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Mechanical damage to graphite electrodes

Release time:2019-11-04 From: Zhengnai Industry in China

Causes of wire expansion in electrodes and nipples

Because the contact resistance in the connection area is always higher than the body resistance, the nipple temperature is always higher than the temperature of the electrode at the same horizontal position. When the temperature rises, the electrode and the connector will expand linearly.
 

The amount of expansion is an important cause of loose connections

The expansion amount is the product of linear expansion coefficient and temperature, and the gap is caused by the difference of expansion amount caused by the difference of radial linear expansion coefficient and temperature, which is an important reason for connection looseness and tripping.


 

Linear expansion is divided into axial linear expansion and radial linear expansion.

Linear expansion is divided into axial linear expansion and radial linear expansion. The radial linear expansion coefficient is always greater than the axial linear expansion coefficient for both electrodes and nipples (extruded products). After calculating the radial and axial linear expansion of electrode body and nipple, the possibility of longitudinal cracking or transverse fracture and loosening and shedding of graphite electrode and nipple at a certain mass level can be predicted.
 
 

The temperature at which the electrode is used on an electric furnace

When the electrode is used on an electric furnace, it is generally connected with 2~3 electrodes, so there are 1~2 connection areas. The temperature of the electrode near the bottom of the electrode holder is generally 500~1200 °C, and it is inserted into the furnace. The electrode often reaches 1500~2000 °C, and the end of the electrode that produces the arc can reach 3000 °C. The electrode temperature is proportional to the current density through the electrode.

Parameter of RP Graphite electrode

Item Unit RP
300-500mm 550-800mm
Electric resistivity Electrode μΩm 7.5-8.6 7.5-8.8
Nipple 4.5-6.0
Bending strength Electrode Mpa ≥7.5 ≥6.5
Nipple ≥16.0
Elastic modulus Electrode Gpa ≤9.0
Nipple ≤13.0
Bulk density Electrode g/cm3 1.54-1.62
Nipple 1.70-1.78
CTE(100-600℃) Electrode 10-6/℃ 2.20-2.60
Nipple 1.50-1.90
Ash % ≤0.3

 

There are 3 kinds of forces when static

Tensile stress of electrode column.
The stress generated when the connector is screwed into the electrode connector hole.
Superimposed stress of tightening torque and electrode weight.
 

3 kinds of mechanical damage common to graphite electrodes

There are three main mechanical damages caused by the difference of linear expansion amount under thermal load and graphite electrode under the action of external force :
1. Another important reason is the gap between the upper and lower electrodes due to the difference in the amount of axial expansion of the electrode and the nipple.
2. The hole of electrode nipple is cracked.
3. The connection thread is loose and the lower electrode falls off due to the unbuckle.


 

Mechanical damage to graphite electrodes

Loose back is a cause of the lower electrode falling off
Electrode connected to the nipple should be enough contact area, friction coefficient and contact pressure, if the machining accuracy is not enough when the connection is not very good or steel furnace operators to clean surface connection or connection of tightening torque is not enough, will not reach the contact area and contact pressure, resulting in loose back clasp, serious when can cause nipple or fall off the bottom electrode.
 

Electrode nipple hole bursting and its causes

Electrode nipple hole burst is also the electrodes and the nipple of linear expansion is not the result of the match, due to the radial expansion nipple of outside diameter is larger than electrode nipple hole diameter, the nipple is compressive stress, electrode produce radial expansion nipple hole wall stress, when the expansion stress accumulated to a certain degree will produce axial crack in nipple hole wall.


 

What cause the electrode to fall off ?

The amount of wire expansion of the nipple is smaller than that of the nipple hole, which may cause the electrode to fall off.
When the linear expansion of the nipple at the same temperature is less than the linear expansion of the nipple hole, the reserved clearance will increase. When the difference between the nipple diameter and the nipple hole diameter is less than 99.5%, it is easy to loosen and loosen to a certain extent. The electric furnace vibrates strongly and the lower electrode may fall off.
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