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CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to roller cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to a cutter element having a bowed crest geometry which provides for a more uniform stress distribution.  
           [0005]    2. Background Art  
           [0006]    The success of rotary drilling enabled the discovery of deep oil and gas reserves. The roller cone rock bit was an important invention that made that success possible. The original roller-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, was able to drill the hard caprock at the Spindletop field, near Beaumont, Tex.  
           [0007]    That invention, within the first decade of the twentieth century, could drill a scant fraction of the depth and speed of modem rotary rock bits. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.  
           [0008]    Roller-cone rock bits typically are secured to a drill string, which is rotated from the surface. Drilling fluid or mud is pumped down the hollow drill string and out of the bit. The drilling mud cools and lubricates the bit as it rotates and carries cuttings generated by the bit to the surface.  
           [0009]    Roller-cone rock bits generally have at least one, and typically three roller cones rotatably mounted to a bearing on the bit body. The roller cones have cutters or cutting elements on them to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during drilling operation. These stresses cause the rock to fail, resulting in disintegration and penetration of the formation material being drilled.  
           [0010]    Operating in the harsh down hole environment, the components of roller-cone rock bits are subjected to many forms of wear. Among the most common forms of wear is abrasive wear caused by contact with abrasive rock formation materials. Moreover, the drilling mud, laden with rock chips or cuttings, is a very effective abrasive slurry.  
           [0011]    Many wear-resistant treatments are applied to the various components of the roller-cone rock bit. Among the most prevalent is the application of a welded-on wear-resistant material or “hardfacing.” This material can be applied to many surfaces of the rock bit, including the cutting elements.  
           [0012]    U.S. Pat. No. 4,262,761 discloses a milled steel tooth rotary rock bit wherein one or more holes are drilled into the crest of the tooth-shaped cutting structure. Tungsten carbide rods are positioned in the holes and hardfacing is applied to the tooth. The hardfacing is applied across the top of the tooth crest and acts to hold the tungsten carbide rods in place. The rods are inserted in holes parallel and close to one flank of the tooth so that the entire length of the carbide rods can be attached to the hardfacing by burning the hardfacing through to the carbide rods. Wear on the tooth will proceed along the side of the tooth not reinforced with the carbide rods and a self-sharpening effect is enhanced by the strength of the carbide rods. The carbide rods and holes therefore can be relatively inexpensive, since close tolerance finishing is not required.  
           [0013]    U.S. Pat. No. 5,152,194 discloses a milled tooth roller cone rock bit consisting of chisel crested milled teeth with generously radiused corners at the ends of the crest. A concave depression is formed in the crest between the radiused ends. A layer of hardfacing material formed over each tooth is thicker at the corners and in the concave depressions in the crest to provide a means to inhibit wear of the hardfacing as the bit works in a borehole.  
           [0014]    U.S. Pat. No. 5,311,958 discloses an earth-boring bit that is provided with three cutters, two of the three cutters are provided with heel disk cutting elements defined by a pair of generally oppositely facing disk surfaces that generally continuously converge to define a circumferential heel disk crest. One of the two cutters having heel disk elements is further provided with an inner disk A cutting element.  
           [0015]    U.S. Pat. No. 5,492,186 discloses an earth boring bit rotatable cutter having a first hardfacing composition of carbide particles selected from the class of cast and macrocrystalline tungsten carbide dispersed in a steel matrix deposited on the gage surface of at least some of the heel row teeth. A substantial portion of these particles are characterized by a high level of abrasion resistance and a lower level of fracture resistance. A second hardfacing composition of carbide particles selected from the class of spherical sintered and spherical cast tungsten is dispersed in a steel matrix deposited over at least the crest and an upper portion of the gage surface to cover the corner that tends to round during drilling. A substantial portion of the particles of this composition are characterized by a high level of fracture resistance and a lower level of abrasion resistance.  
           [0016]    U.S. Pat. No. 5,868,213 discloses a steel tooth, particularly suited for use in a rolling cone bit, includes a root region, a cutting tip spaced from the root region and a gage facing surface therebetween. The gage facing surface includes a knee, and is configured such that the cutting tip is maintained at a position off the gage curve. So positioned, the cutting tip is freed from having to perform any substantial cutting duty in the comer on the borehole comer, and instead may be configured and optimized for bottom hole cutting duty. The knee on the gage facing surface is configured and positioned so as to serve primarily to cut the borehole wall. It is preferred that the knee be positioned off gage, but that it be closer to the gage curve than the cutting tip.  
