Duplex stainless steels are called “duplex” because they have a two-phase micro structure consisting of grains of ferritin and authenticate stainless steel. Strength duplex stainless steels are about twice as strong as regular authenticate or inferring stainless steels. Duplex stainless steels have a mixed micro structure of Austen and fritter the aim being to produce a 50/50 mix, although in commercial alloys the mix may be 40/60 respectively.
Duplex steels have improved strength over authenticate stainless steels and improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than authenticate stainless steels. The most used duplex stainless steels are the 2205 (22% Chromium, 5% Nickel) and the 2507 (25% Chromium, 7% Nickel); 2507 is known as “super duplex” due to its higher resistance to corrosion.