Isothermal formation of alfa martensite at sub-zero Celsius temperature in AISI 632 stainless steel

A semi-austenitic stainless steel of AISI 632 grade was austenitized according to industrial specifications and thereafter subjected to isothermal treatments at sub-zero Celsius temperatures. Isothermal austenite-to-martensite transformation at sub-zero Celsius temperatures was monitored in situ by magnetometry and data are used to construct a TTT diagram for transformation. As an alternative treatment, the material after austenitization was additionally immersed in boiling nitrogen and upquenched in water prior to be subjected to sub-zero Celsius investigation. Magnetometry shows that the additional thermal step yields a negligible increase of the martensite content in the material, but has a noteworthy effect on the kinetics of isothermal martensite formation at sub-zero Celsius temperatures. Data are interpreted in terms of athermal nucleation of martensite clusters followed by their time dependent growth.