Discoloration of Commercial Bronze During Heat Treatment Operations

A medal is a minted product that is used to commemorate significant events or recognize outstanding efforts from individuals. Various nonferrous materials are used for medals due to ease of striking. Additional manufacturing processes may assist with lowering hardness for further improvement of struck images. Heat treatment of commercial bronze was studied to determine hardness relationships with time, temperature, atmosphere, and other various factors during furnace operations. Medals are typically a “bronze” color but would occasionally result in a pink hue, which is not acceptable for an artistic product. Metallurgical cross-sections were analyzed for microstructure and chemical analysis was performed on the surface. The amount of zinc on the surface decreased with higher temperatures and longer processing times. An alpha brass such as commercial bronze is not typically susceptible to dezincification, but testing confirmed that there was selective dealloying during certain heat treatment arrangements. Using this knowledge, heat treatment parameters were optimized to provide the best material and mechanical properties for striking while avoiding unwanted discoloration.