Large precision casting shell preheating device and method

Abstract

The invention provides a large precision casting shell preheating device and method, the device comprises a sand box, a heating power supply and a temperature sensor, sand is contained in the sand box, and the outer surface of a precision casting shell is coated with a silicon carbide layer; the heating power supply is connected with the silicon carbide layer through a heating cable, the silicon carbide layer provides heat for heating, and the heating power supply is electrically connected with a temperature control system; and the temperature sensors are respectively arranged at the central position, the edge position and the thickness change position of the precision casting shell. The method comprises the steps that a large precision casting shell is formed, and the surface of the precision casting shell is treated; the silicon carbide coating is attached to the surface of the precision casting shell; embedding the precision casting shell into the sand in the sand box, and arranging a temperature sensor on the precision casting shell; and the temperature is preset, and the precision casting mold shell is rapidly heated. The silicon carbide layer is used for coating the outer surface of the precision casting shell and is buried in the sand, and when the large precision casting shell is heated through the silicon carbide layer, heating is uniform, heat conduction is fast, and temperature rise can be fast.

Description

Technical Field

[0001] The invention relates to the technical field of casting, and in particular to a large precision casting shell preheating device and method based on silicon carbide coating heating. Background Art

[0002] At present, in the process of large-scale precision casting, shell preheating is an extremely critical link. Traditional shell preheating technology has many shortcomings. For example, the external heating method is difficult to ensure that the inside of the shell, especially the complex cavity, is evenly heated, which can easily lead to defects in the casting; and some existing internal heating methods, such as some heating elements, are difficult to place inside the complex cavity, or are inconvenient to remove after heating, affecting the integrity and subsequent use of the shell. In actual application cases, in order to prevent the large shell from breaking during heating and pouring, it is necessary to bury the large shell in sand for protection. However, in the process of heating with a traditional heating furnace, the heat transfer speed in the sand is slow. In order to prevent the temperature inside the shell from being uneven, the heating time is extended and the temperature gradient is reduced. In the end, it took 7 days to heat the shell to 800°C. The heating efficiency is very low and the temperature uniformity is not easy to control. In summary, an innovative shell preheating technology is needed to solve the heating efficiency and temperature control accuracy problems of shell preheating technology.

[0003] Therefore, how to quickly and evenly heat the large precision casting shell is a problem that the industry urgently needs to solve. Summary of the invention

[0004] A main purpose of the present invention is to overcome at least one defect of the above-mentioned prior art and to provide a large precision casting shell preheating device and method capable of quickly and evenly heating and raising the temperature of the large precision casting shell.

[0005] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0006] According to one aspect of the present invention, a large-scale precision casting shell preheating device is provided, which is used to preheat a large-scale precision casting shell, wherein the precision casting shell has a gate and a runner, and comprises:

[0007] A sandbox, wherein the sandbox contains sand, the precision casting shell is contained in the sand, and the outer surface of the precision casting shell is coated with a silicon carbide layer;

[0008] A heating power supply, wherein the heating power supply is connected to the silicon carbide layer through a heating cable, the silicon carbide layer provides heat for heating, and the heating power supply is electrically connected to a temperature control system, and the temperature control system controls the output power of the heating power supply;

[0009] The temperature sensor is multiple and is respectively arranged at the center, edge and thickness variation position of the precision casting shell. The temperature sensor is communicatively connected to the temperature control system and sends the collected temperature data to the temperature control system.

[0010] According to another aspect of the present invention, a method for preheating a large precision casting shell is provided, using the large precision casting shell preheating device described above, comprising the following steps:

[0011] Step 1: forming a large precision casting shell and treating the surface of the precision casting shell;

[0012] Step 2: The silicon carbide coating is attached to the surface of the precision casting shell;

[0013] Step 3: bury the precision casting shell in the sand in the sandbox, and arrange temperature sensors at the center, edge and thickness change positions of the precision casting shell;

[0014] Step 4: Preset the temperature and quickly heat the precision casting shell. During the heating process, compare the temperature with the preset temperature and control the temperature within a 10% error of the preset temperature.

