Abstract:
A manufacturing method and drill bit having either a preformed steel powder blank or machined steel core and abrasion and erosion resistant material components attached thereon.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method of manufacturing drill bits and other drilling-related structures generally used for drilling subterranean formations, and more specifically to a method of manufacturing a drill bit or drilling-related structure having a porous sintered steel powder core and a powdered tungsten carbide (WC) shell commonly infiltrated with other metals binder having cutter segments thereon or a sintered powder tungsten carbide (WC) core with other metals having cutter segments thereon. In a preferred embodiment, a sintered, preformed blank is formed and placed in a mold configured as a bit or other drilling-related structure, the preformed blank being sized to provide space between the blank and the mold wall to accommodate a layer of WC powder therebetween. Separately, cutter segments are formed having cutters thereon which are attached to the tungsten carbide shell or to the tungsten carbide bit body. 
         [0003]    2. State of the Art 
         [0004]    A typical rotary drill bit includes a bit body secured to a steel shank having a threaded pin connection for attaching the bit body to a drill string, and a crown comprising that part of the bit fitted with cutting structures for cutting into an earth formation. Generally, if the bit is a fixed-cutter or so-called “drag” bit, the cutting structures include a series of cutting elements formed at least in part of a super-abrasive material, such as polycrystalline diamond. The bit body is generally formed of steel, or a matrix of hard particulate material such as tungsten carbide (WC) infiltrated with a binder, generally of copper alloy. 
         [0005]    In the case of steel body bits, the bit body is typically machined from round stock to the desired shape. Internal watercourses for delivery of drilling fluid to the bit face and topographical features defined at precise locations on the bit face may be machined into the bit body using a computer-controlled five-axis machine tool. Hardfacing for resisting abrasion during drilling is usually applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the bit face, generally by inserting the proximal ends of studs, on which the cutting elements are mounted, into apertures bored in the bit face. The end of the bit body opposite the face is then threaded, made up and welded to the bit shank. 
         [0006]    In the case of a matrix-type bit body, it is conventional to employ a preformed, so-called bit “blank” of steel or other suitable material for later attachment to the shank, or threaded end of the bit body matrix. The blank may be merely cylindrically tubular, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on and extending from the bit face. Other preformed elements or displacements comprised of cast resin-coated sand, or in some instances graphite may be employed to define watercourses and passages for delivery of drilling fluid to and away from the bit face, well as cutting element sockets, ridges, lands, nozzle displacements, junk slots and other external topographic features of the bit. The blank and other displacements are placed at appropriate locations and orientations in the mold used to cast the bit body. The blank is bonded to the matrix upon cooling of the bit body after infiltration of the tungsten carbide with the matrix alloy in a furnace. The other displacements are removed once the matrix has cooled. The upper end of the blank is then threaded, made up with a matingly threaded shank, and the two are welded together. The cutting elements (typically diamond, and most often a synthetic polycrystalline diamond compact or PDC) may be bonded to the bit face during furnacing of the bit body if thermally stable PDC&#39;s, commonly termed TSP&#39;s (Thermally Stable Products) are employed, or may be subsequently bonded thereto, usually by brazing or mechanical affixation. 
         [0007]    As may be readily appreciated from the foregoing description, the process of fabricating a matrix-type drill bit is a somewhat costly, complex multi-step process requiring separate fabrication of an intermediate product (the mold) before the end product (the bit) can be cast. Moreover, the blanks and preforms employed must be individually designed and fabricated. 
         [0008]    The mold used to cast a matrix body is typically machined from a cylindrical graphite element. For many years, bit molds were machined to a general bit profile, and the individual bit face topography defined in reverse in the mold by skilled technicians employing the aforementioned preforms and wielding dental-type drills and other fine sculpting tools. In more recent years, many details may be machined in a mold using a computer controlled five-axis machine tool. 
         [0009]    Both batch and conveyor-type continuous furnaces, induction heating coils, and other heating methods known in the art may be used to supply the heat necessary for sintering and or infiltrating to occur. It is well recognized in the art to use sintering techniques to sinter and forge mixtures of cobalt powder and tungsten carbide to form inserts for rock-cutting bits, such as the method disclosed in U.S. Pat. No. 4,484,644 to Cook et al. It has also been recognized in the art to replace at least a portion of the hard metal material (WC) of a typical bit with a tougher, more ductile displacement material, such as iron, steel, or alloys thereof. As described in U.S. Pat. No. 5,090,491 to Tibbitts et al., it is desirable to substitute a less expensive displacement material (such as steel at about 50 cents per pound) for the more expensive hard metals like tungsten carbide (at about ten dollars per pound) to provide a finished bit with improved toughness and ductility as well as impact strength. However, this reference provides that the displacement material should preferably be a mesh size of at least 400 mesh (approximately 0.001 inches) and also states that very fine powdered materials (i.e., less than 0.001 inches in diameter) such as iron may sinter and shrink during fabrication; it being undesirable for the powder to shrink substantially during the heating process. Likewise, in GB 1,572,543 to Holden, the use of relatively inexpensive materials to provide the metal matrix of a bit, such as iron powder bonded with a copper-based alloy, is disclosed. Nowhere, however, do any of these references suggest that a powdered steel blank be sintered or otherwise preformed, then subsequently infiltrated along with a layer of tungsten carbide powder to form a bit or drilling-related structure. 
