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 batch treatment process 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 present continuous furnace treatment process employs a plurality of zones with each zone including an oxygen-free controlled gas atmosphere including hydrogen gas in concentrations between about 1.0 percent to 10.0 percent that varies across the different zones, a hydrocarbon gas, such as propylene, in concentrations of between about 0.1 percent and 10.0 percent that varies across the different zones, with the balance of the gas atmosphere within each zone being nitrogen. The presently disclosed oxygen-free heat treatment processes, preferably carburization processes, use 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:
 heating the metal part in the atmospheric pressure furnace having a gas atmosphere substantially free of oxygen, the gas atmosphere comprising a carbon containing gas, a reducing gas, and an inert gas;   controlling the gas concentrations of the carbon containing gas or reducing gas 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 treated metal part from the atmospheric pressure furnace.   
     
     
         2 . The method of  claim 1  wherein the atmospheric pressure furnace is a continuous-type furnace having a plurality of atmospheric zones and the step of controlling the gas concentrations further comprises maintaining the gas concentrations of the carbon containing gas in each atmospheric zone at the same or different levels to maintain a carbon saturation factor of between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation. 
     
     
         3 . The method of  claim 1  wherein the atmospheric pressure furnace is a batch-type furnace and wherein the step of controlling the gas concentrations further comprises varying the volume concentration of the carbon containing gas as a function of time to maintain the carbon saturation factor in the metal part between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation. 
     
     
         4 . The method of  claim 1  wherein the carbon containing gas is a hydrocarbon gas and the reducing gas is hydrogen. 
     
     
         5 . The method of  claim 4  wherein hydrocarbon gas is selected from the group consisting of: propylene, butene, butadiene, ethylene, ethane, propane, and acetylene. 
     
     
         6 . The method of  claim 1  wherein the carbon containing 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. 
     
     
         7 . The method of  claim 1  wherein a volume concentration of the carbon containing gas is between about nil percent and 10.0 percent. 
     
     
         8 . The method of  claim 1  wherein the volume concentration of the reducing gas is between about nil and 10.0 percent. 
     
     
         9 . The method of  claim 1  further comprising the step of cooling or quenching the metal part in an atmosphere that is substantially free of oxygen. 
     
     
         10 . The method of  claim 1  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.