Heat treating and inspection of metals Page: 30
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27-28 AIR CORPS
O=-Dead soft or fully annealed.
H= Fully cold-worked or hard-wrought.
W= Condition after solution treatment and befoi aging.
T= Fully heat-treated and aged condition.
RT= Temper resulting from cold working after
heat treatment and aging.
A, B, or C preceding the figure indicates a modification of the
(2) Cast aluminum alloys are represented by numbers and letters
which are divided into two parts, separated by a hyphen. The first
part of this identification symbol represents the composition as in the
case of the wrought alloys, while the second part represents the
condition of temper.
b. (1) Several of the aluminum alloys respond readily to heat treat-.
ment. In general, this treatment consists of heating thealloy to a
known temperature, holding this temperature for a definite time, then
quenching the part to room temperature or below. During the heat-
ing process a greater number of the constituents of the metal are put
into solid solution. Rapid quenching retains this condition which
results in a considerable improvement in the strength characteristics.
(2) The heating of aluminum alloy should be done in an electric
furnace or molten salt bath. The salt bath generally used is a mixture
of equal parts of potassium nitrate and sodium nitrate. Parts heated
by this method must be thoroughly cleansed after treatment by
washing in warm water, followed by rinsing in two baths of cold
water. The temperature of the warm water must be held below
1500 F., to prevent reduction of the corrosion resistance of the ma-
terial. The salt bath method of heating should never be used for
complicated parts and assemblies that cannot be easily washed free
of the solution.
28. Heat-treating procedure.-a. Types.-Two types of heat
treatment are applicable to aluminum alloys: the "solution" treatment
and the "precipitation" treatment. Certain alloys develop their
full strength from the solution treatment while others require both
treatments for maximum strength.
(1) The solution treatment consists of heating the metal to the tem-
perature required to cause the constituents to go into solid solution,
holding or soaking the parts at this temperature for a sufficient time
to complete the solution, followed by rapid quenching in cold water
to retain the condition. The time of transfer from the furnace to the
quench tank must not exceed seven seconds.
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/32/: accessed March 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.