1.The content of carbon, silicon, manganese, rare earth and magnesium must be appropriate. High carbon content can reduce the tendency of shrinkage cavity and porosity, but high carbon content will lead to graphite floating. For thin-walled castings, when the content of carbon and silicon is low, it is easy to produce free carbide. For thick wall castings, a lower carbon content and an appropriate increase in silicon content can be used. Manganese is easy to form carbonization and to promote the formation of shrinkage porosity. Therefore, we should strive to reduce manganese content, especially for as Cast Ferritic Nodular Cast iron.
On the premise of spheroidization, the residual amount of magnesium and rare earth is not too high.
2.The slope of the cooling curve of the molten iron with small shrinkage porosity and shrinkage tendency should be smaller, and the degree of supercooling should be smaller. During eutectic solidification, the expansion is smaller and the secondary shrinkage is smaller. If curve C is converted into a, the following conditions shall be met:
(1) Slow cooling
(2) With high carbon equivalent, the tendency of graphite precipitation is large.
(3) There are many effective graphite cores in molten iron.
(4) Good inoculation effect.
3.Mold rigidity for wet sand mold, mold hardness should be more than 90 (type B hardness tester), hoping to achieve 40 MPa tensile strength. In addition, the fastening of the sand box is also very important. The riser free casting can be realized when the metal mold is covered with sand (the thickness of sand is about 8mm) and the self hardening sand is used to make large castings. The conditions to be met are:
In the formula, G is the expansion of molten iron caused by graphitization of carbon;
Liquid shrinkage of α 1-Liquid iron
α 2-solidification shrinkage
β – mold expansion.
4.In order to prevent shrinkage cavity and porosity, the liquid shrinkage should be reduced, and low pouring temperature is advantageous. However, for thin-walled (10 mm) castings, carbonization is easy to occur. At this time, riser feeding is difficult to play a role. Therefore, the proper pouring temperature also depends on the structure and wall thickness of the casting.
5.The pouring system adopts the sequential solidification method, which is an effective measure to prevent shrinkage and porosity for the design and placement of castings, risers, riser necks, internal sprues and cross sprues, as well as the setting of external chill and the use of metal mold when necessary.