INFLUENCE OF A CYCLIC LOADING ON THE INITIATION AND PROPAGATION OF PWSCC IN WELD METAL 182

摘要

PWSCC (Primary Water Stress Corrosion Cracking) of nickel based weld metal 182 is now a major issue in Pressurised Water Reactors, since significant cracking was observed in several welds of the primary circuit in some countries. Many questions arise about the stress (or strain) threshold for initiation and on the crack growth rates of SCC, with significant differences depending on the orientation of the loading axis with regard to the dendrites. The stress threshold for initiation was determined at the level of 350 Mpa with constant load tests on as-welded material. This threshold was not significantly modified by a ripple loading at 360℃. The strain threshold for initiation, determined by interrupted SSRTs for various elongations, was found to range between 1.25 and 1.5%. rnPreliminary data of crack growth rates (CGRs) at 325℃ provided a dependence on (K-9)~(0.4) for the average values of the CGR_(max). The strongly oriented microstructure led to faster (x 2 to 3) CGRs in the direction of the dendrites. The influence of a trapezoidal loading (R = 0.7, 2.8 10~(-4) Hz) on CGRs depended on the orientation : no significant modification was found with respect to a pure static loading in the direction TL perpendicular to the dendrites, but some significant increase (x 5 to 7) could be observed in the direction of the dendrites TS. A stress relief treatment (610℃) could lower the CGRs by a factor 2 to 3.5 with respect to the as-welded condition. The results are discussed with regard to available data from literature and compared to the background on alloy 600 : the chemical composition of the weld metal could lead to some variability (factor 3) in the CGRs (4 welds with significantly different chemical compositions). Tests are scheduled to account for the influences of temperature and composition of the weld metal on the stress threshold and on the crack velocities.