Duplex steel flanges are forged or rolled piping components manufactured from duplex stainless steel — a family of alloys characterised by a two-phase microstructure of approximately equal proportions of austenite and ferrite. This dual-phase structure, maintained by precise control of Cr (21–26%), Ni (4–8%), Mo (0–4%), and N (0.05–0.33%) content, delivers a combination of mechanical and corrosion-resistance properties that neither single-phase austenitic (300-series) nor ferritic stainless steels can match individually. Duplex steel flanges achieve approximately twice the yield strength of AISI 316L, excellent resistance to pitting and crevice corrosion (PREN 25–42+ depending on grade), and outstanding resistance to chloride stress corrosion cracking (SCC) — the failure mode that makes austenitic grades unreliable in warm chloride environments above 60°C.
Tesco Steel & Engineering manufactures ASTM A182 / ASME SA182 duplex steel flanges in all standard grades — from lean duplex F54 (UNS S32304) through standard duplex F51/F60 (UNS S31803/S32205) to super duplex F53/F55 (UNS S32750/S32760) and hyper duplex F57 (UNS S39277) — in NPS ½″ through 60″, Class 150 through 2500, in all facing and end connection types. All flanges are forged per ASTM A182, dimensioned per ASME B16.5 / B16.47 / EN 1092-1, and supplied with full material test reports, PMI, and third-party inspection.
Duplex stainless steels are classified into four categories based on alloy content and PREN (Pitting Resistance Equivalent Number = %Cr + 3.3×%Mo + 16×%N):
| Grade (ASTM) | UNS | Common Name | Cr (%) | Ni (%) | Mo (%) | N (%) | Min PREN | Category |
|---|---|---|---|---|---|---|---|---|
| F54 | S32304 | Lean Duplex 2304 | 22.0–24.0 | 3.5–5.5 | 0.05–0.60 | 0.05–0.20 | ~24 | Lean |
| F51 | S31803 | Duplex 2205 (original) | 21.0–23.0 | 4.5–6.5 | 2.5–3.5 | 0.08–0.20 | ~30.5 | Standard |
| F60 | S32205 | Duplex 2205 (controlled) | 22.0–23.0 | 4.5–6.5 | 3.0–3.5 | 0.14–0.20 | ~34.1 | Standard |
| F52 | S32550 | Ferralium 255 | 24.0–27.0 | 4.5–6.5 | 2.9–3.9 | 0.10–0.25 | ~36 | Super |
| F53 | S32750 | Super Duplex 2507 | 24.0–26.0 | 6.0–8.0 | 3.0–5.0 | 0.24–0.32 | ≥41 | Super |
| F55 | S32760 | Zeron 100 | 24.0–26.0 | 6.0–8.0 | 3.0–4.0 | 0.20–0.30 | ≥40 | Super |
| F57 | S39277 | Hyper Duplex | 24.0–26.0 | 6.5–8.0 | 3.0–4.0 | 0.23–0.33 | ≥48 | Hyper |
| F59 | S32520 | UR52N+ | 24.0–26.0 | 5.5–8.0 | 3.0–4.0 | 0.20–0.35 | ≥46 | Hyper |
| F61 | S32550 | Ferralium 255 variant | 24.0–27.0 | 4.5–6.5 | 2.9–3.9 | 0.10–0.25 | ~36 | Super |
PREN = %Cr + 3.3×%Mo + 16×%N. Minimum PREN calculated from minimum specification composition values.
| Grade | 0.2% Yield Strength (MPa min) | UTS (MPa min) | Elongation (% min) | Hardness (HBW max) | vs 316L Yield |
|---|---|---|---|---|---|
| 316L (F316L) — reference | 170 | 485 | 30 | 217 | — |
| F54 (S32304) | 400 | 600 | 25 | 290 | +135% |
| F51 (S31803) | 450 | 620 | 25 | 290 | +165% |
| F60 (S32205) | 450 | 620 | 25 | 290 | +165% |
| F53 (S32750) | 550 | 795 | 15 | 310 | +224% |
| F55 (S32760) | 550 | 750 | 25 | 310 | +224% |
| F57 (S39277) | 620 | 795 | 25 | 310 | +265% |
Source: ASTM A182 / ASME SA182. The high yield strength of duplex grades often allows one pressure class reduction compared to 316L, significantly reducing material weight and cost in offshore and structural applications.
The Pitting Resistance Equivalent Number (PREN) is the single most important parameter for comparing corrosion resistance in chloride environments. A PREN ≥35 is the minimum threshold generally accepted for seawater immersion service.
