High performance bearing steels
This invention relates to high performance bearing steel and manufacture of same and particularly to an improved corrosion resistant, wear and elevated temperature resistant bearing steel with superior rolling contact fatigue life. 
     There has long been a demand in the aerospace industry and in other industries as well where corrosion resistance, wear and elevated temperatures are a problem for a bearing steel having superior rolling contact fatigue life. 
     There have been, in the past, several steels suggested and used in this demanding area. Unfortunately, however, each of the suggested steels has been lacking in one or more of the properties desired for optimum performance in critical applications. These desired properties are: high hardness, good hot hardness, retention of hardness after exposure to elevated temperatures, good corrosion and oxidation resistance, good wear resistance and, perhaps most difficult to obtain, superior rolling contact fatigue life. 
     The prior art steel which probably most nearly embodied all of these properties is that disclosed in U.S. Pat. No. 3,167,423. However, it clearly lacked ample rolling contact fatigue properties. 
     We have discovered an alloy which combines all of the properties required for aircraft bearings and the like operating at elevated temperatures and/or in corrosive environments, particularly superior fatigue life. Advanced manufacturing techniques in melting and working the alloy are employed to further promote and improve the required product properties. Processing characteristics are also satisfactory, so the alloy may be readily formed, machined and heat treated to the final high hardness. 
     The composition range of this alloy includes the following elements: 
     
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C            about      0.8 - 1.6%                                        
Si           about      0.50%       Max.                                  
Mn           about      0.50%       Max.                                  
S            about      0.10%       Max.                                  
P            about      0.015%      Max.                                  
Cr           about      12 - 20%                                          
Mo           about      2 - 5%                                            
W            about      Up to 3.0%                                        
V            about      0.5% - 3.0%                                       
Ti           about      Up to 0.5%                                        
Al           about      0.03%       Max.                                  
Ni           about      0.50%       Max.                                  
Co           about      0.50%       Max.                                  
Cu           about      0.50%       Max.                                  
B            about      0.05%       Max.                                  
N            about      0.05%       Max.                                  
Fe + incidental                                                           
             Balance                                                      
  impurities                                                              
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     In practice the composition of the alloy is preferably maintained within the following considerably narrower range: 
     
