The consumption mechanism of graphite electrodes

Graphite electrodes, as conductive materials in electric arc furnace smelting, have a proportional relationship with the consumption of electrical work. Modern electric arc furnace steelmaking uses electrical and chemical energy as thermal energy to achieve the goals of four separations (P, C, O, S), two separations (gas, impurities), and two adjustments (temperature, composition) during the steelmaking process. The performance of graphite electrodes is mainly reflected in their suitability and consumption among users, and the consumption of electrodes is directly related to their own quality. The consumption of graphite electrodes in electric arc furnace smelting mainly consists of the following parts.
1. Consumption of graphite electrode ends and outer surface inside the electric furnace
The electric arc generated by graphite electrodes in electric arc furnaces can be divided into long, medium, and short arcs, while the melting of furnace materials and heating depend on the arc power. The arc length is directly proportional to the secondary voltage and inversely proportional to the secondary current and heating rate. In order to improve the smelting speed and significantly shorten the smelting time, high chemical energy operations of forced oxygen blowing are adopted, which puts higher requirements on the oxidation resistance and thermal shock resistance of graphite electrodes. The end consumption of graphite electrodes in smelting includes sublimation generated during high arc temperature and chemical reactions generated during contact with molten steel and slag. The oxidation loss of graphite electrodes accounts for about two-thirds of the total consumption. Its oxidation loss is the product of unit oxidation rate and area and is proportional to time. The longer the heating time in smelting, the greater the consumption. Therefore, it is necessary to install a water-cooled spray system on the electrodes of the electric arc furnace. In normal smelting, the carbon content of graphite electrodes entering the molten steel is generally around 0.01%, and the non conical consumption switch at the end is a normal phenomenon.
2. Residual consumption of graphite electrodes generated during smelting
Residual consumption refers to the non productive consumption of the bottom electrode that falls into the furnace and becomes the final waste during smelting, leaving the production process. The generation of residues is not only related to the intrinsic quality of the joints and electrodes, but also directly related to factors such as the distribution of materials in the furnace, the atmosphere in the furnace, and the power transmission operation, which are directly related to magnetic cards. The main appearance phenomena include: there are "human" shaped cracks at the bottom of the residual body and large longitudinal or splitting cracks; The connection is not tight enough, causing the joint to oxidize first and fall off or break; Loose or broken due to inadequate connection or poor fit; The electrode breaks at the bottom of the joint or hole due to external force; The electrode is subjected to external force, causing the joint or hole end to break; Unreasonable material distribution in the furnace can lead to a large area of material collapse after passing through the well, or improper operation of the power transmission curve can cause serious electrode breakage; The electrode itself has poor quality, etc. This part of the loss is not significant in normal production while ensuring electrode quality, but direct users attach great importance to it.
3. The electrode surface is oxidized and peeled off, accompanied by consumption of rows and falling blocks
In normal smelting production, if the surface of graphite electrodes is uneven or accompanied by peeling and falling off, there is a problem of carbon increase in the molten steel. This phenomenon reflects, on the one hand, the poor antioxidant and thermal shock resistance of the electrode; On the other hand, excessive horizontal oxygen blowing time or amount during smelting can cause severe oxygen enrichment in the furnace and above, resulting in increased electrode peroxidation losses; The second is that if there is serious detachment, the electrode issue must also be considered. This abnormal consumption is a test of the inherent quality and technical service level of the product.
4. Direct losses caused by broken graphite electrodes during smelting
The breakage of graphite electrodes in all electric furnace smelting is a common phenomenon and the main factor affecting consumption. Continuous consumption and occasional breaking in complex environments is a normal phenomenon, but continuous breaking is abnormal. The reason for this is related to many factors. Overall, it can be divided into: artificial fracture and mechanical fracture. Human breakage mainly includes: bumps, scratches during lifting, inadequate connection or improper methods, improper sliding in the leveler, hard collisions or poor sensitivity of transmission control. In mechanical fracture, apart from mechanical faults, electrode quality issues and operational issues often coexist and are difficult to distinguish. There are mainly the following phenomena.
(1) The phenomenon of electrode body breaking
One is that the electrode may have structural defects and lower strength; Secondly, there is a significant lateral impact force due to the short arc operation after piercing in the smelting process, resulting in significant material collapse; The third issue is that the three-phase electrodes on the furnace are severely not vertical and there are phenomena such as hanging furnace slag or scraping the furnace cover. When broken, the sound is very crisp and loud.
