Analysis of Graphitization Cracks of Graphite Products
Graphitization is one of the main processes of heat treatment in the production process of carbon-graphite products. The Acheson graphitization furnace is the main furnace type for graphitization production of current carbon-graphite products. It is a special resistance furnace that is intermittently operated by direct heating and intermittent operation of the products and resistance materials charged in the furnace. The space in which 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 electric energy is converted into Thermal energy makes the product reach the highest temperature of graphitization and completes the graphitization process. It 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 various parts in the furnace core is unbalanced.
After the Acheson graphitization furnace is energized, it relies on the heat generated by the resistance material to heat the product, so that the temperature of the furnace core gradually rises, and the temperature rise throughout the furnace core is very uneven, and the temperature distribution varies greatly. The graphitization furnace The temperature difference between the central part of the core and the two sides of the furnace core near the insulation material can be hundreds of degrees Celsius, and the temperature difference between the upper and lower parts of the furnace core can also reach 100 degrees Celsius. Therefore, the uneven heating temperature distribution in the core of the same graphitization furnace is the main cause of cracks in the core products.
Based on many years of experience in graphitization production, I have briefly summarized and analyzed the causes of cracks and waste in carbon-graphite products during the graphitization production process; here I will discuss with carbon engineers and technicians in order to reduce carbon-graphite products. The purpose of cracking waste products in the process of graphitization production, improving the yield of the graphitization process, reducing the production cost of the graphitization process, and improving economic benefits.
Causes of Graphitization Cracks in Products
In the process of graphitization, the internal factor for the product to produce cracks is that the quality of the product is not high, and the heat resistance is poor; the external factor is that the temperature of the furnace core rises too fast during the process of graphitization, and the temperature difference between the top and bottom of the product is also Increasing along with it, the corresponding increase in thermal stress is the main cause of cracks in the product.
1. The graphitization process system is unreasonable
Furnace loading method
Acheson graphitization furnace products usually adopt the vertical installation method to install the furnace, and the vertical installation method has two forms of formal installation and wrong installation. When the furnace core product is being installed, for any product, there is only one high-density current heating zone. The wider the heating zone, the more uniform heating of the product, on the contrary, the heating is very uneven. In the case of wrong installation, there are two high-density current heating belts for each product, and the heating of the product should be more even than that of the normal installation. Therefore, the choice of the furnace loading method for graphitization furnace products is improper. During the graphitization process, the temperature rise rate around the product varies greatly, and the thermal stress generated by the product exceeds the thermal stress that the body can withstand, which is extremely easy to cause cracks in the product.
Unreasonable power system
The temperature change curve of the Acheson graphitization furnace core is controlled by the power curve of constant power distribution. If the graphitization furnace energization system is unreasonable, the determined graphitization furnace energization curve starts to have too much power and rises too fast. The internal and external temperature gradient of the product is too large during the electrification process, and the thermal stress generated greatly exceeds the resistance of the product, resulting in cracks. Especially when the furnace temperature is between 1300 and 1800 degrees, it is a stage of strictly controlling the furnace temperature rise. At this stage, the physical structure and chemical composition of the product begin to change greatly, and the graphitization of amorphous carbon has not started. In fact, the chemical reaction is Mainly, the combined hydrogen, oxygen, nitrogen, sulfur and other elements in the microcrystalline structure of amorphous carbon continue to escape. As a result of the escape, the impurity elements at the edge of the amorphous carbon microcrystalline structure are continuously reduced, and several lattice defects remain. At the same time, it promotes the relative concentration of thermal stress, which is very easy to cause cracks in the product.
Resistance of resistance material
The resistance of the graphitization furnace core is composed of the resistance of the product and the resistance of the resistance material in series. When the graphitization furnace starts to be energized, the resistance of the resistance material accounts for about 99% of the resistance of the furnace core, and the resistance of the resistance material after the energization is completed. About 97%, it can be seen that in the entire graphitization process, the heat generated by the current passing through the resistive material heats the product. When the resistance of the resistive material differs greatly from the resistance of the product, the heat generated by the resistive material is far greater during the graphitization process. It is much larger than the heat of the product itself, and the temperature difference between the inside and outside of the product is too large, which will cause excessive thermal stress and cause the product to crack and waste.
2. The quality of graphitization operation is not high
The quality of the furnace installation is not high
The operation of the graphitization furnace does not meet the requirements of the process technology standard, the furnace core products are not neatly arranged when the furnace is installed, the spacing of the product groups is inconsistent, the resistance material is filled unevenly, and even the phenomenon of "expansion" of the resistance material occurs, which will appear in the graphitization furnace During the power transmission process, the current distribution throughout the furnace core is very uneven, resulting in uneven heating and temperature rise speed of the product, and the internal temperature difference of the product is too large, and the thermal stress generated will cause the product to crack and waste.
