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 Heavy Hex Nuts (UNS N09925)
Companion to stud bolts and heavy hex bolts — ASME B18.2.4.6M / B18.2.2 / DIN 6915
Incoloy 925 heavy hex nuts (UNS N09925) are the high-load companion to stud bolts and heavy hex bolts in API 6A wellhead, Christmas tree, BOP and pressure-vessel bolted joints. Unlike a standard hex nut (ASME B18.2.2), a heavy hex nut has a thicker height and larger across-flats (AF) dimension, for a 1" diameter bolt, the heavy hex nut is 1-5/8" AF × 31/32" thick (vs the standard 1-1/2" AF × 7/8" thick). The thicker nut height engages more thread length, which (a) distributes the bolt preload across more thread engagement, reducing localized thread stress, (b) tolerates the higher preload required to compress sour-service gaskets without thread stripping, and (c) gives more wrench surface contact for accurate torque application. TorqBolt manufactures Incoloy 925 heavy hex nuts in M6 to M64 (1/4" to 4") thread sizes ASME B18.2.4.6M (metric) / ASME B18.2.6 (inch) / DIN 6915, in the solution-annealed and aged condition (per heat-treatment specification) with 100% hardness inspection ensuring the 35 HRC NACE MR0175 sour-service cap.
Need Incoloy 925 heavy hex nuts? Email info@torqbolt.com with thread size, thread series (UNC / UNF / 8UN / metric), quantity, applicable standards (ASME vs DIN, NACE MR0175 stamp). WhatsApp +91-22-66157017.
Heavy Hex Nut Variants Manufactured
| Variant | Description | Standard | Use |
|---|---|---|---|
| Heavy hex nut (standard) | Larger AF + thicker height than B18.2.2; chamfered both ends | ASME B18.2.6 (inch) / B18.2.4.6M (metric) | API 6A bolting, structural-steel pretensioning, pressure-vessel bolting |
| Heavy hex jam nut | Half-height heavy hex; used as locking nut against full-height nut | ASME B18.2.2 | Vibration-loosening prevention; secondary locking |
| Heavy hex thick nut | Even thicker than standard heavy hex; 1.5× bolt-diameter height | ASTM A194 dimensional reference | Maximum-thread-engagement service; slim-section bolted joints |
| Heavy hex slotted nut | Slots cut across nut for cotter-pin retention | ASME B18.2.2 (slotted hex) | Locked-position joints requiring positive mechanical retention |
| DIN 6915 heavy hex nut | European structural-pattern heavy hex (matched to DIN 6914 bolt + 6916 washer) | DIN 6915 / EN 14399-4 | European structural-steel HV-system bolting |
Standard Size Range & Heavy-Series Dimensions
| Thread Size | UNC / 8UN | Metric Coarse | Heavy AF | Heavy Nut Height | Standard Hex Height (for comparison) |
|---|---|---|---|---|---|
| 1/4" / M6 | UNC 20 | M6 × 1.0 | 1/2" / 13 mm | 15/64" / 6 mm | 7/32" / 5 mm |
| 3/8" / M10 | UNC 16 | M10 × 1.5 | 11/16" / 17 mm | 23/64" / 9 mm | 21/64" / 8 mm |
| 1/2" / M12 | UNC 13 | M12 × 1.75 | 7/8" / 22 mm | 31/64" / 12 mm | 7/16" / 11 mm |
| 5/8" / M16 | UNC 11 | M16 × 2.0 | 1-1/16" / 27 mm | 39/64" / 15 mm | 35/64" / 14 mm |
| 3/4" / M20 | UNC 10 | M20 × 2.5 | 1-1/4" / 32 mm | 47/64" / 18 mm | 41/64" / 16 mm |
| 7/8" / M22 | UNC 9 | M22 × 2.5 | 1-7/16" / 36 mm | 55/64" / 22 mm | 3/4" / 19 mm |
| 1" / M24 | UNC 8 / 8UN | M24 × 3.0 | 1-5/8" / 41 mm | 31/32" / 24 mm | 7/8" / 22 mm |