         [0010]    It is known in the art that although hard, the ductility of cemented hard-carbide articles are almost always inferior to those obtained by casting or forging steel. Thus, it would be advantageous to provide a method of manufacturing a bit or other drilling-related structure that is a relatively simple process and that reduces the cost of producing the structure by replacing a significant amount of the bit matrix material of a typical drilling structure with a sintered steel powder blank without sacrificing the bit&#39;s resistance to erosion and abrasion. Moreover, it would be advantageous to provide such a drilling structure that has improved toughness and impact strength over similar structures manufactured by prior art methods. 
       SUMMARY OF THE INVENTION 
       [0011]    A manufacturing method and drill bit having either a preformed steel powder blank or machined steel core and abrasion and erosion resistant material components attached thereon. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The features and advantages of the present invention can be more readily understood with reference to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings wherein: 
           [0013]      FIG. 1A  is a partially cross-sectioned schematic view of a first embodiment of a drill bit manufactured in accordance with the present invention; 
           [0014]      FIG. 1B  is a partially cross-sectioned schematic view of a second embodiment of a drill bit manufactured in accordance with the present invention; 
           [0015]      FIGS. 2A-2E  illustrate a method of forming a shell for the bit body of the earth-boring rotary drill bit shown in  FIG. 1A  or  1 B; 
           [0016]      FIG. 3A  is a partially cross-sectioned schematic view of another embodiment of a drill bit manufactured in accordance with the present invention having segments on the blades; 
           [0017]      FIG. 3B  is a partially cross-sectioned schematic view of another embodiment of a drill bit manufactured in accordance with the present invention having segments on the blades; 
           [0018]      FIG. 4  is a portion of a segment on a blade of the drill bit; 
           [0019]      FIG. 5  is a portion of a segment on a blade of the drill bit; and 
           [0020]      FIG. 6  is a partially cross-sections schematic view of another embodiment of a drill bit manufactured in accordance with the present invention having segments and or a shell on the blades. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    A drill bit  10  manufactured in accordance with the present invention is illustrated in  FIG. 1A . The drill bit  10  has a typical rotary drag bit configuration and is generally comprised of a bit body  12  including a plurality of longitudinally extending blades  14  defining junk slots  16  between the blades  14 . Each blade  14  defines a leading or cutting face  18  that extends from proximate the center of the bit face around the distal end  15  of the drill bit  10 , and includes a plurality of cutting elements  20  oriented to cut into a subterranean formation upon rotation of the drill bit  10 . The cutting elements  20  are secured to and supported by the blades  14 . Between the uppermost of the cutting elements  20  and the top edge  21  of the blade  14 , each blade  14  defines a longitudinally and radially extending gage portion  22  that corresponds to approximately the largest-diameter-portion of the drill bit  10  and, thus is typically only slightly smaller than the diameter of the hole to be drilled by cutting elements  20  of the bit  10 . The proximal end  23  of the bit  10  includes a threaded portion or pin  25  to threadedly attach the drill bit  10  to a drill collar or downhole motor, as is known in the art. Preferably, the threaded pin portion  25  may be machined directly into the proximal end  23  of the combination shank and blank  34  that is attached and formed into the body  12  of the drill bit  10 . 
         [0022]    As further illustrated by the cut-away portion of  FIG. 1A , the bit  10  is further comprised of either a machine steel core or a porous sintered blank or core  26  comprised of steel or other metal interlocked with the blank  34 , formed of any suitable material, such as steel, titanium, tungsten carbide (WC), etc., and a shell of abrasion-resistant material  28 , such as tungsten carbide (WC), infiltrated with a common metal to form a matrix of tungsten carbide and the metal. In addition, the plenum  29  longitudinally extend from the proximal end  23  to the distal end  15  or crown end  15 , substantially through the blank  34  and core  26 , terminating at shell  28 . As illustrated in  FIG. 1B , the core  26 ′ may have a topographical exterior surface configuration  30  substantially similar to the topography  32  of a completed bit  10 ′, but smaller in size, or be different such that the shell  28  occupies a larger volume of the bit  10  (see  FIG. 1A ). Thus, except for the detailed topography of and surrounding the cutting elements  20 , the core  26 ′ generally may follow the contour of the drill bit  10 ′ defined by its surface topography  32 . This similarity in shape between the core  26 ′ and the topography  32  is a result of a preferred bit manufacturing method of the present invention. Moreover, the plenum  29 ′ may only extend partially through the core  26 ′ such that any waterways connecting the plenum  29 ′ to the nozzle ports  62  and  64  must extend through material of both the core  26 ′ and shell  28 ′. 
         [0023]    The cutters  20  may be bonded to the blades  14  by brazing, mechanical affixation, or adhesive affixation. Alternatively, the cutters  20  may be provided within the mold and bonded to the blades  14  of the shell  28  during infiltration or furnacing of the shell  28  if thermally stable synthetic diamonds, or natural diamonds, are employed. 
