Cause Analysis on Fracture of Bolts for Hydraulic Turbine Brake

The bolts for hydraulic turbine brake of a hydropower station fractured. The fracture causes of the bolts were analyzed by macro observation, micro analysis of fracture surface, chemical composition analysis, metallographic examination, hardness test and so on. The results show that there were some processing defects such as folding and micro cracks on the thread surface of the bolts, where the stress was concentrated. And the improper control of heat treatment process in the production process, the microstructure of bolts is not tempered micro structure, and there is Widmanstatten structure, which reduced the mechanical properties of bolts. When subjected to large impact force, the crack initiated from the processing defects and propagated rapidly, led to fracture of the bolt.