Graphite electrodes are used as conductive materials in electric arc furnace steelmaking. The consumption of graphite electrodes in EAF steelmaking depends not only on the quality of the electrodes, but also on the operation and management level of steelmaking. In this paper, the problems of using graphite electrode in our factory for many years are analyzed one by one.
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Electric arc furnace steelmaking is in the ascendant in iron and steel production enterprises. At present, the proportion of electric arc furnace steelmaking in open hearth, converter and electric arc furnace steelmaking is about 30%. Graphite electrodes are used as conductive materials in electric arc furnace steelmaking. The consumption of graphite electrodes in EAF steelmaking depends not only on the quality of the electrodes, but also on the operation and management level of steelmaking. In order to benefit the operation of graphite electrode in electric arc furnace steelmaking, the problems accumulated in the service of graphite electrode users in our plant over the years are analyzed one by one
01
What are the main factors affecting the consumption of graphite electrodes in EAF steelmaking?
There are mainly: (1) the quantity and mode of charging. (2) Feeding time and power-off time. (3) smelting cycle. (4) Exhaust gas discharge and dust removal system. (5) The quality of electrode adjustment. (6) Load regulation quality. (7) Oxygen blowing operation. (8) The quality of electrode connection. (9) Mass of electrode joint. (10) Machining accuracy of electrode joint hole and joint.
02
What should be paid attention to in the storage of graphite electrodes in steel works?
Electrodes and joints should be stored on clean cement floor to avoid electrode damage or sticking to soil; temporarily unused electrodes should not be removed from the packaging to prevent dust and debris from falling on the joint threads or on the electrical extreme surface and the thread in the electrode hole. The electrodes should be placed neatly in the warehouse. The two ends of the stack should be padded well to prevent slip stacking. The stacking height of the electrodes should not exceed two meters. Stored electrodes should be rain-proof and moisture-proof so as to avoid cracking and accelerating oxidation of electrodes during steelmaking. Keep electrode joint away from high temperature to prevent thrombolysis overflow.
03
How to avoid electrode breaking and tripping in steelmaking process?
In steelmaking process, the following measures can effectively avoid electrode breakage and release: (1) correct electrode phase sequence, counterclockwise direction. (2) The scrap is evenly distributed in the furnace, and the large scrap is placed at the bottom of the furnace as far as possible. (3) Avoid the existence of non-conductive materials in scrap steel. (4) The electrode pillar is aligned with the furnace top hole, and the electrode pillar is parallel. The furnace top hole wall should be cleaned regularly to avoid the accumulation of residual steel slag and forcing the electrode off. (5) Keep the tilting system in good condition and keep the tilting stable. (6) The electrode gripper should avoid clamping at the electrode joint and the hole of the electrode joint. (7) Choose joints with high strength, high processing accuracy and high quality. (8) The moment applied to the connection of the electrodes should be appropriate. (9) Before and during the connection of the electrode, the thread of the electrode hole and the thread of the joint are prevented from being mechanically damaged. (10) Prevent steel slag or foreign body from embedding into the electrode hole and joint to influence the spinning.
04
What should we pay attention to when using graphite electrodes in steel works?
Whether using forklift truck or crane to transport electrodes, careful operation is required. In the process of electrodes hoisting, the damage of electrode ends and threads will cause serious problems for the use of electrodes, especially to protect the threads of threaded holes and joints. When lifting the electrode, it is necessary to have a cushion so as not to damage the end face of the electrode and the thread of the joint.
05
How to connect the electrodes correctly?
When connecting, use compressed air to blow out the hole, the end face of the electrode and the joint, no dust and foreign matter can be embedded. The joint should be kept clean and flat. When the two electrodes are rotated to a certain extent (the gap is about 10 mm), the compressed air is used to blow once again, and then the electrodes are tightened and tightened with moment clamps. The moment should be appropriate. If there is a gap in the connection after tightening, the connection must be withdrawn and reconnected until there is no gap.
06
On the Correct Holding Position of Electrode Holder
The electrode holder can not be clamped at the connection of the electrode and the thread hole of the electrode. It should be clamped between the white wires at both ends of the electrode. At the same time, before clamping the electrode, the electrode surface and the holder should be blown clean by compressed air so as to ensure good conduction of current and heat current between the electrode and the holder and prevent arcing. The gripper is damaged, thus prolonging the service life of the gripper.
07
What measures can be taken to reduce the consumption of electrode oxidation in EAF steelmaking?
The main measures are: (1) reducing the oxidation consumption around the electrode, strengthening the sealing of the furnace and reducing the air intrusion into the furnace; minimizing the exposure time of red-hot electrodes outside the furnace and standardizing the oxygen blowing operation. (2) For smelting furnaces, if conditions permit, spray cooling technology can effectively reduce the side oxidation consumption of electrodes. (3) Spraying antioxidants on the surface of electrodes in steel plants or using antioxidant impregnation technology before the electrodes leave the factory can improve the antioxidant performance of the electrodes.
