Effect of internal hydrogen on low- and high-cycle fatigue life properties and the role of strain-induced martensitic transformation and solid solution strengthening

<p>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, σ_a, and number of cycles to failure, N_f) of each steel was higher in hydrogen-charged specimen than in uncharged specimen. The increase in fatigue limit, Δσ_w, 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 ε_ta-N_f curve (i.e., relationship between total strain amplitude, ε_ta, and number of cycles to failure, N_f) of 68 MPa hydrogen-charged specimen was nearly coincident with that of uncharged specimen in SUS316L, whereas 68 MPa hydrogen-charging markedly lowered ε_ta-N_f 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, f_mH, was 1 % in tensile tests. On the other hand, the f_mH value was 9% in low- and high-cycle fatigue life tests. The increase in fatigue limit due hydrogen-induced solid solution strengthening, Δσ_w, in high-cycle fatigue life tests was expressed as Δσ_w (MPa) = 15.4 × 237H, where H is the hydrogen content (mass %). In addition, the hydrogen-induced strengthening of stress amplitude, Δσ_a, and 0.2% proof strength, Δσ_0.2, in low-cycle fatigue life tests was expressed as Δσ_a+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.</p>