What is duplex stainless steel?
Duplex stainless steel is a unique category of stainless steel with a two-phase microstructure, containing approximately 50% ferrite and 50% austenite. This balanced structure offers excellent strength, superior corrosion resistance, and enhanced durability while minimizing its weaknesses. We supply a range of high-quality duplex steel products to industrial standards, guaranteeing longevity across a range of applications.
History of duplex stainless steel
The concept of duplex stainless alloys was first introduced in the 1920s. However, production of the first duplex alloys did not begin until the 1930s.
These earliest duplex alloys were mostly limited to cast production and specific uses, as they contained a relatively high amount of carbon.
By the late 1960s, decarburization allowed metal foundries to create low-carbon steels with high chromium and nickel content while further balancing the ferrite and austenite structure. This development led to a new generation of duplex stainless steels with improved performance and fewer drawbacks.
These duplex stainless steels share many similarities with the modern alloys in production today
Benefits of Duplex Stainless Steel
High Strength
It’s nearly twice as strong as regular stainless steel, which means you can use thinner sections without losing performance. That saves weight and cost in construction and fabrication.
Excellent Corrosion Resistance
Handles exposure to salt, moisture, and harsh chemicals better than many other steels, making it ideal for marine, offshore, and industrial use.
Long-Term Cost Efficiency
Even with a higher initial cost, it lasts longer and needs less maintenance. That reduces repairs, downtime, and overall project costs.
Great Weldability
Duplex stainless steel grades can be welded with standard techniques while keeping good mechanical and corrosion properties, making fabrication simpler.
Resistant to Stress Cracking
Unlike many stainless steels, it doesn’t crack under high pressure or heat, especially in chloride rich environments.
Better Fatigue Strength
It handles repeated stress well, which makes it perfect for pressure vessels, bridges, and piping systems under load.
Lower Alloy Content
Uses less nickel and molybdenum than some other stainless steels, which helps keep material costs down without sacrificing performance.
Chemical composition of Duplex stainless steel
| UNS No. | C% | Cr% | Ni% | Mo% | N% | Mn% |
|---|---|---|---|---|---|---|
| S31803 | <0.03 | 21.0–23.0 | 4.5–6.5 | 2.5–3.5 | 0.08–0.20 | <2.0 |
| S32205 | <0.03 | 22.0–23.0 | 4.5–6.5 | 3.0–3.5 | 0.14–0.20 | <2.0 |
Carbon (C ≤ 0.03%): Low carbon helps prevent carbide formation during welding, which protects against intergranular corrosion.
Chromium (Cr 21.0–23.0%): Chromium forms a passive oxide layer on the surface giving strong resistance to rust and general corrosion.
Nitrogen (Cr 0.08–0.20%): Nitrogen increases strength and improves resistance to localized corrosion like pitting and stress cracking.
Nickel (Ni 4.5–6.5%): Nickel adds toughness and helps keep the duplex structure stable, improving performance in extreme temperatures.
Molybdenum (Mo 2.5–3.5%): Molybdenum boosts resistance to pitting and crevice corrosion, especially in seawater and acidic environments.
Manganese (Mn ≤ 2.0%): Manganese helps in deoxidation during steelmaking and improves hot workability and strength
Mechanical Composition of Duplex Stainless Steel
| Grade | YS (MPa) | UTS (MPa) | % Elongation | Hardness (BHN) |
|---|---|---|---|---|
| S31803 | 420 min. | 620 min. | 25 min. | 293 max. |
| S32205 | 450 min. | 645 min. | 26 min. | 293 max. |
Yield Strength (YS): 420 MPa (S31803), 450 MPa (S32205): Indicates the stress level at which the material begins to deform permanently. Higher YS in S32205 means better load-bearing capacity before permanent deformation.
Ultimate Tensile Strength (UTS): 620 MPa (S31803), 645 MPa (S32205): Defines the maximum stress the material can withstand before breaking. A higher UTS in S32205 provides improved resistance to breakage under tension.
