Microstructural Characteristics and Fracture Behavior of the Rotor Magnetic Pole Screw in an Industrial Synchronous Motor

The microstructural characteristics and fracture behavior of a magnetic pole screw were investigated here. The screw threads were produced by cold thread rolling. Microstructural analysis (OM, SEM, EBSD), mechanical testing (tensile, hardness, fastening), and fracture morphology observation were conducted. The results indicate that work hardening and microstructural deformation were introduced by the gradient plastic deformation in the screw thread. The elongated microstructure of ferrite and pearlite was obtained in the deformation zones, resulting in increased hardness and decreased plasticity. The thread root subsurface experienced severe localized indentation deformation and exhibited the highest hardness. The distinct forming stress states led to a notable difference in the hardened layer depth between the thread crest and root. The torsional overload fracture was initiated at the stress-concentrated thread root, where the work-hardened microstructure exhibited a limited capacity to accommodate large plastic deformation. The crack propagation was influenced by the gradient microstructure, following three primary propagation paths: transgranular through ferrite, along the ferrite–pearlite phase interface, and cracking through lamellar pearlite. The results provide theoretical support for material design and process optimization to achieve the production of high-performance screws with high strength and hardness at the thread surface and high plasticity in the center.