Research On High-temperature Low Cycle Fatigue Performance Of 22Cr15Ni3.5CuNbN Steel

With the increase of the working temperature of the ultra supercritical unit,higher requirements are put forward for the performance of materials.22Cr15Ni3.5CuNbN steel is developed for the ultra supercritical boiler which is working at 620-650?,it has good mechanical properties at elevated temperature.In the operation of the thermal power unit,the start-up and fluctuation load of the power plant boiler will cause the boiler structure to bear the large alternating load,so the high-temperature fatigue performance of the material of the component is very important for the stability and reliability of the boiler.In this paper,the low-cycle fatigue,microstructure evolution and life prediction methods of 22Cr15Ni3.5CuNbN austenitic heat-resistant steel at different strain amplitudes at 650 °C were studied.According to the test results,the steel exhibited a cyclic hardening behavior at all strain amplitudes.The degree of hardening reached the maximum value at strain amplitude of 0.50%.The cyclic hardening behavior was related to the increase in dislocation density.DSA effect was observed in both tensile test and LCF tests,which is usually caused by interactions between dislocations and solute atoms.DSA enhanced with the increase of strain amplitude and decreased with the increase of the cycle number.SEM analysis of the fracture surface showed that the fracture surface can be divided into three regions: fatigue crack initiation sites,fatigue crack propagation zone and transient rupture zone.There were multiple crack initiation sites when the strain amplitude was high,especially at strain amplitude of 0.60%.EBSD observations showed that there was no remarkable texture formed after LCF tests.Furthermore,cavities were observed in grain boundaries,twinned grain boundaries and trigeminal grain boundaries.The level of local misorientation around cavities and cracks is much higher than that at other positions.TEM observations showed that dislocations would entangle with each other around the precipitates,and dislocationfree zones can be observed near dislocation tangles.Besides,an energy-based prediction model was applied to predict the fatigue life of the steel.