The effect of the strain path on the work hardening of austenitic and ferritic stainless steels in axisymmetric drawing

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. INTRODUCTION

THE ability of metals to be shaped by cold plastic deformation (cold forming) is one of their most important technological characteristics. This type of processing also changes the mechanical properties of the material, usually involving an increase in strength (work hardening) and a decrease in ductility. The magnitude of these effects depends on the material being formed and on the process variables (geometry, strain, strain rate, temperature, and strain path). The work-hardening characteristics of metals (and, thus, their constitutive laws) deeply affect the analytical and numerical analyses of forming processes and especially the end mechanical properties of the products manufactured. The effects of strain, strain rate, and temperature on work hardening have received wide attention in the literature, but the role of the strain path has been far less studied, except for sheet-metal forming.[1–6] Considering the von Mises effective stress (␴e)—plastic-effective strain (␧e) curve of materials (heretofore called the ES-ES curve), two types of sheet behavior have been identified. Type I is observed in brass and some aluminum alloys and is associated with a lower hardening under plane strain than under pure tension. P.R. CETLIN, Professor, and E.C.S. CORREˆA, Doctoral Student, Department of Metallurgical and Materials Engineering, and M.T.P. AGUILAR, Associate Professor, Department of Materials Engineering and Construction, are with the Federal University of Minas Gerais, Minas Gerais, Brazil. Contact e-mail: [email protected] or [email protected] Manuscript submitted November 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

The opposite behavior is found in type II materials (lowcarbon steel and pure aluminum.[3,5]) Similar results were also found for biaxial straining[5] or rolling,[7] instead of plane strain, and for tension at different angles to the rolling direction.[1,2,4] The importance of the preceding phenomena derives from the fact that regions of the sheets undergo a sequence of strain states during forming, especially when a series of separate drawing steps are employed. In addition, the final mechanical properties of formed sheets are usually evaluated in pure tension, which represents a strain-path change in relation to the previous processing. Figure 1 illustrates the situation for a type II material, where a specimen was initially subjected to plane strain up to ␧e1 (curve AB), unloaded, and then subjected to pure tension. If the work-hardening behavior in this last step followed the curve corresponding to only pure tension, its ES-ES behavior would be that described by the curved arrow starting at C and conforming to the pure-tension curve. Experimental results, however, indicate that there is a transient behavior after the strainpath change. The initial flow stresses are higher than if all the processing had occurred under tension but, after some straining, tend to the values typical of full tension. In addition, the initial work-hardening rates are very high and positi

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