Editing Case-hardening (section)

== Chemistry ==

Carbon itself is solid at case-hardening temperatures and so is immobile. Transport to the surface of the steel was as gaseous [[carbon monoxide]], generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon but too slowly to be workable. Although oxygen is required for this process it is re-circulated through the CO cycle and so can be carried out inside a sealed box (the "case"). The sealing is necessary to stop the CO either leaking out or being oxidised to CO<sub>2</sub> by excess outside air.

Adding an easily decomposed carbonate "energiser" such as [[barium carbonate]] breaks down to BaO + CO<sub>2</sub> and this encourages the reaction:
:C (from the donor) + CO<sub>2</sub> <—> 2 CO
increasing the overall abundance of CO and the activity of the carburising compound.<ref name="Higgins, Engineering Metallurgy, energiser" >{{cite book
  |last=Higgins  |first=Raymond A.
  |title=Part I: Applied Physical Metallurgy
  |series=Engineering Metallurgy
  |publisher=Hodder & Stoughton
  |edition=5th
  |year=1983
  |isbn=0-340-28524-9
  |ref=Higgins, Engineering Metallurgy
  |pages=474
}}</ref>

It is a [[common knowledge]] fallacy that case-hardening was done with bone but this is misleading. Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates but is mainly calcium phosphate (as [[hydroxylapatite]]). This does not have the beneficial effect of encouraging CO production and it can also introduce [[phosphorus]] as an impurity into the steel alloy.