Austenitisation of Steels | Heat Treatment | Metallurgy

The first step in the true heat treatment cycle of steel is the austenitisation, i.e. to get a homogeneous austenite by heating it to a predetermined temperature in the austenite stability range. Normally, the initial structure of carbon steels, before this heat treatment, is ferrite + pearlite (in hypo-eutectoid steels), pearlite (eutectoid steel), or cementite + pearlite (hyper eutectoid steels). The Fe-Fe₃C phase diagram does not show any change unless the steel is heated to temperatures above Ae₁ (except dissolution of tertiary cementite).

Ordinarily, it is true that with these microstructures, steels when heated for short periods up to below the lower critical temperature do not show any change. However, the prolonged heating at just below Ae₁ temperature, can change lamellar pearlite to globular pearlite, particularly in high carbon steels. The driving force being the reduction in interface area between cementite and ferrite.

In some steels having silicon, particularly those killed with aluminium, the prolonged heating below Ae₁ and in an atmosphere containing traces of oxygen, cementite decomposes into ferrite and graphite, turning the steel to scrap. Normally, this transformation does not occurs.

As the temperature is raised above Ae₁, it is the pearlite which transforms to austenite first. When all the pearlite has changed to austenite, this austenite grows consuming increasing amount of free ferrite (in hypo-eutectoid steels), or free cementite (in hypereutectoid steels) as the temperature is raised above Ae₁ to above Ae₃, or above A_cm respectively, when the steel is completely austenite Alloy carbides in alloy steels, if present, invariably are the last to dissolve in austenite.

Consider the austenitisation of eutectoid steel having 100% pearlite, when heated slightly above Ae₁ temperature, say at T₁ (Fig. 2.11 a), where pearlite becomes metastable phase and must change to austenite- the stable phase. Pearlite is a lamellar mixture of two phases, the ferrite and the cementite.

Austenite forms by the reaction:

Looking at Fe-Fe₃C diagram, ferrite itself can transform to austenite only above about 910°C, and cementite itself is stable up to its melting point (_~ 1227°C). Thus, on heating above Ae₁ (727°C), neither ferrite, nor cementite is metastable with respect to austenite, but their mixture-pearlite is. Thus, the nucleation of austenite inside ferrite, or cementite is thermodynamically impossible [unless above 910°C in ferrite of pearlite or free ferrite in hypo-eutectoid steels. Experimentally, the nucleation of austenite has been seen to take place at the interfaces of ferrite and cementite lamellae within a pearlite colony but, primarily at the intersections of pearlite colonies.

Some possible nucleation sites for austenite in ferrite + cementite mixture, or only ferrite structures above 910°C. The Finer the pearlite, more is the interface area between ferrite and cementite, more are the potential sites for nucleation (all sites may not be active). Thus, the rate of nucleation of austenite is increased by increasing the interfacial area between ferrite and carbide, i.e. the rate is more in fine lamellar pearlite than coarse lamellar pearlite, which has more rate than the globular pearlite.

Austenite nuclei have FCC crystal structure and can have a carbon concentration between C_P and C_Q [Fig. 2.11 (a) at temperature T₁] as compared to ferrite (0.02% C and BCC structure) and cementite (6.67% C and orthorhombic structure). Thus, during nucleation of austenite, change in the crystal structure as well as carbon diffusion must take place. This nucleation at the interface is supposed to take place by concentration and structural fluctuations.

As in any solid state phase transformation, here too all the three factors:

(i) Volume free energy difference between the old and product phases,

(ii) Increase in surface energy due to interfacial surface of the nucleus, and

(iii) The strain energy due to the difference in volume of the product phase and old matrix—control the energetics and kinetics of the nucleation of austenite.

Once, the austenite has nucleated at the interface of ferrite and cementite, it grows consuming both the ferrite and the cementite of pearlite. Fig. 2.11 (d) shows schematically, the carbon profile in austenite along z – z section of Fig. 2.11 (c).

The eutectoid steel at temperature T₁, assuming local carbon equilibrium at austenite/cementite and the austenite/ferrite interfaces, has austenite having carbon concentration difference as given by points P and Q [Fig. 2.11 (a), (c) and (d)], where, P is a point in austenite close to austenite/ferrite interface, and Q is close to austenite/cementite interface.

