Abstract:
A wear insert comprised of cemented metal carbide is welded to a workpiece made of steel or cemented metal carbide without causing fracturing of the carbide or creating residual stresses that reduce the impact resistance of the part. The part is fabricated using a microwave sintering process prior to welding.

Description:
BACKGROUND 
     Cemented metal carbides and other cermets, polycrystalline diamond (PCD), and cubic boron nitride (CBN), and combinations of them, have been used for many years for cutting tools, hard facing, wear inserts, cutting inserts, and other wear parts and surfaces in various types of tools because of their desirable properties of hardness, toughness and wear resistance. Cemented metal carbide refers to a carbide of one of the group IVB, VB, or VIB metals which is pressed and sintered in the presence of a binder of cobalt, nickel, or iron and the alloys thereof. The most common example of a cemented metal carbide used in downhole applications is tungsten carbide (WC). Polycrystalline diamond is made by sintering powdered diamond in the presence of a catalyst, such as a cobalt alloy or nickel, resulting in intercystalline bonding between individual diamond crystals. The diamond can be synthetic or natural diamond, cubic boron nitride, or wurtzite boron nitride as well as combinations thereof. PCD is typically utilized in wear applications as a crown layer attached to a base comprised of cemented WC. Such an insert is sometimes referred to as a polycrystalline diamond compact (PDC). 
     Drill bits, rock mills and other earth boring tools used in oil and gas exploration are examples of tools that make use of wear resistant inserts for surfaces that will be subject to substantial abrasion and wear. Examples of inserts with abrasion resistant wear surfaces include abrasive jet nozzles, long life wear parts, carbide cutting tools, carbide wire drawing dies, cold heading dies, valve components (including seats), scuff plates, saw blades, deflector plates, milling tools, finishing tools, and various types of components for down hole tools, such as cutters and other inserts for earth boring bits (including rotary and drag bits) and bearing wear surfaces, such as mud-lubricated radial bearings and thrust bearings. An example of diamond bearing comprising a composite having a crown formed of PCD on a substrate of cemented carbide is as described by U.S. Pat. No. 4,729,440. Examples of cutters, bearings, and other types of inserts made from cemented metal carbides and PCD, and methods of manufacturing them, can be found in U.S. Pat. Nos. 6,500,226; 6,315,066; 6,126,895; 6,066,290; 6,063,333; 6,011,248; 6,004,505; 5,848,348; 5,816,347. 
     Inserts made from cemented metal carbide, PCD and cermets are joined to other components of a tool by either press fitting or brazing the insert. Brazing involves melting between two work pieces a filler metal having a melting point below the melting point of each of the work pieces, thereby forming a bond between the two work pieces. Examples of filler metal used for brazing are various alloys of cobalt. Brazing does not cause melting of either of the work pieces. Welding, on the other hand, requires heating adjacent portions of two work pieces above their respective melting points to form a pool of molten material, called weld pool, resulting in material from each piece inter-diffusing to form a bond that joins the pieces when cooled. Welding can be done either with or without the presence of a filler material. 
     Generally speaking, welding cemented metal carbide is not feasible or recommended due to stresses caused by heating of the cemented metal carbide. Although cemented metal carbides are very hard, tough, and resistant to wear, they are also relatively brittle. A small amount of strain can lead to its fracture. Furthermore, the more wear resistant, or harder, cemented tungsten carbide is made, the less tough and resistant to fracture it is. Uneven heating of a cemented metal carbide part leads to large temperature gradients across the part, which induces substantial stress across the part due to different degrees of thermal expansion caused by the uneven heating. Additionally, metal carbides also have a substantially different coefficient of thermal expansion as compared to stainless steel, which is the type of metal of which the bodies of and moving parts of down hole tools are fabricated due to is corrosion-resistance, strength and machinability. Heating a cemented metal carbide part and a steel part hot enough to melt the respective materials at the boundary between the two pieces creates substantial stress on the cemented metal carbide part when it cools. The stress typically leads to fracturing of the cemented metal carbide part during welding. If it does not immediately fracture, the residual stress within the part leads to substantially heightened susceptibility to fracturing when loaded, making the part not feasible for use, especially on downhole tools likely to experience high impact loads. 
