ISO 15156 (jointly issued as ANSI/NACE MR0175 / ISO 15156) is the international standard for materials used in oil and gas production environments containing hydrogen sulfide (H2S). The standard is published in three parts: Part 1 (general principles for cracking-resistant material selection), Part 2 (carbon and low-alloy steels), and Part 3 (cracking-resistant CRAs and other alloys, covers Incoloy 925 in Annex B). Incoloy 925 (UNS N09925) is one of the most widely deployed nickel-iron-chromium alloys for sour service because it combines age-hardened high yield strength (≥110 ksi / 758 MPa) with full ISO 15156-3 Annex B qualification.
Need ISO 15156-3 compliant Incoloy 925 bolting? Email info@torqbolt.com with size + project sour-service envelope, 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 ISO 15156 Covers
ISO 15156 was first published in 2001 as the international harmonisation of NACE MR0175. The two are now jointly maintained as a single binding standard. Operators including Saudi Aramco, ADNOC, Equinor, Shell, ExxonMobil, ONGC, Petrobras and the broader IOGP signatory group write ISO 15156-3 compliance into every sour-service material requisition for upstream oil & gas. The standard mandates which materials may be used at which pH2S, chloride, pH and temperature, and at which hardness limits, to prevent sulfide stress cracking (SSC), chloride stress cracking (Cl-SCC) and hydrogen-induced cracking (HIC).
The companion standard NACE TM0177 defines the C-ring, U-bend and tensile test methods used to qualify alloys against ISO 15156-3 Annex B. Materials are listed in Annex B only after passing TM0177 testing in NACE-mandated solutions (NACE Solution A, 5% NaCl + 0.5% acetic acid saturated H2S; or autoclave variants for HPHT envelopes). See companion API 6A wellhead bolting standard which references ISO 15156-3 / NACE MR0175 as the sour-service compliance baseline for PSL-3 / PSL-4 procurements.
Incoloy 925 Listing in Annex B
Incoloy 925 (UNS N09925) is listed in ISO 15156-3 Annex B (Nickel-base alloys) as a qualified material for downhole, wellhead and Christmas-tree applications. The qualified service envelope:
| 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 with project qualification |
| Maximum hardness | 35 HRC | 46 HRC (project-specific qualification) |
| Chloride concentration | application-specific | tested to 25% NaCl per NACE TM0177 |
| CO2 partial pressure | application-specific | tested to 100 psi (autoclave) |
| Service temperature | up to ~232°C (450°F) | up to ~260°C (500°F) |
| Sulfur tolerance | limited | tested with elemental sulfur at 232°C |
For ISO 15156-3 compliance the typical default is the age-hardened condition with hardness controlled to ≤ 35 HRC. The cold-worked + aged condition extends the qualified envelope substantially but requires project-specific qualification testing per /sour-service. Special Metals datasheet Tables 8, 9 and 10 document the C-ring and autoclave test results, see /datasheet for the consolidated reference data.
Test Data, How Incoloy 925 Earned Annex B Listing
Per Special Metals datasheet Tables 8 and 9, Incoloy 925 has been tested in the four standard NACE solutions defined by ISO 15156-3 / NACE TM0177. None of these tests showed sulfide stress cracking unless explicitly noted:
- NACE Solution A (5% NaCl + 0.5% acetic acid saturated H2S, room temperature, C-ring at 100% yield), no SSC up to 46 HRC after 42 days in cold-worked + aged condition
- 15% NaCl + 200 psi H2S + 100 psi CO2 + 1 g/L sulfur at 232°C, SCC observed in CW+aged 40.5 HRC condition; defines upper service envelope
- 25% NaCl + 200 psi H2S + 100 psi CO2 + 1 g/L sulfur at 204°C, no SSC up to 46 HRC after 15 days
- 15% NaCl saturated H2S + 1000 psi gas (1% H2S, 50% CO2, 49% N2) at 260°C, no SCC up to 46 HRC after 42 days
General-corrosion data (Special Metals Table 10), 15-day autoclave on unstressed coupons:
| Solution | Corrosion rate (mpy) | Corrosion rate (mm/yr) |
|---|---|---|
| 15% NaCl + 200 psi H2S + 100 psi CO2 + 1 g/L sulfur, 232°C | 1.1 | 0.028 |
| 25% NaCl + 200 psi H2S + 100 psi CO2 + 1 g/L sulfur, 232°C | 1.2 | 0.030 |
Why Incoloy 925 vs Other ISO 15156-3 Annex B Alloys
Annex B lists multiple nickel-base CRAs, each qualified for its own envelope. Incoloy 925 sits at the cost-effective sweet spot for high-strength bolting:
| Alloy | UNS | Yield (min) | HRC limit | Cost vs B7M |
|---|---|---|---|---|
| ASTM A193 B7M (low-alloy steel, Annex A.1) | — | 80 ksi (552 MPa) | 22 | 1.0× |
| Incoloy 925 | N09925 | 110 ksi (758 MPa) | 35 | 4–6× |
| Incoloy 825 (annealed only) | N08825 | 45 ksi (310 MPa) | 35 | 3–4× |
| Inconel 718 | N07718 | 150 ksi (1034 MPa) | 40 | 10–12× |
| Inconel 725 | N07725 | 120 ksi (827 MPa) | 43 | 9–11× |
| Inconel 625 (annealed) | N06625 | 60 ksi (414 MPa) | 35 | 6–8× |
| Monel K500 | N05500 | 90 ksi (621 MPa) | 35 | 7–8× |
For most HPHT sour-gas wellhead bolting Incoloy 925 closes the strength budget cost-effectively. Operators step up to Inconel 718 only when the application requires > 110 ksi yield. Incoloy 825 is preferred when the strength budget allows annealed condition (lower cost), but its yield strength is too low for most fastener applications. See Incoloy 925 vs Monel K500 for marine-vs-sour decision logic.
