KINETICS OF MARTENSITIC-TRANSFORMATION IN PARTIALLY BAINITIC 300M STEEL

The kinetics of athermal martensitic transformation have been studied in a high silicon steel (300 M), beginning with samples which were first partially transformed to bainitic ferrite. It is found that the way in which the volume fraction of martensite increases with undercooling below the martensite start temperature is not greatly influenced by the presence of bainitic ferrite, when any carbon enrichment in the residual austenite is taken into account. the martensitic transformation obeys, within the limits of experimental errors, the same law irrespective of the presence or absence of bainitic ferrite prior to transformation. A new relationship, which takes some account of autocatalysis, has been derived to rationalize the athermal kinetics of martensitic reactions and, within the context of certain approximations, is found to be in reasonable agreement with experimental data. The role of chemical composition variations, of the type normally present in commercial steels, seems to be mainly to extend the temperature range over which most of the martensite reaction occurs in the heterogeneous samples, relative to samples which were given a homogenizing heat treatment. © 1990.