Austempering/marquenching powder metal parts

Abstract

A powder metal part is made by compaction at room temperature or an elevated temperature followed by sintering, a secondary densification, heat treating, and optional secondary operations. The particulate materials preferably include iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, and 0-1.5 wt % silicon. At least one secondary densification is applied to the part after compaction and pre-sinter/sinters steps to achieve medium to high density. The secondary densification is part of a double-press double-sinter (DPDS) or is a mechanical working depending on the application requirements. The powder metal is heat treated by austempering or marquenching followed by tempering. A unique composite microstructure is achieved from austempering by controlling the powder chemistry and the holding time at an elevated temperature. The combination of a secondary densification and austempering or marquenching produces a high performance powder metal part for demanding applications.

1 . A method of producing a powder metal part comprising the steps of: 
 a) performing a primary densification on a powder metal composition;    b) sintering on the powder metal composition;    c) performing a secondary densification on the powder metal composition; and    d) heat treating the powder metal composition comprising austempering or marquenching the powder metal composition.    
     
     
         2 . The method of  claim 1 , wherein the powder metal composition comprises iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, 0-1.5 wt % manganese, and 0-1.5 wt % silicon.  
     
     
         3 . The method of  claim 1 , wherein step a) comprises compacting the composition at a pressure of 25 to 65 TSI to result in a green density of 6.4 to 7.4 g/cc.  
     
     
         4 . The method of  claim 1 , wherein step b) comprises heating the composition to 1200° F. to 1800° F. or 2000° F. to 2400° F. for 15 to 90 minutes.  
     
     
         5 . The method of  claim 1 , wherein steps a) through c) comprise a double press double sinter.  
     
     
         6 . The method of  claim 5 , wherein step a) comprises compacting the composition at a pressure of 30 to 65 TSI to result in a green density of 6.4 to 7.4 g/cc, step b) comprises heating to 1200° F. to 1800° F. for 15 to 90 minutes, and step c) comprises the substeps of: 
 e) re-pressing the powder metal composition at 30 to 60 TSI; and    f) high-temperature sintering the powder metal composition at 2100° F. to 2400° F. for 20 to 60 minutes.    
     
     
         7 . The method of  claim 1 , wherein the secondary densification is a mechanical working.  
     
     
         8 . The method of  claim 7 , wherein the mechanical working is selected from the group consisting of: 
 a) extrusion;    b) swaging;    c) roll burnishing;    d) rolling; and    e) shot peening.    
     
     
         9 . The method of  claim 1 , wherein the heat treatment is austempering.  
     
     
         10 . The method of  claim 9 , wherein austempering comprises the substeps of: 
 e) heating the composition to a temperature of 1500° F. to 2000° F. for 20 to 150 minutes;    f) cooling the composition at a rate of 150° F. to 250° F. per minute to a temperature of 450° F. to 800° F.; and    g) holding the composition at a temperature of 450° F. to 800° F. for 20 to 120 minutes.    
     
     
         11 . The method of  claim 10 , wherein substep g) is performed in a molten salt medium or in a gas furnace.  
     
     
         12 . The method of  claim 10 , wherein the temperature in step g) is selected to be slightly above a martensite starting temperature of the composition such that a phase transformation to a bainite microstructure occurs in substep g).  
     
     
         13 . The method of  claim 1 , wherein the heat treatment is marquenching.  
     
     
         14 . The method of  claim 13 , wherein marquenching comprises the substeps of: 
 f) austenitizing the composition at a temperature of 1500° F. to 2000° F. for 20 to 90 minutes;    g) quenching the composition to a uniform temperature slightly above or below a martensite starting temperature of the composition; and    h) cooling the composition at a rate of 40° F./min to 150° F./min to prevent a drastic temperature gradient in the composition.    
     
     
         15 . The method of  claim 14 , wherein substep g) is performed in a hot oil medium or in a gas furnace.  
     
     
         16 . The method of  claim 14  further comprising the step of tempering the composition at a temperature of 300° F. to 1000° F. for 20 to 90 minutes.  
     
     
         17 . The method of  claim 1  further comprising the step of performing at least one secondary operation on the composition after the step of performing a heat treatment.  
     
     
         18 . The method of  claim 17 , wherein the at least one secondary operation is selected from the group consisting of: 
 a) honing;    b) broching;    c) deburring;    d) drilling; and    e) turning.    
     
     
         19 . A powder metal part made by the method of  claim 1 .  
     
     
         20 . The powder metal part of  claim 19  further comprising a bainite microstructure and an overall density of greater than 7.3 g/cc or a density of greater than 7.4 g/cc.