The connection of graphite electrodes is realized through graphite electrode nipples. How to tighten the graphite electrode nipple? Graphite electrode manufacturers suggest that the torque required to screw the joint into the electrode screw hole to achieve a tight connection is well controlled.
RS Graphite Electrode with Nipples
The contact resistance of the graphite electrode with a smooth surface varies with the applied pressure. As the pressure increases, the contact resistance decreases. In order to reduce the contact resistance of the contact parts during electrode connection as much as possible, in addition to the two important conditions of graphite electrode material and processing accuracy, which must meet the technical standards, sufficient tightening torque must be applied when connecting electrodes in steelworks. If the tightening torque is insufficient when connecting, the contact resistance of the contact surface will increase significantly. Even high-quality electrodes will have redness and accelerated oxidation at the contact surface. Or loosening occurs in a strong vibration, increasing the chance of nipple breaking. The larger the electrode specification, the greater the tightening torque required. But the tightening torque is not as big as possible, but to reach a certain torque.
There are two main factors that affect the tightening torque not reaching the specified level. (1) The electrode processing quality is not good. If the processing surface is rough or the clearance of the thread is not well matched, it is difficult to screw into the joint, or it is still loose when it is screwed to the bottom of the screw hole, so it cannot be tightened. (2) The steel mill operator did not tighten the electrode and the joint when adding the new graphite electrode nipple, that is, the required tightening torque was not reached. In order to ensure that the required tightening torque can be achieved when connecting electrodes with different specifications, you can choose to use a graphite electrode wrench.
When tightening graphite electrodes in the smelting industry in some areas, the most common use is to clamp a graphite electrode with two iron clips for fastening operations, and there is also a “soil wrench” without a torsion structure. However, this type of tightening method not only needs to rely on the experience of master craftsmen but also faces the challenge of uneven quality of graphite electrodes from different manufacturers. A slight difference in the tightening force will make the current flow unstable. It leads to a series of problems such as the electrode is not tightened, the electrode is broken, and so on. Not only does it affect production, but it also consumes electricity, and the work efficiency and graphite electrode utilization rate are relatively low, which increases production costs.
The special graphite electrode wrench can perfectly solve these problems. It is widely used in the smelting industry in other regions. The special wrench for graphite electrodes is not only a tool for tightening graphite electrodes but also a kind of torque tool. Its appearance ended the smelting industry’s production history of relying on experience and technology when installing and replacing graphite electrodes. It provides a reliable guarantee for safe production, energy saving, and high efficiency, and cost reduction in the smelting industry.
Rongsheng graphite electrode manufacturer specializes in the production and sales of various types of graphite electrodes including graphite electrode with preset nipple products and can customize graphite electrodes and electrode nipples according to the actual needs of customers. Contact us for a free trial.
Vacuum furnaces process hard zinc and recover zinc metal. The normal working conditions of the vacuum distillation furnace are the vacuum degree of 500~2000Pa and the temperature of 950~1050℃. Under this condition, the vapor pressure of the graphite electrode is very low, and its volatilization loss is negligible, so its service life depends on its degree of oxidation and fracture caused by mechanical and electrical failures. Specifically, what are the reasons for the loss of graphite electrodes in vacuum furnaces?
Graphite Electrodes in RS Supplier
Oxidation loss of graphite
The oxidation of graphite is divided into dry oxidation (air, water, carbon dioxide gas) and wet oxidation (acid, alkali). Graphite is easily oxidized by air, water and carbon dioxide gas (CO2) at high temperature to form carbon monoxide or carbon dioxide. The product of its oxidation varies with the ambient temperature.
2C + O2 = 2CO (above 1 000°C)
C + O2 = CO2 (below 1000℃)
Graphite oxidation products are easy to escape and cannot form a dense oxide film protective layer like on some metal surfaces, so the oxidation reaction is continuous. The temperature and reaction speed at which graphite starts to oxidize are different in various cases. If the temperature at which 1% of the original weight is lost within 24 hours is set as the oxidation start temperature, the oxidation starts temperature of graphite is as follows.
In air: 420 ~ 460 ℃;
In carbon dioxide gas: about 900 °C;
In water vapor: about 700°C.
Generally speaking, graphite with a large porosity, especially a large open porosity, has a large surface area participating in the reaction, and of course the oxidation rate is also fast. The higher the degree of graphitization of graphite products, the better the oxidation resistance. The aggregate of graphite products also has an important influence on oxidation resistance. Graphite products made of aggregates such as petroleum coke and natural graphite with good graphitization properties have excellent oxidation resistance. The presence of trace metal impurities will obviously promote the dry oxidation of graphite.
In the production process of graphite, metals that are usually easy to exist in the form of impurities include sodium, potassium, magnesium, calcium, iron, vanadium, copper aluminum, titanium, etc. Among them, the existence of sodium, potassium, vanadium and copper can catalyze the oxidation reaction of graphite. As the oxidation reaction proceeds, these metal impurity particles move through the material and form defects or pores. This phenomenon is more obvious in the oxidation reaction at lower temperature. According to the data, when some metal impurities are artificially added, such as adding 20 ~ 40μg/g of sodium, potassium, vanadium or copper, the oxidation rate at 550 °C can be increased by 5 times.
