In the production and casting process of ductile iron castings, in addition to general casting defects, there are also some unique defects, such as shrinkage porosity, slag inclusion, subcutaneous pores, poor spheroidization, and recession. These defects affect the performance of castings and increase the scrap rate of castings.
In order to prevent the occurrence of these defects in ductile iron castings, it is necessary to analyze them, summarize various influencing factors, and propose preventive measures in order to effectively reduce the occurrence of defects, thereby improving the mechanical properties and production efficiency of castings.
Before proposing effective preventive measures, it is necessary to first have a specific understanding of the causes of shrinkage and porosity in ductile iron castings. Firstly, it is related to the carbon content. Increasing the carbon content increases the graphitization expansion, which can reduce shrinkage and porosity. In addition, increasing the carbon content can also improve the fluidity of ductile iron, which is beneficial for shrinkage.
When producing ductile iron castings, if the phosphorus content in the molten iron is too high, it will expand the solidification range. At the same time, the low melting point phosphorus eutectic cannot be replenished during the later solidification, and it will weaken the casting shell. Therefore, there is a tendency to increase shrinkage and porosity. Therefore, the phosphorus content in the molten iron should be strictly controlled to be less than 0.08%.
Excessive residual rare earth content can deteriorate the shape of graphite and reduce the spheroidization rate, so the rare earth content should not be too high. Magnesium, on the other hand, is an element that strongly stabilizes carbides and hinders graphitization. From this, it can be seen that the residual magnesium content and residual rare earth content will increase the tendency of ductile iron to whiten, reducing graphite expansion. Therefore, when their content is high, it will increase the tendency of shrinkage and porosity in ductile iron castings.
When a hard shell is formed on the surface of ductile iron castings, the higher the temperature of the molten metal inside, the greater the liquid shrinkage. As a result, the volume of shrinkage and porosity not only increases, but also its relative value increases. In addition, if the wall thickness changes too suddenly, isolated thick sections cannot be compensated, resulting in an increased tendency for shrinkage and porosity. In addition, it is also related to the pouring temperature, the compactness of the sand mold, the pouring riser, and the cold iron, and needs to be adjusted from here.
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