Science of Heat Treatment of Metals | Metallurgy

In this article we will discuss about:- 1. Introduction to Heat Treatment 2. Types of Phase Diagrams in Heat Treatment 3. Benefits of Alloy Hardening 4. Variables.

Introduction to Heat Treatment:

The properties of metals and alloys which are all so important to the engineer and the user, are intimately related to the micro-structure. Micro-structure is defined in a broad sense not only by the microscopically visible structure, i.e. number, type, amount, size, shape and distribution of phases, but also, by features such as, solute concentration and defect structure.

The control of micro-structure is done by controlling the:

(a) Composition, which controls the phases present, their proportion and morphology through, for example, transformation characteristics.

(b) Heat treatment, which also affects the amount, size, shape, number and distribution or phases together with the grain size, composition of phases by an equilibrium, or non-equilibrium partitioning, dislocation structure and defect structure.

(c) Hot and cold working, which affect some of the above features along with the crystallographic textures developed by the phases in the structure.

Thus, the science of heat treatment deals with the factors and mechanisms, which control the micro-structures of metals and alloys, and also to develop an understanding between the micro-structure & the resulting properties. In the last 30 years, much progress has been made quantitatively relating the micro- structure to the mechanical properties, particularly the strength, which can now be predicted from the composition and micro-structure with reasonable assurance.

In more complex structure like tempered martensite, some semi-empirical approach had been adopted for the time being, but computers could be now used to predict the properties, or choose the composition to obtain properties within reasonable limits.

It is necessary to emphasize here that heat treatment is used more frequently to produce metastable phases than stable phases. In reality, almost all commercial materials are used in the form of metastable state. For example, martensite in hardened steel is a metastable phase, and the equilibrium ferrite and cementite aggregate is of limited use. Similarly, all age-hardened micro-structures correspond to metastable states.

The stable phases and associated configurations usually possess relatively poor mechanical properties. Alloys which are useful in the annealed state, that is, in almost stable state, are exceptions rather than the rule, examples being electrical conductor, soft magnetic alloys, etc. The possibility of changing the micro-structure and the morphology of phases to various states is widely exploited in heat treatment. A metastable phase can exist for an indefinite period at room temperature.

Alloying a metal is essential to derive the major benefits of heat treatments, like easily manageable high hardness and strength. A pure metal can be hardened but only a bit by freezing vacancies and the process is difficult. There is no heat treatment method to increase hardness of pure metals (except by grain refinement if the metal shows polymorphism), though heat treatments like recrystallization annealing, stress-relieving annealing, etc. are practised but these normally decrease the hardness of pure metals.

To derive the benefits of heat treatments, alloying is essential for two main reasons:

I. Thermodynamic:

Alloying a metal generally increases the possibility of having more number of phases in the solid alloys. For example, each of the two schematic binary phase shows presence of two phases in most of the alloys at room temperature instead of one as in a pure metal. Alloying also increases the possibility of having metastable phases by heat treatments.

Heat treatment could be used to control and change the nature, amount, number, shape, size and distribution of phases or metastable phases. Thus the micro-structure of the alloys could be changed and controlled thereby resulting in changes in properties of the materials. For example, by adding carbon in iron, a two phase mixture of lamellar pearlite can be obtained. By varying the heat treatment, austenite can be made to decompose to lamellar pearlite-one coarse and the other fine pearlite with hardness of steel Rc 15 and Rc 40 respectively.

Because of smaller interlamellar spacing in fine pearlite, the motion of dislocation in ferrite is more easily and more frequently blocked by the cementite plate of pearlite (smaller the spacing, more quick is the pile up of dislocations) i.e. the dislocation finds resistance to its motion, i.e. there is resistance to deformation which means, the hardness and strength of fine pearlite is more than of coarse pearlite.

It is possible to change the shape of the pearlite by heat treatment, that is, lamellar pearlite can be changed to spheroidised pearlite [Fig. 1.1 (c)] where spheres of cementite are embedded in ferrite matrix. This change in micro-structure reduces the strength and hardness of the steel but the steel becomes more ductile and machinable.

