast steel in moulds begins in the surface layers adjacent to the internal

walls of the mould and proceeds in the opposite direction to the heat flow, i.e. perpendicular to

the walls themselves.

When solidification occurs, it is possible to distinguish three different zones in a cross-section of the

ingot:

the skin zone, which consists of very fine-grained steel due to the very rapid solidification of the

metal in contact with the mould walls

the intermediate zone of columnar crystallisation, which consists of dendritic crystals elongated

towards the centre and perpendicular to the walls of the mould

the central zone, which consists of globular crystals with equal axes.

Due to the effects of the different rates of solidification, the steel components with the lowest

solidification point (generally impurities) are concentrated in the central part of the ingot, which

solidifies last, and cause the phenomenon of macro-segregation.

Furthermore, due to the effect of contraction in volume that occurs in the course of cooling, and to

a greater extent with solidification, a shrinkage cone is formed in the centre of the ingot with

consequent problems for its subsequent use.

To reduce the size of the cone and its harmful effects, a supplementary generally insulated part,

called the head, is placed above the mould to thermally isolate the upper part and slow the

cooling process, thereby producing a shallower shrinkage cone and collecting the bulk of the

impurities and oxides in that zone, which can then be removed.

Continuous Casting

The continuous casting system is actually the most commonly used due to its manufacturing andeconomic advantages (less material lost and elimination of the primary lamination phase in

converting ingots into slabs and billets).

The possibility of using continuous casting depends on the maximum dimensions of the slabs and

billets that can be produced, and also on the final thickness required for the finished product. In

fact, to obtain a good, homogenous finished product it is always necessary to have a reduction in

the thickness of the rolling mill of at least 6 to 1.

The continuous casting process can be summarised as follows (Fig. 1.41, 1.42, 1.43).

The molten steel in the ladle is poured into an intermediate container called a tundish, which

controls the steel flow into the machine and, if necessary, divides it between the different tubes

that make up the machine.

The steel is poured from the tundish into bottomless copper moulds with a vertical oscillation

mechanism and is cooled by circulating water. A regulating device, called a false slab, is inserted

into the mould to serve as the base before the casting.

The oscillating mould, whose length is calculated to permit the formation of a thin skin layer to

enable the steel to support itself, accompanies the solidifying bar in its descent towards the

bottom.

Below the mould there are casting, extractor rollers that begin to pull the false bottom away just

seconds after casting begins, permitting the semi-finished product to move inside the mould; the


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steel begins to solidify at its contact points with the mould walls, assuming the mould form and

finishing the solidification process in an underlying zone where it is subjected to jets of water.

Under the extractor rollers the half-product goes through an oxygen-acetylene station where it is

cut into the required lengths.

Fig. 1.41

Fig. 1.42 Fig. 1.43

There are three types of continuous casting machines:

- vertical machines- vertical machines with bending- curved machines


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The steel solidifies in the same way as that described for mould casting.

Due to the continuous feeding of the molten steel, shrinkage cones cannot form and less material is

wasted.

However, as there is less waste material with its concentration of impurities, it is necessary to pay

particular attention to the phenomenon of macro-segregation, which tends to concentrate all the

impurities in the centre of the bar and could cause delamination of the laminated product.

To avoid this phenomenon, electromagnetic stirring in the mould has been introduced by placing

an electric winding around the mould. In this way a better distribution of the crystallisation nuclei is

obtained with a subsequent reduction in macro-segregation and the creation of a structure with

more-or-less equal axes for most of the thickness.

Furthermore, the steels used have a low level of carbon or impurities.

Rolling

The rolling process aims to elongate the product by a reduction imposed on the transverse sectionusing pressure rollers that form a lamination train (Fig. 1.44 & 1.45).

The products obtained by rolling are the following:

- plates: hot laminates with thickness 3 mm

- sheets: hot or cold laminates with thickness < 3 mm

- long products: section bars, rails, rods, etc- pipes

In this article we will only deal with plates.

