Duplex, Super Duplex and Hyper Duplex

The origins of duplex stainless steels trace back to the 1930s. Aggressive process environments in pulp and paper mills in Sweden spurred the development of more corrosion-resistant grades by local manufacturer Avesta, where higher temperatures and pressures with aggressive sulphite solutions proved onerous.

With PREN values (pitting resistance equivalent number) now an accepted indication of general pitting resistance, it is clear that increasing chromium improves performance. Duplex stainless steels were established around a chromium content of about 22%, compared with 16-18% for standard austenitic stainless steels.

As their name suggests, duplex grades have two distinct microstructural phases, austenite and ferrite. Each is a type of crystal structure that informs its properties. In duplex, 'islands' of ferrite form within a matrix of austenite, so the grade takes the best of both: higher strength, good toughness, relative ease of fabrication, good corrosion resistance and resistance to stress corrosion cracking. The key is to maintain a near 50:50 balance, achieved through composition and process conditions.

The Schaeffler diagram is interesting even for non-metallurgists. A number of elements act like chromium, encouraging a ferritic microstructure, while others act like nickel and encourage austenite. By calculating chromium and nickel 'equivalents', it is possible to predict the final microstructure. The progression from duplex to super duplex to hyper duplex then makes more sense: chromium (and chromium equivalent) increases, raising pitting resistance, while nickel (and nickel equivalent) must also increase to maintain the right balance of microstructure and properties.

Although duplex stainless steels have existed in some form since the 1930s, their uptake only improved in the 1970s. Better steelmaking allowed tighter control of composition, particularly nitrogen, and higher nitrogen improves pitting resistance cost-effectively compared with more expensive elements. Improved understanding of weldability, maintaining the duplex microstructure through the heat-affected zone, meant it could be used without the risk of premature failure from poor fabrication.

At around the same time, super duplex stainless steels were invented. Led initially by Langley Alloys' Ferralium^® 255 in 1969, it was followed by Zeron 100 (UNS S32760) in 1980 and SAF2507 (UNS S32750) in 1988. All three are based on a 25% chromium content and are now formulated to achieve a minimum PREN of 40, a requirement of the NORSOK standard (M-001).

Hyper duplex alloys are now being developed as potential alternatives to more expensive nickel alloys. Alleima developed its SAF 2707 HD grade, pushing chromium beyond 27% and PREN to about 48. This has been exceeded by SAF 3207 HD, with a chromium level of 32% and a PREN of about 50. These changes bring an equivalent increase in yield strength, over 100ksi, compared with 80ksi for S32750/S32760 and 85ksi for Ferralium^® 255-SD50.

To date, these highly-alloyed grades have only been available commercially as extruded seamless tubes, typically for umbilicals, heat exchangers, condensers and seawater coolers. Producing forged bar and rolled plate has proven more difficult, with harder processing, lower yield and manufacturing defects. Matching the corrosion performance of certain nickel alloys is possible, and offering a cost-effective alternative is viable, particularly if nickel prices rise in level or volatility.

Duplex stainless steels:

• Sanmac 2205 - Alloy 2205, UNS S32205, DIN 1.4462, F60, UNS S31803, F51

Super duplex stainless steels:

• Ferralium^® 255-SD50 - Alloy 255, UNS S32550, DIN 1.4507, F61
• SAF2507 - Alloy 32750, UNS S32750, DIN 1.4410, F53
• S32760 - Alloy 32760, UNS S32760, DIN 1.4501, F55, Zeron 100

Hyper duplex stainless steels:

• Alleima SAF 2707 HD - UNS S32707, DIN 1.4658
• Alleima SAF 3207 HD - UNS S33207

Quick comparison