Super-Duplex Stainless Steels and their characteristics
Super-Duplex Stainless Steels, known for their exceptional corrosion resistance and high-performance attributes, trace their origins back more than 70 years to Sweden, where the first-generation Duplex stainless steels were developed for combating corrosion in the sulfite paper industry. Characterized by a mixed microstructure of ferrite and austenite, these steels were designed to withstand aggressive chemical environments, particularly those containing chlorides. In the 1980s, the term "Super-Duplex" emerged to describe a highly alloyed variant with superior pitting resistance, often surpassing traditional stainless steels and nickel-based alloys in demanding applications.
By leveraging high chromium, nickel, molybdenum, and nitrogen contents, Super-Duplex steels offer robust resistance to acids, chlorides, and caustic solutions, bolstering their appeal in industries ranging from chemical and petrochemical to pulp and paper. Recent advancements have further extended their utility, enabling the production of high-strength, corrosion-resistant coils and wires for diverse industrial applications.
Super Duplex Fittings
Super-Duplex falls under the Duplex stainless steel grouping. Duplex stainless steels are graded for their corrosion performance depending on their alloy content. Today, modern Duplex stainless steel can be divided into four groups:
• Lean Duplex such as 2304, which contains no deliberate Mo addition;
• 2205, the work-horse grade accounting for more than 80% of duplex usage;
• 25 Cr duplex such as Alloy 255 and DP-3;
• Super-Duplex; with 25-26 Cr and increased Mo and N compared with 25 Cr grades, including grades such as 2507, Zeron 100, UR 52N+, and DP-3W
Benefits of Super-Duplex Stainless Steel
Super-duplex stainless Steels offer a myriad of benefits that make them indispensable in demanding industrial applications:- High Strength: Provides robust structural integrity.
High Resistance to Pitting and crevice Corrosion: Ensures prolonged durability in corrosive environments.
- High Resistance to Stress Corrosion Cracking, Corrosion Fatigue, and Erosion
- Excellent Resistance to Chloride Stress-Corrosion Cracking
- High Thermal Conductivity
- Low Coefficient of Thermal Expansion
- Good Sulfide Stress Corrosion Resistance
- Lower Thermal Expansion and Higher Heat Conductivity than Austenitic Steels
Applications for Super-Duplex Stainless Steel
Super-Duplex Stainless Steels find versatile applications across various industries, including:
- Heat exchangers, tubes, and pipes for the production and handling of gas and oil.
- Heat exchangers and pipes in desalination plants.
- Mechanical and structural components.
- Power industry FGD (Flue Gas Desulfurization) systems.
- Pipes in process industries handling solutions containing chlorides.
- Utility and industrial systems, rotors, fans, shafts, and press rolls where high corrosion fatigue strength is crucial.
- Cargo tanks, vessels, piping, and welding consumables for chemical tankers.
- High-strength, highly resistant wiring for critical applications.
Composition of Super- Duplex Stainless Steel
The table lists the duplex stainless steels covered in ASTM specifications for plate, sheet, and bar products.
a) Weight percent, maximum unless otherwise noted.
b) Unless otherwise indicated, a common name, not a trademark, widely used, not associated with any one producer, as listed in ASTM A 240.
c) W 0.50-1.00; Cr+3.3Mo+16N=40 min.
d) AISI designation
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