Failure Analysis on Early Fatigue Fracture of a 10.9 Grade Bolt

After undergoing the bumpy fatigue experiment for 2×10⁴ times, the fracture was happened to a 10.9 grade bolt that was assembled on the car seat after quenching and tempering treatment. The fracture reasons of the bolt were analyzed from the aspects of chemical composition, fracture morphology, hardness and microstructure. The results show that:the transition of the thread root was rough with gap, and cracks would be easily generated by the stress concentration; the decarburization in the thread surface layer increased the cracking tendency; micro-cracks formed as a result of the action of alternating stress in the fatigue test, causing the bolt early fatigue fracture.