焼戻しマルテンサイト鋼の水素脆化特性と破壊形態

抄録

Hydrogen embrittlement behavior of tempered martensitic steels has been investigated using a conventional strain rate test (CSRT) for circumferential notched specimens with various notch tip radii. For smaller notch tip radii (below 0.8 mm), hydrogen-charged specimens initially fractured near the notch tip. In contrast, for larger notch tip radii (above 1 mm), hydrogen-charged specimens initially fractured near the center of the specimen; the crack then propagated and failure occurred. The cracks of hydrogen-charged specimens with smaller notch tip radii occurred on the load-displacement curves of uncharged specimens, and the load remained constant regardless of the increase in displacement. These specimens showed quasi-cleavage (QC) and/or intergranular (IG) fracture morphologies. The results indicated that QC and IG fracture modes were stable. Although fracture morphologies changed from dimple to QC to IG with increasing hydrogen content, the critical hydrogen content was identical regardless of the notch tip radii. Fractography of hydrogen-charged specimens with larger notch tip radii unloaded just after the maximum tensile load clearly indicated that QC fracture was stable since it originated at several points, propagated and then coalesced. The relationship between stress triaxiality and critical equivalent plastic strain used for dimple failure could also be considered applicable to QC fracture. The presence of hydrogen in specimens decreased markedly under the critical equivalent plastic strain in this relationship.