Effect of pre-existing martensite on the isothermal transformation kinetics below the M_s temperature in a low-chigh-Si steel

Abstract

Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (Ms). Investigations show that the presence of martensite prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. The aim of this study is to determine whether the presence of a small volume fraction of martensite affects the nucleation kinetics of the subsequent transformation, as well as to qualitatively and quantitatively determine this contribution. For this purpose, dilatometry experiments were performed at different temperatures above and below Ms in a lowcarbon high-silicon steel. The isothermal product formed both above and below Ms has been identified as bainitic ferrite. The transformation kinetics analysis demonstrated that the bainitic ferrite formation is much faster below Ms than above Ms, with a difference of two orders of magnitude.