Case-hardening
Steel which has a carbon content greater than ~0.25% can be direct-hardened by heating to around 600°C, and then quickly cooling, often by immersing in water or oil, known as quenching.
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] As smelting techniques improved, higher furnace temperatures could be achieved which were sufficient to fully melt 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.
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.
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, providing a degree of corrosion resistance, as well as an attractive finish.
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.
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.
The steel work piece is placed inside a case packed tight with a carbon-based case-hardening compound.
Small items may be case-hardened by repeated heating with a torch and quenching in a carbon rich medium, such as the commercial products Kasenit / Casenite or "Cherry Red".
[2][3] Flame or induction hardening are processes in which the surface of the steel is heated very rapidly to high temperatures (by direct application of an oxy-gas flame, or by induction heating) then cooled rapidly, generally using water; this creates a "case" of martensite on the surface.
Typical uses are for the shackle of a lock, where the outer layer is hardened to be file resistant, and mechanical gears, where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure.
The result of the hardening process is controlled by four factors: Carburizing is a process used to case-harden steel with a carbon content between 0.1 and 0.3 wt% C. In this process iron is introduced to a carbon rich environment at elevated temperatures for a certain amount of time, and then quenched so that the carbon is locked in the structure; one of the simpler procedures is repeatedly to heat a part with an acetylene torch set with a fuel-rich flame and quench it in a carbon-rich fluid such as oil.
In these cases, the surfaces requiring the hardness may be hardened selectively, leaving the bulk of the part in its original tough state.
