Nickel White Silicide Hard Spots
While casting with white gold alloys containing silicon, there may be a potential for a defect known as "silicide hard spots". These spots are a combination of silicon & nickel that form a hard nodule either on or below the surface. The silicide is formed upon cool down of the flask. Certain conditions listed below will practically guarantee that the silicides will form. It is possible to have perfect conditions and still create silicides. We currently recommend silicon free alloys for large surface area castings, due to the problem potential.
Some of the causes may be:
- Incomplete burnout
- Investment left on re-melted scrap
- Reactions with crucible materials (loose carbon or impurities)
- Overuse of scrap in the melts
- Iron contamination of the fine gold used
To resolve these problems you can:
- Be sure your burnout temperature reaches 732° C / 1350° F for at least 4 - 5 hours to completely burn carbon residues in the flask, unless you are casting stones in place in which steam de-waxing should be used.
- Be sure all investment is removed from the scrap to be re-melted.
- Be sure to remove any loose material from the crucible prior to melt.
- Limit the reuse of white gold to approximately 1/3 that of yellow gold.
- If refining your own scrap, do not introduce ferrous sulfate to precipitate the fine gold.
Refine any scrap with "hard spots" immediately. Do not reuse any scrap with "hard spots", it will contaminate the next melt.
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