PERCUSSION ROCK BIT

Abstract: A percussion rock bit comprises a steel body (10) having a pin (12) for connection to a drill string at one end and having a plurality of inserts (16) at the other end for crushing rock at the bottom of a hole being drilled. The inserts (16) have a cemented tungsten carbide body (20) partially embedded in the steel bit and at least two layers at the protruding drilling portion of the insert. The outermost layer (26) contains polycrystalline diamond. The remaining layers adjacent the polycrystalline diamond layer (26) are transition layers (28,30) containing a composite of diamond crystals and precemented tungsten carbide, the composite having a higher diamond crystal content adjacent the polycrystalline diamond layer (26) and a higher precemented tungsten carbide content adjacent the tungsten carbide body (20).

Description: The present invention relates to percussion rock bits for example for drilling blastholes or the like, and having polycrystalline diamond-tipped inserts for drilling rock formation. Percussion tool bits (as described, for example, in EP-A-0 122 242) are employed for drilling rock, for instance as in drilling wells, or more frequently for drilling blastholes for blasting in mines and construction projects. The bits are connected to a drill string at one end and typically have a plurality of cemented tungsten carbide inserts embedded in the other end for drilling rock formations. In use, the drill string and bit body are moved up and down rapidly, striking the rock being drilled in a percussive motion. A typical air hammer for percussion bits operates at about 2000 blows per minute and rotates at about 60 r.p.m. Some percussion bits are driven through hydraulic action. The percussion bit hammers the inserts against the rock being drilled, shattering it by repeated blows. Compressed air pumped through the bit removes chips of fractured rock from the bore hole being drilled. Such bits range in size from 75 millimetres to more than 25 centimetres in diameter. Rock bits wear out or fail in such service after drilling many metres of bore hole. The cost of the bits is not considered so much in the cost of the bit, per se, as much as it is considered as the cost of drilling per length of hole drilled. It is considered desirable to drill as much length of bore hole as possible with a given bit before it is used to destruction. It is also important that the gauge diameter of the holes being drilled remain reasonably near the desired gauge. Thus, wear of the bit that would reduce the hole diameter is undesirable. Further, wear of the inserts in the bit during drilling reduces their protrusion from the surface of the steel bit body. The protrusion has a strong influence on the drilling rate. Thus, as the inserts wear out, the rate of penetration may decrease to the extent that it becomes uneconomical to continue drilling. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation, both for reducing bit costs and for maintaining a reasonable rate of penetration of the bit into the rock. When a rock bit is drilling a bore hole, it is important that the diameter or gauge of the bore hole be maintained at the desired value. The outermost row of inserts on a rock bit is known as the gauge row. This row of inserts is subjected to the greatest wear. As the gauge row inserts wear, the diameter of the bore hold being drilled may decrease below the original gauge of the rock bit. The rate of penetration of a rock bit into the rock formation being drilled is an important parameter for drilling. Clearly, it is desirable to maintain a high rate of drilling since this reduces the time required to drill the borehole, and such time can be costly because of the fixed costs involved in drilling. The rate of penetration decreases when the inserts in the bit become worn and do not protrude from the surface to the same extent as they did when drilling commenced. The worn inserts have an increased radius of curvature and increased contact area on the rock. This reduces the rate of penetration. Thus, it is important to maximize the wear resistance of the inserts in a rock bit to maintain a high rate of penetration as long as feasible. It is particularly important to minimize wear of the gauge row inserts to maximize the length of hole drilled to full gauge. Wear resistance of conventional inserts of cemented tungsten carbide may be enhanced by increasing the proportion of tungsten carbide and decreasing the proportion of cobalt in the composite material. This increases the hardness and wear resistance of the cemented tungsten carbide but reduces its toughness so that the inserts are more susceptible to breakage than inserts with high cobalt content. In exemplary embodiments, the cobalt content of inserts for use in rock bits ranges from about six percent to sixteen percent by weight cobalt. Another factor that influences wear resistance and toughness is particle size of the tungsten carbide phase. Exemplary particle size in an insert is in the range of from three to seven microns. This particle size