% Elongation: 25% (S31803), 26% (S32205): Measures the ductility or stretchability of the material before fracture. S32205 slightly exceeds S31803, offering better formability and toughness.
Hardness (BHN): ≤ 293 for both grades: Reflects the resistance to surface indentation and wear. Same maximum hardness in both grades ensures uniform strength and wear performance.
Various grades of Duplex Stainless Steel
We offer various grades of Duplex stainless steel to meet diverse industrial demands. Known for their excellent strength, corrosion resistance, and cost-efficiency, these alloys are ideal for critical applications in aggressive environments.
Regular Duplex
The duplex stainless steel 2205 is the most widely used of the Duplex Stainless Steels, accounting for over 80% of the Duplex Stainless Steel market. This alloy offers superior corrosion resistance in environments where 316L is typically used, along with the benefit of significantly higher yield strength
Lean Duplex
Lean Duplex Stainless Steels possess high strength coupled with excellent corrosion resistance, especially when compared to austenitic grades like 316L. UNS S32101 offers a stable cost advantage due to its low nickel and molybdenum content, making it ideal for structural and general-purpose applications.
UNS S32304
This grade provides better mechanical strength and corrosion resistance than 304/316L stainless steel, particularly in mildly aggressive environments. The duplex stainless steel composition ensures cost-efficiency while maintaining performance in a variety of industrial applications.
Super Duplex (UNS S32750, S32760)
These grades offer excellent resistance to pitting, crevice corrosion, and stress corrosion cracking. With higher strength and durability, they are well-suited for offshore, chemical, and marine applications.
Applications of Duplex Stainless Steel
Duplex stainless steel is used in many industries because it combines strength and corrosion resistance.
Oil & Gas: Duplex is a top choice for offshore platforms and underwater piping. It handles tough fluids like those with chlorides and hydrogen sulfide. It's used in risers, flowlines, subsea manifolds, umbilicals, and valves, places where strength, corrosion resistance, and weight savings matter.
Chemical / Petrochemical Processing: It’s widely used in chemical plants for tanks, heat exchangers, pumps, and piping. Duplex holds up well against harsh acids, caustics, and chlorides. In pulp and paper plants, it's used for bleach tanks, digesters, and liquor tanks, offering longer life and lighter weight than standard grades.
Desalination / Water Treatment: Duplex steels are perfect for brine-heavy systems like desalination plants. They’re used in evaporators, tanks, and piping, where corrosion is intense. The high strength allows thinner walls, saving material and welding work.
Food, Beverage & Storage Tanks: Duplex grades such as S32101 are utilized for large tanks and vessels in the food and beverage industry. Its resistance to cleaning chemicals, alcohols and organic acids make it ideal for use in wineries, breweries, dairies and sugar plants, while reducing material costs.
Architecture & Infrastructure: Duplex steel is showing up in bridges, railings, and building facades. It’s strong, holds up to weather, and keeps a clean look even near saltwater.
Pulp & Paper: One of the duplex’s uses was in pulp mills. It’s still used today in digesters and bleach plants because it resists the chemicals and avoids corrosion issues that damaged older stainless grades.
Other Sectors: It is found in duplex in nuclear power plants (heat exchangers), marine hardware, medical tools, and even car exhaust systems. It’s used in places where high strength and corrosion resistance are a must.
Embrittlement and Phase Stability
Duplex stainless steels are strong and tough, but like all alloys, they can break down if exposed to certain temperatures for too long. The two main risks are 475°C embrittlement and sigma phase formation—both of which affect strength, toughness and corrosion resistance.
475°C Embrittlement (Alpha Prime – α′ Phase)
This happens when duplex steel is held for long periods between 300°C and 500°C. In this range, the ferrite phase can break down and form a hard, brittle structure called alpha prime (α′). This makes the steel less tough and more likely to crack. It also hurts corrosion resistance. Duplex steels are less sensitive to this than pure ferritic steels, because only half the structure is ferrite. Still, all duplex grades can suffer from this over time. It’s worst in high-molybdenum grades, and less of an issue in lean grades. To avoid it, don’t heat or stress-relieve duplex steel in this range. Most design rules limit long-term use to under 315°C (600°F) to be safe.
