Effect of defect acuity on the fracture behavior of alloy 82H welds in low temperature water

Nickel-base welds experience a severe reduction in fracture resistance in low temperature (<150oC) water due to a hydrogen-induced intergranular cracking mechanism. In this study, fracture toughness testing of Alloy 82H welds was performed in 24oC air and 54oC hydrogenated water to characterize the effects of defect acuity on the low temperature crack propagation (LTCP) behavior. Defect acuity effects were evaluated by conducting JIC tests on weld specimens with either as-machined notches or natural weld defects and comparing the fracture response with that obtained using fatigue precracked specimens. Intergranular LTCP did not initiate directly from an as-machined notch; however, once a transgranular tear formed at a notch, it served as a sharp crack from which LTCP initiated. Although intergranular LTCP did not initiate at the notch, the environment caused a substantial reduction in initiation toughness that was associated with a hydrogen-induced transgranular faceting mechanism. A transition from transgranular faceting to intergranular cracking coincided with the onset of rapid crack extension. To evaluate the fracture toughness behavior of welds with crack-like weld defects, compact tension specimens were machined from partial penetration welds such that a natural weld root extension defect served as a precrack. Weld root extension defects are solid state cracks that form in the root region of partial penetration welds due to weld shrinkage stresses. Fracture toughness testing in 54oC hydrogenated water demonstrated that rapid intergranular cracking initiated directly from these natural weld defects. Indeed, the LTCP response for weld root extension defects was indistinguishable from that for fatigue precracks. (author)