A Lead Extrusion Phenomenon

Abstract

IT has been known for the past two decades that in the extrusion of lead cable sheaths and pipe on the hydraulic press, the presence of oxides may result in the existence of irregularities in the structure which are brought to light by suitably etching a polished section. In this case, the feature originates in the oxidation of a surface of the metal exposed at an earlier stage in the process, and the layer of oxide, formed at right angles to the direction of flow, may be folded over, and appear afterwards as a radial line of weakness in the finished pipe. An important distinguishing feature in this type of irregularity is the fact that the crystal structures on the two sides of the feature are quite separate and distinct; there is no growing of the crystals across the line of oxide (see Fig. 1).