Analysis on Reason of Fracture of Clamp Bolt in Multiple Units

The fracture morphology, microstructure and chemical composition of fracture bolts were detected and analyzed by scanning electron microscope, optical microscope and inductively coupled plasma spectrometer. The results show that the chemical composition and microstructure of the broken bolt are not abnormal. The bolt section shows obvious high temperature oxidation traces and coarse grain profiles, and the melt local grain boundaries that connected with the surface at the rounded corners. Fine grains can be seen inside the original austenite grain and other parts of the bolt. Combined with the bolt making process, it can be inferred that the local over-burning of the bolt is caused by improper heating temperature control during the hot heading process. Through the hot upsetting test at different temperatures, reasonable suggestions for technological improvement are put forward.