Duplex stainless steels have a structure consisting of about 50 % ferritic and about 50 % austenitic microstructure. To achieve this mixed structure, ferrite formers (Cr, Mo, Si, W…) and austenite formers (Ni, N, Mn) are brought into a balanced ratio. The ferrite-austenite balance is not only determined by the alloy composition, but also by the heat treatment. Duplex stainless steels are classified according to their corrosion resistance, which in turn depends on the alloying elements. The most common key figure for this is the Pitting Resistance Equivalent Number (PREN). The PREN value is calculated according to the formula Cr + 3.3 Mo + 16-30 N.
Categories of duplex steels
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 EN 1.4462/ UNS S32205) have a PREN value between 28 and 38, contain 22 % chromium and 3 % molybdenum. They cover the middle range of corrosion resistance.
– Super duplex steels (e.g. the grade EN 1.4507/UNS S32520) have a PREN value in the range of 39 to 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 and are intended for particularly aggressive conditions, mostly in the oil and gas industry.
Duplex steels usually have a yield strength twice as high as standard austenitic grades and are magnetisable in proportion to the ferrite content. Compared to the standard austenitic grades, duplex steels have a higher resistance to stress corrosion cracking, mainly due to the lower Ni content.
Due to the strong tendency to precipitate and the associated embrittlement, the permanent operating temperature of duplex steels is limited to approx. max. 300°C and size/wall thickness restrictions must therefore be taken into account during production.
