Encyclopedia
Steel is a
metal alloy whose major component is
iron, with
carbon content between 0.02% and 1.7% by weight. Carbon is the most cost effective alloying material for iron, but many other alloying elements are also used. Carbon and other elements act as a hardening agent, preventing
dislocations in the iron atom
crystal lattice from sliding past one another. Varying the amount of alloying elements and their distribution in the steel controls qualities such as the
hardness, elasticity, ductility, and
tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. The maximum solubility of carbon in iron is 1.7% by weight, occurring at 1130° Celsius; higher concentrations of carbon or lower temperatures will produce cementite which will reduce the material's strength. Alloys with higher carbon content than this are known as
cast iron because of their lower melting point.. As carbon-rich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, and resulting in a cementite-ferrite mixture. Cementite is a stoichiometric phase with the chemical formula of Fe
3C. Cementite forms in regions of higher carbon content while other areas revert to ferrite around it. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as
pearlite due to its
pearl-like appearance, or the similar but less beautiful
bainite.
Perhaps the most important allotrope is
martensite, a chemically
metastable substance with about four to five times the strength of ferrite. A minimum of 0.4 wt% of carbon is needed in order to form martensite. When the austenite is quenched to form martensite, the carbon is "frozen" in place when the cell structure changes from FCC to BCC. The carbon atoms are much too large to fit in the interstitial vaccancies and thus distort the cell structure into a Body Centered Tetragonal structure. Martensite and austenite have an identical chemical composition. As such, it requires extremely little thermal
activation energy to form.
The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in
water or
oil, cooling it so rapidly that the transformation to ferrite or pearlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy.
Martensite has a lower density than austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, these internal stresses can cause a part to shatter as it cools; at the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.
At this point, if the carbon content is high enough to produce a significant concentration of martensite, the result is an extremely hard but very brittle material. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fracture-resistant metal. Because time is so critical to the end result, this process is known as tempering, which forms tempered steel.
Other materials are often added to the iron-carbon mixture to tailor the resulting properties.
Nickel and
manganese in steel add to its tensile strength and make austenite more chemically stable,
chromium increases the hardness and melting temperature, and vanadium also increases the
hardness while reducing the effects of
metal fatigue. Large amounts of chromium and nickel are added to
stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand
sulfur,
nitrogen, and
phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing.
When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be "worked" at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc. It is then heat-treated to produce a desirable crystal structure, and often "cold worked" to produce the final shape. In modern steelmaking these processes are often combined, with ore going in one end of the
assembly line and finished steel coming out the other. These can be streamlined by a deft control of the interaction between work hardening and tempering.
History of iron and steelmaking
Iron was in limited use long before it became possible to smelt it. The first signs of iron use come from
Ancient Egypt and
Sumer, where around 4000 BC small items, such as the tips of
spears and ornaments, were being fashioned from iron recovered from
meteorites . About 6% of
meteorites are composed of an iron-
nickel alloy, and iron recovered from meteorite falls allowed ancient peoples to manufacture small numbers of iron artifacts.
Meteoric iron was also fashioned into tools in precontact
North America. Beginning around the year 1000, the
Thule people of
Greenland began making
harpoons and other edged tools from pieces of the Cape York meteorite. These artifacts were also used as trade goods with other Arctic peoples: tools made from the Cape York meteorite have been found in archaeological sites more than 1000 miles away. When the
American polar explorer
Robert Peary shipped the largest piece of the meteorite to the
American Museum of Natural History in
New York City in 1897, it still weighed over 33 tons.
The name for iron in several ancient languages means "sky metal" or something similar. In distant antiquity, iron was regarded as a precious metal, suitable for royal ornaments.
The Iron Age
Beginning between 3000 BC to 2000 BC increasing numbers of smelted iron objects appear in
Anatolia,
Egypt and
Mesopotamia . The oldest known samples of iron that appear to have been smelted from
iron oxides are small lumps found at copper-smelting sites on the
Sinai Peninsula, dated to about 3000 BC. Some iron oxides are effective fluxes for copper smelting; it is possible that small amounts of metallic iron were made as a by-product of copper and bronze production throughout the Bronze Age.
