quench bare 2017 and 2024 due to the effect on their
reached. Further aging (overaging) causes the strength
corrosion resistance.
to decline until a stable condition is obtained. The
strengthening of the material is due to the uniform
Annealing
alignment of the molecule structure of the aluminum
and alloying element.
Annealing serves to remove the strain hardening
Artificially aged alloys are usually slightly
that results from cold working and, in the case of the
"overaged" to increase their resistance to corrosion,
heat-treated alloys, to remove the effect of the heat
especially the high copper content alloys. This is done
treatment. Annealing is usually carried out in air
to reduce their susceptibility to intergranular corrosion
furnaces, but salt baths may be used if the melting point
caused by underaging.
of the bath is low enough. A bath made up of equal parts
by weight of sodium nitrate and potassium nitrate is
Natural aging alloys can be artificially aged;
satisfactory.
however, it increases the susceptibility of the material
to intergranular corrosion. If used, it should be limited
ANNEALING OF WORK HARDENED
to clad sheet and similar items.
MATERIAL.--Annealing of material that was
initially in the soft or annealed condition but which has
Quenching
been strain-hardened by cold working, such as 1100,
3003, 5052, etc., is accomplished by heating the metal
The basic purpose for quenching is to prevent the
to a temperature of 349 5C (660 10F). It is only
immediate re-precipitation of the soluble constituents
necessary to hold the metal at this temperature for a
after heating to solid solution. To obtain optimum
sufficient length of time to make certain that the
physical properties of aluminum alloys, rapid
temperature in all parts of the load has been brought
quenching is required. The recommended time interval
within the specified range. If the metal is heated
between removal from the heat and immersion is 10
appreciably above 354C (670F), there is a partial
seconds or less. Allowing the metal to cool before
solution of the hardening constituents, and the alloy
quenching promotes intergranular corrosion and
will age harden while standing at room temperature
slightly affects the hardness. There are three methods
unless it has been cooled very slowly. If the temperature
employed for quenching. The one used depends upon
is not raised to 343C (650F), the softening may not be
the item, alloy, and properties desired.
complete. The rate of cooling from the annealing
COLD WATER QUENCHING.--Small parts
temperature is not important. However, a slow cool is
made from sheet, extrusions, tubing, and small fairings
desirable in case any part of the load may have been
are normally quenched in cold water. The temperature
heated above the recommended temperature range.
before quenching should be 85F or less. Sufficient
A N N E A L I N G O F H E AT- T R E AT E D
cold water should be circulated within the quenching
ALLOYS.--The heat-treatable alloys are annealed to
tanks to keep the temperature rise under 20F. This type
remove the effects of strain hardening or to remove the
of quench will ensure good resistance to corrosion, and
effects of solution heat treatment.
is particularly important when heat-treating 2017 and
2024 alloys.
To remove strain hardening due to cold work, a
1-hour soak at 640 to 660F, followed by air-cooling,
HOT WATER QUENCHING.--Large forgings
is generally satisfactory. This practice is also
and heavy sections can be quenched in hot or boiling
satisfactory to remove the effects of heat treatment if
water. This type of quench is used to minimize
the maximum of softness is not required.
distortion and cracking, which are produced by the
unequal temperatures obtained during the quenching
To remove the effects of partial or full heat
operation. The hot water quench will also reduce
treatment, a 2-hour soak at 750 to 800F, followed by a
residual stresses, which improves resistance to stress
maximum cooling rate of 50 per hour to 500F, is
corrosion cracking.
required to obtain maximum softness.
SPRAY QUENCHING.--Water sprays are used
To remove the effects of solution heat treatment or
to quench parts formed from alclad sheets and large
hardening due to cold work, the high zinc-bearing alloy
sections of most alloys. Principal reasons for using this
7075 should be soaked 2 hours at 775F, air cooled to
method are to minimize distortion and to alleviate
450, and soaked 6 hours at 450. The stabilizing
quench cracking. This system is not usually used to
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