Industrial Process of Martensite Fornation Doc

R. V. COLLEGE OF ENGINEERING,

BANGALORE.

MATERIAL SCIENCE AND METALLURGY

ASSIGNMENT ON

INDUSTRIAL PROCESS OF MARTENSITE

FORMATION

By

Abhishek Kumtakar (1RV11ME004)

Amit Vijay Dodmani (1RV11ME016)

Brian Nikil Quadras (1RV11ME036)

G K Shivakumar (1RV11ME044)

3rd

Sem A Section

I n d u s t r i a l P r o c e s s O f M a r t e n s i t e F o r m a t i o n P a g e | 2

Contents:

Introduction Properties Terminology Tempering in steel Quenched steel Welded steel Quench and self tempering Blacksmithing Martempeting Interrupted Quenching Quench Hardening Effect of Quench Hardening


Introduction

Martensite, named after the German metallurgist Adolf Martens (18501914), most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any

crystal structure that is formed by displacive transformation. It includes a class of hard minerals

occurring as lath- or plate-shaped crystal grains. When viewed in cross section, the lenticular

(lens-shaped) crystal grains are sometimes incorrectly described as Acicular (needle-shaped).

Martensite in AISI 4140 steel

In the 1890s, Martens studied samples of different steels under a microscope, and found that

the hardest steels had a regular crystalline structure. He was the first to explain the cause of the

widely differing mechanical properties of steels. Martensitic structures have since been found in

many other practical materials, including shape memory alloys and transformation-toughened

ceramics.

microstructue

Properties


The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure. This martensitic

reaction begins during cooling when the austenite reaches the martensite start

temperature (Ms) and the parent austenite becomes mechanically unstable. At a constant

temperature below Ms, a fraction of the parent austenite transforms rapidly, then no

further transformation will occur.

When the temperature is decreased, more of the austenite transforms to martensite. Finally, when the martensite finish temperature (Mf) is reached, the transformation is

complete. Martensite can also be formed by application of stress (this property is

frequently used in toughened ceramics like yttria-stabilized zirconia and in special steels

like TRIP steels (i.e. transformation induced plasticity steels). Thus, Martensite can be

thermally induced or stress induced.

One of the differences between the two phases is that martensite has a body-centered tetragonal (BCT) crystal structure, whereas austenite has a face-centered cubic (FCC)

structure. The transition between these two structures requires very little thermal

activation energy because it is a diffusionless transformation, which results in the subtle

but rapid rearrangement of atomic positions, and has been known to occur even at

cryogenic temperatures.

Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume. Of considerably greater importance than the

volume change is the shear strain which has a magnitude of about 0.26 and which

determines the shape of the plates of martensite.

Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is not an equilibrium phase. Equilibrium phases form by slow cooling rates

allowing sufficient time for diffusion, whereas martensite is usually formed by swift

cooling rates. Since chemical processes (the attainment of equilibrium) accelerate at

higher temperature, martensite is easily destroyed by the application of heat. This process

is called temperingaa.

In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead.

Since quenching can be difficult to control, many steels are quenched to produce an

overabundance of martensite, then tempered to gradually reduce its concentration until

the right structure for the intended application is achieved. Too much martensite leaves

steel brittle, too little leaves it soft.


Crystal

Structure

FCC BCC BCT

0.35%C Steel, water-quenched from 870C

Tempering

Tempering is a process of heat treating, which is used to increase the toughness of iron-based

alloys. It is also a technique used to increase the toughness of glass. For metals, tempering is

usually performed after hardening, to reduce some of the excess hardness, and is done by heating

the metal to a much lower temperature than was used for hardening.

The exact temperature determines the amount of hardness removed, and depends on both the

specific composition of the alloy and on the desired properties in the finished product. For

instance, very hard tools are often tempered at low temperatures, while springs are tempered to

much higher temperatures. In glass, tempering is performed by heating the glass and then quickly

cooling the surface, increasing the toughness.


Differentially tempered steel. The various colors produced indicate the temperature to which the steel was

heated. Light-straw indicates 204 C (399 F) and light blue indicates 337 C (639 F).

Photomicrograph of martensite, a very hard microstructure formed when steel is quenched. Tempering

reduces the hardness in the martensite by transforming it into various forms of tempered martensite.

Tempering is a heat treatment technique applied to ferrous alloys, such as steel or cast iron,

to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness is

usually accompanied by an increase in ductility, thereby decreasing the brittleness of the metal.

Tempering is usually performed after quenching, which is rapid cooling of the metal to put it in

its hardest state. Tempering is accomplished by controlled heating of the quenched work-piece to

a temperature below its "lower critical temperature".

