Property Class 12.9 Steel Heavy Hex Structural Bolts are supplied to ISO 898-1 property class 12.9. Corrosion behaviour is requires plating or black-oxide protection, the material is magnetic, and the usable service range is commonly limited to approx. +300 °C; confirm against the coating and the load. In practice this grade is the highest common strength class, for high-load machine assembly.
- Grade
- ISO 898-1 property class 12.9
- Corrosion resistance
- Requires plating or black-oxide protection
- Magnetic response
- Magnetic
- Service temperature
- commonly limited to approx. +300 °C; confirm against the coating and the load
Property values are typical for the grade. Certified mechanical test data is available on request for production orders.
Property Class 12.9 Steel Heavy Hex Structural Bolts are supplied to ASTM F3125 Grade A325. The grade provides requires plating or black-oxide protection and is magnetic; the practical service range is commonly limited to approx. +300 °C; confirm against the coating and the load. Property data, grade comparison and the full size table follow.
Why Choose Property Class 12.9 Steel for Heavy Hex Structural Bolts
The highest common strength class, for high-load machine assembly. For heavy hex structural bolts, this matters when requires plating or black-oxide protection is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 898-1 property class 12.9 |
|---|
| Magnetic response | Magnetic |
|---|
| Service temperature | commonly limited to approx. +300 °C; confirm against the coating and the load |
|---|
| Corrosion characteristic | Requires plating or black-oxide protection |
|---|
Corrosion Behavior in Service
Carbon and alloy steel heavy hex structural bolts require a coating for corrosion protection. Zinc plating (with or without passivation) is the most common; hot-dip galvanizing is used for outdoor structural service; black-oxide is mainly a tarnish-prevention finish for tool-room use.
Galvanic & Galling Considerations
For heavy hex structural bolts in steel, the dominant compatibility issue is hydrogen embrittlement at high property classes (10.9 and above) when electroplated. Plating must be followed by an appropriate baking treatment, or a non-electroplated finish (mechanical zinc, zinc flake) should be substituted.
Sizes of Heavy Hex Structural Bolts Available in Property Class 12.9 Steel
| Size | Threads per inch | Width across flats (in) | Head height (in) | Thread length (in) | Minimum tensile (kN) | Mating nut | Reference standard |
|---|
| 1/2" | 13 | 7/8 | 5/16 | 1.00 | 91 | A563 DH | ASTM F3125 |
| 5/8" | 11 | 1-1/16 | 25/64 | 1.25 | 145 | A563 DH | ASTM F3125 |
| 3/4" | 10 | 1-1/4 | 15/32 | 1.375 | 215 | A563 DH | ASTM F3125 |
| 7/8" | 9 | 1-7/16 | 35/64 | 1.50 | 297 | A563 DH | ASTM F3125 |
| 1" | 8 | 1-5/8 | 39/64 | 1.75 | 389 | A563 DH | ASTM F3125 |
| 1-1/8" | 7 | 1-13/16 | 11/16 | 2.00 | 432 | A563 DH | ASTM F3125 |
Dimensions are independent of the material: every size above is available in Property Class 12.9 Steel.
Custom manufacturing · Made to drawing
Need this bolt in a grade that is not listed?
Grade 12 9 Steel is one of the grades the catalog describes. Thread size and pitch, length, property class, drive form, head geometry and head marking are all specifiable — the standard part is where it starts, not where it stops. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
Custom bolt manufacturingSend a drawing
Applications & Industries
Heavy Hex Structural Bolts are bought for a small number of well-defined jobs. These are the ones we ship them for. Stocked from 1/2" to 1-1/8" (screw size).
Industries that specify Heavy Hex Structural Bolts
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Bridges & heavy civils
Slip-critical connections with hardened washers and a specified tensioning method.
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Crane & lifting structures
Bolted connections in gantries, masts and runway beams.
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Structural steelwork
Beam-to-column, splice and bracing connections, tensioned and inspected on site.
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Automotive & transport
Chassis, driveline and body fixings that see vibration and are assembled to a controlled preload.
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Steel fabrication
Bolted connections in frames, enclosures, walkways and weldments, on the bench and on site.
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Energy & infrastructure
Tower, pipework, switchgear and plant fixings specified for long service life and inspectable joints.
Typical applications
- slip-critical connections tensioned by turn-of-nut or DTI washer
- supplied as a matched assembly with a heavy hex nut and hardened washer
- bolting structural steel members where the shear plane falls on the plain shank
- joints where the property class is stated on the drawing
- clamping two machined faces to a calculated preload
- bolting brackets, motors and gearboxes to a frame
Service conditions to specify against
- Marine and coastal
- Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
- High load
- Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
- Torque and preload
- The joint is designed around bolt tension, so lubrication, coating and the friction coefficient all change the torque figure — state them together.
Where to use something else
- Not an alternative to an ordinary hex bolt on a machine: the short thread and large head suit plate connections, not tapped holes.
- Do not use a machine thread into unthreaded sheet or timber; use a tapping screw, rivet nut or insert made for that parent material.
Full application guide: where Heavy Hex Structural Bolts are used, how to specify them, and the cases they are wrong for
