Causes and prevention methods of graphitization cracks in graphite products

Graphitization is one of the main processes of heat treatment in the production process of carbon-graphite products. Acheson graphitization furnace is the main furnace type for the current graphitization production of carbon-graphite products. The space where the products and resistance materials are loaded in the furnace is called the furnace core. The cross-sectional area of the furnace core is usually 3-6M2. A strong current is passed into the graphitization furnace. With the help of the furnace core resistance of the graphitization furnace, the electrical energy is converted into The thermal energy makes the product reach the highest temperature of graphitization and completes the graphitization process, which follows the Joule-Lenz law.
It can be seen that the temperature at different points in the graphitization furnace core is different, and at the same point, the temperature is different at different times. It can be seen that the temperature of the graphitization furnace core is not only a function of space, but also a function of time. Therefore, the temperature distribution of each part in the furnace core is unbalanced.

Product graphitization crack prevention

The graphitization process should be reasonable

1. Selection of furnace loading method
In the production process of the Acheson graphitization furnace, a reasonable furnace loading method is the guarantee for the graphitization of the products. Whether the product adopts the vertical installation method or the horizontal installation method, whether it is a formal installation or a wrong installation, should be determined according to the variety, specifications, quality standards and process parameters of the equipment to ensure that the heating of the product in the furnace core is relatively uniform, so as to reduce the Thermal stress, reducing cracks in the product during graphitization. For large-sized products, the dislocation 1/2D furnace charging method can reduce product cracks and achieve good graphitization effect. For products with high graphitization crack rejection rate and unstable quality, furnace core flow equalization measures can also be taken.

2. Determine a reasonable power-on system
The temperature of the graphitization furnace core is controlled by the power curve of the constant power distribution. The correct and reasonable formulation and application of the graphitization furnace power-on system are very important for improving the yield, saving energy and shortening the graphitization cycle of products. significance. The determination of the energization system of the graphitization furnace should not only take into account factors such as furnace structure, product variety and specifications, quality information, resistance materials, thermal insulation effect, parameters of the power distribution system, etc. Different stages have different requirements for the temperature rise rate.
A reasonable power-on system for the graphitization furnace should be a “fast-slow-fast three-stage power curve to meet the different requirements of the product in the three stages of the temperature rise process, and the furnace core should have a faster temperature rise speed. Reduce the heat dissipation loss of the graphitization furnace, and prevent the temperature gradient of the furnace core from being too large to cause cracks in the products. For products with unstable graphitization quality, it is necessary to strictly control the temperature rise rate of the furnace core in the temperature rise stage to avoid excessive temperature rise. To cause cracks in the product, the rising power of the power transmission curve should be adjusted appropriately to form a four-stage power transmission curve of “fast-slow-slow-fast”.

3. Determine the appropriate resistor material
The Acheson graphitization furnace mainly heats the product by the heat generated by the current passing through the resistance material. The resistance material is closely related to the temperature change of the furnace core. From the perspective of increasing the temperature of the graphitization furnace core, the resistance of the resistance material is required to be larger. Especially in the later stage of power transmission, the secondary output current of the transformer has reached the maximum value, and the furnace core resistance is large at this time, which can maintain a high electrical efficiency; but the resistance of the resistance material is too large, which is unreasonable. Therefore, when determining the resistance material, it is necessary to consider not only the performance of the equipment, but also the variety and specification of the product and the power transmission curve, so that the resistance of the product and the resistance of the resistance material cannot be too different. For small and medium-sized products, metallurgical coke can be used as resistance material. Even if higher starting power and faster rising power are used, cracks are generally not produced in the product; for large-sized products, mixed coke or graphitized coke is used as resistance material. It is more appropriate, so that the resistance difference between the product and the resistance material is small, and the temperature difference between the inside and outside of the product is also reduced. Even if a faster rising power is used, it will not cause cracks in the product.
Second, the quality of operation must meet the standard. In the process of graphitization production operation, the furnace loading operation is the key. Since the products loaded into the graphitization furnace are both heating resistors and objects to be heated, the furnace core resistance is formed together with suitable resistance materials. Core resistance is a necessary condition for graphitization of products. First of all, the condition of the furnace body of the graphitization furnace, the short bus network, and the equipment of the power supply system should be in good condition. When installing the furnace, the section of the furnace core should be symmetrical with the conductive section to prevent the furnace core from drifting. The furnace installation operation should meet the requirements of the process technical regulations. The products should be arranged horizontally and vertically in the furnace core, the group spacing of the products should be consistent, and the resistance material should be well filled to avoid the phenomenon of hanging, and ensure that the temperature distribution of the furnace core is balanced during the power transmission process of the graphitization furnace. Secondly, the ratio of resistance materials should meet the requirements of production process technical standards, and the quality should be stable and uniform, so as to avoid the uneven temperature distribution of the furnace core during the power transmission process of the graphitization furnace. Thirdly, the graphitization furnace should transmit power according to the given power transmission curve, and the power fluctuation should be controlled within the normal range to avoid abnormal fluctuation of the transmission power, so as to ensure that the temperature of the furnace core rises evenly.

4. Add an appropriate amount of inflation inhibitor to the ingredients
The irreversible expansion cracking phenomenon caused by the presence of sulfur in the graphitization process cannot be completely eliminated, but it must be controlled. At present, the most effective way is to control the escape rate of sulfur during the graphitization process of the product. The most practical way is to add an appropriate amount of inflation inhibitor during batching, usually 1%-2% Fe2O3 powder.

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