Process for producing a sintered product of copper-infiltrated iron-base alloy and a two-layer valve seat produced by this process
TECHNICAL FIELD 
     The present invention relates to a process for producing a sintered product of copper-infiltrated iron-base alloy by simultaneous sintering and cooper infiltration. The invention also relates to a two-layer valve seat produced by this process. 
     BACKGROUND OF THE INVENTION 
     Valve seats, ball seats, ratchets and racks that require high resistance to corrosion and impact are made of Cu-infiltrated sintered Fe-base alloys which are produced by the following procedure: 
     (1) preparing a powder mix providing a single or double formulation (in order to produce a sintered material having both high wear resistance and good toughness); 
     (2) pressing the mix into a compact of a single phase (if the mix has a single formulation) or a double phase (in upper and two layers if the mix has a double formulation); 
     (3) sintering the compact by holding it in vacuum or a non-oxidizing atmosphere at a temperature in the range of 1050° to 1200° C.; 
     (4) placing the sintered compact on or under a separately prepared compact of an infiltrant primarily consisting of Cu powder or Cu alloy powder; and 
     (5) holding the assembly in vacuum or non-oxidizing atmosphere at a temperature in the range of 1100° to 1200° C. so that the infiltrant enters and closed open pores in the sintered compact. 
     Internal combustion engines such as automotive engines conventionally use a valve seat made of an Fe-base sintered material the porous area of which is infiltrated with Cu or Cu alloy in order to provide increased corrosion resistance and thermal conductivity, or an Fe-base sintered material the porous area of which is infiltrated with both Cu or Cu alloy and Pb or Pb alloy in order to provide good lubricating properties, as well as improved corrosion resistance and thermal conductivity. 
     However, the conventional process for producing a sintered product of Cu-infiltrated iron-base alloys is costly because a compact of infiltrating material must be prepared separately from the compact of iron-base alloy and the step of Cu infiltration must be effected after the sintering step. 
     SUMMARY OF THE INVENTION 
     Therefore, the present inventors have made studies to develop an economical one-step process for producing a sintered product of Cu-infiltrated Fe-base alloys without involving a separate Cu infiltration step. As a result, they have found that their object can be accomplished by the following procedure: 
     (1) preparing two powder mixes, one having a predetermined formulation with a powder of an infiltrating material, preferably in an amount of 10 to 40 wt. %, that contains Cu powder or Cu alloy powder as the primary component, and the other having a predetermined formulation free from a powder of infiltrating material; 
     (2) pressing the two mixes into a compact having said powder of infiltrating material present locally; 
     (3) sintering the compact under ordinary conditions, such as heating in a cracked ammonia gas atmosphere at a temperature in the range of 1000° to 1150° C. 
     As a result, not only is the compact sintered but also the infiltrating material moves by capillary action and enters all pores in the infiltrant-free area of the compact to thereby produce a sintered material which consists of two parts, one being Cu-infiltrated and pore-free and the other having open pores left after the departure of the infiltrant. One advantage of the process of the present invention is that it can produce a sintered product which is infiltrated with Cu or Cu alloy only in the area that is required to have a particular property. 
     A two-layer valve seat using the sintered material made of a Cu-infiltrated Fe-base alloy can be produced by the following procedure: 
     (1) preparing an Fe-base powder mix which after sintering forms the principal part of a valve seat said mix further including (i) 5 to 35 wt. % of Cu, and optionally 0.01 to 4.0 wt. % of either P or Sn or both, or (ii) 5 to 35 wt. % of Cu, 0.5 to 12 wt. % of Pb, and optionally 0.1 to 4.0 wt. % of either P or Sn or both; 
     (2) preparing another Fe-base powder mix which after sintering forms a layer to contact the valve, said mix optionally including 0.01 to 4.0 wt. % of either P or Sn or both; 
     (3) pressing the two powder mixes to form a two-layer compact wherein one layer is composed of the first powder mix and the other layer is composed of the second powder mix. 
     (4) sintering the two-layer compact. 
     As an alternative method, separate compacts may be prepared from the two powder mixes and assembled together to form a two-layer compact. 
     During the sintering stage, the Cu or Cu alloy, preferably together with Pb or Pb alloy, in the Fe-base compact which is to form the principal part of a valve seat migrates (infiltrates) by capillary action into open pores in the Fe-base compact which is to form a layer which will be in contact with the valve, and as a result, the number of open pores in the latter Fe-base compact is reduced to provide a denser Cu- or Cu/Pb-infiltrated layer. 
