Property Class 12.9 Steel Square-Head 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 (ISO 898-1 property class 12.9) is the highest common strength class, for high-load machine assembly. When square-head bolts are specified in this grade, the dimensions remain per ASME B18.2.1 / SAE J429 Grade 5; the material determines corrosion behavior (requires plating or black-oxide protection) and the service temperature range (commonly limited to approx. +300 °C; confirm against the coating and the load). Both are covered below with the size table.
Why Choose Property Class 12.9 Steel for Square-Head Bolts
The highest common strength class, for high-load machine assembly. For square-head 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 |
|---|
Galvanic & Galling Considerations
For square-head 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.
Comparable Grades
For Square-Head Bolts, class 8.8 is the comparable strength step. Move up when the joint preload approaches the proof load of the current class; move down when the joint is over-specified. For corrosion-critical service, switch to stainless A2 or A4.
Sizes of Square-Head Bolts Available in Property Class 12.9 Steel
| Size | Thread form | Head width (in) | Head height (in) | Tensile strength (psi) | Material / grade | Reference standard |
|---|
| 1/4-20 | fully or partially threaded | 3/8 | 11/64 | 120000 | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
| M6 / M8 / M10 / M12 / M14 / M16 / M20 / M24 / M30 | fully or partially threaded | | | | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
| 5/16-18 through 1-1/4-7 | fully or partially threaded | | | | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
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. Lengths and thread lengths outside the standard table are the commonest request, followed by a drive the standard does not offer and a head marked with your own part number. 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
What Square-Head Bolts are specified for, industry by industry. Stocked from 1/4-20 to 5/16-18 through 1-1/4-7 (screw size).
Industries that specify Square-Head Bolts
-
Steel frames
Partial or full thread options fit different grip lengths.
-
Maintenance work
Metric and inch sizes cover mixed legacy equipment.
-
Fabricated machinery
Square heads provide wrench faces when a hex head is not desired.
-
Agricultural structures
Grade 5 steel covers general structural and equipment joints.
-
Machine building & OEM
The default fastener on production equipment: covers, brackets, motor mounts and sub-assemblies tightened to a specified torque.
-
Automotive & transport
Chassis, driveline and body fixings that see vibration and are assembled to a controlled preload.
Typical applications
- matching the Grade 5 material to the joint requirement
- using the stated ASME and SAE designations when ordering
- tightening a square head with an open wrench
- choosing full or partial thread from the grip
- assemblies that are dismantled for service and re-torqued
- joints where the property class is stated on the drawing
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
- Do not assume the square head provides the same wrench clearance as a standard hex head; check the actual envelope.
- 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 Square-Head Bolts are used, how to specify them, and the cases they are wrong for
![[Ruoteng] M8 M10 M12 Stainless Steel 201 304 316 Hex Head Bolts with Nuts Flat Washers and Lock Washers](https://img.engineeringhardware.com/Ruoteng-M8-M10-M12-Stainless-Steel-201-304-316-Hex-Head-Bolts-with-Nuts-Flat-Washers-and-Lock-Washers.webp)
