A category of stainless steel with high amounts of chromium and moderate nickel content. The duplex class is so named because it is a mixture of austenitic (chromium-nickel stainless class) and ferritic (plain chromium stainless category) structures. This combination was originated to offer more strength than either of those stainless steels. Duplex stainless steels provide high resistance to stress corrosion cracking (formation of cracks caused by a combination of corrosion and stress) and are suitable for heat exchangers, desalination plants, and marine applications. 

Superior corrosion resistance than a 316.  Good Resistance to stress corrosion cracking in a chloride environment. 

Duplex materials have improved over the last decade; further additions of Nitrogen have been made improving weldability. 

Typically twice the yield of austenitic stainless steels.  Minimum Specified UTS typically 680 to 750N/mm2 (98.6 to 108ksi).  Elongation typically > 25%.

Because of the complex nature of this material it is important that it is sourced from good quality steel mills and is properly solution annealed.  Castings and possibly thick sections may not cool fast when annealed causing sigma and other deleterious phases to form. 

The material work hardens if cold formed; even the strain produced from welding can work harden the material particularly in multi pass welding.  Therefore a full solution anneal is advantageous, particularly if low service temperatures are foreseen.