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 is a complex process stage. Here are just a few methods commonly used in our country for casting refractory bricks:
1. Casting method
1.1 Ordinary casting method (code name: China PT, Corning, Xu, Toshiba RC, Xipu RN): Castings are cast with ordinary risers, and the risers are removed when they are hot. The section is divided into two parts, one part is solidified first , The crystallization is fine, this area accounts for 40%-50% of the thickness of the casting, and the other part is solidified afterwards, and there are shrinkage holes and coarse crystals. Bricks cast in this way are relatively inexpensive, and are mostly used in the upper structure of the kiln, the wall of the clarification tank, etc.
1.2 Tilt casting method (code: China QX, Corning TA, Xu TC, Toshiba TCL, Xipu RO): Tilt casting method is to angle the mold before casting and place the riser on one end of the mold for casting In this way, a dense zone can be obtained in the T section, and a higher accuracy can be obtained in the T direction by using a common mold. Therefore, when the pool wall is built with this kind of casting, its height can be used.
1.3 Casting without shrinkage cavity (code: China WS, Corning VF, Asahi VF, Toshiba DCL, casting, concentrate the shrinkage cavity in a certain area, after annealing, remove it with a diamond saw, the remaining useful parts are uniform in composition, and the structure is dense , The average bulk density is close to the theoretical density; the other is the cutting leg method: starting from reducing the cutting area, casting the casting into an "L" shape, so that most of the shrinkage cavity is concentrated on the smaller "L" leg The volume of this leg occupies 60% of the total volume of the casting. The entire casting is always buried in the insulation material during annealing, and is kept tilted to promote the shrinkage cavity to concentrate on the leg. This process is expensive because of the high cutting cost-diamond saw cutting costs generally It is higher than the price of the casting body, and Gu is only used in individual cases.
2. Characteristics of the casting process
The casting process has a great impact on the quality of the casting, not only affecting the integrity of the casting shape, but also directly affecting the internal quality of the casting. The characteristics of the casting process are as follows:
1) During the casting process, there is a violent heat exchange process and chemical reaction process between the molten liquid and the mold. During casting, the temperature of the molten liquid is very high, and there is a great temperature difference between the molten liquid and the mold. Therefore, during the casting process, the molten liquid is continuously cooled and the temperature decreases, while the mold is heated, and the composition of the mold material decomposes and vaporizes. Some chemical reactions increase the air pressure in the cavity, which is not good for filling. In severe cases, swelling will appear, which is due to defects such as porosity layer or insufficient casting.
2) The molten liquid casting process is an unstable process. The impact of the casting stream and the uneven flow, etc., will cause the casting to produce defects such as bulge, cold barrier, and hollow shell on the surface of the casting port.
3) The process of filling the sand mold with the melt is similar to filling the porous container, because the sand mold wall has certain air permeability. If the pressure in the cavity is lower than the air pressure in the mold wall, the melt will inhale external air and cause defects such as pores. On the contrary, the molten liquid will be pressed into the pores of the mold wall, causing serious sand adhesion.
4) The length of the casting process has a significant impact on the temperature distribution of the casting.
3. Casting process
The casting process includes casting temperature, casting speed, casting time and supplemental casting
3.1 Casting temperature
The casting temperature is the temperature at which the melt in the furnace is poured into the mold. Generally, the temperature of the stream near the furnace nozzle is measured with an optical pyrometer. When AZS brick is 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 temperature plays an important role. The higher the temperature of the melt, the lower its viscosity, and thus the better the fluidity and filling. The stronger the ability. But it is not that the higher the casting temperature, the better. 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 speed will increase. While the shrinkage stress increases, the initial grains are coarsened and the components are segregated. It is very easy to produce hot cracks when the core part of the casting finally solidifies, especially for the castings after the second year. 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 be specified to prevent insufficient filling capacity.
