Case-Hardening is a term applied to the process of converting the external surface of articles or masses of iron into steel, with the view of combining the hardness of the latter with the toughness and comparative cheapness of the former. See Iron.

This is a process for converting the surface only of articles made of malleable iron into steel, in order that they may afterwards receive a high polish. The process is extensively applied to the pokers, tongs, and shovels of our domestic fire-grates, and to an infinite variety of our iron manufactures. The following mode, recommended by Mr. Gill, editor of the Technological Repository, deserves confidence, from the extensive practical knowledge of that gentleman in the treatment of iron and steel. This is effected, he says, by inclosing the articles in carbonaceous compounds, either animal or vegetable, and exposing them to heat in close vessels, until the change is completed, and until the surface at least of the articles is converted into steel. For this purpose, bones, from which the ammonia has been extracted by distillation at a high temperature, and which are afterwards ground to a coarse black powder, are chiefly used. The articles being surrounded with this powder, contained in cast-iron vessels, are exposed to a high red heat, in an open fire-place, for several hours, until the surfaces of the iron articles are sufficiently changed to steel; when, if large enough, they may be taken out, whilst hot, and quenched in water, or if too small and numerous, the whole contents of the vessel, bone-dust and all, may be poured into the water.

Any parts of the articles which are required to remain iron after this operation, may be guarded from the action of the carbon, by coating them with clay or loam. Sometimes the water is covered with a layer of oil, two or three inches in depth, to prevent the small steel articles from being cracked in quenching; and it is very convenient in this case to have a wire sieve suspended in the water, at a proper depth beneath its surface, to suffer the bone ashes to fall through, but to detain the small articles. Other substances are employed in case-hardening; leather, burnt till it can be pulverised, is considered a good agent; also the hoofs and horns of animals, heated in an oven until they can be beaten to a coarse powder: the latter are preferred by gunsmiths for their work.