This is the oldest method of making steel; it is known as the cementation process and practically is superseded at the present time. The process consists of heating wrought iron in a packing of charcoal to a high enough temperature so that the solid solution of cementite in iron will gradually diffuse through the entire metal. As formerly carried out to a limited extent, especially at Sheffield, England, long bars of wrought iron were packed in large receptacles, the charcoal filling up between and around the bars. This whole receptacle - enclosed within still another wall, as in a furnace - was brought up to heat gradually and was maintained at something over 700° C. during 7 to 10 days longer, depending on the grade of steel to be produced. Though the process is extremely inefficient, and the product equally varied, it survives in its modern application as casehardening.
Casehardening is the production of a thin layer of steel on the outside of a much lower carbon metal. Recent practice does not confine itself to using charcoal alone, but, depending on the material and purposes, the carbonizing medium may be either charcoal and highly carbonaceous materials, or carbon-bearing gases, or even molten solids which may give to the iron object part of their carbon.
Casehardened objects are fairly common in ordinary life and have especially useful application in that the center of the material may be strong and tough although its exterior will be hard and brittle and take a fine polish. Such objects are common in all sorts of machines, such as bicycles, automobiles, and wherever ball bearings and wearing surfaces are found.