Rapid nitriding mechanism of titanium alloy by gas blow induction heating

This work characterized the surface-modified layers generated on a titanium alloy during gas blow induction heating (GBIH), so as to elucidate the rapid nitriding mechanism. The surface microstructures of GBIH nitrided specimens were analyzed by X-ray diffraction, micro-Vickers hardness testing, scanning electron microscopy and X-ray photoelectron spectroscopy. The data indicate that applying this process for only several minutes can forms nitrided layers on Ti-6Al-4V alloy surfaces, the characteristics of which are similar to those of layers produced by ordinary gas nitriding for several hours. The temperature inside the material was determined to be higher than that on the surface during GBIH nitriding. However, the thickness of the nitrided layer exceeded the estimated value based on the diffusion coefficient at this high internal temperature. This especially thick nitrided layer can possibly be attributed to eddy currents generated in the metal by the application of a high-frequency current. Passivation films on the surface of the Ti-6Al-4V alloy were found to be removed during GBIH nitriding, which is ascribed to the diffusion of oxygen atoms in the passivation film into the substrate. Thus, the data show that GBIH nitriding is able to modify a Ti-6Al-4V alloy within a short period of time based on the effect of an eddy current and the disappearance of the passivation film.