Induction furnace casting crankshaft

It is a casting process that the inner cavity of the iron mold of the crankshaft mold is evenly covered with a layer of 5-8mm heat curing molding sand to form a mold. Due to the good rigidity of sand coated iron mold, thin sand coating, fast cooling speed and fine sand particle size (generally 200 mesh), the dimensional accuracy, compactness, surface quality, grain refinement and spheroidization rate of castings can be significantly improved, and riser free casting can be realized.

Production process of iron mold sand coating line. At present, there are many forms of process layout of iron mold sand coating line, most of which are open line layout. Because it can meet the requirements of different cooling time in crankshaft production, and can realize the mechanization and automation control of iron mold movement and iron mold propulsion in the production process.

1. Sand molding

Sand covered molding process is the key process of iron mold sand covered casting, which directly affects the whole process of production and the yield of crankshaft. At first, the hot box core shooter was used in the sand coating of iron mold, so the mechanization degree was low. At present, many foundry machinery manufacturers in China have produced special sand covering equipment for iron mold. Some of them can realize the whole process of PLC touch screen full-automatic control from mold closing to sand removal after shooting.

(1) Mold temperature

The mold temperature is generally controlled at 230 ℃ in normal production process. When it is too low, the coated sand can not be completely solidified, the shell strength is low and the gas generation is large; when it is too high, the shell is burnt crisp or the coated sand has been solidified in the process of flow, and the destruction of the shell strength is easy to cause sand sticking and sand inclusion on the crankshaft. There are generally two heating methods for the first use of iron mold: one is to heat a batch of iron mold in the core kiln, and then mold them one by one; the other is to heat the iron mold one by one through the pattern and the template (the template is equipped with an electric heating tube) to mold the cold iron mold. This method is relatively slow for the first production of new iron mold. In the normal production process, the residual heat of iron mold can be used for sand coating without heating alone.

Figure 3 production process of iron mold sand coating line

(2) Sand shooting

Sand shooting is a process in which low pressure compressed air (0.4MPa) is used to blow the flow coated sand into the mold cavity, and the sand shooting time is only 1 ~ 2S. In this process, there are two key links: one is exhaust, the other is sand.

When the coated sand is injected into the mold cavity, the compressed air will also enter the mold cavity. If the iron mold exhaust is not smooth, it will cause insufficient injection, air drum and other defects. The common solutions are as follows: ① for the residual gas near the parting surface, make four bumps 0.1-0.2mm higher than the template on the template to form a gap between the template and the iron mold to discharge the residual gas; ② add exhaust plug at individual dead corner or carve exhaust groove on the template to discharge the residual gas. These two methods are quite effective in actual production.

During sand shooting, the sand shooting hole on the sand shooting head is aligned with the sand shooting hole of the iron mold, the lower plane of the sand shooting plate is pressed with the upper plane of the iron mold, and then sand shooting is carried out. But in the actual production, due to the stress deformation after the iron mold casting, the deformation caused by the continuous heating and cooling of the iron mold and the collision of the upper surface of the iron mold, there will be a gap between the upper surface of the iron mold and the lower plane of the sand shooting plate during sand shooting, and the sand will run out of the gap during sand shooting. Sand running not only wastes sand and worsens working environment, but also causes waste products due to inaccurate injection of mold cavity. At the same time, it is easy to cause personal injury to operators due to too fast sand flow rate. At present, the common methods are as follows: 1) inlay a silica gel pad 5mm higher than the plane on the lower surface of the sand shooting board; 2) make the nozzle of the sand shooting board into a movable compressible type, and install a compression spring on the back. In this way, the sand shooting nozzle can be in close contact with the surrounding of the iron mold sand shooting hole to avoid sand running. When the mold is seriously deformed, not only the upper surface of the mold runs away, but also there is a large gap between the lower surface of the mold and the molding board. At this time, the upper and lower sides of the mold can only be planed before reuse.

2. Iron mold cooling

In the actual production, in order to ensure the production efficiency, it is impossible to cool the crankshaft to about 230 ℃ and then open the box (usually 15-20 minutes after pouring). Because the sand coating is very thin, the temperature of the iron mold rises sharply after pouring. The temperature of the iron mold after a cycle is about 350-400 ℃, and it will become higher and higher, resulting in the abnormal production. Because the natural cooling is too slow, if the mold is watered in order to speed up the cooling rate of the mold, it is very easy to cause the mold cracking. At present, the domestic manufacturers of iron mold sand coating line have not produced special equipment for iron mold cooling. In order to solve the problem of mold cooling, we designed a cooling device according to the actual situation.

Fig. 4 iron mold cooling device

The nozzle structure is shown in Figure 5. After the compressed air enters the pipe, the air flow speed is accelerated through the reducing sleeve, and a greater negative pressure is formed at the copper pipe mouth. After the other end of the copper pipe is connected with an oxygen pipe and put into the water in the water tank, the water in the copper pipe is sucked out, and the high-speed air flow atomizes the water. Through the cone of the nozzle and the cone below, the fog is sprayed to the iron surface in the shape of a cone. The cooling device adopts multi head atomization cooling, which makes the iron mold cool evenly, reduces the deformation and cracking of the iron mold, and improves the service life of the iron mold. At the same time, when the water mist is sprayed on the surface of the iron mold, a layer of high-temperature water vapor film is quickly formed on the surface of the iron mold. The high-speed air flow of compressed air blows away the high-temperature water vapor film in time, which speeds up the cooling speed of the iron mold and meets the needs of continuous production of the production line.

