Duplex stainless steel naming rules and common grades

Duplex stainless steel is a stainless steel with both austenite and ferrite phases.

Due to its high strength and excellent corrosion resistance (especially resistance to chloride ion stress corrosion), it is widely used in chemical industry, marine engineering, pressure vessels, bridge construction and other fields.

Common grades such as 2205, 2304, 1.4462, S32205, etc., their naming rules contain the logic of material composition, performance level and standard system.

This article will systematically analyze the naming method of duplex stainless steel from the aspects of classification, naming rules, international standard comparison and typical grade analysis.

1. Introduction

The quest for materials that can withstand increasingly aggressive environments while offering structural integrity and economic viability has led to significant advancements in metallurgy.

Among these, duplex stainless steel represents a significant leap forward, offering a synergistic blend of properties that often surpasses those of traditional single-phase stainless steels.

1.1 Definition of Duplex Stainless Steel

Duplex stainless steel (DSS) is a family of stainless steels characterized by a two-phase microstructure consisting of approximately equal proportions of austenite (face-centered cubic lattice) and ferrite (body-centered cubic lattice).

Typically, this balance is aimed at a 50/50 ferrite/austenite ratio after solution annealing and quenching, though practical ranges can vary (e.g., 40-60% of each phase).

This dual-phase structure is achieved through careful control of chemical composition, primarily with high chromium (typically 19-32%), moderate nickel (typically 3-8%), and often additions of molybdenum (up to 7.5%) and nitrogen (up to 0.4%).

Other elements like manganese, copper, silicon, and tungsten can also be present to fine-tune specific properties.

The unique microstructure of duplex stainless steel is the key to its enhanced mechanical and corrosion resistance properties compared to conventional austenitic (e.g., 304 stainless steel, 316) or ferritic (e.g., 430) stainless steels.

1.2 A Brief History of Duplex Stainless Steel

The development of duplex stainless steel (DSS) has been an evolutionary journey spanning nearly a century:

• Early Concepts (1930s): Initial experimental duplex grades emerged, primarily as castings, aiming to combine ferritic strength with some austenitic benefits, but faced weldability and embrittlement issues.
• First Generation (Post-WWII – 1960s): These alloys, with higher carbon and no intentional nitrogen, had limited applications due to poor weld HAZ properties and toughness.
• Second Generation Breakthrough (1970s-1980s): The intentional addition of nitrogen revolutionized DSS. Nitrogen stabilized austenite, improved strength, enhanced corrosion resistance, and critically, improved weldability. This era saw the rise of grades like Alloy 2205 (UNS S31803/S32205), which became the industry workhorse.
• Third Generation & Beyond (1980s-Present):
  • Super Duplex: Driven by aggressive offshore demands, grades with higher Cr, Mo, and N (e.g., Alloy 2507 – UNS S32750) were developed for superior corrosion resistance (PREN ≥40).
  • Lean Duplex: To offer cost-effective alternatives to austenitic grades, alloys with lower Ni and Mo but balanced with Mn and N (e.g., Alloy 2304 – UNS S32304) emerged, providing good strength and moderate corrosion resistance.
  • Hyper Duplex: Ongoing development focuses on even higher alloyed grades for extreme environments.

1.3 Importance of Duplex Stainless Steel

The importance of duplex stainless steel (DSS) in modern industry is substantial, stemming from its unique and highly advantageous combination of properties:

• Superior Strength: Duplex grades typically offer yield strengths approximately double that of common austenitic stainless steels (like 304/316). This allows for significant weight reduction in structures and components through thinner material sections.
• Excellent Corrosion Resistance: DSS exhibits exceptional resistance to various forms of corrosion, notably chloride stress corrosion cracking (CSCC)—a common failure mode for austenitics—and excellent resistance to pitting and crevice corrosion, especially in molybdenum-bearing and super duplex grades.
• Balanced Toughness and Ductility: While not matching austenitics at cryogenic temperatures, duplex steels provide a good balance of toughness and ductility for a wide range of service temperatures.
• Good Weldability: Modern nitrogen-alloyed duplex grades are readily weldable, provided appropriate procedures are followed to maintain their critical phase balance.
• Cost-Effectiveness (Life Cycle): Although the initial material cost can be higher than some conventional stainless steels, the superior strength (allowing less material) and enhanced corrosion resistance (leading to longer service life and reduced maintenance) often make duplex stainless steel a more economical choice over the component’s lifetime, particularly in aggressive environments.

