retains this condition, which results in a considerable
the bath, the parts are quenched in water. The case
improvement in the strength characteristics.
obtained in this manner is due principally to the
formation of carbides on the surface of the steel. The
The heating of aluminum alloy should be done in
use of a closed pot is required for cyaniding, as cyanide
an electric furnace or molten salt bath. The salt bath
vapors are extremely poisonous.
generally used is a mixture of equal parts of potassium
nitrate and sodium nitrate. Parts heated by this method
Nitriding.--This method of case hardening is
must be thoroughly washed in water after treatment.
advantageous because a harder case is obtained than by
The salt bath method of heating should never be used
carburizing. Nitriding can only be applied to certain
for complicated parts and assemblies that cannot be
special steel alloys, one of the essential constituents of
easily washed free of the salt.
which is aluminum. The process involves the soaking
of the parts in the presence of anhydrous ammonia at a
Heat Treating Procedures
temperature below the critical point of the steel. During
the soaking period, the aluminum and iron combine
There are two types of heat treatment applicable to
with the nitrogen of the ammonia to produce iron
aluminum alloys. They are known as solution and
nitrides in the surface of the metal. Warpage of work
precipitation heat treatment. Certain alloys develop
during nitriding can be reduced by stress-relief
their full strength from the solution treatment, while
others require both treatments for maximum strength.
no higher than 538C (1,000F). Growth of the work is
similarly prevented, but cannot be entirely eliminated,
The NA 01-1A-9 lists the different temper
and some parts may require special allowance in some
designations assigned to aluminum alloys and gives an
dimensions to take care of growth.
example of the alloys using these temper designations.
The temperature required for nitriding is 510C
SOLUTION HEAT TREATMENT.--The solu-
(950F), and the soaking period from 48 to 72 hours.
tion treatment consists of heating the metal to the
An airtight container must be used, and it should be
temperature required to cause the constituents to go
provided with a fan to produce good circulation and
into a solid solution. To complete the solution, often the
even temperature throughout. No quenching is
metal is held at a high temperature for a sufficient time,
required, and the parts may be allowed to cool in air.
and then quenched rapidly in cold water to retain this
condition. It is necessary that solution heat treatment of
HEAT TREATMENT OF NONFERROUS
aluminum alloys be accomplished within close limits in
METALS (ALUMINUM ALLOYS)
reference to temperature control and quenching. The
temperature for heat-treating is usually chosen as high
Aluminum is a white, lustrous metal, light in
as possible without danger of exceeding the melting
weight and corrosion resistant in its pure state. It is
point of any element of the alloy. This is necessary
ductile, malleable, and nonmagnetic. Aluminum
to obtain the maximum improvement in mechanical
combined with various percentages of other metals,
properties. If the maximum specified temperature is
generally copper, manganese, and magnesium, form
exceeded, eutectic melting will occur. The consequence
the aluminum alloys that are used in aircraft
will be inferior physical properties, and usually a
construction. Aluminum alloys are lightweight and
severely blistered surface. If the temperature of the heat
strong, but do not possess the corrosion resistance of
treatment is low, maximum strength will not be
pure aluminum and are generally treated to prevent
obtained.
deterioration. "Alclad" is an aluminum alloy with a
PRECIPITATION (AGE) HARDENING.--The
protective coating of aluminum to make it almost equal
precipitation treatment consists of "aging" material
to the pure metal in corrosion resistance.
previously subjected to solution heat treatments by
Several of the aluminum alloys respond readily to
natural (occurs at room temperature) or artificial aging.
heat treatment. In general, this treatment consists of
Artificial aging consists of heating aluminum alloy to a
heating the alloy to a known temperature, holding this
specific temperature and holding for a specified length
temperature for a definite time, then quenching the part
of time. During this hardening and strengthening
to room temperature or below. During the heating
operation, the alloying constituents in solid solution
process, a greater number of the constituents of the
precipitate out. As precipitation progresses, the
metal are put into solid solution. Rapid quenching
strength of the material increases until the maximum is
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