Abstract: The screw of a melting extruder imported from Germany fractured after being put into use for 7 a (annual). The causes of fracture were analyzed by means of macroscopic analysis, chemical composition analysis, tensile test, fracture microscopic analysis, metallographic examination, micro-hardness test and so on. The results show that the main reasons for the initiation of the crack source in the heat affected region of the screw were the uneven width of heat affected region of roller hardfacing layer, microstructure deformation, and the stress concentration caused by the brittle inclusions in the material under external force. The cracks gradually expanded along the hardfacing layer under the action of torsion stress of the screw, and fatigue fracture eventually occurred.