TRICONE MINING ROCKBITS

BACKGROUND OF INVENTION

Field of the Invention

[0001] Embodiments disclosed herein relate generally to an improved rotary mining bit for boring a borehole in an earthen formation. In particular, the present disclosure relates to an improved weld overlay material for use in mining bits or other earth boring cutting tools.

Background Art

[0002] In mining a borehole for minerals or in search of oil or gas, earth-boring drill bits (or rock bits) are commonly used. Typically, an earth-boring drill bit is mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface. With weight applied to the drill string, the rotating drill bit engages an earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.

[0003] The borehole environment in which a rotary rock bit is operated is extremely harsh and may decrease the life of the bit. Pressurized air, although there is usually some water present, may be circulated downwardly through passages in the bit to the load bearing areas to cool or lubricate or otherwise condition the bearings and also through jetting ports in the bit to remove cuttings from the borehole.

[0004] Rotary bits typically include three cone-shaped members adapted to connect to the lower end of a drill string. Roller cone bits include one or more roller cones rotatably mounted on steel journals or pins integral with the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g. tungsten carbide insert bit, TCI) and "milled tooth" bits. The bit bodies and roller cones of roller cone bits are conventionally made of steel. In a milled tooth bit, the cutting elements or teeth are steel and conventionally integrally formed with the cone. In an insert or TCI bit, the cutting elements or inserts are conventionally formed from tungsten carbide, and may optionally include a diamond enhanced tip thereon. 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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. Additionally, 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.

[0009] 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 a spindle 34 extending from the leg 33 that forms bearing races 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. The spindle 34 forms a radially disposed main bearing face 39 from which a spindle bearing 40 extends. A spindle thrust bearing disc, or "thrust plug," generally designated as 41, is pressed into a leg bearing cavity or socket 42 formed in spindle bearing 40. Cone 38 includes an internal cavity adapted to receive the spindle 34 and the bearings 35, 36, and 37. The cone cavity includes cylindrical surfaces 43 and 44, ball bearing race 37a, 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 cone bearing cavity or socket 45 formed in the cone 38. As discussed in greater detail below, the cone thrust bearing disc 47 engages the spindle thrust bearing disc 41, with the interface therebetween forming a thrust bearing.

[0010] Referring 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 plug 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 plug 41. Air contacting base (not shown) of thrust button 41 serves to cool thrust plug 41 and adjacent cone thrust button 47.

[0011] 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.

[0012] 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 to 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.

[0013] 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 external surfaces of the bit body, cutting elements, and the roller cones on roller cone bits. These exterior parts 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.

[0014] Various hardfacing materials and methods are known in the art for minimizing wear on various parts of a drill bit. For example, U.S. Patent Nos. 4,836,307 issued to Keshavan et al., and 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.

[0015] Materials used in combination with the hardened steel surfaces in bit journal bearings 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 pressures and velocities (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 ⁰ F. Because such high surface temperatures are not uncommon in bit thrust bearings, especially as drilling speeds have increased, if included on bit thrust surfaces, stellites have been the structural inlay material of choice for journal surfaces.

[0016] 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. [0017] Although the prior art hardfacing application techniques are capable of providing improved wear resistance to rock bits, there still exists a need for other techniques that can provide longer lasting rock bits.

SUMMARY OF INVENTION

[0018] In one aspect, embodiments disclosed herein relate to a rock bit, comprising 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, wherein each journal bearing has at least one radial bearing surface and at least one axial bearing surface; at least one roller cone mounted on each journal bearing, wherein each roller cone has at least one radial bearing surface and at least one axial bearing surface; wherein the radial and axial bearing surfaces of each roller cone are substantially mating with the radial and axial bearing surfaces of each journal bearing; at least one cutting element disposed on the at least one roller cone; and a wear resistant weld overlay on at least a portion of the bearing surfaces of the at least one roller cone, wherein the wear resistant weld overlay comprises a steel nanocrystalline material having at least one metal borocarbide precipitant dispersed therein.

