Profiling Microstructure Evolution Roadmap in Heat-Affected Zones of EH36 Shipbuilding Steel Under Controlled Thermal Si
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eldability of thick steel plates is necessitated for the construction of large-scale container ships. Various high-input welding methods, such as submerged arc welding, electroslag welding, and electrogas arc welding, have been employed to improve welding productivity,[1,2] which, in turn, increase shipbuilding efficiency. Unfortunately, high-input welding will deteriorate microstructures in heat-affected zones (HAZs). For one thing, thermal cycles incurred by high heat input welding will generally coarsen austenite grains. For another, brittle microstructures, including ferrite side plate (FSP), grain boundary ferrite (GBF) and
XIAODONG ZOU is with the School of Metallurgy, Northeastern University, Shenyang 110819, China and also with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. JINCHENG SUN is with the School of Metallurgy, Northeastern University. HIROYUKI MATSUURA is with the Department of Materials Engineering, The University of Tokyo. CONG WANG is with the School of Metallurgy, Northeastern University and also with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted December 10, 2019. Article published online May 14, 2020 3392—VOLUME 51A, JULY 2020
upper bainite (Bu), may pose concerted effects reducing the toughness of HAZ.[3–5] The formation of acicular ferrite (AF) in association with non-metallic inclusions is a solution to improve the toughness of HAZ. Generally, a microstructure predominately comprised of interlocking AF is usually superior in mechanical performances inherited from high-angle grain boundaries.[6–8] It needs to be pointed out that numerous factors, such as inclusions chemistry, size distribution, as well as cooling rate, may affect the nucleation of AF.[9–11] For certain steel grades, cooling rate, which is usually specified by the cooling time between 1073 K and 773 K (Dt8/5), is the most important welding parameter, as it dictates final microstructure and properties of HAZ.[12,13] More specifically, the microstructure usually changes (i) from martensite and/or bainite at low Dt8/5, (ii) to GBF with FSPs and/or AF at medium Dt8/5, and (iii) to mainly polygonal ferrite (PF) and/or pearlite at high Dt8/5.[10] Therefore, the evolution of microstructure at different Dt8/5 should be investigated to determine the appropriate cooling rates. In general, Dt8/5 can be correlated to welding heat input. Higher heat input could lead to longer cooling time and lower cooling rate. In a separate study, the effect of heat input on the nucleation and growth of ferrites in HAZ of EH36 shipbuilding steel, one of the typical steels for ship construction, has been well documented, and it has been articulated by in situ observation that AFs prefer to form at the low heat input.[11] However, different thermal cycles may have an influence on the characteristics of inclusions due to the diffusion-controlled growth during cooling and thus affect the final microstructure.[14] Ther
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