The graphitization process of graphite electrodes is actually a temperature control process.
How to control the graphitization temperature of graphite electrodes?
► 1) Repeat the roasting stage
Room temperature to 1250°C is the repeated firing stage. The carbon billet fired at about 1250°C has certain thermoelectric properties and thermal shock resistance. A faster heating rate is used to enable the calcined product to complete the preheating transition stage at the initial stage of graphitization. The structure of the carbon billet will not change greatly. The product itself will not crack.
► 2) Strictly control the heating stage
1250～1800℃ is the key control stage of temperature rise. In this critical temperature range for graphitization, the physical structure and chemical composition of the carbon billet have undergone great changes. The disordered layer structure of amorphous carbon has a tendency to gradually transform to the graphite crystal structure, accompanied by the amorphous carbon microcrystalline structure. The unstable low-molecular-weight hydrocarbons and impurity element groups bound to the edges continue to decompose and escape, and produce structural defects, which also promotes the relative concentration of thermal stress, which is very easy to produce crack waste. In order to slow down the effect of thermal stress, prevent excessive concentration of thermal stress, avoid cracks in the carbon billet, and at the same time, in order to maintain a certain temperature maintenance time, the heating rate at this stage should be strictly controlled.
► 3) Free heating stage
1800°C to the highest graphitization temperature is the free heating stage. In this temperature range, the prototype graphite crystal structure of the carbon material has basically formed, and the temperature continues to rise, which promotes its graphitization degree to further increase. The degree of perfection of graphite crystals mainly depends on the maximum temperature. The effect of the temperature maintenance time is already small, and the heating rate at this stage can be accelerated.
► For example, the Acheson graphitization furnace, due to the large furnace resistance at the beginning of the power transmission, and the insulation material and the resistance material contain a certain amount of moisture, which requires energy, the actual temperature rise in the first stage is not fast, and the heating rate can only reach 30 at the beginning. ～40℃/h, and then gradually speed up to 70~80℃/h. The heating rate of the second stage is required to be controlled at 30-50°C/h. If it is not properly controlled, a large number of cracked waste products will be produced. In fact, the temperature rise in the second stage is the most difficult to control, which is the main reason why the curve actually used is much more conservative than the calculated curve. After the furnace core temperature exceeds 1800℃, it can accelerate the heating rate in the late stage of graphitization. However, due to the increase of heat loss at high temperatures, it is difficult to increase the actual heating rate.