         [0024]    The illustrations presented herein are not meant to be actual views of any particular material, apparatus, system, or method, but are merely idealized representations which are employed to describe the present invention. Additionally, elements common between figures may retain the same numerical designation. 
         [0025]    The term “green” as used herein means unsintered. 
         [0026]    The term “green bit shell” or “green segment” as used herein means an unsintered structure comprising a plurality of discrete particles held together by a binder material, the structure having a size and shape allowing the formation of a shell body suitable for use in an earth-boring drill bit from the structure by subsequent manufacturing processes including, but not limited to, machining and densification. 
         [0027]    The term “brown” as used herein means partially sintered. 
         [0028]    The term “brown shell body” or “brown segment” as used herein means a partially sintered structure comprising a plurality of particles, at least some of which have partially grown together to provide at least partial bonding between adjacent particles, the structure having a size and shape allowing the formation of a bit body suitable for use in an earth-boring drill bit from the structure by subsequent manufacturing processes including, but not limited to, machining and further densification. Brown shell bodies may be formed by, for example, partially sintering a green shell body. 
         [0029]    The term “sintering” as used herein means densification of a particulate component involving removal of at least a portion of the pores between the starting particles (accompanied by shrinkage) combined with coalescence and bonding between adjacent particles. 
         [0030]    As used herein, the term “[metal]-based alloy” (where [metal] is any metal) means commercially pure [metal] in addition to metal alloys wherein the weight percentage of [metal] in the alloy is greater than the weight percentage of any other component of the alloy. 
         [0031]    As used herein, the term “material composition” means the chemical composition and microstructure of a material. In other words, materials having the same chemical composition but a different microstructure are considered to having different material compositions. 
         [0032]    As used herein, the term “tungsten carbide” means any material composition that contains chemical compounds of tungsten and carbon, such as, for example, WC, W 2 C, and combinations of WC and W 2 C. Tungsten carbide includes, for example, cast tungsten carbide, sintered tungsten carbide, and macrocrystalline tungsten carbide. 
         [0033]    The particle-matrix composite material of the shell  28  may include a plurality of hard particles randomly dispersed throughout a matrix material. The hard particles may comprise diamond or ceramic materials such as carbides, nitrides, oxides, and borides (including boron carbide (B 4 C)). More specifically, the hard particles may comprise carbides and borides made from elements such as W, Ti, Mo, Nb, V, Hf, Ta, Cr, Zr, Al, and Si. By way of example and not limitation, materials that may be used to form hard particles include tungsten carbide, titanium carbide (TiC), tantalum carbide (TaC), titanium diboride (TiB 2 ), chromium carbides, titanium nitride (TiN), aluminium oxide (Al 2 O 3 ), aluminium nitride (AlN), and silicon carbide (SiC). Furthermore, combinations of different hard particles may be used to tailor the physical properties and characteristics of the particle-matrix composite material. The hard particles may be formed using techniques known to those of ordinary skill in the art. Most suitable materials for hard particles are commercially available and the formation of the remainder is within the ability of one of ordinary skill in the art. 
         [0034]    The matrix material of the particle-matrix composite material may include, for example, cobalt-based, iron-based, nickel-based, iron and nickel-based, cobalt and nickel-based, iron and cobalt-based, aluminum-based, copper-based, magnesium-based, and titanium-based alloys. The matrix material may also be selected from commercially pure elements such as cobalt, aluminum, copper, magnesium, titanium, iron, and nickel. By way of example and not limitation, the matrix material may include carbon steel, alloy steel, stainless steel, tool steel, Hadfield manganese steel, nickel or cobalt superalloy material, and low thermal expansion iron or nickel based alloys such as INVAR®. As used herein, the term “superalloy” refers to an iron, nickel, and cobalt based-alloys having at least 12% chromium by weight. Additional exemplary alloys that may be used as matrix material include austenitic steels, nickel based superalloys such as INCONEL® 625M or Rene 95, and INVAR® type alloys having a coefficient of thermal expansion that closely matches that of the hard particles used in the particular particle-matrix composite material. More closely matching the coefficient of thermal expansion of matrix material with that of the hard particles offers advantages such as reducing problems associated with residual stresses and thermal fatigue. Another exemplary matrix material is a Hadfield austenitic manganese steel (Fe with approximately 12% Mn by weight and 1.1% C by weight). 
         [0035]    In the present invention, the particle-matrix composite material may include a plurality of −400 ASTM (American Society for Testing and Materials) mesh tungsten carbide particles as the hard particle component of the particle-matrix composite material. For example, the tungsten carbide particles may be substantially composed of WC. As used herein, the phrase “−400 ASTM mesh particles” means particles that pass through an ASTM No. 400 mesh screen as defined in ASTM specification E11-04 entitled Standard Specification for Wire Cloth and Sieves for Testing Purposes. Such tungsten carbide particles may have a diameter of less than about 38 microns. The matrix material forming another component of the particle-matrix composite material may include a metal alloy comprising about 50% cobalt by weight and about 50% nickel by weight. The tungsten carbide particles may comprise between about 60% and about 95% by weight of the particle-matrix composite material, and the matrix material may comprise between about 5% and about 40% by weight of the particle-matrix composite material. More particularly, the tungsten carbide particles may comprise between about 70% and about 80% by weight of the particle-matrix composite material, and the matrix material may comprise between about 20% and about 30% by weight of the particle-matrix composite material. 
