Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 vs Ferralium 255, S32750 vs S32550
Super Duplex 2507 (UNS S32750) and Ferralium 255 (UNS S32550) sit in adjacent rows of the duplex family table but solve different problems. 2507 is a 25Cr nitrogen-enhanced super duplex with PREN 41 to 43, designed for high-chloride pitting and crevice service. Ferralium 255 (Langley Alloys trademark, base UNS S32550) is a 25Cr duplex with intentional copper at 1.5 to 2.5 percent, optimized historically for sulfuric acid duty and abrasion-corrosion resistance in pump and valve service. The forging specs are F53 and F61 respectively, and they are not interchangeable on a typed bill of materials.
Side-by-Side Comparison
| Property | Super Duplex 2507 (S32750) | Ferralium 255 (S32550) |
|---|---|---|
| UNS designation | S32750 | S32550 |
| Werkstoff (EN) | 1.4410 | 1.4507 (X2CrNiMoCuN25-6-3) |
| Forging spec (ASTM A182) | F53 | F61 |
| Trademark | SAF 2507, UR 52N+, Avesta 2507 | Ferralium 255 (Langley Alloys) |
| Chromium | 24.0 to 26.0 | 24.0 to 27.0 |
| Nickel | 6.0 to 8.0 | 4.5 to 6.5 |
| Molybdenum | 3.0 to 5.0 | 2.9 to 3.9 |
| Nitrogen | 0.24 to 0.32 | 0.10 to 0.25 |
| Copper | 0.50 max (residual, not intentional) | 1.5 to 2.5 percent intentional |
| PREN (Cr + 3.3 Mo + 16 N) | 41 to 43 | 38 to 42 |
| Yield strength minimum | 550 MPa | 550 MPa |
| Tensile strength minimum | 795 MPa | 760 MPa |
| Sour service hardness limit | 28 HRC max | 28 HRC max (when supplied to NACE) |
| Cost index (S32750 = 1.00) | 1.00 | 1.05 to 1.20 (trademarked, smaller producer base) |
| Niche advantage | Highest PREN in mainstream duplex, broad availability | Sulfuric acid resistance, abrasion-corrosion in pumps |
PREN reference: PREN formula explained. Ferralium 255 sits at the upper end of standard duplex / lower end of super duplex; some heats clear PREN 40 and qualify formally as super duplex by NORSOK M-630, others sit just below.
When to Choose Super Duplex 2507 (S32750)
- Project specification names F53. F61 is a different grade.
- Highest practical chloride exposure: offshore seawater splash zone, FGD seawater scrubbers, desalination brine piping.
- Sour service per NACE MR0175 / ISO 15156-3 with H2S partial pressure at the upper end of the SSC envelope.
- Standard NORSOK M-630 / NORSOK M-650 procurement on Norwegian Continental Shelf scope.
- Welding heavy. The higher nitrogen content of 2507 stabilizes austenite reformation in the HAZ during multi-pass welding more reliably than Ferralium 255's lower N range.
- Broad supplier base. Sandvik, Outokumpu, Industeel, Aperam all roll S32750. Ferralium 255 is a Langley Alloys trademark with a narrower supplier list.
When to Choose Ferralium 255 (S32550)
- Project specification names F61. This is binding.
- Dilute sulfuric acid streams in pump impellers, valve trim, mixer shafts. The 1.5 to 2.5 percent intentional copper meaningfully improves general corrosion resistance in reducing acid environments compared with 2507.
- Phosphoric acid plants, fertilizer industry pumps, and certain hydrometallurgy duties (nickel laterite leach, copper SX-EW circuits) where Cu-bearing duplex has a documented service history.
- Abrasion-corrosion in slurry service (mineral processing, FGD limestone slurry). Ferralium 255's strain hardening behavior gives slightly better wear resistance under cavitation.
- Replacement of legacy Cu-bearing duplex castings on existing skids where the inspection database, repair welding procedures, and spares carry forward only if the exact grade is matched.
Substitution Rules
- F53 supplied against an F61 PO: not acceptable in most cases. F61's reason for being is the intentional Cu addition for sulfuric acid duty. Removing it deletes the alloy's design intent. Engineering deviation required, almost never granted.
