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.[1] Steel is also to be distinguished from wrought iron with little or no carbon, usually less than 0.035%. It is common talk today to talk about 'the iron and steel industry' as if it were a single thing; it is today, but historically they were separate products.

Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired. A more recent definition is that steels are iron-based alloys that can be plastically formed (pounded, rolled, etc.).

Iron and steel

Iron, like most metals, is not found in the Earth's crust in a native state. Iron can be found in the crust only in combination with oxygen or sulfur. Typically Fe2O3— the form of iron oxide (rust) found as the mineral hematite, and FeS2—Pyrite (fools gold). Iron oxide is a soft sandstone-like material with limited uses on its own. Iron is extracted from ore by removing the oxygen by combining it with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over 1000 °C, while tin melts around 250 °C. Both temperatures could be reached with ancient methods that have been used for at least 6000 years (since the Bronze Age). Since the oxidation rate itself increases rapidly beyond 800 °C, it is important that smelting take place in a fairly oxygen-free environment.

Contents

Iron alloy phases

• Austenite (?-iron; hard)
• Bainite
• Marten site
• Cementite (iron carbide; Fe3C)
• Ferrite (a-iron; soft)
• Pearlite (88% ferrite, 12% cementite)

Types of Steel

• Plain-carbon steel (up to 2.1% carbon)
• Stainless steel (alloy with chromium)
• HSLA steel (high strength low alloy)
• Tool steel (very hard; heat-treated)

Other Iron-based materials

Cast iron (>2.1% carbon)

Wrought iron (almost no carbon) ductile iron The old steel cable of a colliery winding tower Iron ore pellets for the production of steel Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, or allotropes, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure ferrite or a-iron, a fairly metallic material that can dissolve only a small concentration of carbon (no more than 0.021 wt% at 910 °C).

Above 910 °C ferrite undergoes a phase transition from body centered cubic to a face-centered cubic (FCC) structure, called austenite or ?-iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.03

Wt% carbon at 1154 °C)

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 Fe3C.

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 marten site, 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 Marten site. When the Austenite is quenched to form Marten site 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 vacancies and thus distort the cell structure into a Body Centered Tetragonal (BCT) structure.

Marten site 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 marten site, by contrast, occurs almost immediately, due to lower activation energy.

Marten site 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 marten site 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 its carbon content is high enough to produce a significant concentration of marten site, and is extremely hard but very brittle.

Often, steel undergoes further heat treatment at a lower temperature to destroy some of the marten site (by allowing enough time for cementite, etc., to form) 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 (often 18% and 8%, respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the Iron-carbon phase diagram, showing the conditions necessary to form different phases' formation of cementite, allowing marten site 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 (see Iron: History). 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 (1600 km) 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, 2000 BC increasing numbers of smelted iron objects (distinguishable from meteoric iron by their lack of nickel) appear in Anatolia, Egypt and Mesopotamia (see Iron: History).

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 Tutankhamen 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 was 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 axe head from Swedish Iron Age, found at Gotland, Sweden 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 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 small 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 (202 BC–AD 220), 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. (In Chinese, the process was called chao , literally, stir frying.)

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

Earliest evidence of steel making comes to us from Samanalawewa area in Sri Lanka where thousands of sites were found. (Ref. Juleff, 1996) These early furnaces were powered by Monsoon winds and have been dated to 300 B.C. using radiocarbon dating techniques. 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 In this system, high-purity wrought iron, charcoal, and glass were mixed in crucibles and heated until the iron melted and absorbed the carbon. The resulting high-carbon steel, called ?___ ( pulâd ) in Persian and wootz by later Europeans was exported throughout much of Asia. A solid pillar of rust-resistant steel forged in 4th century AD and now standing for many centuries next to the Qutab Minar in Delhi is a Testimony of the steel manufacturing skills of Indian artisans. The famous Damascus swords were possibly made of steel imported from India.

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 (Damascus steel). 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 micro segregation 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 (3 feet) tall, capable of smelting up to 150 kg (350 lb) 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 (16 feet) and smelting as much as 350 kg (750 lb) 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 (a city in modern-day Belgium) 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 (called cementation ) 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 (recently excavated) 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 (or Oregrounds) iron from Sweden. Although it was not understood at the time, the ore from the Dannemora mine (from which it was made) had very low phosphorus content compared to most ores (notably those in

England), 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 industrialized 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 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 Hands worth 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 Abbey dale 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 Abbey dale site. An improvement on crucible steel was the Cementation process.

Iron making 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 (without charcoal), 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

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 (Up to hundreds of tons per batch, compared to tens of tons for the Bessemer process), 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

Schematic drawing of a puddling furnace Schematic drawing of a Bessemer converter A huge vat of molten steel being poured into an Voest-Alpine works in 1952; a modification of the basic Bessemer process, it lances oxygen from above the steel (Instead of bubbling air from below), 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 ton was Produced in 1867, 500 thousand in 1870, 1 million in 1880 and 28 million by 1900. In 2005, total world crudeSteel production was 1,107.2 million metric tons (mmt) 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 (or is hardly 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 Open hearth furnace at the Jones and Laughlin Steel

Company, Pittsburgh, May 1942 Steel - 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 meltingIron, into which other metals could be thrown and mixed.