Patent Publication Number: US-2005123432-A1

Title: Method of producing powder metal parts

Description:
REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of parent application Ser. No. 10/697,344 filed Oct. 30, 2003 entitled, “METHOD OF PRODUCING POWDER METAL PARTS” which claims the benefit of provisional application No. 60/432,823, filed Dec. 12, 2002, entitled “METHOD OF PRODUCING POWDER METAL PARTS”. The aforementioned applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention pertains to a method to produce a material from a metallurgical powder. More particularly, the invention pertains to a method of producing a material from a metallurgical powder comprising iron and carbon.  
      2. Description of Related Art  
      Sinter hardening is a process used to produce a high martensite content material without using a conventional heat treatment process, such as batch heat treating or induction hardening. The sinter-hardening process comprises the steps of sintering the compacting at an elevated temperature, rapidly cooling the compact at the end of the sinter furnace to induce martensite transformation.  
      Another process that is commonly used in the field to produce powder metal parts is double press double sinter (DPDS). In this process, a mixture of powder is compacted, pre-sintered, sized, put through high temperature sintering, and then heat treated. One problem associated with this process is that it is time consuming and high in cost.  
      Kosco&#39;s U.S. Pat. No. 6,338,747 provides another series of steps for forming power metal parts. In the first step of Kosco, metal powders are mixed. The powders are compressed at a pressure of 20 to 70 tsi to form a green compact in a second step. The green compact is then heated between 2000° F.-2400° F. for 25-30 minutes and then holding the compact at the sintering temperature for a sufficient time to ensure that the individual alloy elements diffuse throughout the compact, forming a generally homogeneous iron based alloy in a third step. The sintered compact is then cooled in a fourth step at a rate of no greater than 60° F./minute, bringing the compact to room temperature. The compact is then deformed to increase the density of at least a portion of the compact to at least 7.6 g/cc in a fifth step. In the sixth step, the deformed compact is then heated again between a range of 2050° F.-2400° F. for 25-30 minutes. The compact is then cooled at a rate of 120° F.-400° F./minute to increase the surface hardness to greater than RC 25 in step 7. In step 8, the compact is tempered between 300′-1350° F. for 30-120 minutes.  
      In Example 2 of Kosco&#39;s U.S. Pat. No. 6,338,747 a powder composition is compressed into a green compact, sintered at 2050° F.-2080° F. for 30 minutes, cooled to room temperature and then dip coated in a graphite slurry. After the compact has been coated, the compact is induction heated for three minutes and then transferred to a compact held at 600° F. and slowly cooled in the die to room temperature. The compact is induction heated to allow for hot forming. Hot forming is applying a mechanical deformation to a part that has been heated to a high temperature. The heating of the part is provided by the induction heating and the mechanical deformation is achieved by placing the compact in the die also held at a warm temperature.  
     SUMMARY OF THE INVENTION  
      A method of producing parts from powdered metal comprising the steps of providing a metallurgic powder, compressing the powder at a pressure of 25 to 65 tsi to provide a green compact with a density if 6.4 g/cc to 7.4 g/cc. The compact is then sintered at a high temperature of 2100° F. to 2400° F. or at regular sinter temperatures of 1650° F. to 2100° F., held between 1000° F. to 1900° F. for 5 to 60 minutes so that the microstructure of the compact becomes mainly Pearlite, and then cooled to room temperature. Then, the compact is selectively densified to greater than 7.6 g/cc. The compact is sinter hardened to obtain a mainly Martensite microstructure. The compact can be directly sinter hardened if selective densification is not necessary. Material made by this method is also disclosed.  
