Steel is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels, stainless steels (>10.5%) and heat resisting CrNi steels (>18%). Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment.
The Fe-C Phase Diagram
The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature.
We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures.
Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite.
In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names:
A1, the so-called eutectoid temperature, which is the minimum temperature for austenite
A3, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary
Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C boundary
The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended holding close to A1.
Fig. 1. The Fe-Fe3C diagram.
The Fe-C diagram in Fig 1 is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop.
If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed. It suffices here to mention that
all important alloying elements decrease the eutectoid carbon content,
the austenite-stabilizing elements manganese and nickel decrease A, and
the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1.
Transformation Diagrams
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature. That is when the diffusion of carbon and alloying elements is suppressed or limited to a very short range.
Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing.
The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevated-temperature structure of austenite or austenite-carbide is also of importance in the heat treatment of steel.
One can conveniently describe what is happening during transformation with transformation diagrams. Four different types of such diagrams can be distinguished. These include:
Isothermal transformation diagrams describing the formation of austenite, which will be referred to as ITh diagrams
Isothermal transformation (IT) diagrams, also referred to as time-temperature-transformation (TTT) diagrams, describing the decomposition of austenite
Continuous heating transformation (CRT) diagrams
Continuous cooling transformation (CCT) diagrams
Isothermal Transformation Diagrams
This type of diagram shows what happens when a steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope.
ITh Diagrams (Formation of Austenite). During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume decreases with the formation of the dense austenite phase. From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived.
IT Diagrams (Decomposition of Austenite). The procedure starts at a high temperature, normally in the austenitic range after holding there long enough to obtain homogeneous austenite without undissolved carbides, followed by rapid cooling to the desired hold temperature. The cooling was started from 850°C (1560°F). The A1 and A3 temperatures are indicated as well as the hardness. Above A3 no transformation can occur. Between A1 and A3 only ferrite can form from austenite.
CRT Diagrams
In practical heat treatment situations, a constant temperature is not required, but rather a continuous changing temperature during either cooling or heating. Therefore, more directly applicable information is obtained if the diagram is constructed from dilatometric data using a continuously increasing or decreasing temperature.
Like the ITh diagrams, the CRT diagrams are useful in predicting the effect of short-time austenitization that occurs in induction and laser hardening. One typical question is how high the maximum surface temperature should be in order to achieve complete austenitization for a given heating rate. To high a temperature may cause unwanted austenite grain growth, which produces a more-brittle martensitic microstructure.
CCT Diagrams
As for heating diagrams, it is important to clearly state what type of cooling curve the transformation diagram was derived from.
Use of a constant cooling rate is very common in experimental practice. However, this regime rarely occurs in a practical situation. One can also find curves for so-called natural cooling rates according to Newton’s law of cooling. These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from the quenched end.
Close to the surface the characteristics of the cooling rare can be very complex. Each CCT diagram contains a family of curves representing the cooling rates at different depths of a cylinder with a 300 mm (12 in.) diameter. The slowest cooling rate represents the center of the cylinder. The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The M, temperature is unaffected.
Fig.2. CCT (a) and TTT (b) diagrams.
It should be noted, however, that transformation diagrams can not be used to predict the response to thermal histories that are very much different from the ones used to construct the diagrams. For instance, first cooling rapidly to slightly above Ms and then reheating to a higher temperature will give more rapid transformation than shown in the IT diagram because nucleation is greatly accelerated during the introductory quench. It should also be remembered that the transformation diagrams are sensitive to the exact alloying content within me allowable composition range.

THE ELECTRIC ARC FURNACE The electric arc furnace as the name suggests is a furnace in which heat is generated with the aid of electric arc produced by graphite electrodes. The main components of the electric arc furnace are the furnace shell with tapping device and work opening, the removable roof with the electrodes and a tilting device. The furnace shell is circular and with a refractory lining. The work opening and the tapping device are arranged opposite each other for tapping purposes, the complete furnace is tilted to an angle of about 42 degrees. Normally, the furnace is charged with its roof removed. When scrap is added, a charging bucket travels over the furnace, the bottom opens and the scrap is charged into the furnace within a few minutes. During the process, a control system advances the slow burning electrodes. High voltage is transformed into low voltage and high amperage. The most important parameter for the efficiency of an electric arc furnace is the "specific apparent power of the transformer" - in terms of 1 t of charge. Values range from 300 to 750 kVA/t (kilo-volt-ampere per tonne). In some cases, as much as 1,000 kVA/t has been installed. THE MELTING PROCESSThe electric arc furnace process generally follows the following pattern.