This is also called the lower transformation temperature or lower arrest (A1) temperature;

the temperature at which the crystalline phases of the alloy, called ferrite and cementite, begin

combining to form a single-phase solid solution referred to as austenite.

Precise control of time and temperature during the tempering process is critical to achieve the

desired balance of physical properties. Low tempering temperatures may only relieve some of

the internal stresses, decreasing brittleness while maintaining a majority of the hardness. Higher

tempering temperatures tend to produce a greater reduction in the hardness, sacrificing some

yield strength and tensile strength for an increase in elasticity and plasticity.

However, in some low alloy steels, containing other elements like chromium and molybdenum,

tempering at low temperatures may produce an increase in hardness, while at higher

temperatures the hardness will decrease. Many steels with high concentrations of these alloying

elements behave like precipitation hardening alloys, which produce the opposite effects under

the conditions found in quenching and tempering, and are referred to as maraging steels.


In carbon steels, tempering alters the size and distribution of carbides in the martensite, forming

a microstructure called "tempered martensite". Tempering is also performed on normalized steels

and cast irons, to increase ductility, machinability, and impact strength.

Steel is usually tempered evenly, called "through tempering," producing a nearly uniform

hardness, but it is somethimes heated unevenly, referred to as "differential tempering," producing

a variation in hardness. In tempered glass, tempering is accomplished by creating internal

stresses in the amorphous structure, to increase both impact resistance and safety in the event of

breakage.

Terminology

In metallurgy, one may encounter many terms that have very specific meanings within the field,

but may seem rather vague when viewed from outside. Terms such as "hardness," "impact

resistance," "toughness," and "strength" can carry many different connotations, making it

sometimes difficult to discern the specific meaning. Some of the terms encountered, and their

specific definitions are:

Strength: Also called rigidity, this is resistance to permanent deformation. Strength, in

metallurgy, is still a rather vague term, so is usually divided into yield strength (resistance

to compression), shear strength (resistance to transverse, or cutting forces), and tensile

strength (resistance to stretching).

Toughness: Resistance to fracture, as measured by the Charpy test. Toughness often

increases as strength decreases.

Hardness: Hardness is often used to describe strength or rigidity but, in metallurgy, the

term is usually used to describe resistance to scratching or abrasion.

Brittleness: Brittleness describes a material's tendency to break before bending or

deforming either elastically or plastically. Brittleness increases with decreased toughness,

but is greatly affected by internal stresses as well.

Plasticity: The ability to mold, bend or deform in a manner that does not spontaneously

return to its original shape. This is proportional to the ductility or malleability of the

substance.

Elasticity: Also called flexibility, this is the ability to deform, bend, compress, or stretch

and return to the original shape once the external stress is removed. Elasticity is related to

the Young's modulus of the material.

Impact resistance: Usually synonymous with high-strength toughness, it is the ability

resist shock-loading with minimal deformation.

Wear resistance: Usually synonymous with hardness, this is resistance to erosion,

ablation, spalling, or galling.

Structural integrity: The ability to withstand a maximum-rated load while resisting

fracture, resisting fatigue, and producing a minimal amount of flexing or deflection, to provide a maximum service life.


Tempering in steel

Very few metals react to heat treatment in the same manner, or to the same extent, that steel

does. Steel can be softened to a very malleable state through annealing, or it can be hardened to a

state nearly as rigid and brittle as glass by quenching. However, in its hardened state, steel is

usually far too brittle, lacking the structural integrity to be useful for most applications.

Tempering is a method used to decrease the hardness, thereby increasing the ductility of the

quenched steel, to impart some springiness and malleability to the metal. This allows the metal to

bend before breaking. Depending on how much temper is imparted to the steel, it may bend

elastically (the steel returns to its original shape once the load is removed), or it may bend

plastically (the steel does not return to its original shape, resulting in permanent deformation),

before fracturing.

Tempering is used to precisely balance the mechanical properties of the metal, such as shear

strength, yield strength, hardness, ductility and tensile strength, to achieve any number of a

combination of properties, making the steel useful for a wide variety of applications. Tools such

as hammers and wrenches require good resistance to abrasion, impact resistance, and resistance

to deformation. Springs do not require as much rigidity, but must deform elastically before

breaking. Automotive parts tend to be a little less rigid, but need to deform plastically before

breaking.

Except in rare cases where maximum rigidity and hardness are needed, such as the untempered

steel used for files, quenched steel is almost always tempered to some degree. However, steel is

sometimes annealed through a process called normalizing, leaving the steel only partially

softened. Tempering is sometimes used on normalized steels to further soften it, increasing the

malleability and machinability for easier metalworking. Tempering may also be used on welded

steel, to relieve some of the stresses and excess hardness created in the heat affected zone around

the weld.