           [0017]    U.S. Pat. No. 6,206,115 discloses an earth-boring bit having a bit body with at least one earth disintegrating cutter mounted on it. The cutter is generally conically shaped and rotatably secured to the body. The cutter has a plurality of teeth formed on it. The teeth have underlying stubs of steel which are integrally formed with and protrude from the cutter. The stubs have flanks which incline toward each other and terminate in a top. A carburized layer is formed on the flanks and the top to a selected depth. The stub has a width across its top from one flank to the other that is less than twice the depth of the carburized layer. A layer of hardfacing is coated on the tops and flanks of the stub, forming an apex for the tooth.  
           [0018]    U.S. Pat. No. 6,241,034 discloses a cutter element for a drill bit. The cutter element has a base portion and an extending portion and the extending portion has either a zero draft or a negative draft with respect to the base portion. The non-positive draft allows more of the borehole bottom to be scraped using fewer cutter elements. The cutter elements having non-positive draft can be either tungsten carbide inserts or steel teeth.  
           [0019]    Referring now to FIG. 1, which illustrates a milled tooth roller cone rock bit generally designated as  10 . The bit  10  consists of bit body  12  threaded at pin end  14  and cutting end generally designated as  16 . Each leg  13  supports a rotary cone  18  rotatively retained on a journal, optionally cantilevered from each of the legs (not shown). The milled teeth generally designated as  20  extending from each of the cones  18  may be milled from steel. Each of the chisel crested teeth  20  forms a crest  24 , a base  22 , two flanks  27 , and tooth ends  29 .  
           [0020]    Hardfacing material may be applied to at least one or each of the teeth  20 . In one embodiment, the application of hardfacing is applied only to the cutting side of the tooth as opposed to the other flanks  27  and ends  29  of the teeth  20 . In another embodiment, the hardfacing may be applied to all the flanks  27  and ends  29  of the teeth  20 .  
           [0021]    The rock bit  10  may further include a fluid passage through pin  14  that communicates with a plenum chamber (not shown). In one embodiment, there are one or more nozzles  15  that are secured within body  12 . The nozzles direct fluid from plenum chamber (not shown) towards a borehole bottom. In another embodiment, the rock bit  10  has no nozzles  15 . In another embodiment, the upper portion of each of the legs may have a lubricant reservoir  19  to supply a lubricant to each of the rotary cones  18  through a lubrication channel (not shown).  
           [0022]    Turning now to the prior art of FIGS. 2A and 2B, conventional hardfaced chisel crested teeth generally designated as  40 , when they operate in a borehole for a period of time, wear on the corners  44  of the teeth. The prior art tooth consists of a crown or crest  41  having hardfacing material  42  across the crest and down the flanks  43  terminating near the base  45  of the tooth  40 .  
           [0023]    [0023]FIG. 2C shows the prior art tooth of FIG. 2A with a typical axial stress distribution. The prior art teeth ( 40 ) typically have a concave axial stress distribution ( 50 ) as shown in FIG. 2C.  
           [0024]    As heretofore stated the hardfacing material  42  transitioning from the crest  41  towards to the flanks  43  may be very thin at the comers of the conventional teeth  40 . Consequently, as the tooth wears, the hardfacing, since it may be very thin, may wear out quickly, and thus expose the underlying steel  47  of the tooth  40 . Consequently, erosion voids (not shown) could invade the base metal  47  since it is usually softer than hardfacing material  42 .  