A plant for the production of plates consists of:

1. heating furnaces

2. rolling mill3. hot levellers

4. hot cropping shears to trim the ends of the plates

5. cooling beds and control

6. edging shears to trim the sides of the sheet7. shears to cut the product to the required dimensions.

Fig. 1.44 Fig. 1.45


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Lamination trains can differ according to the number and dimensions of the lamination cylinders:

two, three, four.

The most commonly used rolling mill for the

production of plates is thereversible four-roller

one (Fig. 1.46).

This consists of four cylinders vertically

positioned one on top of the other. Only the

two middle, work cylinders are operated by

the motor and are smaller than the other two

on which they rest. This arrangement is based

on the principle that in lamination, the rate of

elongation increases as the diameter of the

cylinders decreases, which makes it possible

to impose high pressure because the stress

imposed on the working cylinders is absorbed

by the robust support cylinders. It is called

reversible because it can work in two

directions.

Fig. 1.46

Rolling Methods

There are three types of rolling processes (Fig. 1.47).

Standard Rolling: used when high mechanical characteristics, especially resistance, are notrequired and it is desired to contain the costs of production. This process usually finishes at quite

a high temperature to reduce the energy required in the hot plastic lamination deformation. As

the process cannot be controlled there could be a certain variability in the characteristics of the

product.

Normalising Rolling (also called controlled rolling): the rolling process in which the final

deformation occurs within the temperature interval for standard thermal normalisation

treatment so that the structure of the material is generally the same as that obtained with

normalisation. The required values for the mechanical characteristics are kept even after any

subsequent normalisation treatment.Thermo-mechanical Rolling: the lamination process that provides strict control of the temperature

of the plate and the degree of lamination. In general, most of the lamination is obtained around

the temperature of Ar3, which means to say that it could cause the finishing of the lamination

towards the lower part of the temperature interval of the inter-critical bi-phase zone. The properties

conferred by thermo-mechanical rolling are not maintained in the event of a successive

normalisation treatment or another thermal treatment.


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METALLURGY, WELDING AND NDT page 33

Fig. 1.47

Recrystallisation

The different structures and characteristics coming from the different rolling processes are determined, forthe most part, by the recrystallisation processes.

During the high-temperature rolling, austenite grains are strongly deformed, fractured and elongated by the

lamination rollers. As already stated, due to the high temperature and energy of the stored deformation,

the material tends to recover the state it had previously and to recrystallise.

At a higher temperature and/or greater deformation, recrystallisation occurs during the deformation phase

itself (dynamic recrystallisation): due to the very high temperature the austenitic grains grow back afterrecrystallisation, although they do not regain their original dimensions.


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At a lower temperature or a weaker deformation, recrystallisation only occurs at the end of the deformation

phase (static recrystallisation) and the dimensions of the recrystallised grains are smaller.

If rolling occurs at an even lower temperature, recrystallisation does not occur, or only partially, and the

austenitic grains remain deformed.

The temperature for dynamic or static recrystallisation, partial recrystallisation or non-recrystallisation

basically depends on the degree of deformation and the type of steel. Certain micro-alloying materials

such as niobium slow recrystallisation: that is, they increase the temperature or the temperature time interval

and so recrystallisation may occur.

The dimension of the ferritic grain obtained after the transformation is conditioned by recrystallisation. Infact, the nucleation of ferrite begins at the edges of the recrystallised austenitic grains and proceeds

towards the inside of the latter. Instead, in deformed austenitic grains the nucleation of the ferrite not only

takes place along the edge of the grain but also along the deformation band formed inside the grains,

producing a finer ferrite grain.

Continuing the rolling after the beginning of the austenitic transformation (gamma-alpha field) further

deforms the residual austenitic grains and deforms and fractures the ferritic grains already formed, forming

sub-grains and, consequently, a very fine final structure.

