Practical experience of creep failure of components in service are discussed in relation to the creep rupture properties of HK40 (25Cr-20Ni-0.4C) and HP40 (25Cr-35Ni), which are considered using traditional and new data analysis procedures. The new approach is shown to offer not only a sensible mechanistic interpretation of the observed behaviour patterns but also a reasonable method for predicting long-term fracture strengths by extrapolation of short term creep life values.

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Creep damage leading to failure in low alloy ferritic steels at service operating conditions results from: (i) structural coarsening leading to a continuous reduction in creep strength during exposure, and (ii) intergranular creep cavitation. A mechanistic model is presented for describing tertiary creep which accounts for both of these factors. By assuming a limiting creep ductility corresponding to a maximum cavitation potential a lower bound life predictive approach is given. The significant structural parameters are then the background solid solution strength, the interparticle spacing (λ) and the rate of increase of λ, although the long-term, creep-rupture strength is shown to be dictated largely by the latter. Consideration is given to the determination of these kinetics by means of hardness change measurements: good correlations are found with experimental data. A methodology for characterizing the remanent creep life of component material using hardness measurements and post-exposure aging is developed on this basis.

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