Duplex Steel Flanges: Materials, Grades (2205/2507), Standards & Uses

Duplex steel flanges are used to support the current industrial piping systems whereby standard stainless steel (e.g., 316L) is not strong enough. These flanges offer the best corrosion strength and resistance combined with high yield strength to meet the specific requirements of chloride-rich, high-pressure and sour environments. It is, however, important to choose the appropriate material. It will be either the desalination of seawater or the offshore production of oil and gas, but the particular grade of your duplex steel flanges will dictate the life and the security of the system. Here we deconstruct the most prevalent materials, grades, and standards in the production of these important parts.

What Are Duplex Steel Flanges?

Duplex steel flanges are high-performance components produced using a special stainless steel family that comprises 50% of austenite and 50% of ferrite as a dual-phase microstructure. This balance of metallurgy enables them to provide 2 times the yield strength of the normal 300-series stainless steels. In addition to strength, they offer excellent resistance against pitting, crevice corrosion and stress corrosion cracking (SCC). Moreover, their reduced nickel composition guarantees them that they are more stable in costs amid market fluctuation, making them a consistent and economical option to the intensive industrial piping system.

Materials Used in Duplex Steel Flanges

When sourcing Duplex Steel Flanges, you will primarily encounter three categories of materials.

Duplex 2205 Flanges (UNS S32205 / S31803)

This is the “workhorse” of the industry. It offers a balanced chemistry of 22% Chromium and 5% Nickel.

• Best for: General chemical processing, brackish water, and onshore oil and gas.
• Key Benefit: Most cost-effective upgrade from SS316.

Super Duplex 2507 Flanges (UNS S32750 / S32760)

For extreme conditions, Super Duplex is the standard. It contains 25% Chromium and higher Molybdenum/Nitrogen.

• Best for: Seawater systems, desalination, and subsea pipelines.
• Key Benefit: Virtually immune to chloride-induced stress corrosion cracking.

Lean Duplex 2101 Flanges (UNS S32101)

A low-nickel, cost-focused alternative.

• Best for: Structural piping, water treatment, and moderate corrosion environments.

Duplex Grade Comparison

Duplex Steel Flanges Material Selection Guide

Choosing the right grade depends on your environment’s “Pitting Resistance Equivalent Number” (PREN) requirement.

The PREN Formula

Engineers use the PREN formula to calculate the corrosion resistance of Duplex Steel Flanges:

$$PREN = \%Cr + 3.3(\%Mo + 0.5\%W) + 16\%N$$

• Rule of Thumb: For marine or seawater service, always specify a grade with a PREN > 40 (Super Duplex).

Applications of Duplex Steel Flanges

Because Duplex Steel Flanges offer a superior strength-to-weight ratio, they are frequently used in heavy industries to reduce wall thickness and overall project costs without sacrificing safety.

Oil & Gas (Upstream & Downstream)

• Subsea Pipelines & Manifolds: High yield strength allows for thinner, lighter flanges in deep-sea environments.
• Sour Service: Resistance to sulphide stress cracking makes them ideal for “sour” oil and gas wells (NACE MR0175 compliance).

Marine & Desalination

• Seawater Cooling Systems: Super Duplex 2507 flanges are the standard for resisting pitting in high-salinity water.
• RO (Reverse Osmosis) Plants: Used in high-pressure piping to handle brine and seawater intake.

Chemical & Petrochemical

• Corrosive Media Handling: Used in heat exchangers and pressure vessels handling organic acids, caustic solutions, and chlorides.
• Storage Tanks: Flange connections for large-scale chemical storage where durability is a priority.

Pulp & Paper

• Bleaching Systems: Excellent resistance to the aggressive bleaching chemicals used in the papermaking process.

Standards & Specifications for Duplex Steel Flanges

To ensure global compatibility, ourDuplex Steel Flanges are manufactured and tested according to the following international frameworks:

Technical Specification 

Common Types of Duplex Steel Flanges

Depending on the piping connection, we manufacture several types:

• Weld Neck (WN): Best for high-pressure, critical systems.
• Slip-On (SO): Easier to align, ideal for low-pressure applications.
• Blind Flange: Used to seal the end of a piping system.
• Socket Weld (SW): Used for small-diameter, high-pressure pipes.
• Lap Joint (LJ): Used in systems that require frequent dismantling for cleaning.

Quality Control: Tests to Ask For

When buying Duplex Steel Flanges, always request the following certifications to ensure material integrity:

• PMI (Positive Material Identification): Confirms the alloy composition.
• EN 10204 3.1/3.2 Certificate: Essential for traceability.
• Hardness Testing: Crucial for NACE/Sour service compliance (typically < 28 HRC for 2205).
• Phase Balance Check: Ensures the 50/50 Austenite-Ferrite ratio was maintained during forging.