Method for improving porosity of cast iron sand

Compared with other casting methods, wet castings are more prone to defects such as sand sticking, sand holes, sand inclusion and air holes. These defects can be reduced or avoided if the foundry pays attention to controlling the quality of wet sand. The following examples illustrate the relationship between molding sand properties and casting surface defects.

I. Sticking sand

The research work shows that the sand sticking defects of general green sand castings, whether cast steel or cast iron, belong to mechanical sand sticking, not chemical sand sticking. There are many reasons for mechanical sand sticking, and the most common examples are as follows:

1. If the sand is too coarse and the air permeability is too high, the molten metal will easily drill into the pores between the sand particles, making the casting surface rough or wrapping and fixing the sand particles on the surface. A large amount of 30/50 mesh coarse-grained core sand is continuously mixed into the old cast iron sand of a foreign-funded factory in Jiangsu, which makes the permeability of molding sand reach above 220 and the surface of the casting is extremely rough. Small and medium-sized castings are produced by pneumatic microseismic molding machine in casting workshop of a factory in Inner Mongolia. Using 40/70 coarse-grained quartz sand mainly concentrated in 40 meshes as molding sand, the surface of castings is seriously adhered to sand. Usually, the permeability of molding sand is not tested, and it is considered that the requirements of process specification ≥80 have been met. In order to find out the cause of sand sticking, it is found that the air permeability is as high as 1070, which shows that this is the cause of sand sticking. Therefore, the permeability of molding sand must have an upper limit, and the particle size and permeability of molding sand should be within a suitable range. The most suitable sand size for single sand molding of general vibratory compactor is 70/ 140 mesh, and the air permeability is about 70~ 100. The optimum sand size for high-density molding is 50/ 140 or 100/50, and the air permeability is 80~ 140. Some foundries producing engines use 50/ 100 mesh coarse sand as sand cores. When falling sand, it is continuously mixed with old sand, so that the permeability of molding sand may reach more than 180. Therefore, fine sand with 100/ 140 mesh should be added, or the fine particles in the cyclone separator should be returned to the old sand to correct the particle size of molding sand.

2. The content of pulverized coal in cast iron molding sand is insufficient or the quality of pulverized coal is poor. A foundry in Beijing produces brake discs for high-speed trains. The casting material meets the requirements, but the surface is seriously bonded with sand, so it needs to be polished before leaving the factory. The coal powder used in molding sand comes from a small private supplier with close ties in the suburbs. The reason of sand sticking may be that the quality of pulverized coal is too poor or the effective amount of pulverized coal in molding sand is not enough. The "pulverized coal" used in a valve factory in Anhui Province is the waste produced by coke washing, and its ash content is as high as 76%. After use, the performance of the whole molding sand was destroyed, and more than half of the castings were scrapped. Foundry should strengthen the inspection of the quality of purchased pulverized coal. High-quality pulverized coal requires ash ≤ 10%, volatile matter 30~37% and coke residue characteristics 4~5. The effective pulverized coal content of molding sand can be detected by gas production method. The amount of gas produced by using ordinary pulverized coal molding sand per kloc-0/g is about 22~26mL, which is equivalent to 6 ~ 7% of ordinary quality effective pulverized coal, or 5 ~ 6% of high-quality pulverized coal, or 4 ~ 5% of synergistic pulverized coal. The gas production of high compactness molding sand is about 18~24mL, which is equivalent to 3~4% of the content of synergistic pulverized coal. LOI is used by most foreign foundries in China to evaluate the sand sticking resistance of green sand for cast iron. For example, the static pressure molding line of an automobile foundry in Jiangsu stipulates that the burning loss of surface sand is 4. 10 0.30%. There are many modeling materials companies in China that supply all kinds of "coal powder substitutes". The foundry should first carry out pouring tests to compare the surface effect of high-quality pulverized coal or synergistic pulverized coal on castings, the change of molding sand performance and the production cost of castings, and then decide whether to choose them.

