It is generally known that grain boundary cavitation is the main damage mechanism in austenitic stainless steels under tensile hold creep-fatigue interaction condition. Cavities are nucleated during cyclic loading and grown during the tensile hold time by grain boundary diffusion of vacancies. However, if the applied stress is removed when the materials is at high temperature, the cavity would be expected to collapse by losing vacancies to the grain boundary because of its surface tension forces. This annihilation of cavity means the recovery of the materials from the damage formed during creep-fatigue interaction and the recovery of the materials results in the extension of their lives.
The creep-fatigue life extension of AISI 316 austenitic stainless steel by heat treatment for cavity annihilation was investigated. Cavities formed during creep-fatigue tests were found to be annihilated during solution heat treatment, and the cavity annihilation led the materials to have longer creep-fatigue lives.
Fatigue cracks formed during creep-fatigue loading were found on the surface of the specimen when the creep-fatigue test were interrupted to conduct the heat treatment for life extension. Because of the surface cracks, the materials were not recovered completely with the heat treatment. These cracks act as the damage to reduce the creep-fatigue lives after heat treatment and changed the fracture mode. The lives can be extended more when the surface cracks are removed because the crack propagation is retarded till the cavities are nucleated and grown enough to failure.
The change of microstructure and the tensile properties was also investigated. The grain size was increased slightly after heat treatment. When the creep-fatigue test were interrupted, the yield strength of the specimens was increased because of the dislocation structure and the ductility was decreased due to the cavities formed on grain boundary. By heat treatment, the change of the tensile properties were recovered even though the strength and the ductility were slightly lower than those of the virgin specimen.
Different heat treatments were conducted to find better conditions which could enhance the effect of treatments for the life extension. Having different heat treatments, it can also be suggested that the extension of the creep-fatigue life can be maximized by changing the aging condition, after solution treatment, to have the lower density of grain boundary carbides that serve as nucleation sites for the cavities. The specimen aged at higher temperature which has the lower value of cavity nucleation factor, P, has longer extended creep-fatigue life. In this specimen, cavity re-generation during the loading after annihilation of previous cavities is retarded owing to the lower density of the grain boundary carbide.
