Abstract:
The present invention relates to a method of forming a powder rod core comprising injecting a powder mix material into a mold, moving the mold to a curing station, and injecting material into a second mold.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/782,341 filed Feb. 13, 2001 by Harold A. Sreshta and Eric F. Drake, now U.S. Pat. No. 6,361,739. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to heat treatable hardfacings. In particular, this invention relates to a fabrication process for a powder composite core for subsequent use in a sheathing/compaction process for making a high-density powder composite rod for weld-applied hardfacing. 
     2. Description of the Related Art 
     Hard metal overlays are employed in rock drilling bits and other downhole tools to form wear and deformation resistant cutting edges and faying surfaces. These overlays comprise composite structures of hard particles in a metal matrix. Such hard metal overlays are normally formed by brazing or weld deposition of composite rod, producing a metal alloy matrix solidified from a melt containing hard particles that remain at least partially solid. Early examples of hardfacing rods for welding are shown in U.S. Pat. No. 1,757,601 and 3,023,130. 
     Hard metal overlays used on steel-toothed rolling cutter drill bits are subjected to extreme loads and prolonged scraping action. Therefore, the strongest, most wear resistant of fused hard metals are used in these cutting structures. Typically, such hard metal composites utilize sintered pellets or grains of cemented tungsten carbide/cobalt as the primary hard phase. 
     In addition to steel tooth rolling cutter drill bits, other types of down-hole tools also benefit from a strong wear resistant hardfacing material. For example, fixed cutter type earth boring drill bits and stabilizers often utilize welded hardfacing to protect gauge, blade, or watercourse surfaces. In a relatively recent development, tools made to steer drill bits during the drilling operation provide amongst the most demanding applications for hardfacing materials. 
     The formulation of composite rod filler metal entails fabrication and process considerations in addition to constituency selection. Typically, a tubular construction has been employed wherein a metal sheath is formed to enclose a particulate mixture comprising hard particles phases and additives including binders and de-oxidizers. In such a rod, the sheath metal combines with substrate melt, if any, to provide substantially all of the matrix phase of the final composite. The constituency of the hardmetal deposit is dependent on deposition process parameters as well as on the raw material formulation. 
     The management of thermal inputs during weld deposition is critical to deposit soundness and performance. Insufficient substrate heating and/or insufficient filler metal superheat can cause poor bonding, porosity, and irregular deposit configuration. Excess substrate heating, and/or excess filler metal superheat, and/or prolonged molten time produces substrate dilution and hard-particle degradation. Substrate dilution reduces carbide fractions, while sintered particle degradation causes softening and matrix embrittlement. 
     As the carbide loading of the rod and surface area of the application substrate increase, weld temperature and time control become increasingly critical. Composite rod with more than about  60 % by weight carbide fill is problematic to weld without substrate penetration and dilution, especially on large substrates. Deposition dynamics are strongly influenced by the thermal transfer, melting, and flow characteristics of the composite rod. 
     Melting dynamics can be accelerated by incorporating the metal matrix components of the composite rod as a powder rather than as a solid sheath. This approach exploits the high specific surface area of the particulate material to speed up melting, while eliminating transport and mixing dynamics. However, sheath elimination also entails the loss of its structural contributions to handling strength and melt progression. U.S. Pat. Nos. 4,836,307; 4,944,774; and 5,051,112 (all incorporated by reference herein for all they disclose) disclose the sintering of a powdered composite rod as a means of replacing the mechanical strength of the sheath. Such sintered pre-forms develop a strong, porous structure which acts to impede heat flow prior to melt collapse into the weld pool. As a result, some melting speed is sacrificed and melt progression becomes more difficult to control, resulting in operator-induced thickness and composition variation in the hardfacing. 
     In U.S. Pat. No. 4,699,848, a wire-reinforced powder rod is disclosed, replacing external sheath with solid metal filler at the center of the rod, the location least easily melted by thermal flow from external heat sources. This construction exacerbates weldability and control limitations, compared with conventional practice. 
