Abstract:
Hardfacing is applied on gage surfaces of bit blades, the leading and trailing edges of bit blades, and on carbide inserts. The gage surfaces contains natural diamonds, synthetic diamonds, thermally stable polycrystalline (TSP) diamonds and carbide inserts, and the hardfacing is applied over at least a portion of them. As primary cutters on the bit blades are worn down during drilling, the gage surfaces of the bit blades are also worn down. A hardfacing is applied to the worn gage surfaces of the bit blades, thereby allowing the drill bit to drill deeper and longer without requiring replacement.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/938,741, filed May 18, 2007. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to diamond earth-boring drill bits and, in particular, to a method of repairing a matrix body diamond bit. 
     BACKGROUND OF THE INVENTION 
     Rolling cone bits may have teeth machined from the steel bodies of the cones. Rolling cone bits may also have tungsten carbide inserts press-fit into mating holes in the cones. Hardfacing has been employed on the gage surfaces of both types of rolling cone bits, as well as on portions of steel bit bodies for many years to resist abrasive wear. Hardfacing is also applied to the machined teeth. However, hardfacing is not applied to tungsten carbide inserts. 
     The hardfacing typically comprises granules of tungsten carbide located within a steel alloy binder. One method of applying the hardfacing to rolling cone bits has been to use an oxy-acetylene torch to melt a hardfacing tube or rod onto the steel. The hardfacing rod is typically a steel tube containing a filler comprising tungsten carbide granules. The temperature to melt the tube and bond the hardfacing to the steel of the bit in a prior art method for rolling cone bits may be in excess of 1500° C. 
     Another type of bit, often called a diamond bit, has a cast metal-matrix body and polycrystalline diamond cutting elements attached to the body, rather than rolling cones. The metal-matrix material typically comprises tungsten carbide powder and a binder of a metal, such as copper. The metal-matrix material may also contain diamond grit in certain areas. Carbide elements may be attached to the body at various points to resist abrasive wear. Thermally stable polycrystalline (TSP) diamond members may also be attached to the body to resist abrasive wear, such as along the gage surface. 
     Hardfacing has normally not been applied to matrix body diamond bits. The high temperature for the prior art hardfacing process excessively melts the binder of the bit body metal-matrix material. Also, hardfacing has not typically been employed on diamond bit abrasive elements, such as cemented tungsten carbide inserts or tungsten carbide bricks. The high hardfacing temperature melts the binder of these members, which is typically cobalt, and also can cause the members to crack during cool down. In addition, if natural diamonds and/or diamond grit are employed in the metal-matrix of the body, the high temperatures of iron-based hardfacing causes the natural diamonds and synthetic diamonds to revert to carbon and form a carbon dioxide gas. The carbon dioxide gas creates a poor hardfacing layer. The high temperature for iron-based hardfacing has thus precluded its use as a hardfacing for a crown of a diamond bit. 
     Diamond bits have complex shapes and are very costly. Normally, after the bits are used in drilling, they become worn and require repair in order to be re-used. This repair might involve replacing any damaged or missing polycrystalline diamond cutting elements as well as replacing missing abrasive elements. The repair process can be time consuming and expensive. 
     SUMMARY 
     The present invention provides a method for repairing diamond earth-boring bits whereby hardfacing is applied on the gage surface of bit blades. The gage surface may contain natural diamonds, synthetic diamonds, thermally stable polycrystalline (TSP) diamonds, and/or carbide inserts. As the primary cutters on the bit blades are worn down during drilling, the gage surface of the bit blade is also worn down. A hardfacing is applied to the worn gage surfaces of the bit blade, thereby allowing the bit to drill deeper and longer without requiring replacement. Embodiments of the present invention include a method of applying hardfacing over carbide inserts set in the bit blades. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a diamond bit that is worn. 
         FIG. 2  is an enlarged perspective view of a portion of the diamond bit of  FIG. 1 . 
         FIG. 3  is a perspective view of the diamond bit of  FIG. 1  after repair to a gage area of the bit by hardfacing and grinding the hardfacing to the gage diameter. 
         FIG. 4  is a perspective view of the diamond bit of  FIG. 1 , after some repairs have been done to the bit by hardfacing but before grinding. 
         FIG. 5  is an enlarged perspective view of a portion of the diamond bit of  FIG. 1  illustrating a tungsten carbide insert on the bit that has been repaired by hardfacing. 
         FIG. 6  is a perspective view of another portion of the diamond bit of  FIG. 1 , showing hardfacing applied to the blade for repair but before grinding. 
         FIG. 7  is a schematic sectional view of a portion of one of the gage areas of the diamond bit of  FIG. 1 . 
