Duplex steel stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. Duplex grades are characterized into groups based on their alloy content and corrosion resistance.
Standard duplex is 22% chromium with UNS S31803/S32205 known as 2205 being the most widely used.
Super duplex is by definition a duplex stainless steel with a Pitting Resistance Equivalent Number (PREN) > 40, where PREN = %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N. Usually super duplex grades have 25% chromium or more and some common examples are S32760 (Zeron 100 via Rolled Alloys), S32750 (2507) and S32550 (Ferralium),.
