Incoloy 925 (UNS N09925) is one of the most widely deployed corrosion-resistant alloys for sour-service oil and gas applications. The alloy combines age-hardened high strength (110 ksi minimum yield per ASTM B805) with documented resistance to sulfide stress cracking (SSC), stress-corrosion cracking (SCC) and general corrosion in H₂S-bearing environments. It is listed in NACE MR0175 Table A.10 and ISO 15156-3 Annex B for sour-service applications up to 1000 psi H₂S partial pressure at 400°F, an envelope that covers virtually every commercial sour gas well in operation. Special Metals C-ring test data (Tables 8–10) provides the underlying qualification basis. This page summarises the qualified envelope, the supporting test data, and how TorqBolt documents sour-service compliance on every order MTC.
Need NACE-stamped Incoloy 925? Email info@torqbolt.com with product type, size, quantity, and the service H₂S partial pressure / temperature for envelope verification. WhatsApp +91-22-66157017.
NACE MR0175 Qualified Envelope
| Parameter | Qualified Range (Table A.10) | Notes |
|---|---|---|
| H₂S Partial Pressure | Up to 1000 psi (6.9 MPa) | Effectively unrestricted, covers all commercial sour wells |
| Maximum Temperature | 400°F (204°C) | Higher T requires project-specific qualification |
| Chloride Concentration | Up to saturation | No chloride restriction |
| pH | Any | Including acidic completion brines |
| Hardness Cap (CW + Aged) | 35 HRC max | Single most-watched value on every MTC |
| Sulfur (Free) Tolerance | Qualified per Special Metals Table 10 | Tested in elemental sulfur slurry |
| API 6A Material Class | FF / HH (sour), PSL 1/2/3/3G/4 | Wellhead + Christmas tree applications |
C-Ring Sulfide Stress Cracking Data (Special Metals Table 8)
Room-temperature C-ring tests at 100% of yield strength, exposed to NACE TM0177 Solution A (5% NaCl + 0.5% acetic acid saturated with H₂S at 1 bar), all specimens galvanically coupled to carbon steel for worst-case test conditions:
| Condition | Hardness | 0.2% YS, MPa | Test Duration | Result |
|---|---|---|---|---|
| Age-hardened | 38 HRC | 786 | 42 days | No SSC |
| Cold-worked (no age) | 35.5 HRC | 958 | 42 days | No SSC |
| Cold-worked + aged | 43.5 HRC | 1214 | 42 days | No SSC |
| Cold-worked + aged (high) | 46 HRC | 1282 | 42 days | No SSC |
All four specimen conditions, including the highest 46 HRC + 1282 MPa yield combination, survived 42-day NACE TM0177 Method A exposure with zero sulfide stress cracking observed. This data supports the project-specific extension of the NACE MR0175 35 HRC cap to higher hardness values (up to 46 HRC) for room-temperature sour service when project qualification is performed.
High-Temperature Sour Stress-Corrosion Test Data (Special Metals Table 9)
Autoclave C-ring tests at 100% yield, 15-day duration, simulating the most aggressive HPHT sour-service environments:
| Test Medium | Composition | Temperature | Result |
|---|---|---|---|
| Medium A | 15% NaCl + 200 psi H₂S + 100 psi CO₂ + 1 g/L free sulfur | 232°C (450°F) | SCC observed in CW+aged 40.5 HRC, defines envelope upper limit |
| Medium C | 15% NaCl saturated; 1000 psi gas: 1% H₂S / 50% CO₂ / 49% N₂ | 260°C (500°F) | No SCC up to 46 HRC after 42 days |
The Medium A failure at 40.5 HRC + 232°C + free elemental sulfur defines the upper boundary of the qualified envelope. The Medium C survival at 260°C (without elemental sulfur) shows that the alloy's limitation is the combination of high temperature + free sulfur, not high temperature alone. Project qualification testing per ISO 15156-3 Annex B is required for service in Medium A–equivalent environments.
General Corrosion Rate (Special Metals Table 10)
15-day autoclave, unstressed coupons, simulating sustained exposure to severe sour brine:
| Test Environment | Temperature | Corrosion Rate |
|---|---|---|
| 15% NaCl + 200 psi H₂S + free sulfur | 232°C (450°F) | 1.1 mpy (0.028 mm/yr) |
| 25% NaCl + 200 psi H₂S + free sulfur | 232°C (450°F) | 1.2 mpy (0.030 mm/yr) |
General corrosion rates of 1–1.2 mpy (~0.03 mm/yr) at 232°C give a 30-year wall-thickness loss of approximately 1 mm even in the worst test conditions. For most downhole tubing applications with 5–15 mm wall thickness, the wall-loss budget is more than sufficient for the well's design life.
Why Hardness Control Matters, The 35 HRC Cap
Sulfide stress cracking is a hydrogen-embrittlement-driven failure mechanism: H₂S dissociates at the metal surface to release atomic hydrogen, which diffuses into the grain boundary network and reduces ductility. The susceptibility to SSC scales sharply with hardness above ~35 HRC for nickel CRAs. NACE MR0175 Table A.10 caps qualified hardness at 35 HRC for UNS N09925 in cold-worked + aged condition; this is the boundary at which Special Metals' 42-day NACE TM0177 testing transitions from "no failures observed" to "occasional failures observed." TorqBolt 100% hardness-tests every Incoloy 925 piece pre-shipment with values reported on the MTC.
