A method of carburizing ferrous metal parts that includes the steps of providing a furnace for heating the metal parts and a process chamber, purging the air atmosphere from the process chamber and heating said process chamber to a temperature of at least 1100° F., feeding a gas to the process chamber and a source of air at a constant flow rate to the process chamber, boosting the process chamber by feeding an enriching gas to the process chamber to create a carbon potential of at least 0.5%, and more preferably of at least 1.4%, cleaning the process chamber by decreasing the flow of enriching gas to the process chamber to create a carbon potential of less than about 0.25%, repeating the boosting step after performing said cleaning step, and repeating the cleaning step after performing said boosting step.