Sigma Phase and Other Intermetallics
At higher temperatures, roughly 600°C to 1000°C, a different issue can occur. Duplex steels may form sigma phase, a brittle compound rich in chromium and molybdenum. This phase drastically reduces both toughness and resistance to pitting corrosion. Sigma forms when the steel stays too long in this temperature range, especially during slow cooling or incorrect heat treatment. Modern duplex grades (like 2205) are designed to resist sigma phase, but they must still be quenched quickly after solution annealing to prevent it.
Nitrides and Carbides
Duplex steels are made with very low carbon, so they’re not prone to carbide problems like some stainless steels. However, chromium nitrides (Cr₂N) may still form if the steel gets too hot or isn't cooled fast enough, usually between 800°C and 900°C. These are rare and usually minor, but can lower corrosion resistance in spots if not managed.
Corrosion Resistance of Duplex Stainless Steel
Duplex stainless steel offers excellent corrosion resistance in a wide range of environments, including salty, acidic, and chemically aggressive conditions. This makes it suitable for use in various industries where long-term durability is essential.
Stress Corrosion Cracking (SCC)
Due to its ferritic content, duplex stainless steel demonstrates superior resistance to stress corrosion cracking compared to austenitic grades. This is particularly important in high-temperature, chloride-rich, or pressurized environments.
Resistance to Acid
Duplex grades also perform well in acidic environments, offering good resistance to common industrial acids such as sulfuric and nitric acid. This acid resistance makes them ideal for use in industries where exposure to harsh acidic solutions is routine.
Localized Corrosion (Pitting and Crevice)
Duplex stainless steel is highly resistant to localized corrosion, such as pitting and crevice attack. This is due to its high levels of chromium, molybdenum, and nitrogen, which enhance the passive film on the surface. As a result, duplex grades are well-suited for use where stagnant fluids or crevices can pose a risk to traditional stainless steels.
Duplex Stainless Steel Microstructure
Duplex stainless steel is characterized by a mixed microstructure composed of approximately 50% austenite and 50% ferrite phases. This dual-phase structure is achieved through careful control of alloying elements such as chromium, nickel, molybdenum, and nitrogen during production. The austenite phase provides good toughness and corrosion resistance, while the ferrite phase contributes to high strength and resistance to stress corrosion cracking. This balanced structure allows duplex stainless steel to offer an excellent combination of mechanical properties and corrosion resistance, making it suitable for demanding industrial environments.
Duplex steel vs stainless steel
Duplex stainless steel is characterized by a mixed microstructure composed of approximately 50% austenite and 50% ferrite phases. This dual-phase structure is achieved through careful control of alloying elements such as chromium, nickel, molybdenum, and nitrogen during production. The austenite phase provides good toughness and corrosion resistance, while the ferrite phase contributes to high strength and resistance to stress corrosion cracking. This balanced structure allows duplex stainless steel to offer an excellent combination of mechanical properties and corrosion resistance, making it suitable for demanding industrial environments.
Duplex steel vs stainless steel
Strength:
Duplex Steel: Offers roughly twice the yield strength of standard austenitic stainless steels.
Stainless Steel: Lower strength compared to duplex, suitable for general structural applications.
Corrosion Resistance:
Duplex Steel: Superior resistance to stress corrosion cracking, pitting, and crevice corrosion, especially in chloride-rich or aggressive environments.
Stainless Steel: Good corrosion resistance, but less effective in harsh chemical or saltwater conditions.
Weldability and Fabrication:
Duplex Steel: More challenging to weld and form due to its dual-phase nature and need for precise heat control.
Stainless Steel: Easier to fabricate and weld, widely used in standard industrial and consumer applications.