These combined attributes make duplex stainless steel an indispensable solution for demanding applications in sectors like oil and gas, chemical processing, pulp and paper, desalination, and marine engineering, where conventional materials may fall short in performance or longevity.

2. Classification of Duplex Stainless Steel

Duplex stainless steel grades are not a single entity but a family of alloys.

They can be classified based on various criteria, most commonly their alloying content (which correlates with corrosion resistance) or their intended use.

2.1 Classification by Composition Ratio / Corrosion Resistance (PREN)

This is the most prevalent method of classifying duplex stainless steel, often linked to their Pitting Resistance Equivalent Number (PREN).

A higher PREN generally indicates better resistance to localized pitting corrosion.

Lean Duplex Stainless Steel:

• PREN: Typically in the range of 22-27.
• Composition: Characterized by lower nickel and molybdenum content compared to standard duplex grades. Often utilize manganese and increased nitrogen to stabilize austenite and enhance strength.
• Examples: UNS S32101 (LDX 2101®), UNS S32304 (Alloy 2304), UNS S82011.
• Properties: Offer better strength and stress corrosion cracking resistance than 304L/316L austenitic grades, with general corrosion resistance often comparable to 304L or slightly better. More cost-effective due to lower Ni and Mo.
• Purpose: Designed as a cost-effective alternative to 300-series austenitics in moderately corrosive environments where higher strength is beneficial.

Standard Duplex Stainless Steel:

• PREN: Typically in the range of 30-38 (often >32 is used as a common threshold).
• Composition: Contain moderate levels of chromium (21-23%), nickel (4.5-6.5%), molybdenum (2.5-3.5%), and nitrogen.
• Examples: UNS S31803 / S32205 (Alloy 2205) – This is the most widely used duplex grade and is often considered the workhorse.
• Properties: Excellent combination of high strength, good toughness, good weldability, and very good resistance to pitting, crevice corrosion, and chloride stress corrosion cracking. Significantly better than 316L in many aspects.
• Purpose: General-purpose duplex for a wide range of applications in moderately to highly corrosive environments.

Super Duplex Stainless Steel:

• PREN: Typically ≥40.
• Composition: Higher levels of chromium (24-27%), nickel (6-8%), molybdenum (3-5%), and nitrogen (0.24-0.35%). Some may contain tungsten (W) or copper (Cu).
• Examples: UNS S32750 (Alloy 2507), UNS S32760 (Zeron® 100), UNS S32550 (Ferralium® 255).
• Properties: Offer superior strength and outstanding corrosion resistance, especially in highly aggressive chloride-containing environments like seawater, and resistance to sour gas (H₂S).
• Purpose: For highly demanding applications where standard duplex or austenitic grades are insufficient, particularly in offshore oil and gas, chemical processing, and desalination.

Hyper Duplex Stainless Steel (Ultra Duplex):

• PREN: Typically >45, some definitions use >48 or even >50.
• Composition: Even higher alloying content than super duplex, pushing the limits of chromium, molybdenum, nitrogen, and sometimes tungsten.
• Examples: UNS S32707, UNS S33207.
• Properties: Designed for extreme corrosion resistance in the most aggressive environments.
• Purpose: Niche applications where maximum performance is required, often still under development or for very specific projects. Less widely available and more expensive.

2.2 Classification by Use (Less Formal, Often Overlaps with PREN Categories)

While the PREN-based classification is dominant, sometimes duplex stainless steel types are discussed in terms of their primary application drivers:

Duplex Stainless Steel for Structure:

• This often refers to lean duplex or standard duplex grades where their high yield strength is a key advantage, allowing for lighter constructions.
• Examples: Architectural supports, bridges, storage tanks.

