Editing Steel (section)

==Material properties==
===Origins and production===
[[File:FeC-phase-diagram--multilingual.svg|thumb|upright=1.75|An iron-carbon [[phase diagram]] showing the conditions necessary to form different phases]]
[[File:Blacksmithing at the 2015 Fort Ross Festival - Fort Ross State Historic Park - Jenner, California - Sarah Stierch.jpg|thumb|An [[Incandescence|incandescent]] steel workpiece in a [[blacksmith]]'s art]]
Iron is commonly found in the Earth's [[crust (geology)|crust]] in the form of an [[ore]], usually an iron oxide, such as [[magnetite]] or [[hematite]]. Iron is extracted from [[iron ore]] by removing the oxygen through its combination with a preferred chemical partner such as carbon which is then lost to the atmosphere as carbon dioxide. This process, known as [[smelting]], was first applied to metals with lower [[melting]] points, such as [[tin]], which melts at about {{convert|250|C|F|abbr=on}}, and [[copper]], which melts at about {{convert|1100|C|F|abbr=on}}, and the combination, bronze, which has a melting point lower than {{convert|1083|C|F|abbr=on}}. In comparison, cast iron melts at about {{convert|1375|C|F|abbr=on}}.<ref name="Smelting">{{cite book |chapter=Smelting |title=[[Encyclopædia Britannica]] |edition=online |date=2007 |chapter-url= https://www.britannica.com/technology/smelting}}</ref> Small quantities of iron were smelted in ancient times, in the solid-state, by heating the ore in a [[charcoal]] fire and then [[welding]] the clumps together with a hammer and in the process squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily.{{Cn|date=January 2024}}

All of these temperatures could be reached with ancient methods used since the [[Bronze Age]]. Since the oxidation rate of iron increases rapidly beyond {{convert|800|C|F}}, it is important that smelting take place in a low-oxygen environment. Smelting, using carbon to reduce iron oxides, results in an alloy ([[pig iron]]) that retains too much carbon to be called steel.<ref name="Smelting" /> The excess carbon and other impurities are removed in a subsequent step.{{Cn|date=January 2024}}

Other materials are often added to the iron/carbon mixture to produce steel with the desired properties. [[Nickel]] and [[manganese]] in steel add to its tensile strength and make the [[austenite]] form of the iron-carbon solution more stable, [[chromium]] increases hardness and melting temperature, and [[vanadium]] also increases hardness while making it less prone to [[metal fatigue]].<ref name="materialsengineer">{{cite web |title=Alloying of Steels |publisher=Metallurgical Consultants |date=28 June 2006 |url= http://materialsengineer.com/E-Alloying-Steels.htm |access-date=28 February 2007 |url-status=dead |archive-url= https://web.archive.org/web/20070221070822/http://www.materialsengineer.com/E-Alloying-Steels.htm |archive-date=21 February 2007}}</ref>

To inhibit corrosion, at least 11% chromium can be added to steel so that a hard [[Passivation (chemistry)|oxide]] forms on the metal surface; this is known as [[stainless steel]]. Tungsten slows the formation of [[cementite]], keeping carbon in the iron matrix and allowing [[martensite]] to preferentially form at slower quench rates, resulting in [[high-speed steel]]. The addition of [[lead]] and [[sulphur]] decrease grain size, thereby making the steel easier to [[lathe|turn]], but also more brittle and prone to corrosion. Such alloys are nevertheless frequently used for components such as nuts, bolts, and washers in applications where toughness and corrosion resistance are not paramount. For the most part, however, [[Block (periodic table)|p-block]] elements such as sulphur, [[nitrogen]], [[phosphorus]], and lead are considered contaminants that make steel more brittle and are therefore removed from steel during the melting processing.<ref name="materialsengineer" />

===Properties===
[[File:Steel_Fe-C_phase_diagram-en.png|thumb|upright=1.5|Fe-C phase diagram for carbon steels, showing the A<sub>0</sub>, A<sub>1</sub>, A<sub>2</sub> and A<sub>3</sub> critical temperatures for heat treatments]]
The [[density]] of steel varies based on the alloying constituents but usually ranges between {{convert|7750|and|8050|kg/m3|lb/ft3|abbr=on}}, or {{convert|7.75|and|8.05|g/cm3|oz/cuin|abbr=on}}.<ref>{{cite web |last=Elert |first=Glenn |title=Density of Steel |url= http://hypertextbook.com/facts/2004/KarenSutherland.shtml |access-date=23 April 2009}}</ref>

Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At [[room temperature]], the most stable form of pure iron is the [[body-centred cubic]] (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at {{Convert|0|C|F|abbr=on}} and 0.021 wt% at {{convert|723|C|F|abbr=on}}. The inclusion of carbon in alpha iron is called [[Allotropes of iron|ferrite]]. At 910&nbsp;°C, pure iron transforms into a [[face-centred cubic]] (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%,<ref>Sources differ on this value so it has been rounded to 2.1%, however the exact value is rather academic because plain-carbon steel is very rarely made with this level of carbon. See:
* {{harvnb|Smith|Hashemi|2006|p=363}}—2.08%.
* {{harvnb|Degarmo|Black|Kohser|2003|p=75}}—2.11%.
* {{harvnb|Ashby|Jones|1992}}—2.14%.</ref> (38 times that of ferrite) carbon at {{convert|1148|C|F|abbr=on}}, which reflects the upper carbon content of steel, beyond which is cast iron.<ref>{{harvnb|Smith|Hashemi|2006|p=363}}.</ref> When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe<sub>3</sub>C).{{Cn|date=January 2024}}

When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the [[austenitic]] phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to [[precipitate]] out of solution as [[cementite]], leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, cementite and ferrite, precipitate simultaneously producing a layered structure called [[pearlite]], named for its resemblance to [[mother of pearl]]. In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite [[grain boundaries]] until the percentage of carbon in the [[Grain (metal)|grains]] has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form within the grains until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries in hypoeutectoid steel.<ref>{{harvnb|Smith|Hashemi|2006|pp=365–372}}.</ref> The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate.{{Cn|date=January 2024}}

As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel. At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centred austenite and forms [[martensite]]. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a [[body-centred tetragonal]] (BCT) structure. There is no thermal [[activation energy]] for the transformation from austenite to martensite.{{clarify|date=April 2016}} There is no compositional change, so the atoms generally retain their same neighbours.<ref name="smith&hashemi">{{Harvnb|Smith|Hashemi|2006|pp=373–378}}.</ref>

Martensite has a lower density (it expands during the cooling) than does 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 [[physical compression|compression]] on the crystals of martensite and [[tension (mechanics)|tension]] on the remaining ferrite, with a fair amount of [[shear stress|shear]] on both constituents. If quenching is done improperly, the 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 steel is water quenched, although they may not always be visible.<ref>{{cite web |title=Quench hardening of steel |url= http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=12 |access-date=19 July 2009 |work=keytometals.com |url-status=dead |archive-url= https://web.archive.org/web/20090217103241/http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=12 |archive-date=17 February 2009}}</ref>

===Heat treatment===
{{Main|Heat treating}}
There are many types of [[heat treatment|heat treating]] processes available to steel. The most common are [[annealing (metallurgy)|annealing]], [[quenching]], and [[tempering (metallurgy)|tempering]].

Annealing is the process of heating the steel to a sufficiently high temperature to relieve local internal stresses. It does not create a general softening of the product but only locally relieves strains and stresses locked up within the material. Annealing goes through three phases: [[recovery (metallurgy)|recovery]], [[recrystallization (metallurgy)|recrystallization]], and [[grain growth]]. The temperature required to anneal a particular steel depends on the type of annealing to be achieved and the alloying constituents.<ref>{{harvnb|Smith|Hashemi|2006|p=249}}.</ref>

Quenching involves heating the steel to create the austenite phase then quenching it in water or [[oil]]. This rapid cooling results in a hard but brittle martensitic structure.<ref name="smith&hashemi" /> The steel is then tempered, which is just a specialized type of annealing, to reduce brittleness. In this application the annealing (tempering) process transforms some of the martensite into cementite, or [[spheroidite]] and hence it reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel.<ref>{{harvnb|Smith|Hashemi|2006|p=388}}.</ref>