     For the purpose of the present invention, the Fe-base compact that is to be sentered to form the principal part of a valve seat contains 5 to 35 wt. % of Cu or both 5 to 35 wt. % of Cu and 0.5 to 12 wt. % of Pb. If the Cu content is less than 5 wt. % or the Pb content is less than 0.5 wt. %, the desired infiltration of open pores with Cu or Pb is not accomplished. If the Cu content and Pb content exceed 35 wt. % and 12 wt. %, respectively, the excessive amount of the liquid phase obstructs complete sintering and the resulting valve seat is low in strength and may deform easily. 
     According to the present invention, either the Fe-base compact which is to form the principal part of the valve seat, or the Fe-base compact which is to form a layer which will be in contact with the valve, or both types of Fe-base compact may contain P and/or Sn. These elements are effective in improving the fluidity of Cu or Cu alloy or Pb or Pb alloy in the molten state, with the result that infiltration of open pores with Cu or Pb or both is appreciably promoted. If the Sn or P content is less than 0.01 wt. %, their infiltration promoting effect is not obtained, and if their content exceeds 4.0 wt. %, a strong and tough valve seat is not obtained by sintering. Therefore, for the purpose of the present invention, the P and Sn content is preferably within the range of 0.01 to 4.0 wt. %. 
    
    
     Two examples of the process of the present invention are described below. 
     EXAMPLE 1 
     The following powders were prepared: graphite powder with an average particle size of 10 μm; Cu powder of under 200 mesh; Fe powder, Co powder, Mo powder, Sn powder, Fe-Mo alloy powder (60 wt. % Mo), Fe-Cr alloy (60 wt. % Cr), martensitic stainless steel powder equivalent to SUS 410 (0.13 wt. % C, 0.82 wt. % Si, 0.91 wt. % Mn, 0.03 wt. % P, 0.02 wt. % S, and 13.1 wt. % Cr), Fe-Cr-Mo alloy (1.0 wt. % Cr and 0.3 wt. % Mo) powder, Fe-Ni-Cu-Mo alloy (1.5 wt. % Ni, 0.5 wt. % Cu and 0.5 wt. % Mo) powder, Cu-Sn alloy (10 wt. % Sn) powder and Cu-Fe-Mn alloy (4.1 wt. % Fe and 6.8 wt. % Mn) powder, all having a particle size of under 100 mesh. Using these powders, samples of two-layer disk compacts having the formulations indicated in Table 1 and measuring 11.3 mm in diameter and 10 mm high were produced at pressures between 4 and 6 tone/cm 2 . In each sample, the upper layer was composed of a powder mix having no infiltrant, and the lower layer was composed of a powder mix containing an infiltrant made of Cu powder, Sn powder, Cu-Sn alloy powder and Cu-Fe-Mn alloy powder. The thicknesses of the respective upper and lower layers are indicated in Table 1. The compacts were sintered in a modified propane gas atmosphere at temperatures between 1050° and 1200° C. for 30 minutes, thereby producing Cu-infiltrated Fe-base sintered samples No. 1 to No. 9. 
     As a control, a conventional sample was prepared by the following procedure: a powder mix having the formulation indicated in Table 1 was sintered under the same conditions as described above; the sintered product was overlaid with a compact of infiltrant (Cu powder) measuring 8 mm in diameter and 2.5 mm high; the assembly was subjected to the treatment of Cu infiltration, wherein it was held in a cracked ammonia gas atmosphere at 1120° C. for 10 minutes. 
     The upper and lower layers of samples Nos. 1 to 9, as well as the comparative sample, were checked for their percentage density, and the results are shown in Table 1. 
     
                                           TABLE 1                                 
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                                            Percentage Density            
Sample of                                                                 
      Upper layer of compact                                              
                         Lower layer of compact                           
                                            of sintered material          
sintered            thickness          thickness                          
                                            upper                         
                                                 lower                    
material                                                                  
      composition (wt %)                                                  
                    (mm) composition (wt %)                               
                                       (mm) layer                         
                                                 layer                    
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Sample of the present invention                                           
1     stainless steel: 15,                                                
                    5    C: 1, Cu: 15, Fe: bal.                           
                                       5    95   85                       
      C: 1, Fe: bal.                                                      
2     stainless steel: 15,                                                
                    4    C: 1, Cu: 20, Fe: bal.                           
                                       6    96   83                       
      C: 1, Fe--Mo alloy: 13,                                             
      Fe: bal.                                                            
3     Fe--Ni--Cu--Mo alloy: 50,                                           
                    2    C: 1, stainless steel: 4,                        
                                       8    96   83                       
      C: 1, Ni: 1, Fe: bal.                                               
                         Cu: 10, Fe: bal.                                 
