Zeron 100 heavy hex nuts are the structural counterpart to A1082 stud bolts, thicker across-flats and taller than a standard hex nut, sized to develop the full tensile capacity of a matching super duplex stud without thread strip-out. We forge or hot-head them from solution-annealed S32760 bar (Werkstoff 1.4501), tap to 6H/2B class, and PMI every nut. The 25Cr-7Ni-3.5Mo-0.7Cu-0.7W-0.25N composition gives PREN ≥ 40 with the same chloride pitting and sour-service envelope as the studs they pair with, one bill-of-material, one alloy, no galvanic mismatch at the bolted joint.
Hardness ≤ 28 HRC · Proof Load matched to A1082 stud yield · NORSOK M-650 + NACE MR0175 sour-service
Standard pattern is heavy hex per ASME B18.2.2 (inch) or ASME B18.2.4.6M (metric); thread tolerance per ASME B1.1 class 2B / ISO 965 6H. Thickness and across-flats follow the heavy series, ~9% taller than a finished hex nut and ~12% wider, which is what gives the engaged thread length to develop full A1082 stud strength under NACE MR0175 hardness limits. Used opposite Zeron 100 stud bolts on every flange where seawater, sour service, or chloride-SCC are in scope: API 6A wellhead bonnets, NORSOK M-650 topside flanges, FPSO turret connections, subsea manifold blind flanges, desalination RO membrane vessels, FGD scrubber stack flanges.
Heavy hex nuts use a thicker, wider pattern than the finished hex series. The extra metal is what lets the nut develop the full A1082 stud yield without root strip-out, critical for super duplex where the rolled-thread hardness sits near the 28 HRC NACE ceiling.
| Thread (inch) | Width across flats (in) | Thickness (in) | Standard |
|---|---|---|---|
| 1/2-13 UNC | 7/8 | 31/64 | ASME B18.2.2 heavy |
| 5/8-11 UNC | 1-1/16 | 39/64 | ASME B18.2.2 heavy |
| 3/4-10 UNC | 1-1/4 | 47/64 | ASME B18.2.2 heavy |
| 7/8-9 UNC | 1-7/16 | 55/64 | ASME B18.2.2 heavy |
| 1-8 UN | 1-5/8 | 63/64 | ASME B18.2.2 heavy |
| 1-1/4-8 UN | 2 | 1-7/64 | ASME B18.2.2 heavy |
| 1-1/2-8 UN | 2-3/8 | 1-15/64 | ASME B18.2.2 heavy |
| Thread (metric) | Width across flats (mm) | Thickness (mm) | Standard |
|---|---|---|---|
| M16 | 27 | 17.1 | ASME B18.2.4.6M heavy |
| M20 | 34 | 20.7 | ASME B18.2.4.6M heavy |
| M24 | 41 | 24.3 | ASME B18.2.4.6M heavy |
| M30 | 50 | 30.7 | ASME B18.2.4.6M heavy |
| M36 | 60 | 36.6 | ASME B18.2.4.6M heavy |
| M42 | 70 | 42.7 | ASME B18.2.4.6M heavy |
| M48 | 80 | 48.4 | ASME B18.2.4.6M heavy |
A heavy hex nut must develop the full ultimate tensile of the matching A1082 stud before the engaged threads strip. With Zeron 100 (FG supply state, ~870 MPa UTS) on M24 stud, the nut needs roughly 320 kN of tensile load before failure. Standard finished hex (thickness ~21.0 mm at M24) is borderline; the heavy series (24.3 mm thick) gives a 15% engagement-length margin and ensures stud-tensile failure precedes thread strip, required for predictable joint behaviour at offshore design loads. Proof-load testing per ASTM F606 / ISO 898-2.
