Time Temperature Transformation (TTT) of Austenite | Metallurgy

In this article we will discuss about:- 1. Introduction to Time Temperature Transformation of Austenite 2. Types of Transformation 3. Types 4. Factors Effecting TTT Curves.

Introduction to Time Temperature Transformation of Austenite:

To appreciate the character of transformation of austenite, and the phases obtained, Davenport and Bain first introduced the isothermal transformation approach, and showed that by studying the transformation isothermally at a series of temperatures below A₁, a characteristic, time-temperature transformation, TTT, curve can be obtained.

The diagrams that illustrate the transformation of austenite as a function of time at a constant temperature is a TTT, or isothermal transformation (IT) diagram. In the simplest form (in plain carbon, or low alloy steels), these curves have a well-defined ‘C’ or ‘S’ shape.

Each steel composition has its own different ‘S’ curve. Though grain size of austenite and the presence of inclusions, or other inhomogeneities can change the diagram of a given steel composition, but frequently these factors are neglected.

The TTT curves are now available in atlas form for a variety of steels, which could be used for the selection of steels and the design of the heat treatments. Fig. 3.1 illustrates an idealized TTT curve which is more frequent in alloy steels, whereas Fig. 3.3 (b) illustrates for eutectoid composition (0.89% carbon, 0.29% manganese).

A number of small specimens of the steel of which TTT curve is to be drawn, for example, a steel with 0.89% carbon and 0.29% manganese, are heated in salt bath furnace (Fig. 3.2 a) at a temperature above its upper critical temperature (885°C) long enough to obtain homogeneous austenite. Another salt bath (Fig. 3.2 b) is maintained at a sub-critical temperature (say, 700°C). One by one, the specimens from austenitising temperature bath are removed and plunged in the second bath, and kept here, each for a definite time but different than other specimens.

For example, the first specimen may be kept for one second, the second specimen for 5 seconds, the third specimen for 10 seconds, etc., and then quenched in water bath (Fig. 3.2 c). Every specimen has homogeneous austenite in bath (a). If no transformation occurs in a piece during its stay for a definite time in bath (b), it transforms to 100% martensite when put in water. If some transformation occurs in bath (b), then the remaining austenite transforms to martensite in water-bath. Here, the interest lies in the amount of transformation which takes place when kept in bath (b).

After quenching in water, each specimen is prepared for examination by microscopic method, (magnetic and dilatometric studies are also used) to find out the amount of transformation (non-martensitic product) which occurred in bath (b). A graph (Fig. 3.3 a) is plotted between % transformation and time of start of transformation to time of completion of transformation. To have easy detection, 1% transformation may be taken as start and 99% transformation as the completion of transformation.

For example, this steel (0.89% C, 0.29% Mn) has 1% pearlite after 10⁴ seconds, and, 10⁵ seconds when 99% pearlite is obtained [Fig. 3.3 (a)] for holding temperature of bath, 700°C. These points are plotted as in Fig. 3.3 (b). Similar procedure is used, but the temperature of bath is kept different every time, say, at temperature 600°C, the beginning time is 1 second and completion time is 7 seconds [data of Fig. 3.3 (a) is transferred to Fig. 3.3 (b)]. Experiments at, other sub-critical temperatures down to about 230°C, show different times for the start and finish of the transformation.

Below temperature 230°C, the transformation appears to be independent of time, and is only a function of temperature. A curve is drawn joining the points for the times at which transformation begins, and another for the times of finish of transformation as in Fig. 3.3 (b). For points below 230°C, the sub- critical bath is kept at that temperature. The specimen is transferred from 885°C to this bath to be kept for a few seconds.

It is then heated to higher temperature and then put in water. These heating tempers the martensite obtained when put in salt bath and remaining austenite becomes martensite, when put in water. Microscopic studies can distinguish even, 1% of tempered martensite from 99% of martensite to obtain M_s as the temperature of the sub-critical bath. The temperature at which martensite starts forming is designated as M_s and at which finish of the transformation takes place is designated as M_f temperature.

The rate of transformation of austenite to pearlite or bainite is:

(i) Practically nil just at A₁ and B_s temperatures (the curves are tangent to these temperatures) (Fig. 3.1). because austenite is in thermodynamic equilibrium with pearlite and bainite respectively, i.e., austenite at A₁ has free energy practically equal to the free energy of pearlite, and thus does not transform.

(ii) The rate increases at first as temperature falls below A₁ (or B_s) and reaches a maximum as the free energy change increases as the under-cooling increases. The latter also increases the rate of nucleation because the critical nucleus-size decreases. As undercooling Δt, increases, Δg, the difference in free energy of new and old phases increases.

(iii) The rate decreases as undercooling increases further, because the diffusion becomes sluggish as temperature decreases.

