High-Temperature Annealing Significantly Enhances Intrinsic Hot Workability of the As-cast High-Nitrogen M42 High-Speed Steel

One important challenge in producing high-speed steels (HSSs) by conventional route is how to improve their hot workability. In this work, the effect of high-temperature annealing on the microstructure evolution and hot deformation behavior of the as-cast high-nitrogen M42 HSS was studied. The results indicated that the as-cast microstructure of high-nitrogen M42 HSS mainly consisted of the dendrite cells of matrix, interdendritic network of M 2 C eutectic carbides, primary M(C,N) carbonitrides, and intradendritic fine M 2 C secondary carbides. After high-temperature annealing, M 2 C eutectic carbides were transformed into M 6 C and M(C,N) products via a diffusion-driven reaction of M 2 C + matrix → M 6 C + M(C,N), and M 6 C carbides were the dominant products. Initially, M 6 C and M(C,N) nucleated at the M 2 C/matrix interface, and then grew inward from outside. With increasing annealing temperature and time, the average length–width ratio of interdendritic mixed carbides of M 2 C and M 6 C gradually decreased. The full decomposition and breaking of M 2 C eutectic carbides in the as-cast high-nitrogen M42 HSS could be achieved by annealing at 1100 °C for 8 hours. Meanwhile, high-temperature annealing led to the decreased amount of intradendritic M 2 C secondary carbides and the formation of M 6 C carbides within the intradendritic region. The high-temperature annealed steel exhibited higher flow stress than the as-cast steel at most deformation conditions, but lower deformation activation energy. The macroscopic morphologies showed that the hot workability of the high-temperature annealed steel was evidently better than that of the as-cast steel, which could be attributed to its better deformation compatibility and more sufficient dynamic recrystallization (DRX). Moreover, high-temperature annealing treatment led to a more uniform carbide distribution after hot compression. Therefore, high-temperature annealing treatment may provide an opportunity to further maintain or even improve the competitiveness of HSS produced by conventional route compared with those by powder metallurgy.