In
Anatolia, smelted iron was occasionally used for ornamental weapons: an iron-bladed dagger with a bronze hilt has been recovered from a Hattic tomb dating from 2500 BC. Also, the
Egyptian ruler
Tutankhamun died in 1323 BC and was buried with an iron dagger with a golden hilt. An
Ancient Egyptian sword bearing the name of pharaoh
Merneptah as well as a
battle axe with an iron blade and gold-decorated bronze haft were both found in the excavation of
Ugarit . The early
Hittites are known to have bartered iron for
silver, at a rate of 40 times the iron's weight, with
Assyria.
Iron did not, however, replace bronze as the chief metal used for weapons and tools for several centuries, despite some attempts. Working iron required more fuel and significantly more labor than working bronze, and the quality of iron produced by early smiths may have been inferior to bronze as a material for tools. Then, between 1200 and 1000 BC, iron tools and weapons displaced bronze ones throughout the near east. This process appears to have begun in the
Hittite Empire around 1300 BC, or in
Cyprus and southern
Greece, where iron artifacts dominate the archaeological record after 1050 BC.
Mesopotamia was fully into the Iron Age by 900 BC, central Europe by 800 BC. The reason for this sudden adoption of iron remains a topic of debate among archaeologists. One prominent theory is that warfare and mass migrations beginning around 1200 BC disrupted the regional tin trade, forcing a switch from bronze to iron.
Egypt, on the other hand, did not experience such a rapid transition from the bronze to iron ages: although Egyptian smiths did produce iron artifacts, bronze remained in widespread use there until after Egypt's conquest by
Assyria in 663 BC.
Iron smelting at this time was based on the bloomery, a furnace where
bellows were used to force air through a pile of iron ore and burning
charcoal. The
carbon monoxide produced by the charcoal reduced the iron oxides to metallic iron, but the bloomery was not hot enough to melt the iron. Instead, the iron collected in the bottom of the furnace as a spongy mass, or
bloom, whose pores were filled with ash and slag. The bloom then had to be reheated to soften the iron and melt the slag, and then repeatedly beaten and folded to force the molten slag out of it. The result of this time-consuming and laborious process was
wrought iron, a malleable but fairly soft alloy containing little carbon.
Wrought iron can be
carburized into a mild steel by holding it in a charcoal fire for prolonged periods of time. By the beginning of the Iron Age, smiths had discovered that iron that was repeatedly reforged produced a higher quality of metal. Quench-hardening was also known by this time. The oldest quench-hardened steel artifact is a knife found on
Cyprus at a site dated to 1100 BC.
Developments in China
Archaeologists and historians debate whether bloomery-based ironworking ever spread to China from the Middle East. Around 500 BC, however, metalworkers in the southern state of Wu developed an iron smelting technology that would not be practiced in Europe until late medieval times. In Wu, iron smelters achieved a temperature of 1130°C, hot enough to be considered a
blast furnace. At this temperature, iron combines with 4.3% carbon and melts. As a liquid, iron can be
cast into
molds, a method far less laborious than individually forging each piece of iron from a bloom.
Cast iron is rather brittle and unsuitable for striking implements. It can, however, be
decarburized to steel or wrought iron by heating it in air for several days. In China, these ironworking methods spread northward, and by 300 BC, iron was the material of choice throughout China for most tools and weapons. A mass grave in
Hebei province, dated to the early third century BC, contains several soldiers buried with their weapons and other equipment. The artifacts recovered from this grave are variously made of wrought iron, cast iron, malleabilized cast iron, and quench-hardened steel, with only a few, probably ornamental, bronze weapons.
During the
Han Dynasty , Chinese ironworking achieved a scale and sophistication not reached in the West until the eighteenth century. In the first century, the Han government established ironworking as a state monopoly and built a series of large blast furnaces in
Henan province, each capable of producing several tons of iron per day. By this time, Chinese metallurgists had discovered how to
puddle molten pig iron, stirring it in the open air until it lost its carbon and became wrought iron.
Also during this time, Chinese metallurgists had found that wrought iron and cast iron could be melted together to yield an alloy of intermediate carbon content, that is, steel. According to legend, the sword of
Liu Bang, the first Han emperor, was made in this fashion. Some texts of the era mention "harmonizing the hard and the soft" in the context of ironworking; the phrase may refer to this process.
Steelmaking in India and Sri Lanka
Perhaps as early as 300 BC, although certainly by AD 200, high quality steel was being produced in southern
India also by what Europeans would later call the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in crucibles and heated until the iron melted and absorbed the carbon. One of the earliest evidence of steel making comes to us from Samanalawewa area in
Sri Lanka where thousands of sites were found. .
