INFLUENCE OF HIGH PRESSURE HYDROGEN PRE-EXPOSURE ON CRACK GROWTH UNDER MONOTONIC AND CYCLIC LOADING

Low alloy steels exposed to gaseous hydrogen atmospheres at high temperatures and pressures are prone to a mode of environmental degradation known as hydrogen attack, which occurs when carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles. In the present study, the role of such damage in influencing crack initiation/crack growth toughness and fatigue crack propagation behavior is examined in a highly susceptible Mn-Mo-Ni pressure vessel steel. It is shown that, whereas the fracture toughness and tearing modulus are severely degraded, even during the incubation stages of hydrogen attack, fatigue crack propagation rates are relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary voids, which promotes crack growth, and the resulting tortuous crack paths and decarburization-induced softening, which can retard crack growth under cyclic loading. The degradation in toughness is related to changes in fracture ductility through consideration of a simple stress-modified critical strain model for microvoid coalescence. Fatigue behavior, conversely, is modeled in terms of the role of crack tip shielding in reducing the effective stress intensity range, locally experienced at the crack tip. Quantitative estimates of the magnitude of such shielding are derived using two-dimensional models of crack deflection and roughness-induced crack closure.