Abstract:
A drill bit including a bit body having an upper end adapted to be detachably secured to a drill string and at least one leg at its lower end, each leg having a downwardly and inwardly extending journal bearing, at least one roller cone mounted on each journal bearing, at least one cutting element disposed on the at least one roller cone; and a hardfacing overlay on at least a portion of at least one of an inner surface of the at least one roller cone and a surface of the journal bearing, wherein a composition of the hardfacing overlay proximate an outside surface of the hardfacing overlay is different from a composition of the hardfacing overlay proximate an interface between the hardfacing overlay and the at least a portion of at least one of the inner surface of the at least one roller cone and the surface of the journal bearing is disclosed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S. Provisional Application Ser. No. 60/695,975, filed on Jul. 1, 2005. That application is incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to hardfacing coatings on a metallic work piece. In particular, the present invention relates to hardfacing coatings on drill bits. 
   2. Background Art 
   Rotary drill bits are generally well known in the art. These bits typically include three cone-shaped members adapted to connect to the lower end of a drill string. One example of such a drill bit is shown in  FIG. 1 . The bit  10  includes three individual arms  11  that extend downward from the bit body  19  at an angle with respect to the bit axis. The lower end of each arm  11  is shaped to form a spindle or bearing pin (shown as  16  in  FIG. 2 ). A cone cutter  12 , which includes a plurality of cutting elements  14 , is mounted on each spindle and adapted to rotate thereon. As the drill string rotates, the cones  12  roll on the borehole bottom and rotate on about their respective spindles, thereby disintegrating the formation to advance the borehole. 
     FIG. 2  shows a partial, longitudinal cross section of a leg of a rock bit. Each leg includes a journal pin  16 , on which a roller cone  12  is attached. During drilling, the roller cone  12  rotates around the journal pin  16 . The rotation may cause the roller cone  12  to grind against the journal pin  16 . Therefore, wear resistant materials are often included in critical areas on both the journal pin  16  and the inside of the roller cone  12  to minimize wear damage. In addition, bearing systems are provided to allow rotation of the cone cutter and serve to maintain the cone cutter on the spindle. These bearing systems may comprise roller bearings, ball bearings or friction bearings, or some combination of these. 
   As shown in  FIG. 2 , the journal pin  16  includes a cylindrical bearing surface having a hard metal insert  17  on a lower portion of the journal pin  16 , while an open groove  18  is provided on the upper portion of the journal pin  16 . Groove  18  may, for example, extend around 60% of the circumference of the journal pin  16 , and the hard metal  17  can extend around the remaining 40%. The journal pin  16  also has a cylindrical nose  19  at its lower end. 
   The cavity (or inside surface) in the roller cone  12  typically contains a cylindrical bearing surface including an aluminum bronze insert  21  deposited in a groove in the steel of the roller cone  12  or as a floating insert in a groove in the roller cone  12 . The aluminum bronze insert  21  in the roller cone  12  engages the hard metal insert  17  on the journal pin  16  and provides the main bearing surface for the roller cone  12  on the bit body. A nose button  22  is disposed between the end of the cavity in the roller cone  12  and the nose  19  of the journal pin and carries the principal thrust loads of the roller cone  12  on the journal pin  16 . A bushing  23  surrounds the nose and provides additional bearing surface between the roller cone  12  and journal pin  16 . 
   As shown in  FIG. 2 , a plurality of bearing balls  24  are fitted into complementary ball races in the cone and on the journal pin. The bearing surfaces between the journal pin and cone are lubricated by a grease composition. The balls  24  carry any thrust loads tending to remove the roller cone  12  from the journal pin  16  and thereby retain the roller cone  12  on the journal pin  16 . 
   In addition, the interface between each spindle and its cone cutter may include a device (thrust bearing) to transmit thrust (axial) forces from the cone cutter to the spindle and thence to the bit. For description of various thrust bearings, see U.S. Pat. No. 5,868,502 issued to Cariveau et al. This patent is assigned to the assignee of the present invention and is incorporated by reference in its entirety. 
