Thermomechanical fatigue and damage mechanisms in Sanicro 25 steel

Heat resistant austenitic Sanicro 25 steel was subjected to in-phase and out-of-phase thermomechanical fatigue (TMF) loading conditions with different amplitudes of mechanical strain in a wide interval of temperatures (250-700 °C). Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were evaluated for both type of loadings. Scanning electron microscopy combined with FIB cutting and EBSD imaging in longitudinal sections containing cracks revealed the mechanisms responsible for fatigue crack initiation and growth. Fatigue cracks developed rapidly in oxidized grain boundaries during in-phase loading and intergranular crack growth resulted in short fatigue life. Multiple cracks in out-of-phase loading perpendicular to the stress axis have arisen only afterwards oxide layer was formed. The delayed initiation and transgranular growth led to longer fatigue life. The effect of grain boundary oxidation and surface oxide cracking on fatigue life was discussed.