Duplex steel

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Duplex steel is a steel with a two-phase structure, which consists of a ferrite (α-iron) matrix with islands of austenite .

Metallurgical basis

This structure formation is achieved by alloying the steel with ferrite formers such as chromium (Cr) and austenite formers such as nickel (Ni), nitrogen (N), manganese (Mn), whereby the content of austenite formers is adjusted so that the entire structure is not added Room temperature can become austenitic (for example less than 8% Ni). After hot forming , a special heat treatment ensures that the austenite formers are concentrated in individual areas, which are then stable austenitic down to room temperature .

Material properties

Mechanical properties

Duplex steels combine the properties of stainless chromium steels ( ferritic or martensitic ) and stainless chromium-nickel steels (austenitic). They have higher strengths than stainless chrome-nickel steels, but are more ductile than stainless chrome steels. Case of alternating force, they show up to an austenite of about 40% as opposed to pure austenitic nor fatigue strength . In contrast to ferritic or even martensitic stainless steels, the progression of the impact work (A v -T diagram) in duplex steels does not have a steep drop in energy absorption, i.e. there is an upper and a lower position, but the transition is above stretched over a wide temperature range. The lower the test temperature, the lower the energy absorption, as the material becomes brittle .

Corrosion properties

The duplex steels are among the rust and acid-resistant steels (DIN-EN 10088 T1 to T3). Some duplex stainless steels have been developed since the 1970s. A nitrogen-alloyed variant, which is listed under the designation 1.4462 (X2CrNiMoN 22-5-3) according to DIN EN 10088/2, is widespread today. Compared with corrosion-resistant, purely austenitic steels, duplex materials contain less nickel (around 4 to 8%), but usually more chromium. Because of the nickel content, which is very important for the mechanical properties, but has an unfavorable effect on the PREN index (Pitting Resistance Equivalent Number), they are at risk for intergranular corrosion . To counteract this, nitrogen is sometimes added as an austenite former in exchange for a certain proportion of nickel. Another possibility to further improve the corrosion resistance is the addition of molybdenum .

An important advantage with regard to the corrosion properties results from the two-phase nature of the structure. At the phase boundaries, especially at the transition from ferrite to the more ductile austenite, cracks such as those caused by stress corrosion cracking come to a standstill for reasons of fracture mechanics .

Risk of confusion

Duplex steel and dual-phase steel differ on the one hand in the volume proportions of the structural phases and on the other in the structure composition. Duplex steels have a ferritic-austenitic structure, whereas dual-phase steels have a ferritic-martensitic structure. In duplex steels, the volume proportions of ferritic and austenitic structure are the same, dual-phase steels consist of around 80–90% ferrite and only around 10–20% of martensite or other harder phases.

Nickel-reduced duplex steel

In 2005 it was possible to develop a reliable duplex steel with a reduced nickel content of 1% instead of 6% nickel. Because nickel is an expensive raw material, it can save costs.

See also

Web links

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  1. ↑ Murat Mola in: Numerical alloy development of nickel-reduced ferritic-austenitic duplex steels , European University Press ; January 2005