Crystallography of Martensite in Friction-Stir-Welded 12Cr Heat-Resistant Steel
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THE 12Cr steel belongs to a recent generation of heat-resistant ferritic steel. Due to an excellent combination of high creep-rupture strength and good corrosion and oxidation resistance, it is often considered as a very promising structural material for the power industry.[1–5] Unfortunately, the unique properties of this material are totally degraded during conventional fusion welding due to hydrogen-induced cracking, porosity, formation of delta-ferrite, as well as dissolution of strengthening precipitates associated with the solidification process.[6–12] Friction-stir welding (FSW), being an innovative solid-state joining technique, enables
PENG HUA is with the School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, P.R. China, and also with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-602 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, Japan. Contact email: [email protected] SERGEY MIRONOV is with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, and also with the Belgorod National Research University, Pobeda 85, Belgorod 308015, Russia. YUTAKA S. SATO is with the Department of Materials Processing, Graduate School of Engineering, Tohoku University. HIROYUKI KOKAWA is with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, and also with the School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, P.R. China. SEUNG HWANG C. PARK and SATOSHI HIRANO are with the Hitachi Research Laboratory, Hitachi Ltd, Omika 7-1-1, Hitachi 319-1292, Japan. Manuscript submitted November 29, 2018. Article published online April 22, 2019 3158—VOLUME 50A, JULY 2019
avoiding (or minimizing, at least) these problems and thus principally improving the welding quality. The first works in this area have demonstrated excellent feasibility of this technique for joining of 12Cr steel,[13–20] thus necessitating a more fundamental understanding of the underlying physical processes. Specifically, it has been found that the peak FSW temperature normally exceeds the A3 point; therefore, the final weld microstructure is produced by the martensitic transformation occurring during the weld cooling cycle (e.g., Reference 14). For such transformations, the properties of the resulting material are known to be essentially influenced by a crystallographic relationship between the high-temperature and low-temperature phases. In friction-stir-welded 12Cr steel, however, crystallography of the martensitic structure has not been examined so far, to the best of the authors’ knowledge. It has been suggested that the unique character of the FSW process, which involves very large strains at high temperature, and high strain rate may exert an essential influence on the martensitic transformation. Attempting to improve the basic physical understanding of the microstructural processes in this case, the present work f
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