A new methodology for analysis of creep and creep fracture data for 9–12% chromium steels

The approaches commonly used to quantify creep and creep fracture behaviour are critically reviewed. Their limitations are illustrated by reference to information openly available for three martensitic steels, namely, grades 91, 92 and 122. Adopting traditional procedures, the creep design strengths of these 9–12% chromium steels have been reduced substantially as continuing experimental programmes have increased the maximum test durations from ∼30 000 towards 100 000 h. Moreover, even when applied to comprehensive long-term data sets, the estimated 100 000 h strengths vary considerably, depending on the detailed method selected to perform the calculations. In contrast, by normalising the applied stress through the appropriate ultimate tensile strength (UTS) value, new relationships allow the multi-batch stress rupture properties at various creep temperatures to be superimposed onto sigmoidal 'master curves' using the activation energy for lattice diffusion in the alloy steel matrixes (300 kJ mol−1). In this way, the latest 100 000 h strengths determined from the large scale test programmes are predicted accurately by extrapolation of creep life measurements lasting less than 30 000 h, with reasonable estimates obtained even for failure times up to only 5000 h. Validation of the new methodology by analysis of results already produced for different steels and other creep-resistant alloys would therefore limit the scale and costs of current procedures for acquisition of long-term engineering design data.