4.1.2. Elevated Temperature T = 300 ◦C

Single step total-strain control fatigue tests in the LCF regime at 300 ◦C were performed with total strain amplitudes 0.8% ≤ εa,t ≤ 1.6%, a load frequency f = 0.01 Hz and triangular waveform. Figure 7 presents the development of the stress amplitude σ<sup>a</sup> versus the number of cycles N. At 300 ◦C the cyclic deformation behavior of the investigated steel was characterized by initial cyclic hardening, followed by slight cyclic softening before the final stress amplitude drop associated with macro crack propagation. The microstructural changes due to cyclic plastic deformation are described elsewhere [46,63]. At these load parameters, no α´-martensite formation occurred and, compared to ambient temperature, the stress amplitude was significantly lower (compared Figure 6a with Figure 7).

**Figure 7.** Development of stress amplitude σ<sup>a</sup> versus load cycles N during total-strain controlled LCF test at T = 300 ◦C of batch A.