Export
The resulting high-carbon steel, called ????? in
Persian and
wootz by later Europeans, was exported throughout much of Asia. The famous
Damascus swords were possibly made of steel imported from India.
Delhi iron pillar
A solid
pillar of curiously rust-resistant iron—often mistakenly characterized as being made of steel— forged or cast in the
4th century AD, and which has stood for many centuries next to the
Qutab Minar in the
Qutb complex in
Delhi, is a testimony of the metallurgical skills of Indian artisans. The metal is variously described as cast iron or wrought iron. Its resistance to oxidation is theorized to be due to the formation of a protective
patina catalyzed by the a residue of phosphorus in the ore.
Steelmaking in the Middle East
By the
9th century, smiths in the
Abbasid caliphate had developed techniques for forging wootz to produce steel blades of unusual flexibility and sharpness .
has established strong evidence supporting the theory that the distinct surface patterns on Damascus steel blades result from a carbide-banding phenomenon produced by the microsegregation of minor amounts of carbide-forming elements present in the wootz ingots from which the blades were forged. Further, it is likely that wootz Damascus blades with damascene patterns may have been produced only from wootz ingots supplied from those regions of India having appropriate impurity-containing ore deposits.
Ironworking in medieval Europe
The middle ages in Europe saw the construction of progressively larger bloomeries. By the 8th century, smiths in northern
Spain had developed a style that become known as a Catalan forge, a furnace about 1 meter tall, capable of smelting up to 150 kg of iron in each batch. In succeeding centuries, smiths in the
Frankish empire and later the
Holy Roman Empire scaled up this basic design, increasing the height of the flue to as tall as 5 meters and smelting as much as 350 kg of iron in each batch. These larger furnaces required more draft than could be provided by human power, and forging the large blooms that resulted was also beyond the capabilities of a single man. To this end,
waterwheels were employed to power the bellows and hammers.
Eventually, the scaling up of the bloomery reached a point where the furnace was hot enough to produce cast iron. Although the brittle cast iron may initially have been a nuisance to the smith, as it was too brittle to be forged, the spread of
cannons to Europe in the 1300s provided an application for iron casting: cast iron cannonballs.
The oldest known blast furnace in Europe was constructed at Lapphyttan in
Sweden, sometime between 1150 and 1350. Other early European blast furnaces were built throughout the
Rhine valley: blast furnaces were in operation near Liège in the 1340s, and at Massevaux in
France by 1409.
The first English blast furnace was not built until 1491, when Queenstock furnace was built at Buxted, followed by one commissioned
Henry VII at Newbridge, in 1496 in a part of
Sussex known as the Weald. Despite this late start, the production of English iron foundries rapidly grew, in no small part due to foreign craftsmen hired by Henry to bring the craft of iron casting to England. In 1543, William Levett, an English rector who doubled as a Wealden ironmaster , and Peter Baude, a
French craftsman in
Henry VIII's employ, cast the Weald's first one-piece iron cannon. English iron cannons gained a reputation for being superior to, and less expensive than, the bronze cannons made elsewhere in Europe, and at least initially, efforts to copy them outside the Weald failed. The superiority of English cannons over Spanish ones has been credited as one factor in England's 1588 defeat of the
Spanish Armada.
In 1619, Jan Andries Moerbeck, a
Dutch ironmaster, began importing Wealden iron ore for comparison to the ore available on the Continent. One difference he observed was that the English ore contained some calcareous material, and soon after, Dutch ironmasters introduced the use of limestone as a flux in the blast furnace. This practice improved the separation of slag from the cast iron and improved the quality of Continental cast iron.
Steelmaking in early modern Europe
In the early
17th century, ironworkers in western Europe had found a means to carburize wrought iron. Wrought iron bars and charcoal were packed into stone boxes, then held at a red heat for up to a week. During this time, carbon diffused into the iron, producing a product called
cement steel or
blister steel . One of the earliest places where this was used in England was at
Coalbrookdale, where Sir Basil Brooke had two cementation furnaces . For a time in the 1610s, he owned a patent on the process, but had to surrender this in 1619. He probably used Forest of Dean iron as his raw material.
