Improved mechanical and wear properties of H13 tool steel by nitrogen-expanded martensite using current-controlled plasma nitriding

Nitrogen-expanded martensite is a crystalline structure formed by incorporating nitrogen atoms into the interstitial sites of martensitic steels. It has recently gained significant industrial attention due to its excellent mechanical and wear-resistance properties. However, the major challenge in synthesizing nitrogen-expanded martensite is obtaining a phase free of iron or chromium nitrides precipitates. In this study, the effects of varying temperature (460, 480, and 500 °C) and plasma current density (0.5, 1.0, and 1.5 mA/cm2) during plasma nitriding of H13 tool steel were investigated to evaluate their influence on crystalline phases. The results of X-ray diffraction analysis indicate that expanded martensite free of nitride precipitates can be obtained at a temperature of 480 °C and low current density. Moreover, wear analysis using a ball-on-disk wear tester showed that the lowest wear rates were achieved under similar conditions. Grazing incidence X-ray diffraction analysis revealed that the outer region of the nitrided zone had a disordered structure, which could be attributed to a nano crystallization process. The nanoindentation analysis demonstrated that the expanded martensite phase has rigid-elastic properties characterized by high elastic energy (144.5 nJ) and high resistance to plastic deformation