## 1 Introduction  
Because of the current dramatic changes in electricity generation and supply, conventional thermal power plants are operated in a more fluctuating way, resulting in a combination of creep and cyclic loading of the hot section components. As shown by [PERSON] et al. [1], this leads to accelerated crack propagation, and hence, reduces lifetime compared to pure creep loadings. For combined loading conditions, preliminary work at materials for thermal power plant applications [2] has shown, that 22 wt.% Cr, fully ferritic stainless steel Crofer(r)22 H exhibits a higher lifetime compared to advanced ferritic-martensitic (AFM) steel P91. While strengthening of P91 is mainly achieved by M\({}_{23}\)C\({}_{6}\) (M: Mo, Cr)- and MX (M: V, Nb; X: N)-particles, as well as solid solution and sub-grain hardening [3], the fully ferritic Crofer(r)22 H, can be strengthened by a combination of solid solution hardening and intermetallic Laves phase (Fe,Cr,Si)\({}_{2}\)(Nb,W) precipitates [4], whereby Crofer(r)22 H has a limited solubility of C and N in the ferritic matrix. For Crofer(r)22 H, the decisive factor for hardening in (thermomechanical) fatigue loading is the thermomechanically induced precipitation of the Laves phase, which takes place during application in the temperature range from 600 to 700 \({}^{\circ}\)C [5]. Moreover, Crofer(r)22 H exhibits a stable grain structure under thermomechanical fatigue (TMF)-loading, which is in contrast to the AFM steels, showing unstable martensite lath structure and \"polygonization\" at TMF-loading conditions [5]. Additionally, in Reference [5], a higher damage tolerance of Crofer(r)22 H compared to P91 is indicated, leading to a higher TMF-lifetime for Crofer(r)22 H. Investigations at another fully ferritic, Laves phase strengthened steel [6] show a higher amount of hardening and additionally a sub-grain formation in front of the crack tip, resulting in crack branching or deflection at sub-grain boundaries and crack deflection at Laves phase particles [6]. Furthermore, it is assumed, that plastic deformation or high dislocation densities in the vicinity of a crack tip, as well as sub-grain boundaries, can boost the Laves phase precipitation, because both can act as nucleation sites [6]. As shown in Reference [5], the differences in fatigue behavior of both steel types are caused by different microstructural evolution. To quantify the microstructural evolution caused by cyclic loading, several experimental methods can be applied. Besides common microstructural characterization methods, e.g., scanning electron microscopy or transmission electron microscopy [7; 8], further techniques, such as measuring the development of hardness or electrical resistivity at different fatigue states [8; 9], can be used. In this context, [PERSON] et al. [10] showed for the bainitic steel 24 CrMoV55, as well as for the ferritic-pearlitic steel S355J0, that measuring hardness during fatigue tests serves as a suitable indicator to detect fatigue-induced microstructural changes. In general, indentation tests constitute an efficient method to determine various mechanical properties, such as hardness or elastic modulus [11; 12]. Furthermore, a material's cyclic properties can be characterized using cyclic indentation tests (CITs) [13; 14; 15]. As shown by Reference [15] for 100 Cr6 and [13] for two copper alloyed steels with carbon contents of 0.005 and 0.21 wt-%, respectively, using CITs enables the determination of the material's cyclic hardening potential, which correlates with the amount of cyclic hardening detected in uniaxial fatigue tests. Additionally, preliminary work [16] demonstrates at differently heat-treated 42 CrMo4, that the deformation behavior measured in CITs is in good accordance with the cyclic deformation behavior in uniaxial fatigue tests with compressive stresses (\(R=-\infty\)). Moreover, a strong correlation of cyclic hardening potential determined in CITs with the materials damage tolerance was found in References [17; 18].