SPRING 2024

158
THE DISTRIBUTOR’S LINK
ROB LaPOINTE FASTENER SCIENCE: THE ROLE OF CARBON IN STEEL from page 132
of high carbon (> 0.76% C) steel is pearlite (-Fe + Fe3C)
being held together by proeutectoid cementite, which is
a much harder phase of cementite. It is principally this
difference in the microstructures of low alloy steel that
produces a much harder alloy. As the carbon content goes
up, more proeutectoid cementite structures surround
pearlite structures creating a harder metal.
FIGURE 7 CEMENTITE (FE3C). IRON ATOMS ARE PICTURED IN
GREEN AND CARBON IN BLACK.
As we start to add carbon to the iron to form an iron
alloy, we see a third structure form with the carbon atoms
sitting in the spaces between the iron atoms (Figure 7).
This third structure is called cementite, which has the
chemical structure Fe3C. Cementite is very hard and
brittle. The cementite structure has a carbon percentage
of 6.67, which we see on the far-right side of Figure 5.
Thus, at low temperatures, an iron-carbon mixture with
6.67% C would be pure cementite (Fe3C).
If we focus our attention on the region below the
eutectoid point (0.76% C) at 727°C (1341°F), we can
see the area on the phase diagram which represents
low alloy steel. Carbon concentrations less than 0.76
percent represent low and medium carbon steel. Carbon
concentrations greater than 0.76 percent represent high
carbon steel. The microstructure (Figure 8) of low and
medium carbon steel is ferrite (-Fe) with laminations of
pearlite (-Fe + Fe3C). Pearlite is a cementite and ferrite
structure. For low and medium carbon steels, the pearlite
structures are held together with soft ferrite structures
called proeutectoid ferrite. The microstructure (Figure 9)
FIGURE 8 MICROSTRUCTURE OF LOW AND MEDIUM CARBON STEEL.
FIGURE 9 MICROSTRUCTURE OF HIGH CARBON STEEL.
A Fortune or Misfortune Cookie Message
We now understand, at least superficially, how
increasing amounts of carbon produces steel that is
harder and stronger while sacrificing the ductility and
toughness of softer steel with less carbon. As I thought
about my fortune cookie message, I wondered if it was
truly profound or if it was just another fortune cookie
attempt to lift my spirits by believing something good
is about to happen or bolstering my ego. I wondered if,
with the addition of character, a person grows harder
and stronger, but with less resilience. I thought about
the meaning of character building, as in, what builds
a person’s character? Difficult situations? Hardships?
Accomplishments? All these things, I suppose. As we
experience difficulty and resistance in life and get to
the other side of it by pushing and fighting and making
difficult decisions, we build our character. We build our
wisdom. We build a lack of tolerance for meaningless
or idle things or situations that will take us in the wrong
direction. We build a hardness that enables us to resist
being deformed, deflated, or destroyed. This seems like
a good thing, but their formation also makes us rigid and
inflexible. Rigidity and inflexibility prevent us from seeing
and experiencing new ideas, new perspectives, and new
realizations. I do think that my fortune cookie message
was truly profound. As I thought more about it though, I’m
not sure if it was defining the better path or an admonition
for correction toward the better path.
ROB LaPOINTE / EXPEDITE TESTING SERVICE