Soon after that it was found that the best steel could only be produced by buying expensive örgrund iron from
Sweden. Although it was not understood at the time, the ore from the Dannemora mine had very low phosphorus content compared to most ores , which allowed for a finer and stronger crystal structure. Sales of Swedish iron generated considerable trade income, and local development helped the country become the industrialised nation it remains to this day. This Swedish iron provided the main basis for English steelmaking until the 1850s
Benjamin Huntsman in the 1740s found a method of producing a more homogeneous steel. This was done by melting pieces of blister steel in crucibles. This was cast into ingots of crucible steel. He made this discovery at
Handsworth in
England. While producing steel superior to cement steel, the crucible steel process remained relatively expensive in both time and fuel, and could not be used in any sort of modern industrial scale. The strong steels produced were however in high demand for specialty products such as
cutlery and weapons.
Sheffield's
Abbeydale Industrial Hamlet has preserved a waterwheel powered, scythe-making works dating from Huntsman's times. It is still operated for the public, several times per year, using crucible steel made on the Abbeydale site. An improvement on crucible steel was the Cementation process.
References
- K. Barraclough, Steelmaking before Bessemer .
- P. King, 'The cartel in oregrounds iron' Journal of Industrial History 6 , 25-48.
Ironmaking in early modern Europe
From the
16th century to the
18th century, most iron was made by a two-stage process involving a
blast furnace and finery forge, using charcoal as fuel. Production was however limited by the supply of wood for making charcoal.
By the 18th century,
deforestation in western Europe was making ironworking and its charcoal-hungry processes increasingly expensive. In 1709 Abraham Darby began smelting iron using coke, a refined
coal product, in place of charcoal at his
ironworks at
Coalbrookdale in
England. Although coke could be produced less expensively than charcoal, coke-fired iron was initially of inferior quality compared to charcoal-fired iron. It was not until the 1750s, when Darby's son, also called Abraham, managed to start selling coke-smelted
pig iron for the production of wrought iron in finery forges.
Another 18th century European development was the invention of the
puddling furnace. In particular, the form of coal-fired puddling furnace developed by the British ironmaster Henry Cort in 1784 made it possible to convert cast iron into wrought iron in large batches , rendering the ancient finery forge obsolescent. Wrought iron produced using this method became a major raw material in the English midlands' iron manufacturing trades.
Industrial steelmaking
The problem of mass-producing steel was solved in 1855 by
Henry Bessemer, with the introduction of the
Bessemer converter at his steelworks in
Sheffield,
England. . In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and then air was blown through the molten iron from below, igniting the dissolved carbon from the coke. As the carbon burned off, the melting point of the mixture increased, but the heat from the burning carbon provided the extra energy needed to keep the mixture molten. After the carbon content in the melt had dropped to the desired level, the air draft was cut off: a typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour.
In 1867, the
German-
British engineer
Sir William Siemens introduced an improved puddling furnace – the regenerative furnace – that used brick
heat exchangers to preheat the incoming air and conserve fuel. The next year Pierre and Émile Martin, French ironmasters who had licensed Siemens' furnace design, developed a method for measuring the carbon content of molten iron. Thus, the decarburization could be stopped at the steel stage rather than proceeding all the way to wrought iron. This open-hearth process coexisted in industrial practice with the Bessemer process for many years, but eventually proved more economical and displaced it. Reasons for this include its ability to
recycle scrap metal in addition to fresh pig iron, its greater scalability , and the more precise quality control it permitted.
Initially, only ores low in phosphorus and sulfur could be used for quality steelmaking; ores rich in those elements yielded brittle metals little better than cast iron. This problem was solved in 1878 by Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas at the ironworks at Blaenavon in
Wales. Their modified Bessemer process used a converter lined with
limestone or
dolomite, and additional lime was added to the molten metal as a
Flux. This added basic material removed phosphorus and sulfur from the steel as insoluble calcium or magnesium phosphates and sulfates. This development expanded the range of iron ores that could be used to make steel, especially in
France and
Germany, where high-phosphorus ores abounded.
Finally, the basic oxygen process was introduced at the Voest-Alpine works in 1952; a modification of the basic Bessemer process, it lances oxygen from above the steel , reducing the amount of nitrogen uptake into the steel. The basic oxygen process is used in all modern steelworks; the last Bessemer converter in the U.S. was retired in 1968. Furthermore, the last three decades have seen a massive increase in the mini-mill business, where scrap steel only is melted with an
electric arc furnace. These mills only produced bar products at first, but have since expanded into flat and heavy products, once the exclusive domain of the integrated steelworks.
