2-15. At elevated temperatures, the iron matrix exists in a form called "austenite" which is capable of dissolving carbon
in solid solution. At ordinary temperatures the iron exists as "ferrite", in which carbon is relatively insoluble and
precipitates, as described in the preceding paragraph, in the form of carbide particles. The temperature at which this
change from austenite to ferrite begins to occur on cooling is called the "upper critical temperature" of the steel, and
varies with the carbon content; up to approximately 0.85 percent carbon, the upper critical temperature is lowered with
increasing carbon content; from 0.85 to 1.70 percent carbon the upper critical temperature is raised with increasing
carbon content. 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 approximately 28 to 56C (50 to 100 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 steel.
2-16. Successful hardening of steel will largely depend upon the following factors after steel has been selected which
has harden ability desires:
a. Control over the rate of heating, specifically to prevent cracking of thick and irregular sections.
b. Thorough and uniform heating through sections to the correct hardening temperatures.
c. Control of furnace atmosphere, in the case of certain steel parts, to prevent scaling and decarburization.
d. Correct heat capacity, viscosity, and temperature of quenching medium to harden adequately and to avoid
e. In addition to the preceding factors. the thickness of the section controls the depth of hardness for a given steel
composition. Very thick sections may not harden through because of the low rate of cooling at the center.
2-17. 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 be judged approximately by its
color. The accuracy with which temperatures may be judged by color depends on the experience of the workman, the
light in which the work is being done, the character of the scale on the steel, the amount of radiated light within the
furnace, and the emissivity or tendency of steel to radiate or emit light.
2-18. A number of liquids may be used for quenching steel. Both the medium and the form of the bath depend largely
on the nature of the work to be cooled. It is important that a sufficient quantity of the medium be provided to allow the
metal to be quenched without causing an appreciable change in the temperature of the bath. This is particularly
important where many articles are to be quenched in succession.
Aerators may be used in the Quench Tanks to help dissipate the vapor barrier.
2-19. QUENCHING PROCEDURE.
The tendency of steel to warp and crack during the quenching process is difficult to overcome, and is due to the fact that
certain parts of the article cool more rapidly than others. Whenever the rate of cooling is not uniform, internal stresses
are set up on the metal which may result in warpage or cracking, depending on the severity of the stresses. Irregularly
shaped parts are particularly susceptible to these conditions although parts of uniform section size are often affected in a
similar manner. Operations such as forging and machining may set up internal stresses in steel parts and it is therefore
advisable to normalixe articles beform attempting the hardening process. The following recommendations will greatly
reduce the warping tendency and should be carefully observed:
a. An article should never be thrown into quenching media/bath. By permitting it to lie on the bottom of the bath it
is apt to cool faster on the top side than on the bottom side, thus causing it to warp or crack.
b. The article should be slightly agitated in the bath to destroy the coating of vapor which might prevent it from
cooling rapidly. This allows the bath to remove the heat of the article rapidly by conduction and convection.
Change 17 2-4