Cyclic plastic response and damage mechanisms in superaustenitic steel Sanicro 25 in high temperature cycling - Effect of tensile dwells and thermomechanical cycling

The cyclic plastic response and damage evolution in isothermal high temperature constant strain rate cycling and during in-phase thermomechanical fatigue in superaustenitic Sanicro 25 steel have been studied both with and without introduction of dwells at maximum strain. Fatigue hardening/softening curves and fatigue life curves are reported for all types of fatigue tests. Rapid hardening and a tendency to saturation has been found primarily in isothermal cycling with dwells and in thermomechanical cycling. Study of the surface damage evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries perpendicular to the stress axis. Introduction of dwells in maximum tension leads to the enlargement of the plastic strain amplitude and to additional creep damage in the form of cavities along grain boundaries and internal cracks. Fatigue hardening rate in thermomechanical cycling is higher than in constant strain rate cycling and fatigue life decreases substantially in in-phase thermomechanical cycling. These findings are discussed in the perspective of effectiveness of dominant damage mechanisms relevant to individual types of cyclic straining.