range. Specifically, the carburizing steels are those that
Steel is usually subjected to the annealing process
contain no more than 0.20 percent carbon. The lower
for the following purposes:
the carbon content in the steel, the more readily it will
absorb carbon during the carburizing process.
and brittleness.
The amount of carbon absorbed and the thickness
2. To refine the crystalline structure and remove
of the case obtained increase with time; however, the
residual stresses. Steel that has been cold worked is
carburization progresses more slowly as the carbon
usually annealed to increase its ductility.
content increases during the process. The length of time
Assuming that the part to be annealed is heated to
required to produce the desired degree of carburization
the proper temperature, the required slow cooling may
and depth of the case depend upon the composition of
be accomplished in several ways, depending on the
the metal, the kind of carburization material used, and
metal and the degree of softness required.
the temperature to which the metal is subjected. It is
apparent that in carburizing, carbon travels slowly from
Normalizing, although involving a slightly
the outside toward the center; therefore, the proportion
different heat treatment, may be classed as a form of
of carbon absorbed must decrease from the outside to
annealing. This process removes all strains due to
the center.
machining, forging, bending, and welding. Normal-
izing can only be accomplished with a good furnace,
A common method of carburizing is called "pack
where the temperatures and the atmosphere may be
carburizing." When carburizing is to be done by this
closely regulated and held constant throughout the
method, the steel parts are packed with the carburizing
entire operation. A reducing atmosphere will normalize
material in a sealed steel container to prevent the solid
the metal with a minimum amount of oxide scale, while
carburizing compound from burning and retaining the
an oxidizing atmosphere will leave the metal heavily
carbon monoxide and dioxide gases. The container
coated with scale, thus preventing proper development
should be placed in a position to allow the heat to
of hardness in any subsequent hardening operation. The
circulate entirely around it. The furnace must be
articles are put in the furnace and heated to a point
brought to the carburizing temperature as quickly as
above the critical temperature of the steel. After the
possible, and held at this heat from 1 to 16 hours,
parts have been held at this temperature for a sufficient
depending upon the depth of the case desired and the
time to allow the heat to penetrate to the center of the
size of the work. After carburizing, the container should
section, they must be removed from the furnace and
be removed and allowed to cool in the air, or the parts
cooled in still air. Drafts will result in uneven cooling,
removed from the carburizing compound and quenched
which will again set up strains in the metal.
in oil or water. The air-cooling, although slow, reduces
warpage, and is advisable in many cases.
Prolonged soaking of the metal at high tem-
peratures must be avoided, as this practice will cause
In another method of carburizing, called "gaseous
the grain structure to enlarge. The length of time
carburizing," a carbonaceous material is introduced
required for the soaking temperature will depend upon
into the furnace atmosphere. When the steel parts are
the mass of metal being treated.
heated in this carburizing atmosphere, carbon
monoxide combines with the iron to produce results
CASE HARDENING.--In many instances, it is
that are practically the same as those described under
desirable to produce a hard, wear-resistant surface or
the pack carburizing process.
"case" over a strong, tough core. Treatment of this kind
is known as "case hardening." This treatment may be
Cyaniding.--Steel parts may be surface hardened
accomplished in several ways; the principal ways being
by heating while in contact with a cyanide salt,
carburizing, cyaniding, and nitriding.
followed by quenching. Only a thin case is obtained by
this method; therefore, it is seldom used in connection
Carburizing.--When steel is heated, the pores of
with aircraft construction or repair. However, cyaniding
the metal expand, allowing it to absorb any gases to
is a rapid and economical method of case hardening,
which it is exposed. By heating steel while it is in
and may be used in some instances for relatively
contact with a carbonaceous substance, carbonic gases
unimportant parts. The work to be hardened is
given off by this material will penetrate the steel to an
immersed in a bath of molten sodium or potassium
amount proportional to the time and temperature.
cyanide from 30 to 60 minutes. The cyanide bath
should be maintained at a temperature of 760 to 899C
The carburizing process may be applied to plain
carbon steels provided they are within the low-carbon
(1,400 to 1,650F). Immediately after removal from
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