How to solve the phenomenon of carbon increase in the endiksyon chofaj founo chofaj metal?

The workpiece that heats the metal in the induction heating furnace often has a carburizing phenomenon on the surface or part of the surface. This is because the heating of the metal in the induction heating furnace is not completely heated but the surface is carbonized. Once the carbonization of the metal workpiece occurs, it will cause the mechanical Processability deteriorates.