| Standard | What it covers | Why it matters |
|---|---|---|
| ASTM A1082 | Stainless stud, bolt, screw, threaded rod (covers nuts by reference) | Material spec for S32760 super duplex fasteners; mechanical envelope, hardness, intergranular corrosion test |
| ASME B18.2.2 | Square and hex nuts (inch series) | Heavy hex pattern dimensions, thickness, across-flats, chamfer angle |
| ASME B18.2.4.6M | Heavy hex nuts, metric series | Metric heavy hex pattern (M16-M48 typical, M64+ on request) |
| ASME B1.1 / ISO 965 | Thread tolerance, class 2B (inch) / 6H (metric) | Tap class for stud-nut clearance; super duplex galls if interference fit is used |
| ASTM F606 / ISO 898-2 | Proof-load testing | Witnessed nut proof-load to confirm thread-strip margin over A1082 stud yield |
| NACE MR0175 / ISO 15156-3 | Sour-service hardness limit | Tapped-thread root hardness ≤ 28 HRC required for H2S service |
| NORSOK M-650 / M-630 MDS 055 | North Sea super duplex acceptance | QTR'd manufacturer + NDT + traceability for Equinor / Aker BP topside flange bolting |
| API 6A PSL 3 / 3G | Wellhead and Christmas tree equipment | Wellhead bonnet nuts qualified for HPHT + sour service |
| API 20E BSL-3 | Alloy and CRA bolting traceability | Heat-traceable from mill to nut + witnessed mechanical testing |
| EN 10269 / EN 10222-5 | European fastener / forging spec | PED 2014/68/EC equipment buyers (BASF, BAYER, Aker Solutions) |
| Property | SA (typical) | FG (typical) | FLT (typical) | Test method |
|---|---|---|---|---|
| 0.2% Yield Strength | 620-720 MPa | 700-800 MPa | 620-720 MPa | ASTM A370 (witness coupon) |
| Ultimate Tensile | 800-870 MPa | 870-940 MPa | 800-870 MPa | ASTM A370 (witness coupon) |
| Proof Load | per A1082 + B18.2.2 | per A1082 + B18.2.2 | per A1082 + B18.2.2 | ASTM F606 |
| Hardness (body) | 22-27 HRC | 24-28 HRC | 22-27 HRC | ASTM E18 |
| Hardness (tapped root) | 23-27 HRC | 25-28 HRC | 23-27 HRC | HV converted per E140 |
| Charpy V-notch @ -46°C | 60-110 J | 50-90 J | 80-130 J | ASTM E23 |
| Ferrite content | 40-52 % | 40-52 % | 40-52 % | ASTM E562 |
| CPT (G48 Method E) | ≥ 50°C | ≥ 50°C | ≥ 50°C | ASTM G48-E |
| Sigma-phase (A923 Method C) | Pass | Pass | Pass | ASTM A923 Method C |
| Service | Typical pairing | Why heavy pattern |
|---|---|---|
| API 6A wellhead bonnet flange | FG stud + FG heavy hex nut | HPHT torque-tension envelope; thread strip margin critical |
| NORSOK M-650 topside flange (ANSI 600/900/1500) | FG stud + FG heavy hex nut | Cold-worked yield needs heavy nut to fully engage threads |
| Subsea manifold blind / spool flange | FLT stud + FLT heavy hex nut | Charpy-controlled at -46°C for deepwater service |
| FPSO turret / swivel / mooring fasteners | FLT stud + AFP-coated FLT nut | Cyclic load + repeated-makeup; AFP coating prevents galling |
| LNG export terminal cryogenic flanges | FLT stud + FLT heavy hex nut | L7-equivalent toughness without carbon steel SCC concern |
| Desalination RO membrane vessel + brine line | SA stud + SA heavy hex nut | Seawater + concentrated brine, PREN ≥ 40 + 0.7Cu addition |
| FGD scrubber + chimney bolting | SA stud + AFP-coated SA nut | Wet-flue-gas chloride + sulfate; AFP for periodic disassembly |
| Chemical process, HCl, H2SO4, dilute organics | SA stud + SA heavy hex nut | Cu+W content gives reducing-acid resistance superior to 2507 |
A heavy hex nut is ~9% taller and ~12% wider across-flats than a finished hex of the same thread. The extra thickness gives more engaged threads, which is required to develop the full ultimate tensile of an A1082 stud bolt before thread strip. Because Zeron 100 sits near the 28 HRC NACE limit, you can't compensate for thinner nuts with harder material, the only path is more thread engagement, which is exactly what the heavy pattern provides.
Mechanically possible, but it gives up the entire reason to use Zeron 100. B7M studs are AISI 4140 alloy steel; pairing them with super duplex nuts means the stud is the corrosion weak link in the joint. For sour service or seawater, both stud and nut should be Zeron 100 or A1082-equivalent. Use B7M studs only on flanges where corrosion isn't in scope.
Standard supply is class 2B (ASME B1.1, inch) or 6H (ISO 965, metric), the loose nut classes. Super duplex galls when interference-fit threads are torqued, so we never tap closer than 2B/6H. For repeated-makeup applications request the AFP supply state, the coating reduces friction coefficient from ~0.30 to ~0.10 and makes torque-to-tension predictable across multiple disassembly cycles.
Yes, AFP supply state covers Xylan 1014/1070/1400/1424, Xylar 2/101, SermaGard 1105/1280, Dacromet 320/500, TakeCoat 1000, MolyKote, Magnigard. Coating thickness controlled per ASTM F1137; pre-coat NDT on uncoated nut, post-coat dry-film thickness verified on every lot. Friction coefficient measured per ASTM F2328 if buyer specifies.
Stock sizes (M16, M20, M24, M30, M36 metric; 5/8", 3/4", 7/8", 1", 1-1/4" UNC) ship in 5-7 working days with 3.1 MTC, 10-14 days with witnessed 3.2 MTC. NORSOK M-650 QTR'd lots take 14-21 days because of third-party witness scheduling. AFP coating adds 5-7 days post-tap. Specials cut from solution-annealed bar within 5 working days from RFQ approval.
Send the spec (thread + supply state + standards / MDS to comply with + coating if any + quantity + destination port) to info@torqbolt.com or +91-22-66157017 / WhatsApp. We reply with stock availability, mill route, lead time, MTC tier, and price within one business day (Mumbai IST).
