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Primary Steelmaking for Beginners

Steelmaking history

Steelmaking has existed for nearly a thousand years, with modern techniques introduced in the 19th century. The process of manufacturing steel involves removal of impurities such as sulfur and silicon, with introduction of chromium and nickel to produce different grades of steel.

The history of steel making is very old, with it being found in ancient China, India, Iran and even Rome. Before the invention of the Bessemer process in 1850s, steel was only produced in small quantities, suitable for demands of small cities and states. The industrial revolution fueled the need of large scale production, with the Bessemer the only method of making steel in such large volumes.

Primary steel making either involves the use of pig iron or converted into steel, or the use of electric arc furnaces to melt steel scrap and recycle the material.

Oxygen Steelmaking

For using pig iron, the basic oxygen steelmaking method is used. This involves blowing oxygen in a carbon heavy melted iron. The oxygen is blown inside the furnace using a hollow pipe called the lance. The lance is liquid cooled to prevent its melting and its mouth is placed a few feet above the surface of the molten iron. The pressurized oxygen reacts with the carbon. The reaction ignites the carbon and an exothermic reaction raises the temperature to around 1,600 Celsius. Burnt lime or dolomite is introduced, which reacts with other impurities such as silicon and forms a layer on the top of the molten liquid, called slag.

After the purification is complete, the vessel is tilted and the molten iron is poured into another ladle furnace, where other metals and chemicals are added (nickel, chromium etc.) and mixed to produce the exact grade of steel required.

Electric Arc Furnace

Electric arc furnaces are usually used for melting scrap iron and steel. The process involves the use of a lined vessel that is fed with scrap and a three graphite rods are lowered, touching the surface. A high voltage electrical current is passed, which creates arcs and produce heat, melting the steel. To assist the process, some pre melted steel may also be added prior to the arcing and sometimes even gas burners are used to bring the temperature up to speed.

As in all steel making, after the melting is complete, the vessel is tilted to remove the liquefied steel, ensuring that the impurities that are floating on top stay behind to be removed later.

Sources

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y

https://en.wikipedia.org/wiki/Steelmaking#cite_note-3

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

 

 

 

 

Electric Arc Furnace for Beginners

Long History of Electric Arc Furnace Steel Making

Electric Arc Furnaces (EAF) have been used since the 19th century to melt iron. Different attempts were made but the first successful electric arc furnace was developed and patented by James Burgess Readman in 1888. The furnace was specifically crafted for the production of phosphorus.

EAFs also played a pivotal role during World War 2, used primarily of the production of different steel alloys. After the war, the adoption of a mini mill concept integrated with EAFs had helped many European war ravaged countries to start production of electric steel.

Melting Metal with Graphite Electrodes

EAFs are basically a huge electrical circuit that produces heat to melt metal. The construction consists of heat and corrosive resistant vessel with a lid that has three graphite electrodes.

The vessel is “charged” with scrap, light metal pieces sandwiching heavy pieces. The electrodes are lowered and when they touch the metal, low electric voltage is passed. The arc is struck and the graphite electrodes press down, going into the scrap and the electrical energy creating enormous amounts of heat to melt the scrap metal.

As the electrodes bore further into the metal, the electrical voltage is increased since the arcs formed will not be able to touch the sides of the vessel and damaging it. The electrodes are then raised back slightly, allowing a space in which the molten metal can pool up easily.

Advantages of Electric Arc Furnace Steelmaking

Once the impurities that float on the top are removed, the vessel is tilted to pour out the purified liquid metal into pre heated ladles to be cooled off.

EAFs offer advantages over other methods as steel can be made from 100% scrap. The overall energy requirements from making steel from ores is minimal. Another advantage is that unlike ore production of steel, EAFs can be rapidly started or shut down, allowing for batch operations. There is not much wear and tear involved, with usually the graphite electrodes being worn away. The electrodes are manufactured in a modular way where more pieces of electrodes are added as the old ones erode.

 

Sources for Electric Arc Furnace Steelmaking 

United States Patent and Trademark office: %252Fnetahtml%252FPTO%252Fpatimg.htm

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

Modeling and Control of an Electric Arc Furnace, Benoit Boulet, Gino Lalli and Mark Ajersch, Centre for Intelligent Machines, McGill University, 3480 University Street, Montréal, Québec, Canada H3A 2A

Click to access TM18-4_as_published.pdf

http://www.industrialmetalcastings.com/foundries_electric_arc_furnace.html#:~:text=Advantages%3A%20Electric%20arc%20furnaces%20are,required%20to%20produce%20the%20steel.

https://en.wikipedia.org/wiki/Arc_welding

 

The Electric Arc Furnace – Part 4 (EAF vs the others)

This is the fourth part of our EAF-series. It is about the differences and communalities of EAF, Indiction Furnaces, Ladle furnaces, Plasma Furnaces & Vacuum Arc Furnaces.

EAF vs other electrical furnaces

Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc and the current in the furnace terminals passes through the charged material.

