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.2 of 25 microstructure, by a formation of equiaxed dislocation cells and carbide coarsening, was observed, after only few hours of fatigue loading [5].In the following decades, the fatigue behavior and microstructure evolution have been investigated in more detail in P91 [6,7], as well as in other 9-12% Cr-steels [8][9][10]. The topic of combined loading (creep-fatigue) has been extensively studied for P91 [11]. Generally, these studies confirmed the above-mentioned trends and indicated qualitatively similar behaviors for all ferritic-martensitic steel grades. However, the number of studies dealing with fatigue and creep-fatigue of these materials is still quite limited, compared to the great efforts spent on characterizing the creep behavior [12], especially when considering the large parameter fields which would need to be covered for a thorough analysis of the material behavior, under the different combined loadings. Consequently, the amount of available fatigue data is quite restricted, as is reflected, for example, in the number of creep data and fatigue data sheets provided by NIMS in Japan [13].Recently, combined loading involving creep and cyclic fatigue loads has received increased attention due to the more flexible operation of power plants in many countries. Consequently, reliable data on the fatigue and creep-fatigue behavior are required for current construction materials. Since cyclic operation may especially affect thick-walled components (due to the build-up of thermal stresses [1]), the common material grades for live steam piping, headers, etc., need to be comprehensively tested. One current candidate material is P92, an optimized version of the 9% Cr grades with an increased tungsten content. Due to its enhanced creep resistance, compared to earlier grades, P92 can be used up to maximum operation temperatures of 620 • C [14]. A number of studies on the fatigue behavior of P92 have been published recently [15][16][17][18]. It was found that P92 exhibits a considerable cyclic softening, as it is known from P91. No quantitative comparison of the effect was given but it was confirmed that softening depends on a number of parameters, like the amount of plastic s...
