Microstructure and Mechanical Properties of Internal Thread during Cold Extrusion for Q460 High Strength Steel

According to the forming process of internal thread, the microstructure, microhardness and mechanical properties of internal thread surface layer were investigated. The microstructure change and deformation mechanism, harden ability and depth of hardening layers on the crest, the flank and the root of the thread along the layer depth direction were obtained. The results showed that the grains on the surface layers of the thread were refined after the cold extrusion. Fibrous structure of the metal surfaces on the root of thread was pressed to distinguish the grains difficultly, and formed a layer of flown line and smoothly rounded the root to flow from the flank to the crest. The grains on the root were the smallest, followed by the flank and the crest. In the extrusion processes, the microhardness and the tensile strength of the material were increased significantly, and the plasticity and the toughness were decreased. The work-hardening phenomenon greatly increased the surface texture and the mechanical strength of internal thread with cold extrusion.