Heat treatment of Steels

HEAT TREATMENT

OF STEELS

HEAT TREATMENT

Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.

Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.

Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state.

In heat treatment, the processing is most often entirely thermal and modifies only structure. Thermomechanical treatments, which modify component shape and structure, and thermochemical treatments which modify surface chemistry and structure, are also important processing approaches which fall into the domain of heat treatment.

Iron carbon diagram form the basis of heat treatments:

Various types of heat treatment processes are used to modify the following properties or conditions of the steel:

Improve the toughness Increase the hardness Increase the ductility Improve the machinability Refine the grain structure Remove the residual stresses Improve the wear resistance

TYPES OF HEAT TREATMENT PROCESSES

ANNEALING…

ANNEALING Annealing is a heat process whereby a

metal is heated to a specific temperature, held at that temperature and then allowed to cool slowly.

GENERAL EFFECTS OF ANNEALING When a metal is cold-worked, by any of the

many industrial shaping operations, changes occur in both its physical and mechanical properties.

While the increased hardness and strength which result from the working treatment may be of importance in certain applications, it is frequently necessary to return the metal to its original condition to allow further forming operations (e.g. deep drawing) to be carried out of for applications where optimum physical properties, such as electrical conductivity, are essential.

The treatment given to the metal to bring about a decrease of the hardness and an increase in the ductility is known as annealing.

GENERAL EFFECTS OF ANNEALING This usually means keeping the deformed metal

for a certain time at a temperature higher than about one-third the absolute melting point.

Cold working produces an increase in dislocation density; for most metals ρ increases from the value of 1010–1012 lines m-2 typical of the annealed state, to 1012–1013 after a few per cent deformation, and up to 1015–1016 lines m-2 in the heavily deformed state.

Such an array of dislocations gives rise to a substantial strain energy stored in the lattice, so that the cold-worked condition is thermodynamically unstable relative to the undeformed one.

Consequently, the deformed metal will try to return to a state of lower free energy, i.e. a more perfect state.

GENERAL EFFECTS OF ANNEALING In general, this return to a more equilibrium

structure cannot occur spontaneously but only at elevated temperatures where thermally activated processes such as diffusion, cross slip and climb take place.

Like all non-equilibrium processes the rate of approach to equilibrium will be governed by an Arrhenius equation of the form:

Rate = A exp [-Q/kT] where the activation energy Q depends on impurity content, strain, etc.

GENERAL EFFECTS OF ANNEALING The formation of atmospheres by strain-ageing

is one method whereby the metal reduces its excess lattice energy but this process is unique in that it usually leads to a further increase in the structure sensitive properties rather than a reduction to the value characteristic of the annealed condition.

It is necessary, therefore, to increase the temperature of the deformed metal above the strain-ageing temperature before it recovers its original softness and other properties.

STAGES OF ANNEALING The removal of the cold-worked condition,

or in other words, the annealing process, may be divided into three stages:

Recovery Recrystallization Grain growth

RECOVERY This is primarily a low temperature process,

and the property changes produced do not cause appreciable change in microstructure or the properties, such as tensile strength, yield strength, hardness and ductility.

The principal effect of recovery is the relief of internal stresses due to cold working and this prevents stress corrosion cracking and minimizes the distortion produced by residual stresses.

Electrical conductivity is also increased appreciably during the recovery stage.

Commercially, this low temperature treatment in the recovery range is known as stress relief annealing or process annealing.

RECOVERY This process describes the changes in the

distribution and density of defects with associated changes in physical and mechanical properties which take place in worked crystals before recrystallization or alteration of orientation occurs.

The recovery stage of annealing is chiefly concerned with the rearrangement of the cold worked dislocations to reduce the lattice energy and does not involve the migration of large-angle boundaries.

This rearrangement of the dislocations is assisted by thermal activation.

RECOVERY Mutual annihilation of dislocations is one

process. When the two dislocations are on the same slip

plane, it is possible that as they run together and annihilate they will have to cut through intersecting dislocations on other planes, i.e. ‘forest’ dislocations.

This recovery process will, therefore, be aided by thermal fluctuations since the activation energy for such a cutting process is small.

When the two dislocations of opposite sign are not on the same slip plane, climb or cross-slip must first occur, and both processes require thermal activation.

RECOVERY One of the most important recovery

processes which leads to a resultant lowering of the lattice strain energy is rearrangement of the dislocations into cell walls.

This process in its simplest form was originally termed polygonization whereby dislocations all of one sign align themselves into walls to form small-angle or subgrain boundaries.

RECOVERY

RECOVERY During deformation a region of the lattice is

curved, as shown in Figure 7.50a, and the observed curvature can be attributed to the formation of excess edge dislocations parallel to the axis of bending.

On heating, the dislocations form a sub-boundary by a process of annihilation and rearrangement. This is shown in Figure 7.50b, from which it can be seen that it is the excess dislocations of one sign which remain after the annihilation process that align themselves into walls.

POLYGONIZATION Polygonization is a simple form of sub-

boundary formation and the basic movement is climb whereby the edge dislocations change their arrangement from a horizontal to a vertical grouping. This process involves the migration of vacancies to or from the edge of the half-planes of the dislocations .

The removal of vacancies from the lattice, together with the reduced strain energy of dislocations which results, can account for the large change in both electrical resistivity and stored energy observed during this stage, while the change in hardness can be attributed to the rearrangement of dislocations and to the reduction in the density of dislocations.

RECRYSTALLIZATION Recrystallization takes place by a combination of

nucleation of strain free grains and the growth of these nuclei to absorb the entire cold worked material.

The term recrystallization temperature does not refer to a definite temperature below which recrystallization will not occur, but refers to the approximate temperature at which a highly cold worked material completely recrystallizes in one hour.

Pure metals have low recrystallization temperatures as compared with alloys.

Zinc, tin and lead have recrystallization temperatures below room temperature. This means that these metals cannot be cold worked at room temperature since they crystallize spontaneously, reforming a strain free structure.

The recrystallization temperatures of several metals and alloys are listed as in the table below:

Material Recrystallization Temperature (oF)

Copper ( 99.99 %) 250

Copper ( 5 % Zinc) 600

Aluminum (99.99 %) 175

Aluminum alloys 600

Low carbon steel 1000

Zinc 50

Tin 25

Lead 25

RECRYSTALLIZATION In the primary recrystallization stage the

deformed lattice is completely replaced by a new unstrained one by means of a nucleation and growth process, in which practically stress free grains grow from nuclei formed in the deformed matrix.

The orientation of the new grains differs considerably from that of the crystals they consume, so that the growth process must be regarded as incoherent,i.e. it takes place by the advance of large-angle boundaries separating the new crystals from the strained matrix.

RECRYSTALLIZATION During the growth of grains, atoms get

transferred from one grain to another across the boundary. Such a process is thermally activated as shown in Figure

Variation in free energy during grain growth.