is an average particle size of a powder mixture that includes larger and smaller particles. For example, when the average particle size is five microns, there are submicron size particles present as well as particles as large as seven or eight microns. Generally speaking, toughness increases with larger particle size and so does wear resistance. A common grade of cemented tungsten carbide for rock bit inserts has an average tungsten carbide particle size of about six microns and contains about ten to fourteen percent by weight cobalt. Toughness of the inserts is important in a rock bit since the inserts are subjected to impact loads, as well as wear by rubbing against the rock formation. Breakage of inserts can be a substantial problem since it not only results in reduced drilling activity, but the fragments of a broken insert may damage other inserts. It is therefore desirable to provide inserts that are hard, to resist wear, and tough, to resist breakage. According to the present invention there is provided, a rock bit having a steel body with means at one end for connecting the bit to a drill string and a plurality of inserts at the other end for exerting percussive force on the bottom of a bore hole, and thus crushing the rock to be drilled. At least some of those inserts comprise a cemented tungsten carbide body having a shank portion embedded in the head of the bit and a converging end portion protruding from the surface of the head. The converging portion of the insert includes at least one outer layer containing polycrystalline diamond and at least one transition layer between the polycrystalline diamond layer and the cemented carbide body. The transition layer comprises a composite material containing diamond crystals, cobalt and precemented tungsten carbide particles. The diamond crystal content of the composite material is greatest adjacent the polycrystalline diamond layer and lowest adjacent the cemented tungsten carbide body. Percussion rock bits embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Fig. 1 is a fragmentary longitudinal cross-section of the exemplary rock bit; Fig. 2 is a cross-section of one of the inserts of the rock bit shown in Fig. 1; and Fig. 3 is an enlarged cross-section along line 3-3 in Fig. 2. Fig. 1 is a fragmentary longitudinal cross-section of an exemplary percussion rock bit. The bit comprises a hollow steel body 10 having a threaded pin 12 at the upper end of the body for assembly of the rock bit on to a drill string for drilling oil wells or the like. The body includes a cavity 32 and holes 34 communicating between the cavity and the surface of the body. The holes divert the air pumped through the bit by the air hammer, out of the cavity into the borehole to provide cooling and remove rock chips from the hole. The lower end of the body terminates in a head 14. The head is enlarged relative to the body 10 and is somewhat rounded in shape. A plurality of inserts 16 are provided in the surface of the head for bearing on the rock formation being drilled. The inserts provide the drilling action by engaging and crushing subterranean rock formation on the bottom of a bore hole being drilled as the rock bit strikes the rock in a percussive motion. The outer row of inserts 18 on the head is referred to as the gauge row since these inserts drill the gauge or outer diameter of the bore hole. An exemplary insert is illustrated in Fig. 2. Such an insert comprises a precemented tungsten carbide body 20 having a cylindrical grip length extending along a major portion of the insert. At the lower end of the body, protruding from the face of the bit, the insert has a converging portion 24. The converging portion illustrated in Fig. 2 has the preferred hemispherical shape; however, the converging portion may have any of a variety of shapes depending on the desired cutting structure. The converging portion may also be described as a projectile shape, a cone with a rounded end, a chisel shape, or any of a variety of shapes known in the art. Typically, the inserts are embedded in the bit by press-fitting or brazing into the head 14 of the bit. The head has a plurality of conical bottomed holes on its outer surface. The inner end of the insert is conical to fit closely to the bottom of the hole and distribute stresses uniformly to the steel bit body 10. An exemplary hole has a diameter about 0.13 mm smaller than the diameter of the grip of an exemplary insert. The insert is pressed into the hole in the steel head with many thousand pounds of force. This press fit of the insert into the bit tightly secures the insert in place and prevents it from being dislodged during drilling. Preferably, the body 20 of the insert is made of two pieces of cemented tungsten carbide bonded together. In the exemplary embodiment, the border between the two pieces of carbide is located at about the base of the converging portion 24, that is, at about the end of the portion of the insert that protrudes from the surface of the head. The inner carbide piece 22 embedded in the head of the bit comprises 614 grade