Duplex & Super Duplex Stainless Steel Pipe: 2205 vs 2507 Guide

Quick Answer

Duplex stainless steels have a two-phase microstructure of roughly 50% ferrite and 50% austenite, which gives them about double the yield strength of 316L plus far better resistance to chloride stress corrosion cracking (SCC) — the failure mode that limits austenitic 300-series stainless in seawater and chloride-bearing service. The two main grades are duplex 2205 (UNS S32205, the modern tighter-composition version of S31803: ~22% Cr, 5% Ni, 3% Mo, 0.15% N, PREN ≥35, minimum yield 450 MPa / 65 ksi) and super duplex 2507 (UNS S32750, and S32760 which adds tungsten and copper: ~25% Cr, 7% Ni, 4% Mo, 0.25% N, PREN ≥40, minimum yield 550 MPa / 80 ksi). PREN = %Cr + 3.3(%Mo) + 16(%N) predicts pitting and crevice resistance; standard duplex exceeds 30, super duplex exceeds 40, and PREN ≥40 is the accepted threshold for warm seawater. Pipe is supplied to ASTM A790 (A789 covers tubing), always in the solution-annealed and quenched condition. The critical limits: do not use duplex above about 250–300°C sustained (sigma phase and 475°C embrittlement destroy ductility), and welding requires ER2209 filler, argon-nitrogen shielding, and interpass temperature control (typically max 150–200°C) to maintain the ferrite/austenite balance.

A piping engineer on a North Sea platform faced a recurring problem: 316L firewater piping in the splash zone needed replacing every seven years because of chloride pitting. The specification team proposed duplex 2205. Procurement rejected it — duplex cost around 40% more per metre. The argument that eventually won was not corrosion resistance but arithmetic: duplex's doubled yield strength allowed a thinner wall, and the service life extended far beyond seven years. Lifecycle cost, not purchase price, made the case.

That is the duplex proposition in one story. It is not a universal replacement for 316L — in mild service it is simply more expensive steel. But where chlorides, stress, and temperature combine to attack austenitic stainless, duplex changes the economics entirely, and super duplex extends that into direct seawater and subsea service where nothing cheaper survives.

The trade-off is that duplex is less forgiving. Its properties depend on maintaining a balanced two-phase microstructure, and that balance can be destroyed by bad welding practice or by operating outside its temperature window. Specify it correctly and it lasts decades; weld it like ordinary stainless and it can embrittle in service.

For piping engineers, materials specialists, and procurement teams working in offshore, desalination, chemical, and marine projects — this guide covers duplex and super duplex pipe: the grades and what distinguishes them, PREN and corrosion behaviour, ASTM A790 requirements, welding and temperature limits, and how to specify.

For related materials, see Stainless Steel Pipe, Stainless Steel Plate, and Heat Exchanger Tube Materials.

What Makes Duplex Different

Austenitic stainless (304, 316L) has a single-phase austenitic structure. Ferritic stainless has a single-phase ferritic structure. Duplex has both — approximately equal parts, typically requiring not less than 30% of the minor phase.

This dual-phase structure is achieved by controlling chromium, nickel, molybdenum, and nitrogen content, then solution annealing and quenching to lock in the balance.

The result combines the strengths of both families:

The strength advantage matters commercially. Double the yield strength means thinner walls for the same pressure — lighter equipment, less weld metal, lower support loading, and often a cost offset against the higher price per tonne. On offshore topsides, weight saving alone can justify the change.

The Grades

Lean Duplex (2304 / S32304)

Lower alloy content, lower cost, PREN around 24–26. Used where moderate strength improvement over 304/316 is wanted but chloride severity is low — storage tanks, structural, some water applications.

Duplex 2205 (S32205 / S31803)

The workhorse grade and by far the most commonly specified.

