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For lidt kulstofindhold gør jernet forholdsvis blødt, smidigt og svagt. Er kulstofindholdet derimod højere end i stål, giver det en legering der almindeligvis kaldes [[råjern]], som er for sprød og skrøbelig til at den let kan bearbejdes. Andre legeringer med et kulstofindhold over 2,1%, alt efter behandling og indhold af andre grundstoffer, kendes som [[støbejern]]. Støbejern er ikke bearbejdeligt selv når det er varmt, men det kan formes ved [[støbning]] da det blandt andet har et lavere [[smeltepunkt]] end stål.<ref name=EM2/> Nogle former for støbejern, som bevarer evne til at smeltes og støbes, kan varmebehandles efter støbningen når man vil skabe genstande af tempergods eller [[aducérgods]]. Stål skelnes også fra [[smedejern]] (nu stort set forældet), som kan indeholde en lille mængde kulstof men store mængder [[slagge]].
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== Egenskaber ==
[[File:Steel pd.svg|thumb|420px|Dette [[fasediagram]] for jern og kulstof viser under hvilke betingelser forskellige faser dannes.]]
 
Jern findes i Jordens [[skorpe (geologi)|]] i form af forskellige slags [[malm]], oftest et [[jernoxid]] såsom magnetit, hæmatit, osv. Jern udvindes af [[jernmalm]] ved udsmeltning hvor et kemisk stof såsom kulstof tilsættes og forbinder sig med ilten, der så udskilles som kuldioxid. Denne proces blev oprindelig anvendt på metaller med et lavere [[smeltepunkt]], såsom [[tin]], som smelter ved omkring 250°C, [[kobber]], som smelter ved omkring 1100°C, og kombinationen af de to, [[bronze]], som er flydende ved 1083°C. Til sammenligning smelter støbejern ved omkring 1375°C.<ref name="Smelting">{{cite|title=Smelting|encyclopedia=Encyclopædia Britannica|year=2007}}</ref> Små mængder jern kunne udsmeltes i oldtiden, i fast form, ved at varme malmen i en ild af [[trækul]] og svejse klumperne sammen med en hammer så at urenhederne klemmes ud. Med omhu kunne kulstofindholdet kontrolleres ved at bevæge det rundt i ilden.
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<!--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. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. 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.
 
Other materials are often added to the iron/carbon mixture to produce steel with 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=2006-06-28|url=http://materialsengineer.com/E-Alloying-Steels.htm|accessdate=2007-02-28}}</ref>
 
To inhibit corrosion, at least 11% chromium is added to steel so that a hard [[Passivation (chemistry)|oxide]] forms on the metal surface; this is known as [[stainless steel]]. Tungsten interferes with the formation of [[cementite]], allowing [[martensite]] to preferentially form at 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 steel melt during processing.<ref name="materialsengineer"/>
 
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|accessdate = 2009-04-23}}</ref>
 
Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of 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-centered 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 [[ferrite (iron)|ferrite]]. At 910&nbsp;°C pure iron transforms into a [[face-centered cubic]] (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called [[austenite]]. The 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).
 
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 that is able to hold the carbon in solution. The two, ferrite and cementite, 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 and then when the composition left behind is eutectoid, the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form until the remaining composition is 0.8% at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries.<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.
 
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 center austenite and forms [[martensite]]. Martensite is 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-centered tetragonal]] (BCT) structure. There is no thermal [[activation energy]] for the transformation from austenite to martensite.{{clarify|date=April 2016}} Moreover, there is no compositional change so the atoms generally retain their same neighbors.<ref name="smith&hashemi">{{Harvnb|Smith|Hashemi|2006|pp=373–378}}.</ref>
 
Martensite has a lower density (it expands) 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|accessdate=2009-07-19}}</ref>-->
 
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