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概要

Modified 9Cr-Mo steel is a candidate structural steel for demonstration fast breeder reactors (FBR's). Due to low thermal activatio n, the steel can maintain steady-state deformation during long-term creep. The creep strength of its weldment is lower than that of the base metal. Because of different creep strain rate in the solidified weld metal and heat-affected zone (HAZ), creep deformation in HAZ can be lowered but maximum principal stress augmented in fine-grained HAZ to cause detrimental Type IV cracks in this zone. In this study, FEM analyses were made on long-term creep of over 60 years in double U groove welded modified 9Cr-Mo steel plates with typical microstructures in the welded region of the steel plates taken into consideration. The typical welded region of the plates consists of solidified weld, coarse grain HAZ, fine grain HAZ, interfacial HAZ and base metal. These microstructures were modeled as polycrystal structures having slip systems approximated by orthotropic deformation properties. The results obtained were compared with existing experimental results and those obtained by the authors and found to agree with the experimental results. The results were also compared with computational results obtained by a model having hexagonal crystal grains having the same size and a model having two isotropic materials. The results revealed that the equisized-grain model gives conservative creep life prediction whereas the isotropic model unconservative predictions.

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