PSI - Issue 13

Hugo Wärner et al. / Procedia Structural Integrity 13 (2018) 843–848 Hugo Wärner et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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2. Results and discussion 2.1. Cyclic hardening and life evaluation

The softening/hardening curves for the TMF-tests are shown in Fig. 2. It is apparent that tests without dwell time and lower temperature range (Fig. 2a), show cyclic hardening until failure and maximum stress amplitudes increasing with strain amplitudes ( ε a ). Tests with 5-minute dwell time and higher temperature range (Fig. 2b), show both softening and hardening for the virgin material and moderate hardening near the end of life for the pre-aged material. Comparing the different testing conditions in Fig. 2 it is clear that, the tests without dwell time and lower temperature ranges show a more prominent hardening behavior and shorter fatigue life. However, the aging drastically lowers the fatigue life (see Fig. 2b) and this testing condition even shows shorter fatigue life than the test configuration corresponding to Fig. 2a. From the work of Heczko et al. (2018), the prominent hardening of Sanicro 25 is mainly due to precipitation of Cu-rich nanoparticles and Nb-rich precipitates, which impede the dislocation motion during high temperature cyclic condition. The Cu-rich nanoparticles precipitate homogeneously throughout the matrix and are coherent with the austenitic matrix. But the key contribution to the obstruction of dislocation movement is by the strain induced NbC, NbN and Nb(C,N) precipitates, which acts as incoherent dispersoids. For a more in-depth description of this strengthening behavior, see Heczko et al. (2018).

(a)

(b)

Fig. 2. Cyclic hardening/softening curves in TMF. Stress amplitude vs. number of cycles; (a) no dwell time in temperature range of 250-700 °C (Petrá š , 2016); (b) 5 min dwell time in temperature range of 100-800 °C (Wärner, 2017).

In Fig. 3, the CF results are shown. For the virgin material (Fig. 3a) an increase in strain amplitude yields higher stress amplitude, shorter saturation before failure and overall shorter fatigue life. In addition, longer dwell times correspond with lower maximum stress amplitudes and shorter fatigue cycle life. For the pre-aged materials in Fig. 3b, the fatigue life increased compared to the virgin material with the same strain amplitudes and dwell times. The hardening behaviour for the pre-aged material is similar to the pre-aged TMF material, with steady-state hardening until a rapid increase at the end of life. The influence of the different aging temperatures does not have a big impact on the fatigue life for the tests with dwell time of 600 s, but for the tests with dwell time of 300 s the 700 °C pre-aged test showed a significantly higher fatigue life compared to the 650 °C pre-aged test. In spite of the indication of a positive influence of aging for the fatigue life during CF conditions at these temperatures, a more comprehensive study must be performed in order to define the order and validity of this effect. The cyclic stress-strain curves (CSSCs) i.e. the stress amplitude vs. plastic strain amplitude at half-life (Fig. 4a) and the Coffin-Manson i.e. curves plastic strain amplitude vs. number of cycles to fracture (Fig. 4b), for all the different test conditions are shown in Fig. 4. Both curves are represented in bilogarithmic scales and all plots are approximated and fitted by respective power law functions to experimental data using least squares procedure (for more details see Petrá š et al. (2016)).