     SUMMARY 
     The invention relates to a process and apparatus for welding a part, such as a wear insert, made of cemented metal carbide to a workpiece made of an alloy of a Group VIII transitional metal (such as steel) or cemented metal carbide without causing fracturing of the carbide or creating residual stresses that reduce the impact resistance of the part. The part is fabricated using a microwave sintering process prior to welding to have a higher modulus of elasticity as compared to a part containing the same materials fabricated using conventional high temperature and high pressure sintering methods. 
     In one example a wear surface insert for a bearing for an earth boring tool is fabricated by sintering an insert made from cemented tungsten carbide in a microwave furnace. The insert is welded to a workpiece forming part of a downhole tool. In another example, a bearing of an earth boring tool is fabricated by sintering in a microwave furnace a compact formed of layer particles of diamond on a substrate of tungsten carbide, the resulting sintered compact being joined to a steel component of the tool by welding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart illustrating a process for manufacturing a tool with a wear surface insert made from microwave sintered PCD, cemented carbide or a cermet. 
         FIG. 2  is a schematic diagram of a microwave furnace for sintering cemented metal carbide inserts. 
         FIG. 3A  schematically illustrates one configuration of a resistance welder for welding a cemented carbide insert to a stainless steel part. 
         FIG. 3B  schematically illustrates one configuration of a resistance welder for welding a polycrystalline diamond compact (PDC) insert to a stainless steel part. 
         FIG. 3C  schematically illustrates an alternate configuration for welding a PDC to a stainless steel part. 
         FIG. 4A  is a photograph of a polished cross section taken through a microwave-sintered, cemented tungsten carbide insert welded to a stainless steel part at 50× magnification. 
         FIG. 4B  is a photograph of the polished cross section of  FIG. 4A  at 200× magnification. 
         FIGS. 5A ,  5 B and  5 C illustrate an example of a thrust bearing for a downhole tool, such as a motor, turbine, rock mill, or a tri-cone rotary drill bit.  FIG. 5A  is a plan view,  FIG. 5B  is a cross-section of  FIG. 5A , taken along section line  5 B- 5 B, and  FIG. 5C  is an exploded, perspective view. 
         FIGS. 6A ,  6 B and  6 C illustrate an example of a radial bearing for a downhole tool, such as a motor, turbine, rock mill, or a tri-cone rotary drill bit.  FIG. 6A  is a plan view,  FIG. 6B  is a cross-section of  FIG. 6A , taken along section line  6 B- 6 B, and  FIG. 6C  is an exploded, perspective view. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In the following description, like numbers refer to like elements. 
       FIG. 1  illustrates the basic steps comprising a process  100  for welding a portion of an insert made at least in part of cemented carbide to an element of a tool, or a component of a tool, made of steel. 
     At step  102 , loose grains of metal carbide and a metal binder are combined to form a homogenous mixture, which is then shaped or formed into a “green” part that has very near the dimensions and shape of a desired cemented metal carbide part. The green part is formed, for example, by compacting the powders into a mold by cold pressing. It may also be precast with a sacrificial wax if necessary. One example of a metal carbide is tungsten carbide. Typically, the metal binder is a metal alloy containing about 80 to 96% cobalt. Additional materials can also be added. 
     After it is formed, the part is then sintered at step  104  using microwave radiation to heat the part to a point that is below the melting temperature of the metal carbide, but high enough to cause the metal binder to melt throughout the matrix of metal carbide grains, resulting in the particles of carbide fusing or adhering to one another to thereby form a single, solid mass. 
