METALLURGY
The metallurgical properties of a rotor determines its strength, noise, wear and braking characteristics. The casting process must be carefully controlled to produce a high quality rotor. You can’t just dump molten iron into a mold and hope for the best. The rate at which the iron cools in the mold must be closely monitored to achieve the correct tensile strength, hardness and microstructure.

When iron cools, the carbon atoms that are mixed in with it form small flakes of graphite which help dampen and quiet noise. If the iron cools too quickly, the particles of graphite don’t have as much time to form and are much smaller in size, which makes for a noisy rotor.

The rate of cooling also affects the hardness of a rotor. If a rotor is too hard, it will increase pad wear and noise. Hard rotors are also more likely to crack from thermal stress. If a rotor is too soft, it will wear too quickly and may wear unevenly increasing the risk of pedal pulsation and runout problems.

The composition of the iron must also be closely controlled during the casting process to keep out impurities that may form "inclusions" and hard spots. One rotor manufacturer says they sample the molten iron every 15 seconds to make sure the composition is correct. The molten metal is also poured through ceramic filters that trap contaminants. Even the sand that’s used to make the molds is specially treated to control moisture content. This helps keep the sand in place and prevents core shifts that can affect porosity, dimensional accuracy and balance.

The grade of cast iron that’s used in a rotor may even be changed to suit a particular application. One aftermarket rotor manufacturer uses a special grade of "dampened iron" to make replacement rotors for 1997-2002 Chevrolet Malibu and its sister vehicles (Olds Alero, Olds CUtlass and Pontiac Grand Am). In this case, the original OEM rotors turned out to be too noisy so General Motors switched to a dampened grade of iron to cure the problem.