Oxidizing Annealing

When tempering, a thin oxide layer forms on the surface, changing color depending on temperature and duration – from straw yellow through blue to gray. You can best recognize the correct color when the workpiece is clean and free of grease. If heated too hot or too long, the layer becomes too thick and flakes off, and the part loses hardness. Keep the heating even and let the workpiece cool in air, then it'll be fine.