The TTT diagram assumes a characteristic ‘C’ shape. The region of fastest transformation is called nose or knee of the curve. The nucleation of new phase is preceded by a period of incubation. Incubation period is that period in which transformation does not proceed because enough diffusion has not taken place in austenite for the transformation to start.

Larger is the incubation period, greater is the stability of austenite and slower is the rate of austenite decomposition. Compare two curves in Fig. 3.3 (a). The ‘C’ shape illustrates that stability of austenite first decreases sharply to reach a minimum and then increases again.

Types of Transformation in TTT Diagram of Austenite:

A TTT diagram illustrates three different types of the transformation products, each in a different tem­perature range, depending on the degree of super cooling of austenite before it transforms, and are:

(i) Pearlitic range (also called, high temperature transformation range)

(ii) Bainitic range (intermediate temperature transformation range)

(iii) Martensitic range (low temperature transformation range)

The pearlitic range in carbon steels extends over the temperature interval below A₁ to the knee (nose) of the TTT diagram (≈ 550°C). Austenite in this range transforms to lamellar mixture of ferrite and cementite, called pearlite. There takes place diffusion of carbon as well as alloying elements (if present) during this transformation to partition between the two product phases. Just below A₁, coarse pearlite forms with a hardness of HRC 10 (for eutectoid steel).

As the temperature of transformation is lowered, fine and finer pearlite (relative ratio of width of ferrite and cementite lamellae still remains 8:1) forms to get a hardness of HRC 40 at the temperature of the nose of the curve (≈ 550°C).

The lower the temperature of transformation, larger is the driving force for the transformation (i.e., larger is the free energy difference between new and old phase with increased undercooling), finer spacing of pearlite forms as the extra free energy is used to create more interfacial area between ferrite and cementite as it becomes finer.

As now, the distance of diffusion is less, the growth rate of pearlite can increase, i.e., not only the rate of formation of pearlite increases, but the interlamellar spacing too becomes smaller.

Near the nose of the curve, both pearlite and bainite can form, unless there are separate ‘C’ curves for each, as in case of most of the alloy steels. Below this range (nose of the curve or B_s) and above M_s temperature, isothermal transformation product of austenite is bainite. This transformation has many features common to both, the pearlitic (diffusion-based) and the martensitic (diffusion less) transformations. Bainite is an aggregate of ferrite (over-saturated with carbon) and carbide. Ferrite forms by martensitic mechanism and diffusion of only carbon takes place.

The diffusion of alloying elements (if present in steels) is impossible during bainitic formation. The nature of bainite changes with the temperature of formation. The upper bainite-feathery bainite-which forms in upper range of the temperatures, and lower bainite-acicular (needle like) bainite-which forms in lower range of temperatures.

Although, the initial nuclei are ferrite plates at the grain boundary of austenite, but carbide particles form between the plates of ferrite in upper bainite due to faster diffusion of carbon, but form in the ferrite plates as these lag behind the formation of ferrite plates due to sluggish diffusion of carbon in lower bainite.

When austenite is super-cooled by large degree to M_s temperature, the driving force for the transfor­mation of austenite becomes so strongly large that martensitic transformation takes place without diffusion of carbon (and alloying elements, if present). As the amount of carbon present is much more than can be held in solid solution, the product phase, martensite acquires a distorted form of body-centred-cubic lattice, called body-centred tetragonal.

The tetragonality of martensite increases with the carbon content, and thus, the hardness also depends strongly on the carbon content of the steel. The principal cause of high hardness and the strength of martensite is the elastic distortions produced during the formation of martensite plates from M_s—start of martensite formation temperature.

Martensite forms instantly (in 10^-7 seconds) as soon as a temperature (between M_s – M_f) is attained, and its amount increases as the temperature decreases (being athermal transformation) till at M_f (finish of martensite formation), the steel is essentially all martensite (99% or so). Carbon and most other alloying elements lower both the M_s and M_f temperatures, so that in certain steels even the M_s temperature is below room temperature.

Hardening consists of cooling the steel from austenitic state at a rate fast enough to suppress pearlitic and bainitic transformations and let it transforms to martensite. Critical cooling rate is defined as the slowest rate of cooling which produces fully martensitic structure (see Fig. 3.3).

Critical cooling rate is not same for all steels. Carbon steels have high critical cooling rates, 800 to 200°C/s, eutectoid steel having the minimum and so has coarse-grained steel. Alloying elements generally decrease critical cooling rate. 1% Cr reduces to half of 1% C steel; 0.4% Mo reduces from 200°C/sec to 50°C/s.

TTT diagrams are able to provide a lot of information about the nature and the rate of transformation, the phases, the temperature of start, finish of transformation, and hardness of the transformed phases with microstructures.

