hardening, in metallurgy, treatment of metals to increase their resistance to penetration. A metal is harder when it has small grains, which result when the metal is cooled rapidly. Sometimes small areas on the surface of a casting are given a fine-grain structure by chill hardening; metal pieces (chills) are inserted in the wall of a sand mold. The area next to the chill cools faster and becomes harder than the surface next to the sand. Metals worked cold, as by being rolled into thinner pieces, become hardened, partly by reducing grain size and partly by distorting the shape of the grains so that they increasingly resist further distortion. Alloying may harden a metal by changing its chemical composition. In hardening by precipitation, one constituent of a supersaturated solid solution separates from the solution. Usually the process is carried out at above room temperature. At room temperature the process takes longer; it is then known as age-hardening. Aluminum-copper alloys are hardened by precipitation. Iron-carbon alloys, steel and cast iron, for example, respond well to heat treatments. By varying the percentage of carbon and the rate of cooling from a high temperature, many gradations of hardness, softness, toughness, and other properties are achieved. To impart hardness the metal is rapidly cooled from a high temperature by quenching in water, oil, or molten salt. Later heat treatment by tempering or annealing modifies the metal slightly to give other desirable qualities. Steels with a low percentage of carbon can be given a hard surface by increasing the amount of carbon at the surface so that they will respond to heat treatment, a process known as carburizing, or casehardening. One way to do this is to pack steel in charcoal and then heat it. Another way is to heat the metal in a furnace with a hydrocarbon gas atmosphere; still another is to heat the metal in a molten-salt bath containing potassium and sodium cyanides. If the salt bath cited is of a lower temperature, the steel surface will also pick up nitrogen, which helps harden it; the process is then called cyaniding. At even lower temperatures the steel picks up only nitrogen, and is nitrided.
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