MICRO-CHARACTERIZATION OF DISSIMILAR METAL WELD JOINT FOR CONNECTING PIPE- NOZZLE TO SAFE-END IN GENERATION III NUCLEAR POWER PLANT

Abstract

The dissimilar metal weld joint(DMWJ) in primary water system of pressurized water reactors(PWRs) has been proven to be a vulnerable component owing to its proneness to different type of flaws. Thus,maintaining integrity of such joint in case of defect presence is of great importance to the design and safe management of nuclear power plants(NPPs). For a reliable integrity analysis of DMWJ, it is essential to understand the microscopic characteristics in all regions of the joint. In this work, OM, TEM, SEM, durometer, AFM, MFM and SKPFM were utilized to investigate the microstructure, micro- hardness and the distribution of main elements, grain boundary characteristic and residual strain in the A508/52M/316 L DMWJ that used for connecting the pipe safeend and the nozzle of reactor pressure vessel in PWRs, and a comparative analysis about the microstructure and property along the radical direction of the DMWJ was obtained. The results showed that there was no region that differed from the other part of the weldment in terms of the microstructure and micro-hardness dramatically. A layer of fine grain resulting from unmelted filler metal was found in the backing weld part of the joint. The residual strain in the heat affected zone(HAZ) of 316 L was higher than that in other regions. Meanwhile, drastic variations in the microstructure, chemical composition distribution and grain boundary character distribution(GBCD) in both the 316L/52 Mw and the 52Mb/A508 interface regions were observed. The analyses using TEM and MFM test showed that a large number of chromium and molybdenum- rich precipitates particles distributed both along the grain boundaries and inside grains in the 316 L base metal, which were identified to be precipitates with complex elementary composition rather than the normal string delta ferrite in 316 L austenitic stainless steel. The SKPFM test result indicated that these precipitates were more prone to be corroded than the base metal. Therefore, further investigation about the cause of deformation and the impacts to the corrosion resistance, particularly the stress corrosion cracking(SCC) sensitivity of the precipitates needs to be carried out.