The structures formed during the continuous cooling of steel from above Ac₃ can be understood best by studying the constant-temperature (isothermal) transformation of austenite, thus separating the two variables: time and temperature.

One method consists of heating small specimens above Ac₃ to form austenite, then quenching into a suitable bath (e.g. liquid tin) at some constant sub-critical temperature. After holding for selected periods of time, the specimens are withdrawn from the bath and rapidly quenched in cold water. This converts any untransformed austenite into martensite the volume of which can be estimated microscopically. Another method consists in measuring length changes caused by the decomposition of austenite at the constant temperature by means of a dilatometer.

When carbon steel is quenched in the baths at constant temperatures, the velocity of austenite transformation is found to depend on temperature. The time for the beginning and completion of the transformation of the austenite is plotted against the temperature to give the Bain "S-curve", shown in Fig. 1, now called TTT-curve (time-temperature-transformation).

Figure 1. Ideal TTT-curve for 0,65% carbon steel depicting time interval required for beginning, 50% and 100% transformation of austenite at a constant temperature A= Austenite F= Ferrite P = Pearlite B = Bainite

The logarithmic scale of time is used to condense results into a small space. Ae1 and Ae3 lines represent the equilibrium transformation temperatures. Austenite is completely stable above Ae3 and partially unstable between Ae3 and Ae1. Below Ae1 austenite is completely unstable and transforms in time. Two regions of rapid transformation occur about 550° and 250°C. The form of each of the curves and their positions with respect to the time axis depend on the composition and grain size of the austenite which is transforming.

The TTT-curve is most useful in presenting an overall picture of the transformation behaviour of austenite. This enables the metallurgist to interpret the response of steel to any specified heat-treatment, to plan practical heat-treatment operations and to control limited hardening or softening and the time of soaking.

The decomposition of austenite occurs according to three separate but sometimes overlapping mechanisms and results in three different reaction products: pearlitic, bainitic, martensitic.

Pearlitic

The upper dotted curve in Fig. 1 represents the beginning of the formation of ferrite. The curve just below it indicates the beginnings of the breakdown of the austenite remnant into a ferrite-carbide aggregate. In the high-temperature pearlitic range in Fig. 1 the process resembles the solidification of crystals from a liquid by the formation and growth of nuclei of carbide followed by ferrite by side nucleation with side and edge growth, Fig. 2a and b.

At 700°C the formation of nuclei is slow (i.e. incubation period), then growth proceeds rapidly to form large pearlite colonies covering several austenite grains in some cases. As the transformation temperature is lowered to 500°C the incubation period decreases and the pearlite becomes increasingly fine.

Large numbers of nuclei form in the austenite boundaries, but growth is slower and this produces nodular troostite, Fig. 2a. In the case of medium carbon steels the excess ferrite decreases in volume and begins to show an acicular or Widmanstätten type of distribution. The relative amounts of free ferrite to be expected after a given heat-treatment is indicated by the size of the "austenite and ferrite" field and by the temperature interval between Ae₁ and Ae₃.

Bainitic

Between about 500° and 350°C initial nuclei are ferrite which is coherent with the austenite matrix. Cementite then precipitates from the carbon-enriched layer of austenite, allowing further growth of the ferrite as shown in Fig. 2c.

The carbides tend to lie parallel to the long axis of the bainite needle to form the typical open feathery structure of upper bainite. Below 350°C coherent ferrite, supersaturated with carbon, forms first and is then followed by the precipitation of carbide within the ferrite needle, transversely at an angle of 55°. A proportion of the carbide is Fe2,4C and the ferrite contains a little dissolved carbon. This lower bainite structure is somewhat similar to lightly tempered martensite (Fig. 2d).

Figure 2. (a) Effect of different speeds of nucleation and growth on formation of pearlite colonies; (b), (c), (d) diagrammatic representation of formation of pearlite, upper bainite and lower bainite

Martensitic

In quenching down to about 250°C, the temperature drops rapidly through the interval in which "nucleation" could take place, to a temperature so low that the molecular mobility, i.e. diffusion, becomes too small for the formation of nuclei.

In the third stage, therefore, the austenite changes incompletely into a distorted body-centred structure, with little or no diffusion of the carbon into particles of cementite, to form martensite the plates of which are formed at a high speed (less than 0,002 sec). This suggests that the mechanism of formation of this structure is not nucleation and growth but a shearing process. This resembles the process of mechanical twinning and involves very little atomic movement, but considerable internal stress due to the shear and to the position of the carbon atoms.

As the temperature decreases the elastic energy increases and eventually causes a shear in a part of the matrix, which stabilises the rest. Further shear can only occur when the temperature is lowered and more energy gained. The amount of martensite formed, therefore, is practically independent of time and depends principally on the temperatures at which the steel is held. Hence a proportion of austenite is usually retained in quenched steel which can be reduced in amount by a decrease in temperature. This fact is used in sub-zero quenching.

The temperature at which martensite begins to form (Ms) is progressively lowered as the carbon content of the steel increases, e.g.

C %	0,02	0,2	0,4	0,8	1,2
Ms °C	520	490	420	250	150

The temperature is also affected by the alloy content, but the following empirical formula (Steven and Haynes) can be used for calculating Ms from the chemical analyses, provided all carbides have been dissolved in the austenite:

Ms in °C = 561 - 474 (% C) - 33 (% Mn) - 17 (% Ni) - 17(% Cr) - 21 (% Mo).

Mf is about 215°C below the Ms.

Plastic and elastic stresses promote the formation of martensite, but it is retarded when cooling is interrupted. When cooling is resumed after such a stabilisation arrest martensite only begins to form again after cooling to a lower temperature.

The rate and extent of stabilisation (depression) depend on the temperature and time of holding, amount of prior transformation and alloy content.

Two forms of martensite have been identified depending on carbon content. In low carbon steels laths containing many dislocations are found, while in high carbon steels the plates are heavily twinned, Fig. 3(a) and (b).

Figure 3. (a) Lathe martensite formed in 0,08°C steel quenched in brine from 100°C (x20000),	b) Twinned martensite in Fe30%Ni (x110000)

Two groups of phase transformation are now given the name civilian, in which atoms move in a random manner (e.g. pearlite) and military because of its orderly disciplined manner, e.g. martensite. Martensite transformations also occur in non-ferrous alloys often differing greatly from the rather special case in steel.