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 for Sour Service (NACE MR0175 / ISO 15156)
Super Duplex 2507 (UNS S32750 / EN 1.4410) is a qualified material for sour service per NACE MR0175 / ISO 15156-3 within a defined envelope of H2S partial pressure, chloride concentration, temperature, and pH. The qualification rests on solution-annealed condition, hardness 28 HRC max, and ferrite content within 35 to 65 percent. Outside that envelope, sulfide stress cracking (SSC), stress-oriented hydrogen-induced cracking (SOHIC), and chloride pitting can defeat the alloy.
TorqBolt supplies 2507 forgings, fasteners, pipe, fittings, and flanges with full NACE MR0175 / ISO 15156-3 documentation: hardness survey, ferrite count per ASTM E562 or E1245, ASTM A923 sigma-phase screening, and EN 10204 Type 3.1 mill test certificates as standard (Type 3.2 third-party-witnessed available).
Why Super Duplex 2507 in Sour Service
Sour service is the combination of an aqueous phase, dissolved H2S, chloride, and stress. The qualifying drivers for 2507:
- Sulfide stress cracking (SSC) resistance at 28 HRC maximum hardness, when the ferrite stays within 35 to 65 percent. The duplex microstructure absorbs hydrogen far more slowly than ferritic / martensitic steels.
- Chloride pitting resistance from PREN above 41, well clear of the threshold for typical formation brine chloride.
- SOHIC and HIC tolerance for low-strength applications, governed by ISO 15156-3 Annex C.
- Strength margin at 550 MPa minimum yield, which lets designers stay within the API 6A wall-thickness rules without going to nickel-base CRA.
Super Duplex 2507 Sour Service Design Data
| Parameter | Value | Reference |
|---|---|---|
| Sour-service classification threshold | 0.05 psi (0.0035 bar) H2S partial pressure | NACE MR0175 / ISO 15156-1, Section 4 |
| Hardness limit | 28 HRC max (base, weld, HAZ) | ISO 15156-3 Table A.30 |
| Ferrite content | 35 to 65 percent (base and weld combined envelope) | ISO 15156-3, NORSOK M-630 |
| Material condition | Solution annealed at 1040 to 1100 degrees Celsius, water quenched | NORSOK M-630, ASTM A479 / A790 / A182 |
| Sigma-phase screening | ASTM A923 Method A (etch) and Method B (Charpy at minus 40) | ASTM A923-14 |
| PREN | 41 to 43 | Cr + 3.3 times Mo + 16 times N |
| Maximum service temperature (sour) | Per ISO 15156-3 zone classification, typically up to 232 degrees Celsius (450 deg F) | ISO 15156-3 Table A.30 |
| Charpy impact (low MDMT applications) | Minimum 45 J at minus 46 degrees Celsius | NORSOK M-630 |
ISO 15156-3 Zone Classification for 2507
ISO 15156-3 Table A.30 specifies the qualified envelope for solution-annealed 25Cr super duplex (UNS S32750) by zone. The table breaks the application space into bands of H2S partial pressure, chloride, temperature, and pH. The headline limits used for high-level material screening:
- H2S partial pressure as low as 0.05 psi (0.0035 bar) marks the transition from sweet to sour service.
- Chloride concentration in the aqueous phase: typical envelope rows allow chloride up to about 50,000 mg/kg subject to other constraints.
- Maximum temperature for the most permissive H2S row of Table A.30 is on the order of 232 degrees Celsius (450 deg F); more aggressive H2S partial pressures pull the temperature ceiling down.
- pH bands shape the H2S-vs-chloride envelope at the lower temperature end.
Always check the actual project envelope row by row against ISO 15156-3 Table A.30 of the current edition. The summary above is for orientation; the published table is the controlling document and its limits change between editions.
Welding 2507 for Sour-HAZ Performance
- Filler ER2594 (overmatching, slightly higher PREN than the base) per AWS A5.9 is preferred for sour-service items. ER2553 (matching) is acceptable for less aggressive duty.
- Heat input 0.5 to 2.5 kJ per millimetre. Below 0.5 kJ/mm the cooling rate is too fast and ferrite climbs above 65 percent. Above 2.5 kJ/mm cooling slows and sigma-phase / nitride precipitation begin.
- Interpass temperature below 150 degrees Celsius to avoid extended residence in the 600 to 1000 degrees Celsius sigma-phase formation band.
- Shielding gas argon plus 2 to 5 percent nitrogen for GTAW root pass to maintain weld-metal nitrogen and suppress ferrite over-balance.
- Hardness survey on every weld, HAZ, and base material per ISO 15156-3. Maximum 28 HRC. Survey points per ASME IX QW-462.
- Ferrite count per ASTM E562 point counting on a representative section of weld, HAZ, and base. 35 to 65 percent envelope across all three.
- Sigma-phase screening per ASTM A923 Methods A (oxalic etch) and B (Charpy impact at minus 40 degrees Celsius, 54 J minimum). Failed Method B implies sigma-phase precipitation in the heat-treatment cycle.
Qualification and Certification Documents
- Material certificate EN 10204 Type 3.1 (mill QA / QC) as standard. Type 3.2 with DNV / Lloyd's / BV / ABS witness available on request.
- NACE MR0175 / ISO 15156-3 compliance statement on each lot, citing the specific zone of Table A.30 against which the material is qualified.
