BBBBflt] «BlJIUrIrlr - Clpdigital.org

December, 1925
contact with a material from which carbon may be
absorbed. The inner portion, the carbon content of
which is not materially increased, is called the "core"
When such a piece is suitably heat treated, its surface
will have the hardness and resistance to wear of high
carbon steel, while the core retains the ductility and
toughness of low carbon steel.
Steel of low carbon content lends itself more readily
to manufacturing and fabricating operations such as
700
"PEARLITE PLUS FERRITE
riypo-Euteotoic/
1 I I I I I I
P£r7C£HT CF7r?30F.
FIG. 110—Critical point diagram.
rorging- Stamping - float Treating
PEARLITE PLUS CEMEMTiTE
Hyper- Euteotoid
TT7TTT 14 IS
forging, rolling, drawing, machining, stamping, forming,
etc., than does high carbon steel. It also is lower
in first cost, although this saving will be partially offset
or even exceeded by the cost of the carburizing
operation. For many purposes, where parts are required
to withstand wear, a hard surface of moderate
depth is sufficient. Such parts may therefore be made
from low carbon steel, and case hardened, whereas
they would otherwise have to be made from high carbon
steel, with its attending higher manufacturing
costs. In addition to a hard surface for resisting wear,
it is often desirable that a part have a tough and ductile
interior, which will have a high resistance to shock,
and possess the ability to adjust itself to moderate de-
421
other reasons, a very large amount of work is casehardened
today. See Figs. 138 and 139.
Unfortunately, the metallurgical principles underlying
the process of case-hardening have not always
received the attention they deserve, even in plants
which have reduced other heat treating processes to
a scientific basis. This neglect must necessarily result
in a sacrifice in quality or an excessive cost of the
finished product, or both. Case-hardening is one of
the most complex problems which the steel treater
has to handle, and it merits far more careful consideration
than is usually given to it in practice.
The case-hardening process is practically indispensable
for many purposes, but it is not a cure all,
and has numerous draw-backs and pit-falls. Before
FIG. 139a, b, c—Fractures of carburized parts. Rectangular
bar at lower right, C, has been hardened; note refinement.
(Courtesy American Gas Furnace Co.)
undertaking to produce parts by case-hardening, it
is well to make sure that the desired results could not
be obtained more economically by the use of a higher
carbon or alloy steel, with a simple heat treatment.
A comprehensive discussion of case-hardening
would, of course, require much more space than is
FIG. 138—Typical case-hardened parts—spindles, gages, wrist
pins and gears. (Courtesy American Gas Furnace Co.)
available here.
stricted chiefly
The present discussion will be re­
to the fundamental metallurgical
principles involved. Those who wish to make a
formation or overstrain in service, without fracture. further study of the subject, will find much valuable
Parts which require a hard surface are often of such published literature (and also not a little that is based
design that they would be likely to crack in heat more on fancy than fact). The references mentioned
treatment, if made from steel containing enough car­ at the beginning of this section will be found especially
bon to impart the necessary hardness. For these and helpful.