Standards
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Incoloy 925 Stud Bolts (UNS N09925)
Full-thread + double-end stud bolts per ASTM A193 / B805 / API 6A — sour-service NACE MR0175 stamped
Incoloy 925 stud bolts (UNS N09925) are the workhorse fastener that holds together every API 6A wellhead, Christmas tree, BOP stack and high-pressure flanged joint in sour H₂S oilfield service. A stud bolt is a fully-threaded rod (or double-end with two threaded zones and a plain shank) used with two heavy hex nuts; it differs from a hex bolt (which has an integral head) in that both ends are threaded, allowing preload to be applied by tightening either nut. TorqBolt manufactures Incoloy 925 stud bolts in the solution-annealed and aged condition (per heat-treatment specification) with 100% hardness inspection guaranteeing the 35 HRC NACE MR0175 sour-service cap is never exceeded. Sizes 1/4" to 4" diameter, lengths to 1500 mm, in UNC / UNF / 8UN / metric (M6 to M64) thread series. Every order ships with EN 10204 3.1 manufacturer's MTC documenting actual heat-lot chemistry, mechanical results and heat-treatment cycle vs ASTM B805, ASTM A193 and API 6A requirements; 3.2 with TPI witness on request.
Need Incoloy 925 stud bolts? Email info@torqbolt.com with diameter × length, thread series (UNC / UNF / 8UN / metric), quantity, applicable standards (API 6A PSL level, NACE MR0175 stamp, NORSOK MDS A30) and any thread compound / coating requirements. WhatsApp +91-22-66157017.
Stud-Bolt Designs Manufactured
| Design | Description | Thread Standard | Typical Use |
|---|---|---|---|
| Full-thread stud (continuous thread) | Threaded the entire length; both ends accept nuts | ASME B1.1 / B1.13M | Flange bolting; replaceable joints |
| Double-end stud (Type 1, plain shank) | Threaded zones at both ends, smooth shank in middle | ASME B18.31.2 | Stud-and-nut joints with through-bolt configuration |
| Double-end stud (Type 2, reduced shank) | Reduced-diameter shank between threaded zones | ASME B18.31.2 | Designed-stretch / fatigue-resistant joints (BOP, Christmas tree) |
| Tap-end stud | One end threaded for blind tapping into component; other end for nut | ASME B18.31.2 | Engine block / pressure-vessel jacket bolting |
| Through-bolt stud (long) | Lengths exceeding 8× diameter; threaded both ends | Customer drawing | Compact-flange bolting; deep-section pressure vessels |
Standard Size Range & Thread Series
| Diameter | UNC / UNF (inch) | 8UN (inch >1") | Metric (mm) | Length Range |
|---|---|---|---|---|
| 1/4" / M6 | UNC 20 tpi / UNF 28 tpi | M6 × 1.0 | 25 to 200 mm | |
| 3/8" / M10 | UNC 16 / UNF 24 | M10 × 1.5 | 25 to 250 mm | |
| 1/2" / M12 | UNC 13 / UNF 20 | M12 × 1.75 | 30 to 350 mm | |
| 5/8" / M16 | UNC 11 / UNF 18 | M16 × 2.0 | 40 to 400 mm | |
| 3/4" / M20 | UNC 10 / UNF 16 | M20 × 2.5 | 50 to 500 mm | |
| 7/8" / M22 | UNC 9 / UNF 14 | M22 × 2.5 | 50 to 600 mm | |
| 1" / M24 | UNC 8 / UNF 12 | 8UN 8 tpi | M24 × 3.0 | 50 to 700 mm |
| 1-1/8" / M30 | UNC 7 / UNF 12 | 8UN 8 tpi | M30 × 3.5 | 75 to 800 mm |
| 1-1/4" / M33 | UNC 7 / UNF 12 | 8UN 8 tpi | M33 × 3.5 | 75 to 900 mm |
| 1-3/8" / M36 | UNC 6 / UNF 12 | 8UN 8 tpi | M36 × 4.0 | 100 to 1000 mm |
