Bainite and martensite start temperature calculated with exponential carbon dependence

Abstract

Analysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0?1-1?9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and M s temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the M s temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter am controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of M s and am, the volume fraction of retained austenite at room temperature has been calculated for Fe-C alloys. The calculated fraction of retained austenite as a function of carbon content is found to be in good agreement with published data, which gives confidence in the proposed models for M s and αm. © 2012 Institute of Materials, Minerals and Mining.