Marine High Strength Bolts - ISO 898-1 & ASTM A193 Alloy Steel Bolting
Marine High Strength Bolts
High strength hex bolts and stud bolts per ISO 898-1 Class 8.8 / 10.9 / 12.9 and ASTM A193 B7 / A320 L7, supplied for marine hull structure, engine room and deck equipment applications from M12 to M64.
What Are Marine High Strength Bolts?
Amarine high strength bolt is a fastener with a minimum tensile strength of 800 MPa or above, designed for structural and pressure-critical connections on ships and offshore installations. These bolts are manufactured from medium carbon alloy steel, heat treated to achieve the required mechanical properties, and supplied with full material traceability.
High strength bolts for marine use are governed by ISO 898-1 (property classes 8.8, 10.9, 12.9), ASTM A193 B7 (high temperature service), ASTM A320 L7 (low temperature service), and EN 14399 (structural preloading). Selection depends on service temperature, clamping force requirement, and whether the joint is structural or pressure-containing.
Standards and Property Classes
Marine high strength bolts are classified by their property class, which defines minimum tensile strength, yield strength, and proof load. The most common grades used in shipbuilding and marine engineering are:
| Standard | Grade / Class | Min. Tensile (MPa) | Min. Yield (MPa) | Typical Application |
|---|---|---|---|---|
| ISO 898-1 | Class 8.8 | 800 | 640 | General structural connections, deck equipment mounting |
| ISO 898-1 | Class 10.9 | 1040 | 940 | Preloaded structural joints, engine foundation bolts |
| ISO 898-1 | Class 12.9 | 1220 | 1100 | High clamping force applications, alloy steel socket head cap screws |
| ASTM A193 | B7 | 860 | 725 | High temperature bolting, engine room piping flanges up to 454°C |
| ASTM A320 | L7 | 860 | 725 | Low temperature bolting down to -101°C, cryogenic piping systems |
| EN 14399-3 / -4 | HR 10.9 | 1040 | 940 | Preloaded structural bolt assemblies for hull and deck steelwork |
Materials and Heat Treatment
High strength marine bolts are produced from alloy or medium carbon steels. The material selection and heat treatment process directly determine the final mechanical properties:
| Property Class | Typical Material | Heat Treatment |
|---|---|---|
| 8.8 | Medium carbon steel (e.g. C35, C45) | Quench and temper |
| 10.9 | Alloy steel (e.g. 35CrMo4, 42CrMo4) | Quench and temper |
| 12.9 | Alloy steel (e.g. 42CrMo4, SCM440) | Quench and temper |
| ASTM A193 B7 | Cr-Mo alloy steel (AISI 4140) | Quench and temper |
| ASTM A320 L7 | Cr-Mo alloy steel (AISI 4140) | Quench and temper, Charpy impact tested at -101°C |
All materials are supplied with EN 10204 3.1 mill certificates. Petrochemical applications requiring NACE MR0175 / ISO 15156 sour service compliance are available on request with appropriate heat treatment and hardness control.
Size Range and Dimensions
We supply marine high strength bolts in the following size range:
| Thread Diameter | Length Range | Thread Form |
|---|---|---|
| M12 - M24 | 40mm - 300mm | Metric coarse (ISO 4014 / ISO 4017) |
| M24 - M36 | 80mm - 500mm | Metric coarse (ISO 4014 / ISO 4017) |
| M36 - M64 | 100mm - 1000mm+ | Metric coarse (ISO 4014 / ISO 4017) |
| 3/4"- 4" | Custom | UNC / 8UN per ASTM A193 |
Surface Treatment and Coating
Marine environment demands effective corrosion protection. High strength bolts are supplied with the following surface treatments based on service conditions:
- Hot dip galvanized per ISO 1461 - standard protection for general marine exposure
- Zinc flake coating per ISO 10683 - preferred for high strength bolts as it avoids hydrogen embrittlement risk
- PTFE / Xylan coating - for flange bolting where controlled friction coefficient is required
- Cad plated - for specific naval and offshore specifications
For high strength bolts above Class 10.9, zinc flake coating or PTFE coating is recommended over hot dip galvanizing to minimize hydrogen embrittlement risk per ISO 3315.
Quality Control and Testing
Every batch of marine high strength bolts undergoes the following inspections per applicable standards:
- Tensile testing per ISO 4014 / ISO 4017 / ASTM A193
- Proof load testing per ISO 898-1 or ASTM F606
- Hardness testing per ISO 898-1 (surface and core)
- Dimensional inspection per ISO 4759-1
- Surface coating thickness verification per ISO 10683 / ISO 1461
- Charpy impact testing for ASTM A320 L7 and low temperature applications
Third party inspection by classification societies (DNV, ABS, LR, CCS) or petrochemical inspection agencies (SGS, Bureau Veritas, Lloyd's EMI) can be arranged. Materials are traceable to heat number via EN 10204 3.1 certificates.
Applications in Marine and Petrochemical Engineering
Marine high strength bolts are used across all critical shipboard systems:
- Hull structure: Structural bolted connections per EN 14399 for deck and hull steelwork
- Engine room: Piping flange bolts per ASTM A193 B7 for high temperature steam and fuel lines
- Deck equipment: Cranes, winches, and davit mounting bolts per Class 10.9
- Petrochemical tankers: Cargo piping and pressure vessel bolts per ASTM A193 B7 / A194 2H, complying with NACE MR0175 for sour service environments
- Mooring and anchor systems: High strength foundation bolts for bollards and windlasses
Frequently Asked Questions
Class 10.9 bolts have higher tensile strength (1040 MPa vs 800 MPa) and are made from alloy steel with quench and temper heat treatment. Class 8.8 uses medium carbon steel. Use Class 10.9 for preloaded structural joints and engine foundation bolts where higher clamping force is required.
Hot dip galvanizing involves immersion in molten zinc at approximately 450°C, which can cause hydrogen absorption in high strength steels above 1000 MPa tensile strength. This creates hydrogen embrittlement risk. Zinc flake coating per ISO 10683 or PTFE coating are the preferred alternatives for Class 10.9 and 12.9 bolts per ISO 3315 guidance.
Yes. ASTM A193 B7 bolts can be manufactured in compliance with NACE MR0175 / ISO 15156 requirements for sour service environments in petrochemical and offshore applications. This includes controlled hardness (maximum 22 HRC per NACE), specific heat treatment, and documented testing. Provide the service environment details when requesting a quotation.
Standard sizes (M12-M36, Class 8.8/10.9) are typically available from stock or within 15-20 working days. Large diameter bolts (M42-M64) and special grades (ASTM A320 L7, NACE compliant) require 25-35 working days depending on material availability and quantity.
For marine high strength bolt inquiries, contact our engineering team:
Email: sales@sinofastener.com
Website: www.sinofastener.com