Quenched-steel

Tempering is most often performed on steel that has been heated above its upper critical (A3)

temperature and then quickly cooled, in a process called quenching, using methods such as

immersing the red-hot steel in water, oil, or forced-air. The quenched-steel, being placed in, or

very near, its hardest possible state, is then tempered to incrementally decrease the hardness to a

point more suitable for the desired application.

The hardness of the quenched-steel depends on both cooling speed and on the composition of

the alloy. Steel with a high carbon-content will reach a much harder state than steel with a low

carbon-content. Likewise, tempering high-carbon steel to a certain temperature will produce steel

that is considerably harder than low-carbon steel that is tempered at the same temperature. The

amount of time held at the tempering temperature also has an effect.


Tempering at a slightly elevated temperature for a shorter time may produce the same effect as

tempering at a lower temperature for a longer time. Tempering times vary, depending on the

carbon content, size, and desired application of the steel, but typically range from a few minutes

to a few hours.

Tempering quenched-steel at very low temperatures, between 66 and 148 C (151 and 298 F),

will usually not have much effect other than a slight relief of some of the internal stresses.

Tempering at higher temperatures, from 148 to 205 C (298 to 401 F), will produce a slight

reduction in hardness, but will primarily relieve much of the internal stresses. Tempering in the

range of 260 and 340 C (500 and 644 F) causes a decrease in ductility and an increase in

brittleness, and is referred to as the "tempered martensite embrittlement" (TME) range.

This range is usually avoided. Steel requiring more strength than toughness, such as tools, are

usually not tempered above 205 C (401 F). When increased toughness is desired at the expense

of strength, higher tempering temperatures, from 370 to 540 C (698 to 1,004 F), are used.

Tempering at even higher temperatures, between 540 and 600 C (1,004 and 1,112 F), will

produce excellent toughness, but at a serious reduction in the strength and hardness. At 600 C

(1,112 F), the steel experiences another stage of embrittlement, called "temper embrittlement"

(TE), so heating above this temperature is also avoided.

.

Welded steel

Steel that has been arc welded, gas welded, or welded in any other manner besides forge welded,

is affected in a localized area by the heat from the welding process. This localized area, called

the heat-affected zone (HAZ), consists of steel that varies considerably in hardness, from

normalized steel to steel nearly as hard as quenched steel near the edge of this heat-affected

zone. The uneven heating and cooling also creates internal stresses in the metal, both within and

surrounding the weld. Tempering is sometimes used in place of stress relieving to both reduce

the internal stresses and to decrease the brittleness around the weld. Tempering temperatures for

this purpose are generally around 205 C (401 F) and 343 C (649 F).

Quench and self-temper

Modern reinforcing bar of 500 MPa strength can be made from expensive microalloyed steel or

by a quench and self-temper (QST) process. After the bar exits the final rolling pass, where the

final shape of the bar is applied, the bar is then sprayed with water which quenches the outer

surface of the bar. The bar speed and the amount of water are carefully controlled in order to

leave the core of the bar unquenched. The hot core then tempers the already quenched outer part,

leaving a bar with high strength but with a certain degree of ductility too.

Blacksmithing

Tempering was originally a process used and developed by blacksmiths (forgers of iron). The

process was most likely developed by the Hittites of Anatolia (modern-day Turkey), in the


twelfth or eleventh century BC. Without knowledge of metallurgy, tempering was originally

devised through a trial-and-error method.

Because few methods of precisely measuring temperature existed until modern times,

temperature was usually judged by watching the tempering colors of the metal. Tempering often

consisted of heating above a charcoal or coal forge, or by fire, so holding the work at exactly the

right temperature for the correct amount of time was usually not possible. Tempering was

usually performed by slowly, evenly overheating the metal, as judged by the color, and then

immediately cooling, either in open air or by immersing in water.

This produced much the same effect as heating at the proper temperature for the right amount of

time. However, although tempering-color guides exist, this method of tempering usually requires

a good amount of practice to perfect, because the final outcome depends on many factors,

including the composition of the steel, the speed at which it was heated, the type of heat source

(oxidizing or carburizing), the cooling rate, oil films or impurities on the surface, and many other

circumstances which vary from smith to smith or even from job to job. The thickness of the steel

also plays a role. With thicker items, it becomes easier to heat only the surface to the right

temperature, before the heat can penetrate through. However, very thick items may not be able to

harden all the way through during quenching.

