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Casting answers & advice.

Q We have seen some metalcasting facilities adding lime to the charge in the melting process as a flux to keep the molten metal liquid and improve flowability. We have little information on this process. Can you help us better understand how it may affect our furnace refractory?

A In cupola melting of iron, fluxes are added to the furnaces to form a fluid slag which may easily be removed from the surface of the liquid metal. The slag is made up of metal oxides, such as Si[O.sub.2], [Al.sub.2][O.sub.3], MgO and FeO. The presence of a fluid slag covering the metal also may prevent further oxidation of the molten metal. Every cupola adds limestone in the form of calcium carbonate or dolomite, a calcium-magnesium carbonate where it readily converts to Cad (also known as lime), which reacts with the other metal oxides and lowers the viscosity of the slag. Low viscosity slags that contain Cad also can remove sulfur and phosphorous from the liquid iron. Other fluxes also may be used instead of limestone, such as fluorspar (calcium fluoride), soda ash (sodium carbonate), borax (sodium borate) and calcium carbide, but these additions are the rare exception today. Lime does not effect the metal; its effects are on the slag.

The Effects of Lime

Using lime as flux is helpful in cupola melting to help move the slag out of the furnace. But in induction melt operations, it can have a negative effect on the furnace silica refractory. Lime additions can be particularly hard on induction melting or holding furnace refractories. Although it will create a more fluid and viscous slag, the calcium is very hard on the silica refractory and will shorten the lining life in both the melting and holding furnaces.

When using any flux, remember to use as little as necessary to obtain a fluid slag. If the slag composition is not managed, its fluidity will likely be insufficient at the prevailing temperature to flow through the tap hole. Too low an addition will produce a viscous slag. Too high an addition will cause excessive attack on the refractory lining. In addition, correct additions of flux will prevent the build-up of slag on the walls of the furnace. Adding flux to a furnace with heavy build-up on the walls may actually remove the build-up by dissolving it into slag.

For more information on using fluxing materials, see the AFS Cupola Handbook or visit the AFS library at www.afslibrary.com.

Q We are starting to use chromite sand in our furan nobake core room. What is a good tensile strength, typical amount of fines, recommended binder level and recommended binder and acid ratio?

A When converting to chromite sand, a metalcasting facility should start by using a binder percentage and binder acid ratio comparable to what is currently used for silica sand. Most silica sand systems have addition levels at 1-1.5% binder (based on sand weight) and 25-30% catalyst (based on resin weight). The binder level and ratio depend on a number of factors, such as the surface area of the sand (which in turn depends on the grain shape and fines), the strength that is needed for the core, mold strength, and production requirements including how fast the operation requires the core or mold to set up.

For nominal 50 AFS silica sand and nominal 50 AFS chromite sand with similar surface areas, the heavier weight of the chromite sand would lead to a difference in density--typically a ratio of about 1:1.6. One ton of chromite sand has a volume approximately 35% less than 1 ton of silica sand. If both sands have similar sub-angular grain shapes, for chromite sand to achieve the same strengths as silica sand, the binder and catalyst levels should be reduced by 35%. In practice, this reduction in binder additions is not only affected by sand surface area and grain shape but also by mixer efficiency when additions are low. Differences in acid and demand levels also can affect catalyst additions. Given these variables, it is normal for casting facilities to reduce additions by 30% when compared to silica sand to achieve similar strengths.

The strength will be directly related to how much binder is used and usually depends on the core geometry and configurations, as well as casting-conditions such as storage, humidity, shakeout, etc. Using less binder in molds and more with cores is typical. Furan systems generally produce higher tensile strengths in comparison to other nobake systems (300-400 psi after 24 hours) but take longer to set up or reach their ultimate strength. The AFS grain fineness number for chromite sand and the amount of fines are dependent on sand grade. For consistent performance in the sand system, the chromite sand should be close to the silica it replaces in all sand values since low silica and acid demand levels are important to performance.

Answers provided by AFS Molding Methods & Materials Division 4.
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Publication:Modern Casting
Date:Oct 1, 2009
Words:834
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