Two. sand hole

Sand holes and slag holes on the surface of castings are usually collectively referred to as "sand holes". Slag holes are mostly formed by using straw ash or dry sand as slag aggregating agent. The causes of sand holes are as follows:

1. A joint venture foundry in Tianjin produces small and medium-sized gray iron castings such as motor casings by hand molding. The main defect is that scattered sand grains can be seen on the upper surface of the whole casting. It is considered that the sand hole is caused by sand washing, which is formed by scattered sand particles falling from the gating system and molten iron floating on the liquid surface washing the cavity. Factories usually do not control the quality of molding sand. According to the cloud, the wet compressive strength is only 25kPa. The optimum wet compaction strength of artificial molding sand is about 70~80kPa, and the molding of vibration compactor should be 90 ~ 120 kPa. For high-density molding, the wet compressive strength of molding sand can reach 140~ 180 kPa. Large pieces can be higher. In order to improve the wet compressive strength of molding sand, inferior bentonite should be avoided, and the blue absorption of 0.2g bentonite should be above 35mL. Molding sand also needs to contain enough effective bentonite. For example, the blue absorption capacity of 5g of high-density molding sand is mostly 55~65mL. 6-7% of high-quality effective bentonite.

2. There is only one vibration compression molding machine in a foundry in Shandong. After going to work, make a lower mold to cover the ground and the lower core. After half a day, change the formwork to make the upper mold and the closed mold, and prepare for pouring. Defects such as sand holes often appear in castings. The reason is that the surface strength of green sand mold drops sharply after dehydration and drying, and the surface sand particles are easily eroded and fall into molten iron. The phenomenon of "air drying" is more serious in dry season. The opening time of the wet sand box should not exceed half an hour. If there are signs of drying and dehydration on the surface of the sand mold, spray water on the surface of the sand mold with a sprayer before modeling to restore the wet state. A Japanese-funded automobile engine factory in Tianjin used to spray the surface of the cavity with imported surface enhancer, but now it also uses water spray.

3. An automobile parts foundry in Sichuan uses hydrostatic molding machine assembly line to produce cylinder block and cylinder head, and there are many sand holes on the casting surface. In this factory, inferior bentonite and pulverized coal are used as molding sand, and the used sand is not regularly dusted, resulting in the mud content of the used sand sometimes rising to 24%. In order to keep the water content of molding sand at about 4.0% and prevent the formation of porosity defects, the compactness of molding sand must be kept in the range of 27 ~ 32%. The green compressive strength of molding sand is not low, ranging from 170 ~ 2 10 kPa, which is not the cause of sand holes. Due to the high ash content, low compactness and insufficient toughness of molding sand, the crushing index is only about 65 ~ 75%. This kind of molding sand is too brittle, its demoulding performance is poor, and its edges and corners are easily broken, thus causing sand holes. The factory should switch to high-quality bentonite and pulverized coal; Dust removal equipment for used sand should also be used to control the mud content of used sand below 12% and that of molding sand below 13%. Control the crushing index of molding sand at 80 ~ 85%. The compaction rate of molding sand at the molding site is increased to 35 ~ 38%, and the water content is 3.2~3.6%, so that the ratio of (compaction rate)/(water content) is within the range of 10~ 12. This can improve the toughness of molding sand and reduce sand holes. Several factories in Shanghai, Beijing and Harbin added a small amount of α -starch into sand to improve the toughness of molding sand, reduce the friction resistance of mold drawing and enhance the surface air-drying strength. It is beneficial to prevent sand holes and improve the surface smoothness of castings.

4. The engine foundry branch of a tractor factory in Henan Province is mixed with a large number of cold box sand cores, which makes the molding sand brittle and the demoulding performance worse and worse. Not only the edge of sand mold is fragile, but also the hanging sand is easy to break. According to the factory regulations, the cycle time of the grinding wheel sand mixer is only 3 minutes, so the sand mixing time cannot be extended, so as not to affect the sand demand of the molding machine. Later, the sand mixing cycle was extended as much as possible 1min, and it was found that the feel of molding sand changed and the demoulding property was improved. This shows that the original sand mixing time is too short to produce excellent molding sand performance

3. Sand inclusion (scarring and peeling)

Due to the supply of high-quality activated bentonite by many domestic companies, the sand inclusion defects on the surface of wet castings are greatly reduced. However, some green mold foundries will occasionally produce sand inclusion defects.

