In this study, M50NiL steel was carburized (C), nitrided (N), and compound-carburized then nitrided (C + N). Vein-like grain boundaries (VLGBs) were observed in the diffusion layers of both the N and C + N states due to the limited opportunity for diffusion. Transmission electron microscopy (TEM) observation revealed that the VLGB organization differed in the N and C + N states. The VLGB organization consisted mainly of Fe₄N in the N state and Fe₃C and Fe₄N in the C + N state. When the C state was pre-modified by a 200 MPa water jet and then nitrided (C + 200P + N), the increase in dislocation density resulted in a dislocation entanglement phenomenon that split the grains to form subcrystals. The increases in grain boundaries and dislocation density promoted the diffusion of atoms, and thus the VLGB structure was not observed in the diffusion layer of the C + 200P + N state.

This study presents the fabrication of a Cr₃C₂-25NiCr/NiCr coating on Co₃W₃ steel utilizing high-velocity oxygen fuel (HVOF) spraying. The effects of the vacuum heat treatment process on the microstructures, mechanical properties, and wear mechanisms of the coating were systematically analyzed. The results indicated that the microstructure became denser following heat treatment. During the spraying procedure, decarburization resulted in transformation of the metastable phase structure into a stable one. In comparison to the sprayed coating, there was a 93.8% reduction in porosity. The precipitation of nano-secondary carbides shifted the mechanism of solid-solution strengthening to precipitation strengthening, resulting in a 29.1% increase in microhardness. Meanwhile, the thermal softening effect led to a 114.3% increase in fracture toughness. Wear experiments demonstrated that the friction-induced amorphous structure effectively mitigated stress concentration and inhibited crack initiation. The polycrystalline interface transition region between the nano-secondary carbides and the matrix facilitated the shedding of nano-secondary carbides, forming abrasive particles that generated a rolling effect, which significantly reduced the coefficient of friction. The semi-coherent interface between secondary carbides and NiCr decreased the interfacial energy and enhanced the bonding strength, effectively preventing the shedding of carbides during the wear process. Consequently, a dense microstructure, the type of interface, and high hardness and toughness were critical factors in enhancing its wear resistance.