Metallographic characterization of a gold-steel composite

Abstract

A gold-steel composite material for decorative applications was manufactured by forging. This procedure can be referred to as modified damascene technique. Sheets made of steel K110 (C-rich cold work steel) and gold sheets were assembled and forged together at approx. 800 °C. The bar was subsequently twisted to obtain the typical torsion pattern. The end portions of this bar were subjected to a metallographic examination. The steel K110 shows broken carbides which were formed during the forging process. The carbides can be etched using Murakami's reagent, whereas a 3 % Nital solution can be used to etch the steel matrix. Gold etching was challenging because many etchants suitable to etch gold preferentially attack the steel. In reference to gold extraction by cyanide leaching, a KCN solution was used as etchant. It could be shown that atmospheric oxygen acting as an oxidant is already sufficient to etch gold. However, using a KCN-H₂O₂ mixture provides a more efficient etching effect. Gold grains in the gold layers can reach a size of up to 1 mm. A diffusion zone with a thickness of approx. 10 μm can be observed at the steelgold interfaces.