Microstructural evolution and kinetics of the bainite transformation in a silicon-alloyed medium-carbon steel via two-step quenching and partitioning treatment

This study investigates the isothermal bainite transformation kinetics in a silicon-alloyed, medium-carbon steel subjected to a two-step quenching and partitioning (Q&P) treatment, aimed at optimizing the stability and distribution of retained austenite (RA). The designed thermal pathway enabled decoupling of martensite formation and carbon partitioning, offering enhanced control over phase evolution. Dilatometric analysis and Avrami modeling were employed to quantify the transformation behavior during partitioning at 245 °C and 310 °C, following quenching to 140 °C and 180 °C. Microstructural analysis using laser scanning microscopy and EBSD revealed the coexistence of tempered martensite, bainite, RA, and secondary martensite. XRD quantification revealed that the maximum values of RA fractions were near 12–13%, and the average carbon content rose from 0.66 to 1.35 wt.% as partitioning progressed. A notable divergence between increasing RA carbon content and decreasing RA fraction at prolonged partitioning times highlights the competing effects of austenite stabilization, bainitic transformation, and possible carbide precipitation. Arrhenius-derived activation energies indicated lower transformation barriers in samples quenched to 140 °C (53.5–26.8 kJ/mol), confirming accelerated bainite formation due to higher pre-existing martensite content. These findings contribute to a better understanding of microstructural evolution during Q&P processing and may support further improvements in the thermal design of advanced high-strength steels.