(1) Carbon equivalent: increase the amount of carbon, increase the graphitization expansion, and reduce shrinkage and shrinkage. In addition, increasing the carbon equivalent can also increase the fluidity of the ductile iron and facilitate the feeding. The empirical formula for producing high quality castings is C%+1/7Si%>3 9%. However, when the carbon equivalent is increased, the casting should not cause other defects such as graphite floating.
(2) Phosphorus: The phosphorus content in the iron liquid is too high, so that the solidification range is expanded, and the low-melting-point phosphorus eutectic crystals are not replenished at the final solidification, and the casting shell is weakened, so that the shrinkage cavities and shrinkage are increased. The tendency to produce. The general plant control phosphorus content is less than 0 08%.
(3) Rare earth and magnesium: If the residual amount of rare earth is too high, the graphite shape will be deteriorated and the spheroidization rate will be lowered, so the rare earth content should not be too high. Magnesium, on the other hand, is a strongly stable carbide element that hinders graphitization. It can be seen that the amount of residual magnesium and the amount of residual rare earth increase the tendency of the white iron of the ductile iron to decrease the expansion of the graphite, so when their content is high, the tendency of shrinkage and shrinkage is also increased.
(4) Wall thickness: When the surface of the casting is formed into a hard shell, the higher the temperature of the internal molten metal, the larger the liquid shrinkage, and the volume of shrinkage and shrinkage not only increases the absolute value, but also increases the relative value. In addition, if the wall thickness changes too suddenly, the isolated thick section cannot be replenished, and the tendency to cause shrinkage shrinkage increases.
(5) Temperature: high pouring temperature is conducive to feeding, but too high will increase the amount of liquid shrinkage, which is unfavorable for eliminating shrinkage and shrinkage. Therefore, the pouring temperature should be reasonably selected according to the specific conditions, generally 1300~1350 °C is suitable. .
(6) The compactness of the sand type: If the compactness of the sand type is too low or uneven, the phenomenon of cavity expansion occurs under the action of metal static pressure or expansion force after casting, so that the original metal is not enough to be fed. This causes the casting to produce shrinkage and shrinkage.
(7) pouring riser and cold iron: if the pouring system, riser and cold iron are improperly set, the molten metal cannot be solidified in sequence; in addition, the number and size of the riser and the connection with the casting are not affected, which will affect the filling of the riser. Shrink effect.
Preventive measures for shrinkage and shrinkage of ductile iron
(1) Control the composition of molten iron: maintain a high carbon equivalent (>3 9%); reduce the phosphorus content as much as possible (<0 08%); reduce the amount of residual magnesium (<0 07%); use rare earth magnesium alloy to treat, The residual amount of rare earth oxide is controlled at 0 02% to 0 04%.
(2) The process design should ensure that the casting can continuously replenish the high-temperature molten metal from the riser during solidification, and the size and quantity of the riser should be appropriate, and strive to achieve sequential solidification.
(3) If necessary, use cold iron and subsidies to change the temperature distribution of the casting to facilitate sequential solidification.
(4) The pouring temperature should be 1300 ~ 1350 ° C, the pouring time of a pack of molten iron should not exceed 25 min, so as to avoid spheroidization decline.
(5) Improve the compactness of the sand type, generally not lower than 90; uniform sand impact, moisture content should not be too high, to ensure that the mold has sufficient rigidity.https://www.xihuafoundry.com/