Effect of Initial Austenite Grain Size on Microstructure Development and Mechanical Properties in a Medium-carbon Steel Treated with One-step Quenching and Partitioning

Fe-0.4C-1.2Si-0.8Mn (mass%) alloys austenitized at different temperatures, ranging from 1103 to 1473 K, were subjected to interrupted quenching (IQ) at 473 K and then maintained at that temperature to induce the partitioning of carbon from martensite to austenite (one-step quenching and partitioning (Q&P) process). The initial austenite grain size before the IQ was varied from 20 to 573 µm. As the initial austenite grain size becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. The reasons for the increased carbon enrichment were explained by the effective carbon partitioning as well as the promoted bainitic transformation, which were both caused by the increase in the area of the martensite/austenite interface. Tensile tests of the specimens with different initial austenite grain sizes revealed that the mechanical properties of the one-step Q&P specimens improved in both strength and elongation by the refinement of the initial austenite grains.