What is the difference between duplex and super duplex stainless steels?

There are several key differences between Super Duplex Stainless Steels and Duplex Stainless Steels. Super Duplex stainless steels are based around an alloying addition of 25% chromium, whereas duplex stainless steels are based around an alloying addition of 22% chromium. By increasing the chromium content, the level of pitting corrosion resistance is also increased. However, there are other variables than these.

Duplex Stainless Steel

Duplex Stainless Steel is named as such because its metallurgical structure consists of two duel phases – austenitic (face-covered cubic lattice) and ferrite (body-centred cubic lattice). Unlike traditional austenitic stainless steels, super duplex stainless steels have a 20 – 28% higher chromium content, up to a 5% molybdenum content, 9% lower nickel, and only 0.05 – 0.5% Nitrogen.

Super Duplex Stainless Steel

The Pitting Resistance Equivalent Number (PREN) is calculated as PREN = %Cr + 3.3x %Mo + 16x %N, where Cr=chromium, Mo=molybdenum and N=nitrogen. By increasing the chromium content, the PREN increases from 34 to >40, indicating a superior resistance to pitting corrosion over a broad range of environments. Super duplex grades are the default choice for subsea and marine applications due to their recognised long life in seawater.

Cost-efficient use of Alloying Additions

Ordinarily, increases in chromium content would require the addition of expensive alloying elements such as nickel in order to retain the favourable balance of austenitic and ferritic microstructures. However, nitrogen serves this purpose in super duplex stainless steels, resulting in a very attractive combination of high strength, excellent corrosion resistance and competitive pricing against most other corrosion-resistant grades with far higher alloying contents.

Super Duplex stainless steels have a ‘duplex’ two-phase microstructure consisting of both austenitic and ferritic grains that give them a combination of attractive properties. In general, they are twice as strong as austenitic stainless steel. They achieve good toughness and ductility, somewhere between austenitic and ferritic grades.  One further advantage over austenitic stainless steels is their resistance to stress corrosion cracking.

So-called hyper duplex stainless steels are now being developed, based around a composition of 27% and 29% chromium, increasing the corrosion resistance further. However, they are more difficult to manufacture and only available in tube form, and rarely from stock.

Duplex and Super Duplex From Langley Alloys

Langley Alloys has been closely associated with super duplex stainless steels for more than 50 years, having developed and patented the original Ferralium 255 product (UNS S32550, F61, 1.4507). This grade still provides a higher yield strength than other super duplex grades, whilst its higher copper content of 2.0% ensures excellent resistance to corrosion from a range of acids including sulphuric acid, nitric acid and phosphoric acid. We now stock the most complete range including S32760 (F55, 1.4501), S32750 (F53, 1.4410, SAF2507) as well as the 22% Cr duplex grade Alloy 2205 (S32205, F51, 1.4462).

Our stock range extends to 40 different sizes of super duplex solid bar, from 0.5″ (12.7mm) to 16″ (406.4mm) in diameter. We also stock plate up to 3″ (76.2mm) thickness in a range of grades, plus seamless pipes.

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If you have any more questions about duplex and super duplex stainless steels, please contact us today.