Casting
After the batch material reaches the casting standard after the melting and refining stage in the electric arc furnace, the operation process of pouring the molten liquid directly into the mold from the electric furnace is called casting. Although this process is short, each step is related to the quality of the final product and it is a complex process stage. Here are just a few common methods for casting refractory bricks in my country:
01 Casting method
Ordinary casting method (code: China PT, Corning, Asahi, Toshiba RC, Xipu RN): The casting is cast with ordinary risers, and the risers are removed in the hot state. The cross section is divided into two parts. One part solidifies first and the crystals are fine. This area accounts for 40%-50% of the thickness of the casting. The other part solidifies later and has shrinkage holes and coarse crystals. Bricks cast in this way are relatively low in price and are mostly used in the upper structure of the kiln, the wall of the clarifier, etc.
Tilt casting method (code: China QX, Corning TA, Asahi TC, Toshiba TCL, Xipu RO): Tilt casting method is to make the mold into an angle before casting, and put the riser at one end of the mold for casting, so that a dense area can be obtained in the T part, and a higher precision can be obtained in the T direction using ordinary molds. Therefore, when using this casting to build the pool wall, its height can be used.
Shrinkage-free casting (code: China WS, Corning VF, Asahi VF, Toshiba DCL, casting, concentrate the shrinkage cavities in a certain area, cut it off with a diamond saw after annealing, the remaining useful part has uniform composition and dense structure, and its average volume density is close to the theoretical density; the other is the cutting casting leg method: starting from reducing the cutting area, the casting is cast into an "L" shape, so that most of the shrinkage cavities are concentrated on the smaller "L" leg, and the volume of this leg accounts for 60% of the total volume of the casting. The entire casting is buried in the insulation material during annealing and remains tilted to promote the shrinkage cavities to concentrate on the leg. This process is only used in individual cases due to the high cutting cost - the diamond saw cutting cost is generally higher than the price of the casting body.
02 Characteristics of the casting process
During the casting process, There is a violent heat exchange and chemical reaction process between the molten liquid and the mold. The temperature of the molten liquid is very high during casting, and there is a large temperature difference between the molten liquid and the mold. Therefore, during the casting process, the molten liquid is continuously cooled and the temperature is reduced, while the mold is heated, and the components of the mold material are decomposed and vaporized, and undergo certain chemical reactions with the molten liquid, which increases the air pressure in the mold cavity, which is not conducive to filling the mold. In severe cases, mold expansion may occur, and the casting may have defects such as a porous layer or insufficient casting.
The molten liquid casting process is an unstable process. The impact of the casting stream and the uneven flow rate may cause defects such as bulging, cold shut, and hollow shell on the surface of the casting mouth.
The process of filling the sand mold with molten liquid is similar to filling a porous container. Because the sand mold wall has a certain air permeability, if the pressure in the mold cavity is lower than the air pressure in the mold wall, The melt will absorb external gas and cause defects such as pores. Conversely, the molten liquid will be pressed into the pores of the mold wall, causing serious sand attachment.
The length of the casting process has a significant impact on the temperature distribution of the casting.
03 Casting process
Including casting temperature, casting speed, casting time and re-pouring
1. Casting temperature
The casting temperature is the temperature of the molten liquid in the furnace when it is poured into the mold. Generally, an optical pyrometer is used to measure the stream temperature near the furnace nozzle. When AZS bricks are strengthened and melted, the casting temperature can reach 1820-1840 degrees. The viscosity of the melt depends on the chemical composition and temperature of the melt, and the composition of the melt is determined by the formula, so the temperature plays an important role. The higher the temperature of the melt, the lower its viscosity, and thus the better the fluidity and the stronger the filling ability. But it is not the case that the higher the casting temperature, the better. If If the casting temperature is too high, the temperature difference between the casting and the model will decrease, the width of the solidification zone from the surface to the inside will increase, and the solidification shrinkage rate will accelerate. While the shrinkage stress increases, the initial grains will coarsen and the composition will segregate. When the core part of the casting finally solidifies, hot cracking is very likely to occur, especially for the second-largest casting. Therefore, an upper limit of the casting temperature should be specified according to the size and shape of the casting to prevent cracking, and a lower limit should also be specified to prevent insufficient filling capacity.
2. Casting speed and casting time
The casting speed determines the casting time. Every casting has an optimal casting time. Improper casting time will gradually produce many defects. If the casting speed is too fast, the flow will be coarse and the flow rate will be fast. The impact force on the mold will be large, and part of the mold will be broken or melted, causing protrusions in this part of the casting. In addition, when the coarse melt is quickly poured into the mold, a part of the gas is brought into the mold and quickly rises to the top cover of the model. At this time, the melt contacting the top cover has formed a thin shell, which is filled with gas under the thin shell, forming the so-called empty shell. At the same time, the gas brought in is also easy to form bubbles in the casting mold. In addition to gas, the coarse stream poured at high speed may also bring the raw material in the nozzle area into the melt, forming inclusions in the melt. On the contrary, if the casting speed is too slow, defects such as loose corners, knots, sand inclusions and insufficient pouring will also occur. When the pouring speed is slow and the stream is very thin, the melt poured into the model first solidifies into small balls and fills the corners, causing loose corners. If the melt poured first has solidified into a thin shell, it shrinks inward. The melt poured later enters the gap between the thin shell and the model to form surface scars.
At the same time, if the stream is too thin, the melt has solidified before reaching the corners, causing insufficient pouring. Moreover, due to the long casting time and the long baking time of the mold cover, it is easy to peel off and fall into the melt to cause sand inclusions.
3. Refilling
After the casting is completed, the shrinkage cavity that appears is filled with molten liquid after a period of cold shrinkage. This operation process is called replenishing. Usually small bricks solidify very quickly and cannot be re-cast, medium-sized bricks can be re-cast with a shorter interval, and only large bricks need a longer re-casting time. Re-casting is one of the effective means to reduce shrinkage holes in castings and increase bulk density. It is actually equivalent to expanding the volume of the riser. The key to the operation is to control the most suitable time for re-casting. Realizing continuous multiple re-casting is an important method to increase the bulk density of products.
HOME