These developments increased the availability and decreased the price of steel; 22 thousand tonnes were produced in 1867, 500 thousand in 1870, 1 million in 1880 and 28 million by 1900. In 2005, total world crude steel production was 1,107.2 million metric tons . The were China , Japan and the United States .
Until these
19th century developments, steel was an expensive commodity and only used for a limited number of purposes where a particularly hard or flexible metal was needed, as in the cutting edges of tools and springs. The widespread availability of inexpensive steel powered the second industrial revolution and modern society as we know it. Mild steel ultimately replaced
wrought iron for almost all purposes, and
wrought iron is not now made. With minor exceptions, alloy steels only began to be made in the late 19th century.
Stainless steel was only developed on the eve of the
First World War and only began to come into widespread use in the
1920s. These alloy steels are all dependent on the wide availability of inexpensive iron and steel and the ability to alloy it at will.
Steel is currently the most recycled material in the world, the industry estimates that of new metal produced each year some 42.3% is recycled material. All steel that is available is currently recycled, the long service life of steel in applications such as construction means that there is a vast 'store' of steel in use that is recycled as it becomes available. But new metal derived from raw materials is also necessary to make up demand.
Types of steel
Alloy steels were known from antiquity, being
nickel-rich iron from
meteorites hot-worked into useful products. In a modern sense, alloy steels have been made since the invention of furnaces capable of melting iron, into which other metals could be thrown and mixed.
Historic types
- Damascus steel, which was famous in ancient times for its durability and ability to hold an edge, was created from a number of different materials , essentially a complicated alloy with iron as main component.
- Blister steel - steel produced by the cementation process
- Crucible steel - steel produced by Benjamin Huntsman's crucible technique
- Styrian Steel, also called 'German steel' or 'Cullen steel' was made in Styria in Austria by fining cast iron from certain manganese-rich ores.
- Shear steel was blister steel that was broken up, faggotted, heated and welded to produce a more homogeneous product
Contemporary steel
- Carbon steel, composed simply of iron and carbon accounts for 90% of steel production.LA steel]] have small additions of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.
- Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections.
- Stainless steels and surgical stainless steels contain a minimum of 10% chromium, often combined with nickel, to resist corrosion . Some stainless steels are nonmagnetic.
- Tool steels are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening, allow precipitation hardening and improve temperature resistance.
- Advanced High Strength Steels
- Complex Phase Steel
- Dual Phase Steel
- TRIP steel
- TWIP steel
- Maraging steel
- Eglin Steel
- Ferrous superalloys
- Hadfield steel or Manganese steel, this contains 12-14% manganese which when abraded forms an incredibly hard skin which resists wearing. Some examples are tank tracks, bulldozer blade edges and cutting blades on the jaws of life
...
.
Though not an alloy, there exists also galvanized steel, which is steel that has gone through the chemical process of being hot-dipped or electroplated in zinc for protection against rust. Finished steel is steel that can be sold without further work or treatment.
Modern steel
- TMT Steel : Thermo mechanically treated Steel: It is one of the latest developements in the history of Steel. The steel manufacturing process is improved & thereby the properties of this steel to suit to the RCC Construction work has been achieved. The steel wires are passed through the cold water just after drawing from the extruder. This helps in rapid cooling of the skin & the heat starts flowing from center to the skin once the wire is out from water. This acts as a heat treatment process on the steel. The relatively soft core helps in ductility of the steel while treated skin has good weldability to suit to the construction requirements.
Production methods
Historical methods
- bloomery
- pattern welding
- catalan forge
- wootz steel : developed in India, used in the Middle East where it was known as Damascus steel.
- Cementation process used to convert bars of wrought iron into blister steel. This was the main process used in England from the early 17th century.
- crucible technique, similar to the wootz steel, independently redeveloped in Sheffield
...
by Benjamin Huntsman in c.1740, and Pavel Anosov in
Russia in 1837. Huntsman's raw material was blister steel.
Modern methods
- Bessemer process, the first large-scale steel production process for mild steel.
- The Siemens-Martin process, using an Open hearth furnace
- Basic oxygen steelmaking
Uses of steel
Historically
Steel was expensive and was only used where nothing else would do, particularly for the cutting edge of knives, razors, swords, and other tools where a hard sharp edged was needed. It was also used for springs, including those used in clocks and watches.
Since 1850
Steel has been easier