The concentration gradient in austenite is then, C_Q – C_P/x, where x is the distance over which carbon diffuses. Higher is the concentration gradient of carbon is austenite, faster is the carbon diffusion in it higher is the growth rate of austenite.

The carbon concentration difference, (C_Q – C_P) increases approximately proportional to the amount of superheating above Ae₁ (T – Ae₁) i.e., increases with the rise of temperature of austenitisation. The increase of temperature also increases exponentially the diffusivity of carbon in austenite, i.e. increases the diffusion of carbon in austenite.

The concentration gradient also increases if ‘x’, the diffusion distance decreases. The value of ‘x’ depends on the original structure of steel—being small if the structure is fine lamellar pearlite. But fine lamellar pearlite also provides high rates of nucleation of austenite too. As more nuclei are formed, the diffusion distance ‘x’ decreases further, resulting in much higher concentration gradient and ultimately increases the growth rate of austenite.

The rate of movement of austenite-boundary into ferrite and the cementite phases is not equal. This rate is inversely proportional to the concentration jump at the interface. The concentration jump at austenite/cementite interface is from Q to 6.67% (Fig. 2.11 a), and at austenite/ferrite interface is from P to C_a (≈ 0% C) (Fig. 2.11 a), i.e. the former has concentration jump of an order of magnitude higher than the latter. Thus, austenite boundary moves much faster into ferrite phase, completely consuming it to austenite, but still has some undissolved cementite particles. These undissolved cementite particles too get dissolved in austenite in some time.

At this time, when the whole of the pearlitic structure has transformed to austenite, there is non-uniformly of carbon in austenite, i.e., it has higher carbon content at the sites formerly occupied by the cementite and lowest at the centre of former ferrite lamellae. Additional time is needed to obtain a homogeneous austenite.

In hypoeutectoid steels, the size of the proeutectoid ferrite grains is much larger (one, or two order of magnitude) than the thickness of the ferrite lamellae in pearlite. Thus, the lime for complete disappearance of free ferrite exceeds the time needed for the disappearance of pearlite. The complete disappearance of proeutectoid ferrite occurs, if temperature of heating is above temperature. As the end of this transformation too, the austenite is inhomogeneous in carbon, having lowest carbon at the sites where the free ferrite transformed last.

Hypoeutectoid steels too need additional heating over eutectoid steel to get homogeneous austenite. The Fe-Fe₃C diagram illustrates that in hypoeutectoid steels, A₃ temperature decreases as the carbon content of the steel is increased to 0.77%, i.e., the rate of austenitisation increases due to increasing amount of pearlite present in structure. This is because there is increasing amount ferrite-cementite interface area present.

In hypereutectoid steels, the proeutectoid cementite dissolves in austenite after the pearlite has transformed completely to austenite, but at a much slower rate. There is inhomogeneity in austenite as the carbon content is high at sites where free cementite dissolved last, needing additional time, more than even for hypoeutectoid steels to obtain homogeneous austenite.

If a hypoeutectoid steel is austenitised at a higher temperature more than 910°C, above which austenite is more stable than ferrite, the austenite nuclei can nucleate now even inside the proeutectoid ferrite as well as in ferrite of pearlite. Austenite formed has the same composition as of the ferrite (≈ 0% C) from which it formed. This transformation is quite fast as long range large diffusion of carbon is not needed. The left-out cementite of pearlite then dissolves in austenite formed from ferrite.

This austenite, when the cementite has just dissolved in it (formed in a way out of whole pearlite) has large heterogeneity of carbon, but becomes homogeneous in short period of time, because the diffusion distances are small (maximum is half the inter lamellar spacing) to attain uniform carbon content of eutectoid point (0.77% C).

Steel in this state is still in- homogeneous in carbon as austenite (formed from pearlite) has eutectoid carbon content but zero carbon at site which had formed from proeutectoid ferrite. Further homogenisation has to be done to get uniform austenite by the diffusion of carbon over long distances. Hypereutectoid steels also austenitise similarly.