         [0036]    In another embodiment of the present invention, the particle-matrix composite material may include a plurality of −635 ASTM mesh tungsten carbide particles as the hard particle component of the particle-matrix composite material. As used herein, the phrase “−635 ASTM mesh particles” means particles that pass through an ASTM No. 635 mesh screen as defined in ASTM specification E11-04 entitled Standard Specification for Wire Cloth and Sieves for Testing Purposes. Such tungsten carbide particles may have a diameter of less than about 20 microns. The matrix material may include a cobalt-based metal alloy comprising substantially commercially pure cobalt. For example, the matrix material forming another component of the particle-matrix composite material may include greater than about 98% cobalt by weight. The tungsten carbide particles may comprise between about 60% and about 95% by weight of the particle-matrix composite material, and the matrix material may comprise between about 5% and about 40% by weight of the particle-matrix composite material. 
         [0037]      FIGS. 2A-2E  illustrate a method of fonning the shell  28 , which is substantially formed from and composed of a particle-matrix composite material. The method generally includes providing a powder mixture, pressing the powder mixture to form a green body, and at least partially sintering the powder mixture. 
         [0038]    Referring to  FIG. 2A , a powder mixture  78 , which forms the particle-matrix composite material that includes a hard particle component and a matrix material component, may be pressed with substantially isostatic pressure within a mold or container  80 . The powder mixture  78  may include a plurality of the previously described hard particles and a plurality of particles comprising a matrix material, as also previously described herein. Optionally, the powder mixture  78  may further include additives commonly used when pressing powder mixtures such as, for example, binders for providing lubrication during pressing and for providing structural strength to the pressed powder component, plasticizers for making the binder more pliable, and lubricants or compaction aids for reducing inter-particle friction. 
         [0039]    The container  80  may include a fluid-tight deformable member  82 . For example, the fluid-tight deformable member  82  may be a substantially cylindrical bag comprising a deformable polymer material. The container  80  may further include a sealing plate  84 , which may be substantially rigid. The deformable member  82  may be formed from, for example, an elastomer such as rubber, neoprene, silicone, or polyurethane. The deformable member  82  may be filled with the powder mixture  78  and vibrated to provide a uniform distribution of the powder mixture  78  within the deformable member  82 . At least one displacement or insert  86  may be provided within the deformable member  82  for defining features of the bit body  52  such as, for example, the longitudinal bore  40  ( FIG. 2 ). Alternatively, the insert  86  may not be used and the longitudinal bore  40  may be formed using a conventional machining process during subsequent processes. The sealing plate  84  then may be attached or bonded to the deformable member  82  providing a fluid-tight seal therebetween. 
         [0040]    The container  80  (with the powder mixture  78  and any desired inserts  86  contained therein) may be provided within a pressure chamber  90 . A removable cover  91  may be used to provide access to the interior of the pressure chamber  90 . A fluid (which may be substantially incompressible) such as, for example, water, oil, or gas (such as, for example, air or nitrogen) is pumped into the pressure chamber  90  through an opening  92  at high pressures using a pump (not shown). The high pressure of the fluid causes the walls of the deformable member  82  to deform. The fluid pressure may be transmitted substantially uniformly to the powder mixture  78 . The pressure within the pressure chamber  90  during isostatic pressing may be greater than about 35 megapascals (about 5,000 pounds per square inch). More particularly, the pressure within the pressure chamber  90  during isostatic pressing may be greater than about 138 megapascals (20,000 pounds per square inch). In alternative methods, a vacuum may be provided within the container  80  and a pressure greater than about 0.1 megapascals (about 15 pounds per square inch) may be applied to the exterior surfaces of the container (by, for example, the atmosphere) to compact the powder mixture  78 . Isostatic pressing of the powder mixture  78  may form a green powder component or green shell body  94  shown in  FIG. 3B , which can be removed from the pressure chamber  90  and container  80  after pressing. 
         [0041]    In an alternative method of pressing the powder mixture  78  to form the green shell body  94  shown in  FIG. 2B , the powder mixture  78  may be uniaxially pressed in a mold or die (not shown) using a mechanically or hydraulically actuated plunger by methods that are known to those of ordinary skill in the art of powder processing. 
         [0042]    The green shell body  94  shown in  FIG. 2B  may include a plurality of particles (hard particles and particles of matrix material forming the particle-matrix composite material) held together by a binder material provided in the powder mixture  78  ( FIG. 2A ), as previously described. Certain structural features may be machined in the green bit body  94  using conventional machining techniques including, for example, turning techniques, milling techniques, and drilling techniques. Hand held tools also may be used to manually form or shape features in or on the green shell body  94 . By way of example and not limitation, blades  14 , junk slots  16  ( FIGS. 1A ,  1 B), and any surfaces may be machined or otherwise formed in the green shell body  94  to form a shaped green shell body  98  shown in  FIG. 2C . 
