Investigation of Short Crack Nucleation in Cold Worked Alloy 690 and its Weld Metals under Constant Load in Simulate PWR Primary Water

Abstract

The Pacific Northwest National Laboratory is performing constant load stress corrosion crack (SCC) initiation testing on high-Cr, Ni-base Alloy 690 and its weld metals Alloy 52(M)/152(M) in 360°C simulated PWR primary water to evaluate their SCC initiation resistance based on exposure time and microstructural evolution. Ongoing studies of Alloy 690 have identified a crack initiation mechanism that is preceded by grain boundary cavity formation. This process occurred first in thermally treated, highly cold-worked Alloy 690 materials tested at materials yield stress. During recent test interruptions, small IG cracks were found in both moderately cold worked Alloy 690 and Alloy 52(M)/152(M) after a total exposure of ~4.9–5.7 years. Scanning electron microscopy and focused ion beam trenching were employed to characterize the near-surface grain boundary microstructure of selected cracks in detail. In this paper, key results from the latest characterization will be summarized, and the morphology of the cracks found in Alloy 690 and Alloys 52(M)/152(M) will be compared and discussed.