Abstract
Although highly cold-worked UNS N06690 has been shown to be susceptible to stress corrosion crack growth in pressurized water reactor primary water, it is unclear whether stress corrosion cracking initiation occurs under constant load test conditions. To evaluate the stress corrosion cracking initiation response, constant load tensile and blunt-notch compact tension testing were performed in 360°C simulated pressurized water reactor primary water. High-resolution examinations were conducted mid- and post-test to record the evolution of grain boundary precursor damage and intergranular crack nucleation by focused ion beam milling, scanning electron microscopy and transmission electron microscopy. In particular, focused ion beam/ secondary electron microscopy (FIB/SEM) imaging was conducted to record evolution of intergranular cavities, local corrosion and shallow cracks at the specimen surface. Nanometer- sized cavities were observed, associated with grain boundary carbides, in all specimens. Specimens with 30% cold-work consistently exhibited shallow intergranular cracks with oxidized crack flanks and a higher density of cavities ahead of the oxide front. The distribution of carbides and cavities in the plane of the cracked grain boundaries was analyzed to gain mechanistic insights into the processes that lead to crack initiation in highly cold-worked UNS N06690.
