Surface Deformation Nanostructures and Stress Corrosion Crack Precursors Originating from Surface Grinding
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Surface Deformation Nanostructures and Stress Corrosion Crack Precursors Originating from Surface Grinding Matthew J. Olszta, Larry E. Thomas, Stephen M. Bruemmer Battelle Pacific Northwest Division, Richland, WA 99352 USA ABSTRACT Stress corrosion cracking (SCC) in light water reactor components has long been studied from a post mortem perspective, yielding insights into water chemistries and effects on crack propagation. Analysis of a cracked component does not effectively provide information on the corrosion events or on SCC initiation. It is important that microstructures of these early stages be understood because the original surface of the component formed during fabrication is often not the final surface condition that is exposed to reactor water. Pre-service grinding of reactor components and welds is performed for a variety of reasons, from aesthetics to preparation for non-destructive testing. It is this final surface microstructure that often controls SCC initiation. Surface and near-surface characteristics have been investigated in 304SS metal coupons on which controlled grinding was performed. These examinations indicate the extent of subsurface microstructural damage before high-temperature water exposure. Analytical electron microscopy techniques have been used to gain insights into possible surface precursors to corrosion damage and SCC initiation. Nanocrystalline grains were commonly found at the surface in lightly ground to heavily abraded materials within the first ~0.5-10 µm along with high dislocation densities, twinning and lath structures. INTRODUCTION Stress corrosion cracking (SCC) of structural components is a paramount issue impacting the performance, safety and life extension of current light water reactors both in the United States and internationally. Degradation management programs have been established to identify specific concerns aimed at understanding long-term performance and ensure reliability of critical components. Decades of research have provided improved confidence to predict SCC crack growth, but limited knowledge exists for initiation processes. It remains a key missing link for practical behavior prediction in service components. Improved fundamental understanding of surface microstructures in prototypic reactor materials will help provide the necessary underpinning to investigate SCC initiation processes. Recent characterization results on components removed from BWR service [1-3] have highlighted the need for a quantitative understanding of surface deformation-induced substructures in reactor components. This research followed extensive high-resolution characterization of stress-corrosion crack and crack type in components removed from lightwater reactor service [e.g., 4-7]. A layer of highly deformed and often recrystallized metal has been detected in subsurface region that appears to be a direct function of processing and alloy composition. The extent of this damage was quite unexpected particularly for lightly ground or polished materials. Subsequent e
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