Features: the surface of the casting has straight or curved cracks. There are two kinds of cracks: hot crack and cold crack. Most of the hot cracks are zigzag and irregular, and the surface of the fracture is light black with deep oxidation color. The crack of cold crack is relatively straight, the surface of fracture surface of casting has metallic luster and is relatively clean, sometimes there is slight oxidation color.
The main reason of casting crack is that the internal stress is produced when the cooling solidification shrinkage is blocked. When the internal stress is greater than the strength of the metal material, the casting cracks and forms cracks.
(1) Strictly control the chemical composition of the molten iron. Among them, sulfur energy causes “hot brittleness” and hot cracking of castings. Therefore, w (s) in grey cast iron should be lower than 0.12%, but not too low (not lower than 0.05%). If sulfur is too low, the inoculation effect will be affected. The most suitable w (s) is 0.05% – 0.12%. Phosphorus energy causes “cold brittleness” and cold cracking of castings, so w (P) in gray iron is better than 0.15%, and w (P) in nodular iron is better than 0.08%.
(2) Adjust the cooling speed of each part of the casting to avoid local overheating of the casting. Place cold iron on the thick section or hot spot of the casting, and properly disperse the internal sprue to make the temperature of each part of the casting tend to be uniform.
(3) After the casting is poured, do not open the mold too early, do not spray the high temperature casting with cold water, and the appropriate opening time is when the temperature of the casting in the mold is lower than 600 ℃.
(4) When conditions permit, change the structure of the casting to prevent the casting from cracking. If stiffeners are set, the intersection of the two sections is changed from right angle to round corner to reduce stress concentration.