On the precipitation of secondary phases in duplex stainless steels and welds

Abstract Duplex stainless steels (DSSs), containing roughly equal amounts of ferrite and austenite, offer an attractive combination of high strength and good corrosion resistance. These alloys exhibit good resistance to localized corrosion including stress corrosion cracking. This study builds on previous work using computational thermodynamics to investigate the precipitation of secondary phases in DSSs. The focus lies on austenite and the detrimental sigma phase and nitrides; phases critical for corrosion resistance and mechanical properties. Simulations were carried out for the duplex grades LDX 2101, 2304, 2205 and 2507, covering lean to super duplex compositions, as well as a 2509 filler to examine the influence of slightly increased nickel content on austenite formation. Reduced nitrogen levels were also studied to capture the effect of nitrogen loss during processing and welding. Austenite formation was assessed through a new approach, defining critical transformation boundaries based on a 30 vol.-% austenite threshold, relevant for weld metal performance. Nitrogen content, cooling rate, and cell size were found to significantly affect the transformation ferrite to austenite. Nitrogen loss, as can occur during welding, delayed austenite formation and shifted phase stability, especially in lower-alloyed DSSs. The onset of Cr 2 N nitride precipitation was closely tied to insufficient austenite formation, with the critical austenite fraction varying by alloy composition and cell size. Sigma phase simulations based on realistic ferrite composition profiles revealed that both nitrogen content and microstructure refinement accelerate formation. A model extension allowed concurrent simulation of Cr 2 N precipitation during austenite growth, improving insight into the competitive phase evolution.