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

(1)甲烷气体处理：在1198 K的Ar-5%CH_4混合气体中处理10.8 ks后，Ti-6A1-4V合金的最大表面硬度为850Hv，硬化层深度为230gm。Ti-4.5A1-3V-2Fe-2Mo （SP-700）合金在Ar-5%CH_4混合气中1073 K热处理21.6 ks后，表面硬度达到最大880 Hv，硬化层深度达到130gm。结果表明，甲烷气体对钛材料的硬化是有效的。然而，溶解在钛合金中的氢的量不足以使晶粒细化。(2)甲烷-氢混合气处理两种合金均在CH_4:H_2=1:1的气氛中，在1023 K下处理1.8 K，然后进行时效和脱水处理。然而，经过这些处理后，两种合金都没有得到晶粒细化，尽管氢含量达到了1质量%。为了了解钛与氢的相互作用，研究了在1023 K下氢溶1.8 K、523 ~ 873时效28.8 ks、973 ~ 1173脱水3.6 ks条件下钛合金的显微组织变化。在773 K时效和973 ~ 1173 K脱水条件下，Ti-6A1-4V合金晶粒细化。另一方面，SP-700合金中没有细小的α晶粒，而是出现了针状α相。SP-700的β透射比Ti-6A1-4V低100 K。因此，SP-700合金在氢溶解和时效过程中引入的应变不足以再结晶或恢复。氢溶解条件有待进一步研究。

(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.