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August, 1925
with that of Fig. 4 (the overheated bar) it can be
seen that the dark constituent has apparently segregated
and melted out. This results in a lack of cohesion
between the crystal grains and a very light force
is sufficient to rupture the metal completely. It is
therefore absolutely essential, in forging duralumin,
to take every precaution to ensure that the bar, billet
or forging never exceeds a temperature of 500 deg. C.
On the other hand, it is possible to forge duralumin
at temperatures well below 500 deg. C. in a fairly
satisfactory manner. A piece of bar iy in. round was
heated to 490 deg. C, and drawn at the one heat to
1 in. square, and then to the thickness of a knife blade.
The piece was fashioned to the shape of a knife, Fig.
5. A quick light blow was used, and the test was
made in order to see how far the bar could be reduced
before fracture took place.
Cold work hardens the alloy considerably and if
carried to extreme would eventually break it up. In
the production of duralumin forgings, therefore, it is
very necessary that a temperature of 500 deg. C,
should never be exceeded, but that any variation from
the correct temperature of 490 deg. C. should be on
the cold side. The correct range for forging may be
stated to be 470 deg.-500 deg. C. This has given satisfactory
results in practice.
Furnace and Temperature.
The design and regulation of furnaces to heat bars,
billets and forgings of duralumin in the range for forging,
4700 deg. C. to 500 deg. C. is not at all a simple
matter. Moreover, forgers are not accustomed to
handle steel at such low temperatures and require
some time to get into the knack of working. At a
temperature of 500 deg. C. no visible light is emitted.
Even a dull red glow indicates temperature of 600 deg.
C, which is far and away too high for duralumin.
After due consideration of all aspects of the question
an oil fired muffle furnace was chosen as suitable for
heating duralumin and has so proved in practice.
Forging- Stamping - Heat Treating
261
For small work a fireclay muffle 16 in. on the sill,
by 2 in. high by 18 in. from sill to back, with walls 2
in. thick was used. This was contained in, a suitable
chamber about 3 ft. by 2 ft. (outside measurements)
solidly constructed and bound with iron, and supported
at a convenient height. The roof was slightly
arched and made removable.
For small muffles it is sufficient to use one burner
of the fine needle-valve type, with both oil and air
under pressure, and this proves quite up to the work.
It is set in the side of the furnace above the level of
the muffle so as to avoid the direct impinging of the
hot flame on one end of the muffle. The flame passes
along the top of the muffle, down one side, along the
bottom, and out by two vertical flues leading from the
bottom of the furnace to just above the roof. Although
no definite provision is made for heating the
end of the muffle nearest the burner, it is found that
by radiation the heat becomes even all over the muffle.
With large sized muffles more difficulty is experienced
in obtaining an even heat, but the problem
is not incapable of solution.
With all oil fired furnaces it is better to have the
air pipe leading down to the burner instead of up to it,
so that, if any oil leaks at any time through the valve,
it cannot run into the air pipe and collect with the re­
FIG. 1—Photomicrograph of duralumin immediately on
sult that when air is turned on, a large quantity of
quenching. X 75 diams. Etched with hot 25 per cent
oil is blown along with it.
nitric acid.
The furnace temperature must be over 500 deg.
Once the fusible constituent has melted, the metal is C. in order to avoid waste of time waiting until the
ruined and it is, therefore, of no use to allow an over­ bars attain that temperature, and so some relation
heated bar to cool down to 500 deg. C. before forging must be established experimentally between the fur­
under the belief that all will be well. It is simply imnace temperature and the length of time required to
possible to do anything with the metal. In this re­ heat certain sized stock to 500 deg. C. It is essential
spect duralumin and burnt steel are similar.
to know the furnace temperature to provide a bas;s on
FIG. 2—Photomicrograph of duralumin "aged." X 75 diams.
Etched with hot 25 per cent nitric acid. The small "lakes"
of the dark constituent (CuAL) have diffused somewhat
into the light ground (Al).
which to work. This is accomplished by fitting a
base-metal thermo-couple enclosed in a silica sheath
giving direct temperature readings on a galvanometer.
In order that any temperature change shall be recorded
immediately it is advisable to dispense with the
usual iron pyrometer sheath which causes "lag" between
furnace temperatures and the galvanometer. By
placing the couple in one corner of the muffle alrng
the floor, with the head only projecting a short distance,
it is possible to avoid breaking the sheath, al-