Abstract
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.
Highlights
The growing share of renewable energy sources in the electricity markets has forced many power plants into a more “flexible” operation with frequent load shifts and shutdowns
Due to the temperature dependency of the mechanical properties the maximum stresses are highest at 300 ◦ C, Figure 5b
While a strong dependence was obvious at the lower applied strains, only a little further variation was obtained at the total strain amplitudes of 0.4% or higher, after reaching a softening ratio of about 20% and 30%, respectively
Summary
The growing share of renewable energy sources in the electricity markets has forced many power plants into a more “flexible” operation with frequent load shifts and shutdowns. Comprehensive analyses of the mechanical behavior of 9–12% Cr steels have been carried out during the market introduction of these steels [2,3], but they focused on creep and creep-rupture testing, in line with the constant operation modes of the power plants at these times. For P91, the seminal work of Kim and Weertman [5] identified a number of key effects of fatigue loading on subsequent mechanical behavior and microstructure evolution. As a most obvious feature of fatigue on P91, substantial initial softening of the specimens was observed in strain-controlled testing. It continued throughout the fatigue experiments, though with decreasing rates, leading to reductions of the stress range exceeding 250 MPa [5]. Transmission electron microscopy (TEM) revealed that aging had no obvious effect on the ferritic–martensitic microstructure, while a rapid loss of the original lath type
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