Bainitic transformation during the two-step quenching and partitioning process in a medium carbon steel containing silicon

A study of 40SiMnNiCr steel subjected to a two-step quenching and partitioning process (Q&P) is presented. The result suggests that strength variation of Q&P steels during the two-step Q&P process was a cumulative effect of increase of retained austenite fraction, decrease of carbon supersaturation of virgin martensite, and particularly much of lower bainite formation. A trade-off between high strength and good ductility of two-step Q&P steels can be tailored and adjusted by controlling lower bainite fraction. The final amount of austenite was influenced by the transformation kinetics of lower bainite during the partitioning process.

Research highlightsIn this paper, SEM and TEM were used to characterize microstructure of Q&P steels with different partitioning time at 300 °C. The interesting phenomena were discovered and discussed:1.Lower bainite (bainitic ferrite plus ɛ-carbide) rather than carbide-free bainite was observed during partitioning process.2.The mechanical properties of Q&P steels can be tailored and adjusted through balance volume fraction of retained austenite and lower bainite during partitioning process.3.The final amount of austenite was influenced by the transformation kinetics of lower bainite during partitioning process.According to the analysis, it can be concluded that associated with carbon partitioning from martensite to austenite, lower bainite transformation inevitably occurred. More importantly, lower bainite transformation seriously affected the mechanical properties of Q&P steels and final amount of austenite.