Deformation behaviour and microstructural evolution of EUROFER97-2 under low cycle fatigue conditions

Low cycle fatigue (LCF) behaviour of EUROFER97 version 2 was studied in normalized plus tempered conditions within a wide total strain range of 0.4–1.8% at room temperature (RT) and 550 °C. At the lowest strain (0.4%) EUROFER97-2 shows very low softening behaviour at both temperatures. However, by increasing the strain amplitude, the material showed an instant hardening followed by a sharp softening up to rupture and fatigue life was reduced. In-depth microstructural characterization by STEM in thin foils and also in a carbon extraction replica was performed on selected samples strained at 0.4, 0.8, 1 and 1.8% at both RT and 550 °C. The microstructural development is based on the rearrangement and/or annihilation of dislocations, which lead to a sub-grain formation and eventually growth, specially enhanced at 550 °C. The microstructural modifications, in regard to dislocations and sub-grain structures, are more intense as the strain amplitude applied is increased. No significant changes in composition, size, and distribution of M23C6 and MX precipitates were observed. Consequently, it is demonstrated that the fatigue life of EUROFER97-2 at room temperature and at 550 °C is strongly dependent on dislocation evolution.