Hard spots in Kupfer-Zink-Legierungen

The two copper zinc alloys, Cu39Zn3Pb and Cu40Zn2Pb, produced by melting of the recycled scrap, are shown very high tendency to the formation of hardspots due to presence of many impurities. This phenomenon is easily proceed through the processes of melting, casting, heat treatment and forming which are essential for the production of copper-zinc alloys (continuous casting, extrusion and pre-heating of billets and slabs). It was found that the heating for a long period of time for these slabs can mange to get rid of these hard spots due to diffusion of these impurities (Fe, Si and Cr). These hard spots are difficult to observe according to their very small size. On the other hand, it is possible to recognize them in the dimples of the fracture surface after tensile testing. At the same time it is possible to analysis these hard spots by using energy-dispersive X-ray analysis if their size is big enough for this type of testing. Also, these hard spots can be produced by the addition of some elements like iron and silicon to the pure melt of copper and zinc, even in the concentration which is lower than the permissible to the standard percentage. This can be proved the great tendency of formation of these hard spots in copper-zinc alloys in the industrial production plants. In conclusion, it was found that the hard spots are very difficult to disappear completely by using fluxes or proper covering materials on the surface of the molten alloys. It is noticeable that the density of the hard spots is decreased but because of dezincification.