STUDY ON CREEP--RUPTURE PROPERTY ASSESSMENT METHOD FOR 9%---12%Cr FERRITIC HEAT--RESISTANT STEELS

Abstract

Long-term creep-rupture properties are usually evaluated from short-term data by time-temperature parameter(TTP) method,such as Larson-miller parameter(LMP) and Orr-Sherby-Dorn (OSD) methods.However,the conventional TTP methods sometimes overestimate long-term creep rupture properties if the prediction is based on their short-term test data for 9%-12%Cr ferritic steels.The following concepts/methods are thus proposed in this paper in order to reduce the property overestimation tendency caused by the conventional TTP methods and to obtain a better agreement of the predicted property values with the observed ones.They include the C-value optimization and the multi-C region analysis,the long-term(5×10~3-1×10~5 h) creep rupture property prediction using short term test data(≤5×10~3 h),the optimization of function used for property prediction,and the effect of d[g(σ)]/d(P) vs P on the stability of steel properties based on the improved LMP method. All the data sets for the 9%-12%Cr steels are from NIMS database for the related calculations and analyses.The results show that the C value in LMP is not only different from steel to steel type but also varies with the multi-region stress levels,and the new approach to rupture life prediction proposes procedures for extrapolations of the short-term results,with rupture time measurements from tests lasting up to only 5×10~3 h providing reasonable estimates of 10~5 h rupture strengths,as well as the variation tendency of d[f(σ)]/d(P) vs P can reflect directly the long-term property stability of the steels investigated.Therefore,the concepts/methods proposed could improve effectively the accordance of predicted property values with observed ones and overcome obviously the overestimation tendency of 10~5 h strengths,which are more suitable and easily realized to assess the long-term creep-rupture properties of the advanced high Cr ferritic steels.