What Is Duplex Steel?

What is Duplex Steel?

Duplex Steel is a type of stainless steel that has two different grain phases, known as Austenite and Ferrite, present in the microstructure. The Austenite and Ferrite phases have different atomic structures and exhibit different physical properties, and as Duplex Steel contains roughly 50% of each phase it can exhibit beneficial traits from both material types.

These properties include:

• Relatively high strength when compared to other stainless steel products
• Improved ductility and toughness when compared to ferritic steel products, however these values remain less than austenitic products
• Excellent corrosion resistance

How is Duplex Steel Made?

Like all types of Stainless Steel, Duplex Steel is smelted by combining the raw materials containing nickel, iron ore, chromium, molybdenum, and other alloying elements in a furnace to be melted down and mixed together. Compared to other stainless steels, Duplex Steel contains a higher chromium content of around 25%, a reduced nickel content of below 9% and a small about of nitrogen. The alloy content of Duplex Steel is defined in ranges and are typically designated by their chromium content.

The molten Duplex Steel mix solidifies as a completely ferritic solid, however as it cools to room temperature half the material changes from the ferritic phase to the austenite phase.

What is Duplex Steel used for?

The high strength, high corrosion resistance and relatively low alloy content (especially nickel) typically makes Duplex Steel a low-cost option for high strength, high corrosion risk applications. They are used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc and in the petrochemical industry in the form of pipelines and pressure vessels.

Although duplex stainless steels have high corrosion and oxidation resistance they cannot be used at elevated temperatures due to the formation of brittle phases in the ferrite at relatively low temperatures. These phases have a significant effect on the toughness of the steel and these applications should be avoided.

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