Low Cycle Fatigue Behavior and Cyclic Softening of P92 Ferritic-martensitic Steel

The low cycle fatigue (LCF) behavior of P92 martensitic steel was investigated under different controlled strain amplitudes at room and high temperatures (873 K). The cyclic stress responses at all temperatures and strain amplitudes exhibited obviously rapid softening behavior at the early stage of fatigue life, and there was no saturated stage at high temperature. The fracture surfaces of the fatigue samples were observed by scanning electron microscopy (SEM) and optical microscopy. It was shown that crack initiation and propagation occurred transgranularly at both testing temperatures. A typical character was the high density crack branches or secondary cracks along fatigue striations at high temperature, which initiated from the oxidized inclusions and grain boundaries. Further investigation by transmission electron microscopy (TEM) showed that the softening behavior was attributed to the microstructure evolution during fatigue life, such as annihilation of dislocations and migration of martensite laths as well as carbide coarsening, especially for samples tested at high temperature.