MICRO-CHARACTERIZATION OF DISSIMILAR METAL WELD JOINT FOR CONNECTING PIPE- NOZZLE TO SAFE-END IN GENERATION III NUCLEAR POWER PLANT
DING Jie1,2(), ZHANG Zhiming1,2, WANG Jianqiu1,2, HAN En-Hou1,2, TANG Weibao3,4, ZHANG Maolong3,4, SUN Zhiyuan3,4
1 Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 2 Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 3 Shanghai Electric Nuclear Power Equipment Co. Ltd., Shanghai 201306 4 Shanghai Research Center for Weld and Detection Engineering Technique of Nuclear Equipment, Shanghai 201306
Cite this article:
DING Jie, ZHANG Zhiming, WANG Jianqiu, HAN En-Hou, TANG Weibao, ZHANG Maolong, SUN Zhiyuan. MICRO-CHARACTERIZATION OF DISSIMILAR METAL WELD JOINT FOR CONNECTING PIPE- NOZZLE TO SAFE-END IN GENERATION III NUCLEAR POWER PLANT. Acta Metall Sin, 2015, 51(4): 425-439.
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/316L DMWJ that used for connecting the pipe safe-end 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 316L 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/52Mw 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 316L base metal, which were identified to be precipitates with complex elementary composition rather than the normal string delta ferrite in 316L 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.
Table 1 Main chemical compositions of different materials in the dissimilar metal weld joint (DMWJ)
Fig.1 Macro-morphology (a) and sampling scheme (b) of DMWJ (Positions 1 and 8 represent the inner wall and outer wall of the joint, respectively)
Fig.2 OM images of weld alloy 52Mw (a) and buttering alloy 52Mb (b) in the DMWJ
Fig.3 OM images of sample in position 4 in the DMWJ weld (a) and the fine equiaxed grains at the backing weld region of sample in position 3 (b) (The arrow in Fig.3b indicates the fine equiaxed grains)
Fig.4 OM image of the cladding on the surface of A508 inner wall
Fig.5 OM images of 316L base metal (a) and precipitate morphology (b)
Fig.6 SEM image (a), MFM image (b) and Volta potential map (c) on the surface area of large precipitate particle in 316L base metal and a Volta potential profile along a line as indicated in Fig.6c (d)
Fig.7 SEM image (a), TEM image (b) and EDS line scanning curves in Fig.7b (c) of small precipitate particle in the 316L base metal (The inset in Fig.7b shows the SAED pattern)
Fig.8 OM image of the heat affected zone (HAZ) in 316LSS
Fig.9 OM images of A508 base metal with low (a) and high (b) carbon contents
Fig.10 OM images of the HAZ in the A508 (a), fusion zone with ferrite and coarse grain zone with martensite and bainite (b), fine grain zone with ferrite and fine matensite (c) and tempered zone (d) (I—fusion zone, II—coarse grain zone, III—fine grain zone, IV—tempered zone)
Fig.11 OM image of A508 HAZ adjacent to the cladding in the inner wall
Fig.12 Microstructure of 316L/52Mw fusion boundary and the unmixed zone
Fig.13 OM images of the 52Mb/A508 fusion boundary (a), Type-I and Type-II boundaries (b)
Fig.14 Micro-hardness distributions in the joint of inner wall sample in position 1 (a), middle part sample in position 5 (b), outer wall sample in position 8 (c) and HAZ of A508 (d) (FB—fusion boundary)
Fig.15 EDS line scan across the 316L/52Mw FB without (a) and with (b) unmixed zone (UZ) of sample in position 4 (TZ—transition zone)
Fig.16 EDS line scan across the 52Mb/A508 fusion boundary without (a) and with (b) Type-I and Type-II boundaries of sample in position 4
Fig.17 EBSD images of grain boundary character distribution of the 316L/52Mw fusion boundary region (a1~a7: inner wall, b1~b7: backing weld, c1~c7: outer wall; 1, 2, 3 indicate the areas of 316L that are 7, 3, 1 mm apart from the fusion boundary, respectively; 4 indicates the fusion boundary area; 5, 6, 7 indicate the areas of 52Mw that are 1, 3, 7 mm apart from the fusion boundary respectively; black, blue and red lines represent the random high angle grain boundary (RGB), low angle grain boundary (LGB) and coincidence site lattice grain boundary (CSL), respectively)
Fig.18 Grain boundary character distribution (GBCD) across the 316L/52Mw fusion boundary at inner wall (a), backing weld (b) and outer wall (c) regions
Fig.19 EBSD images of grain boundary character distributions of the 52Mb/A508 fusion boundary region of sample in position7 in outer wall (a, b indicate the areas of 52Mb that are 3, 1 mm apart from the fusion boundary; c indicates the fusion boundary area; d, e, f indicate the areas of A508 that are 1, 2, 4 mm apart from the fusion boundary, respectively)
Fig.20 GBCD across the 52Mb/A508 fusion boundary at inner wall (a) and outer wall (b) regions
Fig.21 EBSD images of Kernel average misorientation (KAM) distributions across fusion boundary of inner wall in position 1 (a1, a2, a3 indicate the areas of 316L that are 7, 3, 1 mm apart from the fusion boundary, respectively; a4 indicates the fusion boundary area; a5, a6, a7 indicate the areas of 52Mw that are 1, 3, 7 mm apart from the fusion boundary, respectively; b1, b2 indicate the areas of 52Mb that are 3, 1 mm apart from the 52Mb/A508 fusion boundary, respectively; b3 indicates the 52Mb/A58 fusion boundary area; b4, b5, b6 indicate the areas of A508 that are 1, 2, 4 mm apart from the 52Mb/A508 fusion boundary, respectively)
Fig.22 Kernel average misorientation (KAM) distribution across fusion boundaries in the DMWJ
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