For steelmaking, direct current (DC) arc furnaces are used, with a single electrode in the roof and the current return through a conductive bottom lining or conductive pins in the base.

The advantage of DC is lower electrode consumption per ton of steel produced, since only one electrode is used, as well as less electrical harmonics and other similar problems.

The size of DC arc furnaces is limited by the current carrying capacity of available electrodes, and the maximum allowable voltage. Maintenance of the conductive furnace hearth is a bottleneck in extended operation of a DC arc furnace.

In a steel plant, a ladle furnace (LF) is used to maintain the temperature of liquid steel during processing after tapping from EAF or to change the alloy composition. The ladle is used for the first purpose when there is a delay later in the steelmaking process.

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. Unlike a scrap melting furnace, a ladle furnace does not have a tilting or scrap charging mechanism.[citation needed]

Electric arc furnaces are also used for production of calcium carbideferroalloys and other non-ferrous alloys, and for production of phosphorus. Furnaces for these services are physically different from steel-making furnaces and may operate on a continuous, rather than batch, basis.

Continuous process furnaces may also use paste-type, Søderberg electrodes to prevent interruptions due to electrode changes. Such a furnace is known as a submerged arc furnace because the electrode tips are buried in the slag/charge, and arcing occurs through the slag, between the matte and the electrode. A steelmaking arc furnace, by comparison, arcs in the open.

The key is the electrical resistance, which is what generates the heat required: the resistance in a steelmaking furnace is the atmosphere, while in a submerged-arc furnace the slag or charge forms the resistance. The liquid metal formed in either furnace is too conductive to form an effective heat-generating resistance.

Amateurs have constructed a variety of arc furnaces, often based on electric arc welding kits contained by silical blocks or flower pots. Though crude, these simple furnaces can melt a wide range of materials, create calcium carbide, etc.

A plasma arc furnace (PAF) uses plasma torches instead of graphite electrodes. Each of these torches consists of a casing provided with a nozzle and an axial tubing for feeding a plasma-forming gas (either nitrogen or argon), and a burnable cylindrical graphite electrode located within the tubing. Such furnaces can be referred to as “PAM” (Plasma Arc Melt) furnaces. They are used extensively in the titanium melt industry and similar specialty metals industries.[10]

Vacuum arc remelting (VAR) is a secondary remelting process for vacuum refining and manufacturing of ingots with improved chemical and mechanical homogeneity.

In critical military and commercial aerospace applications, material engineers commonly specify VIM-VAR steels. VIM means Vacuum Induction Melted and VAR means Vacuum Arc Remelted. VIM-VAR steels become bearings for jet engines, rotor shafts for military helicopters, flap actuators for fighter jets, gears in jet or helicopter transmissions, mounts or fasteners for jet engines, jet tail hooks and other demanding applications.

Most grades of steel are melted once and are then cast or teemed into a solid form prior to extensive forging or rolling to a metallurgically sound form. In contrast, VIM-VAR steels go through two more highly purifying melts under vacuum. 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.

The solidified ingots then head for a vacuum induction melting furnace. This vacuum remelting process rids the steel of inclusions and unwanted gases while optimizing the chemical composition. The VIM operation returns these solid ingots to the molten state in the contaminant-free void of a vacuum.

This tightly controlled melt often requires up to 24 hours. Still enveloped by the vacuum, the hot metal flows from the VIM furnace crucible into giant electrode molds. A typical electrode stands about 15 feet (5 m) tall and will be in various diameters. The electrodes solidify under vacuum.

For VIM-VAR steels, the surface of the cooled electrodes must be ground to remove surface irregularities and impurities before the next vacuum remelt. Then the ground electrode is placed in a VAR furnace. In a VAR furnace the steel gradually melts drop-by-drop in the vacuum-sealed chamber.

Vacuum arc remelting further removes lingering inclusions to provide superior steel cleanliness and further remove gases such as oxygen, nitrogen and hydrogen. Controlling the rate at which these droplets form and solidify ensures a consistency of chemistry and microstructure throughout the entire VIM-VAR ingot.

This in turn makes the steel more resistant to fracture or fatigue. This refinement process is essential to meet the performance characteristics of parts like a helicopter rotor shaft, a flap actuator on a military jet or a bearing in a jet engine.

For some commercial or military applications, steel alloys may go through only one vacuum remelt, namely the VAR. For example, steels for solid rocket cases, landing gears or torsion bars for fighting vehicles typically involve the one vacuum remelt.

Vacuum arc remelting is also used in production of titanium and other metals which are reactive or in which high purity is required.

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Sources

https://en.wikipedia.org/wiki/Electric_arc_furnace

Degner, M. et ali, Steel Institute VDeh (2008), Steel Manual, Düsseldorf Verlag Stahleisen GmbH.

httpswww.shutterstock.comdeimage-vectormetallurgy-iron-steel-production-electric-arc-1391332520

The Electric Arc Furnace – Part 3 (Operation)

This is the third of the series of EAF for steelmaking. It focuses on the operation part of EAF production.

