Martensite tempering kinetics: Effects of dislocation density and heating rates

In this study, the combined effect of dislocation density and rapid thermal cycling on the tempering kinetics of fully martensitic steel has been investigated by straining the sheets to different deformation levels. It was found that the strain-induced dislocations created more nucleation sites for carbide precipitation, and decreased carbide growth time. Strain-enhanced precipitation caused the carbide morphology to change from elongated to small quasi-spherical precipitates. Plastic deformation also accelerated the decomposition of carbides and film-like retained austenite in the as-received martensite laths. After tempering the deformed martensite had higher microhardness than tempered unstrained martensite, resulting from finer and semi-coherent precipitates, a high retained dislocation density, and the presence of untempered and partially tempered martensite blocks.