Evaluation of reduction in creep strength based on fracture energy in CSEF steels

高クロム鋼の破壊エネルギー変化によるクリープ強度低下評価

永江 勇二 ; 浅山 泰 

Nagae, Yuji; Asayama, Tai

In power plants operated at elevated temperatures, the operating life of structural materials increases. Therefore, it is very important to be able to predict creep strength in long term above 100,000 h. Furthermore, it has been reported that the in the long term, the actual creep strength is lower than the predicted life. Although this problem has been analyzed, the reasons remain unclear. In this study, a fracture energy model is used to evaluate the mechanisms of the creep strength reduction for martensitic steels. In the model, changes in fracture energy with rupture time are expressed by a power law. The energy density rate is calculated using stress, rupture elongation, and rupture time. The model indicates two mechanisms of creep strength reduction. One is the increase in rupture elongation, which leads to reduction in creep strength with ductility; the other is the decrease in reduction of area, which leads to reduction in creep strength with brittleness. Difference between the two mechanisms affects creep-fatigue strength. The study also shows that the equation based on the fracture energy model for creep-fatigue life can be obtained by a parallel translation of that for creep.