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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