RECRYSTALLIZATIONIt is well known that the rate of recrystallization depends on several important factors, namely:

(1) the amount of prior deformation (the greater the degree of cold work, the lower the recrystallization temperature and the smaller the grain size),

(2) the temperature of the anneal (as the temperature is lowered the time to attain a constant grain size increases exponentially)

(3) the purity of the sample (e.g. zone refined aluminium recrystallizes below room temperature, whereas aluminium of commercial purity must be heated several hundred degrees).

(4)Increasing the annealing time decreases the recrystallization temperature for the start of recrystallization. .

MECHANISM OF RECRYSTALLIZATION

Measurements, using the light microscope, of the increase in diameter of a new grain as a function of time at any given temperature can be expressed as shown in the figure. The diameter increases linearly with time until the growing grains begin to impinge on one another, after which the rate necessarily decreases.

Figure 7.52 Variation of grain diameter with time at a constant temperature.


The classical interpretation of these observations is that nuclei form spontaneously in the matrix after a so-called nucleation time t0, and these nuclei then proceed to grow steadily as shown by the linear relationship.

The driving force for the process is provided by the stored energy of cold work contained in the strained grain on one side of the boundary relative to that on the other side.

Such an interpretation would suggest that the recrystallization process occurs in two distinct stages, i.e. first nucleation and then growth.


During the linear growth period the radius of a nucleus is R= G(t-to), where G, the growth rate, is dR/dt assuming the nucleus is spherical, the volume of the recrystallized nucleus is

If the number of nuclei that form in a time increment dt is N dt per unit volume of unrecrystallized matrix, and if the nuclei do not impinge on one another, then for unit total volume


This equation is valid in the initial stages when

f<<1. When the nuclei impinge on one another the rate of recrystallization decreases and is related to the amount untransformed (1 – f) by

where, for short times, equation reduces to last equation. This Johnson–Mehl equation is expected to

apply to any phase transformation where there is random nucleation, constant N and G and small t0.


In practice, nucleation is not random and the rate not constant so that equation will not strictly apply.

For the case where the nucleation rate decreases exponentially, Avrami developed the equation

where k and n are constants, with n≈ 3 for a fast and n≈ 4 for a slow, decrease of nucleation rate. Provided there is no change in the nucleation

mechanism, n is independent of temperature but k is very sensitive to temperature T; clearly from equation k=∏NG3/3 and both N and G depend on T.


The process of recrystallization may be pictured as follows.

After deformation, polygonization of the bent lattice regions on a fine scale occurs and this results in the formation of several regions in the lattice where the strain energy is lower than in the surrounding matrix; this is a necessary primary condition for nucleation.

During this initial period when the angles between the sub-grains are small and less than one degree, the sub-grains form and grow quite rapidly.

However, as the sub-grains grow to such a size that the angles between them become of the order of a few degrees, the growth of any given sub-grain at the expense of the others is very slow.


Eventually one of the sub-grains will grow to such a size that the boundary mobility begins to increase with increasing angle. A large angle boundary, θ≈(30–40°), has a high mobility because of the large lattice irregularities or ‘gaps’ which exist in the boundary transition layer.

The atoms on such a boundary can easily transfer their allegiance from one crystal to the other.

This sub-grain is then able to grow at a much faster rate than the other subgrains which surround it and so acts as the nucleus of a recrystallized grain.


The further it grows, the greater will be the difference in orientation between the nucleus and the matrix it meets and consumes, until it finally becomes recognizable as a new strain-free crystal separated from its surroundings by a large-angle boundary.Whether recrystallization nucleus it grows to become a strain-free grain depends on three factors:

(1) the stored energy of cold work must be sufficiently high to provide the required driving force,

(2) the potential nucleus should have a size advantage over its neighbours

(3) it must be capable of continued growth by existing in a region of high lattice curvature (e.g.transition band) so that the growing nucleus can quickly achieve a high- angle boundary.

VARIABLES AFFECTING RECRYSTALLIZATION

Prior deformation, for example, will control the extent to which a region of the lattice is curved.

The larger the deformation, the more severely will the lattice be curved and, consequently, the smaller will be the size of a growing sub-grain when it acquires a large-angle boundary.

The importance of impurity content on recrystallization temperature is also evident from the effect impurities have on obstructing dislocation sub-boundary and grain boundary mobility.

The intragranular nucleation of strain-free grains, as discussed above, is considered as abnormal subgrain growth, in which it is necessary to specify that some sub-grains acquire a size advantage and are able to grow at the expense of the normal subgrains.

VARIABLES AFFECTING RECRYSTALLIZATION

It has been suggested that nuclei may also be formed by a process involving the rotation of individual cells so that they coalesce with neighbouring cells to produce larger cells by volume diffusion and dislocation rearrangement

Segregation of solute atoms to, and precipitation on, the grain boundary tends to inhibit intergranular nucleation and gives an advantage to intragranular nucleation, provided the dispersion is not too fine.

Small, finely dispersed particles retard recrystallization by reducing both the nucleation rate and the grain boundary mobility, whereas large coarsely dispersed particles enhance recrystallization by increasing the nucleation rate.

GRAIN GROWTH When primary recrystallization is complete

(i.e. when the growing crystals have consumed all the strained material) the material can lower its energy further by reducing its total area of grain surface.

In this stage the tensile strength and hardness continue to decrease but at a much less rate than the recrystallization stage.

With extensive annealing it is often found that grain boundaries straighten, small grains shrink and larger ones grow.

The general phenomenon is known as grain growth; the major change observed during this stage is the growth of the grain boundaries and reaching the original grain size .

GRAIN GROWTH The most important factor governing the process is

the surface tension of the grain boundaries. A grain boundary has a surface tension, T (= surface-

free energy per unit area) because its atoms have a higher free energy than those within the grains.

Consequently, to reduce this energy a polycrystal will tend to minimize the area of its grain boundaries.

Second-phase particles have a major inhibiting effect on boundary migration and are particularly effective in the control of grain size.

The pinning process arises from surface tension forces exerted by the particle–matrix interface on the grain boundary as it migrates past the particle.

Nevertheless, even after growth has finished the

grain size in a specimen which was previously severely cold-worked remains relatively small, because of the large number of nuclei produced by the working treatment.

Exaggerated grain growth can often be induced, however, in one of two ways, namely:

(1) by subjecting the specimen to a critical strain- anneal treatment, or

(2) by a process of secondary recrystallization.

By applying a critical deformation (usually a few per cent strain) to the specimen the number of nuclei will be kept to a minimum, and if this strain is followed by a high-temperature anneal in a thermal gradient some of these nuclei will be made more favourable for rapid growth than others.

The only driving force for secondary recrystallization is the reduction of grain boundary-free energy, as in normal grain growth. Special conditions are necessary. In some regions the grain boundaries become free (e.g. if the inclusions slowly dissolve or the boundary tears away) and as a result the grain size in such regions becomes appreciably larger than the average.

EFFECT OF ANNEALING ON

TENSILE STRENTH, HARDNESS, DUCTILITY AND GRAIN SIZE

Annealing results in modification of the properties of steel. The figure in the next slide shows the change in some of these properties on annealing.

