Phase transformations during the decomposition of austenite below Ms in a low-carbon steel

The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (Ms) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the Ms temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.