Prediction of Creep-fatigue Lives of Cr Mo Steels with Diercks Equation

The use of the Diercks equation, which was developed for assessing the creep-fatigue lives of AISI 304 (JIS SUS 304), for predicting the creep-fatigue lifetimes of Cr-Mo steels was examined in comparison with the linear cumulative damage rule and the strain range partitioning method. Following conclusions were reached: (1) The differences in the fatigue and the creep-rupture strengths existing between the SUS 304 and the Cr-Mo steels can be accounted for by modifying two factors in the original Diercks equation: for the fatigue lifetime ratio a, the relative lifetime ratio, αr, which is a ratio of the “pure” fatigue life of SUS 304 to that of the Cr-Mo steel concerned, should be used, while for the temperature Tc, the equivalent temperature, Te, which is a temperature that will give rise to the same creep-rupture lifetime for the Cr-Mo steel as for 304 for the stress concerned, should be used. (2) The Diercks equation, modified as in (1) above, predicts the creep-fatigue lives of Cr-Mo steels to a factor of 2, i.e., no more than twice if overestimated and no less than one half if underestimated. This is the same accuracy that the strain range partitioning method features. (3) The linear cumulative damage rule should not be applied to Cr-Mo steels not only because it can give mutually contradicting evaluations for different strain waveforms, but because the accuracy of prediction is unacceptably large. (4) Since the method proposed herein for formulating the modified Diercks equation is free of the complexities in experimentation and in data analysis that characterize the strain range partitioning method, it should be taken as a better means, to the first approximation at least, of assessing the creep-rupture properties of Cr-Mo steels.