Category: CASE-HARDENING OR SURFACE-CARBURIZING
Carburizing, commonly called case-hardening, is the art of producing
a high-carbon surface, or case, upon a low carbon steel article.
Wrenches, locomotive link motions, gun mechanisms, balls and ball
races, automobile gears and many other devices are thereby given
a high-carbon case capable of assuming extreme hardness, while
the interior body of metal, the core, remains soft and tough.
The simplest method is to heat the piece to be hardened to a bright
red, dip it in cyanide of potassium (or cover it by sprinkling
the cyanide over it), keep it hot until the melted cyanide covers
it thoroughly, and quench in water. Carbon and nitrogen enter the
outer skin of the steel and harden this skin but leave the center
soft. The hard surface or case varies in thickness according to
the size of the piece, the materials used and the length of time
which the piece remains at the carburizing temperature. Cyanide
case-hardening is used only where a light or thin skin is sufficient.
It gives a thickness of about 0.002 in.
In some cases of cyanide carburizing, the piece is heated in cyanide
to the desired temperature and then quenched. For a thicker case
the steel is packed in carbon materials of various kinds such as
burnt leather scraps, charcoal, granulated bone or some of the
many carbonizing compounds.
Machined or forged steel parts are packed with case-hardening material
in metal boxes and subjected to a red heat. Under such conditions,
carbon is absorbed by the steel surfaces, and a carburized case is
produced capable of responding to ordinary hardening and tempering
operations, the core meanwhile retaining its original softness and
Such case-hardened parts are stronger, cheaper, and more serviceable
than similar parts made of tool steel. The tough core resists breakage
by shock. The hardened case resists wear from friction. The low cost
of material, the ease of manufacture, and the lessened breakage
in quenching all serve to promote cheap production.
For successful carburizing, the following points should be carefully
The utmost care should be used in the selection of pots for carburizing;
they should be as free as possible from both scaling and warping.
These two requirements eliminate the cast iron pot, although many
are used, thus leaving us to select from malleable castings, wrought
iron, cast steel, and special alloys, such as nichrome or silchrome.
If first cost is not important, it will prove cheaper in the end
to use pots of some special alloy.
[Illustrations: FIGS. 27 to 30.--Case-hardening or carburizing boxes.]
The pots should be standardized to suit the product. Pots should be
made as small as possible in width, and space gained by increasing
the height; for it takes about 1-1/2 hr. to heat the average small
pot of 4 in. in width, between 3 and 4 hr. to heat to the center
of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in.
box; and the longer the time required to heat to the center, the
more uneven the carburizing.
The work is packed in the box surrounded by materials which will
give up carbon when heated. It must be packed so that each piece
is separate from the others and does not touch the box, with a
sufficient amount of carburizing material surrounding each. Figures
27 to 31 show the kind of boxes used and the way the work should be
packed. Figure 31 shows a later type of box in which the edges can
be easily luted. Figure 30 shows test wires broken periodically to
determine the depth of case. Figure 28 shows the minimum clearance
which should be used in packing and Fig. 29 the way in which the
outer pieces receive the heat first and likewise take up the carbon
before those in the center. This is why a slow, soaking heat is
necessary in handling large quantities of work, so as to allow
the heat and carbon to soak in equally.
While it has been claimed that iron below its critical temperature
will absorb some carbon, Giolitti has shown that this absorption
is very slow. In order to produce quick and intense carburization
the iron should preferably be above its upper critical temperature
or 1,600 deg.F.,--therefore the carbon absorbed immediately goes into
austenite, or solid solution. It is also certain that the higher
the temperature the quicker will carbon be absorbed, and the deeper
it will penetrate into the steel, that is, the deeper the case.
At Sheffield, England, where wrought iron is packed in charcoal and
heated for days to convert it into blister steel, the temperatures
are from 1,750 to 1,830 deg.F. Charcoal by itself carburizes slowly,
consequently commercial compounds also contain certain energizers
which give rapid penetration at lower temperatures.
The most important thing in carburizing is the human element. Most
careful vigilance should be kept when packing and unpacking, and the
operator should be instructed in the necessity for clean compound
free from scale, moisture, fire clay, sand, floor sweepings, etc.
From just such causes, many a good carburizer has been unjustly
condemned. It is essential with most carburizers to use about 25 to
50 per cent of used material, in order to prevent undue shrinking
during heating; therefore the necessity of properly screening used
material and carefully inspecting it for foreign substances before
it is used again. It is right here that the greatest carelessness
is generally encountered.
Don't pack the work to be carburized too closely; leave at least
1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the
top of pots, and for a 6-hr. run, have the pieces at least 1/2
in. apart. This gives the heat a chance to thoroughly permeate
the pot, and the carburizing material a chance to shrink without
allowing carburized pieces to touch and cause soft spots.
Good case-hardening pots and annealing tubes can be made from the
desired size of wrought iron pipe. The ends are capped or welded,
and a slot is cut in the side of the pot, equal to one quarter of
its circumference, and about 7/8 of its length. Another piece of
the same diameter pipe cut lengthwise into thirds forms a cover
for this pot. We then have a cheap, substantial pot, non-warping,
with a minimum tendency to scale, but the pot is difficult to seal
tightly. This idea is especially adaptable when long, narrow pots
When pots are packed and the carburizer thoroughly tamped down,
the covers of the pot are put on and sealed with fire clay which
has a little salt mixed into it. The more perfect the seal the
more we can get out of the carburizer. The rates of penetration
depend on temperature and the presence of proper gas in the required
volume. Any pressure we can cause will, of course, have a tendency
to increase the rate of penetration.
If you have a wide furnace, do not load it full at one time. Put
one-half your load in first, in the center of the furnace, and
heat until pots show a low red, about 1,325 to 1,350 deg.F. Then fill
the furnace by putting the cold pots on the outside or, the section
nearest the source of heat. This will give the work in the slowest
portion of the furnace a chance to come to heat at the same time
as the pots that are nearest the sources of heat.
To obtain an even heating of the pots and lessen their tendency
to warp and scale, and to cause the contents of the furnace to
heat up evenly, we should use a reducing fire and fill the heating
chamber with flame. This can be accomplished by partially closing
the waste gas vents and reducing slightly the amount of air used
by the burners. A short flame will then be noticed issuing from
the partially closed vents. Thus, while maintaining the temperature
of the heating chamber, we will have a lower temperature in the
combustion chamber, which will naturally increase its longevity.
Sometimes it is advisable to cool the work in the pots. This saves
compound, and causes a more gradual diffusion of the carbon between
the case and the core, and is very desirable condition, inasmuch
as abrupt cases are inclined to chip out.
The most satisfactory steel to carburize contains between 0.10
and 0.20 per cent carbon, less than 0.35 per cent manganese, less
than 0.04 per cent phosphorus and sulphur, and low silicon. But
steel of this composition does not seem to satisfy our progressive
engineers, and many alloy steels are now on the market, these,
although more or less difficult to machine, give when carburized
the various qualities demanded, such as a very hard case, very tough
core, or very hard case and tough core. However, the additional
elements also have a great effect both on the rate of penetration
during the carburizing operation, and on the final treatment,
consequently such alloy steels require very careful supervision
during the entire heat treating operations.
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