Influence of microstructure on centerburst development in steel extrusions
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I.
INTRODUCTION
CENTERBURSTS are occasionally produced during deformation processing as, for example, in extrusion or wire drawing (Figure 1). Two factors are highly relevant to the initiation of centerbursts: the existence of a tensile stress along the axis of the sample, in which the fault initiates, and the microstructure in this region. Theoretical analyses have been performed to obtain the stress distribution in an extrusion specimen by using either the slip-line method tl,2] or the upper bound theorem, t3,4] In such analyses, the material is generally considered to be isotropic and perfectly plastic. Under these assumptions, the centerburst phenomenon can be related only to tool geometry and friction during the forming process. A more complex theory tSI accounts for strain hardening in the deforming material but continues to treat the material as isotropic. In these theories, the influence of strain rate and temperature on the mechanical properties of the material has not been taken into account, although their influence is highly relevant for centerburst formation. Experimental studies I6] have shown that by changing the ram speed and specimen temperature, it is possible to produce or suppress centerburst formation. In addition, the microstructure of the material has also been shown to have an important effectI6.7] on this process, mainly through variation of the material fracture stress. In addition, radial variation of microstructure through the material produces a flow stress distribution that modifies the tensile stress on the centerline of the sample. The fact that these variables, which have a great influence on centerburst formation, are not accounted for in current theories is due to the necessarily approximate nature of these theories. In principle, the problem could OSCAR E. QUILODRAN ALARCON, Faculdade de Engenharia, and RICARDO ENRIQUE MEDRANO, lnstituto de Fisca, are with UNICAMP, CP 6165, 13081 Campinas, SP, Brazil. PETER P. GILLIS is with the Department of Materials Science and Engineering, University of Kentucky, Lexington, KY 40506. Manuscript submitted February 5, 1990. METALLURGICAL TRANSACTIONS A
be solved by introducing these variables into the theoties. However, this is not an easy task, because during the forming process, the material has very complex strain, strain-rate, and temperature distributions. Therefore, it is very difficult to know, point to point, the microstructure of the material and its mechanical properties under the actual local conditions. Another difficulty is actually measuring the strain, strain-rate, and temperature distributions in the forming processes in order to make significant comparisons with theoretical calculations. Extrusion is done within closed containers, and, generally, the only measured parameters are the loads on the piston and the die t8] and the overall kinematics. Strain and strain-rate distributions can be obtained by the visioplasticity method, t9] In this technique, prior to drawing or extruding, an axisymmetric specimen of the material is
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