Microstructure-hardness relationship in quenched and partitioned medium-carbon and high-carbon steels containing silicon

We describe here the effects of quenching and partitioning (Q&P) process on the evolution of microstructure and consequent changes in hardness in a set of medium-carbon and high-carbon steels containing varying percentage of chromium, manganese, and silicon, with the aim to advance our understanding of Q&P process. The study suggests that in medium-carbon steels, higher partitioning of carbon from martensite to retained austenite and stabilization of austenite occurs when martensite has a higher supersaturation of carbon after quenching, which is obtained at low quench temperature. Another important aspect that emerges from the study is that the transition (ɛ) carbide decreases hardness and its formation is promoted in medium-carbon steels with higher silicon-content such that the precipitation occurs at lower temperature of 250 °C. However, in contrast to medium-carbon steels, the high chromium content in high-carbon steels has a negative impact on the Q&P process because of the formation of large cementite during the spheroidizing treatment that reduces the ability of austenite to be enriched with carbon. The decrease in hardness in high-carbon steels during partitioning is a cumulative effect of austenite stabilization, softening of martensite, and decrease in carbon supersaturation of martensite.