Effect of pouring temperature on casting
Defects easily caused by high-temperature pouring include burning sand, iron inclusion, shrinkage cavity, shrinkage porosity, hot crack, misfire, local oxidation, size unqualified, reactive porosity, and so on. The high pouring temperature can also cause the sand mold to expand, especially the gray iron casting with a complex sand core. When the pouring temperature ≥1420 °c, the waste products increase, and when the pouring temperature is 1460 °c, the waste products reach 50%. In production, the temperature of molten iron can be well controlled by induction furnace melting.
Possible defects at too low temperature
1)Manganese sulfide pore
This kind of blowhole is located below the surface of gray iron casting and often appears after machining. The diameter of the BLOWHOLE is about 2 ~ 6mm. Sometimes there is a little slag in the hole. Metallographic study shows that the defect is caused by MnS segregation and slag mixing, because of low pouring temperature and high Mn and s content in molten iron.
Such as content and appropriate MN content of 0.5% ~ 0.65% can significantly improve the purity of liquid iron, thus effectively prevent such defects.
2)Gas porosity in sand cores
Porosity and multi-porosity porosity is often caused by poor sand core venting. Because the core is usually hardened in the core box, this often leads to a lack of vent holes in the core. In order to form the vent hole, the core can be hardened after the additional drilling.
After processing, small holes with a diameter of 1 ~ 3mm are found under the surface of gray iron castings. In some cases, there are only 1 or 2 small holes. The metallographic study showed that these holes appeared with a small amount of liquid slag, but no s segregation was found there. The results show that the defects are related to the pouring temperature. When the pouring temperature is higher than 1380 °c, the defects are not found in the castings. It is worth mentioning that the gating system’s design has not been changed to eliminate the defect, which can be attributed to the low pouring temperature and the formation of molten iron in the micro-reduction atmosphere.
The most common cause of low pouring temperature is that the molten iron has been transported and stayed in the ladle for a long time before pouring. The heat loss can be significantly reduced by using a ladle cover with an insulating material.