Compositions and methods for sintering powdered metal compacts using little or no hydrogen

Abstract

Metal powder compositions for pressed powder metallurgy, and methods of forming metal parts using the metal powder compositions. The metal powder compositions include a metal powder and a lubricant system, which are pressed to form a green compact. When the green compact is heated during a delubing process, the lubricant system provides a reactive carbon coating on the surface of the metal powder, which upon subsequent sintering, reacts with metal oxides present on the surface of the metal powder to form gas that is easily removed from the system. Removing the metal oxides from the surface of the metal powder provides a more dense metal part, and allows for sintering of the green compact without the need for hydrogen gas, which would otherwise be needed to remove the metal oxides on the surface of the metal powder.

1 . A method of forming a metal part comprising:
 providing a composition comprising metal powder and at least 0.5 wt % of a lubricant system, the lubricant system including
 10-30 wt % stearic acid, 
 0.1-5 wt % guanidine material, 
 0.1-0.8 wt % antioxidant, 
 5-15 wt % microcrystalline wax, 
 10-30 wt % polyethylene/polypropylene copolymer wax, and 
 40-60 wt % ethylene bis(stearamide); 
   applying pressure to the composition to thereby form a green compact;   delubing the green compact in a delubing atmosphere that contains less than 5 volume % hydrogen gas; and   sintering the delubed green compact to thereby form the metal part.   
     
     
         2 . The method according to  claim 1 , wherein the delubing atmosphere is free of intentionally added hydrogen gas. 
     
     
         3 . The method according to  claim 1 , wherein the delubing atmosphere consists essentially of nitrogen gas. 
     
     
         4 . The method according to  claim 1 , wherein the composition is free of lauric acid. 
     
     
         5 . The method according to  claim 1 , further comprising arranging the composition in a die cavity before applying pressure to the composition. 
     
     
         6 . The method according to  claim 5 , further comprising removing the green compact from the die cavity before delubing and sintering the green compact. 
     
     
         7 . The method according to  claim 1 , wherein metal oxides are present on outer surfaces of the metal powder prior to delubing. 
     
     
         8 . The method according to  claim 7 , wherein during delubing, the lubricant system decomposes and leaves a carbon residue on the outer surfaces of the metal powder. 
     
     
         9 . The method according to  claim 8 , wherein the green compact is sintered in a sintering atmosphere that contains less than 5 volume % hydrogen gas. 
     
     
         10 . The method according to  claim 9 , wherein during sintering, the metal oxides present on the outer surfaces of the metal powder are removed in situ in a reaction with the carbon residue. 
     
     
         11 . The method according to  claim 9 , wherein the sintering atmosphere is free of intentionally added hydrogen gas. 
     
     
         12 . The method according to  claim 11 , wherein the delubing atmosphere consists essentially of nitrogen gas. 
     
     
         13 . The method according to  claim 1 , wherein the composition includes 0.5-0.6 wt % of the lubricant system. 
     
     
         14 . The method according to  claim 1 , wherein the guanidine material is a mixture of guanidine stearate and guanidine ethyl hexanoate. 
     
     
         15 . The method according to  claim 1 , wherein the antioxidant is tris (2,4-di-tert-butylphenyl) phosphite. 
     
     
         16 . The method according to  claim 1 , wherein the metal powder includes iron. 
     
     
         17 . A method of forming a metal part comprising steps of:
 providing a composition comprising metal powder and at least 0.5 wt % of a lubricant system;   applying pressure to the composition to thereby form a green compact;   delubing the green compact in a delubing atmosphere that contains less than 5 volume % hydrogen gas; and   sintering the delubed green compact to thereby form the metal part;   wherein the lubricant system transforms from a solid phase material to a viscous liquid phase material during the applying pressure step and thereby spreads onto outer surfaces of the metal powder,   wherein after the delubing step a carbon residue is left on the outer surfaces of the metal powder due to decomposition of the lubricant system, and   wherein during sintering, the metal oxides present on the outer surfaces of the metal powder are removed in situ in a reaction with the carbon residue.   
     
     
         18 . A lubricant system comprising:
 10-30 wt % stearic acid,   0.1-5 wt % guanidine material,   0.1-0.8 wt % antioxidant,   5-15 wt % microcrystalline wax,   10-30 wt % polyethylene/polypropylene copolymer wax, and   40-60 wt % ethylene bis(stearamide),   wherein when the lubricant system is combined in an amount of at least 0.5 wt % with metal powder and compressed into a green compact, and the green compact is delubed and sintered in an atmosphere that contains less than 5 volume % hydrogen gas to thereby form a metal part.   
     
     
         19 . The lubricant system according to  claim 18 , wherein the composition is free of lauric acid. 
     
     
         20 . The lubricant system according to  claim 18 , wherein particles of the metal powder have outer surfaces comprising metal oxide.