   The above described examples are greased bearing bits. The wear situation is even worse in non-lubricated open bearing bits.  FIGS. 3 and 4  show partial, longitudinal cross sections of a leg of an open-bearing air bit. Referring to  FIG. 3 , a typical mining, roller bearing, air cooled rotary cone rock bit generally designated as  30 , includes spindle  34  extending from the leg  33  forms bearing races  31  and  32  for roller bearings  35  and  36 . Intermediate roller bearings  35  and  36 , a plurality of ball bearings  37  rotatably retain the cone  38  on the spindle  34 . Spindle  34  forms a radially disposed main bearing face  39  from which a spindle bearing  40  extends. A spindle thrust bearing disc, or “thrust button,” generally designated as  41 , is pressed into a bearing cone cavity or socket  42  formed in cone spindle bearing  40 . Cone  38  includes an internal cavity adapted to receive spindle  34  and the bearings  35 ,  36 , and  37 . The cone cavity includes cylindrical surfaces  43  and  44 , ball bearing race  37 a, and socket  45 . The radial end face  46  of spindle bearing  40  extends into the cone cavity adjacent cylindrical surface  44 . A cone thrust bearing disc, or “thrust button,” generally designated as  47 , is pressed into a bearing cone cavity or socket  45  formed in cone  38 . As discussed in greater detail below, cone thrust disc  47  engages spindle thrust disc  41 , with the interface therebetween forming a thrust bearing. 
   Referring now to  FIGS. 3 and 4 , spindle  34  includes a main air fluid passage  48  formed in leg  33 . Secondary air passages  49  direct air from main passage  48  to the main bearing face  50 . An axially aligned air passage  51  directs air to a cross channel  52  that is formed in the radial end face  53  of the spindle  34 . Cross channel  52  intersects and passes beneath, in this embodiment, a hardened steel bearing thrust button generally designated as  41  that is interference fitted or pressed into socket  45  formed in spindle  34 . Air passes from central passage  51  into channel  52 , thereby contacting base (not shown) of spindle thrust button  41 . Air contacting base (not shown) of thrust button  41  serves to cool thrust button  41  and adjacent cone thrust button  47 . 
   During operation of an open bearing, air bit, such as the one illustrated in  FIGS. 3 and 4 , the weight of the drill string places a load on the lower face of the cone  38 . The axial component of this load generally causes contact between the radial end face or thrust face  46  of the spindle bearing  40  and the cone cavity or socket  45  formed in cone  38  on the lower, or load, side. The friction resulting from this contact between the cone  38  and the stationary support spindle  34  causes wear on the contacting surfaces that limits the useful life of the drill bit. 
   In greased bearing bits, the use of a lubricant on the contacting surfaces slows the rate of surface wear. However, in open bearing air bits, air is pumped through the drill pipe and through passages in the drill bit to the bearings for cooling and for keeping the bearings clean, rather than a lubricant. While air cools the outer roller bearings adequately, air cooling does not work as well in the nose area of the bit, which is subjected axial loads. The lack of lubrication and cooling on the thrust face increases heat generated by friction thereby promoting galling of the spindle and often causing premature failure of the spindle. 
   In addition to bearings and journal pins, the exposed, exterior parts of drill bits may also be subjected to wear. Some wear-susceptible exterior components of the drill bit include the exterior surfaces of the bit body, external surfaces of the cutting elements, and external surfaces of the roller cones on roller cone bits. 
   These parts, such as bit body, roller cones, and cutting elements, contact the formation during drilling and are subjected to abrasive actions. To prolong the life of a drill bit, these wear-prone surfaces should preferably be coated with a hardfacing material. 
   Various hardfacing materials methods are known in the art for minimizing wear on various parts of a drill bit. For example, U.S. Pat. Nos. 4,836,307 issued to Keshavan et al., and U.S. Pat. Nos. 5,944,127 and 6,659,206 both issued to Liang et al. disclose various hardfacing material compositions and particle size distributions suitable for use in hardfacing inserts, teeth, or roller cones. In addition, various methods have been developed for applying hardfacing coatings to wear prone surfaces on rock bits or inserts. These methods, for example, include thermal spraying, plasma arc welding, laser cladding, or other conventional welding methods. 