Operation

A mid-sized modern steelmaking furnace would have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage between 400 and 900 volts and a secondary current in excess of 44,000 amperes. In a modern shop such a furnace would be expected to produce a quantity of 80 metric tonnes of liquid steel in approximately 50 minutes from charging with cold scrap to tapping the furnace.

In comparison, basic oxygen furnaces can have a capacity of 150–300 tonnes per batch, or “heat”, and can produce a heat in 30–40 minutes. Enormous variations exist in furnace design details and operation, depending on the end product and local conditions, as well as ongoing research to improve furnace efficiency. 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 metric tonnes and fed by eight 32MVA transformers for 256MVA total power.

To produce a ton of steel in an electric arc furnace requires approximately 400 kilowatt-hours per short ton or about 440 kWh per metric tonne; the theoretical minimum amount of energy required to melt a tonne of scrap steel is 300 kWh (melting point 1520 °C/2768 °F).

Therefore, a 300-tonne, 300 MVA EAF will require approximately 132 MWh of energy to melt the steel, and a “power-on time” (the time that steel is being melted with an arc) of approximately 37 minutes. Electric arc steelmaking is only economical where there is plentiful electricity, with a well-developed electrical grid.

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

The scrap is loaded into large buckets called baskets, with “clamshell” doors for a base. Care is taken to layer the scrap in the basket to ensure good furnace operation; heavy melt is placed on top of a light layer of protective shred, on top of which is placed more shred.

These layers should be present in the furnace after charging. After loading, the basket may pass to a scrap pre-heater, which uses hot furnace off-gases to heat the scrap and recover energy, increasing plant efficiency.

The scrap basket is then taken to the melt shop, the roof is swung off the furnace, and the furnace is charged with scrap from the basket. Charging is one of the more dangerous operations for the EAF operators.

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.

In some twin-shell furnaces, the scrap is charged into the second shell while the first is being melted down, and pre-heated with off-gas from the active shell. Other operations are continuous charging—pre-heating scrap on a conveyor belt, which then discharges the scrap into the furnace proper, or charging the scrap from a shaft set above the furnace, with off-gases directed through the shaft. Other furnaces can be charged with hot (molten) metal from other operations.

After charging, the roof is swung back over the furnace and meltdown commences. The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of shred at the top of the furnace.

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. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt.

This enables a molten pool to form more rapidly, reducing tap-to-tap times. Oxygen is blown into the scrap, combusting or cutting the steel, and extra chemical heat is provided by wall-mounted oxygen-fuel burners. Both processes accelerate scrap meltdown. Supersonic nozzles enable oxygen jets to penetrate foaming slag and reach the liquid bath.

Once the scrap has completely melted down and a flat bath is reached, another bucket of scrap can be charged into the furnace and melted down, although EAF development is moving towards single-charge designs.

After the second charge is completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping. More slag formers are introduced and more oxygen is blown into the bath, burning out impurities such as

Removal of carbon takes place after these elements have burnt out first, as they have a greater affinity for oxygen. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone, such as introducing the direct reduced iron and pig iron mentioned earlier.

A foaming slag is maintained throughout, and often overflows the furnace to pour out of the slag door into the slag pit. Temperature sampling and chemical sampling take place via automatic lances. Oxygen and carbon can be automatically measured via special probes that dip into the steel, but for all other elements, a “chill” sample—a small, solidified sample of the steel—is analysed on an arc-emission spectrometer.

Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace. For plain-carbon steel furnaces, as soon as slag is detected during tapping the furnace is rapidly tilted back towards the deslagging side, minimising slag carryover into the ladle.

For some special steel grades, including stainless steel, the slag is poured into the ladle as well, to be treated at the ladle furnace to recover valuable alloying elements. During tapping some alloy additions are introduced into the metal stream, and more lime is 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. During and after tapping, the furnace is “turned around”.

The slag door is cleaned of solidified slag, the visible refractories are inspected and water-cooled components checked for leaks, and electrodes are inspected for damage or lengthened through the addition of new segments; the taphole is filled with sand at the completion of tapping.

For a 90-tonne, medium-power furnace, the whole process will usually take about 60–70 minutes from the tapping of one heat to the tapping of the next (the tap-to-tap time).

The furnace is completely emptied of steel and slag on a regular basis so that an inspection of the refractories can be made and larger repairs made if necessary. As the refractories are often made from calcined carbonates, they are extremely susceptible to hydration from water, so any suspected leaks from water-cooled components are treated extremely seriously, beyond the immediate concern of potential steam explosions.

Excessive refractory wear can lead to breakouts, where the liquid metal and slag penetrate the refractory and furnace shell and escape into the surrounding areas.

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Sources

https://en.wikipedia.org/wiki/Electric_arc_furnace

Degner, M. et ali, Steel Institute VDeh (2008), Steel Manual, Düsseldorf Verlag Stahleisen GmbH.

httpswww.shutterstock.comdeimage-vectormetallurgy-iron-steel-production-electric-arc-1391332520