Graphitization is one of the primary heat treatment processes in the production of carbon-graphite products. The Acheson graphitization furnace is the predominant furnace type currently used for graphitization of carbon-graphite products. It is a special resistance furnace that operates intermittently, using the products and resistor material within the furnace as an "internal heat source" for direct heating. The space within the graphitization furnace where the products and resistor material are placed is called the furnace core, with a cross-sectional area typically ranging from 3 to 6 square meters.
A strong current is passed through the furnace, and the core's resistance converts electrical energy into heat, bringing the products to the maximum graphitization temperature and completing the graphitization process. This process follows the Joule-Lenz law.
As can be seen, the temperature at different points within the graphitization furnace core varies, and even at the same point, the temperature varies over time. Therefore, the temperature of the graphitization furnace core is a function of both space and time, resulting in uneven temperature distribution within the core.
When the Acheson graphitization furnace is powered on, the heat generated by the resistor material heats the product, gradually raising the furnace core temperature. This temperature rise is highly uneven across the core, resulting in significant temperature variations. The temperature difference between the center of the furnace core and the insulation on either side can reach hundreds of degrees Celsius, and the temperature difference between the top and bottom of the furnace core can also reach hundreds of degrees Celsius. Therefore, this uneven temperature distribution within the same graphitization furnace core is the primary cause of cracks in the products within the core.
Based on years of graphitization production experience, we have briefly summarized and analyzed the causes of cracking and rejects in carbon-graphite products during the graphitization process. This discussion, conducted in conjunction with carbon engineering technicians, aims to reduce cracking and rejects in the graphitization process, improve the yield of the graphitization process, reduce production costs, and enhance economic efficiency.
Causes of Graphitization Cracks in Products
During the graphitization process, internal factors contributing to product cracking are low product quality and poor heat resistance. External factors include the rapid rise in temperature within the furnace core during graphitization, which increases the temperature differential between the top and bottom of the product and around the product. This, in turn, increases thermal stress, which is the primary cause of product cracking.
1. Irrational Graphitization Process
① Loading Method
Acheson graphitization furnaces are typically loaded using the vertical loading method, which can be either upright or staggered. When loading the product in the furnace in the upright position, each product is exposed to a single high-density current heating zone. The wider this zone, the more evenly heated the product; otherwise, the heating is highly uneven. When loading the product in the staggered position, each product is exposed to two high-density current heating zones, resulting in more even heating compared to upright loading. Therefore, improper loading methods can lead to significant differences in the temperature rise rates around the product during graphitization, resulting in thermal stresses exceeding the product's tolerance, making it highly susceptible to cracking.
② Irrational Power Supply System
The temperature profile of the Acheson graphitization furnace core is controlled using a constant power distribution power curve. If the power supply system is not optimized, the initial power of the graphitization furnace may be too high and increase too rapidly, resulting in excessive temperature gradients between the interior and exterior of the product during the power supply process. This generates thermal stresses that greatly exceed the product's resistance and cause cracks. This is especially true when the furnace temperature is between 1300°C and 1800°C, a critical temperature rise stage. During this stage, the physical structure and chemical composition of the product begin to undergo significant changes. Graphitization of the amorphous carbon has not yet begun; instead, chemical reactions are predominant. Elements such as hydrogen, oxygen, nitrogen, and sulfur bound to the amorphous carbon microcrystalline structure continuously escape. This release reduces the number of impurity elements at the edges of the microcrystalline structure, leaving behind several lattice defects. This also leads to a relatively concentrated thermal stress, making it highly susceptible to cracking.
③ Resistance of the Resistor Material
The resistance of the graphitization furnace core is composed of the resistance of the product and the resistance of the resistor material in series. When the graphitization furnace is initially energized, the resistance of the resistor material accounts for approximately 99% of the furnace core resistance, and after energization ends, the resistance of the resistor material still accounts for approximately 97%. Therefore, throughout the graphitization process, the heat generated by the current flowing through the resistor material primarily heats the product. If the resistance of the resistor material differs significantly from that of the product, the heat generated by the resistor material during the graphitization process will be much greater than the heat generated by the product itself. This creates a significant temperature difference between the inside and outside of the product, leading to excessive thermal stress and cracks in the product, resulting in scrap.
