Steel will glow Blood Red at 566 degC. (1050 degF.) and will turn into an Orange glow at 941 degC. (1725 degF.). In between these two temperature levels there are various glowing colours ranging from Cherry Red to Salmon. I would say these glowing discs are at a temperature of 700 to 750 degC. (1280 degF to 1380 degF) Steel will melt at 1370 degC. (2500 degF.)
If you look at pics of 911's racing that are shot low from the rear many times you can see the turbos glowing red in the underbody, makes for pretty cool pics.
Sweet pics, saw lots of glowing disks at the Monteray sports car Championship at Laguna. Sorry dont have any pics.