FRACTURE-MECHANICS DATA AND MODELING OF ENVIRONMENTAL CRACKING OF NICKEL-BASE ALLOYS IN HIGH-TEMPERATURE WATER

Environmental cracking of ductile nickel-base alloys has occurred both in pressurized water reactors (e.g., steam-generator tubing) and boiling water reactor components such as pressure-vessel safe ends, weld butters, and filler metals for joining nickel-base alloys or dissimilar metals, and attachment welding pads on pressure vessels. Accurate assessment of the interrelated effects of material, environment, and mechanics on environmentally assisted crack growth rates is required for life prediction of nuclear power reactor components. The objective of this study is to understand and predict the environmental cracking behavior of ductile nickel-base alloys in 288-degrees-C water. Experiments were performed on 1T CT specimens of Alloy 600 base metal, and Alloy 182 and Alloy 82 weld metals to evaluate the effects of water chemistry, heat treatment, and stress intensity on crack growth rate. Fracture mechanics, crack growth data on ductile nickel-base alloys were also throughly surveyed to provide an overview of their environmental cracking behavior. While some differences were observed, the crack growth data on nickel-base alloys were in good agreement with crack growth predictions based on stainless steels as a function of water chemistry, material chemistry (i.e., grain boundary chromium concentration), and stress intensity. Differences were more pronounced in studies that evaluated crack incubation and "short" crack growth, in which much more severe water chemistry and/or stressing conditions were required to achieve a similar response in nickel-base alloys as in stainless steels. However, in both cases, once cracks grew beyond almost-equal-to 50-mu-m, the rates were identical to "long" crack growth rates.