     In U.S. Pat. No. 5,740,872 (incorporated by reference herein for all it discloses) a powder composite rod is disclosed with a thin metal sheath wherein the ratio of powder metal to sheath metal exceeds 2.5. The fabrication of such a bound powder metallurgy composite rod for weld-deposited hard surfacing as described in this patent has been conducted by extrusion and curing of rod cores, followed by sheath attachment using a wrapping mill. The rod core produced in this process has a void volume of about 40 vol %, relying on the binder for green handling strength. The sheath is wrapped with a simple overlap and attached to the core by a silicate adhesive that partially infiltrates the porous core, providing additional handling strength and preventing core movement within the sheath. The silicate adhesive becomes a liquid slag during weld application that must be manipulated out of the deposit, slowing application rates and demanding greater operator skill. Difficulties associated with management of this slag lend to adverse deposit effects, including pellet degradation, porosity, inclusions, and reduced thickness control. Although the thin sheath extruded rod filler metal and application process provide net improvements in application productivity, quality, and in the hardfacing performance as compared with conventional practice, its utility is limited by silicate adhesive effects and also by the relative brittleness of low-density methylcellulose-bound powder cores. 
     As mentioned above, rod cores may be fabricated by extrusion into a trough or channel. The extruded material then is cured, such as by partial de-binding by desiccation. It has been found that during the extrusion and desiccation of rod cores, distortions can occur which include stretching, buckling, and sagging. The resulting cores exhibit bends and non-uniform section, causing difficulties in wrapping translating to finished rod variation that contributes to application variation. 
     An alternative fabrication method involves injection molding of cores into conventional split dies. This approach can substantially eliminate distortion but entails reduced productivity associated with prolonged core residence within the die necessary to achieve sufficient handling strength for removal. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for fabricating a core for incorporation into a composite rod for hardfacing. 
     According to the present invention there is provided a fabrication process for a rod core comprising injecting rod core material into an elongate tubular mold, removing the mold to a curing station, and extracting the cured rod core from the mold. 
     By injecting the material into a closed rigid mold, rather than extruding it into a trough or channel, the buckling and stretching distortions are almost entirely eliminated. The use of a remote curing station allows material to be injected into a second mold while the material within the first mold is setting up, thus speeding up the manufacturing process. 
     The rod core material preferably comprises a powder mix of carbide and metal powders, a fugitive binder, and other additives. The mold may be of, for example, circular, oval, square, hexagonal or star-shaped cross-section. 
     The invention also relates to a method of fabrication of a hardfacing rod comprising encasing a rod core manufactured in accordance with the method defined hereinbefore in a metal sheath. The method may include a step of isostatically compacting the encased rod core to densify the core thereof and mechanically secure the sheath in position. 
     It is contemplated that the method has application to use hardfacing used in downhole tools including both fixed cutter and rolling cutter drill bits, bias pads for downhole rotary steerable systems, stabilizers, and other tools requiring strong and wear resistant hardfacings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a high density hardfacing rod made by the process of the present invention. 
     FIG. 2 is a perspective view of a rolling cutter steel tooth drill bit with hardfacing made by the process of the present invention. 
     FIG. 3 is a perspective view of a fixed cutter drill bit with a hardfacing made by the process of the present invention. 
     FIG. 4 is a cross-section view of a tooth of a rolling cutter steel tooth drill bit with a hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     FIG. 5 is a side view of a downhole stabilizer having a hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     FIG. 6 is a side view of a rotary steerable tool with a bias unit having a hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     FIG. 7 is a partial section view cross-section view of a bottomhole assembly of a drill string with tools having hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     FIG. 8 is a partial section view of a drill string with tools having hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     FIG. 9 is a diagrammatic view illustrating an apparatus for use in the formation of rod cores. 
