Effect of Isothermal Ageing on Microstructure and Corrosion Behavior of Nickel and Molybdenum-Free High Nitrogen Austenitic Stainless Steel

This study investigates the influence of isothermal ageing (550–850 °C) and time (30, 60, 180, 360, 720, and 1440 minutes) on microstructure and corrosion behavior of nickel and molybdenum-free high nitrogen austenitic stainless-steel (HNASS). In solution-annealed conditions, the HNASS has a single-phase austenite structure with annealed twins at the grain boundaries. HNASS treated to the aging range at lower temperatures and shorter duration resulted in various fine, discrete, and globular chromium-nitride (Cr₂N) precipitation at grain boundaries. Whereas increased aging temperature and prolonged exposure duration led to the lamellar Cr2N pearlite-like microstructure formed within the austenite grain when treated at 650 °C, 750 °C, and 850 °C after 360 min, 180 min, and 60 min, respectively. The data obtained in potentiodynamic polarization (PDP) and double loop electrochemical potentiokinetic reactivation (DL-EPR) studies show a higher susceptibility to pitting and intergranular corrosion (IGC) with increased aging temperature and exposure duration. However, resistance to IGC and pitting corrosion of HNASS is correlated with the shape and size of Cr₂N precipitation formed at the grain boundaries and extended into the matrix. The U-bend stress corrosion cracking (SCC) studies showed that the HNASS specimen aged at 750 °C for 1 hour showed no visible cracks. However, some random pit initiation and growth were observed over a period of time when exposed for 150 hours to boiling MgCl₂ solution.