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ANTIMONIAL LEAD ALLOYS AND THE EFFECTS OF TIN
Tin holds arsenic in solution in an antimonial lead alloy.
If the tin content in an antimonial alloy is reduced to .005% or less, the arsenic will come out of solution at high temperatures. When this happens during grid casting, arsenic trioxide forms which appears as a white substance on the cork surface of the mold. This white substance forms a hard surface on the cork and will block up the pores making venting more difficult. It will also plug vent bars.
When the arsenic is leaving solution and forming arsenic trioxide, it will also gas, looking like a white smoke coming from the mold gate. This gas is harmful and should not be inhaled.
Another consequence of gassing is the formation of holes in the gate section.
When casting under these conditions, the lead will hit the mold gate and form balls or shot which will bounce around the gate. The lead tends to separate and does not want to form strings or wires.
If the arsenic continues to come out of solution and reaches a low of 0.01%, antimony will then begin to come out of solution. The arsenic level should be no less than 0.05% to prevent antimony escape.
If an alloy has no tin, it does not flow well and it will be more difficult to cast or fill the grid. As you run and the white surface builds up on the mold, venting will get worse thereby causing more difficulty in filling.
Grid cracking can also become a problem which may be helped by the use of grain refiners.
In 4-1/2% antimony lead, sulfur should be .003% to help prevent cracking. Copper is also a refiner and its presence in the alloy will help cracking.
In 2-3/4% antimony lead, sulfur should be increased to .007% and copper to .075% to help a cracking problem. Introduction of selenium will also help since it too is a grain refiner.
If you find it necessary to cast a low or no tin alloy, the following is recommended:
1. Use a porous or coarse cork and apply to get a rough surface.
2. Drop the ladle, feedline and pot temperatures to as low as possible.
3. Increase the mold temperatures to as high as possible.
4. The ladle should be stream or orifice pour to help venting. No sheet pour.
5. The ladle should pour slowly.
6. Adjust machine speeds accordingly.
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