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

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% a phase precipitation gradually increases with increasing N content. Increasing the N content substantially reduces the quantity of the a phase at 8/? interfaces. Within the first 5 min of aging, the growth rate of the a 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 a phase is attributed to three aspects: (i) N reduces the quantity of favorable nucleation sites by increasing the fraction of 8/? interfaces with low Kurdjumov?Sachs orientation relationship deviation angles; (ii) N diminishes the tendency of Cr and Mo to diffuse from the 8 to ? phase by decreasing the natural partitioning coefficients; and (iii) N decreases the a 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.