SAF Duplex 2205 Stainless Steel (UNS S32205 S31803) Alloy Material SAF Duplex 2205 stainless steel (UNS S32205 formerly known as UNS S31803 and commonly known as "alloy 2205") has a mixed microstructure of austenite and ferrite, and is a two-phase alloy based on the Fe-Cr-Ni series.