Fatigue crack growth retardation due to crack closure in high-strength steel under high-pressure hydrogen

• 1. Fatigue crack growth (FCG) behavior in 1500 MPa-class steel in 90 MPa hydrogen gas. • 2. Significant FCG retardation at low Δ K due to crack closure in hydrogen. • 3. Crack closure due to combined effects of fracture surface roughness and plasticity. • 4. Hydrogen weakened block boundaries depending on plastic zone size and inclination. • 5. FCG at low Δ K showed cycle-dependence even in ultra-high strength steel. The influence of hydrogen on fatigue crack growth (FCG) was investigated in 90 MPa hydrogen gas using a 3Mn-0.2C martensitic steel with a tensile strength of 1500 MPa. Surprisingly, this steel exhibited significant crack closure in hydrogen gas under specific loading conditions, resulting in considerably lower FCG rates compared to those in air. The roughness- and plasticity-induced crack closure markedly retarded the FCG in hydrogen in a relatively low stress intensity factor regime. However, intrinsic FCG resistance, excluding the effect of crack closure, was indeed degraded by hydrogen. Further analysis revealed that the reduction in cohesive strength of block boundaries depended on the plastic zone size and the inclination angle between the longitudinal axis of the blocks and the macroscopic FCG direction. Consequently, hydrogen-enhanced plasticity-mediated decohesion led to cycle-dependent acceleration of FCG, even in such high-strength steel under high-pressure hydrogen environments.