A duplex stainless steel is one with a "duplex" microstructure - a balance of austenitic and ferritic grains. That balance combines the physical and mechanical properties of both phases, making duplex grades an attractive and cost-effective choice.
Austenitic stainless steels generally offer excellent corrosion resistance, good mechanical properties across a wide temperature range including low-temperature toughness, and are non-magnetic. Ferritic stainless steels are generally lower in corrosion resistance and cost, but resist stress corrosion cracking. Duplex grades combine these traits to deliver high strength and excellent corrosion resistance while staying cost-effective and resistant to stress corrosion cracking.
The first duplex grades were based on 22% chromium, giving significantly better corrosion resistance than a standard Alloy 316L austenitic stainless steel, with up to twice the strength. The original S31803 duplex stainless steel has since been refined into S32205, whose more tightly controlled composition improves corrosion resistance.
Duplex and super duplex
Super duplex stainless steels carry a higher content of alloying elements such as chromium, nickel and molybdenum, and are based on 25% chromium. This gives them even greater corrosion resistance and higher strength, suiting them to more demanding environments.
The first commercially produced super duplex stainless steel was Ferralium® 255 (S32550, 1.4507), developed by Langley Alloys. It contains up to 2% copper for increased resistance to sulphuric acid, and remains 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), strength and corrosion resistance increase further still, though they are challenging to manufacture and so limited to seamless pipe.
Our duplex range
We carry one of the most complete ranges of duplex and super duplex stainless steels, in up to 40 different sizes and in quantities sufficient for the largest projects.
Ferralium® 255 (S32550) is available only from Langley Alloys, as round bar from 1/2"-14" (12.7-355.6mm), plate up to 3" (76.2mm), pipe, flanges and fittings.
We stock S32205 (2205) as round bar from 5/8"-20" diameter (15.87-508mm) and plate up to 3" (76.2mm) thickness, along with pipe in scheduled sizes, flanges and fittings. S32750 and S32760 are available ex-stock in a similar size range.