予ひずみを付与したオーステナイト系ステンレス鋼SUS304の疲労挙動に及ぼす加工誘起マルテンサイト変態の影響

Abstract

The fatigue behavior of prestrained type 304 stainless steel was studied. Rotating bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using specimens subjected to the tensile-prestrains of 15%, 30% and 60%. A particular attention was paid to the stress-induced martensite transformation during stress cycling. The fatigue strength of the prestrained specimens increased with increasing prestrain in both environments, but decreased significantly in 3%NaCl solution compared with in laboratory air. The strain-induced martensite transformation occurred in the prestrained specimens, and martensite phase increased with increasing prestrain and with stress cycling at the fatigue limit stress in the 30% and 60% prestrained specimens. The coaxing effect took place remarkably in the unprestrained specimen, but decreased with increasing prestrain. The increase in fatigue strength of the prestrained specimens in laboratory air and the coaxing effect were attributed to both work hardening and the strain-induced martensite transformation, where the contributions of the former and the latter decreased and increased with increasing prestrain, respectively. Since corrosion pits were present at the crack initiation site in the large prestrained specimens, the decrease in fatigue strength in 3%NaCl solution was due to larger environmental susceptibility of martensite phase.