Stainless steel is one of the widely used metals due to its high strength and flexibility, and it can be used in construction, automotive, chemical processing, and marine industries. Duplex and super duplex stainless steel are two of the highest grades of stainless steel, and they have exceptional strength, corrosion, and toughness. Comparing and analyzing those two types of gears, it is possible to identify the major differences and peculiarities that make one or the other most appropriate for particular use.
Understanding Duplex Stainless Steel
Duplex stainless steel is a type of steel that combines both ferritic and austenitic structures in roughly equal proportions. This distinctive grain structure makes duplex stainless steel to have a combination of high strength and corrosion resistance. It usually consists of 22-25% chromium, 4-6% nickel, with lesser proportions of molybdenum and nitrogen. The most frequently used variety of duplex stainless steel is the UNS S31803, or the more commonly known 2205.
This makes the structure of the material more effective in conditions of increased stress: the presence of ferrite gives the material great strength, and the presence of austenite, conversely, makes it more ductile. It can be utilized in pitting, crevice corrosion, and stress corrosion cracking applications of various industries. It also used in heat exchangers, pressure vessels, piping systems and in marine structures and vessels.
What Makes Super Duplex Stainless Steel Different?
Super duplex stainless steel is actually an advancement of the conventional duplex steel. It has a higher percentage of alloying additives like chromium, molybdenum and nitrogen which enhance greatly its mechanical characteristics and its resistance to corrosion. Super duplex grades typically contain 25-30% chromium, 6-7% nickel and around 4% molybdenum, with nitrogen levels up to 0.3%. The most well-known grades of super duplex stainless steel are UNS S32750 and UNS S32760.
The higher chromium content of super duplex stainless steel gives it improved resistance to both general corrosion and more aggressive forms of corrosion, such as pitting and crevice corrosion. It is also more resistant to stress corrosion cracking, making it a preferred material for industries dealing with highly corrosive environments, like the oil and gas industry, offshore structures and chemical processing plants.
Key Differences Between Duplex and Super Duplex Stainless Steel
Corrosion Resistance:
Duplex: The duplex stainless steel has good corrosion resistance but is more sensitive to certain highly aggressive forms of corrosion in extreme conditions.
Super Duplex: Super duplex stainless steel has improved resistance to both pitting and crevice corrosion as compared to other steel products. It contains higher percentages of moles and chromium, and that makes it perfect for use in chloride exposure stations.
Strength and Toughness:
Duplex: Duplex stainless steel is strong, offering good tensile and yield strength, making it suitable for structural applications. However, its strength is lower than a super duplex.
Super Duplex: Super duplex stainless steel is stronger than the regular duplex steel and has higher tensile strength and yield strength for use in applications that are demanding.
Cost:
Duplex: Due to its slightly lower alloy content, duplex stainless steel is typically more affordable than super duplex steel.
Super Duplex: Super duplex stainless steel is more expensive due to its higher alloy content and improved performance in highly corrosive environments.
Applications:
Duplex: Commonly used in applications like wastewater treatment, pulp and paper and chemical processing, where moderate corrosion resistance and high strength are required.
Super Duplex: Due to its enhanced properties, super duplex is used in more challenging environments, such as deep-sea exploration, offshore oil rigs and high-pressure systems.
Conclusion
The main differences between the two are in composition, corrosion resistance, strength, and cost of the material. Although the duplex stainless steel has a good combination of both strength and corrosion resistance for most applications, the super duplex stainless steel is better suited for harsh or more corrosive environments and high end applications. However, when choosing between the two the decision will be based on the requirements of the project and the conditions that the material will be exposed to.