ASTM A194 Heavy Hex Nut | Grade 2H / 7 Marine Flange Nut
ASTM A194 Heavy Hex Nut | Grade 2H / 7 Marine Flange Nut
ASTM A194 / ASME SA194 | Grade 2H / 2HM / 7 / 8 / 8M | M6–M64 | Plain / Zinc Plated / Hot-Dip Galvanized
ASTM A194 heavy hex nut for marine engine room high-temperature flange connections. Grade 2H pairs with ASTM A193 B7 stud bolts; Grade 7/8/8M pairs with B8/B16 stainless and alloy steel studs. Full EN 10204 3.1 certification available.
Product Overview
TheASTM A194 Heavy Hex Nut is a high-strength nut specifically designed to mate with ASTM A193 alloy steel and stainless steel stud bolts in high-temperature and high-pressure bolted flange joints. Manufactured from carbon steel or chromium-molybdenum alloy steel per ASTM A194 specification, these nuts deliver proof load values that ensure the nut assembly never becomes the weak point in the bolted connection.
In marine engine rooms, A194 nuts secure main steam line flanges, boiler feedwater connections, heat exchanger closures, turbine casing joints, and exhaust manifold assemblies at operating temperatures reaching 400°C. Each grade is metallurgically matched to its corresponding stud bolt grade to ensure compatible mechanical properties under thermal cycling and vibration.
Grade Selection & Stud Pairing Guide
| A194 Grade | Material | Proof Load (MPa) | Hardness (HRC) | Paired Stud | Service Temperature |
|---|---|---|---|---|---|
| Grade 2H | Medium Carbon Steel, Q&T | ≥ 1,034 (150 ksi) | 26–36 | ASTM A193 B7 | −29°C to +400°C |
| Grade 2HM | Medium Carbon Steel, Modified | ≥ 1,034 (150 ksi) | 26–32 (tighter range) | ASTM A193 B7 | −29°C to +400°C |
| Grade 7 | Cr-Mo Alloy Steel (AISI 4140) | ≥ 1,034 (150 ksi) | 26–36 | ASTM A193 B7 / B16 | −29°C to +400°C |
| Grade 8 | Austenitic Stainless SS304 | ≥ 517 (75 ksi) | RB 90 max | ASTM A193 B8 Class 1 | −196°C to +800°C |
| Grade 8M | Austenitic Stainless SS316 | ≥ 517 (75 ksi) | RB 90 max | ASTM A193 B8M Class 1 | −196°C to +800°C |
Note: Grade 2HM differs from 2H by tighter hardness range for improved resistance to stress corrosion cracking in sour service environments per NACE MR0175.
Technical Specifications
| Parameter | Value |
|---|---|
| Standard | ASTM A194 / ASME SA194 |
| Nut Type | Heavy Hex Nut (per ASME B18.2.2); Heavy Hex Jam Nut available |
| Size Range | M6 – M64 (1/4"– 2-1/2"Imperial available) |
| Thread | Metric Coarse (ISO 261) / UNC / UNF |
| Thread Tolerance | Class 6H (Metric) / Class 2B (Imperial) |
| Surface Finish | Plain / Zinc Plated / Hot-Dip Galvanized (≥50 μm) |
| Marking | Grade symbol raised or indented on nut face for identification |
| Inspection | 100% thread check with GO/NO-GO gauges; dimensional and hardness testing per ASTM A194 |
| Certification | EN 10204 3.1 material certificate; third-party inspection available (LR / DNV / BV) |
Dimensional Data (Heavy Hex Nut – Selected Sizes)
| Thread | Across Flats (mm) | Across Corners (mm) | Height (mm) | Weight (kg/100 pcs) |
|---|---|---|---|---|
| M6 | 10 | 11.05 | 5.2 | 0.20 |
| M8 | 13 | 14.38 | 6.8 | 0.45 |
| M10 | 16 | 17.77 | 8.4 | 0.80 |
| M12 | 18 / 19 | 20.03 / 21.10 | 10.8 | 1.20 |
| M16 | 24 | 26.75 | 13.5 | 2.50 |
| M20 | 30 | 33.53 | 16.0 | 4.30 |
| M24 | 36 | 40.05 | 19.0 | 7.20 |
| M30 | 46 | 51.42 | 23.5 | 13.5 |
| M36 | 55 | 61.31 | 28.0 | 22.0 |
| M42 | 65 | 72.61 | 33.0 | 35.0 |
| M48 | 75 | 83.91 | 37.0 | 52.0 |
| M64 | 95 | 106.03 | 50.0 | 120.0 |
Note: M12 heavy hex nuts may use 18 mm or 19 mm across-flats depending on standard edition. Dimensions per ASME B18.2.2. Contact us for full dimensional tables and Imperial size equivalents.