• Composition: ~22% Cr, 4.5–6.5% Ni, 2.5–3.5% Mo, plus nitrogen
• S31803 is the original designation; S32205 is the modern, tighter-composition version with controlled nitrogen (~0.14–0.15%)
• The higher, controlled nitrogen in S32205 raises PREN above 34–35 and improves both pitting resistance and weld-zone toughness compared with S31803
• Minimum yield ~450 MPa (65 ksi) — roughly double 316L
• Roughly twice the yield strength of 316L with far better chloride SCC resistance

Specify S32205 rather than S31803 for new work — it is the better-controlled grade and generally available at the same price. State the exact UNS designation on the requisition.

Applications: offshore platform topside piping, chemical tankers, process piping, chemical and petrochemical plant, desalination, pulp and paper, storage.

Super Duplex 2507 (S32750) and S32760

• Composition: ~25% Cr, ~7% Ni, ~4% Mo, ~0.25% N
• PREN ≥40, minimum yield ~550 MPa (80 ksi)
• S32760 adds tungsten and copper for extra resistance in aggressive chloride and acidic media; the tungsten addition supports a modified PREN ≥40

Super duplex is rated as more resistant than 904L and approaching the 6% molybdenum "super austenitic" grades, at lower cost than nickel alloys.

Applications: direct and warm seawater systems, subsea manifolds and flowlines, hot seawater cooling, sour gas service, desalination high-pressure sections, aggressive chemical processing.

Upgrade from 2205 to 2507 when chloride severity, temperature, or crevice conditions exceed what standard duplex can handle — particularly direct seawater contact.

PREN: What It Does and Doesn't Tell You

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3(%Mo) + 16(%N)

Higher PREN means better resistance to pitting and crevice attack in chloride environments.

• Standard duplex (2205): PREN ~34–35
• Super duplex (2507/2760): PREN 40+
• PREN ≥40 is the accepted threshold indicating good pitting and crevice resistance in warm seawater

The caveat that matters: PREN compares alloy chemistry only. Actual corrosion performance also depends on temperature, chloride concentration, pH, deposits, welding quality, and surface condition. A high-PREN alloy welded badly, or left with heat tint and embedded iron, can pit where the number says it should not. PREN is a screening tool, not a guarantee.

ASTM A790 and Ordering Requirements

ASTM A790 / A790M covers seamless and straight-seam welded duplex and super duplex stainless pipe for pressure fluid conveyance, including grades 2205, 2507, S32760, S32520 and S32550.

ASTM A789 covers tubing — the sizing conventions and typical design use differ. Do not use them interchangeably.

Every A790 pipe must be delivered in the solution-annealed and quenched condition. This is not optional — it is what establishes the phase balance.

Related specifications:

• A182 F51 (2205), F53 (2507), F55 (S32760) — forgings, flanges, fittings
• A815 — wrought fittings in duplex grades
• A240 — plate and sheet
• EN 10216-5 — European seamless equivalent

Sizes and Schedules

• Commonly NPS ½" to 24", seamless or welded
• Schedules per ASME B36.19M: 5S, 10S, 40S, 80S
• Ends: plain end, bevelled end, threaded as ordered

See Pipe Schedule Chart for dimensional reference.

What to Verify on the MTC

Buyers typically check, in order:

1. Chemistry — Cr, Mo, Ni, and N ranges (nitrogen is the one most often overlooked and it drives PREN and weld toughness), plus C, Mn, Si, P, S limits
2. Ferrite count — confirming phase balance
3. Corrosion testing — ASTM G48 pitting test where specified
4. Heat treatment records — solution annealing temperature and quench rate
5. Mechanical properties — yield, tensile, elongation, impact
6. Heat/lot traceability consistent through delivery, cutting, and fabrication staging

A complete example specification:

ASTM A790/A790M, UNS S32205 duplex stainless steel seamless pipe, NPS 4, Schedule 40S, 6000 mm length, bevelled ends, solution heat-treated and pickled condition, hydrostatic testing, PMI and EN 10204 3.1 certificate, for chloride-containing chemical service.

