CHILL HARDENING, the process of hardening the surface of a metal by rapidly cooling it while it is molten or at a very high temperature. Usually the chilling is done by plunging the object in a bath of water or oil. In the case of railway and other wagon and car wheels, a "chill mould" is used, consisting of a sand mould with the outer part (which forms the rim) made of cast iron. When the wheel is cast, the rim cools very rapidly be cause the iron part of the mould absorbs and conducts away the heat from it, and the consequent chilling hardens the rim. Chill hardening is sometimes combined with case hardening. (See FOUNDING; CASE HARDENING; IRON AND STEEL.)