API Specification 6A (ANSI/API Spec 6A / ISO 10423) is the petroleum industry standard for wellhead and Christmas tree equipment. It defines material, dimensional, performance, qualification, marking, and quality-control requirements for valves, hangers, chokes, flanges, and the bolting that holds them together. Bolting in sour-gas wellhead service is increasingly specified in Incoloy 925 (UNS N09925), an age-hardenable nickel-iron-chromium-molybdenum alloy that combines B7-class yield strength with full NACE MR0175 / ISO 15156-3 sour-service compliance. TorqBolt manufactures the complete Incoloy 925 API 6A bolting set (stud bolts, heavy hex nuts, washers, body bolts, bonnet studs) for PSL-1 through PSL-4 documentation.
Need API 6A bolting in Incoloy 925? Email info@torqbolt.com with size + PSL level + project H₂S envelope, or call +91-22-66157017. Datasheet PDF + sample MTC available on request.
What API 6A Covers
API 6A applies to surface and subsea wellhead equipment in oil and gas production: surface flange-and-stud connections, christmas-tree valves, chokes, casing-head housings, tubing-head spools, hangers, mandrel hangers, and the fasteners (studs, nuts, cap screws, body bolts) that load these connections. The standard harmonises with ISO 10423 for international procurement and references companion API standards including API 6FA / 6FB (fire test), API 6FC (hot test), API 17D (subsea), API 14A (subsurface safety valves), and API 7-1 (rotary drill stem).
Bolting in API 6A equipment is critical because it carries both pressure-end loads and seal-energising preload. Loss of preload → loss of seal → potential blow-out. The standard mandates traceable mechanical properties at design temperature (not just room temperature), which is why high-temperature sour-service alloys like Incoloy 925 are preferred over carbon-steel B7 in HPHT (high-pressure high-temperature) wells. See also our oil & gas downhole applications page for the full bolting envelope.
PSL Levels, How They Drive Documentation Depth
API 6A defines four Product Specification Levels (PSL-1 through PSL-4). Each higher PSL adds stricter inspection, testing and traceability without changing the material's mechanical properties. PSL is the documentation+QA framework, not the alloy spec. Operators select PSL based on service severity, well depth, and consequence-of-failure analysis.
| PSL | Typical service | Documentation requirement | NDE | Heat-lot tracking |
|---|---|---|---|---|
| PSL-1 | onshore conventional | EN 10204 3.1 (manufacturer) | visual + dimensional | per heat |
| PSL-2 | HP onshore + shallow offshore | EN 10204 3.1 + retained samples | + MT or PT (selected lots) | per heat |
| PSL-3 | HPHT, sour service, deep offshore | EN 10204 3.2 (third-party witnessed) | UT + MT + PT every item | per piece |
| PSL-4 | critical sour HPHT, subsea | EN 10204 3.2 + project-specific QA plan | UT + MT + PT + Charpy at design temp | per piece + serial numbers |
For Incoloy 925 bolting on PSL-3 and PSL-4, TorqBolt supplies the full witness package: third-party inspection (TÜV Rheinland, Lloyd's Register, Bureau Veritas, SGS or buyer-nominated TPI), datasheet-grade chemistry per EN 10204 3.2 MTC, hardness survey across the full heat lot, and Charpy V-notch impact testing at the design temperature specified in the project's material requisition. See /datasheet for sample MTC content.
Why Incoloy 925 vs B7 / B7M for API 6A
For sweet (non-H₂S) onshore wells, ASTM A193 Grade B7 (or B16) carbon-alloy steel bolting is the lowest-cost option. The moment wellhead service goes sour, H₂S partial pressure ≥ 0.05 psia per NACE MR0175 / ISO 15156-3, B7 is disqualified for sulfide stress cracking risk. Operators move to B7M (hardness-tempered B7, 22 HRC max). B7M passes mild sour service but has lower strength than B7. For HPHT sour-gas wells the strength penalty of B7M is unacceptable, and the alloy options narrow:
| Bolting alloy | UNS | Yield (min) | NACE qualified | Cost vs B7 (×) |
|---|---|---|---|---|
| ASTM A193 B7 | — | 105 ksi (724 MPa) | No (sour disqualified) | 1.0 |
| ASTM A193 B7M | — | 80 ksi (552 MPa) | Yes (mild sour, 22 HRC max) | 1.2 |
| Incoloy 925 | N09925 | 110 ksi (758 MPa) | Yes (full envelope, 35 HRC max) | 4–6 |
| Inconel 718 | N07718 | 150 ksi (1034 MPa) | Yes | 10–12 |
| Inconel 725 | N07725 | 120 ksi (827 MPa) | Yes | 9–11 |
Incoloy 925 sits at the cost-effective sweet spot: B7-class yield strength + full sour-service NACE compliance at one-third to one-half the cost of Inconel 718. For most HPHT sour applications the strength budget closes within Incoloy 925's envelope, making it the default bolting alloy on Saudi Aramco, ADNOC, Petrobras, Pemex, ONGC and most North Sea operator specifications. See Incoloy 925 vs Inconel 718 for a side-by-side comparison.
