Machinery Repairman

These various temperature arrests on cooling are
caused by evolutions of heat. On heating, the arrests
occur in reverse order and are caused by absorptions of
heat. The critical points may be detected also by sudden
changes in other physical properties, for instance,
expansivity or electrical conductivity.
IRON-CARBON PHASE DIAGRAM
The complete iron-carbon phase diagram
represents the relationship between temperatures,
compositions, and structures of all phases that may be
formed by iron and carbon under an equilibrium
condition (very slow cooling). Figure 15-11 illustrates
a portion of this diagram for alloys ranging up to 6.7
percent of carbon. The left-hand boundary of the
diagram represents pure iron (ferrite), and the
right-hand boundary represents the compound iron
carbide, Fe 3C, commonly called cementite.
The beginning of freezing (change in state of metal
from liquid to solid) of the various iron-carbon alloys
is shown by line ABCD (fig. 15-11), termed the
LIQUIDUS LINE. The ending of freezing is given by
line AHJECF, termed the SOLIDUS LINE. The
freezing point of iron is lowered by the addition of
carbon (up to 4.3 percent) and the resultant alloys freeze
Figure 15-13.—Phase diagram for carbon steels.
15-14
over a range in temperature instead of at a constant
temperature as does the pure metal iron. The alloy
containing 4.3 percent carbon, called the eutectic alloy
of iron and cementite, freezes at a constant temperature
as indicated by point C (fig. 15-11). Eutectic is defined
as an alloy or solution having its components in such
proportions that the melting point is the lowest possible
for this combination of components. Not all alloys are
eutectic forming. The formation of a eutectic occurs
when a molten alloy or solution of the proper
composition freezes. This temperature (in iron) is
2,065°F, considerably below the freezing point of pure
iron.
Carbon has an important effect upon the transformation
temperatures of iron; it raises the A 4
temperature and lowers the A 3 temperature. This effect
on the A 3 temperature is very important in the heat
treatment of carbon and alloy structural steels, while the
effect on the A 4 temperature is important in the heat
treatment of certain high-alloy steels, particularly
stainless types.
Solid iron can absorb various amounts of carbon,
depending on the crystal structure of the iron and the
temperature to which the iron is heated. The
body-centered iron (alpha or delta) can absorb very little
carbon, whereas the face-centered (gamma) iron can