ANNEALING (From M. Engl. anclen, 0. F. necler, Fr. nieller, to enamel, from Lat. nigel lus, blackish). The process by which glass and certain metals arc heated and then slowly cooled to make them more tenacious and less brittle. The rationale of annealing has been most studied, perhaps, in connection with steel manufacturing. Important steel castings are nearly always annealed, and it is a eommon re quirement for steel forgings. in drawing steel wire, annealing is necessary at frequent inter vals, and it is a common practice to anneal steel plates for the best marine boiler work. The hardening and tempering of steel are analogous processes to annealing, there being a close inter relation between the three phenomena. Steel is hardened by sudden cooling from a high tem perature, usually at or above red heat, by plunging it into oil, water, etc. To temper steel means in its specific sense to mitigate. or to mod erate, the effects of previous hardening. It is usually 'performed by gently reheating the pre viously hardened steel to a much lower tempera ture than red heat and then eooling it. generally suddenly. but sometimes slowly. While temper ing somewhat moderates the effects of previous hardening, annealing aims nearly completely to eliminate them. Annealing of steel is usually effeeted by slow cooling from a temperature at or above red heat. Thns steel is in its hardest and most brittle state when hardened; in its softest and toughest when annealed; and in an intermediate condition when tempered.
In hardening, the steel articles, if small, are heated in boxes or pans filled with charcoal dust, and placed in reverberatory furnaces. Larger ar ticles are heated in the furnace proper, which is often made of a special shape to fit the form of the article, such, for instanee, as a long gun tube. In general, the more rapid the cooling, the harder and more brittle is the steel. Mer cury is the most rapid cooling agent, and water, rapeseed oil, tallow, and coal-tar follow next in the order named. Steel castings and forgings
for guns, marine engine-shafts, and armo•-plate, where strength is more important than hard ness, are usually cooled in oil; while steel for cutting-tools. where extreme hardness is the im portant thing, is ordinarily hardened in water.
in tempering hardened steel articles, they are slowly heated by contact with hot iron bars, plates or rings, on the surface of melted lead or other fusible metal, in hot sand, in burning char coal, or in special furnaces, to a temperature of from 428° F. to 600' F. The temperature re quired for razors is from 446° F. to 469° F.; for shears and scissors, 491° F.; for woodwork ing tools, 531° F.; for swords and coiled springs, 550° F.; for handsaws, 600° F. The heated ar ticle is cooled by plunging it into a bath of water or oil.
In annealing, the article is heated uniformly in a furnace, without direct contact with the flames, to the temperature generally of bright cherry red. The common method of cooling is to withdraw the fire from the furnace and to close all apertures, allowing the furnace slowly to cool down. Cooling is sometimes accomplish ed by burying the heated article in ashes, lime, or other slow conductors of heat, and allowing it to become cool by the radiation of its heat. Boiler and ship plates are often cooled by sim ply withdrawing them from the furnace and throwing them on the mill floor to cool by ra diation. When medals are repeatedly struck by the die-stamper, the gold or other metal, by the concussion, becomes brittle, and requires to be heated and annealed at intervals. Annealing is necessary in gold-beating and in rolling, ham mering, and stamping shect-meta Is generally. Ar ticles of tin. lead, and zinc, which are metals with a low inciting temperature, are annealed in boil ing water, which is allowed to cool with the ar ticle immersed. Malleable iron is east-iron an nealed by being covered with powdered hematite ore and heated and then slowly cooled.