ISO 15156-3 vs NACE MR0175, Same Standard
ISO 15156 and NACE MR0175 are the same standard with two issuing bodies. The standard is now formally published as ANSI/NACE MR0175 / ISO 15156, single document, harmonised technical content, dual logos. Operators reference whichever name is conventional in their region:
- North America: typically references "NACE MR0175" (legacy preference)
- Europe / Norway / Middle East / Asia: typically references "ISO 15156-3"
- International / multi-region projects: references both, e.g. "Per ISO 15156-3 / NACE MR0175 Annex B"
TorqBolt MTCs include both designations on the conformance stamp to satisfy any operator's documentation convention. See /nace-mr0175 for the same standard discussed under its NACE designation.
TorqBolt ISO 15156-3 Documentation Package
Every Incoloy 925 sour-service order ships with the following document set, audit-ready against any operator's material requisition:
- 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)
- ISO 15156-3 / NACE MR0175 conformance stamp on the MTC face
- Heat-lot identifier on every piece
- 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 (yield, tensile, elongation, RoA)
- Hardness survey across the heat lot (≤ 35 HRC for sour service) per ASTM E18
- 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
- Project-specific marking and packaging per buyer requisition
Common ISO 15156-3 Compliant Incoloy 925 Items
| Product | Standard | Sizes | TorqBolt page |
|---|---|---|---|
| Stud bolts (with 2× heavy hex nuts) | ASTM A193 equivalent | 1/2" – 4" | /stud-bolts |
| Heavy hex nuts | ASME B18.2.2 | 1/2" – 4" | /heavy-hex-nuts |
| Hex bolts | ASME B18.2.1 | 3/8" – 2" | /hex-bolts |
| Bar & forgings | ASTM B805 | 8 mm – 350 mm OD | /bars · /forgings |
| Pipe / tube | ASTM B829 / B704 | 1/8" – 24" NPS | /pipe · /tube |
| Sheet / plate | ASTM B983 | 0.5 mm – 100 mm | /plate · /sheet |
Frequently Asked Questions
Are ISO 15156-3 and NACE MR0175 the same? Yes, they are jointly published as a single standard with dual issuing bodies (NACE/AMPP and ISO). Compliance with one IS compliance with the other. See /nace-mr0175.
What is the maximum H2S for Incoloy 925? Per Annex B, 0.2 bar (3 psi) partial pressure in the standard age-hardened condition. Higher envelopes possible in cold-worked + aged condition with project qualification per NACE TM0177.
Why 35 HRC max? Above 35 HRC, sulfide-stress-cracking susceptibility rises substantially per the C-ring test data (Special Metals Tables 8, 9). ISO 15156-3 sets 35 HRC as the qualified threshold for the standard age-hardened condition.
Does TorqBolt supply the cold-worked + aged variant? Yes, for HPHT projects requiring > 0.2 bar pH2S envelope. Project-specific NACE TM0177 qualification testing is required and adds 4–8 weeks to lead time. Standard age-hardened (≤ 35 HRC) supply is the default.
Request a Quote
Send your ISO 15156-3 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, hardness control limit (35 HRC max default), delivery destination. Datasheet PDF + sample NACE-stamped MTC available immediately on request.
References: ISO 15156-3. NACE MR0175 / AMPP. Special Metals INCOLOY alloy 925 Technical Bulletin (UNS N09925), Tables 8, 9, 10. /nace-mr0175. TorqBolt parent-site sour-service bolting.
