Why is the bit garbage after getting red hot? Is it because of annealing?
It will screw up the grain structure of the end mill. Carbide either needs zero coolant, or constant coolant. Going back and forth will break them. The rule with machining is the only thing that should ever glow is the chips. If the bit, or the part is hot enough to emit it’s own light, you’re doing it wrong.
Fun fact, the stuff starts out as a powder.