Corrosion Resistance Used Duplex Stainless Steel:

• This primarily encompasses standard duplex, super duplex, and hyper duplex grades chosen for their ability to withstand specific corrosive media.
• Examples: Chemical processing equipment, offshore components, desalination plants.

Duplex Stainless Steel for Low Temperature:

Austenitic stainless steels are generally better suited for truly cryogenic applications.

Some duplex grades, especially standard duplex grades such as 2205, retain good toughness down to -50°C (-60°F) or slightly lower.

This makes them suitable for some cryogenic process applications where ferritic grades would be too brittle.

If the cryogenic conditions are not extreme, ferritic grades may be stronger than austenitic grades in this range.

This use-based classification is more qualitative and often reflects the primary reason a particular PREN category of duplex is chosen for an application.

3. Naming Rules of Duplex Stainless Steel

The naming and designation of duplex stainless steel grades follow various national and international standards.

Understanding these systems is crucial for accurate specification and procurement.

3.1 Naming Rules of China National Standard (GB)

Chinese (GB) standards often use a system that combines letters and numbers reflecting composition or characteristics.

For duplex stainless steels, designations often start with “S” or incorporate digits related to their UNS numbers.

Example for a grade similar to 2205: Might be designated as S22053 (aligning with UNS S32205) or a specific GB grade like 00Cr22Ni5Mo3N.

00Cr22Ni5Mo3N breakdown (approximate):

• 00: Indicates very low carbon content.
• Cr22: ~22% Chromium.
• Ni5: ~5% Nickel.
• Mo3: ~3% Molybdenum.
• N: Contains Nitrogen.

It’s important to consult the specific GB standard (e.g., GB/T 20878 for stainless steel bars, GB/T 12771 for welded stainless steel tubes) for precise designations.

3.2 Naming Rules of American Standard (ASTM/ASME)

The most widely recognized international system for designating stainless steels, including duplex stainless steel, is the Unified Numbering System (UNS).

UNS System: Duplex stainless steels are typically designated with an “S” prefix followed by five digits.

• The “S” indicates stainless steel (or heat and corrosion-resistant steels).
• The digits often, but not always, give some hint about the composition, though this is not a strict rule.
• Examples:
  • UNS S31803 (older designation for 2205)
  • UNS S32205 (common designation for 2205, with slightly tighter composition control than S31803)
  • UNS S32304 (for lean duplex 2304)
  • UNS S32750 (for super duplex 2507)
  • UNS S32760 (for another super duplex grade)

ASTM and ASME standards (e.g., ASTM A240 for plate, sheet, and strip; ASTM A789/A790 for seamless and welded pipe) will refer to these UNS numbers.

Common trade names (like “2205,” “2507,” “LDX 2101®,” “Zeron® 100”) are also widely used in industry but the UNS number is the definitive specification.

3.3 Naming Rules of European Standard (EN)

European standards (EN), such as EN 10088 (Stainless steels), use a system of numerical designations and name designations.

Numerical Designation (Steel Number): A format like 1.xxxx.

• 1. indicates steel.
• The next digit often indicates the main alloying group (e.g., 4 for stainless steels).
• The following digits are sequential or encoded.
• Examples:
  • 1.4462 (for duplex grade equivalent to UNS S32205 / 2205)
  • 1.4362 (for lean duplex grade equivalent to UNS S32304 / 2304)
  • 1.4410 (for super duplex grade equivalent to UNS S32750 / 2507)
  • 1.4501 (for another super duplex grade, similar to UNS S32760)

Name Designation (Steel Name):

A format typically starting with “X” followed by numbers indicating carbon content, then symbols and numbers for key alloying elements.