[0015] According to a specific embodiment of the present invention, the large precision casting shell is a truncated cone complex cavity shell with a diameter of 1000 mm and a height of 650 mm. The most obvious difference between the large precision casting shell and the small shell is the size. Its outer dimensions are large, and it can cast large precision parts, such as large aircraft engine blades, large ship propellers, etc. Due to its large weight, it must withstand various external forces during transportation, roasting and pouring. For example, when pouring molten metal, the large shell needs to withstand huge liquid impact and pressure, so it needs to have higher room temperature strength and high temperature strength to prevent the shell from breaking. The large precision casting shell is used to cast precision parts, and the requirements for dimensional accuracy are very strict. It is necessary to ensure that the inner cavity size of the shell is accurate so that the casting meets the designed dimensional tolerance range and ensures the quality and performance of the casting. Large precision casting shells often have complex internal structures, including various reinforcing ribs, support structures and complex cavities. These complex structures are to meet the molding requirements of large castings and ensure the stability of the shell during the casting process. During the pouring process of the large precision casting shell, the shell needs to allow the gas in the cavity to be discharged smoothly. If the large mold shell has poor air permeability and gas cannot be discharged smoothly, it is easy to cause defects such as pores in the casting, affecting the quality of the casting.

[0016] According to a specific embodiment of the present invention, in step 1, after using a chemical cleaning agent to remove surface oil, a sandblasting device is used at 2kg / cm 2 Sand blasting is performed with left and right pressure and 40 mesh sandblasting particle size.

[0017] According to a specific embodiment of the present invention, in step 2, a slurry coating method is used to mix silicon carbide powder with a high temperature resistant binder to form a slurry, and the shell is immersed in the slurry and then taken out, and the silicon carbide coating is attached to the surface of the shell after drying and high temperature curing. The thermal conductivity of the silicon carbide layer is as high as 120-180W / mK, which is several times or even more than ten times that of traditional metal materials. It can quickly transfer heat from the high temperature area to the low temperature area, effectively ensuring the normal operation of the equipment under high temperature environment. At the traditional service temperature, the thermal conductivity of the silicon carbide layer gradually decreases with the increase of temperature, but the thermal conductivity of the silicon carbide single crystal increases with the increase of temperature from room temperature to 500°C, and decreases with the increase of temperature above 500°C. The thermal conductivity of the silicon carbide layer is affected by factors such as pores and grain boundaries. The thermal conductivity decreases significantly when the porosity increases or the number of closed pores increases; its thermal conductivity coefficient can be improved by controlling the grain size and distribution, material density and chemical composition, etc. For example, special processes such as hot pressing sintering and spark plasma sintering can be used to produce silicon carbide ceramics with high thermal conductivity, but the size is limited. Although processes such as pressureless sintering can produce silicon carbide ceramics with large size and complex structure, the thermal conductivity is relatively low.

[0018] According to a specific embodiment of the present invention, the high temperature resistant adhesive is silica sol. Silica sol can withstand high temperatures of 1500°C-1600°C. Under high temperature conditions, its internal silicon oxygen bond structure is stable and not easy to decompose and destroy, which can ensure the strength and stability of the bonding parts and is suitable for high temperature industrial kilns, aerospace and other fields. The silicon dioxide particles in silica sol are fine and can fully fill the pores of the bonded objects. After drying, they form a strong silicon oxygen bond, which greatly improves the bonding strength and enhances the overall structural strength of refractory materials. Silica sol has stable chemical properties and is not easy to react chemically with other substances in harsh environments such as high temperature, acid and alkali. It can resist chemical erosion, ensure the durability and reliability of the bonding effect, and extend the service life. Silica sol has a low viscosity, fine particles, good fluidity and permeability, and can be evenly dispersed on the surface and inside of the bonded object to form a continuous and dense bonding layer, which improves the uniformity and integrity of the bonding. Silica sol is non-toxic, odorless, and contains no volatile organic compounds. It is harmless to the environment and human health during production, use, and disposal, and meets environmental protection requirements. It is a green and environmentally friendly binder. Silica sol can enhance the strength, wear resistance, and oxidation resistance of refractory materials, and can also improve the density and thermal shock resistance of materials, and enhance their comprehensive performance at high temperatures. When using silica sol as a binder, there is no need for complex curing processes and the addition of other curing agents. After drying, a strong bond can be formed, which simplifies the production process, improves production efficiency, and reduces costs.

[0019] According to a specific embodiment of the present invention, in step 2, thermal spraying or cold spraying technology is used to mix silicon carbide powder with a carrier gas and spray it onto the mold shell surface through a spray gun. In thermal spraying, a high-temperature flame or plasma is used to melt or semi-melt the powder and then impact the mold shell to form a coating; cold spraying uses a high-speed airflow to make the powder impact the mold shell surface in a solid state and adhere.

