Incoloy 925 (UNS N09925) is one of the few CRAs that combines chloride stress-corrosion cracking (Cl-SCC) immunity with the high yield strength (110 ksi minimum aged) required for fastener service. The 44% minimum nickel content places the alloy above the threshold for Cl-SCC immunity (typically >30% Ni); the 2.5–3.5% molybdenum + 1.5–3.0% copper additions extend pitting and crevice resistance into seawater service. The result is the workhorse alloy for North Sea / Gulf of Mexico subsea fasteners, FPSO turret components, splash-zone bolting and pump shafting where lower-cost duplex stainless steels lack the strength + Cl-SCC envelope. TorqBolt manufactures Incoloy 925 marine bolting and bar/forging stock per ASTM B805, NORSOK M-650 qualification, and the major North Sea oilfield project material data sheets (MDS A30).
Need NORSOK-qualified Incoloy 925 for North Sea / FPSO? Email info@torqbolt.com with project name, MDS number, size schedule and any TPI witness requirements. WhatsApp +91-22-66157017.
Why Incoloy 925 for Marine Service
- Chloride-SCC immunity: 44% minimum nickel places the alloy in the immune range for Cl-SCC. By comparison, Duplex 2205 (~5% Ni) is only resistant; 304/316 stainless are susceptible.
- Pitting + crevice resistance: PREN ~32 (Cr + 3.3·Mo) plus copper benefit gives the alloy a qualified crevice envelope to 131°F (55°C) per Special Metals data, extends to 149°F (65°C) with appropriate surface treatment.
- High strength: 110 ksi YS aged carries marine bolt loads without down-rating. Incoloy 825 (~45 ksi YS) requires significantly larger bolt cross-section for the same preload, increasing assembly weight and cost.
- Non-magnetic: Relative permeability ~1.005, preferred for components near magnetic-instrument compass / inertial systems.
- NORSOK M-650 qualified: TorqBolt is registered manufacturer for Incoloy 925 to NORSOK M-650 mill qualification, suitable for direct supply to Norwegian Continental Shelf projects.
Marine & Offshore Components Manufactured in Incoloy 925
| Component | Standard / Project Reference | Service |
|---|---|---|
| Subsea stud bolts | ASTM B805 / NORSOK M-630 MDS A30 | Subsea Christmas tree, BOP, riser flange |
| Heavy hex nuts | ASTM B805 / ASME B18.2.4.6M | Companion to subsea stud bolts |
| Splash-zone bolting | ASTM B805 / NORSOK | Riser flanges, J-tube clamps, hang-off plates |
| Pump shafting | ASTM B805 round bar | Seawater lift pumps, firewater systems, FPSO offloading |
| FPSO turret swivel components | Customer drawing + NORSOK | High-load swivel internals + bolting |
| Subsea control-line tubing | ASTM B983 | Hydraulic + chemical injection lines |
| Boat-landing brackets, padeyes | Customer + class society | Topside marine fittings |
| Hyperbaric welding consumables (filler bar) | AWS | Subsea repair welding base material |
| Mooring chain end-fittings (high-load) | Customer + class society | Permanent / mobile mooring |
Marine Service Envelope, Compared to Adjacent Alloys
| Property | Incoloy 925 | Monel K-500 | Duplex 2205 | Super-duplex 2507 |
|---|---|---|---|---|
| 0.2% Yield, ksi (MPa) | 110 (760) | 85 (586) | 65 (448) | 80 (550) |
| Cl-SCC resistance | Immune | Immune | Resistant | Resistant |
| PREN (pitting index) | ~32 | N/A (no Cr) | ~35 | ~42 |
| Crevice corrosion limit | ~131°F (55°C) | ~150°F (65°C) | ~95°F (35°C) | ~140°F (60°C) |
| NORSOK M-650 qualified | Yes | Yes | Yes | Yes |
| Magnetic permeability | ~1.005 (non-magnetic) | ~1.001 (non-magnetic) | 30–100 (magnetic) | 30–100 (magnetic) |
| Relative cost | 1.0× | ~1.8× | ~0.30× | ~0.50× |
For high-strength marine bolting where Cl-SCC must be ruled out, Incoloy 925 is the cost-optimal choice. Monel K-500 wins on crevice envelope but costs ~80% more. Duplex / super-duplex are cheaper but cannot meet 110 ksi YS for high-preload bolting and are strongly magnetic (disqualifies them from compass-sensitive applications).