   Materials used in combination with the hardened steel surfaces in bit journal bearings, in provided, have included precipitation-hardened copper-beryllium (shown in U.S. Pat. Nos. 3,721,307 and 3,917,361), spinodally-hardened copper-tin-nickel (shown in U.S. Pat. No. 4,641,976), aluminum bronzes (shown in U.S. Pat. No. 3,995,917), and cobalt-based stellite alloys (shown in U.S. Pat. No. 4,323,284). These materials offer suitable ambient temperature yield strengths for use as structural elements or inlays, and acceptable anti-galling properties against hardened steel. However, at elevated PVs they can undergo a transition to high-friction operation, and except for the stellites, these alloys typically exhibit a rapid reduction in yield strength at temperatures above about 500° F. Because such high surface temperatures are not uncommon in bit thrust bearings, especially as drilling speeds have increased, if included on bit thurst surfaces, stellites have been the structural inlay material of choice for journal surfaces. 
   However, the effectiveness and durability of hardfacing depend on the compositions of the hardfacing materials. In addition, the compositions of the hardfacing materials also affect the strength of the bonding between the hardfacing layers and the underlying substrates. Most hardfacing compositions comprise wear-resistant particles (e.g., carbides) and a matrix metal (or alloy). Generally, altering a composition to enhance the wear resistance of the hardfacing overlay, typically results in a decrease of the fracture toughness of the overlay and reduction in the bonding strength between the hardfacing and the substrate. On the other hand, altering a composition to enhance the fracture toughness and bonding strength between the hardfacing and the substrate, typically results in a decrease in the wear resistance of the hardfacing overlay. Thus, the hardfacing materials used in the protection of drill bits or roller cones often represent a compromise between the desired properties, i.e., wear resistance, fracture toughness, and bonding strength. 
   Although the prior art hardfacing application techniques are capable of providing improved wear resistance to drill bits, there still exists a need for other techniques that can provide longer lasting drill bits. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention relates to a drill bit including a bit body having an upper end adapted to be detachably secured to a drill string and at least one leg at its lower end, each leg having a downwardly and inwardly extending journal bearing, at least one roller cone mounted on each journal bearing, at least one cutting element disposed on the at least one roller cone; and a hardfacing overlay on at least a portion of at least one of an inner surface of the at least one roller cone and a surface of the journal bearing, wherein a composition of the hardfacing overlay proximate an outside surface of the hardfacing overlay is different from a composition of the hardfacing overlay proximate an interface between the hardfacing overlay and the at least a portion of at least one of the inner surface of the at least one roller cone and the surface of the journal bearing. 
   In another aspect, the present invention relates to an open bearing drill bit that includes a bit body having an upper end adapted to be detachably secured to a drill string and at least one leg at its lower end, each leg having a downwardly and inwardly extending journal bearing, each journal bearing having an axial bearing surface and a radial bearing surface, at least one roller cone mounted on each journal bearing, at least one cutting element disposed on the at least one roller cone, and a hardfacing overlay on at least a portion of the axial bearing surface of the journal bearing, wherein a composition of the hardfacing overlay proximate an outside surface of the hardfacing overlay is different from a composition of the hardfacing overlay proximate an interface between the hardfacing overlay and the at least a portion of the axial bearing surface. 
   In another aspect, the present invention relates to a cutting tool for earth formation removal that includes a hardfacing overlay on at least a portion of at least one of a radial and axial load surface of the cutting tool, wherein a composition of the hardfacing overlay proximate an outside surface of the hardfacing overlay is different from a composition of the hardfacing overlay proximate an interface between the hardfacing overlay and the at least a portion of the surface of the cutting tool. 
   In yet another aspect, the present invention relates to a method for applying hardfacing on a cutting tool that includes forming a hardfacing overlay on at least a portion of at least one of a radial and axial load surface of the cutting tool such that a composition of the hardfacing overlay proximate an outside surface is different from a composition of the hardfacing overlay proximate an interface between the hardfacing overlay and a surface of the cutting tool. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows an example of a conventional milled tooth drill bit. 
       FIG. 2  shows a partial cross sectional view of a leg of a conventional drill bit, illustrating the interface between a journal pin and a roller cone. 
       FIG. 3  shows a partial cross sectional view of a leg of a conventional air-cooled drill bit. 
       FIG. 4  is an end view taken through  4 - 4  of  FIG. 3  illustrating the air fluid passages formed in the leg and journal bearing. 
