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 Duplex 2205, S32750 vs S31803/S32205
Super Duplex 2507 (UNS S32750) and Duplex 2205 (UNS S31803 / S32205) are the two volume workhorses of the duplex stainless family. 2205 is the "standard duplex" with 22Cr, 5Ni, 3Mo and PREN 34 to 38. 2507 is "super duplex" with 25Cr, 7Ni, 3.5Mo, 0.3N and PREN 41 to 43. The PREN 40 line is the formal threshold (NORSOK M-630, ISO 17782). Above 40 the alloy is super duplex. Below 40 it is duplex. The two grades are not interchangeable on a piping class, and the substitution rules run one direction only: 2507 can sometimes fill a 2205 PO with customer permission, but 2205 cannot fill a 2507 PO under any normal circumstance.
Side-by-Side Comparison
| Property | Super Duplex 2507 (S32750) | Duplex 2205 (S31803 / S32205) |
|---|---|---|
| UNS designation | S32750 | S31803 (original) / S32205 (modified, tighter N) |
| Werkstoff (EN) | 1.4410 | 1.4462 |
| Forging spec (ASTM A182) | F53 | F51 (S31803) / F60 (S32205) |
| Trademark | SAF 2507, UR 52N+, Avesta 2507 | SAF 2205, UR 45N, Avesta 2205 |
| Chromium | 24.0 to 26.0 | 21.0 to 23.0 |
| Nickel | 6.0 to 8.0 | 4.5 to 6.5 |
| Molybdenum | 3.0 to 5.0 | 2.5 to 3.5 |
| Nitrogen | 0.24 to 0.32 | 0.08 to 0.20 (S31803), 0.14 to 0.20 (S32205) |
| Copper | 0.50 max (residual, not intentional) | (no specification) |
| PREN (Cr + 3.3 Mo + 16 N) | 41 to 43 (super duplex) | 34 to 38 (duplex, below super duplex line) |
| Yield strength minimum | 550 MPa | 450 MPa |
| Tensile strength minimum | 795 MPa | 620 MPa |
| Critical pitting temperature (ASTM G48 A) | 40 to 50 deg C | 20 to 30 deg C |
| Sour service hardness limit | 28 HRC max | 28 HRC max (NACE MR0175) |
| Cost index (S32205 = 1.00) | 1.30 to 1.60 | 1.00 |
| Typical chloride service ceiling | Seawater (full strength) | Brackish to moderate chloride; not seawater above 30 deg C |
When to Choose Super Duplex 2507 (S32750)
- Project specification names F53 on the bill of materials.
- Chloride above ambient seawater concentration (FGD seawater scrubber, MSF / RO desalination brine, evaporator concentrator).
- Service temperature above 40 deg C in seawater. 2205 starts to pit reliably above its CPT in this band; 2507 retains margin.
- Sour service per NACE MR0175 with H2S partial pressure at the upper end of the ISO 15156-3 envelope.
- Higher mechanical loading: 2507 yield is 550 MPa vs 2205 at 450 MPa, which means thinner walls and lighter forgings for the same pressure rating.
- API 6A / 17D wellhead and subsea hardware where the project specification calls for super duplex explicitly.
When to Choose Duplex 2205 (S31803 / S32205)
- Project specification names F51 or F60.
- Brackish water, produced water, or moderate chloride streams below the alloy's CPT.
- Chemical processing where 316L is too thin on the corrosion margin but full super duplex is overspecified. Pulp and paper white liquor, urea synthesis loops, dilute acid heat exchangers.
- Storage tanks and structural members where high-strength duplex saves weight and steel cost compared with carbon steel plus cladding.
- Cost-driven scope. 2205 is 30 to 60 percent cheaper than 2507 on a like-for-like piping basis. When the corrosion environment fits, the saving is real.
- Established supply chain. 2205 is the most widely produced duplex grade globally; lead times and stock availability beat 2507 for non-specialist sizes.
Substitution Rules
- 2507 (F53) supplied against a 2205 (F51/F60) PO: often technically acceptable (every property minimum of 2205 is met or exceeded by 2507) but commercial premium is significant and the customer's QA must agree to the up-grade in writing. Re-stamping and MTC re-issue may be required.
