Surface Hardening Treatment for Titanium and Titanium Alloys in Use of CO2 Gas
Y.-Z. KIM, Ryoji SAHARA, Takayuki NARUSHIMA, Yasutaka IGUCHI, Chiaki OUCHI
Surface hardening treatment of C.P. titanium, α+β type Ti-4.5%Al-3%V-2%Mo-2%Fe (SP-700 alloy) and β type Ti-15%V-3%Al-3%Cr-3%Sn alloy in use of Ar-20%CO2 gas was studied at temperatures between 973 to 1123K in comparison of Ar-20%O2 gas. Weight gain of the specimen during heating was measured by an electric balance, and surface oxide layer thickness was also measured. Surface hardening was evaluated by measurement of micro-hardness distribution profile in the surface layer. Surface hardening due to Ar-20%O2 gas yielded the larger maximum surface hardness value and thicker hardening layer thickness in all three titanium materials compared with Ar-20%O2 gas. The largest surface hardness value was obtained in C.P. titanium, and β type titanium alloy resulted in the most thick hardening layer thickness although the surface hardness value was the smallest among three titanium materials. The maximum surface hardness and hardening layer thickness obtained in use of Ar-20%O2 gas was larger than these of Ar-20%O2 gas in C.P. titanium, nevertheless the former gas yielded higher oxidation rate over the latter gas. The oxidation rate was the smallest in SP-700 alloy among three titanium materials in both gases. Surface hardening due to Ar-20%O2 gas in C.P. titanium was brought about by solid solution hardening due to oxygen and carbon.
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