· Charging
· Melting
· Oxidising
· Deoxidising or refining
Besides scrap or sponge iron, the charge also includes the ores, fluxes (lime, flourspar), reducing agents (carbon) and alloying elements in the form of ferroalloys. These can be added through the work opening before or during oxidizing. Process begins with the ignition of the electric arc. After melting, further scrap can be added. An additional injection of oxygen or some other fuel-gas mixture can accelerate the melting phase. The maximum transferable electric power and the heat stability of the refractory lining determine the time needed for melting. The most up-to-date furnaces with a hi specific apparent power (UHP furnaces) achieve melting periods of about 40 to 60 minutes and tap-to-tap times of about 1.5 hours. During the refining stage, iron oxides included in the slag react with the carbon of the bath. This gives rise to the gaseous carbon monoxide, which causes the heat to boil, and rinses impurities such as phosphorus, hydrogen, nitrogen and non-metallic compounds from the heat. These impurities escape as gases or are included in the slag. Sulphur cannot be completely eliminated. The advantages of steelmaking in the electric arc furnace are :
· All possible grades of steel can be melted
· Low capital outlay
· The melting process can be programmed and automated
· Good efficiency
But it has some shortcomings as well. Because it uses scrap , the EAF route can only be used to produce steel grades with low purity requirements. Major new developments in steel making have taken place in EAF based steel making. Innovations such as DC arc technology, scrap preheating, post combustion, oxygen and carbon injection etc have led to a tremendous increase in productivity and a decrease in electric consumption
The first electric arc furnaces were developed by Paul Héroult, of France, with a commercial plant established in the United States in 1907. Initially "electric steel" was a specialty product for such uses as machine tools and spring steel. Arc furnaces were also used to prepare calcium carbide for use in carbide lamps.
In the 19th century, a number of men had employed an electric arc to melt iron. Sir Humphry Davy conducted an experimental demonstration in 1810; welding was investigated by Pepys in 1815; Pinchon attempted to create an electrothermic furnace in 1853; and, in 1878 - 79, Sir William Siemens took out patents for electric furnaces of the arc type. The Stessano electric furnace is an arc type furnace that usually rotates to mix the bath. The Girod furnace is similar to the Héroult furnace.
Different from the arc type of electrothermic furnace is the induction type furnace. The Kjellin furnace and the Röchling-Rodenhauser furnace are two. The Grönwall furnace produced steel at Trollhattan, in Scandinavia.
While EAFs were widely used in World War II for production of alloy steels, it was only afterwards that electric steelmaking began to expand. The low capital cost for a mini-mill - around US$140-200 per ton of annual installed capacity, compared with US$1,000 per ton of annual installed capacity for an integrated steel mill - allowed mills to be quickly set up in war-ravaged Europe, and also allowed them to successfully compete with the big United States steelmakers, such as Bethlehem Steel and U.S. Steel, for low-cost, carbon steel 'long products' (structural steel, rod and bar, wire and fasteners) in the U.S. market. When Nucor - now one of the largest steel producers in the U.S.[citation needed] - decided to enter the long products market in 1969, they chose to start up a mini-mill, with an EAF as its steelmaking furnace, soon followed by other manufacturers. Whilst Nucor expanded rapidly up and down the Eastern U.S., the companies that followed them into mini-mill operations concentrated on local markets for long products, where the use of an EAF allowed the plants to be flexible with production, according to local demand. This pattern was also followed in countries around the world, with EAF steel production primarily used for long products, while integrated mills, using blast furnaces and basic oxygen furnaces, cornered the markets for flat products - sheet steel and plate. In 1987, Nucor made the decision to expand into the flat products market, still using the EAF production route. The fact that an EAF uses scrap steel as feedstock, instead of raw iron, has impacted on the quality of the flat product made from EAF steel, because of the limited amount of control over the impurities that are contained within the scrap.
