Method of making powder metal parts by surface densification

Abstract

A method of producing parts from powdered metal having a first step of providing a powder, which is compressed at a pressure of 25 to 65 tsi to provide a green compact. The compact is then sintered at 2000° F. to 2400° F. for 20 to 60 minutes and cooled. After the compact has been cooled, the density of the compact is increased to greater than 7.4 g/cc. The compact is austenitized in an atmosphere of gas containing propane and a carbon potential of 0.8%. In one embodiment, the gas also contains ammonia. The compact is heated in the atmosphere at a temperature of 1600° F. for 40 minutes. Immediately following the heating, the compact is quenched in oil a temperature between 120° F. and 150° F. for 10 to 15 minutes. Lastly the compact is tempered at a temperature between 300° F. and 1000° F. for 30 to 90 minutes.

1 . A method of producing parts from powdered metal comprising the steps of: 
 a) providing a metallurgic powder;    b) compressing the metallurgic powder at a pressure of 25 tsi to 65 tsi to provide a green compact;    c) heating the compact to a temperature of 2000° to 2400° F. for 20 to 60 minutes;    d) cooling the compact to ambient temperature;    e) increasing the density of at least a portion of the compact to greater than 7.4 g/cc;    f) placing the compact in an atmosphere of endothermic gas containing propane and a carbon potential of 0.4% to 0.9%;    g) heating the compact in the atmosphere of endothermic gas at a temperature of 1600° F. for 40 minutes;    h) quenching the compact in oil heated to a temperature between 120° F. and 150° F. for 10 to 15 minutes; and    i) heating the compact to a temperature between 300° F. and 1000° F. for 30 to 90 minutes.    
   
   
       2 . The method of  claim 1 , wherein the metallurgic powder is comprised of iron, 0.15 to 0.9 weight percent graphite, and 0.5 to 1.0 weight percent molybdenum, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       3 . The method of  claim 2 , wherein the metallurgic powder further comprises up to 0.5 weight percent manganese, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       4 . The method of  claim 2 , wherein the metallurgic powder further comprises up to 1.5 weight percent silicon, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       5 . The method of  claim 2 , wherein the metallurgic powder further comprises up to 4.5 weight percent nickel, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       6 . The method of  claim 2 , wherein the metallurgic powder further comprises up to 2.0 weight percent copper, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       7 . The method of  claim 2 , wherein the metallurgic powder further comprises up to 4.0 weight percent chromium, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       8 . The method of  claim 1 , wherein the green compact of step b) has a green density of 6.4 g/cc to 7.4 g/cc.  
   
   
       9 . The method of  claim 1 , further comprising heating the compact to a temperature of 1560° F. for 35 minutes between steps d) and e).  
   
   
       10 . The method of  claim 1 , wherein the step of increasing the density of at least a portion of the compact in step e) is selected from a group consisting of sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging, cold forming, and hot forming.  
   
   
       11 . The method of  claim 1 , wherein the atmosphere comprises 400 cfh of endothermic gas and 10 cfh of propane.  
   
   
       12 . The method of  claim 11 , wherein the atmosphere further comprises 10 cfh of ammonia.  
   
   
       13 . The method of  claim 1 , wherein the compact in step i) is heated to a temperature of 400° F. for 60 minutes.  
   
   
       14 . The method of  claim 1 , wherein the compact is not cooled below the temperature of step f) between steps f) and g).  
   
   
       15 . The method of  claim 1 , wherein the compact in step c) is heated to a temperature of 2000° F. to 2100° F. for 20 to 60 minutes.  
   
   
       16 . The method of  claim 1 , wherein the compact in step c) is heated to a temperature of 2070° F. for 30 minutes.  
   
   
       17 . The method of  claim 1 , wherein the compact in step c) is heated to a temperature of 2100° F. to 2400° F. for 20 to 60 minutes.  
   
   
       18 . The method of  claim 1 , wherein the compact in step c) is heated to a temperature of 2300° F. for 30 minutes.  
   
   
       19 . A method of producing parts from powdered metal comprising the steps of: 
 a) providing a metallurgic powder;    b) compressing the metallurgic powder at a pressure of 25 tsi to 65 tsi to provide a green compact;    c) heating the compact to a temperature of 2000° F. to 2400° F. for 20 to 60 minutes;    d) cooling the compact to ambient temperature;    e) increasing the density of at least a portion of the compact to greater than 7.4 g/cc;    f) placing the compact in an atmosphere of endothermic gas containing propane, ammonia, and a carbon potential of 0.4% to 0.9%;    g) heating the compact in the atmosphere of endothermic gas at a temperature of 1600° F. for 40 minutes;    h) quenching the compact in oil heated to a temperature between 120° F. and 150° F. for 10 to 15 minutes; and    i) heating the compact to a temperature between 300° F. and 1000° F. for 30 to 90 minutes.    
   
   
       20 . The method of  claim 19 , wherein the metallurgic powder is comprised of iron, 0.15 to 0.9 weight percent graphite, and 0.5 to 1.0 weight percent molybdenum, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       21 . The method of  claim 20 , wherein the metallurgic powder further comprises up to 0.5 weight percent manganese, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       22 . The method of  claim 20 , wherein the metallurgic powder further comprises up to 1.5 weight percent silicon, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       23 . The method of  claim 20 , wherein the metallurgic powder further comprises up to 4.5 weight percent nickel, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       24 . The method of  claim 20 , wherein the metallurgic powder further comprises up to 2.0 weight percent copper, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       25 . The method of  claim 20 , wherein the metallurgic powder further comprises up to 4.0 weight percent chromium, wherein the weight percentages are calculated based on a total weight of the powder.  
   
   
       26 . The method of  claim 19 , wherein the green compact of step b) has a green density of 6.4 g/cc to 7.4 g/cc.  
   
   
       27 . The method of  claim 19 , further comprising heating the compact to a temperature of 1560° F. for 35 minutes between steps d) and e).  
   
   
       28 . The method of  claim 19 , wherein the step of increasing the density of at least a portion of the compact in step e) is selected from a group consisting of sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging, cold forming, and hot forming.  
   
   
       29 . The method of  claim 19 , wherein the atmosphere comprises 400 cfh of endothermic gas and comprises 10 cfh of propane.  
   
   
       30 . The method of  claim 19 , wherein the compact in step i) is heated to a temperature of 400° F. for 60 minutes.  
   
   
       31 . The method of  claim 19 , wherein the compact is not cooled below the temperature of step f) between steps f) and g).  
   
   
       32 . The method of  claim 19 , wherein the compact in step c) is heated to a temperature of 2000° F. to 2100° F. for 20 to 60 minutes.  
   
   
       33 . The method of  claim 19 , wherein the compact in step c) is heated to a temperature of 2070° F. for 30 minutes.  
   
   
       34 . The method of  claim 19 , wherein the compact in step c) is heated to a temperature of 2100° F. to 2400° F. for 20 to 60 minutes.  
   
   
       35 . The method of  claim 19 , wherein the compact in step c) is heated to a temperature of 2300° F. for 30 minutes.