Microstructural evolution of copper clad steel bimetallic wire

We investigated the microstructure of two different bimetallic wires of Copper Clad Low Carbon Steel Wire (LCSW), which had a 1006 steel core, and Copper Clad High Carbon Steel Wire (HCSW), which had a 1055 steel core. The HCSW generally showed higher hardness than LCSW because of the pearlitic grain structure. A low temperature annealing at 720 °C to the drawn HCSW caused a significant reduction of hardness, which was as low as that of an annealed LCSW. In general, both LCSW and HCSW showed strong global textured features after drawing, with the steel having a strong 〈1 1 0〉 fiber texture and the copper having a 〈1 1 1 〉–〈1 1 2〉 deformation direction. At the interface, a grain size discrepancy at the steel–copper interface was observed. Post-drawing, the LCSW copper grains exhibited refined grain sizes near the interface and has been explained in terms of shear strain gradient. The HCSW did not exhibit this copper grain size distribution but did exhibit a coarsening of the steel grains near the interface after a subsequent 720 °C heat treatment. This is attributed to the large localized stress concentration at the perimeter of the steel region during the drawing process. The strain induced regions at the steel–copper interface have been simulated by finite element modeling. These grain size discrepancies caused the smooth variation in nanohardness across the interface.