The melting period stands as the core of EAF operations, with the EAF having evolved into a highly efficient melting apparatus. Modern designs prioritize maximizing the melting capacity of the furnace. Melting is achieved by supplying energy to the furnace interior, which can be electrical or chemical in nature.
Electrical energy, primarily supplied through graphite electrodes, is usually the largest contributor to melting operations. Initially, an intermediate voltage tap is selected until the electrodes bore into the scrap. To facilitate this bore-in process, light scrap is typically placed on top of the charge. During the initial bore-in period, approximately 15% of the scrap is melted. After a few minutes, once the electrodes have sufficiently penetrated the scrap, a long arc (high voltage) tap can be employed without fear of radiation damage to the roof. The long arc optimizes power transfer to the scrap, leading to the formation of a liquid metal pool in the furnace hearth.
At the start of melting, the arc is erratic and unstable, characterized by wide swings in current and rapid electrode movement. As the furnace atmosphere heats up, the arc stabilizes. Once the molten pool forms, the arc becomes quite stable, and the average power input increases.
Chemical energy is supplied through various sources, including oxy-fuel burners and oxygen lances. Oxy-fuel burners utilize natural gas burned with oxygen or a blend of oxygen and air. Heat is transferred to the scrap via flame radiation and convection from the hot combustion products, as well as through conduction within the scrap itself. Larger scrap pieces take longer to melt into the bath compared to smaller ones.
In some operations, oxygen is injected via a consumable pipe lance to "cut" the scrap. The oxygen reacts with the hot scrap, burning iron to generate intense heat for cutting purposes. Once a molten pool of steel forms in the furnace, oxygen can be directly lanced into the bath. This oxygen reacts with several bath components, including aluminum, silicon, manganese, phosphorus, carbon, and iron. All these reactions are exothermic, generating additional heat to aid in scrap melting. The resulting metallic oxides end up in the slag. The reaction of oxygen with carbon in the bath produces carbon monoxide, which either burns within the furnace if sufficient oxygen is present or is exhausted through a direct evacuation system, where it is burned and conveyed to the pollution control system. Auxiliary fuel operations are discussed in greater detail in the section on EAF operations.
Once enough scrap has been melted to accommodate the second charge, the charging process is repeated. After the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. Consequently, the voltage must be reduced to prevent damage. Alternatively, creating a foamy slag allows the arc to be buried, protecting the furnace shell. Additionally, more energy is retained in the slag and transferred to the bath, enhancing energy efficiency.
Once the final scrap charge is fully melted, flat bath conditions are achieved. At this point, a bath temperature and sample are taken. Analyzing the bath chemistry enables the melter to determine the amount of oxygen to be blown during refining. The melter can also start arranging for bulk tap alloy additions, with final quantities determined after the refining period.
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