Cold microforming of low carbon steel: wire drawing and hydrostatic extrusion

The paper presents the experimental investigation and relevant results of two different cold microforming processes on the same low carbon steel: the wire drawing and the hydrostatic extrusion. Reduction of the cross section of the original wire is the same in both processes, but in wire drawing both lubricated and dry interface conditions are considered. In hydrostatic extrusion only the lubricated condition is investigated. In both cases the required forming load is really higher as expected in non-micro forming operations and the microhardness distribution in the cross sections of the formed material presents higher levels in the external layer of the wire, which can not be explained by the traditional strain hardening reinforcement. The measured reinforcement can be due to a sort of cold grain refinement in the external layer of the wire as detected by the grain size analysis in the cross section of the wire. For this reason the material behavior should take in account this effect including a contribution to material strength which depends on the grain size as suggested by the Hall-Petch relationship and leads to a sort of dual definition of material rheology relevant to the external layer and the core respectively.