Residual stresses in cold-extruded rods

Abstract

Twelve different combinations of extrusion ratio and die angle have been used to produce mild steel rods, the residual stresses in which have been determined by the Sachs bore-out method. The longitudinal residual stresses are found to be tensile near the surface, and it is shown that these can be modified to become compressive by an additional small reduction of about 2% following the main reduction in area in extrusion. The experimental residual stress patterns have been compared with theoretical patterns derived from the stress distribution at the exit boundary of the region of plastic deformation by the flow-function method. In general, selection of the velocity fields to give the minimum work of deformation under frictionless conditions does not give the best agreement with experiment. It is, however, possible to select other velocity fields giving more satisfactory agreement, and these are found to be displaced, in general, from the minimum-energy frictionless fields in a manner expected as a result of friction at the die surface. It therefore appears that residual stresses may be expected to be affected by the detailed mechanism of metal flow in extrusion.