tungsten carbide (6 micron tungsten carbide particles; 14 percent by weight cobalt content) and the outer carbide piece 23 adjacent the inner carbide piece comprises 406 grade tungsten carbide (4 micron tungsten particles; 6 percent by weight cobalt content). The converging portion 24 of the exemplary insert includes an outer layer 26 for engaging rock and two transition layers, outer transition layer 28 and inner transition layer 30, between the outer layer 26 and the cemented tungsten carbide body of the insert. The outer layer 26 comprises polycrystalline diamond (PCD) that includes a mixture of diamond crystals and between one and ten percent by volume (preferably six percent by volume) cobalt. In the exemplary embodiment, the outer layer 26 is 130 microns thick. A range of thickness between 80 microns and 1300 microns is suitable. An outer layer thinner than 80 microns is likely to show thin spots, decreasing its drilling effectiveness. The transition layers 28 and 30 comprise a composite of diamond crystals, cobalt and precemented tungsten carbide containing a greater proportion of diamond crystals near the outer PCD layer 26 and a greater proportion of precemented tungsten carbide particles near the insert body 22. While the currently preferred embodiment comprises two distinct transition layers, any number of transition layers can be used, as long as the proportion of diamond crystals decreases inwardly towards the cemented tungsten carbide body. A single transition layer with an inwardly decreasing gradient of diamond crystals can also be used. The exemplary outer transition layer 28 adjacent the outer layer 26 comprises a mixture of diamond crystals, cobalt and precemented tungsten carbide particles. Preferably, the layer comprises 75 percent by volume preferred diamond crystals and 25 percent by volume precemented tungsten carbide. The preferred diamond portion includes between one and ten percent (preferably six percent) cobalt. Consequently, in a preferred embodiment, the outer transition layer 28 includes 70.5 percent diamond crystals, 4.5 percent cobalt and 25 percent precemented tungsten carbide. It will be recognised that the precemented tungsten carbide moiety also includes cobalt. The exemplary outer transition layer is approximately 250 microns thick; a range of thickness between 50 microns and 5000 microns is suitable. An outer transition layer thinner than 50 microns is likely to show thin spots. The exemplary inner transition layer 30 includes 50 percent by volume preferred diamond crystals and 50 percent by volume precemented tungsten carbide. Again, the preferred diamond portion includes between one and ten percent (preferably six percent) cobalt. Consequently, the preferred embodiment of layer 30 includes 50 percent by volume precemented tungsten carbide, 3 percent by volume cobalt and 47 percent by volume diamond crystals. The exemplary inner transition layer is approximately 380 microns thick; a range of thickness between 50 microns and 5000 microns is suitable. An outer transition layer thinner than 50 microns is likely to show thin spots. In an alternative preferred embodiment, the outer transition layer 28 includes 60 percent by volume preferred diamond, that is, 3.6 percent by volume cobalt, and 56.4 percent by volume diamond, and 40 percent by volume precemented tungsten carbide. The inner transition layer 30 includes 40 percent by volume preferred diamond, that is, 2.4 percent by volume cobalt and 37.6 percent by volume diamond, and 60 percent by volume tungsten carbide. As used in the specification, the term polycrystalline diamond along with its abbreviation "PCD", refers to the material produced by subjecting individual diamond crystals to sufficiently high pressure and high temperature that intercrystalline bonding occurs between adjacent diamond crystals. Exemplary minimum temperature is about 1300 °C and an exemplary minimum pressure is 35 kilobars. The minimum sufficient temperature and pressure in a given embodiment may depend on other parameters, such as the presence of a catalytic material, such as cobalt, with the diamond crystals. Generally, such a catalyst binder material is used to assure intercrystalline bonding at a selected time, temperature and pressure of processing. As used herein, PCD refers to polycrystalline diamond that may also include residual cobalt. Sometimes, PCD is referred to in the art as "sintered diamond". In an exemplary embodiment, the outer layer of PCD is made from a mixture of diamond crystals and cobalt powder, with 13 percent by weight or six percent by volume of cobalt in the total mixture. Preferably, the catalyst metal is present in the range of from one to ten percent by volume. In some embodiments, other catalyst metals such as iron and nickel may be used. The diamond crystal size can range upwardly from submicron sizes. Preferably, about 65 percent of the diamond crystals are in the range of 4 to 8 microns; the other 35 percent of the diamond crystals are in the range of one-half to one micron. The diamond crystals may be either