(2) The phenomenon of electrode body hole bottom breaking
One is that the electrode end structure is loose or has dark lines, the joint and hole are not properly matched, or the material difference does not match the linear expansion coefficient; The second is that the electrodes of the entire phase are not concentric, and the electrode stroke is too long or not sensitive to lifting and lowering; The third reason is that the fabric inside the furnace is unreasonable, and there are non-conductive materials below the electrodes. When broken, the sound is not loud but the tilt is heavy.
(3) Irregular phenomenon of joint breakage
One is that there is a difference in the taper of the joint processing or the elliptical shape of the joint hole is too large; The second reason is that there is a lot of dust inside the hole during connection, which causes excessive contact resistance and rapid oxidation of the local threads of the joint; Thirdly, the electrode connection is not in place and does not meet the torque requirements, resulting in looseness; The fourth is that the controller is tilted, and the electrode is not concentric with the furnace cover hole. When broken, the sound is crisp and low.
(4) Regular phenomenon of joint breakage
Firstly, there is a significant difference in the quality of the joints themselves, and the strength of the joints cannot meet the requirements of the smelting furnace conditions; Secondly, improper tolerance fit between electrode holes and joints or failure to meet the required coupling torque may result in buckle retraction; Thirdly, the fluctuation range of secondary current in power supply is too large or there is a sudden increase, with the maximum instantaneous current far exceeding the rated value by more than 1.2 times; The fourth issue is that the thermal vibration generated when the output and power are too high, and the electrode connection will intermittently turn red and display excessive resistance. The sound becomes muffled when broken.
Factors affecting graphite electrode consumption in smelting
In the steelmaking process, electric arc furnace smelting can be divided into rough smelting and refining, but both are carried out by graphite electrodes to transfer electrical energy into heat energy and achieve the purpose of heating and melting solid furnace materials. Rough refining is an oxidation process aimed at melting scrap steel and discharging harmful gases and impurities, while refining is a reduction process that adjusts steel grade elements and removes gases and impurities by increasing temperature. From this, it can be seen that electric arc furnace smelting uses graphite electrodes to adjust the furnace temperature to achieve oxidation-reduction reactions and meet the requirements of steelmaking technology. The consumption of graphite electrodes in smelting can be divided into active consumption, natural consumption, or technical consumption. Reactive power consumption refers to tangible consumption that has not participated in the heating process or has not fully participated in the heating process, and is also non-technical consumption.
1. The main factors affecting the consumption of graphite electrodes
The consumption of graphite electrodes in electric furnace steelmaking runs through the entire smelting process, and its magnitude is influenced by many factors. Here, we will only analyze the operation of the smelting process.
(1) Poor quality or improper proportion of scrap steel leads to poor slag making effect and prolongs smelting time; The increase in feeding frequency and drilling data has increased the probability of damage and breakage at the bottom of the electrode; The prolonged smelting time is a direct factor leading to an increase in electricity consumption and electrode consumption and loss.
(2) The power supply equipment of the electric furnace does not match the specifications and varieties of the electrodes
Electric arc furnace power transmission is a high current and low voltage operation. If the capacity of the power supply equipment is too large and exceeds the electrode limit load, there will be a red phenomenon from bottom to top in the electrode within 5-10 minutes of arcing, and the boundary line at the connection point is very obvious and most of the incidents of breakage occur; The current is too high or fluctuates too much, and the breaking frequency of the connection is increased, and the bottom consumption is cone-shaped. If the power supply capacity is low and the furnace temperature does not meet the process requirements within the effective heating time, it will require delayed operation. Overloading and overtime operations have the greatest impact on the loss and consumption of graphite electrodes.
(3) Forced increase of chemical energy and improvement of smelting intensity during the oxidation period
Forcefully increasing the oxygen blowing rate (generally less than 45M3/T) during smelting to achieve rapid melting and increase furnace temperature can easily lead to poor furnace conditions and a rich state inside and on the furnace, causing each phase of the electrode to be in a high-temperature flame. When the electrode is in such a furnace environment, there is often delamination and severe surface oxidation.
(4) Technical aspects of the configuration and operation of electric furnaces
There is a fundamental difference between modern electric arc furnaces and traditional electric furnaces. The emergence of ultra high power large-scale electric arc furnaces and hot charging steelmaking processes has improved smelting intensity and production capacity, thus placing higher technical requirements on the quality of graphite electrodes. The selection of power transmission curve and gear during operation, control of arc starting and stable voltage and current, coordination of long, medium, and short arcs, configuration and application of water cooling system all play a crucial role in the service life and consumption of electrodes.