      If high surface durability, high rolling contact fatigue, and high precision are not needed, and a medium or high performance application is not needed, the parts are sinter hardened and then tempered. If a medium or high performance application is needed and the furnace does not have a fast cooling capacity the compact is high temperature sintered, then sinter hardened, and tempered. If a medium or high performance application is needed and the furnace has fast cooling capacity, the compact is high temperature sinter hardened in one step and then tempered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  shows is a block diagram showing the steps of the present invention to produce powder metal parts.  
       FIGS. 2   a  &amp;  2   b  shows prior art temperature diagrams for annealing.  
       FIGS. 2   c  &amp;  2   d  show temperature diagrams for the method of the present invention for annealing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention provides a method for producing powder metal parts with higher mechanical properties while having a minimum amount of heat treated distortion occur. The method is quicker and involves fewer steps than regular sinter and heat treatment as done in the prior art.  
       FIG. 1  is a block diagram that shows the method of producing powder metal parts. In the first step the individual powders of a composition (A), (B), (C), or (D) as shown in Table 1 of metallurgical powder are mixed together. The metallurgical powder composition (A) consists of: 0.4-0.9 weight percent carbon, preferably 0.65 weight percent, 0.5-04.5 weight percent nickel, preferably 4.4 weight percent, 0.5-1.0 weight percent molybdenum, preferably 0.85 weight percent, 0.1-0.5 weight percent manganese, preferably 0.12 weight percent, 0.1-1.5 weight percent silicon, preferably 0.7 weight percent, and a balance of iron.  
      The metallurgical powder composition (B) consists of: 0.4-0.9 weight percent carbon, preferably 0.65 weight percent, 0.1-2.0 weight percent nickel, preferably 1.4 weight percent, 0.1-1.5 weight percent molybdenum, preferably 1.25 weight percent, 0.1-0.5 weight percent manganese, preferably 0.42 weight percent, 0.1-1.5 weight percent copper, preferably 1.0 weight percent, and a balance of iron.  
      The metallurgical powder composition (C) consists of: 0.4-0.9 weight percent carbon, preferably 0.60 weight percent, 0.5-1.0 weight percent molybdenum, preferably 0.5 weight percent, 0.1-4.0 weight percent chromium, preferably 3.0 weight percent, and a balance of iron.  
      The metallurgic powder composition (D) consists of: 0.4-0.9 weight percent carbon, preferably 0.60 weight percent, 0.5-1.0 weight percent molybdenum, preferably 0.50 weight percent, 0.1-4.0 weight percent chromium, preferably 3.0 weight percent, 0.5-04.5 weight percent nickel, 0-0.5 weight percent manganese, 0-1.5 weight percent silicon, and 0-2.0 weight percent copper.  
                                               TABLE 1                       Powder                                       Compositions   Fe   C   Ni   Mo   Mn   Si   Cu   Cr                  (A)   Bal.   0.4-0.9   0.5-4.5   0.5-1.0   0.1-0.5   0.1-1.5   —   —               prefer   prefer   prefer   prefer   prefer               0.65   4.4   0.85   0.12   0.70       (B)   Bal.   0.4-0.9   0.1-2.0   0.1-1.5   0.1-0.5   —   0.1-1.5   —               prefer   prefer   prefer   prefer       prefer               0.65   1.4   1.25   0.42       1.0       (C)   Bal.   0.4-0.9   —   0.5-1.0   —   —   —   0.1-4.0               prefer       prefer               prefer               0.60       0.50               3.0       (D)   Bal.   0.4-0.9   0.5-4.5   0.5-1.0     0-0.5     0-1.5     0-2.0   0.1-4.0               prefer       prefer               prefer               0.60       0.50               3.0                  
 
