2021 年 87 巻 895 号 p. 20-00439
In order to clarify the effect of internal hydrogen on the fatigue life properties of SUS304, SUS316 and SUS316L, tensile tests and low- and high-cycle fatigue life tests were carried out in air at room temperature using 10, 68 and 100 MPa hydrogen-charged specimens. High-cycle fatigue life tests demonstrated that S-N curve (i.e., relationship between stress amplitude, σa, and number of cycles to failure, Nf) of each steel was higher in hydrogen-charged specimen than in uncharged specimen. The increase in fatigue limit, Δσw, with internal hydrogen was 40 MPa in 100 MPa hydrogen-charged specimens, 20 or 30 MPa in 68 MPa hydrogen-charged specimens, and 0 or 10 MPa in 10 MPa hydrogen-charged specimens. Low-cycle fatigue life tests manifested that εta-Nf curve (i.e., relationship between total strain amplitude, εta, and number of cycles to failure, Nf) of 68 MPa hydrogen-charged specimen was nearly coincident with that of uncharged specimen in SUS316L, whereas 68 MPa hydrogen-charging markedly lowered εta-Nf curve in SUS304. The fraction of strain-induced martensite was measured on specimens fractured by tensile tests and low- and high-cycle fatigue life tests. The critical value of the martensite fraction below which 68~100 MPa hydrogen-charging does not cause hydrogen embrittlement, fmH, was 1 % in tensile tests. On the other hand, the fmH value was 9% in low- and high-cycle fatigue life tests. The increase in fatigue limit due hydrogen-induced solid solution strengthening, Δσw, in high-cycle fatigue life tests was expressed as Δσw (MPa) = 15.4 × 237H, where H is the hydrogen content (mass %). In addition, the hydrogen-induced strengthening of stress amplitude, Δσa, and 0.2% proof strength, Δσ0.2, in low-cycle fatigue life tests was expressed as Δσa+0.2 (MPa) = 15.4 × 296H. The results inferred that the contribution of hydrogen to solid solution strengthening was about 10 times larger than that of carbon and nitrogen when compared at the same mass concentration.
