Furnace loading method

Graphite products for the Acheson graphitization furnace are typically loaded using the vertical loading method, which comes in two forms: upright loading and staggered loading. In upright loading, each product has only one high-density current heating band. The wider this heating band is, the more uniformly the product will be heated; conversely, if the heating band is narrow, the heating will be highly uneven. In staggered loading, each product has two high-density current heating bands, resulting in relatively more uniform heating compared to upright loading. Therefore, if the loading method for graphite products in the graphitization furnace is chosen improperly, significant differences in temperature rise rates around the products will occur during the graphitization process. As a result, the thermal stresses generated in the products may exceed the material’s ability to withstand such stresses, making them highly susceptible to cracking.

The power-on system is unreasonable.

The temperature profile of the Acheson graphitization furnace core is controlled by a power curve that employs constant-power distribution. If the energization regimen of the graphitization furnace is improperly designed, the established energization curve may start with excessively high power and an overly rapid power rise, causing an excessive temperature gradient between the inner and outer parts of the product during energization. The resulting thermal stress can far exceed the product’s resistance, leading to cracking. Particularly during the temperature range from 1300 to 1800 degrees Celsius, strict control over the furnace temperature rise is essential, as this stage marks significant changes in the product’s physical structure and chemical composition. At this point, graphitization of the amorphous carbon has not yet begun; instead, chemical reactions dominate. Elements such as hydrogen, oxygen, nitrogen, and sulfur, which are chemically bonded within the amorphous carbon’s microcrystalline structure, continuously escape. As these elements escape, the concentration of impurity elements at the edges of the amorphous carbon’s microcrystalline structure gradually decreases, leaving behind certain lattice defects. Simultaneously, this process leads to a relative concentration of thermal stress, making the product extremely susceptible to cracking.

The resistance of resistive material

The resistance of the graphite furnace core is composed of the series connection of the resistance of the product itself and the resistance of the resistive material. At the start of the graphitization process, the resistance of the resistive material accounts for approximately 99% of the total furnace core resistance; even after the power is turned off, it still accounts for around 97%. Thus, throughout the entire graphitization process, most of the heat required to heat the product comes from the heat generated by the current passing through the resistive material. When there is a significant difference between the resistance of the resistive material and that of the product, during the graphitization process, the heat generated by the resistive material far exceeds the heat generated by the product itself. This leads to an excessive temperature difference between the inner and outer parts of the product, which in turn causes thermal stress that is too severe, resulting in cracks and defective products.

Poor furnace loading quality.

The graphitization furnace charging operation does not meet the requirements of the process and technical standards. During charging, the core products are arranged unevenly, the spacing between product groups is inconsistent, and the resistance material is filled unevenly—indeed, there are even instances of “bulging” in the resistance material. As a result, during the energization of the graphitization furnace, the current distribution throughout the core becomes highly uneven, leading to drastically varying heating rates and temperature rises among the products. This causes excessive temperature differences within the products, and the resulting thermal stresses can lead to cracks and defective products.

The resistance material has uneven quality.

When using mixed coke as the resistance material in graphitization furnaces, since metallurgical coke has a resistivity 5 to 8 times higher than that of graphitization coke, if the metallurgical coke and graphitization coke are not uniformly mixed, the distribution of electrical resistance throughout the furnace core will be highly uneven. This results in significantly different heating rates across various parts of the furnace core during energization, causing excessive temperature differences between the top and bottom as well as around the sides of the product. Consequently, thermal stresses increase dramatically, making the product prone to large-scale cracking and scrap.

