Case-Hardening, a process of hardening the surface of iron by converting it into steel. For this purpose the articles are placed in an iron case, together with animal or vegetable charcoal, and subjected to the process of cementation. The carbon absorbed does not, in the short time allowed for the operation, penetrate beneath the surface. From two to eight hours is the usual time that the articles are exposed to a dull red heat. They are then taken out of the burnt bone dust or other carbonaceous substance, and further hardened by quenching them in oil or cold water. Sometimes they are left to cool in the case, and are afterward tempered. Prussiate of potash has in various ways been found a very useful material for affording its carbon to iron for producing steel. Being a combination of two atoms of carbon and one of nitrogen with one of potash, it offers no solid residue that interferes with the progress of the chemical change, or impairs the quality of the steel. In case-hardening, it is sprinkled or rubbed upon the iron heated to dull red, and this, after being put in the fire for a few minutes, is taken out and tempered in water. The process is a convenient one where small articles are to be exposed to much wear, these being easily made of soft iron, and then externally hardened.
It is also conveniently applied to give a good surface to small articles which are desired to receive the high polish of which steel is susceptible.