Ratchetting behavior of advanced 9–12% chromium ferrite steel under creep-fatigue loadings

The ratchetting behavior of advanced 9–12% chromium ferrite steel was investigated by cyclic loading tests with various hold times and stress ratios at elevated temperature of 873 K. Particular attention was paid to the effect of hold time on the whole-life ratchetting deformation and failure mechanism. Results indicate that the total ratchetting strains under creep-fatigue loading can be decomposed into two parts, i.e., cyclic accumulated creep strain produced during the peak stress hold time (εR1)(εR1), cyclic accumulated inelastic strain produced during the stress change process (εR2εR2). A transition in ratchetting components and rupture behavior with the increase of hold time was observed. In the long hold time domain, a quick shakedown of ratchetting strain εR2εR2 occurs after the very first few cycles and the rupture behavior is fully controlled by the time-dependent creep damage. In the short hold time domain, ratchetting strain εR2εR2 increases till the specimens fails and a mixed damage mode is responsible for the failure. An attempt is made to explain the existence of these two domains in terms of the evolutions of three internal stress components (back stress, isotropic stress and viscous stress) measured at the end of the holding period.

► Two domains with different ratchetting failure mechanisms were observed. ► Creep ratchetting interaction and internal stress occurred merely at the origin of these domains. ► Ratchetting formed in stress change can be ascribed to the back stress variation.