Fracture Location Shift of Dissimilar Metal Welds Under Coupled Thermal-Stress Effect

doi:10.11900/0412.1961.2020.00140

Acta Metall Sin

2020, Vol. 56

Issue (11): 1463-1473 DOI: 10.11900/0412.1961.2020.00140

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Fracture Location Shift of Dissimilar Metal Welds Under Coupled Thermal-Stress Effect

LI Kejian¹^,², ZHANG Yu¹^,², CAI Zhipeng¹^,²^,³^,⁴(

)

1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Tsinghua University, Beijing 100084, China
3 State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
4 Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing 100084, China

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LI Kejian, ZHANG Yu, CAI Zhipeng. Fracture Location Shift of Dissimilar Metal Welds Under Coupled Thermal-Stress Effect. Acta Metall Sin, 2020, 56(11): 1463-1473.

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Abstract

Dissimilar metal welds (DMWs) between high-Cr martensitic heat-resistant steels and nickel-based alloys with nickel-based filler metals are widely used in fossil-fired power plants. Reports of premature failures of DMW joints have attracted considerable attention as such occurrences in the field can lead to significant economic loss and safety issues. Moreover, a comprehensive understanding of the high-temperature performance of new types of DMWs is lacking. In this work, creep tests were conducted over a stress range of 140~260 MPa at of 600 and 620 ℃. A shift in the fracture location with variations in stress was observed, with three typical failure modes. At high stress levels (240~260 MPa), the DMW fractured in the base metal (BM) of martensitic steel, accompanied by a large degree of plastic deformation. At intermediate stress levels (200~240 MPa), the DMW fractured in the fine-grained heat-affected zone (FGHAZ) and the inter-critical heat-affected zone (ICHAZ), with creep cavities around coarsened carbides, indicating a typical type IV crack. At low stress levels (140~200 MPa), the DMW fractured in a mixed mode involving three stages. First, a crack initiated at the interface between the nickel-based weld metal and the martensitic steel, which was attributed to the interaction between the oxidation behavior of the martensitic steels and the thermal stress arising from the mismatch in the coefficients of thermal expansion. Second, the crack deflected into the FGHAZ/ICHAZ and developed into a type IV crack mode. Finally, the crack propagated into the adjacent BM, featuring significant plastic deformation. In addition, in the stress-LMP (Larson-Miller parameter) plot, the creep life at a relatively low stress level was shorter than that predicted by linear extrapolation of the data obtained at a high stress level, indicating a premature failure tendency at low stress. This premature failure tendency can be attributed to microstructure degradation in HAZs and preferential oxidation at the interface at low stress levels.

Key words: dissimilar metal weld martensitic heat resistant steel nickel-based alloy type IV crack oxidation

Received: 06 May 2020

ZTFLH:

TG407

Fund: National Natural Science Foundation of China(51775300);National Natural Science Foundation of China(51901113)

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	Kejian LI
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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00140 OR https://www.ams.org.cn/EN/Y2020/V56/I11/1463

Fig.1 Schematic of the DMW and sampling method of creep specimens (BM—base metal, WM—weld metal, DMW—dissimilar metal weld)

Table 1 Chemical compositions of base metals and filler metal of the DMW

Fig.2 OM images of BM (a), weld interface between WM and HAZ (b), HAZ about 1.5 mm away from the interface of the high Cr martensitic steel (c), and nickel-based WM (d) (HAZ—heat-affected zone)

Fig.3 Linear plot of stress and LMP in three segments of DMWs between high Cr martensitic heat resistant steel and nickel-based alloys with nickel-based filler material (IF—interfacial failure, LMP—Larson-Miller parameter, T—temperature, t_f—failure life)

Fig.4 Creep specimens fractured at high Cr martensitic steel BM (a) and HAZ (b), and crack initiated at the interface between WM and BM then deflected into HAZ and BM (c)

Fig.5 Temperature dependence of failure life for DMWs at the stress of 200 MPa (Q—diffusion activation energy, R—gas constant)

Fig.6 Stress dependence of failure life for DMWs at the temperatures of 600 and 620 ℃

Fig.7 Cross section morphologies of failed DMWs fracturing in BM mode (a), HAZ mode (b), magnification of the zone in dashed rectangle in Fig.7b (c), and IF mode (d) (σ—tensile stress)

Fig.8 Fracture morphologies of DMWs failed in BM mode (a, b), HAZ mode (c, d), and IF mode (e, f)

Fig.9 Hardness profiles across the weld interface of DMWs before and after creep exposure

Fig.10 Cross section SEM images of the area adjacent to facture surface of DMW failed in FGHAZ/ICHAZ (FGHAZ—fine-grained HAZ, ICHAZ—inter-critical HAZ)
(a) macroscopic image showing creep cavity (arrows)
(b) microscopic image showing creep cavity mainly initiated around coarsened carbides

Fig.11 Chemical composition profile across the interface between WM and HAZ in high Cr marten-sitic steel

Fig.12 Formation process of oxide notch at interface
(a) a step resulting from uncoordinated deformation at 620 ℃, 160 MPa for 100 h
(b) interfacial debonding at 620 ℃, 160 MPa for 300 h
(c) an oxide notch formed at interface at 620 ℃, 160 MPa for 800 h

Fig.13 Development of the mixed fracture mode involving three stages for creep specimens failed in IF mode

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