Large-scale Precision Casting Shell Preheating Device and Method

Abstract

The present invention provides a large-scale precision casting shell preheating device and method. The device includes a sand box, a heating power supply, and a temperature sensor. Sand is contained in the sand box, and a silicon carbide layer is coated on the outer surface of the precision casting shell; the heating power supply is connected to the silicon carbide layer through a heating cable, and the silicon carbide layer provides heat for heating. A temperature control system is electrically connected to the heating power supply; the temperature sensors are respectively arranged at the central position, the edge position, and the position where the thickness changes of the precision casting shell. The method includes forming a large-scale precision casting shell and treating the surface of the precision casting shell; attaching a silicon carbide coating to the surface of the precision casting shell; burying the precision casting shell in the sand in the sand box and arranging temperature sensors on the precision casting shell; presetting a temperature and rapidly heating the precision casting shell. The present invention uses a silicon carbide layer coated on the outer surface of the precision casting shell and buried in sand. When the large-scale precision casting shell is heated through the silicon carbide layer, the heat is evenly distributed, and the heat conduction is fast, so that the temperature can be rapidly increased.

Description

Technical Field

[0001] The present invention relates to the field of casting technology, and particularly to a large-scale precision casting shell preheating device and method based on silicon carbide coating heating. Background Art

[0002] Currently, in the process of large-scale precision casting, the preheating of the casting shell is a crucial step. There are many deficiencies in traditional casting shell preheating technologies. For example, external heating methods are difficult to ensure uniform heating of the inside of the casting shell, especially in complex cavity parts, which easily leads to defects in the castings. And for some existing internal heating methods, such as some heating elements are difficult to be placed inside complex cavities, or it is inconvenient to take them out after heating, which affects the integrity of the casting shell and subsequent use. In practical application cases, in order to prevent large casting shells from cracking during heating and pouring, it is necessary to bury the large casting shells in sand for protection. However, during the heating process using a traditional heating furnace, the heat transfer speed in the sand is slow. In order to prevent non-uniform temperature inside the casting shell, the heating time is extended and the temperature gradient is reduced. Finally, it takes 7 days to heat the casting shell to 800 °C, and the heating efficiency is very low, and the temperature uniformity is also difficult to control. In summary, an innovative casting shell preheating technology is needed to solve the problems of heating efficiency and temperature control accuracy of the casting shell preheating technology.

[0003] Therefore, how to quickly and uniformly heat up a large-scale precision casting shell is an urgent problem to be solved in the industry. Summary of the Invention

[0004] A main object of the present invention is to overcome at least one defect of the above-mentioned existing technologies, and to provide a large-scale precision casting shell preheating device and method that can quickly and uniformly heat up a large-scale precision casting shell.

[0005] To achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0006] According to one aspect of the present invention, there is provided a large-scale precision casting shell preheating device for preheating a large-scale precision casting shell, where the precision casting shell has a gate and a runner, and includes:

[0007] A sand box, in which sand is contained, the precision casting shell is placed in the sand, and a silicon carbide layer is coated on the outside of the precision casting shell;

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

[0009] Temperature sensors, and a plurality of the temperature sensors are respectively arranged at the central position, edge position and thickness change position of the precision casting shell. The temperature sensors are communicatively connected to the temperature control system and send the collected temperature data to the temperature control system.

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

[0011] Step 1: Form a large-scale precision casting shell and treat the surface of the precision casting shell;

[0012] Step 2: Attach a silicon carbide coating to the surface of the precision casting shell;

[0013] Step 3: Bury the precision casting shell in the sand in the sand box, and respectively arrange temperature sensors at the central position, edge position and thickness change position of the precision casting shell;

