This was a simple method used from the Iron Age to medieval times where iron ore and charcoal were placed in a small furnace. This was initially little more than a hollow in the ground but later furnaces were typically about 3ft in diameter and 3ft high, constructed of clay or stone with a clay lining. Air was forced into the flames by bellows worked by hand or foot. This kept the charcoal burning but the air supply was kept low to make sure that the charcoal did not burn fully. The effect of this was to produce carbon monoxide gas and, as this passed over the iron ore, it removed the oxygen becoming carbon dioxide in the process. The molten iron separated from the rock it was in and the latter flowed to the bottom of the furnace and out through a hole to a hollow in the ground where it cooled and set to form slag. The iron was left behind in the furnace and, because it was dispersed throughout the mass of ore, it remained as a sponge-like lump called the bloom, about the size of a football.
It was removed from the furnace but still with some slag trapped in its internal spaces. This slag was removed by reheating the bloom to soften it, when it was hammered to consolidate the metal and to force out the slag. This was wrought iron that could then be forged to any required shape. The bloomery process could be carried out in a convenient location near the ore source. At a later date, waterwheel-powered hammering of the bloom increased efficiency and iron smelting became increasingly located where water power could be exploited.
This process was first developed in the 13th Century but did not rival bloomery production, which continued in most places. Pig iron was melted in a hearth that was narrow and deep. The hearth had a charcoal fire blown with bellows through a tuyere and, as the iron melted, the drops fell though the blast and congealed. They were then lifted with an iron bar into the blast. As they melted, they were caught on the end of a large staff, held in the fire and turned rapidly so that the drops spread out, forming a ball. The resultant ball was often forged into bar iron in a hammer mill. The Osmond Process was gradually replaced from the 15th Century by the Finery Process.
From the 15th Century, finery forges were introduced that used pig iron to produce bar iron. They consisted of two hearths. In the Finery Hearth, a Finer used charcoal to burn the carbon off the pig iron, producing a bloom. This was then consolidated using a water-powered hammer. It was then placed in the Chafery Hearth, where a Hammerman used either charcoal or coal to reheat the bloom and then beat it with a hammer to drive the molten slag out of it. He then drew the bloom out into a bar to produce what was known as bar iron.
The first record of a Blast Furnace in Britain is in 1496, when they were introduced into the Weald. They began to spread to other parts of the country during the 1550s and many were built thereafter. The Blast Furnaces made pig iron from iron ore and the basic principle was similar to that of the bloomery in that oxygen was removed from the iron oxide by carbon monoxide from burning charcoal. The smelting temperature was higher, however, and all of the iron and rock was melted. A “charge” of iron ore, coal and limestone was poured in from the top and is heated and dried by the hot gases being blown through the furnace from below. Lower down, the iron ore melts as the carbon starts to burn and from just below the middle of the furnace, molten iron drips down through the remaining fuel onto the hearth at the very bottom of the furnace. In the lower part of the furnace, the limestone acts as a flux and draws together many impurities together into a layer of slag. The air blast from the bellows is introduced a little way above the hearth and must be strong enough to stop the burning contents of the furnace stack dropping into the hearth but also not be so strong as to blow the contents out of the top. The slag is the lighter material so it floats on top of the molten iron and is drained through a hole called the “slag notch” by knocking out a clay plug. The iron was removed in a similar manner by the removal of a plug in the “tap hole” at the bottom of the furnace.
The blast furnace is more efficient than a bloomery because it permits continuous production. The furnace remains in operation throughout, with slag and metal being tapped off as required and more iron ore, limestone flux and fuel being added as necessary. The process also permits the scaling of the furnaces to almost any size, the larger furnaces giving even greater improvements in efficiency. Early furnaces were usually located on sloping ground, close to a stream. Water was used to drive the early bellows to create the draught, while the slope helped to provide a near level roadway onto the top of the furnace. Early blast furnaces used charcoal but from 1709 a major change took place when Abraham Darby used coke as a fuel at his Coalbrookdale furnaces. Within 100 years, the use of coke was almost universal. Another major improvement was In 1828, when James Neilson introduced the Hot Blast. In this, the waste gases from the furnace were burnt and used to preheat the air blown into the furnace. Fuel consumption was cut by one-third using coke or two-thirds using coal, while furnace capacity was also significantly increased. Medieval Blast Furnaces were about 10ft tall and made of fireproof brick, with forced air being provided by hand-operated bellows. Modern Blast Furnaces can be 120ft high, 45ft diameter and produce 10,000 tons per day.
