## 3 Results and Discussion  
### Thermomechanical Fatigue Behavior  
Owing to the different alloying philosophies, the two materials yield decisive differences in the initial and damage phase as well as in fatigue life, but almost comparable stress range in the stable phase of the fatigue life curve.  
Figure 3a displays typical thermomechanical fatigue life curves of the ferritic-martensitic grade 91 and ferritic Crofer(r)22 H steels from oop cycles with complete (100%) obstruction of thermal strain (i.e., the specimen faces compressive stress during heating up to the hot end of the cycle and tensile stress during cooling down to the cold end). Figure 3b in detail depicts the initial phases of both the materials up to 1200 cycles. In Figure 3c,d the associated stress-strain hysteresis loops are given.  
Representative for AFM steels [4; 5; 6; 33], grade 91 is characterized by a comparatively high initial strain range, which is caused by increased dislocation density from martensitic transformation during heat treatment. However, during the first appr. 1000 TMF cycles, the recorded stress range continuously decreases by typically (cf. grade 92 and MarBN-Type steel in [33]) one third of the initial value (Figure 3b). The main part of this drop takes place at the hot ends (i.e., at compressive stress) of the first 10 cycles, which is clearly observable from the hysteresis loops (cf. cycles # 1 vs. # 10 in Figure 3c). Up to 250 cycles, the further decrease localizes almost symmetrically to both ends of the loops (cf. cycles # 10 vs. # 250 in Figure 3c). Towards higher cycle numbers, just slight further changes of the hysteresis loops become evident (cf. cycles # 250 vs. # 1250 in Figure 3c) until a stable phase of slightly negative slope is entered at around 1000 cycles. The material finally reaches the damage phase (Figure 3a: \(>\)1962 cycles, Table 2), after which failure is characterized by a comparatively steep drop in stress range.  
During the initial phase, Crofer(r)22 H strengthens by appr. 25% (Figure 3a,b) and reaches a peak in stress range at about 35 cycles (Figure 3d), then drops down by about 5% up to appr. 400 cycles, where it enters the stable stage, which lasts up to about N\({}_{\text{d}}\) = 1993 cycles. Despite of the lower effective strain range, Crofer(r)22 H reaches a slightly higher stable stress range (Figure 3) and mean stress (Table 2) levels than P91. In the damage phase, it exhibits diminished decrease in stress range per cycle (Figure 3a), which in consequence leads to prolonged TMF lifetime (N\({}_{\text{f}}\) = 4091 cycles) in comparison to P91. To consider this difference in tolerance to damage (represented by the slope difference of the fatigue curves of both materials in the failure range, cf. Figure 3a) fatigue lifetime was determined from the intersection of linear approximations to the stable (deviation of the fatigue curve from this approximation determines N\({}_{\text{d}}\)) and the failure curve sections of the fatigue curve (cf. Crofer(r)22 H curve in Figure 3a).  
A comparison of damage ranges (i.e., the relation of technical lifetime N\({}_{\text{f}}\) to damage initiation N\({}_{\text{d}}\), Table 2) demonstrates the more forgiving character of the ferritic, Laves phase-strengthened Crofer(r)22 H steel concerning initiation and propagation of short cracks in thermomechanical fatigue loading. Mathematical integration of the fatigue hysteresis loop (Figure 3c,d) yields the cumulated deformation energy induced into the material. In the stable range of the fatigue curve, a single cycle in the case of P91 calculates to 1.145 MJ/m\({}^{3}\) (@ N\({}_{\text{f}}\)/2 = 1486 cycles). Despite the higher stress range and mean stress, Crofer(r)22 H with 1.081 MJ/m\({}^{3}\) (@ N\({}_{\text{f}}\)/2 = 2045 cycles) yields lower deformation energy per cycle, because of its lower thermal expansion coefficient. Damage is induced later than in martensitic P91 and it reaches 28% longer fatigue lifetime along with 21% higher cumulated deformation energy until failure (Table 2).