Vacuum carburizing

The method of subatmospheric gaseous carburizing or carbonitriding workpieces in a load chamber within a vacuum furnace comprising the steps of placing a load of the workpieces to be carburized in the load chamber, sealing said furnace and evacuating said chamber to a first subatmosphric pressure for removing most of the air and adsorbed gases and surface contaminants from the workpieces, then bringing said workpieces up to treatment temperature in the range of 1500.degree. F. to 1900.degree. F., holding said workpieces for a sufficient time period at said treatment temperature and at said first subatmospheric pressure for temperature stabilization throughout the load, back filling carburizing gaseous treatment material into said furnace until the pressure in said chamber has been raised to a second subatmospheric pressure greater than said first subatmospheric pressure while maintaining said treatment temperature within said chamber, said treatment temperature being that temperature within the range of 1500.degree. F. to 1900.degree. F. at which the carburizing gaseous treatment material will carburize the workpieces, positively uniformly circulating said gaseous treatment material through the load of workpieces in the load chamber by a hot circulating fan within said furnace, said second subatmospheric pressure being at least 300 torr absolute for enabling the hot circulating fan to effectively circulate said gaseous treatment material in said furnace throughout said load of workpieces in said load chamber, at said second subatmospheric pressure providing an input flow of the gaseous treatment material into said furnace at a predetermined constant flow rate and adjusting the output pumping from said furnace for holding the pressure in said chamber at said second subatmospheric pressure which is at least 300 torr absolute, continuously circulating the gaseous treatment material through the load by the hot circulating fan, and continuing for the desired carburizing time said treatment temperature, said input flow, said output pumping, and said uniform circulating by the hot circulating fan.