If we were told that it was possible to combine the advantages of ferritic and austenitic steels in one alloy, perhaps this would lead us to believe that we would be dealing with a stainless steel with a very high added value. This family of stainless steels exists and they are known as duplex stainless steels.
The basis of duplex
These steels are a combination of austenitic and ferritic properties.
Before discussing the characteristics of these steels. Let us review the definition of the two solutions present in the austenitic and ferritic steel compositions. On the one hand, austenite is very sensitive to stress corrosion, but ferrite is almost immune to it. On the other hand, ferrite is quite brittle, but austenite is tough. Work hardening is much more pronounced in ferritic than in austenitic due to the difference in atomic structure. Ferritic stainless steels are difficult to weld, whereas austenitic stainless steels are easy to weld.
What is the definition of duplex stainless steel?
This “fusion” makes it possible to avoid the Achilles’ heel of both alloys. For example.
If austenite is sensitive to stress corrosion cracking, the duplex structure is virtually immune to it. In addition, the mechanical strength is usually twice that of austenite and ferrite because of its small effective grain size, which is difficult to obtain in single-phase materials.
Another characteristic of duplex steels is their high chromium content (20.1 – 25.4 %), but low nickel content (1.4 – 7 %) compared to austenite. Due to their low nickel content, they are more stable in price. The composition of duplexes also contains molybdenum (0.3 – 4 %) and nitrogen, which are added to improve strength. Manganese can sometimes be found in some duplex grades as a partial replacement for nickel and to improve the solubility of nitrogen in the material.
Applications
Duplex steels are suitable for applications where a combination of corrosion resistance, mechanical strength and weldability is required. Duplex steels include low-alloyed and high-alloyed variants. Low-alloyed duplex steels are used for less demanding applications. In highly corrosive applications, high alloyed duplex steels, also known as super duplex steels, are required.
Applications include heat exchangers, storage tanks, pressure vessels, tubes for the oil and gas industry and various components for the urea industry. In the latter industry, a key stage in a urea plant is the production of ammonium carbamate, a highly corrosive compound. Urea is used as a fertiliser and is a very valuable substance as it provides nitrogen to plants. The production of this substance is essential in modern agriculture.