Duplex steel is a kind of steel with a two-phase molecular structure consisting of a ferrite (δ-iron) matrix with austenitic (γ-iron) "islands".

This structure is achieved by alloying the steel with ferrite-forming elements (e.g. Cr) and austenite-forming elements (e.g. Ni, N, Mn, …), where the percentage of austenite-forming elements is controlled in a way that prevents the entire microstructure becoming austenitic at room temperature (e.g. Ni < 8%). After hot forming, a special heat treatment is applied to ensure that austenite-forming elements concentrate in certain areas and these remain austenitically stable down to room temperature.

Mechanical properties
Duplex steels are characterised by their combinations of properties that are a blend of the characteristics of stainless chromium steels (ferritic or martensitic) and those of stainless chrome-nickel steels (austenitic). They have greater strength than stainless chrome-nickel steels and have better ductility than stainless chromium steels. When subjected to varying stresses, they have fatigue strength up to an austenitic percentage of approx. 40%, compared with purely austenitic steels. The results curve from the notched bar impact test (also referred to as the temperature-absorbed energy curve) shows there is no curve inversion in the energy absorption curve in duplex steels, in contrast to ferritic and even martensitic stainless steels, which means that, although there is a peak and a trough, energy absorption remains at a high level across a broad temperature range. The lower the testing temperature, the lower the energy absorption, since the material becomes more and more brittle.

Corrosion characteristics
Duplex steels are among the most rust and acid resistant steels (DIN EN 10088 1 to 3). Since the 1970s, several stainless duplex steels have been developed. One very common one is a nitrogen-alloyed version, known as 1.4462 (X2CrNiMoN 22-5-3) to DIN EN 10088/2. Compared with purely austenitic corrosion resistant steels, duplex materials contain less nickel (approx. 4% to 8%), but usually have a significantly higher chromium content. As a result of the nickel content, which is an important contributor to mechanical properties but has a negative effect on the PREN number, these steels are liable to intergranular corrosion (IGC). To counteract this, nitrogen is added in some cases as an austenite-forming element as a replacement for a certain proportion of nickel. Another method of improving corrosion resistance is to add molybdenum.
The fact that the molecular structure consists of two phases is an important advantage regarding corrosion properties. Any cracks, such as stress-corrosion cracking, will stop at the phase boundaries for reasons governed by the mechanics of cracking, in particular at the transition from ferrite to the more ductile austenite.

Not to be confused
The difference between this and dual-phase steel is, on the one hand, the volume fraction of the structural phases and, on the other hand, the composition of the structure. In duplex steels, there is a ferritic-austenitic microstructure, while dual-phase steels have a ferritic-martensitic microstructure. In addition, duplex steels consist of equal volume fractions of ferritic and austenitic microstructures, while dual-phase steels consist of approx. 80–90% ferrite and only around 10–20% martensite or other harder phases.

Super duplex

Stainless ferritic-austenitic steel. Very high corrosion resistance and good strength properties. Good weldability, heat treatment after welding is not necessary. Outstanding resistance to general corrosion, stress cracking corrosion, corrosion fatigue, pitting corrosion, crevice corrosion and erosion corrosion. Ambient temperature for long-term stresses max. 280 °C (momentary transgression up to 300 °C is permitted).
Necessary surface condition: pickled or machined.

Components liable to contact with seawater, such as heat exchangers, feed pumps and injection pumps, prop shafts, heavy-duty parts in chemicals and waste water plants and for crude oil and natural gas conveyance (e.g. distributors), separators, turbines and compressor blades, low-pressure compressor components.