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What is duplex steel?21st May 2020
A duplex steel is one that has a mixture of two different metallurgical phases within its microstructure. Although many different grades of steel possess two or more different phases, the term duplex is used almost exclusively for a family of stainless steels that combine austenitic and ferritic phases.
By combining two different phases it is possible to achieve far better properties than would otherwise be achieved in a purely austenitic or ferritic stainless steel. Although there are compromises too, the overall gain in strength, corrosion resistance and cost-effectiveness make duplex stainless steels incredibly interesting to product designers and purchasing managers.
Austenitic stainless steels generally have good corrosion resistance, good impact toughness and are considered straightforward to fabricate and weld. More highly alloyed austenitic stainless steels are available, such as XM-19 and 254SMO, with improved corrosion resistance due to their added chromium and molybdenum. However, in order to retain the required austenitic microstructure, it is necessary to increase the nickel content too, which helps to raise their cost.
Ferritic stainless steels are resistant to stress corrosion cracking, have good strength but the more commonly used grades are less alloyed and therefore less corrosion resistant.
Duplex steels were developed to exploit the desirable properties of both austenitic and stainless steels, namely high strength; excellent corrosion resistance; resistance to stress corrosion cracking and value for money compared with more expensive super austenitic stainless steels and nickel-based alloys. Although nominally a 50:50 mix, in practice, the ferrite content can vary between 35-55% and still achieve acceptable properties. The microstructure appears as grains or islands, of austenite within a matrix of ferrite.
The only limitations of duplex steel grades are their range of operating temperatures and their ease of welding. In common with ferritic steels, low-temperature impact toughness is restricted, so applications below -50degC are not common. Also, there is a risk of impairing the corrosion resistance and impact toughness if exposed long-term to operating temperatures above 250degC.
Duplex stainless steels were first developed in the 1930s. Based around a 22% chromium content, they achieve pitting resistance equivalent number (PREN) of greater than 34, which is a significant uplift from Alloy 316L with a PREN of just 25. As such, it can be used in more aggressive industrial and marine environments. With approximately twice the yield strength of more standard grades, there is the potential to reduce the size of components for the same application, saving metal and saving money. Langley Alloys stocks Sanmac 2205 (1.4462, F51, F60), Sandvik’s product with ‘enhanced machinability as standard’. Our stock of UNS S32205 is dual-certified to meet UNS S31803, from ¾” (19.05mm) to 16” (406.4mm).
Super duplex stainless steels were developed by Langley Alloys. The original super duplex is Ferralium 255 (UNS S32550, F61, 1.4507), and is still the highest strength grade available, with a minimum yield strength of 85ksi (586N/mm2) compared with 80ksi (550N/mm2) for subsequently developed grades. It also provides superior corrosion resistance, particularly in acidic solutions due to the addition of 2% copper in its composition. Super duplex grades are based around a composition of 25% chromium, and also exploit nitrogen in order to push up the PREN to greater than 40. Super duplex stainless steels are the default choice for applications in seawater, where they have been proven to provide long-term performance. As well as Ferralium 255, Langley Alloys also stock UNS S32750 (SAF2507, F53, 1.4410) and UNS S32760 (Zeron 100, F55, 1.4501) in solid bars from ¾” (19.05mm) to 16” (406.2mm).
Hyper duplex stainless steels are being developed with even higher chromium content, such as 27% and 32%. At this level, they achieve a PREN greater than 50, and so rival far more expensive super austenitic and nickel-based alloys.
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