During the recovery stage the decrease in stored energy and electrical resistivity is accompanied by only a slight lowering of hardness.

The greatest simultaneous change in properties occurs during the primary recrystallization stage.

During recrystallization stage, there is a significant drop in tensile strength, hardness and a large increase in the ductility of the material which is illustrated in the figure

EFFECT OF ANNEALING ON TENSILE

STRENTH, HARDNESS, DUCTILITY AND GRAIN SIZE

BENEFITS OF ANNEALING The benefits of annealing are:

Improved ductility Removal of residual stresses that

result from cold-working or machining

Improved machinability Grain refinement

At the same time, annealing also has a few disadvantages as it reduces the hardness, yield strength and tensile strength of the steel

TYPES OF ANNEALING

I. FULL ANNEALINGII.HOMOGENISING ANNEALINGIII.RECRYSTALLIZATION ANNEALINGIV.SPHEROIDISATION ANNEALINGV.STRESS-RELIEF ANNEALINGVI.ISOTHERMAL ANNEALING

FULL ANNEALING Full annealing is the process by which the

distorted cold worked lattice structure is changed back to one which is strain free through the application of heat. This process is carried out entirely in the solid state and is usually followed by slow cooling in the furnace from the desired temperature.

The austenitising temp is a function of carbon content of the steel and can be generalised as:

For hypoeutectoid steels and eutectoid steelAc3+(20-40oC) [to obtain single phase

austenite] For hypereutectoid steels

Ac1+(20-40oC) [to obtain austenite+ cementite]

AIMS OF FULL ANNEALING To refine the grain size of steel castings,

or of hot worked steels to improve the mechanical properties.

To soften the steel To relieve internal stresses To improve machinability It also reduces some defects like aligned

sulphide inclusions, or bands in steels.

The three important parts of full annealing are:Proper austenitising temperatureSoaking timeVery slow cooling through A1(critical

temperature)

The formation of austenite destroys all structures that have existed before heating. Slow cooling yields the original phases of ferrite and pearlite in hypoeuetectoid steels and that of cementite and pearlite in hypereutectoid steels.

HOMOGENISING ANNEALING The subsequent heating, soaking and hot

working homogenises the structure to a large extent since diffusion of C is very fast at high temp and the simultaneous plastic deformation breaks the dendrites with different portions moving in relation to each other which also facilitate diffusion.

The main aim of homogenising annealing is to make the composition uniform, i.e to remove chemical heterogeneity

The alloy steel ingots are homogenised at higher temp like 1150-1200oC for 10-20 hours as the diffusion of substitutional solid solution forming elements is very low

The impact energy and ductility of the steel increase as the homogenizing temperature increases and the hardness, yield strength and tensile strength decrease with an increase in the homogenizing temperature.

Homogenising annealing has a few shortcomings as well. It results in:

Grain coarsening of austenite, thereby impairing the properties

Thick scales on the surface of steels It is an expensive process

RECRYSTALLIZATION ANNEALING Also known as process annealing or sub-critical annealing. Recrystallization annealing consists of heating the cold

worked steel above its recrystallization temp, soaking at this temp, and then cooling thereafter.

Process Annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing.

The main aims of recrystallization annealing are: To restore ductility To refine coarse grains To improve electrical and magnetic properties in

grain-oriented Si steels.

No phase change takes place and the final structure consists of strain-free, equiaxed grains of fine ferrite produced at the expense of deformed elongated ferrite grains.

Recrystallization temp(Tr) is given by:Tr= (0.3-0.5)Tm.p

As little scaling and decarburisation occurs in recrystallization annealing, it is preferred over full annealing.

However It would produce very coarse grains if the steel has undergone critical amount of deformation. In such cases, full annealing is preferred.

SPHEROIDISATION ANNEALINGo Spheroidisation annealing consists of heating,

soaking and cooling, invariably very slowly to produce spheroidal pearlite or globular form of carbides in steels.

o Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels.

o To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied.

o This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes the machining easy.

o Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite.

SPHEROIDISING METHODS: Heating the steel to just below Ac1 temp,

holding at this temperature for a very long period followed by slow cooling.

Austenitise the steel at a temp not more than 50oC above A1, and cool very slowly through A1 to transform this inhomogeneous austenite at a temperature not more than 50oC below A1 temperature

Spheroidizing process applied at a temperature below the LCT.

Spheroidizing process applied at a temperature below and above the LCT.

ADVANTAGES OF SPHEROIDISATION

ANNEALING:

minimum hardness maximum ductility maximum machinability maximum softness

ISOTHERMAL ANNEALING Commonly used for alloy steels to soften

the steel In isothermal annealing, steel

austenitised at a temp 20-40oC above Ac3, is cooled quickly to the temp of isothermal holding( which is below A1 temp in the pearlitic range), held there for the required time so that complete transformation of austenite takes place and then normally cooled in air

The closer the temp of isothermal holding is to A1, coarser is the pearlite, softer is the steel, but longer is the time of isothermal transformation.

ADVANTAGES OF ISOTHERMAL

ANNEALING: As cooling can be done in air after the

transformation is complete, total time of heat treatment and the cost of annealing is less.

The productivity of the furnace is high The microstructure obtained is more uniform

and thus better control over hardness can be obtained as transformation takes place at a constant temperature.

Improved machinability with good surface finish

Reduced warping in subsequent hardening processes.

STRESS-RELIEF ANNEALING• Stress-Relief Annealing is useful in removing

residual stresses due to heavy machining or other cold-working processes.

• It is usually carried out at temperatures below the LCT, which is usually selected around 1000oF.

• The main aims of stress-relief annealing are:• To relieve the internal stresses, and thus, allow

higher external loads to be applied• Increase fatigue life and prevent intercrystalline

corrosion.• To reduce chance of warpage or cracking, or risk

of distortion in cracking• To increase impact resistance and lower

susceptibility to brittle fracture• No change of dimensions in service life

NORMALIZING

NORMALIZING The normalizing of steel is carried out by

heating above the UCT (Upper Critical Temperature) to single phase austenitic region to get homogeneous austenite, soaking there for some time and then cooling it in air to room temperature.

The austenitising temperature range are: For hypoeutectoid steels and

eutectoid steel• Ac3 + (40-60oC)

For hypereutectoid steels• Acm + (30-50oC)

During normalising we use grain

refinement which is associated with allotropic transformation upon heating γ→α

Parts that require maximum toughness and those subjected to impact are often normalized.

When large cross sections are normalized, they are also tempered to further reduce stress and more closely control mechanical properties.

The microstructure obtained by normalizing depends on the composition of the castings (which dictates its hardenability) and the cooling rate.