Standard Rolling

As already said, standard rolling is carried out at a sufficiently high temperature to cause complete

recrystallisation of the grains and their subsequent growth. As it is not controllable, it can cause a certainvariability in the characteristics of the product in the same laminate and, more probably, between rolled

products of the same casting and thickness.

This process is used for steel plates of ordinary quality such as hull structural steel - A and steels for high-

temperature uses.

Normalising Rolling

Normalising rolling permits control of the temperature of the last rolling pass and the thickness reduction andlamination cycle methods immediately preceding the last pass.

The final rolling pass is performed in a completely austenitic field but just above the gamma-alpha A3

transformation line so that only static recrystallisation of the structure occurs, without an increase in the grainor only a partial recrystallisation of the deformed grains. In this case it is important that the final rolling passes

are sufficiently strong (high percentage of thickness reduction) to affect the entire thickness and avoid the

formation of a mixed structure of large grains of undeformed austenite and small grains of deformed and

partially recrystallised austenite.

Thermo-mechanical Rolling

The rolling is carried out in two cycles:

- a first (roughcast) cycle at high recrystallisation temperature and, after a suitable waiting period,

- a second (finishing) cycle at a lower non-recrystallisation temperature that can also conclude in

the gamma-alpha interval.

This method is based on the following principles:


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to benefit from the formation and presence of carbonitriding materials, in particular Nb, but also other

micro-alloying materials (V, Ti, B) that are so stable at high temperatures that they can block the growth

of austenitic grains during the recrystallisation phase during the waiting period following the first cycle

and during the second lamination cycle.

to perform a part of the rolling at a temperature at which austenite does not recrystallise further andmake the ferrite in the austenite grain nuclear at the deformation band with the consequent

formation of a finer ferritic grain. The presence of Nb in the form of carbonitriding materials in the non-

precipitated part at the edge of the grain and still dissolved in the austenite raises the non-

recrystallisation temperature.

lastly, to carry out a part of the lamination in the austenitic-ferritic zone in such a way as to crush theferritic grains into sub-grains.

Due to the high-temperature refining of the grain, which is not possible with the normalising process, this

type of rolling makes it possible to obtain products with high resistance and toughness.

However, as these characteristics are the consequence of thermo-mechanical treatment, which is the result

of a delicate equilibrium between the crystalline structure obtained (extra-fine grain) and the carbonitriding

materials precipitated and their state of dispersal in the structure, the product is very sensitive to actions that

can alter or destroy that equilibrium, especially prolonged high temperatures for thermal treatments, hot

moulding and high-temperature welding.

In general, the characteristics of plates produced with thermo-mechanical procedures are guaranteed only

for heat treatments at a temperature not exceeding 580C.

Classification of Steels

Steels can be classified as follows:

- carbon and carbon-manganese steels;- low alloy steels, i.e. steels containing up to 5% of alloying elements;

- high alloy steels, i.e. steels containing over 5% of alloying elements.

Normally, high alloy steels are stainless steels or special steels, produced for specific purposes, whereas

carbon steels and low alloy steels are employed as construction steels and for general uses.

Carbon and carbon-manganese steels

These are the most commonly used and the least highly regarded; they are easily worked and readilyweldable. However, they are subject to corrosion, above all at high temperatures, at which they also lose

mechanical resistance, while at very low temperatures they become brittle.

They are divided into mild steels, with carbon content less than 1%, and hard steels, with higher carbon

content: mild steels are very ductile and malleable, they are easily worked and have excellent tensile and

compressive strength; they are also very resilient.

Ordinary and high strength steels for hulls belong to this family of steels.

Hard steels are less resilient, that is they are more likely to fracture if subjected to violent impact, but they

also have much greater surface hardness. Also, they are very good for hardening, unlike mild steels. On theother hand, they are obviously less easily worked.


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Quenched and tempered high strength steels

These are normally low alloy steels whose chemical composition is richer than ordinary C-Mn steels due tothe addition of elements (Cr, Ni, Mo, V, Nb, B) favouring their hardening. The quenching and tempering to

which they are subjected substantially increases their tensile strength and toughness.

