Stainless steel is one of the most widely used structural alloys in various industries. However, not all stainless steels are the same. Some, such as austenitic, exhibit excellent corrosion resistance but limited mechanical strength. Others, such as ferritic, exhibit acceptable strength but exhibit lower ductility and corrosion resistance. To overcome these limitations, a special class of steel called duplex stainless steels has been developed, which combines austenitic and ferritic microstructures.
These alloys exhibit high corrosion resistance and excellent mechanical strength , making them ideal for use in the oil and gas, petrochemical, desalination and corrosive environments.
What is duplex steel?
Duplex steel is a type of stainless steel whose microscopic structure consists of two main phases:
The ratio of these two phases is typically close to 50/50 , but the actual ratio varies between 40% and 60% . This combination gives duplex steel the superior properties that distinguish two different steel grades (austenite and ferrite).
Chemical composition of duplex steel
The chemical composition of duplex steels is designed for a balanced ratio of austenite and ferrite. The most important alloying elements in this group are:
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Chromium (Cr): The content is 18% to 28% to improve corrosion resistance.
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Nickel (Ni): about 4–8%, used to stabilize the austenite phase.
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Molybdenum (Mo): Its content is 2% to 5% and is used to improve resistance to local corrosion (pitting and crevice corrosion).
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Nitrogen (N): Its content is very low (about 0.2–0.3%), but it increases the strength and stability of austenite very effectively.
This combination gives duplex steel high resistance to chlorides and corrosive environments.
Microstructure and crystalline properties
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Ferritic (BCC): has high strength and resistance to stress cracking.
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Austenitic (FCC): has better ductility and higher corrosion resistance.
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By combining these two phases, duplex steel has a tensile strength about twice that of austenitic steel while maintaining acceptable ductility.
Mechanical and chemical properties of duplex steel
1. Mechanical properties
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High tensile strength: about twice that of 300 series austenitic steel.
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Medium hardness: However, slightly lower than that of austenitic steels.
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Excellent resistance to stress chloride cracking (SCC).
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Stronger than conventional stainless steel.
2. Corrosion resistance
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General corrosion resistance: Similar to or better than steel 316.
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Resistance to pitting and crevice corrosion: significantly better than austenitic steels due to the presence of molybdenum and nitrogen.
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Stress corrosion resistance: significantly higher than that of austenitic steel.
Classification of duplex steel
Depending on the content of alloying elements, duplex steel is divided into several categories:
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Low alloy duplex stainless steel (low alloy):
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Reduce the nickel and molybdenum content.
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More economical.
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Suitable for building structures and general applications.
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Standard duplex:
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Good balance between mechanical properties and wear resistance.
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The most commonly used type.
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Super Duplex:
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It contains 25 to 28% chromium, 3 to 5% molybdenum and a large amount of nitrogen.
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High resistance to aggressive environments.
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Used in the oil, gas and water desalination industries.
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Super Duplex:
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Latest generation products with the highest possible mineral composition.
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Excellent resistance in highly aggressive chloride environments.
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Advantages of double-walled steel
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Tensile strength and yield strength are twice as high as those of austenitic steel.
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Has excellent resistance to chloride stress cracking.
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Very high resistance to local corrosion.
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Low demand for nickel → Production is cheaper than for high-alloy austenitic steel.
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Acceptable welding and machining skills.
Limitations and disadvantages
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The hardness at low temperatures is lower than that of austenitic steel.
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The welding process is more sensitive due to the possibility of the formation of harmful phases such as sigma.
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At high temperatures (over 300 °C) the crystal structure can become unstable.
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It is more expensive than regular carbon or stainless steel.
Industrial applications of duplex steel
1. Oil and gas industry
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Pipelines that transport sour oil and gas (with H₂S).
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Equipment for offshore drilling.
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Pressure vessels and heat exchangers .
2. Water supply and wastewater disposal
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Thermal and reverse osmosis desalination plants.
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Seawater pipeline.
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Equipment for treatment plants.
3. Chemical and petrochemical industry
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Acid reactors and storage tanks.
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Plants for the production of fertilizers and corrosive chemicals.
4. Maritime Affairs and Shipbuilding
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Ship’s propeller.
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Columns and bearings.
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Land and sea structures.
5. Pulp and paper industry
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Appliances that have come into contact with chlorine and bleach.
Heat treatment of duplex steel
Heat treatment plays an important role in the production of these steels. The most important processes include:
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Solid solution annealing: Heating to 1040–1100 °C, followed by rapid cooling → homogenization of the microstructure.
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Stress relief: Reduces residual stresses after welding.
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Note: Annealing at moderate temperatures (400–900 °C) is not recommended as it may lead to the formation of sigma phases and reduced corrosion properties.
Comparison of duplex steel with austenitic and ferritic steel
| Special feature | Austenite | ferrite | two-phase |
|---|---|---|---|
| Tensile strength | half | half | very large |
| Corrosion resistance | Great | half | very good. |
| Resistance to stress corrosion cracking | weak | Yes | Great |
| Low-temperature hardness | Great | weak | half |
| It’s worth it | higher | fewer | half |
| Application in chloride-containing environments | weak | weak | Great |
Finally
Duplex stainless steel is a sophisticated combination of ferrite and austenite, offering outstanding properties such as high mechanical strength , excellent corrosion resistance , and long service life in aggressive environments . Therefore, this steel grade plays an important role in the oil and gas, petrochemical, desalination, marine, and chemical industries.
Although the cost and weld sensitivity of these steels are higher than conventional steels, their use is very cost-effective in many strategic projects due to the longer life of the equipment and lower maintenance costs .