Figure below shows the normalizing temperatures for hypoeutectoid and hypereutectoid steels

AIMS OF NORMALIZING

To produce a harder and stronger steel than full annealing

To improve machinability To modify and/or refine the grain structure To obtain a relatively good ductility without

reducing the hardness and strength Improve dimensional stability Produce a homogeneous microstructure Reduce banding Provide a more consistent response when

hardening or case hardening

EFFECT OF SOAKING TIME ON THE

MICROSTRUCTURE:

EFFECT OF NORMALIZING TIME

AND TEMPERATURE

ON MICROSTRUCTURE OF HOT ROLLED

SAMPLE

MICROSTRUCTURE AT THE STRIP SURFACE NORMALIZED AT 860oC




NORMALIZING VS

ANNEALING

COMPARISON OF ANNEALING AND

NORMALIZING The metal is heated to a higher temperature and then removed

from the furnace for air cooling in normalizing rather than furnace cooling.

In normalizing, the cooling rate is slower than that of a quench-and-temper operation but faster than that used in annealing.

As a result of this intermediate cooling rate, the parts will possess a hardness and strength somewhat greater than if annealed.

Fully annealed parts are uniform in softness (and machinability) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part.

Internal stresses are more in normalizing as compared to annealing.

Grain size obtained in normalizing is finer than in annealing. Normalizing is a cheaper and less time-consuming process.

COMPARISON OF TEMPERATURE RANGES IN

ANNEALING AND NORMALIZING

COMPARISON OF TIME-TEMPERATURE CYCLES FOR NORMALIZING AND FULL ANNEALING

The slower cooling of annealing results in higher temperature transformation to ferrite and pearlite and coarser microstructures than does normalizing.

EFFECT OF ANNEALING AND NORMALIZING ON DUCTILITY OF

STEELSAnnealing and normalizing do not present a significant difference on the ductility of low carbon steels. As the carbon content increases, annealing maintains the % elongation around 20%. On the other hand, the ductility of the normalized high carbon steels drop to 1 to 2 % level.

EFFECT OF ANNEALING AND NORMALIZING ON THE TENSILE STRENGTH AND YIELD POINT OF

STEELS The tensile strength and the yield point

of the normalized steels are higher than the annealed steels.

Normalizing and annealing do not show a significant difference on the tensile strength and yield point of the low carbon steels.

However, normalized high carbon steels present much higher tensile strength and yield point than those that are annealed. This can be illustrated from the figures.

EFFECT OF ANNEALING AND NORMALIZING ON THE HARDNESS

OF STEELSLow and medium carbon steels can maintain similar hardness levels when normalized or annealed. However, when high carbon steels are normalized they maintain higher levels of hardness than those that are annealed.

ADVANTAGES OF NORMALIZING OVER ANNEALING

Better mechanical properties Lesser time-consuming Lower cost of fuel and operation

ADVANTAGES OF ANNEALING OVER NORMALIZING

Greater softness Complete absence of internal stresses

which is a necessity in complex and intricate parts

HARDENING

HARDENING It is the process of heating the steel to

proper austenitizing temperature , soaking at this temperature to get a fine grained and homogeneous austenite , and then cooling the steel at a rate faster than its critical cooling rate.

OBJECTIVES OF HARDENINGThe aims of hardening are:

1. Main aim of hardening is to induce high hardness. The cutting ability of a tool is proportional to its hardness.

2. Many machine parts and all tools are hardened to induce high wear resistance higher is the hardness , higher is the wear and the abrasion resistance .For example ,gears, shaft.

3. The main objective of hardening machine components made of structural steel is to develop high yield strength with good toughness and ductility to bear high working stresses.

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS The steel is first heated to proper austenising

temperature to obtain a homogeneous and fine grained austenite. This temperature depends on the composition(carbon as well as alloying elements).

The austenitising temperature of plain carbon steels depends on the carbon content of the steel and is generalised as :

For hypo-eutectoid steels :Ac3 + (20 — 40°C) For hyper-eutectoid steels and eutectoid

steel:Ac1 + (20 — 40°C)

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS Hypereutectoid steels, when heated in above

temperature range, to obtain homogeneous and fine-grained austenite which on quenching transforms to fine-grained (very fine needles/plates), and hard martensite as is desired to be obtained.

Heating these steels only up to critical range (between Ac3 and Ac1) is avoided in practice.

Steel then has austenitic and ferrite. On quenching, only austenite transforms to

martensite, and ferrite remains as it is, i.e., incomplete hardening occurs .

The presence of soft ferrite does not permit to achieve high hardness, if that is the objective.

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS If the aim is to get high strength by the

process of tempering ferrite does not permit this as it has low tensile and yield strengths .

In fact, ferrite forms the easy path to fracture. Quenching of hypoeutectoid steels from

temperatures much above the proper temperatures , when austenite has become coarse, results in coarse acicular form of martensite.

Coarse martensite is more brittle, and a unit or two lower in hardness. It lowers the impact strength even after tempering, and is more prone to quench-cracking.

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS Hypereutectoid steels, when heated in the above

range, i.e., just above Ac1 have fine grains of austenite and proeutectoid cementite.

On quenching austenite transforms to fine martensite and cememtite remains unchanged.

As the hardness of cementite (≈ 800 BHN) is more than that of martensite (650-750 BHN), its presence increases the hardness, wear and abrasion resistance as compared to only martensitic structure.

If temperature of austenitisation is much higher than Ac1 but still below Acm temperature, a part of proeutectoid cementite gets dissolved to increase the carbon content of austenhlc(> 0.77%)

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS On quenching as-quenched hardness is

less, because :1. Lesser amount of proeutectoid

cementite is present.2. Larger amount of soft retained

austenite is obtained as the dissolved carbon of cementite has lowered the Ms and Mf temperature.

3. A bit coarser martensite has lesser hardness.

AUSTENISING TEMPERATURE FOR PEARLITIC STEELS Heating hypereutectoid steels to a

temperature higher than Acm results in 100% austenite . It is very coarse austenite as very rapid grain-growth occurs due to dissolution of restraining proeutectoid cementite .

The as-quenched hardness is low because of:1) Absence of harder cementite.2) As more carbon has dissolved in austenite,

more retained austenite is obtained.3) Coarser martensite is a bit less hard and more

brittle. Thus, these temperatures are avoided in

carbon steels

PROCESS OF QUENCHING When a heated steel object (say at 840°C) is plunged

into a stationary bath of cold it has three stages as:

Stage A -vapour-blanket stage: Immediately on quenching, coolant gets vapourized as

the steel part is at high temperature, and thus, a continuous vapour- blanket envelopes the steel part.

Heat escapes from the hot surface very slowly by radiation and conduction through the blanket of water vapour.

Since the vapour-film is a poor heat conductor, the cooling rate is relatively low (stage A in fig ). This long stage is undesirable in most quenching operations.

PROCESS OF QUENCHING

PROCESS OF QUENCHINGStage B-Intermittent contact stage (Liquid-

boiling stage): Heat is removed in the form of heat of

vaporization in this stage as is indicated by the steep slope of the cooling curve.

During this stage, the vapour-blanket is broken intermittently allowing the coolant to come in contact with the hot surface at one instant, but soon being pushed away by violent boiling action of vapour bubble.

