The Ultimate Guide to ASTM A490 (F3125) Heavy-Hex Bolts
SUMMARY: ASTM A490 (also designated F3125 Grade A490) are high-strength, heavy-hex structural bolts used where superior tensile strength and reliable connection performance are required — commonly in bridges, heavy steel buildings, wind towers, mining, and infrastructure. For projects that demand higher strength than A325 bolts, A490 is typically the specified upgrade. Contact California Fastener to discuss custom sizes and lead times.
ASTM A490 / F3125 — Spec basics
What the ASTM F3125 standard covers
ASTM F3125 unifies several bolt grades under one specification; when you see “A490” in structural specs, it refers to bolts manufactured and tested to the F3125 requirements for high-strength, quenched and tempered alloy steel heavy-hex bolts.
Type notes (Type 1 vs Type 3)
Type 1 — standard (carbon/alloy) heavy-hex bolts.
Type 3 — weathering alloy steel (used in atmospheres where corrosion resistance from the base material is useful).
Head profile & dimensions
A490 bolts use a “heavy-hex” head profile (larger across-flats than standard hex heads). Common sizes range through standard structural bolt diameters; check project drawings and the ASTM spec for exact dimensional tables.
Mechanical properties & performance
Below is a qualitative comparison to help engineers and spec writers choose between common options. For precise numerical values (tensile, yield, and hardness), please consult the ASTM F3125 specification or your project engineer.
| Property | A490 (F3125) | A325 | A354 BD | SAE Grade 8 |
|---|---|---|---|---|
| Relative tensile strength | Highest among typical structural bolt grades | High | Very high (heat treated) | Very high (automotive) |
| Typical use case | Heavy structural connections, infrastructure | General structural bolts | Special high-strength applications | Mechanical/automotive, sometimes used in industrial fasteners |
| Heat treatment | Quenched & tempered | Generally heat treated | Quenched & tempered | Quenched & tempered |
| Corrosion considerations | Requires compatible coatings/finishes | More coating flexibility | Depends on alloy & coating | Varies by finish |
Materials & heat treatment
Alloy composition & process
A490 bolts are produced from alloy steel and are quenched and tempered to achieve their high strength. This heat-treatment process is critical — it defines mechanical performance and must be verified on mill certs.
When to use Type 3 (weathering)
Choose Type 3 for applications where the base alloy’s weathering properties reduce corrosion risk (e.g., unpainted structures in certain environments). Type 3 is not a replacement for protective coatings where long-term corrosion protection is required.
Coatings, finishes & what NOT to do
Engineers often ask which finishes are compatible with A490 bolts. Below are practical guidelines — again, have Aaron confirm any brand-specific coatings before calling them out.
Permitted / commonly used finishes
Electroplated zinc / zinc flake (Geomet® / Dacromet®): Often acceptable if specified per the coating manufacturer and project requirements.
Mechanical plating: Occasionally used where hot processes are not permitted.
Paint systems / primers: Common in field-coated applications.
Cautions / frequently prohibited approaches
Hot-dip galvanizing: Often not recommended for A490 without specific approval — the high temperatures and hydrogen exposure during HDG can adversely affect mechanical properties. Refer to project specs.
Improper post-treatment: Any coating or process that risks hydrogen embrittlement or reduces verified mechanical properties should be avoided.
Bolt & nut pairing
Use nuts specified to match the bolt class/grade (e.g., ASTM A563 or equivalent). Mismatched coatings or incompatible heat treatments can lead to connection failures.
Compatibility table (quick reference)
| Coating / Finish | Typical suitability for A490 | Notes |
|---|---|---|
| Zinc flake (Geomet®) | Suitable with spec'd application | Low hydrogen risk, thin coating |
| Electroplated zinc | Suitable if process is controlled | Verify thickness & adhesion |
| Hot-dip galvanizing | Not typically recommended | Confirm with engineer/spec |
| Paint / primer systems | Suitable as surface protection | May require primer + topcoat |
Specification comparisons — when to pick A490 vs alternatives
A490 vs A325
Use A490 where higher tensile strength is required (heavier, more demanding structural connections). A325 is common for general structural bolts; A490 is the “upgrade” for greater strength.
A490 vs A354 BD
A354 BD is a high-strength bolt grade used in some specialized mechanical applications. A490 is tailored for structural connections and is generally more commonly specified by structural engineers.
Quick decision checklist
Need higher tensile strength than A325? → Consider A490.
Is the connection exposed to severe corrosion and requires galvanizing? → Revisit spec; A490 may require a special coating strategy or an alternative material.
Does the project require weathering alloy? → Consider Type 3 or alternate alloys.
Typical industry applications & California Fastener advantages
Where A490 is used:
Bridges and heavy civil structures
High-rise steel structures and heavy-steel buildings
Wind-turbine towers and heavy infrastructure
Mining and heavy equipment mounting
What California Fastener offers:
Made in USA: domestic manufacturing for traceability and lead-time control.
Custom capabilities: per-print manufacturing and fast turnaround for non-standard sizes.
Spec and material range: ability to source and produce bolts to exact project specs.
Responsive service: direct communication and mill-cert traceability for QA.
Installation, torque & inspection best practices
Tensioning vs torque
Tensioning (preferred for critical connections): Directly controls elongation/pretension and is the most accurate method for ensuring required clamp force.
Torque: Easier in the field but influenced by friction; use calibrated torque charts and only when acceptable per project specs.
Practical steps for installers
Verify bolt & nut markings and mill certs before install.
Use specified washers & nuts (per spec).
Follow the specified pre-tensioning or torque procedure; avoid substitute methods without engineering approval.
Inspect threads for damage and ensure proper lubrication if specified.
Inspection checklist
Confirm material & heat-treatment certificates.
Verify coating/finish matches spec.
Check head markings and lot traceability.
Confirm proper washer, nut type, and installation method.
Document torque/tension readings and retain records.
Sourcing, specifying & QA tips (what engineers should request)
What to ask suppliers
Mill certificates and heat-treatment records (traceability).
Coating test reports and application method.
Traceability to lot/batch numbers and packaging documentation.
Any special testing (hardness, tensile) required by the contract.
FAQs
Q: Can A490 bolts be hot-dip galvanized?
A: Hot-dip galvanizing is frequently avoided for A490 due to potential effects on mechanical properties. Always follow project specs and consult the supplier/engineer before specifying HDG for A490 bolts.
Q: What coatings are acceptable for A490?
A: Zinc-flake systems (e.g., Geomet®) and controlled electroplated zinc processes are commonly used; choice depends on required corrosion life, environment, and project approval.
Q: Can A490 be used in seismic designs?
A: Yes — A490 is commonly used in structural connections that require high strength. The engineer must confirm connection design and applicable code requirements.
Q: Are there metric equivalents?
A: There is no direct one-to-one metric equivalent; conversion requires comparing mechanical properties and specified dimensions. Confirm with the project engineer.
Next steps
ASTM A490 heavy-hex bolts are the specification of choice when structural connections require high tensile strength and reliable performance. When specifying or sourcing A490, verify heat treatment, coating compatibility, and installation method — and request full mill and QA documentation.
Ready to proceed? Talk to an Expert to start a quote and confirm technical details.
