In-Situ Observation of Growth Behavior of Niobium Carbide during Dual-Phase Rapid Solidification of SUS347H
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ODUCTION
THE solidification cracking susceptibility of austenitic stainless steel weld metals increases by a primary solidification of c-austenite, well-known as A mode.[1] In the case of austenitic stainless steel, the microstructure of the dual phase of d-ferrite and c-austenite, which is formed by the austenite-ferrite (AF) solidification mode or ferrite-austenite (FA) solidification mode, improves the solidification cracking susceptibility.[2,3] Some groups have reported that the solidification cracking susceptibility of fully austenitic weld metal depends on the solidification temperature range and terminal solidification events that take place at the grain boundaries and interdendritic region.[4–6] Further, niobium carbides as a precipitate typically form in a eutectic-type morphology with austenite matrix in A mode solidification.[7,8] The distortion of the eutectic interface might cause cracking. Especially, it is an important factor in the welding phenomenon with directional solidification. Our interest is in the details of the growth process of niobium carbide in the directional solidification of stainless steel under rapid cooling. Our group has observed a solidification and a phase transformation process by an in-situ observation technique of the directional solidification, which results in dendrite growth during rapid cooling.[9–12] In this work, a displacement of diffraction patterns of a dual-phase solidification mode stainless steel with crystallization of niobium carbides during rapid cooling is investigated by MITSUHARU YONEMURA, HIROYUKI HIRATA, and KAZUHIRO OGAWA, Senior Researchers, and TAKAHIRO OSUKI, Researcher, are with the Corporate Research and Development Laboratories, Sumitomo Metal Industries, Ltd., Amagasaki, 660-0891 Japan. Contact e-mail: [email protected] Manuscript submitted February 25, 2007. Article published online December 4, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
in-situ two-dimensional time-resolved X-ray diffraction. Then, we discuss the growth behavior of the dendrite with crystallization of niobium carbides.
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EXPERIMENT
The specimens were prepared for alloy SUS347H with the composition of Fe-18 pct Cr-8 pct Ni-1 pct Nb-0.2 pct C, which has low solidification crack susceptibility. The phase transformation sequence was calculated from equilibrium thermodynamic relationships using Thermocalc software version Q with the iron-based alloy database in order to compare it with the experimental results. The time-resolved X-ray diffraction (TRXRD) was performed on the BL46XU undulator beam line at the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Hyogo, Japan). The experimental setup of the TRXRD technique for welding is summarized schematically in Figure 1. The timeresolved diffraction patterns in the solidification process were measured, while a torch moves vertically to an X-ray incident beam at the speed of 1.0 mm/s using an automatic stage. Solidification microstructures have a preferred orientation of h100i along the growth directi
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