Uniaxial creep and stress relaxation behavior of modified 9Cr-1Mo steel

An attempt has been made to establish a correlation between the creep strains rates predicted from a stress relaxation test with that directly obtained from a creep test results on a commercial grade of steel (P91) at three different temperatures. Tests were performed on a set of specimens made from the same plate to exclude the effect of material variability on the test data. These were used to explore if short term stress relaxation test (SRT) performed on the same grade of steel could predict its creep rupture strength. The results suggest that the magnitude of activation energy and stress exponent obtained from SRT is significantly lower than those obtained from the creep test data. This is primarily due to the microstructures that evolved during the two variants of tests which had difference in thermal exposures, and leads to the conclusion that material parameter set obtained from SRT cannot be used to predict creep rate of the steel at any given stress and temperature. It needs an additional conversion factor for the prediction of minimum creep rate.