         [0043]    The shaped green shell body  98  shown in  FIG. 2C  may be at least partially sintered to provide a brown bit body  102  shown in  FIG. 2D , which has less than a desired final density. Prior to partially sintering the shaped green bit body  98 , the shaped green bit body  98  may be subjected to moderately elevated temperatures and pressures to burn off or remove any fugitive additives of any binder used that were included in the powder mixture  78  ( FIG. 2A ), as previously described. Furthermore, the shaped green bit body  98  may be subjected to a suitable atmosphere tailored to aid in the removal of such additives. Such atmospheres may include, for example, hydrogen gas at temperatures of about 500° C. 
         [0044]    The brown shell body  102  may be substantially machinable due to the remaining porosity therein. Certain structural features may be machined in the brown shell body  102  using conventional machining techniques including, for example, turning techniques, milling techniques, and drilling techniques. Hand held tools also may be used to manually form or shape features in or on the brown shell body  102 . Tools that include superhard coatings or inserts may be used to facilitate machining of the brown shell body  102 . Additionally, material coatings may be applied to surfaces of the brown shell body  102  that are to be machined to reduce chipping of the brown bit body  102 . Such coatings may include a suitable fixative material or other suitable polymer materials or their like. 
         [0045]    By way of example and not limitation, internal fluid passageways  29 , cutter pockets  36  and blades  14  ( FIGS. 1A ,  1 B) may be machined or otherwise formed in the brown bit body  102  to form a shaped brown shell body  106  shown in  FIG. 2E . Furthermore, if the drill bit  10  is to include a plurality of cutters integrally formed with the shell  28 , the cutters may be positioned within the cutter pockets  36  formed in the brown shell body  102 . Upon subsequent sintering of the brown bit body  102 , the cutters may become bonded to and integrally formed with the shell body  52 . 
         [0046]    The shaped brown bit body  106  shown in  FIG. 2E  then may be fully sintered to a desired final density to provide the previously described shell  28  shown in  FIG. 1A  or  FIG. 1B . As any sintering involves densification and removal of porosity within a structure, the structure being sintered will shrink during the sintering process. In an un-infiltrated structure, a structure may experience linear shrinkage of between 10% and 20% during sintering from a green state to a desired final density. As a result, dimensional shrinkage must be considered and accounted for when designing tooling (molds, dies, etc.) or machining features in structures that are less than fully sintered. 
         [0047]    During all sintering and partial sintering processes, refractory structures or displacements (not shown) may be used to support at least portions of the bit body during the sintering process to maintain desired shapes and dimensions during the densification process. Such displacements may be used, for example, to maintain consistency in the size and geometry of the cutter pockets  36  and the internal fluid passageways  42  during the sintering process. Such refractory structures may be formed from, for example, graphite, silica, or alumina. The use of alumina displacements instead of graphite displacements may be desirable as alumina may be relatively less reactive than graphite, thereby minimizing atomic diffusion during sintering. Additionally, coatings such as alumina, boron nitride, aluminum nitride, or other commercially available materials may be applied to the refractory structures to prevent carbon or other atoms in the refractory structures from diffusing into the bit body during densification. 
         [0048]    In alternative methods, the green shell body  94  shown in  FIG. 2B  may be partially sintered to form a brown bit body without prior machining, and all necessary machining may be performed on the brown shell body prior to infiltrating the brown shell body and fully sintering the brown bit body to a desired final density. Alternatively, all necessary machining may be performed on the green bit body  94  shown in  FIG. 2B , which then may be infiltrated and fully sintered to a desired final density. 
         [0049]    The sintering processes described herein may include conventional sintering in a vacuum furnace, sintering in a vacuum furnace followed by a conventional hot isostatic pressing process, and sintering immediately followed by isostatic pressing at temperatures near the sintering temperature (often referred to as sinter-HIP). Furthermore, the sintering processes described herein may include subliquidus phase sintering. In other words, the sintering processes may be conducted at temperatures proximate to but below the liquidus line of the phase diagram for the matrix material forming a portion of the particle-matrix composite material. For example, the sintering processes described herein may be conducted using a number of different methods known to one of ordinary skill in the art such as the Rapid Omnidirectional Compaction (ROC) process, the Ceracon™ process, hot isostatic pressing (HIP), or adaptations of such processes. 
