Resistivity, bulk density, mechanical strength, coefficient of thermal expansion, elastic modulus, and other indicators are crucial for evaluating the quality of graphite electrodes. The oxidation resistance and thermal shock resistance of these electrodes are directly linked to the aforementioned indicators. Additionally, the precision of machined products and the reliability of their connections are also vital assessment criteria.
The quality of graphite electrodes is influenced by factors such as the quality of raw materials, process technology, equipment used in production, and management practices; among these, the performance of raw materials stands out as the most significant. Ordinary power graphite electrodes, made from standard petroleum coke, exhibit inferior physical and mechanical properties, like higher resistivity, a larger linear expansion coefficient, and lower thermal shock resistance, which limits their allowable current density.
Resistivity denotes a conductor's resistance to electric current flow. It is defined numerically as the resistance of a conductor that is 1 meter long and has a cross-sectional area of 1 m² at a specified temperature. This measurement is crucial for reducing energy consumption during operation. Typically assessed via the voltage drop method, resistivity can also indicate the degree of graphitization in graphite electrodes—lower resistivity reflects higher thermal conductivity and better oxidation resistance.
High-power graphite electrodes are crafted from premium petroleum coke or low-grade needle coke, yielding superior physical and mechanical characteristics compared to ordinary power graphite electrodes, thus permitting greater current density. Conversely, ultra-high power graphite electrodes must utilize high-grade needle coke for production. The bonding quality of high power and ultra-high power electrodes is critical; the joint must exhibit lower electrical resistivity and linear expansion coefficient than the electrode body while also having greater tensile strength and thermal conductivity. The reliability of these connections can be bolstered by using connecting bolts.
Enhanced bulk density aids in reducing porosity, improving mechanical strength, and bolstering oxidation resistance. However, excessive packing density can negatively impact thermal shock resistance, necessitating additional strategies to counteract this limitation. For instance, increasing the graphitization temperature can improve electrode thermal conductivity, while employing needle coke as a raw material can reduce the thermal expansion coefficient of the final products.
The mechanical strength of graphite electrodes is categorized into three types: compressive strength, bending strength, and tensile strength, with bending strength being the primary focus. This attribute is vital, as it affects the likelihood of graphite electrode fractures during use. In electric furnaces, electrodes may suffer breakage from contact with non-conductive objects, collisions, or strong vibrations. Therefore, electrodes with high bending strength are less susceptible to breakage.
The modulus of elasticity is a key mechanical property that gauges a material's capacity for elastic deformation. It refers to the stress-strain ratio within the elastic deformation range—the higher the modulus, the greater the stress required to achieve the desired elastic deformation. Brittle materials typically exhibit a higher modulus of elasticity, while flexible materials have a lower value.
The thermal expansion coefficient of graphite serves as a pivotal parameter that influences the thermal performance of electrodes. A lower coefficient value indicates greater thermal stability, enhanced oxidation resistance, reduced performance degradation, and lower consumable expenses.more
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