In Electric Arc Furnace (EAF) steelmaking, arc ignition and boring (also known as penetration) during the melting stage are critical operations that directly affect melting efficiency, electrode consumption, and overall furnace stability. A clear understanding of the physical mechanisms behind these two phenomena, as well as their corresponding power-on practices, is essential for achieving safe, stable, and high-efficiency steelmaking.
1. What Is “Arc Ignition” in the Electric Arc Furnace Steelmaking Process?
At the beginning of EAF steelmaking, the electrodes are lowered under the control of the regulating system until they come into contact with the scrap steel in the furnace. When the electrode tip touches the scrap, a short circuit is formed, and a high-temperature electric arc is generated and maintained between the electrode and the scrap. This phenomenon is referred to as arc ignition.
Arc ignition marks the formal start of the melting period in electric arc furnace steelmaking. At this stage, the furnace is still filled with solid charge materials, and the arc is relatively exposed. As a result, arc stability is poor and the thermal load on the furnace roof is relatively high, making arc ignition a phase that requires careful control of both electrical input and electrode movement.
2. Characteristics of the Arc Ignition Stage and Power-On Principles
During the initial power-on and arc ignition stage, the furnace is fully charged with scrap, and the distance between the arc and the furnace roof is small. If excessive power or overly high voltage (i.e., an excessively long arc) is applied at this time, the furnace roof—especially small furnace covers—can be seriously damaged. Therefore, the lowest voltage tap is typically selected for power-on during arc ignition to ensure safe and stable operation.
The arc ignition stage is generally short, lasting approximately 1 to 5 minutes. In cases where a large proportion of light and thin scrap is charged in the upper layer, it may be acceptable to apply higher power from the beginning in order to accelerate scrap melting.
At the moment of initial power-on, the metal beneath the electrode is subjected to sudden high-temperature impact, which may cause localized bursting or spalling. In addition, the arc is highly unstable at this stage and arc interruptions occur frequently. Furthermore, due to gaps between individual scrap pieces, electron emission and bombardment can occur between metal surfaces after energization. These factors together result in extremely high noise levels during the early arc ignition period. After several minutes of operation, as the arc becomes buried in the scrap, arc stability improves significantly and the noise level gradually decreases.
3. What Is “Boring (Penetration)” in the Electric Arc Furnace Steelmaking Process?
Following arc ignition, the scrap surrounding the arc beneath the electrode melts rapidly under intense arc heating. With the assistance of the electrode regulating system, the electrode maintains an appropriate distance from the charge and continuously descends as the scrap melts.
As melting progresses, a hole larger than the electrode diameter gradually forms beneath the electrode. In AC electric arc furnaces, three such holes are typically formed, while in DC electric arc furnaces, a single hole is created. This process, in which the arc effectively “bores” through the scrap charge, is known as boring or penetration. When the electrode approaches the furnace bottom and a molten bath begins to form, the electrode starts to rise dynamically under the action of the regulator, marking the end of the penetration stage.
4. Power-On Characteristics and Operational Considerations During the Penetration Stage
A defining feature of the penetration stage is that the electric arc is completely surrounded by scrap material. Under these conditions, nearly all of the arc’s thermal energy is absorbed by the charge, and direct thermal impact on the refractory lining is minimal. As a result, maximum power input is typically applied during the penetration stage to fully exploit the high power density and high melting efficiency of electric arc furnace steelmaking.
It should be noted that scrap collapse occurs frequently during penetration, leading to rapid changes in arc length and electrical conditions. Consequently, current and voltage become highly unstable, and the ammeter needle on the control panel often fluctuates violently. This behavior is characteristic of the penetration stage and requires both responsive automatic regulation systems and attentive operation to maintain overall furnace stability.
