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Causes of Defects in Liquid Metal Filtration in Casting:
1. Classification of metal inclusions
Various liquid casting alloys will produce inclusions during the smelting and pouring process. Metal inclusions can be divided into two categories according to their sources:1.1 Foreign inclusions. It comes from the erosion of furnace lining and ladle refractory materials, slag or scum formed by reaction with air, erosion of molding sand, or any other erosion of materials in contact with molten metal.
1.2 Endogenous inclusions. Such inclusions are formed by reactions in the metal melt, such as magnesium-sulfur inclusions. Magnesium-sulfur inclusions are formed by the reaction in molten iron after adding magnesium-silicon-iron alloy during the spheroidizing process.
2. Xinyuanzhu concluded that the causes of slag inclusion are as follows:
2.1 Silicon: Silicon oxide is also the main component of slag inclusion, so the silicon content is as low as possible.
2.2 Sulfur: The sulfide in molten iron is one of the main reasons for the formation of slag inclusion defects in ductile iron. The melting point of sulfide is lower than that of molten iron. During the solidification of molten iron, sulfide will precipitate out of molten iron, which increases the viscosity of molten iron and makes it difficult for slag or metal oxides in the molten iron to float up. Therefore, when the sulfur content in the molten iron is too high, the castings are prone to slag inclusions. The sulfur content of nodular cast iron raw molten iron should be controlled below 0.06%. When it is between 0.09% and 0.135%, the slag inclusion defects of cast iron will increase sharply.
2.3 Rare Earth and Magnesium: In recent years, studies have suggested that the slag inclusion is mainly caused by the oxidation of magnesium, rare earth and other elements, so the residual magnesium and rare earth should not be too high.
2.4 Pouring temperature: When the pouring temperature is too low, the metal oxides in the molten metal are not easy to float to the surface and remain in the molten metal due to the high viscosity of the molten metal; when the temperature is too high, the slag on the surface of the molten metal becomes Too thin, not easy to remove from the surface of the liquid, and often flows into the mold with the molten metal. In actual production, too low pouring temperature is one of the main reasons for slag inclusion.
2.5 Gating system: The design of the gating system should be reasonable, with slag blocking function, so that the molten metal can fill the mold smoothly, and strive to avoid splashing and turbulence.
2.6 Molding sand: If excess sand or paint adheres to the surface of the molding sand, they can synthesize slag with the oxides in the molten metal, resulting in slag inclusion; the compactness of the sand mold is uneven, and the surface of the molding wall with low compactness is easy It is corroded by molten metal and forms compounds with low melting points, resulting in slag inclusions in castings.
3. Measures to prevent slag inclusion
3.1 Control the composition of the molten iron: try to reduce the sulfur content in the molten iron as much as possible (<0 06%), and add a proper amount of rare earth alloy (0 1% to 0.2%) to purify the molten iron, and reduce the silicon content and residual magnesium as much as possible quantity.
3.2 Smelting process: Try to increase the temperature of molten metal from the furnace, and leave it in a proper place to facilitate the floating and accumulation of non-metallic inclusions. Pick up the slag on the surface of the molten iron, and put a covering agent (perlite, plant ash, etc.) on the surface of the molten iron to prevent the molten iron from oxidizing. Choose a suitable pouring temperature, preferably not lower than 1350℃.
3.3 The pouring system should make the molten iron flow smoothly, and set up a slag collecting bag and a foam ceramic filter slag blocking device.
3.4 The compactness of the mold should be uniform and the strength should be sufficient; the sand in the mold should be blown off when the box is closed.
4. Stomatal defects
Porosity is usually a common defect in castings and often accounts for a high proportion of casting waste. Under modern production conditions, reactive stomata and precipitation stomata are relatively rare, and intrusive stomata are more common.
4.1 Causes of stomata
4.11 Insufficient exhaust of cavity, and the total exhaust cross-sectional area is too small
4.12 Pouring temperature is lower
4.13 Pouring speed is too slow; molten iron filling is not stable and gas is involved
4.14 Moisture of molding sand is high; ash content in molding sand is high, molding sand has poor air permeability
4.15 Improper design of pouring system, molten iron entrapped in gas
4.16 The inoculant has not been dried and has an improper particle size; the molten iron is not sufficiently slag removed, and the slag is not blocked during pouring, causing slag pores
4.17 The fire was not ignited in time during pouring.
4.2 Countermeasures to prevent blowholes
4.21 The higher part of the model is equipped with sufficient number of vent needles or vents with appropriate cross-sections. Generally, the vent cross-section should be about 1.5-1.8 times the total cross-sectional area of the inner runner.
4.22 The gating system should be set according to the principle of semi-open and semi-closed, and add foam ceramic filter device in the runner. The foam ceramic filter has a rectifying effect, and the molten iron is relatively stable when filling the mold, and will not impact the mold or cause splashing or curling. People gas. The section size of the pouring system is more appropriate to calculate with a pouring speed of 8-10kg/s.
4.23 The smelting temperature of molten iron should not be lower than 1500℃, and the pouring temperature of the end box should be controlled at about 1400~C during manual pouring (depending on the size and wall thickness of the casting, it can be adjusted appropriately). It is best to use automatic pouring, and the pouring temperature error should be within 20°C.
4.24 A good sand processing system suitable for high-pressure molding should control the moisture content of the molding sand at 2.8-3.2%, its compaction rate should be between 36-42%, and the temperature and pressure strength should reach 180-220kpa (all refer to the molding process Sampling and testing at the machine). In order to achieve these indicators, it is necessary to monitor the ash content of the molding sand, the amount of auxiliary materials added, the appropriate raw sand particle size, the temperature of the circulating sand and the sand mixing efficiency.
4.25 Pay attention to the work of removing slag from molten iron, blocking slag and ignition during pouring, and drying of inoculant.
5. Cold barrier defects
5.1 Causes of cold barrier defects
5.11 Pouring temperature is too low, molten iron filling ability becomes weak
5.12 The air permeability of the sand mold is poor, the gas pressure in the mold sand is too large, and the molten iron cannot be filled in time
5.13 The design of the gating system is unreasonable, and the cross-sectional area of the runner and the inner runner is small
5.14 Residue adheres to the pouring cup, which reduces the pouring temperature.
5.2 Countermeasures for cold barrier defects
5.21 Optimize the design of the pouring system, increase the area of the foam ceramic filter, and increase the flow of molten iron
5.22 Improve the air permeability of the molding sand and increase the exhaust duct
5.23 Increase casting temperature
5.24 Clean the part of the sprue cup.
Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.
