Hydrostatic Extrusion of Steel

Abstract

A theoretical method for analysing the plastic flow of metals through a simple cone die based on the total work of deformation is discussed. An apparent strain is defined by equating an area under the equivalent stress strain diagram to the work done per unit volume of material. The strain range covered by this area is called the ‘apparent strain’ which is the equivalent strain corresponding to the total external work done and not the actual mean equivalent strain undergone by the material during deformation. The apparent strain theory is based on Siebel’s and Pugh’s methods and includes assumptions which permit its application to processes which involve friction and work hardening materials. It is assumed in this theory that the mean equivalent strain induced by the plastic flow of metals through a die depends only on the geometry of the process and not the way in which the forming loads are applied. This permits its application to all forms of extrusion and drawing. The theory has given good agreement with experimental results obtained by hydrostatic extrusion in which the forming loads consist of combinations of oil pressure and direct loads applied to either the billet or product. The experimental work has shown the feasibility of extruding steel bar and tube by simple hydrostatic extrusion [in which the billet is extruded by the sole actions of the oil pressure] proportionally augmented hydrostatic extrusion [in which the extrusion effect of the oil pressure is increased] and augmented hydrostatic extrusion [in which the oil pressure is aided by an externally applied force]. The technological difficulties associated with these methods are discussed and their possible fields of application are outlined. It has been shown that product augmented hydrostatic extrusion, in which the oil pressure is assisted by a load applied to the product, is a useful and versatile metal forming process which has application to the manufacture of steel tube. This method permits large reductions at product speeds comparable with those obtained by conventional drawing. The limits to the extrusion ratio which can be achieved are fully discussed.