On Jun 21, 7:12*pm, David Lesher wrote:
A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?

All my experience has been with cast IRON rotors, porosity is a
manufacturing defect, all too common on off-shored parts. I've never
seen steel rotors, the friction coefficient will be different and they
probably won't last as long. The graphite pockets in the cast iron
help with longevity. Only hardening that cast iron gets is chill from
the molds, if incorrectly cast, you can get hard spots. This leads to
problems if and when you try to get them turned. Stresses left from
casting will also lead to warping after getting heated up with hard
braking. Unfortunately, most of the aftermarket brake parts you can
get are from the Far East, mostly China and are made to the lowest
possible price. Leads to the situation where it's cheaper to go get
new rotors than to try to get the old ones turned true. Sounds like
you're not going to have that problem, new ones are in the cards no
matter what.

Stan