Evolution of microstructure and tensile properties of Fe-18Ni-12Cr based AFA steel during aging at 700 °C

The effects of aging at 700 °C on the microstructure and tensile properties of an alumina-forming austenitic (AFA) stainless steel were investigated. With increasing aging time, B2-NiAl and Laves phase precipitated first on grain boundaries (GB) and then in the grain interior. The GB precipitate coverage reached 74% after aging for 1000 h. The GB precipitates not only suppressed grain coarsening during aging, but also influenced the tensile fracture mode at 700 °C by partitioning stress concentration on triple junctions. Moreover plate-like and spherical NbC particles precipitated during aging. Spherical NbC with size of around 5 nm were stable, while plate-like NbC grew to 89 nm after aging for 1000 h. These precipitates played an important role on the tensile strength. Age hardening contributed to the increasing tensile strength at RT with aging time, while the softening mechanism of dynamic recovery dominated the tensile tests at 700 °C.