Electrode short circuit or crushed
The vacuum furnace uses graphite electrodes as heating elements, and requires good contact between electrode components during installation. Keep the vertical direction horizontal, and reserve an expansion gap around the installation window at the end of the electrode nut. At the same time, both ends of the electrode assembly and the furnace shell maintain good insulation performance. Due to the poor sealing of the installation window, the zinc vapor in the feeding trolley enters the electrode nut or the electrode connection card through the gap of the sealing material, and condenses into a zinc block. With the continuous growth of the condensed zinc block, it will eventually contact with the iron shell of the furnace body, causing the electrode to short-circuit and damage the electrode. After the furnace shell and electrode support plate are deformed, the expansion gap at the end of the graphite electrode nut is usually insufficient. After the power is heated up, the electrode nut is squeezed by the furnace wall around the installation window to varying degrees, and the electrode is quickly broken.
The electrode processing quality and installation quality also have a very important influence on the service life of the electrode. The connection thread of the electrode and the electrode nipple is poorly processed or the tolerance matching does not meet the standard, which will cause the connection between the electrode and the joint to be too loose or too tight. Too loose will cause poor contact and arcing will burn the electrodes and nipples; too tight will easily damage the threads during assembly. When assembling the electrode, during the screwing process of the electrode and the electrode joint, the action is rude, which will also cause damage to the thread and cause the electrode to be scrapped.
In addition, when the water-cooled electrode is short of water, the heat of the electrode connection card cannot be taken away in time. It will also cause poor contact between the electrode and the card board, thereby burning the electrode connection card board. In severe cases, arcing will burn the electrode and the electrode connection card.
It can be seen that the graphite electrode is still easily worn out in the process of use. Therefore, it is helpful to purchase high-quality graphite electrode products and standardize the operation and installation procedures to reduce the loss of graphite electrodes.
At present, in electric arc steelmaking, as the discharge power and electrode diameter increase, the heat generated by the working current will increase rapidly, and the requirements for joint connection will be more stringent. To realize the ideal connection of the electrode, in addition to improving the processing quality of the joint and the joint hole, and improving the joint material, there are also appropriate requirements for the tightening torque. Make the pre-tightening force appropriate to avoid too much pre-tightening force causing breakage from the joint during work. Or the pre-tightening force is too small, causing the electrode to fall off during work.
Graphite Electrodes Connection for Steelmaking
The Relationship between “Skin Effect” and Electrode Connection
When direct current passes through a conductor, the current in the cross-section of the conductor is uniform. When an alternating current passes through the conductor, the current will concentrate on the surface of the conductor. This phenomenon is called the skin effect of alternating current. The higher the frequency of the current, the more obvious the skin effect. The extent to which the current leaves the center of the conductor section and concentrates on the surface can be expressed by the depth of the skin effect.
The expression of the skin effect depth is:
Formula (1)
In the formula, f is the frequency of the alternating current, in Hz; k is a constant, which is related to the resistivity of the conductor.
It can be seen from formula (1) that the depth of the skin effect decreases as the current frequency increases.
Due to the skin effect, when the electrode is working, under normal circumstances, about 80% of the current flows through the annular belt between the electrode joint and the outer surface, and about 20% of the current flows through the central electrode joint part. This is undoubtedly beneficial to the electrode connection. If the end faces can fit well when the electrodes are connected, the skin effect will be fully utilized, so that most of the current flows through the outer loop. Reduce the resistance heat of the joint part and avoid burning and breaking due to heat. At the same time, the good fit also makes the contact resistance very small and reduces the power consumption.
From the above, in order to make full use of the skin effect, when connecting the electrodes, it is necessary to make the end faces fit as much as possible. At the same time, when the electrode is in discharge, the connection between the electrode and the connector is easily loosened due to electromagnetic vibration. Therefore, it is necessary to apply pretension.
When the electrode connection threaded hole is processed, there are strict requirements on the perpendicularity of the end face to the thread axis, and the end face should be concave by 0.05~0.1mm, which needs to be determined according to the elastic modulus of the electrode material. The ideal connection state of the electrode is to apply an appropriate pre-tightening force to make the end face elastically compressed and deform, overcome the concave, and achieve full fit. At the same time, it is necessary to control the size of the pre-tightening force. It cannot be too big or too small. If the yield strength of the joint is exceeded, failure fracture occurs. The pre-tightening force is small, and it cannot provide enough positive pressure and friction between the threads, causing looseness.
Graphite electrodes are mainly used as conductive materials in electric smelting furnaces. Compared with other conductive materials, the biggest advantage of graphite electrode material is that it has good electrical and thermal conductivity and better toughness, can accept the impact of larger current and does not soften or melt at high temperatures. It is used as a conductive material in the steel-making electric arc furnace, and the heat energy is transferred to the charge through arc discharge to melt the steel scrap. In the submerged arc furnace for smelting yellow phosphorus and industrial silicon, the electric energy is transferred into the charge through the electrode, and the charge itself is melted by the electric resistance heating of the charge.
