Patent Publication Number: US-5298052-A

Title: High temperature bearing alloy and method of producing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high temperature bearing alloy for use in high temperature oxidizing atmosphere and also to a method of producing the same. 
     2. Description of the Related Art 
     Hitherto, materials of a type composed of a base metal such as copper or iron and containing a solid lubricant such as graphite dispersed or embedded therein have been used as materials of bearings which are used in high temperature oxidizing atmosphere These known materials, however, cannot be used satisfactorily at temperatures exceeding 500° C. or so, due to rather inferior heat-resistance exhibited by the base metal and the solid lubricant. In order to cope with demands for higher sliding characteristic, materials have recently been proposed which are produced by dispersing hard particles of Co-Mo-Cr-Si alloy in heat-resistant steels by using powder metallurgical technique Such materials are usable, as disclosed in Japanese Patent Unexamined Publication No. 3-24249, as bearing materials which are required to operate at high temperatures up to 700° C. or so. 
     In recent years, there is a demand for bearing materials used at still higher temperatures exceeding 700° C., such as the bearings for supporting, for example, exhaust gas control valves used in gasoline or diesel engines, gas control valves used in gas turbines, and so forth. Unfortunately, known bearings could not fully meet such demand because they are safely usable only at temperatures not higher than 700° C. 
     Attempts have been made to use heat-resistant alloys or ceramics as the bearing materials, but such attempts also are unsuccessful due to high friction coefficients exhibited by such types of materials. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a high temperature bearing alloy which can be used in oxidizing atmosphere at high temperatures up to 900° C. and which exhibits superior sliding characteristic. 
     Another object of the present invention is to provide a method of producing such a bearing alloy. 
     To these ends, according to one aspect of the present invention, there is provided a high temperature bearing alloy for use in high temperature oxidizing atmosphere, consisting, by weight, of 9-30% Cr, 5-19% Fe, 0.1-1.5% Si, 2-22% Co, 1.4-11.0% Mo, and the balance Ni and incidental impurities, said alloy having a structure in which hard particles of a Co-Mo-Cr-Si alloy are uniformly dispersed. BN not more than 5.0% may be added to the alloy to improve sliding characteristics at high temperature. 
     Powder metallurgical process using Ni-Cr-Fe alloy powder encounters with difficulty due to high hardness of this alloy powder. According to the method of the invention, therefore, powders are prepared separately: namely, pure Ni and Fe-Cr alloy powders. These material powders are mixed with Co-Mo-Cr-Si powder and, if necessary, a BN powder, and the thus obtained mixture powder is compacted and sintered into product. In order to further increase the density of the sintered material, as well as anti-oxidation characteristic of the sintered material at high temperatures, hot pressing or hot isothermal pressing may be effected on the sintered material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, the contents of the respective constituents are limited as follows for the reasons stated below. 
     (1) Cr: 9 to 30 wt % 
     Cr content below 9 wt % cannot provide the matrix alloy with sufficient heat-resistance On the other hand, Cr content not less than 30 wt % impairs the sinterability of the material. The content of Cr, therefore, is determined to be from 9 to 30 wt %. 
     (2) Fe: 5 to 19 wt % 
     Fe content below 5 wt % impairs strength at high temperature, whereas, when Fe content is more than 19 wt %, the hardness of the material powder is increased to reduce powder compactibility. For these reasons, Fe content is determined to be from 5 to 19 wt %. 
     (3) Co-Mo-Cr-Si alloy hard particles. 
     Preferably, the content of these hard particles is from 5 to 35 wt % with respect to the whole alloy material, because a content below 5 wt % does not provide any appreciable effect in improving sliding characteristic, while content exceeding 35 wt % excessively hardens the alloy, resulting in inferior powder compactibility and a risk of damaging of a mating sliding material. The Co-Mo-Cr-Si alloy hard particles preferably contain 26 to 30 wt % Mo, 7.5 to 9.5 wt % Cr, 2.4 to 2.6 wt % Si and the balance substantially Co. At the same time, the contents of Co, Mo and Si in the whole alloy material are preferably determined to be from 0.1 to 1.5 wt %, from 2 to 22 wt % and from 1.4 to 11.0 wt %, respectively, in order to form fine hexagonal closed-packed crystalline particles which contribute to improvement in sliding characteristic. 
     (4) Mn: 2.0 wt % or less 
     This element is added for the purposes of strengthening the matrix alloy and stabilizing the structure. Addition of this element in excess of 2.0 wt %, however, causes an embrittlement of the alloy, so that the content of this element is limited to be 2 0 wt % or less. 
     (5) C: 0.15 wt % or less 
     This element also strengthens the matrix alloy the content of this element, however, is limited to be 0.15 wt % or less, because, when this limit exceeds this value, embrittlement of the alloy is caused due to formation of Cr carbides. 
     (6) BN: 5.0 wt % or less 
     This component is optional and improves sliding characteristic at high temperatures. The content of this component, however, should be limited to 5.0 wt % or less, because the strength of the material is extremely lowered when this content exceeds the upper limit. 