       FIG. 2  is an example of a furnace for a continuous microwave sintering process. Electromagnetic waves generated by microwave energy generator  210  are transmitted through waveguide  212  to chamber  214 . One or more parts  215  to be sintered are placed inside crucibles  216 . The green parts are placed or stacked in each crucible. The crucibles are then transported through chamber  214 , where they are subjected to microwave energy. The crucibles are preferably made from a material that has a very low coupling with microwave energy and thus is somewhat transparent to the microwaves that are used to heat the material from which the parts are made. Examples of such materials are silicon nitride, alloys of silicon nitride, including an alloy composed of silicon nitride and aluminum oxide called “sialon,” hexagonal boron nitride, and low thermal expansion ceramics like sodium zirconium phosphate. 
     In the illustrated example, gravity is used to transport the crucibles through the microwave by stacking them vertically and moving the stack through chamber  214  by removing the bottom-most crucible one at a time. A vertical tube  218  or other structure may be used to keep the crucibles stacked and provide an enclosed environment for an appropriate atmosphere. Crucibles are conveyed into an opening at the top of the tube using a conveyer  220  or any other type of transport or conveyance means. The crucibles exit an opening in the bottom of the tube onto conveyor  222 . An inert or reducing gas is introduced into the tube near the bottom of the tube and exits the tube near the top of it, as indicated by arrows  224  and  226 . A structure  228  functions to pass the crucibles from the tube while preventing air from entering the tube and gas from spilling out of the tube. A similar structure  230  is located at or near the top end of the tube for allowing crucibles to be inserted into the tube while keeping air out of it. Additional details of this type of continuous process system are contained in U.S. Pat. No. 6,004,505 and related patents, which are incorporated herein by reference. 
     Microwave heating to sinter metal carbides offers several advantages. It shortens sintering times. Shorter sintering times result in less chemical and phase change in the metal binder, which is typically cobalt or an alloy containing cobalt. Typical conventional high pressure, high temperature (“HP/HT”) or hot isostatic pressure (“HIP”) sintering (see, for example, U.S. Pat. No. 4,684,405) require temperatures of 1400 degrees centigrade for as long as 12 hours, whereas microwave sintering involves sintering times lasting on the order of 2 to 5 minutes. Shorter sintering times also result in smaller changes in the size of the grains. Smaller changes in the grain size yield more predictable and consistent carbide grain structures. More even heating is possible with microwave, which results in more uniform shrinkage of the part and more uniform distribution of the binder during cooling. Microwave sintering also allows for uniform cooling after sintering, which allows for better management of stresses within the part and better phase control of the metal binder. A microwave sintered metal carbide part typically possesses higher modulus of elasticity, yield strength, and impact strength and greater thermal and electric conductivity as compared to a part having the same starting materials sintered using conventional HP/HT and HIP methods. 
     A polycrystalline diamond compact insert that is comprised of a microwave-sintered, metal carbide substrate and a crown or working surface layer made of polycrystalline diamond can be made in any one of several ways. 
     In a first way, a body of cemented metal carbide and a crown of PCD, CBN or WBN are separately sintered and joined by brazing. The cemented carbide body is sintered using microwave sintering, as described above. The crown is formed from micron-sized diamond, CBN or WBN crystals engineered for specific properties such as abrasion resistance and impact strength that are blended to a controlled distribution and then sintered using HP/HT, HIP or microwave radiation. The two pieces are then bonded by brazing. The brazing of the crown to the substrate could, optionally, occur after the cemented carbide base is welded to a steel substrate. 
     In the second way, a layer of powdered carbide and metal binder and a layer of micron-sized diamond, CBN or WBN crystals are placed in the same mold. The layer of crystals may or may not include a metal binder. Between the layer of crystals and the carbide layer is optionally placed one or more transition layers that are comprised of a mixture of diamond particles and metal carbide, with or without the presence of a metal binder between the layer of crystals and the carbide layer. The molded part is then sintered using microwave radiation. 
