Compréhension et modélisation des mécanismes de lubrification lors du tréfilage des aciers inoxydables avec des savons secs

摘要

Dry wire-drawing is a cold metal working process during which one reduces the wire diameter by plastic deformation, by pulling it trough a die, with a powdered lubricant, called soap, which is a mixture of fat matter and mineral additives. Numerous reductions and high speeds lead to hard work conditions for the soap: a non-optimized lubrication induces a soap thickness which is too thin and produces scratches on the wire, wire rupture and increases the die wear. This work is a study of lubrication mechanisms which occur during stainless steel wire drawing with sodium and calcium soaps. It's focused on experiments set on a single wire-drawing device and on the development of methods allowing experimental and theoretical characterization. Residual weight of soap film, force and die temperature measurements and also drawn wire surface SEM observations allow the analysis of the influence of soap compositions (nature of additives and fat matter) and process parameters (speed, die geometry and roughness, coating and roughness of the wire) on lubrication. Results show the interest of using well-filled sodium soap. They also confirm that high die and wire roughness, a low die angle and a pressure device have positive effects on lubrication contrary to speed which is a critical parameter. Soap characterization (DSC, capillary rheometer, mixer, low-speed wire drawing) is used to study the evolution of the soaps' behaviour with temperature but the link between physical and rheological properties and their performances during process is only partially established. A force and temperature thermomechanical model based on experimental results allows to estimate wire plastic deformation work, die and wire heating and friction shear stresses. The finite-elements code, Forge2®, is used to validate this approach. A lubrication analysis allows to estimate soap film thickness for plastic (low temperature) or viscoplastic (high temperature) behaviour. It allows to interpret the influence of process parameters on results. On another side, one shows how the developed method can be applied to the study of industrial multipass wire drawing.