The rapid cooling in this stage soon brings the metal surface below the boiling point of the coolant.

PROCESS OF QUENCHING The vaporization then stops. Second stage

corresponds to temperature range of 500◦ to 100◦c , and this refers to nose of the CCT curve of the steel , when the steel transforms very rapidly ( to non martensite product ).

Thus, the rate of cooling in this stage is of great importance in hardening of steels.

Stage C-Direct-Contact stage (Liquid-cooling stage): This stage begins when the temperature of steel

surface Is below the boiling point of coolant. Vapours do not form. The cooling is due to

convection and conduction through the liquid. Cooling is slowest here.

QUENCHING MEDIUMS As the aim is to get martensite, the coolant should

have quenching power to cool austenite to let it transform to martensite. The following factors effect the quenching power of the coolant :

The cooling rate decreases as the temperature of water and brine increases, i.e., it increases stage ‘A’, i.e., helps in persistence of the vapour blanket stage.

The increased temperature brings it closer to its boiling point, and thus, requires less heat to form vapour, specially above 60°C.

Good range of temperature for water as coolant is 20-40°C.

Oils in general, show increased cooling rates with the rise of temperature, with optimum cooling rates in range 50°—80°C.

QUENCHING MEDIUMS In oils, the increase of temperature increases the

persistence of vapour-blanket, but this resulting decrease in the cooling rate is more than compensated by the decrease of viscosity (with the rise in temperature) to result in increase of rate of heat removal through the oil.

If the boiling point of a coolant is low, vapours form easily to increase the ‘A’ stage of cooling. ¡t is better to use a coolant with higher boiling point. A coolant with low specific heat gets heated up at a faster rate to form vapours easily.

A coolant with low latent heat of vapourisation changes into vapour easily to promote ‘A’ stage, i.e., decreases the cooling rate.

A coolant with high thermal conductivity increases the cooling rate. Coolants with low viscoity provide faster cooling rates and decrease the ‘A’ stage.

QUENCHING MEDIUMS A coolant should be able to Provide rate of cooling

fast enough to avoid transformation of austenite to pearlite and bainite . Plain carbon steel invariably require çooling in water or brine. Whereby alloy steels are quenched normally in oils.

But milder the cooling medium , lesser the internal stresses developed , and thus lesser the danger of distortion , or cracks . An ideal quenching medium is one which is able to provide very fast cooling rate near the nose of the curve ( 650 -550°C)and at the same time it should provide very considerable slower rate if cooling within the range of martensitic transformation( 300 - 200°C) to minimize internal stresses .

WATER The oldest and still the most popular quenching medium,

water meets the requirements of low cost ,general easy availability, easy handling and safety.

The cooling characteristics change more than oil with the rise of temperature, specially there is a rapid fall in cooling capacity as the temperature rises above 60°C, because of easy formation of vapour-blanket.

The optimum cooling power is when water is 2O-4O°C. Thc cooling power of water is between brine and oils. Water provides high cooling power to avoid the

transformation of austenite to pearlite/bainite, but the major draw back is that it also provides high cooling rate in the the temperature range of martensite formation.

At this stage, the steel is simultaneously under the influence of structural stresses (non-uniform change in structure) and thermal stresses which increase the risk of crack formation.

BRINE Sodium chloride aqueous solutions of about 10% by weight

are widely used and are called brines. The cooling power is between 10% NaOH aqueous solution

and water. These are corrosive to appliances. The greater cooling efficiency of brines, or other aqueous

solutions is based as : In brine heating of the solution at the steel surface causes

the deposition of crystal of the salt on hot steel surface . This layer of solid crystals disrupts with mild explosive

violence, und throws off a cloud of crystals. This action destroys the vapour-film from the surface, and thus permits direct contact of the coolant with the steel surface with an accompanying rapid removal of heat.

Brines are used where cooling rates faster than water are requited.

OILS Oils have cooling power between water at 40°C to water at

90°C. In oil-quench, considerable variation can be obtained by the

use of animal, vegetable, or mineral oil, or their blends. Oils should be used at 50- 80°C when these are more fluid, i.e

less VISCOUS, which increases the cooling power. As the oils used generally have high boiling points, moderate

increase of temperature of oil does not very much increase the vapour blanket stage. However, oils in contrast to water, or brine, have much lower quenching power .

Its this relatively slow cooling rate in the range of martensitic formation is atlvantageous as it helps in minimsing the danger to crack formation.

Oils with high viscosity are less volatile, and thus have decreased vapour-blanket stage (increase thecooling rate). As lesser volatile matter is lost, their cooling power is not affected much with use.

POLYMER QUENCHANTS polymer quenchants cool rapidly the heated

steel to Ms temperature, and then rather slowly when martensite is forming .

Polymer quenchants are water-soluble organic chemicals of high ,molecular weights, and are generally polyalkylene glycol-based, or polyvinyl pyrolidene-based.

Widely different cooling rates can be obtained by varying the concentration of Organic additives in water; higher the additions, slower is the cooling rate of solution.

There are little dangers of distortions and cracks.

SALT BATHS It is an ideal quenching medium for a

steel of not very large section but with good hardenabilty.

Addition of O.3-O.5’% water almost doubles the cooling capacity. Normally holding time is 2-4 minutes/cm of section thickness.

Salt baths used for austenitising keeping the steel clean.

INTERNAL STRESSES S DURING QUENCHING internal stresses are produced due to non-uniform

plastic deformation. In quenching of steels ,this may be caused by thermal stresses, structural stresses, or both, or even premature failure of part in service.

Cooling during quenching lakes place non-uniformly, i.e., causes temperature gradient across the section.

Surface layers contract more than the central portion. Contraction of surface is resisted by the central portion,

and this puts the central portion under the compressive stresses, and the surface layers in tension .

If the magnitude of stress becomes more than the yield stress of steel,deformation occurs.

These stresses that develop in a quenched part as a result of unequal cooling are called thermal stresses.

INTERNAL STRESSES DURING QUENCHING Structural stresses are the stresses which develop due to

due to phase change (mainly austenite to martensite), and at different times.

Structural stresses are developed due to two main reasons:

1. Austenite and its transformation products have unequal specific volume i.e. change in volume occurs when transformation occurs.

2. Phase changes occur at different times in the surface and in centre.

Under right conditions, both types of stresses get superimposed to become larger than the yield strength to cause warping. but when the tensile internal stresses become larger than the tensile strength cracks appear.

If an austenitised steel is quenched, it contracts thermally till Ms temperature is attained .

INTERNAL STRESSES S DURING QUENCHING

INTERNAL STRESSES DURING QUENCHING figure(a) illustrates this in stage 1 As surface cools

faster than centre, i.e., contracts more than centre distribution of stresses across the section is illustrated in fig (b), i.e, the surface is under tensile nature of stress, while centre is under compressive stresses.

Only thermal Stresses are produced in stage 2 , surface having attained Ms temperature, transforms to martenSite, and thus expands, while the centre is still contracting as it is getting cooled.