2. Poor Graphitization Operation Quality
① Poor Furnace Loading Quality
Graphitization furnace loading operations do not meet process and technical standards. During loading, products are not arranged neatly in the furnace core, the spacing between product groups is inconsistent, the resistor material is unevenly filled, and even the resistor material "bulges." This results in uneven current distribution throughout the furnace core during power supply to the graphitization furnace, leading to uneven heating and temperature rise rates for the products. This leads to large temperature differences within the products, and the resulting thermal stress causes cracks and scrap.
② Uneven Resistor Material Quality
When using mixed coke as the resistor material in a graphitization furnace, the resistivity of metallurgical coke is 5-8 times higher than that of graphitized coke. If the metallurgical coke and graphitized coke are not evenly mixed, the resistance distribution throughout the furnace core will be highly uneven, resulting in inconsistent temperature rise rates across the furnace core when power is applied. This leads to large temperature differences between the top and bottom and around the products, increasing thermal stress and causing a large number of cracked products.
③ Graphitization Furnace Core Current Deviation
According to the electrical heating laws of an Acheson graphitization furnace, the temperature distribution within the graphitization furnace core is closely related not only to the core resistance but also to the current flowing through it. When a core current deviation occurs in an Acheson graphitization furnace due to various reasons, the current flowing through the core varies significantly, resulting in significant variations in the core temperature distribution. When the core current distribution varies significantly, areas with high current generate more heat, causing the product temperature to rise more rapidly. Areas with low current generate less heat, causing the product temperature to rise more slowly. Consequently, the core temperature distribution varies significantly, leading to large temperature differences within the product and increased thermal stress, which can cause cracks and result in scrap.
3. Quality of the Calcinated Product
① Internal Cracks in the Calcinated Product
References indicate that the temperature ranges of 350-500°C and 700°C and above during the calcination process are the most dangerous for carbon material failure. When the product's outer surface temperature reaches 800°C and the maximum radial temperature difference is 10.7°C, the area with a radius of 50-65mm determines the material's strength. Within a radius of 65mm from the center of the blank, a dangerous tensile stress zone forms. At temperatures of 700°C or higher, the stress in this area far exceeds the material's fracture strength limit, leading to the development of longitudinal straight cracks in the product. These cracks generally do not extend to the product's outer surface, resulting in internal cracks.
② Product Homogeneity
The uniformity of the density distribution of carbon-graphite products, and the uniformity of both the radial and axial density distributions are closely related to the quality of the product during the graphitization heat treatment. In areas where the product density is unevenly distributed, thermal stress during the graphitization heat treatment can easily generate internal stress in the product. Consequently, the distribution of internal stress is uneven, which can easily cause cracks in the product, resulting in cracked products and rejects during the graphitization process.
③ High Product Bulk Density
The bulk density of carbon-graphite products varies primarily with the production raw materials and process conditions. The product's flexural strength, elastic modulus, and thermal conductivity increase with increasing bulk density. High bulk density increases the elastic modulus and brittleness, leading to poor thermal shock resistance. During the graphitization heat treatment, the thermal stress generated by the high temperature far exceeds the product's inherent stress tolerance, resulting in a significant difference between internal and external stresses, leading to cracks and rejects.
④ Unstable Production in Previous Processes
Because graphitization is the final heat treatment step in carbon-graphite product production and also the highest-temperature heat treatment, it is generally believed that instability or quality fluctuations in previous processes will be most prominently revealed during the graphitization process. If the calcining temperature is low, the pitch softening point is substandard, the roasting temperature is low, or the impregnation weight gain rate is substandard, the product will experience secondary or uneven shrinkage during the high-temperature graphitization process, making it very likely to crack and become scrapped.
