Investigation of precipitation behavior in a weld deposit of 11Cr-2W ferritic steel
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I. INTRODUCTION
HIGH Cr ferritic heat-resistant steels are potential candidate materials for ultra-supercritical (USC) fossil-fired power plant.[1–8] As typical steels, ASME P92 and P122 are strengthened by solid solution hardening and precipitation hardening by adding a large amount of alloying elements such as Cr, W, and Mo. However, the high working temperature of USC plant accelerates the growth and coarsening of precipitates in the steels, and the growth of precipitates can decrease the effect of solid solution hardening and precipitate coarsening can decrease the effect of precipitation hardening. In addition, coarse precipitates may act as sites of initiation of creep voids.[9] Welding processes are necessary for the construction of pipes of reheater, superheater and so on in thermal power plant. However, welding may lead to the deterioration of creep properties.[10–13] Creep tests of welded joints showed that at high temperature and low stress creep fracture often occurs at the fine-grained heat-affected zone (FGHAZ) of welded joints, which is referred to as Type IV cracking.[14] One of the reasons for Type IV cracking is considered to be the deterioration of creep properties due to coarsened precipitates in the FGHAZ. Since the weld thermal cycle induces different material properties in the welded joint, it can be considered that there is a relationship between welding therDEJUN LI, Postdoctoral Fellow, is with the Institute for Materials Science, Welding and Forming, Technical University Graz, A-8010 Graz, Austria. Contact e-mail: [email protected] KENJI SHINOZAKI, Professor, is with the Department of Mechanical Systems Engineering, Graduate School of Engineering, Hiroshima University, Hiroshima 739-8527, Japan. HIDEMASA HARADA is with the Engineering Department, BabcookHitachi K.K., Japan. KENJI OHISHI is with the Engineering Department, Suzuki Motor Corporation, Japan. Manuscript submitted October 14, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
mal cycle and precipitate growing and coarsening. However, few studies involved the effect of welding thermal cycle on the precipitation behaviour during long time aging (or creep) for the steels, especially for FGHAZ region.[15,16,17] This study is aimed at investigating the precipitation behaviour at high working temperature for the welded joints of high Cr ferritic heat resistant steels and identifying the difference in precipitation behavior in FGHAZ, coarse-grained heat-affected zone (CGHAZ), and base metal regions. The precipitation behavior in the welded joint has been investigated in our previous study.[18,19] However, the widths of CGHAZ, FGHAZ are so narrow in an actual welded joint that it is difficult to obtain accurate results for precipitation behavior based only on the actual welded joint specimen. Therefore, simulated HAZ (SHAZ) specimens were used in the study, whose thermal cycles were controlled to be the same as those of one-pass TIG welded specimen.[18,19] In addition, the growth of Laves phase was simulated in the study. II. EXPE
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