Microstructural Effects on SCC Initiation in PWR Primary Water for Cold-Worked Alloy 600
SCC initiation results in simulated PWR primary have been obtained on one mill annealed alloy 600 plate heat in the cold-worked condition. Twelve specimens with similar cold work levels were tested at constant load and three showed much shorter SCC initiation times (<400 h) than the nine others (>1200 h). Post-test examinations revealed that these three specimens all feature an inhomogeneous microstructure where the primary crack always nucleated along the boundary of large elongated grains protruding normally into the gauge. In contrast, such microstructure was either not observed or did not extend deep enough into the gauge in the other specimens exhibiting ~3–7× longer initiation times. In order to better understand the role of this microstructural inhomogeneity in SCC initiation, high-resolution microscopy was performed to compare carbide morphology and strain distribution between the long grains and normal grains, and their potential effects on SCC initiation are discussed in this paper.
KeywordsAlloy 600 PWSCC Crack initiation Cold work Banded microstructure Large elongated grain
The authors gratefully acknowledge the financial support from the Office of Nuclear Energy, U.S. Department of Energy through the Light Water Reactor Sustainability Program. In addition, support is recognized from the U.S. Nuclear Regulatory Commission for additional SCC initiation testing. Nicole Overman at PNNL is acknowledged for conducting the EBSD analysis. Key technical assistance from Robert Seffens, Clyde Chamberlin, Anthony Guzman and Ryan Bouffioux is acknowledged for SCC initiation testing and materials preparation activities.
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