During silica sol precision casting, there are many dense shallow dots on the surface of the casting, which are called pitting defects. Pitting usually occurs on stainless steel castings. Sometimes in local thick sections, sometimes on the entire surface of the casting. The shallow pit has a diameter of 0.3-0.8mm and a depth of 0.3-0.5mm. The uncleaned pit is filled with slag. Pitting significantly reduces the rust and heat resistance of stainless steel, and the polished surface cannot achieve the same level of brightness.
Cause of formation: By analyzing the metal and slag components of the defect, it is believed that the spot is formed by a chemical reaction between the oxide in the liquid metal and the oxide in the mold shell material. Through spectral analysis, the silicon content in the defective metal increases, while the manganese content is low. The petrographic analysis of slag indicates that it contains iron silicate, manganese silicate, and cobalt silicate. Due to changes in the chemical composition of the defective part, the rust resistance and heat resistance of the area have significantly decreased.
Preventive measures for silica sol precision casting:
1: Reasonably select refractory materials for the outer shell. When making stainless steel castings, neutral refractory materials such as corundum powder should be used as the surface layer, and acidic refractory silica sand powder should not be used.
2: Prevent and reduce metal oxidation. Try to use rapid melting to reduce metal oxidation. If the melting and pouring temperatures are appropriately reduced, the molten steel can be fully deoxidized. Pouring is carried out in a reducing atmosphere. Vacuum melting and casting can be used if necessary.
3: Appropriately increase the roasting temperature and casting temperature of the shell, and appropriately reduce the casting temperature of the molten metal. Improved the pouring system and reduced the thermal effect of metal liquid on the mold shell.