1.1 Mechanical sand and chemical sandThe mechanical sand sticking on the surface of the sand casting is that the metal liquid is directly drilled into the pores between the sand particles, and is connected with the sand particles by the encircling and hooking of the metal, without chemical reaction. The reason for the production of chemical sand is that the high temperature metal liquid may be oxidized to produce metal oxides, the main product is ferrous oxide FeO, and its melting point is 1370℃. FeO reacts with SiO2 of molding sand to produce ferrous silicate (i.e. ferriolivine FeO•SiO2), the chemical reaction is as follows:
SiO2 + 2FeO 2FeO•SiO2
The melting point of ferrous silicate is very low, only 1220 ° C, so the fluidity is very good, even if the casting surface has a solidified shell, the newly generated ferrous silicate is still liquid, easy to penetrate into the sand pores. The condensed ferrous silicate has a strong adhesion to the casting and molding sand, and can firmly adhere the molding sand to the surface of the casting to form a chemical adhesive sand.
The production of cast iron with wet sand generally only forms mechanical sand, and does not form chemical sand. This is because the iron liquid contains a lot of carbon, will not produce a lot of iron oxide and other metal oxides. The sand mold also contains a considerable amount of pulverized coal, and the reducing atmosphere generated during pouring can prevent metal oxides. The low SiO2 content of raw sand is not an inevitable condition for the formation of chemical sand in wet cast iron. The results show that the Yellow River aeolian sand with SiO2 content only about 82% is not found to have chemical sand in wet cast iron production.
It is difficult to distinguish the two kinds of sticky sand with the naked eye, and can usually be distinguished by the following methods:
(1) Microscopic inspection: knock a small piece from the sticky sand layer, fix it with liquid resin and grind it into a sample, and observe it with a metallographic microscope. If the sand is mechanically glued, the individual sand grains can be clearly seen sandwiched between the metal. There is a clear dividing line between the infiltrated metal and the sand grain, and there is no chemical reaction product. The infiltrated metal metallographic configuration is consistent with the metallographic structure of the casting body (see Figure 2). If the sand is chemically bonded, it can be seen that there is a new phase in the sand layer that binds the casting to the sand particles (see Figure 3).
(2) Electrical measurement: The bonding material in the mechanical sand is metal and has good electrical conductivity. Turn the knob of the multimeter to the resistance setting and contact the casting with one electrode and the sand area with the other electrode. If the resistance is close to zero, it indicates that the adhesive sand is a mechanical adhesive sand formed by metal wrapping sand particles. If there is a huge resistance, it indicates that non-conductive ferrous silicate has been formed in the sand sticking part, which is a chemical sand sticking.
Chemical identification: Use a flat shovel to chisel down a small piece of sticky sand and immerse it in a test tube containing concentrated hydrochloric acid. If bubbles occur slowly, the color of the liquid changes from colorless transparent to brownish-red overnight. At the end of the reaction, the sticky sand block disappeared, leaving a few single sand grains at the bottom of the test tube, indicating that the mechanical sticky sand, and the iron part has been dissolved into ferric chloride by hydrochloric acid. The chemical reaction formula is:
2Fe + 6HCl 2FeCl3 +3H2↑
If it is chemically cemented sand, there is little bubble production and no significant change in acid. The final residue is a porous mass flocculent.
1.1.1 Influence of various factors on mechanical sand adhesion
The actual production experience shows that the weight of the wet casting is generally not more than one or two hundred kilograms, the wall thickness is mostly not more than 50mm, the water in the sand causes the chilling effect of the casting shell to cool and solidified quickly, and the heating effect of the sand is not too serious. Although the raw sand for cast iron contains quartz (melting point 1715 ° C), it also contains a considerable amount of feldspar (melting point 1170 ~ 1550 ° C), mica (melting point 1150 ~ 1400 ° C) and other minerals with a low melting point, but at the same time, the coal powder contained in the wet cast iron sand inhibits the formation of iron oxide, so it does not cause chemical reactions. Production experience shows that wet steel castings are generally mechanically cemented rather than chemically cemented. This is because the wet cast steel parts are not thick and large castings, and the silica sand used contains high SiO2, the thermal effect of the casting on the molding sand is not serious, and the iron olivine is not produced significantly.
