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Cracks in cold heading parts are a common quality issue in fastener manufacturing, especially around bolt heads, head-to-shank transition areas, upsetting sections, and extrusion zones. Cracks can affect product appearance and dimensional accuracy, and may also reduce mechanical strength, fatigue life, and reliability.
So, what causes cracks in cold heading parts?
Common raw material problems include:
Small defects on the wire surface may expand during upsetting and extrusion as the material undergoes repeated tension and shear, eventually developing into visible cracks.
(1) Excessive deformation in a single operation:
If one forming stage takes on too much deformation, strain concentration can occur, causing cracks when the material exceeds its plastic capacity. For complex or high-deformation parts, the total deformation should be distributed across multiple stations.
A key advantage of multi-station cold heading is distributing the total deformation gradually, reducing the forming load on each station.
(2) Incorrect blank dimensions:
An excessive cut length or oversized blank may overfill the die cavity, preventing excess metal from flowing properly and increasing extrusion stress, which can lead to cracking.
(3) Unbalanced forming speed:
Excessively high forming speeds may cause rapid strain accumulation and insufficient material flow. Excessively low speeds, however, can increase friction and local heating between the blank and die, potentially damaging the surface and generating fine cracks.
The die determines how material flows during forming and therefore plays an important role in crack prevention.
| Die Factor | Possible Effect |
| Small die radius | Stress concentration and restricted material flow |
| Improper cavity design | Excessive local deformation |
| Die wear | Dimensional changes and abnormal loading |
| Die misalignment | One-sided loading and eccentric deformation |
| Poor assembly accuracy | Local abnormal stress |
(1) Poor lubrication:
During cold heading, significant friction occurs between the material and die. Proper lubrication reduces friction and helps material flow smoothly. An incomplete lubricant film or unsuitable lubricant can increase friction and forming loads.
(2) Inadequate surface preparation:
Oxide scale, rust, oil, or other contaminants left on the blank before forming may interfere with uniform deformation. They can create local stress concentrations and contribute to microcracking.
If the cold heading machine has station misalignment, unstable feeding, excessive vibration, or incorrect die installation, the material may be subjected to uneven loading. When cracks repeatedly appear on the same side of the part, equipment alignment, die installation, and feeding accuracy should be checked first.
| Crack Appearance | Priority Inspection |
| Continuous cracks on raw material surface | Raw material surface quality |
| Cracks around the head edge | Material ductility and upsetting deformation |
| Cracks at the head-to-shank transition | Die radius and forming sequence |
| Cracks consistently on one side | Die alignment and machine accuracy |
| Cracks in extrusion areas | Extrusion deformation and lubrication |
| Cracks disappear after changing material | Raw material quality |
| Cracks decrease after changing dies | Die wear or die design |
A single crack pattern may result from several interacting factors, so visual inspection alone cannot always determine the root cause.
Use material suitable for cold heading and inspect its dimensions, hardness, surface quality, and microstructure before production. For critical products, establish batch control to avoid mixing materials of different quality levels.
For parts requiring large deformation, distribute the deformation across multiple stations. This allows the material to form gradually and reduces excessive deformation in any single operation.
Optimize die radii, cavities, and transition areas according to the part geometry. Maintain high die accuracy and inspect die wear regularly to prevent abnormal forming caused by dimensional changes.
Select a lubrication system compatible with the material and forming process. Keep the lubricant film uniform and stable, and regularly inspect surface pretreatment and lubrication performance.
Monitor feeding, stroke, forming speed, station synchronization, and die installation accuracy. When cracks occur, analyze production data and adjust the relevant parameters rather than changing a single setting without diagnosis.
