Carburization-induced microstructure evolution and hardening mechanism of 18CrNiMo7-6 steel

Abstract

The microstructural evolution and hardening mechanism caused by carburizing heat treatment must be determined to improve the in-service functional performance of components used in advanced equipment for aerospace and other industrial applications. Carburizing heat treatment causes the microstructure to exhibit heterogeneous characteristics in the depth direction. In this study, the effects of carburizing heat treatment on the microstructure and hardening mechanism of 18CrNiMo7-6 steel were investigated. The experimental results show the residual austenite (RA) content in the carburized surface reached 56.2% and the RA content in the core was 0.1% after carburizing heat treatment. The kernel average misorientation (KAM) value, grain size, low angle grain boundaries (LAGBs) content, and dislocation density caused by carburizing heat treatment increased with increasing depth. After the carburizing heat treatment, considerable grain refinement occurred, and the minimum grain size of the carburized surface was 3.19 μm. The microhardness of the carburized layer first rose and then reduced, achieving a maximum of 686 HV, which was 57.7% higher than that of the quenching and tempering (uncarburized) samples. In addition, it was found that grain boundary and solution strengthening played a prominent position in the hardening mechanism of the carburized layer. This study has deepened the current understanding of the hardening mechanism of 18CrNiMo7-6 steel during carburizing heat treatment.