Duplex Stainless Steels for Storage Tanks

The Numbering System for Duplex Stainless Steels: A Practical Guide

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In industries such as oil and gas, chemical processing, desalination, and marine engineering, the choice of materials is critical. Among the most trusted materials are duplex stainless steels (DSS)—known for their excellent combination of strength and corrosion resistance. However, selecting the right grade can be confusing, especially with the various numbering systems used globally. This guide aims to demystify the numbering systems for duplex stainless steels, helping engineers, procurement specialists, and technical buyers make informed decisions based on standardized identifiers.

What Are Duplex Stainless Steels?

Duplex stainless steels are a family of stainless steels characterized by a dual-phase microstructure consisting of both austenite and ferrite. This unique structure gives them:

  • Higher strength than austenitic stainless steels (e.g., 304 or 316)
  • Improved stress corrosion cracking resistance
  • Good weldability and formability
  • High resistance to pitting and crevice corrosion

There are various subcategories:

  • Lean Duplex (e.g., UNS S32304)
  • Standard Duplex (e.g., UNS S31803, S32205)
  • Super Duplex (e.g., UNS S32750, S32760)
  • Hyper Duplex (e.g., UNS S32707)

Understanding the Numbering Systems

The most common numbering systems used for duplex stainless steels include:

  1. UNS (Unified Numbering System)

The UNS system, widely used in North America and globally, assigns a six-character alphanumeric code starting with “S” for stainless steels. It is recognized by ASTM and SAE.

Format: SXXXXX

Examples:

UNS Number Common Name Description
S31803 Duplex 2205 First widely used duplex grade
S32205 2205 (Modified) Improved version of S31803 with tighter composition control
S32750 Super Duplex 2507 High strength and excellent corrosion resistance
S32760 Zeron 100 (common trade name) Designed for aggressive environments

Key Insight: The UNS system is ideal for global procurement and specifications because it is material-composition-specific and not tied to any manufacturer.

  1. EN (European Standard – EN 10088 / Werkstoffnummer)

Europe uses both a name-based system and the Werkstoffnummer (W-Nr.), especially in Germany and central Europe.

Name-Based Example:

  • 1.4462 → EN 1.4462 corresponds to UNS S32205
  • 1.4410 → EN 1.4410 corresponds to UNS S32750

Format:

  • Werkstoffnummer: 1.xxxx (numeric code)
  • Name-based: XxCrXNiXMo (e.g., X2CrNiMoN22-5-3)

Comparison Table:

EN Number UNS Number Common Trade Name
1.4462 S32205 2205
1.4410 S32750 2507
1.4501 S32760 Zeron 100

Tip: When dealing with European mills or certs, look for EN 10204 3.1 or 3.2 certificates, and cross-reference the EN/W.Nr. with the corresponding UNS.

  1. ASTM/ASME Standards

ASTM and ASME standards provide specifications rather than identifiers, but they often reference UNS numbers in material standards.

Common ASTM Standards:

  • ASTM A240 – For plate, sheet, and strip
  • ASTM A790 – Seamless and welded pipe
  • ASTM A789 – Tubing (seamless and welded)
  • ASTM A182 – Forgings

Example:

  • ASTM A790 UNS S32750 – Duplex pipe specification
  • ASTM A182 F51 – Forging grade equivalent to UNS S31803
  • ASTM A182 F53 – Equivalent to S32750
  • ASTM A182 F55 – Equivalent to S32760

Note: ASTM F51, F53, and F55 are grade names for forgings, often used in valves, flanges, and fittings.

How to Interpret a Duplex Stainless Steel Grade

To illustrate, let’s interpret UNS S32205:

  • “S” – Stainless steel category
  • “32” – Indicates a duplex grade (austenitic-ferritic)
  • “205” – Specifies the exact composition and distinguishes it from similar alloys (S31803)

When comparing this to EN 1.4462, we can see they are the same alloy under different standards.

Duplex Stainless Steels

Duplex Stainless Steels

Why This Matters in Oil & Gas and Chemical Applications

For engineers and buyers in corrosive environments, especially those involving:

  • Chloride-rich media (e.g., seawater, chemical slurries)
  • Sour gas (H₂S-containing environments)
  • High-pressure/high-temperature service (HPHT)

…it’s critical to specify the correct material grade using the appropriate numbering system to:

  • Avoid material substitution errors
  • Ensure compliance with client/project standards
  • Prevent premature failure due to incorrect metallurgy

Best Practices for Material Selection and Procurement

  1. Always specify by UNS number in international procurement – It ensures clarity regardless of local naming systems.
  2. Cross-check with EN/Werkstoffnummer when dealing with European mills.
  3. Match the product form to the applicable ASTM/ASME specification.
  4. Request MTCs (Mill Test Certificates) showing:
    • Exact composition
    • Mechanical properties
    • Corrosion test results (e.g., PREN, pitting resistance)
  5. Consult with material engineers if the application involves sour service or extreme conditions.

Conclusion

Understanding the numbering system for duplex stainless steels is essential for material engineers, buyers, and project managers alike. It ensures not only technical compliance but also operational reliability in some of the world’s harshest environments.

As global supply chains become increasingly interconnected, having fluency in systems like UNS, EN/Werkstoffnummer, and ASTM standards will empower you to source correctly, avoid costly errors, and ensure long-term performance in critical applications.

Need help choosing the right duplex stainless steel grade for your project? Our technical sales team is ready to assist with grade selection, documentation support, and fast delivery worldwide. Contact us at [email protected] today for technical consultation or a customized quote.