Graphite Electrodes Application
The Advantages of Electric Arc Furnace for Steelmaking
Compared with other steelmaking methods, electric arc furnace steelmaking has its unique advantages. Electric arc furnace steelmaking is heated by an electric arc, and its temperature can be as high as 2 000 ℃ or more. This exceeds the maximum temperature that other steelmaking furnaces can reach when burning and heating with general fuel. The thermal efficiency is higher than that of the open hearth and converter steelmaking methods. Heating with electric energy can also accurately control the temperature and can use various elements (including aluminum, iron, and other elements that are easily oxidized) to alloy steel. Various types of high-quality steel and alloy steel are smelted, such as ball-bearing steel, stainless acid-resistant steel, high-speed tool steel, electrical steel, heat-resistant steel and alloys, and magnetic materials.
At present, electric furnace smelting adopts the traditional alkaline electric arc furnace steelmaking method. The process consists of “refilling the furnace-charging-smelting (including melting period and oxidation period)-refining outside the furnace (reduction period)-tapping-continuous casting” and so on.
According to relevant data, with the continuous improvement of ultra-high power electric arc furnaces and related technologies, the electrode consumption dropped from 6.5 kg/t to 1.2 kg/t from 1965 to 2000. The power consumption is reduced from 630 kW. h/t to 290 kW. h/t and the tapping time is reduced from 180 min to 40 min.
High-Quality Graphite Electrodes for Steelmaking
Characteristics Requirements of Graphite Electrode Materials for Steelmaking Electric Arc Furnaces
When investigating the adaptability of graphite electrodes to steel-making electric arc furnaces, the physical and chemical properties of the materials, the matching accuracy of the electrode and the joint, the reliability of the connection, and the length of the adaptable equipment (the up and down stroke of the electrode) should be analyzed one by one. Among them, the pear blossom properties of graphite electrode materials include mechanical properties, electrothermal properties, electrical conductivity, and oxidation resistance. In addition, it is necessary to understand and master the power transmission mode, smelting method, and process of the equipment.
It is very important to correctly understand the basic characteristics of graphite electrode materials. The characteristics of different grades of graphite electrodes are basically determined by the raw materials and manufacturing methods used.
Graphite electrodes should have at least the following properties on the smelting furnace. (1) Must be able to withstand the arc current required by the steelmaking process. (2) Continuous arc generation must be maintained.
In addition, the electrode should be able to be used in a wide temperature range (that is, it can be used in a state of rapid cooling and rapid heating). In the comprehensive evaluation of the performance of the graphite electrode, the thermal shock resistance index is used as the basic standard to measure the resistance of the electrode tip.
Thermal shock resistance refers to the ability of a material to resist damage under rapid cold and rapid heat. It is a comprehensive reflection of the performance of graphite electrodes.
Compared with other conductive materials, graphite electrodes have some excellent or irreplaceable characteristics under high-temperature conditions. Graphite electrodes can be used at relatively high temperatures (sublimation temperature of 3650°C), and are the only high-temperature conductive materials that can withstand high temperatures. There is no other material that can replace them in actual use. The strength of graphite increases as the temperature rises at high temperatures. Compared with other metals, graphite has the lowest coefficient of thermal expansion. When studying the characteristics of graphite electrodes, especially the thermal stress and vibration of the connection part, the coefficient of thermal expansion and the electrical resistivity is regarded as one of the most important indicators. Therefore, it is very important to choose petroleum coke with a low CTE value to produce graphite electrodes.
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The Influence of Steelmaking Technology Progress on the Consumption of Graphite Electrodes
With the large-scale and ultra-high power of steelmaking electric arc furnaces, many improvements have been made to domestic UHP electric arc furnaces. For example, molten steel is reserved at the bottom of the furnace, and long-arc bubble slag submerged arc operation is used for drainage (the power transmission system is low current and high voltage). The foaming slag operation mainly reduces the radiation of the electric arc to the furnace wall and creates conditions for the long arc operation. The long-arc operation can effectively utilize power, that is, increase power factor, increase thermal efficiency, shorten smelting time and reduce electrical energy consumption. The improvement of thermal efficiency shortens the time for melting scrap steel and smelting. The use of long arc operation increases the vibration caused by the arc but can basically avoid the collapse of the electrode during the good penetration. After the current is reduced, the tip consumption of the electrode can be reduced. In addition, shortening the smelting time in an oxidizing atmosphere means that the side consumption of the electrode is also reduced. Coupled with the auxiliary electrode spray cooling, the electrode consumption of the UHP arc furnace is greatly reduced. From the level of over 2kg/t at the end of the last century to the level of 1.5 kg/t. Sometimes, it even reaches the level of 1.2~1.3 kg/t, which is especially in line with the characteristics of high production and low consumption in China’s steel industry in recent years.