| 1-1/2" / M39 | UNC 6 / UNF 12 | 8UN 8 tpi | M39 × 4.0 | 100 to 1100 mm |
| 1-3/4" / M45 | 8UN 8 tpi | M45 × 4.5 | 125 to 1200 mm | |
| 2" / M48 | 8UN 8 tpi | M48 × 5.0 | 150 to 1300 mm | |
| 2-1/4" / M52 | 8UN 8 tpi | M52 × 5.0 | 150 to 1400 mm | |
| 2-1/2" / M56 | 8UN 8 tpi | M56 × 5.5 | 175 to 1500 mm | |
| 3" / M64 | 8UN 8 tpi | M64 × 6.0 | 200 to 1500 mm | |
| 4" / M100 | 8UN 8 tpi | M100 × 6.0 | 250 to 1500 mm |
Thread series convention: for diameters above 1", the 8UN thread series (8 threads per inch regardless of diameter) is the petroleum-industry standard and the API 6A default, UN-thread series tabulated in ASME B1.1. Thread mark on the bolt shank reads: 1-3/4"-8UN (no space before series). Thread tolerance class 2A (external) for studs; 2B (internal) for matching nuts, ASME B1.1 / B1.13M. Tighter tolerance class 3A available for high-precision applications.
Mechanical Property Requirements
| Property | Aged Condition (ASTM B805 + API 6A min) |
|---|---|
| Tensile Strength | 165 ksi (1140 MPa) min |
| 0.2% Yield Strength | 110 ksi (760 MPa) min |
| Elongation in 4D | 18% min |
| Reduction of Area | 25% min |
| Hardness | 32–35 HRC (NACE sour-service cap) |
| Charpy V-notch at -46°C | 40 J min (NACE sour service) |
| Charpy V-notch at -101°C | 27 J min (API 6A PSL-3G cryogenic) |
See full mechanical properties page for elevated-temperature tensile data and the cold-worked + aged Charpy envelope per Special Metals Tables 8–9.
Applicable Standards
- ASTM B805, UNS N09925 forgings & forging stock (the bar / billet input for stud-bolt manufacture)
- ASTM A193 Grade B8N (alloy 925 variant), Bolting Materials for High-Temperature or High-Pressure Service (UNS N09925 entry)
- ASTM A320 (low-temp service), cryogenic stud-bolt material specification
- ASTM F2281, Standard Specification for Stainless Steel and Nickel-Alloy Bolts for High-Pressure / High-Temperature Service
- API Specification 6A, Wellhead and Tree Equipment, PSL-1 to PSL-4 documentation
- API 20E, Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries (BSL-1 / 2 / 3 quality levels)
- NACE MR0175 / ISO 15156-3, Table A.10 sour-service qualification (35 HRC max)
- NORSOK M-630 MDS A30 / NORSOK L-005, Norwegian Continental Shelf bolting
- ASME B18.31.2, dimensional standard for double-end stud bolts
- ASME B1.1 / B1.13M, thread tolerance and series
- EN 10204 3.1 / 3.2, manufacturer / third-party witnessed certification
API 6A PSL Documentation Levels
| PSL | Pressure Class | Charpy Test Temperature | NDE Required |
|---|---|---|---|
| PSL-1 | 2,000–5,000 psi | Not required | Visual |
| PSL-2 | 5,000–10,000 psi | Not required | UT or MT (per heat lot) |
| PSL-3 | 10,000–15,000 psi | 27 J at -29°C (-20°F) | 100% UT |
| PSL-3G | 10,000–15,000 psi (gas / cryogenic) | 27 J at -46°C / -101°C | 100% UT + grain size |
| PSL-4 | 15,000–20,000 psi | 40 J at -46°C | 100% UT + 100% MT + TPI witness |
Manufacturing Sequence
- Bar input: Incoloy 925 round bar per ASTM B805 in the annealed condition, sourced from approved primary mills (Special Metals, Sandvik, VDM, ATI) with mill MTCs.
- Cut to length: Bar cut to stud-bolt length + threading allowance, ends faced perpendicular to bolt axis.
