Lost Foam Casting is a near-no-cut machining process with high casting precision, flexible design, no sand core, clean production environment, low investment and low production cost. This technology has developed rapidly at home and abroad, and all relevant technologies have made great progress. Lost foam casting technology has been well applied in cast aluminum, cast copper, cast iron and other materials, but it is relatively lagging in the application of cast steel production. Although there have been reports of successful application in high manganese steel, wear resistant steel and other steel grades, in the actual production process, the flow front of the molten metal is a pyrophoric lost foam product (gas) during the casting filling process. And liquid), will react with the molten metal and affect the quality of the molten metal. If the pyrolysis product cannot be discharged smoothly, it will easily cause defects such as pores, carbonation, hydrogenation, etc., which limits the application of the technology. The common defect of lost-molded steel castings is the pores. Therefore, it is necessary to analyze the causes and propose corresponding prevention measures. It is necessary to provide better guidance for the lost foam casting technology to be applied to the production of steel castings.
I. Product Status and Stomatal Causes
The labyrinth ring of steel castings produced by our factory (see Figure 1) is made of ZG270-500 with a mass of 35kg.
Production conditions: 10 pieces per box, polystyrene foam plastic density of 22g / cm3, using water-based paint, drying room temperature is about 45 ° C, drying time is 48h, riser with AB glue bonding, vertical placement The sprue is 45mm×45mm, the sprue is 40mm×25mm, and the sprue is 20mm×15mm. The bottom note has a negative pressure of 0.05 MPa, a pouring temperature of 1550 ° C, and a pouring time of 13 to 17 s. After pouring for 3 minutes, the vacuum is stopped, and after 30 minutes, the sand is dropped.
After machining, a large number of dispersive pores were found in the feeding riser, appearing inside the casting and on the surface of the casting (see Figure 1). The largest diameter is 3mm, the depth is 4mm, and the unqualified product rate is 60%. We systematically collated and analyzed the stomatal defects. It is believed that the pores formed are mainly invasive pores and precipitated pores, and the causes are discussed in depth.
The formation of pores is derived from gas. The pores on the lost-foam casting cast steel labyrinth ring are mainly intrusive pores formed by the intrusion of gas generated by vaporization of the foam into the molten metal, and the precipitated pores brought in the molten steel.
1. Causes of invasive stomata
Invasive pores are the main form of voids in lost-molded steel castings, and the reasons for their formation are as follows.
(1) During the pouring process, the pouring time is too short, the filling is too fast, the foam plastic pattern cannot be rapidly vaporized, and the cracked liquid product enters the molten metal. After the casting is solidified, the gas cannot be discharged, and the pores are formed in the casting.
(2) The amount of gas generated by the foam plastic pattern increases sharply with the increase of the pouring temperature. Figure 2 shows the change of gas pressure with pouring time during pouring. It can be seen that the gas pressure at the moment of pouring increases sharply due to casting of cast steel. The temperature is high, the amount of gas generated at the time of pouring is large, and the gas cannot be discharged in time, especially when the molten steel is entangled in the turbulent state to form a pore.
(3) Improper use of adhesives in bonded foam plastics is an important cause of intrusive pores. In actual production, it has been found that steel castings often produce a large number of pores in the foam molding joint. If a foamed plastic pattern is attached by a plug or a steel stud, no air holes are formed there. This is because the joint uses more adhesive, whether it is 851 super glue or AB glue, its density is several times or even several times that of foam, its vaporization speed is much slower than foam and the vaporization temperature is high. When the glue is not vaporized, the molten metal rises and wraps the glue, and is continuously vaporized during the subsequent cooling process, thereby forming pores in the casting.
(4) Moisture is also one of the factors that form the pores of steel castings. The source of moisture during pouring mainly has the following aspects: First, the coating is not completely dried, or because the water-based coating contains CMC, polyvinyl alcohol, white latex. Organic binders such as starch, if the pattern of the coating is dry, cannot be poured in time, it will absorb the moisture in the air. In addition, the suspension agent such as bentonite of the coating mostly contains structural water and combined water. Since the drying temperature of the pattern is generally only about 40 ° C, the structural water and the combined water generally remain in the coating, and this part of the moisture is under the action of the high temperature molten steel. Vaporization; Second, due to insufficient drying of the foam plastic pattern, more water vapor remains in the foam beads; the third is from the moisture in the molding sand.
2. Causes of precipitated pores
Precipitating pores refer to the inhalation of molten metal during the smelting process, and the formation of pores in the multi-metal during the solidification of the metal. When the molten metal absorbs water vapor, under the action of the heat of the high-temperature molten metal, the water is cracked to generate hydrogen, and the hydrogen is dissolved and then cracked into hydrogen atoms. These reasons are re-polymerized during the solidification of the molten metal to generate hydrogen, which cannot be discharged, and is formed. Precipitated stomata defects. The characteristics of the pores: the gas wall is relatively smooth, has a metallic luster, and the pores are relatively small but a large number.
Second, the measures to solve the porosity of steel castings
It can be seen from the above analysis that during the pouring process, the pouring time is too short, the filling is too fast, the foam plastic pattern cannot be rapidly vaporized, and the adhesive foam molding is improperly used by the binder, which is the main cause of the porosity of the steel casting. In addition, the design of the gating system is unreasonable, as well as factors such as appearance, coating and casting.
