CREEP-FATIGUE EVALUATION OF NORMALIZED AND TEMPERED MODIFIED 9CR-1MO

Creep-fatigue is a fatal failure mode of the high temperature structur al materials of liquid metal fast breeder reactors (LMFBRs). In this r eport, two important issues are discussed for creep-fatigue evaluation of normalized and tempered modified 9Cr-1Mo (modified 9Cr-1Mo(NT)) st eel which is a promising structural material for the steam generator o f large-scale LMFBRs in Japan. Several evaluation methods based on the ductility exhaustion concept are discussed for the prediction of tens ion strain hold creep-fatigue damage of this material. A time-fraction type of linear damage summation concept based on a new ductility exha ustion theory is proposed from the point of view of its appropriate co nservatism for time extrapolation and its simplicity. Also, a life red uction mechanism of low cycle fatigue with strain hold at the compress ion side is discussed, based on the data observed by a scanning type e lectron microscope. Creep damage or the tension mean stress caused by compression strain hold hardly reduce the low cycle fatigue life of th is material. A new concept based on the location of oxidation on the t est specimen surface can explain the reduction in low cycle fatigue li fe of modified 9Cr-1Mo(NT) steel.