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Super Duplex Stainless Steel
Super duplex stainless steel is a high–quality stainless steel alloy that has elevated quantities of chromium, molybdenum, and nitrogen. These elements increase the corrosion resistance, mechanical strength, and durability of the alloy. The material encompasses the benefits of both austenitic and ferritic stainless steels. This makes it highly desirable in ultra–demanding applications, especially within marine, oil and gas, and chemical processing environments.
Super duplex stainless steel has a unique microstructure that adds to its mechanical properties and its resistance to several corrosion types by increased resistance to localized corrosion found with exposure to high chloride environments.
Key Characteristics of Super Duplex Stainless Steel
- Exceptional Corrosion Resistance
- Super duplex stainless steel has exceptional resistance to pitting, crevice corrosion, and chloride stress corrosion cracking, making it a highly dependable material under conditions in saltwater, acid, and aggressive atmospheres.
- High Mechanical Strength
- The yield strength of super duplex stainless steel is approximately two times greater than that of austenitic stainless steels such as 304 and 316, allowing thin and light–weight components without losing the strength of the material and reducing the cost of materials and fabrication.
- Enhanced Fatigue Resistance
- Due to its dual-phase microstructure, super duplex stainless steel can endure high-cycle fatigue and is suitable for applications with varying stress.
- Good Weldability
- Welding is possible with super duplex, but due to the microstructure consisting of two phases, it requires careful control of heat, followed by a post-weld heat treatment to ensure it maintains properties that are resistant to embrittlement.
- Cost-Effective and Sustainable
- While it can be slightly more expensive initially than other offerings, super duplex has a reduced need for maintenance and replacement due to its performance characteristics, meaning its overall cost–effectiveness is better than other options. As well as being suitable to repair, duplex materials are recyclable, thus adding to their sustainability.
Grades of Super Duplex Stainless Steel
- UNS S32750 (2507)
- The most commonly used super duplex grade is S32750, which contains 25% chromium, 7% nickel and an additional 3.5% molybdenum. The properties of these alloys provide optimal strength and corrosion resistance, with specific engagements in extremely corrosive environments.
- UNS S32760
- A grade that incorporates copper and tungsten as additional alloying elements, S32760 has even better corrosion resistance yet is used in the most extreme environments, which often involve chemical processing and offshore oil and gas.
- UNS S39274
- This grade contains extra nitrogen, providing it with additional strength and pitting resistance, especially in high temperature and pressure applications, while remaining similar to the S32760 grade.
Applications of Super Duplex Stainless Steel
- Oil and Gas Industry: Super duplex stainless steel is designed for many uses in offshore and subsea applications, especially around pipelines, valves and manifolds where there is often a high level of exposure to seawater and corrosive substances.
- Chemical Processing: Suitable for aggressive chemicals, acids, and chlorides, super duplex is used for the chemical storage tanks, heat exchangers, and reactors.
- Desalination Plants: Super duplex has resistance to the high salinity and corrosive conditions within desalination plants, particularly in the seawater intake and brine discharge areas.
- Marine Environments: Ideal for seawater applications, such as shipbuilding, seawater cooling, and coastal structures.
- Pulp and Paper Industry: Super duplex resists chemicals used in fluxing and bleaching processes and helps extend the life cycle of equipment found in paper processing plants.
Benefits of Super Duplex Stainless Steel
- Enhanced Service Life: Super duplex has higher corrosion resistance as well as greater strength, which allows it to last longer even under harsh conditions.
- Lower Maintenance Costs: With an increase in longevity and a lower chance of corrosion, super duplex requires less frequent maintenance and replacement.
- Lightweight Design: The high strength-to-weight ratio allows for an overall reduction in material thickness, which saves weight and ultimately material and costs, but without sacrificing integrity.
- Sustainability: As a super duplex is 100% recyclable, it is environmentally friendly and adaptable with sustainable engineering and industrial practices.
Properties of Super Duplex Stainless Steel
- High Chromium Content: Allows for oxidation resistance and a protective passive layer to be formed on the surfaces.
- Molybdenum and Nitrogen Additions: Stronger resistance against pitting, crevice corrosion, and localized corrosion.
- Low Thermal Expansion: Like its counterpart, carbon steel, the super duplex offers low thermal expansion, which makes it perfect for temperature application-sensitive areas.
- Higher Tensile and Yield Strength: Provides structural support in applications with higher stress, yet allows for thinner material.
Standards and Specifications
Super Duplex stainless steels have recognized and accepted manufacturing standards that ensure reliability and performance:
- ASTM A790: This specification covers super duplex seamless and welded piping
- ASTM A182: This specification covers forged fittings and flanges for high-stress applications
- ASME B16.9 and B16.5: Standard for fittings and flanges dimensional reference
- NACE MR0175/ISO 15156: Certification for use in sour (hydrogen sulfide-rich) environments, common in oil and gas.
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- Oil & Gas industries
- Chemical process industries
- Pump and valve in high pressure component
- Food industries
- Pulp and paper industry
- Aerospace industry
- Power plant
- Mechanical component
- Sugar industry
- Cement industry