High temperature components with notches, defects, and flaws can introduce crack initiation and crack propagation under service conditions. Fracture mechanics procedures are needed to study crack problems and to support an advanced remnant life evaluation. Since a more flexible service mode of steam power plants causes a higher number of start-up and shut-down events, creepfatigue crack behavior is decisive for life assessment and integrity of components. Usually, fracture mechanics experiments are carried out under air conditions, although in cases of internal cracks they are not in contact with air. Therefore, it is of interest to realize the degree to which environmental conditions, e.g., crack-tip oxidation, can influence crack initiation and crack growth behavior.
In order to reveal problems related to high temperature components, the crack initiation time and crack growth rate were determined in air environment and in a gas with controlled atmosphere on lCrMo(Ni)V- and lOCrMoWVNbN-steels. Crack initiation and propagation under creep-fatigue conditions have been described with the fracture mechanics parameters C*, KI and ΔKI C*. A modified “Two-Criteria-Diagram” was used to describe creep-fatigue crack initiation.