China Foundry Machinery Leader.
China Foundry Association director unit
Causes of porosity in castings:
1、Control the quality of iron liquid
1.1 Control the amount of residual aluminum wet ductile iron parts of the dangerous residual aluminum amount of 0.03%-0.05%, at this time will appear subcutaneous pores, less than 0.03%, generally will not appear. Under the premise of not affecting the metallographic organization, adding more than 0.2% of aluminum before pouring can eliminate the subcutaneous pores. However, the aluminum in cast iron is mainly from the breeding agent, wet breeding of ductile iron parts, when too much ferrosilicon breeding agent is added to the ferrofluid, it is one of the causes of subcutaneous pores in castings.1.2 Control the amount of titanium in cast iron residual aluminum and residual titanium are sometimes, excessive residual titanium will make cast iron produce serious subcutaneous porosity. Ductile iron parts residual aluminum amount is less than 0.03%, generally do not appear subcutaneous porosity, if the residual titanium content of more than 0.01% at this time, it will produce subcutaneous porosity. The joint action of titanium and aluminum intensifies the reduction of water and gas at the interface, which makes the hydrogen content of the interface ferrofluid higher and more likely to form subcutaneous pores. Residual titanium mainly from the melting furnace charge pig iron ingot, should pay attention to the pig iron ingot titanium content, high titanium content with the use of low, control the amount of titanium.
1.3 Reduce sulfur content of manganese and sulfur For wet ductile iron parts, from the perspective of preventing subcutaneous porosity, elemental sulfur is a harmful element. When the sulfur content exceeds 0.094%, it is easy to produce subcutaneous porosity, and the higher the sulfur content is, the more serious the defects will be. In addition, the H2S produced may make the defects more serious. The oxide and sulfide slag produced after spheroidizing treatment is cleaned up. Otherwise produce at the interface to produce H2S gas will also form subcutaneous pores, this subcutaneous pores around the graphite spheroidization is poor.
1.4 Adding rare earth elements can deoxidize, desulfurize and improve the surface tension of cast iron liquid, which can effectively prevent the generation of subcutaneous pores. Ductile iron generally use rare-earth magnesium-silicon iron alloy as nodulizing agent, add tellurium (Te) 0.002% (20ppm) to the iron liquid before pouring, adding a small amount can play an obvious effect.
1.5 Control the pouring temperature to prevent the ductile iron parts from producing subcutaneous pores, choose the correct pouring temperature to avoid falling into the dangerous pouring temperature range (dangerous pouring temperature 1310℃-1250℃).
1.6 Control the original hydrogen content of the molten iron. It is proved that when the hydrogen content of the molten iron reaches 4-5ppm, it is easy to produce subcutaneous porosity at this time, and generally the original hydrogen content of the molten iron is controlled below 2-2.5ppm, and within 1.5-3.5 hours after opening the furnace, the high hydrogen content is not suitable for pouring.
2、Control of casting factors
2.1 Control the moisture content of the moulding sand Moisture, dead clay and additions in wet moulding sand Moisture should be less than 5%, when the moisture is from 4.5% to 6.5%, the chance of subcutaneous porosity in ductile iron will increase 7-10 times. Adding various additions such as coal dust (4%-6%), hematite powder (2%), ammonium difluoride (2%-2.5%) to the sand mould is beneficial to prevent subcutaneous porosity.
2.2 sand temperature assembly line production, for the subcutaneous pores are very sensitive ductile iron, can not be used to bubble hot air temperature of more than 35 ℃ hot sand modeling, otherwise it is very easy to appear in the casting overheating parts subcutaneous pores, do not solve the problem of hot sand modeling, the effect of other preventive measures is not ideal. Therefore, in the sand processing system design on the old sand reuse sand cooling device, sufficient capacity of the sand storage bucket and other equipment and facilities, the process of adding new sand and reasonable wet type sand wall thickness, etc., to ensure that the production of manufacturing does not appear hot sand molding problems.
2.3 Shake the surface of the cavity to lay molten powder commonly used molten powder is cryolite powder or sodium fluoride powder, according to relevant statistics, shake the laying of sodium fluoride powder compared with the non-shake laying, can make ductile iron subcutaneous pore defects from 25% to 5%, favorable reduction of subcutaneous pores.
2.4 Adding the right amount of coal dust in the sand sand to join the coal dust 4-5% when pouring coal dust in the metal casting interface to form a reducing film, not only to prevent the castings sticky sand, and may resist the reaction of the interface water and gas, but also to prevent subcutaneous porosity. Also can add asphalt 2% or wood chip powder 2%-3% to prevent subcutaneous porosity.
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