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C                 about    1.0%-1.2%                                      
Si                about    0.2-0.4%                                       
Mn                about    0.3-0.5%                                       
S                 about    0.010% Max.                                    
P                 about    0.015% Max.                                    
Cr                about    14%-16%                                        
Mo                about    1.5%-2.5%                                      
W                 about    2.3%-2.7%                                      
V                 about    1%-1.5%                                        
Ti                about    Up to 0.5%                                     
Al                about    0.03% Max.                                     
Ni                about    0.50% Max.                                     
Co                about    0.50% Max.                                     
Cu                about    0.50% Max.                                     
B                 about    0.05% Max.                                     
N                 about    0.05% Max.                                     
Fe + incidental   Balance                                                 
   impurities                                                             
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     In order to obtain the unique characteristics of this invention, it is necessary to maintain the level of oxygen, hydrogen, aluminum and silicon as low as possible in order to provide superior microcleanliness and to avoid formation of delta ferrite, the presence of which would significantly reduce the attainable heat treated hardness. It is also essential to maintain a low level of manganese, nickel, cobalt, copper and nitrogen in order to reduce the presence of retained austenite, the presence of which in objectionable quantities (&gt;5%) is likely to result in dimensional instability that cannot be tolerated in critical applications. 
     We have also discovered that tungsten is beneficial and can be substituted for molybdenum at a ratio of about 1:1.5. The presence of tungsten enhances the rolling contact fatigue properties, refines the grain size, even at increased hardening temperatures, provides a desirable balance with regard to the various carbides formed in the alloy system and finally aids in reducing the retained austenite level. 
     We have also discovered that the method of production of this alloy is critical to obtaining its unique properties, particularly its high level of rolling contact fatigue. We have found that primary vacuum induction melting of this high performance bearing alloy is an absolute necessity to achieve consistently the superior fatigue properties required in aircraft and other high load bearing applications operating at elevated temperatures. This melting process permits the production of the alloy under well controlled and highly reproducible conditions to quality standards not attainable with air melting practices. In particular, the content of non-metallic macro- and microinclusions is drastically reduced in the electrodes cast for subsequent vacuum arc remelting and the alloy composition is maintained within the narrow limits required for optimum property response. 
     Proper selection of the crucible refractory and furnace pressure is mandatory to minimize undesirable reactions between the high carbon alloy and the oxide refractory of the induction crucible; otherwise, such reaction leads to rapid refractory erosion and poor cleanliness in the master alloy. We have found that magnesia and magnesia-alumina spinel of high purity are the most suitable refractory materials. Pressure levels up to 50 mm Hg may be employed to suppress metal-refractory reactions during a major portion of the melting process. Casting of the electrodes is generally carried out under a partial pressure of inert gas to control any boiling reaction in the molds. 
     Vacuum arc remelting of the vacuum cast electrodes, properly heat treated and ground, is performed under a partial inert gas atmosphere at a pressure level permitting stable arc conditions. Because of the strong tendency of the alloy to macro-segregation, the vacuum arc remelting parameters must be selected and maintained with utmost care. We have discovered that a pressure range of 1 to 20 mm Hg is satisfactory for melt rates of 2-7 kg/min., giving an ingot structure free of macrosegregation. The inert gas for partial pressurization of the furnace atmosphere is preferably selected from the family of noble gases. After completion of the constant burn-off cycle, the current is adequately reduced for the hot topping step to minimize the formation of shrinkage cavities in the top of the ingot; this is common practice in vacuum arc melting. The ingot is then removed from the crucible, slowly cooled and annealed prior to forging and rolling to bar products. 
     Thermo-mechanical processing of the alloy is carried out in the temperature range of 1050° to 1190° C. after adequate thermal homogenizing of the ingot. Initial forging on a hydraulic press must proceed slowly in small reduction steps, and frequent reheating of the material is required to reduce surface tearing and material losses due to cracking. A total forging reduction of at least 5:1 is recommended before further working on a cogging or rolling mill is performed. Blooms and billets are thoroughly ground at appropriate sizes to remove surface defects prior to further working. Subsequent finish rolling to final bar product follows, using working temperatures in the range of 1000°-1150° C. 
     In the foregoing general description, we have set out certain objects, purposes and advantages of our invention. Other objects, purposes and advantages will be apparent from the following description and examples and from the drawing which is a graph showing a comparison of fatigue results of the alloy of this invention with prior art alloys. 
    
    
     EXAMPLE I 
     A tungsten-bearing alloy &#34;MB&#34; of the following composition, according to this invention: 
     
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       C             1.07%                                                
       Si            0.35%                                                
       Mn            0.44%                                                
       W             2.56%                                                
       Cr            14.72%                                               
       V             1.18%                                                
       Mo            1.75%                                                
       Ni            0.19%                                                
       Cu            0.05%                                                
       Al            0.01%                                                
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     was produced by double vacuum melting and subjected to rolling contact fatigue testing. The heat treatment of the 10 mm rd. specimens consisted of the following steps: 
     
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Austenitize                                                               
           2100° F.                                                
                     (1149° C.)                                    
                                /30 min.                                  
Quench to  1050° F.                                                
                     (566° C. )                                    
Air cool                                                                  
Stress relieve                                                            
           300° F.                                                 
                     (149° C.)                                     
                                /1 hr.                                    
Refrigerate                                                               
           -100° F.                                                
                     (-74° C.)                                     
                                /15 min.                                  
Temper     970° F.                                                 
                     (521° C.)                                     
                                /2+2 hrs. +                               
           975° F.                                                 
                     (524° C.)                                     
                                /2 hrs +                                  
           985° F.                                                 
                     (529° C.)                                     
                                /2 hrs.                                   
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     The following test conditions were employed in RCF testing: 
     
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Max. Hertz Stress                                                         
                4826 MPa (700 ksi)                                        
Speed           6.3 m/sec. (25 000 cycles/min)                            
Lubricant       MIL-L-7808                                                
Temperature     Room - ambient                                            
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     Alloy of U.S. Pat. No. 3,167,423 was prepared by double vacuum melting and treated according to the teachings of the patent. 
     The results of these tests appear in Table 1. 
     
