Method of vacuum carburizing

Abstract

A method and apparatus of vacuum carburizing by changing the surface chemistry of a metal article through absorption and thermal diffusion of carbon, wherein oxidizing gases and other impurities are initially removed from the chamber in which the metal article is treated by evacuation and the temperature in the chamber is maintained at a level above normal carburizing temperatures, but at a point that is compatible with the physical characteristics of the chamber and the article; whereafter a source of carbon is introduced into the evacuated chamber in accordance with a preselected cycle and at a concentration that is controlled by the absolute pressure of the carbon in the chamber, so that the carbon is absorbed and diffused into the metal article at a controlled rate.

What is claimed is: 
     
       1. A method of carburizing a metallic article in a subatmospheric environment to produce a preselected amount of surface absorbed carbon and diffused gradient of carbon in said article, comprising the steps of heating the article in a heating chamber in a subatmospheric environment having a pressure in the range of approximately 0.2 to 10 -10  mm. Hg, wherein adsorbed gases and other impurities are removed from said article and the surfaces thereof are cleaned, periodically introducing a source of carbon into said chamber and thereafter evacuating said chamber to generate one or more pulses of the carbon source in said chamber so as to remove unwanted waste reactants produced by the source of carbon and to promote effective circulation of carbon vapor around said article, each of said pulses including backfilling said chamber with a carburizing gas to a pressure that is maintained at subatmospheric conditions, the carburizing gas defining the source of carbon and forming active carbon vapor and waste reactants in said chamber after being admitted therein, each pulse including the removal of the waste reactants from the chamber during the period of carbon absorption into said article and following each backfilling operation of admitting the carburizing gas into said chamber, controlling the amount of carburizing gas admitted into said heating chamber in each backfilling operation in proportion to the partial pressure of the carbon vapor in said chamber, wherein the partial pressure of the carbon vapor is maintained at a value from zero up to and including the saturation point of the carbon vapor in said chamber, so that effective absorption and diffusion of carbon into said article is obtained at a controlled rate to produce a uniform case or preselected carbon potential in said article. 
     
     
       2. A method of carburizing as claimed in claim 1, said carburizing gas being methane (CH 4 ). 
     
     
       3. A method of carburizing as claimed in claim 1, the pressure in said heating chamber after backfilling with said carburizing gas being maintained in the range of approximately 20-80 mm. Hg. 
     
     
       4. A method of carburizing as claimed in claim 1, the temperature in said heating chamber during the carburizing of said article being in the range of approximately 1300° F. to 4000° F. .Iadd. 5. A method of carburizing a metallic article in a subatmospheric environment to produce a preselected amount of surface absorbed carbon and diffused gradient of carbon in said article, comprising the steps of heating the article in a heating chamber in a subatmospheric environment, wherein adsorbed gases and other impurities are removed from said article and the surfaces thereof are cleaned, periodically introducing a source of carbon into said chamber and thereafter evacuating said chamber to generate one or more pulses of the carbon source in said chamber so as to remove unwanted waste reactants produced by the source of carbon and to promote effective circulation of carbon vapor around said article, each of said pulses including backfilling said chamber with a carburizing gas to a pressure that is maintained at subatmospheric conditions, the carburizing gas defining the source of carbon and forming active carbon vapor and waste reactants in said chamber after being admitted therein, each pulse including the removal of the waste reactants from the chamber during the period of carbon absorption into said article and following each backfilling operation of admitting the carburizing gas into said chamber, controlling the amount of carburizing gas admitted into said heating chamber in each backfilling operation in proportion to the partial pressure of the carbon vapor in said chamber, so that effective absorption and diffusion of carbon into said article is obtained at a controlled rate to produce a uniform case of preselected carbon potential in said article. .Iaddend.