naturally occurring diamonds or synthetic diamonds produced by a high temperature and high pressure process. The raw materials for making the PCD layer are preferably milled together for a sufficient time to thoroughly coat the diamond particles with cobalt. Milling in a ball mill lined with cemented tungsten carbide and using cemented tungsten carbide balls is preferred to avoid contamination of the diamond. An attritor or planetary mill may be used if desired. Such milling should be sufficiently energetic to "smear" the cobalt but should avoid appreciable comminution of the diamond particles. One or two days of ball milling is appropriate. Intercrystalline bonding between adjacent diamonds occurs and a unitary solid polycrystalline diamond matrix is formed when the milled materials are subjected to high temperature, and at a sufficient pressure that the diamond is thermodynamically stable. The precemented tungsten carbide is made by blending tungsten carbide powder and cobalt powder in a ball mill or the like. The blended powders are compacted and sintered near the melting point of cobalt. The resultant compact is comminuted to the desired particle size for use in making the composite material of the transition layers 28 and 30. In an exemplary embodiment a grit size of -325 US mesh (about 44 microns) precemented tungsten carbide is used in the composite material of the transition layers. The precemented tungsten carbide grit can have a variety of tungsten carbide particle sizes and shapes, and various cobalt contents. The cobalt content can be in the range of from five to sixteen percent by weight and the tungsten carbide particles are preferably in the range of from four to fifteen microns. In an exemplary embodiment the particle size is six microns and the cobalt content is fourteen percent by weight. When making the composite material of the transition layers, it is preferred to thoroughly blend the powders. Preferably, the diamond crystals and cobalt powder are ball milled together as hereinabove described. After initial milling, precemented tungsten carbide grit is added, with or without additional cobalt, and the mixture is further milled for thoroughly blending and smearing the mixture. For example, diamond and cobalt powders may be ball milled together for up to a day before addition of tungsten carbide grit. This mixture is then all milled for another one or two days. Forty-eight hours is an exemplary ball milling time. The composite material of each transition layer is made separately as described above. The procedure is the same for each transition layer; the only variation is in the relative proportions of precemented tungsten carbide grit and diamond crystals and cobalt powders. The blended powders for making each of the layers on the insert are sintered and bonded to a rock bit insert blank comprising a cylindrical cemented tungsten carbide body having a converging portion at one end. The converging portion of the blank has the geometry of the completed insert (e.g. hemispherical), less the thickness of the layers to be formed thereon. The assembly is formed in a deep drawn metal cup which preferably has double walls. There is an inner cup, the inside of which is formed to the desired net shape of the end of the rock bit insert to be formed. The inner cup is zirconium sheet having a thickness of 50 to 125 microns. The outer cup, which mates around the inner cup, is molybdenum with a thickness of 250 microns. A conical zirconium sheet and conical molybdenum sheet close the assembly at the top. The zirconium "can" thus formed protects material within it from the effects of nitrogen and oxygen. The molybdenum can protects the zirconium from water which is often present during the high pressure, high temperature pressing cycle used to form the rock bit insert. To make such an assembly, blended diamond powder, including cobalt, may be placed in the cup and spread into a thin layer by rotation and pressing with an object having the same shape as the insert blank when the blank is axisymmetric. If desired, the insert blank can be used to spread the powder. Powder to make the outer layer is spread first, then powder to make the first transition layer is added and spread on the outer layer. Additional transition layers are formed in the same manner. Finally, the insert blank is put in place and the metal sheets are added to close the top of the assembly. A small amount of paraffin wax may be included in the blended powders to aid distribution and retention of the powder in thin layers. Alternatively, layers can be built up on the end of the insert blank before insertion into the cup. In another embodiment, sufficient wax may be included with the powders to form self-supporting "caps" of blended powder to be placed on the insert blank or in the cups. After assembly is made by one of such techniques, it is preferable to press the assembly through a die to swage the cups tightly against their contents. One or more of such assemblies is then placed in a conventional high pressure cell for pressing in a belt press