ROTOR HEAT TREATING AND CRACKING:
Another question has arisen regarding “heat-treating” of rotors, and also regarding crossdrilled rotor cracking--the cracks that you've heard can appear around the holes in crossdrilled rotors.
Per our factory reps at ATE and Brembo, the rotors come from the factory already heat-treated. This is what is commonly known as "tempering"--a process usually done by a slow "baking" type of action in either a large conveyered convection-type of oven, or on racks in a thermal-exposure type facility. We consulted a metallurgical engineering firm a few miles down the road from our shop (that actually has, among other things, a convection heat-treating facility) with regards to additional heat-treating of the rotors after crossdrilling. We were told in no uncertain terms that any further heat-treating will begin a process called "annealing". Annealing of the metal does in fact make the metal "harder", yes----but it also makes the surface and substrata more brittle and prone to cracking--which is the LAST thing you want a rotor to do.
The fact is that the rotors are heat-treated from the factory to a specific level PURPOSELY to strike the right balance between hardness and the natural, inherent amount of flex or "give" needed to prevent cracking over long periods of time---especially under hard braking and with extended exposure to the constant heat and cold changes and the elements. In addition, the crossdrill holes milled on all RotorLab crossdrilled rotors are chamfered to .5 to mitigate stress and cracking.