⑤ Gas Bloating
The graphitization process causes a certain degree of irreversible volume expansion in the product. This is primarily due to the rapid and concentrated release of sulfur during the graphitization process. The extent of this irreversible expansion increases with increasing sulfur content and faster heat treatment rates. This irreversible expansion behavior is known as "gas swelling."
As we all know, the content of non-carbon elements such as hydrogen, oxygen, and nitrogen in petroleum coke calcined at 1350°C is generally less than 0.1%. However, sulfur is so tightly bound to the carbon atoms of aromatic hydrocarbons that the C-S bonds do not begin to break until temperatures above 1400°C, forming sulfur and sulfur-carbon compounds. At higher temperatures, primarily between 1500°C and 1800°C, these sulfur and sulfur-carbon compounds are rapidly released from the product as gases, generating significant internal stress and forming tiny pores and cracks within the product. When the sulfur content reaches a certain level, it often causes cracks in the product during the graphitization process.
4. Preventing Graphitization Cracks in Products
a. Reasonable Graphitization Process
① Selecting the Furnace Loading Method
In the Acheson graphitization furnace production process, a reasonable furnace loading method is crucial for ensuring the successful graphitization of the product. Whether products are loaded vertically or horizontally, and whether they are loaded upright or staggered, should be determined based on the product type, specifications, quality standards, and equipment process parameters. This ensures relatively uniform heating of the products within the furnace core, reducing thermal stress and cracking during the graphitization process. For large-sized products, staggered loading (1/2D) can reduce cracking and achieve better graphitization results. For products with high scrap rates due to graphitization cracking and unstable quality, current distribution measures can also be implemented within the furnace core.
② Determine a Reasonable Power Supply System
The temperature of the graphitization furnace core is controlled using a power curve with constant power distribution. Correctly formulating and implementing the graphitization furnace power supply system is crucial for improving yield, saving energy, and shortening the graphitization cycle. The graphitization furnace power supply system must not only consider factors such as the furnace structure, product type and specifications, quality information, resistor materials, insulation performance, and power distribution system parameters, but more importantly, it must meet the product's varying temperature rise requirements at different stages within the graphitization furnace.
A reasonable power-on system for the graphitization furnace should be a "fast-slow-fast" three-stage power curve to adapt to the different requirements of the three stages of the product temperature rise process. The furnace core should be kept at a faster temperature rise rate to reduce the heat loss of the graphitization furnace without causing the temperature gradient of the furnace core to be too large, which could cause cracks in the product. For products with unstable graphitization quality, the temperature rise rate of the furnace core in the temperature rise stage should be strictly controlled to avoid excessive temperature rise and cracks in the product. At this time, the power supply curve should be adjusted. The ramp-up power should be appropriately adjusted to form a four-stage power transmission curve: "fast-slow-slow-fast."
③ Determine the Suitable Resistor Material
The Acheson graphitization furnace primarily heats the product through the heat generated by the current passing through the resistor material. The resistor material is closely related to the temperature fluctuations in the furnace core. To increase the temperature of the graphitization furnace core, the resistor material requires a higher resistance, especially in the later stages of power transmission, when the transformer's secondary output current reaches its maximum. This allows for a higher core resistance and maintains high electrical efficiency. However, excessively high resistor material resistance is also inappropriate. Therefore, when selecting the resistor material, it is important to consider both the equipment performance and the product type, specifications, and power transmission curve to ensure that the product resistance and the resistor material resistance do not differ significantly. For small and medium-sized products, metallurgical coke can be used as the resistor material. Even with a higher starting power and a faster ramp-up power, the product generally does not crack. For large products, mixed coke or graphitized coke is more suitable as the resistor material, ensuring that the product and resistor material resistances are comparable. The temperature difference is smaller, and the temperature difference between the inside and outside of the product is also reduced. Even with a faster power increase, cracks in the product will not occur.
