NACE MR0175 (now harmonised internationally as ISO 15156) is the petroleum industry's binding specification for materials used in oil and gas production environments containing hydrogen sulfide (H2S). Operators including Saudi Aramco, ADNOC, Shell, Chevron, ExxonMobil, ONGC, Petrobras and the Norwegian operator group write NACE MR0175 compliance into every sour-service material requisition. Incoloy 925 (UNS N09925) is listed in ISO 15156-3 Annex B (Nickel-base alloys) as a qualified bolting and component material, it combines age-hardened high strength with intrinsic resistance to sulfide stress cracking, making it the cost-effective default sour-service CRA for downhole, wellhead and Christmas-tree applications.
Need NACE MR0175-compliant Incoloy 925 bolting? Email info@torqbolt.com with size + project H2S envelope + chloride concentration, or call +91-22-66157017. NACE-stamped EN 10204 3.1 / 3.2 MTC supplied with every order. Datasheet PDF + sample MTC available on request.
What NACE MR0175 / ISO 15156 Covers
NACE MR0175 was first issued in 1975 and is now jointly maintained as ANSI/NACE MR0175 / ISO 15156. The standard is published in three parts: Part 1 (General principles for selection of cracking-resistant materials), Part 2 (Cracking-resistant carbon and low-alloy steels), and Part 3 (Cracking-resistant CRAs and other alloys, covers Incoloy 925 in Annex B). Compliance is mandatory in most upstream sour-service contracts and is enforced via the EN 10204 3.1 / 3.2 MTC issued with every shipment.
The standard defines the maximum H2S partial pressure, in-situ pH, chloride concentration, temperature, and hardness limit for each qualified material. Materials NOT meeting these limits, or supplied at higher hardness than the standard allows, are disqualified from sour-service procurement. See companion standard ISO 15156-3 for the international harmonisation, plus API 6A wellhead bolting which references NACE MR0175 as the sour-service compliance baseline.
Sour-Service Test Data, Incoloy 925
Per Special Metals datasheet Tables 8, 9 and 10, Incoloy 925 has been qualified through extensive C-ring and autoclave testing in NACE-mandated solutions. Test results, none cracked unless noted:
| Test condition | Yield (MPa) | Hardness (HRC) | Days | SSC? |
|---|---|---|---|---|
| 5% NaCl + 0.5% acetic + sat. H2S (Table 8) | 786 | 38 | 42 | No |
| 5% NaCl + 0.5% acetic + sat. H2S | 958 | 35.5 | 42 | No |
| 5% NaCl + 0.5% acetic + sat. H2S (CW+aged) | 1214 | 43.5 | 42 | No |
| 5% NaCl + 0.5% acetic + sat. H2S (CW+aged) | 1282 | 46 | 42 | No |
| 15% NaCl + 200 psi H2S + 100 psi CO2 + S at 232°C (Table 9) | 1145 | 40.5 | 15 | Yes |
| 15% NaCl saturated H2S + 1000 psi gas at 260°C (Table 9) | 1214 | 43.5 | 42 | No |
The C-ring data show that Incoloy 925 in the standard age-hardened condition (≤ 38 HRC) passes the most aggressive room-temperature sour-service tests. At elevated temperature with elemental sulfur the envelope tightens, project-specific qualification is required for HPHT-with-sulfur service. See /sour-service for the full test matrix and pH-vs-pH2S envelope curves.
Qualified Service Envelope (ISO 15156-3 Annex B)
| Parameter | Standard age-hardened (≤35 HRC) | Cold-worked + aged (project-qualified) |
|---|---|---|
| Maximum H2S partial pressure | 0.2 bar (3 psi) | up to ~3.5 bar (qualified per project) |
| Maximum chloride concentration | per project | tested to 25% NaCl |
| CO2 partial pressure | per project | tested to 100 psi |
| Maximum service temperature | ~232°C (450°F) | ~260°C (500°F) |
| Hardness limit | 35 HRC max | 46 HRC max (project qualified) |
| Sulfur tolerance | limited (project-specific) | tested with elemental S, 232°C |
Why Incoloy 925 vs Other NACE-Qualified Alloys
NACE MR0175 lists multiple nickel-base CRAs in Annex B. Incoloy 925 sits at the cost-effective sweet spot for high-strength sour-service bolting:
| Alloy | UNS | Yield (min) | Hardness limit | Cost vs B7M (×) |
|---|---|---|---|---|
| ASTM A193 B7M (carbon-alloy) | — | 80 ksi (552 MPa) | 22 HRC | 1.0 |
| Incoloy 925 | N09925 | 110 ksi (758 MPa) | 35 HRC | 4–6 |
| Incoloy 825 (annealed) | N08825 | 45 ksi (310 MPa) | 35 HRC | 3–4 |
| Inconel 718 | N07718 | 150 ksi (1034 MPa) | 40 HRC | 10–12 |
| Inconel 725 | N07725 | 120 ksi (827 MPa) | 43 HRC | 9–11 |
| Monel K500 | N05500 | 90 ksi (621 MPa) | 35 HRC | 7–8 |
For typical wellhead bolting where the strength budget closes within 110 ksi yield, Incoloy 925 is the standard NACE-qualified pick. See Incoloy 925 vs Inconel 718 when higher strength is required, or Incoloy 925 vs Incoloy 825 when the application allows lower strength.
