Study on fracture toughness of 617 Ni-based alloy welded joint under different elevated temperatures
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Study on fracture toughness of 617 Ni-based alloy welded joint under different elevated temperatures Yuan Gao1, Chendong Shao1,b), Haichao Cui1, Ninshu Ma2, Fenggui Lu1,a),b) 1
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China 2 Joining & Welding Research Institute, Osaka University, Osaka 567-0047, Japan a) Address all correspondence to this author. e-mail: [email protected] b) These authors contributed equally to this work. Received: 7 April 2020; accepted: 1 June 2020
The fracture toughness of 617 Ni-based weld metal (WM) under different elevated temperatures was tested with a novel method and its fracture mechanism was investigated in this paper. It was found that the fracture toughness of WM was lower than that of base metal (BM) at the same temperature, which was mainly due to the coarse columnar structure, differences in misorientation, and precipitated phases. For both BM and WM, the fracture toughness was lower at elevated temperature due to decreased strength. Much more micro-voids caused by Ti(C, N) and M23C6 inside grains of BM could be observed adjacent to the crack path, which accounted for the dramatically decreased fracture toughness of BM at elevated temperature. In comparison, fewer micro-voids could be observed in WM due to the lack of those second particles. As a result, the J0.2 value and propagation path morphology both showed that the WM had more stable microstructure even though possessing lower toughness.
Introduction In order to decrease the CO2 emission in thermal power plant, the efficiency of turbines can be improved with higher steam pressure and temperature, which is a big challenge for materials used to manufacture the turbine rotor [1, 2]. Ni-based alloy becomes a candidate rotor material in ultra-supercritical steam turbines for its higher fracture toughness, creep strength, and better fatigue resistance property [3, 4, 5, 6, 7]. Among these properties, fracture toughness represents the ability of the material to withstand large loads in the case of preexisting crack, which is an important parameter for the safe operation of steam turbines. However, the current researches on fracture toughness are mostly conducted at room temperature (RT), which is unable to provide reliable data support for the safety assessment of steam turbine rotors operating at elevated temperature. To study the fracture toughness of materials at elevated temperature, it is necessary to consider the change in mechanical properties, evolution of microstructure, and precipitated phases. It has been reported that the primary particles in Ni-based alloy are Ti(C, N), M23C6, and M6C, and γ’ is a common interstitial phase in Ni-based alloy [8, 9, 10, 11, 12, 13, 14]. Li et al. [15] reported that the formation of oxides at elevated temperature can directly deteriorate the properties of
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Ni-based alloy. According to Oh et al. [16], the formation of micro-voi
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