     Preferred Embodiment of the Invention 
     The following powders (a) to (d) were prepared: (a) pure Ni powder of grain sizes -250 mesh (not greater than 60 μm); (b) Fe-Cr alloy powder of grain sizes -250 mesh (not greater than 60 μm) and composed of 44.5 wt % Cr, 17.5 wt % Ni, 4.1 wt % Mo, 1.5 wt % Si, 0.5 wt % Mn, 0.1 wt % C and the balance substantially Fe; (c) Co-Mo-Cr-Si alloy powder of grain sizes -100 meshes (not greater than 150 μm) and composed of 28 wt % Mo, 8.5 wt % Cr, 2.5 wt % Si and the balance substantially Co; and (d) BN powder of a mean particle size of 10 μm. These powders (a) to (d) were mixed at varying ratios as shown in Table 1, whereby a powder of a compositions shown in Table 2 were obtained. 
     In order to improve the formability, zinc stearate was added by 1 wt % to the powder mixture, and the particles of the Co-Mo-Cr-Si alloy powder were fined to -100 meshes (not greater than 150 μm) so as to enable fine and uniform dispersion of this powder in the matrix. The mixture powder was then formed into test pieces of 22 mm diameter and 50 mm length at a forming pressure of 6 ton/cm 2 . The test pieces were then heated at 480° C. for sufficient removal of zinc stearate and were sintered for 1 hour in an (H 2  +N 2 ) atmosphere at 1150° C. Subsequently, the test pieces were hot-pressed at 6 ton/cm 2  at 900° C. for attaining higher density. The rod-shaped test pieces of 22 mm diameter and 50 mm length thus obtained were processed and subjected to tensile tests conducted at the room temperature, 500° C. and 900° C., respectively. The results of the tensile tests are shown in Table 3. In order to examine the anti-oxidation characteristic at high temperature, the rod-shaped test pieces were processed into the form of bearings having an outside diameter of 16 mm, inside diameter of 10 mm and the length of 20 mm. The thus obtained bearings were exposed in an oxidizing atmosphere of 900° C. and increments of weight were measured at 5 hours, 20 hours, 50 hours and 100 hours after the commencement of the exposing. The results of the measurements are shown in Table 4. In order to examine the sliding characteristics, flat plate-shaped test pieces were produced from the rod-shaped test pieces. The plate-shaped test pieces were then subjected to a friction/wear test conducted at high temperatures, using an Ni-Cr type heat-resistant alloy pin as the sliding mating member. More specifically, the test was conducted by reciprocately sliding the pin of 4 mm diameter under a load of 2 kgf at a mean sliding velocity of 1 m/min for 1 hour. The friction coefficients and amounts of wear of the plates are shown in Table 5. 
     As will be understood from the foregoing description, the bearing alloy in accordance with the present invention employs a matrix formed of Ni base alloy which is superior in resistance to heat, whereby oxidation weight increment is reduced when this bearing alloy is used in high temperature oxidizing atmospheres. Furthermore, the results of the sliding test shows that the bearing alloy of the invention also excels in friction/wear resistance even at high temperature around 900° C., by virtue of addition of a suitable amount of Co-Mo-Cr-Si alloy and/or BN. 
     Thus, the bearing alloy in accordance with the present invention exhibits superior performance when used in high temperature oxidizing atmosphere. 
     
                       TABLE 1                                                     
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          CONTENTS (wt %)                                                 
            Pure                                                          
            Ni                          BN                                
            pow-   Fe--Cr   Co--Mo--Cr--Si                                
                                        pow-                              
       No.  der    powder   powder      der                               
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Alloys of the                                                             
         1      50     30     20          --                              
Invention                                                                 
         2      47     30     20          3                               
         3      60     35      5          --                              
         4      40     25     35          --                              
         5      57     35      5          3                               
         6      37     25     35          3                               
         7      45     30     20          5                               
         8      63     17     20          --                              
         9      22     58     20          --                              
Comparative                                                               
         10     65     15     20          --                              
materials                                                                 
         11     15     65     20          --                              
         12     62     35      3          --                              
         13     38     25     37          --                              
         14     44     30     20          6                               
         15     Composition No. 1 without hot pressing                    
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                                           TABLE 2                                 
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          CONTENTS (wt %)                                                 
       No.                                                                
          Ni Fe Cr Co Mo  Si                                              
                            Mn  C  BN                                     
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Alloys of                                                                 
       1  Bal.                                                            
             9.5                                                          
                15.1                                                      
                   12.2                                                   
                      6.8 1.0                                             
                            0.2 0.03                                      
                                   --                                     
the    2  Bal.                                                            
             9.5                                                          
                15.1                                                      
                   12.2                                                   
                      6.8 1.0                                             
                            0.2 0.03                                      
                                   3                                      
Invention                                                                 
       3  Bal.                                                            
             11.1                                                         
                16.0                                                      
                    3.1                                                   
                      2.8 0.7                                             
                            0.2 0.04                                      
                                   --                                     
       4  Bal.                                                            
             8.0                                                          
                14.1                                                      
                   21.4                                                   
                      10.8                                                
                          1.3                                             
                            0.1 0.03                                      
                                   --                                     
       5  Bal.                                                            
             11.1                                                         
                16.0                                                      
                    3.1                                                   
                      2.8 0.7                                             
                            0.2 0.04                                      
                                   3                                      
       6  Bal.                                                            
             8.0                                                          
                14.1                                                      
                   21.4                                                   
                      10.8                                                
                          1.3                                             
                            0.1 0.03                                      
                                   3                                      
       7  Bal.                                                            
             9.5                                                          
                15.1                                                      
                   12.2                                                   
                      6.8 1.0                                             
                            0.2 0.03                                      
                                   5                                      
