Micromechanism of Decrease of Impact Toughness in Coarse-Grain Heat-Affected Zone of HSLA Steel with Increasing Welding
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INTRODUCTION
ADVANCED heavy plates of high-strength lowalloy (HSLA) steels with higher strength, enhanced impact toughness and improved weldability have been developed since 1970s. Heavy thickness, extra-width, and extra-length steel plate with yield strength higher than 460 MPa is widely used for ship-building.[1–5] Due to the lower carbon content these steels show less sensitivity to hydrogen-induced cold crack (HICC) and can be welded without preheating, therefore reduce the time and cost of fabrication. However, for these steels, welding is still a critical process, which produces the coarse-grain heat-affected zone (CGHAZ) and deteriorates the property of the welded joint. It was found that CGHAZ had a higher susceptibility to HICC than the weld metal (WM) and the HICC cracking rate decreased notably with the increasing heat input.[6] In most cases, the CGHAZ showed the lowest ductility and toughness,[7–12] which was related to coarsening grain size in the CGHAZ and coarse martensite-austenite (M–A) constituent. Therefore, more studies were concentrated on studying microstructures and properties of the heat-affected zone (HAZ) by varying peak temperatures, heat inputs, and alloy compositions.[7–19] In Reference 12 coarse austenite grain size associated with coarse M–A constituent along grain boundary was revealed as the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent was favorable in R. CAO and J.H. CHEN, Professors, J. LI, Graduate, and J.Y. MA, Engineer, are with the State Key Laboratory of Gansu Advanced Processing and Recycling of Non-ferrous Metallic Metals and Key Laboratory of Non-ferrous Metal Alloys of the Ministry of Education, Lanzhou University of Technology, Lanzhou 730050, P.R. China. Contact e-mail: [email protected] D.S. LIU, Senior Researcher, Manager, is with the Institute of Research of Iron and Steel, Shasteel, Zhangjiagang, 215623 Jiangsu, P.R. China. Manuscript submitted December 3, 2014. Article published online April 28, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
obtaining high toughness. Good toughness was obtained upon refining the prior austenite grain size in the CGHAZ in the first pass and hence in the intercritical coarse-grain heat-affected zone (ICCGHAZ) in the second pass. Banerjee et al.[13] found that compared with the heating rate, the peak temperature of thermal cycle at simulated HAZ of 980 pipeline steel played a dominant role on final austenite grain size. Zhang and Kannengiesser et al.[14] studied the microstructure and hardness in simulated HAZs of micro-alloyed HSLA steels. They found that the peak temperature affected microstructure and hardness, and the heat simulated results were consistent with that of real weld HAZ. In Reference 15 a newly developed high-strength hightoughness steel E690 for offshore structures was studied. It was revealed that a high heat input welding followed by a fast cooling cycle was a promising welding process to improve the efficiency and toughness of offshore steels beca
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