Surface hardening using solute oxygen formed by dissociation of oxidation film on C.P. (commercially pure) titanium, α+β type Ti-4.5Al-3V-2Fe-2Mo (SP-700) alloy and β type Ti-15Mo-5Zr-3Al (Ti-15-53) alloy was investigated. This method consists of two processes at 1073 K; surface hardening in use of Ar-5%CO gas with a short period of time and subsequent heat treatment under vacuum. The maximum surface hardness and hardening layer depth of C.P. titanium obtained by surface hardening for 1.8 ks were 420 Hv and 30 μm, respectively and these values increased to 820 Hv and 70 μm, respectively, by post heat treatment for 14.4 ks. The extra-surface hardening obtained by post heat treatment was yielded by solid solution hardening of oxygen with the following steps; the solute oxygen is continuously formed at oxide/titanium interface by the dissociation of oxidation film formed by the surface hardening treatment, and then it diffuses into titanium matrix. The maximum surface hardness was the highest in C.P. titanium and the lowest in Ti-15-53. On the other hand, the hardening layer depth was the deepest in Ti-15-53 and the shallowest in C.P. titanium. These results could be explained by the difference of solubility and diffusivity of solute oxygen between titanium α and β phases. This two-step process seems to be a beneficial industrial surface hardening method for titanium materials because it enables to remove the oxide film, achieving the very high maximum surface hardness comparable to that obtained by one-step surface hardening under the same total processing time.