      The addition of nickel to the metal powder compositions (A), (B), (C), and (D) provides solid solution strengthening and increased hardenability. The addition of silicon to the metal powder compositions (A) and (D) forms the liquid phase during high temperature sintering, increases sintering kinetics, and adds some strengthening effects. The addition of copper to the metal powder compositions (B) and (D) increases the hardenability. Copper also acts as a brazing agent and as an age-hardening agent, which causes considerable increases in strength and hardness. The addition of molybdenum, manganese, and chromium increases the material&#39;s strength and hardenability.  
      The second step of the method is to compact or compress the mixture of powders. The powders are compacted with a compaction pressure in the range of 25 to 65 tsi, preferably, 40 to 45 tsi, resulting in a green compact with a green density of 6.4 to 7.4 g/cc.  
      The third step varies based on whether the part being made will require high surface durability, high rolling contact fatigue, and/or high precision in profile or not. If high surface durability, high rolling contact fatigue, and/or high precision in profile are required, the compact is sintered in the third step and annealed in the fourth step, followed by surface densification. The term sintered throughout this application refers to high temperature sintering or regular sintering and is dependent on the composition of the metallurgic powder being used.  
      If high surface durability, high rolling contact fatigue, and/or high precision profile is not required and the parts produced will not be used for medium to high performance applications, the compact is sintered hardened and then tempered. If high surface durability, high rolling contact fatigue, and/or high precision profile is not required and the parts produced will be used for medium to high performance applications, the compact is either high temperature sintered, sinter hardened, and then tempered or high temperature sinter hardened in one step and then tempered, depending on whether the furnace has fast cooling capabilities.  
      If high temperature sintering is carried out, then compositions (A), (B), (C), or (D) may be used. If regular sintering is carried out, then compositions (B), (C), or (D) may be used.  
      In a preferred embodiment, the above qualities are necessary and the green compact is either regularly sintered or high temperature sintered. For high temperature sintering, the green compact is heated between 2100° F. to 2400° F., preferably 2300° F. The compact is held at the high temperature between 20 and 60 minutes, preferably 30 minutes. Holding the compact at the sintering temperature for a sufficient time period is important to ensure that the individual alloy elements diffuse through out the compact. For regular sintering, the green compact is heated between 1650°-2100°, preferably 2070° F. for 20 to 60 minutes, preferably 30 minutes.  
      Annealing is a heat treatment process where the microstructure is modified to lower material hardness and improve formability. Annealing is used to soften metals but does not include any mechanical deformation. Annealing consists of heating to and holding at a suitable temperature, followed by cooling at a suitable rate. In Powder Metallurgy, annealing of the compact usually takes place during the cooling step of the high temperature sintering or regular sintering. In the prior art, for example, in Kosco&#39;s U.S. Pat. No. 6,338,747, a method of producing powder metal parts uses a slow cooling step as shown in prior art  FIG. 2   a . The slow cooling step is difficult to control. Prior Art  FIG. 2   b  shows the annealing step being present after the part has cooled down to room temperature. While this technique is acceptable, it takes a significant amount of time and energy since the part needs to cool down to room temperature and then heat up again to get to the annealing temperature.  
      In a first preferred embodiment, as shown in  FIG. 2   c , annealing of the compact takes place during the cooling of the furnace directly after sintering. The compact is not allowed to cool down to room temperature, instead the furnace is allowed to cool down on its own, until the furnace temperature is about 50° F. below the critical temperature of steel, which ranges from 1000° F. to 1900° F., and is preferably 1450° F. The critical temperature will vary slightly depending on the chemical composition of powders used. The compact is then held for 5 to 60 minutes, preferably 15 minutes at 1450° F. for annealing to occur, improving the formability of the powder metal parts for densification. The cooling rate is not the primary factor in controlling the resultant hardness of the compact after transformation is completed during annealing. After the compact has undergone annealing, the compact is cooled to room temperature prior to densification. After annealing, the microstructure may be either mainly spheroidized Pearlite or mainly Pearlite.  
