Causes of fracture of 304 stainless steel bolt for motor train unit

The 304 austenitic stainless steel bolt for a motor train unit fractured after about 2 months of use. The fracture causes were analyzed by means of macroscopic observation, chemical composition analysis and microscopic analysis. The results show that the raw material of the bolt was rolled, and the microstructure was austenite + ferrite + carbide, which led to the directionality of the mechanical properties of the raw material of the bolt, and the radial plasticity and toughness of the material were obviously reduced. In the process of thread rubbing, there were many processing cracks near the root of the thread. The textured structure made these source cracks more likely to expand under the action of low-cycle radial stress, which eventually led to fatigue fracture of bolts.