In stage Il, centre may get plastically deformed ,as it is still ductile austenite.

In stage 3, martensite of surface and austenite of centre continue contracting leading to slight increase in stress levels.

INTERNAL STRESSES DURING QUENCHING In stage IV, centre has attained M5

temperature, and begins to expand as it forms martensite, while surface is still contracting.

The centre, as it expands, puts the surface in higher stress levels .

The surface has little deformation as it consists of brittle martensie.

It is during this stage, the greatest danger of cracking exists.

Thus, stress levels are highest not in the beginning of the quench, but when the centre is transforming to martensite.

INTERNAL STRESSES DURING QUENCHING However, higher is the Ms temperature of

the steel, lesser is the expansion, there is reduced danger of quench-cracking.

Increase of carbon and alloying elements lower the Ms temperature making the steel more prone to quench cracking.

RETAINED AUSTENITE Martensitic transfomiation is essentially an athermal

transformation. Austenite begins to transform to martensite at Ms, and the

amount of martensite formed increases as the temperature decreases to complete at Mf temperature.

Less than 1 % of austenite may not transform because of unfavourable stress conditions.

The Ms and Mf temperatures are lowered as the amounts of carbon content and alloying elements(except cobalt and aluminium) increase in the steel.

In a quenched steel, the amount of martensite formed depends on the location of Ms and Mf and the temperature of the coolant (which is normally room temperature. As long as room temperature lies between Ms and Mf temperatures, austenite does not transform completely to martensite as it has not been cooled below Mf temperature.

This untransformed austenite is retained austenite.

ADVANTAGES OF RETAINED AUSTENITE 10% retained austenite is normally desirable

as its ductility relieves some internal stresses developed during hardening. This reduces the danger to distortion or cracks.

The presence of 30-40% retained austenite makes straightening of components possible after hardening.

Non distorting tools owe their existence to retained austenite . It tries to balance transformational volume changes during heating as well as cooling cycles of heat treatment to produce little overall change in size of the tools.

DISADVANTAGES1) The presence of soft austenite decreases the

hardness of hardened steels.2) As retained austenite may transform to lower

bainite, or martensite later in service ,increase in dimensions of the part occurs.

3) This creates problems in precision gauges or dies.

4) Stresses may develop in the part itself as well as in adjacent pans. Grinding-cracks are mainly due to retained austenite transforming to martensite.

5) Austenite is non-magnetic, decreases the magnetic properties of the steels.

ELIMINATION OF RETAINED AUSTENITE Retained austenite is generally undesirable. It is

eliminated by one of the methods:1. Sub-Zero treatment (cold treatment): It Consists of cooling the hardened steel (having retained

austenite) to a temperature below 0°C or its Mf temperature.

There is no reason to cool a steel much below its Mf temperature.

Sub-zero treatment is more effective if it is done immediately after the quenching operation (normally done to room temperature).

Sub-zero treatment is done in a low temperature-cooling unit, which consists of double-walled vessel.

The interior is made of copper in which the parts to be deep-frozen are kept, and the exterior is made of steel provided with good heat-insulation.

ELIMINATION OF RETAINED AUSTENITE The space in between the vessels is filled with

some coolant. The sub-zero coolants could be, dry ice (Solid

CO2) + acetone (— 78°C); Ice + NaCl (—23°C); liquid air (—183°C); liquid N2 (— 196°C); Freon (— 111°C).

Total time of cooling in this unit is 1/2 to 1 hour.

As this treatment transforms austenite to martensite, steels after sub-zero treatment have high hardness, wear and abrasion resistance, and have no danger of grinding-cracks.

ELIMINATION OF RETAINED AUSTENITE The stresses are increased further and thus,

tempering should be done immediately after sub-zero treatment.

Carburised steels, ball-bearing steels, highly alloy tool steels, are normally given cold treatment.

2. Tempering: The second stage of tempering eliminates the

retained austenite in most steels. In high alloy steels, multiple tempering is able to

eliminate the retained austenite during cooling from the tempering temperature.

DEFECTS IN HARDENING The main defects produced during hardening are:1. Mechanical properties not up to specifications: The common defect in hardened tools, or

component is low surface hardness. One or more of the followings could be the cause

of such a defect. Insufficient fast cooling due to overheated or even

polluted coolant could be responsible for a defect. The presence of scale, or oil, etc. on the surface

also decreases the cooling rate. Circulation of coolant, or agitation of component

may also result in such defect.

DEFECTS IN HARDENING A shorter austenitising time can also cause such a

defect. Lower austenitising temperature can also result in such a defect.

Decarburisation can also result in low surface hardness. If too high temperature had been used, which produces larger amount of retained austenite can result in low surface hardness.

2. Soft Spots: Soft areas on the hardened surface are called ‘soft spots’. The adhering scale, or decarburisation of some areas or

prolonged vapour-blanket stage due to overheated coolant or insufficient agitation or circulation of coolant, or rough surface could cause presence of soft spots surface.

ELIMINATION OF RETAINED AUSTENITE3. Quench Cracks: Quench cracks form as a result of internal stresses developed

of tensile nature exceeding the tensile strength of the steel. Steel with lower Ms temperature due to higher contents of

alloying elements are more prone to quench cracks. Higher carbon also results in more brittle martensite.

Quench cracks can form if there is more time lag between the process of quenching and tempering.

Overheating of steel or a wrong coolant which gave much faster rate of cooling, or there is faulty design of the component with sharp corners and sharp transition between sections, or a wrong steel has been chosen.

Presence of large amounts of retained austenite causes grinding cracks.

The other defects could be distortion and warpage; change in dimensions; oxidation and decarburisation

TEMPERING

Martensite is a very strong phase, but it is normally very brittle so it is necessary to modify the mechanical properties by heat, treatment in the range 150—700°C.

Essentially, martensite is a highly Supersaturated solid solution of carbon in iron which, during tempering, rejects carbon in the form of finely divided carbide phases.

The end result of tempering is a fine dispersion of carbides in an α-iron matrix which often bears little structural similarity to the original as-quenched martensite.

TEMPERING OF PLAIN CARBON STEELS In the as-quenched martensite

structure,the laths or plates are heavily dislocated to an extent that individual dislocations are very difficult to observe in thin-foil electron micrographs.

A typical dislocation density for a 0.2 wt% carbon steel is between 0.3 and 1.0 x 1012 cm cm-3. As the carbon content rises above about 0.3 wt%, fine twins about 5—10 nm wide are also observed.

Often carbide particles, usually rods or small plates, are observed (Fig. 9.1).

TEMPERING OF PLAIN CARBON STEELS

TEMPERING OF PLAIN CARBON STEELS These occur in the first-formed

martensite, i.e. the martensite formed near Ms, which has the opportunity of tempering during the remainder of the quench.

This phenomenon, which is referred to as autó-tempering, is clearly more likely to occur in steels with a high Ms.

