## 2 Materials and Methods
During the fatigue tests, a magnetic field sensor was used to record changes in the tangential magnetic field strength due to the progressively forming ferromagnetic phase fractions, which is generated based on the deformation-induced \(\alpha^{\prime}\)-martensite formation of the AISI 347, as well as on microstructural changes due to dislocation reactions, as well as crack initiation and propagation processes at different load levels. \(\varepsilon\)-martensite could also be formed, but this is not detectable due to its paramagnetic properties. The probe consists of a Hall sensor encapsulated with epoxy resin in a PTFE housing. For magnetization, a constant direct current was applied to the specimen. Magnetic measurements up to elevated temperatures of about 200 \(\mathrm{\SIUnitSymbolCels  
The detection of the ferromagnetic portions by means of a Feritescope was able to confirm the previously described metallographic finding of \(\alpha^{\prime}\)-martensite. This ferromagnetic content increases from the cross-section edge to the center and reaches a maximum of 0.8 vol.-%. \(\delta^{\prime}\)-ferrite was not detected in any of the metallographic investigations carried out. In addition, micro hardness measurements were performed with a Fisher-scope HM 2000 and macro hardness measurements with a conventional hardness tester (Figure 2b). The Vickers micro hardness profile with a testing force of 0.98 N (HV0.1) shows a cup-shaped profile from approx. 340 HV0.1 at the sample edge to approx. 180 HV0.1 in the center. In comparison, macro hardness measurements were performed with a testing force of 98.07 N (HV10). In comparison to the micro hardness distribution, this curve shows an approximately narrower U-shaped progression. Here, the macro hardness is approx. 190 HV10 in the edge area and approx. 145 HV10 in the center. The gradient, which can be seen in both the micro and macro hardness measurements, can be related to the finer-grained microstructure in the edge region. The earlier hardness increase in the macro hardness measurements is attributed to the additional increase in \(\alpha^{\prime}\)-martensite formation due to the 100 times larger testing force. In this project, specimens for two material conditions representing the initial condition and the aged condition are considered. Testing of the aged specimens served to qualify the NDT methods used to characterize the damage evolution during the tests. For aging, the specimens in the initial condition are set to an artificially defined aged condition by means of a practice-oriented thermo-mechanical loading. The parameters for the setting of the artificially aged condition were selected based on existing practice-relevant applications in nuclear technology [24; 25]. These parameters for aging in air include a mechanical loading with a total strain amplitude of 0.3 \(\times\) 10\({}^{-2}\), a strain rate of 4 \(\times\) 10\({}^{-3}\)/s, and a thermal superposition with a temperature of 240 \({}^{\circ}\)C, which represents the mean value of the temperature range from 150 to 325 \({}^{\circ}\)C.  
To provide a data base required for the aging process with regard to the determination of the load cycles required to pre-damage, several fatigue specimens were loaded with the thermal and mechanical parameters up to a load drop \(L\) of 25%. Using statistical methods, the achieved lifetimes were evaluated, allowing load cycles (up to the failure criterion) to be calculated for different probabilities of survival. At a survival probability of 95%, the obtained load cycle for \(L\)25% is 31,450 cycles. Here, the aged condition was defined by 0.5 \(\times\)\(L\)25%. Thus, the mechanical pre-damage results in 15,725 load cycles. The parameters examined experimentally and statistically ensured that the material AISI 347 is already transferred by the thermo-mechanical loading to a material condition that deviates significantly from the initial condition without already exhibiting macroscopically detectable damage characteristics.