Tempering colors

Pieces of through-tempered steel flatbar. The first one, on the left, is normalized steel. The second is

quenched, untempered martensite. The remaining pieces have been tempered in an oven to their

corresponding temperature, for an hour each. "Tempering standards" like these are sometimes used by

blacksmiths for comparison, ensuring that the work is tempered to the proper color.

If steel has been freshly ground, sanded, or polished, it will form an oxide layer on its surface

when heated. As the temperature of the steel is increased, the thickness of the iron oxide will also

increase. Although iron oxide is not normally transparent, such thin layers do allow light to pass

through, reflecting off both the upper and lower surfaces of the layer.

This causes a phenomenon called thin-film interference, which produces colors on the surface.

As the thickness of this layer increases with temperature, it causes the colors to change from a

very light yellow, to brown, then purple, then blue. These colors appear at very precise


temperatures, and provide the blacksmith with a very accurate gauge for measuring the

temperature. The various colors, their corresponding temperatures, and some of their uses are:

Faint-yellow 176 C (349 F) engravers, razors, scrapers Light-straw 205 C (401 F) rock drills, reamers, metal-cutting saws Dark-straw 226 C (439 F) scribers, planer blades Brown 260 C (500 F) taps, dies, drill bits, hammers, cold chisels Purple 282 C (540 F) surgical tools, punches, stone carving tools Dark blue 310 C (590 F) screwdrivers, wrenches Light blue 337 C (639 F) springs, wood-cutting saws Grey-blue 371 C (700 F) and higher structural steel

Beyond the grey-blue color, the iron oxide loses its transparency, and the temperature can no

longer be judged in this way. The layer will also increase in thickness as time passes, which is

another reason overheating and immediate cooling is used. Steel in a tempering oven, held at 205

C (401 F) for a long time, will begin to turn brown, purple or blue, even though the

temperature did not exceed that needed to produce a light-straw color. Oxidizing or carburizing

heat sources may also affect the final result. The iron oxide layer, unlike rust, also protects the

steel from corrosion through passivation.[12]

DIFFERENTIAL TEMPERING

Differential tempering is often used for tempering cutting tools, such as this cold chisel.

Differential tempering is a method of providing different amounts of temper to different parts of

the steel. The method was often used in bladesmithing, for making knives and swords, to provide

a very hard edge while softening the spine or center of the blade. This increased the toughness

while maintaining a very hard, sharp, impact-resistant edge, helping to prevent breakage. This

technique was more often found in Europe, as opposed to the differential hardening techniques

more common in Asia, such as in Japanese swordsmithing.

Differential tempering consists of applying heat to only a portion of the blade, usually the spine,

or the center of double-edged blades. For single-edged blades, the heat, often in the form of a

flame or a red-hot bar, is applied to the spine of the blade only. The blade is then carefully

watched as the tempering colors form and slowly creep toward the edge. The heat is then

removed before the light-straw color reaches the edge. The colors will continue to move toward

the edge for a short time after the heat is removed, so the smith typically removes the heat a little

early, so that the pale-yellow just reaches the edge, and travels no farther. A similar method is

used for double-edged blades, but the heat source is applied to the center of the blade, allowing

the colors to creep out toward each edge.[


Interrupted quenching

Interrupted quenching methods are often referred to as tempering, although the processes are

very different from traditional tempering. These methods consist of quenching to a specific

temperature that is above the martensite start (Ms) temperature, and then holding at that

temperature for extended amounts of time. Depending on the temperature and the amount of

time, this allows either pure bainite to form, or holds-off forming the martensite until much of

the internal stresses relax. These methods are known as austempering and martempering.

MARTEMPERING

Martempering is similar to austempering, in that the steel is quenched in a bath of molten metal

or salts to quickly cool it past the pearlite-forming range. However, in martempering, the goal is

to create martensite rather than bainite. The steel is quenched to a much lower temperature than

is used for austempering; to just above the martensite start temperature. The metal is then held at

this temperature until the temperature of the steel reaches an equilibrium.

The steel is then removed from the bath before any bainite can form, and then is allowed to air-

cool, turning it into martensite. The interruption in cooling allows much of the internal stresses to

relax before the martensite forms, decreasing the brittleness of the steel. However, the


martempered steel will usually need to undergo further tempering to adjust the hardness and

toughness.

Physical processes

Tempering involves a three-step process in which unstable martensite decomposes into ferrite

and unstable carbides, and finally into stable cementite, forming various stages of a

microstructure called tempered martensite. The martensite typically consists of laths (strips) or

plates, sometimes appearing acicular (needle-like) or lenticular (lens-shaped). Depending on the

carbon content, it also contains a certain amount of "retained austenite."

