ABSTRACT
Consider the eutectoid reaction g ! aþb. Pearlite consists of alternate lamellae of a and b growing synchronously and cooperatively into the g phase as illustrated in Figure 7.1.
As a result of kinetic considerations, proeutectoid a or proeutectoid b can still form at temperatures below that of the eutectoid. Proeutectoid reaction products form independently of each other. Eutectoid structures, on the other hand, are essentially, by definition, the consequence of mutually assisted precipitation of the a and b phases. The morphology of the eutectoid decomposition product has been found to take two extremum forms: the lamellar eutectoid, i.e., pearlite, and the nonlamellar eutectoid, which is defined here as bainite* (see Figure 7.2). Although these two definitions quite adequately cover a large proportion of the eutectoid microstructures observed, it must be noted that a considerable range of intermediate structures can develop. Such structures are usually termed degenerate pearlite or a variant of bainite, depending upon the viewpoint of the beholder.