• X indicates alloy steel (high alloy content).
• The number following X divided by 100 gives the nominal carbon content (e.g., X2 means ~0.02% C).
• Chemical symbols followed by numbers indicating their percentage.
• Example for 1.4462 (2205): X2CrNiMoN22-5-3
  • X2: Carbon content ≤ 0.03% (target ~0.02%).
  • CrNiMoN: Main alloying elements Chromium, Nickel, Molybdenum, Nitrogen.
  • 22-5-3: Nominal percentages for Cr (~22%), Ni (~5%), Mo (~3%). Nitrogen is implied by the “N” but its percentage isn’t always explicitly in the name part.

3.4 Naming Rules of Japanese Standard (JIS)

Japanese Industrial Standards (JIS) also have their own system for designating stainless steels.

• JIS designations for stainless steel often start with “SUS” (Steel Use Stainless).
• Following “SUS,” there might be numbers similar to AISI grades for austenitics (e.g., SUS304, SUS316) or specific designations for other types.
• For duplex stainless steel, JIS often adopts or aligns closely with UNS numbers or has specific SUS designations.
  • Example for a grade similar to 2205: SUS329J3L (older designation) or may refer to SUS S32205 (adopting UNS).
  • Example for a super duplex grade similar to 2507: SUS329J4L (older designation) or may refer to SUS S32750.
    The “L” often indicates low carbon. The “J” can denote a Japanese-specific modification or standard.

Understanding these different naming conventions is vital when dealing with international projects, sourcing materials globally, or interpreting specifications from various origins.

The UNS system is often the most universal reference point.

4. Comparison of International Standards for Duplex Stainless Steel

While the UNS system provides a common language, specific property requirements, testing procedures, and delivery conditions for duplex stainless steel products are detailed in various international and national standards.

When specifying or procuring duplex, it’s essential to refer to the relevant product form standard (e.g., for plate, pipe, bar).

When specifying duplex stainless steel, clearly stating the required UNS number and the relevant product form standard (e.g., “Duplex Stainless Steel UNS S32205 according to ASTM A240”) is best practice to avoid ambiguity.

5. Analysis of Common Duplex Stainless Steel Grades

Several duplex stainless steel grades are prominent in industrial use, each offering a distinct balance of properties and cost. Understanding their key features is crucial for proper selection.

• Alloy 2205 (UNS S31803 / S32205): The Workhorse
  • Type: Standard Duplex (PREN ~35-36).
  • Key Features: Excellent balance of high strength (approx. twice that of 316L), very good resistance to pitting, crevice corrosion, and chloride stress corrosion cracking (CSCC). Good weldability.
  • Typical Uses: Widely used in oil & gas, chemical processing, pulp & paper, and structural applications.
• Alloy 2304 (UNS S32304): The Lean Option
  • Type: Lean Duplex (PREN ~25-26).
  • Key Features: Higher strength than 304L/316L, good CSCC resistance. Corrosion resistance generally comparable to 304L, but with structural benefits of duplex. Cost-effective due to lower Ni and Mo.
  • Typical Uses: Storage tanks, structural components, hot water systems; an alternative to 300-series austenitics in moderately corrosive environments.
• Alloy 2507 (UNS S32750): The Super Performer
  • Type: Super Duplex (PREN ≥42.5).
  • Key Features: Very high strength and outstanding corrosion resistance, especially in aggressive chloride environments (e.g., seawater). Excellent pitting and CSCC resistance.
  • Typical Uses: Demanding applications in offshore oil & gas industry, desalination plants, chemical processing with highly corrosive media.

Important Note on High-Performance Austenitic Comparators:

It’s vital to distinguish duplex grades from high-performance austenitic stainless steels, which are sometimes considered for similar applications but have different microstructures and property balances:

• 254SMO® (UNS S31254 / EN 1.4547): Super Austenitic, Not Duplex
  • Key Features: PREN ~42-45. Excellent pitting/crevice corrosion resistance (comparable to super duplex), superior formability and cryogenic toughness typical of austenitics. Lower yield strength than super duplex.
  • Typical Uses: Seawater systems, pulp/paper bleaching, chemical processing; often an alternative to super duplex where high formability is beneficial.
• 1.4435 (AISI 316L Modified): High-Purity Austenitic, Not Duplex
  • Key Features: PREN ~28-31. Enhanced corrosion resistance over standard 316L, very low ferrite, often produced for high surface finish quality.
  • Typical Uses: Pharmaceutical, biotech, and high-purity food/beverage applications requiring excellent cleanability and corrosion resistance.