Duplex stainless steels are weldable but require strict thermal controls to maintain the correct austenite:ferrite phase balance (35–65% ferrite) in the weld metal and heat-affected zone (HAZ). Deviation from these controls risks sigma phase embrittlement (too slow cooling in 600–1000°C range) or excessive ferrite (too fast cooling or too high nitrogen loss), both of which degrade toughness and corrosion resistance:
| Parameter | Standard Duplex (F51/F60) | Super Duplex (F53/F55) |
|---|---|---|
| Preheat | Not required (ambient) | Not required (ambient) |
| Max interpass temperature | 150°C | 100°C |
| PWHT required? | No — prohibited unless sigma suspected | No — prohibited |
| Solution anneal (if needed) | 1020–1080°C + water quench | 1050–1100°C + water quench |
| GTAW filler (TIG) | ER2209 | ER2594 |
| SMAW electrodes | E2209 | E2594 |
| Shielding gas | Ar or Ar + 2–3% N₂ | Ar + 2–3% N₂ (N₂ essential) |
| Ferrite content (weld) | 35–65% (Fischer Feritscope) | 35–65% (Fischer Feritscope) |
| Impact test | −40°C or −46°C (offshore) | −46°C (per NORSOK) |
When welding super duplex grades (F53/F55), pure argon shielding allows nitrogen to escape the weld pool, shifting the microstructure toward excess ferrite and reducing PREN by up to 4 points. Always use Ar + 2–3% N₂ shielding and backing gas. Additionally, austenitic over-matching filler is sometimes used to compensate for weld-pool dilution — confirm with your project welding engineer.
| ASTM Grade | UNS Number | Common Name | Min PREN | Primary Application |
|---|---|---|---|---|
| ASTM A182 F51 | S31803 | Duplex 2205 (original) | ~30.5 | Oil & gas, chemical processing, offshore topsides |
| ASTM A182 F52 | S32550 | Ferralium 255 | ~36 | Seawater, desalination, chemical plant |
| ASTM A182 F53 | S32750 | Super Duplex 2507 | ≥41 | Seawater injection, subsea, offshore FPSO |
| ASTM A182 F54 | S32304 | Lean Duplex 2304 | ~24 | Moderate Cl⁻ service, cost-optimised structures |
| ASTM A182 F55 | S32760 | Zeron 100 | ≥40 | Offshore, deepwater, seawater cooling systems |
| ASTM A182 F57 | S39277 | Hyper Duplex | ≥48 | Extreme chloride, deep subsea, aggressive process |
| ASTM A182 F59 | S32520 | UR52N+ | ≥46 | Hot seawater, FGD, chlorine environments |
| ASTM A182 F60 | S32205 | Duplex 2205 (controlled comp.) | ~34.1 | NORSOK offshore, oil & gas, preferred over F51 |
| ASTM A182 F61 | S32550 | Ferralium 255 variant | ~36 | Marine, chemical, high-strength corrosive service |
| Standard | Scope |
|---|---|
| ASTM A182 / ASME SA182 | Material standard for forged duplex steel flanges — covers all grades F51–F61, chemistry, mechanical properties, heat treatment, and testing requirements |
| ASME B16.5 | Pipe flanges and flanged fittings — NPS ½″ to 24″, Class 150 to 2500. Dimensional tolerances, pressure-temperature ratings (P-T tables for each material group) |
| ASME B16.47 | Large diameter flanges — NPS 26″ to 60″, Class 75 to 900 (Series A/MSS SP-44 and Series B/API 605) |
| EN 1092-1 | European flange standard — PN 2.5 to PN 400, types 01 (blind) through 13 (weld neck) |
| NORSOK M-630 | Material data sheets (MDS) for offshore — D47 specifies UNS S32205 (F60); D48 specifies S32750 (F53); D49 specifies S32760 (F55) |
| ASTM A923 | Detecting detrimental intermetallic phases in duplex stainless steel — Method A (etch), Method B (Charpy impact), Method C (CPT corrosion test) |
| ASTM E562 | Determining volume fraction by systematic manual point count — used to verify 35–65% ferrite content in weld |
| API 6A / ISO 10423 | Wellhead and Christmas tree equipment — covers flanges for high-pressure well service (BX/RX ring joint, 2000–20000 PSI) |
| ISO 9001:2015 | Tesco Steel & Engineering quality management system certification |
Topsides piping, seawater lift systems, subsea trees, HIPPS valve flanges, riser flanges, flare header connections. F60/F51 for topsides; F53/F55 for subsea and seawater injection.
MSF and RO plant high-pressure feed piping, brine heater nozzles, inter-stage headers. F53 (S32750) is the preferred grade for seawater at high temperature and pressure.
Acid production, acetic acid plants, urea synthesis loops, chloride-containing process streams, FGD (Flue Gas Desulfurization) systems. F53 and F55 for aggressive acid chloride service.
