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

Ferritic-austenitic duplex stainless steels (DSSs), containing roughly equal amounts of ferrite and austenite, offer an attractive combination of high strength and good corrosion resistance. Commercially available since the 1930s, DSSs exhibit very good resistance to localised corrosion including stress corrosion cracking. Their lower nickel content also presents a cost advantage compared to austenitic stainless steels. This study builds on previous work using computational thermodynamics (Thermo-Calc and DICTRA) to investigate the precipitation of secondary phases in DSSs. The focus lies on austenite (precipitating from ferrite) as well as the detrimental sigma phase and chromium nitrides – phases critical for determining corrosion resistance and mechanical properties. Approximately 50% austenite is considered optimal, while sigma phase and nitrides should be avoided. 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 metal 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 simulation approach, defining critical transformation boundaries based on a 30% austenite threshold, relevant for weld metal performance. Nitrogen content, cooling rate, and cell size were found to significantly affect transformation kinetics. Nitrogen loss, as can occur during welding, delayed austenite formation and shifted phase stability, especially in lower-alloyed grades. The onset of Cr ₂ N chromium nitride precipitation was closely tied to insufficient austenite reformation, 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 its formation. A model extension allowed concurrent simulation of Cr ₂ N precipitation during austenite growth, improving insight into the competitive phase evolution during thermal processing.