Complementary Thermal Analysis Protocols for the Investigation of the Tempering Reactions of a Carbide-Free Bainitic Steel

Austenite corresponds to a major constituent of carbide-free bainitic steels required to obtain enhanced deformability assisted by transformation-induced plasticity. It is, therefore, of foremost importance to retain sufficient amounts during processing and heat treatment. In order to analyze and describe the tempering reactions occurring during heating a material consisting of bainitic ferrite and C-enriched austenite, a complementary thermal analysis protocol has been applied using dilatometry and differential scanning calorimetry. Thereby, kinetic parameters were determined and interpreted in the view of literature and supporting calculations. Austenite decomposition was observed using both methods in the temperature regime of about 500°C. The reaction’s signature was revealed by calculation of the transformation enthalpy. Dilatometry suggests that decomposition of highly C-enriched austenite predominates at lower temperatures and, on further heating, regions with lower C concentrations follow. Additionally, dilatometry revealed C redistribution in bainitic ferrite already occurring at temperatures between 230°C and 330°C.