Characteristics and remedy of pinhole defects in sand casting

Defect characteristics and detection methods

It is a small round or fly shaped hole of different sizes with needle tip on the casting, and its surface is smooth. Most of them are produced in the thick part of the post solidification casting. It was found in X-ray fluoroscopy, macroscopical examination and machining. On the X-ray film, it is a small black spot

Causes of formation
  1. Raw materials and return materials are not dried and contain moisture
  2. The crucible, ladle and tools for smelting are not dried, bringing in water
  3. The air humidity is high during smelting, and there is too much wind during pouring
  4. The water content of chloride and chlorine in the refining agent exceeds the standard
  5. The melting time is too long, the melting temperature is too high, the time from refining to pouring is too long, and it is inhaled and oxidized twice
  6. The water content in flux and coating exceeds the standard, or the coating is not dried
  7. The humidity of molding sand is too high
Prevention methods and remedial measures
  1. Before entering the furnace, the raw materials and return materials shall be blown with sand, and the oxides and dirt on the surface shall be removed and preheated to about 500 ℃ before entering the furnace for melting
  2. The crucible should be preheated to about 700 ℃ (dark red) before charging and melting
  3. The furnace charge with serious pinholes shall be remelted and refined and confirmed to be qualified before use
  4. Control the melting temperature not to exceed 750 ℃, the whole melting time not to exceed 4H (refer to start melting → pouring completion), the time of primary refining not to exceed 1H; the time of secondary refining not to exceed 1.5h
  5. Strictly control the water content of the refining agents such as chloride and chlorine not to exceed the standard (0.06% ~ 0.1%). When refining with chlorine, the refining temperature should be lower
  6. Control the pouring temperature of alloy liquid within the lower limit specified in the process specification
  7. When the absolute humidity of the air is high, in order to reduce pinholes, secondary refining can be carried out or the alloy can be melted with covering agent
  8. If chlorine salt is used for refining, it shall be dried thoroughly according to the regulations before use. After refining, keep the temperature for 5-8min, then remove the slag and pour. The refining temperature of alloy liquid is better than its melting point of 75-100 ℃
  9. Strictly control the water content of molding sand within the lower limit of the specified standard
  10. Materials with high heat capacity and thermal conductivity are used as mold materials to speed up the cooling speed of castings
  11. For large area castings without pinholes (such as refrigerator liner blister castings), tens to thousands of cold irons can be placed on the mold cavity to obtain the surface layer of fine grains without pinholes in rapid solidification
  12. If necessary, some important castings can be converted to metal mold casting or pressure casting