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 Inconel 625, S32750 vs N06625
Super Duplex 2507 (UNS S32750) and Inconel 625 (UNS N06625) belong to different alloy families and answer different procurement questions. 2507 is a 25Cr-7Ni-3.5Mo nitrogen-enhanced super duplex stainless. PREN around 41 to 43, austenite-ferrite microstructure, yield 550 MPa, copper at 0.50 max (residual, not intentional). Inconel 625 (Special Metals trademark, base UNS N06625) is a Ni-base solid solution alloy: roughly 60 percent nickel, 21 to 23 percent chromium, 8 to 10 percent molybdenum, 3.15 to 4.15 percent niobium, with PREN around 50 and CPT near 80 deg C in ASTM G48 testing. Inconel 625 is the right answer where 2507's chloride limit is exceeded; 2507 is the right answer almost everywhere it qualifies, because it costs roughly one-quarter to one-third of N06625 on a like-for-like basis.
Side-by-Side Comparison
| Property | Super Duplex 2507 (S32750) | Inconel 625 (N06625) |
|---|---|---|
| UNS designation | S32750 | N06625 |
| Alloy family | Super duplex stainless steel | Nickel-base solid solution |
| Forging spec | ASTM A182 F53 | ASTM B564 N06625 |
| Chromium | 24.0 to 26.0 | 20.0 to 23.0 |
| Nickel | 6.0 to 8.0 | 58.0 minimum (typically 60 to 63) |
| Molybdenum | 3.0 to 5.0 | 8.0 to 10.0 |
| Iron | Balance (around 65 percent) | 5.0 maximum |
| Niobium (Nb + Ta) | (none) | 3.15 to 4.15 |
| Nitrogen | 0.24 to 0.32 | (not specified) |
| PREN (Cr + 3.3 Mo + 16 N) | 41 to 43 | around 50 |
| Yield strength minimum | 550 MPa | 415 MPa (Grade 1 annealed) |
| Tensile strength minimum | 795 MPa | 825 MPa |
| Critical pitting temperature (G48 A) | 40 to 50 deg C | around 80 deg C |
| Useful service temperature ceiling | around 300 deg C (avoid 475 deg C embrittlement) | 980 deg C (oxidising), 650 deg C (long-term creep) |
| Sour service per NACE MR0175 | Qualified, hardness 28 HRC max | Qualified (broader envelope, more PPP H2S permitted) |
| Cost index (S32750 = 1.00) | 1.00 | 3.0 to 4.5 |
When to Choose Super Duplex 2507 (S32750)
- Project specification names F53 or super duplex per NORSOK M-630.
- Service temperature below 50 deg C in seawater. The CPT margin is sufficient.
- Sour service within the NACE MR0175 / ISO 15156-3 envelope. 2507 qualifies and the duplex microstructure offers higher yield strength than annealed N06625.
- Cost-driven scope. The 3 to 4.5 times price differential vs Inconel 625 is significant on heavy piping classes, large flanges, and bulk fastener orders.
- Higher mechanical loading: 2507 yield 550 MPa is meaningfully higher than annealed Inconel 625 yield 415 MPa, allowing thinner walls and lighter forgings at the same pressure rating. The trade-off is service temperature: 2507 should not see continuous duty above 300 deg C because of 475 deg C embrittlement risk.
- Strain hardening or fatigue-loaded components. The duplex microstructure work-hardens predictably and resists fatigue better than the more ductile Ni-base solid solution.
When to Choose Inconel 625 (N06625)
- Service temperature above 50 to 60 deg C in seawater or chloride-bearing process streams. 2507 is at or beyond its CPT limit; Inconel 625 has 30 deg C of additional margin.
- High-temperature service from 300 to 980 deg C: hydrocarbon furnace tubing, ethylene cracker pyrolysis tubes, high-temperature combustion-chamber components, exhaust ducts above 600 deg C.
- Severe sour service beyond the NACE MR0175 envelope for super duplex. Inconel 625 has a broader H2S partial-pressure permission in ISO 15156-3 Table A.16.
- Reducing acid streams (HCl below 30 percent, H2SO4 mixed with HF) where duplex stainless general-corrosion rate is unacceptable. The high Mo and Ni content of N06625 gives broad-spectrum acid resistance.
- Cryogenic service to liquid helium temperatures. The face-centred cubic Ni-base structure has no DBTT; duplex stainless has a useful Charpy floor at minus 46 to minus 80 deg C but is not qualified for LNG / cryogenic duty.
- Subsea control line tubing, downhole tubing in deep sour wells, and specific subsea connectors where the qualifying agency (DNV, BV, ABS) requires N06625.
Substitution Rules
- Inconel 625 supplied against an F53 (S32750) PO: commercially impractical (3 to 4.5 times the cost) but technically acceptable in almost every chloride or sour service. Customer approval required.
