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66 Fbrging-Stamping - Heat Treating
perature have been known for some time but information
on these points at the temperatures of operation is
meager or entirely lacking. It has generally been assumed
that the temperature coefficient of the alloy was
uniform throughout the whole range of temperatures
used, an assumption which, as will be seen from the
following tables, is very far from the truth.
For convenience in classification the alloys are divided
into three sub-groups:
1. Nickel-chromium alloys.
2. Nickel-iron-chromium alloys.
3. Iron-chromium alloys.
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200 400 600 600 1000
Temperature, deg. Cent.
FIG. 2—Showing the variation of electrical resistance of several
alloys at various temperatures.
Alloy No. 193 and Monel values plotted are given in Table I,
chemical compositions in Table II.
Nickel-Iron-Chromium Alloy.—Values plotted are for Alloy
No. 4, Table IV; for chemical composition see Table V.
Iron-Chromium Alloy.—Values plotted are for Alloy No. 1,
Table VI; for chemical composition see text.
Nickel-Chromium Alloys.
The simplest and best combination is the binary
alloy of nickel and chromium. Alloys containing as
high as 25 and 30 per cent of chromium have been made.
It is commercially practicable to make alloys containing
15 and 20 per cent of chromium. In general it may
be said that the resistance to oxidation bears a direct
relation to the amount of chromium in the wire. This
is certainly true for chromium contents up to 20 per
cent and probably holds for higher percentages. For
temperatures below 1000 deg. C. alloys of the Nichrome
III class containing 15 per cent of chromium
have given good service. For the range between 1000
deg. C. and 1100 deg. C. alloys of the Nichrome IV
class, containing 18 to 20 per cent of chromium, are
February, 1925
advisable. While these alloys may be used for short
periods at temperatures above 1100 deg. C. their life
under these conditions is exceedingly short.
The variations in resistance of some of these nickelchromium
wires and their specific resistances at room
temperatures are given in Table III. Typical resistance-temperature
curves are plotted in Fig. 3. These
results indicate an interesting peculiarity which seems
to be inherent in all nickel-chromium wires. In the
case of the Nichrome III alloys (15 per cent chromium)
the resistance-temperature curve rises at a uniform
rate up to 500 deg. C. From 500 to 700 deg. C,
the curve runs practically parallel to the temperature
axis, indicating a zero temperature coefficient over this
range. Above 700 deg. C. the resistance rises sharply
again. In the Nichrome IV series (18 to 20 per cent
chromium) the same general trend is followed except
that between 500 and 700 deg. C. the wire has a marked
negative temperature coefficient. This can be readily
seen in Fig. 2 on which the values for the two representative
nickel-chromium wires are plotted.
The reason for this change in slope of the resistance
temperature curve at 500 deg. C. is not at present understood.
It is not a magnetic transformation point
since the materials themselves are non-magnetic at
room temperatures. In the absence of a better explanation
we may conclude that it is due* to a change
in the molecular configuration of the elements in the
wire.
Nickel-Iron-Chromium Alloys.
Of all the alloys in this class, the alloy known as
Nichrome has received the most extended application.
This alloy contains approximately 60 per cent of
nickel, 26 per cent of iron and 12 per cent of chromium.
TABLE V.—CHEMICAL COMPOSITIONS OF THE NICK­
EL-IRON-CHROMIUM ALLOYS REPORTED
IN TABLE IV.
All values are per cent.
Alloy Nickel Iron Chromium Manganese
Nichrome
No. 2 59.29 28.70 10.75 1.54
No. 3 59.02 27.86 10.90 1.54
No. 8 59.57 26.97 11.05 1.69
No. 12 .... 61.75 24.97 11.45 1.19
No. 9 62.00 24.35 11.70 1.46
No. 7 61.20 24.88 12.05 1.44
Nichrome II
No. 1 .... 19.5
No. 2 69.35 10.53 17.95 1.58
Other Alloys
No. 3 53.58 13.77 31.35 0.00
No. 4 27.62 48745 21.10 0.85
It has a higher specific resistance than a straight nickel-chromium
alloy and a somewhat higher temperature
coefficient. It resists oxidation satisfactorily at temperatures
up to 900 cleg. C. For temperatures in excess
of this, it should be replaced by a nickel-chromium
alloy substantially free from iron.
Nichrome II is a nickel-iron-chromium alloy of intermediate
composition. It has less iron and more
chromium than Nichrome.
Attempts have been made from time to time to
diminish the nickel content of this class of materials.
But under these conditions, to maintain the high re-