Correlation and extrapolation of creep-rupture data: A case study using 9Cr-1Mo steel data

The various parametric methods used to extrapolate data from shorter-time creep tests to longer times as required for design purposes are reviewed. The limitations on the applicability of the various methods are discussed, and data obtained from an experimental test program on 9Cr-1Mo steel, together with recent published reference data, are used to demonstrate the variation in the predicted life at longer times on the basis of a series of parametric methods. It is emphasized that the most appropriate parameter for the particular data set and material in question must be used. The choice of parameter should not be made blindly or substantial differences between predicted allowable stress and actual allowable stress can result. The data for the 9Cr-1Mo steel are discussed in terms of creep mechanisms and precipitation effects, and the potential problem of lower stress than predicted for long-term behaviour is examined.