Initiation of SCC crack in nickel based weld metals : influence of microstructure features

Abstract : Alloy 82, a nickel based alloy, is used as deposited metal in dissimilar metal welds in Pressurized Water Reactors (PWR). Previous studies dealing with PWSCC (Primary Water Stress Corrosion Cracking) crack growth rate measurement in Alloy 82 highlighted that the crack front consists of isolated intergranular cracks. These uneven crack fronts introduce the question of the effect of the microstructural features on Stress Corrosion Cracking (SCC) behavior. The influence of several parameters, including the chemical composition, metallurgical condition and welding process, on the SCC behavior is studied in order to understand which microstructural parameters are critical for SCC initiation. Microstructural and local mechanical behavior characterization was performed using Electron Back Scattered Diffraction (EBSD) and Digital Imaging Correlation (DIC), and further correlated with SCC initiation sites, showing that SCC cracks are intergranular and propagate perpendicular to the loading direction and that Alloy 82 with lower chromium and carbon contents is more susceptible to SCC. The DIC performed on Alloy 82 welds reveals that the strain field on the surface of U-bend specimens is heterogeneous at several scales (strain field distributions and localization pattern).