08
How does the phase sequence of electrodes affect the use of electrodes?
The discount and breakage of the positive and negative electrodes of the electrode phase sequence during the use of EAF steelmaking have a great influence. If the electrode phase sequence is clockwise, the electrodes will be loosened after a period of electrification, which will easily lead to the loosening of the electrodes or the fracture of the joints. The correct electrode phase sequence should be counterclockwise. In this way, the electrodes will be loosened after a period of electrification. Joints will become more and more tight in use.
09
Why do phase electrodes need to be in parallel and aligned with the top hole of the furnace cover in EAF steelmaking?
In dealing with the electrode pillar and the top hole of the furnace cover, the friction between the electrode pillar and the furnace cover should be avoided. Otherwise, the friction between the electrode pillar and the furnace cover will cause the furnace cover to extrude the electrodes when it is lifted or lowered. For the AC furnace, the three-phase electrode pillar should be kept as parallel as possible.
10
How to apply the moment when the electrode is switched?
Torque applied during electrode rotation should be appropriate, and operation should be continuous. Torque too small will cause joint thermal loosening. Torque too large will cause hole bracing of electrode joint. Special tool for electrode rotation should be used during rotation. Do not tighten or loosen too tightly. If the end contact is found to be cleared after tightening, it must be removed and cleaned before re-spinning.
11
Why is a graphite hanger better than a metal hanger?
Although the metal hanger is durable and not easy to be damaged, the thermal expansion of the metal hanger is easy to crack the electrode hole after being heated in use. At the same time, the thread in the electrode hole is easy to be damaged when the metal hanger is connected, resulting in a large area scraping of the thread in the hole, which makes the electrode easy to trip off. Graphite hanger has the same thermal expansion as the electrode. The performance and hardness of the graphite hanger will not cause the above-mentioned bad use, but the graphite hanger has a short service life and is easy to be damaged. If serious damage is found, it should be replaced in time.
12
How to select the correct electrode in EAF steelmaking?
According to the design characteristics of the electric arc furnace, the electrode suitable for the production of the electric arc furnace should be selected reasonably, and the product with the best cost performance ratio should be selected. It is very necessary to carefully select the suitable electrode for each furnace. The special performance of the furnace, charging mode, maximum current intensity, length of the electrode column under the clamp, distance between the furnace side wall and the circumference of the electrode, etc. are all factors that must be considered when selecting the electrode for the electric arc furnace.
13
What is the effect of resistivity on the use of electrodes in steelmaking?
The resistivity of graphite electrodes is a physical index reflecting the conductivity of the electrodes, which is related to the manufacturing process of the electrodes. The resistivity of graphite electrodes of different specifications and varieties is defined qualitatively by the state. Generally speaking, when choosing certain specifications of the electrodes, steel mills must choose the resistivity specifications specified in the national metallurgical standards. In the perimeter, the high resistivity will make the electrodes red and hot when electrified, and increase the consumption of electrode oxidation.
14
What is the effect of bulk density on the use of electrodes in steelmaking?
The volume density of graphite electrode reflects the dense state of the electrode and is closely related to the manufacturing process of the electrode. The volume density of graphite electrodes of different specifications and varieties is regulated by the state. The products with low volume density show that the overall structure of the product has higher porosity, the oxidation speed of the product is faster at high temperature, and the consumption of electrodes is easy to increase. Generally speaking, the volume density of electrodes is better in the specified value when the steel mill chooses the electrodes, but the higher the volume density, the better, because some volume density is too high. Sometimes, due to the poor thermal shock resistance of electrodes, surface peeling, debris and cracks are prone to occur during steelmaking, which will affect steelmaking on the contrary.
15
When using graphite electrodes, why should steel mills prevent multiple products from mixing?
Graphite electrodes used in steel mills are often supplied by many manufacturers. When many products are mixed in steelmaking, it will not only make it difficult for steel mills to make statistics on consumption of individual products, but also because of the different raw materials and manufacturing processes adopted by each manufacturer, the physical and chemical properties and processing tolerances of the electrodes and joints of each manufacturer are different. This is the case. Therefore, the matching tolerance produced in the mixed use can easily lead to the phenomenon of electrodes falling off and breaking. The correct way of use is to use the products of one manufacturer alone, and then continue the products of another manufacturer after the end. To reduce the number of electrodes replaced by different manufacturer, the electrodes of the same manufacturer should use the matching contacts with the manufacturer. Prevent mixing.
16
What are the characteristics of needle coke?
