MING Hongliang1,2, ZHANG Zhiming1, WANG Jianqiu1,, HAN En-Hou1, SU Mingxing3
1 Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials, Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
3 Shanghai Research Center for Weld and Detection Engineering Technique of Nuclear Equipment, Shanghai 201306, China
Correspondent: WANG Jianqiu, professor, Tel: (024)23893723, E-mail: wangjianqiu@imr.ac.cn通讯作者 王俭秋,wangjianqiu@imr.ac.cn,主要从事核电关键材料力学化学交互作用研究
Supported by National Natural Science Foundation of China (No.51301183), Science and Technology Commission of Shanghai Municipality (No.14DZ2250300) and Key Research Program of Frontier Sciences, CAS (No.QYZDY-SSW-JSC012);
Dissimilar metal weld joints (DMWJ) widely exist in the nuclear power plants to join the different parts which are made of different structural materials. Among these DMWJs, safe-end DMWJ has attracted much attention of researchers and operating enterprises, as premature failures, mainly stress corrosion cracking failures, have occurred in these kinds of joints. However, DMWJ with 52M as filler metal in the nuclear power plants has no in-service experience. To ensure the structural integrity of the weld joint and the safe operation of the future plants, the microstructure and local properties of a domestic safe-end DMWJ by using hot-wire gas tungsten arc welding (GTAW) technology was studied in detail by OM, SEM, micro-hardness testing, local mechanical tensile testing and slow strain rate tests. The tensile tests were performed at room temperature with the tensile speed of 5 μm/s while the slow strain rate tests were conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃. A large amount of type I boundaries and type II boundaries which are susceptible to stress corrosion cracking (SCC) exist in 52Mb near the SA508/52Mb interface and result in the highest SCC susceptibility of this interface. Microstructure transition was found in the SA508 heat affected zone (HAZ). In 316LN HAZ, increasing the distance from the fusion boundary, the number fraction of CSL boundaries increase while the residual strain decreases, resulting in the second-highest SCC susceptibility of 316LN HAZ. In 52M, residual strain distributes randomly but not uniformly, the residual strain is prone to accumulate at the grain boundaries. Dramatic changes of mechanical properties are observed across the joint, especially at the SA508/52M interface. The differences of the local microstructure and chemical composition lead to the differences of the local properties of the weld joint.
核能作为一种清洁、经济、高效的能源,受到各国的广泛重视,我国已经成为世界上在建核电站最多的国家。但是核电站的建设极其复杂,用到的材料种类也多种多样,不同材料间的相互焊接会导致许多问题,如C迁移、焊接残余应力产生、焊缝金属的稀释等,这些均成为造成众多核电关键设备失效的重要原因。大量的研究及运行经验表明,压水堆核电站一回路系统中连接低合金钢压力容器壳体管嘴与奥氏体不锈钢安全端的异种金属焊接接头(dissimilar metal welded joint,DMWJ),是一回路循环系统的薄弱部件,服役时间远低于设计寿命,应力腐蚀开裂（SCC）是其主要的失效形式之一[1]。
本工作的研究对象为以镍基合金52M为填充金属,以低合金钢SA508 Gr.3 Cl.2 (简写为SA508)及奥氏体不锈钢SA 336 Gr F316LN (简写为316LN)为母材的国产核电安全端异种金属焊接接头。各种材料的化学成分如表1所示。焊接工艺流程为：首先采用钨极氩弧焊(GTAW)工艺在SA508管嘴堆焊隔离层,焊丝为直径1.2 mm的52M,堆焊时焊丝要进行预热;隔离层堆焊完成后进行焊后热处理以消除焊接残余应力;再次采用GTAW工艺,以直径为0.9 mm的52M为填充金属将带有隔离层的管嘴与安全端过渡管(316LN)进行对接焊;对接焊完成后不再进行焊后热处理。在本文中,隔离层及对接焊缝区分别记为52Mb和52Mw。焊接件的局部切割件及焊缝截面示意图如图1a和b所示。
表1
Table 1
表1
表1
焊接件不同部位主要化学成分
Table 1
Chemical composition of materials in dissimilar metal weld joint (DMWJ) (mass fraction / %)
Material
C
Si
Mn
Cr
Ni
S
P
Fe
N
Mo
316LN
0.014
0.624
1.576
17.34
10.84
<0.001
0.026
Bal.
0.116
2.210
SA508
0.170
0.210
1.360
0.16
0.80
0.001
0.006
Bal.
