When the internal stress of gravity casting exceeds the strength of metal, the casting will produce cracks. Cracks are serious casting defects. We must try to prevent them. According to the temperature range of crack formation, it is divided into hot crack and cold crack, which are introduced in detail below.
1. Thermal cracking
It is generally formed at the end of solidification when the metal is at a high temperature near the solidus. Its shape is characterized by short crack, wide crack, tortuous shape, and oxidation color in the crack. Hot cracking can be caused by unreasonable casting structure, large alloy shrinkage, poor concession of mold (core) sand and unreasonable gravity casting process. Sulfur and phosphorus in steel and iron reduce the toughness of steel and iron and increase the tendency of hot cracking.
Reasonable adjustment of alloy composition, strict control of sulfur and phosphorus content in steel and iron, reasonable design of casting structure, adoption of the principle of simultaneous solidification and improvement of the yield of mold (core) sand are all effective measures to prevent hot cracking.
2. Cold crack
Cold crack is the sum of thermal stress and shrinkage stress when the casting is cooled to low temperature and in elastic state. If it is greater than the strength of the alloy at this temperature, cold crack will occur. The cold crack is formed at a lower temperature. The crack is small, continuous and linear, clean in the crack, and sometimes slightly oxidized. Castings with large wall thickness difference and complex shape, especially large and thin castings, are prone to cold cracking.
All measures to reduce the internal stress of gravity casting or reduce the brittleness of alloy can prevent the formation of cold crack. For example, phosphorus in steel and cast iron can significantly reduce the impact toughness of the alloy, increase brittleness and prone to cold cracking. Therefore, it should be strictly limited in metal smelting.
Editing: WMM