Chromite sand is chosen for specific foundry applications due to its advantageous properties:
- 1.High Thermal Conductivity: This is its most significant advantage. Chromite sand draws heat away from the solidifying metal much faster than silica sand.
- •Benefit: Promotes directional solidification (from the casting inward to the risers), reduces the risk of shrinkage defects, and allows for finer grain structure in the cast metal, improving mechanical properties.
- 2.High Refractoriness: It has a very high melting point of approximately 2,150°C (3,902°F), which is significantly higher than silica sand (approx. 1,650°C / 3,002°F).
- •Benefit: Resists sintering and metal penetration when casting high-temperature alloys like steel, stainless steel, and high-alloy irons. It does not break down at these temperatures.
- 3.Low Thermal Expansion: Chromite sand has a very low, linear coefficient of thermal expansion (about 1/3rd that of silica sand).
- •Benefit: Dramatically reduces the occurrence of veining, finning, and rat-tailing defects caused by the expansion of the mold wall during metal pouring. This leads to better surface finish and fewer casting repairs.
- 4.Basic pH (Chemical Neutrality): It is chemically basic, which makes it resistant to basic slags. This is a key advantage over acidic silica sand when using basic slag-forming binders.
- •Benefit: Reduces burn-on and chemical reactions between the mold and the molten metal, resulting in cleaner castings that are easier to clean.
- 5.High Density / Bulk Density: It is much denser and heavier than silica sand.
- •Benefit: Provides excellent mold and core stability, reducing the risk of mold wall movement or collapse, especially for large cores. However, this also means molds are heavier.
How to Use Chromite Sand in the Foundry
Chromite sand is rarely used to make an entire mold. Due to its higher cost, it is typically used in one of two ways:
- 1.Facing Sand: The most common method. A layer of chromite sand (usually 1-2 inches / 25-50 mm thick) is placed against the pattern to form the mold cavity surface. The backing sand behind it is a cheaper, standard silica sand. This gives all the benefits at the metal-mold interface without the cost of a full chromite mold.
- 2.Core Sand: Used for making cores, especially those that are complex, thin, or are surrounded by a large volume of hot metal (e.g., cylinder head water jacket cores). Its high refractoriness and stability prevent core failure.