Abstract: Apparatus and method for carburizing and carbonitriding ferrous metal work in a continuous furnace having aligned heating, carburizing and diffusing zones. Each zone is provided with fan means in at least one side wall of the furnace, with both the inlet and the outlet of the fan at a level below the work trays which are conveyed through the furnace so that the gas atmosphere is pulled downwardly through the work at one or more pier positions within a processing zone and pushed upwardly through the work at the remaining pier positions within the zone. By proper refractory design and fan locations a more uniform and controllable flow pattern is provided throughout the length and width of each processing zone.