Fig. 1.1 illustrates that for the same carbon content of the steel, hardness of spheroidised pearlite is always less than the steel with coarse lamellar pearlite (in annealed state). The spheroidised structure offers less resistance to motion of dislocations as the dislocations bypass the spheres of cementite similar to illustration in Fig. 1.7. Thus, the possibility of having more phases by alloying and by just changing the size and shape of phases, properties can be varied.

 

Alloying a metal may give rise to a binary phase diagram in which eutectoid reaction takes place. Eutectoid reaction is the main basis of heat treatment in steels. It is the polymorphic change in iron (where alpha iron changes to gamma iron on heating and vice versa on cooling) and the difference in solid solubilities of carbon in gamma iron and in alpha iron which are responsible for various heat treatments in steels.

Alloying a metal may give rise to a binary phase which shows a decrease of solid solubility with the fall of temperature. This is the main requirement for an alloy system in which hardening takes place by precipitation hardening. This method of hardening is the base of heat treatment of non-ferrous alloys, specially the aluminium alloy.

Order hardening can occur only in alloys, where a heat treatment produces fine-scale domain structure. Though order hardening is not commonly used but it is an important base in some alloys of noble metals.

II. Kinetics of Process:

In a broad sense, it can be said that most of the heat treatments are based on the following two principles:

1. Precipitation hardening

2. Eutectoid transformation

In both of them, heating is done of a duplex structure to obtain a single homogeneous phase at a predetermined high temperature. The rate of decomposition of high temperature phase to the equilibrium duplex structure during and after quenching is quite slow due to the difficulties in the nucleation and also due to slower diffusion at lower temperatures, that is, as the kinetics of the duplex-phase-formation is slow, it can be retarded, or avoided, and new metastable phase or phases can be made to form.

Alloying helps in slowing this kinetics of duplex-phase-formation. Thus, a much thicker-part can be leisurely-quenched to get supersaturated solid solution or metastable phase, and thus useful heat treatment cycles are possible.

For example, 0.77% carbon steel having 100% pearlite in micro-structure on heating properly to 760°C has now fine-grained homogeneous austenite. As the rate of transformation of austenite to pearlite is a relatively slow process, and thus, it can be retarded or even avoided by having faster cooling rates than the equilibrium cooling rate to obtain other metastable-phases like martensite or even, bainite.

Table 1.3 illustrates various phases (stable or metastable) obtained by different heat treatments of 0.77% carbon steel with their resulting properties like hardness and strength. It is mainly the alloying of iron with carbon, which makes the metastable-phase martensite to be very hard, and the presence of other alloying elements facilitates its formation in steels.

Types of Phase Diagrams in Heat Treatment:

The type of phase-diagram of an alloy-system plays a very important role in the heat treatment rather; it is the basis for analysis of heat treatment of alloys. The phase-diagram should show either, decrease of solid solubility with the fall of temperature or should show eutectoid transformation along with the allotropic changes to get feasibility of heat treatments.

The phase diagram helps to know:

1. Types of heat treatment possible in the alloy-system.

2. The alloy compositions most suitable for the heat treatment.

3. Correct temperature range of the heat treatment cycles.

4. Overheating and burning temperature ranges.

Fig. 1.3 (a) illustrates a schematic phase-diagram having a solvus TP, which shows decrease of solid solubility with the fall of temperature. Such a phase-diagram illustrates the feasibility of heat treatment of some of the alloys by precipitation and dispersion-hardening methods. All the alloys to the left of point P, say, a composition like X does not undergo any change on heating (except unwanted grain growth) and thus cannot be heat treated by precipitation-hardening.

All the alloys to the right of point P (when dissolution of excess theta phase in alpha solid solution takes place on heating) can be obtained by quenching from high temperature in a supersaturated solid solution state, and which on subsequent ageing causes precipitation of the excess metastable phase/phases causing thereby precipitation-hardening.

All alloys between the compositions of point P and S can fully dissolve excess of theta on heating (just above solvus temperature) to form homogeneous alpha solid solution, and thus, every section of the micro-structure changes by heat treatment.

Ageing also causes precipitation in every section of microstructure. In alloys to the right of point S, a portion of theta phase remains undissolved on heating to the highest point without melting (fusion is not allowed in heat treatments) and does not change in the heat treatment. The beneficial effects of heat treat­ment shall be most pronounced in the alloy of composition at point, S.

