Duplex Steel

Duplex stainless steels typically contain 50% austenite and 50% ferrite (Figure 9a-c). The two-phase mixture also leads to a marked refinement in the grain size of both the austenite and ferrite. This, together with the presence of ferrite, makes the material about twice as strong as common austenitic steels.

They contain only about half the nickel concentration of typical austenitic stainless steels; they are therefore less expensive and less sensitive to the price of nickel.

With their high chromium concentration, they have excellent pitting and crevice corrosion resistance, and to chloride stress corrosion. The two phase mixture also reduces the risk of intergranular attack; for the same reason, they are not prone to solidifcation cracking during welding.

Chemical Composition of Duplex Steel

In general, the toughness of stainless steels increases in the order ferritic, duplex and austenitic stainless steels. Duplex stainless steels, because of their high Cr concentration, are prone to the 475°C embrittlement so their application is frequently confined to temperatures below about 300°C.

The superduplex stainless steels have a higher chromium and molybdenum concentration to enhance pitting corrosion resistance; these ferrite stabilising elements are balanced using a higher nickel and nitrogen concentrations (austenite stabilisers) in order to maintain about equal amounts of ferrite and austenite. One definition is that a superduplex stainless steel must have a pitting index which is greater than 40.