Stress corrosion cracking (SCC) is a continuingaging management concern for pressurized waterreactors. Stress corrosion cracking has been observed ina variety of materials and components across nuclearpower plants. In this investigation, A286 reactor coolantpump turning vane bolts with stress corrosion crackswere identified as part of routine plant maintenanceactivities, and laboratory investigations were performed.The A286 material is susceptible to SCC in thepressurized water reactor primary coolant environment,and stress is provided by preload on the bolt.The investigations included fractography,metallography, electron back-scatter diffraction, andhardness testing. Intergranular cracks were identified inthe head-to-shank region of the bolt, which is the expectedfailure location due to this being the highest stress area.The cracks exhibited extensive branching. Cracks werenot observed to propagate completely through the bolt,indicating either some relief of stress as the crackprogressed or slow crack propagation. Shallowintergranular cracks were also identified on the seatingsurface of the bolt. As this area is expected to be incompression, this was not expected, and so indicates somestrong cracking susceptibility at shallow, localized areasof the material.The microstructure of the A286 material was foundto be non-homogenous. An apparently bi-modal grainstructure was observed and etching with 2% sulfuric acid,intended to identify gamma prime precipitation, createdbanded patterns throughout the material cross-section.The inhomogeneous microstructure may result inlocalized chromium depletion and offer a possibleexplanation of the shallow cracks on the bolt seatingsurface.
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