Effects of heterogeneity in dissimilar metal welds (DMWs) on the micromechanical properties of materials for nuclear power plants (NPPs)

Sammanfattning

Approximately 30% of all energy production in Sweden comes from nuclear power plants (NPPs), with an increase expected due to future energy demands. To ensure the structural integrity of the operating power plants, it is important to understand and evaluate the risk for certain degradation mechanism to occur under relevant operating conditions. One such mechanism, stress corrosion cracking (SCC), has been observed in dissimilar metal welds (DMWs) in the reactor coolant pressure boundary (RCPB), challenging the structural integrity and thus the availability of the NPPs. Research regarding the micromechanical behavior and mechanical properties of the interface between the low alloy steel (LAS) reactor pressure vessel (RPV) and the currently used nickelbased alloy weld metal (WM) Alloy 52 (A52) when subjected to long-term aging is scarce. The present study characterizes the microstructure of the crystallographically mismatched fusion boundary (FB) interface between the LAS RPV and Ni-based WM A52 by means of scanning electron microscopy (SEM) including secondary (SE) and backscattered electrons (BSE) imaging. Micromechanical properties are examined using nanoindentation, where as received (AR) and thermally aged (TA) mock-up specimens provided by Finnish TVO and Swedish Ringhals are compared to understand how subjection to involuntary long-term aging in the area of significance across the FB affects the micromechanical structure. Results show a microstructure consisting of clear bainitic grains on the LAS side, with significantly larger austenitic grains on the WM side. The area most adjacent to the FB on the LAS side show finer grains that differ from the bainitic characteristics in Ringhals TA condition in comparison to AR condition, and can be presumed to be a carbon depleted zone (CDZ). Micromechanical tests that include the heat affected zones (HAZ) alongside the LAS FB show that the material exhibits a hardness peak on the FB, with hardness adjacent to the FB being significantly lower in comparison to the FB itself. The WM side showcases relatively slightly higher hardness values than the LAS side. TVO samples displayed higher hardness in TA condition along the HAZ in comparison to AR condition, and lower hardness than AR in the WM. Ringhals sample data exhibited more noise, and comparisons between AR and TA conditions did not display significant differences in the HAZ. However, the location of the hardness peak differed slightly on the WM side of the FB, with the TA sample displaying two peaks; one at the FB, and one at the same x-position as the AR sample. Similar general trends between AR and TA conditions for TVO and Ringhals can be observed.