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転位密度の影響

The effect of dislocation density on hydrogen embrittlement resistance of tempered martensite in low alloy steels was investigated quantitatively. The various samples of which dislocation density was from 0.7×1014 m−2 to 4.5×1014 m−2 were prepared by changing tempering temperature and C content. Then, the yield stress of the samples was between 780 MPa and 1020 MPa. Especially, the change of the C content made various dislocation density samples even though the yield stress is almost same. The hydrogen embrittlement resistance was estimated with four-point bend test. The hydrogen embrittlement resistance decreases with increasing the dislocation density and the yield stress. Although the critical yield stress was changed dramatically by changing C content, the critical dislocation density was almost same (about 2.0×1014 m−2) even though the C content was different. In addition, the absorbed hydrogen content correlated with the dislocation density, and did not depend on the C content in this research. This means that the dislocation density is higher, the hydrogen trapping ability is higher. As the result, when the absorbed hydrogen content reach the critical diffusible hydrogen content, hydrogen embrittlement cracking occurs probably.