3. Packing

Because the coated sand uses phenolic resin as curing agent, a large amount of gas will be produced after pouring hot metal into the mold cavity. If the gas can not be removed in time, it will cause blowholes and insufficient pouring. For this reason, in addition to the design of vent holes in the upper mold pattern, exhaust through the sand covering about 5mm from the sand shooting needle to the upper mold surface, 0.5mm iron sheets should be padded at the four corners of the parting surface when closing the box, so as to make the parting surface produce clearance and exhaust gas.

Melting, spheroidizing, inoculation and pouring of hot metal

Because of the rapid cooling of the casting, the iron mold sand coating process provides favorable conditions for obtaining castings with finer grains, higher spheroidization level, better surface quality and better mechanical properties, but it also puts forward higher requirements for melting, spheroidization, inoculation and pouring of molten iron.

1. Smelting

Because of the rapid cooling speed of the casting, the inclusions and gas in the molten iron entering the mold cavity are not easy to float out, so it is easy to form slag inclusion and porosity, and cold shut is also easy to form. Therefore, high temperature and clean molten iron are required in the production of iron mold sand coating process. Now because of the rising price of coke, the smelting cost of medium frequency induction furnace is no longer higher than that of cupola. At the same time, medium frequency induction furnace can obtain stable chemical composition, high temperature and clean molten iron, and can meet the needs of continuous production. Therefore, medium frequency induction furnace is the best choice for casting crankshaft with iron mold sand coating process. Generally, the melting temperature of medium frequency induction furnace is about 1500 ℃.

Similarly, because of the rapid cooling speed of iron mold with sand coating process and the high undercooling degree of molten iron melted by medium frequency induction furnace, the tendency of casting white spot is large, and the casting is easy to form free cementite after cooling. Therefore, the carbon silicon equivalent in proportioning is higher than that in clay sand molding process, and the common chemical composition is shown in the table.

Chemical composition of casting ingredients

2. Spheroidization

Spheroidizing treatment is an important link in the production of nodular cast iron crankshaft. The shape of graphite directly affects the strength performance of crankshaft. At present, the domestic nodularizing agent mainly uses the rare earth magnesium silicon iron composite nodularizing agent. Which brand of nodularizing agent to choose depends on the absorption rate and whether the reaction is stable. Because the iron mold sand coating process requires a higher temperature of hot metal discharged from EAF, it is better to choose a lower grade nodularizing agent, such as fesimg8rt3. In order to reduce the secondary slag formed by the interaction of sulfur and magnesium, the residual rare earth magnesium should not be too high; when the sulfur content of the original hot metal is too high, the porous plug ladle can be used for desulfurization, and then poured into the electric furnace to raise the temperature.

3. Inoculation and pouring

Due to the rapid cooling of iron mold sand coating process and the high undercooling of hot metal in electric furnace, the tendency of chill in the solidification process of castings is large. Therefore, it is necessary to inoculate the molten iron sufficiently. Generally, two inoculation treatments are carried out: the first is to put 0.2% ～ 0.3% ferrosilicon into the ladle during spheroidizing treatment; the second is to add 0.1% flow inoculation during pouring, and to use composite high-efficiency inoculant containing calcium and barium, which can effectively increase the graphite core, refine the grain, and delay the decline time of inoculation. Generally, the particle size of inoculant is 60 mesh, so it should not be placed for a long time to avoid moisture and oxidation.

The pouring temperature is generally about 1420 ℃, and attention should be paid to slag retaining during pouring. Due to the rapid solidification of molten iron and the difficulty of inclusion floating, filters are usually placed in the gating system.

Auxiliary alloying of crankshaft after heat normalizing

Rapid solidification of castings and high temperature pure molten iron in medium frequency melting furnace provide favorable conditions for grain refinement and spheroidization level improvement of castings. Under normal production conditions, the pearlite content is generally 65% ～ 75%, the tensile strength is 680 ～ 750N / mm2, and the elongation is 4% ～ 6%. In the molten iron treatment process, add 0.5% to 6% Cu and trace Sb which strongly promotes pearlite transformation. The continuous mechanization or automation control of the iron coated sand line makes the open box work easier to control. When the casting temperature is about 860~900 degrees Celsius, the box is opened, and the casting is rapidly transferred to the cooling chamber for spray cooling. In this way, the nodular cast iron crankshaft blank with pearlite content of 85% ~ 90% and mechanical properties of qt800-3 can be obtained. It can not only improve the high temperature environment of the cleaning workshop, reduce the round-trip transportation and one-time cleaning, but more importantly, it can eliminate the huge heat treatment cost and effectively reduce the production cost. It has become an inevitable trend for crankshaft production to replace the present normalizing heat treatment by normalizing with waste heat.

Iron mold sand coated crankshaft casting simplifies the process of clay sand, overcomes the inherent weakness of clay sand (huge sand treatment, dirty and poor working environment and low material performance), and can realize continuous and controllable production, which provides favorable conditions for mechanization and automation of high-quality crankshaft production. This advanced technology will be more and more widely used in foundry production.

Yiphee Electric Furnace Co.,Ltd specializes in producing induction smelting furnace.Tel(WhatsApp):+86-13450756789, yiphee@yiphee.cn