       8  Bal.                                                            
             5.4                                                          
                 9.3                                                      
                   12.2                                                   
                      6.3 0.8                                             
                            0.1 0.02                                      
                                   --                                     
       9  Bal.                                                            
             18.4                                                         
                27.5                                                      
                   12.2                                                   
                      8.0 1.4                                             
                            0.3 0.06                                      
                                   --                                     
Comparative                                                               
       10 Bal.                                                            
             4.8                                                          
                 8.4                                                      
                   12.2                                                   
                      6.2 0.7                                             
                            0.1 0.02                                      
                                   --                                     
materials                                                                 
       11 Bal.                                                            
             20.7                                                         
                30.6                                                      
                   12.2                                                   
                      8.3 1.5                                             
                            0.3 0.07                                      
                                   --                                     
       12 Bal.                                                            
             11.1                                                         
                15.8                                                      
                    1.8                                                   
                      2.3 0.6                                             
                            0.2 0.04                                      
                                   --                                     
       13 Bal.                                                            
             8.0                                                          
                14.3                                                      
                   22.6                                                   
                      11.4                                                
                          1.3                                             
                            0.1 0.03                                      
                                   --                                     
       14 Bal.                                                            
             9.5                                                          
                15.1                                                      
                   12.2                                                   
                      6.8 1.0                                             
                            0.2 0.03                                      
                                   6                                      
       15 Bal.                                                            
             9.5                                                          
                15.1                                                      
                   12.2                                                   
                      6.8 1.0                                             
                            0.2 0.03                                      
                                   --                                     
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                       TABLE 3                                                     
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Tensile Strength MPa                                                      
             Measuring temp.                                              
               Room                                                       
          No.  temp.      500° C.                                  
                                  900° C.                          
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Alloys of the                                                             
            1      296        221   56                                    
Invention   2      211        162   53                                    
            3      276        203   51                                    
            4      314        235   56                                    
            5      202        153   50                                    
            6      267        180   56                                    
            7      198        148   53                                    
            8      327        240   62                                    
            9      254        179   48                                    
Comparative 10     321        241   59                                    
materials   11     260        153   41                                    
            12     271        190   49                                    
            13     310        233   53                                    
            14     179        130   44                                    
            15     216        187   49                                    
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                       TABLE 4                                                     
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Oxidation weight increment ratios                                         
           Exposure time (at 900° C.)                              
       No.   10 Hr    20 Hr    50 Hr  100 Hr                              
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Alloys of the                                                             
         1       0.4      0.4    0.5    0.5                               
Invention                                                                 
         2       0.4      0.5    0.5    0.5                               
         3       0.4      0.5    0.4    0.4                               
         4       0.5      0.6    0.6    0.6                               
         5       0.6      0.6    0.7    0.8                               
         6       0.6      0.7    0.9    1.0                               
         7       0.6      0.7    1.0    1.1                               
         8       0.5      0.5    0.6    0.5                               
         9       1.1      1.2    1.3    1.3                               
Comparative                                                               
         10      0.6      0.6    0.7    0.7                               
materials                                                                 
         11      1.9      2.3    2.8    3.3                               
         12      0.4      0.5    0.5    0.5                               
         13      0.5      0.6    0.6    0.6                               
         14      0.7      0.9    0.9    1.2                               
         15      1.8      2.8    3.6    3.7                               
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 {(W.sub.A - W.sub.B)/W.sub.B } × 100                               
 W.sub.A : Weight after exposure                                          
 W.sub.B : Weight before exposure                                         
 
    
     
                       TABLE 5                                                     
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Sliding test                                                              
           Friction    Amount of wear of                                  
           coefficient plate (mm.sup.3)                                   
       No.   500° C.                                               
                      900° C.                                      
                               500° C.                             
                                      900° C.                      
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Alloys of the                                                             
         1       0.63     0.41   0.55   0.32                              
Invention                                                                 
         2       0.78     0.62   0.68   0.55                              
         3       0.81     1.13   0.77   1.02                              
         4       0.51     0.36   0.38   0.29                              
         5       0.88     1.20   0.86   1.21                              
         6       0.58     0.59   0.42   0.36                              
Comparative                                                               
         11      0.62     1.26   0.60   1.30                              
materials                                                                 
         12      1.01     1.58   0.98   1.53                              
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