3.2 Casting speed and casting time
The casting speed determines the casting time. Every casting has an optimal casting time, and improper casting time will gradually produce many defects. If the casting speed is too fast, the stream will be thick, the flow rate will be fast, and the impact force on the mold will be large, and a part of the mold will be broken or melted, and the part of the casting will produce protrusions. In addition, when the coarse melt is quickly poured into the mold, a part of the gas is taken into the mold and quickly rises to the top of the mold. At this time, the melt contacting the top cover has formed a thin shell, and the thin shell is filled with gas to form a so-called empty shell. At the same time, the introduced gas is also easy to form bubbles in the mold. In addition to gas, the coarse stream poured at high speed may also bring the raw material in the furnace nozzle area into the melt and form 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, resulting in loose corners. If the melt poured in first has solidified into a thin shell, shrink inward. The later poured melt 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, resulting in insufficient pouring. Moreover, because the casting time is too long and the baking time of the mold cover is too long, it is easy to peel off and fall into the melt to cause sand inclusion.
3.3 Refill
After the casting is finished and cold-shrinked for a period of time, the shrinkage cavity appears to be filled with melt again. This operation is called supplemental pouring. Usually small bricks solidify quickly and cannot be re-poured, medium-sized bricks can be re-poured with a short interval, and only large-scale bricks have a longer time. Supplementary pouring is one of the effective means to reduce the shrinkage of castings and increase the bulk density. It is actually equivalent to expanding the volume of the riser. The key to operation is to control the most appropriate time for supplementary pouring. Realizing continuous multiple pouring is an important method to increase product bulk density.
4. The relationship between casting and porosity in casting
It is normal for ordinary cast AZS refractory bricks to have shrinkage cavities and shrinkage porosity, but it is often found that there are many pores. Obviously, the existence of any pore will directly reduce the quality of the casting.
There are two types of pores in castings, one is microscopic pores, which can only be seen under a microscope when made into thin slices; the other is macroscopic pores, which are visible to the naked eye, which is what we often call pores. They come from four aspects: charge, melting process, casting material, casting process.
Here we discuss the influence of mold and casting on porosity.
4.1 Porosity caused by casting material
There are two common pores on the edge of the casting in contact with the mold:
1) The dense subcutaneous honeycomb pores perpendicular to the mould wall are caused by the wet sand working surface. Water is a gas-producing substance. A unit volume of water is heated to 1000 degrees and becomes water vapor. When the pressure remains the same, the volume increases by 1700 times. If heated to the casting temperature, it may reach nearly ten thousand times. Therefore, the water on the surface of the sand mold suddenly generates so much gas that will form a high pressure, which will cause the gas to invade the solidified edge melt and extend to the direction of low resistance, so it becomes a long strip. Therefore, it is forbidden to use wet molds during casting operations.
2) The subcutaneous round pores are mostly single round bubbles within 10 mm from the edge. This is due to the decomposition of the sand adhesive when it comes into contact with the melt to generate a large amount of gas, part of which escapes from the gap of the sand, and part of it due to transient The cavity is not well exhausted and the pressure invades the melt, so it is very important to pay attention to the cavity exhaust during casting. Of course, improve the air permeability of the sand mold (for example, use round sand to clean the dust in the sand, and the air permeability on the back is greater than the working surface的, etc.) means that it is more important to expand from the inside out.
4.2 Porosity caused by casting operation
There are three types of such pores: the hollow shell of the casting port, the pinholes in the foam layer, etc.
4.2.1 Empty shell
During rapid casting, the cavity is not well vented, so an air pocket is formed at the dead corner of the brick top surface. After solidification, it becomes a large air pocket layer with only a thin shell, which is called an empty shell in the waste analysis. In order to prevent the formation of hollow shells, in addition to the time limit of the casting operation, it is also specified that the casting stream should be thick and then thin, and fast and then slow. But when it is close to the full type, it must be faster, otherwise there will be insufficient pouring at the upper corners, and the type cover must be pressed by heavy objects or manpower to avoid being pushed open by the static pressure formed by the melt.
4.2.2 Foam layer
The sundries such as insulation materials fall into the mold, and react with the first segment of the molten liquid falling into the cavity to form a rigid bubble layer, which floats on the liquid surface and floats up to the top surface of the cavity, forming a nozzle surface after solidification It is necessary to check and keep the cavity clean before casting.
