The effect of phosphorus on the shrinkage defects of castings is shown in two aspects: one is that phosphorus, as a graphitization promoting element, can increase the graphite expansion in cast iron, and it can also improve the fluidity of molten iron, both of which can improve the feeding performance of molten iron; the other is that phosphorus can expand the range of solidification temperature of cast iron, so as to increase the probability of shrinkage porosity of castings. But at last, the eutectic structure of phosphorus determines that phosphorus can promote the production of shrinkage porosity.
When the phosphorus content is more than 0.05%, there will be binary or ternary eutectic structure with low soldering point in the grain boundary of cast iron. This kind of structure is mainly distributed near the eutectic group and the hot spot of the casting, which leads to the formation of shrinkage porosity.
However, the increase of phosphorus content can reduce the probability of sand sticking, and zhycasting Zhang Ruifen and others believe that the reason for the above facts lies in the presence of phosphorus eutectic structure. He thinks that the sticking sand of grey cast iron is caused by the expansion of graphite in the solidification process, which results in the hot metal being squeezed into the molding sand, while the existence of phosphorus eutectic makes the solidification temperature range of cast iron expand, thus offsetting the extrusion pressure of hot metal caused by the expansion of graphite.
However, the structure of phosphorus eutectic also has some influence on the mechanical properties of cast iron. If the content of phosphorus in cast iron increases, the distribution form of phosphorus eutectic structure will change into isolated block, uniform distribution and intermittent network distribution in turn, resulting in the decrease of tensile strength and the increase of the probability of crack defects in castings. At the same time, due to the large atomic radius of phosphorus in gray iron, the lattice of gray iron matrix is changed, which increases the ability of the matrix to resist external forces, so as to improve the hardness of gray iron.
Based on the above considerations, for the iron castings with high requirements for mechanical properties, materials with anti leakage requirements and easy to crack, the phosphorus content is generally controlled between 0.03% and 0.06%, and the lower limit is often taken. However, for castings with low requirements for compactness and mechanical properties and often easy to be bonded with sand, the phosphorus content is controlled at about 0.06% – 0.12%.