Digesters, bleach plant piping, black liquor evaporators. Duplex resists SCC and pitting from hot chloride-containing bleaching liquors that cause rapid failure in 316L.
Seawater cooling lines, ballast water treatment systems, propeller shaft seals, offshore structure nozzles. F52 and F53 for direct seawater contact; F51/F60 for above-waterline service.
Urea synthesis and stripping equipment, carbamate piping, ammonia scrubbers. The SCC resistance of duplex at high temperature and under residual stress is critical in urea plant environments.
A duplex steel flange is a piping component manufactured from duplex stainless steel — an alloy with a two-phase microstructure of approximately 50% austenite and 50% ferrite. This dual structure gives duplex flanges roughly twice the yield strength of standard austenitic grades (316L) and superior resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride-containing environments. They are covered by ASTM A182 / ASME SA182 and are dimensioned per ASME B16.5, B16.47, or EN 1092-1.
The key difference is the PREN (Pitting Resistance Equivalent Number): standard duplex (F51/S31803, F60/S32205) achieves PREN ~30–36; super duplex (F53/S32750, F55/S32760) achieves PREN ≥40. Super duplex achieves this through higher Cr (24–26%), Mo (3.5–4%), and N content. In practice, standard duplex is used for oil & gas topside and process piping in moderate chloride service; super duplex is required for seawater injection, subsea equipment, and highly aggressive chloride environments.
PREN = %Cr + 3.3×%Mo + 16×%N. It predicts a steel's resistance to localised pitting corrosion in chloride environments. A PREN ≥35 is the minimum threshold generally required for seawater immersion service. Standard duplex S31803 (F51) achieves minimum PREN ~30.5 — marginal for chlorinated seawater. S32205 (F60) achieves ~34.1. Super duplex S32750 (F53) achieves ≥41, comfortably above the seawater threshold. Always use the minimum PREN from minimum specification composition — not the nominal — when checking suitability.
For offshore topsides and process piping in non-seawater chloride service, ASTM A182 F60 (UNS S32205) is the preferred baseline — NORSOK M-630 MDS D47 specifies S32205 rather than S31803, because F60 guarantees a higher minimum Cr and Mo content and therefore a higher guaranteed minimum PREN (~34.1 vs ~30.5). For seawater injection lines, subsea trees, topside seawater cooling, and splash zone applications, F53 (UNS S32750, PREN ≥41) or F55 (UNS S32760, PREN ≥40) is required per NORSOK MDS D48/D49.
No — PWHT is not required and is in fact prohibited for duplex steel flanges. Heating duplex steels above 600°C precipitates sigma phase (a brittle FeCr intermetallic) which destroys both toughness and corrosion resistance. The correct post-weld procedure is: control interpass temperature to ≤150°C (standard duplex) or ≤100°C (super duplex), use matching nitrogen-alloyed filler (ER2209 for 2205; ER2594 for 2507), use Ar + N₂ shielding gas for super duplex, and verify ferrite content (35–65%) on the weld. If sigma phase is detected by ASTM A923, solution anneal at 1020–1100°C + immediate water quench restores properties.
Duplex steel (F51/S31803 and F60/S32205) has a minimum yield strength (0.2% proof stress) of 450 MPa, versus 316L (F316L) at 170 MPa minimum. This means duplex is approximately 2.6× stronger in yield than 316L. Practically this allows: thinner wall flanges and pipe for the same design pressure, one pressure class reduction (e.g., Class 300 instead of Class 600) in many cases, and significant weight reduction in offshore structural applications — where duplex is increasingly preferred over 316L even when the corrosion requirement is moderate.
Both F51 (UNS S31803) and F60 (UNS S32205) are 22Cr-5Ni-3Mo 2205-type duplex grades — F60 is the controlled-composition (tighter-specification) version of F51. The key differences: F60 specifies Cr minimum 22.0% (vs 21.0% for F51) and Mo minimum 3.0% (vs 2.5% for F51). These tighter lower bounds raise the guaranteed minimum PREN from ~30.5 to ~34.1. For any project referencing NORSOK or specifying S32205, order F60 (not F51) — the UNS numbers are different and the minimum corrosion performance is meaningfully higher.
Sigma phase (σ) is a hard, brittle intermetallic compound (FeCr, approximately 45% Cr) that forms in duplex stainless steels when held at 600–1000°C. It precipitates preferentially within the ferrite phase, causing: impact energy reduction by >90% at ambient temperature; reduced ductility (elongation may fall below 5%); degraded pitting and crevice corrosion resistance (PREN reduction). Sigma phase forms rapidly at 700–900°C — a flange in a furnace for 30 minutes at 850°C can be severely embrittled. Detection is by ASTM A923 Method A (oxalic acid etch) and Method B (Charpy impact test at −40°C).