4     stainless steel: 10,                                                
                    8    C: 1, Cu: 40, Fe: bal.                           
                                       2    95   85                       
      C: 1, Co: 4, Mo: 2,                                                 
      Fe--Mo alloy: 5, Fe: bal.                                           
5     Fe--Cr--Mo alloy: 50,                                               
                    6    C: 1, Cu--Sn alloy: 25,                          
                                       4    97   82                       
      stainless steel: 12,                                                
                         Fe: bal.                                         
      C: 1, Fe: bal.                                                      
6     stainless steel: 15,                                                
                    5    C: 1, Cu--Fe--Mn alloy: 25,                      
                                       5    95   84                       
      C: 1, Fe--Cr alloy: 3,                                              
                         Fe: bal.                                         
      Fe: bal.                                                            
7     Fe--Ni--Cu--Mo alloy: 50,                                           
                    6    C: 1, Cu: 30, Sn: 2,                             
                                       4    96   82                       
      Co: 2, C:1, Fe: bal.                                                
                         Fe: bal.                                         
8     C: 0.5, Fe: bal.                                                    
                    5    Cu: 30, Fe: bal.                                 
                                       5    96   84                       
9     C: 0.5, Fe: bal.                                                    
                    5    Cu: 30, Sn: 3, Fe: bal.                          
                                       5    97   82                       
Conven-                                                                   
      stainless steel: 15, C: 1, Fe: bal. (prepared by infiltrating       
                                            96e                           
tional                                                                    
      sintered material with Cu)                                          
sample                                                                    
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     EXAMPLE 2 
     The following powders were prepared: reduced Fe powder of under 100 mesh; carbonyl Ni powder of under 350 mesh; graphite powder with an average grain size of 10 μm; Mo powder of under 350 mesh; Co powder, Cu powder, Fe-Cr alloy (60wt. % Cr) powder, Fe-Mo alloy (60 wt. % Mo) powder, Fe-Nb alloy (60 wt. % Nb) powder, Fe-W alloy (77 wt. % W) powder, Fe-V alloy (80 wt. % V), Fe-S alloy (1 wt. % S) powder, Cu-P alloy (0.1 wt. % P) powder, Cu-P alloy (10 wt. % P) powder, Sn powder, Cu-Sn alloy (0.1 wt. % Sn), and Cu-Sn alloy (10 wt. % Sn) powder, all having a size of under 150 mesh; Cu-Pb alloy (30 wt. % Pb) powder and Pb powder of under 250 mesh; Fe-Si alloy (52 wt. % Si) and Fe-Al alloy (51 wt. % Al) powders of under 100 mesh; and Fe-Mn alloy (60 wt. % Mn) powder of under 200 mesh. Using these powders, samples of a two-layer compact wherein the upper layer would form a layer to contact the valve and the lower layer would form the principal part of a valve seat were produced at a pressure of 6 tons/cm 2 . The formulations and thickness of the respective layers are indicated in Table 2 below. The samples of the compact were sintered in a cracked ammonia gas at a temperature between 1100° and 1180° C. for 30 minutes. The sintered samples were cut to lengths and trimmed to form 23 ring-shaped samples of the two-layer valve seat of the present invention measuring 34.35 mmφ (O.D.), 27.0 mmφ (I.D.) and 7.4 mm high. 
     Conventional valve seat samples (A) and (B) were prepared from the formulations indicated in Table 2 by sintering them under the same conditions as above except that the sintered products were infiltrated with Cu or Cu-Pb alloy (30 wt. % Pb) across the entire thickness. 
     The respective valve seat samples were set in gasoline engines (displacement: 1600 cc) with valves made of JIS SUH-3 and subjected to a wear test by running the engines on leaded gasoline (for samples Nos. 1 to 11) or leadless gasoline (for samples Nos. 12 to 23) at 6000 rpm for 50 hours. After the test, the valve-contacting faces of the samples and the valves were checked for maximum depth of wear. The results are listed in Table 2. The Cu and Pb contents in the valve-contacting layer (1 mm from the top) and the underlying layer (2 mm from the bottom) and the percentage density of each area are also shown in Table 2. 
