Patent Publication Number: US-2006018782-A1

Title: Media mixture for improved residual compressive stress in a product

Description:
RELATED APPLICATION  
      The present application is a continuation in part of U.S. patent application Ser. No. 09/965,162, filed Sep. 27, 2001 which claims priority from U.S. Provisional Application Ser. No. 60/236,001, filed Sep. 28, 2000. Both applications are incorporated herein by reference in their entirety. The present application is also related to and claims priority from U.S. Provisional Application 60/604,555, filed Aug. 26, 2004, entitled “Process for Forming Spherical Components”, which is also incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to a media mixture for forming a product with a surface layer having increased residual compressive stress.  
     BACKGROUND  
      Residual stress is a stress that exists within a part without any external load, such as an applied force or thermal gradient. These stresses are induced during the manufacturing process, for example, due to working of the part, surface treatment of the part or temperature changes during part formation. Residual stresses within a part can be tensile or compressive. Most conventional manufacturing processes tend to induce residual tensile stresses into the component being fabricated.  
      It is well known that tensile stress tends to reduce the mechanical performance of materials. For example, cracks that form within a part tend to propagate more readily under the influence of tensile residual stresses in the part. These stresses act upon the crack causing the crack tip to extend through the part. Fatigue or cyclic loading in the vicinity of the crack can accelerate the crack propagation, which could lead to catastrophic failure of the component. It is also known that residual tensile stresses can cause corrosion in a part to propagate into cracking. Thus, residual tensile stresses in a component are generally not favorable.  
      One known way of reducing crack propagation in component is by inducing a compressive residual stress in the vicinity of a crack tip. The compressive stress tends to inhibit crack growth, thus improving the fatigue life of the component. However, to date, the processes for introducing compressive residual stress in a part have been limited to shot and hammer peening. The drawback with both these processes is that they produce relatively large, inconsistent compressive stress spots in the component.  FIGS. 1A-1D  illustrate the effect of conventional peening on a metallic surface.  FIG. 1A  schematically illustrates the granular arrangement in a manufactured part. The grains are generally under a tensile residual stress. As a shot hits the surface, it deforms it locally, inducing compression into the material. See  FIG. 1B . The force of the shot causes localized plastic deformation, resulting in a small area of subsurface residual compression.  
      Due to the size of the shots used in conventional shot peening, the localized areas of compression are not consistent. As such, the resulting product will generally include mixtures of tensile and compressive residual stresses at the surface. See  FIG. 1C  As such, in the event that a crack develops in between shot peened zones, those cracks can propagate.  
      In products that have tight internal radii, for example at the root of gear teeth and in notched materials (which is generically shown in  FIG. 1D ), it is generally not possible for the surface to be completely peened. As such, the very points where there are high stress concentrations (e.g., notches and radiused corners) are the same places where shot peening cannot reach and, thus, cannot assist in reducing crack propagation.  
     SUMMARY OF THE INVENTION  
      A media mixtures is disclosed for imparting compressive residual stress on a work piece in a high energy centrifugal processor. The media mixture includes carrier media, preferably made from a fractured grain product. The carrier medium having an average size of at least about 0.05 inches and a hardness. A plurality of abrasive particles are mixed with the carrier media, Each particle has a size that is smaller than the carrier medium and has a hardness that is greater than the hardness of the carrier medium.  
      The abrasive particles preferably covering on average at least about 50% of the carrier medium. The abrasive particles preferably having a size of at least approximately 1000 grit. The abrasive particles being attached to the carrier medium preferably with a lubricant.  
      The abrasive particles are preferably selected from a group consisting of aluminum oxide, silicon carbide, tungsten carbide and diamond.  
      In one embodiment of the invention, a plurality of solid additives are added to the media. The solid additives each have a size that is larger than the abrasive and a density that is greater than the carrier medium. The solid additives are preferably substantially spherical in shape.  
      The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For the purpose of illustrating the invention, the drawings show a form of the invention which is presently preferred. However, it should be understood that this invention is not limited to the precise arrangements and instrumentalities shown in the drawings.  
       FIGS. 1A-1D  schematically illustrate the application of conventional peening on a material surface.  
