ASTM A490 Heavy Hex Structural Bolt F3125 Marine
ASTM A490 Heavy Hex Structural Bolt — F3125 Grade A490, 1/2"–1-1/2"
TheASTM A490 heavy hex structural bolt — now consolidated under ASTM F3125 Grade A490 — is a quenched and tempered alloy steel bolt engineered exclusively for structural steel-to-steel connections in high-load applications. Unlike standard hex bolts that serve general machinery fastening, the A490 heavy hex bolt features a significantly larger and thicker hexagon head per ANSI/ASME B18.2.6, providing a wider bearing face that reduces clamping stress on connected steel plates and enables the higher preload values essential for slip-critical and bearing-type structural joints.
sinofastener supplies ASTM A490 heavy hex structural bolts in two types, covering imperial sizes 1/2"through 1-1/2"with metric equivalents M12 through M36. Each bolt is hot-forged as a single piece, full-length quenched and tempered to achieve a minimum tensile strength of 150 ksi (1030 MPa).
A490 Type 1 vs Type 3: Material Selection
| Property | Type 1 (Cr-Mo Alloy Steel) | Type 3 (Weathering Steel) |
|---|---|---|
| Steel Chemistry | Chromium-Molybdenum alloy (similar to AISI 4140/4142) | Low-alloy weathering steel (Cu-Cr-Ni-P composition) |
| Tensile Strength | 150 ksi (1030 MPa) min | 150 ksi (1030 MPa) min |
| Hardness Range | HRC 33–38 | HRC 33–38 |
| Corrosion Protection | Requires external coating: HDG, Dacromet, or Geomet | Self-protecting — develops stable oxide patina (no coating needed) |
| Patina Formation | N/A | Stabilized rust-like oxide layer forms over 18–36 months of wet/dry cycling |
| Marine Application | Coated for offshore jacket nodes, topside framing, crane mast splices | Exposed topside structures where coating maintenance is impractical |
| Limitation | Coating damage exposes bare steel to corrosion | Not suitable for continuously submerged or splash-zone service |
Why A490 Heavy Hex Head Geometry Matters
| Feature | A490 Heavy Hex | Standard Hex Bolt |
|---|---|---|
| Head Width (across flats) | Larger per B18.2.6 — wider wrench size for same diameter | Standard per B18.2.1 |
| Head Height | Taller — provides greater bearing area on steel plate | Lower profile |
| Bearing Stress | Distributed over larger area — reduces plate deformation under high preload | Higher localized stress under preload |
| Identification | Head stamped with"A490"or"F3125"+ manufacturer mark | Grade marking varies by specification |
| Design Purpose | Structural steel connections requiring high preload | General machinery and equipment fastening |
Marine and Offshore Structural Applications
- Offshore Jacket Structures — brace-to-leg node connections where slip-critical joints prevent relative movement under cyclic wave loading
- Topside Module Framing — beam-to-column and girder splice connections in deck modules and living quarter platforms
- Flare Boom Supports — structural brackets securing flare towers to platform edge cantilevers
- Shipyard Crane Mast Splices — bolted column connections on deck cranes handling heavy cargo loads
- Dock and Quay Structures — steel pile cap plates, bollard foundation girders, and fender frame connections
- Bridge-to-Platform Transitions — structural splice plates connecting gangway support beams to fixed platform structures
Structural Bolting Assembly: Bolt + Nut + Washer
A490 structural connections require matched assemblies — the bolt, nut, and washer must be specified as a system to ensure compatible hardness and load distribution:
| Component | Specification | Function |
|---|---|---|
| Bolt | ASTM F3125 Grade A490 (Heavy Hex) | Primary load-bearing element, 150 ksi tensile |
| Nut | ASTM A563 Grade DH (Heavy Hex) | Proof load tested to match A490 bolt strength; beveled bearing surface |
| Washer | ASTM F436 (Hardened) | Distributes clamping force; provides smooth rotation surface for nut |
Heat Treatment and Mechanical Properties
All A490 bolts are hot-forged to the heavy hex profile, then subjected to full-length quench and temper heat treatment:
- Austenitizing — heated above critical temperature (approximately 870–900°C)
- Quenching — rapid cooling in oil or polymer solution to form martensite
- Tempering — reheated to 425–595°C to achieve target hardness HRC 33–38 and restore ductility
- Result — tempered martensite microstructure providing 150 ksi tensile with adequate toughness for structural service
Surface Treatment Options (Type 1 Only)
| Coating | Corrosion Resistance | Application Notes |
|---|---|---|
| Hot-Dip Galvanized (HDG) | Excellent — zinc provides sacrificial anode protection | For Type 1 only; requires hydrogen embrittlement relief bake within 4 hours of plating |
| Dacromet | Very high — zinc-aluminum flake coating, no hydrogen risk | Preferred for high-strength structural bolts; eliminates embrittlement concern |
| Geomet | Very high — zinc-aluminum flake coating, thin film | Lower coating thickness than Dacromet; suitable where dimensional tolerance is tight |
| Type 3 — Bare | Self-protecting weathering patina | No coating applied; patina forms naturally in atmospheric wet/dry cycling |
Standards Reference
All A490 heavy hex structural bolts are manufactured per ASTM F3125/F3125M (Standard Specification for High Strength Structural Bolts, Steel and Alloy Steel, Heat Treated, 120 to 150 ksi Minimum Tensile Strength, for Structural Steel Joints). Head dimensions per ANSI/ASME B18.2.6 (Heavy Hexagon Structural Bolts). Thread dimensions per ASME B1.1 (UNC thread series for structural bolts). Material certification per EN 10204 3.1. Third-party inspection coordinated on request.
sinofastener — 36 years of fastener manufacturing. Annual capacity 50,000 tonnes. Matched A490 + A563 DH + F436 assemblies available for project procurement.
Material: Type 1 (Cr-Mo Alloy Steel, quenched & tempered) / Type 3 (Weathering Steel)
Standard: ASTM F3125 Grade A490
Size Range: 1/2"–1-1/2"(M12–M36)
Tensile Strength: 150 ksi (1030 MPa) min
Hardness: HRC 33–38
Thread: UNC (Unified National Coarse)
E-mail: sales@sinofastener.com | SINOFASTENER
