Diffusion is a controlling factor in the development of microstructure. Another factor is solubility, which is a measure of how much of a particular element can be accommodated by the crystal lattice before it is rejected. In metals when two or more elements are soluble in the crystal lattice, a solid solution is created (somewhat analogous to a liquid solution of sugar in hot coffee). 
For example, when added to iron, carbon has very limited solubility in ferrite but is about 100
times more soluble in austenite, as seen in the iron–carbon diagram in Fig. 2 (a limited
version of the diagram in Fig. 1). The maximum solubility of carbon in ferrite is about
0.022% C at 727 C while the maximum solubility of carbon in austenite is 100 times more,
2.11% C at 1148 C. At room temperature the solubility of carbon in iron is only about
0.005%. Any amount of carbon in excess of the solubility limit is rejected from solid solution
and is usually combined with iron to form an iron carbide compound called cementite. This
hard and brittle compound has the chemical formula Fe3C and a carbon content of 6.7%.
This is illustrated in the following two examples. The first example is a microstructure of a
very low carbon steel (0.002% C), shown in Fig. 3a. The microstructure consists of only
ferrite grains (crystals) and grain boundaries. The second example is a microstructure of a
low-carbon steel containing 0.02% C, in Fig. 3b. In this microstructure, cementite can be
seen as particles at the ferrite grain boundaries. The excess carbon rejected from the solid
solution of ferrite formed this cementite. As the carbon content in steel is increased, another
form of cementite appears as a constituent called pearlite, which can be found in most carbon
steels. Examples of pearlite in low-carbon (0.08% C) and medium-carbon (0.20% C) steels
are seen in Figs. 4a and 4b. Pearlite has a lamellar (parallel-plate) microstructure, as shown
at higher magnification in Fig. 5, and consists of layers of ferrite and cementite. Thus, in
these examples, in increasing the carbon level from 0.002 to 0.02 to 0.08 to 0.20%, the
excess carbon is manifested as a carbide phase in two different forms, cementite particles
and cementite in pearlite. Both forms increase the hardness and strength of iron. However,
there is a trade-off; cementite also decreases ductility and toughness.
Pearlite forms on cooling austenite through a eutectoid reaction as seen below:
Austenite ↔ Fe C ferrite 3
A eutectoid reaction occurs when a solid phase or constituent reacts to form two different
solid constituents on cooling (a eutectic reaction occurs when a liquid phase reacts to form
two solid phases). The eutectoid reaction is reversible on heating. In steel, the eutectoid
reaction (under equilibrium conditions) takes place at 727 C and can be seen on the iron–
carbon diagram (Fig. 1) as the ‘‘V’’ at the bottom left side of the diagram. A fully pearlitic
microstructure forms at 0.77% C at the eutectoid temperature of 727 C (the horizontal line
on the left side of the iron–carbon diagram). Steels with less than 0.77% C are called
hypoeutectoid steels and consist of mixtures of ferrite and pearlite with the amount of pearlite
increasing as the carbon content increases. The ferrite phase is called a proeutectoid phase
because it forms prior to the eutectoid transformation that occurs at 727 C. A typical example
of proeutectoid ferrite is shown in Fig. 6. In this photomicrograph, the ferrite (the whiteappearing
constituent) formed on the prior austenite grain boundaries of hypoeutectoid steel
with 0.60% C. The remaining constituent (dark appearing) is pearlite. Steels between
0.77% C and about 2% C are called hypereutectoid steels and consist of pearlite with proeutectoid
cementite. Cementite forms a continuous carbide network at the boundaries of the
prior austenite grains. Because there is a carbide network, hypereutectoid steels are characterized
as steels with little or no ductility and very poor toughness. This means that in the
commercial world the vast majority of carbon steels are hypoeutectoid steels.
Thus, according to the iron–carbon diagram, steels that are processed under equilibrium
or near-equilibrium conditions can form (a) pure ferrite at very low carbon levels generally
under 0.005% C, (b) ferrite plus cementite particles at slightly higher carbon levels between
0.005% C and 0.022% C, (c) ferrite plus pearlite mixtures between 0.022% C and 0.77% C,
(d) 100% pearlite at 0.77% C, and (e) mixtures of pearlite plus cementite networks between
0.77% C and 2% C. The higher the percentage of cementite, the higher the hardness and
strength and lower the ductility and toughness of the steel.

Mechanical Engineers’
Handbook: Materials and Mechanical Design,
Volume 1, Third Edition.
Edited by Myer Kutz


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