PSI - Issue 52

Jiri Dvorak et al. / Procedia Structural Integrity 52 (2024) 259–266 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

262

4

Modified creep curves (Fig. 3(b,c)) better showed a primary creep region, where the creep rate decreases with time, and of a tertiary or acceleration creep region, where the creep rate increases with time after reaching a minimum creep rate. Furthermore, it is clear from Fig. 3(c) that the tertiary creep stage covers the dominant duration of the creep exposure up to fracture of the specimen. Prolonged tertiary creep in terms of time and strain can be attributed to loss of solid solution strengthening due to microstructural instability (Abe and Nakazawa, 1992).

Fig. 3. Standard (a) and modified (b,c) creep curves of P92 steel at 600 °C and different applied stresses.

3.3. Analyses of operating creep mechanisms Stress dependence of the minimum creep rate ̇ at constant temperature and at constant grain size can be described by the power-law relationship of the form: ̇ ≅ , (1) where A is a constant independent of σ , and n is the stress exponent of the creep rate which can characterize the various creep deformation mechanisms in materials. Similarry, the variations of rupture life ( t r ) with applied stress σ also obeyed power law as ≅ − (2) Fig. 4 and Fig. 5 shows the variations of minimum creep rate ̇ and time to fracture t f with applied stress σ as double logarithmic plots. The slopes and, therefore, the apparent stress exponent of the minimum creep rate was evaluated as n = 17.7. Simultaneously, the stress exponent of the time to fracture illustrated by Fig. 5 was obtained a slightly lower m = 16.2. Similarity of these values indicates that both the creep deformation and fracture are controlled by the same mechanism(s) and all creep tests were performed in the region of the power-law (dislocation) creep (Cadek, 1988; Sklenička et al., 2015, 2003; Sklenička and Kloc, 2011) where creep process is controlled by climb of dislocations. (Maruyama et al., 1986) pointed out that the stress exponent n depends on applied stress. The values determined in this work are also in coincidence with previous published data executed on P92 steells at the same temperature (Abe et al., 1992; Kimura et al., 2008; Sakthivel et al., 2015; Sklenicka et al., 2018) in high stress region. Lower values was observed by (Sklenička et al., 2003) for long -term ageing P92 steel and by (Kral et al., 2018) for ultra-fine grained P92 steel.