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

[0015] According to a specific embodiment of the present invention, the large-scale precision casting shell is a frustum-shaped complex cavity shell with a diameter of 1000 mm and a height of 650 mm. The most obvious difference between the large-scale precision casting shell and the small-scale shell is the size. Its external dimensions are large, and it can cast large-scale precision parts, such as large aero-engine blades, large marine propellers, etc. Due to its large own weight, it has to bear various external forces during handling, roasting and pouring processes. For example, when pouring molten metal, the large-scale shell needs to bear huge liquid impact force and pressure, so it needs to have high normal temperature strength and high temperature strength to prevent the shell from cracking. The large-scale precision casting shell is used to cast precision parts, and has strict requirements for dimensional accuracy. It is necessary to ensure the accurate internal cavity size of the shell so that the casting meets the designed dimensional tolerance range and ensures the quality and performance of the casting. The large-scale precision casting shell often has a complex internal structure, including various reinforcing ribs, supporting structures and complex cavities. These complex structures are to meet the forming requirements of large-scale castings and ensure the stability of the shell during the casting process. During the pouring process of the large-scale precision casting shell, the gas in the cavity needs to be discharged smoothly. If the air permeability of the large-scale shell is poor and the gas discharge is not smooth, 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 removing the surface oil stain with a chemical cleaning reagent, sandblasting treatment is carried out by using a sandblasting device with a pressure of about 2 kg / cm 2 and a sandblasting particle size of 40 mesh.

[0017] According to a specific embodiment of the present invention, in step two, the slurry coating method is adopted. Silicon carbide powder is mixed with a high-temperature resistant binder to form a slurry. After the mold shell is immersed in the slurry and taken out, it is subjected to drying and high-temperature curing treatment so that the silicon carbide coating adheres to the surface of the mold shell. The thermal conductivity of the silicon carbide layer is as high as 120 - 180 W / 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 in a high-temperature environment. At traditional service temperatures, the thermal conductivity of the silicon carbide layer gradually decreases with the increase of temperature. However, for silicon carbide single crystals, from room temperature to 500 °C, the thermal conductivity increases with the increase of temperature, and above 500 °C, it decreases with the increase of temperature. The thermal conductivity of the silicon carbide layer is affected by factors such as pores and grain boundaries. When the porosity increases or the number of closed pores increases, the thermal conductivity decreases significantly. By controlling the grain size and distribution, material density and chemical composition, etc., its thermal conductivity coefficient can be improved. For example, special processes such as hot pressing sintering and spark plasma sintering can be used to prepare silicon carbide ceramics with high thermal conductivity, but the size is limited; while processes such as pressureless sintering can produce silicon carbide ceramics with large sizes and complex structures, but the thermal conductivity is relatively low.

[0018] According to a specific embodiment of the present invention, the high-temperature resistant binder is silica sol. Silica sol can withstand high temperatures of 1500 °C - 1600 °C. In a high-temperature environment, the silicon-oxygen bond structure inside it is stable and not easily decomposed and damaged, which can ensure the strength and stability of the bonding part and is suitable for fields such as high-temperature industrial kilns and aerospace. The silica particles in silica sol are fine, which can fully fill the pores of the bonded object. After drying, a firm silicon-oxygen bonding bond is formed, greatly improving the bonding strength and enhancing the overall structural strength of refractory materials, etc. Silica sol has stable chemical properties and is not easily chemically reacted with other substances in harsh environments such as high temperature, acid and alkali, which can resist chemical erosion, ensure the durability and reliability of the bonding effect, and extend the service life. The viscosity of silica sol is low, the particles are fine, and it has good fluidity and permeability. It can be evenly dispersed on the surface and inside of the bonded object to form a continuous and dense bonding layer, improving the uniformity and integrity of the bonding. Silica sol is non-toxic, odorless, and free of volatile organic compounds, and is harmless to the environment and human health during the production, use and disposal processes, meeting environmental protection requirements, and is a green and environmentally friendly binder. Silica sol can enhance the strength, wear resistance, oxidation resistance and other properties of refractory materials, and can also improve the density and thermal shock resistance of materials, enhancing their comprehensive performance at high temperatures. When using silica sol as a binder, no complex curing process and other curing agents are required, and a firm bond can be formed after drying, simplifying the production process, improving production efficiency, and reducing costs.

[0019] According to a specific embodiment of the present invention, in step two, thermal spraying or cold spraying technology is adopted. After mixing silicon carbide powder with a carrier gas, it is sprayed onto the surface of the investment shell through a spray gun. During thermal spraying, high-temperature flames or plasma are used to melt or semi-melt the powder, and then it impacts the investment shell to form a coating; during cold spraying, a high-speed gas flow is used to make the powder impact the surface of the investment shell in a solid state and adhere to it.

