So long as there is a ready supply of scrap steel and enough electrical power, an electric arc furnace can be located anywhere
RAYMOND R. SHEPARD
In this, the third of a four-part series, the author provides an overview of the electric arc furnace (EAF) process and how it is used to make low-carbon and specialty steels. The capital required to start a traditional, comprehensive steel mill is extraordinary. Because of the economics facing the steel industry today and the fact EAFs can directly accept scrap, electric arc furnaces are emerging as a potential solution for steel production in the United States. Inspectors will benefit from a basic understanding of this process. The next issue will detail the continuous casting process and the rolling of steel.
tilization of the electric arc furnace (EAF) for steel pro - duction has dramatically increased during the last thirty Uyears. Although we may think of the process as new, it has been in existence about one hundred years. Initially, factors such as cost, availability of electricity, and technological issues relegated the process to small batches of steel or for the pro - duction of specialty steels. After sixty years, the process and economics evolved to where it was economically feasible to use EAFs on a large scale. The early 1970s saw the EAF process take hold, and the proportion of steel produced by this process has continuously increased. At age 23, Paul Louis Heroult became famous as the codis - coverer of the electrolytic process for aluminum production. Fig. 1 — Sorted scrap is used to charge EAFs. Some furnaces Aluminum had been a very expensive metal to produce, but the are able to accept 100% scrap, making EAFs efficient recyclers. new process made it available on an industrial scale for the first Small scrap is usually used first to protect the refractory material time. Heroult’s work with the electrolytic process laid the foun - from damage by larger pieces. dation for experiments he performed to develop the electric arc furnace, and a few years later, around 1900, the first EAF was Today, EAFs recycle millions of pounds of scrap steel in the pro - produced. The first furnaces were very small, having no more duction of low-carbon steel and high-quality alloy steel and stain - than a few yards capacity. Heroult eventually came to the United less steels. States and worked to establish a few EAFs here. He saw great potential for this process and worked diligently in an attempt to Steelmaking with an Electric Arc Furnace see it gain a foothold in the steelmaking industry. Due to eco - nomic factors, he never saw his dream of massive EAF steel pro - The EAF is a stout, round vessel. Large carbon electrodes duction come to fruition during his lifetime and his discovery of are lowered from above the vessel until they almost touch the the electrolytic process for aluminum overshadowed his work in metal. An electric arc is then initiated that delivers significant steel until after his death. heat input. Operating at voltages in the range of 100–600 volts, The EAF process was ahead of its time. However, economics and 20–80 kA (AC furnaces), an EAF consumes enough elec - change. Enormous capital is required to start a conventional, tricity to power a small town. integrated ironworks for producing pig iron and the manual To put the amperage (heat) in perspective, it is roughly the labor necessary is staggering. The EAF gained a foothold equivalent of 670 welders, using E7018 electrodes, striking arcs in because of its lower labor costs and ability to process scrap the same place at the same time. In fact, the process is similar to a directly. Processing scrap meant an EAF did not have to be shielded metal arc welding power source set up to perform air car - located near a blast furnace for the delivery of pig iron but bon arc gouging, albeit on a much more massive scale. The trans - instead could be located anywhere that had a supply of scrap former produces high amperage that is delivered via cables to the and the proper electrical generating capacity. The term “mini carbon electrode. The electrode melts the steel it contacts, but mill” was coined to describe small EAFs that started to appear instead of blowing the steel away as in air carbon arc gouging, the in areas that were not traditional steel production regions. furnace holds and controls the molten steel.
RAYMOND R. SHEPARD ( [email protected] ) is Quality Specialist/Training Coordinator, Kakivik Asset Management, Anchorage, Alaska.
SPRING 2004 • 27 assembly are then moved back into place. Actuation may be accomplished through the use of hydraulics or electric motors and cables. Charging the furnace may take several buckets of scrap — Figs. 1, 2. To reduce downtime, several processes have been developed where scrap is fed to the furnace continuously or in increments, eliminating the need for bucket charging. These systems save money as there is no off-time during which the furnace is not producing steel. In the Consteel Process, scrap is fed to the EAF though a conveyor. The off-gas from the furnace is directed over a sealed portion of the conveyor. This off-gas preheats the scrap and increases the efficiency of the EAF process.
