Role of tempering cooling rate on impact toughness of 2CrMoV weld metal
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Role of tempering cooling rate on impact toughness of 2CrMoV weld metal Tao Fang1, Xia Liu2, Chendong Shao1, Haichao Cui1,a), Fenggui Lu1,b) 1
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China 2 Department of Technology Research and Development, Shanghai Turbine Plant of Shanghai Electric Power Generation Equipment Co. Ltd., Shanghai 200240, People’s Republic of China a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] Received: 13 November 2019; accepted: 21 February 2020
Tempering cooling rate plays a significant role in the impact toughness of 2CrMoV weld metal. Three different tempering cooling rate experiments were carried out; it is found that the impact toughness of weld metal improved from 44.61 to 117.49 J as the cooling rate increased from 5 to 40 °C/h. Microstructure characterization revealed that the large blocky M–A constituents and cluster precipitation were considered to act as stress concentration sources and cleavage fracture initiators at a cooling rate of 5 °C/h. Under the cooling rate of 20 °C/ h, the decrease of blocky M–A constituents as well as homogeneous distribution of precipitation induced the transition from cleavage to interfacial decohesion. The chance of crack propagation in intragranular ferrite matrix was increased, which needed to absorb more energy and improve impact toughness. When the tempering cooling rate reached at 40 °C/h, the cracks mainly propagated in the ferrite matrix; meanwhile, fine and homogeneous distribution of precipitation greatly inhibited crack propagation and led to higher impact toughness.
Introduction Nuclear power acting as a clean energy alternative to thermal power attracts much more attention attributing to its energy structure adjustment and power generation efficiency [1, 2, 3]. As the key constituents of nuclear power equipment, low pressure welded rotors in steam turbines produce large impact load when starting and stopping, which have higher requirements on the toughness of materials and their safety under service [4, 5]. NiCrMoV steel is thus used in the manufacture of steam turbine rotors as a suitable material [6, 7]. For large-scale rotors in steam turbines, narrow gap submerged arc welding (NG-SAW) method is chosen to manufacture welding rotor for its cost-effective and high production efficiency, in which multilayer and multi-pass are utilized to improve deposition efficiency and weld quality [8, 9, 10]. However, for large structural parts with high strength operating at low and normal temperatures, impact toughness has become one of the main problems affecting the reliability of equipment [11, 12, 13, 14]. A large number of studies have been performed on the reliable evaluation of NiCrMoV steel and impact toughness test of weld metal in the past. According
ª Materials Research Society 2020
to our group previous research, the impact toughness of NiC
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