[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 easily involved in chemical reactions with other substances under conditions such as high temperature and high pressure. This can avoid the generation of impurities due to chemical reactions during the carrier gas process, which may interfere with experimental results or damage equipment. Nitrogen has high purity and is easily obtained. High-purity nitrogen can ensure the quality of the carrier gas and reduce the adverse effects brought by impurities. Moreover, there are mature air separation technologies in industry to produce nitrogen in large quantities, and the cost is relatively reasonable. The diffusion rate of nitrogen is moderate. This enables it to carry the sample relatively stably as a carrier gas and transport the sample to the designated position. It will not be too fast so that the sample passes through the detection area before it has time to react and be detected, nor will it be too slow to affect the detection efficiency. In terms of safety, nitrogen is a non-combustible gas, reducing the risk of fire and explosion during use and allowing it to be used with confidence in various safety-required occasions.

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

[0022] According to a specific embodiment of the present invention, in step four, the preheating temperature range of the investment shell for precision casting is set to 900 - 1000 degrees through a temperature control system; the heating power supply is started, and the silicon carbide coating generates heat to preheat the investment shell for precision casting. The temperature sensor monitors the internal temperature of the investment shell in real time and feeds it 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 investment shell for precision casting within the 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; if the temperature is lower than the lower limit value, 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.

[0023] As can be seen from the above technical solutions, the advantages and positive effects of the large-scale investment shell preheating device and method of the present invention are as follows:

[0024] The present invention uses a silicon carbide layer to coat the outer surface of the investment shell for precision casting and buries it in sand. When the large-scale investment shell for precision casting is heated through the silicon carbide layer, the heat is evenly distributed and the heat conduction is fast, enabling rapid heating, thereby improving the preheating efficiency and preheating quality of the large-scale investment shell for precision casting. Description of the Drawings

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

[0026] Explanation of drawing numbers:

[0027] 1. Heating power supply;

[0028] 2. Temperature control system;

[0029] 3. Precision casting shell;

[0030] 4. Sprue and runner;

[0031] 5. Silicon carbide layer;

[0032] 6. Heating cable;

[0033] 7. Temperature sensor;

[0034] 8. Sand box;

[0035] 9. Sand. Detailed implementation manners

[0036] Now, the exemplary embodiments will be described more fully with reference to the accompanying drawings. However, the exemplary embodiments can be implemented in various forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. Like reference numerals in the figures denote the same or similar structures, and thus their detailed descriptions will be omitted.

[0037] In the following description of the different examples of the present invention, reference is made to the accompanying drawings which form a part of the present invention and in which are shown, by way of illustration, different exemplary structures, systems, and steps by which aspects of the present invention can be implemented. It should be understood that other specific solutions 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 for convenience only, for example, according to the orientation 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] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those conditions not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can be obtained as conventional products commercially available.

[0039] Figure 1 This is a schematic structural diagram of the preheating device for large-scale precision casting molds of the present invention.

[0040] As Figure 1 shown, the preheating device for large-scale precision casting molds of the present invention is used to preheat a large-scale precision casting mold 3, and the precision casting mold 3 has a gate and a runner 4, and includes:

[0041] A sand box 8, in which sand 9 is contained, the precision casting mold 3 is contained in the sand 9, and a silicon carbide layer 5 is coated on the outer surface of the precision casting mold 3; the sand 9 is filled therein to protect the precision casting mold 3, and the silicon carbide layer 5 is coated on the outer surface of the precision casting mold 3 for rapid heating and heat conduction.

[0042] A heating power supply 1, the 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. A temperature control system 2 is electrically connected to the heating power supply 1, 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 to make the heating temperature of the silicon carbide layer 5 stable near a preset temperature. The heating cable 6 is directly connected to the silicon carbide layer 5, and the precision casting mold 3 is heated through the silicon carbide layer 5, and the heat conduction is fast and uniform.

[0043] Temperature sensors 7, a plurality of the temperature sensors 7 are respectively arranged at the central position, the edge position and the position where the thickness changes of the precision casting mold 3. The temperature sensors 7 are communicatively connected to the temperature control system 2 and send the collected temperature data to the temperature control system 2, and the temperature control system 2 controls the power of the heating power supply 1 according to these data. The temperature sensors 7 can adopt thermocouple sensors, or other forms of sensors can be adopted according to needs.

