hot tear

A hot tear is a fracture that appears in a metal casting while it is still cooling through the solid-liquid transition zone, typically between 1000 and 1400 degrees Celsius depending on the alloy. The crack forms because the metal shrinks as it cools, but the mold or surrounding solidified metal prevents free contraction. This generates tensile stress that exceeds the tensile strength of the partially solid material, which at that temperature is weak and unable to deform plastically without breaking.

Hot tears most commonly appear in aluminum, magnesium, and steel castings. They follow the grain boundaries or run through dendrite arms in the last regions to solidify. The fracture surfaces often show a jagged, branching pattern. In sand castings, hot tears may be visible on the surface, or they may be internal, revealed only during machining or X-ray inspection. The defect is distinct from cold cracking, which occurs after the casting has cooled to near ambient temperature.

Hot tearing is driven by three factors working together: the coefficient of thermal contraction of the alloy, the degree to which the mold or surrounding metal restrains shrinkage, and the strength and ductility of the semi-solid material. Alloys with wide freezing ranges, such as aluminum-copper or aluminum-silicon-copper systems, are more prone to hot tearing than those that solidify quickly over a narrow range. Certain elements, including sulfur, phosphorus, and iron, increase susceptibility in steels and aluminum alloys.

Prevention relies on mold design and process control. Reducing mold stiffness allows modest shrinkage movement. Tapered or stepped sections ease stress concentration. Removing hot spots through chilling and careful gating directs solidification to proceed from thin sections toward thick ones, leaving the strongest metal to endure final contraction strains. Alloy composition adjustments, such as grain refinement or reduced impurity levels, improve ductility in the semi-solid state. Preheating the mold reduces temperature gradients and thermal shock.

The term is literal and descriptive: the tear occurs while the metal is hot, during the transition from liquid to solid. It is distinguished from other casting defects such as shrinkage cavities, porosity, and segregation, each of which requires different corrective measures. Hot tearing has plagued foundries since the earliest days of metal casting and remains a persistent quality issue in complex, thick-walled, or highly restrained castings.