Phase Transformations in Isothermal Treatments Below the Ms temperature

Thermomechanical processing is of crucial importance for the design of Advanced Multiphase High Strength Steels. Heat treatments often include an isothermal holding around the martensite start temperature (M_s) in order to obtain a multiphase matrix. This matrix is generally formed by non-equilibrium phases, such as bainite, martensite, and retained austenite. When the cooling from austenitisation is stopped between the martensite start (M_s) temperature and the martensite finish (M_f) temperature, a predetermined fraction of athermal martensite is formed previous to the isothermal treatment. Investigations show that the presence of martensite accelerates the kinetics of the subsequent transformation. However, which is the qualitative and quantitative contribution of this martensite to the transformation kinetics? After the mentioned initial cooling, the untransformed austenite can decompose into other product phases during the subsequent isothermal treatment. Although bainite has been reported as a product phase from the decomposition of austenite in isothermal treatments below M_s, there is still discussion about the nature of the isothermal product obtained in those treatments. This product can be identified as bainite or martensite due to similarities with both product phases. The combination of characterization techniques, such as SEM and EBSD, can give us a better insight about the nature of the product phases formed. The contribution of prior martensite to the subsequent transformation kinetics as well as the structural and morphological differences identified in the product phases formed in treatments below M_s will be highlighted.