1. Reduce the amount of gas generated by the foam plastic pattern and reduce the gas generation rate
(1) Under the premise of ensuring the strength and rigidity of the pattern, minimize the density of the pattern, thereby reducing the amount of gas generation; make the sprue, riser and thick parts into hollow plastic patterns (the density of foam used in our factory is currently 0.018~0.020g/cm 3 ).
(2) Reduce the size of the gating system. The inner runner is changed from the original 20mm × 15mm to 15mm × 15mm, so that the pouring time is increased to about 20s. The casting speed is lowered to reduce the gas generation rate of the polystyrene, so that the generated gas can be discharged through the coating layer in time without entering the molten steel. The sufficient vaporization time of the pattern also increases the vaporization rate, reduces the liquid residue accumulated at the interface between the molten steel and the coating layer, and avoids the formation of pores on the side of the rim.
2. Use a low-gas binder
The less the amount of binder, the better. It is best to use the riser and the product as a whole, or use a plug or steel nail connection, or a binder in combination with a steel nail.
3. Guarantee paint quality
(1) Select a coating with high temperature and good permeability to increase the gas permeability and wettability of the coating, and reduce the air pressure of the air gap cavity to ensure the smooth filling of the molten metal. In the dry sand lost foam casting, due to the use of dry sand molding, the dry sand permeability is much higher than the permeability of the coating, and the permeability of the coating mainly depends on the permeability of the coating. When the permeability of the coating is small, the gas permeability of the coating is much smaller than the pyrolysis speed of the pattern. The pressure in the air gap before the flow of the alloy is high, and the flow front of the liquid metal is unstable, causing gas and suction.
(2) The coating should be evenly coated and have high high temperature strength to reduce the thickness of the coating. Generally, it can be controlled at 0.5~1mm.
(3) The coating should be fully dried, and the dried pattern should be poured in time to avoid moisture.
We improved the original paint by the above three measures: the particle size of silica sand was changed from 0.071mm (200 mesh) to 0.080mm (180 mesh); the latex was changed from 25kg to 20kg; The 4kg is changed to 5kg. The improved coating enhances high temperature strength and gas permeability and is less prone to cracking.
4. Set slag and exhaust riser
The gas decomposition products of the polystyrene foam pattern have to escape through the coating cavity. Due to the rapid filling speed of the molten metal, the liquid pyrolysis product is not enough to be fully cracked and vaporized, so that liquid products or residues tend to accumulate in the dead angle of the casting. Or the top makes these parts easy to form pores. Therefore, small risers should be placed in these parts, and the top coating of the riser should be filled with a plurality of small holes to facilitate the discharge of the liquid when the accumulated liquid product is finally vaporized. The size of the riser can be set according to the size of the product, and the small riser is easier to clean. Practice has proved that these small risers can effectively eliminate slag inclusions and porosity defects in castings.
5. Choose the right casting process
(1) The pouring position should be such that the important large surface is in a vertical or inclined state, which is conducive to the smooth filling of the molten metal and the gradual vaporization of the plastic pattern, so that the air gap pressure is small and balanced.
(2) The pouring system is preferably a closed bottom injection, step casting or side injection to ensure the smooth flow of molten metal and to avoid a large amount of air entering the cavity during the pouring process.
(3) Increasing the pouring temperature of the molten metal and reducing the pouring speed are beneficial to the full vaporization and rapid discharge of the foam plastic pattern. Practice has proved that increasing the pouring temperature can effectively reduce a series of casting defects such as pores. Casting temperature of steel castings should be above 1600 °C, at this time should ensure that the coating has a high temperature strength, otherwise it will lead to more serious sand.
(4) Adhere to the "slow-fast-slow" pouring principle. First use a small flow of metal liquid to pass through the sprue, and use a large flow to quickly fill the sprue cup to close the sprue to ensure the vacuum until the pouring cup metal liquid is not sinking. When the amount of molten steel poured is estimated to reach the top surface of the casting, the casting speed is slowed down, the flow rate of the molten metal is reduced, and the gas in the molten metal or possibly invading is allowed to escape or precipitate from the molten metal.
6. Fully degassing the molten steel before pouring
(1) Cleaning the charge, especially the rust with serious rust should be treated by rust removal.
(2) Rapid melting to shorten the high temperature melting time.
(3) Fully deoxidized, the amount of aluminum used is generally 0.04% to 0.06%, and not more than 0.15%.
In addition, the molding sand should be dried without the use of moisture or wet molding sand. The vacuum negative pressure of steel castings should generally be controlled at 0.05 to 0.06 MPa.
Third, the conclusion
Through the implementation of the above comprehensive measures, not only the pore defects of the labyrinth ring of the steel castings are basically solved, but also the phenomenon of gas shrinkage of other steel castings is reduced, and the yield rate is over 90%.
references:
[1] Chen Guobiao, Xiao Keze, Jiang Buju. Handbook of Defects and Countermeasures for Castings [M]. Beijing: Mechanical Industry Press, 2003.
[2] Du Qixin, Li Sanyu, Zhang Zhou. Lost Foam Casting High Manganese Steel Bend Pipe[J]. Founding Technology, Vol.2003 (in) 563-564.
[3] Liu Lizhong, Fu Tongshun et al. Solid casting of wear-resistant steel elbow [J]. Casting: 1998 (4): 41-43.
About the author: Zeng Xiangquan, mechanical engineer, technical department of Jiangxi Xingang Machinery Manufacturing Co., Ltd.
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