水素ガス雰囲気中における低炭素鋼S10Cの疲労き裂粒界進展機構の検討

抄録

In order to investigate the mechanism for the intergranular fatigue crack propagation of a low carbon steel in a low pressure hydrogen gas environment, two kinds of examinations were carried out. One was the examination on the effect of cyclic prestrain on the crack growth behavior. The other was the in-situ observation of intergranular fatigue crack propagation in hydrogen gas environment. The main results are as follows. (1) SEM observation on the surface morphology of a plain specimen fatigued in a hydrogen gas showed that a gap at the grain boundary induced by slip behavior but not in nitrogen gas. (2) A specimen cyclic-prestrained in hydrogen gas showed the light influences on the increase in crack propagation rate. (3) Mating intergranular facets on the fatigue fracture surfaces showed the matching of striation-like pattern in the manner anticipated from the damaging mechanism of (1). (4) Environment-change test from hydrogen to nitrogen during the fatigue test showed that intergranular facets appeared even in nitrogen. The results (3) and (4) means the damaging process of (1) is valid in actual fatigue crack. (5) In-situ observation of a crack propagation behavior in hydrogen gas showed that a crack propagated faster when along the grain boundary. However, no visible discontinuous crack advance appeared, a large number of load repetitions were required. Phenomena considered to be damage process (1) appeared ahead of a crack tip. From the results, a convincing mechanism for the intergranular fatigue crack propagation process is as follows. Grain boundaries just ahead of a crack tip are damaged due to a large number of hydrogen-enhanced slip repetition, therefore the fatigue crack becomes easier to propagate along the grain boundary in hydrogen.