| 1-1/8" / M30 | UNC 7 / 8UN | M30 × 3.5 | 1-13/16" / 46 mm | 1-3/32" / 28 mm | 63/64" / 25 mm |
| 1-1/4" / M33 | UNC 7 / 8UN | M33 × 3.5 | 2" / 50 mm | 1-7/32" / 31 mm | 1-7/64" / 28 mm |
| 1-3/8" / M36 | UNC 6 / 8UN | M36 × 4.0 | 2-3/16" / 55 mm | 1-11/32" / 34 mm | 1-7/32" / 31 mm |
| 1-1/2" / M39 | UNC 6 / 8UN | M39 × 4.0 | 2-3/8" / 60 mm | 1-15/32" / 37 mm | 1-11/32" / 34 mm |
| 1-3/4" / M45 | 8UN | M45 × 4.5 | 2-3/4" / 70 mm | 1-23/32" / 44 mm | |
| 2" / M48 | 8UN | M48 × 5.0 | 3-1/8" / 75 mm | 1-31/32" / 50 mm | |
| 2-1/2" / M56 | 8UN | M56 × 5.5 | 3-7/8" / 85 mm | 2-29/64" / 62 mm | |
| 3" / M64 | 8UN | M64 × 6.0 | 4-5/8" / 95 mm | 2-29/32" / 74 mm | |
| 4" | 8UN | 6" | 3-29/32" |
Why the thicker nut matters: heavy hex nut height is approximately equal to bolt diameter (vs ~7/8 D for standard hex), giving full-thread-engagement strength matched to the bolt's ultimate tensile capacity. A standard hex nut on a 110 ksi YS Incoloy 925 stud risks thread strip-out under full preload, the heavy hex nut prevents this by distributing the load over more threads.
Mechanical Property Requirements
| Property | Aged Condition (ASTM B805 / ASTM A194 spec) |
|---|---|
| Proof Load (per ASTM A194) | Matched to 165 ksi UTS bolt grade (full proof per A194 Table 3) |
| Tensile Strength (parent material) | 165 ksi (1140 MPa) min |
| 0.2% Yield Strength | 110 ksi (760 MPa) min |
| Hardness | 32–35 HRC (NACE sour-service cap) |
| Charpy V-notch at -46°C | 40 J min (NACE sour service) |
Heavy hex nuts are proof-load tested per ASTM A194 (Carbon and Alloy Steel Nuts for Bolts for High-Pressure or High-Temperature Service), the equivalent provisions apply to nickel-alloy nuts. Sample proof-load test per heat-treatment lot included on the MTC.
Applicable Standards
- ASTM B805, UNS N09925 forging stock (input bar)
- ASME B18.2.6, Heavy hex nuts (inch series, structural)
- ASME B18.2.4.6M, Heavy hex nuts (metric series)
- ASME B18.2.2, Square and Hex Nuts (general nut dimensional)
- DIN 6915 / EN 14399-4, European heavy hex nut for HV-system structural assemblies
- ASTM A194, Carbon and Alloy Steel Nuts for Bolts for High-Pressure or High-Temperature Service (proof-load reference)
- ASTM F3125, High-strength structural bolt assemblies
- API Specification 6A, Wellhead and Tree Equipment, PSL-1 to PSL-4
- NACE MR0175 / ISO 15156-3, Table A.10 sour-service qualification
- NORSOK M-630 MDS A30, Norwegian Continental Shelf bolting
- ASME B1.1 / B1.13M, thread tolerance (2B internal for nuts)
- EN 10204 3.1 / 3.2, certification
API 6A PSL Documentation Levels
| PSL | Pressure Class | Charpy | NDE |
|---|---|---|---|
| PSL-1 | 2,000–5,000 psi | Not required | Visual |
| PSL-2 | 5,000–10,000 psi | Not required | Sample MT |
| PSL-3 | 10,000–15,000 psi | 27 J at -29°C | 100% MT or PT |
| PSL-3G | 10,000–15,000 psi gas | 27 J at -46°C / -101°C | 100% MT + grain size |
| PSL-4 | 15,000–20,000 psi | 40 J at -46°C | 100% MT + 100% PT + TPI witness |
Manufacturing Sequence
- Bar input: Incoloy 925 hex bar (or round bar for small quantities) per ASTM B805 in annealed condition. Hex bar feedstock is cut to nut blank length.
