What is the reason for the soft spots on the surface of the graphite ring mold?

Reasons for soft spots on the surface of the graphite ring mold:
(1) The surface of the graphite ring mold has oxide scales, rust spots and local decarburization before heat treatment.
(2) After the graphite ring mold is quenched and heated, the cooling and quenching medium is not properly selected, and there are too many impurities or aging in the quenching medium.
Precaution:
(1) The oxide skin and rust spots should be removed before heat treatment of the graphite ring mold, and the surface of the graphite ring mold should be properly protected during quenching and heating. Vacuum electric furnace, salt bath furnace and protective atmosphere furnace should be used for heating as much as possible.
(2) When the graphite ring mold is cooled after quenching and heating, a suitable cooling medium should be selected, and the cooling medium used for a long time should be filtered frequently or replaced regularly.