4.2.3 Pinhole
Pinholes often occur in the first brick cast. This is because the furnace nozzle area is not cleaned up, mixed with raw materials or graphite furnace nozzle oxidation outcrop powder, etc., which react sharply with the melt and then poured into the mold. Due to the viscosity of the melt Large, these bubbles can't float up, so they stay in the melt irregularly. Therefore, keeping the melt channel clean during casting cannot be ignored. The most common pores caused by other conditions are the dampness of the charge and the insufficient melting temperature, which makes the casting become the shrinkage cavity dispersion type or bread type. In addition, there are operations that are not noticed by people. For example, (1) the electrode was moved in series before casting, causing a large amount of graphite powder to fall into the furnace, resulting in the inclusion of pores in the casting and not being dense; (2) live casting, melting at the electrode When the liquid continues to react, the material liquid is not dense, and obviously the casting will not be dense; (3) Casting with raw material, especially when there is raw material in the front area of ??the furnace mouth, the raw material enters the casting, in addition to destroying the lithofacies structure In addition, pores are also generated.
5. Defects and prevention measures in the casting process
5.1 Defects in the casting process
5.1.2 Shrinkage of castings
The formation of shrinkage cavity, shrinkage porosity, internal stress, cracks, deformation and other defects in castings are all related to the shrinkage that occurs when the melt is solidified. The shrinkage of castings can be divided into three stages: liquid shrinkage, solidification shrinkage, and solid state shrinkage. For AZS with a fixed composition, the liquidus temperature is constant, so the higher the casting temperature, the greater the liquid volume shrinkage. The solidification shrinkage is manifested as the continued decline of the liquid level in the cavity, so the solidification shrinkage plus the liquid shrinkage is the basic reason for the shrinkage of the casting. From this difference in specific gravity, it is known that the volume shrinkage of this part of AZS can reach 12%-15%.
5.1.3 The formation of shrinkage cavity in castings and its influencing factors
The formation of shrinkage cavity: when the melt starts to solidify layer by layer from the mold wall in the mold, if the melt caused by liquid shrinkage and solid shrinkage is reduced and cannot be supplemented, concentrated shrinkage will occur where the casting finally solidifies hole.
When the edge temperature of the casting drops below the solidus temperature, the surface of the casting transforms into a hard shell, forming a sealed container containing the solution. When it is further cooled, the melt in the shell shrinks in liquid form due to the continuous decrease in temperature on the one hand, and solidifies and shrinks due to the thickening of the hard shell. In addition, the solid shell also shrinks due to the decrease in temperature, which reduces the outer size of the casting. Since the liquid shrinkage and solidification shrinkage of the AZS melt greatly exceed the solid state shrinkage of the shell, under the action of gravity, the page will separate from the top surface of the shell and shrinkage holes will appear. As the solidification continues, the hard shell will continue to thicken and the liquid level will continue to drop. After the melt has completely solidified, a shrinkage cavity will be formed in the core part under the casting opening.
If the amount of gas in the solution in the hard shell is small, when the liquid level separates from the top surface of the shell, the shrinkage cavity will form a vacuum, and the thin shell on the upper surface may sink in the direction of the shrinkage cavity under the action of atmospheric pressure. The shrinkage cavity should include two parts: the outer shrinkage cavity and the inner shrinkage cavity.
The factors that affect the shrinkage cavity volume include: the melt itself shrinks too much, the shrinkage cavity volume is large; the melt itself solidifies and shrinks, the shrinkage cavity volume is large; the melt itself shrinks in solid state, the shrinkage cavity volume decreases; the more the casting speed Slowly, the smaller the shrinkage cavity volume; the greater the chilling capacity of the mold, the smaller the shrinkage cavity volume.
5.2 The formation of shrinkage porosity in castings and its influencing factors
Shrinkage porosity refers to the scattered small cavities or dense coarse crystals, or coarse crystal groups under the shrinkage cavities of castings. What can be seen by the naked eye is called shrinkage porosity. The formation of shrinkage porosity is mainly related to the composition of the melt, the characteristics of crystal solidification and the solidification sequence of the casting. When the crystallinity of the casting is strong, the final solidified shrinkage cavity becomes free to grow coarse crystals, and the part of AZS is often coarse. The columnar corundum crystals have hexagonal zirconia crystals on the crystal surface. When the periphery of a slender casting is solidified and it is difficult to feed the melt, the lower part will produce axial shrinkage.
