Visualization and Analysis of Variant Grouping in Continuously Cooled Low-Carbon Steel Welds
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NTRODUCTION
THE microstructure of welds in steel are affected by the spatial distribution of thermal cycles[1] and continuous cooling[2]; furthermore, mixed microstructures form. To assess the mechanical properties of welds, it is critical to understand the morphology and crystallographic properties of the weld microstructures. Recent studies have shown the Kurdjumov–Sachs orientation relation (K–S OR) for classifying the microstructure and determining the properties of low-carbon steel and steel alloys, and analyzing the variant grouping. Lambert-Perlade et al.[3] showed that a crystallographic packet of bainite in low-carbon steel containing Cu, Mo, Nb, and V comprised variants belonging to a single Bain zone.[3–6] In the martensitic phase, the variant group formed in the same close-packed plane (CP) is known as the CP group.[7–9] Furthermore, the effects of carbon content[10] and transformation temperature[9] on variant grouping were systematically studied in 9Ni steel and lowcarbon steel, respectively, from the viewpoint of transformation strain.[9] The results enable the quantitative classification of steel microstructures and help understand the crystallographic properties of microstructural units. Few studies have focused on the variant grouping behavior of welds under continuous cooling. Furthermore, there are fewer studies on the time-series analysis of variant grouping. One of us has observed and analyzed variant grouping behavior in Cu-Ni-Cr-MoV steel under extremely slow cooling rates that formed only bainite.[11,12] HIDENORI TERASAKI, Associate Professor, YUTARO SHINTOME, Graduate Student, ATSUSHI TAKADA, Ph.D. Student, and YU-ICHI KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Osaka, Japan. Contact e-mail: [email protected]. ac.jp KOJI MORIGUCHI, Senior Researcher, and YUSAKU TOMIO, Researcher, are with the Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, Tokyo, Japan. Manuscript submitted December 19, 2013. Article published online March 26, 2014 3554—VOLUME 45A, JULY 2014
In this study, using the same steel, we analyzed the variant grouping behavior under continuous rapid cooling using time-series analysis. We also clarified the link between variant grouping behavior and the final nested microstructure.
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EXPERIMENTAL PROCEDURE
Low-carbon steel with composition Fe-0.1C-0.3Si0.25Cu-1.0Ni-0.4Cr-0.4Mo-0.039V (mass%) was used. It was austenitized at 1623 K (1350 °C) for 30 seconds and then cooled at 12 K/s to room temperature, which simulated the heat-affected zone (HAZ) of a weld with small heat input. Hereafter, the thermal cycle is referred to as ‘‘CGHAZ-25,’’ as it takes 25 seconds to go from 1073 K to 773 K (800 °C to 500 °C) in the cooling cycle. In contrast, in a previous study, variant grouping in the same steel under significantly slow cooling rate (1.67 K/s) was analyzed,[11] (hereafter referred to as ‘‘CGHAZ180’’). The evolutions of microstructures in CGHAZ180 and CGHAZ-25 are com
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