Graphitization furnace core flow deviation

According to the electrical heating law of the Acheson graphitization furnace, the temperature distribution within the furnace core is not only related to the core’s resistance but also closely linked to the magnitude of the current flowing through the core. When the furnace core of an Acheson graphitization furnace experiences current imbalance for various reasons, the current flowing through different parts of the core varies significantly, leading to substantial differences in the temperature distribution across the core. When the current distribution within the core differs markedly, areas with higher current generate more heat, causing the products in those regions to heat up more rapidly; conversely, areas with lower current produce less heat, resulting in slower temperature rise for the products in those regions. Consequently, the temperature distribution within the furnace core becomes highly uneven, which in turn leads to significant temperature differences inside the products and greatly increases thermal stresses. This can ultimately cause cracks and defects in the products.

3. Quality of the baked product itself

Internal cracks in the calcined product

According to available data, during the firing process, the temperature ranges of 350–500°C and 700°C and above represent the most hazardous temperature zones where carbon materials are likely to suffer damage. When the outer surface temperature of the product reaches 800°C and the maximum radial temperature difference is 10.7°C, the region with a radius of 50–65 mm determines the material’s strength characteristics. Within the central zone of the green body, extending to a radius of 65 mm, a dangerous tensile stress zone is formed. At temperatures of 700°C or higher, the stresses in this zone far exceed the material’s ultimate tensile strength—this is precisely the reason why longitudinal, straight cracks develop in the product. Such cracks generally do not propagate to the outer surface of the product; rather, they remain as internal cracks.

Homogeneity of the product

The uniformity of the density distribution in carbon-graphite products, as well as the degree to which the radial and axial density distributions are uniform, are closely related to the quality of the products during the graphitization heat treatment. In areas where the density distribution is uneven, thermal stresses generated during the graphitization heat treatment can easily induce internal stresses within the product. Consequently, the distribution of these internal stresses will also be uneven. Such uneven internal stresses tend to cause cracks in the product, leading to defective items with cracks during the graphitization process.

The product has a relatively high bulk density.

The bulk density of carbon-graphite products varies primarily depending on the differences in raw materials and process technology conditions. As the bulk density increases, the flexural strength, elastic modulus, and thermal conductivity of the products all rise. However, when the bulk density is excessively high, the elastic modulus of the product increases while its brittleness also rises, resulting in poorer resistance to thermal shock. During graphitization heat treatment, the thermal stresses generated at high temperatures greatly exceed the stress that the product itself can withstand, leading to significant differences between internal and external stresses and ultimately causing cracks and defects in the product.

The preceding process is unstable.

Since graphitization is the final heat treatment process in the production of carbon-graphite products and also the one involving the highest temperature, it is generally believed that any instability or quality fluctuations occurring in previous processing steps will become particularly evident during the graphitization stage. If the calcined material has a low temperature, the pitch softening point fails to meet specifications, the baking temperature is too low, or the impregnation weight gain rate is unsatisfactory, the resulting products, when subjected to the high-temperature graphitization treatment, may experience secondary shrinkage or uneven shrinkage, making them highly susceptible to cracking and producing defective items.

Bloating phenomenon

During the graphitization process of products, a certain degree of irreversible volume expansion occurs. The primary cause of this expansion is the rapid release of sulfur concentrated within the product during graphitization. The extent of this irreversible expansion increases with rising sulfur content and faster heating rates. This irreversible expansion behavior is referred to as the "gas swelling phenomenon."

As is well known, after calcination at 1350°C, the content of non-carbon elements such as hydrogen, oxygen, and nitrogen in petroleum coke generally falls below 0.1%. However, sulfur and aromatic hydrocarbons are tightly bonded to carbon atoms, so that the C—S bond begins to break only above 1400°C, yielding sulfur and sulfur-carbon compounds. At even higher temperatures—typically between 1500°C and 1800°C—the sulfur and sulfur-carbon compounds formed are released rapidly from the product in gaseous form, generating significant internal stresses within the product and leading to the formation of tiny pores and cracks. When the sulfur content reaches a certain level, it often results in cracking during the graphitization process.

4. Prevention of Graphitization Cracks in Products

a. The graphitization process must be reasonable.

