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Residual Stresses in Punches for Backward Extrusion

The paper deals with the influence of residual stresses acting in a subsurface zone of cylindrical punches on their durability during extrusion. Residual stresses were induced in new punches by surface finishing and evolved during punch service. Punches were made of American Iron and Steel Institute M2 high-speed steel, finished by gas nitriding. Punches served for mass extrusion of closed-end tube zinc anodes applied in popular R6 size batteries. The lifetime of punches was expressed as a number of zinc tubes extruded up to a given loss in punch roll diameter. Residual stresses were measured in new and exploited punches on punch roll in a several-microns-thick subsurface zone adjacent to the working face by the X-ray sin2Ψ technique. Measurements were performed in two orientations of a rectangularshaped X-ray beam (circumferential and axial) and in six angular locations around a punch working face. It was found that the residual stress sign and magnitude depended essentially on the punch finishing and orientation of the X-ray beam during measurement. The most favorable compressive stresses were measured after nitriding in the axial direction. Punch durability appeared connected with residual stress sign and magnitude. In exploited punches of the highest durability, low tensile and even compressive stresses still remained. Cracking was observed under the working face of punches of low durability in which tensile stresses were measured.

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