Fig. 1.3 (b) illustrates a schematic phase-diagram showing polymorphic change and eutectoid reaction (γ on cooling gives α + β). Such a phase-diagram is the basis of heat treatments in steels. The high temperature phase, gamma may not be allowed to transform to equilibrium phases by faster cooling such as obtained by water-quenching. For example in steels, austenite is made to transform to martensite, which is defined as a supersaturated solid solution of carbon is ferrite, and which can be tempered to obtain desired combination of mechanical properties in the steels.

Pure metals and alloys can be given some heat treatments (not involving phase transformation, or phase dissolution) such as recrystallisation annealing or stress-relieving annealing etc. but none of them increases hardness and strength. The chemical-thermal treatments can be analysed based on some variation of diagrams as illustrated in Fig. 1.3 (a) and (b). The thermo-mechanical treatments imply a combination of heat treatment and mechanical deformation, and are used to derive certain benefits.

 

Benefits of Alloy Hardening in Heat Treatment:

Alloying a metal is essential to derive the benefits of heat treatments. It has been used in many different ways to strengthen the metals. The most important general method is to obstruct the motion of dislocations by a fine dispersion of foreign particles, which are uniformly distributed throughout the matrix and thus produce increased strength and hardness in alloys.

These particles may be present in the following forms:

(a) Single solute atoms – This causes solid solution strengthening.

(b) Large clusters of solute atoms, or even precipitates of separate phases, which may be coherent, semi-coherent, or incoherent (or a mixture of them) with the matrix as happens in precipitation hardening and dispersion hardening.

The main features of the generalised method of alloy hardening can be understood by analysing the particular cases of hardening (by heat treatment) taking place in plain carbon steels and in aluminium alloys. In steels, ferrite-carbide mixture is heated to fine grained homogeneous austenite, which on quenching produces a supersaturated solid solution, called martensite. Martensite is very hard and little further hardening occurs on tempering (which is quite similar to ageing process that is, keeping martensite at a temperature for a definite time), which brings the supersaturated carbon out of solution as fine carbide precipitates.

The steel becomes soft again if tempering is continued to cause coagulation and coarsening of carbide particles. In aluminium alloys, duplex phase mixture (α + θ) is heated above solvus to get a solid solution (single phase alpha) and then quenched. The quenched supersaturated alpha solid solution is quite soft and ductile and thus can be cold worked (rivets made of such alloys are upset in this state).

But very large hardening (by a factor of five) takes place on ageing as coherent and semi-coherent fine precipitates are formed. Aluminium alloy also softens if over-ageing takes place due to coagulation and coarsening of precipitates.

Let us compare salient features of these two cases. Steel is hard in quenched state, whereas aluminium alloy is soft in quenched state (as it hardens only on ageing), although both are in supersaturated solid solution state after cooling cycle of the heat treatments. The reason appears to depend on several effects.

One important effect is based on the analysis of solid solution hardening. Measurements of solid solution hardening have shown that alloying atoms, which produce tetragonal, or, other non-spherical lattice distortions, harden the alloy much more strongly than those that produce spherically symmetrical distortions.

This fact becomes very clear as the increase in shear yield stress, σ with atomic fraction c of alloying atom, that is, d σ/d c in dilute solid solutions is of order 3 G for the former and 0.1 G for the latter, where G is the shear modulus. Thus, interstitial solute atoms like carbon in BCC-iron (as well as in other transition metals) produce great solid solution hardening, whereas the substitutional solute such as copper in quenched aluminium alloys does not. In steels, carbon atoms (radius of 0.077 nm) in BCC-iron has been seen to sit in octahedral sites (radius of 0.019 nm), which is smaller in size than tetrahedral site (radius of 0.036 nm).

The octahedral site is non-symmetrical, that is, out of six nearest iron atoms, two are much closer than the remaining four (Fig. 1.4 a). When carbon sits in the octahedral site, it displaces each of these two iron atoms (atoms X and Y of Fig. 1.4 a) by 0.053 nm in one of the <100> directions causing tetragonal distortion of the lattice (Fig. 1.4 b), where only one carbon atom has been shown to displace the iron atoms. With higher carbon concentration of martensite, more interstitial sites are filled.

