Research on formation mechanism of cracks in connection bolt of turbine's large shaft

The connection bolt material of turbine's large shaft of a hydropower station is S35C steel and cracking occurred after operation for 65 thousand hours.By means of optical microscope,direct reading spectrometer,spectrum analyzer,microscopic hardness meter,tensile testing machine and impact test machine etc.,the macro feature,chemical composition,hardness,mechanics under indoor temperature,metallic phase,energy spectrum and electronic microscope are researched.The research results show that the bolt structure is abnormal reticular ferrite,which results in obvious decrease of anti-impact function of the bolt;foreign impurity and micro holes are existed in bolt,which results in local stress concentration and the holes become larger and longer under tensile stress action and forming larger cracks in the last.