SUMMARY
In continuous casting processes, the loading phases of the raw material and any machine downtime are critical factors, especially in conjunction with the use of crucibles and dies that are too worn or of poor quality. In these conditions, in fact, there is a high probability of the metal clinging to the graphite with consequent dragging of carbonaceous inclusions inside the profile. These inclusions are then the source of breakage, especially in the case of subsequent mechanical processing.
PROBLEM
In the case under consideration, there were problems with the profile made of 18 ct white gold which showed breakage phenomena during deformation. More in-depth scanning electron microscopy analyses have detected carbon in high concentrations in the rupture sections (see figures 1 and 2).
The presence of carbon was not evident immediately after the casting. Only during mechanical deformation and annealing was a deterioration of the mechanical properties of the metal noted, also linked to the reduction of the resistant section. As a consequence of these phenomena, the ruptures in question then occurred.
There are several hypotheses about the possible causes of these inclusions. Crucibles of not high enough quality or too worn may have been used. The phases of machine downtime or metal loading may also have been crucial: the metal may have clung to the die bringing with it some traces of graphite that would then have formed the inclusions in question.
SOLUTION
The machine stop phases are the most critical for any adhesion of the profile to the walls of the die (especially in white gold). For this reason, it is of fundamental importance to pay close attention:
- the quality of the crucible and the supply chain
- to avoid using crucibles and dies that are too worn due to graphite which, with use, tends to flake off
- to limit machine downtime as much as possible
- to use inert gas that can protect crucibles and dies from excessive contact with oxygen.