Needle coke is a kind of high-quality carbon raw material, which is divided into coal and oil series. Its surface shows obvious stripe pattern. When broken, it is mostly long needle-shaped fragments. Fibrous structure can be observed under a microscope, so it is called needle coke. Needle coke is easily graphitized at high temperatures above degrees. Graphite electrodes made from needle coke have low resistivity, high bulk density and low coefficient of thermal expansion. They are the necessary raw materials for the production of ultra-high power electrodes and high power electrodes. The price of needle coke is much higher than that of ordinary coke, which is about 5-8 times higher at present.
17
Will the vacuum system on the electric arc furnace affect the consumption of electrodes?
The fan used in the vacuum system produces a certain negative pressure when it works, which increases the air velocity around the red-hot electrodes in steelmaking, thus increasing the oxidation consumption of the electrodes. In steelmaking, a well-regulated vacuum system maintains a good working environment and stabilizes the consumption of the electrodes.
18
How to avoid increasing electrode consumption in steelmaking?
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To avoid the increase of electrode consumption in steelmaking, it is necessary to: (1) maintain a good power supply state and supply electricity within the allowable current intensity range of the electrode according to the design requirements of the electric furnace. (2) Prevent the arcing point from immersing in the molten pool. (3) Prevent the increase of carbon by immersing the electrodes in molten steel. (4) If conditions permit, spray cooling technology is used for the electrodes. (5) Setting up the correct exhaust emission system. (6) To adopt the correct oxygen blowing system.
19
How long does the production cycle of graphite electrodes take?
The production process and corresponding time of a batch of ultra-high power or high power graphite electrodes are as follows: electrode pressing (3 days) &#; roasting (25 days) &#; impregnation (4 days) &#; roasting (15 days) &#; graphitization (10 days) &#; machining, quality inspection (2 days) &#; packaging and delivery of finished products (1 day). From feeding to product delivery, the fastest production cycle will be 60 days, and the fastest production cycle will be 90 days due to the two leaching and three roasting treatment of the electrode joint more than that of the electrode.
20
What are the characteristics of the electrodes produced in series graphitization furnace?
The development direction of graphitization furnace is internal heat series graphitization furnace. Because the current density of the series column is the same, the difference of the electrode resistivity is very small; secondly, the resistivity of the two ends of the inner series graphitized product is slightly lower than that of the middle part (the resistivity of the two ends of the Acheson furnace graphitized product is higher than that of the middle part), which is conducive to reducing the resistance of the connection and alleviating the reddening phenomenon of the connection when users use it. Therefore, the quality uniformity of electrodes produced by series graphitization furnace is better than that of Acheson furnace, and it is more suitable for the production requirements of electric arc furnace steelmaking.
21
Why does the quality of electrode joint play an important role in EAF steelmaking?
Joints play a key role in the connection of electrodes in steelmaking. The quality of joints is directly related to the use of electrodes in electric furnace steelmaking. No matter how good the quality of electrodes is, if there is no good joint with it, problems will arise in steelmaking. According to the relevant data, more than 80% of the accidents of using electrodes in EAF steelmaking are caused by the breaking and loosening of the joints. Therefore, the selection of high quality electrode joints is the guarantee condition for the normal use of EAF steelmaking electrodes.
22
What quality indicators of graphite electrode (joint) products have an impact on electric furnace steelmaking?
23
What are the consequences of serious oxidation at the electrode end of the upper end of the electrode gripper?
During steelmaking in smelting furnace, scrap is burned in the furnace. At the same time, because of oxygen blowing in the furnace, the flame column height is often higher than the electrode end of the upper end of the gripper. It is easy to oxidize the electrode end surface. If the oxidation is serious, the deformation of the electrode end surface will change from plane to inclined surface. When the upper end is connected with the new electrode, the oxidation deformation of the lower electrode end surface will occur. The new electrode can not be well contacted, and the gap of the electrode is large, which can easily cause oxidation and fracture of the internal joints. Without changing steelmaking conditions, the best preventive measure is to install a protective cover on the electrode end of the upper end of the gripper to block flame and air, so as to achieve the purpose of protecting the electrode end.
Figure 1: An example of a graphite electrode.
Figure 6: Copper also can be used for photo engraving and acid etching.
Figure 2: Some new high-speed mills are specially designed to machine graphite - and can even cut square internal corners.
Figure 3: The finish on the graphite should be as good as you need in the mold.
Figure 5: Copper can be used for coining and forging.
Figure 4: Copper is useful in small cavities, where it is difficult to polish.
When it comes time to decide whether to use graphite or copper electrodes in your shop, it's important to look at the big picture. According to Stu Haley, regional manager of Belmont Technologies, Inc., a provider of EDM supplies, tooling, accessories and machines, "To say which electrode works best is very difficult, it is totally applications driven. So much depends on what you have to work with on your shop floor in the way of support equipment. Both copper and graphite provide approximately the same end result. The difference is time to EDM the work and electrode manufacturing time and cost."