      In an alternative embodiment, as shown in  FIG. 2   d  annealing occurs at the critical temperature. As in the previous embodiment, the furnace cools to the critical temperature at the cooling rate of the furnace. The compact is held between 1050° F. to 1900° F., preferably 1510° F. for 5 to 60 minutes, preferably 15 minutes. Again, the cooling rate is not the primary factor in controlling the resultant hardness of the compact after phase transformation is completed. The compact is then cooled after transformation. After annealing, the microstructure may be either mainly spheroidized Pearlite or mainly Pearlite. Annealing improves the formability of the powder metal parts for subsequent densification. After the compact has undergone annealing, the compact is cooled to room temperature prior to densification.  
      The fourth step of densification. Densification may follow the annealing step of either embodiment and utilizes mechanical working or some other deformation technique to increase the density of all or a desired portion or region of the sintered compact to greater than 7.6 g/cc. Examples of mechanical working include sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging and hot forming. Other techniques know to one skilled in the art may also be used.  
      The fifth step is sinter hardening. The compact is held at 1650° F. to 2100° F. for 20 to 80 minutes and then cooled at a rate between 150 to 250° F./min.  
      The sixth step is to temper the sintered compact at 300° F.-1000° F. for 30 to 90 minutes. The final microstructure of the part made from any of the compositions, (A), (B), (C), or (D) is mainly tempered Martensite.  
      If there is no high surface durability and high rolling contact fatigue requirements, then annealing and densification is not necessary. If medium to high performance applications are not required, the compact is sinter hardened. For sinter hardening, the compact is held at 1650° F. to 2100° F. for 20 to 80 minutes and then cooled at a rate between 150 to 250° F./min.  
      If medium to high performance applications are necessary and the furnace being used does not have fast cooling capabilities, the compact is high temperature sintered, sinter hardened, and then tempered. High temperature sintering is comprised of the step of heating the compact to 2100° F. to 2400° F. for 20 to 60 minutes. For sinter hardening, the compact is heated again to 1650° F. to 2100° F. for 20 to 80 minutes and then cooled at a rate of 150° to 250° F. per minute to room temperature. The compact is then tempered by the temperature and time disclosed above.  
      If medium to high performance applications are necessary and the furnace being used has fast cooling capabilities, then the compact is high temperature sinter hardened and then tempered. High temperature sinter hardening is comprised of the steps of heating the compact to 2100° F. to 2400° F. for 20 to 60 minutes and then cooling at a rate of 150° F. to 250° F. per minute to room temperature. The compact is then tempered by the temperature and time disclosed above.  
      The final microstructure is mainly tempered martensite, 0-20% bainite, and less than 5% retained austenite and the metal parts produced have a hardness of 27 to 65 HRC.  
     EXAMPLE 1  
      A powder including 0.60 wt % carbon, 0.7 wt % silicon, 0.13 wt % manganese, 4.4 wt % nickel, 0.85 wt % molybdenum, and the balance being iron was combined by blending, see Table 2. A green compact was formed by molding the powder between 25 to 65 tsi. The green density of the compact was 6.95 g/cc. The green compact was then sintered at 2300° F. for 40 minutes. In the next step the compact is sinter furnace hardened at 1850° F. with fast cooling for 25 minutes. Lastly, the compact was tempered at 400° F. for 60 minutes. The end product, a 25-teeth sprocket displayed an apparent hardness of 37 to 39 HRC and an overall tooth density of 7.07 g/cm 3 . The tooth rupture of the sprocket were tested to see how much load may be applied before the teeth fail or rupture. In this example the test was conducted using three 0.200″ diameter pins. The result was 7300 lbf to 8300 lbf in comparison to the same part by MPIF FN-0208 powder being made by the double pressed double sintered method and heat treated with induction hardening. The powder that was double press double sintered resulted in 5000 lbf to 6500 lbf being applied before tooth rupture occurred. By using the metallurgical powder and the method of the present invention, a higher tooth rupture strength was achieved even with a lower tooth density. The results are summarized in Table 3.  
                                               TABLE 2                       Powder                                       Compo-       sitions   Fe   C   Ni   Mo   Mn   Si   Cu   Cr                  (A)   Bal.   0.6   4.4   0.85   0.13   0.7   —   0.03       MPIF FN-   Bal.   0.6-0.9   1.0-3.0   —   —   —   0-2.5   —       0208                  
 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
               
