Method of optimizing an oxygen free heat treating process

Abstract

A method of controlling an oxygen-free heat treating process in an atmospheric pressure furnace is disclosed. The present method employs an oxygen-free 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 carburization process uses a precisely controlled gas atmosphere to minimize inter-granular oxidation, eliminate the formation of soot and cementite, and avoid hydrogen embrittlement.

1 . A method of controlling an oxygen free heat treating process using a heat treating model that optimizes the concentrations of gases in a controlled gas atmosphere of an atmospheric pressure furnace as a function of time, the controlled gas atmosphere including a reducing gas and a carbon containing gas and is substantially free of oxygen, the method comprising the steps of;
 inputting parameters into the heat treating model, the parameters including desired case depth, target temperature, and alloy composition of the part to be treated;   ascertaining a prescribed minimum concentration of the reducing gas necessary to inhibit formation of metal-oxides in the part at the target temperature; and   ascertaining a concentration of the carbon containing gas as a function of time to achieve the desired case depth yet inhibit formation of cementite in the part at the target temperature.   
     
     
         2 . The method of  claim 1  wherein the reducing gas is hydrogen gas and the carbon containing gas is hydrocarbon gas. 
     
     
         3 . The method of  claim 2  further comprising the step of treating the part in accordance with the ascertained hydrogen gas concentration and the ascertained hydrocarbon gas concentration as a function of time. 
     
     
         4 . The method of  claim 2  wherein the step of ascertaining a prescribed minimum concentration of hydrogen gas further comprises the steps of:
 determining the oxidation potential of hydrogen at the target temperature and level of residual oxygen in the furnace for a range of hydrogen gas concentrations;   comparing the oxidation potential of metals within the part to be treated at the target temperature with the oxidation potential of hydrogen at the target temperature; and   selecting the prescribed minimum concentration of hydrogen gas that yields a greater oxidation potential than the oxidation potential of metals within the part to be treated.   
     
     
         5 . The method of  claim 2  wherein the step of ascertaining a profile of hydrocarbon gas as a function of time further comprises the steps of:
 selecting a desired carbon saturation factor;   selecting a desired carbon concentration at the surface of the metal part;   estimating the diffusion of carbon in the part as a function of time at the target temperature based on the alloy composition of the part and a desired case depth;   determining a desired carbon flux from the hydrocarbon gas to the surface of the part as a function of time that yields the desired carbon saturation factor; and   selecting the concentration of hydrocarbon gas as a function of time that produces the desired carbon flux from the hydrocarbon gas to the surface of the part as a function of time.   
     
     
         6 . The method of  claim 2  wherein the controlled gas atmosphere includes the hydrogen gas and the hydrocarbon gas with a balance of the controlled gas atmosphere being substantially nitrogen gas. 
     
     
         7 . The method of  claim 2  wherein the controlled gas atmosphere includes a maximum volume concentration of hydrogen gas of about 5.25 percent. 
     
     
         8 . The method of  claim 2  wherein the step of inputting parameters into the heat treating model further comprises measuring the temperature of the furnace and inputting the temperature into the heat treating model. 
     
     
         9 . The method of  claim 3  wherein the step of inputting parameters into the heat treating model further comprises sensing the level of residual oxygen present in the furnace and inputting the level of residual oxygen into the heat treating model. 
     
     
         10 . The method of  claim 2  wherein the profile of 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 as a function of time to a minimum hydrocarbon gas concentration over a second prescribed duration; and   maintaining the minimum hydrocarbon gas concentration for a third prescribed duration.   
     
     
         11 . The method of  claim 10  wherein the step of reducing the hydrocarbon gas concentration from the maximum hydrocarbon gas concentration to a minimum hydrocarbon gas concentration further comprises gradually decreasing the hydrocarbon gas concentration as a function of time from the maximum hydrocarbon gas concentration to the minimum hydrocarbon gas concentration. 
     
     
         12 . The method of  claim 10  wherein the hydrocarbon gas is selected from the group consisting of propylene, butadiene, ethylene, butane, ethane, propane and acetylene. 
     
     
         13 . The method of  claim 10  wherein the minimum volume concentration of hydrocarbon gas between about 0.2 percent and about 1.5 percent. 
     
     
         14 . The method of  claim 10  wherein the maximum volume concentration of hydrocarbon gas between about 1.0 percent and about 5.0 percent. 
     
     
         15 . The method of  claim 2  wherein the treatment of the part is carburization and the volume concentration of hydrocarbon gas is between about 5.0 percent and 1.0 percent. 
     
     
         16 . The method of  claim 2  wherein the treatment of the metal part is neutral hardening and the volume concentration of hydrocarbon gas is between about 1.0 percent and 0.2 percent. 
     
     
         17 . A method of controlling oxide formation in a controlled atmosphere, oxygen free heat treating process, the method comprising the steps of:
 identifying selected parameters including target temperature, level of residual oxygen in a heat-treating atmospheric pressure furnace, and alloy composition of the metal part to be treated; and   ascertaining a prescribed minimum concentration of a reducing gas necessary to inhibit formation of metal-oxides in the part at the target temperature based on the level of residual oxygen in the furnace, and the alloy composition of the metal part to be treated.   
     
     
         18 . The method of  claim 17  wherein the reducing gas is hydrogen gas. 
     
     
         19 . The method of  claim 18  wherein the step of ascertaining a prescribed minimum concentration of hydrogen gas further comprises the steps of:
 determining the oxidation potential of hydrogen at the target temperature and the level of residual oxygen in the heat-treating atmospheric pressure furnace for a range of hydrogen gas concentrations;   comparing the oxidation potential of selected metals within the part to be treated at the target temperature with the determined oxidation potential of hydrogen; and   selecting the prescribed minimum concentration of hydrogen gas that yields a greater oxidation potential than the oxidation potential of selected metals within the part to be treated.