Heat treating and inspection of metals Page: 18
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9 AIR CORPS
tion of carbon absorbed must decrease from the outside to the center.
(3) Solid, liquid, and gas carburizing methods are emloyed.
(a) The simplest method of carburizing consists of soaking the
parts at an elevated temperature while in contact with solid carbo-
naceous material such as wood charcoal, bone charcoal, charred
(b) Liquid carburizing consists of immersing the parts in a liquid
salt bath heated to the proper temperature to which amorphous carbon
has been added. The carbon penetrates the pores of the steel as in
the solid method, producing the desired case.
(c) Gas carburizing consists of heating the parts in a retort and
subjecting them to a carbonaceous gas such as carbon monoxide, or
the common fuel gases. This process is particularly adaptable to
certain engine parts.
(4) When carburizing, the parts are packed with the carburizing
material in a sealed steel container to prevent the solid carburizing
compound from burning and retain the carbon monoxide and
dioxide gases. Nichrome boxes, capped pipes of mild steel, or welded
mild steel boxes may be used. The former are most economical for
production because they withstand oxidation. The latter two are
useful only as substitutes. The container should be so placed as to
allow the heat to circulate entirely around it. The furnace must be
brought to the carburizing temperature as quickly as possible and
held at this heat for from 1 to 16 hours depending upon the depth of
case desired and the size of the work. After carburizing, the con-
tainer should be removed and allowed to cool in air or the parts re-
moved from the carburizing compound and quenched in oil or water.
The air cooling, although slow, reduces warpage and is advisable in
many cases. Carburizing temperatures for the various steels are
given in table III.
(5) Carburized steel parts are rarely used without subsequent heat
treatment, which consists of several steps to obtain optimum hardness
in the case, and optimum strength and ductility in the core. Grain
size of the core and case is refined.
(a) Refining the core is accomplished by reheating the parts to a
point just above the critical temperature of the steel. After soaking
for a sufficient time to insure uniform heating, the parts are quenched
in oil. The temperatures required are given in table III.
(b) The hardening temperature for the high carbon case is well
below that of the core. It is, therefore, necessary to again heat the
parts to the critical temperature of the case and quench them in oil
to produce the required hardness. A soaking period of 10 minutes is
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/20/: accessed April 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.