Mechanism and morphology of pearlite formation

8.3.2.2 Mechanism and morphology of pearlite formation

The growth of pearlite from austenite clearly involves two distinct processes: (1) a redistribution of carbon (since the carbon concentrates in the cementite and avoids the ferrite) and (2) a crystallographic change (since the structure of both ferrite and cementite differs from that of austenite). Of these two processes it is generally agreed that the rate of growth is governed by the diffusion of carbon atoms, and the crystallographic change occurs as readily as the redistribution of carbon will allow. The active nucleus of the pearlite nodule may be either a ferrite or cementite platelet, depending on the conditions of temperature and composition which prevail during the transformation, but usually it is assumed to be cementite. The nucleus may form at a grain boundary as shown in Figure 8.20a, and after its formation the surrounding matrix is depleted of carbon, so that conditions favour the nucleation of ferrite plates adjacent to the cementite nucleus (Figure 8.20b). The ferrite plates in turn reject carbon atoms into the surrounding austenite and this favours the formation of cementite nuclei, which then continue to grow. At the same time as the pearlite nodule grows sideways, the ferrite and cementite lamellae advance into the austenite, since the carbon atoms rejected ahead of the advancing ferrite diffuse into the path of the growing cementite (Figure 8.20c). Eventually, a cementite plate of different orientation forms and this acts as a new nucleus as shown in Figures 8.20d and 8.20e.

Homogeneous austenite, when held at a constant temperature, produces pearlite at a constant rate and with a constant interlamellar spacing. However, the interlamellar spacing decreases with decreasing temperature, and becomes irresolvable in the optical microscope as the temperature approaches that corresponding to the knee of the curve. An increase in hardness occurs as the spacing decreases. Zener explains the dependence of interlamellar spacing

Figure 8.20 Nucleation and growth of pearlite nodules. (a) Initial Fe 3 C nucleus; (b) Fe 3 C plate full grown, ˛-Fe now nucleated; (c) ˛-Fe plates now full grown, new Fe 3 C plates nucleated; (d) Fe 3 C nucleus of different orientation forms and original nodule grows; (e) new nodule at advanced stage of growth (after Mehl and Hagel; courtesy of Pergamon Press) .

on temperature in the following way. If the interlamellar spacing is large, the diffusion distance of the carbon atoms in order to concentrate in the cementite is also large, and the rate of carbon redistribution is correspondingly slow. Conversely, if the spacing is small the area, and hence energy, of the ferrite-cementite interfaces become large. In consequence, such a high proportion of the free energy released in the austenite to pearlite transformation is needed to provide the interfacial energy that little will remain to provide the ‘driving force’ for the change. Thus, a balance between these two opposing conditions is necessary to allow the formation of pearlite to proceed, and at a constant temperature the interlamellar spacing remains constant. However, because the free energy change, G , accompanying the transformation increases with increasing degree of undercooling, larger interfacial

areas can be tolerated as the temperature of transformation is lowered, with the result that the interlamellar spacing decreases with decreasing temperature.

The majority of commercial steels are not usu- ally of the euctectoid composition (0.8% carbon), but hypo-eutectoid (i.e. <0.8% carbon). In such steels, pro-eutectoid ferrite is first formed before the pearlite reaction begins and this is shown in the TTT curve by a third decomposition line. From Figure 8.18b it can be seen that the amount of pro-eutectoid ferrite decreases as the isothermal transformation temperature is lowered. The morphology of the precipitated fer- rite depends on the usual precipitation variables (i.e. temperature, time, carbon content and grain size) and growth occurs preferentially at grain boundaries and on certain crystallographic planes. The Widmanst¨atten

278 Modern Physical Metallurgy and Materials Engineering pattern with ferrite growing along f1 1 1g planes of the

parent austenite is a familiar structure of these steels.