Types of TTT Diagrams of Austenite:

Broadly, there are following types of TTT diagrams:

1. Separate Curves for Pearlitic and Bainitic Transformation:

A large number of alloy steels have two noses of the TTT curves, i.e., two minimums for the stability of super-cooled austenite-upper (higher temperature) one for pearlitic, and the lower one for bainitic transformation as illustrated in Fig. 3.5. TTT diagrams of plain carbon steels (Fig. 3.6) and some alloy steels have single knee. Actually, single knee curve is a particular case where super-imposition of temperature intervals for pearlitic and bainitic transformations takes place.

(a) Bainitic transformation is complete (Fig. 3.5 a). For example, steel with C = 0.4%; Mn = 0.8%; Cr = 1.1%; Mo = 0.2%.

(b) Bainitic transformation is incomplete (Fig. 3.5 b). It is quite common in higher carbon and high alloy steels. For example, in steel with C = 1.5%, Cr = 11.5 %; Mo = 0.8%; V = 0.2% (Alloying elements are carbide-formers)

(c) Bainitic transformation is absent (Fig. 3.5 c). For example, in steel with C = 0.24%; Cr = 13.32%; Ni = 0.32%. Such steels may be air-hardening steels.

(d) Pearlitic transformation is absent (Fig. 3.5 d). The rate of transformation is too slow to be observed experimentally. For example, in steel with C = 0.2%; Cr = 0.30%; Mo = 0.50%; B = 0.001 – 0.003%.

2. Single (Overlapping) Curve for Pearlitic and Bainitic Transformations:

Such ‘S’ curves are obtained in plain carbon and low alloy steels (C-Co, low Mn, Mo, Si, or Ni-steels).

(a) Without proeutectoid transformations- For example, eutectoid steel and low alloy steels. Fig. 3.6 (b) illustrates for eutectoid steel.

(b) With proeutectoid transformations- For example, hypoeutectoid steels and hypereutectoid steels [Fig. 3.6 (a) and (c)]. During isothermal transformation in the steels, before pearlite is formed, proeutectoid ferrite separates in hypoeutectoid steels, and proeutectoid cementite precipitates in hypereutectoid steels. There is an extra curve (Fig. 3.7 a) for such precipitation from austenite in TTT diagrams.

The amount of proeutectoid ferrite formed decreases in a steel as the temperature of isothermal transformation is lowered (Fig. 3.7 a illustrates at temperature T₁ and T₂) in hypoeutectoid steels.

Thus, pearlite, then, does not contain 0.77% carbon but less carbon in hypoeutectoid steels as the remaining free ferrite is present distributed in pearlite. Such a pearlite is called pseudo-pearlite. Also, the carbon content of pearlite in hypereutectoid steel too contains more than 0.77% carbon if precipitation takes place at lower temperatures.

Factors Effecting TTT Curves:

The shape of the ‘S’ curves, i.e., the kinetics of transformation of austenite, is effected by:

1. Composition of Steel- (a) Carbon, (b) Alloying elements

2. Grain size of austenite

3. Heterogeneity of austenite

1. Composition of Steel:

Carbon and alloying elements affect the transformation of austenite in many ways. As the amount of carbon and most alloying elements increase in steel, the lower part of the curves is progressively lowered because except for cobalt and aluminium, all the elements lower the M_s temperature.

The austenite stabilisers lower the Ac₃ as well as Ac₁ temperatures, i.e. these elements (Ni, Mn, C etc.) lower the upper part of the ‘S’ curves. This probably is the reason of having overlapping ‘C’ curves for pearlitic and bainitic transformations in plain carbon steel as well as in steels having Ni, etc. [Even 3.41% Ni steel has single ‘C’ curve as shown in Fig. 3.7 (b)].

Ferrite stabilisers raise Ac₃ as well as Ac₁ temperatures. As the ‘S’ curve for such steels get raised upward as well as gets lowered down ward, there are invariably two ‘C’ curves-one for pearlitic and the other for bainitic transformations. For example, presence of 0.8% chromium and 0.33% molybdenum in steel yield two ‘C’ curves. Fig. 3.7 (c) illustrates this.

Andrew suggests the effect of the elements on Ac₃ by equation:

Ac₃ = 910 – 203 √%C – 15.2 (% Ni) + 44.7 (% Si) + 104 (%V) + 31.5 (% Mo) + 13.1 (%W)..(3.1) and on Ac, by equation-

Ac₁ = 727 – 10.7 (% Mn) – 16.9 (% Ni) + 29.1 (% Si) + 290 (% As) + 16.9 (% Cr) + 6.38 (% W)

(a) Effect of Carbon:

Fe-Fe₃ C diagram illustrates (also, as given in equation 3.1) that Ac₃ temperature decreases as the carbon content of the steel increases upto 0.77%, and hence the driving force for the transformation of austenite decreases, i.e. carbon increases the stability of austenite as its content increases up to 0.77%. As the carbon content increases in hypoeutectoid steels, the amount of free ferrite decreases, i.e. nucleation and growth rate of ferrite at a given temperature decreases. Ferrite is nuclei for pearlite transformation in hypoeutectoid steels.

