Page 363 - Materials Science and Engineering An Introduction

9.18 The Iron–Iron Carbide (Fe–Fe C) Phase Diagram • 335
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The austenite, or g phase, of iron, when alloyed with carbon alone, is not stable
below 727 C (1341 F), as indicated in Figure 9.24. The maximum solubility of carbon
in austenite, 2.14 wt%, occurs at 1147 C (2097 F). This solubility is approximately 100
times greater than the maximum for BCC ferrite because the FCC octahedral sites are
larger than the BCC tetrahedral sites (compare the results of Problems 4.8a and 4.9), and,
therefore, the strains imposed on the surrounding iron atoms are much lower. As the
discussions that follow demonstrate, phase transformations involving austenite are very
important in the heat treating of steels. In passing, it should be mentioned that austenite
is nonmagnetic. Figure 9.25b shows a photomicrograph of this austenite phase. 2
The d-ferrite is virtually the same as a-ferrite, except for the range of temperatures
over which each exists. Because the d-ferrite is stable only at relatively high tempera-
tures, it is of no technological importance and is not discussed further.
Cementite (Fe 3 C) forms when the solubility limit of carbon in a-ferrite is exceeded
below 727 C (1341 F) (for compositions within the a + Fe 3 C phase region). As indicated
in Figure 9.24, Fe 3 C also coexists with the g phase between 727 C and 1147 C (1341 F
and 2097 F). Mechanically, cementite is very hard and brittle; the strength of some steels
is greatly enhanced by its presence.
Strictly speaking, cementite is only metastable; that is, it remains as a compound in-
definitely at room temperature. However, if heated to between 650 C and 700 C (1200 F and
1300 F) for several years, it gradually changes or transforms into a-iron and carbon, in the
form of graphite, which remains upon subsequent cooling to room temperature. Thus, the
Tutorial Video: phase diagram in Figure 9.24 is not a true equilibrium one because cementite is not an equilib-
Eutectic Reaction rium compound. However, because the decomposition rate of cementite is extremely sluggish,
Vocabulary and virtually all the carbon in steel is as Fe 3 C instead of graphite, and the iron–iron carbide phase
Microstructures diagram is, for all practical purposes, valid. As will be seen in Section 11.2, addition of silicon
Eutectoid to cast irons greatly accelerates this cementite decomposition reaction to form graphite.
Reaction Terms
The two-phase regions are labeled in Figure 9.24. It may be noted that one eutectic
exists for the iron–iron carbide system, at 4.30 wt% C and 1147 C (2097 F); for this
eutectic reaction,
Eutectic reaction for cooling
the iron–iron carbide L m g + Fe 3 C (9.18)
system heating
the liquid solidifies to form austenite and cementite phases. Subsequent cooling to room
temperature promotes additional phase changes.
It may be noted that a eutectoid invariant point exists at a composition of 0.76 wt%
C and a temperature of 727 C (1341 F). This eutectoid reaction may be represented by

Eutectoid reaction cooling
for the iron–iron g(0.76 wt% C) m a(0.022 wt% C) + Fe 3 C(6.7 wt% C) (9.19)
carbide system heating
or, upon cooling, the solid g phase is transformed into a-iron and cementite. (Eutectoid
phase transformations were addressed in Section 9.14.) The eutectoid phase changes
described by Equation 9.19 are very important, being fundamental to the heat treatment
of steels, as explained in subsequent discussions.
Ferrous alloys are those in which iron is the prime component, but carbon as well
as other alloying elements may be present. In the classification scheme of ferrous alloys
based on carbon content, there are three types: iron, steel, and cast iron. Commercially
pure iron contains less than 0.008 wt% C and, from the phase diagram, is composed
almost exclusively of the ferrite phase at room temperature. The iron–carbon alloys

2 Annealing twins, found in alloys having the FCC crystal structure (Section 4.6), may be observed in this photomi-
crograph for austenite. They do not occur in BCC alloys, which explains their absence in the ferrite micrograph of
Figure 9.25a.