Selection of Furnace Loading Method

In the production process of the Acheson graphitization furnace, a rational loading method is essential to ensure the successful graphitization of the products. Whether to use a vertical or horizontal loading method, as well as whether to load the products in a straight or staggered arrangement, should be determined based on the product’s type, specifications, quality standards, and the furnace’s process parameters. This ensures relatively uniform heating within the furnace core, thereby reducing thermal stress and minimizing cracking during the graphitization process. For large-sized products, adopting a staggered loading method with a 1/2D offset can effectively reduce cracking and achieve excellent graphitization results. For products with high defect rates due to graphitization cracks and unstable quality, measures can also be taken to ensure uniform gas flow within the furnace core.

Determine a reasonable energization schedule.

The temperature of the graphite furnace core is controlled by a power curve with constant power distribution. Properly formulating and implementing the power-on schedule for the graphite furnace is of great significance for improving the yield of finished products, conserving energy, and shortening the graphitization cycle of the products. Determining the power-on schedule for the graphite furnace requires consideration not only of factors such as furnace design, product specifications and grades, quality information, resistance materials, insulation performance, and parameters of the power distribution system, but more importantly, it must take into account the varying requirements for heating rate at different stages of the product’s journey through the graphite furnace.

A reasonable power supply regimen for graphitization furnaces should follow a three-stage power curve—“fast—slow—fast”—to accommodate the varying requirements of the product during its temperature-rise process. The furnace core should maintain a relatively rapid temperature-raising rate, which not only reduces heat loss from the graphitization furnace but also prevents an excessive temperature gradient within the furnace core that could lead to cracking in the product. For products with unstable graphitization quality, it is especially important to strictly control the rate of temperature rise in the furnace core during the heating phase, so as to avoid rapid temperature increases that might cause cracking. In such cases, the rising power profile of the power supply curve should be appropriately adjusted to form a four-stage power supply curve: “fast—slow—slow—fast.”

Determine the appropriate resistor material.

The Acheson graphitization furnace primarily heats the products by generating heat through the resistance of the resistive material via electric current. The resistance of the resistive material is closely related to the temperature of the furnace core. From the perspective of raising the furnace core temperature, it is desirable for the resistive material to have a relatively high electrical resistance—especially during the later stages of power supply, when the secondary output current of the transformer has reached its maximum value. At this point, the furnace core exhibits higher resistance, which helps maintain high electrical efficiency. However, excessively high resistance in the resistive material would also be impractical. Therefore, when selecting the resistive material, it is essential to take into account both the performance of the equipment and the specific product specifications as well as the power supply curve, ensuring that the resistance of the product itself and that of the resistive material do not differ too greatly. For small- and medium-sized products, metallurgical coke can be used as the resistive material; even with relatively high initial power levels and rapid power ramp-up rates, such products generally will not develop cracks. For large-sized products, it is more appropriate to use a mixture of cokes or graphitizing coke as the resistive material. This approach ensures that the resistance difference between the product and the resistive material is smaller, thereby reducing the temperature gradient both inside and outside the product. Even under rapid power ramp-up conditions, this configuration effectively prevents cracking in the product.

b. The operational quality must meet the standards.

In the graphiteization production process, the furnace-charging operation is critical. Since the products loaded into the graphiteization furnace serve both as heating resistors and as objects to be heated, they, together with appropriately selected resistance materials, form the core resistance of the furnace. An adequately controlled core resistance is an essential prerequisite for the successful graphiteization of the products. First, the furnace body itself, the busbar short network, and the equipment of the power supply system must all be in good condition. During furnace charging, the cross-section of the furnace core should be symmetrical with respect to the conductive cross-section, thus preventing current imbalance within the core. The furnace-charging procedure must comply with the requirements specified in the process technical regulations. Products inside the furnace core must be arranged horizontally and vertically, with uniform spacing between product groups. Resistance materials must be properly packed to avoid any voids, ensuring a balanced temperature distribution throughout the furnace core during the power-on phase. Second, the proportioning of resistance materials must meet the requirements of the production process’s technical standards, and their quality must be stable and uniform, thereby avoiding uneven temperature distribution within the furnace core during the power-on phase. Third, the graphiteization furnace must be powered according to the prescribed power-on curve, with power fluctuations kept within normal limits. Any abnormal fluctuations in power delivery must be avoided to ensure that the furnace core temperature rises uniformly.

c. Master quality information from preceding processes.

