Hydrogen-enhanced fatigue crack growth of martensitic stainless steel: A predictive model and experimental validation

Atomic hydrogen (H) diffusion in metallic structures affects the mechanical properties of steel alloys. However, the effect of hydrogen embrittlement on the fatigue crack propagation rate (FCGR) is not fully understood, and serious uncertainties can affect the design and inspection schedules of hydraulic turbine runners based on the defect-tolerance approach. The FCGR in a complex microstructure such as tempered martensitic stainless steel is governed by the H content, stress distribution, and microstructure at the crack tip. The synergetic interactions between these parameters should be kinetically studied as the crack propagates. In this study, an original model was proposed to predict the influence of H on the fatigue crack propagation rate. The model was developed specifically for tempered martensitic stainless steels containing austenite. Subsequently, it was validated through experiments performed on 415 martensitic stainless steel containing 20% of reformed austenite (RA). The ma-terial's FCGR was tested in its raw condition, also after charging with H. Compact tension (CT) specimens were tested at two constant stress intensity factor ranges (Delta K = 8 MPa.m0.5 and 15 MPa.m0.5) and three cyclic loading frequencies (f = 35 Hz, 3.5 Hz, and 0.35 Hz). The results show a very good agreement with the predictions of the model. Moreover, both the model and the experimental results reveal that there is a critical Delta K that is dependent on the loading frequency at which the impact of H on the FCGR is maximum. Moreover, as predicted by the model, a decrease in the loading frequency led to an increase in the susceptibility of the FCGR to H.