Heat treating and inspection of metals Page: 17
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HEAT TREATING AND INSPECTION OF METALS 8-9
scale, thus preventing outside hardness in any subsequent hardening
operation. The articles are put in the furnace and heated to a point
above the critical temperature of the steel. After the parts have been
held at this temperature for a sufficient time to allow the heat to
penetrate to the center of the section, they must be removed from the
furnace and cooled in still air. Drafts will result in uneven cooling,
which will again set up strains in the metal. Prolonged soaking of
the metal at high temperatures must be avoided, as this practice will
cause the grain structure to enlarge. The length of time required for
the soaking temperature will depend upon the mass of metal being
9. Case hardening.-In many instances, it is desirable to pro-
duce a hard wear-resistant surface or "case" over a strong tough core.
Treatment of this kind is known as "case hardening." This treat-
ment may be accomplished in several ways, the principal ways being
carburizing, cyaniding, and nitriding.
a. Carburizing.-When steel is heated, the pores of the metal ex-
pand allowing it to absorb any gases to which it is exposed. By heat-
ing steel while in contact with a carbonaceous substance, carbonic
gases given off by this material will penetrate the steel to an amount
proportional to the time and temperature. For example, if mild or
soft steel is heated to 1,3500 F. in an atmosphere of carbonic gases, it
will absorb the gas until approximately 0.50 percent of carbon content
has been attained at the surface; this being the saturation point of the
steel for the particular temperature. By increasing the heat to
1,6500 F., the same steel will absorb the gas until a 1.50 percent carbon
content has been attained, which is the normal saturation point for the
(1) The carburizing process may be applied to both plain carbon
and alloy steels provided they fall within the low carbon range. Spe-
cifically, the carburizing steels are those containing not more than 0.20
percent carbon and are listed in table III. The lower the carbon con-
tent in the steel, the more readily it will absorb carbon during the
(2) The amount of carbon absorbed and the thickness of the case
obtained increases with time; however, carburization progresses
more slowly as the carbon content increases during the process. The
length of time required to produce the desired degree of carburiza-
tion and depth of case depends upon the composition of the metal, the
kind of carburization material used, and the temperature to which the
metal is subjected. It is apparent that in carburizing, carbon travels
slowly from the outside toward the center and therefore, the propor-
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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/m1/19/: accessed April 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.