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Acta Metall Sin  2020, Vol. 56 Issue (8): 1057-1066    DOI: 10.11900/0412.1961.2019.00449
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Effect of Dilution Ratio of the First 309L Cladding Layer on the Microstructure and Mechanical Properties of Weld Joint of Connecting Pipe-Nozzle to Safe-End in Nuclear Power Plant
ZHANG Maolong1,2, LU Yanhong1, CHEN Shenghu3, RONG Lijian3(), LU Hao2
1 Shanghai Electric Nuclear Power Equipment Co. Ltd. , Shanghai 201306, China
2 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
3 Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

The transition joint between austenitic stainless steel pipe and low alloy steel nozzle of the pressure vessel has attracted much attention due to the occurrence of failure during application. Usually, the low alloy steel vessel nozzle should be firstly buttered with several layers of austenitic stainless steel and then welded to the austenitic stainless steel pipe. Cracking phenomenon in the austenitic cladding layer sometimes occurs during fabrication of the transition joint, and the cracking mechanism is not very clear. It is worth noting that microstructure in the first buttering layer is largely dependent on the welding condition, because the variation of the buttering welding parameters would lead to different dilution ratios in the cladding layer. Therefore, it is essential to investigate the effect of dilution ratio of the cladding layer on the mechanical properties of the weld joint. In this work, microstructure of the 309L cladding layer under two kinds of buttering welding parameters was analyzed using OM, SEM, XRD, EPMA and EBSD, and its effects on the mechanical properties of the weld joints were further studied. The results show that duplex microstructure (austenite+martensite) are present in the 309L cladding layers under two kinds of buttering welding parameters, but the dilution ratio could determine the morphology and amount of martensite phase. Microstructure consisting of austenite and lath martensite is found in the 309L cladding layer with a lower dilution ratio. A higher dilution ratio could increase the amount of lath martensite. The formation of needle-like martensite occurs when the dilution ratio exceeds a critical value. The dilution ratio in the 309L cladding layers directly affects the mechanical properties of weld joint. For the weld joint with a lower dilution ratio, no cracking phenomonen is observed during three-point bending test, and the specimens fracture at the weld fusion zone after tensile test. For the weld joint with a higher dilution ratio, cracking phenomenon initiated at the 309L cladding layer is present during three-point bending test, and a significat reduction in the tensile strength and elongation is observed. During deformation, the strain incompatibility between needle-like martensite and austenite is produced, leading to the formation of microcracks at the interfaces. The preferential cracking at the 309L cladding layer with a higher dilution ratio leads to the degradation of mechanical properties of the weld joint.

Key words:  buttering welding      dilution ratio      microstructure      mechanical property      cracking mechanism     
Received:  25 December 2019     
ZTFLH:  TG44  
Fund: National Natural Science Foundation of China(51871218);Program of Key Laboratory of Nuclear Materials and Safety Assessment, Chinese Academy of Sciences(2019NMSAKF03)
Corresponding Authors:  RONG Lijian     E-mail:  ljrong@imr.ac.cn

Cite this article: 

ZHANG Maolong, LU Yanhong, CHEN Shenghu, RONG Lijian, LU Hao. Effect of Dilution Ratio of the First 309L Cladding Layer on the Microstructure and Mechanical Properties of Weld Joint of Connecting Pipe-Nozzle to Safe-End in Nuclear Power Plant. Acta Metall Sin, 2020, 56(8): 1057-1066.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00449     OR     https://www.ams.org.cn/EN/Y2020/V56/I8/1057

MaterialCSiMnPSNiCrMoCuCoAlTiNFe
SA5080.2100.221.350.0040.0030.870.21-0.04-0.02-0.01Bal.
316LN0.0140.381.100.0240.00411.216.22.13-0.01--0.11Bal.
EQ309L0.0120.371.630.0130.00313.224.00.060.030.04-0.0070.04Bal.
EQ308L0.0120.341.630.0130.00310.120.60.090.030.04-0.0060.04Bal.
ERNiCrFe-70.0240.160.240.0030.00160.428.90.010.01<0.010.70.50.019.09
Table 1  Chemical compositions of each material in the safe-end joint
Fig.1  Schematic of the dissimilar metal welded safe-end joint (a) and macro-morphologies of austenitic stainless steel cladding layer of S1 (b) and S2 (b)
Fig.2  Schematics of bending sample (a) and tensile sample (b)
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Fig.3  OM (a~d) and SEM (e, f) images of microstructures of the 309L cladding layer of S1 (a, c, e) and S2 (b, d, f) (The red dashed lines in Fig.3c and d show the interfaces of martensite and austenite)
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Fig.4  XRD spectra of the 309L cladding layers of S1 and S2
Fig.5  EPMA results of the 309L cladding layers of S1 (a) and S2 (b)
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Fig.6  Macro-morphologies of the weld joint of S1 (a) and S2 (b) after the 180° bending test
Fig.7  Tensile stress-strain curves of the weld joints of S1 and S2 at room temperature
Fig.8  Morphologies of side surfaces of the fractured samples of S1 (a) and S2 (b), and fracture surfaces of the weld joint of S2 (c~f)
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Fig.9  Low (a) and high (b) magnified OM images of cross-sectional microstructures near the tensile fracture surface of the weld joint of S2 (The arrows show the micro-cracks)
Fig.10  Prediction of microstructure for the cladding layer according to the Schaeffler diagram (The microstructures of the 309L cladding layer are indicated by black arrows with the increased dilution ratio (D), and microstructures of the 309L cladding layer of S1 and S2 according the chemical composition are indicated; A—austenite, M—martensite, F—ferrite)[18]
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Fig.11  EBSD phase distributions of the 309L cladding layers of S1 (a) and S2 (b)
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Weld jointMass fraction of element / %

Dilution ratio

%

CSiMnNiCr[Ni][Cr]
S10.0760.571.388.8115.7211.7816.5836
S20.0910.511.337.8613.6911.2614.4645
Table 2  Alloy element contents of 309L cladding layer of weld joints of S1 and S2 and their calculated dilution ratios
Fig.12  Microhardness distributions across the 316LN/NiCrFe-7/309L along the dissimilar weld joints of S1 and S2
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