Process for treating steel alloys

Abstract

The process includes heating of steel alloy parts in the presence of hydrogen prior to introduction of, for example, carburizing gas, preferably acetylene, in the presence of a diluent carrier gas. The process uses a continuous cycle involving only one carburizing (boost) step and one diffusion step, and high velocity gas quenching. The process is advantageously carried out in a unique, versatile vacuum furnace (also having high internal pressure capability) that provides very high velocity, continuous flow gas quenching, and a furnace design, including a long, low profile work zone configuration and quench gas recirculation equipment and flow pattern that facilitates high velocity gas flow. The entire process is accomplished in a single self-contained chamber of the furnace.

1 . A process for treating (in a specific chamber of a heat treating vacuum furnace having low vacuum capability, high pressure capability, and very high gas-circulating capability), said chamber having gas transport lines for providing gas to and drawing gas from said chamber), surfaces of steel alloy work pieces, said process comprising: 
 (a) drawing a very low pressure vacuum to evacuate gas from said chamber;    (b) allowing hydrogen to flow through a gas line into said chamber to a pressure not exceeding 10 torr;    (c) heating said chamber to a temperature between 871 and 1038 degrees C. and soaking the work pieces in that heat for at least 15 minutes, and then adjusting the pressure to at least 7.6 torr by adding gas as necessary;    (d) while maintaining the pressure at a level of at least 7.6 torr, and at a temperature at least as high as 871 C adding to said chamber through at least one said gas line, gas having a capability of desirably affecting said surfaces; and    (e) shutting of the gas having a capability of desirably affecting said surface and while maintaining the temperature at least as high as 871 C allowing nitrogen to flow through a gas line to a pressure not exceeding 10 torr and allowing the work pieces to diffuse and then    (f) shutting off the heating mechanism and very rapidly quenching by releasing large quantities of quench gas at high pressure into said chamber and recycling the quench gas at a high rate of speed.    
   
   
       2 . A process in accordance with  claim 1  wherein said gas having a capability of desirably affecting said surfaces includes at least one gas capable of affecting the alloy metal to form hardened surfaces thereon and at least one diluent gas.  
   
   
       3 . A process in accordance with  claim 2  wherein said gas having a capability of desirably affecting said surfaces includes at least one carburizing gas.  
   
   
       4 . A process in accordance with  claim 3  wherein said gas having a capability of desirably affecting said surfaces comprises a member of the group consisting of propane, ethylene and acetylene.  
   
   
       5 . A process in accordance with  claim 3  wherein said gas having a capability of desirably affecting said surfaces comprises acetylene.  
   
   
       6 . A process in accordance with  claim 3  wherein the carburizing gas is acetylene and is added to said chamber as a mixture of acetylene and a diluent that is a member of the group consisting of hydrogen and nitrogen.  
   
   
       7 . A process in accordance with  claim 1  wherein said gas having a capability of desirably affecting said surfaces comprises a carburizing gas.  
   
   
       8 . A process in accordance with  claim 2  wherein said diluent comprises hydrogen or nitrogen.  
   
   
       9 . A process in accordance with  claim 8  wherein said diluent is hydrogen.  
   
   
       10 . A process in accordance with  claim 7  wherein said diluent is nitrogen.  
   
   
       11 . A process in accordance with  claim 1  wherein said quench gas is nitrogen.  
   
   
       12 . A process in accordance with  claim 1  wherein said quench gas is helium.  
   
   
       13 . A process in accordance with  claim 1  wherein said quench gas is hydrogen.  
   
   
       14 . A process in accordance with  claim 1  wherein the internal pressure during the last 10 minutes of the quench is at least 8 bar.  
   
   
       15 . A process in accordance with  claim 1  wherein the internal pressure during the last 20 minutes of the quench is at least 9 bar.  
   
   
       16 . A process in accordance with  claim 1  wherein the internal pressure during the last 10 minutes of the quench is at least 10 bar.