Stabilizing the Stress State of Wire in Drawing under Countertension with Variable Frictional Coefficient

Abstract

Abstract—In wire drawing, instability of the deformation parameters produces variable axial stress. The contact friction changes within an extremely short period and depends on many factors, such as the uniformity of the workpiece’s physicomechanical properties over its length; lubricant quality; stability of the tensile force on entering the deformation zone; and the cooling rate for the drawplates, tension drums, disks, and guide rollers. To assess the influence of the frictional coefficient on the stability of the drawing force (stress), the first derivative of the drawing stress with respect to the frictional coefficient is considered. Specifically, its relative value is considered: in other words, the ratio of the derivative and the initial yield point of the wire is monitored. That permits comparison of the derivatives with different initial yield points of the wire. The derivative is calculated by means of an equation for the axial stress in which the following factors are taken into account: the countertension; the change in yield point along the length of the plastic-deformation zone; and the decrease in radial stress in the calibration band as a function of the increase in axial stress in the drawplate’s working cone. The relative value of the first derivative is expressed in terms of the countertensile stress and the frictional coefficient, with different models of strengthening and different deformation parameters. The derivative declines with increase in countertensile stress. Consequently drawing with countertension is expedient in order to stabilize the drawing force and decrease the fatigue wear of the drawplate’s working surface. The behavior of the derivative is plotted on the basis of the formula obtained. Drawing is considered in the case where the frictional coefficient is constant in the calibration band and variable in the drawplate’s working cone. For that process, the relative value of the first derivative of the drawing stress with respect to the frictional coefficient is determined for the axial stress. The calibration band stabilizes the drawing stress (force) if the frictional coefficient is constant in that band. The instability of the drawing stress with deviation in the frictional coefficient depends on the other deformation parameters. For example, the derivative increases with decrease in the drawing angle. The steady influence of the deformation parameters on the stability of the drawing stress with variation in the frictional coefficient is consistent with Brovman’s results and also with the results of calculating the axial stress with the constant and fluctuating frictional coefficient. Greater stability of the drawing stress improves the wire quality and the working life of the components in the drawing system—for example, on account of decrease in the fatigue wear of the working surfaces of drawplates, drums, guide rollers, and adjustment rollers. The stress state in the zone where the wire is deformed may be improved by drawing with countertension and using special drawplate designs.