Martensite is a highly supersaturated solid solution of carbon in ferrite. The crystal structure gets one axis, the C-axis elongated as compared to the other two parameters ‘a’ of the unit cell and thus becomes body-centered tetragonal, BCT (when carbon in steel is more than 0.2%). With still more carbon in steel, tetragonality increases.

Thus, carbon in martensite causes tetragonal distortion, or non-spherical distortion. The main reason, probably, of high hardness of steel in quenched state, that is. high hardness of martensite is that the non-spherical distortions (due to carbon in it) can interact with shear stress fields as well as hydrostatic stresses of screw dislocations and edge dislocations, that is, it obstructs the motion of both screw and edge dislocations very strongly.

On the other hand, the spherically symmetrical distortion (such as produced by substituting copper atoms in the quenched aluminium alloys) can relieve hydrostatic stresses of edge dislocations only, that is, it obstructs the motion of edge dislocations only, leaving the screw dislocations to move relatively freely and thus, cannot cause high hardness, and that is, probably, why quenched supersaturated solid solution of aluminium-copper alloys are soft and ductile.

Another important additional effect in quench-hardening of steel is that by shear mechanism of transformation, a large number of dislocations are produced in martensite (density of dislocations in Lath type of martensite is 10¹¹ to 10¹² cm^-2) during quenching which also adds to the hardness of martensite.

Aluminium alloy is soft in as-quenched state but hardens on ageing. Duralumin (2024 – T6) alter ageing has tensile strength of 500 MN m-2 ( ~ 50 kgf/mm2) as compared to tensile strength of aluminium alloy of 100 MNm-2. An increase in strength by a factor of five makes it and other alloys such as Al-Zn alloy (7075 – T6, having a strength of 550 MN m-2) very suitable as air-craft-structural materials.

Aluminium containing up to 2.5% lithium is being used in military aircrafts as lithium having relative density of only 0.534, and the alloys containing it, have relative density of low value 2.54 with increase in specific modulus of up to 25%. Such alloys shall form 30% of the aircraft-structural materials (the hardening in these alloys in due to coherent phase δ’ – Al₃ Li) in coming years.

Ageing causes clustering of solute atoms, which are called Guinier Preston (GP) Zones, or, formation of fine intermediate precipitates, which form more quickly than equilibrium precipitate. The greatest precipitation-hardening is usually associated with large volume fraction of most finely dispersed form of GP zones having particle density of 1017 to 1018 cm-3 or intermediate precipitates, or a mixture of them (10 – 100 A° in size) without precipitate free zones.

Obstacles to the motion of dislocations are the internal strains around precipitates. The more complex the crystal lattice of the strengthening phase, and more its com­position differs from the main solid solution, greater is the strengthening effect of age-hardening.

Several factors cause hardening by ageing-process, but two most general ones are:

1. A moving dislocation cuts through the particles lying across its path in the slip plane. Fig. 1.5 illus­trates schematic cutting of particles by a moving dislocation, whereas an electron-micro­graph of such a particle. Additional interface (which may include interfacial dislocation) is created between the particles and the matrix, or an increase in number of solute-solvent bonds across the slip planes takes place reversing the clustering process.

Additional work must be done by the applied stress for this to occur, the magnitude of which is controlled by factors such as relative atomic sizes of concerned atoms and the difference ill Stacking-fault-energy between the matrix and the precipitate.

The stronger the GP zones (or intermediate precipitates) and greater their modulus of elasticity, more difficult it is to cut them through by the moving dislocations, and higher becomes the strength and hardness of the alloy. Maximum age-hardening, in an alloy occurs when there is present a critical dispersion of GP zones, or intermediate precipitates or both.

2. When the particles become large sized (with ageing) and more widely spaced (about 1000 A in size), the dislocation line forms loops from one particle to the next so that the particles can present their full resistance to the dislocation motion. Fig. 1.7 illustrates schematically, as to how a dislocation first bends and then, at high stresses passes around the particles. At increasing stresses, the dislocations form closed dislocation loops around the particles (Fig. 1.7 c). The dislocation continues moving further leaving loops around the particles (Fig. 1.7 e). These loops, or rings naturally impede the motion of new incoming dislocations.