- Thread cutting / rolling: Threads applied in the SOLUTION-ANNEALED condition (annealed material is more forgiving for both single-point thread cutting on lathes and roll-forming). Thread rolling is preferred for fatigue-loaded fasteners, cold-rolled threads have residual compressive stress at the root that increases fatigue life by 2–3×.
- Solution anneal + age cycle: Per heat-treatment specification, 1900°F WQ + 1365°F × 8h FC + 1150°F × 8h AC. Threads cut before ageing maintain hardness uniformity at the root (vs cutting after ageing, which leaves a hardness gradient and SSC initiation sites).
- Final inspection: 100% Rockwell hardness per ASTM E18 (within 32–35 HRC band); thread gauging ASME B1.3 (Go / No-Go ring gauges); dimensional check ASME B18.31.2; sample tensile + Charpy per ASTM E8 / E23.
- Marking: Each stud stamped (low-stress die) with heat number, UNS N09925, applicable standard reference, NACE MR0175 stamp (sour-service orders), API 6A PSL level (wellhead orders).
Companion Components
- Incoloy 925 heavy hex nuts, companion to stud bolts ASME B18.2.4.6M
- Incoloy 925 washers, flat / lock washers ASME B18.22.1
- Thread compound, sulphur-free, copper-free anti-seize per API RP 7A1 (Loctite 8009 nickel anti-seize, PetroStar Premium Plus, Henkel 8023)
- Belleville / disc washers in Incoloy 925 for high-load fatigue applications
Application Examples
- API 6A wellhead body / bonnet bolting: 7-1/16" 10K wellhead = 16× 1-1/4"-8UN studs typically
- Christmas tree flange bolting: 6BX gasket on 6° 5K hub = 12× 1-1/2"-8UN through-bolts
- Subsea horizontal tree connector bolting: Compact-flange designs with high-preload double-end studs
- BOP stack lockdown bolting: 18-3/4" 15K BOP = ring of 1-3/4"-8UN through-bolts
- Manifold flange bolting: Sour-service production / injection manifold flange sets
- Heat exchanger flange bolting: Sour-duty shell-and-tube exchangers in refining / petrochemical
- Compact-flange bolting: ABB/Vector/Steelhead/Galperti compact flanges in HPHT systems
- Pressure-vessel manway bolting: Sour-service vessel inspection openings
Standard Coatings & Finishes
- Bare (passivated): Incoloy 925 in oxide-free condition per ASTM A380 / A967, standard sour-service finish
- PTFE coating: Xylan 1424 / 1070 / Whitford™ for low-friction make-up, resists galling on first installation
- Phosphate + oil: Manganese phosphate per MIL-DTL-16232 + light oil for corrosion protection during storage
- Cad plate: NOT recommended for sour service (cadmium causes embrittlement of nickel alloys in H₂S)
- Zn / hot-dip galvanise: NOT applicable (Incoloy 925 is corrosion-resistant; zinc adds galvanic risk in stainless flanged joints)
Frequently Asked Questions
What's the difference between a stud bolt and a hex bolt? A stud bolt is fully threaded (or threaded at both ends with plain shank) and uses two nuts. A hex bolt has an integral hex head and uses one nut. Studs are preferred for permanent / repeatedly-disassembled flanged joints because the same stud can be reused after disassembly with new nuts; hex bolts are preferred for one-time structural connections. API 6A wellhead service uses studs almost exclusively.
Why is 8UN the standard thread for diameters above 1"? The 8UN thread series (constant 8 threads per inch regardless of diameter) was adopted by API as the petroleum-industry standard because it gives consistent thread pitch and load distribution across the wide diameter range used in wellhead bolting. Below 1", UNC (coarse) is standard. Above 1", UNF (fine) is rare in oilfield service, the constant-pitch 8UN provides better preload retention under cyclic / vibrating loads.
Should I order full-thread or double-end studs? Full-thread for replaceable joints where the same stud may rotate position over the joint life (any thread can engage any nut). Double-end with reduced shank (Type 2) for fatigue-critical applications, the reduced shank distributes elastic strain over a longer length, giving higher fatigue life under cyclic loading. Wellhead bolting is typically full-thread; subsea connector bolting and high-cycle compressor bolting is typically double-end Type 2.
