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

Abstract

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.

Keywords

• austenitic stainless steels
• internal hydrogen
• low- and high-cycle fatigue life tests
• strain-induced martensitic transformation
• solid solution strenghening