Temperature Limits — The Critical Constraint

Duplex has a narrower temperature window than austenitic stainless, and exceeding it destroys the properties you paid for.

Upper limit: do not use duplex above roughly 250–300°C in sustained service.

Between about 300–1000°C, several phase transformations occur. The most significant is the eutectoid decomposition of delta-ferrite into sigma (σ) phase and secondary austenite. Sigma phase dramatically reduces ductility and toughness. Additionally, "475°C embrittlement" affects the ferrite phase over long exposure in the 300–500°C range.

Lower limit: continuous service is generally quoted from about −50°C; below that, impact testing verification is required.

Practical rule: duplex is a corrosion-resistance and strength material for ambient-to-moderate temperature service. For high-temperature duty, use austenitic or alloy steels instead — see Alloy Steel Pipe A335.

Welding Duplex

Welding is where duplex projects succeed or fail, because the weld must reproduce the ferrite/austenite balance.

Key requirements:

• Filler metal: ER2209 for 2205 (over-alloyed in nickel to promote austenite formation on cooling); ER2594 for super duplex
• Shielding gas: argon with nitrogen addition helps maintain nitrogen content and austenite formation
• Interpass temperature control: typically maximum 150–200°C — excessive heat input or slow cooling promotes intermetallic phases
• Heat input control: both too low (excess ferrite, poor toughness) and too high (sigma phase) are damaging
• Root protection: full purge to prevent oxidation
• Post-weld: pickling and passivation to restore the passive layer; heat tint must be removed, as it is a pitting initiation site

No PWHT in the carbon-steel sense — duplex is not stress-relieved the way Cr-Mo steels are. If heat treatment is required, it is a full solution anneal and quench, which is usually impractical in the field.

Verify with ferrite measurement on production welds where the specification requires it.

When to Specify Duplex

Consider duplex when:

• Chloride concentration exceeds roughly 1,000 ppm
• Temperature exceeds ~60°C in chloride service (chloride SCC risk rises sharply with temperature)
• 316L is experiencing pitting, crevice corrosion, or SCC in existing service
• Wall thickness reduction delivers meaningful weight or cost savings
• Crevice-prone interfaces exist (flanged joints, gaskets, supports, under deposits)
• Sour service requires both strength and corrosion resistance (verify NACE MR0175 compliance)

Consider super duplex when:

• Direct seawater contact, especially warm seawater
• Subsea service
• Chloride severity or temperature exceeds 2205's window
• Aggressive acidic-chloride media

Do not specify duplex for high-temperature service, or where the extra strength and corrosion resistance deliver no benefit — it is simply expensive steel in mild duty.

Common Specification Mistakes

After 15+ years supplying piping materials to offshore, desalination, and process projects:

Mistake 1: Specifying "Duplex" Without a UNS Number

Requisition says "duplex stainless". Lean duplex, 2205, and super duplex have very different composition, corrosion performance, and cost.

Prevention: State the exact UNS designation (S32205, S32750, S32760) required by the project.

Mistake 2: Accepting S31803 Where S32205 Is Needed

Older S31803 supplied against a 2205 requirement. Its wider composition band can give lower nitrogen, lower PREN, and poorer weld-zone toughness.

Prevention: Specify S32205 explicitly and verify the nitrogen content on the MTC.

Mistake 3: Using Duplex Above Its Temperature Limit

Duplex specified for service around 300°C or higher. Sigma phase forms over time and ductility collapses.

Prevention: Limit sustained service to roughly 250°C (some codes and suppliers cite 300°C maximum). Use austenitic or alloy steel above that.

Mistake 4: Welding with Standard Stainless Practice

308L/316L filler used, or heat input and interpass temperature uncontrolled. The weld ends up excessively ferritic or contains intermetallics — poor toughness and corrosion resistance exactly where the pipe is most vulnerable.

