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. Titanium materials used were aϩb type alloy of Ti-4.5%Al-3%V-2%Mo-2%Fe (SP-700) and b type alloy of Ti-15%V-3%Cr-3%Sn-3%Al . Oxidation accompanied with surface hardening in use of Ar-5%CO gas is much reduced compared with that of Ar-20%CO 2 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 aϩb two phases or phase transformation from b to aϩb two phases, respectively, and surface hardening is primarily controlled by volume fraction of a phase hardened by interstitials enrichment. The other b type titanium alloy of Ti-15%Mo-5%Zr-3%Al yields much marked surface hardening over Ti-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. KEY WORDS: C.P. titanium; SP-700 alloy; Ti-15V-3Cr-3Sn-3Al alloy; surface hardening; CO gas; CO 2 gas; oxidation; maximum surface hardness; hardening layer depth.

Sliding wear resistant properties of C.P. titanium and SP-700 alloy surface-hardened by Ar-5%CO gas were evaluated using a counterpart material of a bearing steel, JIS SUJ-2 steel and Nishihara type of sliding wear testing machine. In the latter, two disk specimens were rotated at different rotating speeds under a given compressive applied load, yielding a sliding ratio of 20 %. Wear tests were repeated intermittently for several times, and a respective test time period in each series of wear tests was primarily varied. The mass loss in both disks was measured after each test. Wear resistance of annealed C.P. titanium without surface hardening was inferior to that of annealed SUJ-2 steel, but surface hardened C.P. titanium resulted in superior wear resistance over quench-tempered SUJ-2 steel with a hardness of 720 in Hv. Observation results of worn surfaces in both disks indicate that preferential wear occurred in the convex region of a furrow-like pattern formed by a lathe machining, resulting in a reduction of surface roughness values with wear progress. When a respective test time period was extended to 21.6 ks, adhesive wear took place between worn surfaces in both specimens, and the mass loss ratio in titanium disk increased at a much higher rate compared with that of a respective test time period of less than 14.4 ks. The steel debris torn off from worn surface of SUJ-2 steel disk was observed to adhere to the worn surface in surface hardened C.P. titanium disk. Wear resistant property of surface hardened SP-700 alloy was also superior to quench-tempered SUJ-2 steel.KEY WORDS: surface hardening; sliding wear resistance; C.P. titanium; SP-700 alloy; SUJ-2 steel; worn surface; adhesive wear; surface roughness. © 2008 ISIJimproved by the increase of hardness in base alloys or in the surface layer of metals or alloys. The former is basically performed by combination of alloy designing and heat treatment, but it is not always useful in titanium materials because the attainable maximum hardness in titanium alloys is very limited compared with steels. The latter is achieved by application of various surface engineering technologies, which have been mostly studied in titanium materials. In fact, various surface engineering technologies such as anodizing, shot peening, nickel or hard chromium plating, various diffusion treatments or plasma spray have been investigated for improvement of wear resistance of titanium or its alloys. 3,8,9) Among them, surface hardening using oxygen gas, which is a diffusion treatment, has been most widely studied in titanium materials, and the wear resistant property of C.P. titanium or Ti-6%Al-4%V alloys surface-hardened by this method was evaluated.10-13) Thermal oxidation for surface hardening adopted in these studies was mostly conducted at 1 173 K, which accompanied with marked oxidation. To solve this problem, Dong et al. developed the oxygen boost diffusion process for the deepcase hardening of titanium alloys, where the oxide layer formed during thermal oxidation was utilized f...

Surface hardening using solute oxygen formed by the dissociation of titanium oxide (TiO 2 ) layer on commercially pure (C.P.) titanium, þ type Ti-4.5Al-3V-2Fe-2Mo (SP-700) alloy, and 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 mm, respectively. After post heat treatment for 14.4 ks, these values increased to 820 Hv and 70 mm, 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 phase and 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.