Intergranular M 23 C 6 Carbide Precipitation Behavior and Its Effect on Mechanical Properties of Inconel 690 Tubes
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TRODUCTION
INCONEL 690 is a high-chromium nickel alloy having good formability, high strength, and excellent corrosion resistance to many corrosive aqueous media and high-temperature atmospheres.[1–3] Owing to these attractive properties, it is used in combustion systems, turbines, and nuclear reactors.[4] These special properties are dependent on the morphology and distribution of precipitates in the matrix.[5] Carbides of M23C6 generally precipitate at grain boundaries in Inconel 690 TAE-HYUK LEE, Ph.D. Student, SIN-HYEONG JOO, Master Course, and HAYK H. NERSISYAN, Research Professor, are with the Graduate School of Department of Advanced Materials Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea, and also with the Rapidly Solidified Materials Research Center, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea. YOUNG-JUN LEE, Ph.D. Student, is with the Rapidly Solidified Materials Research Center, Chungnam National University, and also with the Graduate School of Energy Science & Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea. KYOUNG-TAE PARK, Senior Researcher, is with the Rare Metal R&D Group, Korea Institute of Industrial Technology, 7-47 Songdo-Dong, Yeonsoo-Gu, Incheon 406-130, Republic of Korea. JONG-HYEON LEE, Professor, is with the Graduate School of Department of Advanced Materials Engineering, Chungnam National University, and Rapidly Solidified Materials Research Center, Chungnam National University, and also with the Graduate School of Energy Science & Technology, Chungnam National University. Contact e-mail: [email protected] Manuscript submitted March 23, 2015. Article published online June 13, 2015 4020—VOLUME 46A, SEPTEMBER 2015
during thermal aging.[6] The precipitation of M23C6 carbides in Inconel 690 has been extensively studied for many years.[7–9] The precipitation of second phases, such as M23C6 carbides, is known to occur selectively at a grain boundary. It was found that the morphology and the precipitation kinetics of M23C6 carbides are related closely to grain boundary misorientation.[10] Hong et al. found that the carbide morphology and size are strongly related with grain boundary misorientation in austenitic stainless steel.[11] Liu et al. have shown that in a Ni18Cr-18Fe alloy, precipitates were smaller and more closely spaced on R1, 9 and 29 coincidence boundaries in contrast to random boundaries.[12] Trillo et al. found that carbides tend to precipitate first at random grain high-angle boundaries, and then at non-coherent twin boundaries and failed to observe carbide precipitation at coherent twin boundaries in 304 stainless steels.[13] Up to now, it is still unclear why the morphology of M23C6 is different at different types of grain boundaries in various alloys. The precipitation behavior of M23C6 carbide remains to be further investigated. Several studies on Inconel 690 have been carried out, with most of them focusing on aspects relate
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