CA1256096A

CA1256096A - Rock bit with wear resistant inserts

Info

Publication number: CA1256096A
Application number: CA000517811A
Authority: CA
Inventors: David R. Hall
Original assignee: Smith International Inc
Current assignee: Smith International Inc
Priority date: 1985-10-18
Filing date: 1986-09-09
Publication date: 1989-06-20
Also published as: DE3685083D1; JPS62111093A; IE862386L; EP0219959A3; EP0219959B1; EP0219959A2; IE57504B1

Abstract

ROCK BIT WITH WEAR RESISTANT INSERTS
Abstract A rock bit has three cones with tungsten carbide inserts in the cutting structure of each cone. The gage row inserts have a layer of polycrystalline diamond on the converging portion protruding from the surface of the cone. One or more transition layers are provided between the tungsten carbide insert body and the polycrystalline diamond layer. Such a transition layer comprises diamond and precemented tungsten carbide.

Description

~256096 ROCR BIT WITH WEAR RESISTANT INSERTS

Field of the Invention This invention relates to rock bits for drilling oil wells or the like having polycrystalline diamond (PCD) tipped inserts for drilling a rock formation.

Ba~kground of the Invention ~ eavy duty rock bits are employed for drilling wells in subterranean formation for oil, gas, geothermal steam and the like. Such bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations.

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~ 256~

i -2-1 The cutter cones are mounted on steel journals or pins integral with the body at its lower end. In use the drill string and bit body are rotated in the bore hole and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled.
As such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in su~h severe environments is in the order of 20 to 200 hours for bits in sizes of about 10 6-1/2 to 12-1/~ inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
~ hen a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string 15 for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite 20 desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number 25 of reasons, including wearing out or breakage of the structure contacting the rock formation. The other principal reason for replacing a rock bit on a drill string is that the bearings supporting one or more of the roller cones fail due to excessive wear, loss of lubricant or the like. There are a variety of other minor causes of failure in rock bits in some rock ~.2560~36 1 formations. There is a continual efort to upgrade the performance and lengthen the lifetime of those components of a rock bit that are likely to cause a need for rep7ace-ment. There have been continual improvements in bearings, but few major improvements in the cutting structure of rock bits.
When a rock bit is drilling a bore hole, it is important that the diameter or gage of the bore hole be maintained at the desired value. The outermost row of inserts on each cone of a rock bit is known as the gage row.
This row of inserts is subjected to the greatest wear since it travels furthest on the bottom of the hole, and the gage row inserts also tend to rub on the sidewall of the hole as the cones rotate on the drill bit body. As the gage row inserts wear, the diameter of the bore hole being drilled may decrease below the original gage of the rock bit. When the bit is worn out and removed, a bottom portion of t~e hole is usually under gage. When the next bit is run in the hole, it is therefore necessary to ream that bottom portion of the hole to bring it to the full desired gage. This not only takes substantial time but commences wear on the gage row inserts, which again results in an under gage hole as the second bit wears out. Further, as the bit reams, a side load is applied to the cones, "pinching" the bit and applying a high side load on the bearings, which can cause premature failure of the bearings.
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 well, and such time is quite expensive because of the - fixed-costs involved in drilling. The rate of penetration decreases when the inserts in the cones become worn and do not protrude from the cone surface to the same extent they ~2560g~

1 did when drilling commences. The worn inser-ts have an increased radius of curvature and increased contact area on the rock. It therefore takes greater force to penetrate and this may cause both hearing failure and gage row insert breakage~ Drilling rate is continually observed by the driller and when the inserts are worn to the point that the rate of penetration is unacceptably low, the bit is replaced.
Thus, it i5 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 gage row inserts to maximize the length of hole drilled to full gage.
Wear resistance of conventional inserts o cemented tungsten carbide may be enhanced by increasing the propor-tion of tun~sten 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 higher cobalt content. In exemplary embodiments, the cobalt content of inserts for use in rock bits ranges from about 6~ to 16~ 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. Eor 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 - ~251~~

1 par~icle size of about six microns and contains about 10 to 14% by weight cobalt.
Toughness of the inserts is important in a rock bit since the inserts are subjected to impact loads as the cones rotate, 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.

B ef Summar of the Invention rl Y
A rock bit is therefore provided in practice o~ this invention with a steel body having means at one end for connecting the bit to a drill string, and means at the opposite end for mounting roller cones for rotation around ! an axis transverse to the axis of the hit. Each of the roller cones mounted on the body for rolling on the bottom of a bore hole being drilled includes a plurality of inserts for crushing rock at the bottom of the bore hole. At least a portion of those inserts comprise a cemented tungsten carbide body having a grip length embedded in the cone and a converging end portion protruding from the surface of the cone. A layer containing polycrystalline diamond is provided on the converging end of such carbide bodies with a transition layer between the polycrystalline diamond layer and the carbide body. The transition layer comprises a composite material containing diamond crystals and pre-cemented tungsten carbide particles.