Mechanical Properties at API 6A Design Temperatures
Per Special Metals SMC HA 46 (the OEM's internal acceptance specification), age-hardened Incoloy 925 yield strength ≥ 110 ksi (758 MPa), tensile strength ≥ 140 ksi (965 MPa), elongation ≥ 18% in 4D, reduction of area ≥ 25%, hardness 26–38 HRC, and Charpy V-notch impact ≥ 35 J at room temperature. The age-hardening cycle (1900°F / 1040°C anneal + 1365°F / 740°C age) is detailed on our heat treatment page. At elevated temperature (200–650°F / 95–345°C, typical Christmas-tree thermal range), the alloy retains a substantial fraction of its room-temperature strength per the Special Metals datasheet Figure 1. See /mechanical-properties for the full curve.
Sour-Service Envelope
Incoloy 925 is listed in ISO 15156-3 Annex B (Nickel Alloys) for sour-service oil and gas production. The qualified envelope is:
- H₂S partial pressure: up to 0.2 bar (3 psia) in the standard age-hardened condition with hardness controlled to ≤ 35 HRC
- Higher pH₂S envelopes: possible in cold-worked + aged condition with project-specific qualification per NACE C-ring testing (Special Metals datasheet Tables 8 and 9 show no SSC up to 46 HRC under qualified test conditions)
- Chloride: tested up to 25% NaCl per Table 9
- CO₂: tested at 100 psi partial pressure in NACE C-ring solutions
- Service temperature: room temperature to ~600°F (316°C), covers typical surface and subsea Christmas tree thermal range
Full sour-service test data + envelope curves are on our sour service page. NACE-stamped MTCs are supplied with every Incoloy 925 sour-service order from TorqBolt.
Common Incoloy 925 Bolting Items in API 6A Service
| Bolting item | Standard | Typical sizes | TorqBolt page |
|---|---|---|---|
| Stud bolts (full thread + 2× heavy hex nuts) | ASTM A193 equivalent | 1/2" through 4" diameter | /stud-bolts |
| Heavy hex nuts | ASME B18.2.2 / ASTM A194 grade 7M equivalent | 1/2" through 4" diameter | /heavy-hex-nuts |
| Hex bolts (body bolts, bonnet bolts) | ASME B18.2.1 | 3/8" through 2" diameter | /hex-bolts |
| Heavy hex bolts (BSL-2 / BSL-3 bonnet) | ASME B18.2.6 | 1/2" through 3" diameter | /heavy-hex-bolts |
| Threaded rod / all-thread | ASTM A193 equivalent | up to 12 ft length | /threaded-rods |
| Washers (plain hardened) | ASTM F436M | 1/2" through 4" | /washers |
TorqBolt API 6A Documentation Package
For PSL-3 and PSL-4 procurements, TorqBolt issues the following documentation set with every Incoloy 925 bolting order. Each package is project-specific and audit-ready against the operator's material requisition:
- EN 10204 3.1 manufacturer's MTC (PSL-1/2 minimum) or 3.2 third-party witnessed MTC (PSL-3/4)
- NACE MR0175 conformance stamp on the MTC face
- Heat-lot map showing every piece's heat number and serial number
- 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 plus Charpy V-notch impact at design temperature
- Hardness survey across the heat lot (≤ 35 HRC for sour service)
- NDE certificates: UT (ultrasonic), MT (magnetic-particle), PT (dye-penetrant) per API 6A Annex F
- Heat-treat charts (solution anneal + age) per SMC HA 46 cycle
- Dimensional inspection ASME B18.2.1 / B18.2.2 / B18.2.6 / F436M
- Project-specific marking and packaging per buyer requisition
Frequently Asked Questions
Is Incoloy 925 the only sour-service bolting alloy for API 6A PSL-3? No. Inconel 718, Inconel 725, Inconel 925-equivalents, and certain duplex/super-duplex stainless steels (with hardness control) also qualify. Incoloy 925 is the most common cost-effective choice for projects where the strength budget closes within 110 ksi yield.
What is the minimum hardness for sour service? Per ISO 15156-3 Annex B, Incoloy 925 must be supplied at ≤ 35 HRC in the age-hardened condition. TorqBolt controls aging time/temperature plus cooling rate to keep all delivered material at or below this limit, with a hardness map per heat lot.
How long does PSL-3 / PSL-4 documentation take? Standard lead time for in-stock Incoloy 925 bolting with EN 10204 3.1 MTC is 7–10 working days. Add 5–10 days for third-party witness inspection (TPI scheduling). Expedited service (3–5 days) available on request for stocked sizes.
Does TorqBolt supply other API 6A materials? Yes. Beyond Incoloy 925 we manufacture API 6A bolting in Inconel 718, Inconel 725, Monel K500, super-duplex 2507, A286, and standard B7/B7M carbon-alloy. See TorqBolt API 6A certified bolting on the parent site for the complete material list.
Request a Quote
Send your API 6A bolting RFQ to info@torqbolt.com or WhatsApp +91 22 6615 7017. Include: bolting item type, size, PSL level, sour-service envelope (pH₂S, chloride, temperature), required MTC level, quantity, delivery destination. Datasheet PDF + sample MTC available immediately on request.
References: Special Metals INCOLOY alloy 925 Technical Bulletin (UNS N09925), Tables 1, 6, 7, 8, 9, 10. API 6A standard. ISO 15156-3 Annex B. NACE MR0175.