                       Table 1.                                                    
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ROLLING CONTACT (RC Rig) FATIGUE TEST RESULTS                             
            Alloy &#34;B&#34;  Alloy &#34;MB&#34;                                         
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B-10 Life                                                                 
(× 10.sup.-6 Cycles)                                                
              2.43         2.81                                           
B-50 Life                                                                 
(× 10.sup.-6 cycles)                                                
              6.19         9.04                                           
Weibull Slope 2.01         1.61                                           
Correlation                                                               
Coefficient   .88          .90                                            
Hardness Rc   61.6         61.8                                           
Retained Austenite                                                        
              5.0%         3.0%                                           
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     The increase in fatigue life of the alloy of this invention (MB) over the alloy of U.S. Pat. No. 3,167,423 (B) is significant. 
     EXAMPLE II 
     Five ball fatigue tests of three lots of alloy balls produced via the teachings of this new invention, using the vacuum induction melting technique (VIM) followed by vacuum arc remelting (VAR), was conducted and the resulting data were compared to data on a lot of VIM Alloy &#34;B&#34; balls, produced via the teachings of U.S. Pat. No. 3,167,423. For a second comparison, comparable data generated from VIM-VAR M50 Balls are included. These data clearly illustrate two favorable comparisons for the new invention, namely: 
     1. At a 10% life level, the new invention material is from 14 to 28 times superior to that of material representative of U.S. Pat. No. 3,167,423. 
     2. At a 10% life level the new invention material is from 6-12 times superior to that of premimum quality VIM-VAR M50, which is representative of material that is currently a very popular material for use in critical main shaft jet aircraft bearings. 
     The test data are set out in Table 2 and graphically summarized in the accompanying drawing. The three lots of balls according to this invention are identified as A, B and C. The balls according to U.S. Pat. No. 3,167,423 are identified as D and the balls of M50 steel are identified as E. 
     
                       Table 2.                                                    
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FATIGUE RESULTS WITH 1.27-CM-(1/2-INCH-) DIAMETER                         
BALLS RUN IN FIVE-BALL FATIGUE TESTER                                     
[Maximum Hertz stress,                                                    
5520 MPa (800 000 psi); contact angle, 30°; shaft                  
speed, 10 000 rpm; temperature, 340° K. (150° F.).]         
Fatigue life, millions of                                                 
upper ball stress cycles       Confidence                                 
Mate- 10-Percent                                                          
                50-Percent      Failure                                   
                                       number,                            
rial  life      life      Slope index.sup.a                               
                                       percent.sup.b                      
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A     56.7      120       2.50  31 of 40                                  
                                       97                                 
B     112       164       4.94  20 of 30                                  
                                       --                                 
C     66.4      192       1.77  10 of 21                                  
                                       87                                 
D     3.9       26.3      0.99  29 of 30                                  
                                       &gt;99                                
E     8.9       36.6      1.33  34 of 40                                  
                                       &gt;99                                
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 .sup.a Indicates number of failures out of total number of tests.        
 .sup.b Probability, expressed as a percentage, that lot B (baseline) has 
 10-percent, fatigue life greater than that of the particular lot being   
 considered.                                                              
 
    
     It can be seen from the foregoing test data that the alloy of the present invention provides a solution to the problem of bearing contact fatigue as well as the problems of hardness, corrosion resistance, etc. faced by the aircraft industry. 
     While we have described certain preferred practices and embodiments of our invention in the foregoing disclosure, it will be understood that this invention may be otherwise embodied within the scope of the following claims.