Methods for carburizing steel parts

A method of carburizing steel workpieces in a vestibule furnace having an integral quench vestibule, inner and outer doors and a work chamber without a carrier gas such as endothermic or purified exothermic gas, comprising the steps of heating the work chamber to a temperature of at least 1350.degree. F. while introducing a flow of nitrogen gas into said vestibule at a flow rate of less than 50% of the carrier gas flow rate recommended for said furnace such that the oxygen content in said vestibule is maintained below a level at which combustion is supported;

• opening said outer door and inserting said workpieces into said vestibule;
• closing said outer door and detecting the pressure within said vestibule;
• passing said workpieces from said vestibule to said work chamber and closing said inner door after a positive pressure is detected in said vestibule;
• introducing a gaseous carbon source without a carrier gas therefor into said work chamber while maintaining said nitrogen flow to said vestibule thereby carburizing said workpieces with a substantially lower quantity of said gaseous carbon source than is required for carburizing atmospheres when said carrier gas is utilized in carburizing said workpieces;
• opening said inner door after a predetermined carburization period has elapsed;
• passing said carburized workpieces from said work chamber into said vestibule and closing said inner door;
• quenching said carburized workpieces in said vestibule for a predetermined time period whereby said workpieces are case hardened;
• and then removing said case hardened workpieces from said vestibule.

A method of carburizing steel workpieces in the work chamber of a vestibule furnace having one or more vestibules without a carrier gas such as endothermic or purified exothermic gas comprising the steps of:

• a. heating the work chamber to a temperature of at least 1350.degree. F.;
• b. introducing an inert gas into each vestibule of said furnace at a flow rate of less than 50% of the carrier gas flow rate recommended for said furnace to maintain the oxygen content in each vestibule below levels at which combustion is supported;
• c. placing workpieces to be carburized in said work chamber;
• d. supplying a gaseous carbon source without a carrier gas therefor to said work chamber while maintaining said inert gas flow to said vestibule to carburize said workpieces;
• e. sensing the carbon potential of the atmosphere in said work chamber;
• f. controlling the flow of said gaseous carbon source in response to said sensed carbon potential such that said gaseous carbon source is supplied to said work chamber at the minimal flow rate necessary to maintain a predetermined carbon potential therein;
• g. removing carburized workpieces from said chamber;
• and
• h. quenching said removed carburized workpieces thereby case hardening said removed workpieces.