Haley explains that choosing an electrode material is often a result of where you were born and what type of EDM equipment you use. "For example, graphite was basically developed in the United States back in the early s, so the American EDM equipment manufacturers in those days concentrated on the graphite circuitry when designing their equipment," he says. "Whereas, since the European and Asian EDM equipment manufacturers didn't have access to graphite, they developed copper circuitry.
"If you have newer equipment built after , the electrode material of choice in North America is graphite," he adds. "This is used in 90% of the applications. In Europe and Asia, graphite is becoming more popular as an electrode material because of availability, machinability and speed of cutting."
Following are some of the advantages and disadvantages of each material so that you can decide which is best for your application.
Sold by grades, graphite cuts approximately three times faster than copper, according to Haley. "What makes a good grade or a poor grade is particle size," he explains. "Particle size gives you strength, machinability and greatly influences the metal removal rate, wear and the surface finish. Graphite is made up of carbon particles that are put through a graphitizing process to produce graphite. The smaller the particle size is, the better the graphite. Particle sizes in different grades of graphite can be ." for general-purpose use to ." for the extremely fine detail and superior surface finishes. Graphite can be purchased in big blocks, and then cut up to be machined; or it can be ordered precut or ground into the size you require.
"Graphite machines very easily &#; you can mill it, grind it, turn it, drill it, tap it, even file it to whatever shape you want," Haley continues. "Another advantage of graphite is that it doesn't burr. You can put it on a duplicating machine or a graphite high-speed mill and cut out complex shapes and forms, and once it's cut you are finished &#; with no deburring." (See Figure 1)
Additionally, graphite's high melting temperature results in less wear than other electrode materials, so a mold could be cut with one or two electrodes on a CNC EDM machine with very little wear, Haley adds. "A CNC sinker may need a third or forth electrode to finish the mold," he notes. "It depends on the age of the EDM machines."
If your shop has older fabricating equipment, machining graphite electrodes will result in dust particles on the shop floor and in the nearby machines. However, the new high-speed mills that are sold today are specially designed to machine graphite. "They are totally enclosed and have a vacuum system to remove all of the dust," Haley points out, "and there are some machines that can even cut square internal corners." (See Figure 2)
Another important point to keep in mind is that the finish on any electrode is the finish that will be put in the mold. "So, if you have a lot of cutter or grinding marks on the electrode, you will reproduce that in your mold," Haley says. "Normally, the finish on the graphite should be as good as you need in the mold." (See Figure 3)
Haley notes that copper can be cut on wire EDM machines, but there are only certain graphites that can be cut on a wire machine &#; the particle size should be five microns or less. Copper also is a little more forgiving in a poor flush situation than graphite. "In EDM, if the flush doesn't remove the eroded particles or chips out of the cutting area, there's something that can occur where you get a non-pulsating, direct flow of current from the electrode to the workpiece," Haley explains. "The result of this is a pit in the workpiece. Copper is more forgiving in those applications &#; all metallic electrodes are &#; they won't arc out as fast. But some of the newer EDM power supplies have adaptive logic or fuzzy logic, which eliminates the problem altogether. Copper &#; when used at specific settings with the correct flushing techniques using a CNC machine or a machine with an orbiting system &#; can produce a mirror-like surface finish. This is useful in small cavities where it is difficult to polish." (See Figure 4)
When you add tungsten to copper (copper tungsten), the result is an electrode material that has extremely good wear characteristics but is very difficult to machine, Haley notes. "When EDM'ing carbide, this is the best metallic electrode material to use. The best graphite material would be copper graphite, which is graphite impregnated with copper," he says.
According to Haley, there are alternative ways of machining copper &#; including coining (see Figure 5), forging, photo engraving/acid etching (see Figure 6) and stamping/forming. There are some shops that make a form and then copper plate it.
"Copper is more difficult to machine," Haley says, "and when you mill it, it has the tendency to stick to the cutter. When you grind it, it can clog up the grinding wheel. It heats up quickly and has a tendency to grab the grinding wheel. You'll have burrs no matter how you machine it, so you have to deburr it. Size and weight also can be an issue. A 12" x 7" x 2 3/4" piece of copper weighs 75 pounds, whereas a piece of graphite the same size is 11 pounds.
"The bottom line is use what you think will work best in your shop with the equipment that you have available," Haley continues. "If some electrodes are beyond your machining ability, have someone make them for you. You have to base it on your own experience and the resources you have available. Ask for suggestions from the equipment manufacturer or from your electrode material supplier. Regardless of what electrode material or combination of materials you decide on, be sure that you know the speeds and feeds to machine the material safely. When in doubt, ask."
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