Cold crack refers to the crack caused by the local casting stress greater than the ultimate strength of the alloy when the steel casting is cooled to the elastic state after solidification. Combined with the production characteristics of the hook tongue of the hook body, the cold crack is mainly caused by the collision and extrusion during the machining processes such as sand falling, cleaning, cutting, pouring and riser, and the residual stress causes the cracking of steel castings. For steel castings, the pouring and riser are usually cut by gas cutting, including oxyacetylene flame gas cutting, mechanical gas cutting, etc.
Casting stress and gas cutting thermal stress are intertwined, which is likely to produce cracks. Steel castings with large riser and good hardenability are easy to produce microcracks in the heat affected zone during gas cutting. The cutting pouring and riser process needs to be strictly controlled, and local preheating treatment should be carried out when necessary. In addition, the welding cracks during the welding repair of parts are mainly related to improper welding repair operation, including insufficient preheating before welding, too fast cooling after welding, etc.
The occurrence of pore defects mainly lies in that the water content of molding sand is too high, the flow is interrupted or too fast in the pouring process, the entrained gas can not be fully eliminated, and it is retained in the near upper plane of the hook body. Finally, large pores are often formed, which need to be removed and repaired by welding. The main reason for the sand inclusion defect is that the mold (core) sand on the upper part of the part is heated by the liquid steel for a long time, moisture migration, reduced strength, local camber and cracking on the surface, and the non solidified liquid steel is drilled into the gap to form the sand inclusion defect. If the sand inclusion defect is serious, it needs to be cleaned and removed and repaired by welding, which is time-consuming and labor-consuming. Mechanical sand sticking is mainly caused by the large thermal impact of liquid steel on the surface of the sand core at the sensitive part, poor coating quality or uneven coating thickness on the surface of the sand core. During pouring, the resin film between sand particles is gasified to form a capillary channel. The liquid steel infiltrates into the channel under the action of metal static pressure, vapor pressure and surface tension to form mechanical sand sticking. It needs to be cleaned, and pits appear on the surface, which increases the cost.