Duplex is a category of several stainless steel grades, and is named this way because of the two-phase structure. It consists of a mixture of austenite and ferrite in roughly equal proportions. Unlike the one-phase materials like 316L, which is austenitic, the microstructure of duplex is both austenitic and ferritic. In a nutshell, austenite and ferrite are different phases in which (stainless) steel can exist. It is comparable to water – which can take gas, liquid, frozen or plasmatic form.
Duplex results from the right choice of alloys, and a heat treatment. Under the microscope, ferrite and austenite show different crystalline structures, as is pictured on the left. While heat treated, the duplex is completely ferritic, but after quenching, half of the ferritic grains transform to austenitic islands.
Properties and use
The conditions of austenite and ferrite are different, and so are the properties. For example, the maximum carbon solubility in austenite is 2,03%. For ferrite this is a much smaller amount: 0,02%. The combination of the two phases gives this material a combination of attractive advantages. Compared with 316L, the corrosion resistance is similar, but the yield strength of Duplex is more than twice that of its 316L counterpart. Duplex has improved resistance against stress corrosion, which is due to the ferritic part of the material.
The different varieties of duplex have existed for decades, but its importance has grown significantly these past few years, mainly because of its use in (petro) chemical environments. Duplex stainless steels are subdivided into lean, standard, super, or hyper duplex based on the quantity of alloying elements. For each material grade, there are different definitions, defined by the different standards. The well-known and most-used type of duplex is the 1.4462/ 2205/ UNS31803/ X2CrNiMoN22-5-3/ F51.
Arcus provides a wide range of pipes, flanges and fittings in various types of duplex. Please consult our experienced sales team for any enquiries or questions you might have.