Electric arc furnace

[1][2] Further electric arc furnaces were developed by Paul Héroult, of France, with a commercial plant established in the United States in 1907.

While EAFs were widely used in World War II for production of alloy steels, it was only later that electric steelmaking began to expand.

The electric arc temperature reaches around 3,000 °C (5,400 °F), thus causing the lower sections of the electrodes to glow incandescently when in operation.

[8] The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders.

Large water-cooled cables connect the bus tubes or arms with the transformer located adjacent to the furnace.

Modern furnaces mount oxygen-fuel burners in the sidewall and use them to provide chemical energy to the cold-spots, making the heating of the steel more uniform.

The largest scrap-only furnace (in terms of tapping weight and transformer rating) is a DC furnace operated by Tokyo Steel in Japan, with a tap weight of 420 tonnes and fed by eight 32 MVA transformers for 256 MVA total power.

In many locations, mills operate during off-peak hours when utilities have surplus power generating capacity and the price of electricity is less.

Scrap generally comes in two main grades: shred (whitegoods, cars and other objects made of similar light-gauge steel) and heavy melt (large slabs and beams), along with some direct reduced iron (DRI) or pig iron for chemical balance.

A lot of potential energy is released by the tonnes of falling metal; any liquid metal in the furnace is often displaced upwards and outwards by the solid scrap, and the grease and dust on the scrap is ignited if the furnace is hot, resulting in a fireball erupting.

Lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs.

Oxygen is blown into the scrap, combusting or cutting the steel, and extra chemical heat is provided by wall-mounted oxygen-fuel burners.

Supersonic nozzles enable oxygen jets to penetrate foaming slag and reach the liquid bath.

Slag usually consists of metal oxides, and acts as a destination for oxidised impurities, as a thermal blanket (stopping excessive heat loss) and helping to reduce erosion of the refractory lining.

Another major component of EAF slag is iron oxide from steel combusting with the injected oxygen.

Later in the heat, carbon (in the form of coke or coal) is injected into this slag layer, reacting with the iron oxide to form metallic iron and carbon monoxide gas, which then causes the slag to foam, allowing greater thermal efficiency, and better arc stability and electrical efficiency.

The slag blanket also covers the arcs, preventing damage to the furnace roof and sidewalls from radiant heat.

During tapping some alloy additions are introduced into the metal stream, and more fluxes such as lime are added on top of the ladle to begin building a new slag layer.

Often, a few tonnes of liquid steel and slag is left in the furnace in order to form a "hot heel", which helps preheat the next charge of scrap and accelerate its meltdown.

A typical steelmaking arc furnace is the source of steel for a mini-mill, which may make bars or strip product.

It is called collected dust and usually contains heavy metals, such as zinc, lead and dioxins, etc.

The size of DC arc furnaces is limited by the current carrying capacity of available electrodes, and the maximum allowable voltage.

The ladle furnace consists of a refractory roof, a heating system, and, when applicable, a provision for injecting argon gas into the bottom of the melt for stirring.

Though crude, these simple furnaces can melt a wide range of materials, create calcium carbide, and more.

These typically hide behind slag coverage and can hydrate the refractory in the hearth, leading to a break out of molten metal or in the worst case a steam explosion.

After melting in an electric arc furnace and alloying in an argon oxygen decarburization vessel, steels destined for vacuum remelting are cast into ingot molds.

This vacuum remelting process rids the steel of inclusions and unwanted gases while optimizing the chemical composition.

Still enveloped by the vacuum, the hot metal flows from the VIM furnace crucible into giant electrode molds.

Controlling the rate at which these droplets form and solidify ensures a consistency of chemistry and microstructure throughout the entire VIM-VAR ingot, making the steel more resistant to fracture or fatigue.

For example, steels for solid rocket cases, landing gears, or torsion bars for fighting vehicles typically involve one vacuum remelt.