Low cycle fatigue behavior and microstructure evolution of a novel Fe-22Cr-15Ni austenitic heat-resistant steel

Abstract

22Cr15Ni3.5CuNbN stainless steel is a newly developed heat-resistant austenitic steel for superior ultra-super critical fossil power plants with operating temperature up to 650°C. In this study, low cycle fatigue (LCF) behavior and fatigue failure damage mechanism of this steel were investigated. The LCF tests were conducted at 650°C in air. The microstructures, dislocations and precipitates after the LCF were studied to identify the fatigue damage mechanism. A cyclic hardening behavior was observed, where the cyclic hardening rate increased first and deceased as the applied strain amplitude was greater than 0.50%. This was similar to the variation of the dislocation density. A high strain amplitude induced dislocation annihilation and thus the cyclic hardening rate of 0.60% strain amplitude became lower than that of 0.50%. In addition, dynamic strain ageing occurred and depended on the plastic deformation and in the case of low strain amplitude it gradually disappeared after a few cycles while in the case of high strain amplitude it maintained through the fatigue life. Moreover, the fatigue cracks initiated at the outer surface and mainly in grain boundaries, twin boundaries and triple grain boundaries, owing to the high strain localization and the strong interaction between dislocations and precipitates.