Initiating the Arc
When the arc is first initiated, there is an explosion of sound. The arc is initially unstable as it dances around the assorted scrap. Scrap in direct contact with the arc is instantly melted into liquid that runs through the scrap to the bottom of the furnace. The electrodes are slowly lowered through the scrap, melting a hole as they go, a step called the bore-in period. Once a suffi - cient pool of molten steel has formed beneath the electrodes, the arc stabilizes above the liquid bath. Operators then may increase the current applied to the furnace. The remaining scrap melts as it contacts the ever-increasing molten bath. The arc between the electrode and the molten metal can reach as high as 15,000°C at its hottest region. Depending on the current applied and the arc hold-off length, the arc may be Fig. 2 — As shown here, molten pig iron can also be used to up to approximately six inches across and one foot long. The charge electric arc furnaces. EAF vessel is liquid cooled. Heat is introduced to the furnace at such a fast rate the differential expansion rates of the refractory material and the external steel structure would cause premature Supplying Power failure of the refractory material. The outside skin of the vessel is made of segmented steel panels. At points where heat damage Primary power from electrical utilities is generated in three would be severely detrimental, water-cooled copper panels are phases. This means there are three conductors, 120 degrees out used instead. Water cooling removes heat content from the fur - of phase with each other, providing electricity to the EAF. nace, but reduces expansion of the EAF. This reduction in Electricity moves through the grid at a high voltage and low expansion and contraction during furnace cycles increases the amperage. Amperage for this process is synonymous with heat. life of the refractory material and the frame. A power system designed to carry high amperage would have to One might initially think an electric furnace solely derives its use massive conductors and would not be cost effective. The heat energy from electricity; but that is not correct. An EAF fur - EAF must convert high-voltage, low-amperage power into high- nace derives its energy from a complex interaction of electrical amperage, low-voltage power. This is done in the transformer and chemical energy. An oxygen lance may be a part of the EAF room adjacent to the EAF. process. The introduction of oxygen over the heat allows carbon A step-up transformer changes the low amperage to high- in the molten steel to combine, forming CO. This exothermic amperage power. A transformer is actually a simple electrical reaction adds heat to the melt and helps reduce the required device consisting of a coil of a number of predetermined turns current supplied to the furnace. Since a high level of low-carbon and an inner iron core that may be moved up or down the inte - scrap steel is utilized in the charging of the furnace, carbon may rior of the coil. Power is transferred from the coil to the core via actually be added to the melt. The use of oxygen at levels high - induction. The higher the number of coils, the higher the er than necessary for oxidation to occur significantly reduces amperage output. Thus, the heat input of all three electrodes the amount of current supplied to the furnace. High oxygen lev - can be independently controlled. els do have a side effect though. Carbon monoxide and nitrous Typically there are three transformers, one for each leg of oxide are created, and these gases must be collected and treat - power. The high-amperage current is brought to the EAF ed through a fume system before being released as a benign gas through three massive, liquid-cooled cables. These cables or into the environment. bus bars carry the current out of the transformer room and Electrodes are made of graphite (carbon) and are typically across a movable arm to the top of the oven. The arm assembly more than 20 inches in diameter. The electrode is consumed can weigh up to 20 tons. Care must be taken to not induce cur - through sublimation at the electrode tip, and through oxidation rents in adjacent structural support members, as the intense along its heated sides. (Sublimation is the process where a sub - localized heating could cause catastrophic failure. The three stance changes directly from a solid to a gas without going electrodes are centered over the top of the furnace in a triangu - through a liquid phase.) To help minimize side wall oxidation lar pattern. Because of the configuration, this region is called and increase electrode life, a water spray on the electrode above the delta region. the furnace helps keep the electrode cooler, which slows down To charge the furnace, the roof and electrodes are lifted up the oxidation reaction and increases electrode life. and swung out of the way. A crane with a clam-shell-type bucket Once the scrap has completely melted, a sample is taken deposits scrap steel into the furnace. The roof and electrode from the furnace for chemical analysis. This is used to deter -
28 • INSPECTION TRENDS mine what elements need to be introduced to the bath and how much oxygen needs to be introduced to allow the chemical reac - tions necessary to occur. Slag is added to the furnace to help facilitate removal of elements detrimental to the properties of the steel being produced. These elements may include sulfur, phosphorus, and metallics such as aluminum and manganese. The slag is removed by slightly tilting the furnace and pouring off the slag through the slag door. Analysis of the bath is also conducted. If the bath meets the requirements, the furnace is ready for tapping.
