Duplex and super duplex stainless steels are the two most specified corrosion-resistant alloy families in oil and gas, offshore, desalination, and chemical processing. Both share a dual-phase austenitic-ferritic microstructure that delivers approximately twice the yield strength of standard austenitic grades like 316L — but they are not interchangeable. The choice between them depends on chloride concentration, temperature, H₂S content, and budget.

This guide covers the composition differences, PREN values, mechanical properties, ASTM grade references, and selection criteria that determine which grade is correct for a given application. For related material selection context, see our NACE MR0175 / ISO 15156 guide.

What Is Duplex Stainless Steel?

Duplex stainless steel has a microstructure of approximately 50% austenite and 50% ferrite — the same balanced two-phase structure that gives it both the toughness of austenitic grades and the strength and stress corrosion cracking resistance of ferritic grades.

The standard duplex grade is UNS S32205 (EN 1.4462), also known by the trade designation SAF 2205 or by the legacy UNS S31803 designation. It contains 21–23% chromium, 4.5–6.5% nickel, and 2.5–3.5% molybdenum, giving it a PREN (Pitting Resistance Equivalent Number) of ≥34. At PREN ≥34, duplex S32205 resists pitting corrosion in chloride environments up to approximately 1,000 ppm at moderate temperatures.

Yield strength of 450 MPa minimum — roughly double that of 316L — allows wall thickness reductions in pipework and pressure vessel design, which partially offsets the higher material cost over standard austenitic grades.

What Is Super Duplex Stainless Steel?

Super duplex stainless steel shares the same dual-phase microstructure but with significantly higher alloying content — particularly chromium (24–26%), molybdenum (3.5–4.2%), and nitrogen (0.24–0.32%). The result is a PREN of ≥40–41, which makes super duplex grades suitable for direct seawater service, offshore subsea applications, and chemical environments where duplex S32205 would suffer pitting or crevice corrosion.

The two main super duplex grades are:

• UNS S32750 (EN 1.4410 / SAF 2507): the original high-performance super duplex. 25% Cr, 4% Mo, 0.27% N. PREN typically 42–43. Dominant in offshore oil and gas and desalination.
• UNS S32760 (EN 1.4501 / Zeron 100): adds 0.5–1.0% tungsten and copper to S32750's base composition. Tungsten enhances pitting resistance further; copper improves resistance to reducing acids. PREN ≥40. Preferred in chemical process and mixed-environment service.

Yield strength of 550 MPa minimum — higher than duplex — combined with seawater-rated corrosion resistance makes super duplex the default specification for subsea, topsides, and aggressive chemical plant applications where duplex is on the margin.

Composition Comparison — Duplex vs Super Duplex

PREN = %Cr + 3.3×%Mo + 16×%N. A PREN of ≥34 is the minimum threshold for duplex service. ≥40 is required for seawater and offshore-rated super duplex service.

Mechanical Properties and Corrosion Performance

Both grades are assessed under NACE MR0175 / ISO 15156 for sour service (H₂S + CO₂ environments) in oil and gas. Super duplex S32750 and S32760 are accepted to higher H₂S partial pressures than S32205 under Part 2, and may be qualified under Part 3 as a CRA for the most aggressive sour conditions.

When to Specify Duplex vs Super Duplex

Specify Duplex S32205 when:

• Chloride levels are below ~1,000 ppm at service temperature
• The environment is moderately aggressive — brackish water, process water, mild acid service
• Pressure vessel or structural applications where higher strength than 316L is needed but full seawater resistance is not
• Budget is a primary constraint — S32205 costs significantly less than super duplex grades
• Applications: heat exchangers, pressure vessels, pulp and paper, water treatment, bridges and structures

Specify Super Duplex S32750 or S32760 when:

• Direct seawater service — offshore topsides, subsea pipework, desalination plant piping
• Chloride levels exceed 1,000 ppm or operating temperature is elevated
• Sour service (H₂S + CO₂) at higher partial pressures where S32205 is not accepted under NACE MR0175
• Chemical process streams containing chlorides plus oxidising or reducing acids
• Applications: offshore platforms, subsea valves, chemical injection systems, desalination high-pressure trains, FGD systems
• Specify S32760 specifically when tungsten or copper additions are required for mixed acid or reducing acid environments

ASTM Standards — Duplex and Super Duplex

All supply should be accompanied by an EN 10204 3.1 MTC as minimum, confirming chemical composition, mechanical properties, and heat number traceability. EN 10204 3.2 is required for offshore and classification-governed projects.

Common Specification Mistakes to Avoid

• Specifying 'duplex pipe' without the UNS number — S31803 and S32205 are both 'duplex 2205' grades but have different composition limits; always state UNS S32205 for the tighter chemistry
• Mixing up S32750 and S32760 — both are called 'super duplex' and '2507' loosely in the market, but are different grades with different compositions; state UNS number explicitly
• Ordering to ASTM A790 without specifying seamless or welded — the standard covers both; state seamless (A790) or welded (A928) explicitly
• Omitting the supply condition — duplex and super duplex pipe and fittings must be solution annealed; state explicitly on the purchase order

For a full breakdown of procurement errors in engineered alloy ordering, see our material specification mistakes guide.

Source Certified Duplex and Super Duplex Stainless Steel

Nifty Alloys LLC supplies certified duplex (UNS S32205 / 1.4462) and super duplex (UNS S32750 / S32760) stainless steel in pipe, tube, plate, sheet, bar, and forgings — to ASTM A790, A789, A182, A240, and A276 — with EN 10204 3.1 MTCs as standard. Supply to oil and gas, offshore, chemical processing, desalination, and marine projects across the UAE, GCC, and global markets.

View our duplex stainless steel product range or contact our team for availability, grade confirmation, and certification requirements.

Frequently Asked Questions

What does PREN mean and how is it calculated?
PREN (Pitting Resistance Equivalent Number) is calculated as %Cr + 3.3×%Mo + 16×%N. It's a comparative index for how resistant a stainless grade is to pitting corrosion in chloride environments — higher PREN means better resistance. A PREN ≥34 is the general threshold for duplex service; ≥40 is required for seawater and offshore-rated super duplex.

Is duplex stainless steel magnetic?
Yes, partially. Duplex and super duplex grades contain roughly 50% ferrite (magnetic) and 50% austenite (non-magnetic), so they show a moderate magnetic response — unlike standard austenitic grades like 316L, which are effectively non-magnetic.

Can duplex and super duplex be welded with standard techniques?
Yes, but with tighter heat input control than austenitic grades. Excessive heat input during welding can upset the austenite-ferrite balance, reducing corrosion resistance and toughness in the heat-affected zone. Welding procedures should be qualified specifically for duplex/super duplex, not carried over from austenitic welding practice.

What's the actual difference between S32750 and S32760 super duplex?
Both are PREN ≥40 super duplex grades, but S32760 adds tungsten and copper to its composition — tungsten improves pitting resistance further, and copper improves resistance to reducing acids. S32750 is the more common default for offshore oil & gas and desalination; S32760 is preferred where mixed acid or reducing-acid chemical service is involved.

Do I need EN 10204 3.1 or 3.2 certification for duplex/super duplex?
3.1 is the standard minimum, confirming composition, mechanical properties, and heat traceability. 3.2 adds independent third-party witness/validation and is typically required for offshore or classification-society-governed projects (ABS, DNV-GL).