Wear mechanism of tungsten carbide dies during wet drawing of steel tyre cords

Wet drawing of brass coated steel wires, used for tyre reinforcement, is realised with tungsten carbide (WC) dies sintered with a cobalt binder. Die wear represents an important limitation to the production process and cost savings. Mechanical models of drawing are not sufficient to explain the phenomenon experimentally observed. Experiments were carried out on an analytical drawing machine and coupled with SEM/EDX observations on cut half dies. Wear scars observed, correspond to brittle fractures and removal of grain clusters. Evidence of the existence of a metal transfer is given. It is composed of brass and iron whose presence is associated with die wear. Welding between iron and cobalt at high temperature modifies the binder, weakens the structure and leads to decohesion of grains and grain clusters. Two parameters play a major role on this mechanism. The iron transferred on the die surface and the thermal conditions, generated by both friction and plastic deformation, which accelerate wear. To prevent this decohesion wear, the lubricant must generate thick films to avoid asperity contacts, or extreme-pressure films to provide against iron/cobalt adhesion. Solutions like new die materials also represent an interesting option.