| Steel is a metal alloy whose major
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| | similar but less beautiful bainite.
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| component is iron, with carbon content
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| | Perhaps the most important allotrope is
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| between 0.02% and 1.7% by weight. Carbon
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| | martensite, a chemically metastable
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| is the most cost effective alloying
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| | substance with about four to five times
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| material for iron, but many other
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| | the strength of ferrite. A minimum of 0.4
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| alloying elements are also used.[1]
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| | wt% of carbon is needed in order to form
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| Carbon and other elements act as a
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| | martensite. When the austenite is
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| hardening agent, preventing dislocations
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| | quenched to form martensite, the carbon
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| in the iron atom crystal lattice from
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| | is "frozen" in place when the cell
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| sliding past one another. Varying the
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| | structure changes from FCC to BCC. The
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| amount of alloying elements and their
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| | carbon atoms are much too large to fit in
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| distribution in the steel controls
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| | the interstitial vaccancies and thus
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| qualities such as the hardness,
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| | distort the cell structure into a Body
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| elasticity, ductility, and tensile
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| | Centered Tetragonal (BCT) structure.
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| strength of the resulting steel. Steel
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| | Martensite and austenite have an
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| with increased carbon content can be made
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| | identical chemical composition. As such,
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| harder and stronger than iron, but is
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| | it requires extremely little thermal
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| also more brittle. The maximum solubility
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| | activation energy to form.
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| of carbon in iron is 1.7% by weight,
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| | The heat treatment process for most
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| occurring at 1130° Celsius; higher
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| | steels involves heating the alloy until
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| concentrations of carbon or lower
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| | austenite forms, then quenching the hot
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| temperatures will produce cementite which
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| | metal in water or oil, cooling it so
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| will reduce the material's strength.
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| | rapidly that the transformation to
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| Alloys with higher carbon content than
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| | ferrite or pearlite does not have time to
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| this are known as cast iron because of
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| | take place. The transformation into
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| their lower melting point.[1] Steel is
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| | martensite, by contrast, occurs almost
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| also to be distinguished from wrought
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| | immediately, due to a lower activation
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| iron with little or no carbon, usually
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| | energy.
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| less than 0.035%. It is common today to
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| | Martensite has a lower density than
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| talk about 'the iron and steel industry'
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| | austenite, so that the transformation
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| as if it were a single thing; it is
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| | between them results in a change of
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| today, but historically they were
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| | volume. In this case, expansion occurs.
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| separate products.
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| | Internal stresses from this expansion
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| Currently there are several classes of
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| | generally take the form of compression on
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| steels in which carbon is replaced with
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| | the crystals of martensite and tension on
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| other alloying materials, and carbon, if
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| | the remaining ferrite, with a fair amount
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| present, is undesired. A more recent
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| | of shear on both constituents. If
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| definition is that steels are iron-based
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| | quenching is done improperly, these
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| alloys that can be plastically formed
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| | internal stresses can cause a part to
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| (pounded, rolled, etc.).
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| | shatter as it cools; at the very least,
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| Iron, like most metals, is not found in
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| | they cause internal work hardening and
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| the Earth's crust in an elemental state.
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| | other microscopic imperfections. It is
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| Iron can be found in the crust only in
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| | common for quench cracks to form when
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| combination with oxygen or sulfur.
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| | water quenched, although they may not
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| Typically Fe2O3—the form of iron oxide
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| | always be visible.
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| (rust) found as the mineral hematite, and
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| | At this point, if the carbon content is
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| FeS2—Pyrite (fool's gold). Iron oxide
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| | high enough to produce a significant
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| is a soft sandstone-like material with
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| | concentration of martensite, the result
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| limited uses on its own. Iron is
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| | is an extremely hard but very brittle
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| extracted from ore by removing the oxygen
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| | material. Often, steel undergoes further
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| by combining it with a preferred chemical
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| | heat treatment at a lower temperature to
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| partner such as carbon. This process,
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| | destroy some of the martensite (by
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| known as smelting, was first applied to
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| | allowing enough time for cementite, etc.,
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| metals with lower melting points. Copper
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| | to form) and help settle the internal
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| melts at just over 1000 °C, while tin
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| | stresses and defects. This softens the
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| melts around 250 °C. Steel melts at
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| | steel, producing a more ductile and
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| around 1370 °C. Both temperatures could
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| | fracture-resistant metal. Because time is
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| be reached with ancient methods that have
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| | so critical to the end result, this
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| been used for at least 6000 years (since
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| | process is known as tempering, which
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| the Bronze Age). Since the oxidation rate
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| | forms tempered steel.
