Page 195 - Materials Chemistry, Second Edition

182 3 Metals

this point is a solid rather than a liquid. Hence, the transition is referred to as the
eutectoid. As its name implies, alloys with carbon concentrations greater or less than
the eutectoid are known as hypereutectoid or hypoeutectoid steels, respectively.
The eutectoid mixture of steel consists of a lamellar microstructure of soft/ductile
ferrite and hard/brittle cementite (Fe 3 C). Accordingly, this phase is known as
pearlite, since the interaction of light gives rise to a mother-of-pearl multicolored
pattern when viewed through a light microscope. Cooling of austenitic steel at a
higher cooling rate will yield a ferrite/cementite mixture in the form of needles or
plates, known as bainite. Though the composition is identical to pearlite, the
microstructure is markedly different (non-lamellar), which yields a stronger, more
ductile alloy that is used for applications such as shovels, garden tools, etc.
Figure 3.18 illustrates the microstructural changes when austenitic steel is slowly
cooled. For hypoeutectoid steel, ferrite begins to form along the austenite grain
boundaries. Further cooling results in a ferrite-rich phase, with some remaining

austenite crystals. At the eutectoid point of 723 C, the residual austenite is converted
to pearlite, yielding a phase that contains both ferrite and pearlite crystals upon
further cooling.
By comparison, hypereutectoid steel contains significantly greater carbon con-
centrations; cooling results in the precipitation of the excess carbon in the form of
cementite nuclei that form along austenite grain boundaries. In Chapter 2,we
showed how polycrystalline aggregates, always found in pure metals and alloys,
form grain boundaries due to misaligned crystallites. Since the bonding character of
neighboring atoms is broken across the grain boundary, the diffusion of impurities
occurs more readily in these areas, thus explaining the preferential nucleation and
growth of cementite in these regions.
Considering the atomic weights of Fe and C, pure cementite corresponds to
6.7 wt.% carbon. It has been determined experimentally that the strength of steel
increases with carbon content up to the eutectoid composition, and then begins to
drop as cementite nuclei are formed in the material. It should be noted that other Fe–
C phases exist with greater carbon concentrations than cementite. However,
Figure 3.17 shows only the phases to the left of cementite that are technically useful
for materials applications. Cementite is actually a metastable phase, with graphite
representing the most stable form of carbon at equilibrium. However, it is difficult to
obtain stable graphitic nuclei in steels due to the low concentration of carbon.
As a final note regarding the Fe–C phase diagram, the eutectic temperature

corresponding to the minimum melting point of the Fe–C system is 1,130 C.
As the liquid is cooled at the eutectic temperature, solidification of ledeburite will
occur. The microstructure of ledeburite consists of tiny austenite crystals embedded
in a matrix of cementite. At carbon concentrations less than the eutectic (i.e., 4.3 wt.
% C), ledeburite and austenite will form a solid solution. By contrast, increasing
carbon concentrations will result in ledeburite/cementite solutions. At temperatures
lower than the eutectoid (and carbon concentrations greater than the eutectoid),
ledeburite will still be present alongside cementite or pearlite.
The incorporation of carbon into an iron lattice affects the interactions between
neighboring iron atoms. As carbon is introduced at relatively low concentrations, the
carbon atoms rearrange themselves within interstitial sites of the iron lattice,