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

Abstract

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.

Thermomechanical fatigue of round tube specimens manufactured by precision directional solidification casting method

“…The parallel cracks in a thick oxide layer in OP TMF cycling are responsible for fatigue crack initiation. 47 As shown in Figure 6, compression creep occurs during high-temperature half cycles under OP conditions. However, the compressive creep is not very important, 34 because the compressive creep is much less damaging compared with oxidation, fatigue damage and tensile creep.…”

“…Figure 11B is the local magnified image of Figure 11A, showing the oxide layer on the surface of the specimen was cracked due to the compressive stress at a low temperature under the IP condition. Figure 11D is the local magnified image of Figure 11C, showing the oxide layer cracked perpendicularly to the stress axis 47 under the OP condition. The formed oxides experience a brittle-ductile transition temperature when the temperature drops from the maximum to minimum.…”

“…The parallel cracks in a thick oxide layer in OP TMF cycling are responsible for fatigue crack initiation. 47…”

“…47 The internal damage may significantly influence the growth of the dominant crack initiated on the surface due to the interaction of this crack with the internal cracks that were nucleated closed to the crack tip. 47 Yang et al 1 discovered some visible typical creep failure characteristics, such as dimples and tearing ridges under IP conditions. A similar conclusion is reached in the current work, as shown in Figure 9D.…”