A325 and A490 structural bolts are both heavy hex bolts used in steel-to-steel connections, but they are specified for different strength levels and coating rules. For buyers, the safest approach is to treat them as complete structural bolt assemblies, not as loose bolts.
Quick answer
ASTM F3125 Grade A325 is the standard high-strength structural bolt for many building and bridge connections. ASTM F3125 Grade A490 is a higher-strength alloy steel structural bolt used where the connection design requires greater pretension or slip-critical capacity. A490 is not a simple upgrade from A325; it has stricter handling and coating limitations.
| Item | A325 | A490 |
|---|---|---|
| Typical strength level | 120 ksi class for common diameters | 150 ksi class |
| Common use | General structural steel bolting | Higher-strength slip-critical connections |
| Hot-dip galvanizing | Allowed for suitable Type 1 assemblies | Not allowed because of hydrogen embrittlement risk |
| Matching washer | ASTM F436 | ASTM F436 |
| Matching nut | Often ASTM A563 DH | Often ASTM A563 DH or project-specified equivalent |
Why the head shape matters
Structural bolts use a heavy hex head. Compared with a standard hex bolt head, the heavy hex pattern gives a larger bearing and wrenching area. That matters during installation, because structural bolts are often tightened to a specified pretension rather than simply snugged by feel.
If your drawing says "A325" or "A490", do not substitute a general heavy hex bolt without confirming the full standard, head marking, mechanical property class, and test certificate requirements.
Specify the assembly, not just the bolt
The common purchasing mistake is to write only "A325 bolt" on the RFQ. For structural jobs, the buyer should specify the full assembly:
- Bolt standard and grade: ASTM F3125 Grade A325 or Grade A490
- Type: Type 1 or Type 3, depending on material and corrosion environment
- Diameter, length, thread length, and quantity
- Nut grade, usually heavy hex structural nuts such as A563 DH where applicable
- Washer standard, commonly ASTM F436 hardened structural washers
- Finish: plain, hot-dip galvanized for eligible A325 assemblies, or weathering steel for Type 3
- Required documents: Mill Test Certificate, heat number traceability, and packing list
Coating rules: the part buyers often miss
Hot-dip galvanized A325 assemblies are common, but the zinc thickness changes thread fit. Matching galvanized nuts may need oversize tapping to assemble correctly after coating. That is why bolt, nut, and washer sourcing should be coordinated as one set.
A490 is different. A490 bolts should not be hot-dip galvanized because the higher-strength steel is more sensitive to hydrogen embrittlement. If corrosion protection is required, the project engineer should approve an alternative system rather than letting a supplier choose one casually.
Inspection and documents to request
For export buyers, document control is just as important as unit price. Ask for MTCs that identify chemical composition, mechanical properties, heat lot, and standard compliance. Head markings should match the grade. Packaging labels should identify size, grade, finish, batch, and quantity.
For mixed shipments from Yongnian, HDBolt can source the heavy hex structural bolt, matching nut, and F436 washer from suitable factories and consolidate them under one shipment. For a quotation, send the standard, diameter, length, finish, quantity, and destination port through the contact page.