     FIG. 10 is a side view of a device which encases the cores in a steel sheath. 
     FIG. 11 is a perspective view of a sheet of elastomer material to wrap the sheathed core prior to compaction. 
     FIG. 12 is a perspective view of an elastomer container with a plurality of chambers adapted to hold the undensified cured core-sheath assembly prior to compaction. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, the high density hardfacing rod  10  of the present invention has a core  12  wrapped in a steel sheath  14 . The high density hardfacing rod  10  is particularly suitable for hardfacings in downhole tools including earth boring drill bits  16 , bias pads  18  for downhole rotary steerable systems  20 , and stabilizers  22 , and for other types of tools requiring strong and wear resistant hardfacings. 
     The high density hardfacing rod  10  has a molded core  12  made of a particulate mixture comprising cemented carbide pellets, metal carbides, metal powders, and deoxidizer, with a methylcellulose fugitive binder. 
     The process for making the high density hardfacing rod  10  includes the steps of:preparing a powder mix comprising carbide and metal powders, a fugitive binder, and other additives to render a moldable rheology; forming the powder mix into a powder core  12  encased with a metal sheath  14  to form a core-sheath assembly  110 ; and isostatically compacting the core-sheath assembly  110  densifying to at least 65% of theoretical density and mechanically attaching the sheath. The core-sheath assembly  110  is dried at about 450F. for between 30 minutes and 2 hours, or alternatively heated in a vacuum at 930F. for between 30 minutes and 2 hours to remove the methylcellulose binder. 
     The mixture is encased with a thin (0.001″ to 0.010″) steel sheath  14 . This steel sheath  14  thickness range provides a weight ratio of the sheath to metallic matrix powders in the core of greater than 2.5. In the preferred embodiment, the steel sheath  14  is from 0.002″ to 0.006″ thick to optimize chemistry and melting characteristics. 
     The powder mixture consists of about 27 wt % each of 16/20 mesh sintered WCCo pellets and 20/30 mesh crushed sintered WC/Co; about 9 wt %, 40/100 mesh monocrystalline WC; about 4 wt % 40/100 mesh cast crushed tungsten carbide; about 28 wt % 325 mesh iron powder; about 3 wt % silico-manganese powder and about 1.2 wt % methylcellulose binder. 
     The powder mixture is hydrated to a pH-adjusted moldable rheology and is injection-molded to form a powder core for later encasing with a steel sheath  14 . The cores  12  or core/sheath assemblies  110  are desiccated to remove water of hydration, providing sufficient handling strength for subsequent processing. 
     As shown in FIG. 9, the hydrated powder mixture  122  is injection molded by using an injection unit  124  to inject the powder mixture  122  into a mold  126 . The mold  126  is of elongate tubular form having an internal cross-sectional shape chosen depending upon the application in which the hardfacing material is to be used. For example, the internal cross-sectional shape of the mold  126  may be circular, oval, square, hexagonal or of star-shaped form. The mold  126  is dimensioned to allow for shrinkage of the molded material, and has sufficient wall thickness to withstand the injection pressure used which may be several thousand psi. Alternatively, a thinner wall section may be employed if reinforcement means are employed in the injection unit. Such reinforcement could comprise a close-fitting rigid split die constrained with a closure force sufficient to carry substantially all of the injection stress applied to the mold  126 , or a hydraulically-activated elastomeric bladder surrounding the mold  126  with balancing pressure applied during the injection cycle. 
     Once the mixture  122  has been injected into the mold  126 , the filled mold  126  is moved to a curing station  128  where strengthening of the mixture  122  occurs preferably by desiccation. Such desiccation may be accelerated, if desired, by heating or otherwise controlling the conditions around the mold. Once it has been determined that the material has cured to an extent sufficient to permit handling thereof, the core is extracted from the mold. Although this could be achieved by splitting the mold  126  in two or more parts to permit opening thereof, it is not necessary. In the preferred embodiment, the constituency of the mixture  122  is adjusted such that curing shrinkage provides sufficient clearance between the core and tube mold to release the material for gravity or ejector pin extraction. 