Prevention: Qualify procedures specifically for duplex — ER2209/ER2594 filler, argon-nitrogen shielding, interpass max 150–200°C, controlled heat input, full purge, post-weld pickling.

Mistake 5: Relying on PREN Alone

Material selected purely on PREN, ignoring actual temperature, chloride level, pH, and crevice conditions.

Prevention: Use PREN as a screening tool, then verify against service conditions and, where critical, corrosion test data (ASTM G48).

Mistake 6: Leaving Heat Tint and Embedded Iron

Welds and cut ends not pickled and passivated. Heat tint and iron contamination become pitting initiation sites — the pipe corrodes despite its high PREN.

Prevention: Specify pickled and passivated condition; control fabrication to avoid carbon steel contamination (dedicated tools, stainless brushes).

Mistake 7: Confusing A790 and A789

Tubing ordered against a pipe specification or vice versa; dimensions and design basis do not match.

Prevention: A790 for pipe, A789 for tubing. State the correct one.

Supply from Kasko Makine

Kasko Makine supplies duplex and super duplex stainless steel pipe and components for offshore, desalination, chemical, marine, and process projects:

Grades:

• Lean duplex: S32304
• Duplex: S32205 / S31803 (2205)
• Super duplex: S32750 (2507), S32760, S32520, S32550

Products:

• Seamless and welded pipe to ASTM A790 / A790M, EN 10216-5
• Tubing to ASTM A789
• Fittings to ASTM A815
• Flanges and forgings to ASTM A182 F51 / F53 / F55
• Plate and sheet to ASTM A240

Sizes: NPS ½" to 24" (larger by enquiry); schedules 5S, 10S, 40S, 80S per ASME B36.19M; PE, BE, or threaded ends

Condition: solution annealed and quenched, pickled and passivated

Certification and testing:

• EN 10204 Type 3.1 / 3.2 material test certificates
• Full chemical analysis including nitrogen
• Ferrite count measurement
• ASTM G48 pitting corrosion testing where specified
• Solution annealing temperature and quench rate records
• Hydrostatic testing, PMI, NDT per PO/ITP
• NACE MR0175 / ISO 15156 for sour service
• Heat and lot traceability list

Engineering support:

• Grade selection (2205 vs 2507) against chloride level and temperature
• PREN and corrosion assessment for the service
• Wall thickness optimisation using duplex's higher allowable stress
• Welding procedure guidance (filler, shielding, interpass control)
• Temperature limit review
• Matching fitting and flange specification

Logistics: Duplex and super duplex pipe shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Standard 2205 sizes 8-14 weeks; super duplex and large diameters 12-20 weeks.

Need duplex or super duplex pipe? Send us the exact UNS grade, size and schedule, quantity, service conditions (fluid, chloride level, temperature, pressure), required heat treatment condition, and inspection/testing requirements to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm grade suitability, review the specification, and provide pricing with full certification within 48 hours.

Continue Reading: Pipe & Materials Guides

• Stainless Steel Pipe — 304, 316 and austenitic grades
• Pipe Schedule Chart — Dimensions and B36.19M schedules
• Carbon Steel Pipe — Carbon steel specifications
• Heat Exchanger Tube Materials — Corrosion-driven material selection
• Stainless Steel Plate — Plate grades

Frequently Asked Questions

Q: What is duplex stainless steel?
A: Duplex stainless steel has a two-phase microstructure of approximately 50% ferrite and 50% austenite, typically requiring not less than 30% of the minor phase. This dual-phase structure is achieved by controlling chromium, nickel, molybdenum, and nitrogen content, then solution annealing and quenching to lock in the balance. The result combines advantages of both phases: roughly double the yield strength of 316L austenitic stainless (450 MPa minimum for 2205 versus around 205 MPa for 316L) together with substantially better resistance to chloride stress corrosion cracking, pitting, and crevice corrosion. Duplex pipe is supplied to ASTM A790, always in the solution-annealed and quenched condition.