What thread compound do you recommend for Incoloy 925 studs? Sulphur-free, copper-free anti-seize compounds:
- Loctite 8009 (high-purity nickel anti-seize), recommended for sour service, fully API RP 7A1 compliant
- Henkel Permabond Antiseize 8023, nickel-graphite-free formulation
- PetroStar Premium Plus, PTFE-based, no metal particulate
AVOID: copper-bearing compounds (galvanic with stainless flanges); graphite-bearing compounds (galvanic + sulfide-attracting); sulphur-bearing EP compounds (SSC initiator); zinc-bearing compounds (hydrogen-charging during cathodic protection).
Can Incoloy 925 stud bolts be used for cryogenic LNG service? Yes, per API 6A PSL-3G qualification, Incoloy 925 stud bolts deliver 27 J min Charpy V-notch at -101°C (-150°F). For LNG-specific service to -196°C, project qualification testing is recommended, Special Metals data supports performance at these temperatures with appropriate coupon validation.
What's the typical lead time for Incoloy 925 stud bolts? Standard sizes (1/2" to 2" diameter, lengths to 600 mm): 4–6 weeks ex-bar-stock or 8–10 weeks new-melt. Larger sizes (2-1/2" to 4"): 6–10 weeks. Custom lengths above 1500 mm: 10–14 weeks. The bottleneck is the 18-hour age cycle in calibrated furnaces.
Documentation Package
- EN 10204 3.1 manufacturer's MTC (3.2 with DNV / Bureau Veritas / Lloyd's / TUV / SGS witness on request)
- Heat-lot chemistry vs UNS N09925 limits ASTM B805
- Tensile + RA + elongation results per heat / per heat-treatment lot
- 100% hardness inspection per ASTM E18 (within 32–35 HRC NACE band)
- Charpy V-notch at -46°C / -101°C as applicable
- Heat-treatment furnace chart-recorder traces
- Thread gauge inspection (Go / No-Go) ASME B1.3
- Dimensional inspection ASME B18.31.2
- NACE MR0175 Table A.10 reference (sour-service orders)
- API 6A PSL level reference (wellhead orders) with TPI witness for PSL-4
- Stud marking: heat number + UNS N09925 + standard ref + NACE stamp
Related Products
- Heavy hex nuts, companion to stud bolts ASME B18.2.4.6M
- Hex bolts, integral-head bolts for one-time structural joints
- Threaded rods, long-form continuous-thread for cut-to-length stud manufacture
- Washers, flat / lock washers for load distribution
- Bar stock, raw material for custom stud-bolt manufacture
- Full Incoloy 925 datasheet, chemistry / mech / physical / heat-treat reference
Request a Quote
For Incoloy 925 stud bolts with full MTC documentation:
- Email: info@torqbolt.com
- Phone: +91-22-66157017
- WhatsApp: +91-22-66157017
Specify diameter, length, thread series (UNC / UNF / 8UN / metric), thread tolerance class (2A standard / 3A precision), stud design (full-thread / double-end Type 1 / Type 2), quantity, applicable standards (API 6A PSL level, NACE MR0175 stamp, NORSOK MDS A30, ASTM A193 / A320 / F2281), required MTC level (3.1 / 3.2 with TPI witness) and any coating / finish requirement (bare passivated / PTFE / phosphate). Lead time confirmed at quote stage.
References: ASTM B805; ASTM A193 / A320 / F2281; ASME B18.31.2 (double-end studs); ASME B1.1 / B1.13M (thread series); ASME B1.3 (thread gauging); API Specification 6A Annex M; API 20E (bolting quality levels); NACE MR0175 / ISO 15156-3 Table A.10; NORSOK M-630 / M-650; EN 10204; ASTM A380 / A967 (passivation); MIL-DTL-16232 (phosphate). Special Metals Corporation INCOLOY® alloy 925 Technical Bulletin. INCOLOY® is a registered trademark of Special Metals Corporation.
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