Getting the right material for a disc is quite difficult to get just right, but stainless steel will be too weak and the tips will fold over in a few hits. Hardox will be better, but against another robot with the same material the disc teeth is always going to get damaged.

D2 is a tool steel, often used in large power presses. I usually just call our local supplier and see what they recommend and see if they have any offcuts cheap of free.