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

Abstract

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.