Surface Hardening Treatment for Titanium Materials Using Ar-5%CO Gas in Combination with Post Heat Treatment under Vacuum

Abstract

Surface hardening using solute oxygen formed by the dissociation of titanium oxide (TiO₂) layer on commercially pure (C.P.) 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 steps: surface hardening using Ar-5%CO gas for a short time period and subsequent heat treatment under vacuum. Both treatments were carried out at 1073 K. The maximum surface hardness and hardening layer depth for C.P. titanium obtained by surface hardening in Ar-5%CO gas for 1.8 ks were 420 Hv and 30 μm, respectively. After post heat treatment for 14.4 ks, these values increased to 820 Hv and 70 μm, respectively. The increase of surface hardening achieved by post heat treatment was yielded by solid solution hardening of oxygen via the following steps. Solute oxygen was continuously formed at the oxide layer/titanium interface by the dissociation of the oxide layer formed during surface hardening treatment. Oxygen then diffused into titanium matrix, which resulted in solid solution hardening. The highest and lowest values of the maximum surface hardness were obtained in C.P. titanium and Ti-15-53 alloy, respectively. On the other hand, the hardening layer depth was largest for Ti-15-53 alloy and smallest for C.P. titanium. These results can be explained by the differences in solubility and diffusivity of oxygen between the titanium α phase and β phase. This two-step process appears to be a beneficial industrial surface hardening method for titanium materials because it enables the removal of the oxide layer while yielding surface hardness comparable to that obtained by the one-step process under the same total heat-treating time.