- Hardness report base, HAZ, weld, with Rockwell C indents and survey traverse pattern.
- Ferrite count report per ASTM E562 or E1245.
- ASTM A923 Method A and Method B reports (etch micrograph and Charpy result).
- Charpy impact report at minus 46 degrees Celsius for low MDMT items per NORSOK M-630.
- PMI (positive material identification) per ASTM E1476 on every piece by handheld XRF or OES at the mill or stockholder.
- Heat-treatment chart showing solution-annealing temperature, soak time, and water quench for the entire heat or batch.
Failure Modes Outside the Qualified Envelope
- Sulfide stress cracking (SSC) when hardness is allowed to climb above 28 HRC, typically through cold work, machining, or improper PWHT.
- Sigma-phase embrittlement when slow cooling through 600 to 1000 degrees Celsius is permitted (under-quenched solution anneal, slow weld cooling, or service above 300 degrees Celsius). Charpy at minus 40 falls from 200+ J typical to under 30 J.
- Hydrogen-induced cracking (HIC) on cathodic over-protection. High-strength bolting under sacrificial-anode CP can absorb hydrogen and crack. Isolate CP zones from high-strength fasteners.
- Pitting at chloride / temperature combinations exceeding the PREN envelope. Above CPT (50+ degrees Celsius for 2507) chloride pitting outpaces passive-film recovery.
- Crevice corrosion at flat-sheet gaskets and improperly designed flange faces. Use spiral-wound CG gaskets with PTFE or graphite fill.
Recommended TorqBolt 2507 Products with Sour-Service Documentation
- 2507 stud bolts: ASTM A1082 / API 20F PSL2-PSL4, NACE MR0175 hardness 28 HRC max
- 2507 heavy hex nuts: ASTM A1082 in matching S32750
- 2507 flanges: ASTM A182 F53 weld neck and blind, RTJ for sour gas
- 2507 fittings: ASTM A815 WP-S32750 buttweld
- 2507 pipe: ASTM A790 seamless and welded
- 2507 forgings: ASTM A479 / A182 F53 for tree bodies, valve blocks, connectors
Related Application Pages
- Oil and gas: wellheads, manifolds, surface flowlines
- Marine and offshore: subsea hardware, NORSOK supply
- Seawater applications: sea-chest, fire-water, lift systems
- Chemical processing: dilute HCl, H2SO4, HF service
- NACE MR0175 compliance summary
Super Duplex 2507 Sour Service FAQ
When does service become sour for material selection purposes?
Per NACE MR0175 / ISO 15156-1 Section 4, service is sour when H2S partial pressure in the wetted gas phase reaches 0.05 psi (0.0035 bar) or higher, in the presence of an aqueous phase. Below that threshold, conventional carbon steel and standard CRAs apply without the additional NACE constraints.
What is the hardness limit for 2507 in sour service?
28 HRC maximum on base material, HAZ, and weld metal per ISO 15156-3 Table A.30. The limit must be maintained through machining, cold work, and weld procedure; uncontrolled cold work can locally lift hardness above the limit and trigger SSC. Hardness survey is mandatory on every sour-service weld.
What is the ferrite content envelope for 2507 in sour service?
35 to 65 percent ferrite across base, HAZ, and weld combined. Below 35 percent the alloy loses pitting resistance and approaches austenitic behaviour. Above 65 percent the alloy loses Charpy toughness and SSC resistance. Measured per ASTM E562 (point counting) or ASTM E1245 (image analysis) on representative cross-sections.
How do I check ISO 15156-3 Table A.30 against my application?
Take the design H2S partial pressure, chloride concentration in the aqueous phase, maximum service temperature, and minimum pH at the operating point. Then walk Table A.30 row by row for solution-annealed 25Cr super duplex (UNS S32750) and confirm every parameter sits inside one acceptable row of the table. If any one parameter is out of bounds, the alloy is not qualified for that service. Use the current edition of ISO 15156-3; the table changes between editions.
What happens if 2507 is welded with too high heat input?
Slow cooling through the 600 to 1000 degrees Celsius band causes sigma-phase precipitation. Sigma phase is a chromium-molybdenum-rich intermetallic that depletes the surrounding matrix of the elements that give 2507 its corrosion resistance. The result: Charpy impact at minus 40 falls by 80 percent or more, and pitting resistance drops below 2205 levels. ASTM A923 Method B detects this. Charpy below 54 J at minus 40 fails the test.
Can 2507 be used at H2S partial pressures above 1.5 bar?
It depends on the chloride and temperature combination at the operating point. Higher H2S partial pressures push the qualified temperature ceiling down per ISO 15156-3 Table A.30. At very high H2S partial pressures combined with high chloride and high temperature, the alloy of choice shifts up to Alloy 825, Alloy 925, or Alloy 625. Always validate against Table A.30 of the current edition rather than rely on general rules.
Is sour-service 2507 limited to specific product forms?
No. ISO 15156-3 qualification applies to the alloy chemistry and condition (solution annealed, 28 HRC max, ferrite 35 to 65 percent), not the product form. Bar (ASTM A479), forgings (ASTM A182 F53), pipe (ASTM A790), fittings (ASTM A815), bolting (ASTM A1082, API 20F), and welded fabrications all qualify when made and tested per the controlling product spec. Each lot must carry the NACE MR0175 / ISO 15156-3 compliance statement with the supporting hardness, ferrite, and ASTM A923 evidence.
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