Duplex steel

The term Duplex (lat. for “double”) steel designates the steel grades which have a two-phase micro-structure, consisting of ferrite and austenite. The structure is characterized by a ferrite matrix with islands of austenite. The positive properties of duplex steel grades are greatest in case of a balanced ratio of the proportions of ferrite to austenite. Their properties combine characteristics of chromium steel types and of chromium-nickel steels. They are stronger than chromium-nickel steels, but are characterized by a higher ductility than pure chromium steels at the same time.

Duplex steel

Properties of Alloy Elements of Duplex Steel

Chromium is responsible for the corrosion-resistant properties of duplex steel grades. As a ferrite former chromium is suited to promote the development of space-centered crystal lattices. Nickel is also an austenite former and enlarges the profile in corrosion resistance. Resistance to corrosion can be further increased by molybdenum. Especially the resistance to pitting corrosion is increased by molybdenum. Nitrogen increases the positive corrosion properties as well. Apart from the increased protection against pitting corrosion, it reduces the likelihood of crevice corrosion. Moreover, it increases the strength of duplex steels.

Which Duplex Steel Grades are there?

Duplex steel meets high requirements. In principle, three types of duplex steel can be distinguished: Standard Duplex is the most frequently used version. The most common version of this sort is steel with the material number  S31803 (1.4462). Standard duplex steel is versatile and is particularly used, if both corrosion resistance and strength are required. Standard duplex steels have a proportion of more than 20% of chromium, about 5% of nickel and about 3% of molybdenum. A simpler version is Mager Duplex (also known as Lean Duplex), which is available among others in the grades S32001 (1.4482), S32101 (1.4162), S32202 (1.4062), S32304 (1.4362) and 1.4655.

Lean duplex, in comparison with standard duplex, has lower proportions of nickel and molybdenum. This results partly in lower material costs. In addition, lean duplex has a larger tolerance towards tensile stress. Lean duplex is among others used in the construction industry for these uses, in case of which this grade can use its full potential for weight saving. The third variant is the super-duplex grades. They stand out because of a particularly strong resistance to corrosion and a high strength. The most widespread representative of this group is S32750 (1.4410). Others are S32760 (1.4501), S32750 (1.4507) and 1.4477. Super duplex is suitable for demanding areas of application, such as seawater desalination or construction of chemical tankers. Super duplex grades have an increased proportion of chromium (about 25%), nickel (up to 8%) and molybdenum (up to 4%) compared to standard duplex.

What are Methods to harden Duplex Steels?

Heat treatment processes for duplex steel often result in problems to maintain the properties with respect to corrosion and strength. Using the BORINOX® methoDuplex steel can be hardenendd, duplex grades can be further improved in their positive properties, without the protection against corrosion being lost. Do you need an increase in the degree of hardness for your duplex steel product? Contact us today.