HEAT TREATING AND INSPECTION OF METALS 28
(2) The precipitation treatment consists of "aging" the parts pre-
viously subjected to the solution treatment at a definite temperature
for a considerable time. During the aging period a portion of the al-
loying constituents precipitate out in the form of extremely fine par-
ticles, greatly increasing the strength of the alloy.
b. Effects.-Heat treatment affects not only the mechanical prop-
erties but also the corrosion-resistant properties. It is possible to
obtain the desired mechanical properties upon heat treatment and yet
fail to secure a maximum corrosion resistance by cooling too slowly.
As little as 100 under the specified solution temperature may affect
either the mechanical or corrosion-resistant properties or both.
c. Quenching and aging.--Quenching from the elevated tempera-
ture causes some increase in strength and hardness but the maximum
values are not immediately obtained and the ductility is hardly af-
fected. While in this condition the alloy will stand a considerable
amount of cold forming without danger of rupture. After a period
of approximately 1 hour the effects of aging begin to reduce the
ductility of the material, making it impractical to work the alloy.
Aging'is a natural characteristic of the metal and is dependent on time
and temperature. At temperatures below 32 F., aging is greatly
retarded and the material may be held in a workable condition for sev-
eral days by means of refrigeration. At room temperature, however,
the aging effect will start at once and continue over a period of several
days although it is practically, complete in from 16 to 24 hours.
d. Annealing.-(1) The annealing procedure for aluminum alloys
consists of bringing the alloys to an elevated temperature, holding or
soaking them at this temperature for a length of time, depending upon
the mass of the metal, then cooling them in still air. Annealing
leaves the mental in the best condition for cold working; however, when
prolonged forming operations are involved the metal will take on a
condition known as "mechanical hardness" which will resist further
working. In this case, it may be necessary to anneal the part several
times during the forming process to avoid cracking. Aluminum alloy
must not be used in the annealed state for highly stressed parts or
fittings, and is rehardened for such application.
(2) Alclad parts should be heated as quickly and carefully as pos-
sible, due to the fact that long exposure to heat tends to cause some of
Sthe constituents of the core to diffuse into the pure aluminum coating,
reducing the effectiveness of the pure aluminum as a corrosion-resistant
United States. War Dept. Heat treating and inspection of metals. Washington, D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc96657/. Accessed August 22, 2014.