Dissolution of Cementite and Ferrite for Growth of Austenite:

The concentration gradient (as explained above) causes the growth of austenite by diffusion of carbon atoms in austenite from its boundaries with cementite [from point Q to point P-Fig. 2.11 (c) and (d)] to areas adjoining ferrite. Thus, the carbon in austenite at the interface with cementite decreases from the equilibrium value, say from Q to Q’ (Fig. 2.11 a).

To restore the carbon to its equilibrium content level to Q, dissolution of adjoining cementite in austenite occurs, thus, facilitating its transformation to austenite. Similarly, the diffusing carbon atoms change the carbon content in austenite adjoining to ferrite from P to P’ (Fig. 2.11 a). To restore it to P, ferrite dissolves in austenite undergoing α to γ allotropic transformation too. As larger diffusion is needed, the dissolution of cementite is a slower process than the dissolution of ferrite.

The kinetics of austenite formation can be studied by a transformation diagram for continuous heating transformation such as for a 0.70% carbon steel as illustrated in Fig. 2.12. Three increasing heating rates I, II, III have been drawn on this diagram. The rate of austenite formation increases rapidly with the increase of temperature, such as for example, at 800°C, the process of austenitisation (with some carbides in it) takes 5 seconds.

On continuous heating of steel, pearlite to austenite transformation takes place over a certain temperature interval. Higher is the rate of heating, higher is the temperature at which pearlite starts to transform to austenite, but longer is the temperature interval of this transformation. Curve II, the transformation starts at around 750°C, but complete homogeneous austenite is obtained at 870°C within around 12 seconds.

It is apparent from this diagram that free ferrite dissolves later and at a higher temperature than the transformation of pearlite to austenite. This austenite has undissolved carbide in it. Higher temperature and/or more time are needed to dissolve the carbide. The resultant austenite is in- homogeneous and requires higher temperatures and/or time to obtain completely homogeneous austenite. Such diagrams are of great help in estimating time of heating for getting homogeneous austenite.

It has been seen that at any austenitising temperature, homogeneous austenite is obtained in shorter time if the structure is relatively uniform, such as only pearlite as compared to the duplex microstructure, like ferrite + pearlite, or cementite + pearlite. Austenitisation is quicker in fine pearlite as compared to coarse pearlite [Fig. 2.13 (a) and (c)], and sphervidised pearlite takes longest time [Fig. 2.13 (e) and (f)].

As more austenite grains are formed at same austenitisation temperature if pearlite is finer, diffusion is faster (Fig. 2.14). Slightly tempered martensite (with small dispersion of carbides) takes lesser time than lamellar pearlite.

Homogenisation of carbon steels does not take much time as the diffusion of interstitial carbon is a few orders of magnitude faster as compared to substitutional solid solution forming elements like nickel, chromium, etc.

The presence of alloying elements effects the rate of homogenisation depending on the:

(i) Nature of the elements,

(ii) Their distribution in the starting structure i.e., in ferrite, or and carbide,

(iii) Dispersion of phases.

Proper austenitisation temperatures have to be used as elements like nickel and manganese lower A₁, A₃ temperatures, whereas elements like chromium, vanadium raise them.

Carbide forming elements in alloy steels like chromium, molybdenum, tungsten, vanadium, etc., as compared to non-carbide forming elements, retard austenitisation to greater extent due to their existence as alloyed cementite, or their own carbides, the latter are more difficult to dissolve in austenite than cementite The difference is more marked, more stable is the alloy carbide.

Some very stable carbide like TiC, V₄ C₃, NbC, etc. do not dissolve in austenite until heated to very high temperatures. For example, vanadium carbide dissolves at around 1050°C, whereas, the niobium carbide dissolves only around 1150°C, and these temperatures are much higher than required for cementite dissolution. Depending on the nature of the alloying elements, these are non-uniformly distributed between ferrite and the carbide.

Even on dissolution, the alloying content is not the same throughout the volume of austenite, and their diffusion is slow. Thus, alloy steels need higher austenitising temperatures and/or longer times to form homogeneous austenite, their exact temperature and time is normally determined experimentally.

In high alloy steels, some elements like vanadium as in high speed steel (18/4/i) remains present in structure as carbide and does not go in solution atleast completely, and is allowed to be so intentionally, so that such high temperatures of austenitisation may otherwise develop very coarse grain of austenite—which is not desirable.