Improved relationship between Vickers hardness and yield stress for cold formed materials

Abstract

Cold‐formed metal products are increasingly serving as high duty machine parts. Designers and users need to know their properties as accurately as possible. One such product property is the new yield strength, which can be approximated by the final flow stress of the workpiece material during forming. Vickers hardness measurements provide an easy and inexpensive method of evaluating the new local yield stress in cold‐formed workpieces. The well‐known available models given in literature to convert the measured hardness number into the corresponding yield stress have an error of up to 25 %. This is basically due to the facts that cold formed material experiences large plastic strains in the main forming stage, the hardening behaviour is anisotropic and, moreover, the material properties are inhomogeneous especially at the workpiece surface. The purpose of this study is to improve the accuracy of the well‐known available correlation models between Vickers hardness measurements and yield stress. This is achieved by utilizing finite element simulations of the indentation process. The models currently incorporate only the isotropic strain‐hardening behaviour of the work material. The new suggested model decreases the theoretical conversion error to less than 10 %.The improved model has been verified by experiments. The difficulty in verifying the models is realizing an experiment with a precisely known high plastic strain. In this study, the forward extrusion process was utilized for this purpose. In the forward extrusion process there is one location in the workpiece where the plastic equivalent strain and hence the yield stress is exactly known: the axis of the extrudate. By this method it is possible to obtain strain‐hardening states up to an equivalent plastic strain of 1.6 (!). Hence, making hardness measurements at the axis of extruded workpieces, it was possible to verify the improved relationship up to realistic strain values. The results have shown that the new relationship supplies conversions with a drastically reduced error as compared to the classical relations.