Method for oxygen free carburization in atmospheric pressure furnaces

Abstract

A method of oxygen-free heat treatment of a steel part in an atmospheric pressure furnace is disclosed. The present method employs an oxygen-free controlled gas atmosphere including hydrogen gas in concentrations between about 1.0 percent to 10.0 percent, a hydrocarbon gas, such as propylene, in concentrations of between about 0.1 percent and 10.0 percent that varies as a function of time, with the balance of the gas atmosphere being nitrogen. The presently disclosed oxygen-free heat treatment process, preferably a carburization process, uses a precisely controlled atmosphere to minimize inter-granular oxidation, eliminate the formation of soot and cementite or other metallic carbides, and avoid hydrogen embrittlement.

1 . A method of treating a metal part in an atmospheric pressure furnace comprising the steps of:
 introducing the metal part into the atmospheric pressure furnace having a gas atmosphere substantially free of oxygen, the gas atmosphere comprising a carbon containing gas and a reducing gas;   heating the metal part to a target temperature;   adjusting the gas concentrations of the gas atmosphere to inhibit formation of metal-oxides on the metal part and to maintain a carbon saturation factor of between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation; and   removing the metal part from the atmospheric pressure furnace.   
     
     
         2 . The method of  claim 1  wherein the carbon containing gas is a hydrocarbon gas and the reducing gas is hydrogen. 
     
     
         3 . The method of  claim 2  wherein the step of adjusting the gas concentrations of the gas atmosphere further comprise varying the volume concentration of the hydrocarbon gas in the gas atmosphere as a function of time to maintain the carbon saturation factor of between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation. 
     
     
         4 . The method of  claim 3  wherein hydrocarbon gas is selected from the group consisting of: propylene, butene, butadiene, ethylene, ethane, propane, and acetylene. 
     
     
         5 . The method of  claim 4  wherein the hydrocarbon gas has a free energy of formation per gram-mole of carbon between 64 and 85 kJ over the temperature range of about 850 degrees Centigrade to about 1000 degrees Centigrade. 
     
     
         6 . The method of  claim 4  wherein the hydrocarbon gas is propylene in a volume concentration of no greater than about 3.15 percent. 
     
     
         7 . The method of  claim 2  wherein a volume concentration of the hydrocarbon gas is between about 0.1 percent and 10.0 percent. 
     
     
         8 . The method of  claim 2  wherein the treatment of the metal part is neutral hardening and the volume concentration of the hydrocarbon gas is between about 0.2 percent and 1.0 percent. 
     
     
         9 . The method of  claim 2  wherein the treatment of the metal part is carburization of the metal part and the volume concentration of the hydrocarbon gas is between about 1.0 percent and 5.0 percent. 
     
     
         10 . The method of  claim 2  wherein the volume concentration of the hydrogen gas is between about 1.0 percent and 10.0 percent. 
     
     
         11 . The method of  claim 2  wherein the volume concentration of the hydrogen gas is between about 1.5 percent and 5.25 percent. 
     
     
         12 . The method of  claim 1  further comprising the step of seasoning the atmospheric pressure furnace to reduce the oxygen and moisture content thereof. 
     
     
         13 . The method of  claim 1  further comprising the step of cooling the metal part. 
     
     
         14 . The method of  claim 13  wherein the step of cooling the metal part further comprises cooling the metal part in an atmosphere that is substantially free of oxygen. 
     
     
         15 . A method of treating a metal part in an atmospheric pressure furnace comprising the steps of:
 preparing the atmospheric pressure furnace with a controlled gas atmosphere substantially free of oxygen and including hydrogen gas;   adjusting the concentration of the hydrogen gas above a prescribed minimum threshold to inhibit formation of metal-oxides at a target temperature;   introducing a hydrocarbon gas into the controlled gas atmosphere; and   treating a metal part in the controlled gas atmosphere of the atmospheric pressure furnace at the target temperature.   
     
     
         16 . The method of  claim 15  wherein the controlled gas atmosphere includes hydrogen gas in a volume concentration between about 1.0 percent and 10.0 percent. 
     
     
         17 . The method of  claim 15  wherein the controlled gas atmosphere includes hydrogen in a volume concentration between about 1.5 percent and about 5.25 percent. 
     
     
         18 . The method of  claim 15  wherein the step of preparing the atmospheric pressure furnace with a controlled gas atmosphere further comprises seasoning the atmospheric pressure furnace to reduce the oxygen and moisture content thereof. 
     
     
         19 . The method of  claim 15  wherein the metal part contains metals selected from the group of chromium, manganese, titanium or silicon and any inter-granular oxidation of such metals occurs at a depth of less than or equal to 0.0007 inches. 
     
     
         20 . The method of  claim 15  further comprising the step of cooling or quenching the metal part in an atmosphere that is substantially free of oxygen. 
     
     
         21 . A method of treating a metal part in an atmospheric pressure furnace comprising the steps of:
 preparing the atmospheric pressure furnace with a controlled gas atmosphere substantially free of oxygen and including a hydrocarbon gas substantially free of oxygen;   heating the metal part at a target temperature in the controlled gas atmosphere of the atmospheric pressure furnace for a prescribed duration;   adjusting the concentration of the hydrocarbon gas as a function of time to maintain a carbon saturation factor of between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation throughout the prescribed duration.   
     
     
         22 . The method of  claim 21  wherein the concentration of hydrocarbon gas of the controlled gas atmosphere is adjusted to a volume concentration of between about 10.0 percent and about 0.10 percent. 
     
     
         23 . The method of  claim 21  wherein the concentration of hydrocarbon gas of the controlled gas atmosphere is adjusted to a volume concentration of between about 5.0 percent and about 1.0 percent. 
     
     
         24 . The method of  claim 21  wherein the step of adjusting the concentration of the hydrocarbon gas as a function of time further comprises:
 introducing an initial maximum hydrocarbon gas concentration for a first prescribed duration;   reducing the hydrocarbon gas concentration from the maximum hydrocarbon gas concentration to a minimum hydrocarbon gas concentration over a second prescribed duration; and   maintaining the minimum hydrocarbon gas concentration for a third prescribed duration.   
     
     
         25 . The method of  claim 21  wherein the hydrocarbon gas is selected from the group consisting of: propylene, butene, butadiene, ethylene, ethane, propane, and acetylene. 
     
     
         26 . The method of  claim 25  wherein the hydrocarbon gas has a free energy of formation per gram-mole of carbon between about 64 and 85 kJ over the temperature range of about 850 degrees Centigrade to about 1000 degrees Centigrade. 
     
     
         27 . The method of  claim 21  wherein the hydrocarbon gas is propylene and the propylene gas concentration is no greater than about 3.15 percent by volume. 
     
     
         28 . The method of  claim 21  wherein there is negligible soot formation on the surface of the treated metal part. 
     
     
         29 . A method of treating a metal part in an atmospheric pressure furnace comprising the steps of:
 introducing the metal part into the atmospheric pressure furnace having a gas atmosphere substantially free of oxygen, the gas atmosphere including a carbon containing gas, a reducing gas, and an inert gas;   adjusting the gas concentrations of the carbon containing gas to maintain a carbon saturation factor associated with the metal part in the atmospheric pressure furnace of between about 0.75 and about 1.25;   wherein negligible soot and cementite, or other metallic carbide, is formed on a surface of the metal part.   
     
     
         30 . The method of  claim 29  wherein the carbon saturation factor is maintained in a range of between about 0.90 and about 1.10 with negligible cementite or other metallic carbide formation.