b. Operational quality must meet standards
In the graphitization production process, furnace loading is critical. Since the product loaded into the graphitization furnace serves as both the heating resistor and the object being heated, it is combined with the appropriate resistor material to form the furnace core resistance. Proper furnace core resistance is essential for product graphitization. First, the graphitization furnace body, busbar short network, and power supply system equipment must be in good condition. During furnace loading, the furnace core cross-section must be symmetrical with the conductive cross-section to prevent current deviation in the furnace core. Furnace loading must comply with process technology. Regulations require that products be arranged horizontally and vertically within the furnace core, with consistent spacing between product groups. The resistor material must be properly filled to avoid overhanging parts, ensuring a balanced temperature distribution within the furnace core during power supply to the graphitization furnace. Furthermore, the resistor material ratio must meet production process and technical standards, ensuring consistent quality to avoid uneven temperature distribution within the furnace core during power supply. Finally, the graphitization furnace must deliver power according to the specified power supply curve, with power fluctuations kept within normal limits to avoid abnormal power fluctuations, ensuring a balanced temperature rise within the furnace core.
c. Mastering Quality Information from Previous Processes
Maintaining timely access to production and quality information from previous processes is crucial. Based on the stability and quality specifications of the products from the previous process and the actual production practices of the current process, a practical and feasible graphitization production process and technical specifications should be developed to prevent cracks and rejects during the graphitization process and ensure consistent graphitization quality. During graphitization furnace loading, inspect each product for appearance and quality. Any products that do not meet the requirements of the graphitization process should be removed. Products that do not meet the technical requirements must not be loaded into the graphitization furnace for graphitization and must be promptly returned to the previous process.
d. Adding an appropriate amount of inflation inhibitor to the batching
The irreversible expansion and cracking caused by the presence of sulfur during the graphitization process cannot be eliminated, but it must be controlled. Currently, the most effective approach is to control the rate of sulfur release during the graphitization process. The most practical method is to add an appropriate amount of inflation inhibitor to the batching process, typically 1%-2% Fe2O3 powder.
The mechanism of adding inflation inhibitors is that the inhibitor captures sulfur within the temperature range of the product's graphitization inflation, forming sulfur compounds that are released as gases at higher temperatures. This widens the temperature range for sulfur release, preventing the product from cracking due to excessive internal stress caused by the concentrated and rapidly escaping gas. The most commonly used inflation inhibitor is Fe2O3 powder. Its mechanism of action is that at temperatures above 1000°C, Fe2O3 powder is easily reduced to Iron or carbon-iron compounds are produced. Carbon-iron compounds further decompose into iron and carbon at higher temperatures. The iron formed in this process reacts with the sulfur released from the decomposition of the product, slowly releasing it as iron sulfide. This slows the release of sulfur from the product and acts as a sulfur inhibitor.
Fe2O3 powder not only has a high chemical affinity for sulfur in the product, effectively suppressing sulfur, but is also abundant and inexpensive, and has no adverse effects on the electric furnace steelmaking process. Furthermore, Fe2O3 powder has a strong catalytic effect on the graphitization process of the product, making it an excellent graphitization catalyst. Therefore, for petroleum coke with a high sulfur content, adding an appropriate amount of Fe2O3 powder, a flatulence inhibitor, can have a significant effect on the product. The production of carbon-graphite products can achieve multiple goals at once.
In short, the causes of cracking and rejection in carbon-graphite products during the graphitization heat treatment process are multifaceted and complex. To prevent cracking and rejection in carbon-graphite products during the graphitization heat treatment process, various process and technology improvements must be implemented, with equal emphasis on both the product itself and the product itself. The most critical aspects are ensuring high product quality, excellent heat resistance, and homogeneous production. The quality and technical indicators of the preceding processes must meet the requirements of production process standards, and quality fluctuations must be kept within normal ranges.
Furthermore, during the graphitization heat treatment process, the temperature rise rate of the product in the Acheson graphitization furnace core must be strictly controlled to avoid excessively rapid core temperature increases, which would increase the temperature difference within the product and cause correspondingly increased thermal stress, leading to cracking and rejection.