The primary material standard is ASTM A182 / ASME SA182 — covering all duplex grades F51 through F61 for forged flanges. Dimensional standards: ASME B16.5 (NPS ½–24, Class 150–2500) and ASME B16.47 (NPS 26–60). For European projects: EN 1092-1 (forged flanges, PN class). For offshore: NORSOK M-630 MDS D47 (F60/S32205), D48 (F53/S32750), D49 (F55/S32760) apply additional qualification requirements including impact testing at −46°C and ferrite content verification.
Tesco Steel manufactures duplex steel flanges in NPS ½″ through NPS 24″ per ASME B16.5 in Class 150, 300, 600, 900, 1500, and 2500. Larger sizes NPS 26″ through 60″ are available per ASME B16.47 Series A (MSS SP-44) or Series B (API 605). All flange types: weld neck raised face (WNRF) and ring type joint (WNRTJ), slip-on, blind, socket weld, threaded, lap joint, long weld neck, and spectacle blind — in all duplex grades from F51 through F61.
Below 500 ppm Cl⁻ at ambient temperature: austenitic 316L may be adequate. 500–5,000 ppm Cl⁻: specify standard duplex F51 or F60. Above 5,000 ppm (including seawater at ~19,000 ppm) and/or elevated temperatures: specify super duplex F53 or F55 (PREN ≥40). Above 60°C in any chloride service, austenitic stainless steel is at risk of SCC — duplex is required.
For seawater or aggressive chloride service, require minimum PREN ≥35 (conservative: ≥40 for warm seawater above 25°C). Calculate PREN from the minimum specification composition — not nominal. F60 minimum PREN is ~34.1 (using Cr=22.0, Mo=3.0, N=0.14) — borderline for seawater. F53 minimum PREN is ~41 — confirmed suitable. Do not accept vendor statements of "typical PREN" without checking minimum guarantee.
Standard duplex (F51/F60) is resistant to chloride SCC up to approximately 150°C. Super duplex (F53/F55) resists SCC to approximately 180°C. Above these temperatures in chloride service, Inconel 625 or Hastelloy C-276 should be considered. Also verify that the design temperature does not approach the 250°C lower limit for 475°C embrittlement — a consideration for very high temperature duplex service above 250°C.
Confirm the flange pressure class meets design pressure at operating temperature per ASME B16.5 P-T tables (Group 2.3 for S31803/S32205; Group 3.1 for S32750/S32760). The high yield strength of duplex often permits one pressure class reduction versus 316L — for example, Class 300 duplex vs Class 600 316L for the same pressure. Confirm reduced class is acceptable to the design engineer and project specification before ordering.
Check whether the project references NORSOK M-630 (offshore Norway), Shell DEP, Aramco SAES, or other owner specifications. These commonly add: Charpy impact testing at −46°C per ASTM A923 Method B; ferrite content 35–65% by Fischer Feritscope; PMI (Positive Material Identification) on all heat numbers; solution anneal + water quench heat treatment with full records; hardness ≤28 HRC across weld cross-section. Write the complete specification on the purchase order: ASTM A182 Grade F60, UNS S32205, ASME B16.5, NPS, Class, type, facing, schedule, and supplementary requirements.
All grades F51 to F61 | NPS ½″ to 60″ | Class 150 to 2500 | ASME B16.5 / B16.47 / EN 1092-1 / NORSOK | PMI & 3rd-party inspection | Export to 96 countries
Send Enquiry WhatsApp EnquiryCountries we export to: Duplex Steel Flanges Manufacturers / Suppliers / Stockist in Kuwait, UAE, Germany, Saudi Arabia, West Africa, Iraq, Norway, UK, Netherlands, Qatar, Oman, Bahrain, Canada, USA, Australia, Singapore, Malaysia, Thailand, Indonesia, South Korea, Japan, Brazil, Mexico, South Africa, Nigeria, Egypt, Turkey, India, and across the EU, Middle East, Southeast Asia, and the Americas.
Every duplex and super duplex steel flange dispatched by Tesco Steel & Engineering undergoes individual protection before packing — bore caps and face protectors prevent surface damage to machined faces, plastic stretch film shields against moisture and contact corrosion during transit, and large-diameter flanges receive heavy-duty foam or non-woven wrapping. Finished goods are packed in sea-worthy wooden export crates or palletised in woven bags for container loading. All packages carry packing lists, heat numbers, material certificates, and country-of-origin declarations for smooth customs clearance across all 96 countries we export to.