or cubic press. A variety of known cell configurations are suitable. An exemplary cell has a graphite heater tube surrounding such an assembly and insulated from it by salt or pyrophyllite for sealing the cell and transmitting pressure. Such a cell, including one or more such assemblies for forming a rock bit insert, is placed in a high pressure belt or cubic press and sufficient pressure is applied that diamond is thermodynamically stable at the temperatures involved in the sintering process. In an exemplary embodiment, a pressure of 50 kilobars is used. As soon as the assembly is at high pressure, current is passed through the graphite heater tube to raise the temperature of the assembly to at least 1300 °C and preferably to 1350° to 1400 °C. When the assembly has been at high temperature for a sufficient period for sintering and formation of polycrystalline diamond, the current is turned off and the parts rapidly cooled by heat transfer to the water cooled anvils of the press. When the temperature is below 1300 °C, and preferably below 200 °C, pressure can be released so that the cell and its contents can be ejected from the press. The metal cans and any other adhering material can be readily removed from the completed insert by sand blasting or etching. The grip of the completed insert may be diamond ground to a cylinder of the desired size for fitting in a hole in the cone of a rock bit. The composite layers of diamond crystals and precemented carbide particles are, of course, sintered by the high temperature and pressure and are no longer in the form of discrete particles that could be separated from each other. The layers sinter to each other and the two cemented carbide pieces forming the body of the insert sintered to each other. The transition layers help ensure that the insert is hard enough to drill hard rock formation without suffering undue wear and yet tough enough to avoid breakage or spalling. The outer polycrystalline diamond layer is very hard. It crushes the rock formation being drilled effectively, and it resists wear through abrasion. However, it is also somewhat brittle, having a higher modulus of elasticity than the cemented tungsten carbide body. The presence of transition layers between the outer layer and the cemented tungsten carbide body minimizes a sharp drop in elasticity between the polycrystalline diamond outer layer and the cemented tungsten carbide body, that would otherwise contribute to spalling of the PCD layer from the insert. Because of the progressive decrease in diamond content and increase in carbide content, the modulus of elasticity progressively decreases from the PCD layer to the carbide body. This can be accomplished through the use of discrete transition layers as in the preferred embodiment, the transition layer adjacent the outer layer having a higher proportion of diamond crystals than the transition layer adjacent the cemented tungsten carbide body, or by a gradient of diamond crystals and precemented tungsten carbide, the proportion of diamond crystals decreasing towards the cemented tungsten carbide body. The extra stability provided by the transition layers is especially desirable in light of the force exerted on percussion bits during use. Percussion bits are subjected to greater impact loads than rotary bits which exert force on the rock through rotary rather than percussive motion. Consequently, not only must the drilling portion of the insert be harder to be effective, but the insert is more likely to chip or spall. The combination of an outer diamond layer and one or more transition layers between the diamond layer and the tungsten carbide body provides a drilling surface that is hard and resistant to wear while minimizing the probability of chipping or spalling due to a sharp discontinuity in elasticity. PCD-tipped percussion bits were tested in a sandstone formation as well as a granite formation. Typical footage in sandstone using a percussion bit with standard tungsten carbide inserts and no diamond layer, without resharpening the bit, is 107 to 153 m (350 to 500 feet). Using a PCD-tipped insert percussion bit as herein described, the bits have drilled an average of 763 m (2500 feet). Footage of as much as 1146 m (3758 feet) has been obtained. The bit failed only when the steel eroded and the inserts fell out. In granite, the typical footage for a standard tungsten carbide insert percussion bit without resharpening is 183 m (600 feet) or less. If the bit is resharpened the footage may be as much as 366 m (1200 feet); however, the bit must be resharpened every 46 m (150 feet), or eight times during its useful lifetime. A PCD-tipped insert percussion bit has drilled as much as 440 m (1455 feet) in granite. The degree of wear of PCD-tipped inserts has been compared with the wear of tungsten carbide inserts on the same bit. The PCD-tipped inserts showed no discernible signs of wear even when the tungsten carbide inserts were nearly worn away. This was the case even when the PCD-tipped inserts were located on the gauge row, which usually wears out first.

