Introduction
Duplex stainless steels, also known as ferrite-austenite stainless steel, are highly reliable and corrosion-resistant materials with a two-phase microstructure. The two phases consisted of ferrite and austenite, making duplex stainless steels an exceptional material with both good strength and corrosion resistance comparable to austenitic and ferritic stainless steels. The improved resistance of duplex stainless steels was a result of the higher content of chromium and molybdenum, combined with a low carbon content of generally less than 0.03%. It is important to note that the higher the percentage of alloying elements in the duplex stainless steels, the better its corrosion resistance. Thus far, duplex stainless steels have been commonly used for applications requiring corrosion resistance such as wastewater treatment, chemical and offshore oil and gas processing.
Classification of duplex stainless steel
Duplex stainless steels can be divided into three types based on their composition: lean duplex stainless steels, standard duplex stainless steels, and superduplex stainless steels. Depending on their composition, the mechanical properties and weldability of the three types of duplex stainless steels may differ.
Lean duplex stainless steel
Lean duplex stainless steels have a PREN of between 25 and 29 and contain a lower chromium and molybdenum content. The lower level of alloying elements leads to a lower cost but also results in reduced corrosion resistance and a lower strength-to-weight ratio, making them less suitable for high-stress applications. Typical lean duplex grades include LDX 2101, Avesta 2202, Centaur LDX, and 1.4162.
Standard duplex stainless steel
Standard duplex stainless steels have a PREN range of 30-34 and a higher content of chromium and molybdenum compared to lean duplex stainless steels, providing excellent corrosion and mechanical properties while still being cost effective compared to ferritic or austenitic stainless steels. Common standard duplex grades include 2304, 2205, and Zeron 100.
Super duplex stainless steel
Super duplex stainless steel is an alloy with a PREN of 35 and over and is composed of 25-40% chromium, 7-25% nickel, and 4-25% molybdenum. Due to their higher levels of alloying elements, super duplex stainless steels have superior corrosion and mechanical properties compared to standard duplex stainless steels. Common super duplex stainless steel grades include 2507, Zeron 100X, and 2553.
Conclusion
To summarize, duplex stainless steels can be classified into three types based on their composition - lean duplex stainless steels, standard duplex stainless steels, and superduplex stainless steels. Each type has its own advantages and disadvantages and should be chosen based on the required application. Additionally, the composition and performance of each type of duplex stainless steel can be improved through various additives and treatments.