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

[0045] Mold Shell and Silicon Carbide Coating: The mold shell serves as the basic mold for precision casting, and the silicon carbide coating applied to its outer surface is the core heating component. Silicon carbide has excellent high-temperature resistance and thermal conductivity, enabling efficient conversion of electrical energy into heat energy and transfer to the mold shell. For the coating of the silicon carbide coating, the slurry coating method can be used. Mix silicon carbide powder with silica sol to make a uniform slurry, immerse the mold shell in it and then slowly take it out. By controlling the dipping 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. Or the spraying method can be used. Using thermal spraying or cold spraying technology, mix silicon carbide powder with a carrier gas (such as nitrogen) and spray it onto the surface of the mold shell through a spray gun. In thermal spraying, the powder is melted or semi-melted by a high-temperature flame or plasma and then impacts the mold shell to form a coating. Cold spraying relies on a high-speed gas flow to make the powder impact the surface of the mold shell in a solid state and adhere. By precisely controlling the spraying parameters, the deviation of the coating thickness uniformity can be controlled within ±10%, thus ensuring uniform heating.

[0046] Heating Power Supply: Connected to the silicon carbide coating to provide electrical energy for it. According to the size, shape of the mold shell, as well as 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 a large and thick-walled mold shell, the voltage or current can be appropriately increased to speed up the preheating process, but it is necessary to ensure 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, and at the edge and parts with large thickness changes, so as to comprehensively and accurately obtain the temperature distribution inside the mold shell.

[0048] Temperature Control System: Composed 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 value, 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 value, the output of the heating power supply is increased. Through this closed-loop control method, it is ensured that the temperature inside the mold shell is always stable within the preset range.

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

[0050] Coating Coating and Preparation: First, the surface of the mold shell is pretreated. A cleaning process is used to remove surface oil stains, impurities, etc., and then sandblasting is carried out to make the surface of the mold shell have a certain roughness to enhance the adhesion of the silicon carbide coating to the mold shell. Then, the silicon carbide coating is coated on the outer surface of the mold 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, a silicon carbide coating is prepared by the slurry coating method. Silicon carbide powder and silica sol are mixed in a ratio of 80:20 and stirred thoroughly in a stirrer for 2 - 3 hours to form a uniform slurry. The pre-treated shell mold is immersed in the slurry at a speed of 2 cm / minute, left for 3 hours and then slowly taken out, 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. Or the spraying method, such as thermal spraying, is used. Silicon carbide powder is mixed with a nitrogen carrier gas and thermally sprayed through a spray gun under parameters such as a power of 5 kW and a spraying distance of 30 mm, so that the coating adheres evenly to the surface of the shell mold, and the deviation of the coating thickness uniformity is controlled within ±0.2 mm.

[0056] Next, the heating power supply is connected to the silicon carbide coating, and thermocouple temperature sensors are installed at the center, edge, and thickness-changing parts inside the shell mold. The preheating temperature range is set to 900 - 1000 degrees through the temperature control system.

[0057] The heating power supply is started to begin preheating. During the preheating process, the temperature sensors monitor the temperature in real time and give feedback. For example, when the temperature at the center of the shell mold reaches 950 degrees, the temperature control system reduces the output of the heating power supply according to the preset algorithm to keep the temperature stable within the preset range. If it is monitored that the resistance change of the silicon carbide coating exceeds ±50%, the heating is paused, and the coating is checked for abnormalities such as cracking and peeling. If any, repairs are made or re-coating is carried out before continuing the preheating.

[0058] Through the device and method of the present invention, many problems in the preheating process of large-scale precision casting shell molds can be effectively solved, the preheating quality and production efficiency are improved, and it has a broad application prospect.

[0059] Advantages of the present invention

[0060] 1. Resistance to thermal stress and physical damage: During heating and casting, the outside of the shell mold is affected by environmental factors such as thermal shock and mechanical collision. Coating the outside of the shell mold with silicon carbide can effectively resist thermal stress. When the shell mold is rapidly heated from a lower temperature to a high temperature, the coating can buffer the thermal shock and prevent the shell mold from cracking due to excessive thermal stress. At the same time, it can also protect the shell mold from physical damage such as collision and friction by external objects.

[0061] 2. High heat transfer efficiency: The high thermal conductivity of the coating can conduct the heat of external objects to the shell mold faster, thereby accelerating the heating rate inside the shell mold, especially suitable for large-scale precision casting shell molds with high requirements for heating efficiency.

[0062] 3. High temperature control accuracy: The electronic control system forms a closed-loop control through sensors. Coupled with the thermal response speed of the coating, an accurate constant temperature can be obtained.

