Effect Of Different Carburizing Material

: The Working Of Steel

[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
he 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