High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 1 - pure fatigue, microstructure evolution and damage characteristics

Low cycle fatigue (LCF) behavior of a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel at elevated temperatures is correlated to the microstructural evolution after cyclic straining. The fully reversed strain-controlled tests were conducted in air at 550 °C and 650 °C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. This is due to the fact that highly stable nano-oxide particles act as persistent barriers for dislocation motion which as a consequence slows down the typical microstructural evolution realized in the conventional non-ODS steels. Upon cycling at 550 °C, microstructure exhibits only minor changes. The main modifications are in respect to the dislocations rearrangement and/or annihilation which finally result in their reduced density. At 650 °C, microstructural evolution hastens and becomes prominent mainly in nano-oxides/carbides deficient regions. Here, in addition to the reduced dislocation density, partially eliminated original sub-grain structures, grain growth, M23C6 carbides coarsening and Cr-W enriched Laves phase precipitation were evident. The microstructural modifications, concerning dislocation density and sub-grain structures, intensify even further with increase in applied strain amplitude. Nevertheless, annealing at 650 °C for similar duration has no major influence on microstructure. Damage studies revealed expeditious as well as pronounced damage with increase in applied strain amplitude. Cracks initiation, eventuates at the early stage of the test, and their propagation were further assisted by oxidation. The stable crack growth region manifests secondary cracks, and at higher magnification the classical fine-scale transgranular ductile fatigue fracture features called striations. In addition, at 650 °C, crack path also acquires an intergranular tendency under higher strain amplitude.