The critical stress, i.e. the yield stress, σ_y required to force the dislocation through, is inversely proportional to the distance, R between the particles:

where, b is the burgers vector of dislocation, G is the shear modulus of the matrix. As the distance between the particles is now reduced due to the development of loops or rings around them, the stress required to move the new incoming dislocation between them increases, that is, the yield stress of the material increases. In case of incoherent particles, the dislocations can only pass around them. This is called ‘Orowan’s ripening’ of particles. Relatively, the yield strength of the alloy is low, but rate of work hardening is high. This happens when the alloy is overaged.

Special interesting situation arises, when the precipitates are present which can resist shearing by dislocations and yet be too closely spaced to allow by-passing by dislocations. High levels of both strengthening and work hardening take place.

Age hardening has been extensively used to raise the strength, heat resistance and certain physical properties of many Aluminium-, iron-, copper-, nickel based alloys.

The concept of precipitation arising out of decrease of solid solubility with the fall of temperature has been exploited in many heat treatment processes directly or indirectly.

The strengthening of HSLA (high strength low alloy) steels by micro alloying, having total concentration of solute elements of 0.2% such as of niobium, vanadium, etc., takes place though by obtaining fine ferrite grain size of ASTM No 12-14, but has been possible by precipitation of fine size particles due to fall of temperature during controlled rolling to obtain yield strength of 400-500 MPa; tensile strength of 600 – 650 MPa with 20-22% elongation alongwith good weldability.

In creep resistant super alloys, precipitates form and cause increase of strength at high temperatures such as in nickel-base super alloys, where low interface-energy particles of Ni₃ (Ti, Al) provide creep resistance. Heat treatment obtains these particles. The process of tempering of highly alloyed tool steels causes the precipitation of finely dispersed transitory carbides to provide high hardness at high temperatures called ‘red hardness’ or ‘secondary hardness’. Fine dispersion of alloy nitrides in niter-alloy during nitriding is the reason of high hardness, even at high temperatures (≈ 500°C).

Most of the heat treatments of alloys are based in principles on- (I) eutectoid transformation or (II) precipitation hardening, or some variation of these processes.

The scientific principles of these two methods have been tactfully exploited in most of the heat treatments of materials. The science of heat treatment requires complete understanding of these principles and using them to advantage as per service requirements.

Heat Treatment Variables:

The microstructural and compositional (constitutional) characteristics of an alloy to a large extent control all the structure-sensitive properties. After an alloy has been carefully and rightly designed (the right composition chosen) for a component, based on the required combination of properties for a particular application, the properties in the finished product are not the only criterion, albeit the most important ones, because the product requires to be manufactured.

It is essential to ensure that throughout the whole of the manufacturing process, the microstructure at any given stage is appropriate for the most efficient and effective processing at the next stage and so on to the final product. The economics of every stage in the microstructural design of an alloy must be considered carefully and optimised, so as to produce the best combination of properties at the lowest cost.

The over-design leads to inefficient use of expensive materials and processing techniques, and under-designed alloy docs not give good serviceability. High purity of the alloys and their correct and well-controlled heat treatment constitute the chief criterion in conferring properties confirming within narrows limits to a definite specification.

Any form of heat treatment is directly related to the changes in the nature, form or distribution of the microstructural constituents. These changes result from the effect of temperature on phase equilibrium. The form of micro-constituents is effected by the conditions under which they separate from solid solutions and by the tendency to assume shapes at temperatures that permit diffusion and atomic rearrangement.

The atomic mobility is greatly accelerated by the application of heat. The higher the temperature of heating, the greater and faster will be the tendency of the reaction, provided the temperature of the reaction is higher than the temperature at which equilibrium exists. If the reaction occurs on cooling, the greater the amount of super-cooling, the greater is the rate of transformation and greater the effect on the microstructure though during cooling, diffusion continues to become slower.

The main aim of any heat treatment process, whether at some intermediate stage of processing, or in the final stages, is to produce the desired changes in the microstructure of metals and alloys. There are a number of factors, or heat treatment process variables, which have to be chosen properly and controlled as these affect the attainment of proper microstructures and thus, the ensuing properties in the metals and alloys.