         FIG. 8  is a perspective view of a diamond bit that is worn. 
         FIG. 9  is an enlarged perspective view of a portion of the diamond bit of  FIG. 8 . 
         FIG. 10  is a perspective view of the diamond bit of  FIG. 8  after repair to the gage area of the bit by hardfacing and grinding the hardfacing to the gage diameter. 
         FIG. 11  is a schematic sectional view of a portion of one of the gage areas of the diamond bit of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2 , bit  11  is an earth-boring bit having a shank  13 , normally formed of steel. Shank  13  has a threaded stem  15  on its end for securing to the drill string (not shown). A crown  17  is formed on the end of shank  13  opposite stem  15 . Crown  17  is typically formed of a tungsten carbide metal-matrix material  18 . 
     Crown  17  has a plurality of blades  19  formed thereon. Blades  19  are preferably integrally formed with crown  17  and extend over and down the sides of crown  17 , forming a gage surface  20 . Gage surface  20  is an area located at the maximum diameter of each blade  19  and determines the diameter of the borehole being drilled. Junk slots  21  extend between each blade  19 . One or more nozzles (not shown) are located on the bottom of crown  17  between blades  19  for discharging drilling fluid. The drilling fluid, along with cuttings, flows through junk slots  21  and back up the annulus surrounding the drill string. 
     A number of polycrystalline diamond cutters (PDCs)  23  are mounted on the leading edge of each blade  19 . Some PDC elements  23  may be located on a portion of a blade  19  between the leading and trailing edges, behind those on the leading edges. In some bits, one or more of the PDC elements  23  will be located on the leading edges of part of gage surface  20  of each blade  19 . Each PDC element  23  comprises a disk of polycrystalline diamond bonded to a cylindrical cemented or sintered tungsten carbide base  25  ( FIG. 5 ), which, in turn, is brazed into a hole or receptacle  26  ( FIG. 4 ), which was provided in metal-matrix material  18  of blade  19  while crown  17  was being molded. 
     Bit  11  has a number of wear-resistant members mounted on it to resist wear of crown  17 . These wear-resistant members are harder and more resistant to abrasive wear than the metal-matrix material  18  of crown  17 . For example, the particular bit  11  shown has an optional cemented or sintered tungsten carbide insert  27  mounted to each blade  19  for resisting wear. Insert  27  is dome-shaped and is located approximately midway between the leading and trailing edges of each blade  19  above gage surface  20 . In this example, insert  27  is located directly rearward from one of the PDC elements  23  mounted at the leading edge of blade  19 . 
     In this embodiment, as shown in  FIG. 2 , other wear-resistant members include natural diamonds  28  mounted on each gage surface  20 . Natural diamonds  28  are normally sufficiently large to be easily visible without magnification. Two vertical rows of natural diamonds  28  are shown on each gage surface  20 , but this arrangement can vary. The exposed faces of natural diamonds  28  are generally flush with the surface of metal-matrix material  18 . 
     Other abrasion-resistant members include carbide members  29 , typically called “bricks,” which are mounted on gage surface  20  of each blade  19  alongside the rows of natural diamonds  28 . Carbide bricks  29  are of cemented or sintered tungsten carbide, similar to the material used for carbide base  25  and tungsten carbide insert  27 , but are typically rectangular in shape. The exposed face of each brick  29  is generally flush with the surface of metal-matrix material  18  of gage surface  20 .  FIG. 7  illustrates one of the carbide bricks  29  embedded within metal-matrix material  18  of crown  17 . 
     Also,  FIG. 7  shows that metal-matrix material  18  in this example also contains diamond grit particles  30 , which are exaggerated in size. Diamond grit particles  30  comprise much smaller diamonds than natural diamonds  28  and are not readily visible without magnification. The individual particles of diamond grit  30  may be coated, and are embedded within metal-matrix material  18  at or near the surface. 
     In a different embodiment of bit  11 , as shown in  FIGS. 8 and 9 , thermally stable polycrystalline (TSP) diamonds  39  are mounted on each gage surface  20  to resist wear of gage surface  20 . TSP diamonds  39  are typically larger than natural diamonds  28  ( FIG. 2 ) and are easily visible without magnification. Four offset vertical rows of TSP diamonds  39  are shown on each gage surface  20 , but this arrangement can vary. The exposed faces of TSP diamonds  39  are generally flush with the surface of metal-matrix material  18 . 
