Strain Rate Dependence of Yield Strength and Long-Term Creep Strength Evaluation of ASME Grade 91 Steel

Accuracy of long-term creep life prediction of creep strength enhanced ferritic steels is improved by region splitting analysis method in consideration of 50% of the 0.2% offset yield stress. According to JIS and ASTM standards for tensile test, however, specified strain rates for evaluation of yield strength are slightly different from each other. The conditions specified in JIS G0567 and ASTM E21-03a are 0.3 ± 0.2%/min and 0.5 ± 0.2%/min, respectively. Strain rate influences on yield strength and 0.2% offset yield stress of ASME T91 at 600 and 650°C under strain rate of 0.5%/min was about 10% higher than that under strain rate of 0.3%/min. Influence of difference in strain rate between JIS and ASTM regulations on the long-term creep rupture life prediction by region splitting analysis method was insignificant. With decrease in stress, magnitude of creep strain at the onset of accelerating creep stage decreased from about 2% in the short-term to less than 1% in the long-term. Life fraction of the time to 1% total strain tended to increase with decrease in stress. It was indicated that the initiation of tertiary creep should be a more important parameter for the stress intensity limit, St, than time to a total strain of 1%, since 80% of the minimum stress to cause initiation of tertiary creep was definitely smaller than 100% of the average stress required to obtain a total strain of 1% in the long-term.