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August, 1925
Heat Treatment.
In the heat treatment of duralumin forgings accurate
pyrometer control is absolutely essential. Full
treatment consists of heating to 480 deg. C, quenching'in
water and "ageing." Heating may be carried
out in muffle furnaces, oil or gas fired, but where
electricity is cheap electrically heated muffles would
give very good results. A bath of molten salts (sodium
nitrite and potassium nitrate) may be used, but
blistering may possibly be caused. On the other hand,
the danger of overheating is considerably less with a
salt bath, especially if accurate pyrometric control is
obtainable. A point against the salt bath is the loss
of the fused salts which adhere to the forgings on
removal from the bath. Each method has its advantages
and if properly controlled would give satisfactory
results. "Ageing" can be accelerated by hn.l.ng
the forgings in water. At the end of six hours, "ageing"
is practically complete, so that even a short
boiling assists materially in shortening the time required
for the attainment of the maximum strength.
FIG. 5—Knife forged at one heat from the 1^-in
diameter bar shown.
The annealing operation on duralumin consists of
heating to 350 deg. C, preferably in salt baths owing
to the low temperature required, followed by cooling
in air. Little or no "ageing" takes place after annealing
and the mechanical properties are approximately :
Yield point — tons per sq. in.. . . 7
Maximum stress — tons per sq. in. 16
Elongation — per cent 14
Any machine operations can be carried out on the
forging, either in the annealed or the fully heat-treated
condition without ill-effect provided that the workis
not allowed to get hotter than 100 deg. C. (boiling
water).
Pickling.
The removal of the black surface caused by the
use of forging oil and the restoration of the characteristic
aluminum appearance of a forging is quite simply
done either before or after heat-treatment.
The forgings are placed in a hot strong solution
of caustic soda in water and left for a time. Oil and
grease will be removed, and the aluminum is attacked
by the soda. If the attack is too vigorous, or the
articles are left in too long "pitting" will result. They
are removed soon after the soda has begun to attack
them, rinsed in water and well brushed. Sometimes
Forging- Stamping Heat Treating 26.5
this does not remove the black deposit which is very
adherent. Immersion in diluted (10 per cent) sulphuric
acid for a time, followed by rinsing and vigorous
brushing, will generally prove efficacious. If not,
a further soda and acid treatment will be necessary.
It is hardly necessary to point out that the soda
should not be contained in a metal vessel, as it will
attack it rapidly. An earthenware vessel is most suitable.
After this treatment a thorough rinsing in boiling
water is necessary to remove all acid or soda, otherwise
\ery rapid corrosion of the duralumin occurs.
Examination.
The main defects in duralumin forgings have been
indicated—"laps", "runs", "chops" and forging in
loose metal. Any of these defects may easily be seen
by pickling in soda and acid as above. The macrostructure
and flow of the metal can also be brought
ii]) by long etching in soda, followed by cleansing in
acid as indicated.
An_\- sandblasting must be done with very fine
sand and a low pressure, otherwise a very poor surface,
full of "pits" is obtained. Grinding operations
should not 1>e carried out on duralumin forgings as
(he fine "swarf" is very easily ignited by a spark.
Water should not be used to quench an igr.ited aluminum,
but damp sand is efficacious. Filing, although
slower, is safer than grinding, but aluminum powder
and iron scale (iron oxide) mixed together form the
powerful explosive, thermite. Ignition of thermite is
difficult compared to other explosives, but there is
considerable danger in having such a mixture lying
around, particularly near grinding wheels. Where
steel and duralumin forgings pass through an inspection
department together, precautions must be taken
against mixing aluminum filings and iron scale, where
there is any danger of igniting them. Apart from
these two points there is no clanger with duralumin.
GAS FUEL IDEAL FOR JAPANNING
One of the most popular uses for city gas is for
japanning automobile parts, stoves, ranges, and all
kinds of sheet metal goods for domestic use. A peculiar
feature about japan is that with the same japan the
heat of the oven must be regulated with due consideration
for the temperature to which the metal will be
exposed in use. That is. a baking temperature of 200
deg. F may prove entirely satisfactory when used on
an automobile. However, if no more than that were
used in japanning a baking range, the japan would not
be able to stand up under kitchen usage. The gas
range oven is ordinarily used at a temperature of 400
deg. F. and under.
In the case of enameling, which is also done largely
by gas. even greater care must be taken to bake the
enamel at a much higher temperature than the product
in use will be subjected to. The enamel used on
electric reflectors that are used in outdoor lighting
and for gas ranges has to be baked at from 1,500 to
2.000 deg.
The use of city gas in the japan oven results in
many advantages and economies because it permits of
very close control of the temperature of the oven thus
speeding up the production.