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Ash And Thermal Shock Resistance

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

Definition of thermal shock resistance

The performance that a graphite electrode can withstand a drastic change in temperature when it is used at a high temperature without being destroyed is called thermal shock resistance.
 

Thermal shock resistance is related to 4 indicators

The thermal shock resistance of graphite electrodes is mainly related to its own linear expansion coefficient, tensile strength, thermal conductivity and elastic modulus.


 

3 factors affecting the magnitude of thermal shock resistance

Factors that generate thermal stress.
Factors that buffer thermal stress.
Factors that resist thermal stress.
Relationship between elastic modulus and thermal shock resistance
The elastic modulus is the main factor affecting the thermal stress of the buffer. The smaller the elastic modulus of the electrode, the better the elastic property, and the stronger the ability to buffer thermal stress, that is, the better the thermal shock resistance.
 

2 indicators related to improving thermal shock resistance

The electrode has high tensile strength and shear strength, which can resist the damage of thermal stress in the case of a sharp temperature change, and the electrode can still be used intact. Therefore, improving the tensile strength and shear strength is beneficial to improve the thermal shock resistance of the graphite electrode.


 

2 steps to improve thermal shock resistance

To improve the thermal shock resistance of the graphite electrode.
Firstly, a raw material having a low coefficient of linear expansion (needle coke) and a suitable particle size composition should be selected.
Secondly, the temperature of the graphitization is increased while the mechanical strength is increased by the impregnation method.
 

5 methods for indirect testing of thermal shock resistance

Some research institutes have designed a variety of special instruments for measurement, indirectly testing the thermal shock resistance of graphite electrodes.
 
arc discharge heating method.
electron beam heating method.
high frequency induction heating method.
radial exposure Press method.
Electric heating method.
 

Main components of carbon materials

The main component of carbon materials is carbon, and other impurities include volatiles and ash.

Item Unit UHP HP RP
300-600mm 650-800mm 300-800mm 300-800mm
Electric resistivity Electrode μΩm 4.8-5.8 4.6-5.8 5.8-6.6 7.8-8.8
Nipple 3.8-4.5 3.8-4.5 4.2-5.0 5.0-6.5
Flexural strength Electrode Mpa 10.0-14.0 10.0-14.0 10.0-13.0 7.0-12.0
Nipple 20.0-26.0 22.0-26.0 18.0-22.0 15.0-20.0
Elastic modulus Electrode Gpa 9.0-13.0 10.0-14.0 8.0-12.0 7.1-9.3
Nipple 15.0-18.0 15.0-18.0 14.0-16.0 12.0-14.0
Bulk density Electrode ≥g/cm3 1.68-1.74 1.70-1.74 1.62-1.66 1.53-1.56
Nipple 1.77-1.82 1.78-1.84 1.74-1.80 1.70-1.74
CTE(100-600℃) Electrode x10-6/℃ 1.1-1.4 1.1-1.4 1.6-1.9 2.2-2.6
Nipple 0.9-1.2 0.9-1.2 1.1-1.4 2.0-2.5
Ash   0.3 0.3 0.3 0.5

 

The origin of volatiles

The elements such as nitrogen, hydrogen, oxygen, etc. in the volatile matter escape in a gaseous state after calcination or further heating.
 

The origin of ash

After the ash is burned, the metal and non-metal elements (generally their oxides) remain, mainly silicon, calcium, aluminum, iron, magnesium, copper and other elements. The content of boron and vanadium is not many.
 

Ash component

The element in the ash is mainly iron, but some impurity elements in the ash reduce the oxidation resistance of the electrode. Adding a small amount of iron oxide powder to the ingredients will slightly increase the ash content of the finished electrode.
 

Reasons for limiting boron and vanadium

The formation of high-melting carbides at high temperatures is difficult to remove even at high temperatures of graphitization. Therefore, the production of high-purity graphite has a certain limit on the content of boron and vanadium in the raw materials.
 

Factors affecting ash content

The ash content of the graphite electrode is related to the raw material ash.
It is related to the temperature of graphitization. The higher the graphitization temperature, the more oxides are volatilized and the less ash remaining.
It is related to the amount of purge gas supplied.
Relevant to the ventilation process conditions.


 

Ways to reduce ash

After the carbonaceous raw material is calcined (1300 ° C), the ash content increases due to the escape of volatile matter and a small amount of carbonaceous oxidation. After the graphitization high temperature treatment, most of the impurity elements volatilize, and the ash content can be reduced by an order of magnitude.
 

Graphite electrode produced by petroleum coke has lower ash content

The ash content of the graphite product is mainly affected by the ash size of the raw materials used, and the ash content of the petroleum coke is low, so the ash content of the graphite electrode generally does not exceed 0.5%.
 

The influence of ash in steel making

The ash content of the graphite electrode is less than 1%, which has no obvious effect on steelmaking, but some impurity elements in the ash will reduce the oxidation resistance of the electrode.
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