-
0.490
52Mb
0.019
0.110
0.810
29.77
59.20
<0.0005
0.003
8.73
0.006
0.008
52Mw
0.023
0.110
0.900
29.76
58.80
<0.0005
0.003
8.74
0.006
0.100
Material
Co
Cu
Al
Ti
Cb(Nb)+Ta
V
B
Zr
Other
316LN
<0.050
-
-
-
-
-
-
-
-
SA508
-
0.04
-
-
-
0.005
-
-
-
52Mb
0.010
0.03
0.11
0.17
0.89
-
0.0007
0.010
<0.500
52Mw
0.006
0.02
0.11
0.19
0.89
-
0.0005
0.003
<0.494
Note: 52Mb—Inconel filler metal 52M used for buttering, 52Mw—Inconel filler metal 52M used for welding
表1
焊接件不同部位主要化学成分
Table 1
Chemical composition of materials in dissimilar metal weld joint (DMWJ) (mass fraction / %)
为了便于金相试样的制备,采用线切割工艺将焊缝切片沿厚度方向(径向)分为6块,分别记为H1~H6,如图1b所示。将线切割的试样首先利用水砂纸逐级打磨至2000号,然后机械抛光至2.5 µm,刻蚀后进行金相组织观察。由于材料不同,选用不同的刻蚀液：SA508低合金钢用4% (体积分数)硝酸酒精进行刻蚀;隔离层、对接焊缝区及316LN由16 g FeCl3+80 mL HCl+2 mL HNO3+11 mL H2O 配置的溶液进行刻蚀。利用 Observer. Z1m 光学显微镜(OM)对焊接接头不同部位的金相组织进行观察分析。利用配有能谱仪(EDS)的XL30 场发射环境扫描电镜(ESEM)分析了SA508/52Mb及52Mw/316LN熔合线界面处主要合金元素的分布。应用飞行时间二次离子质谱ToF-SIMS分析SA508/52Mb界面处C元素的分布情况,采用液态金属铋离子枪,加速电压为30 keV,控制软件及分析软件均为Surface Lab 6。
Fig.1
Photograph of the safe-end DMWJ (a) and schematic of the cross-section of the DMWJ and positions for micro-hardness testing, metallographic and EBSD observation (b) (unit: mm)
Fig.7
OM images of the microstructure transition in the SA508 heat affected zone (HAZ) (Figs.7a~h are higher magnification images of the microstructure transition: coarse ferrite+small amounts of carbides (coarse-grained region, Fig.7a)→bainite+fine martensite (fine-grained region, Figs.7b~f)→ferrite+martensite+bainite (partially transformed region, Fig.7g)→bainite (base metal, Fig.7h))
Fig.12
Inverse pole figures (IPFs) (a), kernel average misorientation (KAM) maps (b), grain boundary character distribution (GBCD) maps (c) as a function of the distance from the fusion boundary (x) of 316LN and 52M in sample H1, KAM as a function of the distance from the 52Mw/316LN interface in samples H1, H4 and H6 (d), and the number fractions of low angle boundary (LAB), coincidence site lattice (CSL) boundary and random high angle grain boundary (RGB) as a function of the distance from the 52Mw/316LN interface in sample H1 (e)
Fig.19
Stress-extension curves of local areas in the DMWJ obtained by slow strain rate test (SSRT) conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃
Fig.20
Distributions of angular deviations from the ideal Σ3 misorientation as a function of the distance from the fusion boundary in 316LN of sample H1
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Abstract In this paper, the local mechanical properties (yield/ultimate strength, ductility as well as stress鈥搒train curves) of an Alloy52M dissimilar metal welded joint (DMWJ) in nuclear power systems were determined by using the mini-sized flat tensile specimens, and the local microstructures in the joint were characterized and analyzed. The results show that there are drastic variations in the local mechanical properties in the interface regions between materials. The heterogeneous mechanical properties are mainly related to the local microstructures of the DMWJ. The complex microstructures in the interface regions between materials were mainly caused by the heat flow and element migration during welding process. The local mechanical properties and their mismatches may have significant effect on crack-tip fracture mechanics parameter, plastic deformation behavior, local fracture resistance and crack growth behavior. Therefore, they need to be obtained and used in the integrity assessment of theDMWJs.
