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Steel

2007 Schools Wikipedia Selection. Related subjects: Materials science

   The old Steel cable of a colliery winding tower
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   The old Steel cable of a colliery winding tower

   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. Steel is also to be distinguished from wrought iron with little
   or no carbon, usually less than 0.035%. It is common 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 alloy phases

   Austenite (γ-iron; hard)
   Bainite
   Martensite
   Cementite (iron carbide; Fe[3]C)
   Ferrite (α-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
   Iron ore pellets for the production of steel
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   Iron ore pellets for the production of steel

   Iron, like most metals, is not found in the Earth's crust in an
   elemental state. Iron can be found in the crust only in combination
   with oxygen or sulfur. Typically Fe[2]O[3]—the form of iron oxide
   (rust) found as the mineral hematite, and FeS[2]—Pyrite (fool's 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. Steel
   melts at around 1370 °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 low-oxygen
   environment. 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.
   Iron-carbon phase diagram, showing the conditions necessary to form
   different phases
   Enlarge
   Iron-carbon phase diagram, showing the conditions necessary to form
   different phases

   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
   α-iron, a fairly soft 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 γ-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 Fe[3]C. 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 (BCT) 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 (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 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 (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.

   Presently iron is the most recycled substance on the planet.
   Iron axehead from Swedish Iron Age, found at Gotland, Sweden
   Enlarge
   Iron axehead from Swedish Iron Age, found at Gotland, Sweden

The Iron Age

   Beginning between 3000 BC to 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 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 (see 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 ( 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

   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. (Juleff, 1996).

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 (see
   cementation process). 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.

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
   (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.
   Schematic drawing of a puddling furnace
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   Schematic drawing of a puddling furnace
   Schematic drawing of a Bessemer converter
   Enlarge
   Schematic drawing of a Bessemer converter

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. (An early converter can still be seen
   at the city's Kelham Island Museum). 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.

   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 (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.

   Enlarge

   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 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.

   Here is a table of the types of steel:
   http://claymore.engineer.gvsu.edu/eod/material/material-4.gif

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 (some only in traces), 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' (being
       traded through Cologne) was made in the Styria in Austria (Roman
       province of Noricum) 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.
     * HSLA steel (high strength, low alloy) have small additions (usually
       <2% by weight) 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 (
       rust). 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 (after Sir Robert Hadfield) 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 developments in the history of Steel. The steel
       manufacturing process is improved and thereby the properties of
       this steel to suit to the RCC Construction work has been achieved.
       The steel wires are passed through cold water just after drawing
       from the extruder. This helps in rapid cooling of the skin and heat
       starts flowing from the centre to the skin once the wire is out of
       the water. This acts as a heat treatment. The relatively soft core
       helps in ductility of the steel while the treated skin has good
       weldability to suit construction requirements.

Production methods

Historical methods

     * bloomery
     * pattern welding
     * catalan forge
     * wootz steel ( crucible technique): 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.
     * Puddling

Modern methods

     * Electric arc furnace a form of secondary steelmaking from scrap,
       steel is hard as a resultant of this, though the process can also
       use direct-reduced iron
     * Production of pig iron using blast furnace
     * Converters (steel from pig iron):

    1. Bessemer process, the first large-scale steel production process
       for mild steel.
    2. The Siemens-Martin process, using an Open hearth furnace
    3. 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 to obtain and much cheaper, and it has replaced
   wrought iron for a multitude of purposes. It continues to be used in
   many situations, though the new availabilty of plastics during the 20th
   century has meant that it has ceased to be used for some.

Long steel

     * Wires
     * Rail tracks
     * As girders in building modern tall buildings, bridges

Flat carbon steel

     * For the inside and outside body of cars, trains
     * Major appliances

Stainless steel

     * Cutlery
     * Rulers
     * Wrist Watches

The Royal Canadian Mint produce Dimes (value = $0.10)

     * Composed of 92% Steel
     * Since 2000
     * The Coin Weighs 1.75 g

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