If we are told that it is possible to combine the advantages of ferritic and austenitic steels in one alloy, perhaps this leads us to believe that we are dealing with a stainless steel with very high added value. This family of stainless steels exists and they are called duplex stainless steels.
duplex basics
These steels are a combination of austenitic and ferritic properties.
Before discussing the properties of these steels. Let us review the definitions of the two solutions present in the austenitic and ferritic steel compositions. On the one hand, austenite is very sensitive to stress corrosion, while ferrite is hardly affected by it. On the other hand, ferrite is brittle while austenite is hard. Due to the difference in atomic structure, work hardening is more pronounced in ferrite than in austenite. Ferritic stainless steels are difficult to weld, while austenitic stainless steels are easy to weld.
What is the definition of duplex stainless steel?
This "fusion" avoids the Achilles' heel of the two alloys. E.g.
If austenite is susceptible to stress corrosion cracking, the duplex structure is practically immune to it. In addition, due to its small effective grain size, the mechanical strength is usually twice that of austenite and ferrite, 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 austenitic. Prices are relatively stable due to the low nickel content. The composition of duplex alloys also contains molybdenum (0.3 – 4 %) and nitrogen, which are added to increase strength. Manganese can sometimes be found in some duplex grades as a partial replacement for nickel and increases the solubility of nitrogen in the material.
application
Duplex steels are suitable for applications that require a combination of corrosion resistance, mechanical strength and weldability. Duplex steels include low alloy and high alloy variants. Low alloy dual phase steels are used for less demanding applications. In highly corrosive applications, high alloy duplex steels, also known as super duplex steels, are required.
Applications include heat exchangers, storage tanks, pressure vessels, piping in the oil and gas industry and various components in the urea industry. In the latter industry, a critical stage in urea plants is the production of ammonium carbamate, a highly corrosive compound. Urea is used as a fertilizer and is a very valuable substance because it provides nitrogen to plants. The production of this substance is essential in modern agriculture.
