In Situ Observation of the Nucleation and Growth of Ferrite Laths in the Heat-Affected Zone of EH36-Mg Shipbuilding Stee

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mprove the cruising range of ships and to reduce the building costs, shipbuilding steels are required to carry superior properties such as high strength, outstanding toughness, and excellent weldability. One proven way of enhancing shipbuilding efficiency and reducing cost is to implement high heat input welding during manufacturing via demonstrated techniques such as submerged arc welding and electroslag welding.[1,2] However, high heat input welding usually leads to significant coarsening of austenite grains and formation of microstructures, such as ferrite side plate (FSP) and upper bainite, which invariably lead to the reduced toughness of the heat-affected zone (HAZ).[3–5] Intragranular acicular ferrite (AF) is regarded as the optimal microstructure combining excellent strength and toughness.[6,7] This is due to the fact that AFs, generally nucleating on the potent nonmetallic inclusions in the prior austenite grain, carry chaotic arrangements of laths and fine-grained interlocking

XIAODONG ZOU, JINCHENG SUN, and CONG WANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact email: [email protected] HIROYUKI MATSUURA is with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Manuscript submitted May 3, 2018. METALLURGICAL AND MATERIALS TRANSACTIONS B

microstructure features that effectively divide prior austenite grain into several subgrains and inhibit the propagation of cracks.[8,9] Therefore, AF nucleation on inclusions within coarse austenite grains is a solution to improve the toughness of HAZ for shipbuilding steels.[10,11] It is found that the nucleation of AF is affected by several factors such as the characteristics of inclusions and cooling rate during solidification.[8,9,11–15] Xu et al.[11] reported that Mg-containing inclusions could effectively facilitate the nucleation of AF and improve the toughness of HAZ in EH36 shipbuilding steel, and the fraction of AF increased with the increase of Mg content from 0 to 99 ppm. Yang et al.[15] found that there was an optimal cooling rate for the formation of AF in the medium-carbon steel with 0.025 pct Ti. Sung et al.[16] suggested that the volume fraction of AF in HAZ of API X80 pipeline steels decreased as the heat input increased from 35 to 60 kJ/cm due to decreasing cooling rate during c fi a transformation. Hence, correlation studies involving formation of AF and welding heat input are necessary to derive appropriate welding conditions and microstructures. A high-temperature confocal scanning laser microscope (CSLM) enables observation of the nucleation and growth of AF during welding in situ.[17–19] The present work aims to investigate the effect of welding heat input on the microstructure in the HAZ of EH36-Mg shipbuilding steel. Concurrent to in situ observation of AF nucleation and growth by CSLM, crystallographic orientations of microstructural features in HAZ are performed by electron backscatter diffraction (EBSD) to offer quantitative explanations. Chemical compositions of the t