The slitting mill was invented in Belgium in the 16th Century and the first one in Britain was built at Dartford in 1590. Others quickly followed throughout the country. It was basically a watermill for slitting bars of iron into rods. It consisted of two pairs of rolls turned by water wheels. A piece was cut off the end of a bar with shears, powered by one of the water wheels, and heated in a furnace. This was then passed between flat rolls which made it into a thick plate. It was then passed through the second rolls (known as cutters) which slit it into rods. The cutters had intersecting grooves, which sheared the iron lengthways. The rods then were then passed to nailers who made the rods into nails by giving them a point and head.
In 1614, William Ellyot and Mathias Meysey took out a patent for making steel, which had been developed on the Continent. They then transferred the patent to Sir Basil Brooke Brooke, who built two Cementation Furnaces on his land at Coalbrookdale. He probably used bar iron from the Forest of Dean, where he was a partner in an ironworks. Brooke was forced to surrender the patent in 1619, since a clause in it prohibited the import of steel but he had not been able to supply as much good steel as was needed.
Wrought iron bars were placed in the Cementation Furnace for conversion into cementation or blister steel. The furnace was constructed from sandstone in the form of a large chest called a “pot” with a lid. These were 14ft by 4ft and 3½ft deep and the whole lot was contained within a bottle shaped structure, similar to a pottery kiln, that sheltered the furnaces from the weather and acted as a chimney. Iron bars and charcoal were packed in alternating layers, with a top layer of charcoal and then refractory matter to make the pot or "coffin" airtight. Some manufacturers used a mix of powdered charcoal, soot and mineral salts, called “cement powder”. In larger works, up to 16 tons of iron was treated in each cycle.
Depending on the thickness of the iron bars, the pots were then heated from below for a week or more. Bars were regularly examined and when the correct condition was reached the heat was withdrawn and the pots were left until cool (usually around 14 days). The iron "gained" a little over 1% in mass from the carbon in the charcoal and became heterogeneous bars of blister steel. The bars were then shortened, bound, heated and hammered, pressed or rolled to become shear steel. Alternatively they could be broken up and melted in a crucible with a flux to become crucible steel or cast steel.
In 1678, Sir Clement Clerke introduced the Reverberatory Furnace (also called a Cupola or Open Hearth Furnace). It used coal as a fuel but the burning coal remained separate from the iron ore and thus did not contaminate it with impurities like sulphur and ash. Both were enclosed in a brick structure with an arched roof. The heat from the fire was reflected from the roof down onto the iron ore and the fumes escaped up a chimney.
In the 1740s, Benjamin Huntsman developed a process to produce high quality steel for cutting edge tools and springs. Blister steel bars were cut into small lengths and placed into a small cylindrical vessel made of fireclay called the “crucible”. After being fitted with a lid, the crucible was placed into a coke fire until the steel had completely melted. When removed from the fire, the crucible was tipped to empty the contents into moulds. While crucible steel was very high quality, it was also expensive but was still being produced up until the 1950s for specialist uses.
Potting and Stamping
In 1763, John Wood of Wednesbury and Charles Wood of Egremont patented a process to make bar ion from pig iron which became known as potting and stamping. Pig iron was melted in an oxidising atmosphere. The metal was then allowed to cool, broken up by stamping and washed. The granulated iron was then heated in pots in a reverberatory furnace and the resultant bloom was then drawn out under a forge hammer in the usual way. During the 14 year term of the patent, the process was hardly used except by the inventors. However, from shortly before the patent expired, it was adopted by many ironmasters in the West Midlands.