[0020] According to a specific embodiment of the present invention, the carrier gas is nitrogen. Nitrogen is an inert gas with stable chemical properties and is not easy to react chemically with other substances under conditions such as high temperature and high pressure. This can avoid the interference of experimental results or damage to equipment due to impurities generated by chemical reactions during the carrier gas process. Nitrogen has high purity and is easy to obtain. High-purity nitrogen can ensure the quality of the carrier gas and reduce the adverse effects of impurities. In addition, there is mature air separation technology in the industry to produce nitrogen in large quantities, and the cost is relatively reasonable. The diffusion rate of nitrogen is moderate. This allows it to transport the sample to the specified position more stably when it is used as a carrier gas to carry the sample, and it will not be too fast to make the sample pass through the detection area before it has time to react to the detection, nor will it be too slow to affect the detection efficiency. In terms of safety, nitrogen is a non-flammable gas, which reduces the risk of fire and explosion during use and can be used with confidence in various occasions with safety requirements.

[0021] According to a specific embodiment of the present invention, the coating thickness is 0.5 mm to 2 mm, or the coating thickness uniformity deviation is controlled within ±10%.

[0022] According to a specific embodiment of the present invention, in step four, the preheating temperature range of the precision casting shell is set to 900-1000 degrees through the temperature control system; the heating power supply is started, the silicon carbide coating generates heat to preheat the precision casting shell, and the temperature sensor monitors the internal temperature of the shell in real time and feeds back to the temperature control system; the temperature control system controls the output of the heating power supply according to the feedback information, so that the internal temperature of the precision casting shell is maintained within a preset range of 900-1000 degrees, if the temperature is higher than the upper limit value, the output of the heating power supply is reduced, and if the temperature is lower than the lower limit value, the output of the heating power supply is increased, and the silicon carbide coating adopts a segmented heating method, and the heating amplitude of each segment does not exceed 50 degrees.

[0023] It can be seen from the above technical solution that the advantages and positive effects of the large precision casting shell preheating device and method of the present invention are:

[0024] The present invention uses a silicon carbide layer to coat the outer surface of the precision casting shell and bury it in sand. When the large precision casting shell is heated through the silicon carbide layer, it is heated evenly and has fast heat conduction, and can be heated quickly, thereby improving the preheating efficiency and preheating quality of the large precision casting shell. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1It is a structural schematic diagram of a large precision casting shell preheating device of the present invention.

[0026] Description of the figure number:

[0027] 1. Heating power supply;

[0028] 2. Temperature control system;

[0029] 3. Precision casting shell;

[0030] 4. Gate and runner;

[0031] 5. Silicon carbide layer;

[0032] 6. Heating cable;

[0033] 7. Temperature sensor;

[0034] 8. Sandbox;

[0035] 9. Sand. DETAILED DESCRIPTION

[0036] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in a variety of forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the present invention will be comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar structures, and thus their detailed description will be omitted.

[0037] In the following description of different examples of the present invention, reference is made to the accompanying drawings, which form a part of the present invention and in which different exemplary structures, systems and steps that can implement multiple aspects of the present invention are shown by way of example. It should be understood that other specific schemes of components, structures, exemplary devices, systems and steps can be used, and structural and functional modifications can be made without departing from the scope of the present invention. Moreover, although the terms "top", "bottom", "front", "rear", "side", etc. may be used in this specification to describe different exemplary features and elements of the present invention, these terms are used herein only for convenience, such as according to the direction of the examples described in the accompanying drawings. Nothing in this specification should be construed as requiring a specific three-dimensional orientation of the structure to fall within the scope of the present invention.

[0038] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0039] Figure 1 It is a structural schematic diagram of a large precision casting shell preheating device of the present invention.

[0040] like Figure 1 As shown, the large precision casting shell preheating device of the present invention is used for preheating a large precision casting shell 3, wherein the precision casting shell 3 has a gate and a runner 4, and comprises:

[0041] A sandbox 8, wherein the sandbox 8 contains sand 9, the precision casting shell 3 is contained in the sand 9, and the outside of the precision casting shell 3 is coated with a silicon carbide layer 5; the sand 9 is filled in the sandbox 8 to protect the precision casting shell 3, and the silicon carbide layer 5 is coated on the outer surface of the precision casting shell 3 for rapid heating and heat conduction.