     In a third way, micron-sized diamond, CBN or WBN crystals engineered for specific properties such as abrasion resistance and impact strength are blended to a controlled distribution and placed with a cemented carbide substrate, which has been previously sintered using microwave radiation, in a refractory container called a cell. The cell is placed in a computer controlled press at pressures of approximately one million pounds per square inch and temperatures of about 2600 degrees Fahrenheit. While under high pressure, a current is passed through the cell to create high temperature, and the diamond or CBN crystals fuse together to form an integral, superabrasive, polycrystalline layer bonded to the carbide, with uniform properties in all directions. The polycrystalline diamond layer can optionally be made more thermally stable by either entirely leaching or partially leaching metal catalyst used for sintering the diamond particles. 
     U.S. Pat. Nos. 5,641,921, 5,848,348, 6,004,505, 6,011,248 and 6,500,226, which are incorporated herein by reference, disclose additional information about processes for microwave sintering metal carbides and forming polycrystalline diamond compacts. Other examples of inserts containing at least a region or portion made from microwave sintered carbide include inserts with tungsten carbide bodies reinforced with thermally stable polycrystalline diamond (TSP) and dispersed diamond grit shown in U.S. Pat. No. 6,315,066, which is also incorporated herein. 
     Referring now only to  FIG. 1  the final steps in the process  100  are, at step  106 , positioning a surface of a portion of the insert made of microwave-sintered, cemented carbide adjacent to the surface of the part to which it will be welded, applying heat to the adjoining surfaces to melt the surfaces and cause inter-diffusion of the melted material at step  108 , and then allow the weld to cool at step  110 . 
     In  FIG. 3A  an insert  300  made entirely of microwave-sintered cemented metal carbide is placed during step  106  adjacent to workpiece  302  and held, such as by clamping, adjacent to the workpiece by electrodes  306  and  308  of a resistance welder  310 . The workpiece  302  in this example, is made of stainless steel or any other alloy made from a Group VIII metal or microwave sintered cemented metal carbide. One example of a resistance welding machine suitable for use is the Streamline, LPW Series resistance spot/projection welder sold by the Roueche Company, LLC. The workpiece  302  comprises, for example, a component or body of a downhole tool. A plurality of microwave-sintered, cemented metal carbide parts can be welded to the same work piece. Welding multiple microwave sintered inserts to the same workpiece effectively extends the size of the wear surface area. 
     A pulse of electric current is applied to electrodes by the resistance welder  310 . The resistance to current flow at the boundary between the pieces causes a generation of heat within the immediate vicinity of the boundary, and raises the temperature of the steel and the cemented metal carbide high enough to result in melting of the respective pieces immediately adjacent the boundary. A weld puddle forms between the insert  300  and the workpiece  302 , resulting in the cemented carbide substrate and steel inter-diffusing in a region  304  where the two pieces adjoin, thereby forming a weld once the joint is allowed to cool at step  110 . A filler material, such a cobalt or nickel, or an alloy containing cobalt and/or nickel, may be placed between the two pieces during welding, but it is not necessary. 
     The electrodes can be placed in any position that results in a current flowing across the boundary of the pieces being welded. 
       FIGS. 3B and 3C  illustrate two approaches to welding to a part  302  made of stainless steel to a PDC insert  312  having a microwave-sintered, cemented carbide substrate  314  and a sintered polycrystalline crown  316 . Polycrystalline diamond and similar materials conduct electricity poorly. In  FIG. 3B , microwave sintered, cemented metal carbide insert  300  is welded first to part  302  in the manner shown in  FIG. 3A . The substrate  314  of the PDC  312  is then welded to the insert by placing collet  318  around the substrate  314  and collet  320  around the insert  300  and forming a weld  319  using resistance welding. Collets  318  and  320  preferably encircle the substrate and are connected to the resistance welder  310  in place of the electrodes  306  and  308  ( FIG. 3A ). The PDC insert  312  and the metal carbide insert  300  need to be held together, as indicated by arrows  321 , by a clamp or similar mechanism. As shown in  FIG. 3C , a microwave-sintered, cemented metal carbide substrate  322  of PDC insert  324 , which has a top layer or crown  326  of PCD can also be directly welded to a stainless steel part  302  by placing around the substrate  322  collet  328  and connecting resistance welder  310  to electrode  308  and collet  328 . A clamping force, indicated by arrow  321 , is applied to hold the parts together. The substrate  322  is made thicker to accommodate the collet  328 . Current is appended to form weld  330 . 
