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April, 192S Forging- Stamping - Heat Treating 131
1. Permanence — capability of resisting the
action of the air or of furnace gases at working
temperatures.
2. Uniform scale — continuous and regular increase
of e.m.f. for increase of temperature over the
working range.
3. Power — relatively high e.m.f.
4. Economy — low cost for service rendered.
5. Reproducibility—permitting numerous couples
to be made all having the same characteristics.
6. Constancy of calibration — minimum tendency
to take on impurities which will change the
temperature •—• e.m.f. relation of the couple.
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FIG. 85—End of Thermocouple.
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FIG. 86—Base metal couple, bare.
FIG. 87—Base metal couple, with sheath and head.
In selecting a couple the first thing generally to
be considered is the temperature range over which it
will be required to work- The higher the temperature
the more expensive the metals, as a rule.
For temperatures up to about 900 degrees C, which
cover ordinary heat treating practice, a couple consisting
of a wire of iron and one of constantan (an
alloy of copper and nickel), is extensively used. This
combination has a high e.m.f., is reasonably permanent,
and is moderate in cost.
Another widely used couple is made from two patented
heat resisting alloys known as "chromel" and
"alumel". Chromel is an alloy of chromium and nickel;
alumel an alloy of aluminum and nickel. They
have the property of resisting oxidation at high temperatures.
This couple has a somewhat lower e.m.f.
than the iron-constantan pair, and its first cost is
higher, but its life is longer at ordinary heat treating
temperatures, and it may be used as high as 1100 deg.
C, or for short periods, as high as 1300 deg. C. These
two couples, and others made of common metals, are
called "base metal" couples.
For higher temperatures, up to about 1500 deg. C,
which is the practical limit for the use of thermocouples,
a combination consisting of one wire of pure
platinum, and one of an alloy of 90 per cent platinum
and 10 per cent rhodium, should be employed.
This is known as the Le Chatelier, or "noble metal"
couple. Its e.m.f. is only about one-fifth that of the
iron-constantan couple, and its cost is quite high, but
it is the only couple which will give satisfactory service
above 1100 deg. C.
Protection Tubes and Mounting.
In order to prolong its life, protect it from mechanical
damage and from attack or contamination by
furnace gases or liquid heat treating baths in which it
may be immersed, the portion of a couple which is ex­
posed to high temperatures is usually covered by a
sheath or protection tube. The nature of this sheath
is determined by the condition under which the couple
must operate. For use with iron constantan couples
a wrought iron or low carbon steel tube, securely
plugged or welded at the end, makes a cheap and satisfactory
sheath. For use with base metal couples at
higher temperatures, tubes of special heat resisting alloys
are preferable. Nichrome and chromel tubes are
widely used and other good alloys are available. Various
refractory protection tubes, such as porcelain, fire
clay, graphite, etc., are useful in some cases, but these
have the disadvantage of being easily broken, and as
they are not such good conductors of heat as metals,
they tend to make the couple sluggish in responding to
fluctuations in the furnace temperature.
Platinum is rapidly contaminated by vapors from
other metals at high temperatures, and must be protected
from metallic vapors coming from the charge or
from metal parts of the furnace. Glazed porcelain or
fused silica tubes are usually employed and these must
be gas tight- These tubes are fragile and may be protected
from mechanical damage by an outer protection
tube of heat resisting alloys, carborundum, etc.
The thermocouple itself is generally a separate unit
from the wires or leads which connect it to the measuring
instrument. Base metal couples are usually made
of rather heavy wires (1/16 to y, in., but more frequently
about y in.), of a suitable length to extend
into the furnace to the desired point. They are welded
together at the hot end, by means of an oxyacetylene
welding flame or electric arc, sometimes being
twisted together a few turns for greater strength. In-
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FIG. 88 (left)—Rotary switch for connecting several couples
to indicator.
FIG. 89 (right)—Bus bar for connecting several couples to
recorder.
sulating beads are slipped over each wire to preve
them from short circuiting along their length. This is
illustrated in Fig. 85. These heavy wires terminate
in a "head" outside the furnace where they are connected
to lighter wires, sometimes flexible; which run
to the indicating instrument, see Fig. 86. This head
is usually mounted at the end of the protection tube,
so that the whole is a convenient unit for handling or
replacement, as shown in Fig. 87.
Extension Leads.
For base metal couples, it is customary to run wires
of the same metals but of smaller size from the head
of the couple to the indicating or recording instru-