       FIG. 5  shows a partial cross sectional view of a leg of a drill bit having a hardfacing overlay in accordance with one embodiment of the invention. 
       FIG. 6A and 6B  show a journal bearing surface having a hardfacing overlay in accordance with one embodiment of the invention. 
       FIG. 7  shows a schematic of a prior art automatic hardfacing system. 
       FIG. 8  shows a milled tooth having a hardfacing overlay in accordance with one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention relate to methods for providing hardfacing to surfaces of a metal part that are likely subjected to wear in a graded manner such that the compositions of the hardfacing materials vary as a function of distance from the interface between the hardfacing overlay and the metal object. Some embodiments of the invention relate to metal objects that include graded hardfacing overlays. Being able to generate graded hardfacing overlays on a metal object makes it possible to design wear protection based on selected applications. In accordance with some embodiments of the invention, the hardfacing near the interface may have a composition designed for enhanced bonding to the metal object, while the compositions near the wear surface of the hardfacing overlay may be designed to be more wear resistant. 
   In a particular embodiment, the hardfacing overlay disclosed herein is provided to a bearing surface of a drill bit.  FIG. 5  is a perspective view of a single leg  105  of an open-bearing air roller-cone bit in accordance with one embodiment of this invention. The lower end of leg  105  extended into a journal bearing shaft  111 . Each journal bearing shaft  111  supports a roller cone  113 . The end face  139  of journal bearing shaft  11  extends into the cone cavity adjacent cylindrical surface  141 . The cone  113  is held on the journal bearing shaft  111  by ball elements  115  in this embodiment. A ball passage  117  extends from an outer surface of leg  105  and intersects the upper section of bearing shaft  111 . The ball elements  115  are inserted through the ball passage  117  into the aligned ball grooves  119  once the cone  113  has been placed over the journal bearing shaft  111 . A ball plug  121  then fills the ball passage  117  to retain the ball elements  115  in the grooves  119 . Retaining rings and other retaining systems are common in the field and are also compatible with this invention. 
   Each leg  105  of the bit has a main air passage  123  that leads through the leg  105  to the ball passage  117 . A bearing shaft air passage  127  leads from the ball passage  117  to the end of the journal bearing shaft  111 . Cylindrical roller bearings  131  are located around the journal bearing shaft  111  to reduce the friction between the journal bearing shaft  111  and the cone  113 . The roller bearings  131  are between the journal bearing shaft roller bearing grooves  133  and the aligned cone roller bearing grooves  135 . A thrust bearing  137  may be included at the end of the journal bearing shaft  111  to handle axial loads. These bearings  131 ,  137  are cooled by the compressed air provided from the surface. 
   In one embodiment, a graded hardfacing may be provided on end face (axial bearing surface)  139  of journal bearing shaft  111 , which is subjected to axial loads. In another embodiment, a graded hardfacing may be provided on other bearing surfaces, including for example, journal bearing shaft roller bearing grooves (radial bearing surface)  133  of journal bearing shaft, which is subjected to radial loads. In yet another embodiment, a graded hardfacing may be included on similar, corresponding bearing surfaces of a greased drill bit, or open bearing bits cooled by water, which do not contain air passages. 
   Referring to  FIG. 6A and 6B , an journal bearing assembly of a drill bit according to one embodiment of the present invention is shown. Journal bearing assembly  60  includes journal bearing shaft  62 , cylindrical roller bearings  63  are located around the journal bearing shaft  62 , and ball elements  65  located around the ball groove  64  formed in journal bearing shaft  62 . A hardfacing deposit/overlay  66  is formed in groove  67  of the axial bearing surface  68  of journal bearing shaft  62 . Hardfacing deposit/overlay is a graded hardfacing overlay, where the composition of the hardfacing overlay proximate an outside surface  66   a  of the hardfacing overlay is different from a composition of the hardfacing overlay proximate an interface  66   b  between the hardfacing overlay and groove  67  in the axial bearing surface  68 . 