Thus, the nose of ‘S’ curve becomes more and more on the right hand side, i.e. the incubation period at the nose of the curve increases as carbon increases to 0.77% carbon, but shifts towards left-hand-side as carbon increases more than 0.77% in the hypereutectoid range. In hypereutectoid steels, the already formed pro-eutectoid cementite acts as the nuclei for the pearlitic transformation, i.e. it accelerates pearlitic transformation, thus shifts ‘S’ curve towards left. Ferrite is the nucleus for bainite formation.

As the nucleation and growth rates of ferrite decreases with the increasing carbon content in the whole range of carbon steel compositions, the bainitic transformation is uniformly retarded with the increasing carbon content, i.e. bainitic part of ‘S’ curve uniformly shifts towards the right. Compare TTT diagrams for hypoeutectoid, eutectoid and the hypereutectoid steels in Fig. 3.6. The incubation period is maximum (critical cooling rate is slowest) in eutectoid steel. M_s temperatures are uniformly decreasing in the whole range of carbon. Bainitic part is uniformly shifted to the right.

(b) Effect of Alloying Elements:

Almost all alloying elements (except cobalt) increase the stability of the super-cooled austenite in both pearlitic and bainitic regions, and thus, shift the ‘S’ curve in TTT diagram for the start and the end of the transformation to the right. This is easy to appreciate for austenite stabilising elements (Ni, Mn, etc.) as explained above for carbon. The high stability of austenite due to other elements is better understood by analysing the effect of alloying elements on the mechanism of austenite transformation.

As the single phase alloyed homogeneous austenite changes to ferrite and carbide (or alloy carbide), it requires partitioning of carbon and the alloying elements between ferrite and carbide by the process of diffusion. Carbide forming elements try to join carbide, while others join ferrite.

The increased stability of austenite in the pearlitic range is due to:

(i) Low rate of diffusion of alloying elements in austenite (as these are substitutional solutes),

(ii) Reduced rate of diffusion of carbon, as the carbide forming elements do not easily part carbon away from them. More intense is the retardation, stronger is the carbide forming element,

(iii) The alloyed solute reduce the rate of allotropic change, γ → α, by solute drag effect on γ → α interface boundary.

The effect of alloying elements is less pronounced in bainitic region, as the diffusion of only carbon takes place in the formation of bainite without any redistribution of the alloying elements. Alloying elements reduce the diffusion of the carbon, and thus the bainitic transformation. However, the rate of bainitic transformation is faster in alloy steels with lower carbon content, i.e., bainitic region is more towards the left (as in Fig. 3.5 a), and that of pearlitic transformation is faster in alloy steels with higher carbon content (Fig. 3.5 b). The pearlitic transformation in low carbon alloy steels with carbide forming elements like Cr, W, Mo, may be so slow that it may not occur experimentally (Fig. 3.5 d), whereas in 0.3-0.4% carbon with 10-12% Cr in steel, the bainitic transformation may not occur.

2. Effect of Grain Size of Austenite:

All the decomposition products of austenite nucleate heterogeneously preferentially at the grain boundaries. A fine grained steel has larger grain boundary area than a coarse grained steel, and consequently favours nucleation of pearlite, bainite, ferrite, cementite and thus, reduces the incubation period, that means, the TTT curve of the fine grained steel is more towards left, significantly in the pearlitic range, than a coarse grained steel of same composition.

3. Effect of Heterogeneity of Austenite:

On heating steel to austenite region, the just formed austenite has heterogeneity of carbon as well as alloying elements. Carbon is rich in regions of locations of prior cementite (may be are present some undissolved cementite particles). The presence of alloy-rich and alloy-depleted regions is more likely in alloy steels because the diffusion of substitutional elements is much slower than carbon. More so when a strong carbide forming element is present as undissolved carbide, which impoverish the austenite of carbon.

The undissolved cementite, alloy carbide, or even carbon-rich regions nucleate the pearlite rapidly. The heterogeneity of austenite results in less precise TTT diagram because, the start of the reaction corresponds to the lean portion and the end of the transformation to the richest (dissolved as solid solution) alloy regions. If homogenisation of heterogeneous austenite is done before, then the effective time of isothermal transformation is appreciably reduced, or, the temperature interval of effective transformation during continuous cooling is reduced.

This heterogeneity of austenite, when the steel is heated to temperatures not higher than 50°C above A₁ is put to advantage to obtain spheroidised pearlite by letting it decompose within 50°C below A₁ to improve the ductility and the machinability (of high carbon steels).