Rare earth addition on SAF 2507 Super Duplex Stainless Steel: effect on microstructural evolution and mechanical properties

The manufacturing of large duplex stainless-steel components commonly requires sand casting, which does not ensure sufficiently high cooling rates to prevent the formation of deleterious secondary phases, especially in thick sections. To restore a balanced ferrite-austenite microstructure free of secondary phases, a solution annealing treatment is required. This study investigates the role of rare earth elements as alloying additions in SAF 2507 super duplex stainless steel (X2CrNiMoN25–7–4). Small cerium additions were introduced to modify the precipitation kinetics of secondary phases and reduce segregation during solidification. Their effect on improving the microstructural stability of thick cast components was then evaluated. The first part of the study focused on solution heat treatment, with particular emphasis on the influence of temperature and holding time on microstructure, mechanical properties and corrosion resistance. A SAF 2507 sand casting was produced, and several specimens were subjected to isothermal heat treatments between 950°C and 1150°C with different holding times. Microstructural characterization was performed using optical and scanning electron microscopy, while image analysis was used to quantify phase fractions. Mechanical behaviour was evaluated through tensile, impact and hardness tests, and corrosion resistance was assessed by immersion tests. To investigate the effect of rare earth additions, two additional castings of the same alloy were produced with different cerium contents and subjected to identical heat treatments and characterization procedures, enabling direct comparison with the base alloy. The results confirmed the correlation between σ-phase content and the deterioration of mechanical and corrosion properties. Rare earth additions improved microstructural stability and overall performance by reducing the formation of deleterious phases, demonstrating the potential of combining alloying design with heat treatment optimization to produce large SAF 2507 cast components.