This brief overview highlights the primary characteristics and application niches of these key duplex stainless steel grades and relevant high-performance austenitic alternatives.

For detailed specifications, always consult material data sheets.

Quick comparison of duplex and austenitic stainless steel grades

(Cost factors are very approximate and highly market-dependent).

This analysis shows the progression in properties (and often cost) as one moves from lean to standard to super duplex grades, and also highlights how high-performance austenitic grades like 254SMO® fit into the landscape.

6. Application Areas of Duplex Stainless Steel

The unique combination of high strength, excellent corrosion resistance (especially against chlorides and CSCC), and reasonable toughness makes duplex stainless steel a preferred material in a growing number of demanding industries:

Oil and Gas Industry (Upstream, Midstream, Downstream):

• Offshore Platforms: Piping systems, structural components, separators, scrubbers, heat exchangers (Super Duplex like 2507 or S32760 are common).
• Subsea Equipment: Flowlines, umbilicals, manifolds, Christmas trees.
• Onshore Processing: Pipelines for sour gas (H₂S containing), refining equipment.
• LNG Terminals: Storage tank components, piping.

Chemical Processing Industry (CPI):

• Tanks and Vessels: For storing and processing corrosive chemicals, acids, and chlorides.
• Piping Systems: Transporting aggressive fluids.
• Heat Exchangers, Reactors, and Distillation Columns.
• (Grades: 2205, 2507, and other specialized duplex/super duplex depending on chemical severity).

Pulp and Paper Industry:

• Digesters, Bleach Plant Equipment, Black Liquor Recovery Boilers.
• (Grades: 2205, lean duplex like 2304 or LDX 2101® for less aggressive parts).

Desalination Plants:

• High-Pressure Piping and Pumps in Reverse Osmosis (RO) Systems.
• Evaporators in Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) units.
• (Grades: Super Duplex like 2507 are extensively used due to high chloride seawater).

Marine Industry and Shipbuilding:

• Shafts, Rudders, Propellers.
• Seawater Cooling Systems.
• Cargo Tanks for Chemical Tankers.
• Offshore Wind Turbine Foundations and Structures.

Water and Wastewater Treatment:

• Pumps, Valves, Piping in corrosive water environments.
• Sludge Treatment Equipment.

Mining and Minerals Processing:

• Equipment handling abrasive and corrosive slurries.
• Leaching tanks.

Architecture, Building, and Construction:

• Structural Components: Bridges (pedestrian and road), building facades, roof trusses, support beams where high strength and corrosion resistance are needed (Lean Duplex and Standard Duplex).
• Swimming Pool Structures and Fittings: Resistance to chlorinated water.
• Coastal Structures: Handrails, supports.

Food and Beverage Industry:

• Storage Tanks, Piping, and Processing Equipment where higher strength or specific corrosion resistance beyond 304/316 is required (e.g., handling brines, certain acidic food products). (Lean Duplex often suitable).

Pollution Control Equipment:

• Flue Gas Desulfurization (FGD) Systems: Scrubbers, ductwork, and chimneys exposed to acidic and chloride-containing flue gases.

The selection of a specific duplex stainless steel grade (lean, standard, or super) depends on the severity of the corrosive environment, the required mechanical strength, temperature conditions, and economic considerations.

7. Advantages and Challenges of Duplex Stainless Steel

Duplex stainless steel (DSS) offers compelling benefits but requires careful consideration of its specific characteristics.

Key Advantages:

• Superior Strength: Roughly double the yield strength of common austenitics, enabling lighter designs.
• Excellent Corrosion Resistance: Especially against chloride stress corrosion cracking (CSCC), pitting, and crevice corrosion (high PREN grades).
• Good Weldability & Toughness: With proper procedures, DSS offers good performance in these areas.
• Favorable Life Cycle Cost: High strength and corrosion resistance can offset higher initial material costs through longevity and reduced maintenance.