[0042] A heating power supply 1 is connected to the silicon carbide layer 5 through a heating cable 6, and the silicon carbide layer 5 provides heat for heating. The heating power supply 1 is electrically connected to a temperature control system 2, and the temperature control system 2 controls the output power of the heating power supply 1; the temperature control system 2 controls the heating power supply 1 so that the heating temperature of the silicon carbide layer 5 by the heating power supply 1 is stabilized near a preset temperature. The heating cable 6 is directly connected to the silicon carbide layer 5, and the precision casting shell 3 is heated through the silicon carbide layer 5, and the heat conduction is fast and uniform.

[0043] The temperature sensor 7 is multiple and is respectively arranged at the center, edge and thickness change position of the precision casting shell 3. The temperature sensor 7 is connected to the temperature control system 2 for communication and sends the collected temperature data to the temperature control system 2. The temperature control system 2 controls the power of the heating power supply 1 according to the data. The temperature sensor 7 can be a thermocouple sensor or other forms of sensors as needed.

[0044] The above devices can be summarized as follows:

[0045] Mold shell and silicon carbide coating: The mold shell is the basic mold for precision casting, and the silicon carbide coating coated on its outer surface is the core heating component. Silicon carbide has excellent high temperature resistance and thermal conductivity, and can efficiently convert electrical energy into thermal energy and transfer it to the mold shell. For the coating of silicon carbide coating, the slurry coating method can be used. Silicon carbide powder and silica sol are mixed to form a uniform slurry. The mold shell is immersed in it and then slowly taken out. By controlling the immersion speed, time, and subsequent drying and high-temperature curing treatment conditions, a coating with a thickness between 0.5 mm and 2 mm and tightly bonded to the mold shell can be obtained. Alternatively, a spraying method may be used, using thermal spraying or cold spraying technology to mix silicon carbide powder with a carrier gas (such as nitrogen) and spray it onto the mold shell surface through a spray gun. Thermal spraying uses a high-temperature flame or plasma to melt or semi-melt the powder and then impact the mold shell to form a coating. Cold spraying relies on a high-speed airflow to make the powder impact the mold shell surface in a solid state and adhere to it. By precisely controlling the spraying parameters, the coating thickness uniformity deviation can be controlled within ±10%, thereby ensuring the uniformity of heating.

[0046] Heating power supply: connected to the silicon carbide coating to provide it with electrical energy. Depending on the size and shape of the shell and the required preheating temperature and speed, the heating power supply can adjust the output voltage or current to meet different heating requirements. For example, for large and thick shells, the voltage or current can be appropriately increased to speed up the preheating process, but it must be ensured that it is within the tolerance range of the silicon carbide coating to avoid damage to the coating due to overcurrent.

[0047] Temperature sensor: A thermocouple temperature sensor is used, and its measuring end is installed at key positions inside the mold shell, such as the center of the mold shell to monitor the overall average temperature, the edge parts and parts with large thickness changes, so as to comprehensively and accurately obtain the temperature distribution inside the mold shell.

[0048] Temperature control system: It consists of a controller and a control circuit. The controller receives the temperature signal fed back by the temperature sensor and compares it with the preset temperature range of 900-1000 degrees. If the temperature is higher than the upper limit, the controller reduces the output of the heating power supply through the control circuit to reduce the heating power of the silicon carbide coating; if the temperature is lower than the lower limit, the heating power supply output is increased. Through this closed-loop control method, the internal temperature of the shell is always stable within the preset range.

[0049] The present invention also provides a preheating method for a large precision casting shell, which specifically comprises:

[0050] Coating and preparation: First, pre-treat the shell surface, use a cleaning process to remove surface oil, impurities, etc., and then perform sandblasting to form a certain degree of roughness on the shell surface to enhance the adhesion between the silicon carbide coating and the shell. Then, apply the silicon carbide coating on the outer surface of the shell according to the above-mentioned slurry coating method or spraying method.

[0051] Device connection and setting: Connect the heating power supply to the silicon carbide coating correctly to ensure that the electrical connection is reliable and there is no short circuit or other hidden dangers. Install the temperature sensor at a predetermined position inside the shell and connect it to the temperature control system. Set the shell preheating temperature range to 900-1000 degrees through the operation interface of the temperature control system.

