Fracture Cause of Connecting Flange and Bolts of a 3B Circulating Water Pump in a Power Station

The connecting flange and bolts of a 3B circulating water pump in a power station were fractured. By means of macroscopic observation, chemical composition analysis, microstructure observation, mechanical property test, scanning electron microscopy and energy dispersive spectrometer analysis, the reasons for the fracture of flange and bolts were analyzed. The results showed that in the seawater medium environment, crevice corrosion occurred between the flange joint surface and the nut pressing surface, causing the bolts loosen and fatigue cracks nucleated at the root of screw thread, resulting in fatigue fracture and the stress on the flange was greatly increased. Under the combined action of self-weight of the fluid, the rotation of impeller and fluid disturbance, the flange was eventually be overloaded and fractured.