Key Challenges & Limitations:

• Temperature Sensitivity: Not ideal for prolonged high-temperature service (above ~300-350°C due to embrittlement risk) or true cryogenic applications.
• More Demanding Fabrication: Machining, forming, and welding require more specialized techniques and controls compared to standard austenitic steels due to higher strength and unique metallurgy.
• Higher Initial Cost: Due to significant alloying elements (Cr, Ni, Mo, N).
• Risk of Detrimental Phases: Incorrect heat treatment or welding can lead to embrittlement.

Successful application of duplex stainless steel involves leveraging its strengths while mitigating these challenges through proper design, material selection, and fabrication practices.

8. FAQ: Duplex Stainless Steel

Q1: What makes duplex stainless steel “duplex”?

A: Its two-phase microstructure, containing roughly equal parts of austenite and ferrite, gives it unique properties.

Q2: Is duplex stronger than austenitic stainless steel (e.g., 304/316)?

A: Yes, duplex stainless steels typically have about twice the yield strength of common annealed austenitic grades like 304L or 316L.

Q3: What is PREN and why is it important for duplex grades?

A: PREN (Pitting Resistance Equivalent Number) estimates resistance to pitting corrosion based on Cr, Mo, and N content. A higher PREN indicates better resistance, crucial for selecting grades for corrosive service.

Q4: Are all duplex stainless steels magnetic?

A: Yes. The presence of the ferrite phase makes them inherently magnetic, unlike fully annealed austenitic stainless steels.

Q5: Can duplex stainless steel be used at high temperatures?

A: Duplex grades have limitations. Prolonged use above ~300-350°C (570-660°F) can lead to embrittlement due to deleterious phase precipitation. Austenitic or specialized alloys are better for high-temperature service.

Q6: What’s the main advantage of super duplex (e.g., 2507) over standard duplex (e.g., 2205)?

A: Super duplex grades (like 2507) offer significantly superior corrosion resistance (especially to pitting and crevice corrosion in harsh chloride environments) and higher strength due to increased chromium, molybdenum, and nitrogen content.

Q7: What are “lean duplex” stainless steels?

A: These are more cost-effective grades (e.g., 2304, LDX 2101®) with lower nickel and molybdenum. They use manganese and nitrogen for structure and strength, offering better strength and CSCC resistance than 304L/316L for moderately corrosive environments.

Q8: Does duplex stainless steel need passivation after fabrication?

A: It forms a passive layer naturally. Thorough cleaning after fabrication is usually sufficient. Formal chemical passivation might be specified for critical applications or after significant surface contamination to ensure optimal corrosion resistance.

9. Summary

Duplex stainless steel (DSS) stands as a sophisticated class of materials, uniquely bridging the property gap between ferritic and austenitic stainless steels.

Its hallmark dual-phase microstructure, a balanced blend of ferrite and austenite, delivers an exceptional combination of high mechanical strength, outstanding corrosion resistance—particularly against chloride stress corrosion cracking (CSCC) and localized pitting—and good weldability.

Evolving from early concepts to modern lean, standard (e.g., Alloy 2205), super (e.g., Alloy 2507), and even hyper-duplex variants, these alloys consistently meet the increasing demands of critical industries like oil and gas, chemical processing, and marine engineering.

Effective utilization hinges on understanding DSS classifications (often by PREN value), international naming conventions, and the specific attributes of common grades.

While offering significant advantages in strength and corrosion performance, DSS also presents challenges such as high-temperature limitations and more demanding fabrication procedures compared to simpler austenitic grades.

Therefore, a comprehensive evaluation of service conditions, mechanical requirements, fabrication capabilities, and life-cycle costs is essential for optimal material selection.

In essence, duplex stainless steel provides a powerful suite of solutions for demanding engineering challenges, solidifying its role as a durable, high-performance, and often economically sound choice in the realm of advanced materials.