Effect of Isothermal Transformation Treatment and Tempering on the Microstructure and Hardness of a Medium C and High Si Steels

Abstract

This research work aims to study the effect of 2wt%Mo on bainitic transformation kinetics and bainite morphology in 0.6wt%C-1.2wt%Si-1.0wt%Mn-0.2wt%Cr steel. Specimens are austenitized at 950°C and rapidly cooled in salt bath and isothermally treated between 200°C–300°C for different time intervals. Another set of specimens are rapidly cooled in oil after an austenitization treatment and then tempered in the temperature range of 200°C–550°C. In C–Mn–Si specimens, the amount of retained austenite increases with increasing the amount of bainite but no retained austenite is observed in bainitic C–Mn–Si–Mo specimens. The tempering behavior of C–Mn–Si–Mo alloy is considerably different than that of the Mo free alloy. Mo in a martensitic microstructure of C–Mn–Si–Mo alloy shows the secondary hardening effect peak upon tempering at 500°C. The examination of the secondary hardening would also contribute to the behavior of Mo in tempered bainitic steels. The bainitic specimens do not show a peak hardness but softening is retarded upon tempering at the same temperature range.