Chromium-molybdenum steels

These steels are suitable for the construction of boilers and pressure vessels operated at elevatedtemperatures. The chromium and molybdenum, which are present in the solid solution of the ferrite matrix

or in the form of carbides, reduce the tendency of the structure of the steel to deteriorate at high

temperatures and therefore increase creep resistance. Also, the chromium provides improved oxidation

resistance at elevated temperatures.

Ferritic steels for low temperatures

The following techniques are adopted to improve the performance at low temperatures of C-Mn steels:

- complete deoxidation and denitrification of the steel and uniform distribution of the impurities so as to

avoid local concentrations of segregations and improve the ductility and toughness of the steel

- attainment of structures with a fine grain by means of heat treatments or thermo-mechanical

treatments.For low temperature service less than 60C, nickel additions of increasing amounts (from 1 to 9%) are used

as a function of the minimum operating temperature. The nickel shifts the ductile to brittle transition curve

towards low temperatures without causing an excessive increase in hardness. These steels are

characterised by bainitic/martensitic structures with levels of residual austenite up to 20% in the case of steel

containing 9% Ni.

Stainless Steels

Stainless steels are Fe-Cr or Fe-Cr-Ni type ferrous alloys with chromium content from 12% to 30% and up to25% nickel. Chromium, which is a less noble metal than iron in terms of electrochemical potential, oxidises

forming a tough continuous layer which is impervious to water and highly resistant to many corrosive agents,

thereby protecting the metal beneath. The film self-repairs in the presence of oxygen if the steel isdamaged. In the context of corrosion prevention, the spontaneous formation of this hard, non-reactive

surface film is known as passivaton.

Stainless steels can be divided into the following families:

Austenitic and superaustenitic steels containing Cr-Ni or Cr-Mn-Ni

Martensitic steels containing Cr (11-16%) and Ni (max 4%) Ferritic steels containing Cr (11-25%)

Austenitic-ferritic (duplex) steels containing Cr (18-28%), Ni (4-9%) and Mo (1,5-3%)

Precipitation-hardening steels.


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5. Heat Treatment for Steel

Heat treatments vary in terms of the temperatures reached and the way they are carried out, and are

generally broken down into two categories:

- treatments at temperatures above A3: annealing

normalising

quenching

- treatments at temperatures below A1: temperingstress relieving

Annealing

This heat treatment is carried out by heating the material to above A3. The initial micrographic structure istransformed into an austenitic structure; the less the temperature is pushed beyond A3, the finer this structure

remains. Steps must be taken, however, to make sure that the austenitic transformation takes place

throughout the entire metal mass.

Slow cooling in the furnace can then take place so as to cross the A3and A1lines over a period of sufficient

length to obtain complete separation of ferrite and pearlite.

A heat treatment of this kind can be used to obtain a final micrographic structure that is of a granular type

with equal axes. In this way the coarse solidification structures are eliminated, these being more fragile, or

less plastic, than the transformation structures.In addition, if the time spent in the austenitic range is sufficiently long, annealing can be used to retain the

differences in composition, constitution and structure using diffusion processes, and to eliminate structural

distortions due to plastic working for example.

Normalising

Normalising is a heat treatment that differs from annealing in terms of the greater cooling speed used.

After the time spent in the austenitic field the piece is cooled in still air rather than in the furnace. The A3 A1

interval is passed through more quickly, giving rise to a ferrite + pearlite structure that is still granular but finer

(Fig. 1.48a/b-1.49a/b).

Grain fineness, which is typical of normalised structures, generally gives the metal better mechanical

properties, as we have already seen.

Normalising is also intended to transform a coarse and uneven structure into a homogeneous structure with

fine grains: the figures below show examples of the effects of normalising a casting and a plate.

If the C content is rather high (over 0,30), however, cooling in the air may give rise to initial hardening

phenomena with a significant reduction in ductility. In this case annealing is preferable.


