Question

The capability of moulding sand to withstand higher temperatures of the molten metal, is called:

• A. Refractoriness
• B. Green strength
• C. Permeability
• D. Collapsibility

Hint:

Think of "Refractoriness" as "Resistance" to heat. It is the sand's way of saying, "I won't melt, even when it gets incredibly hot!"

Answer 1

The Correct Option is A

Moulding sand must possess specific physical and chemical properties to produce high-quality castings. Because it comes into direct contact with molten metal, its thermal stability is paramount. 1. Understanding Refractoriness:

Refractoriness is the property of a moulding material (primarily the silica sand and the clay binder) that allows it to withstand high temperatures without melting, fusing, or undergoing chemical breakdown.

2. Why Refractoriness is Essential: When molten metal (often exceeding $1500^{\circ}\text{C}$ for steel) is poured into a sand mould:

• If the sand has low refractoriness, it will fuse to the surface of the metal, causing a defect known as "burn-on" or "sand fusion."

• Fused sand is extremely difficult to remove during the cleaning (fettling) process and damages the surface finish of the casting.

3. Factors Influencing Refractoriness: The refractoriness of a sand mixture depends on the purity of the silica sand and the type of clay used. Impurities like iron oxide or magnesium can lower the fusion point. Pure silica has a high fusion point (approx $1710^{\circ}\text{C}$), making it an excellent refractory material for most industrial metals.

4. Other Sand Properties:

• Green Strength: The strength of the sand when it is still moist (tempered with water).

• Permeability: The ability of the sand to allow gases and steam to escape during pouring.

• Collapsibility: The ability of the sand mould to break apart easily after the metal has solidified to allow for contraction.