         SUMMARY OF THE INVENTION  
         [0025]    One aspect of the invention is a drill bit comprising a bit body, at least one roller cone rotatably mounted on the bit body. The cone has a plurality of milled teeth at selected locations on the cone. At least one of the milled teeth comprises a substrate having a convex crest and a layer of hardfacing applied to said convex crest. The convex crest is adapted to produce at least one of a convex axial stress distribution, a substantially even axial stress distribution, and a substantially smooth axial stress distribution. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a perspective view of a milled tooth rotary cone rock bit with hardfacing material on each tooth;  
         [0027]    [0027]FIG. 2A is a cross-sectional prior art view of a tooth illustrating the crest and hardfacing of the tooth;  
         [0028]    [0028]FIG. 2B is a cross-sectional prior art view of a worn tooth illustrating destructive voids in the hardfacing and base metal material at the corners of the crest of the tooth;  
         [0029]    [0029]FIG. 2C is a cross-sectional prior art view of a tooth illustrating the axial stress distribution, crest, and hardfacing of the tooth;  
         [0030]    [0030]FIG. 3 is a cross-sectional view of an improved hardfaced chisel crested milled tooth;  
         [0031]    [0031]FIG. 4 is a diagrammatic cross-section of a tooth of a 9⅞ inch milled tooth rotary cone rock bit;  
         [0032]    [0032]FIG. 5 is a cross-sectional view of another configuration of an improved hardfaced milled tooth;  
         [0033]    [0033]FIG. 6 is a perspective view of a single chisel crested milled tooth with hardfacing in a thicker layer around rounded comers of the tooth adjacent the flank and end faces of the tooth;  
         [0034]    [0034]FIG. 7 is a cross-sectional view of the axial stress distribution of an improved hardfaced chisel crested milled tooth; and  
         [0035]    [0035]FIG. 8 is a cross-sectional view of the axial stress distribution of another configuration of an improved hardfaced milled tooth; 
     
    
     DETAILED DESCRIPTION  
       [0036]    Turning now to one embodiment illustrated in FIG. 3, the chisel tooth generally designated as  20  consists of, for example, a steel foundation  21 , forming flanks  27 , ends  29  and a crest  24 . Between rounded comers  26  is a convex portion  25  on the crest  24  of the tooth. The convex portion  25  enables hardfacing material  32  to be thicker at the comers  26  of the crest  24 , therefore providing for more durable cutting comers  26 . Each of the comers  26  has a sufficient radius so that the thickness of the hardfacing material is assured as it transitions from the crest  24  towards the ends  29  and the flanks  27  of the tooth  20 . The hardfacing material may terminate at the base  22  of each of the teeth  20 . The base  22  provides a termination point for the hardfacing material  32  as it is applied over the crest ends and flanks of each of the teeth  20 .  
         [0037]    By providing a convex portion  25  or rounded geometry and rounded comers  26  at the end of the crested tooth, the hardfacing material may be applied more generously at the comers  26  of the crest and at a sufficient thickness in the center of the crest to produce a generally flat crest  24 . The geometry at the comers  26  assures a thick application of hardfacing material at a vulnerable area of the tooth.  
         [0038]    One suitable hardfacing material and a method of its application is described in U.S. Pat. No. 4,836,307 to Keshavan et al and is incorporated herein by reference in its entirety.  
         [0039]    Referring now to the cross-sectional example of FIG. 4, a typical tooth  20  formed from a cone of a 9⅞ inch diameter milled tooth roller cone rock bit could, for example, have a tooth height “A” of about 0.5 to about 1.5 inches, in one embodiment, 0.72 inches, and a width “B” of about 0.5 to about 1.0 inches, in one embodiment, 0.62 inches across the chisel crown of the tooth  20 . The radius at the comers  26  may be between about 0.02 and about 0.20 inches, in one embodiment, about 0.08 inches. The convex radius  25  may be between about 0.15 and 1.0 inches, in one embodiment, 0.50 inches. The depth “C” of the convex radius may be between about 0.02 inches and about 0.20 inches, in one embodiment, about 0.05 inches.  
         [0040]    In one embodiment, the crest  24  of the tooth  20  may be substantially flat between radiused comers, the tooth having a varied hardfacing  32  thickness between radiused comers. In another embodiment, the crest  24  of the tooth  20  may be convex between radiused comers, the tooth having a constant hardfacing thickness between radiused comers. In another embodiment, the crest  24  of the tooth  20  may be convex between radiused comers, the tooth having a varied hardfacing  32  thickness between radiused corners, wherein the hardfacing  32  is thicker at the radiused comers.  
         [0041]    The hardfacing  32  may have a thickness along the ends  29 , flanks  27  and comers  26  between about 0.02 and about 0.18 inches, in one embodiment a thickness of about 0.10 inches.  
         [0042]    The thickness of the hardfacing at depth “D” and along the crest  24  may be between about 0.04 and about 0.18 inches, in one embodiment a depth of about 0.10 inches (with respect to the example of FIG. 3).  
         [0043]    [0043]FIG. 5 illustrates an alternative embodiment of the present invention wherein the chisel crest tooth generally designated as  120  forms a crest  124  that transitions into ends  129  and flanks  127 . Crest  124  forms a convex shape  125 , in one embodiment a bow, between corners  126  that allows a substantially uniform thickness of hardfacing material  132  across the crest  124 . The hardfacing material  132  can also maintain a relatively thick layer across the corners  126  and down the ends  129  and flanks  127  towards the cone  18  (shown in FIG. 1). One advantage may be to maintain a uniform axial stress profile across the crest  124 . Another advantage may be to provide a robust or thick hardfacing material across the flanks  124  and ends  126  such that the tooth as it operates in a borehole retains its integrity and sharpness as it works in a borehole.  