               
                   
                 New Method With 
                 DPDS (Double Press Double 
               
               
                 Characteristic 
                 Powder Composition (A) 
                 Sinter) With MPIF FN-0208 
               
               
                   
               
             
            
               
                 Tooth Density 
                 7.07 g/cm 3   
                 7.4 g/cm 3   
               
               
                 Tooth Rupture 
                 7300-8300 lbf 
                 5000-6500 lbf 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2  
      A powder including 0.55 wt % carbon, 0.7 wt % silicon, 0.13 wt % manganese, 4.4 wt % nickel, 0.85 wt % molybdenum, and the balance being iron was combined by blending as shown in Table 4. A green compact was formed by molding the powder between 25 and 65 tsi. The green density of the green compact was 6.95 g/cc. The green compact was then sintered at 2300° F. for 40 minutes. In the next step the compact is sinter furnace hardened at 1850° F. with fast cooling for 25 minutes. Lastly, the compact was tempered at 400° F. for 60 minutes. The end product, a 17-teeth sprocket displayed an apparent hardness of 38.5 HRC and an overall tooth density of 7.05 g/cm 3 . The tooth rupture of the sprocket were tested to see how much load may be applied before the teeth fail or rupture. In this example the test was conducted using three 0.187″ diameter pins. The result was 3350 lbf to 4350 lbf in comparison to the same part by MPIF FN-0208 powder being made by the double pressed double sintered method and heat treated with induction hardening. The powder that was double press double sintered resulted in 2470 lbf to 3660 lbf being applied before tooth rupture occurred. By using the metallurgical powder and the method of the present invention, a higher tooth rupture strength was achieved even with a lower tooth density. The results are summarized in Table 5.  
                                               TABLE 4                       Powder                                       Compositions   Fe   C   Ni   Mo   Mn   Si   Cu   Cr                  (A)   Bal.   0.55   4.4   0.85   0.13   0.7   —   —       MPIF FN-   Bal.   0.6-0.9   1.0-3.0   —   —   —   0-2.5   —       0208                  
 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
               
               
                   
                 New Method With 
                 DPDS (Double Press Double 
               
               
                 Characteristic 
                 Powder Composition (A) 
                 Sinter) With MPIF FN-0208 
               
               
                   
               
             
            
               
                 Tooth Density 
                 7.05 g/cm 3   
                 7.36 g/cm 3   
               
               
                 Tooth Rupture 
                 3350-4350 lbf 
                 2470-3660 lbf 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 3  
      A powder including 0.60 wt % carbon, 0.7 wt % silicon, 0.13 wt % manganese, 4.4 wt % nickel, 0.85 wt % molybdenum, and the balance being iron, was combined by blending as shown in Table 6. A green compact was formed by molding the powder between 25 and 65 tsi. The green density of the green compact was 6.95 g/cc. The green compact was then sintered at 2300° F. for 60 minutes. In the next step the compact is sinter furnace hardened at 1850° F. with fast cooling for 25 minutes. Lastly, the compact was tempered at 400° F. for 60 minutes. The end product, a 26-teeth sprocket displayed an apparent hardness of 40 HRC and an overall tooth density of 7.06 g/cm 3 . The tooth rupture of the sprocket were tested to see how much load may be applied before the teeth fail or rupture. In this example the test was conducted using three 0.187″ diameter pins. The result was 4740 lbf in comparison to the same part by MPIF FN-0208 powder being made by the double pressed double sintered method and heat treated with induction hardening. The powder that was double press double sintered resulted in 810 lbf being applied before tooth rupture occurred. By using the metallurgical powder and the method of the present invention, a higher tooth rupture strength was achieved even with a lower tooth density. The results are summarized in Table 7.  
                                               TABLE 6                       Powder                                       Compositions   Fe   C   Ni   Mo   Mn   Si   Cu   Cr                  (A)   Bal.   0.6   4.4   0.85   0.13   0.7   —   —       MPIF FN-   Bal.   0.6-0.9   1.0-3.0   —   —   —   0-2.5   —       0208                  
 
