CASE HARDENING. The process of imparting to steel or wrought iron an extremely hard surface. It is of great impor tance in cases, and they are many, in which it is necessary to combine, in an article manufactured of iron or steel, tensile strength with resistance to surface attrition. Case hardening secures this by carburizing the outer surface of the steel em ployed. The article to be case hardened is cast, and the casting heated in a suitable furnace in which it is exposed to contact with charcoal at a temperature of Soo' to goo' C. for from a few hours to a few days. The result of this process is very literally to give a "hard case" or skin to the metal ; the body of the article being left unchanged in character. For empirical reasons, the charcoal em ployed is derived from horn, leather, etc. After carburizing, the articles are usually quenched in cold water. The process amounts to a partial application of the principle of cementation. Warships are cased in steel which has undergone various surface hardening processes of this kind; armour plate of the Harvey type is made of ductile steel to which a casing of great hardness is given by the surface being converted into high-carbon steel suddenly cooled.
(See IRON AND STEEL; CEMENTATION.)