Analysis of the causes of graphitization cracking of carbon-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 of carbon-graphite products. It is a special resistance furnace that is directly heated and intermittently operated by the internal heat source made of products and resistance materials loaded in the furnace. The space where products and resistance materials are placed 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 resistance of the graphitization furnace core, the electrical energy is converted into The heat energy makes the product reach the maximum temperature of graphitization, and completes the graphitization process. It follows the Joule-Lentz law.

It can be seen that the temperature at different points in the core of the graphitization furnace is different, and at the same point, the temperature is different at different times. It can be seen that the temperature of the core of the graphitization furnace is both a function of space and time. Therefore, the temperature distribution of each part in the furnace core is uneven.

After being powered on, the Acheson graphitization furnace relies on the heat generated by the resistance material to heat the product, which gradually raises the temperature of the furnace core. The temperature rises throughout the furnace core are very uneven, and the temperature distribution varies greatly. The temperature at the center of the core and the heat insulation material on both sides of the furnace core can differ by hundreds of 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 graphitization production experience, I have briefly summarized and analyzed the causes of cracking of carbon-graphite products in the process of graphitization production; here I will discuss with carbon engineering and technical personnel to reduce carbon-graphite products The cracked waste products in the process of graphitization production process, improve the yield of graphitization process, reduce the production cost of graphitization process, and improve the purpose of economic benefits。

Causes of product graphitization cracks

In the process of graphitization, the internal factor for cracking of the product is that the body 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 graphitization process, and the temperature difference between the upper and lower or the surrounding of the product is also With the increase, the corresponding increase in thermal stress is the main cause of cracks in the product.

1. The graphitization process system is unreasonable

Loading method

Atchison graphitization furnace products are usually installed in the vertical installation method. The vertical installation method has two forms: normal installation and wrong installation. When the furnace core product is being installed, there is only one high-density current heating zone for any one of the products. The wider the heating zone, the more uniform the heating of the product. On the contrary, the heating is very uneven. When wrongly installed, there are two high-density current heating belts for each product, and the product is heated more uniformly than the formal installation. Therefore, the choice of the furnace installation method of the graphitization furnace is improper. In the process of graphitization and power transmission, the temperature rise rate around the product is quite different, and the thermal stress generated by the product exceeds the thermal stress that the body can withstand, which is very likely 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 power supply system is unreasonable, the determined graphitization furnace power supply curve starts with too large power and rising power too fast, so that When the product is energized, the internal and external temperature gradients are too large, and the resulting thermal stress greatly exceeds the resistance of the product and causes cracks. Especially when the furnace temperature is between 1300 and 1800 degrees, in order to strictly control the furnace temperature rise stage, the physical structure and chemical composition of the products at this stage begin to change greatly. The graphitization of amorphous carbon has not begun. In fact, the chemical reaction is Mainly, the hydrogen, oxygen, nitrogen, sulfur and other elements combined in the amorphous carbon microcrystalline structure continue to escape. As a result of the escape, the impurity elements at the edge of the amorphous carbon microcrystalline structure continue to decrease, and some 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 material

The resistance of the graphitized furnace core is composed of the resistance of the product and the resistance of the resistance material in series. When the graphitization furnace is energized, the resistance of the resistance material accounts for about 99% of the resistance of the core, and the resistance of the resistance material after the energization ends. About 97%, it can be seen that the heat generated by the current through the resistance material is mainly used to heat the product during the graphitization process. When the resistance of the resistance material and the resistance of the product are very different, the heat generated by the resistance material during the graphitization process is far It is far greater than the heat of the product itself, causing the temperature difference between the inside and outside of the product to be too large, which will cause thermal stress to produce cracked products.

2. The quality of graphitization operation is not high

The quality of the furnace is not high

The operation of the graphitization furnace does not meet the requirements of technological standards. When the furnace is installed, the core products are not arranged neatly, the spacing of the product groups is inconsistent, the resistance material is unevenly filled, and even the phenomenon of "expansion" of the resistance material appears, which will appear in the graphitization furnace During the power transmission process, the current distribution throughout the furnace core is very uneven, resulting in very uneven heating and temperature rise of the product, the internal temperature difference of the product is too large, and the resulting thermal stress makes the product crack and waste.

