Influence of the cooling rate below Ms on the martensitic transformation in a low alloy medium-carbon steel

Abstract : The influence of cooling rate below the martensite start temperature, Ms, on the kinetics of martensitic transformation in a medium carbon low alloy steel was determined using high resolution dilatometry, optical microscopy and X-ray diffraction techniques. A two-stage transformation was observed for slow cooling rates while at higher cooling rates, martensitic transformation occured through a single stage process. It is shown that the Koistinen–Marburger equation cannot adequately describe the observed two-stage transformation. A new equation is proposed in order to model the evolution of martensitic transformation by considering the influence of post Ms cooling rate. The method considers contributions from both, the initial austenite and the carbon enriched austenite. The underlying mechanisms are discussed and validated with experimental findings.