[0052] Heating process control: Start the heating power supply, the silicon carbide coating starts to heat up and preheat the shell. During the heating process, the temperature sensor monitors the internal temperature of the shell in real time and feeds the data back to the temperature control system. The temperature control system controls the output of the heating power supply based on the feedback information to keep the internal temperature of the shell within the preset range of 900-1000 degrees. In order to avoid thermal stress caused by rapid temperature changes in the shell, the silicon carbide coating adopts a segmented heating method, and the temperature rise in each segment does not exceed 50 degrees. At the same time, during the heating process, the resistance change of the silicon carbide coating is monitored in real time, because the resistance change is closely related to the heating state and uniformity of the coating. If the resistance change exceeds the preset range, it may indicate that the coating is unevenly heated or partially damaged. At this time, the heating power supply can be adjusted in time or the coating can be checked for damage to ensure the smooth progress of the preheating process.

[0053] The following is a detailed description with reference to a specific embodiment.

[0054] First, select a suitable large precision casting shell and pre-treat its surface. For example, a truncated cone complex cavity shell with a diameter of 1000mm and a height of 650mm is cleaned with chemical cleaning agents to remove surface oil stains, and then sandblasted with 2kg / cm 2 Sandblasting is performed with left and right pressure and 40 mesh sandblasting particle size. Sandblasting can effectively remove impurities such as scale and oil on the surface of the workpiece. It can form a uniform roughness on the surface of the workpiece. When spraying or bonding, the rough surface can increase the contact area between the coating or adhesive and the workpiece, thereby improving the adhesion and bonding strength of the coating. The compressive stress layer generated by sandblasting on the surface of the workpiece can offset part of the tensile stress generated during work, delay the generation and expansion of fatigue cracks on the surface of the workpiece, and effectively improve the fatigue strength of the workpiece. For example, after sandblasting, the performance of mechanical parts is improved when subjected to alternating loads. Workpieces of various shapes and sizes can be processed, whether small precision parts or large structural parts can be sandblasted. And it can process local areas of the workpiece to meet different design and processing requirements.

[0055] Then, the silicon carbide coating is prepared by slurry coating method. Silicon carbide powder and silica sol are mixed at a ratio of 80:20, and stirred in a stirrer for 2-3 hours to form a uniform slurry. The pretreated shell is immersed in the slurry at a speed of 2 cm / min, and slowly taken out after staying for 3 hours, dried at 80°C for 30 hours, and then placed in a high-temperature furnace and cured at 200°C for 5 hours to obtain a silicon carbide coating with a thickness of 1 mm. Alternatively, a spraying method, such as thermal spraying, is used to mix silicon carbide powder with nitrogen carrier gas, and thermal spraying is performed by a spray gun at parameters such as power 5kW and spraying distance 30mm, so that the coating is evenly attached to the shell surface, and the coating thickness uniformity deviation is controlled within ±0.2mm.

[0056] Next, connect the heating power supply to the silicon carbide coating, install the thermocouple temperature sensor at the center, edge and thickness change position inside the shell, and set the preheating temperature range to 900-1000 degrees through the temperature control system.

[0057] Start the heating power supply and start preheating. During the preheating process, the temperature sensor monitors the temperature in real time and provides feedback. For example, when the temperature at the center of the shell reaches 950 degrees, the temperature control system reduces the output of the heating power supply according to the preset algorithm to stabilize the temperature within the preset range. If the resistance change of the silicon carbide coating exceeds ±50%, the heating is suspended and the coating is checked for abnormal conditions such as cracking and peeling. If so, repair or re-coat before continuing preheating.

[0058] The device and method of the present invention can effectively solve many problems in the preheating process of large precision casting shells, improve the preheating quality and production efficiency, and has broad application prospects.

[0059] Advantages of the invention

[0060] 1. Resist thermal stress and physical damage: During the heating and casting process, the outside of the shell will be affected by environmental factors, such as thermal shock, mechanical collision, etc. Applying silicon carbide coating on the outside of the shell can effectively resist thermal stress. When the shell is quickly heated from a lower temperature to a high temperature, the coating can buffer the thermal shock and prevent the shell from breaking due to excessive thermal stress. At the same time, it can also protect the shell from physical damage such as collision and friction from external objects.

[0061] 2. High heat transfer efficiency: The high thermal conductivity of the coating can transfer the heat from the external object to the mold shell faster, thereby accelerating the temperature rise inside the mold shell. It is especially suitable for large precision casting mold shells that require high heating efficiency.

[0062] 3. High temperature control accuracy: The electronic control system forms a closed-loop control through the sensor, and the thermal response speed of the coating can obtain accurate constant temperature.

