Bainite and martensite start temperature calculated with exponential carbon dependence

Abstract

Analysis of published data demonstrates that the start temperatures of bainite (B_s) and martensite (M_s) 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 B_s 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 α_m controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of M_s and α_m, 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 α_m.