Generation and Prevention of Subcutaneous Porosity in Ductile Iron Castings Produced by Tidal Coal Powder Sand
Release time:
2012-09-29 15:06
Source:
Subcutaneous porosity is the most common casting defect in the production of ductile iron castings, especially thin and small castings. The causes and how to prevent it are for reference.
(I) causes
Moisture content in the moisture type, sulfur content in the iron liquid, oxygen content spheroidization treatment Mg content interaction occurs the following reactions
Mg S → Mg 2MgS (impurity),Mg O → MgO (impurity) all consume Mg
Mg H2O → Mg 2[H] gas ↑ MgS H2O → MgO [H2S] gas ↑
Hydrogen and hydrogen sulfide gas generated by the reaction products are the main sources of subcutaneous stomata.
Due to the characteristics of pasty solidification of ductile iron, long surface crust time, low surface strength, high mold moisture, low ash permeability and high back pressure, in this case, the gas and reaction generated gas at the molten iron-mold interface are easy to invade the surface of the casting under the condition of high back pressure, and the surface of the ductile iron is easy to oxidize. The produced oxide film will hinder the rapid discharge of the gas precipitated from the molten iron and the external intrusion. Low pouring temperature, thickening of oxide film, more hindered the escape of gas, moist corrosion of furnace charge, moist ferrosilicon and nodulizer, cupola melting high humidity blast, low melting temperature, serious oxidation of molten iron, poor purity of molten iron, resin sand or sand core with high nitrogen content, and ferrosilicon with high Al content can increase the nitrogen, hydrogen, oxygen and other gases in molten iron to form subcutaneous pores or pores.
(II) measures
1. Strictly control the sand moisture (not more than 5%), try to improve the permeability of sand. Some small and medium-sized factory sand is repeatedly used for a long time without dust removal measures, which will accumulate a lot of dead clay (bentonite), dead coal powder and ablated and broken sand fine powder, thus increasing the ash content in the sand over time. This not only greatly reduces the air permeability of the sand, increases the amount of water added when mixing sand (more ash and more water absorption) but also reduces the strength and refractoriness of the sand, and is easy to form pores and subcutaneous pores, sand washing (sand holes), sand sticking and other casting defects. Under this condition, in order to improve the permeability of molding sand and reduce the amount of water added, only regular replacement of molding sand, elimination of half of the old sand, adding new sand or regular use of wind forced dust removal of all the old sand, in addition to debris, improve the quality of molding sand.
2.5 ~ 7% of high-quality pulverized coal is added to the surface sand. Under the action of high-temperature liquid iron, pulverized coal burns in the wall to form CO, which inhibits the direct contact reaction between moisture and magnesium to form H2, which can reduce the appearance of subcutaneous pores.
3. When modeling, the ice crystal stone powder (the principle is the same as above) is used to reduce the formation of subcutaneous stomata. (Before use, the cryocrystalline powder must be baked at about 100 ℃ to remove water, then put into a cloth bag, evenly sprinkle it on the surface of the model, and then fill in sand for modeling. The cryocrystalline powder can stick to the casting mold well and will not be washed away by molten iron to form slag).
4. In the casting process, pay attention to the setting of the gate, and use the open multi-runner pouring system to make the iron liquid flow into the cavity smoothly, prevent turbulence and avoid excessive magnesium vapor. The overflow riser is used to introduce the molten iron (high gas content, serious oxidation of molten iron, lowest temperature of molten iron, and discharge of unfavorable gas) that enters the cavity first into the overflow riser, which can reduce the subcutaneous pores of the casting.
5. Use pig iron with lower sulfur content and relatively pure furnace charge to strengthen desulfurization measures to reduce the sulfur content of the original molten iron as much as possible. After spheroidization, the molten iron should be carefully stripped of scum to reduce the residual amount of magnesium sulfide (MgS), which can reduce the "sulfur return phenomenon" of molten iron and reduce the formation of subcutaneous pores.
6. The spheroidizing agent and inoculant should be transported and stored to prevent rain and moisture absorption. The hot metal bag must be baked and dried to prevent moisture from entering the molten iron. Reduce hot metal oxidation and gettering.
7. During spheroidization treatment, 0.3% of the glacial stone powder (or salt) is put into half of the bottom and half of the molten iron surface, which can combine MgS and other impurities to form low melting point molten slag, which is easy to accumulate and float for removal. Adding glacial stone powder (or salt) can reduce the surface conjunctival temperature of molten iron and melt the oxide film on the surface of molten iron, which is conducive to the discharge of internal gas and forms a dense layer of HF gas, prevent air intrusion, protect molten iron from oxidation, and help reduce the formation of subcutaneous pores.
8. The use of as-cast ductile iron with nodularizer and high-efficiency compound inoculant to reduce the content of MgO, can reduce the amount of Mg in the molten iron, reduce the cooling of the molten iron, improve the purity of the molten iron, reduce the burning loss of magnesium and control of MgS inclusions, reduce the temperature of the molten iron oxide film, reduce the thickness of the oxide film, can effectively reduce the casting defects such as oxide inclusions and subcutaneous pores.
9. No need to get wet in the rain and seriously corroded furnace charge; Cupola dehumidifies the air supply, adjusts the air volume and air pressure, reduces the oxidation of molten iron, increases the melting temperature and the temperature of molten iron coming out of the furnace, and can improve the purity of molten iron and reduce the gas content. Appropriately increasing the pouring temperature should not be lower than 1300 ℃ (easy to form oxide conjunctiva temperature is generally 1250 ℃-1280 ℃) can reduce the formation of casting pores and subcutaneous pores.
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