Stress Corrosion Cracking Behavior in the Transition Region of Alloy 182/Low-Alloy Reactor Pressure Vessel Steel Dissimilar Metal Weld Joints in Light Water Reactor Environments

The stress corrosion cracking (SCC) behavior perpendicular to the fusion line in the transition region between the alloy 182 nickel-base weld metal and the adjacent low-alloy reactor pressure vessel (RPV) steel of simulated dissimilar metal weld joints was investigated under boiling-water reactor normal-water chemistry conditions at different stress intensities and chloride concentrations. A special emphasis was placed on the question whether a fast growing interdendritic SCC crack in the highly susceptible alloy 182 weld metal can easily cross the fusion line and significantly propagate into the adjacent low-alloy RPV steel. Cessation of interdendritic stress corrosion crack growth was observed in high-purity or sulfate-containing oxygenated water under periodical partial unloading or constant loading conditions with stress intensity factors below 60 MPa.m(1/2) for those parts of the crack front that reached the fusion line. In chloride containing water, on the other hand, the interdendritic stress corrosion crack in the alloy 182 weld metal very easily crossed the fusion line and further propagated with a very high growth rate as a transgranular crack into the heat-affected zone and base material of the adjacent low-alloy steel. Tests under hydrogen water chemistry conditions with RPV steels revealed a much higher chloride tolerance, where even 100 ppb of chloride were not sufficient to induce SCC below 60 MPa.m(1/2). Nevertheless, preliminary experiments with dissimilar metal weld joints indicate that minor SCC crack growth into the low-alloy RPV steel heat-affected zone cannot be excluded above 60 MPa.m(1/2) in a primary pressurized water reactor environment.