Causes and Prevention of Casting Cracks (Hot/Cold Cracks)

Casting cracks are the most common fatal defects in casting production, mainly divided into two categories: hot cracking and cold cracking. There are significant differences in temperature, causes, and morphology between the two, and if not prevented and treated in a timely manner, it can directly lead to the scrapping of castings. Mastering the formation laws of two types of cracks is the key to improving the qualification rate of castings.

1、 Causes and characteristics of thermal cracking

Hot cracking occurs during the high-temperature solidification stage of castings, where the temperature approaches the solidus line. The main cause is that when the metal liquid solidifies and shrinks, it is mechanically hindered by the sand mold and core, resulting in tensile stress inside, exceeding the plastic limit of the metal at high temperatures. In addition, unreasonable alloy composition, excessive sulfur and phosphorus impurities, high pouring temperature, uneven casting wall thickness, and small rounded corners can all exacerbate hot cracking. Hot crack patterns are rough and tortuous in shape, often appearing in thick parts of castings, corners, and near gates.

2、 Causes and characteristics of cold cracking

Cold cracking occurs in the low-temperature stage after the casting has completely solidified. The main reasons are uneven cooling of castings, excessive residual stress, deterioration of metal plasticity at low temperatures, and fracture at stress concentration sites. High hardness of sand molds, early or late opening of boxes, large differences in casting wall thickness, and too fast cooling rate can all induce cold cracking. Cold crack patterns are flat and straight, with clean cross-sections, often appearing at the corners and thin-walled transitions of castings.

3、 Comprehensive prevention and control measures

For hot cracking, it is necessary to control the pouring temperature reasonably, avoid overheating, optimize the casting structure, increase the corner radius, and eliminate sudden wall thickness changes. Strictly control impurities in raw materials, optimize the pouring system, ensure uniform solidification of castings, and reduce shrinkage resistance. To address cold cracking, it is necessary to slow down the overall cooling rate, avoid rapid cooling, control the opening time reasonably, and prevent forced demolding. Simultaneously optimizing the molding process, reducing sand mold resistance, releasing casting residual stress through aging treatment, reducing crack generation from the root, and ensuring stable casting quality.