Cost and Application:
Duplex Steel: More cost-effective than high-end stainless steels for demanding environments due to thinner sections and longer lifespan.
Stainless Steel: Lower initial cost and sufficient for less demanding applications like kitchen equipment, architecture, and piping in mild environments.
Guidelines for welding duplex
Selection of Material
Use duplex stainless steel with controlled chemistry to ensure proper phase balance. Choose matching or slightly over-alloyed filler metals (like ER2209 for 2205) to compensate for any ferrite loss during welding.
Cleaning of the Material
Thoroughly clean the material before welding. Remove oil, grease, moisture, oxides, and other contaminants to prevent inclusions, porosity, and loss of corrosion resistance during welding.
Joint Design
Design joints to minimize restraint and support balanced heat input. Use wide enough gaps to allow weld metal flow and to avoid incomplete fusion. Proper joint geometry reduces stress concentration and distortion.
Preheating
Preheating is generally not required for duplex stainless steels. However, in very cold environments (< 5°C), slight preheating (~50°C) may be applied to avoid moisture-induced cracking.
Heat Input and Interpass Temperature
We maintain moderate heat input. typically between 0.5 and 2.5 kJ/mm. Excessive heat input or high interpass temperature, can lead to harmful phase formation (e.g., sigma phase). Keep interpass temperature below 150°C to retain toughness and corrosion resistance.
Postweld Heat Treatment
In most cases, duplex stainless steels do not require postweld heat treatment. However, if sigma or other intermetallic phases are suspected (due to excessive heat or slow cooling), solution annealing followed by rapid quenching may be necessary.
Desired Phase Balance
We aim for a balanced microstructure in the weld zone. Achieving this balance ensures optimum mechanical strength and corrosion resistance. Proper filler selection, heat control, and cooling rate are key to maintaining this phase ratio.
Other Joining Techniques We Use for Duplex Stainless Steel
We employ several advanced joining methods beyond traditional welding to ensure strong, reliable connections with duplex stainless steel:
Brazing
We perform precision brazing when working with thin-walled duplex components or joining dissimilar metals. By carefully selecting compatible filler materials and controlling the temperature, we prevent unwanted intermetallic phase formation and ensure joint integrity.
Mechanical Fastening
For assemblies that require modular construction or future disassembly, we use bolting, riveting, and custom clamps. Duplex’s high mechanical strength supports durable mechanical joints even under dynamic loads.
Explosion Bonding
In applications such as pressure vessels or heat exchangers, we apply explosion bonding to join duplex with dissimilar metals like carbon steel. This method creates a solid metallurgical bond without melting the base metals.
Friction Stir Welding (FSW)
We utilize FSW for select duplex grades, especially lean duplex. This technique keeps heat input low, minimizes distortion, and preserves phase balance—all while producing clean, strong joints.
Electron Beam and Laser Welding
For high-precision, automated joining, we implement electron beam and laser welding. These processes minimize the heat-affected zone and are ideal for thin-walled or high-spec applications.
Adhesive Bonding
In certain architectural or non-structural applications, we use industrial-grade adhesives. With proper surface preparation, this technique delivers clean finishes and corrosion-resistant joints.
Conclusion
Duplex stainless steels such as UNS S31803, S32205, and Super Duplex S32750 are high-performance alloys that combine the best of both austenitic and ferritic stainless steels. They provide nearly twice the strength of 304/316, along with superior corrosion resistance, especially in chloride-rich and high-stress environments. Grades like 2205 duplex help reduce material usage, extend service life, and address stress corrosion cracking challenges in aggressive operating conditions. However, achieving optimal performance requires strict attention to welding practices, heat input, and phase balance, while avoiding exposure to embrittlement-prone temperature ranges.
At Calico Metal Industries, we understand the demands of modern industrial applications. We supply premium-grade duplex and super duplex stainless steel products, precisely processed and quality-checked for critical performance. Reach out to us today for customized solutions, expert support, and reliable duplex stainless steel supply. Let us help you get the most from your materials.
