## 3 Results and Discussion
### Novel Materials for Improved Structural Safety
During the initial 30 cycles, Crofer22 H hardened by 205 MPa, which is equivalent to an increase in strength of 38% (Table 6). After reaching the maximum stress range, Crofer22 H was characterized by a comparatively steep drop in stress range before it entered the stable range (Figure 18) at around 380 cycles. The HiperFer 17 Cr2 steel in contrast continuously hardened within the initial 70 cycles (\(\approx\)5% of technical lifetime), avoiding the steep drop in stress range and directly entering the stable range on a higher stress range level (Figure 18). The hardening range increased to 236 MPa, corresponding to an increase in strength of 42% (Table 6). These differences in behavior were caused by differing precipitation kinetics and phase fractions of Laves phase precipitates. Dislocations induced by cyclic plasticization cause (temporary) hardening. Crofer22 H and HiperFer dislocations were immediately decorated and pinned by nucleating Laves phase particles, i.e., the temporary strengthening effect became quasi-permanent. The chemical composition of HiperFer 17 Cr2 is optimized for high volume fraction of the Laves phase. For this reason, the precipitation process lasts longer (5%/1% of technical lifetime HiperFer/Crofer(r)22 H) and creates more dislocation pinning particles in HiperFer, which causes increased strengthening and thus stabilization of the stress range on a higher level. In Crofer22 H, less dislocations were pinned by precipitates, i.e., a smaller extent of temporary dislocation strengthening became quasi-permanent, which caused a decrease in stress range after the maximum was reached. Figure 18: Comparison of thermomechanical fatigue curves of 316L [18], Crofer22 H, and HiperFer 17 Cr2 (100% out-of-phase cycles (i.e. \(\varepsilon_{\text{mech}}=-\varepsilon_{\text{th}}\). ), \(\Delta\)T = 50–650 \({}^{\circ}\)C, dT/dt = 10 Ks\({}^{-1}\), \(\text{h}=0\) at T\({}_{\text{min}}\) and T\({}_{\text{max}}\)) [18]. Because the stable stress range was lower, damage occurred later in Crofer22 H. Despite the high level of strain, obstruction in the experiments the S-N curves of Crofer22 H and HiperFer decreased moderately, especially in comparison to 316L. The austenitic steel strengthened in the hardening range by only 47 MPa, which corresponded to an increase in strength of only 5.4% (Table 6). 316L failed abruptly, with only a \(\approx\)13.8% damage range. With 29.9% of the lifetime in the damage range, HiperFer was found to be the superior material. Despite the fact that the effective strain range in the austenitic 316L steel (\(\Delta\epsilon_{\text{th. }}\), i.e. \(\Delta\epsilon_{\text{mech.-HiperFer}}=0.71\%\), while \(\Delta\epsilon_{\text{mech.-316L}}=1.14\%\)), i.e., dislocation hardening, was much higher, HiperFer reached a comparably high stable stress range level. The TMF experiments demonstrated the high fatigue resistance potential of HiperFer steel. However, the total lifetime of a component is composed of the sum of the technical lifetime up to the initiation of detectable cracks (characterized for example by TMF life) and the residual lifetime (characterized by FCG). For this reason, it is necessary to develop a material that actively obstructs both the initiation and propagation of cracks in order to exploit the maximum material and component lifetime. HiperFer steels were developed with this objective in mind [18]. A comparison of the ambient temperature FCG data (where thermomechanically induced precipitation in Laves phase-strengthened ferritic steel is not yet effective) of AM316L, conventionally produced 316L, and Crofer22 H in the precipitation annealed state gave an impression of the potential for the obstruction of fatigue cracks, inherent in these steel grades (Figure 19).