         [0044]    In another embodiment of the present invention (not shown), the chisel crest tooth, generally designated as  120  forms a crest  124  that transitions into ends  129  and flanks  127 . Crest  124  forms a convex shape  125 , in one embodiment a bow, between corners  126  that allows a gradually decreasing thickness of hardfacing material  132  across the crest  124 , so that the thickness of the hardfacing material  132  is thickest across the comers and less thick in the middle between the corners. The hardfacing material  132  can also maintain a relatively thick layer across the corners  126  and down the ends  129  and flanks  127  towards the cone  18  (shown in FIG. 1). One advantage may be to maintain a uniform axial stress profile across the crest  124 , or a convex stress profile across the crest  124 . Another advantage may be to provide a robust or thick hardfacing material across the flanks  124  and ends  126  such that the tooth as it operates in a borehole retains its integrity and sharpness as it works in a borehole.  
         [0045]    In another alternative embodiment, the flanks  127  and/or the ends  129  may have a depression or concave portion (not shown) whereby the hardfacing material is thicker at the concave portion thus providing a thicker area along the flanks  127  and/or the ends  129 . In another alternative embodiment, the flanks  127  and/or the ends  129  may have a convex portion (not shown) or a bow, whereby the hardfacing material is either the same thickness or thinner at the convex portion (not shown). Hardfacing may terminate at base  122  at each of the mill teeth  120 . A convex portion on the flanks  127  and/or the ends  129  may provide increased tooth strength due to the larger amount of tooth substrate material. A concave portion on the flanks  127  and/or the ends  129  may provide increased hardfacing thickness and increased tooth durability due to the larger amount of tooth hardfacing material.  
         [0046]    In another alternative embodiment, the tooth may have more than one convex portions, or bows, along the crest, the corners may be rounded in much the same manner as in FIGS. 3, 4, and  5  in order to assure a thickness at the corners of the tooth. In another alternative embodiment, the flanks and/or the ends may have a concave portion, a convex portion, or multiple concave and/or convex portions. Alternatively, the flanks and/or the ends may have a series of depressions to assure a robust layer of hardfacing along the ends and flanks. The hardfacing material may terminate on a groove or shoulder or recess at the base of the tooth.  
         [0047]    [0047]FIG. 6 illustrates a perspective view of one of the chisel crested teeth  320  wherein the comers  330  of the tooth are rounded, so that a minimum thickness of hardfacing material  332  is on the comer  330 , which forms the junctions between the ends  329  and flanks  327 . The steel foundation (not shown) is covered by the hardfacing material  332 . The top of the tooth  320  forms a crest  324 . In one embodiment, the crest  324  is convex, and in an alternative embodiment, the crest  324  is substantially flat. The hardfacing material  332  terminates at the base  322  of the tooth  320 . The base  322  provides a termination point for the hardfacing material  332  as it is applied over the crest ends  329  and flanks  327  of each of the teeth  320 . The hardfacing material  332  is applied with a sufficient thickness over the entire tooth to improve its integrity and durability.  
         [0048]    In an alternative embodiment, a milled tooth with a convex chisel crest converging at both radiused ends could be hardfaced. In one embodiment, the thickness of the hardfacing could remain substantially constant across the crest as illustrated by the specific example of FIG. 5. In another embodiment, the thickness of the hardfacing could vary across the crest as illustrated by the specific example of FIG. 3.  
         [0049]    In an alternative embodiment, a spherical or semi-spherical surface of a milled tooth could be hardfaced as long as the radiuses are within the general parameters set forth in FIG. 4, thereby assuring a minimum thickness of hardfacing and the enhanced durability of the tooth as it works in a borehole.  
         [0050]    In an embodiment such as shown in FIG. 6, each tooth  320 , after the hardfacing  332  is applied, will appear outwardly with relatively straight crest  324 , ends  329 , and flanks  327 , the hardfacing having a uniform termination point at the base  322  of the milled tooth  320 . In another embodiment, one or more of the crest  324 , ends  329 , and flanks  327  may have a rounded appearance.  