The important variables which affect the heat treatment process are:

(a) Temperature of heating (generally austenitising),

(b) Time of heating and soaking,

(c) Rate of heating,

(d) Rate of cooling,

(e) Furnace atmosphere, etc.

The process variables also include variables during multiple heating stages, interrupted heating, cooling to sub-zero temperatures, interrupted quenching, multiple tempering, etc. The chemical composition of alloy, original microstructure of the part, size and shape of the part, ultimate properties desired in the part and economics of the process, or processes control the exact method, or duration of the cycle, or cycles.

The temperature of heating (generally austenitising) is decided by the:

(i) Composition of the steel,

(ii) Type of heat treatment to be given to it but the temperature should be a bit higher than the equilibrium temperature to obtain fast rate of transformation in a reasonable time.

At higher temperatures, there takes place increase in:

(i) Rate of nucleation of austenite,

(ii) Rate of growth of austenite,

(iii) Diffusion, etc.

The importance of increasing austenitising temperature on time of austenite formation in an eutectoid steel having lamellar pearlitic structure. The amount of austenite formed is plotted as a function of austenitising time at two different temperatures, 751°C and 730°C. The austenite formation requires some incubation period for the first nuclei to form and then proceeds at a more rapid rate as more nuclei develop and grow.

The time dependence is expected because the transformation requires lot of diffusion of carbon to produce an austenite containing 0.77% carbon from low carbon ferrite (0.02%C) and high carbon (6.67%) cementite of pearlite. With increasing temperature of austenitisation, the rate of nucleation of austenite grains increases more intensively than the linear rate of growth, resulting in finer grains of austenite.

Hence, by heating quickly to a high temperature and soaking for a short time only at that temperature, it is possible to obtain fine grained austenite, but normally it is difficult to exactly control these conditions in industrial practice. The incubation period is less at high temperature as well as the time to complete transformation to austenite is less as diffusion is faster. Once nucleation has been completed, the extra time of soaking can cause grain coarsening of austenite at that high temperature of austenitisation.

Among the reasons for keeping the austenitising temperature as low as possible are the increased tendencies towards:

(i) Grain growth

(ii) Cracking and distortion

(iii) Oxidation (scale formation) and decarburisation

(iv) Higher cost of fuel.

If temperatures are high:

The total time of heating is the sum of the time for raising the temperature to the austenitising temperature up to the centre of the part (t_h) plus the time of soaking (t_s) at the austenitising temperature:

t_total = t_h + t_s

The heating time depends on:

(i) Size of the part

(ii) Shape of the part

(iii) Thermal conductivity of the steel

(iv) Heat transfer coefficient of the .medium

(v) Temperature of the heating medium

(vi) Arrangement of the parts in the furnace.

The heating time (t_h) is more for thicker parts. Comparing the simple shapes (Fig. 1.9) of the same steel under similar conditions, the relative heating time (t_h) is 1 for the sphere, 2.5 for the parallelopiped, 2 for the cylinder, and 4 for the plate. The thermal conductivity of steels is reasonably good, but depends to a considerable extent on its composition, i.e. amount of carbon and the alloying elements.

As the alloy content increases, the thermal conductivity decreases and thus, stainless steels need more heating time than plain carbon steels. The heating of the components is usually done in gaseous medium (hot air, or in fuel combustion products), molten salts or molten metals. Salts, or liquid baths have generally higher heat transfer coefficients and thus, provides quicker heating. The relative heating time for hardening is 1 in gaseous medium, 0.5 in molten salts and 0.25 in molten metals.

The use of fluidized-bed-heating reduces the heating time of parts quite considerably. Since, the heating rate is a function of the difference of the temperature between the part and the heating medium, rapid heating may be obtained by using a heating medium at a temperature well above the required heating temperature and removing the part from the furnace when the temperature has been attained.

Heating time is also dependent on the arrangement of the parts in the furnace, that is, if parts of the same size and shape are put in a furnace in such a way that one of them is uniformly heated from all sides, the second gets heated up from three sides, and the third is heated from only one side, then the relative heating time shall be in ratio 1:1.5:4.

