Iron Casting Application
This fine, powdered coal has been used to improve the surface finish of iron and copper castings since the 1870s. Used in greensand and dry sand moulding, coal dust protects the mould surface against the action of molten metal. When the metal comes into contact with a mould surface containing coal dust, a gaseous envelope is formed, which resists the fusion of sand to metal. Additives like pitch, cellulose, and silica flour are used, depending on the metal cast.
- Castings cool more slowly, preventing cracking
- Coal dust increases both greensand and dry sand strength
- Reduces expansion, scabbing, and metal penetration
- Prevents sand burn
- Imparts a cleaner finish
- Aids in separation at knockout
- Lessens rat tailing and other expansion defects
- Coal dust contains low concentrations of trace metals, PAH, and sulphates.
- Standard and Low-Ash formulations suitable for foundry use
African Pegmatite has two grades of Coal Dust available.
- 100 Micron
- 23%-25% Volatile Matter
- 14%-20% Ash
- 5% Moisture
- 100 Microns
- 34%-40% Volatile Matter
- 10%-11% Ash
- 5% Moisture
- 6%-1% Sulphur
The quality of coal dust that’s suitable for foundry use should consist of finely crushed bituminous coal that’s free of foreign material.
Contamination of sulphur by the metal at the mould face can cause defects. Sulphur content of foundry coal dust should never be more than 10%.
Optimum levels of Coal Dust should be established by trial and always weighed and metered.
Appropriate proportion by weight of Coal Dust in greensand varies from 2%-3% for small castings and from 7%-8% for heavy work. Higher levels are sometimes used in special cases.
Effects of too much Coal Dust:
- Grazed surfaces
- Gas holes
- Rounded edges
A build-up of coal dust in sand can cause a blue skin on the casting, a decrease in permeability, and an increase in moisture requirement of the sand.
Coal dust is an essential component of iron casting. To put it simply, iron casting involves melting the iron into a liquid state and pouring it into a mould. The mould is removed when the metal solidifies in the desired shape. Other metals employ coal dust in their manufacturing processes, but this page will focus on the iron casting application.
Once the iron is in a molten state, degassers are used to eliminate contaminants that can cause various defects during the casting process. Carbon monoxide is a common degasser for iron casting. If degassing does not eliminate enough of the porosity of the iron, vacuum impregnation may be used to seal pores and leak paths.
Patterns can be made for the moulds from wood, plastic, metal, or wax. When the mould is ready and the metal is molten, the liquid iron is poured into the mould. Today, pouring is usually handled by robots and pouring machines, but some mould might require the use of vacuum or pressurised gases.
The iron is then allowed to solidify inside the mould to retain the shape of the mould cavity. The solid figure is then removed from the mould through techniques known as shaking and tumbling. Once the casting is removed from the mould, heads, runners, gates, and risers are removed from the casting using torches, ceramic cut-off blades, or bandsaws.
The Role of Coal Dust in Iron Casting
Green sand containing coal dust is used for sand casting of ferrous materials like iron and steel. There should never be more than 5% coal dust in green sand because coal dust will undergo combustion when exposed to molten metal and release an off-gassing vapor. Green sand should never be used in the casting of non-ferrous metals because the coal dust will release carbon monoxide, which will cause oxidation of the non-ferrous metals.
Coal dust increases the refractoriness of green sand so that it can withstand the high temperatures required during iron casting. The fusion temperature of coal dust is upwards of 1600° C. The fine particle size of coal dust results in an improved surface finish on the final product.
When the coal dust releases carbon monoxide when exposed to the molten iron, the carbon monoxide acts as a degasser. This eliminates gaseous contaminants within the mould cavity that can cause casting defects and decreases the porosity of the iron. The coal dust also increases the collapsibility, or the ability to be cleanly stripped away from the solidified metal, of the green sand. The coal dust burns out during casting and makes the mould much easier to remove during knockout.
It is recommended that foundry coal dust contain at least 30% volatile matter to achieve a desirable surface finish. The ash content should not exceed 12% and the sulphur content should never be about 1% or casting defects are likely to occur. Chlorine can also cause casting defects and should be kept to a bare minimum.
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