Subtle microstructural changes during prolonged annealing of ODS-Eurofer steel

Abstract

Reduced-activation ferritic-martensitic oxide-dispersion-strengthened (RAFM-ODS) Eurofer steel is a potential candidate material for structural applications in fusion reactors. Microstructural stability during long-term exposure at high temperatures is a key issue. Depending on the amount of prior cold-rolling strain and service temperature, important solid-state restoration reactions occur such as recovery, recrystallization and particle coarsening. ODS-Eurofer steel was cold rolled up to 80% reduction in thickness and annealed at 800 °C for durations up to 4320 h. Changes in microstructure were tracked by X-ray diffraction measurements using synchrotron radiation in post-mortem specimens to estimate dislocation character and density. The volume fraction of recrystallized grains was estimated using grain orientation spread (GOS) maps from electron backscatter diffraction (EBSD). Most of the softening occur in the first hour of annealing and it seems to be closely related to discontinuous recrystallization where a few special grain boundaries overcome Zener-Smith pinning effects caused by fine and stable Y2O3-based particles. M23C6 carbides undergo coarsening upon annealing and, as a result, extended recovery is the predominant softening mechanism as annealing proceeds, although only about 15% softening is noticed after annealing for 4320 h. Using thermodynamic and kinetic calculations, the results were extrapolated to the predicted service temperature of 650 °C. The results suggest that the remarkable microstructural stability of ODS-Eurofer would withstand almost 180 years at high service temperatures without major loss of the mechanical properties of the materials.

Keywords

• ODS-Eurofer steel
• Microstructural stability
• Extended recovery
• Recrystallization
• Electron backscatter diffraction
• Particle coarsening