Effect of Nickel Contents on the Microstructure and Mechanical Properties for Low-Carbon Bainitic Weld Metals
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JMEPEG DOI: 10.1007/s11665-017-2638-2
Effect of Nickel Contents on the Microstructure and Mechanical Properties for Low-Carbon Bainitic Weld Metals Gaojun Mao, Rui Cao, Jun Yang, Yong Jiang, Shuai Wang, Xili Guo, Junjun Yuan, Xiaobo Zhang, and Jianhong Chen (Submitted October 9, 2016; in revised form December 14, 2016) Multi-pass weld metals were deposited on Q345 base steel using metal powder-flux-cored wire with various Ni contents to investigate the effects of the Ni content on the weld microstructure and property. The types of the microstructures were identified by optical microscope, scanning electron microscope, transmission electron microscope, and micro-hardness tests. As a focusing point, the lath bainite and lath martensite were distinguished by their compositions, morphologies, and hardness. In particular, a number of black plane facets appearing between lath bainite or lath martensite packets were characterized by laser scanning confocal microscope. The results indicated that with the increase in Ni contents in the range of 0, 2, 4, and 6%, the microstructures in the weld-deposited metal were changed from the domination of the granular bainite to the majority of the lath bainite and/or the lath martensite and the micro-hardness of the welddeposited metal increased. Meanwhile, the average width of columnar grain displays a decreasing trend and prior austenite grain size decreases while increases with higher Ni content above 4%. Yield strength and ultimate tensile strength decrease, while the reduction in fracture area increases with the decreasing Ni mass fraction and the increasing test temperature, respectively. And poor yield strength in Ni6 specimen can be attributed to elements segregation caused by weld defect. Finally, micro-hardness distribution in correspondence with specimens presents as a style of cloud-map. Keywords
black plane facets, micro-hardness, microstructure, multi-pass weld metals, Ni content, tensile property
1. Introduction Nowadays, advanced steels developed by optimizing the alloy compositions and microstructure from ferrite plus pearlite through tempered martensite to extra-low-carbon lath bainite plus lath martensite can successfully achieve high strength of 1000 MPa and Charpy V toughness of 200 J at 40 C (Ref 1). In addition, strictly thermo-mechanical controlling process, transformation-induced plasticity process, and reasonable heat treatment process for element partitioning further improve the performances (Ref 2). These steels with high strength and high toughness have been widely applied in the field of pipeline, ship building, and various manufacturing industries. However, due to the complexity of welding processes and limitations of thermal process of welding, the toughness of the weld metal in low temperature is much lower than that of the base metal. For X70, X80 pipeline steel, the impact toughness of the matched weld
Gaojun Mao, Rui Cao, Junjun Yuan, Xiaobo Zhang, and Jianhong Chen, State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metal,
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