       FIG. 2A  schematically illustrates the path of a media particle as it interacts with the surface of a work piece in a centrifugal processor.  
       FIGS. 2B and 2C  schematically illustrate the resulting residual compressive stress profile produced in a work piece using a media mixture according to the present invention.  
       FIG. 3  schematically illustrates a carrier medium coated with hard abrasive particles in accordance with one embodiment of the invention.  
       FIG. 4  is a graph comparing the surface roughness between an unprocessed bearing race and a processed bearing race. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      It has been determined that centrifugal processors, such as those available from Mikronite Technologies, Inc., and disclosed in U.S. Pat. Nos. 5,355,638, 5,848,929, 6,599,176, 6,733,375 and PCT/US03/21218, afford an almost perfect slide relationship between the part being processed and the media in the processing vessel that is used to modify the surface of the part. The forces imposed on the media within a centrifugal processor are such that the media contacts the work piece at an angle of incidence that is typically not perpendicular to the work piece surface. This produces a desirable slide relationship since it tends to reduce or eliminate cumulative and inconsistent impact forces that plague the uniformity of surface and sub-surface treatment in other high compression processes such as hammering or peening. The sliding signature of the media along the part surface produces a short (as compared to the total length of the piece) substantially linear scratch on the surface. The angle of incidence of the media results in applied forces that are not only normal to the surface of the part, but also parallel to the surface. In contract, shot peening and hammering are designed to apply a perpendicular or normal force onto the material. Accordingly these methods produces only impact forces with a localized, essentially point compression induced in the part.  
      The scratching of the surface of a material in a centrifugal processor results in substantially entire surface contact. Furthermore, the lateral of side motion of the media on the surface produces molecular movement of the work piece material, as opposed to simple crushing that is produced by peening. It has been determined that when the movement of the media is highly accelerated, the result is that the scratching produces substantially uniform and contiguous surface and subsurface (i.e., layer) of residual compression in the work piece.  FIG. 2A  schematically illustrates the resulting surface layer compression that results in a part subjected to centrifugal processing. An exemplary trajectory T of a media M is shown. The media skips off the surface at high speed, thus producing the highly desirable scratching and subsurface compression. The fluidized media environment created by the centrifugal processors described in the above referenced patents produces multidirectional scratches across the entire surface of the work piece, which produces a substantially uniform surface finish and subsurface compression, see  FIG. 2B , and permits the application of compressive stress in many remote regions, see  FIG. 2C .  
      Experimentation has yielded several media blends that control depth and intensity of compression, resulting in optimal compressive residual stress profiles, and no substantial undesirable direct impacts.  
      In one embodiment, the media  10  is composed of a small grain product  12 . Various grain or similar products can be used in the media, such as organic grain products that can be fractured. The fracturing permits further reach of the grain surfaces into tight corners. Some preferred forms of grain are walnut shells and corn. The weight of the grain product will vary depending on the degree and length of time of processing desired, the material being processed, and size of the product being processed relative to the vessel size, etc. In one exemplary embodiment, the grain has a weight of between about 15 and 35 pounds per cubic foot of weight per volume. More preferably the weight is about 26 pounds per cubic foot of weight per volume. The media at this weight will typically have a size (cross-sectional dimension) of about 0.05 inches or larger, usually graded through screen separation. The maximum preferred media size is 0.50 inches. The preferred media will range in size from about 0.05 inches to about 0.25 inches. The preferred minimum size used in the media mixture is about 0.05 inches. While smaller particles can be used, the result is that the processing time becomes excessively long. However, as will be apparent the size of the part being processed will, to some extent, impact the size of the media selected  
      In order to enhance the surface finishing, the surface of the media  10  is preferably coated with a hard particle (abrasive)  14  of a smaller size, for example about 1000 grit or larger.  FIG. 3  schematically illustrates a carrier medium  12  with abrasive particles  14  on the surface of the grain  12 . Since abrasive is being carried by the grain (i.e., the grain is the carrier)  12 , there is less volume needed of abrasive. Preferably, the abrasive  14  covers, on average, more than 50% of the carrier medium  12  (e.g., grain). More preferably, the abrasive  14  covers 50%-100% of the surface of the carrier medium  12 . In one embodiment, the abrasive covers between about 80% to about 90% of the surface of the carrier medium. The hard particles may be, for example, aluminum oxide, silicon carbide, tungsten carbide, or diamond. The present invention is not, however, limited to this selection of hard materials. On the contrary, any material can be used depending on the processing desired, and more preferably any material that is harder than the carrier can be used. Also, more than one type of abrasive particle and carrier medium can be used in a media mixture.  