1. A small garage in Jiangxi wants to produce aluminum castings for motorcycle engines in wet state. I borrowed two brown paper bags of Qiushan "clay" to mix sand. Later, I went to the materials department and bought two clay bags. However, it was found that the newly purchased clay sand had low adhesion, and the sand mold cracked and peeled after fire baking, resulting in serious sand inclusion defects in the castings. At that time, it was tested whether the mud of two kinds of clay could be thickened by alkali activation under extremely simple conditions. It is proved that the sack is not bentonite, but real clay, which can not be used in green mold casting. The reason for the problem is that weakly acidic calcium-based bentonite is called "acid clay" by the geological department. Many foundries refer to "acid clay" as "clay". Results Bentonite with montmorillonite as the main mineral component is confused with real clay with kaolinite as the main mineral component (that is, ordinary clay). True clay is mainly used for firing ceramics and is not suitable for wet mold casting. Foundries can also identify two different clay minerals by their blue absorption ability. 0.2g bentonite absorbs methylene blue in 25~45mL, while ordinary clay absorbs methylene blue only one tenth of bentonite.

2. Impact of water quality: A Taiwan-funded foundry in Tianjin uses an extrusion molding machine to produce cast iron frying pans for export. Using high-quality activated bentonite mixed sand, the compressive strength of green sand is 200 ~ 250 kPa, the compactness is 35 ~ 38%, and the water content is about 3%. However, there is a sand inclusion defect near the inner runner of the casting, and it is suspected that there is a problem with the well water used for sand mixing. The well pipe used for deep well water mixed with sand in this factory is blocked. In order to save money, the boss drilled a 20-meter shallow well to mix sand and water. The workers found that the water in this well was salty and could not be drunk, and it did not foam when washing hands and rubbing soap. This shallow well water contains a lot of sodium, magnesium and chloride ions, which has a strong anti-activation effect on activated bentonite, and it is easy to produce sand inclusion defects when it is used to produce castings with mixed sand. After drinking water mixed with sand is imported from nearby factories, the influence of the original water quality cannot be completely eliminated. There is a factory in Jiangsu province that produces refrigerator compressor castings by extrusion molding. Mixing sand with river water flowing through a small river outside the factory coincides with the discharge of wastewater from a chemical plant in the upstream, resulting in sand inclusion defects in castings, which is also due to the anti-activation effect of wastewater. If you doubt whether the water quality is suitable for sand mixing, you can take 2g or 3g bentonite and measure the expansion value, or expansion multiple and free expansion with purified water and water to be measured. If the test result of the water to be tested is much lower than that of purified water, it means that the quality of the water to be tested is not available.

Four. air hole

Pore defects in castings are mainly divided into four types: entrainment porosity, invasion porosity, precipitation porosity and reaction porosity. The following examples illustrate the causes and preventive measures of common blowholes in factories.

1. It is not easy to determine the type and cause of pores. According to the production experience, increasing the pouring temperature by 30~50℃ can often reduce the occurrence of any type of blowhole defects. Pay attention to the temperature of the last one or two sand molds poured in each ladle, because the temperature of molten iron in the ladle has dropped at this time, which is easy to cause blowhole defects. A Taiwan-funded foundry in Tianjin produces industrial sewing machine shells. Each molten iron can be poured with 7 sand molds, but only 5 sand molds are poured. The molten iron left in the ladle is poured back into the electric furnace, and then a spoonful of molten iron is poured into the sand mold again to keep the pouring temperature and reduce the porosity defects.

