Investigation of Microstructure, Residual Stress, and Hardness of Ti-6Al-4V after Plasma Nitriding Process with Different Times and Temperatures

The residual stress and hardness generated by the nitriding process are important parameters for increasing the bending fatigue strength to Ti-6Al-4V. Therefore, this research is focused on the analysis of residual stress and hardness, including surface morphology and microstructure generated by the nitriding process at different times and temperatures. The plasma nitriding at temperatures of 750 °C and 800 °C with times of 5 h and 10 h were selected in this research. After plasma nitriding, the material would have residual compressive stress and higher hardness, including changes in the surface morphology and microstructure. The results also indicated that higher temperature and processing times generated more surface roughness and thickness in the compound layer, resulting in higher surface hardness. Moreover, higher time and temperature could generate deeper residual compressive stress and case depth hardness. This research revealed maximum hardness in the cross-sectional analysis of 643 HV and residual compressive stress of −65.3 MPa. In conclusion, the depth of the residual stress and case depth hardness were well compatible with the depth of the diffusion layer of plasma-nitrided Ti-6Al-4V, which confirmed the effect of plasma nitriding.