From the solidification volume change model of nodular iron castings, we can get the following measures to guide the process design of nodular iron castings.
1.Pay attention to the smelting quality of molten iron
Selecting the appropriate carbon equivalent, generally 4.3 to 4.4 is more appropriate, and moderately increasing the carbon silicon ratio can reduce the shrinkage tendency. Controlling the alloy content of molten iron and the amount of spheroidized residual magnesium is also an important factor, because alloy elements can affect the precipitation of austenite and graphite.
2.Pouring temperature
According to the above analysis, the volume shrinkage of nodular iron castings is 1.5% when the temperature drops by 100 ℃. Therefore, in the case of nodular iron castings without cold insulation or insufficient pouring, lower pouring temperature shall be used as far as possible to reduce liquid shrinkage, so as to reduce the volume shrinkage in the whole solidification process and reduce the shrinkage tendency of nodular iron castings.
3.Rigidity of ductile iron mold
The mold stiffness of nodular iron castings is an important condition to realize the riser free process. When the nodular iron castings are graphitized and expanded during solidification, if the mold can withstand the pressure, the mold wall cannot migrate, and the expansion pressure of nodular iron castings turns inward to realize self feeding, so that the riser free nodular iron process can be realized, Therefore, improving the mold strength by some means is also one of the means to reduce the shrinkage porosity of castings. For example, adding refractory brick, graphite block and cold iron in wet mold sand casting can improve the mold stiffness. Of course, the modulus of nodular iron castings is also an important condition for whether the riser free process can be realized. It is generally considered that the modulus of nodular iron castings not less than 2.5cm is one of the conditions for the riser free process.
4.Setting of riser neck and ingate
According to the above analysis, eutectic expansion will occur in nodular iron castings. Therefore, it will have a significant impact on the design of riser neck and inner gate. Due to the existence of eutectic expansion of nodular cast iron, the inner gate should adopt thin internal gate to solidify rapidly after the pouring of nodular cast iron, so that the molten iron will not be pushed back into the sprue when the eutectic expansion occurs, and the length width ratio of the cross section of the sprue is generally greater than 3. At the same time, for the riser neck, it also has the risk that the molten iron will be pushed back when the eutectic expansion of nodular iron castings occurs. Therefore, the modulus of the riser neck should be strictly controlled so that the riser neck has solidified when the eutectic expansion of nodular iron castings occurs, and the nodular iron castings have no risk of back filling the riser. At the same time, pay attention not to cause contact hot joints at the riser neck, resulting in the risk of casting root shrinkage. Due to the solidification characteristics of nodular cast iron, it is found that the solidification of nodular cast iron is far different from the sequential solidification of cast steel and gray cast iron, which has a significant impact on the casting process. Therefore, the in-depth study of the solidification characteristics of nodular cast iron is of great benefit to the design of casting process.