In addition, the application of technologies such as out-of-furnace refining, scrap preheating, and addition of molten iron has contributed to the reduction of electrode consumption. The automatic control of electrode lifting protection makes the phenomenon of electrode breakage caused by human operation less and less.
The advancement of steelmaking technology has brought about a substantial reduction in electrode consumption, making imported electrodes of uniform material competitive advantage. It is a top priority for domestic carbon companies to break through the quality of large-size ultra-high-power electrodes as soon as possible and strive to reduce steelmaking consumption. To learn more about graphite electrodes for steelmaking electric arc furnaces, please contact us.
The nipple connection area in the graphite electrode column is a common part of fracture, and it is also a complex area with large electrical, thermal, and mechanical loads. There is large thermal stress in this area, so it is also a part that needs to be studied.
Graphite Electrode and Nipple
Experiment on the Influence of Temperature Distribution of Graphite Electrode Nipples on Thermal Stress
The calculation model selected in the experiment is the connection area formed by the φ500mm high-power electrode and the nipple(φ298.45X 372.5mm). The two sides are symmetrical with the nipple meridian as the midline, and the length is 780mm. And set the calculation model to be located directly above the furnace cover, with a current intensity of 45kA.
According to the structural characteristics of the graphite electrode nipple, the calculation is handled as an axisymmetric problem, and the load is also applied in an axisymmetric manner. In the calculation, it is assumed that the electrode and the nipple are linear elastic bodies, and the difference in the properties of the electrode and the nipple material and the orthogonal anisotropy characteristics, as well as the current steering in the electrode-nipple connection area are considered. The corresponding changes in physical parameters caused by temperature are not considered.
Part of the triangular unit is inserted, the screw contact area unit is encrypted, and the other parts gradually become thinner.
In order to simulate the actual connection conditions, two temperature calculation models are selected in this experiment.
In model 1, it is considered that the electrode bears the effect of the tightening torque. The end faces of the two electrodes are pressed and contacted normally, and the current flows in from the tapered threaded surface at the upper end of the nipple, flows out from the lower end of the nipple, and flows through the contact end faces of the two electrodes at the same time. Model 1 is used to simulate normal connection conditions.
In model 2, a thin layer of the insulator is added to the connecting end of the two electrodes, so that the current can only flow in from the threaded surface of the upper end of the nipple and flow out from the lower end. Model 2 is used to simulate the working condition when the connection end face is loose. The calculated value of this model may be slightly higher than the actual temperature value, but it can fully explain the temperature distribution and the trend of temperature rise.
The surface temperature of graphite electrodes during steelmaking is actually measured. At the end of steel-making melting, when the electrode reaches a steady-state temperature distribution, a handheld fast infrared thermometer is used to record the electrode surface temperature distribution. The measured result is used as the boundary condition of the finite element method to simulate the physical characteristics of the electrode connection area.
The surface temperature of the graphite electrode is measured as a linear distribution from the electrode clamping end to the furnace cover. The surface temperature of the calculated model is about 820~740℃.
The calculation results of experimental data show that. In steelmaking, the electrode connection area above the furnace cover is subjected to heat conduction from the electrodes in the furnace (with higher temperature), current flow through the heat generation, connection factors, and the surrounding environment, which bears a greater thermal load. The temperature distribution is parabolic along the radial direction, and the highest temperature area is in the nipple and threaded connection area. Model 2 shows a steeper temperature gradient and higher nipple temperature, which is almost double that of Model 1, which shows that the nipple bears a greater thermal load.
The following conclusions are obtained through calculation and discussion. In addition to the electrical and thermal properties of the material itself, and the current load, the connection status has a certain effect on the temperature field of the connection area. A certain torque can reduce the temperature, and excessive torque will damage the thread tooth root. Loosening of the connection surface can cause very high temperatures in the nipple.
Graphite Electrode Nipples
Recommendations on Graphite Electrode Nipples
The temperature distribution has a certain influence on the size and distribution of thermal stress. Improve the temperature distribution to make it more reasonable, which can improve the thermal stress and reduce the consumption of alkali. Based on the calculation results and analysis, this article puts forward several suggestions.
(1) Requirements for material performance
The resistivity should be low. In particular, the electrical resistivity of the nipple is lower than that of the electrode, so that the power (I*R) loss is small and the thermal conductivity is large. The electrodes dissipate quickly and the radial temperature gradient is reduced.
(2) Best connection status
Use a torque wrench to apply a suitable connection torque to ensure the strength of the threaded tooth root, while the contact resistance is minimized, and the temperature of the connection area is reduced.
(3) Sincerely small contact resistance
Coating a certain substance on the connection surface can increase the friction and reduce the contact resistance value, and has good electrical and thermal conductivity. In addition, the contact area can be increased and the contact resistance can be reduced by changing the shape of the nipple.
(4) Use a cooling system to lower the temperature.
