The “anisotropy” of graphite electrodes affects their performance differently in DC and AC electric arc furnaces. The core difference lies in the fact that the skin effect of AC current interacts with the radial/circumferential conductive properties of graphite electrodes, whereas DC current does not have this problem. This difference directly affects the electrode’s current-carrying capacity, thermal stress distribution, and consumption pattern.
The anisotropy of graphite electrodes originates from the preferential orientation of needle coke particles along the axial direction during manufacturing. Its key manifestation is that the axial resistivity is low, about 4.9 μΩ·m, while the radial and circumferential resistivity is high, about 7.3 μΩ·m.
In AC electric arc furnaces, the alternating magnetic field generated by AC current induces the skin effect. Current tends to concentrate and flow in the surface layer of the electrode rather than being uniformly distributed across the entire cross-section. When the electrode diameter is large or the frequency is high, this effect is particularly significant, causing the AC resistance to be much higher than the DC resistance, and the current density to concentrate in the circumferential and radial directions at the outer periphery of the electrode. This direction happens to correspond to the higher-resistivity direction in graphite anisotropy, so the current is forced to flow through a high-resistance path, generating more Joule heat.
In DC electric arc furnaces, the current direction is constant, there is no skin effect, and the current can flow uniformly through the electrode cross-section, mainly along the axial low-resistivity direction. The negative impact of anisotropy is therefore much smaller.
Specifically, in terms of different manifestations, in DC electric arc furnaces the current is uniformly distributed and mainly flows along the axial low-resistance direction, so the influence of anisotropy is relatively small, and the current-carrying capacity is mainly limited by the circumferential thermal stress caused by ohmic heating. In AC electric arc furnaces, there is a pronounced skin effect, especially on large-diameter electrodes, and the current is forced to flow through the high-resistance radial and circumferential directions, increasing the effective resistance and also increasing the risk of local overheating and thermal stress cracking. In terms of electrode configuration, DC furnaces usually use a single electrode together with a bottom electrode, with a larger diameter, up to 32 inches, or 800 mm; AC furnaces usually use three electrodes, with relatively smaller diameters, typically 20 to 28 inches. In terms of electrode consumption, DC furnaces can theoretically reduce it by about 50%, while AC furnaces consume three electrodes separately, resulting in higher total consumption.
From an engineering perspective, graphite electrodes for DC furnaces usually need a larger diameter to carry the entire current, while the three electrodes of an AC furnace can share the current, so the diameter of a single electrode is relatively smaller. In AC furnaces, the skin effect forces the current to flow through the high-resistivity directions of the graphite electrode, increasing the risk of local overheating and thermal stress cracking. The differences in anisotropic material properties in the electrode joint region, that is, the different physical properties of the electrode body and the joint pin, will further aggravate stress concentration and become a potential point of mechanical failure. One of the main reasons for the lower electrode consumption in DC furnaces is precisely that it avoids the additional resistance heating and thermal stress caused by the AC skin effect, allowing the current to more uniformly utilize the axial low-resistance channel of the electrode and reducing electrode fracture and oxidation consumption caused by local overheating.
Post time: Sep-10-2026