Influence of intercritical tempering temperature on impact toughness of a quenched and tempered medium-Mn steel: Intercritical tempering versus traditional tempering

The influence of intercritical tempering temperature on impact toughness of quenched and tempered 0.05C-5.42Mn medium-Mn steel was studied and compared with traditional tempering. The experimental steel had high hardenability because of high Mn-content. Lath-like α'-martensite without retained austenite was obtained over a wide range of quenching rate of 0.5-30 °C/s, and the quenched steel showed high strength but low impact toughness. On intercritical tempering at 625 °C and 665 °C, the impact toughness was enhanced, as compared to traditional tempering at 570 °C. The reversed austenite enriched with Mn and C formed between the martensite laths was the underlying reason for the increased absorbed crack propagation energy, and the ductile-brittle transition temperature (DBTT) was reduced because of increased stability of reversed austenite. Compared to the steel tempered at 625 °C, the steel tempered at 665 °C contained more reversed austenite, but the reversed austenite was less stable because of reduced enrichment of Mn and C. The enrichment or depletion of Mn and C in austenite and martensite was thermodynamically studied by DICTRA. In striking contrast to the steels tempered between 625 and 665 °C, twinned martensite was formed in the steel tempered at high temperature of 700 °C, and the steel exhibited impact toughness lower than the quenched steel.