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Fig. 1.48a coarse casting Fig. 1.48b normalised casting

Fig. 1.49a plate in the as rolled condition Fig. 1.49b normalised plate

Quenching

Quenching is a heat treatment that is carried out by heating the material to a temperature exceeding A 3and then cooling it very quickly in water or oil.

The quick cooling does not allow time for austenite to be transformed into ferrite and pearlite and, as

already indicated in previous paragraphs, bainite type structures are formed or, in the case of very fast

cooling, hard, fragile martensitic structures are formed.

There are many factors that influence the hardening characteristics of a material, including:o cooling speed

o percentage of carbon

o presence of alloying elements

o grain size

o temperature at which austenitic transformation takes place

o presence of unmelted elements

o heating speed.

As we have already said, the cooling speed is a basic factor and as this increases so does the probability of

producing martensitic structures.




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Carbon and alloying elements promote the formation of hardened structures.

Grain size plays an important role. In fact, ferritic-pearlitic transformation comes about due to nucleation at

the edge of the grain and, bearing in mind that everything that inhibits this transformation favours

hardening, the larger the grain is the less space there is available around the grain, and the more easily

hardening will occur.

The higher the temperature at which austenitic transformation occurs, the more the grain is overheated and

the larger its size, which makes the steel easier to harden.

Elements that have not melted gather around the edges of the grains and set up a centre for germination

for pearlitic transformation, thereby limiting the steels susceptibility to hardening.Very quick heating and a very short time in the austenitic-ferritic field (between A1and A3), as occurs in

welding, may lead to a low diffusion and homogenisation of the carbon released by the cementite, thereby

creating zones that are rich in carbon. When these areas are then cooled, they may tend to harden even

when the average composition of the steel would exclude this possibility.

Apart from the rate of cooling, C content and that of other alloying elements are also very important

factors.

There is a formula that includes the various elements and their influence on the hardening capacity of a

material. This is the C equivalent formula:

Ceq= C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Hardening brings about an increase in the hardness of the material and significant reduction in ductility and

toughness. It is therefore very important to try to avoid hardening phenomena, especially following welding

operations.

In some cases hardening treatment is applied voluntarily. This is the case in hardening and tempering that is

used to obtain materials that have notable mechanical characteristics, associated with good toughness

characteristics.

Quenching and tempering involves the combination of two heat treatments: hardening followed by

tempering.

Tempering

Lets look at the action of tempering treatment, which is carried out at temperatures of between 400 and650C.

At these temperatures the acicular structure of the martensite (Fig. 1.50 A) is transformed into a non-laminar

structure of ferrite and cementite (Fig. 1.50 B).


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Fig. 1.50

Hardness is greatly reduced and, what is more, the higher the tempering temperature the more the steels

ductility and toughness increase (Fig. 1.51 & 1.52).

Fig. 1.51 Fig. 1.52

This is caused by the carbon escaping from the distorted tetragonal structure of the martensite, while the

grain size remains unchanged and is very small due to extremely quick cooling.

Stress Relieving

Stress relieving treatment is essentially intended to eliminate or rather reduce as much as possible theresidual tension that is an inevitable result of any welding operation.

For steels commonly used in construction this treatment is carried out at 600 650C, with this temperature

generally being maintained for two minutes for each millimetre of thickness, and a minimum of 30 minutes

(Fig. 1.53).

The efficacy of the treatment is due to the fact that at high temperature the yield strength of the material is

reduced to values that are practically negligible.

Thus, when the entire welded structure is heated to this temperature and kept there sufficiently long, the

tension is released and reduced to the value of the yield strength at that temperature.


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This release of tension may result in plastic deformation and so, after heat treatment, the dimensions of the

structure are changed to a greater or lesser extent.

Naturally, since this heat treatment is carried out at the same temperature as tempering, the beneficial

effects of the two are added to one another and thus things such as hardness points can be eliminated in

specific cases.

Fig. 1.53

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