γ″-Ni3Nb precipitate in Fe–Ni base alloy

For Fe–Ni alloy involving the metastable γ″-Ni3Nb precipitates in the standard chemical composition of Fe–43Ni–15Cr–2W–1Mo–3.8Nb–0.1V–0.1Ti–0.28Al–0.1Zr–0.04C (mass%), hardness measurement and TEM analyses were performed during aging at temperatures 650–750 °C up to 200 h after 20% cold-rolling. The hardness increases due to coherent γ″-Ni3Nb precipitate, but longer aging induces hardness decrease due to its coarsening and dislocation recovery. Aging at 800 °C for 200 h leads to transformation to the stable δ-Ni3Nb. Based on these data, an appropriate aging condition is considered to be at the temperature 700–750 °C for 8 h in order to distribute finely intra-granular γ″ precipitates at the final stage of manufacturing.