         [0050]    Broadly, and by way of example only, sintering a green powder compact using the ROC process involves presintering the green powder compact at a relatively low temperature to only a sufficient degree to develop sufficient strength to permit handling of the powder compact. The resulting brown structure is wrapped in a material such as graphite foil to seal the brown structure. The wrapped brown structure is placed in a container, which is filled with particles of a ceramic, polymer, or glass material having a substantially lower melting point than that of the matrix material in the brown structure. The container is heated to the desired sintering temperature, which is above the melting temperature of the particles of a ceramic, polymer, or glass material, but below the liquidus temperature of the matrix material in the brown structure. The heated container with the molten ceramic, polymer, or glass material (and the brown structure immersed therein) is placed in a mechanical or hydraulic press, such as a forging press, that is used to apply pressure to the molten ceramic or polymer material. Isostatic pressures within the molten ceramic, polymer, or glass material facilitate consolidation and sintering of the brown structure at the elevated temperatures within the container. The molten ceramic, polymer, or glass material acts to transmit the pressure and heat to the brown structure. In this manner, the molten ceramic, polymer, or glass acts as a pressure transmission medium through which pressure is applied to the structure during sintering. Subsequent to the release of pressure and cooling, the sintered structure is then removed from the ceramic, polymer, or glass material. A more detailed explanation of the ROC process and suitable equipment for the practice thereof is provided by U.S. Pat. Nos. 4,094,709, 4,233,720, 4,341,557, 4,526,748, 4,547,337, 4,562,990, 4,596,694, 4,597,730, 4,656,002 4,744,943 and 5,232,522, the disclosure of each of which patents is incorporated herein by reference. 
         [0051]    The Ceracon™ process, which is similar to the aforementioned ROC process, may also be adapted for use in the present invention to fully sinter brown structures to a final density. In the Ceracon™ process, the brown structure is coated with a ceramic coating such as alumina, zirconium oxide, or chrome oxide. Other similar, hard, generally inert, protective, removable coatings may also be used. The coated brown structure is fully consolidated by transmitting at least substantially isostatic pressure to the coated brown structure using ceramic particles instead of a fluid media as in the ROC process. A more detailed explanation of the Ceracon™ process is provided by U.S. Pat. No. 4,499,048, the disclosure of which patent is incorporated herein by reference. 
         [0052]    Furthermore, in embodiments of the invention in which tungsten carbide is used in a particle-matrix composite bit body, the sintering processes described herein also may include a carbon control cycle tailored to improve the stoichiometry of the tungsten carbide material. By way of example and not limitation, if the tungsten carbide material includes WC, the sintering processes described herein may include subjecting the tungsten carbide material to a gaseous mixture including hydrogen and methane at elevated temperatures. For example, the tungsten carbide material may be subjected to a flow of gases including hydrogen and methane at a temperature of about 1,000° C. 
         [0053]    After final sintering of the shell  28 , the shell  28  is attached to the core  26  using any suitable bonding process, such as brazing, individual fasteners, etc. 
         [0054]    As illustrated in  FIGS. 3A and 3B , the drill bit  10  includes the cutters  20  mounted on segments  14 ′ which are attached to the blades  14 . The segments  14 ′ are formed in the same manner as the shell  28  described hereinbefore. The segments  14 ′ may be formed of any desired length for attachment to a blade  14 , such as from the gage of the drill bit through any length of a blade  14 . The segments  14 ′ are formed of particle-matrix composite material which may include a plurality of hard particles randomly dispersed throughout a matrix material. The hard particles may comprise diamond or ceramic materials such as carbides, nitrides, oxides, and borides (including boron carbide (B 4 C)). More specifically, the hard particles may comprise carbides and borides made from elements such as W, Ti, Mo, Nb, V, Hf, Ta, Cr, Zr, Al, and Si. By way of example and not limitation, materials that may be used to form hard particles include tungsten carbide, titanium carbide (TiC), tantalum carbide (TaC), titanium diboride (TiB 2 ), chromium carbides, titanium nitride (TiN), aluminium oxide (Al 2 O 3 ), aluminium nitride (AIN), and silicon carbide (SiC). Furthermore, combinations of different bard particles may be used to tailor the physical properties and characteristics of the particle-matrix composite material. The hard particles may be formed using techniques known to those of ordinary skill in the art. Most suitable materials for hard particles are commercially available and the formation of the remainder is within the ability of one of ordinary skill in the art. 
         [0055]    The matrix material of the particle-matrix composite material may include, for example, cobalt-based, iron-based, nickel-based, iron and nickel-based, cobalt and nickel-based, iron and cobalt-based, aluminum-based, copper-based, magnesium-based, and titanium-based alloys. The matrix material may also be selected from commercially pure elements such as cobalt, aluminum, copper, magnesium, titanium, iron, and nickel. By way of example and not limitation, the matrix material may include carbon steel, alloy steel, stainless steel, tool steel, Hadfield manganese steel, nickel or cobalt superalloy material, and low thermal expansion iron or nickel based alloys such as INVAR®. As used herein, the term “superalloy” refers to iron, nickel, and cobalt based-alloys having at least 12% chromium by weight. Additional exemplary alloys that may be used as matrix material include austenitic steels, nickel based superalloys such as INCONEL® 625M or Rene 95, and INVAR® type alloys having a coefficient of thermal expansion that closely matches that of the hard particles used in the particular particle-matrix composite material. More closely matching the coefficient of thermal expansion of matrix material with that of the hard particles offers advantages such as reducing problems associated with residual stresses and thermal fatigue. Another exemplary matrix material is a Hadfield austenitic manganese steel (Fe with approximately 12% Mn by weight and 1.1% C by weight). 