[0019] In another aspect, embodiments disclosed herein relate to a rock bit, comprising a rock bit, comprising 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, wherein each journal bearing has at least one radial bearing surface and at least one axial bearing surface; at least one roller cone mounted on each journal bearing, wherein each roller cone has at least one radial bearing surface and at least one axial bearing surface; wherein the radial and axial bearing surfaces of each roller cone are substantially mating with the radial and axial bearing surfaces of each journal bearing; at least one cutting element disposed on the at least one roller cone; and a wear resistant weld overlay on at least a portion of the bearing surfaces of the at least one roller cone, wherein the wear resistant weld overlay comprises a steel nanocrystalline material having a hardness of at least 60 HRc. [0020] In another aspect, embodiments disclosed herein relate to a rock bit, comprising a rock bit, comprising 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, wherein each journal bearing has at least one radial bearing surface and at least one axial bearing surface; at least one roller cone mounted on each journal bearing, wherein each roller cone has at least one radial bearing surface and at least one axial bearing surface; wherein the radial and axial bearing surfaces of each roller cone are substantially mating with the radial and axial bearing surfaces of each journal bearing; at least one cutting element disposed on the at least one roller cone; and a wear resistant weld overlay on at least a portion of the bearing surfaces of the at least one roller cone, wherein the wear resistant weld overlay comprises a steel nanocrystalline material having a surface roughness of about 3.2 μm or better (Ra).

[0021] In yet another aspect, embodiments disclosed herein relate to a rock 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, wherein each journal bearing has at least one radial bearing surface, at least one axial bearing surface, and a journal thrust bearing disc; at least one roller cone mounted on each journal bearing, wherein each roller cone has at least one radial bearing surface, at least one axial bearing surface, and a cone thrust bearing disc; wherein the radial and axial bearing surfaces of each roller cone are substantially mating with the radial and axial bearing surfaces of each journal bearing; and wherein the cone thrust bearing disc is substantially mating with the journal thrust bearing disc; at least one cutting element disposed on the at least one roller cone; and wherein at least one of the journal thrust bearing disc and the cone thrust bearing disc comprises: a substrate; and a wear resistant weld overlay on the substrate, wherein the wear resistant weld overlay comprises a steel nanocrystalline material having at least one metal borocarbide precipitant dispersed therein.

[0022] Other aspects and advantages of the invention will be apparent from the following description and the appended claims. BRIEF DESCRIPTION OF DRAWINGS [0023] FIG. 1 shows an example of a conventional milled tooth rock bit.

[0024] FIG. 2 shows a partial cross sectional view of a leg of a conventional rock bit, illustrating the interface between a journal pin and a roller cone.

[0025] FIG. 3 shows a partial cross sectional view of a leg of a conventional air- cooled rock bit.

[0026] 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.

[0027] FIG. 5 shows a partial cross sectional view of a leg of a rock bit having a wear resistant weld overlay on the cone thrust bearing in accordance with one embodiment of the present disclsoure.

[0028] FIG. 6 shows a thrust bearing disc according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

[0029] In one aspect, embodiments disclosed herein relate generally to mining rock bits having a superior wear resistant weld overlay provided on at least an axial bearing surface of the cone. Specifically, providing a wear resistant weld overlay that has improved hot hardness properties and higher wear resistance in comparison with conventional techniques may minimize the wear to the cone and thus extend the life of the bit. Such a weld overlay, in accordance with the embodiments of the present disclosure, which is applied to axial cone thrust bearing surfaces, may include a steel nanocrystalline material, i.e., a steel material having nanocrystalline grain sizes.

[0030] "Steel," as used herein, refers to any iron-based alloy in which no other single element (besides iron) is present in excess of 30 weight percent, and for which the iron content amounts to at least 50 weight percent. Steel generally includes regular arrangements of atoms, with the periodic stacking arrangements forming 3- dimensional lattices which define the internal structure of the steel. The internal structure, also referred to as the microstructure, of conventional steel alloys is always metallic and poly crystalline (consisting of many crystalline grains). Typically, an increase in hardness can be accompanied by a corresponding decrease in toughness.

[0031] Steel is typically formed by cooling a molten alloy. For steel alloys, the rate of cooling will determine whether the alloy cools to form an internal structure that predominately comprises crystalline grains, or, in rare cases a structure which is predominately amorphous (a so-called metallic glass). Generally, it is found that if the cooling proceeds slowly (i.e. at a rate less that about 10 ⁴ K/s), large grain sizes occur, while if the cooling proceeds rapidly (i.e. at rate greater than or equal to about 10 ⁴ K/s, and preferably between 10 ⁴ and 10 ⁵ K/s) microcrystalline (or nanocrystalline) internal grain structures are formed (depending on the composition).