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ABSTRACT In the nuclear power industry, dissimilar metal welding is widely used for joining low alloy steel to austenite stainless steel components with nickel-base filler metals. In this study, attention was paid to the weld metal in multi-pass Alloy 52-A508 dissimilar welds. An approximately 2 mm wide transition zone was observed that consisted of a martensitic layer (10-20um) along the weld interface and the austenite phase region with varying degrees of dilution. After post-weld heat treatment, the microstructures near the weld interface consisted of martensite, carbides and Type II boundaries. The presence of Type II boundaries significantly reduced the resistance to stress corrosion cracking (SCC) and formed intergranular cracking under simulated reactor coolant conditions. Constant extension rate tensile (CERT) tests were performed on the notched tensile specimens in 300掳C water at two extension rates, 3 脳 10-4 and 1 脳 10-6 mm/s. A fast CERT test can be regarded to have no contribution of corrosion, and its results can be used as standards for comparison. In the slow CERT tests, the ductility losses of round-bar specimens with a circumferential notch at various regions in the weld metal were ranked accordingly. The relative susceptibility to SCC in terms of the ductility loss in increasing order of severity was as follows: the undiluted weld metal, the transition zone and the weld interface. SEM fractographic observations were consistent with the SCC results, i.e., an increased ductility loss or SCC susceptibility was associated with more brittle fractures.
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Abstract The microstructure, local mechanical properties and local stress corrosion cracking susceptibility of an SA508-52M-316LN domestic dissimilar metal welded safe-end joint used for AP1000 nuclear power plant prepared by automatic gas tungsten arc welding was studied in this work by optical microscopy, scanning electron microscopy (with electron back scattering diffraction and an energy dispersive X-ray spectroscopy system), micro-hardness testing, local mechanical tensile testing and local slow strain rate tests. The micro-hardness, local mechanical properties and stress corrosion cracking susceptibility across this dissimilar metal weld joint vary because of the complex microstructure across the fusion area and the dramatic chemical composition change across the fusion lines. Briefly, Type I boundaries and Type II boundaries exist in 52 Mb near the SA508-52Mb interface, a microstructure transition was found in SA508 heat affected zone, the residual strain and grain boundary character distribution changes as a function of the distance from the fusion boundary in 316 LN heat affected zone, micro-hardness distribution and local mechanical properties along the DMWJ are heterogeneous, and 52Mw-316LN interface has the highest SCC susceptibility in this DMWJ while 316LN base metal has the lowest one.
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high-temperature water; assisted cracking; alloy 690tt; pure water; growth; behavior; dependence; chemistry; coolant; model
Hu CL, XiaS, LiH, et al.Effect of grain boundary network on the intergranular stress corrosion cracking of 304 stainless steel[J]. Acta Metall. Sin., 2011, 47: 939
Gertsman VY, Bruemmer SM.Study of grain boundary character along intergranular stress corrosion crack paths in austenitic alloys[J]. Acta Mater., 2001, 49: 1589
Abstract Samples of austenitic stainless alloys were examined by means of scanning and transmission electron microscopy. Misorientations were measured by electron backscattered diffraction. Grain boundary distributions were analyzed with special emphasis on the grain boundary character along intergranular stress corrosion cracks and at crack arrest points. It was established that only coherent twin Σ3 boundaries could be considered as “special” ones with regard to crack resistance. However, it is possible that twin interactions with random grain boundaries may inhibit crack propagation. The results suggest that other factors besides geometrical ones play an important role in the intergranular stress corrosion cracking of commercial alloys.
TanL, Allen TR, Busby JT.Grain boundary engineering for structure materials of nuclear reactors[J]. J. Nucl. Mater., 2013, 441: 661
Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic鈥搈artensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys.
West EA, Was GS.IGSCC of grain boundary engineered 316L and 690 in supercritical water[J]. J. Nucl. Mater., 2009, 392: 264
This study evaluated the influence of a high fraction of special grain boundaries on the intergranular stress corrosion cracking susceptibility of 316L stainless steel and nickel base alloy 690 in supercritical water. By thermomechanically processing the alloys to create specimens with largely different special boundary fractions, it was possible to isolate the effects of the grain boundary structure on the intergranular stress corrosion cracking behavior. Constant extension rate tensile experiments were performed in 50002°C deaerated supercritical water, and SEM analysis of the cracking behavior was performed on the gage surfaces of the specimens. Results indicate that the fraction of cracked grain boundary length in the specimens with higher fractions of special boundaries is reduced for 316L and 690 by factors of 9 and 5 at 15% strain, and 3 and 2 at 25% strain, respectively. This reduction is due to the special boundaries, which at 25% strain have a frequency of cracking that is 9–18 times lower than that for a random high angle boundary.
Characterization of microstructure, local deformation and microchemistry in Alloy 600 heat-affected zone and stress corrosion cracking in high temperature water
Numerical simulation of temperature field and residual stress in multi-pass welds in stainless steel pipe and comparison with experimental measurements
Microstructure, local mechanical properties and stress corrosion cracking susceptibility of an SA508-52M-316LN safe-end dissimilar metal weld joint by GTAW
Effect of dissolved oxygen content on stress corrosion cracking of a cold worked 316L stainless steel in simulated pressurized water reactor primary water environment
, 韩恩厚