The following will discuss the various factors affecting the production of mechanical sand in castings:
1.1.1.1 Sand compaction degree
If the tightness degree of manual molding and seismic molding is low, the sand particles on the surface of the sand mold are relatively loose, and the pits and corners of the sand mold cavity are more likely to appear loose. If the liquid metal is drilled into the pores between the sand grains, the surface of the casting will appear rough; Drill deeper and wrap the sand to form mechanical sand. The molding worker can take the finger plug and use the sharp end of the punch hammer to punch the sand part tightly. Whether the high-density molding with high productivity has local porosity depends on the fluidity of the sand, so many factories try to reduce the compaction rate of the sand to improve the fluidity of the sand. In the process of sand filling and compaction, it is very effective to improve the compactness of sand mold by using microvibration. In addition, it also depends on the level of hydraulic or air pressure set by the compaction device. Figure 4 shows a gray iron automobile casting with mechanical sand sticking, using an imported static press molding machine, two pieces in a box. However, the pressure regulation of the hydraulic system is not proper, and the compaction ratio of the sand box is low. And the amount of sand between the two pieces and the sand box is only about 25mm. Sand surface hardness is only 50 ~ 60, edge side hardness is less than 40.
1.1.1.2 Particle size and permeability of molding sand
On the one hand, the thickness of the wet sand should ensure that the exhaust is smooth after pouring, and on the other hand, the permeability of the wet sand should not be too high, so as to avoid the metal liquid easily penetrating into the pores between the sand particles. There are more exhaust holes on the sand mold of small parts produced by hand molding, and the sand is often used. The sand grain can be finer, and the air permeability of the sand is about 40 to 60. The molding sand particle size of wet single sand is roughly 70/140 mesh, and the air permeability is mostly in the range of 60 to 90. High density sand is more dense, which requires high permeability of sand. The particle size is mostly 50/140 or 140/50 mesh, and the air permeability is more concentrated in 100 ~ 140. In many factories, the raw sand size of the sand core is coarser than the sand size, such as the raw sand size of the automobile engine cylinder core is 50/100 mesh, and there will be a lot of core sand mixed with the sand and the sand size will be coarser in the long-term production. As a result, the sand permeability of some factories is as high as 160 or more, or even about 200. Unless the paint is sprayed on the surface of the sand mold, the surface of the casting becomes rough and may even have local mechanical sand. In the United States, a factory adds 5% of 100 and 140 mesh two-screen fine-grained new sand when mixing wet sand to correct the coarser sand phenomenon, so that the sand particle size is maintained at 50/140 four-screen distribution.
1.1.1.3 Liquid metal pressure
The higher the liquid metal pressure, the more serious the mechanical sand sticking. Therefore, the bottom of tall castings is easier to form mechanical sand.
1.1.1.4 Casting temperature and casting wall thickness
The liquid metal has high temperature and good fluidity, and it is easy to penetrate into the pores between the sand particles and produce mechanical sand. However, in order to avoid defects such as porosity and cold insulation, the pouring temperature can not be arbitrarily reduced. The production of complex thin wall castings requires a higher pouring temperature.
1.1.1.5 Sand paint
To produce wet castings with large weight, alcohol-based paint can be sprayed into the cavity of the sand mold, and the core and mold can be cut after ignition. Generally, the upper type can not spray paint, because the metal hydraulic head is smaller than the lower type. Another advantage of the spray coating is to improve the erosion resistance of the sand surface. However, the formula of the wet type coating is different from that of the sand core coating, and its strength cannot be too high, and it must match the strength of the sand mold, otherwise it may crack the coating and warping the skin, and cause the casting to produce sand clamping defects. If there is a resin core or oil sand core in the wet sand mold of general cast iron with low internal cavity requirements, in order to prevent liquid metal from drilling into the sand core, a coating paste composed of oil or other binders and graphite powder, quartz powder or other refractory powder can be applied on the surface of the hardened sand core where the metal liquid is easily penetrated locally, and the core can be lowered after cooling and drying. When producing cast iron parts with high internal cavity cleanliness and finish requirements (such as cylinder heads, body, hydraulic system valves, etc.), the sand core must be immersed or coated as a whole and then the surface is dried. When the cast iron is produced by hand, the clay toner is carefully applied on the surface of the wet sand mold and sand core with a soft brush. Some also spray graphite and water mixed liquid, can be poured after drying. Graphite powder can fill the pores without being wet by liquid iron, which is difficult to drill into the sand. Caterpillar Foundry in the United States uses high pressure modeling to produce large engine cylinder body of construction machinery, and its measure to overcome mechanical sand sticking is to rely on the upper and lower sand mold comprehensive automatic spray base coating. Then automatically spray with a large flame gun to dry the coating and sand surface. The variety and amount of bentonite, pulverized coal and other materials used in the surface drying molding sand, as well as the performance control of the molding sand are different from that of ordinary wet molding sand.
