WINTER 2024

90
THE DISTRIBUTOR’S LINK
ROB LaPOINTE CASE-HARDENING – CREATING FASTENERS WITH A DURABLE SKIN & FLEXIBLE CORE from page 8
You may be wondering why durability and flexibility are
contrasting features. Let’s take a moment to explain the
relationship between durability and flexibility in carbon
steel. When we think of durability, we think of something
that resists change. “A diamond is forever” as De Beers
says. Diamonds may not last forever, but they are very
durable. The durability of diamonds is due to their
hardness and resistance to corrosion. The durability of
carbon steel is also related to its hardness. The harder
the steel, the more durable it is. The harder the steel, the
greater its resistance to being scratched or bent. Along
with this durability due to hardness, comes brittleness.
Brittleness is the tendency to break rather than bend.
High hardness steel can withstand high forces and not
bend or change shape, but when it reaches its breaking
point, it fails by fracture without much bending. This is
why high hardness is desirable in steel that is required
to cut and shape other materials or in parts that must
move against one another with minimum wear.
The harder the steel, the greater its resistance to
being scratched or bent. Along with this durability due to
hardness, comes brittleness. Brittleness is the tendency
to break rather than bend. High hardness steel can
withstand high forces and not bend or change shape,
but when it reaches its breaking point, it fails by fracture
without much bending. This is why high hardness is
desirable in steel that is required to cut and shape other
materials or in parts that must move against one another
with minimum wear.
FIGURE 4. A MAN PULLING TAFFY, SHOWING ITS DUCTILE NATURE
FIGURE 3 SELF-DRILLING SCREWS THAT ARE CASE-HARDENED TO
DRILL THROUGH AND FASTEN SOFTER METAL.
Flexibility is the characteristic of being able to
distort an object’s shape without compromising its
structure. Flexible materials like rubber can be distorted
significantly without breaking. Carbon steel with low
hardness has this feature. It can be bent or distorted to
the point that it changes its original shape and does not
fail due to fracture. It stretches. This is the opposite of
brittleness and is usually called ductility. One can think
of taffy, the sweet and sticky candy (Figure 4), as being
ductile. Taffy can be stretched out thinner and thinner
before it breaks. Soft steel that is flexible will not hold up
very well if it needs to continuously rub against another
part. It will change its shape and wear out.
Some fasteners need this dual nature to properly
perform the fastening job for which they’re intended.
Construction screws need to be durable to penetrate
a variety of materials while remaining flexible to be
stressed or bent without breaking. Thread forming
and tapping screws need to have a durable and hard
cutting edge and still be able to absorb impact without
fracture. The cutting end of a self-drilling screw (Figure
3) is essentially a drill bit on the end of a screw; this
eliminates the need for one of the tools to do the job.
The drill bit and the screw in one. Genius!
CONTINUED ON PAGE 138