   Hardfacing materials typically comprise a metal or alloy matrix and wear-resistant particles (e.g., tungsten carbides or boron nitrides). Hardfacing compositions comprising carbides are more common than boride or nitride-containing hardfacing compositions. For clarity, this description may use “carbides” (e.g., tungsten carbides, other metal carbides, or mixtures thereof) to represent general wear-resistant particles. One of ordinary skill in the art would appreciate that “carbide” particles in the hardfacing compositions may be replaced with other wear-resistant particles (e.g., borides, nitrides, carbides or mixtures) without departing from the scope of the invention. In a hardfacing overlay, the wear resistant particles are suspended in a matrix of metal. The wear resistant particles give the hardfacing overlay hardness and wear resistance, while the matrix metal (or alloy) provides fracture toughness to the hardfacing overlay. In addition, the matrix metal also contributes to the bonding between the hardfacing overlay and the metal object (thrust faces, bearing surfaces, roller cones, or cutters). 
   In accordance with some embodiments of the invention, hardfacing compositions variations may have lower total alloy content near the interface with the metal object, but with higher alloy contents near the wear surface. Examples of suitable alloys include the Stellite™ family of alloys sold by Deloro Stellite Co. (Goshen, Ind.). Stellite™ alloys contain cobalt, tungsten, chromium, carbon, and are known for their wear resistance and corrosion resistance at high temperatures. Alternatively, compositions of the invention may comprise a low carbide content Stellite™ near the interface between the hardfacing overlay and the metal object and change to a Stellite™ alloy with higher carbide content near the wear surface. Other examples of graded hardfacing may include a composition having varying proportions of cast and/or sintered carbide pellets in a Ni—Cr—Si matrix varying as a function of distance from the interface. The above examples of hardfacing overlays having variations in hardfacing compositions are for illustration only. One of ordinary skill in the art would appreciate that many other variations are possible without departing from the scope of the invention. 
   In addition, being able to have graded hardfacing makes it possible to match the thermal expansion coefficients and/or elastic modulus of the hardfacing material at or near the interface with those properties of the metal subject. For example, Stellite™ with lower alloy contents may have a better match of thermal expansion and modulus to the steel of the underlying metal article. With better matched thermal expansion coefficients and/or elastic modulus, the article will have less residual stress. 
   Many factors affect the durability of a hardfacing overlay on a metal object. These factors, for example, include wear resistance of the hardfacing overlay and the strength of the bonding between the hardfacing overlay and the surface of the metal object. These factors are functions of the compositions of the hardfacing materials, i.e., the material compositions and physical structure (size and shape) of the wear resistant particles, the chemical composition and microstructure of the metal or alloy, and the relative proportions of the carbides to the matrix metal or alloy. While higher proportions of the wear-resistant particles (carbide or boron nitride particles) will increase the wear resistance of the hardfacing overlay, unfortunately they decreases the fracture toughness of the hardfacing overlay and weaken the bonding between the hardfacing overlay and the metal object. On the other hand, increasing the proportions of the matrix metal can increase the fracture toughness of the hardfacing overlay and enhance the bonding between the hardfacing overlay and the metal object; however, these benefits come at the expense of the wear resistance of the hardfacing overlay. As a result, prior art hardfacing application often represents a compromise between wear resistance and fracture toughness. 
   In accordance with embodiments of the invention, hardfacing overlay on a metal object, such as thrust bearings, drill bits, roller cones, or cutters, may have an enhanced wear resistance without sacrificing fracture toughness or bonding strength between the hardfacing overlay and the metal object, or have an enhanced bonding between the hardfacing overlay and the metal object without sacrificing the wear resistance of the hardfacing overlay. In some embodiments of the invention, a hardfacing overlay may have both an enhanced wear resistance and an increased bonding to the surface of the metal object. 
   Embodiments of the invention are based on “graded” hardfacing, which has different compositions in regions close to the wear surface (i.e., outside surface) of the hardfacing overlay, as compared to regions close to the interface between the hardfacing overlay and the metal object. As used herein, “graded hardfacing” generally refers to hardfacing overlays having different compositions in regions close to the wear surface, as compared to regions close to the interface. For clarity of description, the “graded hardfacing” may be referred to as having composition variations as a function of distance from the interface between the hardfacing overlay and the metal object. However, one of ordinary skill in the area would appreciate that such variations in the hardfacing compositions may also be referenced to the wear surface or outside surface of the hardfacing overlay, or the like. In accordance with embodiments of the invention, the composition differences as a function of the distance from the interface may be gradual or stepwise. The gradual variations of the compositions may be linear or non-linear (e.g., a monotonic curve). The composition differences may be achieved during the hardfacing application process. 
