Surface hardening process accompanying with grain refinement in titanium materials using methane gas

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(1) Methane gas treatmentMaximum surface hardness and hardening layer depth were 850Hv and 230gm, respectively, for Ti-6A1-4V alloy after the treatment in Ar-5%CH_4 mixture gas at 1198 K for 10.8 ks. As for Ti-4.5A1-3V-2Fe-2Mo (SP-700) alloy, the maximum surface hardness of 880 Hv and the hardening layer depth of 130gm were obtained after the treatment in Ar-5%CH_4 mixture gas at 1073 K for 21.6 ks. It was suggested that the use of methane gas is effective for hardening in titanium materials. However, the amounts of dissolved hydrogen into the titanium alloys were not sufficient for grain refinement.(2) Treatment in methane-hydrogen mixture gasBoth alloys were treated in the atmosphere of CH_4:H_2=1:1 at 1023 K for 1.8 ks, followed by aging and dehydration treatments. However, no grain refinement was obtained for both the alloy after these treatments, though the hydrogen content of 1 mass% was achieved.(3) Treatment in hydrogen gasIn order to understand the interaction between titanium and hydrogen, the microstructure change of the titanium alloys was investigated under the conditions of hydrogen dissolution at 1023 K for 1.8 ks, aging at 523-873 for 28.8 ks and dehydration at 973-1173 for 3.6 ks. The grain refinement was achieved in Ti-6A1-4V alloy with aging at 773 K and dehydration at 973 to 1173 K. On the other hand, acicular alpha phase was observed instead of fine alpha grain for SP-700 alloy. The beta transus of SP-700 is 100 K lower than that of Ti-6A1-4V. Therefore, the strain introduced during hydrogen dissolution and aging processes in SP-700 alloy was not enough for recrystallization or recovery. The further study upon the hydrogen dissolution conditions is required.