As there are a large number of factors, the heating time in practice may vary from 1-2 minutes (for thin sections less than 1 mm in a salt bath) to many hours for large parts in air furnace. A safe rule of heating time is one hour for 25 mm of section. For steels, having alloying elements particularly strong carbide forming elements, the austenitising temperature and time under given condition often have to be experimentally determined.

Soaking time (t_s) depends on the rate of transformation of pearlite to austenite, which itself depends on the amount of superheating above the equilibrium temperature and the dispersion of the ferrite and cementite in pearlite (also on the amount and type of alloying elements if present).

The larger the size of the part, lower should be the rate of heating, to avoid development of internal stresses during heating. Lower heating rates also ensure the surface and the centre of the part to be nearly at the same temperature at any instant, which also ensures almost simultaneous phase change (at the surface as well as at the centre). This reduces the soaking time of the part. If higher heating rates are used for thick parts, there could be large difference of temperatures at the surface and at the centre as heat conduction takes its own time.

Extra time is needed to raise the temperature of the centre of the part. Thus, the soaking time needed is more if higher is the heating rate of thick parts. Normally, intricate-shaped parts, or parts with varying thicknesses or diameters are heated slowly. When a thin-section part is heated at a fast rate, the surface and the centre attain the same temperature almost simultaneously.

The transformation to austenite takes place almost simultaneously. The homogenising of austenite too takes place in a short time, because the changes occur at higher temperature levels, where diffusion and related phenomenon are faster. The soaking time too is small or negligible.

The variable ‘cooling rate’ depends on the heat treatment fixed for that steel. Increasing the cooling rate (for diffusion based transfor­mation of austenite to occur), increases the super-cooling, which increases the rate of nucleation and the growth of new phase, but diffusion becomes slower resulting in finer dispersion of new equilibrium phases, or phases.

Further increasing the cooling rate may partly avoid the new phase formation, or even completely avoid the formation of the equilibrium phases, thus, make high temperature phase metastable at low temperature, or the formation of a new metastable phase or phases may occur. This is being illustrated in Fig. 1.10. It shows the general range of composition and temperature for the occurrence of different diffusional transformations under isothermal conditions as well as for diffusion less transformation after avoiding diffusional transformations.

In the area marked “ferrite” and at temperatures higher than Ae₁, austenite only partially transforms to ferrite as the equilibrium phases at these temperatures are ferrite and austenite, whereas, below Ae₁ austenite initially transforms to ferrite, enriching the remaining austenite in carbon, which at a later stage transforms to pearlite as its composition enters the area marked “pearlite” Similarly, in the area marked “cementite”, the transformation occurs to cementite and untransformed austenite above Ae₁ temperature and to cementite and pearlite below Ae₁.

In the “pearlite” and “Bainite” regions, austenite fully transforms to these products. Below Ms temperature, the super-cooled austenite transforms to martensite athermally. In transformations above 450°C or so, carbon and other alloying elements diffuse but between 450°C to Ms, it is only the diffusion of carbon which occurs. Below Ms, no diffusion, even of carbon occurs.

Slowest cooling rate is obtained when the steel is cooled in the furnace itself, when the furnace is switched off, the process is called annealing. Still faster cooling is provided by air cooling a heated part, the process is then called normalising. Oil-quenching provides still faster cooling.

Water and still faster quenching medium, brine, normally leads to hardening. Faster cooling causes complications like distortion, warpage, cracks due to the development of internal stresses. Slower cooling may not be able to induce required properties in steel.

Various methods like interrupted quenching, martempering, or cooling through water to oil, etc. may be used to obtain the desired properties in plain carbon steels. If the cracks, etc. cannot still be avoided, then costly (if warranted) alloy steels may be used and a slower cooling rate giving medium, generally oil, may be used.

Mention may be made here, that just minimum carbon should be specified for an application, which can give the required hardness and strength and not more than the minimum because carbon is the rogue element and is the main cause of quite a few types of brittleness in steels.

Other alloying elements, if essential, are added in steel at their minimum concentration levels as these elements make the steel expensive and complicate the processing as well as heat treatment cycles. Over designing is inefficient use of expensive alloying elements and processing techniques, which is basically against the principles of heat treatment.