     
       
         
           
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
               
               
                   
                 New Method With 
                 DPDS (Double Press Double 
               
               
                 Characteristic 
                 Powder Composition (A) 
                 Sinter) With MPIF FN-0208 
               
               
                   
               
             
            
               
                 Tooth Density 
                 7.05 g/cm 3   
                 7.36 g/cm 3   
               
               
                 Tooth Rupture 
                 4740 lbf 
                 810 lbf 
               
               
                   
               
            
           
         
       
     
      In producing metal parts using the methods described above, the tooth density of the metal parts produced may vary with compaction pressure and powder compressibility, making the range of tooth density produced using the methods of the present invention between 6.75 g/cc to 7.25 g/cc.  
     EXAMPLE 4  
      A powder including 0.65 wt % carbon, 1.4 wt % nickel, 1.25 wt % molybdenum, 0.42 wt % manganese, 1.0 wt % copper, and the balance being iron was combined by blending. The blending powder was compacted into pucks by 45 tsi with a green density of 6.95 g/cc. The pucks were then sintered at 2070° F. for 30 minutes. The sintered pucks were then annealed at 1510° F. for 15 minutes and cooled to room temperature. After the pucks were cooled, they were machined into rings and rolled in a roller machine. After surface densification, the rings were sinter hardened at 2070° F. for 30 minutes and cooled at a rate of 150° F. per minute. Next, the rings were tempered at 400° F. for 60 minutes.  
      The hardness of the compact after it is annealed is HRB 73.4 and its hardness after surface densification is HRB 93.9, which is a 28% increase. The surface density is increased to 7.7 g/cc and the densification depth to 7.6 g/cc is 0.8 mm. The surface hardness after sinter hardening ranges between HRC 52 to HRC 58 depending upon the degree of densification.  
     EXAMPLE 5  
      A powder including 0.5 wt % carbon, 0.5 wt % molybdenum, 3 wt % chromium and the balance being iron was combined by blending. The blending powder was compacted into pucks by 45 tsi with a green density of 6.95 g/cc. The pucks were then sintered at 2070° F. for 30 minutes. The sintered pucks were then annealed at 1510° F. for 15 minutes and cooled to room temperature. After the pucks were cooled, they were machined into rings and rolled in a roller machine. After surface densification, the rings were sinter hardened at 2070° F. for 30 minutes and cooled at a rate of 150° F. per minute. Next, the rings were tempered at 400° F. for 60 minutes.  
      The hardness of the compact after it is annealed is HRB 74.5 and its hardness after surface densification or rolling is HRB 92.1, which is a 28% increase. The surface density is increased to 7.8 g/cc and the densification depth to 7.6 g/cc is 1.4 mm.  
     EXAMPLE 6  
      A powder including 0.6 wt % carbon, 0.5 wt % molybdenum, 3 wt % chromium and the balance being iron was combined by blending. The blending powder was compacted into pucks by 45 tsi with a green density of 6.95 g/cc. The pucks were then sintered at 2070° F. for 30 minutes. The sintered pucks were then annealed at 1510° F. for 15 minutes and cooled to room temperature. After the pucks were cooled, they were machined into rings and rolled in a roller machine. After surface densification, the rings were sinter hardened at 2070° F. for 30 minutes and cooled at a rate of 150° F. per minute. Next, the rings were tempered at 400° F. for 60 minutes.  
      The hardness of the compact after it is annealed is HRB 76.5 and its hardness after surface densification or rolling is HRB 94.9, which is a 24% increase. The surface density is increased to 7.78 g/cc and the densification depth to 7.6 g/cc is 1.2 mm. The surface hardness after sinter hardening ranges between HRC 59 to HRC 60 depending upon the degree of densification.  
      Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.