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392 rorging - Stamping - Heat Treating November, 1925
temperature is also necessary for some of the alloy
steels. such as silicon sheets used in the manufacture
of motors, transformers, and other electrical apparatus.
Heating Mediums.
Until about four years ago, all commercial annealing
was carried on in fuel-fired furnaces using coal,
coke, oil or gas, and these methods are still in quite
general use. In the past few years, however, there
has developed in the metal work industries a recognition
of the great importance of more accurate and
uniform heat treatment of metals, including annealing.
aging and normalizing.
The tendency is now. where maximum quality and
uniformity- of product are desired, to give consideration
to the electric furnace for these operations. The
actual cost of the electric energy used will, in most
instances, be higher than the bare cost of fuel would
be. but this is frequently justified by the results to be
secured. In general, the electric furnace gives a better
and more uniform product with less loss from
scale, and quite often! savings are realized which
result in a lower over-all cost of product.
A properly designed electric furnace operates with
a verv uniform distribution of heat and at no time is
the temperature of the heat source much above the
annealing temperature. This means that the metal is
FIG. 2—Car type annealing furnace. Inside dimensions 5 ft.
wide, 6 ft. 10 in. long by 4 ft. high. Connected load 120
kw. Operating temperature 1600 deg. F.
heated through to just the right degree and will have
the uniform grain structure which characterizes a perfect
anneal. There are no hard spots to slow down
machining operations and castings are less likelv to
break under strain and. as there is no overheating nor
flow of air through the furnace, scaling will be reduced
to a minimum.
Temperature Control.
The furnace temperature is controlled automatically
and is maintained more closely and accurately
than is possible with the average fuel-heated furnace,
and this is accomplished with a minimum of attendant
labor. Usually the annealing process may be
carried out over night, thus permitting the use of
off peak power purchased at minimum cost. Night
operation does not necessarily require night labor or
FIG. 3—Car bottom direct heat furnace for annealing steel
castings. 1600 deg. F. Weight of charge 8 tons. Inside
dimensions 6y2 ft. wide, 9 ft. deep, 3 ft. high.- Connected
load 200 kw.
attendance because the furnace may be charged late
in the day, after which the power is turned on in the
evening by a time switch which subsequently cuts off
the furnace at the end of a pre-determined time period.
The charge is removed in the morning when the men
have returned to work. Often the same furnace is
used during the day for other heat-treating operations.
Where the work to be annealed must be kept free
from scale, as is the case with punchings, it is necessary
to pack the steel in annealing boxes before loading
into the fuel-fired furnace. These boxes are closed
with a cover to exclude the air and, if made of iron,
they are of heavy construction to resist the oxidation
which results from repeated use. Boxes made of heatresisting
alloys have a longer life but cost much more.
In either case, fuel is consumed in heating the boxes
and( when it is realized that, not infrequently, the
weight of boxes equals that of their contents, it is
obvious to what extent these containers reduce the
capacity of the furnace and increase the amount of
fuel required for annealing.
Characteristics of Electric Heating.
The characteristics of the electric furnace permit a
construction and method of loading which result in
the furnace itself serving as the annealing box or container.
Thus, the electric furnace eliminates the items
of expense for containers, saves the cost of energy for
heating the latter and gives greater productive capacity
with less floor space.
There are other advantages for the electric furnace
which are important, if somewhat intangible. Improvement
of shop conditions is now generally recognized
as beneficial to both employer and employee.
The latter is appreciative and does better work and
more of it, if the surrounding temperature permits
him to work in comfort and the air he breathes is free