HEAT TREATMENT OF FERROUS METALS
HARDENING.--Heat treatment considerably
(STEEL)
transforms the grain structure of steel, and it is while
passing through a critical temperature range that steel
The first important consideration in the heat
acquires hardening power. When a piece of steel is
treatment of a steel part is to know its chemical
heated slowly and uniformly beyond a red heat, its
composition. This, in turn, determines its upper critical
appearance will increase in brightness until a certain
point. When the upper critical point is known, the next
temperature is reached. The color will change slightly,
consideration is the rate of heating and cooling to be
becoming somewhat darker, which may be taken as an
used. Uniform-heating furnaces, proper temperature
indication that a transformation is taking place within
controls, and suitable quenching mediums are used in
the metal (pearlite being converted into austenite).
carrying out these operations.
When this change of state is complete, the steel will
continue to increase in brightness, and if cooled quickly
Principles of Heat Treatment of Steel
to prevent the change from reversing, hardness will be
produced. If, instead of being rapidly quenched, the
Changing the internal structure of a ferrous metal is
steel is allowed to cool slowly, the metal will again pass
accomplished by heating it to a temperature above its
through a change of state, and the cooling rate will be
upper critical point, holding it at that temperature for a
momentarily arrested.
time sufficient to permit certain internal changes to
To obtain a condition of maximum hardness, it is
occur, and then cooling to atmospheric temperature
necessary to raise the temperature of the steel
under predetermined, controlled conditions.
sufficiently high to cause the change of state to fully
At ordinary temperatures, the carbon in steel exists
complete itself. This temperature is known as the
in the form of particles of iron carbide scattered
upper critical point. Steel that has been heated to its
throughout the iron mixture known as ferrite. The
upper critical point will harden completely if rapidly
number, size, and distribution of these particles
quenched; however, in practice, it is necessary to
determine the hardness of the steel. At elevated
exceed this temperature by approximately 28 to 56C
temperatures, the carbon is dissolved in the mixture in
(50 to 100F) to ensure thorough heating of the inside
the form of a solid solution called "austenite," and the
of the piece. If the upper critical temperature is
carbide particles appear only after the steel has been
exceeded too much, an unsatisfactory coarse grain size
cooled. If the cooling is slow, the carbide particles are
will be developed in the hardened steel.
relatively coarse and few. In this condition the steel is
Successful hardening of steel will largely depend
soft. If cooling is rapid, as by quenching in oil or water,
upon the following factors:
the carbon precipitates as a cloud of very fine carbide
particles, and the steel is hardened. The fact that the
1. Control over the rate of heating, specifically to
carbide particles can be dissolved in austenite is the
prevent cracking of thick and irregular sections
basis of the heat treatment of steel. The temperatures at
2. Thorough and uniform heating through
which this transformation takes place are called the
sections to correct hardening temperatures
"critical points," and vary with the composition of the
steel. The element normally having the greatest
3. Control of furnace atmosphere, in the case of
influence is carbon. The various heat-treating
certain steel parts, to prevent scaling and de-
procedures for commonly used aircraft steels are
carburization
outlined in Aerospace Metals--General Data and
4. Correct heat capacity, viscosity, and
Usage Factors, NAVAIR 01-1A-9.
temperature of quenching media, to harden adequately
and to avoid cracks
Forms of Heat Treatment of Steel
When heating steel, you should use accurate
There are different forms of heating ferrous
instruments to determine the temperature. At times,
materials such as steel. The methods covered in this
however, such instruments are not available, and
chapter are hardening, quenching, tempering,
in such cases, the temperature of the steel may be
annealing and normalizing, and case hardening. Terms
judged approximately by its color. The temperatures
such as carburizing, cyaniding, and nitriding are also
discussed.
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