Editing Iron (section)

====Steelmaking====
{{Main|Steelmaking|Ironworks}}
<!-- Several other articles cover the material that might go into this section: please do not expand it excessively. This article concerns all aspects of the element iron, and should thus NOT be overburdened with details of metallurgy... Agree. Should be a summary. -->
The pig iron produced by the blast furnace process contains up to 4–5% carbon (by mass), with small amounts of other impurities like sulfur, magnesium, phosphorus, and manganese. This high level of carbon makes it relatively weak and brittle. Reducing the amount of carbon to 0.002–2.1% produces [[steel]], which may be up to 1000 times harder than pure iron. A great variety of steel articles can then be made by [[cold working]], [[hot rolling]], [[forging]], [[machining]], etc. Removing the impurities from pig iron, but leaving 2–4% carbon, results in [[cast iron]], which is cast by [[foundry|foundries]] into articles such as stoves, pipes, radiators, lamp-posts, and rails.{{sfn|Greenwood|Earnshaw|1997|p=1073}}

Steel products often undergo various [[heat treatment]]s after they are forged to shape. [[annealing (metallurgy)|Annealing]] consists of heating them to 700–800 °C for several hours and then gradual cooling. It makes the steel softer and more workable.<ref name="Verhoeven">Verhoeven, J.D. (1975) ''Fundamentals of Physical Metallurgy'', Wiley, New York, p. 326</ref>
<!--why is this in production of iron?Steel may be hardened by [[cold working]]. The metal is bent or hammered into its final shape at a relatively cool temperature. Cold forging is the stamping of a piece of steel into shape by a heavy press. Wrenches are commonly made by cold forging. Cold rolling, which involves making a thinner but harder sheet, and cold drawing, which makes a thinner but stronger wire, are two other methods of cold working. To harden the steel, it is heated to red-hot and then cooled by quenching it in the water. It becomes harder and more brittle. If it is too hardened, it is then heated to a required temperature and allowed to cool. The steel thus formed is less brittle.

[[Heat treatment]] is another way to harden steel. The steel is heated red-hot, then cooled quickly. The iron carbide molecules are decomposed by the heat, but do not have time to reform. Since the free carbon atoms are stuck, it makes the steel much harder and stronger than before.<ref name="Biddle" />

Sometimes both toughness and hardness are desired. A process called [[case hardening]] may be used. Steel is heated to about 900 °C then plunged into oil or water. Carbon from the oil can diffuse into the steel, making the surface very hard. The surface cools quickly, but the inside cools slowly, making an extremely hard surface and a durable, resistant inner layer.

Iron may be [[Passivation (chemistry)|passivated]] by dipping it into a concentrated [[nitric acid]] solution. This forms a protective layer of oxide on the metal, protecting it from further corrosion.<ref>{{cite book|url=https://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf |title=Picking and passivating stainless steel, Materials and Application Series, Volume 4 |publisher=Euro Inox |year=2007 |edition=2nd |isbn=978-2-87997-224-4}}</ref>-->
<gallery widths="200" heights="150">
File:LightningVolt Iron Ore Pellets.jpg|This heap of iron ore pellets will be used in steel production.
File:Melted raw-iron.jpg|A pot of molten iron being used to make steel
</gallery>