      In one embodiment, the abrasive  14  is attached to the carrier  12 . The attachment can be through various means. For example, in an exemplary arrangement, a lubricant, such as thin viscosity oil is added to the carrier medium  12 . The lubricant is preferably selected so that it soaks into the carrier and/or abrasive, without muting the facets of the hard abrasive particle. The lubricant coats the carrier medium and creates a small degree of surface adhesion which holds the abrasive onto the carrier medium. While the exemplary embodiment discussed above coats the abrasive onto the carrier medium (e.g., grain) it is also contemplated that the abrasive may simply be mixed with the carrier medium.  
      The carrier medium preferably has a jagged shape which forms traction against adjacent carrier media, causing the carrier media to cascade around inside the vessel. The carrier medium provides the support structure for carrying the hard (but typically lighter) abrasive particles into the work piece. As a consequence, the weight of an individual abrasive particle is not germane to calculating the contacting force. Rather the entire multiplier of g forces are the determinant in the ultimate benefit achieved. It should be readily apparent that the carrier media need not be jagged. The hard particles themselves can provide the frictional interface with adjacent pieces of media. Sharper edges are preferred for the hard particles since such edges provide more of an effect on the surface of the work piece.  
      It is preferred that the carrier medium has a lower hardness than the abrasive particles. As such, the carrier medium will bend or flex when it contacts the work piece, thereby limiting damage to the work piece.  
      In testing, this blend of media was used in one of the centrifugal processors described above and processed at speeds that generated accelerations of approximately 16 g&#39;s or more. In one exemplary embodiment, the processor was operated at about 30 g&#39;s on a work piece of steel for at least approximately 20 minutes, and achieved a compressive residual stress of at least approximately 175 ksi substantially across the surface of the work piece and to a subsurface depth of approximately 0.012 inches with a reduction in stress levels as depth increased.  
      Variations in media composition have also been tested. In one embodiment, the media composition was modified to include larger solids and defined non-antagonistic shapes, such as spheres. The solids (or additives) are dense compositions and usually larger then the carrier media. The solids generally do not have any sharp edges so as to minimize damage to the work piece. The solids and/or non-antagonistic shapes do not need to have any particular density, only that they are preferably heavier than the carrier media. The large size of the dense solids has been found to enhance the surface compressive residual stress to over 200 ksi, but the larger size also typically results in a reduction in the depth of the compressive stress that is imposed on the material. This is likely due to a reduced angle of incidence caused by the heavier material, moving closer to 180 degrees or a path that is parallel to the surface of the material.  
      The harder surface benefit helps to create a differentiation in parts operating in an articulating (e.g., intermeshing) manner with parts of similar materials untreated. For example, in untreated gears made from identical materials, as the gears rotate and mesh, there is a chance that, if there is no oxide layer on the surface of the materials, the mating surfaces will weld (cold fuse) to one another. This is sometimes referred to as a “metallic bond”. As the gears continue to rotate, the weld results in a portion of one or both of the gears to break off. To prevent this, some gear trains use gears that are made from different materials. The present invention eliminates this problem by processing one gear so as to produce a density and stress level in the gear surface that is sufficiently distinct from the surface of the mating gear so as to preclude or reduce chemical failure modes (such as galling, cold welding, and tearing) that occurs when identical metals contact one under high pressure and/or temperature conditions.  
      Another problem that the present invention solves is corrosion that can develop from anodic potential in intermeshing or contacting metallic parts. As electrodes transfer from anodes to cathodes, they bring with them oxides. This produces corrosive oxidation. As noted above, the processing of the surface using the present invention to create a substantially uniform electrical surface which reduces or minimizes the occurrence of anodic potential.  
      The larger dense media additives can, if too large, create a relationship of equal inertial tendency to resist movement, resulting in intensified collisions with damaging results. Thus, in order to control the compressive stress being imparted without work piece damage, it is desirable to have the overall weight of the individual pieces of media additive not exceed 10% of the work piece. The total of all additives are not cumulative and tends to have no relationship to destructive effects.  
