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Due to their excellent creep resistance and good oxidation resistance, 9–12% Cr ferritic–martensitic stainless steels are widely used as high temperature construction materials in power plants. However, the mutual combination of different loadings (e.g., creep and fatigue), due to a “flexible” operation of power plants, may seriously reduce the lifetimes of the respective components. In the present study, low cycle fatigue (LCF) and relaxation fatigue (RF) tests performed on grade P92 helped to understand the behavior of ferritic–martensitic steels under a combined loading. The softening and lifetime behavior strongly depend on the temperature and total strain range. Especially at small strain amplitudes, the lifetime is seriously reduced when adding a hold time which indicates the importance of considering technically relevant small strains.
9-12% Cr ferritic-martensitic stainless steels are widely used as high temperature construction materials in power plants due to their excellent creep resistance and good oxidation resistance. Creep resistance is considered as the primary parameter in material selection for base-load power plants, but the growing share of renewable energy sources in power generation forces many installations into more "flexible" operation with frequent load reductions and shutdowns. Under cyclic operation of power plants, temperature gradients occur especially in thick-walled components. Locally, these gradients lead to complex time- and temperature-dependent loading scenarios which may result in superimposed creep deformation/damage, creep-fatigue and thermo-mechanical fatigue. The combination of different damage processes may seriously reduce the lifetimes of respective components. A fundamental understanding of the damage evolution in ferritic-martensitic steels under combined static and cyclic loading is therefore required.
In the present contribution, the thermo-mechanical fatigue and creep-fatigue behavior of two different 9% Cr steel grades (P91 and P92) was investigated. Standard LCF/TMF-tests as well as near-service loading tests with different hold times have been carried out to study the material behavior of the new loading scenarios. Microstructural investigations help to identify the observed deformation characteristics (like e.g. pronounced softening) and the dominating damage mechanisms under different loads. The test programme is complemented by selected tests on service-aged material which demonstrate possible reactions of older components in existing plants to changes in operation profiles. In a next step, the experimental results will be used for parameter identification of a deformation and lifetime model to predict the material behavior.