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Duplex

What is duplex?

Duplex is a type of stainless steel whose microstructure consists of two different phases: austenite and ferrite. The proportion of these phases is usually around 50/50, which is why the material is called duplex steel. This combination gives the steel properties that cannot be achieved with purely austenitic or ferritic stainless steels. Duplex is particularly known for its high strength, good corrosion resistance, and relatively good machinability compared to other stainless steels.

In the engineering workshop industry, duplex steels are an important material group because they combine mechanical strength and chemical resistance in a way that makes them suitable for many demanding applications.

Composition and microstructure of duplex steel

Duplex steels are alloyed steels in which the most important alloying elements are chromium, nickel and often also molybdenum and nitrogen. The chromium content is typically around 22–25%, which provides good corrosion resistance. Nickel content is usually around 4–7%, and its role is to balance the formation of the austenitic and ferritic structures.

The microstructure is the key characteristic of duplex steel. Austenite gives the material toughness and good formability, while ferrite increases strength and improves resistance, for example against stress corrosion cracking. This dual-phase structure makes duplex steel significantly stronger than many conventional austenitic stainless steels.

Strength and corrosion resistance of duplex steel

The yield strength of duplex steels is typically about twice that of conventional austenitic stainless steels. This means that thinner sections can be used without compromising structural integrity. In machine shops, this can be seen, for example, in lighter yet very durable structures.

Corrosion resistance is another major advantage. Duplex steels have excellent resistance to chloride-containing environments and are less susceptible to stress corrosion cracking than many other stainless steels. For this reason, they are often used in structures that come into contact with seawater, chemicals, and other aggressive environments.

Use of duplex steel in the engineering workshop industry

In machine shops, duplex steels are used especially in applications that require both high strength and good corrosion resistance. Typical applications include pump and valve components, piping components, shafts, and a variety of process industry equipment.

Duplex steels are widely used, for example, in the equipment of the chemical industry, energy sector, and pulp and paper industry. In these environments, materials must withstand both mechanical loads and corrosive substances over a long service life.

Machinability of duplex steel

The machining of duplex steel differs in many ways from the machining of conventional stainless steels. The material’s high strength and toughness increase cutting forces and can make chip formation more difficult. In addition, duplex steels work-harden rapidly, which must be taken into account during machining.

To achieve a good machined surface, it is important to use cutting tools suitable for the purpose and optimized machining parameters, such as cutting speed, feed, and depth of cut. Effective use of cutting fluid also improves tool life and chip control.

Weldability of duplex

Duplex steels are generally readily weldable, but in welding it is important to control the temperature and cooling rate. Excessive heat input can alter the balance between austenite and ferrite, which may impair the material’s corrosion resistance and mechanical properties.

Therefore, carefully specified welding procedures and filler materials are often used for duplex steels to keep the microstructure as optimal as possible.

Types of duplex steel

There are several subcategories of duplex steels that differ in alloy content and properties. The most common groups are:

Lean duplex steels contain fewer alloying elements and are cost-effective alternatives. Super duplex steels, on the other hand, offer extremely high corrosion resistance and strength, but their machining can be more demanding.

Summary

Duplex is a stainless steel whose mixed austenitic–ferritic microstructure combines high strength and good corrosion resistance. Thanks to these properties, duplex steels are excellently suited to demanding engineering workshop applications where materials must provide both mechanical durability and long-term corrosion resistance.

The use of duplex steels has increased especially in applications where conventional stainless steels do not provide sufficient strength or chemical resistance. With appropriate machining methods and optimized cutting parameters, duplex is a material that can be machined efficiently and offers very high performance for workshop use.