      In one preferred embodiment, spherical ductile media additives such as metallic balls, can prove most effective when factoring damage aversion as a primary criteria. Small metal balls, having a maximum size which permits contact with the tightest regions of the work piece being processed, have been tested in total media concentrations of 20%, 40%, and 50% (i.e., media concentrations=amount of additives/(amount of carrier+abrasive+additives) All concentrations provided beneficial results. It should be noted that the inclusion of metallic balls in the media mixture increases the temperature during processing due to the agitation. As such, the smallest concentration of the metallic ball additive is desirable to avoid rapid temperature rise within the processing vessel.  
      In cases where heat elevation is desirable, such as for annealing or hardening requirements, larger concentrations of metallic balls can induce those effects without any risk of damage. Conduction of heat in large concentration ball-on-ball contact, and forced convection during agitation of the fluidized media maintain extremely uniform controlled temperatures (albeit elevated). External devices for controlling the internal atmosphere within the processing vessel can also be used to control the process parameters.  
      In one embodiment, grain media was mixed with metallic balls to create the media mixture. The grain is preferably smaller than the metallic balls. In this embodiment, there is preferably no abrasive on the grain, although it can be added if desired. The grain media provides a support geometry for the metallic sphere not possible with the void spaces that would otherwise exist in a sphere-on-sphere media. Spheres, by design, have less surface interlocking then more cubic, random, or polygonal shapes. Although this aids in part slide and roll off of touching pieces, it has a greater likelihood of collapse in a solely ball-stacked-on-ball structure. As such, to use metallic balls without a support media results in reduced stability, inertia, and time of contact, regardless of density or size.  
      Testing has established that use of the high energy centrifugal processors described above in combination with the media can enhance even conventionally processed gears. For example, it has been determined that the present invention can be used to significantly increase the compressive residual stress in conventionally carburized products, including pinion gears. The process is described in detail in co-pending application Ser. No. ______ , entitled “Sub-Surface Enhanced Gear” (Attorney Docket No. 9436-40US (209417)), which is incorporated herein by reference in its entirety.  
      Referring to Test Protocol 1 shown below, a carburized pinion gear was measured before and after application of the high energy finishing process. Prior to application, the pinion had a compressive residual stress at its surface of 122.7 ksi as measured using X-ray diffraction. After application of the process according to the present invention, the compressive residual stress was up to 204.3 ksi. That is a 66% increase. As shown in the chart, the increase in compressive residual stress was consistently measured down to a depth of 0.015 inches. On average, there was a 50% increase in compressive residual stress through the depth.  
      Table 1A shows that the present invention can be used to produce a 67% decrease in the surface roughness of the carburized pinion gear as measured in terms of Ra. Ra is the Arithmetic Mean Deviation of the roughness profile. This is a tremendous improvement in the surface roughness of a conventional pinion gear. As shown the peaks on the surface of a conventional carburized pinion gear were on average 34 μin. When these high peaks break off during use of the pinion, they tend to cause damage within the gearbox. The present invention addresses this problem by significantly reducing the height of the peaks, essentially evening out the surface. This minimizes damage to the pinion gear and the gearbox.  
      Test Protocols 2-5 (shown below) were conducted on cutting flutes made from different materials. These tests show similar beneficial results as Test Protocol 1 discussed above.  
      Test Protocol 6 was performed on a bearing race made from E52100 steel. Again there was an increase in the compressive residual stress from 4% to 72.2%. Also shown in  FIG. 4  is a chart of the Ra on the surface of the bearing race. As can be readily seen, the unprocessed part had significant variations over the surface resulting in an Ra of 10.5 μin. These variations result in potential hot spots where faults (e.g. cracks) can start. Furthermore, the variations in the surface contour generate vibrations that result in vibratory loading on the race and increased acoustic noise. The processed bearing race according to the present invention had a significant decrease in the surface roughness producing an Ra of 0.91 μin.  
      Test Results  
      The present invention has been applied to several specimens. The following summarizes the test results.  
      Test Protocol—1 
      Material—4140 Steel—Carburized Pinion Gear     Processing time—approximately 45 minutes at about 30 g&#39;s     Depth reading—surface to 0.015 inches    

