Microstructural evolution in two variants of NF709 at 1023 and 1073 K

The microstructural state of two grades of the creep-resistant austenitic stainless steels NF709, a Fe20Cr-25Ni (wt pet) based steel, has been studied, in the as-received state, after an additional solution treatment and after static aging at 1023 and 1073 K. Although the two variants are chemically similar, they exhibit different microstructures following identical heat treatment. In particular, the Z phase and sigma are found in largely different quantities. The study suggests that a seldom observed variant of the eta structure, Cr3Ni2SiX, is stabilized by nitrogen. Both steels exhibit a yet undocumented precipitation sequence when compared to more conventional austenitic steels (for which the precipitation sequences have been reviewed by Sourmail (Mater. Sci. Technol., 2001. vol. 17. pp. 1-14)).