Duplex stainless steel offers improved resistance to stress corrosion cracking Duplex stainless steels S31803 1.4462 F51. are a family of stainless steel. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body-centered cubic lattice) in roughly equal proportions. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316. The main difference in composition, when compared with austenitic stainless steel is that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc., and in the petrochemical industry in the form of pipelines and pressure vessels. In addition to the improved corrosion resistance compared with the 300 series stainless steels duplex steel also has higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280 N/mm2, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450 N/mm2, and a super duplex grade a minimum of 550 N/mm2.