[0063] 4. Prevent oxidation and chemical corrosion: Under high temperature environment, the mold shell may undergo oxidation reaction or chemical corrosion with oxygen, water vapor, etc. in the air. The outer coating can act as a barrier to prevent these gases from contacting the mold shell, protect the mold shell material from oxidation and chemical corrosion, and extend the service life of the mold shell.

[0064] The present invention effectively solves the problems existing in the existing preheating technology, such as uneven heating and difficulty in adapting to complex cavities, improves the preheating quality and production efficiency of the precision casting shell, and has significant innovation and application value.

[0065] Those skilled in the art should understand that the specific structures and processes shown in the above specific embodiments are only exemplary and not restrictive. Moreover, those skilled in the art can combine the various technical features shown above in various possible ways to form new technical solutions, or make other changes, which are all within the scope of the present invention.

Claims

1. A large-scale precision casting shell preheating device, used for preheating a large-scale precision casting shell, wherein the precision casting shell has a gate and a runner, and is characterized in that: include: A sandbox, wherein the sandbox contains sand, the precision casting shell is contained in the sand, and the outer surface of the precision casting shell is coated with a silicon carbide layer; A heating power supply, wherein the heating power supply is connected to the silicon carbide layer through a heating cable, the silicon carbide layer provides heat for heating, and the heating power supply is electrically connected to a temperature control system, and the temperature control system controls the output power of the heating power supply; The temperature sensor is multiple and is respectively arranged at the center, edge and thickness variation position of the precision casting shell. The temperature sensor is communicatively connected to the temperature control system and sends the collected temperature data to the temperature control system.

2. A method for preheating a large precision casting shell, characterized in that: The large precision casting shell preheating device according to claim 1 comprises the following steps: Step 1: forming a large precision casting shell and treating the surface of the precision casting shell; Step 2: The silicon carbide coating is attached to the surface of the precision casting shell; Step 3: bury the precision casting shell in the sand in the sandbox, and arrange temperature sensors at the center, edge and thickness change positions of the precision casting shell; Step 4: Preset the temperature and quickly heat the precision casting shell. During the heating process, compare the temperature with the preset temperature and control the temperature within a 10% error of the preset temperature.

3. The method for preheating a large precision casting shell according to claim 2, characterized in that: The large precision casting shell is a truncated cone complex cavity shell with a diameter of 1000 mm and a height of 650 mm.

4. The method for preheating a large precision casting shell according to claim 2, characterized in that: In step 1, after using chemical cleaning agents to remove surface oil stains, sandblasting equipment is used at 2kg / cm 2 Sand blasting is performed with left and right pressure and 40 mesh sandblasting particle size.

5. The method for preheating a large precision casting shell according to claim 2, characterized in that: In step 2, a slurry coating method is used to mix silicon carbide powder with a high temperature resistant binder to form a slurry. The mold shell is immersed in the slurry and then taken out. After drying and high temperature curing, the silicon carbide coating is attached to the surface of the mold shell.

6. The method for preheating a large precision casting shell according to claim 5, characterized in that: The high temperature resistant adhesive is silica sol.

7. The method for preheating a large precision casting shell according to claim 2, characterized in that: In step 2, thermal spraying or cold spraying technology is used to mix silicon carbide powder with carrier gas and spray it onto the mold shell surface through a spray gun. During thermal spraying, high-temperature flame or plasma is used to melt or semi-melt the powder and then impact the mold shell to form a coating; cold spraying uses high-speed airflow to make the powder impact the mold shell surface in a solid state and adhere.

8. The method for preheating a large precision casting shell according to claim 7, characterized in that: The carrier gas is nitrogen.

9. The method for preheating a large precision casting shell according to claim 5 or 7, characterized in that: The coating thickness is 0.5 mm to 2 mm, or the coating thickness uniformity deviation is controlled within ±10%.

10. The large precision casting shell preheating method according to claim 2, characterized in that: In step 4, the temperature control system is used to set the preheating temperature range of the precision casting shell to 900-1000 degrees; the heating power supply is started, the silicon carbide coating generates heat to preheat the precision casting shell, and the temperature sensor monitors the internal temperature of the shell in real time and feeds back to the temperature control system; the temperature control system controls the output of the heating power supply according to the feedback information to keep the internal temperature of the precision casting shell within the preset range of 900-1000 degrees. If the temperature is higher than the upper limit, the output of the heating power supply is reduced, and if the temperature is lower than the lower limit, the output of the heating power supply is increased. The silicon carbide coating adopts a segmented heating method, and the heating amplitude of each segment does not exceed 50 degrees.

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