5.3 Basic methods to reduce shrinkage porosity and shrinkage of castings
Formulate a reasonable casting process to effectively control the solidification process, establish good feeding conditions, and convert the shrinkage porosity into concentrated shrinkage cavities as much as possible, and move it to the place where the casting finally solidifies, so that a risk can be added to it. Mouth, so that the shrinkage cavity finally moves into the riser, so as to obtain a dense product.
The solidification process is divided into two ways: sequential solidification and layer-by-layer solidification: the so-called controlled solidification process is the natural layer-by-layer solidification (the surface layer solidifies first and gradually grows thicker to the center of the casting) artificially from the end area of ??the casting away from the riser to the riser Between the ports, a gradually increasing temperature gradient is established to make the end area solidify first, and then solidify in the direction of the riser in order, that is, to achieve sequential solidification, so as to achieve the purpose of feeding the melt and introducing shrinkage holes into the riser. Sequential solidification tends to cause large temperature differences in different parts of the casting, which leads to defects such as cracks and residual stress in the casting.
5.4 riser
5.4.1 The role of riser and its feeding principle
The additional part of the non-cast body that supplements the solidification shrinkage of the casting is called a riser. The riser has the function of reducing shrinkage and exhaust, and as an observation hole. The size, shape and quantity of the riser must ensure that it is the slowest solidification part, and has enough volume to hold enough melt to supplement the volume shrinkage of the casting during the solidification process.
The basic principle of feeder feeding: During the solidification of the casting, there must be a temperature gradient towards the riser. When the riser area and the end area of ??the casting are connected to each other, a dense casting without shrinkage can be obtained. On the contrary, if the riser area and the end area are separated by a middle area without temperature gradient, the middle area will appear axis shrinkage.
5.4.2 Design of riser
The content of the riser design is the shape, size, quantity and disposal method.
1) Riser shape
According to the feeding principle of the riser, the riser must be the slowest solidification part. Therefore, when designing the riser, it is first required to have a low heat dissipation rate. When the shape should have the smallest surface area under the same volume, the heat loss will be slow. Calculations show that the surface area of ??the sphere is the smallest, followed by the cylinder, the cube, and then increasing sequentially. The riser design also needs to consider the difficulty of making in actual production, so spherical risers are not used in
AZS brick production.
2) Riser size
It mainly refers to its diameter and height. Due to the complex shape and different sizes of the castings, the required risers cannot be the same. Therefore, a feeding modulus is introduced, which is the ratio of the volume of the riser to the volume of the casting, and the size of the riser is selected with the goal of controlling the unit weight of the casting. A large-volume riser is effective for increasing the bulk density of the casting. The large-volume riser is divided into a large riser or two middle risers, or one riser is used up and then replaced with a riser.
Regarding the height of the riser, it is generally believed that the higher the riser, the greater the hydrostatic pressure and the more obvious the feeding effect. In fact, if the riser is too high, it will not be able to withstand the static pressure of the melt and bulge, and the bottom of the casting will have cracks after annealing, so it is not advisable to increase the riser height one-sidedly without improving other process conditions. According to the law that the shrinkage cavity and porosity of AZS occupy half of the casting height, it is believed that the design principle of the riser height should be consistent with the height of the casting, that is, the ratio of the two is 1:1.
3) Number of risers and disposal methods
Generally, one riser is used for one casting, and two risers are used for castings longer than 700 mm in ordinary casting. There are two treatment methods for risers: cold cutting and hot eradication. Cold cutting riser: When the riser and the casting are annealed together, the removal of the riser must be cut with a diamond saw blade. Hot removal of the riser: After the casting is finished, wait for the riser feeding to be completed and immediately remove the riser, cover the nozzle with a sand plate, and then anneal. The time control of hot removal of the riser is very important. Too early, the role of the riser will be reduced, the melt will overflow, and the bulge of the nozzle will increase the amount of cold working: On the contrary, if it is too late, the casting is prone to internal cracks, so according to the size of the brick, The size of the riser is generally controlled for 12-40 minutes. Keep the surface of the nozzle slightly lower than the surface of the casting and smooth and beautiful.
