微細粒オーステナイト系ステンレス鋼の降伏強度と疲労特性に及ぼす初期結晶組織および相変態の影響

抄録

Tensile and rotating bending fatigue tests were conducted on SUS304 and SUS316L specimens with precisely controlled grain size and plastic strain, and both fatigue and static strength data were obtained. The relationship between fatigue properties and static strength was then investigated. It was found that, in specimens with a fully recrystallized microstructure, both static and fatigue strengths followed the Hall–Petch relationship. Moreover, SUS304 exhibited fatigue strength comparable to SUS316L despite its lower static strength. This behavior is considered to result from secondary hardening associated with stress-induced martensitic transformation during the fatigue process. X-ray diffraction analysis confirmed that martensitic transformation occurred in SUS304 after 10⁴ cycles at the stress level corresponding to the fatigue limit, and the tendency for martensitic transformation was independent of grain size. Microstructural changes in the early stage of fatigue process were examined using EBSD analysis, which revealed that plastic flow occurred near the surface while martensitic transformation had not yet initiated. Furthermore, when the S–N data were normalized by yield stress, it was found plastic strain and texture introduced by cold rolling significantly increased static strength, whereas the improvement in fatigue strength was relatively small.