Effect of Intercritical Quenching on the Microstructure and Cryogenic Mechanical Properties of a 7 Pct Ni Steel
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LIQUEFIED natural gas (LNG) has been attracting more attention as a clean and efficient energy source in recent years. Many countries, including China, have constructed large numbers of LNG facilities and plan to construct more such projects, including terminals and trunklines, tanker ships, tank trucks, and pipelines.[1,2] The large number of planned construction projects will lead to a huge demand for 9 pct Ni steel, which has excellent cryogenic mechanical properties and has been extensively used on existing LNG projects. However, because of the demand and the instability of the price of
HONGWEI CAO is with the Department of Materials Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, P.R. China and also with the CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China. XINGHONG LUO and SHI LIU are with the CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] GUOFENG ZHAN is with the Research and Development Center of Wuhan Iron and Steel (Group) Corp, 28 Yejin Road, Qingshan, Wuhan 430080, P.R. China. Manuscript submitted September 28, 2016. Article published online July 6, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
nickel, the cost of 9 pct Ni steel, and therefore the cost of these LNG projects, becomes higher and higher. It would be highly desirable, therefore, to reduce the content of nickel in the steel if it would not harm the cryogenic properties. In this case, 7 pct Ni steel is suggested as a possible lower cost substitute for 9 pct Ni steel for LNG projects.[3] It has been widely accepted that reversed austenite plays a key role in low-carbon, high-nickel steels for cryogenic service. The amount and distribution of this reversed austenite significantly affect the cryogenic mechanical properties of these steels. As one of the most prominent microstructural constituents observed after heat treatment, reversed austenite is usually precipitated along the martensite and prior austenite grain boundaries.[4] Many reports[5–7] have shown that, compared with a conventional quench and temper (QT) process, more reversed austenite can be produced in Ni-containing cryogenic steels by a QLT process. The QLT process consists of austenitizing and quenching, intercritical heating and quenching, and finally tempering. Significant improvements in the cryogenic mechanical properties of these steels result. The crucial difference between the QLT process and the QT process is the addition of an intermediate step of heating at an intercritical temperature and quenching, which plays a key role in VOLUME 48A, SEPTEMBER 2017—4403
optimizing the microstructure and thus the performance of the steels. Thus, in order to ensure that the cryogenic mechanical properties of 7 pct Ni steel can reach those of 9 pct Ni steel, in this study the influence of the intercritical heating
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