What is Composition of Duplex Stainless Steel - Definition

In metallurgy, stainless steel is a steel alloy with at least 10.5% chromium with or without other alloying elements and a maximum of 1.2% carbon by mass. Stainless steels, also known as inox steels or inox from French inoxydable (inoxidizable), are steel alloys, which are very well known for their corrosion resistance, which increases with increasing chromium content. Corrosion resistance may also be enhanced by nickel and molybdenum additions. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 10.5% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties.

Duplex Stainless Steel

Duplex stainless steels, as their name indicates, are a combination of two of the main alloy types. They have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Their corrosion resistance is similar to their austenitic counterparts, but their stress-corrosion resistance (especially to chloride stress corrosion cracking), tensile strength, and yield strengths (roughly twice the yield strength of austenitic stainless steels) are generally superior to that of the austenitic grades.

Composition of Duplex Stainless Steel

In duplex stainless steels, carbon is kept to very low levels (C<0.03%). Chromium content ranges from 21.00 to 26.00%, nickel content ranges from 3.50 to 8.00% and these alloys may contain molybdenum (up to 4.50%). Toughness and ductility generally fall between those of the austenitic and ferritic grades. Duplex grades usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex and superduplex. Superduplex steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. Common uses are in marine applications, petrochemical plant, desalination plant, heat exchangers and papermaking industry. Today, the oil and gas industry is the largest user and has pushed for more corrosion resistant grades, leading to the development of superduplex steels.

The resistance of stainless steels to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 10.5% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties.

Duplex Stainless Steels – SAF 2205 – 1.4462

A common duplex stainless steel is SAF 2205 (a Sandvik-owned trademark for a 22Cr duplex (ferritic-austenitic) stainless steel), which contains typically 22% chromium and 5% nickel. It has excellent corrosion resistance and high strength, 2205 is the most widely used duplex stainless steel. Applications of SAF 2205 are in the following industries:

Transport, storage and chemical processing
Processing equipment
High chloride and marine environments
Oil and gas exploration
Paper machines

References:

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Duplex Stainless Steel

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