     
                                           TABLE 2-1                               
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                   Compact for                            Depth           
Compact for making making the principal                   of max.         
valve-contacting layer                                                    
                   part of the value seat                                 
                                  Cu content (wt %)                       
                                              Percentage                  
                                                          wear at         
Valve         layer          valve-                                       
                                  valve-                                  
                                        principal                         
                                              valve-                      
                                                    principal             
                                                          valve-          
seat          thickness      contacting                                   
                                  contacting                              
                                        part of                           
                                              contacting                  
                                                    part                  
                                                          contact         
sample                                                                    
    composition (wt %)                                                    
              (mm) composition (wt %)                                     
                             (mm) layer valve seat                        
                                              layer valve                 
                                                          face            
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Samples of the present invention                                          
1   C: 1%, Cr: 1.5%,                                                      
              2.0  C: 1%, Cr: 1%,                                         
                             5.4  6.0   12.8  97    88    0.04            
    Ni: 0.5%, Mo: 7%,                                                     
                   Cu: 15%, Fe: bal.                                      
2   CO: 1%, S: 0.1%,                                                      
              3.0  C: 1%, Cr: 1%,                                         
                             4.4  4.2   17.1  95    86    0.02            
3   Fe: bal.  3.7  Cu: 20%, Fe: bal.                                      
                             3.7  4.3   15.1  95    87    0.03            
4             5.0  C: 1%, Cr: 1%,                                         
                             2.4  2.9   19.0  94    85    0.04            
                   Cu: 25%, Fe: bal.                                      
5   C: 1%, CR: 1.5%,                                                      
              3.0  C: 1%, Cr: 1%,                                         
                             4.4  5.0   16.6  96    88    0.03            
                   Cu: 20%, P: 0.04%,                                     
                   Fe: bal. -6                                            
                             Ni: 8%, Mo: 7%,                              
                                        C: 1%, Cr: 1%,                    
                                                    5.1   16.5 96 88 0.03 
                                                          6               
    Co: 5%, Nb: 1%,                                                       
                   Cu: 20%, Sn:                                           
    Fe: bal.       0.04%, Fe: bal.                                        
7                  C: 1%, Cr: 1%, 5.2   16.5  96    88    0.03            
                   Cu: 20%, P: 1.2%,                                      
                   Fe: bal.                                               
8   C: 1.3%, Cr: 6.5%,                                                    
              3.7  C: 1%, Cr: 1%,                                         
                             3.7  9.5   15.5  96    82    0.04            
    Ni: 0.5%, W: 2.5%,                                                    
                   Cu: 25%, Fe: bal.                                      
9   Co: 1%, V: 0.5%,                                                      
                   C: 1%, Cr: 1%, 10.2  14.8  97    81    0.04            
    S: 0.1%, Fe: bal.                                                     
                   Cu: 25%, Sn: 1.5%,                                     
                   P: 1%, Fe: bal.                                        
10  C: 1.2%, Cr: 6.5%,                                                    
                   C: 1%, Cr: 1%, 9.7   15.3  97    80    0.03            
    Ni: 0.5%, W: 2.5%,                                                    
                   Cu: 25%, Sn:                                           
    Co: 2%, S: 0.1%                                                       
                   2.3%, Fe: bal.                                         
    Fe: bal.                                                              
11                 C: 1%, Cr: 1%, 9.8   15.2  97    81    0.03            
                   Cu: 25%, P: 2.3%,                                      
                   Fe: bal.                                               
conven-                                                                   
    C: 1%, Cr: 1.5%, Ni: 0.5%, Mo: 7%                                     
                                  6.2         97          0.03            
tional                                                                    
    Co: 1%, S: 0.1%, Fe: bal.                                             
product                                                                   
(A)                                                                       
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                                           TABLE 2-2                               
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Compact for   Compact for                                                 
making valve- making the principal           Percentage                   
                                                       Depth of           
contacting layer                                                          
              part of the valve seat                                      
                         Cu content (wt %)                                
                                   Pb content (wt %)                      
                                             density   max. wear at       
          layer      layer                                                
                         valve-                                           
                              princi-                                     
                                   valve-                                 
                                        princi-                           
                                             valve-                       
                                                  princi-                 
                                                       valve-con-         
Valve     thick-     thick-                                               
                         con- pal part                                    
                                   con- pal part                          
                                             con- pal part                
                                                       tact face          
seat                                                                      
    composition                                                           
          ness                                                            
              composition                                                 
                     ness                                                 
                         tacting                                          