2. A Japanese factory in Beijing once found a casting with air holes. After sawing, it was found that the pores kept floating out. It is estimated that the back pressure exceeds the static pressure of molten iron at the interface where the pores are generated, which leads to the invasion of pores, but it is impossible to judge what the gas source is. Some factories treat old sand piles as garbage dumps, cigarette butts, popsicles, waste paper balls, melon seed skins ... thrown into old sand and mixed with molding sand, which may form pore defects. Smoking is forbidden in some foreign foundries, which is also one of the effective measures to prevent blowholes.

3. A factory in Shandong produces medium-sized export valve iron castings, which are molded by impact molding machine and made into sand cores by cold box. The factory adopts two-day continuous modeling, falling core modeling, and the cupola is poured every other day. The porosity rejection rate of the castings produced is very high. The analysis reason is that the gas generated by the sand core absorbing moisture enters the molten iron, resulting in the invasion of pores. The cold box sand core is placed in the sand mold with high relative humidity for a long time, which is easy to absorb moisture. When pouring, not only the binder releases gas, but also the water absorbed by the sand core releases a lot of water vapor, which is easy to produce blowhole defects. We should open the furnace every other day as usual, or close the mold after molding, and then open the box, open the core and open the box for pouring on the same day. It can not only prevent the sand core from absorbing moisture, but also reduce the air drying and dehydration of the sand mold, so that the porosity defects are greatly reduced. Open more gas risers to increase the exhaust capacity. Properly increasing pouring speed and quickly establishing static pressure to resist the invasion of interface gas are also beneficial to prevent the invasion of pores.

4. Judging from the blowhole defects in engine casting plants in Henan, Shandong, Liaoning, Jilin and other places. Most of the pores encountered are still pore defects caused by sand core gas, and few are nitrogen pinholes precipitated. Because the binder of the old sand core is changed to resin with low nitrogen content, an appropriate amount of iron oxide is added to the core sand and coating when necessary. Therefore, first of all, we should strengthen the exhaust ability of sand core. An unobstructed vent hole should be opened in the middle of the sand core. For the sand core with thick and large cross section, the sand core can be hollowed out or half-dug into a grid cavity and then bonded. The most common way to exhaust the resin self-hardening sand core is to use nylon braided pipe, which can be easily embedded into the sand core along any shape of the sand core when making the core. The hot core box, the cold core box and the shell core are all injected integrally, so the exhaust pipe cannot be embedded, and the ventilation needle or rod can be placed and taken out before or after coring. But more commonly, after the sand core is hardened, a cemented carbide drill is used to drill holes from the core head to help exhaust. For example, a hydraulic parts factory in Shanxi produces hydraulic valves with extremely complicated shapes, and the core head of the shell core is all drilled with angle drills to help exhaust. There is a foundry in Spain that produces cars. The cylinder head water jacket sand core is made, and the core heads of each cooling water channel of the water jacket sand core are drilled simultaneously from bottom to top with a special multi-head drilling machine. When placing a large sand core, if the gap between the core head and the core seat is too large, molten iron will drill into the sand core vent hole. The core head of the sand core should be sealed with sealing materials such as refractory fiber felt pad, mud strip and asbestos rope. Also pay attention to high-temperature rapid pouring, and quickly establish that the pressure of molten iron exceeds the back pressure of the gas generating point, so that gas can not drill into molten iron and become bubbles. Even if the gas has drilled into the molten iron, it will float and be discharged together with the molten iron entering the exhaust riser. In addition, it is necessary to use low gassing adhesive to prevent blowhole defects. For example, the combustion ring of a British gas stove produced by a factory in Beijing has only one core, so it is difficult to exhaust, so the gas evolution from the shell core should be controlled below 12mL/g as far as possible, and it should be poured quickly at high temperature.

