EFFECTS OF HEATING, HOLDING,
known as pearlite, and the A1 temperature is, therefore,
AND COOLING
frequently referred to as the pearlite point. Since the A1
transformation involves the transformation of austenite
We have seen what happens to the structure of an
to pearlite (which contains cementite--Fe3C), pure iron
iron-carbon alloy containing 0.83 percent carbon when
it is cooled very slowly so all of the transformations
Theoretically, iron must be alloyed with a minimum of
0.03 percent of carbon before the first minute traces of
diagram. In the same manner, we could use the
equilibrium diagram to find the transformation points
If the steel is held at a temperature just below A1 (either
or ranges for other iron-carbon alloys containing
during cooling or heating), the carbide in the pearlite
different percentages of carbon.
tends to coalesce into globules or spheroids; this
If we want to change a metal's properties, we have
phenomenon, known as SPHEROIDIZATION, will be
to create a specific grain structure in the metal. We do
discussed later.
this primarily by (1) heating the metal at a certain rate
A hypoeutectoid steel (less than 0.83 percent
to a certain temperature, (2) holding or soaking it at that
carbon), cooled slowly from a temperature above the A3
temperature for a specified time, and (3) cooling it at a
specified rate. Thus the three major factors involved in
temperature, begins to precipitate ferrite when the A3
all heat-treating processes are (1) rate of heating,
line is reached. This ferrite is often called pro-eutectoid,
(2) holding time (or soaking time) at the specified
since it forms before the eutectoid temperature is
temperature, and (3) rate of cooling. A fourth factor--
reached. As the temperature drops from the A3 to the
the alloy during heat treatment--is important in nearly
and as the amount of the remaining austenite decreases
all treating processes.
progressively, its carbon content increases. At the A1
The rate of heating determines where the changes
temperature the remaining austenite reaches eutectoid
will occur in the material as it is heated. Increasing the
rate of heating raises the temperatures at which the
transformations occur. Within certain limits, the faster
view D). The microstructures of slowly cooled
the material is heated, the higher its transformation
hypoeutectoid steels thus consist of mixtures of ferrite
temperatures will be. The temperature to which the
material is raised and the time it is held at that
carbon content, the higher is the temperature at which
temperature affect the size of the grains in the final
ferrite begins to precipitate and the greater is the amount
structure. The rate of heating also partially determines
of ferrite in the final structure.
whether or not stresses will be set up in the material by
The temperature range between the A1 and A3
the process of heating.
The holding time (or soaking time) at temperature
points is called the transformation range. Theoretically,
is important for a number of reasons. The holding time
the critical points in any given steel should occur at
must be sufficient to allow all parts of the piece to come
about the same temperatures, if the steel is being heated
to a uniform temperature, except in those cases
or cooled very slowly. Practically, however, they do not
involving localized hardening of certain areas. If the
since the A3 and A1 points, affected slightly by the rate
rate of heating is very slow, uniformity of temperature
of heating, are affected tremendously by the rate of
may be reached with a short holding time. But if the rate
cooling. Rapid rates of heating raise these points only
of heating is rapid, a longer holding time will probably
slightly, but rapid rates of cooling lower the
be required. In any case, holding time must be sufficient
temperatures of transformation considerably. To
to allow the required transformations to take place. As
differentiate between the critical points on heating and
we have seen, the transformations do not occur
cooling, the small letters c (for chauffage, meaning
instantaneously but require a definite time for
heating) and r (for refroidissement, meaning cooling)
completion.
are added. The terminology of the critical points thus
The rate of cooling is tremendously important in
becomes Ac3, Ar3, Acl, Arl, and so on. The letter e is
most heat-treating processes. Increasing the rate of
used to designate the occurrence of the points under
cooling lowers the temperatures at which the
conditions of extremely slow cooling on the assumption
transformations occur; and the lower the transformation
temperature, the stronger and harder the final product
will be. Very rapid cooling does not allow time for the
Aec m).
15-16