The transformation in energy systems leads to more pronounced fluctuations in the operating levels of power plants, which results in higher thermomechanical loads and thus, in an accelerated damage. Consequently, power plant materials with optimized thermomechanical and cyclic strengths are required. In this context, fully ferritic high chromium steels show a higher resistance to fatigue crack initiation and growth in relation to conventional advanced ferritic martensitic (AFM) steels. This is predominantly caused by thermomechanically induced precipitation of the finely dispersed intermetallic Laves phase [1]. As the precipitation of the Laves phase depends on the operation temperature and time, as well as on the amount of plastic deformation, a sound knowledge of the relation between the loading conditions, the precipitation state and the resulting fatigue behaviour is required.

Consequently, in this work the influence of temperature, strain rate and stress level on the fatigue behavior of the fully ferritic high chromium steel HiperFer-17Cr2 and, as a reference, the AFM steel P91 is investigated. Therefore, fatigue tests were carried out at different frequencies (0.005 to 5 Hz) and temperatures (600 °C to 650 °C). The results demonstrate a significant reduction in the fatigue strength of the P91 from 600 °C to 650 °C, while the temperature-induced reduction in the fatigue strength of HiperFer-17Cr2 is less pronounced. This can be explained by differences in the cyclic deformation behavior. While the P91 shows cyclic softening, the HiperFer-17Cr2 reveals a strong cyclic hardening, especially in the initial phase of the fatigue tests, which is assumed to be caused by precipitation hardening due to Laves phase formation. As shown in previous work [2], instrumented cyclic indentation tests (CIT) can be used to quantify the cyclic hardening potential, and with this, to evaluate the precipitation state of the Laves phase. Thus, CIT were performed at differently loaded specimens to determine the influence of the loading conditions applied on the precipitation hardening.

[1] Kuhn et al.: Impact of Thermomechanical Fatigue on Microstructure Evolution of a Ferritic Martensitic 9 Cr and a Ferritic, Stainless 22 Cr Steel. 2020. Appl. Sci. 10, 6338

[2] Blinn et al.: Analysis of the Thermomechanical Fatigue Behavior of Fully Ferritic High Chromium Steel Crofer22H with Cyclic Indentation Testing. 2020. Appl. Sci. 10, 6461