English: A timing sprocket for an engine that has been heat treated to a specific hardness, and then flame hardened at only the surface of the teeth. The bluish discoloration around the teeth indicate the area that was quickly heated to red hot and then quenched, hardening only the surface to the highest hardness possible. The sprocket is then used without further tempering, providing a very wear-resistant surface, but providing tougher metal directly underneath. The heat-affected zone lies just inside this discoloration, penetrating beneath the hardened metal, between the teeth and the ring of tempering colors.