MaterialsScienceand Engineering, A129 (1990) 257-272 257 Kinetics of Martensitic Transformation in Partially Bainitic 300M Steel S. A. KHAN and H. K. D. H. BHADESHIA Departmentof MaterialsScienceand Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ (U.K.) (Received January 2, 1990; in revisedform April 2, 1990) Abstract The kinetics of athermal martensitic trans- formation have been studied in a high silicon steel (300M), beginning with samples which were first partially transformed to bainitic ferrite. It is found that the way in which the volume fraction of mar- tensite increases with undercooling below the martensite start temperature is not greatly in- fluenced 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 rela- tionship, which takes some account of auto- catalysis, 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 experi- mental data. The role of chemical composition variations, of the type normally present in com- mercial steels, seems to be mainly to extend the temperature range over which most of the martens- ite reaction occurs in the heterogeneous samples, relative to samples which were given a homogeniz- ing heat treatment. 1. Introduction Mixed microstructures consisting of bainite and martensite are usually a consequence of inadequate heat treatment or the use of steels with insufficient martensite hardenability in applications involving heavy sections. Early research [1-3] suggested that the presence of bainite in an otherwise martensitic microstructure generally leads to a deterioration in ductility, toughness and strength. Contrary to these generally pessimistic indica- tions of the properties of mixed microstructures, more recent work by Tomita and Okabayashi [4, 5] has tended to indicate that in some circum- stances the presence of bainite in a pre- dominantly martensitic microstructure enhances both strength and toughness relative to the single- phase samples. They explained their results by postulating that, as the lower bainite subdivides regions of austenite, there is in effect a refinement of the austenite grain size and consequently a refinement of the martensite packet size on the subsequent transformation of austenite. This leads to a strengthening of the martensite via a grain size effect. In addition, the strength of the bainite is supposed to be enhanced by the con- straint provided to its deformation by the stronger martensite. Thus, mixed microstructures of bainite and martensite are bound to become more prominent commercially but are also of interest from a fundamental point of view, to reveal the influence of partial bainitic reaction on the subsequent formation of martensite. An attempt is therefore made here to model the development of martens- itic transformation in a high silicon, medium carbon steel, with the commercial designation "300M", with the particular aim of rationalizing the formation of martensite in samples which already contain some bainitic ferrite. For this reason, some of the samples studied were quenched directly from the austenitization tem- perature, while others were first transformed iso- thermally to bainitic ferrite, before cooling to ambient temperature to allow some of the residual austenite to decompose to martensite. The studies were carried out both on chemically segregated "as-received" samples and, for com- parison purposes, on samples homogenized by prolonged heat treatment at a high temperature. The work presented here extends earlier research [6, 7], which focused on the develop- ment of the bainite reaction, to the modelling of the subsequent transformation of some of the residual austenite into martensite. The ultimate 0921-5093/90/$3.50 © ElsevierSequoia/Printedin The Netherlands