Abstract: 2205 DSS(duplex stainless steel)were solution treated at 1100℃ followed by aging at 650~900℃ for different time to study the precipitation of σ phase and its influence on microstructures and properties. The results showed that σ phase precipitates in two different ways， one is the precipitation at phase boundaries accompanied by carbides; the other is the precipitation in the ferrite grains or at the ferrite grain boundaries without accompanied carbides. The precipitation of σ phase is the most serious when aged at 850℃. After the precipitation， the segregation of alloying elements Cr and Mo in different phases occurs in different degrees. Small amounts of σ phase have little effect on ductility， but reduce impact toughness significantly. Large amounts of σ phase can reduce both ductility and impact toughness severely. The precipitation of σ phase has little effect on yield strength， and can improve tensile strength slightly.

Key words: 2205 DSS(duplex stainless steel), ageing, σ phase, microstructure, mechanical property