     As an alternative to resistance welding, a piece of microwave-sintered, cemented metal carbide can be welded to another piece of the microwave-sintered, cemented metal carbide or to a Group VIII metal alloy using a capacitive discharge welder or other type of welder that delivers a pulse of electrical current that causes heating in the immediate vicinity of the two surfaces that will be joined by the weld. 
       FIGS. 4A and 4B  are photographs of a polished cross-section of a microwave-sintered, tungsten carbide and cobalt insert  400  welded to a piece of 4140 stainless steel  402  using a streamline, LPW Series resistance welder sold by the Roueche Company and the method described in connection with  FIG. 1 . The photograph of  FIG. 4A  is taken along the weld at a magnification of 50×, and the photograph of  FIG. 4B  is taken along the weld at a magnification of 200×. Before sintering, the insert was comprised of a mixture of 1 to 2 micron tungsten carbide powder and cobalt. The amount of cobalt contained in the body was 13% by weight. The insert was sintered using a microwave furnace substantially as described in connection with  FIG. 2 . After sintering, the tungsten carbide insert was welded to a piece of 4140 stainless steel using a Streamline, LPW Series resistance spot/projection welder sold by the Roueche Company, LLC. 
     Based on the photos, the resulting weld  404  appears to be approximately 2 to 3 microns thick. Cobalt, the cementing metal in the tungsten carbide, appears to have melted and wet the tungsten carbide grains along the boundary of the insert adjacent to the stainless steel, without substantially affecting the integrity of the metal carbide matrix, even in the immediate vicinity of the weld. The melted cobalt appears also to have inter-diffused with a thin layer of melted stainless steel immediately adjacent the boundary between the two pieces. However, the tungsten carbide grains in the sintered part do not appear to have been substantially disturbed, such as by the carbide dissolving into the metal binder and precipitating into the weld or by the melting of the metal binder much beyond the immediate surface of the sintered tungsten carbide. The weld is therefore predominantly of a mixture of cobalt and stainless steel. The part did not fracture during or after welding. 
       FIGS. 5A-5C  and  6 A- 6 C illustrate, respectively, examples of a thrust bearing and of an axial bearing having bearing surfaces, each of the bearing surfaces being comprised of a plurality of inserts  39 . In these embodiments, each of the inserts is comprised of a microwave-sintered cemented metal carbide substrate, for example a tungsten carbide insert cemented with cobalt. Thrust bearing  30  is comprised of two races  40  and  42 . Axial bearing  32  is similarly comprised of two races  36  and  38 . Each of the races is made from stainless steel. The microwave sintered metal carbide inserts are welded to the race in the manner described above in connection with  FIGS. 1 ,  2  and  3 A- 3 C. 
     To achieve the necessary curvature the inserts can be cast with the curvature on the top and bottom and milled as necessary to achieve the desired geometry. Alternately, the insert can be cast with a flat bottom that is set on a complementary flat surface that is machined in the race. 
     Except for the welding of the microwave sintered, cemented metal carbide inserts to the races, the bearings in these figures are substantially similar to bearings found in the prior art, and are only included to be representative of such bearings for downhole tool applications. Another example of bearing surfaces comprised of PCD is a roller cone drill bit described in U.S. Pat. No. 4,729,440. Using the process described herein, a PDC with microwave-sintered, cemented metal carbide substrate is substituted for the polycrystalline diamond compacts used for the bearing surfaces described in this patent, and then welded rather than brazed or mechanically press fitted, to the stainless steel parts of the tool. 
     The foregoing exemplary embodiments employ, at least in part, certain teachings of the invention. The invention, as defined by the appended claims, is not limited to the described embodiments. Alterations and modifications to the disclosed embodiments may be made without departing from the invention. The meaning of the terms used in this specification are, unless expressly stated otherwise, intended to have ordinary and customary meaning and are not intended to be limited to the details of the illustrated structures or the disclosed embodiments.