It is essential to promptly obtain information on the production status and quality of the preceding process. Based on the stability of product production and quality technical indicators from the preceding process, as well as the actual production conditions of this process, we must develop practical and feasible technical specifications for the graphitization process. This will help prevent cracks and defects during graphitization and ensure stable graphitization quality. When loading products into the graphitization furnace, each product must be carefully inspected for both its appearance and internal quality. Products that do not meet the technical specifications for the graphitization process must not be loaded into the furnace for graphitization; instead, they should be promptly returned to the preceding process.

d. Add an appropriate amount of gas expansion inhibitor to the ingredients.

The irreversible expansion and cracking phenomenon caused by the presence of sulfur during graphitization cannot be completely eliminated, but it must be carefully controlled. Currently, a relatively effective approach is to control the rate at which sulfur escapes during the graphitization process of the product. The most practical method is to add an appropriate amount of gas-expansion inhibitor to the raw materials—typically 1% to 2% of Fe2O3 powder.

As for the mechanism of adding an expansion-inhibiting agent, the inhibitor primarily works by capturing sulfur within the temperature range where the graphite in the product undergoes thermal expansion and gas evolution. It forms sulfur compounds during this initial temperature range and then releases these gases at higher temperatures. This broadens the temperature range over which sulfur can escape, preventing the product from experiencing excessive internal stress due to the sudden and concentrated release of gases, thereby avoiding cracking. The most commonly used expansion-inhibiting agent is Fe2O3 powder. Its mechanism of action involves the easy reduction of Fe2O3 powder at temperatures above 1000°C, yielding iron or iron-carbon compounds. At even higher temperatures, these iron-carbon compounds further decompose into iron and carbon. During this process, the iron formed reacts chemically with the sulfur released from the decomposition of sulfur-containing compounds in the product, forming iron sulfide that is slowly released. As a result, the rate of sulfur release from the product is significantly slowed down, effectively inhibiting sulfur evolution. The relevant chemical reaction equation is:

Since Fe2O3 powder not only exhibits a high chemical affinity for sulfur in the products and effectively inhibits sulfur, but also boasts abundant resources and low cost, it does not produce any adverse effects during electric furnace steelmaking. Moreover, Fe2O3 powder exerts a strong catalytic effect on the graphitization process of the products, making it an excellent graphitization catalyst. Thus, for petroleum coke with high sulfur content, adding an appropriate amount of the swelling-inhibiting agent Fe2O3 powder can achieve multiple benefits in the production of carbon-graphite products.

Closing remarks

In summary, the causes of cracking and scrap during the graphitization heat treatment of carbon-graphite products are multifaceted and relatively complex. To prevent the formation of cracked and scrap products during the graphitization heat treatment of carbon-graphite products, it is essential to adopt a variety of process and technological improvement measures, addressing both the symptoms and root causes. Most critically, the intrinsic quality of the products themselves must be high, with excellent thermal resistance and uniformity in production. Moreover, the quality and technical specifications of preceding processes must meet the requirements of the production technology standards, and quality fluctuations must be kept within normal ranges. Additionally, during the graphitization heat treatment, the rate at which the temperature rises in the core of the Acheson graphitization furnace must be strictly controlled, avoiding excessively rapid temperature increases in the furnace core. Such rapid temperature rises can lead to greater internal temperature differences within the products, thereby increasing thermal stresses and ultimately causing cracks and scrap.