Thermal-mechanical fatigue behavior and lifetime prediction of P92 steel with different phase angles

The influence of phase angle varying from 0 degrees (in-phase, IP), 90 degrees (clockwise diamond, CD) and 180 degrees (out-of-phase, OP) on thermal-mechanical fatigue behavior and lifetime of P92 steel has been investigated in present paper. Thermal-mechanical fatigue (TMF) tests were conducted under mechanical strain control whose amplitudes were 0.4%, 0.6% and 0.8% with thermal cycles between 550 degrees C and 650 degrees C. For comparison, isothermal fatigue (IF) tests also have been achieved at maximum circular temperature of 650 degrees C under the three mechanical strain amplitudes. Both thermal-mechanical and isothermal fatigue tests were finished with the equal mechanical strain ratio of R-e = -1 and cycle period of 120 s. Results show that the sequence of failure lifetime is N-f(OP) < N-f(IP) < N-f(CD) < N-f(IF) for a given mechanical strain amplitude. Fractographic and microstructural investigations show that cracks initiate and propagate transgranularly for both TMF and IF tests and the creep relaxation and oxidation phenomenon contribute a lot in the process of microstructural damage. Three life prediction models were selected and compared to predict failure lifetime, in which the Ostergren model related to phase angle was modified to achieve a better predictive precision.