Study on metallographic damage parameter in creep-damage-dominant condition under creep-fatigue loading Original Research Article

The microstructural changes of type 304 stainless steel in the creep-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 interrupted during the periods of life cycle. Cross-section of all tests specimens were observed with a scanning electron microscope. Microstructures such as cavities, precipitate nucleation sites and internal microcrack initiation along grain boundaries were investigated. Cavity segregation phenomena which occur in an angular relationship between cavitated grain boundaries and the loading direction were also studied. Then the mechanism of cavity initiation and growth up to microcrack initiation was analysed. The number of cavities per unit area (NC), mean cavity area (MCA) and area fraction of cavity per unit area (AFC) were measured and studied in respect of their suitability to evaluate the damage under the present condition. Finally a more suitable MDP which should be possible to relate with both cavitation and grain boundary deformation quantitatively is discussed.