Annealing To Relieve Internal Stresses





Work quenched from a high temperature and not afterward tempered

will, if complex in shape, contain many internal stresses which may

later cause it to break. They may be eased off by slight heating

without materially lessening the hardness of the piece. One way

to do this is to hold the piece over a fire and test it with a

moistened finger. Another way is to dip the piece in boiling water

after it has first been quenched in a cold bath. Such steps are

not necessary with articles which will afterward be tempered and

in which the strains are thus reduced.



In annealing steels the operation is similar to hardening, as far

as heating is concerned. The critical temperatures are the proper

ones for annealing as well as hardening. From this point on there

is a difference, for annealing consists in cooling as slowly as

possible. The slower the cooling the softer will be the steel.



Annealing may be done in the open air, in furnaces, in hot ashes

or lime, in powdered charcoal, in burnt bone, in charred leather

and in water. Open-air annealing will do as a crude measure in

cases where it is desired to take the internal stresses out of

a piece. Care must be taken in using this method that the piece

is not exposed to drafts or placed on some cold substance that

will chill it. Furnace annealing is much better and consists in

heating the piece in a furnace to the critical temperature and

then allowing the work and the furnace to cool together.



When lime or ashes are used as materials to keep air away from

the steel and retain the heat, they should be first heated to make

sure that they are dry. Powdered charcoal is used for high-grade

annealing, the piece being packed in this substance in an iron box

and both the work and the box raised to the critical temperature

and then allowed to cool slowly. Machinery steel may be annealed in

spent ground-bone that has been used in casehardening; but tool

steel must never be annealed in this way, as it will be injured

by the phosphorus contained in the bone. Charred leather is the

best annealing material for high-carbon steel, because it prevents

decarbonizing taking place.





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