Duplex steels – materials for the most demanding applications

The growing demand for energy in the markets necessitates increased production, alongside the development of new gas fields. What applies to natural gas is equally relevant to crude oil. Here, too, there is a clear trend towards more intensive production. Furthermore, since February 2022, the provision of fossil fuels has also had to take into account the issue of emerging dependencies on political systems. Efforts to change the current situation are not without consequences for the metal industry. Apart from the dramatic energy shortage, duplex steels are increasingly coming into the spotlight.

This is primarily due to their special material properties, as duplex steels are significantly stronger than other stainless steels. These properties are of particular value not only, but also, in the development of new natural gas and oil sources, as transport takes place via pipelines running deep on the seabed. These are connected to platforms from which ships ultimately transport the energy sources. Waves, currents, external pressure, salt water – duplex steels withstand these conditions and are therefore suitable for applications that place the highest demands on materials.

Remarkable material properties

A closer look reveals that duplex steel consists of a two-phase microstructure comprising a ferrite matrix with islands of austenite. This microstructure is achieved by alloying the ferrite-forming element chromium (Cr) with the austenite-forming elements nickel (Ni), nitrogen (N) and manganese (Mn). The austenite content is adjusted so that the overall microstructure cannot become austenitic at room temperature. This can be achieved, for example, with a nickel content of less than eight per cent. Following hot forming, a special heat treatment ensures that the austenite formers are concentrated in specific areas. These areas then remain austenitic down to room temperature.

The result is steels with remarkable mechanical properties. Duplex steels combine the properties of stainless chromium steels (ferritic/martensitic) with those of stainless chromium-nickel steels (austenitic). This is why duplex steels are considered more ductile than stainless chromium steels, for example. Furthermore, duplex steels also perform well in terms of fatigue strength. The same applies to notch toughness. Duplex steels are classified as corrosion- and acid-resistant materials (DIN EN 10088 T1 to T3). Compared to purely austenitic steels, duplex steels contain less nickel – but more chromium.

Odering blanks for duplex steels

Nickel plays a key role in the mechanical properties of the material. Nickel-reduced duplex steels have been available on the market since 2005. They have a nickel content of between one and six per cent. As nickel is a relatively expensive raw material, this results in cost savings. Given the current supply situation with disrupted supply chains, this is of particular importance. Good to know: To prevent intergranular corrosion, a certain proportion of nickel can be replaced by nitrogen as an austenite former. Another option for improving the corrosion resistance of duplex steels is to add molybdenum.

Duplex steel – bespoke solutions for every application

The advantages of duplex steels become apparent in practice. The material’s high strength makes it possible to reduce material usage. This means that boiler components such as boilers, tanks, pipes and structures can be manufactured with thinner wall thicknesses, for example. This saves on material, transport weight and, above all, welding costs. In general, steels in the duplex family share common properties such as a two-phase microstructure consisting of approximately 50/50% austenite and ferrite, a lower service temperature limit of around -50°C, and higher strength compared to austenitic steels. The duplex family can be subdivided as follows:

  • Lean Duplex Stainless Steel
    The term ‘Lean’ indicates a low proportion of costly alloying elements. Here, the nickel content is lowest and no molybdenum is added.
Duplex grade EN Cmax Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Manganese (Mn) Copper (Cu) Tungsten (W) PREN
2101 1.4162 0,04 21,5 1,5 0,3 0,22 5 ––– ––– 26
2102   0,03 21,5 1,5 0,5 0,21 2,5 ––– ––– 26,5
2202 1.4062 0,03 22,5 2 0,3 0,21 1,3 ––– ––– 27
2304 1.4362 0,03 23 4 0,3 0,12 1 ––– ––– 26
  • Duplex (standard) 
    Slightly higher nickel content and the addition of molybdenum.
Duplex grade EN Cmax Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Manganese (Mn) Copper (Cu) Tungsten (W) PREN
2404 1.4462 0,03 24 3,6 1,6 0,27 3,5 0,4 ––– 33
2003   0,03 21 3,5 1,7 0,16 1,5 ––– ––– 30
2205 1.4462 0,03 22,5 5,5 3,3 0,18 1,9 ––– ––– 35/36
  • Super Duplex
    The chromium content here exceeds 25%, whilst the nickel and molybdenum contents are higher than in standard duplex steel.
Duplex grade EN Cmax Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Manganese (Mn) Copper (Cu) Tungsten (W) PREN
2507 1.4410 0,03 25 7 3,5 0,26 1,0 ––– ––– 41
  • Hyper Duplex 
    Offers the highest resistance to pitting corrosion combined with maximum mechanical strength. These material properties are achieved through the complex alloy composition and an even higher proportion of alloying elements; in particular, nitrogen is added to almost the limit of solubility.
     
Duplex grade EN Cmax Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Nitrogen (N) Manganese (Mn) Copper (Cu) Tungsten (W) PREN
2707   0,03 27 7 4,5 0,4 1,5 1,0 ––– 48
2906 Safurex® 1.4477 0,03 29 6,5 2,3 0,35 1,0 ––– ––– 42

Hyper-duplex steels – outstanding material properties

The use of duplex steels offers a host of advantages. Their material properties ensure that the amount of material required can be significantly reduced. The same applies to labour costs, particularly during fabrication – welding thinner sheets is quicker and easier. In addition, the use of duplex steels leads to reduced transport costs due to their lower weight. The minimised use of material also offers further potential for savings. For example, the lower weight means that foundations can be designed to be correspondingly smaller.

The cutting-edge product in this field is currently known as hyper-duplex. Thanks to its outstanding material properties, it is ideally suited for high pressure, high temperatures and extremely corrosive environments, such as those frequently encountered in the oil and gas industry.

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