Behavior of Grade 91 material specimens with and without defect at elevated temperature

• Creep crack growth rate model of Grade 91 steel was derived.
• Fatigue crack growth rate model of Grade 91 steel was derived.
• The crack growth models in RCC-MRx were compared with test data and conservatism was reviewed.
• The test data from mechanical strength and creep tests for Grade 91 steel were compared with those of RCC-MRx.

In a defect assessment and leak before break analysis under creep-fatigue loading, crack growth models are necessary. The French elevated temperature design code of RCC-MRx provides a fatigue crack growth (FCG) rate model and creep crack growth (CCG) rate model for Mod.9Cr-1Mo steel in Section III Tome 6. In this study, the crack growth models were derived from a number of crack growth tests for Grade 91 steel specimens under fatigue loading and creep loading at elevated temperature. The test data from the experiments of FCG and CCG were obtained, and the test data were compared with those of the RCC-MRx to investigate conservatism of the crack growth models in RCC-MRx. It was shown that the FCG rate model of RCC-MRx was conservative while the CCG model was non-conservative for Grade 91 steel when compared with present test data. In addition mechanical strength tests and creep tests were conducted and the test results were compared with those of the RCC-MRx.