NACE Test Methods Referenced
- NACE TM0177 Method A, Tensile in NACE Solution: Smooth tensile specimen at 80% YS in NACE Solution A for 720 hr. Pass = no failure.
- NACE TM0177 Method C, C-Ring: Bent C-shape coupon at 100% YS in NACE Solution A for 720 hr. Used for plate / wrought material qualification.
- NACE TM0177 Method D, Double Cantilever Beam (DCB): Crack-propagation rate measurement, for fracture-mechanics-based design.
- NACE TM0316, Four-Point Bend: Used for welded coupons to test HAZ and weld metal.
- NACE TM0284, Hydrogen-Induced Cracking (HIC): Slow-strain-rate or stepwise cracking evaluation.
Sour-Service MTC Documentation Package
Every TorqBolt Incoloy 925 sour-service order ships with the following MTC documentation:
- EN 10204 3.1 manufacturer's certificate (3.2 third-party witnessed on request)
- NACE MR0175 Table A.10 reference on the certificate face
- 100% hardness inspection per ASTM E18 with values reported per piece
- Sample tensile + Charpy per heat / per heat-treatment lot
- Heat-treatment chart-recorder traces for solution anneal + age cycles (per heat-treatment specification)
- Optional NACE TM0177 / TM0316 production-coupon test data on request
- NACE stamping on the physical components (low-stress die)
Application-Specific Sour-Service Use Cases
- Downhole completion equipment, production tubing, packers, landing nipples, SSSV bodies
- Surface safety valves (SSV), valve bodies, stems, bonnets, internals
- Tubing & casing hangers, API 6A wellhead components
- Drill-string tool joints, sour-service drilling and workover
- API 6A wellhead bolting, stud bolts and heavy hex nuts to PSL-3 / PSL-3G / PSL-4
- Christmas tree internals, choke valves, manifold bolting, riser-flange bolting
Frequently Asked Questions
Is "NACE MR0175" the same as "ISO 15156"? Functionally yes, the two standards are now harmonised and published as a joint NACE / ISO document. NACE MR0175 is the legacy North-American name; ISO 15156-3 is the international name. The technical content (Table A.10 for UNS N09925) is identical. Most MTCs reference both.
Why does the 35 HRC cap matter when Special Metals tested up to 46 HRC with no failures? The 35 HRC cap is the codified industry limit; service above this requires project-specific qualification per ISO 15156-3 Annex B Section 6 (which Special Metals' Table 8 data supports for room-temperature service). For routine commercial supply, TorqBolt manufactures within the 35 HRC cap; for HPHT projects with project qualification, higher hardness can be supplied with documented test programme.
Can Incoloy 925 be used in environments with elemental sulfur? Yes, with caution. Special Metals Table 10 shows acceptable general corrosion rates (~1 mpy) in 200 psi H₂S + free sulfur at 232°C. SSC behaviour is more sensitive, the Medium A failure at 40.5 HRC indicates that elemental sulfur reduces the qualified hardness envelope at high temperature. For sustained exposure to free sulfur above 200°C, project qualification is recommended.
How does Incoloy 925 compare to Inconel 625 / 725 for sour service? All three are NACE MR0175 qualified. Inconel 625 (Table A.12) has an unrestricted H₂S envelope but lower strength (60 ksi YS). Inconel 725 (Table A.14) combines high strength (~120 ksi YS) with unrestricted H₂S, it is the alternative when 925's 110 ksi YS is insufficient. Inconel 718 is restricted to 15 psi H₂S maximum (Table A.13), not suitable for most sour wells.
Related Pages
- NACE MR0175 standard reference
- ISO 15156-3 standard reference
- Downhole oil & gas applications
- API 6A wellhead bolting
- Full Incoloy 925 datasheet
Request a Quote
For NACE MR0175 stamped Incoloy 925:
- Email: info@torqbolt.com
- WhatsApp: +91-22-66157017
- Sample MTC + NACE TM0177 production-coupon data available on request
Specify product type (stud bolts, nuts, hex bolts, tubing, plate), size, quantity, applicable specs (ASTM B805, API 6A, NACE MR0175), service H₂S partial pressure / temperature / chloride concentration, and any project-qualification requirements (NACE TM0177 / TM0316 production coupon, third-party witness, NORSOK MDS A30).
References: NACE MR0175 / ISO 15156-3 Petroleum and Natural Gas Industries, Materials for use in H₂S-containing environments, Table A.10 (UNS N09925). Special Metals Corporation: INCOLOY® alloy 925 Technical Bulletin, Tables 8 (NACE C-ring), 9 (high-temperature sour SCC), 10 (general corrosion). NACE TM0177 / TM0316 / TM0284 referenced test methods. ASTM B805; API Specification 6A. INCOLOY® is a registered trademark of Special Metals Corporation.