In 1783, Henry Cort developed a process called rolling. In this, the bloom was passed through grooved rollers and flat bars were produced. The bars of wrought iron were of poor quality, called “muck bars” or “puddle bars”. To improve the quality of wrought iron, the bars were cut up, piled and tied together by wires, a process known as faggoting or piling. They were then reheated and rolled again in merchant rolls. The process was repeated several times to get wrought iron of the desired quality. Wrought iron that has been rolled multiple times is called merchant bar or merchant iron. Rolling replaced hammering for consolidating wrought iron and expelling some of the dross as it was 15 times faster than hammering with a trip hammer.
In 1784, Henry Cort devised a method of producing wrought iron from cast iron using a coal fired Reverberatory Furnace. Solid cast iron was heated until it was molten, then the fire was damped down and the iron stirred to bring as much as possible in contact with the air. As wrought iron has a higher melting point than cast iron, if the temperature in the furnace was correct the iron began to solidify as the carbon was removed. Eventually the wrought iron could be worked into a single lump of iron in the centre of the hearth. This wrought iron was not usable in this form because of the slag within the lump so it was lifted from the furnace and forged using a “shingling hammer”. Finally it was rolled into bars or sheet. As most of the slag was squeezed out of the iron under the shingling hammer, this could be a dangerous job as with each drop of the hammer white hot slag would be sprayed across the forge. As the workmen had to hold and move the iron during the forging, there was no option other than for them to dress in heavy protective clothing. An improvement to Cort's process came from Joseph Hall in 1816. He added mill scale (iron oxide formed and broken off during forging and rolling) to the cast iron at the start of the puddling process. Once the iron had melted, the carbon monoxide formed by the mill scale bubbled up through the iron giving the impression of boiling, thus the common name for this refinement “pig boiling”.
In 1830, Gustav Ekman from Sweden introduced a new process in an attempt to allow the Swedish iron industry to compete with Britain. The Lancashire Hearth (so named because Ekman got the idea while visiting ironworks in Lancashire) consisted of a rectangular closed furnace with a 24ft high chimney at one end and a working arch in front of the hearth proper at the other. The hearth itself consisted of a rectangular box of iron plates, the bottom plate being water-cooled. Pig iron was loaded through a door at the foot of the chimney and stacked on an iron-clad bridge so that it could be heated by the waste gases from the hearth. Pre-heated air was blown into the hearth through a single water-cooled tuyere. Surplus slag was removed with a shovel but some was left to help the process. Pig iron stacked on the bridge at the back of the hearth was then pulled forward with a hook and charcoal added. The blast was then turned on and fining began. When the pigs began to melt, a bar was used to stir the iron and another one to lift it back into the blast. Periodically the tuyere had to be cleaned of matter sticking to it with a third bar. Finally, the iron was gathered into a “loop” which was lifted out of the hearth with a heavier bar and tongs, then taken to the shingling hammer. The process was more fuel-efficient and more productive than its predecessors.
In 1856, Henry Bessemer introduced a new steelmaking process, whereby air was blown through molten pig iron to produce mild steel. This made steel much more economical and led to the demise of wrought iron. The Bessemer Converter was a large steel vessel supported on pivots, with a single opening in the top in the form of a spout. The flat bottom had a number of openings through which air was blown during the conversion process. For filling (charging), the converter was tipped onto its back and molten iron poured in through the upturned spout. Scrap iron and steel were also added in solid form. It was then rotated to the upright working position and aligned to a chimney to carry the smoke and fumes out of the building. The conversion process began by increasing the air pressure applied until it was forced vigorously through the molten iron. Each batch took about 30 minute to process. Oxygen from the air blast combined with the carbon to form carbon monoxide, giving a spectacular display of sparks and flames shooting out of the spout.
