What is meant by duplex stainless steel?

A duplex stainless steel is named because it has a ‘duplex’ microstructure, meaning a balance of austenitic and ferritic grains. This means that duplex stainless steels combine physical and mechanical properties of both, making them an attractive and cost-effective proposition.

Austenitic stainless steels generally provide excellent corrosion resistance, good mechanical properties over a wide range of operating temperatures, including low temperature toughness, and are non-magnetic. Ferritic stainless steels are generally of lower corrosion resistance, and lower cost, but do resist stress corrosion cracking. Duplex stainless steels combine many of these properties to achieve high strength and excellent corrosion resistance, remain highly cost-effective and still resist stress corrosion cracking.

The first duplex stainless steels were based around 22% chromium content, which provides significantly better corrosion resistance compared with a more standard Alloy 316L austenitic stainless steel. In addition, the strength is up to twice that of 316L as well. The original S31803 duplex stainless steel has been improved, with S32205 having a more tightly controlled composition, resulting in better corrosion resistance.

Super duplex stainless steels have a higher content of alloying elements, such as chromium, nickel, and molybdenum, compared to duplex stainless steels. They are based upon 25% chromium content. As such, super duplex stainless steels exhibit even greater corrosion resistance and higher strength than duplex stainless steels, making them suitable for more demanding environments.

The very first commercially produced super duplex stainless steel was Ferralium 255 (S32550, 1.4507), which was developed by Langley Alloys. It contains up to 2% copper to support increased resistance to sulphuric acid and is still the only super duplex stainless steel to achieve a minimum yield strength of 85ksi (586N/mm2). Other grades include S32750 (1.4410) and S32760 (1.4501).

More recently, hyper duplex stainless steels have been developed. With 27% chromium (S32707) or 32% chromium (S33207), the strength and corrosion resistance increase even further. However, they are challenging to manufacture, and so limited to seamless pipes.

Langley Alloys carries the most complete range of duplex and super duplex stainless steels, in up to 40 different sizes, and in quantities sufficient to meet the largest of projects.

Ferralium 255 (S32550) is available only from Langley Alloys, as round bar from ½”-14” (12.7-355.6mm), plate up to 3” (76.2mm), pipe, flanges and fittings.

Langley Alloys stocks S32205 (2205) duplex stainless steels as round bars from 5/8”-20” diameter (15.87-508mm), plate up to 3” (76.2mm) thickness, as well as pipes in scheduled sizes, flanges and fittings. S32750 and S32760 are also available in a similar size range ex-stock.

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