     While the mold  126  is located at the curing station  128 , another mold  130  is moved to the injection unit  124  and the mixture  122  injected into the mold  130 . The curing station  128  is preferably designed to receive sufficient molds to enable the manufacturing process to run continuously. 
     A device for wrapping a steel sheath  14  about a core  12  is shown in FIG. 10. A roll of steel strip  100 , is pulled into a set of rolls  102 . Steel sheathing strip  100  may be manufactured from a variety of metal alloys, but is preferably manufactured from annealed low-carbon AISI 1008 steel. The rolls bend the steel strip  100  to form into a ‘U’ shape. A core  104  is then placed into the ‘U’ shaped steel strip. The assembly is further pulled through a folding die  106  and optionally through a reinforcing chamber (or die)  108  to form a core-sheath assembly  110 . The core-sheath assembly  110  is then cut into a convenient length, typically about 28 in., by a cut-off saw  112 . 
     The desiccated core-sheath assembly  110  is then prepared for compaction. Preferably, compaction of the desiccated core-sheath assembly  110  is done in a cold isostatic press. Referring now to FIGS. 11 and 12, in order to achieve sufficient densification, it is necessary to encapsulate and seal the core-sheath assemblies  110  in, for example, an elastomeric material, wrapping a sheet  114  as shown in FIG.  11  and sealing in a secondary container to encapsulate it, or preferably, encapsulating multiple core-sheath assemblies  110  in a common elastomeric multi-cavity mold  116  shown in FIG.  12 . 
     Encapsulated core-sheath assemblies  110  are compacted within a cold isostatic press (not shown) by hydraulic compaction to a pressure between about 30,000 psi (30 ksi) and about 50,000 psi (50 ksi)—preferably 40 ksi. The compaction cycle transforms the core-sheath assembly  110  into a high density hard facing rod  10 . After compaction, the high density hard facing rod  10  reflects a wrinkled appearance due to accommodation-buckling of the sheath as indicated by numeral  120  in FIG.  1 . 
     The high density hard facing rod  10  is then normally dried and stored for use, or it may be treated at higher temperature to remove the residual fugitive binder prior to application. Void content for composite rod with 60 wt % hardmetal is typically 40 vol % before compaction, decreasing to about 31 vol % after CIP-densification. The high density hard facing rod  10  is about 13% more dense, with a tightly adherent sheath, reduced oxidation susceptibility, and increased strength and ductility. 
     As shown in FIGS. 2-8, the high density hard facing rod  10  is designed to be applied to steel substrates, typically the surfaces of drill bits  16  and other downhole tools  56 ,  58 ,  62  by oxy-fuel welding (OFW). When oxygen-acetylene is utilized, flame temperature and reducing characteristics are established through various gas flows with a slightly oxygen reducing (excess acetylene) flame adjustment. Oxygen consumption and application rates vary directly with the surface area, with oxygen flows varying considerably. 
     A hardfacing layer made from the high density hard facing rod  10  exhibits well-preserved cemented tungsten carbide particles as a primary constituent, which retain 90% or more of their original hardness, with monocrystalline and cast WC/WC as secondary phases. Total deposit carbide volume fractions of about 57% are typical, with porosity volume fractions less than 1%. The fraction of the sintered tungsten carbide particles in the deposit with greater than or equal to 90% hardness retention is greater than 84% even on large substrates, and often exceeds 90%. Tungsten carbide particle distribution is uniform, allowing hardfacing optimizations via site and shape control. Deposition rate increases of 50-100% over low-density equivalents are attributed to faster melting and the absence of silicate adhesive complications. 
