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
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.
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.
Fund: National Natural Science Foundation of China(51871218);Program of Key Laboratory of Nuclear Materials and Safety Assessment, Chinese Academy of Sciences(2019NMSAKF03)
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) Color online
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) Color online
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) Color online
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) Color online
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] Color online
Fig.11 EBSD phase distributions of the 309L cladding layers of S1 (a) and S2 (b) Color online
Weld joint
Mass fraction of element / %
Dilution ratio
%
C
Si
Mn
Ni
Cr
[Ni]
[Cr]
S1
0.076
0.57
1.38
8.81
15.72
11.78
16.58
36
S2
0.091
0.51
1.33
7.86
13.69
11.26
14.46
45
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
[1]
Dai P K. Materials and Welding of the Main Equipment in Pressurized Water Reactor [M]. Shanghai: Shanghai Scientific and Technological Literature Press, 2008: 227
Li G F, Yang W. Cracking of dissimilar metal welds in nuclear power plants and methods to evaluate its susceptibility to stress corrosion cracking [J]. Nucl. Saf., 2003, (2): 37
Lundin C D. Dissimilar metal welds—Transition joints literature review [J]. Weld. Res. Suppl., 1982, 61: 58
[4]
Liu Z Q, Liu K. Guide for Welding of Dissimilar Metals [M]. Beijing: China Machine Press, 1997: 7
(刘中青, 刘 凯. 异种金属焊接技术指南 [M]. 北京: 机械工业出版社, 1997: 7)
[5]
Ding J, Zhang Z M, Wang J Q, et al. Micro-characterization of dissimilar metal weld joint for connecting pipe-nozzle to safe-end in Generation III nuclear power plant [J]. Acta Metall. Sin., 2015, 51: 425
doi: 10.11900/0412.1961.2014.00299
Li G F, Li G J, Fang K W, et al. Stress corrosion cracking behavior of dissimilar metal weld A508/52M/316L in high temperature water environment [J]. Acta Metall. Sin., 2011, 47: 797
doi: 10.3724/SP.J.1037.2011.00316
Ming H L, Zhang Z M, Wang J Q, et al. Microstructure and local properties of a domestic safe-end dissimilar metal weld joint by using hot-wire GTAW [J]. Acta Metall. Sin., 2017, 53: 57
doi: 10.11900/0412.1961.2016.00135
Xiong Q, Li H J, Lu Z P, et al. Characterization of microstructure of A508III/309L/308L weld and oxide films formed in deaerated high-temperature water [J]. J. Nucl. Mater., 2018, 498: 227
doi: 10.1016/j.jnucmat.2017.10.030
[9]
Li G F, Congleton J. Stress corrosion cracking of a low alloy steel to stainless steel transition weld in PWR primary waters at 292 ℃ [J]. Corros. Sci., 2000, 42: 1005
doi: 10.1016/S0010-938X(99)00131-6
[10]
Wang H T, Wang G Z, Xuan F Z, et al. Fracture mechanism of a dissimilar metal welded joint in nuclear power plant [J]. Eng. Fail. Anal., 2013, 28: 134
doi: 10.1016/j.engfailanal.2012.10.005
[11]
Joseph A, Rai S K, Jayakumar T, et al. Evaluation of residual stresses in dissimilar weld joints [J]. Int. J. Press. Vessels Pip., 2005, 82: 700
doi: 10.1016/j.ijpvp.2005.03.006
[12]
Wang H T, Wang G Z, Xuan F Z, et al. Local fracture behavior in an alloy 52M dissimilar metal welded joint in nuclear power plants [J]. Nucl. Tech., 2013, 36(4): 040628
Ming H L, Zhu R L, Zhang Z M, et al. Microstructure, local mechanical properties and stress corrosion cracking susceptibility of an SA508-52M-316LN safe-end dissimilar metal weld joint by GTAW [J]. Mater. Sci. Eng., 2016, A669: 279
[14]
Sireesha M, Albert S K, Shankar V, et al. A comparative evaluation of welding consumables for dissimilar welds between 316LN austenitic stainless steel and Alloy 800 [J]. J. Nucl. Mater., 2000, 279: 65
doi: 10.1016/S0022-3115(99)00275-5
[15]
Santosh R, Das S K, Das G, et al. Three-dimensional thermomechanical simulation and experimental validation on failure of dissimilar material welds [J]. Metall. Mater. Trans., 2016, 47A: 3511
[16]
Lei F, Tan J H, Xu C J. Studies on the shedding behavior of the surfacing layer on the inner wall of the safety and nozzle of the steam generator nozzle [J]. Technol. Innov. App., 2019, (5): 49
Lu Y H, Zhang M L, Tang W B, et al. Research on microstructure evolution and interfacial disbonding mechanism during dissimilar weld between 18MND5 and 309L [J]. Press. Vessel Technol., 2017, 34(9): 21
Schaeffler A L. Constitution diagram for stainless steel weld metal [J]. Met. Prog., 1949, 56: 680
[19]
Kotecki D J, Siewert T A. WRC-1992 constitution diagram for stainless steel weld metals: A modification of the WRC-1988 diagram [J]. Weld. Res. Suppl., 1992, 71: 171s
[20]
Ming H L, Zhang Z M, Wang J Q, et al. Microstructural characterization of an SA508-309L/308L-316L domestic dissimilar metal welded safe-end joint [J]. Mater. Charact., 2014, 97: 101
doi: 10.1016/j.matchar.2014.08.023
[21]
Dupont J N, Kusko C S. Technical note: Martensite formation in austenitic/ferritic dissimilar alloy welds [J]. Weld. Res., 2007, 86: 51s
[22]
Chung W C, Huang J Y, Tsay L W, et al. Microstructure and stress corrosion cracking behavior of the weld metal in alloy 52-A508 dissimilar welds [J]. Mater. Trans., 2011, 52: 12
doi: 10.2320/matertrans.M2010294
[23]
Wang Q Y, Chen S H, Rong L J. δ-Ferrite formation and its effect on the mechanical properties of heavy-section AISI 316 stainless steel casting [J]. Metall. Mater. Trans., 2020, 51A: 2998
[24]
Azuma M, Goutianos S, Hansen N, et al. Effect of hardness of martensite and ferrite on void formation in dual phase steel [J]. Mater. Sci. Technol., 2012, 28: 1092
[25]
Jin X J, Chen S H, Rong L J. Effect of Fe2Zr phase on the mechanical properties and fracture behavior of Fe-Cr-W-Zr ferritic alloy [J]. Mater. Sci. Eng., 2018, A722: 173
[26]
Feng G C. Affecting factors of the dilution rate during buttering deposition and its controlling methods [J]. Weld. Technol., 1996, (1): 22