Uneven quality of resistance material
When mixed coke is used as the resistance material in the graphitization furnace, since the resistivity of metallurgical coke is 5-8 times larger than that of graphitized coke, if the mixture of metallurgical coke and graphitized coke is not uniform, the resistance distribution throughout the furnace core is very uneven, resulting in The temperature rise rate of each part of the furnace core is very uneven when the power is turned on, and the temperature difference between the upper and lower parts of the product is too large, and the thermal stress also increases, which is likely to cause a large number of cracked waste products.
Graphitization furnace core bias current
According to the law of electric heating of the Acheson graphitization furnace, the temperature distribution in the graphitization furnace core is not only related to the resistance of the furnace core, but also closely related to the current passing through the furnace core. When the furnace core of the Acheson graphitization furnace has a bias current due to various reasons, the current passing through the furnace core is very different, and the temperature distribution of the furnace core is very different. When the current distribution of the furnace core differs greatly, the part with large current will generate more heat, the product temperature in this area will rise faster, and the part with low current will generate less heat, and the product temperature in this area will rise slowly, so the furnace core temperature The distribution difference is large, so the internal temperature difference of the product is also large, and the thermal stress generated is correspondingly increased, causing the product to produce cracks and waste.
3. The quality of the roasted product
Internal cracks in baked goods
According to information, the temperature range of 350-500 degrees and 700 degrees and above is the most dangerous temperature range in which carbon materials may be damaged during the roasting process. When the temperature of the outer surface of the product is 800 degrees and the maximum radial temperature difference is 10.7 degrees, the area with a radius of 50-65mm determines the strength of the material. A dangerous tensile stress zone is formed within the radius of the blank center of 65mm. When the temperature is 700 degrees or higher, the stress in this area is far greater than the limit of the material's breaking strength. This is the reason why the product produces longitudinal straight cracks, which generally do not extend to the outer surface of the product, that is, the product Internal cracks.
Homogeneity of products
The uniformity of the density distribution of carbon-graphite products, and the uniformity of the radial density and axial density distribution of the products are closely related to the quality of the products during the graphitization heat treatment process. Where the density distribution of the product is uneven, during the graphitization heat treatment process, due to the effect of thermal stress, the product is prone to internal stress. Correspondingly, the distribution of the internal stress of the product is also uneven. This uneven internal stress is likely to cause cracks in the product. As a result, cracks and scraps appear during the graphitization process.
Product bulk density is too high
The bulk density of carbon-graphite products mainly varies with the production materials and technological conditions. The flexural strength, elastic modulus, and thermal conductivity of the products increase with the increase in bulk density. When the bulk density is high, the elastic modulus of the product increases, and the brittleness increases, resulting in poor thermal shock resistance of the product. During the graphitization heat treatment process, the thermal stress generated by the high temperature greatly exceeds the stress that the product itself can withstand. , The internal and external stress is quite different, and the product will produce cracks and scraps.
Unstable production in the previous process
Since graphitization is the last heat treatment process in the production of carbon-graphite products, it is also the heat treatment process with the highest temperature. It is generally believed that when the current process production is unstable or there is quality fluctuation, it will be concentratedly exposed during the graphitization process. If the temperature of the calcined material is low, the softening point of the asphalt is unqualified, the roasting temperature is low, the impregnation weight gain rate is unqualified, etc., the product will cause secondary shrinkage or uneven shrinkage during graphitization high temperature treatment, which is very easy to produce cracked waste products.
A certain degree of irreversible volume expansion occurs during the graphitization process of the product. The main reason is that the product is caused by the rapid escape of sulfur concentration during the graphitization process. The degree of this irreversible expansion increases with the increase in the sulfur content. As the heat treatment speed increases, this irreversible expansion behavior is called "flatulence".
As we all know, the content of non-carbon elements such as hydrogen, oxygen, nitrogen, etc. of petroleum coke after calcination at 1350 temperature is generally less than 0.1%; however, sulfur and the carbon atoms of aromatic hydrocarbons are very tightly combined, so that c-s The bond begins to break when the temperature is above 1400 degrees, forming sulfur and sulfur-carbon compounds; at higher temperatures, mainly at 1500-1800 degrees, the generated sulfur and sulfur-carbon compounds are rapidly released from the product in the form of gas When the sulfur content reaches a certain level, it will often cause cracks in the product during the graphitization process.
4. Product graphitization crack prevention
a. The graphitization process should be reasonable
Choice 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 product. Whether the product adopts the vertical or horizontal installation method, whether it is formal or wrong installation, must be determined according to the product variety, specification, quality standard and equipment process parameters to ensure that the heating of the product in the furnace core is relatively uniform to reduce Thermal stress reduces the cracks of the product during the graphitization process. For large-size products, using the staggered 1/2D furnace loading method can reduce product cracks and have a good graphitization effect. For products with high scrap rate of graphitization cracks and unstable quality, furnace core flow sharing measures can also be taken.