         [0051]    In one embodiment of the invention, as shown in FIG. 1, the teeth  20  have an axial crest  24 . Axial crests  24  are so called because the crest  24  generally is substantially aligned with the axis of rotation of the cone  18  that the tooth is located on. In an alternative embodiment, the teeth  20  may have a circumferential crest (not shown). Circumferential crests (not shown) are so called because the crest (not shown) generally is substantially oriented circumferentially about the cone  18  that the tooth is located on, or substantially aligned with a circumference of the cone  18  that the tooth is located on. A circumferential crest (not shown) would have different loading properties and stress distribution than an axial crest  24  because a circumferential crest has a rolling action with the rock formation downhole where only a portion of the crest interacts with the rock formation at one time, while for an axial crest  24 , substantially the entire crest penetrates the rock formation at the same time. In another embodiment of the invention (not shown), the teeth  20  have a crest  24  that is neither axial nor circumferential, but the crests  24  are substantially aligned with a line that is between the axis of rotation of the cone  18  that the tooth is located on and the circumference of the cone  18  that the tooth is located on. In another embodiment, the crests  24  are substantially aligned with a line that is within about 40° (in any direction) of the axis of rotation of the cone  18  that the tooth is located on. In another embodiment, the crests  24  are substantially aligned with a line that is within about 30° (in any direction) of the axis of rotation of the cone  18  that the tooth is located on. In another embodiment, the crests  24  are substantially aligned with a line that is within about 15° (in any direction) of the axis of rotation of the cone  18  that the tooth is located on.  
         [0052]    [0052]FIG. 7 shows an embodiment of the tooth of FIG. 3 with an axial stress distribution. The tooth ( 20 ) may have a convex axial stress distribution ( 52 ) as shown in FIG. 7. This convex axial stress distribution ( 52 ) provides a higher level of axial stress in the middle of the crest ( 24 ) than at the comers ( 26 ) of the tooth ( 20 ). Advantages of this convex axial stress distribution ( 52 ) may include aggressive penetration of the rock formation while drilling.  
         [0053]    [0053]FIG. 8 shows an embodiment of the tooth of FIG. 5 with an axial stress distribution. The tooth ( 120 ) may have a level axial stress distribution ( 54 ) as shown in FIG. 8. This level axial stress distribution ( 54 ) provides a substantially even level of axial stress in the middle of the crest ( 124 ) as compared to the level of axial stress at the comers ( 126 ) of the tooth ( 120 ). Advantages of this level axial stress distribution ( 54 ) may include favorable tooth wear at the corners ( 126 ).  
         [0054]    In one embodiment, shown in FIG. 7, the crest geometry is adapted and/or designed to produce a convex axial stress distribution. In another embodiment, shown in FIG. 8, the crest geometry is adapted and/or designed to produce a substantially even axial stress distribution. In another embodiment, the crest geometry is adapted and/or designed to gradually increase the thickness of the hardfacing on the crest in relation to the magnitude of the axial stress. In another embodiment, the crest geometry is adapted and/or designed to produce a substantially smooth axial stress distribution; some prior art crest geometries could produce concave, or erratically shaped axial stress distributions.  
         [0055]    Other advantages of the invention may include one or more of the following:  
         [0056]    The larger radius at the comers of a crest of a milled tooth enables a thicker layer of hardfacing at the comers of the crest of the tooth;  
         [0057]    A thicker layer of hardfacing provided along a crest of a chisel type milled tooth between radiused comers enhances the durability of the tooth as it operates in a borehole;  
         [0058]    The radiusing of the comers adjacent the flanks and ends of the chisel crested teeth further strengthens the capability of the tooth to retain its hardfacing during downhole operations;  
         [0059]    A convex substrate crest and a convex hardfacing crest provides a uniform axial stress distribution across the crest;  
         [0060]    A convex substrate crest and a flat hardfacing crest provides a gradual increase in the hardfacing thickness, and thicker hardfacing at the corners;  
         [0061]    A convex substrate crest provides a convex axial stress distribution;  
         [0062]    A convex substrate crest provides a substantially even axial stress distribution;  
         [0063]    A convex substrate crest provides a substantially smooth axial stress distribution;  
         [0064]    A convex substrate crest provides a preferred loading condition; and  
         [0065]    A convex substrate crest provides improved wear characteristics.  
         [0066]    Other advantages of the invention will be apparent from the appended claims.  
         [0067]    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.

Summary:
A drill bit comprising a bit body, at least one roller cone rotatably mounted on the bit body. The cone has a plurality of milled teeth at selected locations on the cone. At least one of the milled teeth comprises a substrate having a convex crest and a layer of hardfacing applied to said convex crest. The convex crest is adapted to produce at least one of a convex axial stress distribution, a substantially even axial stress distribution, and a substantially smooth axial stress distribution.