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| itself increases rapidly beyond 800 °C,
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| | Other materials are often added to the
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| it is important that smelting take place
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| | iron-carbon mixture to tailor the
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| in a low-oxygen environment. Unlike
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| | resulting properties. Nickel and
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| copper and tin, liquid iron dissolves
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| | manganese in steel add to its tensile
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| carbon quite readily, so that smelting
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| | strength and make austenite more
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| results in an alloy containing too much
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| | chemically stable, chromium increases the
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| carbon to be called steel.
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| | hardness and melting temperature, and
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| Even in the narrow range of
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| | vanadium also increases the hardness
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| concentrations that make up steel,
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| | while reducing the effects of metal
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| mixtures of carbon and iron can form into
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| | fatigue. Large amounts of chromium and
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| a number of different structures, or
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| | nickel (often 18% and 8%, respectively)
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| allotropes, with very different
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| | are added to stainless steel so that a
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| properties; understanding these is
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| | hard oxide forms on the metal surface to
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| essential to making quality steel. At
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| | inhibit corrosion. Tungsten interferes
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| room temperature, the most stable form of
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| | with the formation of cementite, allowing
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| iron is the body-centered cubic (BCC)
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| | martensite to form with slower quench
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| structure ferrite or ?-iron, a fairly
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| | rates, resulting in high speed steel. On
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| soft metallic material that can dissolve
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| | the other hand sulfur, nitrogen, and
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| only a small concentration of carbon (no
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| | phosphorus make steel more brittle, so
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| more than 0.021 wt% at 910 °C). Above
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| | these commonly found elements must be
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| 910 °C ferrite undergoes a phase
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| | removed from the ore during processing.
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| transition from body-centered cubic to a
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| | When iron is smelted from its ore by
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| face-centered cubic (FCC) structure,
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| | commercial processes, it contains more
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| called austenite or ?-iron, which is
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| | carbon than is desirable. To become
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| similarly soft and metallic but can
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| | steel, it must be melted and reprocessed
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| dissolve considerably more carbon (as
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| | to remove the correct amount of carbon,
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| much as 2.03 wt% carbon at 1154 °C)[2].
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| | at which point other elements can be
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| As carbon-rich austenite cools, the
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| | added. Once this liquid is cast into
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| mixture attempts to revert to the ferrite
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| | ingots, it usually must be "worked" at
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| phase, resulting in an excess of carbon.
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| | high temperature to remove any cracks or
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| One way for carbon to leave the austenite
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| | poorly mixed regions from the
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| is for cementite to precipitate out of
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| | solidification process, and to produce
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| the mix, leaving behind iron that is pure
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| | shapes such as plate, sheet, wire, etc.
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| enough to take the form of ferrite, and
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| | It is then heat-treated to produce a
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| resulting in a cementite-ferrite mixture.
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| | desirable crystal structure, and often
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| Cementite is a stoichiometric phase with
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| | "cold worked" to produce the final shape.
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| the chemical formula of Fe3C. Cementite
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| | In modern steelmaking these processes are
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| forms in regions of higher carbon content
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| | often combined, with ore going in one end
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| while other areas revert to ferrite
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| | of the assembly line and finished steel
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| around it. Self-reinforcing patterns
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| | coming out the other. These can be
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| often emerge during this process, leading
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| | streamlined by a deft control of the
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| to a patterned layering known as pearlite
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| | interaction between work hardening and
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| due to its pearl-like appearance, or the
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| | tempering.
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