Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints
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HIGH-CR ferritic heat-resisting steels are used for high-temperature components designed to cope with the higher steam temperatures that are increasingly being adopted in ultra-supercritical (USC) thermal power plants. They are also the most promising materials for high-temperature structural components needed in nextgeneration high-temperature nuclear power reactors such as fast breeder reactors and high-temperature gas-cooled reactors. Ferritic heat-resisting steels normally show local fine-grained microstructures in the heat-affected zone (HAZ) of weldment as a result of the welding thermal cycle. Creep fracture along the finegrained HAZ of welds, which is known as Type IV failure,[1–3] is now known to be a serious problem in high-Cr ferritic heat-resisting steels that are used for long periods at high temperatures. A great deal of research on Type IV failure has been conducted using simulated HAZ specimens made by heat treatment or standard creep specimens cut out from welded joints.[4–10] Observations of creep voids and cracks have also been performed in the past, mainly through replica methods using specimen surfaces.[10] Few reports, however, show quantitatively the processes HIROMICHI HONGO and TAKASHI WATANABE, Chief Engineers, are with the Materials Information Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047 Japan. MASAAKI TABUCHI, Group Leader, is with the Creep Resistant Materials Group, Materials Reliability Unit, National Institute for Materials Science. Contact e-mail: [email protected] Manuscript submitted December 9, 2010. Article published online October 28, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
of Type IV failure, which occurs inside weld components, from damage nucleation and growth to final failure, using large-scale welded joint specimens that include all the weld layers. The current paper aims to elucidate the processes and mechanisms of Type IV failure of a Modified 9Cr-1Mo steel (9Cr-1Mo-V-Nb steel, ASME Grade 91 steel) weld, which is used for boiler components in USC thermal power plants at approximately 873 K (600 °C), by conducting creep rupture tests and also by interrupting creep tests. The creep tests using the large-scale welded joint specimens were interrupted at several time steps and the formation and growth of creep damage in the HAZ investigated. The data acquired on Type IV creep damage were compared with the computed creep damage obtained using damage mechanics analysis.
II.
MATERIALS AND EXPERIMENTAL PROCEDURE
A. Material The material used was a 25-mm-thick ASME Gr.91 steel plate. Tables I and II, respectively, show its chemical composition and tensile properties. Using this plate as base metal, a welded joint with a double U groove was made employing gas tungsten arc (GTA) welding. After welding, the welded joint was subjected to post-weld heat treatment (PWHT) for 60 minutes at 1018 K (745 °C). Figure 1 shows the groove configuration and cross-sectional macrostructure in the welded joint. In the next section, the thickness (
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