Why Chromite Sand
Chromite sand has been selected and used in foundries as a result of its unique combination of properties. The properties listed below generally result in a better surface finish and reduced likelihood of casting defects. Scroll down to learn more on each property
High thermal conductivity
The Thermal Conductivity of chromite sand is higher than the other commercially available foundry moulding sands. This results in a mild chilling of the surface and a thicker solidified skin. This thicker skin improves the casting surface and reduces the likelihood of casting defects. Generally, foundries use Chromite Sand for one of the following reasons:
- to promote directional solidification (particularly in smaller/thinner-walled castings) when positioned in specific areas,
- to position around steel chills to significantly reduce the likelihood of cracking around the chills, or
- in the manufacturing of larger castings, to use chromite sand as the moulding material to promote a thicker solidifying skin as the molten metal rises up the mould face. This reduces the risk of it being washed away which allows faster filling times. This high heat abstraction rate also aids in inhibiting penetration and burn on defects.
Solidification time of a 150mm sphere with various mold materials
| Mold Material | Mold Material |
|---|---|
| Copper chill | 4.2 |
| Steel chill | 4.3 |
| Graphite mold | 5.1 |
| Chromite sand mold | 13.4 |
| Zircon sand mold | 13.8 |
| Olivine sand mold | 15.8 |
| Silica sand mold | 17 |
Low thermal expansion
Silica Sand undergoes an alpha-beta phase transformation at approx 350oC – 650oC. This corresponds to the temperature zone that the mould experiences during cooling. This can lead to expansion defects such as veining or cracking in castings where silica sand moulds are used
Chromite has a much lower expansion over the same temperature range. In addition, the Chromite expansion does not have the same stepped expansion; therefore the likelihood of these expansion type defects is significantly reduced.
Lower thermal expansion generally results in improved dimensional stability in finished castings.
Wetability resistance to molten steel
Chromite is used as it has the ability to resist metal penetration by molten steel and oxides in the metal. This is mainly due to the chilling ability of chromite to make the oxides less fluid when in contact with the mould
No-one wants to see these defects!
Basic to neutral pH
The basic to neutral pH of chromite significantly reduces the likelihood of reaction defects. These are most often seen when the acidic nature of the silica sand moulds react with the basic nature of manganese steel castings.
Maintaining a consistent pH is also critical as some modern binder systems cannot cope with a pH that is too high or variable.
High Fusion Point
With our reserves chrome content averaging 47.1%, we expect to see a fusion point at approximately 1850°C (or 3362°F)
Broad sieve distribution
Our standard material is a 4-sieve distribution. This means that 4 sieves contain more than 10% retained. Our current material has more than 90% on the ASTM 40-100 sieves. A typical sieve distribution is shown below.
This has been shown through research to provide an excellent packing density which reduces the binder requirements and improves casting quality. Our processing plant allows us to provide a range of different distributions and sizings according to individual foundry’s requirements.
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