Failure Analysis on Fracture of Cross Recessed Countersunk Flat Head Screws

Batch fracture failure happened to ML30CrMnSiA steel cross recessed countersunk flat head screws during initial assembly. The fracture reasons of the screws were analyzed by means of macro observation,chemical composition analysis, mechanical property test, metallographic analysis, fracture SEM and EDS analysis, hydrogen content determination and so on. The results show that micro cracks formed at the bottom of the screw head cross recess during cold heading or heat treatment. In the process of acid pickling and cadmium plating, hydrogen penetrated into the matrix along the micro cracks. In addition, the strength of the screws was higher than the design requirements. So hydrogen embrittlement cracking occurred. With the propagation of the cracks, the actual loading area of the screw decreased, and finally the screw fractured under the action of assembly stress.