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. Cracks can be divided into hot cracks and cold cracks according to the temperature range of formation, 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, zigzag shape, and oxidation color in the crack. Hot cracking can be caused by unreasonable casting structure, large alloy shrinkage, poor core sand yield 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 contents 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 the thermal stress and shrinkage stress when the casting is cooled to a low temperature and in an elastic state. If it is greater than the strength of the alloy at that temperature, cold crack will occur. The cold crack is formed at a lower temperature. The crack is fine, continuous and linear, clean inside, 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 the brittleness of alloys can prevent the formation of cold cracks. 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.
