Microstructure and Mechanical Properties of an Ultrahigh-Strength 40SiMnNiCr Steel during the One-Step Quenching and Partitioning Process

The quenching and partitioning (Q&P) process is a novel heat treatment for the enhancement of the strength level of steels without a significant deterioration of ductility. In this work, a study of 40SiMnNiCr steel subjected to the one-step Q&P process is presented. The study results suggest that the strength level of the steel subject to one-step Q&P increases at first and subsequently decreases with the partitioning time because of the synergistic effect of the increase in the retained austenite fraction, the decrease in carbon supersaturation in martensite, the change in the dislocation density in martensite, and the formation of transition carbide. The presence of the transition carbide markedly increases the strength level of the one-step quenched and partitioned steel, with the ultimate tensile strength (UTS) over 2400 MPa and the ductility more than 10 pct during partitioning at 453 K (180 °C) for 180 seconds. Isothermal martensite transformation possibly occurred in this medium-carbon ferrous alloy during the one-step Q&P processing. Meanwhile, in the early stages of the low-temperature partitioning process, carbon partitioning from martensite to austenite plays a dominant role in the carbon redistribution competitions. In addition, the relationship between the microstructure and mechanical properties of the one-step quenched and partitioned steel is discussed.