STUDY ON METALLOGRAPHIC DAMAGE PARAMETER IN CREEP-DAMAGE-DOMINANT CONDITION UNDER CREEP-FATIGUE LOADING

The microstructural changes of type 304 stainless steel in the creep-d amage-dominant condition under creep-fatigue loading were investigated and the representativeness of some metallographical damage parameters (MDPs) was studied. A series of creep-fatigue tests was conducted at low total strain range and long strain hold time. Some of these were i nterrupted during the periods of life cycle. Cross-sections of all tes t specimens were observed with a scanning electron microscope. Microst ructures such as cavities, precipitate nucleation sites and internal m icrocrack initiation along grain boundaries were investigated. Cavity segregation phenomena which occur in an angular relationship between c avitated grain boundaries and the loading direction were also studied. Then the mechanism of cavity initiation and growth up to microcrack i nitiation was analysed. The number of cavities per unit area (NC), mea n cavity area (MCA) and area fraction of cavity per unit area (AFC) we re measured and studied in respect of their suitability to evaluate th e damage under the present condition. Finally a more suitable MDP whic h should be possible to relate with both cavitation and grain boundary deformation quantitatively is discussed.