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Detailed table of contents
First section: Furnaces and Baths for Heating Steel
Previous section: Tempering
Next section: Casehardening

Annealing Steel. -- The purpose of annealing is not only to soften steel for machining, but to remove all strains incident to rolling or hammering. A common method of annealing is to pack the steel in a cast-iron box containing some material, such as powdered charcoal, charred bone, charred leather, slaked lime, sand, fireclay, etc. The box and its contents are then heated in a furnace to the proper temperature, for a length of time depending upon the size of the steel. After heating, the box and its contents should be allowed to cool at a rate slow enough to prevent any hardening. It is essential, when annealing, to exclude the air as completely as possible while the steel is hot, to prevent the outside of the steel from becoming oxidized.

The temperature required for annealing should be slightly above the critical point, which varies for different steels. Low-carbon steel should be annealed at about 1650 degrees F., and high-carbon steel at between 1400 degrees and 1500 degrees F. This temperature should be maintained just long enough to heat the entire piece evenly throughout. Care should be taken not to heat the steel much above the decalescence or hardening point. When steel is heated above this temperature, the grain assumes a definite size for that particular temperature, the coarseness increasing with an increase in temperature. Moreover, if steel that has been heated above the critical point is cooled slowly, the coarseness of the grain corresponds to the coarseness at the maximum temperature; hence, the grain of annealed steel is coarser, the higher the temperature to which it is heated above the critical point.

If only a small piece of steel or a single tool is to be annealed, this can be done by building up a firebrick box in an ordinary blacksmith's fire, placing the tool in it, covering over the top, then heating the whole, covering with coke and leaving it to cool over night. Another quick method is to heat the steel to a red heat, bury it in dry sand, sawdust, lime or hot ashes, and allow it to cool. Quick annealing can also be partially effected by heating the piece to a dull black-red and plunging it into hot water. This method is not to be recommended.

Annealing High-speed Steel. -- The following method of annealing high-speed steel is recommended by one of the largest high-speed tool steel manufacturers in America, and corresponds in all important points to the practice of most other manufacturers: Use an iron box or pipe of sufficient size to allow at least one-half inch of packing between the pieces of steel to be annealed and the sides of the box or pipe. It is not necessary that each piece be kept separate from every other piece, but only that the steel be prevented from touching the dies of the annealing pipe or box. Pack carefully with powdered charcoal, fine dry lime or mice (preferably charcoal), and cover with an air-tight cap, or lute with fireclay; heat slowly to a full red heat (about 1475 degrees or 1500 degrees F.) and keep at this heat from 2 to 8 hours, depending upon the size of the pieces to be annealed. A piece 2 by 1 by 8 inches requires about three hours. Cool slowly as possible, and do not expose to the air until cold. A good way is to allow the box or pipe to remain in the furnace until cold.

A series of experiments made to determine the proper temperature to which to heat high-speed steel for annealing, showed the following results: When the steel was heated to below 1250 degrees F. and slowly cooled, as in annealing, it retained the original hardness and brittleness imparted in forging. When heated to between 1250 degrees and 1450 degrees F., the Brinnell test indicated that the steel was soft, but impact tests proved that the steel still retained its original brittleness. However, when heated to between 1475 degrees and 1525 degrees F., the steel became very soft; it had a fine-grained fracture, and all of the initial brittleness had entirely disappeared. In carrying these tests further, to 1600 degrees, 1750 degrees, and 1850 degrees F., it was found that the steel became very soft, but there was a gradual increase in brittleness and in the size of the grain, until at 1850 degrees F. the steel again became as brittle as unannealed steel. Dried air-slaked lime was used as a packing medium in making these tests.

Detailed table of contents
First section: Furnaces and Baths for Heating Steel
Previous section: Tempering
Next section: Casehardening

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