5. A factory in Shanxi produces gray cast iron crankshaft by extrusion molding machine, and there are small pores on the surface and under the casting. Generally, it is a small hole with a diameter of 1~3mm, which mostly exists in the epidermis 1 ~ 3 mm and is exposed during shot blasting cleaning or rough machining. This factory does not use resin sand cores, and there will be no nitrogen holes. Defects should belong to reaction holes. That is, the molten metal reacts with the mold at the interface, and the generated gas is dissolved in the molten metal. When cooling, the solubility decreases and bubbles are precipitated. The casting material is gray iron, which also excludes the reaction between magnesium or rare earth in molten iron and moisture in sand mold. It is suspected that charging materials and inoculants may bring aluminum and titanium into molten iron. Because aluminum and titanium react with water to release the atomic state [H] which is soluble in molten iron. When the layer solidifies, the solubility of hydrogen decreases, which precipitates in the molecular gas phase and grows into hydrogen bubbles. From the analysis report of ferrosilicon inoculant in this factory, it can be seen that the aluminum content reaches 1.86%, which may be the main reason for subcutaneous stomata. The aluminum content of ferrosilicon inoculant is preferably about 65438 0.0%, and the maximum is not more than 65438 0.5%. For the ferrosilicon used in stream inoculation, it is necessary not only to control the low aluminum content, but also to limit the addition amount to 0.08 ~ 0. 10%. In order to prevent aluminum, titanium and other elements from reacting with water, the water content of squeeze molding sand must also be controlled at no more than 4%.

6. After the ductile iron molten iron is poured into the green sand mold, the residual magnesium reacts strongly with the oxygen in the water molecules, resulting in atomic state [H], which is the main reason for the subcutaneous blowhole defect. Melting and technological measures must be taken to prevent the formation of reaction holes. A ductile iron factory in Hebei is a professional factory that produces ductile iron castings such as truck clutch pressure plate, and the castings have no subcutaneous pores. From the point of view of smelting in this plant, the reason for avoiding blowholes is that all indexes are not beyond the conventional range. If high-quality cast coke is used, the discharging temperature of cupola is above 1480℃, the inner cavity depth of spheroidized ladle is 1.5~2 times the diameter, cryolite powder shakes on the surface of ladle and inner cavity, and the molten iron contains 0.03~0.04% sulfur and 0.40~0.46% residual magnesium. But from a technical point of view, the water content of surface sand is as high as 7.0~7.6%, far exceeding the maximum of 5.5%. The reason why there are no pores may be that a large amount of pulverized coal is added to (1) molding sand, and the loss on ignition is as high as 7.0~7.9%. 4~5% more than usual. The gas production of molding sand is unpredictable, estimated to be above 35mL/5g. The surface of the casting exposed after pouring is dark blue, which shows that there is a lot of strong reducing atmosphere in the cavity during pouring, which dilutes the water vapor in the cavity. (2) The permeability of the sand mold is 100, and there are a large number of exhaust holes, so most of the water vapor generated by pouring is discharged from the mold cavity, which reduces the water vapor that may participate in the reaction. (3) Although the water content of surface sand is quite high, the silt content is as high as 2 1%. Therefore, the measured compaction rate is only about 36%, indicating that the molding sand is not wet. Maybe the mud absorbed a lot of water and slowed down the chemical reaction. Therefore, it can be considered that the defect of reaction porosity mainly depends on the compactness (that is, the degree of dryness and wetness of molding sand), rather than the water content.

Discussion on verbs (abbreviation of verb)

1. The condition of obtaining high-quality molding sand is to select high-quality raw materials and apply excellent sand mixing technology at the same time. Some green sand foundries with strict technical management require all the sand in the sand mixer to be removed before the end of each shift. Most major green foundries in the United States require that the distance between the scraper and the chassis of the sand mixer is the thickness of the coin. The sand mixing cycle of several auto parts foundries in Japan using roller sand mixer is 6 minutes. However, in some domestic foundries, the sand in the grinding disc and grinding wheel of sand mixer is not cleaned for a long time, and the wear of scraper is not adjusted, so the grinding wheel can be mixed for up to 3 minutes. How can this be mixed to produce excellent molding sand?