There are generally three molding methods used in the carbon industry, namely compression molding, extrusion molding, and vibration molding methods. Among them, the compression molding method is low due to labor productivity. At present, except for a small number of products with special requirements that use this molding method, the molding process in the carbon industry has basically been withdrawn. From the perspective of my country and the world, the extrusion molding method is the main molding method in the carbon industry. The graphite electrode formed by this method has preferential orientation in the axial direction, which makes the various “physical-mechanical” parameters in the axial direction of the product better than other directions, which is suitable for the use conditions of graphite electrodes and has high labor productivity. However, in order to produce large-diameter graphite electrodes or other carbon products with large cross-sections, a large-tonnage hydraulic press must be used when using this molding method. At present, the production of graphite electrodes with φ400mm in our country generally uses a 2500t hydraulic extruder, and the production of graphite electrodes with a diameter of 500mm or more uses a 3500t hydraulic extruder. Some foreign manufacturers of graphite electrodes also commonly use 4000t and 6000t hydraulic extruders, and the largest hydraulic extruder may be the 12700 hydraulic extruders of National Carbon Co. of the United States. These equipment not only have high extrusion pressure but also have a very long body due to the requirements of the electrode extruder’s molding method, so the bodyweight is very large. For example, a 3550t hydraulic extruder made in the Soviet Union is 36m long and weighs 577t. A 6300t hydraulic extruder made in Austria weighs 700t. In addition, these devices are equipped with high-power main motors, generally 300~400kW. It is conceivable that such equipment requires a large amount of investment and high energy consumption, which is not affordable by ordinary small and medium-sized factories.
Since the French VAW company vigorously introduced the vibration forming method in the 1960s, it has been widely used in the aluminum carbon industry, especially in the production of prebaked anodes, and has gradually been extended to the production of cathode carbon blocks and graphite electrodes. The vibration molding machine used in this molding method has a simple structure, a compact body, a small weight, and a low cost. According to the estimation of the French company KHD, the investment of a vibration forming machine is about 40% of that of a corresponding hydraulic extruder. The total power of the motor is only 37% of that of the extruder, and the molding energy consumption of the product is only 32% of that of the extruder. For some simple vibration forming machines in our country, the investment for one is only about 200,000 yuan, which is only about 5% of the 2500t extruder. Although its labor productivity and single-unit capacity are lower, it can also be used for the molding of large graphite electrodes with diameters above φ300mm and even φ500mm or larger. This is exactly what the small and medium-sized carbon plants hope for.
Although the vibration forming machine has many advantages mentioned above, can high-quality products be obtained by using it for the forming of graphite electrodes? At least can you get products that meet the standards? For this, most people in my country’s carbon industry hold a negative attitude. It is mainly believed that the particles in the vibration molded product are preferably oriented along the transverse direction, which is a bad orientation for graphite electrodes. Secondly, it is considered that the volume density of vibration molded products is not uniform. Because the above two points will affect a series of physical and mechanical properties of the product, people’s denial or suspicion is not unreasonable.
Based on everyone’s skepticism, some people conducted relevant discussions and analyses and finally came to the following conclusions.
1) Vibration molding, as a molding method for producing graphite electrodes, can produce ordinary graphite electrodes that are suitable for my country’s current national standard GB3072-82.
2) If the vibration hydroforming method is adopted, ordinary graphite electrodes of better quality can be produced, and the bulk density can reach more than 1.60g/cm3. If it is combined with vacuuming during the molding process, its bulk density can be further increased.
3) Since the graphite electrode formed by vibration has a higher volume density and a lower porosity, oxidation consumption can be reduced during use.
4) Due to the random orientation of the particles of the vibration-shaped electrode, its physical-mechanical properties have similar values in the axial and radial directions. Therefore, when the axial resistivity is similar to that of the extruded electrode, its radial physical-mechanical parameters are better than those of the extruded electrode.
5) The vibration forming machine, especially the small simple vibration forming machine, has little investment, but it can produce large-diameter graphite electrodes and other large-section graphite products. Moreover, its technology is easy to master, and the forming yield is high, which is suitable for small and medium carbon factories.
6) Although the vibration forming method has been developed in my country since the end of the 1960s. However, it has not been mass-produced for many years, and most manufacturers are limited to using it to produce carbon blocks and regenerated graphite electrodes. Many people hold negative attitudes about whether it can be used in the production of graphite electrodes. Therefore, a lot of work needs to be done. Only when the electrodes produced by them are proved to be at least no worse than extruded electrodes in long-term and large-scale use, can this molding method be recognized by the carbon industry in my country.
The author puts forward a little work and some opinions on the vibration forming method for the reference of colleagues. I also hope to get criticism and corrections from my colleagues. This article is from the Internet. If there is something wrong, please contact the author of this website to delete or modify it. Learn more about the graphite electrode production process.
The graphite electrode body needs the graphite electrode nipple to connect them one by one in series. Therefore, they also have certain connections and differences in performance indicators.