1.1.1.6 Pulverized coal content of molding sand
Pulverized coal is the most important sand additive to prevent sand sticking and improve surface finish of wet cast iron. But the sale of pulverized coal is mixed. Generally, the effective pulverized coal in the production of medium sized cast iron molding sand may be 3.5-7.0%, mainly depending on the quality of pulverized coal and the requirements for the as-cast surface. In order to eliminate the influence of pulverized coal quality, only 1g of molding sand at 900℃ can represent the effective pulverized coal content. For example, the sand gas output of ordinary machine modeling can be between 20~26mL/g, and the sand gas output of high 宻 degree modeling can be in the range of 16~22mL/g. It is commonly used in foreign countries to determine whether the content of pulverized coal in molding sand is enough. For example, some factories require sand burn reduction of 3.0~5.0%. In the actual production, the appearance of the casting can be seen to find out whether the effective amount of coal powder contained in the molding sand is appropriate. If the surface of the casting is rough, and the permeability of the sand and the compactness of the sand are not improper, there may be insufficient effective coal powder or poor coal powder quality. If the surface of the casting is obviously blue, but it is rough, the effective coal powder may be enough, and the sand is high in permeability, or the sand is not compact enough.
At present, there are many kinds of substitutes for coal in our country. Among them, the anti-sand effect of starch material is basically equivalent to that of high quality pulverized coal. However, it is only suitable for the production of gray iron castings, such as the production of ductile iron parts may produce subcutaneous porosity defects, because it cannot produce enough reducing atmosphere. There are also some “coal substitute” goods, the real specific formula is unknown, and the use effect is also very different. Users should rely on the pouring test to judge its actual anti-sand effect. Use the same original sand (not old sand, so as not to interfere with the test results), bentonite, water, and then add different anti-sticking sand materials to mix molding sand. Efforts should be made to keep the sand permeability is the same or close, molding hardness is the same, the pouring temperature is the same. Compare the surface finish of the casting, and then you can make a selection decision.
There are two main types of anti-sticky sand products produced abroad:
(1) Enhanced pulverized coal (high efficiency pulverized coal) : adding 20-40% high softening point petroleum asphalt to pulverized coal, so that its bright carbon content is increased to 12-20%, and the anti-sticky sand ability is greatly improved. At present, there are several companies in China that supply coal with enhanced efficiency. (2) Mixed additives: It is a mixture of high-quality bentonite and high-quality pulverized coal, and can also be added to starch, wood powder and other materials as needed. The characteristics of the products in a production line in a large casting plant are similar, and the ratio of bentonite to pulverized coal does not need to change frequently. The use of mixed additives is easy to control and manage, and the equipment is simplified. The formula is jointly formulated by the engineers of the supply and demand sides according to the production conditions of the casting. It is transported to the workshop by bulk tanker and pneumatic into the material tank. When the user mixes sand, only one additive can be added.
