The ferritic-austenitic microstructure of duplex steels is achieved by balancing the alloying elements Chromium, Nickel, Molybdenum, Manganese and Nitrogen and results in higher yield strengths and improved resistance to stress corrosion cracking. Duplex steels are used in a variety of applications under highly corrosive conditions in a wide range of industries, e.g. Oil/Gas & CPI.
Stainless duplex steels have a structure consisting of around 50 % ferritic and around 50 % austenitic microstructure. In order to achieve this mixed structure, ferrite formers (Cr, Mo, Si, W…) and austenite formers (Ni, N, Mn) are brought into a balanced relationship with one another. The ferrite-austenite balance is determined not only by the alloy composition, but also by the heat treatment.
Duplex stainless steels are categorised according to their corrosion resistance, which in turn depends on the alloying elements. The most common indicator for this is the Pitting Resistance Equivalent Number (PREN) calculated according to the formula %Cr + 3.3 %Mo + 16-30 %N.
Lean duplex grades (such as EN 1.4362/UNS S32304) have a PREN value of 22 to 27, a comparatively low nickel content and are not or only slightly alloyed with molybdenum. They are best suited for less aggressive conditions.
Standard duplex steels (the most common representative is grade EN 1.4462/ UNS S32205) have a PREN value between 28 and 38, contain 22 % chromium and 3 % molybdenum. They cover the medium range of corrosion resistance.
Super duplex steels (e.g. grade EN 1.4507/UNS S32520) have a PREN value between 39 and 45 and are alloyed with 25 % chromium, 3.5 % molybdenum and 0.22 % to 0.3 % nitrogen. Hyper-duplex steels have a PREN value > 45. Super and hyper-duplex steels are intended for particularly aggressive conditions, usually in the oil and gas industry.
Duplex steels usually have twice the yield strength of standard austenitic grades and are magnetisable in proportion to their ferrite content. Compared to standard austenitic grades, duplex steels are generally more resistant to stress corrosion cracking due to their lower Ni content.
Due to the strong tendency to precipitate and the associated embrittlement, the continuous service temperature of all duplex steels is limited to approx. max. 300°C and therefore size/wall thickness restrictions must be taken into account during production.