Determine a reasonable power system
The temperature of the graphitization furnace core is controlled by the power curve of constant power distribution. The correct and reasonable formulation and application of the graphitization furnace power system are very important for improving the yield, saving energy, and shortening the graphitization cycle of products. significance. The determination of the electrification system of the graphitization furnace should not only take into account factors such as furnace structure, product specifications, quality information, resistance materials, insulation effects, and parameters of the power distribution system, but more importantly, conform to the products in the graphitization furnace. Different stages have different requirements for temperature rise speed.
A reasonable graphitization furnace power-on system should be a "fast-slow-fast three-stage power curve to meet the different requirements of the three stages in the temperature rise process of the product. The furnace core should be maintained at a faster temperature rise rate, both Reduce the heat dissipation loss of the graphitization furnace, and prevent the furnace core temperature gradient from being too large, causing the product to crack. For products with unstable graphitization quality, it is necessary to strictly control the temperature rise rate of the furnace core during the temperature rise stage to avoid excessive temperature rise To cause cracks in the product, at this time, the rising power of the power transmission curve should be appropriately adjusted to form a "fast-slow-slow-fast" four-stage power transmission curve.
Determine the appropriate resistance material
The Acheson graphitization furnace mainly heats the product by the heat generated by the electric current 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. At this time, the furnace core resistance is relatively large, which can maintain high electrical efficiency; however, the resistance of the resistor material is too large, which is unreasonable. Therefore, when determining the resistance material, it is necessary to consider not only the equipment performance, but also the product specifications and 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 with higher starting power and faster rising power, the product will generally not crack; for large-size products, use mixed coke or graphitized coke as resistance material It is more appropriate. In this way, 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 power rise is used, it will not cause the product to crack.
b. The quality of operation must meet the standard
In the process of graphitization production operation, the furnace loading operation is the key. Because 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 prerequisite for graphitization of products. First of all, the furnace body condition of the graphitization furnace, the short busbar network, and the equipment of the power supply system should be intact. The furnace core section should be symmetrical with the conductive cross section when the furnace is installed to prevent the furnace core from drifting. The furnace installation operation should meet the requirements of the process and technical regulations. The products should be arranged horizontally and vertically in the furnace core, the product group spacing should be consistent, the resistance material should be filled well to avoid the phenomenon of hanging, and ensure that the furnace core temperature distribution is balanced during the graphitization furnace power supply process. Secondly, the ratio of resistor materials must meet the requirements of the production process and technical standards, and the quality must be stable and uniform to avoid uneven temperature distribution in the furnace core during the graphitization furnace power supply process. 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 fluctuations of the power transmission, so as to ensure that the temperature of the furnace core rises evenly.
c. Master the quality information of the previous process
It is necessary to grasp the production situation and quality information of the previous process in time, aim at the production stability and quality technical indicators of the product in the previous process, and combine the actual production of this process to formulate feasible graphitization production process technical conditions to prevent products from being graphitized. There are cracked waste products to ensure the stable quality of graphitization. In the graphitization furnace, the appearance and body quality of the products must be checked one by one. Products that do not meet the technical conditions of the graphitization process must not be loaded into the graphitization furnace for graphitization, and the material must be returned to the upper process in time.
d. Add proper amount of swelling inhibitor to the ingredients
It is impossible to completely eliminate the irreversible expansion and cracking caused by the presence of sulfur in the graphitization process, but it must be controlled. At present, the most effective way is to control the sulfur escape rate during the graphitization of the product. The most practical way is to add an appropriate amount of swelling inhibitor during the batching, usually 1%-2% Fe2O3 powder.
Regarding the mechanism of adding a gas swelling inhibitor, the main reason is that the inhibitor can capture sulfur in the temperature range of graphitization and gas swelling of the product to generate sulfur compounds and form a gas in a higher temperature range to release it, thereby broadening the sulfur content. The escape temperature range prevents the product from cracking due to the concentration of the rapidly escaping gas and excessive internal stress. The most commonly used gas swelling inhibitor is Fe2O3 powder. The mechanism of action is that at a high temperature above 1000 degrees, Fe2O3 powder is easily reduced to produce iron or carbon-iron compounds. Carbon-iron compounds will be further decomposed into iron and iron at higher temperatures. Carbon, the iron formed in this process reacts with the sulfur released from the decomposition of the product, and is slowly released in the form of iron sulfide, thereby slowing the escape rate of sulfur in the product and inhibiting sulfur. The relevant chemical reaction formula is:
Because Fe2O3 powder not only has a high chemical affinity for the sulfur in the product, and has a good sulfur suppression effect, but also has abundant resources and low price, and it will not produce any adverse effects in the electric furnace steelmaking process. In addition, Fe2O3 powder also has a strong catalytic effect on the graphitization process of products, and is an excellent graphitization catalyst. It can be seen that for petroleum coke with high sulfur content, adding proper amount of swelling inhibitor Fe2O3 powder,