STAGES OF TEMPERING

STAGES OF TEMPERING On reheating as-quenched martensite, the

tempering takes place in four distinct but overlapping stages:

Stage 1, up to 250°C — precipitation of E-iron carbide; partial loss of tetragonality in martensite.

Stage 2, between 200 and 300°C — decomposition of retained austenite .

Stage 3, between 200 and 350°C — replacement of &iron carbide by cementite; martensite loses tetragonality.

Stage 4, above 350°C — cementite coarsens and spheroidizes; recrystallization of ferrite.

TEMPERING — STAGE 1 Martensite formed in medium and high

carbon steels (0.3—1.5 wt% C) is not stable at room temperature because interstitial carbon atoms can diffuse in the tetragonal martensite lattice at this temperature.

This instability increases between room temperature and 250°C, when €-iron carbide precipitates in the martensite (Fig. 9.2).

This carbide has a close-packed hexagonal structure, and precipitates as narrow laths or rodlets on cube planes of the matrix with a well-definedorientation relationship .

TEMPERING — STAGE 1

TEMPERING — STAGE 1 At the end of stage 1 the martensite still possesses a

tetragonality, indicating a carbon content of around0.25 wt%.

It follows that steels with lower carbon contents are unlikely to precipitate €-carhide.

This stage of tempering possess an activation energy of between 60 and 80 kJ mo1, which is in the right range for diffusion of carbon in martensite. The activation energy has been shown to increase linearly with the carbon concentration between 0.2 and 1.5 wt% C.

This would be expected as increasing the carbon concentration also increases the occupancy of the preferredinterstitial sites, i.e. the octahedral interstices at the mid-points of cell edges, and centres of cell faces, thus reducing the mobility of C atoms.


TEMPERING — STAGE 2 During stage 2. austenite retained during

quenching is decomposed usually in the temperature range 230-300°C.

In martensitiC plain carbon steels below 0.5 carbon. the retained austenite is often below 2%, rising to around 6 % at 0.8 wt C and over 30 % at 1.25 wt C.

The little available evidence suggests that in the range 230-300°C, retained austenite decomposes to bainitic ferrte and cementite, but no detailed comparison between this phase and lower bainite has yet been made.

TEMPERING — STAGE 3 During the third stage of tempering,

cementite flrst appears in the microstructure as a Widmanstatten distribution of plates which have a well-defined orientation relationship with the matrix which has now lost its tetragonality and become ferrite.

This reaction commences as low as 100°C and is fully developed at 300°C, with particles up to 200 nm long and 15 nm in thickness.

Similar structures are often observed in lower carbon steels as quenched, as a result of the formation of Fe3C during the quench.

TEMPERING — STAGE 3 During tempering, the most likely sites for the

nucleation of the cementite are the €-iron carbide irterfaces with the matrix (Fig 9.2) and as the Fe3C particles grow, the €-iron carbide particles graduallydisappear.

The twins occurring in the higher carbon martensites are also site for the nucleation and growth of cementite which tends to grow alongthe twin boundaries forming colonies of similarly oriented lath shaped particles (Fig. 9.3) which can be readily ditinguished from the normal Widmanstatten habit.


TEMPERING — STAGE 3 A third site for the nucleation of cementite is

the grain boundary regions (Fig, 9.4)of both the interlath boundaries of martensite andthe original austenite grain boundaries.

The cementite can form as very thin films which are difficult to detect but which gradually sp1eroidise to give rise to welI-defined particles of Fe3C in the grain boundary regions.

There is some evidence to show that these. boundary cementite films can adversely affect ductility. However it can be modified by addition of alloying elements.

TEMPERING — STAGE 3 During the third stage of tempering , the

tetragonality of the matrix disappears and it is then, essentially, ferrite, not supersaturated withrespect to carbon.

Subsequent changes in the morphology of cementite particles occur by process where the smaller particles dissolve in the matrix providing carbon for the selective growth of the larger particles.


TEMPERING — STAGE 4 It is useful to define a fourth stage of tempering in

which the cementite particles undergo a coarsening process and essentially lose their crystallographic morphology, becoming spheroidized.

It commences between 300 and 400◦C, while spheroidizatiun takes place increasingly up to 700◦C.

At the higher end of this range of tempera.ture the martensite lath boundaries are replaced by more equi-axid fèrrite grain boundaries by a process which is best described as recrystallization.

The final result is an equi-axed array of ferritegrains with coarse spheroidized particles of Fe3C (Fig. 9.5), partly, but not exclusively, by the grain boundaries.

TEMPERING — STAGE 4 The spherodisation of the Fe3C is encouraged by the

resulting decrease in surface energy. The particles which preferentially grew and spheroidize

are located mainly at interlath boundaries and prioraustenite boundaries, although some particles remain in the matrix.

The boundary sites are preferred because of the greater ease of diffusion in these regions. Also, the growth of cementite into ferrite is associated with a decrease in density so vacancies are required to accommodate the growing cementite.

Vacancies will diffuse away from cementite particles which are redissolving in the ferrite and towards cementite particles which are growing, so that the rate controlling process is likely to be the diffusion of vacancies.

TEMPERING — STAGE 4 The original martensite lath boundaries remain

stable up to about 600°C, but in the range 350—600°C. there is considerable rearrangement of the dislocations within the laths and at those lath boundaries which are essentially low angle boundaries.

This leads to a marked reduction in the dislocation density and to lath-shaped ferritic grains closely related to the packets of similarly oriented laths in the originalmartensite.

This process, which is essentially one of recovery, isreplaced between 600 and 700°C by recrystallization which results in the formation of equi-axed ferrite grains with spheroidal Fe3C particles in the boundaries and within the grains.

TEMPERING — STAGE 4 This process occurs most readily in carbon

steels. At higher carbon content, the increased

density of cementite particles is much more effective in pinning the ferrite boundaries, so recrystallisation is much more sluggish.

The final process is the continued coarsening of the cementite particles and gradual ferrite grain growth.

ROLE OF CARBON CONTENT Firstly, the hardness of the as-quenched martensite is

largely influenced by the carbon content, as is the morphology of the martensite laths which have a up to 0.3 wt% C, changing at higher carbon contents.

The Ms temperature is reduced as the carbon content increases, arid thus the probability of the occurrence of auto-tempering is less.

During fast quenching in alloys with less than 0.2 wt % C, the majority (up to 90%) of the carbon segregates to dislocations and lath boundaries, but with slower quenching some precipitation of cementite occurs.

On subsequent tempering of low carbon steels up to 200°C further segregation of carbon takes place. but no precipitation has been observed.

ROLE OF CARBON CONTENT

ROLE OF CARBON CONTENT Under normal circumstances it is difficult to

detect any tetragonality in martensitic in steels with less than 0.2 wt% C, a fact which can explained by the rapid segregation of carbon during quenching.

The hardness change; during tempering are also very dependent on carbon content, as shown in figure for steels up to 0.4 wt% C.

Above this concentration, an increase in hardness has been observed in temperature range 50—150°C, as €-carbide precipitation strengthens the martensite.