         [0056]    In the present invention, the particle-matrix composite material may include a plurality of −400 ASTM (American Society for Testing and Materials) mesh tungsten carbide particles as the hard particle component of the particle-matrix composite material. For example, the tungsten carbide particles may be substantially composed of WC. As used herein, the phrase “−400 ASTM mesh particles” means particles that pass through an ASTM No. 400 mesh screen as defined in ASTM specification E11-04 entitled Standard Specification for Wire Cloth and Sieves for Testing Purposes. Such tungsten carbide particles may have a diameter of less than about 38 microns. The matrix material forming another component of the particle-matrix composite material may include a metal alloy comprising about 50% cobalt by weight and about 50% nickel by weight. The tungsten carbide particles may comprise between about 60% and about 95% by weight of the particle-matrix composite material, and the matrix material may comprise between about 5% and about 40% by weight of the particle-matrix composite material. More particularly, the tungsten carbide particles may comprise between about 70% and about 80% by weight of the particle-matrix composite material, and the matrix material may comprise between about 20% and about 30% by weight of the particle-matrix composite material. 
         [0057]    Alternately, the particle-matrix composite material may include a plurality of −635 ASTM mesh tungsten carbide particles as the hard particle component of the particle-matrix composite material. As used herein, the phrase “−635 ASTM mesh particles” means particles that pass through an ASTM No. 635 mesh screen as defined in ASTM specification E11-04 entitled Standard Specification for Wire Cloth and Sieves for Testing Purposes. Such tungsten carbide particles may have a diameter of less than about 20 microns. The matrix material may include a cobalt-based metal alloy comprising substantially commercially pure cobalt. For example, the matrix material forming another component of the particle-matrix composite material may include greater than about 98% cobalt by weight. The tungsten carbide particles may comprise between about 60% and about 95% by weight of the particle-matrix composite material, and the matrix material may comprise between about 5% and about 40% by weight of the particle-matrix composite material. 
         [0058]    The segments  14 ′ are formed in the same manner as the process for forming the shell hereinbefore and illustrated in  FIGS. 2A-2E . Similarly, the cutters  20  are attached to the segments  14 ′ as described hereinbefore with respect to the shell  28 . 
         [0059]    In  FIG. 4 , the segments  14 ′ include one or more protrusions  14 ″ extending therefrom to mate with recesses formed in the blades  14  to provide an accurate location of the segment on the blade  14 . The protrusions  14 ″ may extend from a side and the back of a segment to provide any desired number of locations for the segment  14 ″ on a blade  14 . The segment  14 ′ is attached to a blade  14  in any suitable manner, such as brazing, fasteners, etc. In addition to providing a structure to locate the segment  14 ′ on a blade  14 , the protrusions  14 ″ provide additional surface area to secure the segment  14 ′ to a blade  14  when the segment  14 ′ is attached to the blade  14  by brazing a similar attachment process. The protrusions  14 ″ may be of any desired suitable geometric shape and dimension. 
         [0060]    As illustrated in  FIG. 5 , the segment  14 ′ may extend around the front face, outer edge, and back face of a portion of a blade  14  of the drill bit  10 . In this manner, the blade  14  of the drill bit  10  is protected on all three sides thereof by the segment  14 ′ which is constructed of material having a higher abrasion resistance than that of the blade  14 . The segment  14 ′ may be attached to the blade  14  by any suitable attachment process, such as brazing, fasteners, etc. 
         [0061]    Illustrated in drawing  FIG. 6 , the segments  14 ′, although the segments  14 ′ can be formed as a shell such as shell  106  described herein, and blank  34  are illustrated in a in a pressure chamber  90  such as described hereinbefore. The blank  34  and segments  14 ′ are supported on suitable inserts  86  with a particle-matrix material powder mixture  78 , such as described herein, filling the space in the mold or container  80 , such as described herein, between the mold  80  and segments  14 ′ and blank  34 . A fluid (which may be substantially incompressible) such as, for example, water, oil, or gas (such as, for example, air or nitrogen) is pumped into the pressure chamber  90  through an opening  92  at high pressures using a pump (not shown). The high pressure of the fluid causes the walls of the deformable member  82  to deform. The fluid pressure may be transmitted substantially uniformly to the powder mixture  78 . The pressure within the pressure chamber  90  during isostatic pressing may be greater than about 35 megapascals (about 5,000 pounds per square inch). More particularly, the pressure within the pressure chamber  90  during isostatic pressing may be greater than about 138 megapascals (20,000 pounds per square inch). In alternative methods, a vacuum may be provided within the container  80  and a pressure greater than about 0.1 megapascals (about 15 pounds per square inch) may be applied to the exterior surfaces of the container (by, for example, the atmosphere) to compact the powder mixture  78 . Isostatic pressing of the powder mixture  78  may form a green powder component or green shell body, such as green shell body  94  described herein, shown in  FIG. 3B , which can be removed from the pressure chamber  90  and container  80  after pressing. 