[edit] Construction
An electric arc furnace used for steelmaking consists of a refractory-lined vessel, usually water-cooled in larger sizes, covered with a retractable roof, and through which one or more graphite electrodes enter the furnace. The furnace is primarily split into three sections: the shell, which consists of the sidewalls and lower steel 'bowl'; the hearth, which consists of the refractory that lines the lower bowl; and the roof, which may be refractory-lined or water-cooled, and can be shaped as a section of a sphere, or as a frustum (conical section). The roof also supports the refractory delta in its centre, through which one or more graphite electrodes enter. The hearth may be hemispherical in shape, or in an eccentric bottom tapping furnace (see below), the hearth has the shape of a halved egg. In modern meltshops, the furnace is often raised off the ground floor, so that ladles and slag pots can easily be manoeuvered under either end of the furnace. Separate to the furnace structure is the electrode support and electrical system, and the tilting platform on which the furnace rests. Two configurations are possible: the electrode supports and the roof tilt with the furnace, or are fixed to the raised platform.
A typical alternating current furnace has three electrodes. Electrodes are round in section, and typically in segments with threaded couplings, so that as the electrodes wear, new segments can be added. The arc forms between the charged material and the electrode, and the charge is heated both by current passing through the charge and by the radiant energy evolved by the arc. The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders. The regulating system maintains an approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes while it melts. The mast arms holding the electrodes carry heavy busbars, which may be hollow water-cooled copper pipes, used to convey current to the electrode holders. Modern systems use 'hot arms', where the whole arm carries the current, increasing efficiency. These can be made from copper-clad steel or aluminium. Since the electrodes move up and down automatically for regulation of the arc, and are raised to allow removal of the furnace roof, heavy water-cooled cables connect the bus tubes/arms with the transformer located adjacent to the furnace. To protect the transformer from the heat of the furnace, it is installed in a vault.
The furnace is built on a tilting platform so that the liquid steel can be poured into another vessel for transport in the steel making process. The operation of tilting the furnace to pour off molten steel is called "tapping". Originally, all steelmaking furnaces had a tapping spout closed with refractory that washed out when the furnace was tilted, but often modern furnaces have an eccentric bottom tap-hole (EBT) to reduce inclusion of nitrogen and slag in the liquid steel, with the taphole set in the 'nose' of the egg-shaped hearth. Modern plants may have two shells with a single set of electrodes that can be transferred between the two; one shell preheats scrap while the other shell is utilised for meltdown. Other DC-based furnaces have a similar arrangement, but have electrodes for each shell and one set of electronics.
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 60 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 operations, depending on the end product and local conditions, as well as ongoing research to improve furnace efficiency - the largest furnace (in terms of tapping weight and transformer rating) is in Turkey, with a tap weight of 350 metric tonnes and a transformer of 350 MVA.
To produce a ton of steel in an electric arc furnace requires on the close order of 400 kilowatt-hours per short ton of electrical energy, or about 440kWh per metric tonne; the theoretical minimum amount of energy required to melt a tonne of scrap steel is 300kWh (melting point 1520°C/2768°F). Electric arc steelmaking is only economical where there is a plentiful supply of electric power, with a well-developed electrical grid.
[edit] Operation
Scrap metal is delivered to a scrap bay, located next to the melt shop. Scrap generally comes in two main grades: shred (scrap light enough to have been passed through a shredder) and heavy melt (large slabs and beams), along with some direct reduced iron (DRI) or pig iron for chemical balance. Some furnaces, however, melt almost 100% DRI.
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 to increase plant overall 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. There is a lot of energy generated by multiple 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 that coats the scrap is ignited if the furnace is hot, resulting in a fireball erupting out of the top of the furnace and the slag door. 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. Yet 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. In more modern furnaces, oxygen is also lanced into the scrap, combusting or cutting the steel and burning out carbon, and sometimes chemical heat is provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap meltdown.
