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Deoxidising is the fifth essential.When gas torch melting, deoxidisation is accomplished by the use of scavenging elements and fluxes. Fluxes shield the surface of the metal from exposure to atmospheric oxygen. Scavenging elements dissolve the oxides in the molten metal and become a part of the finished alloy. The Oxy/gas torch must be controlled to limit the amount of oxygen used to feed the gas flame and to maintain a reducing atmosphere. Fluxes used are usually borax (sodium tetraborate), melting point 760°C, or boric acid, melting point 870°C, and powdered or lump charcoal. Charcoal will form a carbon monoxide/dioxide gas cover to the melt. Boric acid is used as it melts easily and forms a protective layer on top of the metal. It also combines with the metallic oxides, thereby keeping the molten metal clean. Although there are many oxygen scavenging elements available, my preference is silicon added in the form of 10% silicon copper. The addition of silicon in this form is easily controlled to minute quantities. Silicon is an efficient scavenger. Minute additions are all that are necessary; indeed in my experience all scavenging agents are only required in minute amounts. |
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