Hardness Control, How TorqBolt Stays Below 35 HRC
The recommended Incoloy 925 heat treatment cycle (solution anneal at 1900°F / 1040°C + age at 1365°F / 740°C for 6–9 hours) typically produces hardness in the 26–38 HRC range. For NACE MR0175 sour-service supply, TorqBolt controls the aging time and furnace cooling rate to keep all delivered material at or below 35 HRC. Process controls:
- Heat-lot hardness map, every piece sampled per ASTM E18 (Rockwell C scale), full survey across the lot recorded on the MTC
- Aging cycle data logger, ramp + soak + cool curve archived per heat lot
- NACE-stamped MTC, explicit conformance statement on the certificate face: "Material complies with NACE MR0175 (Annex B) for sour-service applications"
- Statistical sampling, minimum 10% sample size per heat lot, 100% on critical bolting
- Optional Charpy impact at design temperature, for HPHT projects requiring low-temperature toughness verification
TorqBolt NACE MR0175 Documentation Package
For sour-service procurements, TorqBolt issues the following document set with every shipment. Each is project-specific and audit-ready:
- EN 10204 3.1 manufacturer's MTC (standard) or 3.2 third-party witnessed MTC (TÜV Rheinland, Lloyd's Register, Bureau Veritas, SGS or buyer-nominated TPI)
- NACE MR0175 conformance stamp on the MTC face
- Heat-lot identifier on every piece (heat number + serial where applicable)
- Chemistry per Special Metals SMC HA 46 (Ni 42–46, Cr 19.5–22.5, Mo 2.5–3.5, Cu 1.5–3.0, Ti 1.9–2.4, plus residuals)
- Mechanical-property certification at room temperature
- Hardness survey across the heat lot with min/max/mean values
- Heat-treat charts (solution anneal + age cycle) per SMC HA 46
- Optional NDE certificates (UT, MT, PT) per project requirement
- Optional Charpy V-notch impact testing at design temperature
Common NACE MR0175-Compliant Incoloy 925 Items
| Product | Standard | Sizes | Internal page |
|---|---|---|---|
| Stud bolts (with 2× heavy hex nuts) | ASTM A193 equivalent + B7M-class strength | 1/2" – 4" | /stud-bolts |
| Heavy hex nuts | ASME B18.2.2 / A194 grade 7M equivalent | 1/2" – 4" | /heavy-hex-nuts |
| Hex bolts (body / bonnet) | ASME B18.2.1 | 3/8" – 2" | /hex-bolts |
| Bar & forgings (machined components) | ASTM B805 | 8 mm – 350 mm OD | /bars · /forgings |
| Pipe / tube (downhole tubing) | ASTM B829 / B704 | 1/8" – 24" NPS | /pipe · /tube |
| Sheet / plate | ASTM B983 | 0.5 mm – 100 mm thick | /plate · /sheet |
Frequently Asked Questions
What is the difference between NACE MR0175 and ISO 15156? They are the same standard. ISO 15156 is the international harmonisation of NACE MR0175 (originally a US-only standard). Compliance with one IS compliance with the other. The standard is now published as "ANSI/NACE MR0175 / ISO 15156", single document, dual issuing bodies.
Why 35 HRC max for Incoloy 925 in sour service? Above 35 HRC, sulfide-stress-cracking susceptibility rises substantially per the C-ring test data. ISO 15156-3 Annex B sets 35 HRC as the qualified threshold for the standard age-hardened condition. Higher hardness (up to 46 HRC) is allowed only in the cold-worked + aged condition with project-specific qualification per the operator's material requisition.
Does NACE MR0175 apply only to bolting? No. It applies to all metallic materials in sour-service production: tubing, casing, valves, hangers, packers, completion tools, surface flow lines, separators. Incoloy 925 is qualified across all these forms, see the table above.
How long does NACE-stamped MTC documentation take? Standard 3.1 MTC ships with the order (no lead-time addition). 3.2 third-party witnessed MTC adds 5–7 days for TPI scheduling and witness inspection. Expedited service available on request for stocked sizes.
Request a Quote
Send your sour-service Incoloy 925 RFQ to info@torqbolt.com or WhatsApp +91 22 6615 7017. Include: product type, size, quantity, project H2S envelope (pH2S, chloride, temperature), MTC level (3.1 or 3.2), hardness control limit (35 HRC max default), delivery destination. Datasheet PDF + sample NACE-stamped MTC available immediately on request.
References: NACE MR0175 / AMPP standards. ISO 15156-3 Annex B. Special Metals INCOLOY alloy 925 Technical Bulletin (UNS N09925), Tables 8, 9, 10. TorqBolt parent-site sour-service bolting.