Graphite Electrode with the Nipple
The Performance Index Difference of Graphite Electrode Body and Nipple
(1) Resistivity. The resistivity of ordinary power graphite electrode is not more than 8.5μΩ·m, 9.0μΩ·m, 10.0μΩ·m, and 10.5μΩ·m, and the resistivity of ordinary power graphite electrode nipple is not more than 8.5μΩ·m. The resistivity of the high-power graphite electrode is not more than 6.5μΩ·m and 7.5μΩ·m, and the resistivity of the high-power graphite electrode nipple is not more than 6.5μΩ·m. The resistivity of the ultra-high power graphite electrode is not more than 6.2μΩ·m and 6.5μΩ·m, and the resistivity of the ultra-high power graphite electrode nipple is not more than 5.5μΩ·m.
(2) Allowable current density and current load during electrode use. For graphite electrodes with a diameter of 300~500mm, the allowable current load of ordinary power graphite electrodes is 10000~20000A, and the allowable current density is 1318A/cm2. The allowable current load of the high-power graphite electrode is 13000~48000A, and the allowable current density is 1524A/cm2. The allowable current load of ultra-high power graphite electrode is 15000~55000A, and the allowable current density is 1830A/cm2.
(3) Flexural strength. The flexural strength of the ordinary power graphite electrode is not less than 6.4MPa, 7.8MPa, and 9.8MPa, and the flexural strength of the ordinary power graphite electrode nipple is not less than 13.0MPa. The flexural strength of the high-power graphite electrode is not less than 9.8MPa and 10.5MPa, and the flexural strength of the high-power graphite electrode nipple is not less than 14.0MPa. The flexural strength of the ultra-high-power graphite electrode is not less than 10.0MPa and 10.5MPa, and the flexural strength of the ultra-high-power graphite electrode nipple is not less than 16.0MPa.
(4) Bulk density. The bulk density of ordinary power graphite electrodes is not less than 1.52g/cm3 and 1.58g/cm3, and the bulk density of ordinary power graphite electrode nipples is not less than 1.68g/cm3. The bulk density of the high-power graphite electrode is not less than 1.60g/cm3, and the bulk density of the high-power graphite electrode nipple is not less than 1.70g/cm3. The bulk density of the ultra-high power graphite electrode is not less than 1.64g/cm3 and 1.65g/cm3, and the bulk density of the ultra-high power graphite electrode nipple is not less than 1.70g/cm3 and 1.72g/cm3.
(5) Linear expansion coefficient. In the temperature range of 100~600℃, the linear expansion coefficient of ordinary power graphite electrode is not more than 2.9×10-6℃-1. The coefficient of linear expansion of common power graphite electrode nipples is not more than 2.7×10-6°C-1 and 2.8×10-6°C-1, which are only used as reference indicators. For high-power and ultra-high-power graphite electrodes, the linear expansion coefficient is the main quality assessment index. The linear expansion coefficient of the high-power graphite electrode is not more than 2.4×10-6°C-1, and the linear expansion coefficient of the high-power graphite electrode nipple is not more than 2.2×10-6°C-1. The linear expansion coefficient of the ultra-high power graphite electrode is not more than 1.5×10-6℃-1, and the linear expansion coefficient of the ultra-high-power graphite electrode nipple is not more than 1.4×10-6℃-1.
(6) Consumption of steelmaking electrodes. The electrode consumption of ordinary power graphite electrodes is 46kg per ton of electric furnace steel. The electrode consumption of high-power graphite electrodes is 2.53.5kg per ton of electric furnace steel. The electrode consumption of ultra-high-power graphite electrodes is 1.12.5kg per ton of electric furnace steel.
The above is the performance index of the graphite electrode body and graphite electrode nipple. If you need to buy graphite electrodes and the matching nipple. Please contact us.
The role of the electrode is to conduct electricity and convert electrical energy into heat. Electrodes are divided into three types: carbon electrodes, graphite electrodes, and self-baking electrodes according to their use and production process. In submerged arc furnaces, self-baking electrodes are mainly used, but when producing ferroalloy products with lower carbon content, such as industrial silicon, graphite electrodes are required. So what are the differences among the self-baking electrodes, carbon electrodes, and graphite electrodes?
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First of all, we must clearly distinguish these three electrodes.
The carbon electrode is made of low-ash anthracite, metallurgical coke, pitch coke, and petroleum coke as raw materials, and is composed of a certain proportion and particle size. Add binder asphalt and tar when mixing, stir evenly at the proper temperature, and then press into shape. Finally, it is made by slow roasting in the roasting furnace.
Graphite electrode uses petroleum coke and pitch coke as raw materials to make a carbon electrode. Then put it in a graphitized resistance furnace at a temperature of 2000-2500°C, and make it through graphitization.
The self-baking electrode uses anthracite, coke, pitch, and tar as raw materials. The electrode paste is made at a certain temperature, and then the electrode paste is put into the electrode shell installed on the electric furnace. In the production process of the electric furnace, it relies on the Joule heat generated when the current passes through and the heat transfer in the furnace to self-sinter and coke. This kind of electrode can be used continuously, and sintered to form while connecting the long side, and can be fired into a large diameter.
