Common Causes of Broken Electrodes in Electric Arc Furnaces (Submerged Arc Furnaces)

During the production process of electric arc furnaces, electrodes are often broken due to problems with the quality of the charge, equipment, operation and the electrode itself or the interaction of the above factors. On the one hand, the electrode breakage accident will cause the electric arc furnace to stop intermittently and affect its production efficiency and on-site production organization; on the other hand, it will increase the consumption of electric arc furnace electrodes, thereby increasing the smelting cost of the electric arc furnace.

Therefore, carefully analyzing the problem of electrode breakage in the electric arc furnace, accurately determining the real cause of electrode breakage and effectively guiding on-site production can significantly reduce the number of electrode breakages in the electric arc furnace, thereby greatly improving the production efficiency of the electric arc furnace and reducing the cost of steelmaking.

Analysis of Causes of Broken Electrodes in Electric Arc Furnaces

Electrode breakage usually occurs at the highest joint of the electrode column or the threaded hole of the joint, but the electrode body also occasionally breaks.

1. Charge Causes

The quality of charge not only affects the control level of the metal recovery rate of the electric arc furnace, but also directly affects the safety of the electrode operation during the smelting process of the electric arc furnace. The broken electrode problem caused by charge problems is mainly manifested in the following aspects.

(1) Non-conductive materials such as rubber and wood products are mixed in the charge.

(2) A large amount of non-conductive materials such as refractory materials and mud and sand adhere to the surface of the charge.

(3) The poor conductivity of the arc starting material will also increase the probability of electrode breakage during the arc starting stage.

2. Equipment causes

(1) Electrical control system causes

The failure caused by the control system is usually repeated and inevitable, which poses a great threat to the safety of electrode operation, and is particularly manifested in the following aspects.

① Reverse phase sequence: Under normal circumstances, the tightening direction of the electrode should be consistent with the direction of the electromagnetic force when the electrode is working. However, if the phase sequence is accidentally reversed during debugging or maintenance, it is easy to cause the electrode to loosen from the joint during the smelting process. When the electrode is loose, the gap at the electrode joint increases, the resistance increases, the joint becomes hot and red, the oxidation accelerates and eventually the lower section falls off. In severe cases, it falls off directly, causing the joint thread to be scrapped or the electrode joint to be directly broken during the smelting process. The method of judging the phase sequence current and the electromagnetic force of the electrode is that the positive direction of the current is from the beginning to the end of the wire. The current direction of each phase electrode can be marked according to the regulation that the positive direction of the current is from the beginning to the end of the wire. The right-hand rule is used to determine the direction of the magnetic line of force, and the left-hand rule is used to determine the direction of the electromagnetic force on the outer surface of the electrode. The magnitude of the electromagnetic force: F=I1×I2/d×10⁻⁷[N/m], where I1, I2: phase current [A], d: distance between electrodes [m].

(a) Normal phase sequence

(b) Reverse phase sequence

② Reverse ground detection signal: Usually, the ground signals of each phase of the three-phase electrode are detected and fed back independently. If any two phases of the ground detection signals are accidentally reversed, the electrode will first contact the charge phase during the smelting process. Because it cannot receive its own feedback signal in time, the electrode and the scrap steel will be squeezed and broken. Its fault characteristics: The electrode that first contacts the charge will not stop descending because its signal feedback phase electrode has not yet contacted the charge. It will break due to excessive force during the continued descent.

③ The connection line between the neutral point of the three-phase voltage transformer measuring the voltage on the short-circuit side and the bottom shell of the electric furnace is disconnected or in poor contact. After high-voltage power transmission, the three-phase balance of the secondary short-circuit no-load voltage is normal. During the automatic electrode descent process, when the lower end of a phase electrode contacts the scrap steel in the furnace, the secondary voltage of the phase should drop to less than 20% of the no-load voltage or lower. However, after the connection line between the neutral point of the voltage transformer and the bottom shell of the electric furnace is disconnected, the electrode adjustment system cannot detect the voltage drop of the phase to the ground, and cannot determine that the electrode has touched the charge. The electrode adjustment system controls the electrode to continue to descend, resulting in the electrode and the charge being squeezed and broken. This fault is manifested as no arcing and always breaking the electrode that contacts the charge first, while the secondary voltage of the phase electrode remains basically unchanged.

(2) Causes of mechanical actuators

① Deformation of the rod of the electrode lifting hydraulic cylinder: If the braking force of the hydraulic drive mechanism decreases or the delay coefficient of the system increases, the action and response speed of the electrode actuator will be delayed, and the electrode end may collide with the scrap steel to a large extent, resulting in the electrode breaking. This fault is often manifested as unbalanced, unstable and fluctuating three-phase electrode load current during normal smelting.

