Duplex stainless steels are alloys with metallurgical structures generally regarded as being approximately equal parts of austenite and ferrite, with a 60/40, 40/60-phase distribution being the envelope widely considered as acceptable. The combination of austenite/ferrite produces alloys with twice the strength of conventional austenitic stainless steels.
Duplex stainless steels are virtually immune to stress corrosion cracking (the Achilles’ heel of common austenitic stainless steels) and are highly resistant to pitting and crevice corrosion. Possessing these characteristics, it is not surprising to find the majority of, but by no means all, applications to be seawater related. Duplex stainless steels have many uses in offshore oil and gas production and naval equipment, particularly sub-surface.