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.
BÖHLER A903 (UNS S32205) is the most common stainless ferritic-austenitic Cr-Ni-Mo steel with nitrogen addition. In addition to good strength properties, this steel offers high corrosion resistance, especially against stress corrosion cracking in chloride-containing solutions, and is resistant to intergranular corrosion up to 300°C. The alloy should not be used at temperatures above 300°C due to embrittlement. Heat treatment after welding is not necessary. Required surface finish: pickled, scale-free heat treated or machined. Commonly used in the oil and gas industry, hydroelectric power, pressure vessels, pulp and paper industry, components and chemical tanks, such as parts for separators and heat exchangers and parts in the paper industry, oil and gas extraction , compressors, seawater desalination.
Read MoreBÖHLER A911SA is a ferritic-austenitic stainless steel of Type 25%Cr with PREN of min. 40. It offers excellent corrosion resistance against general corrosion, stress corrosion cracking, corrosion fatigue, pitting, crevice and erosion corrosion and possessing very high mechanical strength properties. However, it contains less copper compared to other super Duplex SS but a deliberate addition of tungsten instead, claimed to selectively improve corrosion resistance. Good weldability, post weld heat treatment not necessary. Max. temperature for long periods of service: 280°C (535°F ) / (300°C/570°F possible for short periods). Surface condition for optimum corrosion resistance: pickled or machined. Components in offshore, waste water, sea water desalination and chemical plants with aggressive chloride-containing media e.g. heat exchangers, separator parts, compressor and pump components, turbine blades.
Read MoreBohler A913 (UNS S32750) is the most widely used super duplex grade on the market and is a 25%Cr ferritic-austenitic stainless steel with PREN min 41. The steel offers highest corrosion resistance and good strength properties and is especially suitable for use in aggressive environments containing chlorides. Good weldability, heat treatment after welding is not required. Excellent resistance to general corrosion, stress corrosion cracking, vibration cracking, pitting and crevice corrosion as well as erosion corrosion. Max. operating temperature for long-term use: 280°C (short-term exceeding up to 300°C permissible). Required surface finish: pickled or machined. Components exposed to seawater such as heat exchangers, feed or injection pumps, propeller shafts, highly stressed parts in chemical and waste water plants and for oil and gas production (e.g. manifolds), separators, turbine and fan blades, low-pressure compressor components.
Read MoreBÖHLER A930 is a stainless ferritic-austenitic steel of the 25%Cr type. Highest corrosion resistance and good strength properties. Good weldability, heat treatment after welding is not required. Excellent resistance to general corrosion, stress corrosion cracking, vibration cracking, pitting and crevice corrosion as well as erosion corrosion. Max. operating temperature for long-term use: 280°C (short-term exceeding up to 300°C permissible). Required surface finish:pickled or machined. BÖHLER A930 (UNS S32550, 1.4507, F61) is the original super duplex stainless steel. As such, it was the first alloy to be called “super duplex” and is based on a chromium content of 25 %. Compared to the later alternatives UNS S32750 and UNS S 32760, it is the only grade with an increased copper content for better pitting corrosion resistance. Components in offshore, waste water, seawater desalination and chemical plants with aggressive chloride-containing media, e.g. heat exchangers, separator parts, compressor and pump components, turbine blades.
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