② Problem of gap between electrode columns: Too large or too small gap between electrode columns will affect the sensitivity of electrode adjustment response. If the gap is too large, it is easy to cause the electrode to shake too much at the moment of starting and break the electrode. If the gap is too small, the electrode adjustment system will react slowly and the electrode will be easily broken when the material collapses or scrap steel is touched.

③ Problem of gap between electrode and small furnace cover: If the gap between electrode and small furnace cover is too small, the electrode will be broken due to the force between electrode and furnace cover during the downward process of electrode.

④ Insufficient clamping force of clamp: If the butterfly spring of clamp is broken or the pre-tightening force is insufficient, it is easy to cause the electrode to fall off from the clamp and break the electrode.

III. Operation reasons

With the popularization of arc furnace smelting related knowledge, steel mills have increased the training of operators, and job operations have gradually become standardized. Therefore, the probability of electrode breakage caused by operation reasons has been significantly reduced, but it is still necessary to further emphasize the operation precautions of some special links, and strive to completely solve the problem of electrode breakage caused by operation reasons. Common operation problems mainly include the following aspects.

(1) Batching and charging process

Failure to strictly follow the batching operation points to batch, resulting in serious batching problems such as heavy materials, large blocks of materials moving up, and non-conductive materials entering the furnace. During the charging process, the furnace charge is seriously skewed after entering the furnace due to the positioning of the overhead crane or human operation, and the electrode is concentrated and broken in the middle and late stages of smelting.

(2) Furnace door cleaning process

When the forklift cleans the furnace door, the electrode is not lifted high enough, or the slag blocks and other materials with poor conductivity are pushed directly under the electrode at the furnace door, which can also cause electrode breakage accidents.

(3) Connection and clamping process

Improper purging, verticality and pre-tightening force during the connection process will seriously affect the quality of electrode connection, resulting in a decrease in the strength of the electrode joint during the use of the electrode on the line, and frequent electrode breakage accidents during the smelting process. If the clamping part is clamped at the white line joint area or close to the white line, it will be broken due to damage to the joint part or excessive lateral shear force on the weaker part of the bottom thread of the hole.

(4) Smelting and slag making process

If lime and other auxiliary materials are added too concentratedly or the feeding speed is too fast during the melting stage, the electrode may be broken due to poor conductivity of the charge under the electrode. In addition, the probability of electrode breakage of phases 1, 2, and 3 should be the same. If one phase is more than the other two, operational problems should be considered. At the same time, the breakage caused by responsibility problems is more common in night shifts than day shifts, and more common in holidays than daily.

IV. Electrode quality problems

Production suspension and pouring interruption caused by graphite electrode quality problems will generate a large amount of direct and indirect costs. Therefore, steel mills have always been sensitive to the problem of electrode breakage in electric furnaces and are particularly concerned about it.

The connection area between graphite electrode and joint is a part with large and complex electrical, thermal and mechanical loads, and is also a common fracture site. As for the quality of the joint itself, the main reasons for the joint breakage are as follows: the joint volume density is low, the strength is generally low, and it is easy to break during use (especially in the furnaces with a large proportion of medium and heavy materials); the resistivity is high, and the temperature of the joint part rises rapidly when the power is turned on, which will cause the joint part of the electrode connection to have a large thermal stress and increase the probability of breakage; the joint's flexural strength is not enough, and the internal crack joint mixed into the finished joint will also cause frequent breakage problems; the unreasonable matching of the joint and the body processing precision is also prone to joint breakage or body longitudinal cracking problems.

Generally, the probability of the electrode body breaking is low. The main reasons for the body breaking are as follows:

① The screw hole of the electrode body has quality defects; the volume density and strength of the electrode body are not enough;

② The electrode body does not match the joint index and processing accuracy; the poor thermal shock resistance of the electrode will also cause deep cracks at the electrode end;

③ Electrodes with internal transverse cracks are mixed into the finished product without detection, and electrode structural defects such as ordinary coke mixed with needle coke are also important factors causing the electrode body to break.

Summary

The various factors and links that lead to electrode breakage are analyzed and summarized, and a series of control measures and on-site solutions are formulated in a targeted manner to effectively solve the problem of electrode breakage caused by furnace materials, equipment, on-site operations and electrode quality problems, and provide a reference for the division of responsibilities after the electrode is broken.