   Embodiments of the invention may use any suitable hardfacing technique(s) known in the art to achieve hardfacing composition variations. Prior art methods that may be used with embodiments of the invention, for example, may include atomic hydrogen welding, oxyacetylene welding, plasma transfer arc (“PTA”), pulsed plasma transfer arc (“PPTA”), gas tungsten arc, shielded metal arc process, laser cladding, or the like. 
   Welding is among the oldest methods for application of hardfacing onto a rock bit. In a typical application, a welding tube is melted by an oxyacetylene or atomic hydrogen welding torch onto the surface of the metal object that is to be protected (e.g., a cutter, roller cone, or drill bit). The welding tube comprises a filler enclosed in a steel (or other alloy) tube, in which the filler mainly comprises carbide particles (or borides or nitrides) but may also comprise deoxidizer for steel, flux, or a resin binder. When melted, the steel (or other alloy) suspends the carbide particles in the hardfacing overlay and also helps to bond the hardfacing layer to the metal object. This steel (or other alloy) may be generally referred to as “matrix metal” or “binder alloy.” In typical applications, the proportions of the filler to the steel tube may be adjusted by controlling the diameter and/or the thickness of the steel tube. 
   In accordance with some embodiments of the invention, the diameter and/or thickness of the steel welding tube may be varied (either gradually or stepwise) to provide different proportions of the carbide (or borides or nitrides) particles to the binder alloy. For example, the starting end of the welding tube may have a thicker wall and/or a smaller inside diameter, as compared to the other end of the welding tube, to provide a composition having a higher proportion of the binder alloy in the beginning. In accordance with other embodiments of the invention, a welding tube may have substantially the same wall thickness and/or inside diameter along its length; however, the filler therein may have different compositions (e.g., different proportions of carbide particles to binder alloy powder) along the length of the welding tube. In accordance with some embodiments of the invention, a welding rod as disclosed in U.S. Pat. No. 5,501,112 issued to Keshavan et al. may be used instead of a welding tube. This patent is assigned to the assignee of the present invention and is incorporated by reference in its entirety. 
   Some embodiments of the invention use laser cladding or plasma transferred arc to deposit hardfacing on the metal object. Examples of the use of laser cladding in applying hardfacing to drill bits may be found in U.S. Pat. No. 4,781,770 issued to Kar. Examples of plasma transfer arc (PTA) techniques may be found in U.S. Pat. No. 6,615,936 issued to Mourik et al., while examples of pulsed plasma transferred arc (PPTA) may be found in U.S. Pat. No. 6,124,564 issued to Sue et al. These patents are assigned to the assignee of the present invention and are incorporated by reference in their entireties. With these techniques, energy beams, i.e., laser or plasma transferred arc, may be directed to a hardfacing composition to melt the hardfacing composition onto the metal object. In accordance with embodiments of the invention, the compositions (e.g., the proportions of the carbides to the binder alloy) of the hardfacing compositions (repeated) may be varied to produce graded hardfacing. The variation in the hardfacing compositions may be gradual or stepwise depending on the desired effects. 
   With laser cladding or plasma transferred arc techniques, the hardfacing compositions are often fed in a powder form. When using powder injection, a mixture of carbide particles (or boride or nitride particles) and a metal matrix powder may be injected into a plasma stream or an arc. In accordance with embodiments of the invention, the hardfacing mixtures used have varying compositions. The varying compositions may be achieved, for example, by gradually or stepwise addition of one of the components (either the carbide particles or the metal matrix powder) into an initial composition, which may comprise a mixture or a single component. Alternatively, the carbide particles and the metal matrix powder may be separately injected using separate powder feeders. With this approach, the rates of the separate powder feeders may be controlled to give the desired variations in the compositions. Powder may be fed through the interior or the exterior of the torch, arc or plasma. With multiple powder feeders, some powders may be used to feed inside the torch, arc or plasma, while the remaining powders may be fed outside the torch, arc or plasma. 
