(1) Silicon: The oxide of silicon is also a major component of slag inclusion, so the amount of silicon is reduced as much as possible.
(2) Sulfur: Sulfide in molten iron is one of the main causes of slag inclusion defects in ductile iron. The melting point of the sulfide is lower than the melting point of the molten iron. During the solidification of the molten iron, the sulfide will precipitate from the molten iron, increasing the viscosity of the molten iron, making it difficult for the molten iron or metal oxide in the molten iron to float. Therefore, when the sulfur content in the molten iron is too high, the casting is liable to cause slag inclusion. The sulphur content of the ductile iron original iron solution should be controlled below 0 06%. When it is between 0 09% and 0 135%, the cast iron slag defects will increase sharply.
(3) Rare Earth and Magnesium: In recent years, it has been considered that slag inclusion is mainly caused by oxidation of elements such as magnesium and rare earth, so residual magnesium and rare earth should not be too high.
(4) Pouring temperature: When the pouring temperature is too low, the metal oxide in the molten metal is too high in viscosity due to the viscosity of the molten metal, and it is not easy to float to the surface and remains in the molten metal; when the temperature is too high, the slag on the surface of the molten metal It becomes too thin to be easily removed from the liquid surface, often with the inflow of molten metal. In actual production, the pouring temperature is too low, which is one of the main causes of slag inclusion. In addition, the choice of casting temperature should also consider the relationship between carbon and silicon content.
(5) Gating system: The design of the gating system should be reasonable, and it has the function of slag blocking, so that the molten metal can smoothly fill the casting mold, and strive to avoid splashing and turbulence.
(6) Type sand: If the surface of the molding sand is adhered with excess sand or paint, they can be combined with the oxides in the molten metal to form slag, resulting in slag inclusion; the compactness of the sand type is uneven, and the wall with low compactness The surface is easily eroded by the molten metal and forms a compound having a low melting point, resulting in slag inclusion in the casting.
Ductile iron slag prevention measures
(1) Control the composition of molten iron: Minimize the sulfur content in the molten iron (<0 06%), add rare earth alloy (0 1% to 0 2%) to purify the molten iron, and reduce the silicon content and residue as much as possible. The amount of magnesium.
(2) Smelting process: It is necessary to increase the temperature of the molten metal as much as possible, and to seduce properly, so as to facilitate the floating and gathering of non-metallic inclusions.扒 Clean the surface of the molten iron, the surface of the molten iron should be covered with a covering agent (perlite, grass ash, etc.) to prevent oxidation of the molten iron. Choose a suitable casting temperature, preferably no lower than 1350 °C.
(3) The pouring system should make the flow of molten iron smooth, and should be provided with slag collecting slag and slag blocking device (such as filter residue net) to avoid sanding of the sprue.
(4) The compactness of the mold should be uniform and the strength is sufficient; the sand in the mold should be blown off when the box is closed.
Ductile iron graphite floating factor
3. 1 Influencing factors
(1) Carbon equivalent: The carbon equivalent is too high, so that a large amount of graphite is precipitated when the molten iron is at a high temperature. Since the density of graphite is smaller than that of molten iron, graphite is floated to the upper part of the casting by the action of magnesium vapor. The higher the carbon equivalent, the more severe the graphite floating phenomenon. It should be pointed out that too high carbon equivalent is the main reason for graphite floating, but it is not the only reason. Casting size and wall thickness are also important factors affecting graphite floating.
(2) Silicon: Under the condition that the carbon equivalent is constant, appropriately reducing the silicon content helps to reduce the tendency of graphite to float.
(3) Rare earth: When the rare earth content is too small, the solubility of carbon in the molten iron will decrease, and the molten iron will precipitate a large amount of graphite, which will increase the graphite floating.
(4) Spheroidization temperature and incubation temperature: In order to improve the absorption rate of magnesium and rare earth elements, domestic experimental studies have shown that the most suitable temperature of molten iron during spheroidization is 1380 to 1450 °C. In this temperature range, as the temperature increases, the absorption rates of magnesium and rare earth increase.
(5) Pouring temperature: In general, the higher the pouring temperature, the greater the tendency of graphite to float. This is because the castings are in a liquid state for a long time to facilitate the precipitation of graphite. A.P.Druschitz and W.W.Chaput found that if the solidification time is shortened, the graphite floatation tends to decrease as the pouring temperature increases.
(6) Residence time: The residence time between the incubation process and the completion of the casting is too long, which provides conditions for the precipitation of graphite. Generally, this time should be controlled within 10 minutes.