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326 Forging-Sfamping - Heaf Treating
These volume changes have an important bearing
on the deformation or cracking of steel parts during
heat treatment, as will be discussed in Chapter VII.
PART 3
IRON-CARBON DIAGRAM
The critical point diagram, Fig. 110, which was
described in the first part of this Chapter, portrays
the changes which take place in steel during slow
heating and cooling, through the critical range. It
is therefore limited to temperatures not much over 900
deg. C. and to iron-carbon alloys containing not more
than about 1.7 per cent carbon. When the carbon content
is much in excess of the latter figure, the material
is classed as cast iron.
When steel or cast iron is heated to the molten
state, or slowly cooled from the molten state, as in
casting, certain structural changes take place, which
have important effects on the character of the metal.
These changes may be studied in connection with the
iron-carbon diagram shown in Fig. 109.
This graph is also known as an "equilibrium" or
"constitution" diagram of iron-carbon alloys, and
sometimes as a Roozeboom diagram, after the originator.
The diagram has been plotted differently by
various authorities and some details are still open to
question. Archer's diagram, reproduced here, is probably
one of the simplest and most accurate as well as
one of the most up-to-date. Archer's description (ref.
12) has already been quoted in connection with the
critical point diagram, and will be followed rather
closely here. It has already been explained that the
diagram is plotted by laying off temperature vertically
and composition (per cent carbon), horizontally. Any
point on the diagram represents the condition of a
definite alloy at a definite temperature. The carbon
content is shown on the horizontal base line, directlv
below the point in question, while the temperature is
shown on the vertical scale, directly opposite the point.
Only 6.67 per cent carbon would cause the alloy to
consist entirely of the compound Fe3C, or cementite.
This is the theoretical limit of the iron carbon diagram.
Practically, a maximum of about 5.0 per cent
carbon is never exceeded.
Constituents and Phases.
We have seen (Part 1 of this Chapter), that those
portions of an alloy which, under the microscope appear
to be definite units in the structure, are called
"constituents." The term is rather indefinite for it is
applied to pearlite, sorbite, troostite and martensite,
although these are really made up of a mixture of two
different things, cementite and ferrite, which may be
distinguished, at least in the first two cases, if the magnification
is high enough.
A more definite term is "phase". A phase is a
portion of an alloy which is physically and chemically
homogeneous (uniform) throughout, and which is separated
from the rest of the alloy by distinct bounding
surfaces. More specifically, a phase is a state or aspect
in which an element or substance may exist.
The following phases occur in the iron carbon
alloys: Molten alloy, austenite, ferrite, cementite
and are not "physically and chemically homogeneous
throughout." It is evident that a phase in an alloy
may be an element (as graphite, which is a form of
September, 1925
carbon), a compound (as cementite, Fe3C), a solid
solution (as austenite or ferrite), or a liquid solution
(as the molten alloy), but not a mixture.
Constitution.
When we say what phases are present in a given
alloy, at a given temperature, and how much there is
of each phase, we completely describe its "constitution".
(Constitution should not be confused with constituents.
By construction we mean the make-up of
an alloy at a given temperature, i.e., what phases of
the materials composing it are present.)
It is sufficient description of the constitution of an
annealed low carbon steel at room temperature, for
example, to say that it contains 3 per cent cementite
and 97 per cent ferrite, although a description of the
structure of this steel might give the added information
that it contained about 22 per cent of pearlite and 78
per cent of free ferrite. The iron-carbon diagram deals
only with the constitution of the iron-carbon alloys,
and not with their structure.
Equilibrium.
The iron-carbon diagram represents these alloys
in a condition known as "equilibrium." We may consider
that a state of equilibrium exists in any alloy at
any given temperature, when exposure to that temperature
for any further period of time does not produce
any change in the constitution, provided the temperature
in question is sufficiently high to allow constitutional
changes to go to completion. The test of
equilibrium is that the same condition is reached, no
matter from which side it is approached, whether heating
or cooling.
Let us now consider the changes that take place
when iron-carbon alloys cool from the molten to the
solid state, and the phases that are produced, as shown
on the diagram.
The Liquidus.
The line "ABD" called the liquidus, represents the
beginning of solidification on cooling and the end of
melting on heating. All points above this line represent
alloys in a completely molten condition. All
points below ABD represent alloys partially or completely
solid.
Solidification.
Pure iron, represented by the point A, and the ironcementite
eutectic, represented by the point B, melt
and solidify at constant temperature. All other alloys
represented in the diagram, melt and freeze over a
range of temperature.
Alloys containing 0 to 4.3 per cent carbon, begin to
solidify on cooling to the line AB by the separation
of austenite crystals from the liquid.
Alloys containing more than 4.3 per cent carbon,
begin to solidify with the separation of cementite from
the liquid, on cooling to the line BD.
The alloy containing 4.3 per cent carbon is the
eutectic alloy, and solidifies entirely at the point B.
with the simultaneous formation of austenite and cementite.
In the case of alloys containing less than 1.7 per
cent carbon, austenite continues to freeze out on cooling
from AB to AE. At AE the alloy is completely
solid and consists of one phase, austenite.
In alloys containing from 1.7 to 4.3 per cent carbon,
austenite freezes out of the liquid from AB to EB. At