After being asked a couple of questions by potential purchasers, I've checked the spec again with the manufacturer and they're a higher spec casting than initially briefed - the casting is GG15HC, some technical info below
Technical details
GG15HC high silicone, high carbon cast iron alloy as the name implies uses higher carbon content during the casting process; approximately double the carbon content than normal brake discs. Carbon content ranges from 2% to approximately 4% and GG15HC is right at the top end. More silicone is also added to GG15HC to encourage the formation of graphite during the cooling process. These graphite flakes add "Plasticity" to the cast iron matrix and are excellent at absorbing shocks and preventing cracks.
Major advantages of GG15HC cast iron:
- The graphite flakes dampen vibrations, reducing brake judder and brake noise
- The graphite flakes conduct heat very well and therefore reduce thermal distortions
- The graphite flakes act as mini stress "focal points" for potential cracking making this material extremely viable for use on drilled and slotted discs, drawing the stress away from the machined slots or holes, preventing cracks.
The discs will also be fully balanced and hubs covered in a black corrosion-resistant coating.