30万時間超使用後の CrMoV 鋳鋼製タービン蒸気弁の脆化評価

抄録

Damage and degradation gradually proceed in high temperature components during long-term operation. Prediction of softening and embrittlement which are related to tensile strength and critical crack length respectively, during operation is important for maintaining reliable operation of the components. So far although studies on temper embrittlement were conducted by using artificially produced materials by heat treatments, evaluation of embrittlement for actual long-term used materials were very limited. No data was obtained for long-term used components operating over 300,000 hours. In this study, micro structure observation and, hardness and Charpy impact tests were conducted using the long-term operated steam valve at high temperature for over 300,000 hours. Additional heat treatment at 585C for 4457 hours equivalent to 100,000 hours at 538C was done for the used material to obtain the aged material. Coarsening of carbide precipitation and decrease of dislocation structures were observed by a transmission electron microscope both in the used and aged materials. The ductile-brittle transition temperature of the aged material is lower than that of the used material indicating ductility of the used material recovers by the heat treatment. Rate of grainboundary fracture area to brittle fracture area of the used material is much larger than that of the aged material. Based on the Charpy impact test results in this and previous studies, change in embrittlement characteristic with operating time is discussed by introducing the matrix and the grainboundary strengths, and the plastic deformation resistance which determine the matrix and grainboundary fracture, and ductile fracture respectively.