The shrinkage rates of several common iron carbon alloys are shown in the table. It can be seen from the table that the shrinkage rate of carbon steel is the largest in the table.
Material Science | Pouring temperature / ° C | C | Si | Mn | P | S | Mg | Shrinkage rate (%) | Metal matrix structure |
Carbon steel | 1540 | 0.24 | 0.01 | 0.05 | 0.05 | 0.04 | — | 6.45 | Ferrite + pearlite |
White cast iron | 1250 | 2.65 | 1.10 | 0.48 | 0.16 | 0.09 | — | 5.70 | Carbide + pearlite |
Grey cast iron | 1270 | 3.40 | 4.12 | 0.60 | 0.09 | 0.025 | — | 1.65 | Ferrite |
Gray ductile iron | 1290 | 3.22 | 3.70 | 0.51 | 0.09 | 0.010 | 0.060 | 5.50 | Pearlite + ferrite |
Shrinkage porosity can be divided into macro shrinkage porosity (abbreviated as shrinkage porosity) and micro shrinkage porosity (micro shrinkage porosity). Shrinkage porosity is often distributed in the middle area of the casting wall, thick parts, riser root and near the ingate. The basic reason for the formation of shrinkage porosity is that the liquid shrinkage and solidification shrinkage of the alloy are greater than the solid shrinkage. However, the basic condition for the formation of shrinkage porosity is that the crystallization temperature range of the alloy is wide and tends to volume solidification, or the temperature gradient of the casting section in the shrinkage porosity area is small and the solidification area is wide. The wider the solidification area of the casting, the easier it is to produce shrinkage porosity.
For alloys with a certain composition, the number of shrinkage cavities and porosity can be transformed into each other, but their total volume is basically the same, that is:
The total shrinkage cavity volume depends on the shrinkage characteristic curve of the alloy and is also affected by other conditions. The solidification and feeding characteristics of castings are related to alloy composition, but also affected by injection conditions, mold properties and feeding pressure.
With the increase of pouring temperature, the liquid shrinkage and shrinkage cavity volume of the alloy increase, but it has little effect on the volume of shrinkage porosity. The chilling energy of the mold is strong, the solidification area of the casting becomes narrow, the shrinkage porosity decreases, the shrinkage cavity volume increases correspondingly, and the total volume of the shrinkage cavity remains unchanged or decreases. Increasing the feeding pressure during solidification can reduce the shrinkage porosity and increase the volume of shrinkage cavity. If the alloy is poured and solidified under high pressure, a compact casting without shrinkage cavity and shrinkage porosity can be obtained.