Linking Grain Boundary Microstructure to Stress Corrosion Cracking of Cold Rolled Alloy 690 In PWR Primary Water

ABSTRACT Grain boundary microstructures and microchemistries are examined in cold-rolled alloy 690 materials and comparisons are made to intergranular stress corrosion cracking (IGSCC) behavior in PWR primary water. Chromium carbide precipitation is found to be a key aspect for materials in both the mill annealed and thermally treated conditions. Cold rolling to high levels of reduction was discovered to produce small IG voids and cracked carbides in alloys with a high density of grain boundary carbides. The degree of permanent grain boundary damage from cold rolling was found to depend directly on the initial IG carbide distribution. For the same degree of cold rolling, alloys with few IG precipitates exhibited much less permanent damage. Although this difference in grain boundary damage appears to correlate with measured SCC growth rates, crack tip examinations reveal that cracked carbides appeared to blunt propagation of IGSCC cracks in many cases. Preliminary results suggest that the localized grain boundary strains and stresses produced during cold rolling promote IGSCC susceptibility and not the cracked carbides and voids. INTRODUCTION Intergranular stress corrosion cracking (IGSCC) of Fe- and Ni-base austenitic stainless alloys has been a continuing problem in commercial boiling-water-reactor (BWR) and pressurized-water- reactor (PWR) nuclear power plants. Grain boundary microstructure and microchemistry has been shown to play a controlling role in the observed cracking for most cases. This was clearly the case for BWR failures in Fe-base 300-series stainless steel components that were often due to Cr-carbide precipitation and concurrent Cr depletion (thermal sensitization) during fabrication.^1,2 Although sensitization remains an issue for Ni-base stainless alloy 600 in oxidizing BWR water environments, it is not a controlling process in PWR primary water. Nevertheless, grain boundary carbide distributions have been shown to influence IGSCC susceptibility with thermally treated material showing improved behavior versus mill-annealed material.^3,4