低合金鋼の焼戻しマルテンサイトにおける耐水素脆性に及ぼすMnの影響

抄録

The effect of Mn content on hydrogen embrittlement resistance of tempered martensite in low alloy steel was investigated. The hydrogen embrittlement resistance was estimated with Double Cantilever Beam (DCB) test in aqueous environment containing hydrogen sulfide. In the DCB test, the specimens with pre-crack were prepared, the crack propagated while the specimens were exposed to aqueous environment containing hydrogen sulfide. The crack propagation route was analyzed into intergranular fracture and transgranular fracture, and intergranular fracture rate was calculated. The fracture toughness decreases from 29 MPa√m to 25 MPa√m with increasing Mn content from 0.5% to 1.5%. Then, the intergranular fracture rate increases from 26.6% to 54.4%, and the absorbed hydrogen content increases from 1.8 mass ppm to 2.3 mass ppm. The decrease of fracture toughness is probably because cohesive energy of grain boundary (2γ_int) decreases with increasing Mn content at the prior austenite grain boundary and increasing absorbed hydrogen.