There are many theoretical and practical reasons for the formation of hot cracks in marine castings, but the fundamental reason is the solidification mode of marine castings and the thermal stress and shrinkage stress of castings during solidification. 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 marine 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. It can be seen from this that the alloys with wide solidification temperature range and paste or sponge network solidification mode are prone to hot cracking.

With the narrowing of the solidification temperature range, the hot cracking tendency of the alloy becomes smaller, and the eutectic alloy solidified at constant temperature is not easy to form hot cracking. The hot crack is formed in the solidification period of the casting, but it does not mean that the hot crack will occur when the casting solidifies. It mainly depends on the thermal stress and shrinkage stress of the casting during solidification. The thermal stress in the solid grain skeleton in the solidification area of the casting is easy to cause hot cracking or subcutaneous hot cracking of the casting; The shrinkage stress caused by external obstacles is the main condition for hot cracking of castings.

The linear shrinkage of the casting shell in the solidified state is hindered by external factors such as sand core, molding sand, friction between the casting surface and the sand mold surface, and there will be shrinkage stress (tensile stress) in the shell. The thin shell formed by the hot joint of the casting, especially the sharp corner at the hot joint, becomes the place where the shrinkage stress is concentrated, and the casting is easy to produce hot cracks in these places.

The causes of hot cracks are reflected in the process and casting structure, including: the wall thickness of marine castings is not uniform, and the internal angle is too small; There are too many bifurcations at the overlapping parts, and the outer frame and rib plate of the casting hinder the normal shrinkage of the casting; The gating and riser system hinders the normal shrinkage of the casting, for example, the gating and riser are close to the box belt or the molding sand strength between the gating and riser is very high, which limits the free shrinkage of the casting; The riser is too small or too large; The linear shrinkage of the alloy is too large; The forming elements of low melting point phase in the alloy exceed the standard, and the content of sulfur and phosphorus in cast steel and cast iron is high; The casting is unpacked too early and cooled too fast.