[0032] Some materials may be referred to in the art as metallic glass or an amorphous metallic material. An amorphous material generally has no long-range order of the positions of the atoms. If the cooling rate is faster than the rate at which molecules can organize into a more thermodynamically favorable crystalline state, then an amorphous solid will be formed. However, devitrification of an amorphous material may result in a crystalline (or morphous) steel material having a nanocrystalline grain size. Such devitrification can be accomplished by heating the metallic glass to a temperature of from about 450 to 700 ⁰ C. Such heating enables a solid state phase change wherein the amorphous phase of a metallic glass is converted to one or more crystalline (or morphous) solid phases. The solid state devitrification of an amorphous enables uniform nucleation to occur throughout the metallic glass to form nanocrystalline grains within the glass. The metal matrix microstructure formed via the devitrification may comprise a steel matrix (iron with dissolved interstitials), with an intimate mixture of ceramic precipitates (transition metal carbides, borides, suicides, etc.). The nanocrystalline scale metal matrix composite grain structure may enable a combination of mechanical properties which are improved compared to the properties which would exist with larger grain sizes or with the metallic glass. Such improved mechanical properties may include, for example, high strength, and high hardness coupled with maintained or improved ductility or toughness. However, it is also within the scope of the present disclosure that such nanocrystalline microstructure (a steel matrix (iron with dissolved interstitials) with an intimate mixture of ceramic precipitates (transition metal carbides, borides, suicides, etc.)) may result without devitrification.

[0033] Desired properties of microcrystalline grains (/. e. , grains having a size on the order of 10 ^"6 meters) can frequently be improved by reducing the grain size to that of nanocrystalline grains (i.e., grains having a size on the order of 10 ^"9 meters). Thus, the steel nanocomposites used as a weld overlay in accordance with the present disclosure include a metallic material having a microstructure with a crystalline grain less than about 10 microns.

[0034] Thus, the steel nano-crystalline materials may be iron based alloys, such as those marketed under the name Superhard Steel Alloys™, available from The Nanosteel™ Company as well as a derivative of such a metallic glass-forming, iron alloy. Additionally, the weld overlay may include other alloys based on iron, or other metals, that are susceptible to forming metallic glass materials at critical cooling rates less than about 10 ⁵ K/s. Accordingly, the alloy may solidify before significant growth of crystalline domains, thereby producing a nano-crystalline microstructure. The nanocrystalline scale metal matrix composite grain structure may advantageously enable a combination of mechanical properties that are improved compared to the properties which would exist with larger grain sizes or a metallic glass. Such improved mechanical properties may include, for example, high strength and high hardness, as well as a maintained or even increased toughness relative to materials comprising larger grain sizes or a metallic glass. Steel nanocrystalline materials that may be used as the weld overlay in embodiments of the present disclosure may include those described in U.S. Patent Nos. 6,689,234 and 6,767,419, and U.S. Patent Publication Nos. 2008/0053274, 2007/0029295, and 2005/0252586, all of which are herein incorporated by reference in their entirety.

[0035] An exemplary alloy may include a steel composition, comprising at least 50% iron and at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, and the class of elements called rare earths including Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and at least one element selected from the group consisting of B, C, N, O, P and S. An exemplary mixture may comprise at least 55% iron, by weight, and comprise at least one element selected from the group consisting of B, C, Si and P. In particular embodiments, the mixture may comprise at least two of B, C and Si.

[0036] In other particular embodiments, the steel nanocrystalline material may include Cr, Mo, Nb, W, Al, B, C, Mn, Si, Fe, and combinations thereof. In a preferred embodiment, the steel nanocrystalline material may include up to about 20 weight percent Cr, up to about 10 weight percent Mo, up to about 10 weight percent Nb, up to about 10 weight percent W, up to about 5 weight percent Al, up to about 5 weight percent B, up to about 5 weight percent C, up to about 5 weight percent Mn, up to about 2 weight percent Si, and a balance of Fe. In another preferred embodiment, the weld overlay may include up to about 18 weight percent Cr, up to about 6 weight percent B, up to about 5 weight percent Al, up to about 5 weight percent Nb, up to about 2 weight percent C, up to about 2 weight percent Mn, up to about 2 percent Si, and a balance of Fe.