- F53 (S32750) supplied against an N06625 PO: generally NOT acceptable. The N06625 specification exists because the service envelope (temperature, chloride concentration, acid content) exceeds super duplex capability. Substitution risks early failure. Customer approval required and rarely granted.
- Galvanic compatibility: N06625 is more noble than S32750 in seawater. A direct couple (N06625 valve trim in a 2507 piping system) puts the 2507 at slight galvanic risk. The risk is small at modest area ratios but should be flagged in the corrosion review.
- Mixed weldments: permissible with qualified WPS. Use Ni-base filler ENiCrMo-3 (matching N06625) or ER NiCrMo-3 for GTAW. Heat input window is wider for N06625 because there is no ferrite balance to maintain. Post-weld solution anneal not normally required for N06625 (annealed condition is standard); for 2507 base material the post-weld ferrite count rule (35 to 65 percent per ASTM E562) still applies.
Welding and Heat Treatment
2507 solution annealing window: 1040 to 1100 deg C, water quench, mandatory after any high-heat-input welding to dissolve sigma and chi phase. Welding consumables ER2594 / ER2553 super duplex per AWS A5.9. Inconel 625 solution annealing: 980 to 1150 deg C depending on grade (Grade 1 annealed for general corrosion service; Grade 2 solution-treated for higher temperature). Welding consumable ENiCrMo-3 / ERNiCrMo-3 per AWS A5.11 / A5.14. Both alloys weld without preheat and without post-weld stress relief in standard sections. The duplex requires heat-input control (0.5 to 2.5 kJ per millimetre); the Ni-base alloy is more forgiving on heat input but must be cleaned of all sulfur, lead and zinc contamination prior to welding to prevent embrittlement.
Related References
- UNS S32750 reference
- ASTM A182 F53 forgings
- NORSOK M-630 super duplex
- Super Duplex 2507 seawater applications
- Super Duplex 2507 sour service
- Super Duplex 2507 vs Zeron 100
- Super Duplex vs Duplex (when to upgrade)
FAQ
Can I substitute Inconel 625 for Super Duplex 2507?
Technically yes in almost every 2507 service envelope, but the cost differential (3 to 4.5 times) makes it commercially impractical except where the buyer wants standardization across a mixed-service skid. Customer approval is still required because the substitution changes the welding consumable, the heat treatment regime, and the galvanic position.
Which alloy is more expensive, 2507 or Inconel 625?
Inconel 625 typically prices 3 to 4.5 times higher than Super Duplex 2507 on a like-for-like basis. The 60 percent nickel content drives the cost; nickel is the dominant raw-material input on a per-tonne basis. Spot pricing tracks the LME nickel index closely.
When does 2507 fail and Inconel 625 survive?
In seawater above 50 to 60 deg C the 2507 CPT is breached and pitting initiates within service hours; Inconel 625 retains margin to roughly 80 deg C. In severe sour service (H2S partial pressure above the 2507 NACE permission) the duplex SSC margin is exhausted while Inconel 625 still qualifies. In high-temperature service above 300 deg C the duplex risks 475 deg C embrittlement; Inconel 625 is rated for continuous duty to 650 deg C creep and 980 deg C oxidation.
What is the PREN difference?
2507: PREN 41 to 43. Inconel 625: PREN around 50 (Cr around 21, Mo 9, no significant N). The PREN difference translates to roughly 30 deg C of additional CPT margin in ASTM G48 testing. PREN is a coarse predictor for Ni-base alloys; the high Mo content carries the pitting resistance even though Cr is similar to 2205 duplex.
Are F53 and N06625 forgings interchangeable?
No. F53 is super duplex stainless (ASTM A182). N06625 forgings are supplied to ASTM B564. Different specifications, different inspection regimes, different welding procedures. The substitution rule is one-directional: N06625 can usually substitute for F53 with customer approval, but F53 cannot substitute for N06625 without engineering deviation, which is not normally granted.
Does NORSOK M-630 cover Inconel 625?
No. NORSOK M-630 covers stainless steel material data sheets. Ni-base alloys including N06625 fall under separate NORSOK M-DP material lists. Project specifications usually cite the appropriate ASTM B-series specification (B564 forgings, B443 plate, B704 tube, B366 fittings) for N06625 directly.
Why not just use Inconel 625 everywhere to be safe?
Cost. The 3 to 4.5 times premium consumes project budget without delivering measurable lifecycle benefit in the temperature and chloride bands where 2507 qualifies. Material standardization on the highest grade is poor procurement practice; the right approach is a fit-for-service review per piping class, with 2507 as the workhorse and N06625 reserved for service envelopes that exceed super duplex capability.
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