I guess the shrinkage cavity of carbon steel mentioned here is aimed at the defect of steel castings-shrinkage cavity, which is repaired by welding. Steel castings are also divided into low carbon, medium carbon and high carbon. To repair low carbon steel castings, the surface of shrinkage cavity defects should be trimmed to white at first. The selection of covered electrode is the same as that of ordinary carbon steel, and no special measures are needed in the process. Welding repair of low carbon steel castings is relatively easy to complete. It is difficult to repair medium carbon steel castings and high carbon steel castings, especially high carbon steel castings. Because of their high carbon equivalent content and high material strength, quenching structures are easy to occur during welding and heating, and cracks and cracks appear in the weld and its vicinity under the action of welding internal stress. However, when necessary, special measures can be taken in the selection of covered electrode and welding process, such as selecting special covered electrode with good toughness, such as stainless steel covered electrode, cleaning the repair welding range, degreasing, dewatering and derusting, slightly higher and wider preheating temperature and range, controlling welding current as small as possible, continuous welding and slow cooling after welding. Especially when the first few layers are welded, if cracks are found in the weld, the cracks in the previous layer should be fully welded when the latter layer is welded. After welding, you can try to beat the weld metal with a hammer, so that the crystals of the weld metal are refined and attached to each other, which is beneficial to crack resistance. This method is witnessed by the author when welding small closed round stainless steel in large rigid structures. Of course, it should be cooled slowly after welding. In short, due to the high carbon equivalent, welding cracks should be taken as the key analysis and research object to avoid the formation of brittle quenching structure and take corresponding measures.