Precursor Evolution and Stress Corrosion Cracking Initiation of Cold-Worked Alloy 690 in Simulated Pressurized Water Reactor Primary Water

Stress corrosion crack initiation of two thermally-treated, cold-worked (CW) Alloy 690 (UNS N06690) materials was investigated in 360 degrees C simulated pressurized water reactor primary water using constant load tensile (CLT) and blunt notch compact tension (BNCT) tests equipped with direct current potential drop (DCPD) for in situ detection of cracking. Stress corrosion cracking initiation was not detected by DCPD for either the 21% or 31% CW CLT specimens loaded at their yield stress for similar to 9,220 h; however, intergranular (IG) precursor damage and isolated surface cracks were observed on the specimens. The two 31% CW BNCT specimens loaded at constant stress intensity after several cyclic loading ramps showed DCPD-indicated crack initiation after 10,400 h of exposure, which resulted from significant growth of IG cracks. The 21% CW BNCT specimens only exhibited isolated small IG surface cracks and showed no apparent DCPD change throughout the test. Post-test cross-section examinations revealed many grain boundary (GB) nanocavities in the bulk of all of the CLT and BNCT specimens particularly for the 31% CW materials. Cavities were also found along GBs extending to the surface, suggesting an important role in crack nucleation. This paper provides an overview of the evolution of GB cavities and discusses their effects on crack initiation in CW Alloy 690.