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Crankshaft
The crankshaft was the most highly stressed part of the entir...

Preventing Cracks In Hardening
The blacksmith in the small shop, where equipment is usually ...

Alloying Elements
Commercial steels of even the simplest types are therefore p...

Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

Robert Mushet
Robert (Forester) Mushet (1811-1891), born in the Forest of D...

Silicon
Silicon prevents, to a large extent, defects such as gas bubb...

Double Annealing
Water annealing consists in heating the piece, allowing it to...

Heat Treatment Of Milling Cutters Drills Reamers Etc
THE FIRE.--Gas and electric furnaces designed for high heats ...

Carburizing By Gas
The process of carburizing by gas, briefly mentioned on page ...

An Automatic Temperature Control Pyrometer
Automatic temperature control instruments are similar to the ...

Chromium
Chromium when alloyed with steel, has the characteristic func...

Rate Of Absorption
According to Guillet, the absorption of carbon is favored by ...

Hardening High-speed Steels
We will now take up the matter of hardening high-speed steels...

Temperature For Annealing
Theoretically, annealing should be accomplished at a tempera...

A Satisfactory Luting Mixture
A mixture of fireclay and sand will be found very satisfactor...

Heat Treatment Of Gear Blanks
This section is based on a paper read before the American Gea...

The Electric Process
The fourth method of manufacturing steel is by the electric f...

For Milling Cutters And Formed Tools
FORGING.--Forge as before.--ANNEALING.--Place the steel in a ...

Machineability
Reheating for machine ability was done at 100 deg. less than ...

Lathe And Planer Tools
FORGING.--Gently warm the steel to remove any chill, is parti...



Effect Of Different Carburizing Material






Category: CASE-HARDENING OR SURFACE-CARBURIZING

[Illustrations: FIGS. 33 to 37.]

Each of these different packing materials has a different effect
upon the work in which it is heated. Charcoal by itself will give
a rather light case. Mixed with raw bone it will carburize more
rapidly, and still more so if mixed with burnt bone. Raw bone and
burnt bone, as may be inferred, are both quicker carbonizers than
charcoal, but raw bone must never be used where the breakage of
hardened edges is to be avoided, as it contains phosphorus and
tends to make the piece brittle. Charred leather mixed with charcoal
is a still faster material, and horns and hoofs exceed even this
in speed; but these two compounds are restricted by their cost
to use with high-grade articles, usually of tool or high-carbon
steel, that are to be hardened locally--that is, pack-hardened.
Cyanide of potassium or prussiate of potash are also included in
the list of carbonizing materials; but outside of carburizing by
dipping into melted baths of this material, their use is largely
confined to local hardening of small surfaces, such as holes in
dies and the like.

Dr. Federico Giolitti has proven that when carbonizing with charcoal,
or charcoal plus barium carbonate, the active agent which introduces
carbon into the steel is a gas, carbon monoxide (CO), derived by
combustion of the charcoal in the air trapped in the box, or by
decomposition of the carbonate. This gas diffuses in and out of
the hot steel, transporting carbon from the charcoal to the outer
portions of the metal:

If energizers like tar, peat, and vegetable fiber are used, they
produce hydrocarbon gases on being heated--gases principally composed
of hydrogen and carbon. These gases are unstable in the presence of
hot iron: it seems to decompose them and sooty carbon is deposited
on the surface of the metal. This diffuses into the metal a little,
but it acts principally by being a ready source of carbon, highly
active and waiting to be carried into the metal by the carbon
monoxide--which as before, is the principal transfer agent.

Animal refuse when used to speed up the action of clean charcoal
acts somewhat in the same manner, but in addition the gases given
off by the hot substance contain nitrogen compounds. Nitrogen and
cyanides (compounds of carbon and nitrogen) have long been known
to give a very hard thin case very rapidly. It has been discovered
only recently that this is due to the steel absorbing nitrogen
as well as carbon, and that nitrogen hardens steel and makes it
brittle just like carbon does. In fact it is very difficult to
distinguish between these two hardening agents when examining a
carburized steel under the microscope.

One of the advantages of hardening by carburizing is the fact that
you can arrange to leave part of the work soft and thus retain
the toughness and strength of the original material. Figures 33
to 37 show ways of doing this. The inside of the cup in Fig. 34
is locally hardened, as illustrated in Fig. 34, spent or used
bone being packed around the surfaces that are to be left soft,
while cyanide of potassium is put around those which are desired
hard. The threads of the nut in Fig. 35 are kept soft by carburizing
the nut while upon a stud. The profile gage, Fig. 36, is made of
high-carbon steel and is hardened on the inside by packing with
charred leather, but kept soft on the outside by surrounding it
with fireclay. The rivet stud shown in Fig. 37 is carburized while
of its full diameter and then turned down to the size of the rivet
end, thus cutting away the carburized surface.

After packing the work carefully in the boxes the lids are sealed
or luted with fireclay to keep out any gases from the fire. The
size of box should be proportioned to the work. The box should
not be too large especially for light work that is run on a short
heat. If it can be just large enough to allow the proper amount
of material around it, the work is apt to be more satisfactory
in every way.

Pieces of this kind are of course not quenched and hardened in
the carburizing heat, but are left in the box to cool, just as in
box annealing, being reheated and quenched as a second operation.
In fact, this is a good scheme to use for the majority of carburizing
work of small and moderate size. Material is on the market with which
one side of the steel can be treated; or copper-plating one side
of it will answer the same purpose and prevent that side becoming
carburized.





Next: Quenching The Work

Previous: Quenching



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