(5) The quality of graphite electrodes
The smelting and process of electric arc furnaces now require increasingly strong resistance to oxidation and thermal shock of graphite electrodes, and the high consumption caused by quality fluctuations is of great concern to direct users. So the balance and stability of graphite electrode quality are the most important factors determining the size of consumption.
2. Comparative analysis of high and low consumption of graphite electrodes
When the process and operating conditions of the smelting electric arc furnace do not change significantly during a certain period of time, the consumption level of graphite electrodes is also basically balanced. It is normal for the consumption of electrodes to fluctuate with the prolongation of furnace life or changes in process. So, it is a common phenomenon for users to raise objections when there are products from the same company on the same electric arc furnace that consume a lot of dust. The quality of any product has fluctuations, but the magnitude of the fluctuations reflects the manufacturer's level of technical equipment and comprehensive management.
(1) Further analysis of graphite electrode breakage in smelting
Occasional electrode breakage in electric arc furnace smelting is a normal phenomenon and cannot be absolutely avoided, while electrodes in large DC, AC, and LF electric furnaces are considered accidents. Dealing with electrode breakage residues is the most arduous task in operation, which inevitably results in high consumption, longer smelting cycles, reduced production, and increased costs. Most of the advanced large DC and AC electric furnaces with advanced political parties equipped with control network systems, which greatly reduce human factors, break electrodes 5-7 times a month for AC electric furnaces with general technical level in China; Advanced large LF furnaces generally do not allow electrode breakage. The reasons for electrode breakage in electric furnace smelting operations are quite complex, and the following five aspects should be of great concern.
(2) Smelting process
A raw material ratio, avoiding large blocks of material and non-conductive materials below the electrode;
When the phenomenon of "bridging" occurs after crossing the well, it is necessary to use a long arc operation to avoid large material collapse and impact;
The lifting of the C electrode and the small furnace cover must be concentric to avoid scratching and breaking during thermal vibration.
(3) Power transmission system
Determine the initial arc starting position and raise it in sequence (each furnace has at least 3 power transmission curves) to avoid excessive current fluctuations and frequent changes in long and short arcs inside the furnace, which may cause excessive thermal vibration;
As the furnace temperature increases, a certain amount of internal stress needs to be released at the electrode connection. After arcing, there is a gradual increase in the current per unit area of the electrode, which is the process of adapting the electrode to the furnace condition.
Overloading operation, the overload capacity of new electric furnaces is generally not more than 20%. If the arc current exceeds the rated value, the joint is most likely to break. When the electrode adapts to the furnace condition, even if overloaded, it will still operate normally, but there is a phenomenon of red joint.
(4) Smelting furnace condition
The feeding and power transmission operations are related to the changes in furnace conditions, but oxygen blowing, nozzle combustion, and fuel oil are the key factors that worsen furnace conditions. The utilization of chemical energy reduces power consumption, but increases the degree of oxidation on the electrode surface and upper end face. Especially when the negative pressure is too high, it accelerates the oxidation rate of the electrode surface and causes the electrode body surface to become cone-shaped.
The steelmaking process is the process of slag making. The use of chemical energy increases the stirring force of molten steel and is more conducive to the production of foam slag. The boiling of molten steel, the thickness of slag layer, the fluidity of slag liquid and the submerged arc effect not only affect the smelting effect, but also have a significant impact on the consumption of carbon at the bottom of the electrode and the consumption of turning around and peripheral surface.
The operation during the deterioration of the C furnace is frequently changing, with vibrations on the electrodes accompanied by left and right oscillations. The frequent changes in current increase thermal vibration, which not only accelerates surface consumption of graphite electrodes in oxygen rich environments, but also tests the strength of the connection parts (joints).
(5) Electrode storage and transportation
On site storage electrodes should avoid contact with liquid media, otherwise fish scale like chunks may appear after being heated.
When storing joints on site, it is important to avoid being close to high-temperature heat sources, otherwise the joint bolts may melt and flow out after being heated.
(6) Electrode quality
For large UHP and HP limit electric furnaces, the provided graphite electrodes must improve the physical quality.
Be sure to eliminate structural defects or insufficient strength issues, otherwise joint breakage accidents may occur within 1-3 minutes of power transmission in the first batch of smelting materials.
The machining accuracy of the C-electrode end face is also crucial. If there are gaps, air permeability will occur and local redness will occur. After about 10 minutes of power transmission, the junction turns noticeably red. After continuous smelting for 2-3 furnaces, the internal buckle is prone to oxidation and may break or fall off.
The issue of tolerance fit must be constantly monitored, whether it is loose or poorly connected, as long as there are gaps, they will break or fall off. From the perspective of usage, most of the problems that occur in certain parts are caused by tolerance fit.