         [0062]    The green bit body formed into a green bit blank  34 , such as generally like green shell body  94  shown in  FIG. 2B  and  FIG. 1B , may include a plurality of particles (hard particles and particles of matrix material forming the particle-matrix composite material) held together by a binder material provided in the powder mixture  78  ( FIG. 2A ), as previously described. Certain structural features may be machined in the green bit body  94  using conventional machining techniques including, for example, turning techniques, milling techniques, and drilling techniques. Hand held tools also may be used to manually form or shape features in or on the green shell body. By way of example and not limitation, blades  14 , junk slots  16  ( FIGS. 1A ,  1 B), and any surfaces may be machined or otherwise formed in the green shell body to form a shaped green shell body, generally such as green shell body  98  shown in  FIG. 2C . 
         [0063]    The shaped green bit body, generally such as green shell body  98  shown in  FIG. 2C  may be at least partially sintered to provide a brown bit body, generally such as brown bit body  102  shown in  FIG. 2D , which has less than a desired final density. Prior to partially sintering the shaped green bit body, the shaped green bit body may be subjected to moderately elevated temperatures and pressures to burn off or remove any fugitive additives of any binder used that were included in the powder mixture  78  ( FIG. 2A ), as previously described. Furthermore, the shaped green bit body may be subjected to a suitable atmosphere tailored to aid in the removal of such additives. Such atmospheres may include, for example, hydrogen gas at temperatures of about 500° C. 
         [0064]    The brown shell body, such as brown body  102  described herein, may be substantially machinable due to the remaining porosity therein. Certain structural features may be machined in the brown shell body using conventional machining techniques including, for example, turning techniques, milling techniques, and drilling techniques. Hand held tools also may be used to manually form or shape features in or on the brown shell body. Tools that include superhard coatings or inserts may be used to facilitate machining of the brown shell body. Additionally, material coatings may be applied to surfaces of the brown shell body that are to be machined to reduce chipping of the brown bit body. Such coatings may include a suitable fixative material or other suitable polymer materials or their like. 
         [0065]    By way of example and not limitation, internal fluid passageways  29  and cutter pockets  36  ( FIGS. 1A ,  1 B) may be machined or otherwise formed in the brown bit body to form a shaped brown bit body. If the drill bit  10  is to include additional cutters or wear knots, the cutters and wear knots may be positioned within the cutter pockets formed in the brown bit body. Upon subsequent sintering of the brown bit body, the cutters may become bonded to and integrally formed with the bit body. 
         [0066]    The shaped brown bit body then may be fully sintered to a desired final density. As any sintering involves densification and removal of porosity within a structure, the structure being sintered will shrink during the sintering process. In an un-infiltrated structure, a structure may experience linear shrinkage of between 10% and 20% during sintering from a green state to a desired final density. As a result, dimensional shrinkage must be considered and accounted for when designing tooling (molds, dies, etc.) or machining features in structures that are less than fully sintered. 
         [0067]    During all sintering and partial sintering processes, refractory structures or displacements (not shown) may be used to support at least portions of the bit body during the sintering process to maintain desired shapes and dimensions during the densification process. Such displacements may be used, for example, to maintain consistency in the size and geometry of the cutter pockets  36  and the internal fluid passageways  42  during the sintering process. Such refractory structures may be formed from, for example, graphite, silica, or alumina. The use of alumina displacements instead of graphite displacements may be desirable as alumina may be relatively less reactive than graphite, thereby minimizing atomic diffusion during sintering. Additionally, coatings such as alumina, boron nitride, aluminum nitride, or other commercially available materials may be applied to the refractory structures to prevent carbon or other atoms in the refractory structures from diffusing into the bit body during densification. 
         [0068]    In alternative methods, the green bit body may be partially sintered to form a brown bit body without prior machining, and all necessary machining may be performed on the brown shell body prior to infiltrating the brown bit body and fully sintering the brown bit body to a desired final density. Alternatively, all necessary machining may be performed on the green bit body  94  shown in  FIG. 2B , which then may be infiltrated and fully sintered to a desired final density. 
         [0069]    The sintering processes described herein may include conventional sintering in a vacuum furnace, sintering in a vacuum furnace followed by a conventional hot isostatic pressing process, and sintering immediately followed by isostatic pressing at temperatures near the sintering temperature (often referred to as sinter-HIP). Furthermore, the sintering processes described herein may include subliquidus phase sintering. In other words, the sintering processes may be conducted at temperatures proximate to but below the liquidus line of the phase diagram for the matrix material forming a portion of the particle-matrix composite material. For example, the sintering processes described herein may be conducted using a number of different methods known to one of ordinary skill in the art such as the Rapid Omnidirectional Compaction (ROC) process, the Ceracon™ process, hot isostatic pressing (HIP), or adaptations of such processes. 
         [0070]    While teachings of the present invention are described herein in relation to embodiments of earth-boring rotary drill bits that include fixed cutters, other types of earth-boring drilling tools such as, for example, core bits, eccentric bits, bicenter bits, reamers, mills, drag bits, roller cone bits, and other such structures known in the art may embody teachings of the present invention and may be formed by methods that embody teachings of the present invention. 
         [0071]    While the present invention has been described herein with respect to certain preferred embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions and modifications to the preferred embodiments may be made without departing from the scope of the invention as hereinafter claimed. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention as contemplated by the inventors. Further, the invention has utility in drill bits and core bits having different and various bit profiles as well as cutter types.