[0063] 4. Prevention of oxidation and chemical corrosion: In a high-temperature environment, the mold shell may undergo oxidation reactions or chemical corrosion with oxygen, water vapor, etc. in the air. The outer coating can act as a barrier to prevent these gases from coming into contact with the mold shell, protecting the mold shell material from oxidation and chemical corrosion and extending the service life of the mold shell.

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

[0065] Those of ordinary skill in the art to which the present invention pertains should understand that the specific structures and process procedures shown in the above specific implementation part are merely exemplary, not restrictive. Moreover, those of ordinary skill in the art to which the present invention pertains can combine the various technical features shown above in various possible ways to form new technical solutions, or make other modifications, and all fall within the scope of the present invention.

Claims

1. A preheating method for a large-scale precision casting shell, characterized in that: The large-scale precision casting shell preheating device is used for preheating a large-scale precision casting shell, and the precision casting shell has a gate and a runner, including: A sand box, in which sand is contained, the precision casting shell is placed in the sand, and a silicon carbide layer is coated on the outer surface of the precision casting shell; A heating power supply, the heating power supply is connected to the silicon carbide layer through a heating cable, and the silicon carbide layer provides heat for heating. A temperature control system is electrically connected to the heating power supply, and the temperature control system controls the output power of the heating power supply; Temperature sensors, there are multiple temperature sensors, which are respectively arranged at the center position, edge position and thickness change position of the precision casting shell. The temperature sensors are communicatively connected to the temperature control system and send the collected temperature data to the temperature control system; The preheating method for the large-scale precision casting shell includes the following steps: Step 1: Form a large-scale precision casting shell and treat the surface of the precision casting shell. The large-scale precision casting shell is a frustum-shaped complex cavity shell with a diameter of 1000 mm and a height of 650 mm; Step 2: Adopt the slurry coating method, thermal spraying or cold spraying, and the silicon carbide coating adheres to the surface of the precision casting shell. The coating thickness is 0.5 mm to 2 mm, or the deviation of the coating thickness uniformity is controlled within ±10%. The coating is arranged on the upper and lower end faces, outer peripheral surface and the wall surface of the central through hole of the large-scale precision casting shell; Step 3: Burry the precision casting shell into the sand in the sand box, and arrange temperature sensors at the center position, edge position and thickness change position of the precision casting shell respectively; Step 4: Preset the temperature and quickly heat the precision casting shell. During the heating process, compare with the preset temperature and control the temperature within 10% error of the preset temperature. The silicon carbide coating adopts a segmented heating method, and the heating amplitude of each segment does not exceed 50 degrees.

2. The large-scale precision casting shell preheating method according to claim 1, wherein: In Step 1, after removing the surface oil stain with a chemical cleaning reagent, use a sandblasting device to perform sandblasting treatment at a pressure of about 2 kg / cm² and a sandblasting particle size of 40 mesh.

3. The large-scale precision casting shell preheating method according to claim 1, characterized in that: In Step 2, mix silicon carbide powder with a high-temperature resistant binder to make a slurry. After the shell is immersed in the slurry and taken out, it is subjected to drying and high-temperature curing treatment to make the silicon carbide coating adhere to the surface of the shell.

4. The large-scale precision casting shell preheating method according to claim 3, characterized in that: The high-temperature resistant binder is silica sol.

5. The large-scale precision casting shell preheating method according to claim 1, wherein: In Step 2, adopt thermal spraying or cold spraying technology. Mix the silicon carbide powder with a carrier gas and spray it onto the surface of the shell through a spray gun. During thermal spraying, use a high-temperature flame or plasma to melt or semi-melt the powder and then impact the shell to form a coating; during cold spraying, use a high-speed air flow to make the powder impact the surface of the shell in a solid state and adhere.

6. The preheating method for a large-scale precision casting shell according to claim 5, characterized in that: The carrier gas is nitrogen.

7. The preheating method for a large-scale precision casting shell according to claim 1, characterized in that: In Step 4, set the preheating temperature range of the precision casting shell to 900 - 1000 degrees through the temperature control system; start the heating power supply, and the silicon carbide coating generates heat to preheat the precision casting shell. The temperature sensors monitor the internal temperature of the shell in real time and feed it 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 value, reduce the output of the heating power supply. If the temperature is lower than the lower limit value, increase the output of the heating power supply.

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