Effect of austenite grain size and bainite morphology on overall kinetics of bainite transformation in steels

The effect of austenite grain size on the evolution of the bainite transformation in steels is not yet certain in spite of many investigations.1–7 Barford and Owen1 reported that the reaction rate was accelerated by decreasing the grain size, because of an increase in the number density of grain boundary nucleation sites. Umemoto et al.2 reported similar results and proposed an equation that described the grain size dependence of overall kinetics. Davenport3 argued that the grain size had no appreciable effect upon the transformation kinetics. By contrast, Graham and Axon4 suggested that because the growth of a bainite plate is resisted by the matrix, a smaller austenite grain size must retard growth. These apparently contradictory studies have yet to be rationalised. It has been demonstrated recently8,9 that different steels can show opposite effects of the austenite grain size on the bainite reaction rate and that the difference in kinetic behaviour is accompanied by obvious distinctions between the bainite microstructures. The purpose of the present work was to derive a general equation describing the reaction rate, taking into account the bainite morphology experimentally observed, with the aim of rationalising the austenite grain size effect.