Characteristics and remedy of cold shut defect in sand casting

Defect characteristics and detection methods

The appearance is the striated or herringbone pits that are not closed by the alloy liquid, and there is oxide scale near them, which generally appears on the top wall of the casting, on the horizontal or vertical surface of the thin wall, at the junction of the thick and thin wall, on the wall surface of the thin rib or at the edges and corners, and at the places where the fillet of the outer edge of the casting is too small and the alloy liquid is not smooth. Most of them are found in macroscopic inspection, magnifying glass inspection, fluorescence inspection and X-ray inspection after sand blasting, corrosion and oxidation treatment.

Causes of formation
  1. The pouring temperature of the alloy liquid is too low and the fluidity is poor, so that the confluence of the two streams of liquid cannot be butted and fused together due to the thick oxide skin of the alloy liquid
  2. The design of pouring system is unreasonable, or the pouring position of casting is unreasonable, so that the confluence of alloy liquid is concentrated in the thin-walled part of casting
  3. The exhaust performance of the mold is not good, and the filling of the alloy liquid is blocked by the air cushion in the mold cavity and cannot be fused
  4. When pouring, the flow is cut off. The first filling alloy liquid has crusted, and the later entering alloy liquid cannot be fused with it
Prevention methods and remedial measures
  1. According to the structural shape of the casting and the characteristics of the alloy, properly increase the pouring temperature of the alloy liquid
  2. Control the water content of sand mold within 5.5%. The compactness of the sand mold shall be uniform, and the lower limit value shall be taken to prevent the gas, reoxidation, confluence and fusion of blocking liquid flow after pouring into the alloy liquid
  3. The sand core shall be dried, and the paste applied for repairing the core shall also be dried, so as to avoid gas and air cushion after the alloy liquid enters
  4. As far as possible, the core material with large gas generation shall not be used
  5. Put vent holes or gas risers on the sand mold or sand core where cold insulation and under casting are easy to occur to prevent the alloy liquid from being well fused due to the obstruction of air cushion pressure