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The maximum thickness of flanging position on the vacuum vessel (VV) window collar of China Fusion Engineering Test Reactor (CFETR) is 160 mm. Electron beam welding can achieve one-time penetration of the thick plate without groove and the deformation after welding is quite small. The cross section of weld was sampled for microscopic observation and mechanical properties tests to explore the difference in microstructure and performances of 160 mm thick 316 L stainless steel electron-beam-welded joints. The results showed that the weld depth-width ratio reached up to 16:1, and the microstructure of weld was composed of ferrite and austenite with different morphologies which was mainly coarse cellular crystal in heat affected zone (HAZ). The equiaxed grain orientation in the root region (denoted as “R region”) was mostly [001] and the columnar grain orientation along the temperature gradient was mostly [101]. The microstructure along the depth direction was quite different, which was mainly related to the solidification mode and composition overcooling. The high strength (YS=715.22 MPa) of R region was due to the smaller temperature gradient and the larger cooling rate, the position of fracture suggested that the strength of weld was higher than that of the base metal. The microhardness measurement results showed that the hardness increased gradually along the depth direction of weld (Top:156.5 HV, Root: 238.7 HV) and fluctuated along the width direction. The tensile properties along the thickness direction exhibited a trend of decreasing first and then increasing, which was mainly related to the characteristics of electron beam deep penetration welding and the difference of grain structure along the thickness direction. Introduction As a core safe component of China Fusion Engineering Test Reactor (CFETR), the vacuum vessel (VV) provides a stable vacuum environment for plasma fusion reaction[1], [2], [3]. The VV serves in a complex mechanical-temperature-electromagnetic coupling environment, requiring all welds to achieve full penetration. Besides, the arc welding defect level realizes the ISO5817: 2014-B quality standard and the electron beam welding (EBW) defect level realizes the ISO13919–1: 2019-B quality standard. The VV has a complex hyperboloid structure with the cross section of double-layer D -shaped, and contains an upper window, a middle window and a lower window for diagnostic systems, replacing internal components and installing auxiliary heating and. etc. As a connecting part between windows and the main body of VV, the window collar has the characteristics of complex contour and large thickness as shown in Fig. 1(a). The flanging is a key position where connects the main body of VV, thus the control of welding deformation is more stringent. The maximum size of the window collar of the CFETR VV is about 4.2 m × 2.7 m. The vacuum EBW can realize the micro-deformation welding of large thickness structure with the advantages of small post-welding deformation, narrow heat affected zone (HAZ) and large aspect ratio. However, the vacuum chamber and welding motion system of electron beam welding machine is unable to weld the large window collar parts directly, and the collar can only move in plane but not rotate. In order to satisfy the full penetration of the flanging part, the back of the weld must be added with a pallet, and the maximum penetration depth is about 160 mm, as shown in Fig. 1(b). The EBW of large-thick plates is a deep penetration welding mode along with keyhole effects. Microstructure inhomogeneity which affects the mechanical properties often occurs along the thickness direction of joint. There are few reports and studies on the microstructure and properties inhomogeneity of EBW joint. Bhanu et al. [4] prepared 8 mm thick EBW dissimilar joints between P91 ferritic / martensitic steel and Incoloy 800HT nickel-based alloy and characterized the microstructure of weld involving nickel-based alloy. The results showed that the microstructure of weld center exhibited a mixed solidification mode with equiaxed dendrites and cellular structures. Maurya et al. [5] studied the microstructure, mechanical properties and corrosion behavior of dissimilar welded joints. There were significant differences in the microstructure of the dissimilar joints at the weld and interface, the microstructure inhomogeneity had a significant effect on the mechanical properties of the welded joints, including microhardness, tensile and impact strength. Wang et al. [6] carried out the EBW of 40 mm thick 316 L and analyzed the microstructure and properties after welding. The study found that the weld microstructures were mainly columnar crystal, fine dendrite and equiaxed crystal. The hardness in the middle of the weld was the highest, the strength of the weld was obviously better than that of