In the production and casting process of nodular iron castings, in addition to general casting defects, There are also some unique defects, such as shrinkage porosity, slag inclusion, subcutaneous porosity, poor spheroidization and deterioration. these defects affect the properties of castings and increase the rejection rate of castings. In order to prevent the occurrence of these defects in nodular iron castings, it is necessary to analyze them, summarize various influencing factors and put forward preventive measures, so as to effectively reduce the occurrence of defects and improve the mechanical properties and production efficiency of castings.
Before the effective preventive measures are put forward, it is necessary to understand the causes of shrinkage cavity and porosity in nodular iron castings. First of all, it is certainly related to the carbon content.
Increasing the carbon content, increasing the graphitization expansion, can reduce the shrinkage porosity. In addition, the increase of carbon content can also improve the fluidity of ductile iron, which is conducive to feeding.
During the production of nodular iron castings, if the phosphorus content in the molten iron is too high, the solidification range will be enlarged, and the low melting point phosphorus eutectic will not be replenished during the post solidification, and the casting shell will be weakened, so there is a tendency to increase the shrinkage cavity and porosity, so the phosphorus content in the molten iron should be strictly controlled to make it less than 0.08%.
If the residual amount of rare earth is too high, the graphite shape will be deteriorated and the spheroidization rate will be reduced, so the content of rare earth should not be too high.
Magnesium is a strong stable carbide element, which hinders graphitization. It can be seen that the amount of residual magnesium and rare earth will increase the white tendency of ductile iron and decrease the graphite expansion, so when their content is high, it will increase the tendency of shrinkage cavity and porosity.
When the hard shell is formed on the surface of nodular iron castings, the higher the temperature of liquid metal and the larger the liquid shrinkage, the volume of shrinkage cavity and shrinkage porosity not only increases, but also its relative value. In addition, if the change of wall thickness is too sudden, the lsolated thick section will not be fed, which will increase the tendency of shrinkage porosity. In addition, it is related to pouring temperature, compactness of sand mold, pouring riser and cold iron, which should be adjusted from here.