How is the pitting of investment castings formed?

There are many concentrated annular shallow spots on the surface of the casting, which are called pitted defects. Pits generally appear on aluminum alloy die castings. Sometimes at a part of thick cross section, sometimes on the surface of all precision EPC castings. The aperture of shallow dent is 0.3-0.8mm, and the height is 0.3-0.5mm. The dents before pretreatment are filled with slag chemicals. The pitting significantly reduces the anti rust treatment ability and temperature resistance of stainless steel plate, and the surface to be polished cannot reach the bright and consistent level.

Cause: After analyzing the metal composition and slag composition of the defect, it is believed that the pitting is caused by the chemical change between the metal oxide in the metal liquid and the oxide in the shell material. After quantitative analysis, the silicon content in the metal at the disadvantage is increased, while the manganese content is very little. Petrographic analysis of the slag shows that the slag contains chemical substances such as magnesium iron chloride, magnesium manganese chloride and cobalt silicate. Due to the change in the composition of the defect location, the rust prevention treatment and temperature resistance of this point are significantly reduced.

Countermeasures to avoid:

1: Shell refractory insulation materials shall be effectively used. Neutral refractory and thermal insulation materials, such as corundum powder and fine sand, shall be used as the overall surface when making EPC castings, and acid alkali refractory and thermal insulation material silica sand powder shall not be used.

2. EPC manufacturers should avoid reducing metal air oxidation. Rapid smelting shall be selected as far as possible to reduce metal air oxidation. If the smelting and pouring temperatures are properly reduced, the molten steel will be further deammoniated. Pour in reducing atmosphere. If necessary, vacuum melting and pouring shall be selected.

3: Moderately increase the shell sintering temperature and pouring temperature to properly reduce the pouring temperature of molten metal. Improve the gating system software to alleviate the thermal effect of molten metal on the mold shell