Causes of pitting and preventive measures

There are many dense round shallow spots on the surface of the casting, which are called pitting defects. Pockmarks usually appear on stainless steel castings. Sometimes on the local thick section, sometimes on the entire lost wax casting surface. The diameter of the shallow depression is 0.3-0.8mm, and the depth is 0.3-0.5mm. The pits were filled with slag material before cleaning. Pockmarks significantly reduce the rust resistance and heat resistance of stainless steel, and the surface to be polished cannot achieve the same degree of brightness.

Cause: After analyzing the metal composition and slag composition at the defect, it is believed that the pitting is formed by the chemical reaction between the oxide in the molten metal and the oxide in the shell material. According to spectroscopic analysis, the silicon content in the metal of the defect increased, while the manganese content was very small. The petrographic analysis of the slag shows that the slag contains compounds such as iron silicate, manganese silicate and cobalt silicate. Because the chemical composition of the defect site has changed, the rust prevention and heat resistance of the defect site have decreased significantly.

• 　　 Preventive measures: 1: Reasonably select refractory materials for the shell. When making stainless steel lost wax casting, neutral refractory materials such as corundum silt should be used as the surface layer, and acid refractory silica sand powder should not be used.
  2: Prevent and reduce metal oxidation. Try to use fast smelting to reduce metal oxidation. Another example is to appropriately lower the melting and pouring temperature, and fully deoxidize the molten steel. Pouring in a reducing atmosphere. Use vacuum melting and pouring when necessary.
  3: Appropriately increase the baking temperature and pouring temperature of the shell to appropriately reduce the molten metal pouring temperature. Improve the gating system to reduce the thermal effect of molten metal on the shell

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