Duplex Stainless Steel
Duplex stainless steel is referred to as a steel which has a two-phase structure consisting of a ferrite ( δ -iron ) with matrix islands of austenite ( γ - iron).
Metallurgical basis
This structure formation is achieved by placing the steel Ferritbildnern (e.g., Cr), and austenite formers (such as Ni, N, Mn, ...) alloy, wherein the amount of austenite is set so that not all of the structure is austenitic at room temperature may (for example Ni < 8%). After hot forming is provided by a special heat treatment that the austenite concentrate in certain areas, the way down to room temperature are then stable austenitic.
Material properties
Mechanical properties
Duplex steels are characterized by their combination of properties that represent a mixture of the properties of stainless chromium steels ( ferritic or martensitic ) and stainless chromium -nickel steels (austenitic ). They have higher strengths than the stainless chromium -nickel steels, but in this case have a higher ductility than chromium stainless steels. Their behavior under varying stress has up to one austenite of about 40 % in contrast to pure austenitic still a fatigue strength. The resulting curve from the impact test ( also known as AVT - curve) has for the duplex in contrast to ferritic or even martensitic stainless steels no steep drop in energy absorption, that is, although there is a high and a low position, the energy absorption is, however, while over a wide temperature range on a high level. The lower the test temperature, the lower the energy absorption of the material becomes brittle because more and more.
Corrosion properties
The duplex stainless steels are among the rust-and acid -resistant steels ( DIN EN 10088 T1 to T3). Since the 1970s, some duplex stainless steels have been developed. Widely today is a nitrogen-alloyed variant that is carried under the name 1.4462 ( X2CrNiMoN 22-5-3 ) according to DIN EN 10088/2. Containing duplex grades, compared with corrosion-resistant pure austenitic steels, nickel less (about 4-8 %), but usually a much higher chromium content. Due to the nickel content, which is very important for the mechanical properties, but has an unfavorable effect on the Pren - index, they are at risk of intergranular corrosion. In order to counteract this, some nitrogen is added as an austenite in exchange for a certain amount of nickel. Another way to improve the corrosion resistance, the addition of molybdenum.
An important advantage with respect to the corrosion properties in turn results from the two-phase nature of the microstructure. At the phase boundaries, especially in the transition from ferrite to austenite ductile, come cracks as they arise in the stress corrosion cracking, fracture mechanics of reasons to stop.
Likelihood of confusion
The difference from the dual-phase steel the one hand the volume fractions of the structural phases, on the other hand, the structural composition. The duplex stainless steels there is a ferritic- austenitic microstructure, while dual-phase steels exhibit a ferritic- martensitic microstructure. In addition, duplex steels consist of the same volume fractions of ferritic and austenitic structure, while dual-phase steels consist of about 80-90 % of ferrite and only about 10-20% of martensite or other harder phases.
Reduced nickel duplex stainless steel
2005 was the first time to develop a reduced to 1/6 nickel content reliable duplex steel. Because nickel is an expensive raw material cost savings can be achieved thereby.