[0037] Additionally, as mentioned above, at least one transition metal carbides, borides, suicides, or borocarbide may precipitate out of the matrix during cooling of the alloy. Additionally, depending on the type of precipitant, it is hypothesized that some precipitants may also function as a grain growth inhibitor, to inhibit grain growth during cooling of the alloy. Such uniform and fine distribution of precipitants within the steels may result in the weld overlay having a smooth finished surface. In a particular embodiment, the weld overlay may have a surface roughness of 3.2 μm or better (Ra), and more preferably of 1.6 μm or better, and even more preferably of 0.8 μm or better.

[0038] Additionally or alternatively, the wear resistant weld overlay may have a porosity of less than about 5%, and more preferably less than 1%. Application techniques such as those described below may allow for the application of the wear resistant weld overlay to be very low in porosity and very high in bond strength. A low porosity (i.e., less than 1%) may be advantageous because it may reduce or prevent the pores from becoming interconnected, as is seen at higher porosity levels and which may behave similarly to a crack in the overlay. Further, interconnected pores may not be desirable particularly in sliding wear environments as they may facilitate a higher rate of material removal and thus a higher rate of wear on the overlay. [0039] Conventional rock bits typically have a hardened or tempered steel on the cone thrust bearing surfaces and a STELLITE™ alloy weld overlay on the journal thrust bearing surfaces. STELLITE™ alloys (sold by Deloro Stellite Co. (Goshen, IN)) contain cobalt, tungsten, chromium, and carbon, and are known for their wear resistance and corrosion resistance at high temperatures. During drilling operations, the hardened or tempered steel on the cone rides against the Stellite™ alloy weld overlay on the leg, resulting in significant wear to the cone thrust surface due to the lower hot hardness value of a conventionally heat treated cone steel relative to that of the Stellite™ alloy. Hot hardness refers to the ability of a material to retain hardness and wear resistance at high temperatures. The severe wear on the cone thrust face may limit the life of a rock bit by premature thrust bearing failure while the radial bearing and cutting structures remain intact.

[0040] Thus, in accordance with embodiments of the present disclosure, a wear resistant steel nanocrystalline weld overlay may be provided to various thrust bearing surfaces of the cone to improve the wear resistance and hardness of the cone surface at elevated temperatures. Improved hot hardness may reduce or prevent deformation and/or degradation of the weld overlay when it is subjected to high temperatures, for example, during its application to surfaces of the cone as well as mining operations, when frictional heat is generated at the thrust bearing surfaces. Additionally, during application of the steel nanocrystalline material as a weld overlay on a cone thrust bearing surface, it may be desirable to pre-heat the cone prior to application. Specifically, in one embodiment, the cone may be pre-heated from about 500 to 850°F, and more preferably to about ~600°F, to facilitate application of the weld overlay to the desired thrust bearing surfaces of the cone, as well as to minimize the thermal stress related cracking which typically occurs in the microstructure of the overlay. Furthermore, improved hot hardness may prevent deformation and/or degradation of the weld overlay during mining operations, thus extending the life of the surfaces to which it is applied. In a particular embodiment, the hot hardness of the weld overlay may decrease by less than ~20% when the temperature is increased from ~73°F to ~932°F. In a particular embodiment, the hot hardness may decrease as little as ~17% (from 70 HRc to 59 HRc) when the temperature is increased from ~73°F to ~932°F, or more preferably as little as -11% (from 72 HRc to 64 HRc) when the temperature is increased by the same amount, compared to Stellite™ alloys which may have a greater decrease (greater than 20%) in hot hardness, for example by as much as 26%, or compared to carburized (or otherwise heat treated) steel which would have an even greater decrease in hot hardness.

[0041] In some embodiments, it may be desirable to use the nanocrystalline steel material as a metallic phase that is used in combination with a hard component phase, i.e., hard particles dispersed in the metallic nanocrystalline steel material. In such an instance, the hard component materials may be selected from, for example, metal oxides, metal nitrides, metal borides, and other metal carbides (and alloys thereof), such as sintered tungsten carbide (e.g., WC-Co), monocrystalline tungsten carbide, macrocrystalline tungsten carbide, multicrystal or polycrystalline tungsten carbide, and, in some embodiments, the additional component of spherical cast tungsten carbide (e.g., a eutectic of WC-W2C), each of which may be crushed in form.

[0042] In one embodiment, the wear resistant weld overlay formed from the steel nanocrystalline material may have a hardness greater than that of the cone surface on which it is disposed as well as the journal bearing surface to which it is opposing. In other embodiments, the wear resistant weld overlay may have a hardness of greater than about 60 HRc; from about 60 to 75 HRc in another embodiment; and greater than about 60, 65, and 70 HRc in various other embodiments.