- F61 supplied against an F53 PO: sometimes technically acceptable for chloride pitting service at moderate temperatures, but PREN may fall below 40 and disqualify the alloy as super duplex per NORSOK M-630. Customer approval required, plus heat-by-heat PREN verification.
- NACE MR0175 sour service: both alloys qualify when ferrite is 35 to 65 percent and hardness is at or below 28 HRC. The intentional Cu in F61 has no defined adverse effect on SSC resistance.
- Mixed grade weldments: permissible with qualified WPS. Use ER2594 or ER2553 super duplex filler. Cu-bearing filler (ER2553 with Cu) may be specified by the buyer when joining F61 to itself in acid service; check the welding procedure document.
Welding and Heat Treatment
Both alloys solution anneal at 1040 to 1100 deg C followed by water quench. Both weld with super duplex consumables ER2594 or ER2553 to AWS A5.9. Heat input 0.5 to 2.5 kJ per millimetre, interpass temperature below 150 deg C. Ferralium 255's lower nitrogen range demands tighter control of the GTAW shield gas (argon plus 2 to 5 percent N2) to prevent ferrite drift in the HAZ. Post-weld ferrite count 35 to 65 percent per ASTM E562. Post-weld solution anneal is mandatory on heavy sections to dissolve sigma and chi phase precipitates in the 700 to 950 deg C danger band.
Related References
- UNS S32750 reference
- ASTM A182 F53 forgings
- NORSOK M-630 super duplex
- Super Duplex 2507 vs Zeron 100
- Super Duplex 2507 vs Duplex 2205
- Super Duplex vs Duplex (when to upgrade)
- PREN formula explained
FAQ
Are F53 and F61 interchangeable on a project?
No. F53 is UNS S32750 with no intentional copper. F61 is UNS S32550 with 1.5 to 2.5 percent intentional copper. The two grades exist for different reasons and the substitution requires customer written approval, which is rarely granted because the alloys are optimized for different corrosion environments.
Which alloy is more expensive, 2507 or Ferralium 255?
Ferralium 255 typically prices 5 to 20 percent higher than Super Duplex 2507. The premium reflects the smaller producer base (Ferralium is a Langley Alloys trademark) and the intentional copper alloying. On a routine seawater piping scope where Cu offers no service benefit, the premium is wasted.
What is the PREN difference?
2507 reliably hits PREN 41 to 43. Ferralium 255 sits at PREN 38 to 42 depending on heat. Some F61 heats clear PREN 40 and qualify as super duplex per NORSOK M-630, but the qualification is heat-specific. 2507 qualifies as super duplex on every heat that meets the chemistry minima.
When does 2507 fail and Ferralium 255 survive?
In dilute sulfuric acid (5 to 50 percent at ambient to moderately elevated temperature) and in mixed sulfuric-phosphoric streams in fertilizer and hydrometallurgy plants, the intentional copper in Ferralium 255 reduces general corrosion rate compared with 2507. The benefit is real and documented in vendor isocorrosion charts. In chloride pitting service Ferralium 255 has no advantage over 2507.
Why does Ferralium 255 have intentional copper?
Copper improves passivity of duplex alloys in reducing acid environments. The 1.5 to 2.5 percent Cu addition in S32550 is the alloy's main differentiator. It was originally developed for pump impellers and valve trim in sulfuric acid duty where 2205 duplex corroded too fast and full austenitic Alloy 20 (N08020) was overspecified.
Can I weld 2507 to Ferralium 255 in the same joint?
Yes, with a qualified WPS. Standard super duplex filler ER2594 or ER2553 per AWS A5.9 is acceptable for chloride service. For a Cu-bearing acid service joint, the WPS may specify Cu-bearing filler (ER2553-Cu variant). Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 deg C, ferrite 35 to 65 percent per ASTM E562.
Does Ferralium 255 qualify for NACE sour service?
Yes, when supplied to NACE MR0175 / ISO 15156-3 with hardness at or below 28 HRC and ferrite content within 35 to 65 percent. The intentional copper does not impair sulfide stress cracking resistance. Heat-specific NACE qualification testing should be on the MTC.
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