   Another method of feeding a hardfacing composition is by use of a wire or a rod, as disclosed in U.S. Pat. No. 5,501,112 issued to Keshavan et al. The wire or rod may be made of a hardfacing composition (i.e., a mixture). Alternatively, the wire may be made of a matrix metal, and the outside of the wire is coated with the carbide particles, or vice versa. In accordance with embodiments of the invention, the compositions of the wires or rods are varied along the length so that the finished hardfacing overlay will have graded compositions. In some embodiments, multiple wires or rods may be used to achieve the variations in the hardfacing compositions. When multiple wires or rods are used, the variation in the hardfacing compositions may be achieved by different rates of feeding separate wires or rods, each of which may comprise a different component or composition, or by using wires or rods having different compositions along their lengths. The wires or rods may be fed inside or outside a hardfacing torch, arc or plasma. 
   Any apparatus adapted to apply hardfacing known in the art may be used with embodiments of the invention. For example, the automated hardfacing system disclosed in U.S. Pat. No. 6,392,190 issued to Sue et al. may be used with methods of the invention. This patent is assigned to the assignee of the present invention and is incorporated by reference in its entirety.  FIG. 7  shows an automatic hardfacing system disclosed in this patent, which includes a computer-controlled robotic arm  72  for positioning a plasma transferred arc welding apparatus  74  (or other welding apparatus). The automatic system  70  can also control the hardfacing powder flow rates to produce the desired hardfacing overlay. This system can also feed multiple wires/rods or vary the wire/rod feeding speeds. 
   A method in accordance with embodiments of the invention may include a step of determining the pattern of hardfacing composition variations desired for the metal object (e.g., a thrust surface, bearing surface, a roller cone or a cutting element) and then applying the hardfacing material according to the desired variations. For example, the composition variations may be gradual or stepwise. The composition variations may produce more binder alloy near the interface as compared to the wear surface. Once the variation pattern is determined, a hardfacing overlay may be deposited onto the metal object according to the pattern of composition variation. To achieve the desired pattern, appropriate hardfacing compositions are used. The forming of the hardfacing overlay may use any techniques known in the art. 
   Embodiments of the present invention may also find use in any downhole cutting application in which there exists metal-to-metal contact that may result in wear failure. Further, while the present disclosure refers to components of a drill bit, it is expressly within the scope of the present invention, that the graded hardfacing overlays disclosed herein may be used in other downhole cutting tools including, for example, reamers, continuous miners, or other components of drill bits. One of skill in the art would recognize that cutting tools that may be provided with the graded hardfacing disclosed herein are not necessarily limited to tools using in oil and gas exploration, but rather include all types of cutting tools used in drilling and mining. For example, some embodiments of the invention relate to cutting elements, roller cones or drill bits having graded hardfacing overlays.  FIG. 8  shows an exemplary cutter  80  having a steel body  82  and a hardfacing overlay  84 . The hardfacing overlay  84  has a composition near the interface  86  that is different from a composition near the wear surface  88 . For example, the composition near the wear surface  88  may be rich in carbides, while the composition near the interface  86  may be rich in matrix metal. Additionally, a graded hardfacing may be applied to cutting elements such as those described in the U.S. Patent Application entitled, “Assymetrical Graded Composite for Improved Drill Bits,” filed concurrently herewith, which is herein incorporated by reference in its entirety. 
   While this example shows a gradual variation of the hardfacing compositions, other embodiments of the invention may have stepwise variations in the hardfacing compositions. Furthermore, while a cutting element is shown for illustration, other embodiments of the invention may include other drill bit components or other cutting toolds having graded hardfacing. 
   Advantageously, embodiments of the present invention provide methods for producing bearing surfaces of drill bits having graded hardfacing overlays. In addition, methods of the invention can provide components of cutting tools and/or drill bits that include graded hardfacing overlays. An axial bearing surface having graded hardfacing overlays may allow for a hardfacing that provides both increased wear resistance and fracture toughness and/or increased bonding of the hardfacing overlays to the steel journal bearing. Methods of the invention permit the use of lower cost material near the interface between the hardfacing overlay and the metal object, reducing the cost of the hardfacing products. Being able to form graded hardfacing overlays makes it possible to tailor the coated substrate to the desired properties, such as enhanced wear resistance, and/or allow for enhanced bonding to the metal object, extending the life of the metal substrate. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.