NORSOK M-650 / M-630 Qualification
TorqBolt is registered as NORSOK M-650 qualified manufacturer for UNS N09925 with the following scope:
- NORSOK M-650: Material manufacturer qualification (mill QTRs valid 5 years, with 180 / 90 / 60 / 30-day re-qualification alerts)
- NORSOK M-630 MDS A30: Material data sheet for nickel-iron-chromium alloys, Incoloy 925 specifically
- NORSOK M-501 / L-005: Surface preparation + coating compatibility for offshore service
- DNV / Bureau Veritas / ABS / Lloyd's witness: Available for project-specific TPI verification
Material data sheets and qualification test reports are issued with every NORSOK-spec order. See parent-site TorqBolt NORSOK M-650 qualified bolting for full qualification scope.
Cathodic Protection & Hydrogen Embrittlement
Subsea components are typically cathodically-protected (CP) by sacrificial aluminium anodes or by impressed-current systems. The CP cathode polarises the steel structure to ~−0.95 to −1.10 V vs Ag/AgCl, which is well within the safe operating range for Incoloy 925 (no hydrogen embrittlement risk above ~−1.10 V).
For applications where stray-current or impressed-current overprotection could drive potentials below −1.10 V, Incoloy 925 stress should be limited to ≤80% specified minimum yield strength to provide hydrogen-embrittlement margin. This is a common design rule for subsea fasteners attached to ICCP-protected structures.
Service Limitations
- Crevice corrosion above 131°F (55°C): Surface deposits, gasket crevices and biological fouling can initiate localised attack above this threshold. For deepwater high-temperature service, surface treatment (electropolish, passivation per ASTM A967) extends the limit.
- Galvanic compatibility: Incoloy 925 is more noble than carbon steel, 13Cr stainless and titanium, in mixed-metal subsea systems, isolation gaskets / sleeves are specified.
- Velocity / erosion: Standard CRA design rules apply, flow velocity below 5 m/s for produced-water service to avoid erosion of the passive film.
- Marine biofouling: Like all CRAs, Incoloy 925 is subject to marine biofouling. Antifouling coatings or periodic cleaning may be required for long-term subsea service.
Frequently Asked Questions
Why not just use 316L stainless for marine bolting? 316L is susceptible to Cl-SCC at temperatures above ~140°F in chloride environments. For splash-zone or fully-immersed service in warm water (Gulf of Mexico, West Africa), 316L bolts have failed by SCC in service. Incoloy 925 is immune to Cl-SCC at any temperature within its qualified envelope, the additional cost is justified for safety-critical applications.
Is Incoloy 925 better than super-duplex 2507 for FPSO? Depends on the application. For piping and tubing where strength is set by wall thickness, 2507 is more cost-effective. For high-load fasteners, Incoloy 925's 110 ksi YS minimum + Cl-SCC immunity wins over 2507's 80 ksi YS + Cl-SCC resistance. The two alloys are complementary, many FPSOs use both: 2507 for structural piping, Incoloy 925 for compact-flange and high-preload bolting.
What thread compound should be used for Incoloy 925 marine bolting? Sulphur-free, copper-free thread compounds, for example Loctite 8009 (high-purity nickel anti-seize) or PetroStar 7 (PTFE-based). Avoid copper-bearing compounds (galvanic), zinc-bearing compounds (hydrogen-charging risk during cathodic protection), and graphite-bearing compounds (galvanic with stainless flanges).
How does TorqBolt verify NORSOK M-650 status? Mill QTRs (Qualification Test Reports) are valid 5 years from the test date; TorqBolt's QA system tracks expiry with 180 / 90 / 60 / 30-day alerts. Each order's MTC references the active QTR; TPI witness can be added at any stage of the manufacturing process for project-specific verification.
Related Pages
- Downhole oil & gas applications
- NACE MR0175 sour-service envelope
- Surface safety valves (SSV)
- Tubing & casing hangers
- Full Incoloy 925 datasheet
Request a Quote
For NORSOK M-650 / DNV / Bureau Veritas / ABS / Lloyd's qualified Incoloy 925 marine bolting and components:
- Email: info@torqbolt.com
- WhatsApp: +91-22-66157017
- NORSOK MDS A30 + sample MTC available on request
Specify project name, applicable MDS (NORSOK M-630 A30 / customer spec), product type (stud bolts, nuts, hex bolts, bar / pump shafting, control-line tubing), size, quantity, applicable standards (ASTM B805, NORSOK M-650 / M-630, API 6A for wellhead bolting), TPI witness requirements (DNV / BV / ABS / Lloyd's), and any cathodic-protection design stress limits.
References: Special Metals Corporation: INCOLOY® alloy 925 Technical Bulletin. ASTM B805; ASTM B983. NORSOK M-650 (material manufacturer qualification); NORSOK M-630 MDS A30 (material data sheet); NORSOK M-501 / L-005. NACE MR0175 / ISO 15156-3. INCOLOY® is a registered trademark of Special Metals Corporation.
