Figure 15-13.--Phase diagram for carbon steels.
over a range in temperature instead of at a constant
These various temperature arrests on cooling are
temperature as does the pure metal iron. The alloy
caused by evolutions of heat. On heating, the arrests
containing 4.3 percent carbon, called the eutectic alloy
occur in reverse order and are caused by absorptions of
of iron and cementite, freezes at a constant temperature
heat. The critical points may be detected also by sudden
changes in other physical properties, for instance,
as indicated by point C (fig. 15-11). Eutectic is defined
expansivity or electrical conductivity.
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
IRON-CARBON PHASE DIAGRAM
eutectic forming. The formation of a eutectic occurs
when a molten alloy or solution of the proper
The complete iron-carbon phase diagram
composition freezes. This temperature (in iron) is
represents the relationship between temperatures,
2,065°F, considerably below the freezing point of pure
compositions, and structures of all phases that may be
iron.
formed by iron and carbon under an equilibrium
Carbon has an important effect upon the trans-
condition (very slow cooling). Figure 15-11 illustrates
formation temperatures of iron; it raises the A4
a portion of this diagram for alloys ranging up to 6.7
temperature and lowers the A3 temperature. This effect
percent of carbon. The left-hand boundary of the
on the A3 temperature is very important in the heat
diagram represents pure iron (ferrite), and the
treatment of carbon and alloy structural steels, while the
right-hand boundary represents the compound iron
effect on the A4 temperature is important in the heat
carbide, Fe3C, commonly called cementite.
treatment of certain high-alloy steels, particularly
The beginning of freezing (change in state of metal
stainless types.
from liquid to solid) of the various iron-carbon alloys
Solid iron can absorb various amounts of carbon,
is shown by line ABCD (fig. 15-11), termed the
depending on the crystal structure of the iron and the
LIQUIDUS LINE. The ending of freezing is given by
temperature to which the iron is heated. The
line AHJECF, termed the SOLIDUS LINE. The
body-centered iron (alpha or delta) can absorb very little
freezing point of iron is lowered by the addition of
carbon (up to 4.3 percent) and the resultant alloys freeze
carbon, whereas the face-centered (gamma) iron can
15-14