圧力容器用Cr-Mo鋼の固溶水素脆化に及ぼす炭化物および介在物の影響

抄録

Internal hydrogen embrittlement at room temperature was investigated for 2 1/4Cr-1Mo and 3Cr-1Mo steels. Hydrogen was charged in hydrogen gas of 100-250 kg/cm² at 400-480°C. The results are as follows:
(1) Ductility loss due to absorbed hydrogen decreased with decreasing tempering parameter until tensile strength reaches 70-75 kgf/mm² and it was found that vanadium addition of above 0.2% reduced the internal hydrogen embrittlement. Both smaller tempering parameter and vanadium addition dispersed finer particles of carbides and, at the same time, increased absorbed hydrogen and markedly decreased hydrogen evolution rate at around room temperature. This suggests that the carbides trap hydrogen and partition of hydrogen to the trap sites increases with lowering temperature. Therefore, hydrogen content in the matrix decreases at room temperature and hydrogen supply to crack initiation sites is reduced, resulting in suppressing the ductily loss.
(2) The ductility loss due to internal hydrogen was caused by the microcracks initiated at inclusions such as Al₂O₃ and MnS. It was affected by the number and size of inclusions.
(3) It was demonstrated that cracks at elongated MnS inclusions were induced without external stress when steels were cooled to room temperature after hydrogenation at high temperature.