Microstructure and Local Properties of a Domestic Safe-End Dissimilar Metal Weld Joint by Using Hot-Wire GTAW

doi:10.11900/0412.1961.2016.00135

Acta Metall Sin

2017, Vol. 53

Issue (1): 57-69 DOI: 10.11900/0412.1961.2016.00135

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Microstructure and Local Properties of a Domestic Safe-End Dissimilar Metal Weld Joint by Using Hot-Wire GTAW

Hongliang MING^1,²,Zhiming ZHANG¹,Jianqiu WANG¹(

),En-Hou HAN¹,Mingxing SU³

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

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Hongliang MING,Zhiming ZHANG,Jianqiu WANG,En-Hou HAN,Mingxing SU. Microstructure and Local Properties of a Domestic Safe-End Dissimilar Metal Weld Joint by Using Hot-Wire GTAW. Acta Metall Sin, 2017, 53(1): 57-69.

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Abstract

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 H₃BO₃and 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.

Key words: dissimilar metal weld joint, microstructure, local mechanical property, stress corrosion cracking susceptibility, residual strain

Received: 13 April 2016

Fund: 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)

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	Hongliang MING
	Zhiming ZHANG
	Jianqiu WANG
	En-Hou HAN
	Mingxing SU

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00135 OR https://www.ams.org.cn/EN/Y2017/V53/I1/57

Table 1 Chemical composition of materials in dissimilar metal weld joint (DMWJ) (mass fraction / %)

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.2 Schematic of the small-sized flat tensile sample used for the local mechanical property test (δ—thickness, unit: mm)

Fig.3 Schematic of U-notched round-bar sample used for slow strain rate test (unit: mm)

Fig.4 Schematic of locations of the 51 small-sized flat tensile samples in the DMWJ (unit: mm)

Fig.5 OM images of SA508 (a), 316LN (b), 52Mb (c) and 52Mw (d)

Fig.6 OM images of SA508/52Mb interface (a) and 52Mw/316LN interface (b)

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.8 OM image of the microstructure transition in the 316LN HAZ

Fig.9 Morphology of SA508/52Mb interface without a martensite zone (a) and EDS analysis along the line in Fig.9a (b)

Fig.10 Morphology of SA508/52Mb interface with a martensite zone (a) and EDS analysis along the line in Fig.10a (b)

Fig.11 Morphology of 52Mw/316LN interface (a) and EDS analysis along the line in Fig.11a (b)

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.13 IQ map (a), IPF (b), KAM map (c) and phase distribution (d) of the SA508/52Mb interface in sampe H4

Fig.14 Microhardness distribution along the DMWJ across L1 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

Fig.15 Microhardness distribution along the DMWJ across L2 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

Fig.16 Microhardness distribution along the DMWJ across L3 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

Fig.17 SEM image of the SA508/52Mb interface (a) and C distribution (b)

Fig.18 Yield strength, ultimate strength and fracture strain of all the 51 samples across the DMWJ at room temperature (as shown in Fig.4)

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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