However, the general trend is an overall softening, as the tempering temperature is raised.

MECHANICAL PROPERTIES OF TEMPERED PLAIN CARBON STEELS The absence of other alloying elements means that

the hardenability of the steels is low, so a fully martensitic structure is only possible in thin sections.

However, this may not be a disadvantage where shallow hardened surface layers are all that is required.

Secondly, at lower carbon levels, the Ms temperature is rather high, so autotempering islikely to take place.

Thirdly, at the higher carbon levels the presenceof retained austenite will influence the results.

Added to these factors, plain carbon steels can exhibit quench cracking which makes it difficult to obtain reliable test results. This is particularly the case at higher carbon levels, i.e. above 0.5 wt% carbon.

TEMPERING OF ALLOY STEELS The addition of allying elements to a steel has a

substantial effect on the kinetics of the y →α transformation, and also of the pearlite reaction.

Most common alloying elements move the TTT curves to longer times, with the result that it is much easier to miss the nose of the curve during quenching.

This essentially gives higher hardenability, since martensite structures can be achieved at slower cooling rates and, in practical terms, thicker specimens can be made fully martensitic.

Alloying elements have also been shown to have asubstantial effect in depressing the Ms temperature.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES It is clear that certain elements, notably silicon

can stabilize the €-iron carbide to such an extent that it is still present in the ‘microstructure after tempering at 400°C in steels with 1-2 wt% Si, and at even higher temperatures if the silicon is furtherincreased.

The evidence suggests that both the nucleation and growth of the carbide is slowed down and that silicon enters into the €-carbide structure.

It is also clear that the transformation of €-iron carbide to cementite is delayed considerably.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES While the tetragonality of martensite

disappears by 300°C in plain carbon steels, in steels containing some alloying elements, e.g. Cr, Mo, W V, Ti, Si, the tetragonal lattice is still observed after tempering at 450°C and even as high as 500°C .

It is clear that these alloying elements increase the stability of the supersaturated iron-carbide solid solution.

In contrast manganese and nickel decrease the stability.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES Alloying elements also greatly influence

the proportion of austenite retained on quenching.

Typically a steel with 4% molybdenum, 0.2%C, in the martensitic state contains less than 2% austenite, and about 5% is detected in a steel with 1% vanadium and 0.2%C.

The austenite can be revealed as a fine network around the martensite laths, by using dark field electron microscopy.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES On tempering each of the above steels at 300°C,

the austenite decomposes to give thin grain boundaryfilms of cementite which, in the case of the higher concentrations of retained austenite, can be fairly continuous along the lath boundaries.

It is likely that this interlath cementite is responsible for tempered embrittlement frequently encountered as a toughness minimum in the range 300—350°C, by leading to easy nucleation of cracks, which then propagate across the tempered martensite laths.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES Alloying elements can also restrain the coarsening of

cementite in the range 400-700°C, a basic process during the fourth stage of tempering.

Several alloying elements, notably silicon, chromium.molybdenum and tungsten, cause the cementite to retain its fine Widmanstatten structure to higher temperatures, either by entering into the cementite structure or by segregating at the carbide-ferriteinterfaces.

Whatever the basic cause may be, the effect is to delaysignificantly the softening process during tempering.

THE EFFECT OF ALLOYING ELEMENTS ON THEFORMATION OF IRON CARBIDES This influence on the cementite dispersion has

other effects, in so far as the carbide particles, by remaining finer, slow down the reorganization of the dislocations inherited from the martensite, with the result that the dislocation substructures refine more slowly.

In plain-carbon Steel cementite particle begin to coarsen in the temperature range 350 -400°C and addition of chromium, silicon, molybdenum or tungsten delays the coarsening to the range 500-550°C.

It should be emphasized that up to 500°C the only carbides to form are those of iron.

THE FORMATION OF ALLOY CARBIDES -SECONDARY HARDENING A number of the familiar alloying elements in

steels form carbides, nitrides and borides which are thermodynamically more stable than cementite.

It would therefore be expected that when strong carbide elements are present in a steel in sufficient concentration, their carbides would be formed in preference to cementite .

Nevertheless during the tempering of all ahoy steels, alloy carbides do not form until the temperature range 500-600°C , because below this the metallic alloying elements cannot diffuse sufficiently rapidly to allow alloy carbides to nucleate.

THE FORMATION OF ALLOY CARBIDES -SECONDARY HARDENING The metallic elements diffuse substitutionally, in

contrast to carbon and nitrogen which move through the iron interstitially.

With the result that the diffusivities of carbon and nitrogen are of several orders of magnitude greater in iron than those of the metallic alloying elements.

Consequently higher temperatures are needed for the necessary diffusion of the alloying elements.

It is this ability of certain alloying elements to form fine alloy carbide dispersions in the range 500—600°C, which remain very fine even after prolonged tempering, that allows the development of high strength levels in many alloy steels.

THE FORMATION OF ALLOY CARBIDES -SECONDARY HARDENING Indeed, the formation of alloy carbides

between 500 and 600°C is accompanied by a marked increase in strength, often in excess to that of the as-quenched martensite.

This phenomenon, which is referred to as

secondary hardening, is best shown in steels containing molybdenum, vanadium, tungsten, titanium, and also in chromium steels at higher alloy concentrations.

THE FORMATION OF ALLOY CARBIDES -SECONDARY HARDENING This secondary hardening process is a type

of age-hardening reaction, in which a relatively coarse cementite dispersion is replaced by a new and much finer alloy carbide dispersion.

On attaining a critical dispersion parameter, the strength of the steel reaches a maximum, and as the carbide dispersion slowly coarsens, the strength drops.

THE FORMATION OF ALLOY CARBIDES -SECONDARY HARDENING The process is both time and temperature

dependent, so both variables are often combined in a parameter:

P = T(k + log t)

where: T is the absolute temperature and t the tempering time in hours, while k is a constant which is about 20 for

alloy steels, usually referred to as the Holloman-Jaffe parameter.

COMPARISON OF

VARIOUS HEAT-TREATMENT

PROCESSES

STRESS VS STRAIN FOR STEEL SUBJECTED TO DIFFERENT HEAT TREATMENT PROCESSES

COMPARISON OF MICROSTRUCTURES

ANNEALED VS HARDENED METALS

PHYSICAL PROPERTIES: Annealed metals are relatively soft and can be cut and shaped more easily. They bend easily when pressure is applied. As a rule they are heated and allowed to cool slowly.

The animation above shows that an annealed metal is usually softer and can be deformed more easily than metals that are not annealed.

PHYSICAL PROPERTIES: Hardened metals are difficult to cut and shape. They are very difficult if not impossible to bend. As a rule they are heated and cooled very quickly by quenching in clean, cold water.

The animation above shows that metals that have not been annealed are very difficult to deform.

THANK YOU…

Ashish Kumar GoudaDept. of Metallurgical & Materials Engg.

NIT,Rourkela,India

Engineering

Heat treatment of Steels