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November, 1925
Forging- Stamping - Heat Treating
H i g h - F r e q u e n c y I n d u c t i o n F u r n a c e
Although Still in Its Early Stages of Development, It Provides an
T H E use of high-frequency currents for induction
heating has added a new series of physical conditions
to those under which metallurgical operations
may be performed. Although still in its earlystages
of development, it provides an unrivalled means
for rapid experimental and research work on alloy
steels. It is being used for several metallurgical
operations of the utmost importance, notably in the
preparation of new special alloys having desirable
electrical characteristics, which offer -nth opportunities
for acceleration in submarine telegraphy that they
will have a marked influence on inter-continental communications
and commerce.
The early work in connection with high-frequency
furnaces was carried out by Dr. Northrup at Princeton
University, whose brilliant studies on the physical
laws governing induction from high-frequency equipment
led to the evolution of the first metal melting
furnaces.
Low-frequency induction furnaces have been
known for 40 years, and 20 years ago it seemed possible
that they would to a large extent replace the
crucible for high grade steel making. Further development
was, however, prevented by the high capital
expenditure necessary on electrical plant to obtain
the very low frequencies required, the impossibility of
making different alloys in succession, owing to the
fact that part of the charge must be left in the furnace
after each heat to start the next, and the advent of the
arc furnace with its greater refining power. No lowfrequency
induction furnaces are now being used for
steel making in Great Britain, and comparatively few
abroad.
Inductive heating has recently found wide application
in the non-ferrous trade in furnaces having a
vertical slot worked on normal commercial frequencies.
But all these furnaces have the disadvantage
of requiring an iron core, as the result of which the
molten metal is contained in small channels surrounding
the iron core as well as in the main bath of the
furnace. This entails wear and tear of the refractory
material, and great length of furnace banks, as compared
with the total weight of the metal contained in
the furnace.
In the case of high-frequency heating, the ideal
form of container is available, a cylindrical vessel holding
a maximum metal with a minimum of radiating
surface and refractory of material exposed to corrosive
action.
Obviously, inductive heating is superior in thermal
efficiency to any arc furnace or fuel-fired equipment,
owing to the fact that the heat is actually generated
in the charge to be melted, and consequently
there is no loss of heat in the passage through refrartory
containers as in crucible furnaces, or in flue gases
as in the case of open hearth or reverberatory furnaces.
•Paper presented before the (British) Iron and Steel Institute,
September, 1925, meeting.
-[•London, England.
Unrivalled Means for Rapid Experimental and
Research Work on Alloy Steels
By DONALD F. CAMPBELLf
401
The high-frequency furnace has an immensely
steep heat gradient between the molten metal and the
outside of the furnace, the distance between molten
platinum at, say, 1900 deg. C, and a water-cooled copper
coil being only y inch in platinum melting furnaces.
This somewhat extraordinary fact does not
produce excessive heat losses owing to the rate of input
of calories into the charge, and the loss.Tieing a
function of time, is only- considerable during the last
few minutes of the melting operation, when the temperature
of the metal is high.
FIG. 1—Sectional view of high frequency crucile furnace.
The great disadvantage of high-frequency melting
lies in the difficulty and expense of obtaining the
necessary^ equipment to produce high-frequency currents,
this problem being still in its early state of development,
but its solution is advancing along several
different lines with great rapidity.
aA. furnace installation consists of a source of highfrequency
current, with suitable power factor correction,
and a furnace of extreme simplicity' of design.
High-frequency currents may be obtained by
means of rotary generators, valves, or a combinationof
spark gaps and condensers. The latter equipment
was first used by Dr. Northrup, and it has been applied
in the case of the many small furnaces used for