hot top

A hot top is a refractory-lined open riser fitted to the top of a casting mold, used principally in ingot casting of steel and other alloys. As molten metal solidifies from the outside in, the interior shrinks; without intervention, this creates a large hollow cavity (a pipe or shrinkage cavity) that weakens the ingot and makes it unsuitable for rolling or forging. The hot top keeps the metal in the riser and the uppermost portion of the ingot molten for as long as possible, allowing the cavity to form there instead of deep inside the ingot, where it would be harder to remove.

The hot top consists of a steel shell lined with ceramic material, typically fireclay or magnesite brick, chosen for its insulation properties and chemical resistance to molten steel. The ceramic lining is thick enough to minimize heat loss through the walls; the entire assembly sits directly above the ingot mold. As the ingot solidifies beneath it, the hot top becomes a riser reservoir where metal can feed downward to compensate for shrinkage in the cooling ingot below.

How it performs in practice

The effectiveness of a hot top depends on maintaining adequate temperature in the riser metal and controlling the solidification rate of the ingot. Exothermic compounds, powders that release heat as they react, are often added to the surface of the molten metal in the hot top to keep it hot longer. Cast iron has sometimes been added as well. The hot top is left uncovered, exposed to the air, hence the term open riser. This contrasts with a blind riser, which is sealed or partially enclosed and relies primarily on the insulation of the mold itself.

Steel ingots cast in sizes above roughly 10 tons almost always use a hot top. Smaller ingots may rely on a blind riser or on no riser at all if the ingot geometry is favorable. The cavity that does form in the hot top is cropped off (removed by cutting) during ingot conditioning before the steel goes to the rolling mill, making the hot top a controlled sacrifice rather than a hidden defect. The method became standard practice in large-scale steel foundries because it dramatically improved ingot quality and reduced the proportion of material lost to defects.

The main variables are the height and diameter of the hot top relative to the ingot, the thickness of the ceramic lining, and the temperature of the incoming molten metal. A hot top that cools too quickly will fail to keep the riser metal molten; one that is too large wastes metal and floor space. Modern practice often combines a hot top with precise mold design and controlled casting speeds to ensure predictable solidification and cavity formation. The hot top itself is a reusable piece of equipment, reconditioned or relined between casts.