Reasons for 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.

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. The temperature rise of the furnace core is very uneven, and the temperature distribution is very different. 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 part and the lower part of the furnace core can also reach 100 degrees Celsius. Therefore, the non-uniform heating temperature distribution in the same graphitization furnace core is the main cause of cracks in the furnace core products.

Causes of graphitization cracks in products

In the process of graphitization, the internal factor of cracks in the product 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 graphitization process, and the temperature difference between the upper and lower sides of the product is also With the increase, the thermal stress also increases accordingly, which is the main reason for cracks in the product.

The graphitization process system is unreasonable

1. Furnace loading method

Acheson's graphitization furnace products are usually installed in a vertical installation method, and the vertical installation method has two forms: normal installation and wrong installation. When the furnace core product is being installed, for any product, there is only one high-density current heating belt. The wider the heating belt, the more uniform the heating of the product. When wrongly installed, there are two high-density current heating belts for each product, and the heating of the product is more uniform than that of the formal installation. Therefore, the loading method of graphitization furnace products is not properly selected. During the process of graphitization and power transmission, 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 very easy to cause cracks in the product.

2. The power-on system is unreasonable

The temperature change curve of the Acheson graphitization furnace core is controlled by the power curve of constant power distribution. If the power-on system of the graphitization furnace is unreasonable, the determined power-on curve of the graphitization furnace starts with too high power and rises too fast, so that the When the product is energized, the temperature gradient inside and outside is too large, and the thermal stress generated greatly exceeds the resistance of the product, resulting in cracks. Especially when the furnace temperature is 1300-1800 degrees, in order to strictly control the furnace temperature rise stage, the physical structure and chemical composition of the product begin to change greatly at this stage, and the graphitization of amorphous carbon does not start. In fact, the chemical reaction is Mainly, the elements such as hydrogen, oxygen, nitrogen, sulfur and other elements bound in the crystallite structure of amorphous carbon continue to escape. As a result of the escape, the impurity elements in the edge part of the crystallite structure of amorphous carbon continue to decrease, and some lattice defects remain. , At the same time, the thermal stress is relatively concentrated, and it is very easy to cause cracks in the product.

3. The resistance of the 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 energize, the resistance of the resistance material accounts for about 99% of the resistance of the furnace core. About 97%, it can be seen that in the whole graphitization process, it is mainly the heat generated by the current passing through the resistance material to heat the product. When the resistance of the resistance material is quite different from the resistance of the product, the heat generated by the resistance material is far away during the graphitization and electrification process. 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 excessive thermal stress to cause cracks in the product.

The quality of graphitization operation is not high

1. The quality of furnace loading is not high

The operation of the graphitization furnace does not meet the requirements of the process technical standards. When the furnace is loaded, the furnace core products are not neatly arranged, the spacing between the product groups is inconsistent, the filling of the resistance material is uneven, and even the phenomenon of "expanded material" of the resistance material occurs, which will appear in the graphitization furnace. In the process of power transmission, the current distribution around the furnace core is very uneven, resulting in uneven heating and temperature rise rates of the product, and the temperature difference inside the product is too large, and the thermal stress generated will cause the product to crack and waste.

2. The quality of the resistance material is uneven

When the graphitized charging furnace uses mixed coke as the resistance material, the resistivity of metallurgical coke is 5-8 times larger than that of graphitized coke. When the furnace core is energized, the temperature rise speed of each part of the furnace core is very uneven, the temperature difference between the upper and lower parts of the product and the surrounding area is too large, and the thermal stress also increases, which is easy to produce a large number of cracked waste products.

3. Bias flow of graphitization furnace core

According to the electric heating law of the Acheson graphitization furnace, the temperature distribution in the graphitization furnace core is not only related to the furnace core resistance, but also closely related to the current passing through the furnace core. When the furnace core of Acheson graphitization furnace is biased 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 difference between the current distribution of the furnace core is large, the part with large current generates more heat, the temperature rise of the products in this area is faster, and the heat generated in the part with small current is less, the temperature rise of products in this area is slower, so the temperature of the furnace core 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 crack and waste.

The quality of roasted products

1. Internal cracks in roasted products

According to information, the temperature range of 350-500 degrees and 700 degrees and above during the roasting process of the product is the most dangerous temperature range where the carbon material may be damaged. When the outer surface temperature 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, and a dangerous tensile stress area is formed within the radius of 65mm from the center of the blank. When the temperature is 700 degrees or higher, the stress in this area is far greater than the limit of the breaking strength of the material, which is why the product produces longitudinal straight cracks, which generally do not extend to the outer surface of the product, that is, the product of internal cracks.

2. Product homogeneity

The uniformity of the density distribution of carbon-graphite products, the uniformity of the radial density and the axial density distribution of the product is closely related to the quality of the product 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 thermal stress, the product is prone to internal stress, and the internal stress distribution of the product is also uneven. This uneven internal stress is easy to cause cracks in the product. As a result, cracked waste products appear during the graphitization process.

3. The bulk density of the product is high

The bulk density of carbon-graphite products mainly varies with the production raw materials and technological conditions. The flexural strength, elastic modulus and thermal conductivity of the products increase with the increase of 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. , There is a large difference between the internal and external stress, and the product produces cracked waste.

4. The pre-process production is unstable

Because 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 production of the current process is unstable or the quality fluctuates, it will be exposed intensively 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, and the weight gain rate of the impregnation is unqualified, etc., the product will cause secondary shrinkage or uneven shrinkage during the high temperature graphitization treatment, and it is very easy to produce cracked waste products.

5. Bloating phenomenon

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 of sulfur content and The increase in the heat treatment rate increases, and this irreversible expansion behavior is called "blowing phenomenon".

As we all know, the content of non-carbon elements such as hydrogen, oxygen, nitrogen, etc. is generally less than 0.1% in petroleum coke after calcination at 1350 temperature; however, the carbon atoms of sulfur and aromatic hydrocarbons are bound so strongly that c-s bond begins to break above 1400 degrees, and sulfur and sulfur-carbon compounds are formed; 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 often leads to cracks in the products during the graphitization process.