Low cycle fatigue and creep-fatigue interaction behavior of C630R ferritic/martensitic heat-resistant steel at high temperature

Abstract

The low-cycle fatigue (LCF) and creep-fatigue interaction (CFI) characteristics of 630R ferritic/martensitic heat-resistant steel at 630 °C have been examined. The LCF tests were conducted within a strain range of 0.3 % to 1.0 %. CFI assessments were conducted using trapezoidal waveforms with varying strain amplitudes and load holding durations. The progression of cyclic stress, hysteresis curves, stress relaxation behavior, fracture mechanism, and microstructure evolution during LCF and CFI loading was analyed. The findings suggest that C630R heat-resistant steel displays significant cyclic softening behavior during both low-cycle fatigue and creep-fatigue testing. In the low-cycle fatigue test, the extent of softening enhance with upper strain amplitudes, the introduction of hold time further accelerates this softening. Increased strain amplitudes during low-cycle fatigue (LCF) testing led to a higher number of crack initiation points (the cracking sources are 2 and 5 at 0.6 % and 1.0 % strain amplitudes, respectively). Fatigue fracture still the vital failure pattern under varying load-holding times, with extended load-holding durations promoting crack propagation, an increased presence of creep voids is observed. The interaction between fatigue and creep effects becomes more evident, which results in the shortening of fatigue lifespan. Under cyclic loading conditions, the martensitic lath structure experiences recovery, which results in cyclic softening. As either the increase of load-holding time and strain amplitude, the microstructure exhibits more uniform coarsening, with the lath structure gradatim transforming into uniform dislocation cell structure. Furthermore, a prominent W-Laves phase developed during the creep-fatigue tests, with the Laves phase increasing in coarsen as the loading period was prolonged.