The process could be judged very accurately by the size and colour of the flames thrown out. As the oxidisation of the carbon released heat, the steel became considerably hotter than the iron that was originally charged. Adding the solid scrap at the beginning of the process helped to keep the temperature under control. When ready, the air blast was reduced and the furnace was tilted for emptying (teaming). A major improvement was the Basic Bessemer Process, in which phosphorus was removed from the iron by using an alkaline lining in the vessel. This allowed the wide use of lower quality iron ores in producing steel for general and specialist use. The Converters produced between 5-30 tons of steel a time and were usually operated in pairs, one being blown while the other was filled or tapped. Bessemer set up his own steel company, which became one of the largest in the world. His process revolutionised steel manufacture by decreasing its cost from £40 per ton to £6 per ton, along with greatly increasing the scale and speed of production of this vital raw material. The process also decreased the labour requirements for steel-making. After 1890, the Bessemer process was gradually supplanted by open-hearth steelmaking and by the middle of the 20th Century was no longer in use.
Siemens-Martin Open Hearth Process
Carl Siemens developed the Siemens regenerative furnace in the 1850s and claimed in 1857 to be recovering enough heat to save 70–80% of the fuel previously used. This furnace operated at a high temperature by using regenerative preheating of fuel and air for combustion. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, but Siemens did not initially use it for that. In 1865, the French engineer Pierre-Emile Martin took out a license from Siemens and used the regenerative furnace for making steel. The process was known as the Siemens-Martin process and the furnace was called an "open-hearth" furnace. It is a batch process and each batch is called a "heat". The furnace is first loaded with light scrap metal and, once it has melted, heavy scrap is added, together with pig iron from blast furnaces. As soon as all the steel has melted, slag forming agents such as limestone are added. The oxygen in iron oxide and other impurities decarburize the pig iron by burning the carbon away, forming steel. To increase the oxygen contents of the heat, more iron ore can be added. The furnace is tapped in the same way as a blast furnace is tapped, ie a hole is drilled on the side of the hearth and the raw steel flows out. Once all of the steel has been tapped, the slag is skimmed away. The raw steel may be cast into ingots (a process called “teeming”) or used in continuous casting for the rolling mill. The process is far slower than a Bessemer Converter and thus easier to control and take samples for quality control. As it is slow, it is not necessary to burn all the carbon away, as in Bessemer process, but the process can be terminated at given point when desired carbon contents has been achieved. The regenerators are the distinctive feature of the furnace and consist of fire-brick flues filled with bricks set on edge and arranged in such a way as to have a great number of small passages between them. The bricks absorb most of the heat from the outgoing waste gases and return it later to the incoming cold gases for combustion.
In 1925, James Aston in America developed a process for manufacturing wrought iron quickly and economically. It involved taking molten steel from a Bessemer Converter and pouring it into cooler liquid slag. The temperature of the steel was about 1,500 °C and the liquid slag was maintained at approximately 1,200 °C. The molten steel contained a large amount of dissolved gas so, when the liquid steel hit the cooler surface of the liquid slag, the gases were liberated. The molten steel then froze to yield a spongy mass, having a temperature of about 1,370 °C. The spongy mass was then finished by being shingled and rolled as described under puddling. Each batch could convert 3-4 tons.
Electric Arc Furnace
The first electric arc furnace was developed by Paul Heroult in France, with a commercial plant established in the United States in 1907. Scrap metal is delivered to a scrap bay, located next to the melt shop. The scrap is loaded into large buckets called baskets, with "clamshell" doors for a base. It is placed on a light layer of shredded metal and another layer of shredded metal is placed on top. After loading, the basket is passed to a scrap pre-heater, which uses hot furnace gases to heat the scrap. The scrap basket is then taken to the melt shop, the roof is swung off the furnace and the furnace is loaded with scrap from the basket. This is one of the more dangerous operations for operators as liquid metal in the furnace is often displaced upwards and outwards by the solid scrap. If the grease and dust on the scrap is ignited, the result is like 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 with pre-heated scrap on a conveyor belt, which then discharges the scrap into the furnace proper.
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 metal 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. Oxygen is blown into the scrap, combusting or cutting the steel, and extra chemical heat is provided by wall-mounted oxygen-fuel burners. As soon as the temperature and chemistry are correct, the steel is tapped out into a preheated ladle by tilting the furnace. The whole process will usually take about 60–70 minutes from the tapping of one heat to the tapping of the next.