Formation Behavior of AlN Precipitates in Super-Duplex Stainless Steel and the Impact on Mechanical Properties

AlN precipitates can be easily formed in Al-killed super-duplex stainless steel (SDSS), and may significantly deteriorate the mechanical properties of the steel. Therefore, three 2507 SDSS ingots containing 290 to 660 ppm Al were prepared, focusing on the characteristics of AlN precipitates, the solubility product of AlN and the mechanical properties. AlN precipitates were observed mainly in ferrite phase, and uniformly distributed over the phase. With increasing temperature from 1223 K to 1473 K, the area fraction of AlN precipitates approximately linearly decreases. However, further increasing temperature to 1573 K, the area fraction exhibits an obvious increasing tendency. Such a phenomenon is reported for the first time, and may be related to rapid variation of austenite and ferrite fractions over the temperature range. Based on a newly developed method, the expression of AlN solubility product as a function of temperature was obtained, and high reliability was demonstrated. The tensile/yield strength of 2507 SDSS are insensitive to Al content. However, the presence of 410 ppm and 660 ppm Al deteriorates the elongation at break and the impact energy, due to the formation of excessive AlN precipitates. This work provides useful guidance for the control of Al levels in 2507 SDSS.