When a single sand is mixed, the amount of pulverization coal is first determined by the quality of the pulverization coal itself, and is also affected by the sand/iron ratio, casting thickness, pouring temperature, cooling time, cleaning method, specific requirements for the surface finish of the casting and other factors. Some factories in Germany say that the unit of pulverized coal supplement is kg of pulverized coal supplement per 100kg of hot metal and per 1% of bright carbon formation (that is, effective pulverized coal). For example, Mettmann casting plant statistics in the production of bright carbon formation (coal) supplemental amount of 0.14 ~ 0.27kg / 1% bright carbon formation / 100kg iron. The example of the German Southern Chemical Company has a sand/iron ratio of 10:1 and an ECOSIL pulverized coal consumption of 18kg/t Fe per ton of iron poured. That is, 10 tons of molding sand per ton of hot metal is poured, and 18kg of ECOSIL pulverized coal is added to the molding sand, which is equivalent to 0.18% of pulverized coal when mixing sand. If the sand/iron ratio of most factories in China is about 6:1, the amount of ECOSIL pulverized coal mixed sand should be 0.30%. According to the casting manual “Modeling Materials” (page 103 ~ 104 of the 2nd edition), China’s Dongfeng Motor Company, FAW Casting Co., LTD., China Yituo Group Company, Shanghai Automobile Engine Company and Nanjing Taixi Cast Iron Co., LTD., 14 kinds of high-density modeling line wet type single sand formula. The maximum amount of pulverized coal added in sand mixing is 3 ~ 4%, and the minimum is 0.3 ~ 0.5%. In addition, FAW, Texix, Shanghai engine plant single percussion molding sand 4 kinds. The highest amount of pulverized coal is 3-5%, and the lowest is 1-1.25%. The reason for the high amount of pulverized coal replenishment in most of the above-mentioned Chinese factories is that the pulverized coal used in these factories is of low quality. The author’s experience of using high quality pulverized coal and enhanced pulverized coal in some factories in China in recent years shows that the additive amount of mixed pulverized coal in single sand of general wet cast iron is between 0.15 ~ 0.3%, and that of individual thick and large pieces is 0.5%. The average ratio of sand to iron in a gas-washed line in Fushun is 11:1, and the average ratio of sand to iron in an extruded line in the same workshop is 7.5:1. The additive amount of pulverized coal added to the sand mixed by the sand treatment system shared by the two lines is only 0.08 ~ 0.12%. It can be seen that even if the price of high-quality and efficient pulverized coal is slightly higher (less than twice of ordinary pulverized coal), the consumption is only a fraction of that of ordinary pulverized coal. After use, not only the production cost is greatly reduced, but also the storage and transportation costs are saved. Moreover, the clay content and water content of molding sand are greatly reduced, and the toughness, air permeability and molding ability are improved. Not only the surface of the casting is smooth, but also the defects such as pores and sand holes must be significantly reduced.
1.2 Explosive sand bonding
In the casting line of the mechanized casting factory, it is often seen that after pouring, an explosion will occur between almost every sand box and the trolley table, which will not cause casting defects. However, sometimes it is also possible to see another explosion inside the cavity that can cause the surface of the casting to stick to sand, which is called explosive sticking to sand. High-density casting may appear this explosive sand defect, and usually mechanical sand appears in the lower surface and hot joint of the casting position is different, explosive sand mostly occurs in the casting position of the upper surface. The reason for the explosion is that the water of the sand mold evaporates and condenses on the upper surface of the mold cavity at a low temperature when the metal liquid level rises and contacts with the upper surface of the mold cavity. When the water evaporates suddenly, the explosion occurs, and the huge gas pressure generated forces the metal liquid to drill into the surface of the sand mold and form sticky sand. Sometimes the explosion is so violent that liquid metal is even spewed out of the riser straight into the roof. It is easy for sand to explode when the sand has high water content and compactness, high coal content, high mold hardness, poor ventilation condition and fast pouring speed.
1.3 Hot sticky sand
Hot bonded sand is a relatively rare type of bonded sand. There are several phenomena:
(1) The SiO2 content of raw sand used for cast iron wet molding sand is low, such as the SiO2 content of the Yellow River aeolian sand and some local river sand or mountain sand is only about 80%, and the sintering temperature of the original sand itself is low. When heavy parts are poured, the surface of the casting is coated with a thick layer of sand. If the molding sand contains sufficient pulverized coal, the sintered sand layer is easy to fall off and be cleaned away, and there is no mechanical sand sticking.
(2) Hebei Province has a household extrusion molding machine production of gray cast iron auto parts factory, most of the surface of the casting can be exposed after sand drop, after a short time of shot blasting cleaning the casting surface is quite clean. However, it was suddenly found that the surface of the casting was covered by a layer of sand after sand fall. After shot blasting, the surface of the casting can be easily exposed, indicating that the iron liquid is not drilled into the sand mold and does not belong to mechanical sand. The abnormal phenomenon is “hot sticky sand” defect. The reason for this will not be the reduction of silica in the original sand, because the plant has always used a stable quality inner coating sand. The casting temperature of liquid iron is not too high. It is suspected that the bentonite company added too much sodium carbonate ingredient when treating activated bentonite. Sodium carbonate itself is a metallurgical flux, which can reduce the sintering point and melting point of silica sand and bentonite and cause hot sticky sand.
Post time: May-10-2024
