diagram indicates equilibrium conditions-you might
say, ideal conditions of very slow and very uniform
heating and cooling. The rate and uniformity of heating
and cooling affect the internal structure of alloys and
alter the relationship between temperature and internal
structure. Therefore, equilibrium diagrams represent
theoretical rather than actual conditions.
TRANSFORMATION TEMPERATURES
If you allow a molten sample of pure iron to cool
slowly and measure the temperature of the iron at
Figure 15-12.--Idealized cooling curve for pure iron.
regular intervals, an idealized (equilibrium) time-
temperature plot of the data will appear as shown in
for steels that tend to crack or distort from more sudden
ature arrests) in this curve are caused by physical
quenches. The final cooling for the temperature of the
changes in the iron.
molten salt bath is accomplished in still air. All traces
The first arrest at 2,800F marks the temperature at
of the salt must be washed from the steel to prevent
which the iron freezes. The other arrests (known as
corrosion.
transformation temperatures or critical points) mark
temperatures at which certain internal changes take
SPECIAL PROTECTIVE ATMOSPHERES are
place in the solid iron. Some of these temperatures are
used for the first-stage cooling of some steels. The
very important in the heat treatment of steel.
protective atmosphere almost entirely eliminates air
As was mentioned before, the atoms in all solid
from around the metal and thus prevents scaling. When
metals are arranged in a definite geometric pattern. The
the steel has cooled enough so there is no further danger
atoms in iron immediately after freezing are arranged
of scaling, the remainder of the cooling is done in still
in the body-centered cubic structure. In this crystal
air.
structure the unit cell consists of a cube with an iron
atom at each of the eight comers and another in the
PRINCIPLES OF HEAT-TREATING
center. Each of the many individual grains (crystals) of
FERROUS ALLOYS
which the solid metal is composed is built up of a very
As we have seen, the properties of a metal or an
large number of these unit cells, all oriented alike in the
same grain. This high-temperature iron is known as
alloy are directly related to the metallurgical structure
delta iron.
of the material. Since we know that the basic purpose
of heat treatment is to CHANGE the properties of the
At 2,550F (the A4 point, fig. 15-12), iron under-
goes an allotropic transformation; that is, the arrange-
materials, let's see how this is done. The following
ment of the atoms in the crystal changes. The new
sections deal with basic considerations in heat
crystal structure is face-centered cubic, and the unit cell
treat-ment--equilibrium diagrams, transformation
again consists of a cube with an iron atom at each of the
temperatures, and the effects of heating, holding at
eight corners, but with an iron atom in the center of each
temperature, and cooling.
of the six faces instead on one in the center of the cube.
This form is known as gamma iron. At 1,670F (the A3
EQUILIBRIUM DIAGRAMS
point), iron undergoes another allotropic transformation
The relationships among the various metallurgical
and reverts to the body-centered cubic system. This
structures that compose alloys and the temperatures at
structure, which is basically the same as the structure of
which these structures exist are shown on existing
delta iron, is stable at all temperatures below the A3
point and is known as alpha iron. The arrest at 1,420F
(the A2 point) is not caused by an allotropic change. It
(also called a phase diagram) for iron-carbon alloys.
marks the temperature at which iron becomes
This type of diagram gives a good overall view of the
ferromagnetic and is, therefore, termed the magnetic
effects of temperature on the structures of various
transition. Above this temperature iron is nonmagnetic.
15-13