4.3.2. Elevated Temperature T = 300 ◦C

High precision stress-strain hysteresis measurement suitable to quantify microplastic deformations at low load amplitudes is, as with ultrasonic fatigue, impossible for tests using servohydraulic systems with a load frequency f = 980 Hz. The S-N Woehler curve resulting from constant amplitude fatigue tests at this frequency at 300 ◦C is given in Figure 12c. Similar to the HCF behavior at 300 ◦C at low stress amplitude, a very low volume fraction of α'-martensite was detected in the specimen which achieved the limit number of cycles Nl = 5 <sup>×</sup> 108. Since the Ferritscope™ sensor cannot be used in situ at this testing temperature, the kinetics of α'-martensite development was analyzed by an interrupted VHCF test with a stress amplitude of 160 MPa. At defined load cycles, ex situ Feritscope™ measurements were taken at the specimen surface at ambient temperature. Figure 11 shows no α´-martensite up to N = 1 <sup>×</sup> 107. The onset of <sup>α</sup>´-martensite formation occurred between N = 1 <sup>×</sup> 107 and N = 1 <sup>×</sup> 108. At N <sup>=</sup> <sup>1</sup> <sup>×</sup> 108 0.12 FE% and at N <sup>=</sup> <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>8</sup> 0.13 FE% were measured without specimen failure. As the measured volume fraction of α´-martensite was very low, further microstructural changes had to play a role in the cyclic hardening of metastable austenitic steel in the VHCF regime at 300 ◦C.

**Figure 11.** Development of α'-martensite during the interrupted VHCF test at σ<sup>a</sup> = 160 MPa, f = 980 Hz and T = 300 ◦C.

## **5. Summary**

Figure 12 summarizes the results from fatigue tests on AISI 347 in LCF, HCF and VHCF regimes at AT and 300 ◦C in the form of S-N curves. The influences of metastability of two different batches of AISI 347 on cyclic deformation behavior are clearly seen in LCF tests at ambient temperature (see Figure 6). The determined total strain-controlled fatigue life in the LCF regime at AT and 300 ◦C is similar (Figure 12a). However, significantly different cyclic deformation behavior was observed. At AT, cyclic hardening due to α´-martensite formation took place and the transformation from a single-phase austenitic to two-phase austenite/α´-martensite microstructure developed. The results epitomize a "dynamical composite material" with changing volume fraction and distribution of "reinforcements" during cyclic loading. At 300 ◦C in the LCF regime, no α´-martensite was measured and cyclic deformation behavior showed slight initial cyclic hardening followed by saturation/softening. Despite significantly different cyclic deformation behavior and resulting stress amplitudes in AT and T = 300 ◦C, respectively, similar fatigue lifetimes were estimated. However, the specimens loaded at AT achieved stresses higher than the ultimate tensile strength. A positive influence of α´-martensite formation on fatigue life was observed in stress-controlled fatigue tests in the HCF regime (Figure 12b) where austenite-α´-martensite transformation caused hardening, which significantly reduced the plastic strain amplitude and led to increased fatigue life. The α´-martensite formation occurred in all fatigue tests at AT. At 300 ◦C, albeit specimens loaded with low stress amplitude showed a very small volume fraction of α´-martensite. These specimens achieved the limit number of cycles without failure. A comparison of the S-N curves from tests at AT with T = 300 ◦C clearly showed a decrease in fatigue strength with an increase of temperature (Figure 12b). Similar behavior was observed for VHCF tests at AT and 300 ◦C (Figure 12c). At AT, higher fatigue strength existed, which correlated with the development of α´-martensite at each load amplitude. At 300 ◦C, only specimens with the formation of a low volume fraction of α´-martensite achieved the limit of the number of cycles without failure.

**Figure 12.** (**a**) εa,t –N curve in LCF regime of batch A at AT and T = 300 ◦C, (**b**) S-N curve in HCF regime of batch A at T = 300 ◦C and batch B at AT, (**c**) S-N curve in VHCF regime of batch A at AT and T = 300 ◦C.
