The most widely produced stainless steels are either austenitic or ferritic in microstructure. At an atomic level, this equates to a different crystalline structure. Never thought of steels as crystalline? Well, depending upon the steels composition, how they are produced, heat treated and cooled then the atoms within the metal can align themselves in many different ways. This crystal structure fundamentally controls the properties of the metal.
So-called austenitic stainless steels share the same generic properties; good toughness and impact strength, easy to weld, can be work hardened, sensitive to stress corrosion cracking. Different types of austenitic stainless steels can provide additional properties, for instance Fermonic 50 (XM-19) has improved strength and corrosion resistance over 316L, but they are both considered easy to fabricate and with toughness across a wider range of operating temperatures.
Ferritic stainless steels have a different crystalline structure which gives a different set of performance traits compared with austenitic alloys; less tough but much more resistant to stress corrosion cracking, difficult to weld and less likely to work harden. In much the same way as austenitic stainless steels, there are multiple different grades, that through alloy content can provide varying levels of corrosion resistance at a cost.
Therefore, duplex stainless steels are a hybrid – nominally a 50:50 mix of austenitic and ferritic microstructures. This is achieved through two main mechanisms:
a) Composition. Of the individual elements that go into each specific grade, some tend to favour the creation of the austenitic structure whilst others favour the creation of the ferritic structure.
b) Thermal history. The processing temperatures and cooling conditions will control the exact proportion of each crystalline structure that forms.
The ‘duplex’ structure exhibits itself as ‘islands’ of austenite grains within a ‘matrix’ of ferrite. Maintaining both a good ration between the two, and a favourable size and distribution of the grains contributes to optimising mechanical properties and corrosion resistance.
As you may now be expecting, the microstructure of a duplex stainless steel manages to combine the favourable properties of either type – so generally acceptable toughness, higher strength, resistance to stress corrosion cracking and able to be fabricated and welded.
The most common duplex steel is 2205 (UNS S32205 / S31803 / F51 / F60 / SAF2205 / Sanmac 2205), based upon a 22% chromium content.
Finally, 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 the content of chromium, molybdenum and nitrogen all helps to increase resistance to pitting corrosion. Our unique Ferralium 255 (UNS S32550 / F61 / 1.4507) was the very first super duplex stainless steel – invented and patented by Langley Alloys – but was followed by UNS S32760 (UNS S32760 / F55 / 1.4501 / Zeron 100) and S32750 (UNS S32750 / F53 / 1.4410 / SAF2507).
In recent years, development work has sought to optimise the available grades further. ‘Lean’ duplex grades have a sightly lesser content of the more expensive alloying elements (typically chromium) without sacrificing performance. However, these are only commercially available as tubes which are produced to order. ‘Hyper’ duplex stainless steels have gone the other way, with chromium contents of 27-29% compared with the 22% of duplex and 25% of super duplex, leading to an increase in corrosion resistance. In this instance, they are proving difficult to manufacture in the wrought form (bars, plates) and are limited to seamless tubes.
For enquiries about duplex stainless steels from Langley Alloys Ltd, please contact us.
SAF2205 and Sanmac are registered trademarks of Sandvik Materials Technology.
Zeron 100 is a registered trademark of Rolled Alloys Inc.
Ferralium and Fermonic are registered trademarks of Langley Alloys Ltd.