Editing Case-hardening (section)

== History ==

Early [[iron]] [[smelting]] made use of [[bloomery|bloomeries]] which converted iron ore into metallic iron by heating it in a furnace which burnt wood and charcoal. Because the temperatures that could be achieved by this method were generally below the melting point of iron, it was not truly smelted, but instead converted into a spongy metallic iron/slag matrix. This matrix then required re-heating and hammering to extract as much of the slag as possible, in order to produce a low-carbon malleable wrought iron which could then be forged into tools etc. Due to its low carbon content, wrought iron is quite soft, so something like a knife blade could not be kept very sharp; it would blunt quickly and bend easily.{{Citation needed|date=October 2024}}

As smelting techniques improved, higher furnace temperatures could be achieved which were sufficient to fully melt iron. However, in the process, the iron picked up carbon from the charcoal or coke used to heat it. This resulted in molten iron with a carbon content of around 3%, which was termed [[cast iron]]. This liquid iron could be cast into complex shapes, but due to its high carbon content, it was very brittle, not at all malleable, and totally unsuitable for something like a knife blade. Further processing was required to remove the excess carbon from cast iron and create malleable [[wrought iron]] (the ultimate developments of this being the [[Bessemer converter]] and the [[Siemens-Martin process|Siemens process]]).

After the removal of almost all carbon from cast iron, the result was a metal that was very [[malleable]] and [[ductile]] but not very hard, nor capable of being hardened by heating and quenching. This led to the introduction of case hardening. The resulting case-hardened product combines much of the malleability and toughness of a low-carbon steel core with the hardness and resilience of the outer high-carbon steel skin.

The traditional method of applying the carbon to the surface of the iron involved packing the iron in a mixture of carbon-rich material such as ground [[bone]] and [[charcoal]] or a combination of [[leather]], [[hoof|hooves]], [[salt]] and [[urine]], all inside a well-sealed box (the "case"). This carburizing package is then heated to a high temperature—but still under the melting point of the iron—and left at that temperature for a length of time. The longer the package is held at the high temperature, the deeper the carbon will diffuse into the surface. Different depths of hardening are desirable for different purposes: sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts like gears might need only shallow hardening for increased wear resistance.

The resulting case-hardened part may show distinct surface discoloration, if the carbon material is mixed organic matter as described above. The steel darkens significantly and shows a mottled pattern of black, blue, and purple caused by the various compounds formed from impurities in the bone and charcoal. This oxide surface works similarly to [[bluing (steel)|bluing]], providing a degree of corrosion resistance, as well as an attractive finish. ''Case colouring'' refers to this pattern and is commonly encountered as a decorative finish on [[firearms]].

Case-hardened steel combines extreme hardness and extreme toughness, which is not readily matched by homogeneous alloys since hard homogeneous steels tend to be brittle, especially those steels whose hardness relies on carbon content alone. Alloy steels containing nickel, chromium, or molybdenum can have very high hardness, strength, or elongation values, but at a greater cost than a case-hardened item with a low-carbon core.