Effect of nitrogen on precipitation behavior of hyper duplex stainless steel S32707

Highlights: • Increasing N content significantly inhibits the precipitation of σ phase. • Raising N content promotes Cr₂N precipitation only after long term aging. • N reduces Cr and Mo diffusion trend, Cr activity, nucleation site and driving force of σ phase. • N raises Cr and N diffusion rate, N activity and driving force of Cr₂N. Thermodynamic calculations, time–temperature–transformation diagram construction, microstructural characterization and precipitation kinetics analyses were conducted to investigate the effect of N content on the precipitation behavior of hyper duplex stainless steel S32707. The results demonstrate that the nose time of 3% σ phase precipitation gradually increases with increasing N content. Increasing the N content substantially reduces the quantity of the σ phase at δ/γ interfaces. Within the first 5 min of aging, the growth rate of the σ phase decreased in the order N0.35, N0.45 and N0.56. Between 5 min and 48 h, the growth rates were essentially the same. The mechanism by which N inhibits the σ phase is attributed to three aspects: (i) N reduces the quantity of favorable nucleation sites by increasing the fraction of δ/γ interfaces with low Kurdjumov–Sachs orientation relationship deviation angles; (ii) N diminishes the tendency of Cr and Mo to diffuse from the δ to γ phase by decreasing the natural partitioning coefficients; and (iii) N decreases the σ phase nucleation driving force and Cr activity. Raising the N content promotes Cr₂N precipitation only after long-term aging. The role of N in promoting Cr₂N is generalized in two aspects: first, N increases the diffusion rate of Cr and N in austenite based on its increased diffusion coefficient with increasing N content; second, N increases the Cr₂N nucleation driving force and N activity.