Fractographic study was conducted for austenitic stainless steels fractured in the five basic creep-fatigue tests, named PP, PC, CP, CC and DC tests. Fracture surface was observed with an SEM and the crack growth behavior was examined in the longitudinal cross section of the specimen with an optical microscope. Fracture morphologies were found to be closely related to the type of test, that, is, the type of inelastic strain cycled to the specimen.
When Δεpp-type of strain was cycled, transgranular crack initiation and its growth at the specimen surface were typical. When Δεpc-type of strain was cycled, dimple formation occured inside the specimen and led to a tensile-like fracture. When Δεcp-type of strain was cycled, the initiation of many cracks in the grain boundaries and their intergranular or transgranular coalescence were typical. When Δεcc-type of strain or δc-type of strain was cycled, intergranular crack initiation and its growth at the specimen were fundamental features of the fracture surface.
The effects of environment, temperature and stress or strain condition on each fracture morphology were discussed. The fundamental mechanism of nucleation of voids in the grainboundary can be used to explain the difference among the fracture morphologies.