[0043] The wear resistant weld overlay disclosed herein may be applied to desired surfaces of the cone using one of several techniques known in the art, including oxyacetylene welding (OXY), atomic hydrogen welding (ATW), gas metal arc welding (GMAW), metal inert gas welding (MIG), gas tungsten arc welding (GTAW), tungsten inert gas welding (TIG), plasma transfer arc welding (PTAW), high velocity oxygen fuel (HVOF), twin wire arc spray (TWAS), laser cladding, or other applicable processes as known by one of ordinary skill in the art.

[0044] The wear resistant weld overlay may be disposed on any desired surface of the cone. The thickness of the wear resistant weld overlay may range from 1 to 3 mm in one embodiment. One of skill in the art would recognize the thickness need not be uniform across all surfaces of the cone; rather, it is within the scope of the present invention that the thickness may be varied to optimize performance. Additionally, during application of the wear resistant weld overlay, multiple layers of the wear resistant weld overlay may be applied to the desired surfaces. If multiple layers of a wear resistant weld overlay are provided, one of ordinary skill in the art would recognize that compositions and resulting properties may be varied across the multiple layers to promote bonding and adhesion of the wear resistant weld overlay to the desired cone surface.

[0045] FIG. 5 is a perspective view of a single leg 105 of an open-bearing air roller- cone bit having a weld overlay incorporated therein, in accordance with one embodiment of the present disclosure. The lower end of leg 105 extended into a journal bearing shaft 111. Each journal bearing shaft 111 supports a roller cone 113. The journal bearing shaft 111 extends into the cone cavity adjacent the cylindrical thrust bearing surface 146 of the cone 113. 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.

[0046] 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 (between a cone thrust bearing disc 47 and the spindle thrust bearing disc 41 pressed into cavities in the cone and leg, respectively) 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. [0047] In one embodiment, a wear resistant weld overlay, such as those described above, may be deposited on one or more bearing surfaces of the cone, the leg, or combinations thereof. For example, the wear resistant weld overlays may be deposited on one or more radial or axial bearing surfaces of the cone, leg, or combinations thereon. Specifically, as shown in FIG. 5, cone bearing surfaces 154, or corresponding leg bearing surfaces 153, which are subjected to axial loads, may be provided with a wear resistant overlay deposited thereon. The axial bearing surfaces that may be coated with the weld overlay of the present disclosure may include the primary and/or secondary thrust bearing. Further, while many bits, including the bit shown in FIG. 5, include a cone thrust bearing disc 47 and a spindle thrust bearing disc 41 to form the primary thrust bearing 137, it is also within the scope of the present disclosure that a weld overlay of the present disclosure may instead be used to fill the cavity (in the journal or cone) typically occupied the bearing discs 47, 41, thus replacing cone thrust bearing disc 47 or spindle thrust bearing disc 41. In yet another embodiment, shown in FIG. 6, a weld overlay 161 of the present disclosure may be applied to a bearing disc substrate 160 to form a layered bearing disc 162. Such layered bearing disc 162 may be used as either cone thrust bearing disc 47 or spindle thrust bearing disc 47. Further, when using such a layered bearing disc, there exists no limitation on the type of material suitable for use as the bearing disc substrate. For example, the substrate may be used from materials such as tungsten carbide, tool steel, etc. Use of such a layered approach may allow for the inversely related properties of hardness / wear resistance and toughness to be simultaneously achieved.

[0048] Further, it is also within the scope of the present disclosure that cone bearing surfaces 135, which are subjected to radial loads, may be provided with a wear resistant overlay, as may the leg bearing surface 133 corresponding to cone bearing surface 135. Thus, there exists no limitation on the number of type of internal bearing surfaces on which the wear resistant weld overlay of the present disclosure may be deposited. Further, while FIG. 5 illustrates an open bearing air roller cone bit, the weld resistant weld overlay may also be deposited on similar bearing surfaces on the leg or cone of a greased drill bit, or open bearing bits cooled by water, which do not contain air passages. [0049] Advantageously, embodiments of the present disclosure provide for a crystalline wear resistant weld overlay to be disposed upon an axial or radial bearing surface of a roller cone. An axial bearing surface having a wear resistant weld overlay may provide both increased wear resistance and fracture toughness and/or increased bonding of the wear resistant weld overlay to the metal cone.

[0050] 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.