Hybrid carburization with intermediate rapid quench

Abstract

A carbon hardened surface is produced in a metal workpiece without forming carbide precipitates by carburizing the workpiece at high temperature carburization conditions, rapidly quenching the workpiece, and then carburizing the workpiece under low temperature carburization conditions.

1 . A process for fanning a carbon hardened surface in a metal workpiece without forming carbide precipitates, the process comprising subjecting the workpiece to both high temperature carburization and low temperature carburization, wherein immediately after high temperature carburization, the workpiece is rapidly quenched to a temperature below which carbide precipitates form. 
   
   
       2 . The process of  claim 1 , wherein the process comprises carburizing the workpiece at high temperature carburization conditions, rapidly quenching the workpiece, and then carburizing the workpiece under low temperature carburization conditions. 
   
   
       3 . The process of  claim 2 , wherein the workpiece is rapidly quenched through a region of conditions where carbide precipitates can form without forming such carbide precipitates. 
   
   
       4 . The process of  claim 2 , wherein during carburization of the workpiece under low temperature carburization conditions, the instantaneous rate of carburization is held essentially constant. 
   
   
       5 . The process of  claim 2 , wherein during carburization of the workpiece under low temperature carburization conditions, the instantaneous rate of carburization is reduced from a higher value during earlier stages of carburization to a lower value during subsequent stages of carburization. 
   
   
       6 . The process of  claim 5 , wherein the instantaneous rate of carburization is reduced by reducing the carburization temperature. 
   
   
       7 . The process of  claim 5 , wherein the instantaneous rate of carburization is reduced by reducing the concentration of carbon in the carburizing gas. 
   
   
       8 . The process of  claim 1 , wherein the process comprises carburizing the workpiece at low temperature carburization conditions, rapidly heating the workpiece to a temperature above which carbide precipitates can form, further carburizing the workpiece under high temperature carburization conditions, and rapidly quenching the workpiece to a temperature below which carbide precipitates can form. 
   
   
       9 . The process of  claim 8 , wherein the workpiece is rapidly quenched through a region of conditions where carbide precipitates can form without forming such carbide precipitates. 
   
   
       10 . The process of  claim 1 , wherein the metal is stainless steel. 
   
   
       11 . A process for altering the surface of a metal workpiece by diffusing an element into the workpiece without forming precipitates of the diffused element in the altered surface, the process comprising contacting the workpiece with a diffusion gas containing the element at a first elevated temperature which is above the temperature at which such precipitates can form and, in addition, contacting the workpiece with a diffusion gas at a second elevated temperature which is lower than the first elevated temperature and which is also below a temperature at which such precipitates can form,
 wherein immediately after the workpiece is contacted with the diffusion gas at the first elevated temperature, the workpiece is rapidly quenched to a temperature below which such precipitates can foam.   
   
   
       12 . The process of  claim 11 , wherein the process comprises contacting the workpiece with a diffusion gas containing the element at a first elevated temperature which is above the temperature at which such precipitates can form, rapidly quenching the workpiece through a region of conditions where such precipitates can form, and then completing diffusion of the element into the workpiece by contacting the workpiece with another diffusion gas at a second elevated temperature lower than the first elevated temperature under conditions which avoid formation of such precipitates. 
   
   
       13 . The process of  claim 12 , wherein during completion of diffusion at the second elevated temperature, the instantaneous rate of diffusion is held essentially constant. 
   
   
       14 . The process of claim  21 , wherein during completion of diffusion at the second elevated temperature, the instantaneous rate of diffusion is reduced from a higher value during earlier stages of diffusion to a lower value during subsequent stages of diffusion. 
   
   
       15 . The process of  claim 14 , wherein the instantaneous rate of diffusion is reduced by reducing the diffusion temperature, reducing the concentration of diffusing element in the diffusion gas, or both. 
   
   
       16 . The process of  claim 11 , wherein the process comprises contacting the workpiece with a diffusion gas at a second elevated temperature below which such precipitates can form, rapidly heating the workpiece to a temperature above which such precipitates can form, contacting the workpiece with a diffusion gas at a first elevated temperature above which such precipitates can form, and rapidly quenching the workpiece to a temperature below which such precipitates can form. 
   
   
       17 . The process of  claim 11 , wherein the metal is aluminum or an alloy of aluminum. 
   
   
       18 . The process of  claim 11 , wherein the metal is titanium or an alloy of titanium.