Retained austenite are crystals which are unable to transform into martensite, even after

quenching below the martensite finish (Mf) temperature. An increase in alloying agents or carbon

content causes an increase in retained austenite, lowering the wear resistance of the steel,

although some or most of the retained austenite can be transformed into martensite by cold and

cryogenic treatments prior to tempering.

The martensite forms during a diffusionless transformation, in which the transformation occurs

due to shear-stresses created in the crystal lattices rather than by chemical changes that occur

during precipitation. The shear-stresses create many defects, or "dislocations," between the

crystals, providing less-stressful areas for the carbon atoms to relocate.

Upon heating, the carbon atoms first migrate to these defects, and then begin forming unstable

carbides. This reduces the amount of total martensite by changing some of it to ferrite. Further

heating reduces the martensite even more, transforming the unstable carbides into stable

cementite.

The first stage of tempering occurs between room-temperature and 200 C (392 F). In the first

stage, carbon precipitates into -carbon (Fe24C). In the second stage, occurring between 150 C (302 F) and 300 C (572 F), the retained austenite transforms into a form of lower-bainite

containing -carbon rather than cementite.

The third stage occurs at 200 C (392 F) and higher. In the third stage, -carbon precipitates into cementite, and the carbon content in the martensite decreases. If tempered at higher

temperatures, between 650 C (1,202 F) and 700 C (1,292 F), or for longer amounts of time,

the martensite may become fully ferritic and the cementite may become coarser or spheroidize.

In spheroidized steel, the cementite recedes into rods or spherical shaped globules, and the steel

becomes softer than annealed steel; nearly as soft as pure iron, making it very easy to form or

machine.

Quenching


Coke being pushed into a quenching car, Hanna furnaces of the Great Lakes Steel Corporation, Detroit,

Michigan, November 1942.

In materials science, quenching is the rapid cooling of a workpiece to obtain certain material

properties. It prevents low-temperature processes, such as phase transformations, from occurring

by only providing a narrow window of time in which the reaction is both thermodynamically

favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase

toughness of both alloys and plastics (produced through polymerization).

In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case

the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite

becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid

temperature becomes much lower, but the kinetic barriers to phase transformation remain the

same.

This allows quenching to start at a lower temperature, making the process much easier. High

speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion

that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly

in air has most of the desired effects of quenching.

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous

metal or "metallic glass".

Quench hardening

Quench hardening is a mechanical process in which steel and cast iron alloys are strengthened

and hardened. These metals consist of ferrous metals and alloys. This is done by heating the

material to a certain temperature, depematerial. This produces a harder material by either surface

hardening or through-hardening varying on the rate at which the material is cooled. The material


is then often tempered to reduce the brittleness that may increase from the quench hardening

process. Items that may be quenched include gears, shafts, and wear blocks.

Process

Quenching metals is a progression; the first step is soaking the metal, i.e. heating it to the

required temperature. Soaking can be done by air (air furnace), or a bath. The soaking time in air

furnaces should be 1 to 2 minutes for each millimeter of cross-section. For a bath the time can

range a little higher. The recommended time allotment in salt or lead baths is 0 to 6 minutes.

Uneven heating or overheating should be avoided at all cost. Most materials are heated from

anywhere to 815 to 900 C (1,500 to 1,650 F).

The next item on the progression list is the cooling of the part. Water is one of the most efficient

quenching media where maximum hardness is acquired, but there is a small chance that it may

cause distortion and tiny cracking. When hardness can be sacrificed, whale, cottonseed and

mineral oils are used. These often tend to oxidize and form a sludge, which consequently lowers

the efficiency. The quenching velocity (cooling rate) of oil is much less than water. Intermediate

rates between water and oil can be obtained with water containing 10-30% UCON from DOW, a

substance with an inverse solubility which therefore deposits on the object to slow the rate of

cooling.

To minimize distortion, long cylindrical workpieces are quenched vertically; flat workpieces are

quenched on edge; and thick sections should enter the bath first. To prevent steam bubbles the

bath is agitated.

Effect of Quench Hardening

Before the material is hardened, the microstructure of the material is a pearlite grain structure

that is uniform and lamellar. Pearlite is a mixture of ferrite and cementite formed when steel or

cast iron are manufactured and cooled at a slow rate. After quench hardening, the microstructure

of the material form into martensite as a fine, needle-like grain structure.[1]

Before using this technique it is essential to look up the rate constants for the quenching of the

excited states of metal ions.

When quenching, there are numerous types of media. Some of the more common

include: air, brine (salt water), oil and water

Bibliography


1) www.industrialheating.com

2) steel.keytometals.com

3) www.scientfic.net

4) www.metalpass.com

5) Image Courtesy:

www.google.co.in

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