Marine Applications
- Main Steam Lines: Grade 2H nuts paired with B7 studs for steam piping flanges at temperatures up to 400°C
- Boiler Systems: Boiler feedwater connections, manway doors, and steam drum access hatches
- Heat Exchangers: Shell-and-tube heat exchanger cover bolting and condenser end plate fastening
- Turbine Casings: Steam turbine casing joint bolting and exhaust diffuser flange connections
- Seawater Service: Grade 8/8M stainless nuts paired with B8/B8M studs for seawater cooling system flanges
- Exhaust Manifolds: Grade 7 nuts for diesel engine exhaust piping where corrosion and temperature combine
Installation & Torque Guidelines
- Inspect nut threads for damage, corrosion, or debris before assembly. Ensure thread engagement matches at least 1.5× bolt diameter for full strength.
- For service above 200°C, apply high-temperature anti-seize compound to prevent galling and enable future disassembly.
- Tighten in a star pattern (criss-cross sequence) when securing flanges with multiple studs to ensure uniform gasket compression.
- For critical flanges, follow ASME PCC-1 bolting procedure: tighten to 30% of target torque, then 60%, then 100% in sequential passes.
- Re-torque after 24 hours of elevated-temperature service to account for thermal relaxation and initial embedding.
- Verify grade marking on nut face matches the paired stud grade — never substitute lower-grade nuts on alloy steel studs.
Why Choose SINOFASTENER
- Full grade range available: 2H, 2HM, 7, 8, 8M — all in stock from M6 to M64
- Heavy hex configuration per ASME B18.2.2 with proper across-flats and height dimensions
- Grade symbol marked on nut face for visual identification during installation
- 100% thread inspection with GO/NO-GO gauges per ISO 965 / ASME B1.2
- EN 10204 3.1 material certificates with heat number traceability
- Annual production capacity 50,000 tons across all fastener categories
FAQ
What nut grade should I pair with ASTM A193 B7 studs?
ASTM A194 Grade 2H is the standard pairing for B7 studs in high-temperature flange applications. Grade 2H provides proof load of 150 ksi (1,034 MPa), ensuring the nut matches or exceeds the strength of the B7 stud. For sour service environments (H₂S exposure), use Grade 2HM with tighter hardness control.
What is the difference between Grade 2H and Grade 7?
Both grades have the same 150 ksi proof load requirement. The difference is material: Grade 2H is medium carbon steel (quenched and tempered), while Grade 7 is chromium-molybdenum alloy steel (AISI 4140). Grade 7 offers better resistance to high-temperature creep and is often specified for critical refinery and marine applications above 350°C.
Can I use Grade 8 stainless nuts with B7 carbon steel studs?
Technically yes, but it is not recommended for high-temperature service. Grade 8 has significantly lower proof load (75 ksi vs 150 ksi) and different thermal expansion characteristics. Use Grade 8 nuts only with B8/B8M stainless steel studs in corrosion-resistant applications.
What is the maximum service temperature for Grade 2H nuts?
Grade 2H nuts are rated for continuous service up to 400°C. Above this temperature, the nut material may experience creep and loss of clamping force. For applications exceeding 400°C, consider Grade 7 (alloy steel) or austenitic stainless grades (8/8M) depending on the environment.
Do you provide third-party inspection reports?
Yes. All shipments include EN 10204 3.1 material certificates. Third-party inspection from Lloyd's Register, DNV, Bureau Veritas, or other classification societies is available upon request for marine and offshore applications.
Contact Us
For ASTM A194 heavy hex nut inquiries, please contact:
E-mail:sales@sinofastener.com
SINOFASTENER — Marine Fastener Supplier | Annual Capacity 50,000 Tons
