Effect of plug on levelling of residual stress in tube drawing

The effect of a plug on residual stress in tube drawing is examined by both numerical analysis and laboratory experiment. In the numerical analysis, finite element method is applied to study the residual stress in tube drawing both with and without a plug. Under the condition that a plug is not used, the axial residual stress is analyzed changing the tube wall thickness. The axial residual stress pattern is almost the same regardless of the wall thickness, and it is tensile near the outer surface and compressive near the inner surface. Based on the results, the mechanism of residual stress generation is assumed and a certain reduction in thickness by a plug is envisaged to be effective for levelling of the residual stress. It is thought that the residual stress would be levelled more efficiently with the increase of reduction in thickness, though the drawing force would increase at the same time. Therefore, it is important to find the minimum bare reduction in thickness to level the residual stress. Successively, a series of numerical experiments was carried out to decide the effective range of thickness reduction by a plug. In order to evaluate the intensity of residual stress, an index, which is the integration of the absolute value of axial residual stress, was introduced. As a result, it is revealed that a certain minimum bare thickness reduction exists, beyond which the intensity of the residual stress is significantly reduced. In order to verify the result of numerical analysis, experiments were also conducted using a drawing machine. In the experiments, the intensity of residual stress was evaluated by a slitting method where the tubes were divided into eight pieces by slitting every 45° and the displacements of the tube ends were measured. The numerical and experimental results are in good agreement.