A duplex steel has a mixture of two different metallurgical phases in its microstructure. Many steels contain two or more phases, but the term duplex is used almost exclusively for a family of stainless steels that combine austenitic and ferritic phases.
Combining two phases gives far better properties than a purely austenitic or ferritic stainless steel can. There are compromises, but the overall gain in strength, corrosion resistance and cost-effectiveness makes duplex stainless steels genuinely useful to designers and buyers.
The two phases
Austenitic stainless steels generally have good corrosion resistance and impact toughness and are straightforward to fabricate and weld. More highly alloyed austenitic grades, such as XM-19 and 254SMO, add chromium and molybdenum for better corrosion resistance, but they also need more nickel to hold the austenitic structure, which raises their cost.
Ferritic stainless steels resist stress corrosion cracking and have good strength, but the commonly used grades are less alloyed and so less corrosion resistant.
Why duplex works
Duplex steels were developed to exploit the best of both: high strength; excellent corrosion resistance; resistance to stress corrosion cracking; and value for money against more expensive super austenitic stainless steels and nickel-based alloys. The mix is nominally 50:50, but in practice the ferrite content can vary between 35-55% and still deliver acceptable properties. The microstructure appears as grains, or islands, of austenite within a matrix of ferrite.
The main limitations are operating temperature range and ease of welding. Like ferritic steels, duplex grades have restricted low-temperature impact toughness, so applications below -50degC are uncommon. Long-term exposure above 250degC risks impairing both corrosion resistance and impact toughness.
Duplex grades
Duplex stainless steels were first developed in the 1930s. Based on a 22% chromium content, they achieve a pitting resistance equivalent number (PREN) greater than 34 - a significant uplift from Alloy 316L, at just 25 - so they suit more aggressive industrial and marine environments. With roughly twice the yield strength of standard grades, components can often be made smaller for the same duty, saving metal and money. We stock Sanmac® 2205 (1.4462, F51, F60), Alleima'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 grades
Super duplex stainless steels were developed by Langley Alloys. The original is Ferralium® 255 (UNS S32550, F61, 1.4507), still the highest strength grade available, with a minimum yield strength of 85ksi (586N/mm2) against 80ksi (550N/mm2) for grades developed later. It also offers superior corrosion resistance, particularly in acidic solutions, thanks to 2% copper in its composition. Super duplex grades are based on about 25% chromium and use nitrogen to push the PREN above 40. They are the default choice for seawater applications, where they have proven long-term performance. Alongside Ferralium® 255, we 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 grades
Hyper duplex stainless steels are being developed with even higher chromium content, such as 27% and 32%. At that level they reach a PREN greater than 50, rivalling far more expensive super austenitic and nickel-based alloys.