                              of valve                                    
                                   tacting                                
                                        of valve                          
                                             tacting                      
                                                  of valve                
                                                       valve              
sample                                                                    
    (wt %)                                                                
          (mm)                                                            
              (wt %) (mm)                                                 
                         layer                                            
                              seat layer                                  
                                        seat layer                        
                                                  seat seat               
                                                           valve          
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Samples of the present invention                                          
12  C: 1  3.7 C: 1, Cu: 23,                                               
                     3.7 5.9  17.1 0.5  1.5  96   82   0.04               
                                                           0.01           
    Cr: 1.5   Pb:2,                                                       
    Mo: 7     Fe: bal.                                                    
13  Co: 1     C: 1, Cu: 20,                                               
                         5.8  14.2 0.4  4.6  97   82   0.04               
                                                           0.01           
    S: 0.1    Pb: 5,                                                      
    Fe: bal.  Fe: bal.                                                    
14            C: 1, Cu: 15,                                               
                         4.4  10.6 3.0  7.0  97   83   0.03               
                                                           0              
              Pb: 10, Sn: 2,                                              
              Fe: bal.                                                    
15            C: 1, Cu: 20,                                               
                         5.8  14.2 1.4  3.6  97   81   0.03               
                                                           0              
              Pb: 5,                                                      
              Mn: 0.8,                                                    
              Al: 0.2,                                                    
              Fe: bal.                                                    
16            C: 1, Cu: 20,                                               
                         5.7  14.3 1.4  3.6  97   81   0.04               
                                                           0              
              Pb: 5, Cr: 1.5,                                             
              Mo: 0.4,                                                    
              Fe: bal.                                                    
17            C: 1, Cu: 20,                                               
                         5.7  14.3 1.4  3.6  97   81   0.05               
                                                           0              
              Pb: 5, Cr: 1,                                               
              Si: 0.7,                                                    
              Fe: bal.                                                    
18  C: 1.2, Cr:                                                           
          3.7 C: 0.6, Cu: 20,                                             
                     3.7 9.0  11.0 0.5  1.5  96   81   0.04               
                                                           0.01           
    6.5,      Pb: 2,                                                      
    Ni: 0.5,  Fe: bal.                                                    
19  Mo: 2.5, V:                                                           
              C: 1, Cr: 1,                                                
                         10.2 14.8 0.4  0.6  97   81   0.04               
                                                           0.01           
    0.5, Co: 1,                                                           
              Cu: 25, Pb: 1,                                              
    S: 0.1    Sn: 1.5, P: 1,                                              
    Fe: bal.  Fe: bal.                                                    
20  C: 1, 5   C: 1, Cr: 1,                                                
                     2.4 2.4  19.9 0.6  3.7  94   85   0.04               
                                                           0.01           
    Cr: 1.5,  Cu: 25, Pb: 5,                                              
    Ni: 8,    Fe: bal.                                                    
21  Mo: 7,                                                                
          3.0 C: 1, Cr: 1,                                                
                     4.4 4.0  17.4 1.2  4.2  96   88   0.03               
                                                           0              
    Co: 5,    Cu: 20, Pb: 5,                                              
    Nb: 1,    P: 0.04,                                                    
    Fe: bal.  Fe: bal.                                                    
22            C: 1, Cr: 1,                                                
                         4.1  17.4 1.2  4.2  96   88   0.03               
                                                           0              
              Cu: 20, Pb: 5,                                              
              Sn: 0.04,                                                   
              Fe: bal.                                                    
23            C: 1, Cr: 1,                                                
                         4.1  17.4 1.3  4.2  97   88   0.03               
                                                           0              
              Cu: 20, Pb: 5,                                              
              P: 1.2,                                                     
              Fe: bal.                                                    
conven-                                                                   
    C: 1, Cr: 1.5, Ni: 0.5, Mo: 7,                                        
                         4.9  4.9  2.1  2.1  97   97   0.03               
                                                           0              
tional                                                                    
    Co: 1, S: 0.1, Fe: bal.                                               
product                                                                   
(B)                                                                       
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     Table 1 shows that the upper layer of each of samples Nos. 1 to 9 produced by the process of the present invention was completely infiltrated with Cu. 
     Table 2 shows that two-layer valve seat samples 1 to 23 according to the present invention were characterized by a valve-containing layer that was as dense as the comparative samples because of infiltration of open pores with the Cu or Cu alloy or Pb or Pb alloy from the principal part of the valve seat. Therefore, samples 1 to 11 proved to be as wear-resistant as comparative sample (A), and samples 12 to 23 exhibited wear resistance and lubricating properties as good as those of comparative sample (B). 
     As will be apparent from the foregoing description, according to the process of the present invention, a sintered material that is infiltrated with Cu or both Cu and Pb only in the necessary area can be produced without requiring a separate Cu or Pb infiltration step. Because of this one-step process and the need to infiltrate only the minimum necessary portion with Cu, this invention will greatly contribute to reducing the cost of the final product as compared with the conventional product.