Tapping the Furnace
Tapping the furnace is a critical step for the quality of the fin - ished steel. Some alloying elements and deoxidizers may be added. Also, as the refractory lining is slowly consumed during regular use, the tapping hole slowly increases in size. While this may seem minor, the larger hole decreases the amount of time it takes to empty the furnace and increases the likelihood of entrapping oxygen and slag during tapping. A slag cover may be placed in the furnace prior to tapping. The cover floats atop the molten metal but beneath the slag, thus separating the two. The cover is designed to impede the flow out the tap hole once the liquid level is low enough. This helps prevent slag from pouring into the ladle. A ladle is made from an outer shell of steel and is lined with a thick layer of refractory material that protects the steel from Fig. 3 — Small batches of high-alloy specialty steels are produced melting. However, if molten steel were introduced to a room- in amounts too small to be continuously cast. These ingots are temperate ladle, the refractory would act as a heat sink and being poured using the bottom pouring technique. This technique remove heat from the steel. This is detrimental because when is less likely to introduce slag and oxygen into the pour than were alloying elements are added, the temperature of the molten earlier methods. steel will decrease. Preserving the temperature during the alloy - ing and degassing process is desirable. Ladles typically will be preheated for 12 hours prior to their use. outlet on the bottom of the ladle. The tube is then lowered into When steel is tapped from the furnace, care is taken to the ingot mold. As the mold fills, the ladle is lifted to ensure the ensure the molten column going to the ladle is as short as pos - pour is as short as possible. This reduces atmospheric entrap - sible and that the pour is smooth. A long column and turbulent ment. If the steel is directed to the continuous caster, the ladle flow introduces higher levels of air into the molten steel. will be carried to a tundish, a vessel that holds molten steel for Traditionally, after the molten steel was poured into the ladle, an intermediate amount of time and controls the flow of molten oxygen and nitrogen bubbled back to the surface in a frothy, vio - metal into the continuous casting machine. The tundish is the lent manner. Steelmakers would add ferro-silicon or other ele - first step in the continuous casting process. ments to facilitate removal of the oxygen. When the reaction Steelmaking with the EAF process is a highly controlled between the oxygen and silicon was complete, the activity process that produces a high-quality product. As the steel indus - stopped, hence the derivative term “killed steel.” try evolves, more steel production utilizing the EAF process can Vacuum degassing is a modern improvement for removing be expected. By utilizing low-carbon scrap steel instead of pig oxygen and nitrogen from the liquid steel. There are several dif - iron, EAFs can produce a quality product competitively. As ferent methods of vacuum degassing. Electrodes may be used inspectors, we need to understand how steel, a material we often to introduce heat. A light slag is added. Alloying elements are take for granted, improves and evolves with technology. O charged to bring the levels to the specification the steelmaker desires. There are different ways to stir the molten steel. One Acknowledgments method utilizes argon bubbled through a porous plug on the bottom of the ladle to stir the steel. Argon is a noble gas; it will All photos are courtesy of the Timken Company. not combine with other elements and make detrimental chemi - cal changes to the steel. Another method uses electromagnetic stirring to mix the alloying elements. These two methods great - Bibliography ly improve the quality of the steel, making a more homogenous product as compared to traditional methods and reducing the 1. Cramb, A. 2003. The Making, Shaping and Treating of Steel likelihood of inherent defects. The ladle is encapsulated, and a 11th Edition — Casting Volume. Pittsburgh, Pa.: The AISE Steel vacuum is applied. The stirring action mixes the alloys and Foundation. brings gases to the surface where the vacuum environment 2. Freuhan, J. R. 1998. The Making, Shaping and Treating of removes them for treatment. The steel is now ready for either Steel 11th Edition — Steelmaking and Refining Volume. casting into ingots, or to be delivered to the continuous caster. Pittsburgh, Pa: The AISE Steel Foundation. Specialty tool steels and other high-alloy steels are made in 3. Linnert, G. 1994. Welding Metallurgy , Volume I. Hilton quantities too small to support a continuous caster. These steels Head Island, S.C.: GML Publications. are cast into ingots — Fig. 3. The highest quality ingots use the 4. Neely, J. E. 2000. Practical Metallurgy and Materials of bottom pouring technique in which a long tube is attached to an Industry. Upper Saddle River, N.J.: Prentice-Hall, Inc.
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