Q: What is the difference between duplex 2205 and super duplex 2507?
A: Duplex 2205 (UNS S32205, or the older S31803) contains approximately 22% chromium, 5% nickel, 3% molybdenum and 0.15% nitrogen, giving a PREN of 35 or above and a minimum yield strength of 450 MPa (65 ksi). Super duplex 2507 (UNS S32750) contains approximately 25% chromium, 7% nickel, 4% molybdenum and 0.25% nitrogen, achieving PREN of 40 or above and minimum yield of 550 MPa (80 ksi). The higher alloy content makes super duplex significantly more resistant to pitting and crevice corrosion in aggressive chloride environments, which is why it is specified for direct and warm seawater, subsea service, and sour gas. UNS S32760 is a super duplex variant adding tungsten and copper for extra resistance in aggressive chloride and acidic media.

Q: What is PREN and what does it mean?
A: PREN stands for Pitting Resistance Equivalent Number, calculated as %Cr + 3.3(%Mo) + 16(%N). It predicts an alloy's resistance to pitting and crevice corrosion in chloride environments — the higher the number, the better the resistance. Standard duplex grades such as 2205 achieve PREN above 30 (typically 34–35), while super duplex grades such as 2507 exceed 40. A PREN of at least 40 is the accepted threshold indicating good pitting and crevice corrosion resistance in warm seawater, rated as more resistant than 904L and approaching the 6% molybdenum super austenitic grades. Importantly, PREN compares alloy chemistry only — actual corrosion performance also depends on temperature, chloride concentration, pH, deposits, welding quality, and surface condition.

Q: What is the maximum temperature for duplex stainless steel?
A: Duplex stainless steel should not be used above approximately 250–300°C in sustained service. Between about 300°C and 1000°C, phase transformations occur — most significantly the eutectoid decomposition of delta-ferrite into sigma phase and secondary austenite. The appearance of sigma phase dramatically decreases ductility and toughness. Additionally, 475°C embrittlement affects the ferrite phase during long exposure in the 300–500°C range. The continuous service range is generally quoted as −50°C to around 250°C, with impact testing verification required below −50°C. Duplex is therefore a corrosion-resistance and strength material for ambient to moderate temperatures; austenitic or alloy steels should be used for high-temperature duty.

Q: How is duplex stainless steel pipe welded?
A: Duplex welding must reproduce the balanced ferrite/austenite microstructure, so it requires specific procedures. Use ER2209 filler metal for 2205 (over-alloyed in nickel to promote austenite formation on cooling) and ER2594 for super duplex. Use argon shielding gas with nitrogen addition to maintain nitrogen content and austenite formation. Control interpass temperature to a maximum of typically 150–200°C, and control heat input carefully — too low produces excess ferrite with poor toughness, too high promotes sigma phase and other intermetallics. Provide a full purge for root protection. After welding, pickle and passivate to restore the passive layer and remove heat tint, which is otherwise a pitting initiation site. Duplex is not stress-relieved like Cr-Mo steel; any heat treatment must be a full solution anneal and quench.

Q: When should duplex be used instead of 316L?
A: Specify duplex when chloride concentration exceeds roughly 1,000 ppm, when temperature exceeds around 60°C in chloride service (chloride stress corrosion cracking risk rises sharply with temperature), when 316L is already experiencing pitting, crevice corrosion or SCC in similar service, when crevice-prone interfaces exist such as flanged joints and supports, or when duplex's doubled yield strength allows a wall thickness reduction that delivers meaningful weight and cost savings. Upgrade further to super duplex 2507 for direct or warm seawater, subsea service, and the most aggressive chemical duties. Duplex is not a universal replacement for 316L — in mild service it is simply more expensive steel, and its narrower temperature window makes it unsuitable above about 250°C.