By Andrew Markel
Editor, Brake & Front End
When
you compare rotors side-by-side, they may look the same. But, the
difference between a comeback and satisfied customer might be at the
microscopic level.
Just
about every rotor is made of cast iron. But what can make or break a
rotor are ingredients like carbon, chromium silica and pearlite. Also,
how a rotor is cast and cooled can determine its final properties. This
is called the science of metallurgy.
Ensuring
that a rotor has the right metallurgy costs the manufacturer time and
money. It is not that the raw material costs more, it is that the
manufacturing process may require more expensive equipment. Also,
increasing energy costs and labor costs may be higher.
You
may be saying, “iron is iron.” This is not true when it comes to brake
rotors. OEMs use different grades of iron to ensure that a vehicle
platform has the right characteristics of wear, noise dampening and
performance. Like brake pads, rotors can be vehicle specific.
Taking
a chance with some “cheap,” “economy” or “inexpensive” rotors that do
not use the right grade of iron or manufacturing process could end up
in a comeback.
Microstructures
Talk
to any metallurgist or materials engineer and they will inevitably
mention microstructures. What they are talking about is how the atoms
and molecules bond or connect with each other to form small cohesive
structures.
Think
of these microstructures as a tiny racecar roll cage. A strong roll
cage joins to the chassis at many points and the bars intersect at many
joints. However, a weak roll cage will have fewer bars and joints. The
stronger roll cage is able to resist being torn apart because it has
more connections and stronger bonds. Also, a well-designed roll cage
can influence the overall performance of the vehicle.
The
same can be said about the cast iron mix that makes up a rotor. How the
iron interacts with other elements like graphite (a form of carbon),
silica and other materials can influence the microstructure of the iron
and influence the final product. These microstructures can influence
properties like wear, coefficient of friction and noise control.
Metallurgists
can fine-tune the mix of iron and element to give a rotor the right
characteristics for a vehicle. If more graphite is added to a mix, it
can improve the noise damping properties of the rotor. But, the
increased graphite content can lower the rotor’s resistance to wear and
abrasion. If the wear characteristics need to be improved, a substance
called pearlite can be added to the mix.
Achieving
the best performance of the rotor is a balancing act. Adding too much
of a component can ultimately affect the rotor’s ability to generate
friction and dissipate heat.
Stress and Heat
Casting
rotors can stress them out. Going from cold raw material to a hot
liquid, then cooling into a new shape causes stress in the raw casting.
The
microstructure of the rotor contracts as it cools. If one area of the
rotor cools faster then an adjoining area, stress is created between
the two areas due to the different rates of contraction. As you can
imagine, rotors with large hats or a large swept area have areas that
cool at different rates.
This
stress can influence the performance of the rotor under heat and
mechanical forces. This includes warping and mechanical failure. In
other words, this stress could cause you the stress of a comeback in
some cases.
To
avoid these problems for some applications, a reputable rotor
manufacturer will perform a process called stress relieving. It is a
process where the fresh cast rotor is kept at a constant temperature
for a certain length of time. Stress relieving is typically performed
in the temperature range of 500 degrees C and 650 degrees C for periods
up to 24 hours. This can cost a brake rotor manufacturer time and
energy costs.
Stress
relieving of cast iron rotors can be performed to minimize rotor
warping or thickness variation that can occur under extreme service
conditions. Stress relieving has no effect on microstructure. While it
may not be required for every aftermarket rotor, for some applications
it is critical.
Cryogenically Treated Rotors
One
way to make sure the metallurgy of a rotor is the best it can be is to
cryogenically treat the rotor after it has been manufactured. Cryogenic
processing involves placing rotors in special refrigeration equipment
that takes the parts down to the temperature of liquid nitrogen — minus
300° F below zero! At such extremely cold temperatures, a lot of really
“cool” things happen. Atoms stop shaking around and settle down into a
more relaxed state. This helps relax residual stresses in the metal for
improved fatigue resistance, tensile strength, abrasion resistance and
hardness.
In
steel alloys, it can actually change the microstructure of the metal
itself. Cast iron contains more carbon than steel, so the
microstructure does not change as dramatically.
Even
so, when done properly, a cryogenically treated rotor will run cooler
(some claim up to 40 percent cooler!), last longer (two to four times
longer), and resist warping and cracking better than any untreated
rotor. The process can be applied to stock OE rotors, aftermarket
rotors, or drilled or slotted rotors, and it treats the part all the
way through, not just the surface.