Surface hardening treatment for CP titanium and titanium alloys in use of Ar-5%CO gas

Surface hardening of C.P. (commercially pure) titanium and titanium alloys in use of Ar-5%CO gas was investigated in the temperature range between 973 K and 1 123 K. Ttanium materials used were alpha+beta type alloy of Ti-4.5%Al-3%V-2%Mo-2%Fe (SP-700) and beta type alloy of Ti-15%V-3%Cr-3%Sn-3%Al (Ti-15-333). Oxidation accompanied with surface hardening in use of Ar-5%CO gas is much reduced compared with that of Ar-20%CO2 gas. Surface hardening was evaluated by both of the maximum surface hardness and hardening layer depth obtained from hardness distribution profiles in the subsurface region, The former is the highest in C.P. titanium and the lowest in Ti-15-333 alloy, and the latter is the deepest in Ti-15-333 alloy and the shallowest in C.P. titanium. Surface hardening in C.P. titanium is caused by solid solution hardening of oxygen and carbon enriched in the subsurface region. Enrichment of these interstitials in the subsurface region of SP-700 or Ti-15-333 alloys causes the increase of a volume fraction in alpha+beta two phases or phase transformation from beta to alpha+beta two phases, respectively, and surface hardening is primarily controlled by volume fraction of alpha phase hardened by interstitials enrichment. The other beta type titanium alloy of Ti-15%Mo-5%Zr-3%Al yields much marked surface hardening over T-15-333 alloy. All of these results were analyzed and discussed based on oxygen and carbon concentration profiles, which were obtained by EPMA, and were also calculated by uni-dimensional diffusion model.