Uneven quality of resistance material

When the mixed coke is used as the resistance material in the graphitizing furnace, the resistance of the metallurgical coke is 5-8 times greater than that of the graphitized coke. If the metallurgical coke and graphitized coke are mixed unevenly, the resistance distribution throughout the furnace core is very uneven, resulting in When the power is turned on, the temperature rise rate of each part of the furnace core is very uneven. The temperature difference between the upper and lower and the surroundings of the product is too large, and the thermal stress is also increased, which is easy to produce a large number of cracked waste products.

Graphite furnace core flow

According to the electrothermal law 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 magnitude of the current passing through the furnace core. When the core of the Acheson graphitization furnace has a bias flow due to various reasons, the current passing through the core is very different, and the temperature distribution of the core is very different. When the difference in the current distribution of the core is large, the heat generated in the part with high current is more, the temperature of the product in this area rises faster, and the heat generated in the part with less current is less, the temperature of the product in this area is slower, so the temperature of the core The difference in distribution is large, so that the internal temperature difference of the product is also large, and the thermal stress generated is correspondingly increased, resulting in cracked waste products.

3. Quality of roasted product

Cracks in the baked product

According to information, the temperature range of 350-500 degrees and 700 degrees and above during the roasting of products is the most dangerous temperature range in which carbon materials may be damaged. 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. Within the range of 65mm of the center radius of the blank, a dangerous tensile stress area is formed. When the temperature is 700 degrees or higher, the stress in this area is much greater than the limit of the fracture strength of the material. This is the reason why the product produces longitudinal straight cracks. Such cracks generally do not extend to the outer surface of the product, that is, the product Internal crack.

Product homogeneity

The uniformity of the density distribution of carbon-graphite products, 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, in the process of graphitization heat treatment, due to the effect of thermal stress, the product is prone to internal stress. Correspondingly, the internal stress of the product is also unevenly distributed. Such uneven internal stress is easy to cause cracks in the product. As a result, cracked waste products occur during the graphitization process.

High volume density of products

The volume 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 of the volume 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 difference between internal and external stress is large, the product produces crack waste.

Pre-production is unstable

Because graphitization is the last heat treatment process for the production of carbon-graphite products, it is also the heat treatment process with the highest temperature. It is generally believed that when the production of the current process is unstable or quality fluctuations occur, it will be exposed intensively during the graphitization process. If the temperature of the calcined material is low, the softening point of asphalt is unqualified, the calcination temperature is low, and the impregnation weight gain rate is unqualified, etc., when the product is graphitized at high temperature, it will cause secondary shrinkage or uneven shrinkage, which is very easy to produce cracked waste products.

Bloating

A certain degree of irreversible volume expansion occurs during the graphitization of products. The main reason is that during the graphitization process, due to the sharp escape of sulfur concentration, the degree of this irreversible expansion increases as the sulfur content increases and The speed of heat treatment increases, and this irreversible expansion behavior is called "air inflation".

As is known to all, the content of non-carbon elements such as hydrogen, oxygen, nitrogen, etc. after being calcined at 1350 temperature is generally less than 0.1%; however, the carbon atoms of sulfur and aromatic hydrocarbons are so strongly bound that c-s The bond begins to break above 1400 degrees, generating 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 it comes out, it will cause a lot of internal stress on the product, forming tiny holes and cracks in the product. When the sulfur content reaches a certain level, it will often cause cracks in the product during the graphitization process.

4. Prevention of product graphitization cracks

 a. The graphitization process should be reasonable

Selection of furnace installation method

In the production process of Acheson graphitization furnace, a reasonable furnace installation method is the guarantee for the graphitization of 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 product is heated relatively uniformly in the furnace core to reduce Thermal stress reduces cracks during graphitization. For large-size products, the use of dislocation 1/2D furnace installation can reduce product cracks and have a good graphitization effect. For products with a high rate of graphitized crack waste and unstable quality, it is also possible to adopt the furnace core current sharing measure.

