Graphite electrodes are divided into ordinary power graphite electrode (RP), high power graphite electrode (HP) and ultra high power graphite electrode (UHP) according to the different physical and chemical indexes of raw materials and finished products.

The main indicators to measure the quality of graphite electrodes are resistivity, bulk density, mechanical strength, thermal expansion coefficient, elastic modulus and so on. The oxidation resistance and thermal shock resistance of the graphite electrode in use are related to the above indicators, and the accuracy of the quality of the machined product and the reliability of the connection are also important inspection items.

● Resistivity

The resistance of a conductor to the passage of current. Numerically equal to the resistance of a conductor with a length of 1m and a cross-sectional area of 1 ㎡ at a given temperature, reducing consumption in use. Usually measured by the pressure drop method, the size of the resistivity can measure the degree of graphitization of the graphite electrode, the lower the resistivity of the graphite electrode, the higher the thermal conductivity, the better the oxidation resistance.

● Bulk density

Increasing the bulk density is beneficial to reduce the porosity, improve the mechanical strength and improve the antioxidant capacity. However, if the bulk density is too large, the thermal shock resistance decreases. Therefore, other measures should be taken to remedy this deficiency. For example, increasing the graphitization temperature to improve the thermal conductivity of the electrode and using needle coke as raw material to reduce the thermal expansion coefficient of the finished product.

● Mechanical strength

The mechanical strength of graphite electrode is divided into compressive strength, bending strength and tensile strength. The bending strength is mainly measured, and the bending strength is the performance index related to fracture in the use of graphite electrode. In the electric furnace, when the electrode is in contact with a non-conductive object, or due to collision collapse, strong vibration and other reasons, the graphite electrode is often in danger of being broken, and the graphite electrode with high bending strength is not easy to be broken.

● Modulus of elasticity

The modulus of elasticity is an important aspect of mechanical properties. It is an index to measure the elastic deformation capacity of a material and refers to the stress-strain ratio within the elastic deformation range. The greater the elastic modulus, the greater the elastic deformation that produces the desired stress, the greater the elastic modulus of the brittle material, and the smaller the elastic modulus of the flexible material.

● Coefficient of thermal expansion

The thermal expansion coefficient of graphite is a very important parameter that affects the thermal performance of the electrode. The lower the value, the stronger the thermal stability of the domestic product, the higher the oxidation resistance, the less the broken performance and the lower the consumption.

The quality of graphite electrodes depends on the performance of raw materials, process technology, management and production equipment, and the performance of raw materials is the primary condition. Ordinary power graphite electrode, produced by ordinary grade petroleum coke, has low physical and mechanical properties, such as high resistivity, large linear expansion coefficient and poor thermal shock resistance, so the allowable current density is low.

High-power graphite electrodes are produced with high-quality petroleum coke (or low-grade needle coke), and their physical and mechanical properties are higher than those of ordinary power graphite electrodes, allowing greater current density. Ultra-high power graphite electrodes must be produced using high grade needle coke. The joint quality of high-power and ultra-high-power graphite electrodes is particularly important. Not only the resistivity and linear expansion coefficient of the joint blank are smaller than those of the electrode body, but also the joint blank should have higher tensile strength and thermal conductivity. In order to enhance the reliability of the electrode connection, the connector should be equipped with connecting bolts.