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December, 1925
Carburizing Processes.
Carburizing processes are usually classified under
three headings, according to the materials used:
1—Solid;
2—Liquid;
3—Gas.
Strictly speaking, there are but two methods of
carburizing, namely by liquid and gas, since solid
carburizing materials owe their action to the gases
which they generate, when heated to the carburizing
temperatures. Carbon may be absorbed by steel from
either a liquid or gaseous material of suitable composition,
with which it is in contact at sufficiently elevated
temperatures, but little or no carbon is absorbed directly
from a solid carbonaceous material. This has
been proved by tests under a vacuum.
The distinction between the solid and gas process
lies chiefly in the fact that, in the former, the parts
are packed in a container together with the solid carburizing
material, and upon heating, the carburizing
gases are generated within the container, and act
on the steel; whereas in the gas process the parts
are placed in a container through which gas generated
from an external source is caused to flow. The use
of solid carburizers is also referred to as "pack
hardening."
In the liquid process, the parts are immersed in a
bath of a molten salt, or combination of salts of suitable
composition, or the salt is applied in granular
form to the previously heated part, whereupon it
melts and spreads over the surface.
Each of these processes, solid, liquid, and gas, has
distinctive characteristics and advantages, which fit
it for certain types of work.
Solid Process.
When carburizing in solid materials, the parts are
packed in a container with a layer of carburizing material
on the bottom and top, and well surrounding each
piece. Although carbon is absorbed from the gas
generated when the compound is heated, and not
directly from the compound itself, it will be found
that parts which are uncovered, in contact with each
other, or not well surrounded by the compound during
treatment, will have an inferior case, or none at all.
The container usually consists of a box or pot
provided with a cover, and may be of cast iron, cast
or pressed steel, or one of a number of special heat
resisting alloys. For emergency work, a satisfactory
container may be made from steel pipe, with a threaded
cap for a cover, or by welding sheet steel into a box.
The cover should be sealed with fire clay, to retard,
insofar as possible, the escape of carburizing gases,
and prevent the entry of air or oxidizing furnace gases.
If screwed on, the cover should have a small hole to
relieve the pressure of the internal expanding gases on
heating, otherwise an explosion may result.
The size and shape of the container should be
adapted to the work in hand. In large containers a
long time is required for the heat to penetrate to the
center, since solid carburizing materials are relatively
poor heat conductors. Very small containers, on the
other hand, require more labor in handling and packing,
and their own weight and heat capacity is large
in comparison with that of the charge they contain.
Round containers are less likely to lose their shape
by warping, than rectangular ones. All containers
should have stubby feet to raise them from the furnace
floor, so as to allow uniform heating all around-
rorging- Stamping - Heat Treating
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The properly packed and sealed containers are
placed in a suitable furnace, whose temperature is then
raised to, or slightly above, that desired for the carburizing
action, and held for a long enough time to
heat the charge uniformly to the desired temperature,
and maintain it at this temperature long enough
to produce a case of the thickness intended.
The time required for the material at the center of
the container of certain size, to reach carburizing temperature,
may be a large part of the total time required
FIG. 143a—Case of carburized machine steel. (35x.) Somewhat
"abnormal" structure, indicating inferior steel. Hypereutectoid
surface at left, core at right. (T. W. Downes.)
FIG. 143b—Same as 143a, showing surface at higher magnification.
(100 x.) Abnormal arrangement of free cementite.
(T. W. Downes.)
for treatment. This time may be determined experimentally
by removing containers after various periods,
and noting when a case has begun to form. A much
better way is to insert a thermocouple, sheathed in a
thin steel tube, to the center of the container, thru a
hole in the wall or cover. (The hole must, of course,
be sealed), and observe the actual rise of temperature.
This temperature is then held long enough to produce
the required depth of case. The furnace tern-