Determine a reasonable power system

Determine the appropriate resistance material

The Acheson graphitization furnace mainly heats the product by the current generated by 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 late stage of power transmission, the secondary output current of the transformer has reached the maximum value. At this time, the resistance of the furnace core is large, which can maintain high electrical efficiency; but the resistance of the resistance material is too large, which is also unreasonable. Therefore, when determining the resistance material, it is necessary to consider not only the performance of the equipment, but also the variety specifications and power transmission curve of the product, so that the resistance of the product and the resistance of the resistance material cannot be too large. For small and medium-sized products, metallurgical coke can be used as the resistance material. Even if a higher starting power and a faster rising power are used, the product generally does not produce cracks; for large-size products, mixed coke or graphitized coke is used as the resistance material It is more appropriate, so that the difference between the resistance of 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 a crack in the product.The temperature of the graphitization furnace core is controlled by the power curve of constant power distribution. It is very important to formulate and use the power system of the graphitization furnace correctly and reasonably to improve the yield rate, save energy and shorten 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 the furnace structure, product variety specifications, quality information, resistance materials, thermal insulation effect, and parameters of the power distribution system, but more importantly, it is consistent with the products in the graphitization furnace Different requirements for the temperature rise rate at different stages.

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 of the product during the temperature rise process, and the furnace core should have a faster temperature rise speed, both Reduce heat dissipation loss of graphitization furnace without excessively large temperature gradient of the core, which will crack the product. For products with unstable graphitization quality, it is necessary to strictly control the temperature rise rate of the core during the temperature rise stage to avoid excessive temperature rise The product is cracked. 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.

b. The quality of operation must meet the standards

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 heated objects, they are combined with appropriate resistance materials to form the core resistance. The appropriate furnace Core resistance is a necessary condition for graphitization of products. First of all, the furnace body condition of the graphitization furnace, the busbar short network, and the equipment of the power supply system should be intact. The furnace core section should be commensurate with the conductive section when installing the furnace to prevent the furnace core from drifting. The furnace installation operation must comply with the requirements of the technical specifications. The products should be arranged horizontally and vertically in the furnace core, the group spacing of the products should be consistent, the resistance material should be filled well, to avoid the phenomenon of floating, to ensure that the temperature distribution of the furnace core is balanced during the power supply of the graphitization furnace. Secondly, the ratio of the resistance material should meet the requirements of the production process technical standards, and the quality should be stable and uniform, to avoid the uneven temperature distribution of the furnace core during the power supply of the graphitization furnace. Third, the graphitization furnace should be powered according to the given power transmission curve, and the power fluctuation should be controlled within the normal range to avoid abnormal fluctuations in the power transmission power 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 status 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, combine with the actual production of this process, formulate practical technical conditions for graphitization production process, and prevent the product from being in the process of graphitization Cracked waste products appear to ensure stable graphitization quality. In the graphitization furnace, the appearance and body quality of the products should be checked one by one. For products that do not meet the technical conditions of the graphitization process, they must not be placed in the graphitization furnace for graphitization, and the materials must be returned to the process in time.

d. Add proper amount of inflation inhibitor to the ingredients

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

Regarding the mechanism of adding a bloating inhibitor, the main reason is that the inhibitor can capture sulfur in the temperature range of the graphitized bloating of the product to form a sulfur compound and form a gas to be released in a higher temperature range, thereby widening the sulfur content. The escaped temperature range prevents the product from cracking due to the concentration of rapidly escaping gas and excessive internal stress. The most commonly used inflation inhibitor is Fe2O3 powder. Its mechanism of action is that when the temperature is above 1000 degrees, Fe2O3 powder is easily reduced to form iron or carbon iron compounds. The carbon iron compounds will further decompose 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 playing a role in inhibiting sulfur. The relevant chemical reaction formula is:

Because Fe2O3 powder not only has a high chemical affinity for sulfur in the product, it has a good effect of suppressing sulfur, but also has abundant resources and low price. It will not produce any adverse effects in the steelmaking process of the electric furnace. In addition, Fe2O3 powder 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 appropriate amount of inflation inhibitor Fe2O3 powder can play a multi-purpose role in the production of carbon-graphite products.

Conclusion

In short, the causes of cracked waste products of carbon-graphite products in the process of graphitization heat treatment are many and complicated. In order to prevent the cracking of carbon-graphite products during the graphitization heat treatment process, various technological improvement measures must be taken, with equal emphasis on specimens. The most important thing is that the quality of the product's body is high, the heat resistance is good, and the homogenized production is achieved. The quality and technical indicators of the previous process must meet the technical standards of the production process, and the quality fluctuation should be controlled within the normal range. In addition, during the graphitization heat treatment process, the rate of temperature rise of the product in the core of the Acheson graphitization furnace should be strictly controlled to avoid the temperature rise rate of the core being too fast, the internal temperature difference of the product increases, and the thermal stress increases accordingly. Large, and cracked waste products.