     Tailoring of matrix properties by core powder additions or steel sheath alloy modifications is straightforward. These property and application advantages result from the minimization of the time that the materials spend in the high temperature, molten state between rod  10  and deposit. This is a direct advantage of using a powdered (but not sintered) core  12  with a very thin steel sheath  14 , and OFW application, which does not gouge the substrate and provides well controlled deposition and cooling. 
     The volume per cent of the cemented carbide primary particles exhibiting 90% or more of their original hardness is at least 85%. These carbide primary particles can be comprised of one or more carbides of the elements W, Mo, Cr, Ta, Nb, and V. 
     Applications for the high density hard facing rod  10  are numerous. Referring now to FIGS. 2-4, one type of earth boring drill bit  16 , a tooth type rolling cutter drill bit, is shown as numeral  24  in FIG.  2 . Typically, tooth bits  16  have a body  26  upon which are mounted rolling cutters  28  with cutting teeth  30 . In operation weight is applied to the bit, forcing the cutting teeth  30  of the cutters  28  into the earth  29 , and, as the bit  24  is rotated, the earth  29  causes the cutters  28  to rotate effecting a drilling action. 
     The teeth  30  are generally wedge shaped with a pair of relatively flat flanks  32  and a crest  34 . During drilling, the crest  34  is forced into the earth formation. By design, the rolling cutters do not allow a true rolling action of the teeth  30  when drilling. 
     Therefore, each tooth  30  is scraped, or plowed in a short distance through the earth formation as it is penetrating the earth. In order to prolong the life of the drill bit a hardfacing  36  is applied to the flanks  32  and crest  34  of the teeth  30  with the high density hard facing rod  10  of the present invention. 
     Another type of drill bit  16 , a fixed cutter drag-type drill bit  38 , is shown in FIG.  3 . The fixed cutter drill bit  38  comprises a bit body  40  machined from steel and having blades  42  formed on the leading face  43  of the bit  38 . Extending side-by-side along each of the blades  42  is a plurality of cutting structures, indicated at  44 . 
     The gauge region  46  of the drill bit  38  must resist the loading and abrasion arising from constant scraping against the borehole wall  39 . Therefore, there is applied to the surface  46  a hardfacing  48  with the high density hard facing rod  10  made by the process of the present invention. 
     Referring now to FIGS. 5 and 6, there are shown other down hole tools utilizing the hardfacing applied from the high density hard facing rod of the present invention. In FIG. 5, a stabilizer  22  is shown with a plurality of blades  50 . Each blade  50  must be able to withstand the severe abrasion and loads it is subjected to during operation. In order to extend the life of these blades, a hardfacing material  52  is often applied. Hardfacing  52  applied by the high density hard facing rod made by the process of the present invention is suitable for this application. 
     Shown in FIG. 6, is a rotary steerable unit  20 , with a bias pad  18 . The bias pad  18  repeatedly engages the sidewall  39  of the borehole  31  in order to push the tool to one side as directed by its control system. Because these bias pads  18  repeatedly apply extreme loads to the borehole wall  39 , they must be coated with, or made from a very abrasion resistant and strong material such as a hardfacing  54  and applied by the high density hardfacing rod  10  made by a process of the present invention. 
     Referring now to FIGS. 7 and 8, are shown other applications utilizing downhole tools  56 ,  58  having a hardfacing applied with a high density hard facing rod made with the process of the present invention. In FIG. 8 a number of different tools  56 ,  58  are shown in the drill string  60 . These tools  56 ,  58  may include, but are not limited to, downhole motors, measuring while drilling tools, logging tools, vibration dampers, shock absorbers, and centralizes. These tools  56 ,  58  benefit from hardfacing applied with high density hard facing rod made by the process of the present invention. In particular, down hole bottom hole assemblies  62 , as shown in FIG. 7, are often operated while gravity is pushing them against the borehole wall  39 . Once again the extreme abrasion and loads applied to the size of these tools make them benefit from a hardfacing applied with a high density hardfacing rod made by the process of the present invention. 
     Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.