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THE DISTRIBUTOR’S LINK
ROB LaPOINTE FASTENER SCIENCE: THE ROLE OF CARBON IN STEEL from page 90
Microstructure
The microstructure of carbon steel is where the
character of the steel is formed. Literally, the character
or properties of the material arise from the atomic and
molecular structures that assemble themselves by the
balancing of electrical force with the actions of the atoms
due to their thermal energy. This balancing act creates
the crystal structure of the metal. To understand the
crystal structure, or microstructure for iron-carbon alloys
at different thermal energies, we rely on experimental
and theoretical research that has been used to develop
a kind of picture of the alloy at a variety of temperatures
and amounts of carbon by weight percent. This picture is
called an equilibrium phase diagram. Equilibrium phase
diagrams represent the material’s uniform physical and
chemical properties charted with respect to the materials
composition and temperature.
Figure 5 shows us an iron-carbon phase diagram. This
phase diagram provides us with a tremendous amount
of information about the structure and properties of
carbon steel. A complete understanding of this diagram
is beyond the scope of this article but focus on a
few aspects of the diagram will serve to illustrate the
properties of low alloy steel and the role carbon plays.
Let’s begin our exploration of the iron-carbon phase
diagram by considering the three structures that pure
FIGURE 5 IRON-CARBON PHASE DIAGRAM.
FIGURE 6 CRYSTAL STRUCTURES OF IRON. IRON ATOMS ARE
PICTURED IN RED.
iron can form as a crystal or solid. To orient yourself to
the iron-carbon phase diagram (Figure 5), percent carbon
is measured on the horizontal axis with 0% carbon
on the left and ranging to 6.67% carbon on the right.
Temperature is measured on the vertical axis with 0°C
on the bottom and ranging up to approximately 1700°C
on the top. Pure iron can be considered to have less
than 0.008% carbon and is situated on the far left of
the diagram. Along the left border of the diagram (Figure
5) we can see the three phases of pure carbon. Above
a temperature of 1536°C (2798°F), iron is a liquid. As
iron cools below 1536°C (2798°F), it solidifies into a
solid called delta phase or delta iron (-Fe). The structure
of -Fe is a body-centered cubic or BCC (Figure 6). In
the body-centered cubic arrangement, an iron atom is
at the center of the cube structure of iron atoms. As
iron continues to cool it transforms
its arrangement to a face-center
cubic (FCC) at 1401°C (2554°F). This
structure is called gamma iron (-Fe) or
austinite. The FCC structure is denser
than the BCC structure. Continued
cooling below 911°C (1672°F) causes
another change in the structure of iron
to alpha iron (-Fe). Alpha iron, also
known as ferrite, is a BCC structure
like delta iron, but is a bit more
compact.
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