     Normally, crown  17  is formed in an infiltration process, which is a long cycle, high temperature, atmospheric pressure process. A graphite mold is formed in the shape of crown  17 . Shank  13  is supported by a fixture, and blanks are placed in the mold to define PDC element receptacles  26  ( FIG. 4 ). Tungsten carbide bricks  29 , natural diamonds  28 , TSP diamonds  39 , and tungsten carbide inserts  27 , if employed, are fixed at appropriate places in the mold. A matrix powder, typically tungsten carbide, is placed in the mold and around shank  13 . The powder may also contain diamond grit  30  in certain places. Binder particles, such as a copper alloy, are placed on an upper surface of the tungsten carbide powder within the graphite mold. The heat melts the binder, causing it to infiltrate down through the tungsten carbide powder, bonding the carbide powder, diamond grit  30 , natural diamonds  28 , TSP diamonds  39 , carbide bricks  29  and tungsten carbide inserts  27 . After removal from the furnace, the PDC elements  23  are subsequently brazed into receptacles  26 . 
     As shown in  FIGS. 1 and 2 , after drilling a number of wells, some of the PDC elements  23  may be broken. In addition, some of the carbide bricks  29  may be cracked and broken. Tungsten carbide inserts  27  may be worn or broken. The leading and trailing edges of blades  19  may also become eroded. If the metal-matrix material  18  erodes too deeply, the carbide bases  25  cannot be reinstalled within receptacles  26  ( FIG. 4 ) and the bit  11  will have to be discarded. As shown in  FIGS. 8 and 9 , after drilling a number of wells, the TSP gage surface  20  may be worn. If gage surface  20  continues to wear, further exposing TSP diamonds  39 , the bit  11  will eventually be discarded. 
     In the method comprised by this invention, hardfacing is employed on several areas of a bit that normally would not be feasible. The hardfacing is preferably a nickel or nickel alloy-based hardfacing. The nickel-based hardfacing melts at a much lower temperature than iron-based hardfacing, such as at a temperature less than 1200° C. This lower temperature is not as detrimental to metal-matrix material  18 , natural diamonds  28 , diamond grit  30 , TSP diamonds  39 , tungsten carbide bricks  29 , and tungsten carbide inserts  27 . The lower temperature does not excessively melt the binder from metal-matrix material  18  nor the binder from sintered tungsten carbide bricks  29  and inserts  27 . 
     One example of a type of suitable alloy is an alloy of nickel, boron, chromium and silicon in the following relative percentages by weight: 
                                                 carbon    .45%           chromium   11.0%           silicon    2.25%           boron    2.5%           iron    2.25%           nickel   balance                        
This alloy has a hardness of about 38-42 Rockwell C and a melting temperature of about 1100° C. The hard abrasive components may be the same as conventionally used on rolling cone bits with iron-based hardfacing. For example, the hardfacing may include monocrystalline tungsten carbide, sintered tungsten carbide, either crushed or spherical, and cast tungsten carbide, either crushed or spherical. The sizes of the particles and the quantity by weight of the particles to the binder may be the same as conventionally used in iron-based hardfacing, but are in no way limited to these parameters. Preferably, a rod is formed containing the nickel alloy mixed with the hard abrasive particles. The rod may be formed in different manners. One way is by liquid phase sintering of the nickel alloy and abrasive particles. Another way is by an extrusion process of the nickel alloy mixed with the abrasive particles, which results in the extruded product being rolled onto a spool. Alternatively, the nickel alloy could be made into a tube and the abrasive particles placed inside.
 
     To repair bit  11 , normally a technician removes PDC elements  23  from their receptacles  26  before applying hardfacing so as to avoid the heat from damaging PDC elements  23 . They are removed conventionally by applying brazing temperature heat to soften the brazing metal. Once elements  23  are removed, the operator then uses an oxy-acetylene torch to apply the nickel-based hardfacing. 
     The technician will apply hardfacing to the worn gage surface  20 , as illustrated in  FIGS. 4 and 10  and indicated by the numeral  31 . Gage hardfacing layer  31  may be applied completely over the cracked and broken carbide bricks  29  ( FIG. 3 ). As shown in  FIGS. 7 and 11 , gage hardfacing layer  31  overlies carbide bricks  29 , metal-matrix material  18 , natural diamonds  28 , TSP diamonds  39 , and exposed diamond grit  30 . Gage hardfacing layer  31  may extend from the leading edge to the trailing edge of each blade  19  and may extend up to the closest PDC element  23  on each blade  19  (not shown).  FIG. 5  shows hardfacing layer  35  applied to the exposed portions of tungsten carbide insert  27  ( FIG. 2 ). 
     After applying the hardfacing, the technician grinds gage surface hardfacing layer  31  to the original gage tolerances ( FIG. 3 ) and grinds the other hardfacing layers where needed. The operator then brazes PDC elements  23  into receptacles  26  ( FIG. 4 ). Tests indicate that the nickel-based hardfacing adheres well to metal-matrix material  18  and is wear resistant.