In stainless steels, "duplex" means a hybrid microstructure - nominally a 50:50 mix of austenite and ferrite - engineered to combine the best properties of each. The result is generally good toughness, higher strength, resistance to stress corrosion cracking, and the ability to be fabricated and welded.
Austenitic and ferritic: the two building blocks
Most stainless steels are either austenitic or ferritic in microstructure. At an atomic level, this is a different crystalline structure. Depending on composition, and on how the steel is produced, heat treated and cooled, the atoms align in different ways, and this crystal structure fundamentally controls the properties of the metal.
Austenitic stainless steels share a generic set of properties: good toughness and impact strength, easy to weld, able to be work hardened, but sensitive to stress corrosion cracking. Different types add further properties; Fermonic® 50 (XM-19), for example, has improved strength and corrosion resistance over 316L, while both remain easy to fabricate with toughness across a wide temperature range.
Ferritic stainless steels have a different crystalline structure and a different set of traits: less tough, but much more resistant to stress corrosion cracking, difficult to weld and less likely to work harden. As with austenitics, alloy content sets the level of corrosion resistance at a cost.
How the duplex structure is achieved
A duplex stainless steel reaches its roughly 50:50 balance through two mechanisms:
- Composition. Some elements favour the austenitic structure, others the ferritic.
- Thermal history. Processing temperatures and cooling conditions control the exact proportion of each structure that forms.
The duplex structure appears as islands of austenite grains within a matrix of ferrite. Maintaining a good ratio between the two, along with a favourable grain size and distribution, optimises both mechanical properties and corrosion resistance.
The most common duplex steel is 2205 (UNS S32205 / S31803 / F51 / F60 / SAF2205 / Sanmac 2205), based on a 22% chromium content.
When is a duplex a super duplex?
Super duplex stainless steels are typically defined as grades with a PREN (pitting resistance equivalent number) greater than 40. Increasing chromium, molybdenum and nitrogen all help to increase resistance to pitting corrosion. Our Ferralium® 255 (UNS S32550 / F61 / 1.4507) was the very first super duplex stainless steel, invented and patented by Langley Alloys. It was followed by S32760 (UNS S32760 / F55 / 1.4501 / Zeron 100) and S32750 (UNS S32750 / F53 / 1.4410 / SAF2507).
Lean and hyper duplex
Recent development work has sought to optimise the available grades further. Lean duplex grades carry a slightly lower content of the more expensive alloying elements (typically chromium) without sacrificing performance, but are only commercially available as tubes produced to order. Hyper duplex stainless steels go the other way, with chromium contents of 27-29% compared with 22% for duplex and 25% for super duplex, increasing corrosion resistance. These are proving difficult to manufacture in wrought form (bars, plates) and are limited to seamless tubes.
| Grade | UNS | DIN |
|---|---|---|
| Ferralium® 255 | S32550 | 1.4507 |
| S32760 (Zeron 100) | S32760 | 1.4501 |
| S32750 (SAF2507) | S32750 | 1.4410 |