The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

Superaustenitic steel Sanicro 25 has been subjected to in-phase and out-of-phase thermomechanical fatigue cycles in the temperature range from 250°C to 700°C. Both constant strain rate cycling and cycling with 10 minutes dwell at peak temperature were applied. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were evaluated. The transmission electron microscopy was used to find modifications of the internal structure and precipitation of the nanoparticles. 10 min dwell at maximum temperature modified substantially the dislocation arrangement. Various nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X-ray spectroscopy in scanning transmission electron microscope. The damage mechanism operating under specific loading conditions was investigated on the surface as well as in the interior of the cycled specimens. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for crack nucleation and propagation. Effect of dwells on fatigue behaviour, modification of internal structure and damage mechanisms are analysed and discussed.