Modeling the fatigue crack growth of X100 pipeline steel in gaseous hydrogen

•FCG tests performed on pipeline steel in air and high-pressure gaseous hydrogen.•Hydrogen-assisted (HA) FCG highly dependent upon hydrogen pressure.•HA FCG falls into two regimes depending upon the crack extension per cycle.•Single relationship proposed to correlate HA FCG of X100 steel. This work proposes a phenomenological fatigue crack propagation (FCP) model for API-5L X100 pipeline steel exposed to high-pressure gaseous hydrogen. The semi-empirical model is predicated upon the hypothesis that one of two mechanisms dominate the fatigue crack growth (FCG) response depending upon the crack extension per cycle (da/dN) and the material hydrogen concentration. For da/dN between approximately 1×10−5mm/cycle and 3×10−4mm/cycle, fatigue crack growth in hydrogen is markedly increased over that in laboratory air, resulting in a Paris exponent over two and a half times that of air and producing a predominately intergranular crack propagation surface. Fatigue crack growth in hydrogen at da/dN above approximately 3×10−4mm/cycle result in FCP rates over an order of magnitude higher than that of lab air. The Paris exponent in this regime approaches that of lab air and the crack morphology is predominately transgranular. Increasing the hydrogen test pressure from 1.7MPa to 20.7MPa increases the FCG rate by as much as two, depending upon the stress intensity factor. It is proposed that the FCG response in hydrogen at da/dN 3×10−4mm/cycle results from fatigue-dominated mechanisms. The proposed model predicts fatigue crack propagation as a function of applied ΔK and hydrogen pressure. Results of fatigue crack growth tests in gaseous hydrogen as well as fracture morphology are presented in support of the proposed model. The model correlates well with test results and elucidates how the proposed mechanisms contribute to fatigue crack propagation in pipeline steel in environments similar to those found in service.