The self-baking electrode has a simple manufacturing process and low cost and is widely used in ferroalloy production. It is usually used to produce ferrosilicon, silicon-chromium alloy, silicon-manganese alloy, high-carbon ferromanganese, low-carbon ferromanganese, high-carbon ferrochrome, low-carbon ferrochrome, silicon-calcium alloy, Ferro tungsten, etc. Self-baking electrodes are easy to increase the carbon of the alloy, and the iron skin of the electrode shell is also easy to bring the alloy into iron. Therefore, the production of iron alloys and pure metals with very low carbon content, such as micro-carbon ferrochrome, industrial silicon, silicon aluminum alloy, metal manganese, etc., uses carbon electrodes or graphite electrodes.
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Graphite Electrodes Applications
Among them, graphite electrodes have excellent properties such as high temperature resistance, oxidation resistance, good electrical conductivity, high mechanical strength, low impurity content, and good thermal vibration resistance. Mainly used as electrode devices in electric arc furnaces and refining furnaces for smelting steel. Submerged electric furnace for producing ferroalloy, industrial silicon, yellow phosphorus, matte, calcium carbide, etc. Graphitization furnace for producing graphite electrodes, furnace for melting glass, electric furnace for producing silicon carbide, etc.
With economic development, the demand for refined steel has increased. Preparation of extruded large-size φ600~φ800mm high-quality GHP graphite electrodes, supporting the application of three-phase 6 and 9 or 12 power supply arrangements in a large ferroalloy submerged arc furnace smelting new technology method. Give full play to the high-quality characteristics of GHP graphite electrodes. Realize high-power, large-capacity, energy-saving and emission reduction, safe and efficient, low-cost smelting production in the ferroalloy submerged arc furnace industry, change the outdated technical level of ferroalloy submerged arc furnace smelting, and provide a new way for technological innovation, upgrading, and transformation of enterprises. Facilitate the virtuous cycle of green economy development.
Technical problems or product problems related to refractories and carbon materials for submerged arc furnaces. Please email us: info@graphelectrode.com.
The physical performance requirements of large-scale, UHP graphite electrodes, with the introduction and continuous expansion of DC electric arc furnaces in my country, are of great significance for the best use of electric arc furnaces and reducing electrode consumption. Therefore, in order to promote the development of the electric furnace steelmaking industry, it is also extremely necessary for the development of large-scale, ultra-high power electrodes. Next, we will learn more about the application and cost-saving of graphite electrodes in electric arc furnaces from two aspects: the best use conditions of the electrodes in the DC arc and the physical properties of large-size graphite electrodes for DC electric arc furnaces.
uhp graphite electordes for Arc Furnaces
The Best Use Conditions of Electrodes in DC Electric Furnace
A certain electrode diameter corresponds to the best current load. Only in this way can the consumption of the electrode be the lowest, the utilization rate is the highest, and the role of the electrode can be fully and effectively played. H. Hagd et al. proposed two calculation criteria for a certain electrode diameter corresponding to the maximum current load. The first criterion is to calculate the current load capacity by considering only the thermal load; the second criterion is to calculate the current load capacity by calculating the mechanical stress through the thermal load.
Figure 5 shows the maximum tangential tensile stress of the φ600mm electrode at different current intensities. The calculation takes into account the conical shape of the electrode tip. The experimental results show that the maximum current load of the φ600mm electrode is about 85kA, and the maximum tangential tensile stress value generated at this time is 22MPa. When the current load is 62kA, the maximum tangential tensile stress on the outer surface of the electrode at the connection is 11.2MPa. This value is the lowest current intensity value at which the connection is broken, the electrode is peeled off, and the crack grows.
Fig. 6 is the upper limit of the current load corresponding to a certain diameter electrode obtained according to the thermal interception load (first criterion). Fig. 7 is the upper limit of the current load corresponding to a certain straight electrode obtained by calculating the mechanical stress (the second criterion) through the thermal load. Comparing the current load limit values obtained by the two criteria, the results are relatively consistent and satisfy the linear relationship. In practical applications, the second criterion is more important when the results obtained are applied to electric furnace electrodes. Because the second criterion is the result derived from thermal stress, and thermal stress is often one of the important reasons for electrode failure and connection damage.
The performance of graphite electrodes and nipples produced in my country is definitely different from that of H.Hagel calculation electrodes. Therefore, our application of this criterion can only use its linear relationship, and the general trend should be consistent.
Physical Properties of Large-Size Graphite Electrodes for DC Electric Solitary Furnace
With the increasing number of high-power and large-capacity DC electric furnaces, the electrode diameter is getting larger and larger, up to φ800mm, and the current interception is getting higher and higher, reaching 150kA. It is very important to choose a suitable electrode quality standard to meet the needs of high-power and large-capacity electric furnaces. All practical and theoretical studies have obtained the same results, and the performance of large-scale graphite electrodes needs to be improved even more. This requires attention to the selection of raw materials and the establishment of advanced production processes.
Higher requirements are put forward on the physical properties of large-size electrodes used in DC electric arc furnaces.
