Guide on Duplex Stainless Steel
Duplex stainless steel is a type of stainless steel products by its unique microstructure, typically containing equal parts of austenite and ferrite phases. Although not officially defined, it is generally understood that the ferritic phase should constitute at least 30% of the volume. For a comprehensive understanding of duplex stainless steel microstructures, further reading in specialized literature is recommended.
Let's explore the benefits, types, and applications of duplex stainless steel through our comprehensive blog.
What Is Duplex Stainless Steel?
Duplex stainless steel is a specialized alloy featuring a balanced blend of two distinct phases: austenite and ferrite. This composition combines the high strength of ferritic stainless steel with the superior corrosion resistance of austenitic stainless steel. Typically composed of 40 to 60% ferrite after solution annealing, duplex stainless steel offers a microstructure with nearly equal proportions of both ferrite and austenite phases. Higher chromium and molybdenum content provide exceptional resistance to pitting and crevice corrosion, especially in aggressive environments. This makes duplex stainless steel an excellent choice for a wide range of industrial applications.
Benefits of Duplex Stainless Steel
Duplex stainless steel offers numerous advantages over traditional austenitic and ferritic stainless steel grades.
Improved Strength: Many duplex steel grades are up to twice as strong as their austenitic and ferritic counterparts.
High Toughness and Ductility: Duplex stainless steel tends to be more formable under pressure compared to ferritic grades, offering greater toughness. While it may have slightly lower toughness than austenitic steels, the unique structure and properties of duplex stainless steel often provide superior overall performance.
High Corrosion Resistance: Depending on the grade, duplex stainless steel can offer corrosion resistance comparable to or even better than common austenitic grades. Alloys with higher nitrogen, molybdenum, and chromium content show excellent resistance to crevice corrosion and chloride pitting.
Cost Effectiveness: Duplex stainless steel combines these benefits while requiring lower levels of expensive elements like molybdenum and nickel, making it a more cost-effective option than many traditional austenitic grades. Additionally, the price of duplex material tends to be less volatile, making cost estimation more predictable. The higher strength and corrosion resistance of duplex stainless steel also allow for the use of thinner materials, further reducing costs.
Types of Duplex Steel Grades
Duplex stainless steel grades are known for their balanced microstructure, combining the best properties of both ferritic and austenitic stainless steels. Here are three key types of duplex steel grades and their applications:
2205 (UNS S32205/S31803) 2205 is a popular duplex stainless steel grade with excellent general corrosion resistance, surpassing 316L in most environments. Its high chromium, molybdenum, and nitrogen content protect against pitting, crevice, and general corrosion. However, it's not suitable for applications requiring prolonged exposure to temperatures above 300°C due to the formation of brittle microstructures. Typical applications include chemical processing equipment, oil and gas industry components, desalination plants, marine structures, and pulp and paper production machinery.
2507 (UNS S32750) 2507, also known as super duplex, offers very high resistance to pitting and crevice corrosion thanks to its high molybdenum and chromium content. It has excellent strength but can become brittle if exposed to high temperatures for too long. This grade is ideal for extreme environments such as desalination plants, seawater systems, and the petrochemical industry. Common uses include chemical processing equipment, oil and gas industry components, and equipment used in seawater desalination and the pulp and paper industry.
ZERON 100 (UNS S32760) ZERON 100 is another super duplex stainless steel with high pitting and crevice corrosion resistance. It also resists sulphide stress corrosion cracking but, like other super duplex grades, can become brittle at high temperatures. This grade is widely used in the oil and gas industry, especially in offshore production, due to its excellent corrosion resistance in seawater and sour gas environments. It's also used in chemical processing plants, desalination equipment, and various applications in the petrochemical, paper and pulp, and marine industries.
In summary, duplex stainless steel grades like 2205, 2507, and ZERON 100 offer a unique combination of strength and corrosion resistance, making them suitable for a wide range of demanding applications across different industries.
Duplex vs Super Duplex
Super Duplex, with 25% chromium, provides superior pitting resistance compared to duplex steel's 22%. Both types need preheating and post-weld heat treatment to address chromium carbide precipitation. While both resist pitting and harsh conditions, super duplex excels due to its higher chromium, nickel, and molybdenum content. Duplex steel is easier to machine, whereas super duplex requires more precision and slower speeds due to its higher alloy content.