- Blank cutting: Cold-saw or shear cut to nut height + finishing allowance. Both faces machined flat and parallel.
- Chamfering: Both top and bottom faces chamfered to 30° ASME B18.2.6 / B18.2.4.6M to remove sharp edges and ease nut starting on the bolt thread.
- Through-hole drilling: Pilot hole drilled centrally, then bored to thread minor diameter.
- Internal thread cutting / tapping: Internal thread cut in solution-annealed condition. Cut threading (single-point or rotary tap) preferred over rolling for internal threads, rolling is impractical for blind / through-hole nut bores.
- Solution anneal + age cycle: Per heat-treatment specification. The thicker nut height (vs standard hex) requires careful furnace loading to ensure uniform soak temperature throughout the section.
- Final inspection: Across-flats and across-corners dimensional check ASME B18.2.6 / B18.2.4.6M; nut height + chamfer angle verification; internal thread gauging Go / No-Go ASME B1.3; 100% Rockwell hardness (within 32–35 HRC); proof-load sample test per ASTM A194 Table 3.
- Marking: Each nut stamped on one face with grade designation (e.g., "925" or "N09925") + heat number + manufacturer ID + NACE stamp (sour orders).
Companion Components
- Incoloy 925 stud bolts, primary mating fastener, two heavy hex nuts per stud
- Incoloy 925 heavy hex bolts, mating heavy-series bolt + single nut configuration
- Heavy structural washers (F436 equivalent), thicker washer matched to heavy nut bearing surface
- DTI (direct-tension-indicator) washers for pretensioned structural joints
- Thread compound, sulphur-free, copper-free anti-seize per API RP 7A1 (Loctite 8009 nickel anti-seize)
Application Examples
- API 6A wellhead body bolting: 2× nuts per stud, one each side of the flange
- Christmas tree bonnet bolting: Heavy hex nut prevents thread strip-out under full preload
- Subsea Christmas tree connector bolting: Subsea-rated heavy hex nut on stud-and-nut joints
- BOP stack flange bolting: Heavy hex nut companion to high-strength stud
- Compact-flange bolting: ABB Vector / Steelhead / Galperti compact flanges with heavy hex companion
- Pressure-vessel manway bolting: Sour-service inspection-port closure with high-preload nut
- Heat exchanger flange bolting: Sour-duty shell-and-tube exchanger bonnet retention
- Topside structural-steel HV-system: EN 14399 European pretensioned structural-steel bolting
Standard Coatings & Finishes
- Bare (passivated): Per ASTM A380 / A967, standard sour-service finish
- PTFE coating: Xylan 1424 / 1070, reduces friction during make-up; matches PTFE-coated stud finish
- Phosphate + oil: MIL-DTL-16232 + light oil for storage protection
- Cad plate / hot-dip galvanise / waxed: NOT applied for nickel-alloy CRA service
Frequently Asked Questions
Why use a heavy hex nut instead of a standard hex nut? Standard hex nuts ASME B18.2.2 have nut height ~7/8 of bolt diameter, insufficient thread engagement to develop the full ultimate tensile strength of a 165 ksi UTS Incoloy 925 stud bolt. Heavy hex nuts have height ~equal to bolt diameter, providing full strength match. For any joint loaded above 50% of bolt yield, heavy hex is the engineering default.
Are heavy hex nuts dimensionally identical to ASTM A194 grade 8 nuts? External dimensions (AF, height, chamfer) are identical, ASTM A194 specifies the proof-load + chemistry, while ASME B18.2.6 / B18.2.4.6M specifies the dimensions. So a heavy hex nut in Incoloy 925 has the SAME wrench-engagement geometry as an A194 grade 7 / 8 nut, but the corrosion + sour-service envelope of the nickel alloy.
