Heat treating and inspection of metals Page: 12
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6 AIR CORPS
austenite is being converted into pearlite. The terms "decalescence"
and "recalescence" may be further defined as follows, sing eutectoid
steels (steels having a 0.85 carbon content) as an example:
(1) Decalescence.-When a piece of steel is heated, the temperature
rises uniformly until it reaches a point between 1,325 and 1,350
F. At this point, the rise in temperature suddenly halts due to the
fact that the metal absorbs the heat necessary for the change of state.
After this halt, the temperature will continue its normal rate of
increase. It is the halting in the temperature range that is termed
decalescence. At the point of decalescence, the carbon and iron is
forming a solid solution, and the steel is passing from its annealed
condition into its hardening condition.
(2) Recalescence.-When steel is slowly cooled from a point above
the critical temperature, the cooling proceeds at a uniform rate until
the piece reaches a point between 1,250 and 1,300 F. At this time,
the cooling is noticeably arrested and the metal acutally rises in
temperature as the change of state again takes place. This change
is the opposite of decalescence and is termed recalescence.
b. In order to obtain a condition of maximum hardness, it is neces-
sary to raise the temperature of the steel sufficiently high to cause the
change of state to fully complete itself. This temperature is known
ais the "upper critical point" and is given for the principal aircraft
steels in a subsequent portion of the text. Steel that has been heated
to its upper critical point will harden completely if rapidly quenched;
however, in practice it is necessary to exceed this temperature by
from 25 to 50 F., to insure thorough heating of the inside of the
piece. If the upper critical temperature is exceeded too much, an
unsatisfactory coarse grain size will be developed in the hardened
c. Successful hardening of steel will largely depend upon the fol-
(1) Control over the rate of heating, specifically to prevent crack-
ing of thick and irregular sections.
(2) Thorough and uniform heating through sections to the correct
(3) Control of furnace atmosphere, in the case of certain steel
parts, to prevent scaling and decarburization.
(4) Correct heat capacity, viscosity, and temperature of quenching
media, to harden adequately and to avoid cracks.
d. When heating steel, the temperature should be determined by the
use of accurate instruments. At times, however, such instruments
are not available, and in such cases the temperature of the steel may
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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/m1/14/: accessed March 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.