Characterization of surface layer formed by gas blow induction heating nitriding at different temperatures and its effect on the fatigue properties of titanium alloy

Gas blow induction heating (GBIH) nitriding was carried out for Ti–6Al–4V alloy at different temperatures, and its effect on surface characteristics, wear resistance and fatigue properties was investigated. The microstructures of the specimen surface were characterized using optical microscopy, laser microscopy, X-ray diffraction, micro-Vickers hardness tester, and nano-indentation tester. It was found that the surface characteristics of titanium alloy varied with increasing the treatment temperature of GBIH-nitriding. In this study, a nitrogen diffusion layer was formed on the titanium alloy surface GBIH-nitrided at 1073 ​K. A nitrogen compound layer and diffusion layer were created at the treatment temperature of 1173 ​K, which suppressed the wear loss to under one tenth that of the substrate. In order to examine the fatigue properties of the treated alloy, fatigue tests under four-point bending were performed. The fatigue strength of the Ti–6Al–4V alloy GBIH-nitrided at 1173 ​K was 234 ​MPa lower than that of the mirror-polished alloy. The decrease in the fatigue strength is due to the formation of the compound layer with a Young’s modulus approximately twice that of the substrate, and the grain coarsening from 7 ​μm to 300 ​μm. The results indicate that a thick nitrided layer with high hardness and the close Young’s modulus to the substrate improved the fatigue strength of the Ti–6Al–4V alloy.