Can heavy hex nuts be matched to DIN 6915 (European) bolts? Yes, DIN 6915 heavy hex nut matches DIN 6914 heavy hex bolt + DIN 6916 washer in the European HV-system structural assembly. We supply matched assemblies on request, per EN 14399 parts 4 (HV-system) certification requirements.
What thread compound for Incoloy 925 heavy hex nuts? Same as for studs / bolts, sulphur-free, copper-free anti-seize per API RP 7A1: Loctite 8009 nickel anti-seize, Henkel Permabond Antiseize 8023 or PetroStar Premium Plus. Apply to nut bearing face + thread engagement. AVOID copper-bearing, graphite-bearing, sulphur-bearing or zinc-bearing compounds.
What's the typical lead time for Incoloy 925 heavy hex nuts? Standard sizes (M16 to M30 / 5/8" to 1-1/8"): 4–6 weeks ex-stock or 6–8 weeks new-melt. Larger sizes (M36 to M64 / 1-3/8" to 3"): 8–12 weeks. Custom dimensions or DIN 6915 European pattern: 8–10 weeks. The bottleneck is the 18-hour age cycle plus internal-thread tapping, which is slower than external thread cutting.
Documentation Package
- EN 10204 3.1 manufacturer's MTC (3.2 with TPI witness on request)
- Heat-lot chemistry vs UNS N09925 limits ASTM B805
- Tensile + RA + elongation results (parent material) per heat / per heat-treatment lot
- Proof-load test per ASTM A194 Table 3 (nut-specific load capacity validation)
- 100% Rockwell hardness inspection per ASTM E18 (32–35 HRC NACE band)
- Sample Charpy V-notch at -46°C / -101°C as applicable
- Heat-treatment furnace chart-recorder traces
- Nut dimensional check (heavy AF, heavy height, chamfer angle) ASME B18.2.6 / B18.2.4.6M
- Internal thread gauge inspection (Go / No-Go) ASME B1.3
- NACE MR0175 Table A.10 reference (sour-service orders)
- API 6A PSL level reference (TPI witness for PSL-4)
- Nut marking: grade + heat number + manufacturer ID + NACE stamp
Related Products
- Stud bolts, primary mating fastener (2 nuts per stud)
- Heavy hex bolts, mating bolt for single-nut configuration
- Standard hex nuts, B18.2.2 thinner companion for low-load joints
- Washers, load distribution under nut bearing face
- Standard hex bolts, smaller AF mating bolt
- Full Incoloy 925 datasheet
Request a Quote
For Incoloy 925 heavy hex nuts with full MTC documentation:
- Email: info@torqbolt.com
- Phone: +91-22-66157017
- WhatsApp: +91-22-66157017
Specify thread size, thread series (UNC / 8UN / metric), dimensional standard (ASME B18.2.6 inch / B18.2.4.6M metric / DIN 6915 European), quantity, applicable standards (API 6A PSL level, NACE MR0175 stamp, NORSOK MDS A30, ASTM A194 proof load), required MTC level (3.1 / 3.2 with TPI witness) and any coating / finish requirement.
References: ASTM B805; ASME B18.2.6 (heavy hex inch); ASME B18.2.4.6M (heavy hex metric); ASME B18.2.2 (general hex nuts); DIN 6915 / EN 14399-4 (European HV system); ASTM A194 (nut proof-load); ASTM F3125 (structural strength); ASME B1.1 / B1.13M (thread series); ASME B1.3 (thread gauging); API Specification 6A Annex M; NACE MR0175 / ISO 15156-3 Table A.10; NORSOK M-630 / M-650; EN 10204; ASTM A380 / A967 (passivation); MIL-DTL-16232. Special Metals Corporation INCOLOY® alloy 925 Technical Bulletin. INCOLOY® is a registered trademark of Special Metals Corporation.
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