      Location—measurement at same point but at different depths  
               TABLE 1                          Compressive Residual Stress v. Depth - Pinion Gear                                 Stress (ksi)                                         Depth   Gear U   Gear M       Percentage       (inches)   (Unprocessed)   (Processed)   Delta   Increase                                         0   −122.7   −204.3   −81.6   66.5%       0.0005   −86.2   −124.6   −38.4   44.5%       0.001   −59.8   −95.7   −35.9   60.0%       0.002   −32.3   −59.8   −27.5   85.1%       0.004   −25.3   −37.6   −12.3   48.6%       0.007   −32.1   −42.7   −10.6   33.0%       0.01   −27   −43.3   −16.3   60.4%       0.015   −49.1   −51.4   −2.3   4.7%                  
 
     
       
         
           
               
             
               
                 TABLE 1A 
               
             
            
               
                   
               
               
                   
               
               
                 Ra on Surface - Pinion Gear 
               
            
           
           
               
               
               
            
               
                 Gear U 
                 Gear M 
                 Percentage 
               
               
                 (Unprocessed) 
                 (Processed) 
                 Decrease 
               
               
                   
               
               
                 34 μin 
                 11 μin 
                 67.6% 
               
               
                   
               
            
           
         
       
     
      Test Protocol—2 
      Material—imported High Strength Steel ½ End Mill 2 Flutes—Solid carbide     Processing time—6 minutes at 30 g&#39;s     Depth reading—surface    

      Location inside flute, 3 measurements at same point, but at different angles  
                                   TABLE 2                                       Unprocessed   Processed   Percentage           Location   (ksi)   (ksi)   Increase                                                            90°   −20   −40.48   39%           45°   −11.2   −32.66   190%            315°    −27.27   −39.36   44%                      
 
      Test Protocol—3 
      Material—American High Strength Cobalt Steel ½ End Mill 2 Flutes—Square end     Processing time—6 minutes at 30 g&#39;s     Depth reading—surface    

      Location inside flute, 3 measurements at same point, but at different angles  
                                   TABLE 3                                       Unprocessed   Processed   Percentage           Location   (ksi)   (ksi)   Increase                                                            90°   −95.11   −115.01   20%           45°   −46.6   −94.71   104%            315°    −125.03   −130.41    4%                      
 
      Test Protocol—4 
      Material—American Solid Carbide 7/16 End Mill 2 Flutes—Solid carbide     Processing time—6 minutes at 30 g&#39;s     Depth reading—surface    

      Location inside flute, 3 measurements at same point, but at different angles  
                                   TABLE 4                                       Unprocessed   Processed   Percentage           Location   (ksi)   (ksi)   Increase                                                            90°   −79.89   −114.58   44%           45°   −114.48   −129.82   13%           315°    −16.25   −68.53   325%                       
 
      Test Protocol—5 
      Material—American Hob Steel 2 Piece Test/(DP40PAZOWD, 057-1°21′), M42 Tin Coated     Processing time—8 minutes at 30 g&#39;s     Depth reading—surface    

      Location—1 measurement  
                                   TABLE 5                                       Unprocessed   Processed   Percentage           Location   (ksi)   (ksi)   Increase                          90°   −8.2   −17.7   115%                      
 
      Test Protocol—6 
      Material—E52100 Steel—Bearing Race     Processing time—6 minutes at 30 g&#39;s     Depth reading—surface to 0.02 inches    

      Location—measurement at same point, but at different depths  
               TABLE 6                          E52100 Steel - Bearing Race                                     Unprocessed       Processed                                     Depth   Stress   Depth   Stress   Percentage       (inches)   (ksi)   (inches)   (ksi)   Increase                                         0   −37.18   0   −57.95   55.8%       0.0005   −40.08   0.0006   −46.2   15.2%       0.001   −37.79   0.001   −39.3     4%       0.0019   −35.55   0.0021   −41.11   15.6%       0.0042   −32.08   0.004   −40.62   26.6%       0.0068   −29.4   0.0069   −30.81    4.8%       0.0104   −16.85   0.0098   −29.02   72.2%       0.0151   −14.79   0.015   −20.53   38.8%       0.02   −9.27   0.021   −12.93   39.5%                  
 
      Accordingly, the present invention results in improved compressive residual stress in parts. This increased compressive residual stress helps to prevent and/or reduce the propagation of cracking in the products. Also, as discussed above, the media produces a product with a very low surface roughness. This results in reduced loading on the part, including thermal loads, as well as reduced vibrations.  
      Although the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.