ガスブローIH窒化処理を施したβ-rich α+β型チタン合金（SP700^®）の軸荷重疲労特性の評価

抄録

Gas blow induction heating (GBIH) nitriding is a rapid nitriding method, but there are few researchs to evaluate its fatigue properties. In this study, hourglass-type fatigue specimen made of beta-rich alpha+beta-type titanium alloy (SP700^®) was treated with GBIH nitriding to evaluate axial loading fatigue properties. GBIH nitriding was performed at 1173 K for 60 s in a controlled nitrogen atmosphere, and its effect on the surface characteristics and fatigue properties was investigated. The formation of the nitrogen compound (TiN) layer with the thickness of approximately 2 μm was clearly observed using an optical microscope and an X-ray diffraction. The formation of the nitrogen diffusion layer (approximately 70 μm) was recognized using a micro-Vickers hardness tester. Axial loading fatigue tests were performed at a stress ratio of -1. The fatigue strength of the GBIH nitrided titanium alloy (GBIH series) exhibited a considerable decrease as compared to that of the mirror-polished alloy (P series). This was attributed to grain coarsening and the formation of the compound layer. Additional fatigue tests were performed for the GBIH+P series in which the compound layer from the GBIH series was removed to determine the effect of the compound layer; however, the fatigue strength was still lower than that of the P series. These results indicated that grain coarsening was the dominant factor in decreasing the fatigue strength.