Poorly tempered cast iron rotor

 Created by: h_lankford
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 Posted: Oct 16, 2019 03:07AM
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blue streak only on inner face of rotor.
No scoring marks to implicate metal to metal contact.
The abnormal sound was not metal screeching either, more like a rubbery groaning noise, only on braking.

 Posted: Oct 15, 2019 08:14PM
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Is the heat mark on the  front of the disc or just on the back?

If it's just on the back is there any possibility that a caliper bolt was fouling the disc? The mark looks in about the right place to line up with a caliper bolt.

 Posted: Oct 15, 2019 06:13PM
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See photo.
I asked yet another person about the discolored rotor . His IMMEDIATE reply was that this was an improperly manufactured, improperly tempered cast iron rotor that became evident after a few hundred miles.  Any comments? 

Here is the story.
Last year ( or two ) I posted about an unexplained noise on braking. Both front 8.4 in rotors, flanges, calipers, hoses and pads had been replaced just the year before and all was well, but after only a few hundred miles a strange rotational almost-rubber-like rubbing sound emanated from the left ONLY WHILE BRAKING. Not a metal screeching like a dust shield scraping the rotor. Only on forward, not in reverse. All speeds. Braking seemed reasonably straight.

Pads changed again,, different brands and composition - no help
Caliper replaced (again) - thinking a piston might be sticking - no help
Wheel bearings good, no play, no rumble
Rotor looked fine -at least the outer side . (The inboard side is covered by dust shield that would require too much trouble to take off.)

Finally, I got around to finding the issue:
I removed the caliper, then castle nut, then rotor.
Photo of inboard side of the rotor shows a bluish circular streak suggesting heat damage. There was no scoring of the metal surface, only the discoloration. Replacing the rotor with a new one (same grade and source) solved the noise problem .

I could not explain this, although admittedly the inboard pad had been tight to fit in, even with the piston pushed all the way in. Presumably, it may have dragged on the first new rotor enough in those first few hundred miles to overheat the rotor, but there was no asymmetrical braking.  Only a part of the rotor was blue and you would think all of its swept area would have been overheated and blue. Whatever the reason, the mystery is resolved. I think the badly-tempered cast iron explanation may be correct. Harvey