EFFECTS OF PREEXISTING GRAIN-BOUNDARY MICROVOID DISTRIBUTIONS ON FRACTURE-TOUGHNESS AND FATIGUE CRACK-GROWTH IN LOW-ALLOY STEEL

The alloy investigated was a Mn-Mo-Ni nuclear pressure vessel steel, ASTM A533B Class 2. Microvoid damage was achieved by prior exposure to gaseous hydrogen atmospheres at high temperatures and pressures where carbon reacts with hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). Whereas the crack initiation and crack growth toughness (i. e. K//1//c and the tearing modulus) were degraded even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remained relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip driving force (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture where the presence of small microvoid clusters promotes coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.