Phase transformations in a corrosion-resistant high-chromium nitrogen-bearing steel

The structure, phase composition, and mechanical properties of an austenitic corrosion-resistant high-chromium nitrogen-bearing (～0.5% N) steel are studied in the as-cast state and after homogenizing heat treatment (HT) followed by quenching. The main structural constituents of the as-cast steel are austenite and the σ phase (12%), which forms as an interdendritic metal during solidification, and δ ferrite and M ₂₃C₆-type chromium carbides are absent. Homogenizing HT at 1100–1200°C leads to the σ → γ transformation through the stage of the formation of intermediate δ ferrite via the restructuring of the tetragonal into the bcc lattice. Upon long-term homogenizing HT, the chromium concentration in ferrite decreases due to diffusion chromium redistribution and the δ → γ transformation takes place. The austenite in both the as-cast steel and the steel subjected to homogenizing HT followed by water quenching contains numerous (Cr, V)N nanoparticles. The twofold yield strength of this steel (～400 MPa) as compared to nitrogen-free 18Cr-10Ni-type steels can be explained by not only the solid-solution hardening of austenite by nitrogen but also by precipitation hardening.