Secondary phase precipitation in ultrafine-grained superduplex stainless steels

Abstract. Ultrafine-grained S322750 superduplex stainless steel has been obtained through high-pressure torsion (HPT) process. Microstructural evolution after the severe plastic deformation (SPD) has shown no phase transformation. Isothermal heat treatment was applied after the deformation process in order to promote secondary phase precipitation. Precipitation was enhanced in the ultrafine-grained stainless steel in comparison to the coarse-grained stainless steel at the same isothermal heat treatment conditions. Mainly sigma phase and chromium nitrides were found as a product of the local diffusion process. The morphology of the secondary phases has been analyzed by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM).