Hot crack is a kind of crack defect that occurs at high temperature during the formation of gray iron castings. The cracks that can be observed on the surface of gray iron castings are called external cracks, and the cracks hidden inside the castings are called internal cracks. Hot cracking is one of the common casting defects of iron castings, especially gray iron castings, but iron castings and some light alloy castings. What causes hot cracking?
- The hot crack develops along the grain boundary, and its shape is zigzag and irregular.
- The surface of the external fracture is oxidized. The external fracture of gray is black, and the crack of aluminum alloy is dark gray, which indicates that the crack is formed and oxidized under high temperature. Because the internal crack occurs in the casting, the oxidation is not obvious, and there are often dendrites.
- Another feature of the outer crack is that the surface is wide and the inner is narrow. The crack extends from the surface to the inside of the casting, sometimes the crack penetrates the whole section, forming a penetrating crack; the shape of the internal crack is the width of the casting center, the closer to the casting surface, the narrower.
- Most of the hot cracking occurred in the pyrolysis part with slow solidification. The external crack often occurs at the corner of gray iron casting, the abrupt change of section thickness, the cross joint of wall and wall, the root of riser, between two external cold irons and the fat part that can produce stress concentration; the internal crack occurs near the center of post solidification inside the casting.
In the total number of scrap parts of gray iron castings to be repaired, hot cracks account for 20%. Because gray iron castings are alloy castings with complex structure and big wall thickness difference, the hot crack rate is higher, so we should pay attention to the occurrence and prevention of hot cracks.