Cracking of ship inspection castings, i.e. thermal cracking, is mainly caused by the following reasons:

After the liquid metal is poured into the mold, the heat loss is mainly through the mold wall, so the solidification always starts from the surface of the casting. When a large number of dendrites appear in the later stage of solidification and overlap into a complete skeleton, solid-state shrinkage begins to occur. However, there is still a layer of liquid metal film (liquid film) between the dendrites at this time. If the shrinkage of the casting is not hindered, the dendrite skeleton can shrink freely without any force. When the shrinkage of dendrite skeleton is hindered by sand mold or sand core, tensile stress will be generated if it cannot shrink freely.

When the tensile stress exceeds the material strength limit, the dendrites will crack. If the dendrite skeleton is pulled apart slowly, and there is enough molten metal around the pulled part to flow into the tensile crack in time and supplement, then the casting will not produce hot cracks. On the contrary, if the crack is not supplemented by molten metal, hot cracks will appear in the casting.

Cracking of ship inspection castings is generally caused by the following reasons:

1. The wall thickness of the casting is uneven, and the forming shrinkage at the junction of thin wall and thick wall and the turning point is blocked.

2. The mold temperature and pouring temperature are low, resulting in poor filling, resulting in incomplete fusion of the metal matrix, insufficient strength after solidification, and easy to crack, especially at the position far from the ingate.

3. The mold retention time of the casting is too long, which makes the grain of the casting coarse.

4. The casting alloy has too high impurity content, which reduces the plasticity of the alloy, especially the iron content is too high or the silicon content is too low.

5. The push rod is not configured properly or the ejection force is not uniform, so that each part of the die casting cannot be demoulded at the same time. During the demoulding process, the demoulding inclination will be caused and the die casting will crack.

6. The strength of the molded parts is not enough, especially the slender core, which will twist and deform during filling, hindering the smooth demoulding of the casting.

7. There are side pits on the surface of the molded parts along the demoulding direction, or because the demoulding slope of the molded parts is too small and the surface grinding is not sufficient, the demoulding resistance increases, and the die casting parts are torn under the larger demoulding resistance.

Measures to prevent the occurrence of hot cracks in ship inspection castings:

1. Improve the casting structure: the wall thickness shall be uniform, and the transition fillet shall be made at the corner to reduce the stress concentration. The spokes of wheel castings can be bent if necessary.

2. Improve the melting quality of alloy materials: adopt refining and degassing processes to remove oxidation inclusions and gases in molten metal. Control the content of harmful impurities and adopt reasonable smelting process to prevent cold cracks.

3. Adopting correct casting process measures: simultaneous solidification of castings is not only helpful to prevent hot cracks, but also helpful to prevent cold cracks. The position and size of the pouring riser shall be reasonably set to make the cooling speed of each part of the casting as uniform as possible and reduce the tendency of cold cracks.

4. Aging heat treatment: Castings with large casting stress shall be subject to aging heat treatment in time to avoid cold cracks caused by excessive residual stress. If necessary, the casting shall undergo an aging heat treatment after cutting the pouring riser or welding repair.