Abstract: The present invention relates to a method of carburizing and carbonitriding steel parts in the work chamber of a vestibule furnace with a substantially reduced consumption of carbon sources by exposing such parts to a gaseous carbon source such as natural gas while supplying an inert gas to the vestibule at a sufficient rate to control the entry of atmospheric decarburizing agents into the work chamber and to maintain the oxygen content in the vestibule at safe levels, below a minimum level required for supporting combustion or for allowing local puffs and explosions. The vestibule furnace may be of the batch or continuous type. In addition, the carbon potential of the atmosphere within the work chamber of the furnace is preferably continuously sensed and in response to such sensed carbon potential, the flow of natural gas to the work chamber is controlled so as to maintain the carbon potential at a predetermined value.