Claims: A percussion rock bit, comprising a steel body (10); means (12) at one end of the steel body (10) for connecting the bit to a drill string, a plurality of inserts embedded in the other end of the steel body (10), characterised in that at least a number of the inserts (16) comprise a cemented tungsten carbide body (20) having a grip portion embedded in the steel body (10) and a converging end 24 protruding from the surface of the steel body (10); a polycrystalline diamond layer (26) on the converging end (24) of the carbide body (20); at least one transition layer (28,30) between the polycrystalline diamond layer (26) and the carbide body (20), such a transition layer (28,30) comprising a composite containing diamond crystals and precemented tungsten carbide particles, wherein the diamond crystal content is greater adjacent the polycrystalline diamond layer (26) and lower adjacent the cemented tungsten carbide body (20). A rock bit as claimed in Claim 1, characterised in that the polycrystalline diamond layer (26) includes from about one to about ten percent by volume cobalt. A rock bit as claimed in Claim 1 or in Claim 2, characterised by two transition layers (28,30) between the polycrystalline diamond layer (26) and the tungsten carbide body (20). A percussion rock bit as claimed in Claim 3 characterised in that the transition layer (28) adjacent the polycrystalline diamond layer (26) has a greater proportion of diamond crystals relative to precemented tungsten carbide than the transition layer (30) adjacent the cemented tungsten carbide body (20). A rock bit as claimed in Claim 3 or in Claim 4, characterised in that the transition layer (28) adjacent the polycrystalline diamond layer (26) comprises about 75 percent by volume diamond crystals and about 25 percent by volume precemented tungsten carbide. A rock bit according to any one of Claims 3 to 5 characterised in that the transition layer (30) adjacent the cemented tungsten carbide body (20) comprises about 50 percent by volume diamond crystals and about 50 percent by volume precemented tungsten carbide particles. A rock bit according to any one of Claims 1 to 3 characterised in that the transition layer (28) adjacent the polycrystalline diamond layer (20) comprises about 60 percent by volume diamond crystals and about 40 percent by volume precemented tungsten carbide. A rock bit according to any one of Claims 1 to 3 and 7, characterised in that the transition layer (30) adjacent the tungsten carbide body (20) comprises about 40 percent by volume diamond crystals and about 60 percent by volume precemented tungsten carbide. A rock bit according to any one of Claims 5 to 8 characaterised in that the diamond crystals include from about one to about ten percent by volume cobalt. A rock bit according to any preceding claim characterised in that the insert comprises a cylindrical body (20) including two pieces of cemented tungsten carbide (22,23) bonded together, an inner carbide piece (22) being embedded in the rock bit and comprising relatively harder tungsten carbide, and an outer carbide piece (23) protruding from the rock bit comprising relatively softer carbide.

Inventors: HALL, DAVID RICHARD
Applicants: SMITH INTERNATIONAL, INC.
Application year: 1986
Main classification: E21B10
IPC codes: B22F7, E21B10, B32B15