the base metal. Chen et al. [7] welded 50 mm thick 304 stainless steel plate at one time via EBW. The solidification mode varied with the solidification rates which were diverse in various depths of the weld, and the grains gradually decreased from top to root. Elmer et al. [8] studied the effects of different alloy compositions and cooling rates on the solidification mode and microstructure of the weld. The alloy only solidified in single-phase ferrite or single-phase austenite at a high cooling rate, the microstructure changed with the partial solid transformation of ferrite to austenite. Through previous work, it was found that the thickness of materials they studied was generally no more than 100 mm, which was because too large thickness of the plate increased the difficulty of welding and the forming quality was difficult to guarantee. However, two 160 mm thick 316 L stainless steel plates were well welded via vacuum electron beam welding in this paper. The weld was sampled from top to root for observing the macro/micro morphology and testing the performances. The change law of microstructure and properties was analyzed, which provided theoretical guidance for the welding of VV window collar applied to CFETR. Section snippets Experimental procedure Two 316 L stainless steel plates (ASTM A240/A240M-16a) with the size of 300×25×160 mm were selected as the experimental material. Welding tests were carried out by the ZD150–60C CV66M vacuum electron beam welding machine with working vacuum of about 1.7 × 10−4 mbar, accelerating voltage of 150 kV and the detailed welding parameters were listed in Table 1. To remove the surface oxide, oil and water stains and reduce the production of welding defects, the 316 L plates were subjected to mechanical Macroscopic morphology characteristics Fig. 3 showed the macroscopic morphology in the weld section of the 160 mm thick 316 L electron-beam-welded joint, which obviously clarified the large weld depth-width ratio of about 16:1 and the weld reinforcement of about 3 mm. The weld width varied greatly along the thickness direction from top to root, where the maximum was about 10 mm (T region) and minimum was about 3 mm (R region). There were no cracks, nail tips and other defects found by phased array ultrasonic testing (UT) after Conclusion In this paper, the microstructure and mechanical properties of 160 mm thick 316 L electron-beam-welded joint were investigated in detail. The following conclusions can be drawn: 1 The optimized welding process parameters were selected to achieve welding through two 160 mm thick 316 L stainless steel plates, resulting in a weld with large depth-to-width ratio and no discernible surface cracks and nail tip defects. 2 The temperature gradient and cooling rate along the depth direction of weld were CRediT authorship contribution statement Jianguo Ma: Methodology, Investigation, Writing – original draft, Conceptualization. Zhiyong Wang: Methodology, Investigation, Writing – review & editing. Zhihong Liu: Writing – review & editing, Supervision. Haibiao Ji: Data curation, Writing – review & editing. Xiaowei Xia: Writing – review & editing, Supervision. Chengwen Li: Writing – review & editing. Zhenfei Liu: Investigation. Zhongtao Zhang: Investigation, Data curation. Jiefeng Wu: Writing – review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was funded by Comprehensive Research Facility for Fusion Technology Program of China [grant number 2018-000052-73-01-001228 ], the Collaborative Innovation Program of Hefei Science Center, CAS [grant number 2022HSC-CIP025 ], Youth Innovation Promotion Association CAS [grant number 2019433 ] and the National Nature Science Foundation of China [grant number 12105185 ]. This work was also supported by the Experimental Advanced Superconducting Tokamak device and Anhui Province Key References (18) H. Ji et al. Analysis of welding deformation on CFETR 1/32 vacuum vessel mockup Fusion Eng. Des. 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Structure–property relationships and corrosion behavior of laser-welded X-70/UNS S32750 dissimilar joint Archiv. Civ. Mech. Eng (2023) Z. Wang, D. Wang, Z. Wen, et al. Study on microstructure and mechanical property of electron beam welding joint of... There are more references available in the full text version of this article. Cited by (0) Recommended articles (0) View full text © 2023 Elsevier B.V. All rights reserved. About ScienceDirect Remote access Shopping cart Advertise Contact and support Terms and conditions Privacy policy We use cookies to help provide and enhance our service and tailor content and ads. By continuing you agree to the use of cookies . Copyright © 2023 Elsevier B.V. or its licensors or contributors. ScienceDirect® is a registered trademark of Elsevier B.V. ScienceDirect® is a registered trademark of Elsevier B.V.
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