(1) The resistivity is further reduced. In this way, the power loss (I2R) is small, and a reasonable temperature gradient can also be obtained. This requires a higher graphitization temperature to ensure a decrease in resistivity.
(2) The coefficient of thermal expansion should be low. Since the graphite electrode and the graphite electrode nipple at the connection site have different thermal expansion properties in the axial and radial directions, the temperature at the nipple is higher and the thermal stress is greater. Under the premise of low thermal expansion coefficient, the axial and radial thermal expansion coefficient of the electrode should be slightly greater than or equal to the nipple and should satisfy αpTp-αNTN ≥ 0. Among them, αp is the thermal expansion coefficient of the electrode; Tp is the temperature of the electrode; αN is the thermal expansion coefficient of the nipple; TN is the temperature of the nipple. In this way, the thermal stress value is reduced. To achieve this, there must be advanced technology, raw materials, and formula to ensure.
(3) The thermal conductivity should be high. The high thermal conductivity allows the electrode to dissipate heat quickly, and the radial temperature gradient is small, which is also beneficial to improve the thermal stress. The thermal conductivity should be above 200W/m·K.
(4) It must have a certain strength. The electrode must have a certain strength, and the strength of the nipple is higher than that of the electrode. Large-size electrodes have a large weight. In order to ensure a reasonable current flow, there should be a higher pressure on the connection end surface. If the strength is insufficient, the connection thread will be damaged, local cracks will occur prematurely, and the failure rate will be accelerated. At the same time, we also hope that the modulus of elasticity should be low, so that the thermal stress σ = EaT is also reduced. Therefore, both strength and elastic modulus should be considered. The strength can be improved through process control, selection of reasonable formulas and raw materials, and multiple dipping processes.
(5) The porosity should be low. For electrodes, excessive surface porosity will increase oxidation and increase electrode consumption. Porosity can be improved by the surface treatment process.
In the configuration of Graphite Electrodes for Steelmaking Electric Arc Furnaces, it is necessary to follow the basic principle of “normal power electric furnaces with ordinary power graphite electrodes, high-power electric furnaces with high-power graphite electrodes, and ultra-high-power electric furnaces with ultra-high-power graphite electrodes“.
Graphite Electrodes for Steelmaking Electric Arc Furnaces
Configuration Scheme of the Graphite Electrode for AC Steelmaking Electric Arc Furnace
For AC steelmaking electric arc furnace,
The 10~30t electric furnace is equipped with graphite electrodes with a diameter of 300~400mm.
The 30-50t electric furnace is equipped with graphite electrodes with a diameter of 450mm.
The 60~80t electric furnace is equipped with graphite electrodes with a diameter of 500mm.
The 100~170t electric furnace is equipped with graphite electrodes with a diameter of 550~600mm.
The 200t electric furnace is equipped with graphite electrodes with a diameter of 600~700mm.
The 250~300t electric furnace is equipped with a 700mm diameter graphite electrode.
RS High-Quality Graphite Electrode
Configuration Scheme of the Graphite Electrodes for DC Steelmaking Electric Arc Furnace
For DC steelmaking electric arc furnace,
The 30t electric furnace is equipped with graphite electrodes with a diameter of 450mm.
The 60t electric furnace is equipped with graphite electrodes with a diameter of 500mm.
70~80t electric furnace is equipped with graphite electrodes with a diameter of 600mm.
The 100~130t electric furnace is equipped with a 700mm diameter graphite electrode.
The 150t electric furnace is equipped with graphite electrodes with a diameter of 750mm.
UHP graphite electrode
Rongsheng Group, one of the graphite electrodes suppliers, provides you with Graphite Electrodes for Steelmaking Electric Arc Furnaces.
FAQ1How to choose the electrode correctly in the electric arc furnace steelmaking?
According to the design characteristics of the electric arc furnace, reasonably select the electrode that meets the production of the electric arc furnace, and select the product with the best cost performance. It is very necessary to carefully select the electrode suitable for each furnace. The special performance of the steelmaking furnace, the feeding method, the maximum current intensity, the length of the electrode column under the holder, the distance between the sidewall of the furnace and the electrode circumference, etc., are all factors that must be considered when choosing the electrode for the electric arc furnace.
FAQ2Why is the electrode required to be parallel and centered with the top hole of the furnace cover when steelmaking in an electric arc furnace?
The electrode column and the top hole of the furnace cover should be centered. The electrode column should avoid friction with the furnace cover, otherwise, friction with the furnace cover during lifting and lowering will cause the furnace cover to break the electrode. For AC furnaces, the three-phase electrode columns should be kept as parallel as possible.
FAQ3What effect does the resistivity performance have on the use of electrodes in steelmaking?
The resistivity of the graphite electrode is a physical index that reflects the conductivity of the electrode and is related to the manufacturing process of the electrode. The country has qualitatively specified values for the resistivity of graphite electrodes of different specifications. Generally speaking, when steel mills choose electrodes of certain specifications, they must choose the resistivity range specified by the national metallurgical standards. Too high resistivity will cause the electrode to become red and hot when it is energized, and increase electrode oxidation consumption.