- 2205 (F51/F60) supplied against a 2507 (F53) PO: not acceptable. PREN falls below the 40 super duplex threshold, mechanical minima are lower, and NORSOK M-630 D45 / D47 lines reject the substitution. No customer normally grants this deviation.
- S31803 vs S32205: S32205 is a tighter chemistry version of S31803 (narrower N range, higher Cr minimum). S32205 always meets S31803 acceptance criteria; S31803 may not always clear S32205 minima. Modern dual-certified material is the norm.
- Mixed 2205 / 2507 weldments: permissible with qualified WPS. Use super duplex filler ER2594 (overmatching, ensures weld PREN clears the higher base metal value).
Welding and Heat Treatment
Both alloys solution anneal at 1020 to 1100 deg C. 2507 sits at the upper end of the window (1040 to 1100 deg C) because the higher alloying content needs a hotter solution treatment to fully dissolve sigma phase. 2205 anneals at 1020 to 1080 deg C. Both weld with duplex / super duplex consumables to AWS A5.9: ER2209 for 2205 base, ER2594 or ER2553 for 2507 base. Heat input 0.5 to 2.5 kJ per millimetre, interpass temperature below 150 deg C. Argon plus 2 to 5 percent N2 shielding for GTAW root. Post-weld ferrite count 35 to 65 percent per ASTM E562. The 2205 family is more forgiving on welding because the lower alloy content slows sigma kinetics and widens the practical heat-input window.
Related References
- UNS S32750 reference
- ASTM A182 F53 forgings
- NORSOK M-630 super duplex
- Super Duplex 2507 vs Zeron 100
- Super Duplex 2507 vs Ferralium 255
- Super Duplex vs Duplex (when to upgrade)
- Super Duplex 2507 seawater applications
FAQ
Are F53 and F51/F60 interchangeable?
No. F53 is super duplex S32750. F51 is duplex S31803, F60 is duplex S32205. They span different PREN bands (above 40 vs below 40) and different mechanical minima. Substitution requires customer written approval. Up-substitution (2507 for 2205) is sometimes granted with a commercial premium. Down-substitution (2205 for 2507) is not normally granted because the alloy fails the super duplex acceptance criteria.
Which alloy is more expensive, 2507 or 2205?
2507 typically prices 30 to 60 percent higher than 2205 on a like-for-like basis. The premium reflects the higher Cr, Ni, Mo and N content and the smaller production volume. Spot pricing tracks the LME nickel index closely.
What is the PREN difference?
2507: PREN 41 to 43 (qualifies as super duplex per the 40 threshold in NORSOK M-630 and ISO 17782). 2205: PREN 34 to 38 (standard duplex, does not qualify as super duplex). The PREN difference translates to a 15 to 20 deg C difference in critical pitting temperature in ASTM G48 Method A testing.
When does 2205 fail and 2507 survive?
In seawater service above 30 to 40 deg C, 2205 typically pits and crevice corrodes within months while 2507 retains margin to 50 deg C and higher. In FGD seawater scrubbers, MSF / RO desalination plants, and offshore splash zone bolting, 2205 is regularly under-specified and 2507 is the default choice. In sour service at the upper end of the NACE H2S envelope, 2205 may also fall short on SSC margin where 2507 still qualifies.
Can I substitute 2507 for 2205 to save inventory complexity?
Sometimes, with customer approval and a price agreement. The technical case is straightforward (every 2205 minimum is exceeded by 2507) but the commercial premium and the QA paperwork (re-stamping, dual MTC, traceability adjustment) often consume the inventory simplification benefit. A standardization study should compare the lifecycle cost, not just the unit price.
Are S31803 and S32205 the same grade?
Closely related but not identical. S32205 is a 1996 modification of S31803 with tighter chemistry (narrower nitrogen range, higher Cr and Mo minima). Modern duplex 2205 is usually dual-certified to both UNS designations. Material certified to S32205 always meets S31803; the reverse is not always true.
Does NORSOK M-630 cover both 2205 and 2507?
Yes, in separate Material Data Sheets. D45 / D47 cover super duplex (S32750, no W). D55 / D57 cover W-bearing super duplex (S32760). Standard duplex 2205 (S31803 / S32205) is covered under separate MDS lines for 22Cr duplex. Project specifications cite the MDS number, not the alloy nickname.
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