Microstructure and Mechanical Properties of a 18Kt 5N Gold Alloy After Different Heat Treatments

In precious metals industry, heat treatments play a key role in the production process, since the type and the sequence of these treatments produce the right combination of hardness and workability. The production sequence usually involves rolling, plastic deformation, heat treating, and surface finishing. In this work, different heat treatments were performed on samples of 18Kt 5N gold–copper–silver alloy, and the evolution of the microstructure and the microindentation hardness was analyzed. The heat treatments were performed after the first and the second rolling process (a softening heat treatment) and before the final surface brushing and polishing (a hardening heat treatment). Two different softening heat treatments were tested: a continuous annealing treatment performed in a belt furnace and a solution heat treatment in static furnace. Two different hardening treatments were studied: an aging heat treatment and a treatment of solubilization followed by an aging treatment. The microstructural observation and mechanical analysis showed that the solution treatment in static furnace produced smaller and more homogeneous grain size than a continuous annealing treatment, whereas the hardness was not reduced. This could be very important because the reduction in the grain size results in a marked decrease in the orange peel defects on the final products. It was also found that between the two hardening treatments, solubilization and aging produced an increase of about 90 HV in the hardness with a significant reduction in the number of superficial defects during the final surface finishing steps. The optimization of the heat treatments, studied in this work, produces a significant decrease in the total number of defects at the end of the production cycle.