Editing Iron (section)

====Types of steels and alloys====
{{See also|Steel}}
[[File:Iron carbon phase diagram.svg|thumb|left|upright=1.8|Iron-carbon phase diagram]]
α-Iron is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).<ref>{{Cite book|url={{Google books|xv420pEC2qMC|page=PA183|keywords=|text=|plainurl=yes}}| page=183| title=Concise encyclopedia of the structure of materials| first=John Wilson|last = Martin| publisher=Elsevier| date= 2007|isbn=978-0-08-045127-5}}</ref> [[Austenite]] (γ-iron) is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of [[stainless steel]] used for making cutlery, and hospital and food-service equipment.<ref name="Metallo" />

Commercially available iron is classified based on purity and the abundance of additives. [[Pig iron]] has 3.5–4.5% carbon<ref name="msts">{{Cite book|last1 = Camp|first1 = James McIntyre|last2 = Francis |first2 = Charles Blaine|title = The Making, Shaping and Treating of Steel|publisher = Carnegie Steel Company |date=1920|location = Pittsburgh|pages = 173–74|url={{Google books|P9MxAAAAMAAJ|keywords=|text=|plain-url=yes}}|isbn = 1-147-64423-3}}</ref> and contains varying amounts of contaminants such as [[sulfur]], silicon and [[phosphorus]]. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of [[manganese]].{{sfn|Greenwood|Earnshaw|1997|p=1073}} Pig iron has a [[melting point]] in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}} Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

"White" cast irons contain their carbon in the form of [[cementite]], or iron carbide (Fe<sub>3</sub>C).{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}} This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron carbide, a very pale, silvery, shiny material, hence the appellation. Cooling a mixture of iron with 0.8% carbon slowly below 723 °C to room temperature results in separate, alternating layers of cementite and α-iron, which is soft and malleable and is called [[pearlite]] for its appearance. Rapid cooling, on the other hand, does not allow time for this separation and creates hard and brittle [[martensite]]. The steel can then be tempered by reheating to a temperature in between, changing the proportions of pearlite and martensite. The end product below 0.8% carbon content is a pearlite-αFe mixture, and that above 0.8% carbon content is a pearlite-cementite mixture.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

In [[gray iron]] the carbon exists as separate, fine flakes of [[graphite]], and also renders the material brittle due to the sharp edged flakes of graphite that produce [[stress concentration]] sites within the material.<ref name="Hashemi">{{Citation | last1 = Smith | first1 = William F. | last2 = Hashemi | first2 = Javad | title = Foundations of Materials Science and Engineering | edition = 4th | year = 2006 | publisher = McGraw-Hill | isbn = 0-07-295358-6 | postscript =. |page=431}}</ref> A newer variant of gray iron, referred to as [[ductile iron]], is specially treated with trace amounts of [[magnesium]] to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.<ref name="Hashemi" />

[[Wrought iron]] contains less than 0.25% carbon but large amounts of [[slag]] that give it a fibrous characteristic.<ref name="msts" /> Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by [[mild steel]], which corrodes more readily than wrought iron, but is cheaper and more widely available. [[Carbon steel]] contains 2.0% carbon or less,<ref name="kts">{{cite web|title=Classification of Carbon and Low-Alloy Steels |url=https://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=62 |access-date=5 January 2008 |url-status=dead |archive-date=2 January 2011 |archive-url=https://web.archive.org/web/20110102110320/http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=62}}</ref> with small amounts of [[manganese]], [[sulfur]], [[phosphorus]], and silicon. [[Alloy steel]]s contain varying amounts of carbon as well as other metals, such as [[chromium]], [[vanadium]], [[molybdenum]], nickel, [[tungsten]], etc. Their alloy content raises their cost, and so they are usually only employed for specialist uses. One common alloy steel, though, is [[stainless steel]]. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed '[[HSLA steel|HSLA]]' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.<ref name="kts" /><ref>{{cite web |title=HSLA Steel |date=2002-11-15 |url=https://machinedesign.com/BasicsOfDesignEngineeringItem/717/65970/HSLASteel.aspx |access-date=2008-10-11 |archive-url=https://web.archive.org/web/20091230082918/https://machinedesign.com/article/hsla-steel-1115 |archive-date=30 December 2009 |url-status=dead}}</ref><ref>{{cite book |last=Oberg |first=E. |title=Machinery's Handbook |place=New York |publisher=Industrial Press |edition=25th |year=1996 |display-authors=etal |pages=440–42 |bibcode=1984msh..book.....R}}</ref>

Alloys with high purity elemental makeups (such as alloys of [[electrolytic iron]]) have specifically enhanced properties such as [[ductility]], [[tensile strength]], [[toughness]], [[fatigue limit|fatigue strength]], heat resistance, and corrosion resistance.

Apart from traditional applications, iron is also used for protection from ionizing radiation. Although it is lighter than another traditional protection material, [[lead]], it is much stronger mechanically.<ref>{{cite web |url=https://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-13033.pdf |title=Radiation Shielding at High-Energy Electron and Proton Accelerators |last1=Rokni |first1=Sayed H. |last2=Cossairt |first2=J. Donald |last3=Liu |first3=James C. |date=January 2008 |access-date=6 August 2016}}</ref>

The main disadvantage of iron and steel is that pure iron, and most of its alloys, suffer badly from [[rust]] if not protected in some way, a cost amounting to over 1% of the world's economy.{{sfn|Greenwood|Earnshaw|1997|p=1076}} [[Paint]]ing, [[galvanization]], [[Passivation (chemistry)|passivation]], plastic coating and [[bluing (steel)|bluing]] are all used to protect iron from rust by excluding [[water]] and oxygen or by [[cathodic protection]]. The mechanism of the rusting of iron is as follows:{{sfn|Greenwood|Earnshaw|1997|p=1076}}

:Cathode: 3 O<sub>2</sub> + 6 H<sub>2</sub>O + 12 e<sup>−</sup> → 12 OH<sup>−</sup>
:Anode: 4 Fe → 4 Fe<sup>2+</sup> + 8 e<sup>−</sup>; 4 Fe<sup>2+</sup> → 4 Fe<sup>3+</sup> + 4 e<sup>−</sup>
:Overall: 4 Fe + 3 O<sub>2</sub> + 6 H<sub>2</sub>O → 4 Fe<sup>3+</sup> + 12 OH<sup>−</sup> → 4 Fe(OH)<sub>3</sub> or 4 FeO(OH) + 4 H<sub>2</sub>O

The electrolyte is usually [[iron(II) sulfate]] in urban areas (formed when atmospheric [[sulfur dioxide]] attacks iron), and salt particles in the atmosphere in seaside areas.{{sfn|Greenwood|Earnshaw|1997|p=1076}}