Nucleation and growth of pearlite If a homogeneous austenitic specimen of eutectoid

8.3.2.1 Nucleation and growth of pearlite If a homogeneous austenitic specimen of eutectoid

composition were to be transferred quickly to a bath held at some temperature between 720 °

C and

C, decomposition curves of the form shown in Figure 8.19a would be obtained. These curves, typical of a nucleation and growth process, indicate that the transformation undergoes an incubation period, an accelerating stage and a decelerating stage; the volume transformed into pearlite has the time-dependence described by the Avrami equation (7.44). When the transformation is in its initial stage the austenite contains a few small pearlite nodules each of which grow during the period A to B (see curve obtained at 690 °

C) and, at the same time, further nuclei form. The percentage of austenite transformed is quite small, since the nuclei are small and their total volume represents only a fraction of the original austenite. During the B to C stage the transformation rate accelerates, since as each nodule increases in size the area of contact between austenite and pearlite regions also increases: the larger the pearlite volumes, the greater is the surface area upon which to deposit further transformation products. At C, the growing nodules begin to impinge on each other, so that the area of contact between pearlite and austenite decreases and from this stage onwards, the larger the nodules the lower is the rate of transformation. Clearly, the rate of transformation depends on (1) the rate of nucleation of pearlite nodules, N (i.e. the number of nuclei formed in unit volume in unit time), and (2) the rate of growth of these nodules, G (i.e. the rate that the radius of the nodule increases with time). The variation of N

and G with temperature for a eutectoid steel is shown in Figure 8.19b.

The rate of nucleation increases with decreasing temperature down to the knee of the curve and in this respect is analogous to other processes of phase precipitation where hysteresis occurs (see Chapter 3). In addition, the nucleation rate is very structure sen- sitive so that nucleation occurs readily in regions of high energy where the structure is distorted. In homo- geneous austenite the nucleation of pearlite occurs almost exclusively at grain boundaries and, for this reason, the size of the austenite grains, prior to quench- ing, has an important effect on hardenability (a term which denotes the depth in a steel to which a fully martensitic structure can be obtained). Coarse-grained steels can be hardened more easily than fine-grained steels because to obtain maximum hardening in a steel, the decomposition of austenite to pearlite should be avoided, and this is more easily accomplished if the grain boundary area, or the number of potential pearlite nucleation sites, is small. Thus, an increase in austen- ite grain size effectively pushes the upper part of the TTT curve to longer times, so that, with a given cool- ing rate, the knee can be avoided more easily. The structure-sensitivity of the rate of nucleation is also reflected in other ways. For example, if the austenite grain is heterogeneous, pearlite nucleation is observed at inclusions as well as at grain boundaries. Moreover, plastic deformation during transformation increases the rate of transformation, since the introduction of dislo- cations provides extra sites for nucleation, while the vacancies produced by plastic deformation enhance the diffusion process.

The rate of growth of pearlite, like the rate of nucle- ation, also increases with decreasing temperature down to the knee of the curve, even though it is governed by the diffusion of carbon, which, of course, decreases with decreasing temperature. The reason for this is that the interlamellar spacing of the pearlite also decreases rapidly with decreasing temperature, and because the

Figure 8.19 Effect of temperature on (a) amount of pearlite formed with time and (b) rate of nucleation and rate of growth of pearlite (after Mehl and Hagel, 1956; courtesy of Pergamon Press) .

Strengthening and toughening 277 carbon atoms do not have to travel so far, the carbon

supply is easily maintained. In contrast to the rate of nucleation, however, the rate of growth of pearlite is quite structure-insensitive and, therefore, is indiffer- ent to the presence of grain boundaries or inclusions. These two factors are important in governing the size of the pearlite nodules produced. If, for instance, the

steel is transformed just below A 1 , where the rate of

nucleation is very low in comparison with the rate of growth (i.e. the ratio N/G is small), very large nodules are developed. Then, owing to the structure- insensitivity of the growth process, the few nodules formed are able to grow across grain boundaries, with the result that pearlite nodules larger than the original austenite grain size are often observed. By compari- son, if the steel is transformed at a lower temperature, just above the knee of the TTT curve where N/G is large, the rate of nucleation is high and the pearlite nodule size is correspondingly small.