Martensitic transformations in AISI 440C stainless steel

AISI 440C stainless steel possesses a low Ms temperature, which is far below room temperature. After subzero treatment in liquid nitrogen, the steel forms plate martensite with significant amounts of retained austenite. Dilatometric experiments with microstructural observation were performed to investigate the tempered martensite and the decomposed retained austenite during multiple tempering treatments. The results indicate that a complete transformation of retained austenite can be more easily achieved by multiple tempering cycles than by a single long-time cycle. The possible mechanism for the decomposition of retained austenite during multiple tempering cycles is attributed to the invariant-plane-strain of the prior martensitic transformation extending accommodation defects to the adjacent retained austenite, which favors further transformations in the subsequent tempering operations.