Duplex Stainless Steel

What is Duplex Stainless Steel?

Duplex stainless steel is a strong and corrosion-resistant material, made from a mix of two stainless steel structures, ferrite and austenite. This dual-phase structure gives it the best of both such as high strength and excellent resistance to chloride-related corrosion, including stress corrosion cracking and pitting. Palgotta Metal supplies high-performance duplex stainless steel products, built for demanding industrial use. It typically contains more chromium and molybdenum and less nickel than standard austenitic grades like 304 or 316. These changes in composition make duplex steel a reliable, long-lasting, and cost-effective choice, especially in harsh environments where both strength and corrosion resistance matter.

A Brief History

The idea of duplex stainless steel first came about in the 1920s. Early versions were mostly cast and used in specific areas due to their high carbon content. In the 1960s, steelmakers developed better processing techniques, mainly decarburization. This allowed the creation of duplex grades with lower carbon, better structure balance, and higher performance. These improvements shaped the modern duplex stainless steels we rely on today, known for their strength, stability, and corrosion resistance.

Benefits of Duplex Stainless Steel

Duplex stainless steel offers a powerful combination of performance, durability, and value. Here's why it stands out:

Corrosion Resistance: Duplex steel resists corrosion much better than standard grades. It's especially strong against chloride stress corrosion and pitting, making it perfect for marine, oil, and chemical environments.

High Strength: With up to twice the strength of common austenitic stainless steels, duplex allows the use of thinner sections, saving weight and cost without sacrificing performance.

Cost Effectiveness: Due to lower nickel content and high strength, duplex steel reduces both material cost and weight. That saves production, transport, and installation costs.

Durability and Pressure Handling: Duplex steel stands up to extreme pressures, and high mechanical stress. It maintains strength over time, even under heavy loads and demanding conditions.

Resistance in Harsh Environments: Whether exposed to seawater, chemicals, or severe temperatures, duplex stainless steel resists cracking, corroding, and weakening. That makes it a go-to choice for critical systems.

Specification of Duplex Stainless Steel

This section covers the core specifications that define duplex stainless steel, its chemical composition, mechanical properties, and its various grades. These variables show how the material performs in real-world conditions, such as corrosion resistance and pressure, and stress tolerance.

Chemical Composition of Duplex Stainless Steel

Duplex stainless steel has a high chromium content (21.0–23.0%) for superior rust resistance and a low carbon content (≤ 0.03%) to prevent corrosion caused by welding. Nitrogen (0.08-0.20%) increases strength and helps prevent pitting. Nitrogen (0.08–0.20%) boosts strength and helps resist pitting. Nickel (4.5–6.5%) adds toughness and keeps the duplex structure stable. Molybdenum (2.5–3.5%) improves resistance to seawater and acidic corrosion, while manganese (≤ 2.0%) supports strength and workability.

Mechanical Composition of Duplex Stainless Steel

Duplex stainless steel offers high yield strength of about 420 MPa for S31803 and 450 MPa for S32205, allowing it to handle heavy loads without deforming. Its ultimate tensile strength varies from 620 to 645 MPa, showing high resistance to breaking. With an elongation of 25-26%, it has good ductility and formability. The hardness is typically ≤ 293 BHN, providing solid wear resistance without making the material brittle.

Various grades of Duplex Stainless Steel

We provide a wide range of Duplex Stainless Steel grades designed to fulfill the various and demanding requirements of modern industries. Known for their superior strength, corrosion resistance, and cost-effectiveness. 

Regular Duplex – UNS S32205 / Duplex 2205

Duplex 2205 is the most used grade among Duplex Stainless Steels, with more than 80% of global usage. It has great corrosion resistance, particularly in chloride-rich environments, where 316L is widely utilized. It features twice the yield strength of standard austenitic stainless steels. This makes it an excellent choice for applications requiring both durability and performance.

Lean Duplex – UNS S32101

Lean Duplex Stainless Steels like UNS S32101 offer an economical alternative with high strength and good corrosion resistance, particularly when compared to conventional austenitic grades such as 316L. This alloy is perfect for general-purpose fabrications, storage tanks and structural components. It offers cost stability because of its low nickel and molybdenum concentration.

UNS S32304

UNS S32304 offers enhanced mechanical strength and better corrosion resistance than 304/316L stainless steels especially in mildly aggressive environments. Its balanced composition makes it an affordable option for a variety of industrial applications, such as chemical processing and water treatment.

Super Duplex – UNS S32750 / S32760

Super Duplex grades like UNS S32750 and UNS S32760 are designed for extreme performance. They provide excellent resistance to pitting, crevice corrosion, and stress corrosion cracking. With significantly higher strength and toughness, Super Duplex alloys are the material of choice for offshore platforms, desalination plants, chemical process industries, and marine environment.

How the Austenite/Ferrite Balance is Achieved?

Precise control of the alloy's chemical composition and thermal processing is necessary to achieve the austenite-ferrite balance in duplex stainless steels. Key alloying elements such as chromium, nickel, molybdenum, and nitrogen are carefully adjusted to promote a microstructure consisting of approximately equal parts of austenite and ferrite. This dual-phase structure is typically established during solidification and further refined through solution annealing, ensuring optimal mechanical strength and corrosion resistance.

Applications of Duplex Stainless Steel

Duplex stainless steel is utilized in many different industries where durability and resistance to corrosion are essential. It is used in chemical pressure vessels, tanks, pipelines, and heat exchangers. The oil and gas sector uses it for pipelines, risers and processing systems. It works effectively in coastal environments and high-chloride places like desalination facilities. In pulp and paper mills, duplex steels are used in digesters and bleach systems. You’ll also find it in food processing equipment, biofuel plants, and cargo tanks for ships and trucks.

Embrittlement and Phase Stability

475°C Embrittlement (Alpha Prime – α′ Phase)

 Duplex stainless steel can lose toughness if held between 300°C and 500°C for too long. In this range, the ferrite phase starts breaking down and forms alpha prime (α′), a brittle structure that weakens the steel and lowers corrosion resistance. This happens slowly, but it gets worse in high-molybdenum grades. Lean duplex grades are less affected. To avoid this issue, don’t stress-relieve or heat-treat duplex steel in this range. To avoid this, set the temperature to 315°C (600°F) as the safe upper limit for continuous use.

Sigma Phase and Phase Stability

If duplex steel sits too long between 600°C and 1000°C, sigma phase can form. This brittle phase eats away at toughness and wrecks corrosion resistance. It forms during slow cooling or poor heat treatment, especially in weld zones or thick sections. Sigma is rich in chromium and molybdenum, two elements vital for corrosion resistance. Once they bond into sigma, the surrounding steel suffers. Newer grades like 2205 resist sigma better, but fast quenching after solution annealing is still key to keeping the structure stable.

Duplex Stainless Steel Microstructure

Duplex stainless steel has a mix of two phases, about half austenite and half ferrite. This blend gives it a strong balance of properties. Austenite adds toughness and helps fight corrosion. Ferrite gives the steel high strength and better resistance to cracking under stress. This dual-phase setup comes from careful control of elements like chromium, nickel, molybdenum, and nitrogen during production. The result is a tough, durable alloy that works well in harsh, demanding environments.