Examination of Cavity Formation in Cold-Worked Alloy 690

摘要

Although stress corrosion cracking (SCC) has notbeen reported in Alloy 690 components after more than30 years of pressure water reactor (PWR) service,susceptibility to SCC growth in PWR primary water hasbeen reported by many labs for highly CW Alloy 690along with isolated observations of crack initiation. Posttestexaminations on SCC initiation specimens suggesteda key role of creep by inducing the formation and growthof grain boundary cavities at carbide interfaces leading tointergranular cracking. However, it is clear that highlevels of cold work can produce grain boundary voids andcomplicates the quantitative evaluation of damageevolution during testing in high-temperature water.Therefore, it is crucial to understand how to properlyexamine and classify this type of damage in the pre-testcondition and establish its evolution with time. Thispaper outlines our observational protocols on how to bestprepare sample surfaces to avoid inducing artifacts, andto perform subsequent examinations utilizing scanningelectron microscopy, transmission electron microscopyand focused ion beam milling to assess the formation anddevelopment of grain boundary cavities. Detailedexaminations will be reviewed on several commercialAlloy 690 heats in the pre-test CW condition and afterlong-term initiation testing at constant load in 360°Csimulated PWR primary water to effectively documentgrain boundary cavity formation and growth leading tocrack initiation.