Grade 12.9 Steel (class 12.9) is the highest common strength class, for high-load machine assembly. When carriage bolts are specified in this grade, the dimensions remain per DIN 603; the material determines corrosion behavior (requires plating or black-oxide protection) and the service temperature range (up to approx. +300 °C). Both are covered below with the size table.
Why Choose Grade 12.9 Steel for Carriage Bolts
The highest common strength class, for high-load machine assembly. For carriage bolts, this matters when requires plating or black-oxide protection is the primary requirement.
Mechanical & Physical Properties
| Grade designation | class 12.9 |
|---|---|
| Magnetic response | Magnetic |
| Service temperature | up to approx. +300 °C |
| Corrosion characteristic | Requires plating or black-oxide protection |
Galvanic & Galling Considerations
For carriage 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 Carriage 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 Carriage Bolts Available in Grade 12.9 Steel
| Size | Pitch (mm) | Head diameter dk (mm) | Head height k (mm) | Square neck width (mm) | Square neck depth (mm) | Common length range (mm) | Reference standard |
|---|---|---|---|---|---|---|---|
| M5 | 0.8 | 13.0 | 3.0 | 5.0 | 2.5 | 16~80 | DIN 603 / ISO 8677 |
| M6 | 1.0 | 16.0 | 3.5 | 6.0 | 3.0 | 16~120 | DIN 603 / ISO 8677 |
| M8 | 1.25 | 20.0 | 4.5 | 8.0 | 4.0 | 20~160 | DIN 603 / ISO 8677 |
| M10 | 1.5 | 25.0 | 5.5 | 10.0 | 5.0 | 25~200 | DIN 603 / ISO 8677 |
| M12 | 1.75 | 30.0 | 7.0 | 12.0 | 6.0 | 30~240 | DIN 603 / ISO 8677 |
| M14 | 2.0 | 35.0 | 8.0 | 14.0 | 7.0 | 40~260 | DIN 603 / ISO 8677 |
| M16 | 2.0 | 40.0 | 9.0 | 16.0 | 8.0 | 40~300 | DIN 603 / ISO 8677 |
| M20 | 2.5 | 50.0 | 11.0 | 20.0 | 10.0 | 50~300 | DIN 603 / ISO 8677 |