An important part of steelmaking is the formation of slag, which floats on the surface of the molten steel. 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. For a furnace with basic refractories, which includes most carbon steel-producing furnaces, the usual slag formers are calcium oxide (CaO, in the form of burnt lime) and magnesium oxide (MgO, in the form of dolomite and magnesite). These slag formers are either charged with the scrap, or blown into the furnace during meltdown. Later in the heat, carbon (in the form of coke) is lanced into this slag layer, partially combusting to form 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, prevents damage to the furnace roof and sidewalls from radiant heat.
Once flat bath conditions are reached, i.e. the scrap has been completely melted down, often another bucket of scrap is charged into the furnace and melted down. 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 lanced into the bath, burning out impurities such as silicon, sulphur, phosphorus, aluminium, manganese and calcium and removing their oxides to the slag. 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 (in the form of a 'chill' - a small, solidified sample of the steel) take place via automatic lances.
Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace. As soon as slag is detected during tapping the furnace is rapidly tilted back towards the deslagging side, minimising slag entering the ladle. During tapping some alloy additions are introduced into the metal stream. 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, repairs may take place, 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).
[edit] Advantages of electric arc furnace for steelmaking
The precise control of chemistry and temperature encouraged use of electric arc furnaces during World War II for production of steel for shell casings. Today steelmaking arc furnaces produce many grades of steel, from concrete reinforcing bars and common merchant-quality standard channels, bars, and flats to special bar quality grades used for the automotive and oil industry.
A typical steelmaking arc furnace is the source of steel for a mini-mill, which may make bars or strip product. The steelmaking arc furnace is generally charged with scrap steel, though if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used for steelmaking.
[edit] Environmental issues
Although the modern electric arc furnace is a highly efficient recycler of steel scrap, operation of an arc furnace shop can have adverse environmental effects. Much of the capital cost of a new installation will be devoted to systems intended to reduce these effects, which include:
High sound levels
Dust and off-gas production
Slag production
Cooling water demand
Heavy truck traffic for scrap, materials handling, and products
Environmental effects of electricity generation
Because of the very dynamic quality of the arc furnace load, power systems may require technical measures to maintain the quality of power for other customers; flicker and harmonic distortion are common side-effects of arc furnace operation on a power system.
[edit] Other electric arc furnaces
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. However, the size of DC arc furnaces is limited by the available electrodes and 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 can be used to maintain the temperature of liquid steel during processing after tapping from the scrap-melting furnace. This also allows the molten steel to be kept ready for use in the event of a delay later in the steelmaking process. The ladle furnace consists of only the refractory roof and electrode system of a scrap-melting furnace, but it has no need for a tilting mechanism or scrap charging.
Electric arc furnaces are also used for production of 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 (Soderberg) electrodes to prevent interruptions due to electrode changes. Such furnaces are usually known as submerged arc furnaces, 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 are capable of melting a wide range of materials and creating calcium carbide etc.
Foamy Slag Practice
The application of the foaming-slag practice to steel manufacture at the electric-arc furnace (EAF) brings benefits in terms of lower electric energy, refractory, electrode consumption and lower noise.The problems of efficient generation of foaming slags include chemical and physical conditions and control of foaming intensity. This latter aspect is critical especially if the foaming slag technology is applied to the manufacture of stainless steels. In this case, a reduced foaming of slag results in a prolonged exposure of furnace walls to radiation from the electric arc with the consequent inevitable damage of the refractory lining. This paper describes a method for controlling slag foaming as applied to the stainless steel making at EAF no. 4 of Acciai Speciali Terni, which produces approx. 800000 t/year of austenitic and ferritic stainless steels. The paper examines the critical aspects involved in the generation of foaming slags for high-chromium steels, the results obtained and technique applied to detect the "arc covered" and "arc uncovered" condition. A device carried out detection of the arc status by two optic sensors of different sensitivity (namely ultraviolet and infrared) and two different modes of signal processing. The system has demonstrated to have an efficient discriminating capability in both the cases and can advise operators whether actions should be taken to recover the electric arc coverage.