Bainite Formation Kinetics During Isothermal Holding in Modified High Cr Ferritic Steel
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INTRODUCTION
NINE to 12 pct Cr Ferritic steels are being considered as the preferred structural materials for elevated temperature applications for fossil-fired[1,2] and future nuclear fission and fusion power plants[3,4] due to their good thermal properties as the outstanding high-temperature endurance, excellent heat conductivity, and low thermal expansion coefficient.[5,6] So far, it has been widely used as a research benchmark for developing new ferritic heat-resistant steel used for power plants in Japan, Europe, and the United States.[7–9] The properties of the steel depend on the microstructure control; in other words, the control of phase transformation is especially important to the final microstructure. It has been recognized that martensite is the typical microstructure of 9–12 pct Cr ferritic steels after normalizing treatment because cooling in air is sufficient to initiate the martensitic transformation because of the high chromium content.[10,11] The nonisothermal martensitic transformation during continuous cooling in high Cr ferritic steels has received considerable attention over the years.[12–14] Yet, the effect of isothermal holding after normalizing on transformation behavior and microstructure in high Cr ferritic steel has rarely been reported. Generally speaking, thermal stabilization of austenite above or below Ms (martensitic transformation starting temperature) during cooling and during isothermal holding may occur, resulting in a decease of Ms or suppression of the course of martensitic transformation.[15] This phenomenon is attributed to the fact that CHENXI LIU, YONGCHANG LIU, DANTIAN ZHANG, ZHIMING GAO, and ZESHENG YAN, Professors, are with the School of Material Science and Engineering, Tianjin Key Laboratory of Advanced Jointing Technology, Tianjin University, Tianjin 300072, P. R. China. Contact e-mail: [email protected] Manuscript submitted December 5, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
the formation of Cottrell atmosphere around dislocations during isothermal holding raises the yield strength of the parent phase (austenite) and, thus, blocks the martensitic transformation.[16,17] Nevertheless, Guliaev[18] found that thermal stabilization would not be found during isothermal holding when isothermal bainitic transformation occurs in various types of carbon steels. Beyond this research, few studies dealt with the effect of isothermal bainitic transformation on thermal stabilization of austenite before martensitic transformation in steels. Furthermore, it can be confirmed that no previous work on isothermal bainitic transformation of 9-12 pct Cr ferritic steel exists because a microstructure of 100 pct martensite is easy to obtain. In this project, the high-resolution dilatometric methods have been adopted to investigate the course of isothermal holding for various temperatures during cooling after austenitizing in a modified high Cr ferritic steel developed by our group. The quantitative investigation of transformation kinetics and the microstructure of samples subjected to differe
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