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~2S60g~

1 Drawings These and other features and advantages of the presentinvention will be appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:
FIG. 1 illustrates in semi-schematic perspective an exemplary rock bit;
FIG. 2 is a partial longitudinal cross section of such a rock bit;
FIG. 3 is a longitudinal cross section of an exemplary insert for such a rock bit;
FIG. 4 is a longitudinal cross section of another embodiment of such insert; and FIG. 5 is a longitudinal cross section of a subassembly for forming such a rock bit insert.

~256096 1 Description ~ n exemplary rock bit comprises a steel body 10 having three cutter cones 11 mounted on its lower end. A threaded pin 12 is at the upper end of the body for assembly of the rock bit onto a drill string for drilling oil wells or the like. A plurality of tungsten carbide inserts 13 are - provided in the surfaces of the cutter cones for bearing on rock formation being drilled.
FIG. 2 is a fragmentary longitudinal cross section of the rock bit extending radially from the rotational axis 14 of the rock bit through one of the three legs on which the cutter cones 11 are mounted~ Each leg includes a journal pin 16 extending downwardly and radially inwardly of the rock bit body. The ~ournal pin includes a cylindrical bearing surface having a hard metal insert 17 on a lower portion of the journal pin. The hard metal insert is typically a cobalt or iron base alloy welded in place in a groove on the journal leg and having a substantailly greater hardness than the steel forming the journal pin and rock bit body. An open groove 18 corresponding to the insert 17 is provided on the upper portion of the journal pin. Such a groove can, for example, extend around 60~ or so of the circu~ference of the journal pin and the hard metal 17 can extend around the remaining 40~ or so. The journal pin also has a cylindrical nose 19 at its lower end.
Each cutter cone 11 is in the form of a hollow generally conical steel body having tungsten carbide inserts 13 pressed into holes on the external surface. The-ou-ter row of inserts 20 on each cone-is referred to as the gage row since these inserts drill at the gage or outer diameter of the bore hole.

~5 ~256096 1 Such tungsten carbide inserts provide the drilling action by engaginy and crushing subterranean rock for~ation on the bottom of a bore hole being drilled as the rock bit is rotated, The cavity in the cone contains a cylindrical beaxing surface including an aluminum bronze insert 21 deposited in a groove in the steel of the cone or as a floating insert in a groove in the cone. The aluminum bronze insert 21 in the cone engages the hard metal insert 17 on the leg and provides the main bearing surface for the cone on the bit body. A nose button 22 is between the end of the cavity in the cone and the nose 19, and carries the principal thrust loads of the cone on the journal pin. A bushing 23 surrounds the nose and provides additional bearing surface between the cone and journal pin.
A plurality of bearing balls 24 are fitted into complementary ball races in the cone and on the journal pin.
These balls are inserted through a ball passage 26 which extends through the journal pin between the bearing races and the exterior of the roc~ bit. A cone is first fitted on the journal pin and then the bearing balls 24 are inserted through the ball passage. The balls carry any thrust loads tending to remove the cone from the journal pin and thereby retain the cone on the journal pin. The balls are retained in the races by a ball retainer 27 inserted through the ball passage 26 after the balls are in place. A plug 28 is then welded into the end of the ball passage to keep the ball retainer in place.
The bearing surfaces between the journal pin and cone axe lubricated by a grease which fills the regions adjacent the bearing surfaces plus various passages and a grease ~2~6096 1 reservoir. The grease reservoir comprises a cavity 2~ in the rock bit body which is connected to the ball passage 26 by a lubricant passage 31. GEease also fills the portion of the ball passaye adjacent the ball retainer, the open 5 groove 18 on the upper side of the journal pin and a - diagonally extending passaye 32 therebetween. Grease is retained in the bearing structure by a resilient seal in the form of an 0-ring 33 between the cone and journal pin.
A pressure compensation subassembly is included in the grease reservoir 29. This subassembly comprises a ~etal cup 34 with an opening 36 at its inner end. A
flexible rubber bellows 37 extends into the cup from its outer end. The bellows is held in place by a cap 3~
having a vent passage 39 therethrough. The pressure co~pensation subassembly is held in the grease reservoir by a snap ring 41.
The bellows has a boss 42 at its inner en~ which can seat against the cap 3~ at one end of the displacement of the bellows for sealing the vent passage 39. The end of the bellows c~n also seat against the cup 34 at the other end of its stroke, thereby sealing the opening 36.
In practice of this invention at least a portion of the cutting structure of the rock bit comprises tungsten carbide inserts that are tipped with polycrystalline diamond. An exemplary insert is illustrated in longitudinal cross section in FIG. 3. Such an insert has a cylindrical grip length 46 ex-tending along a major portion of the insert. At one end there is a converging portion 47 which may have any of a variety of shapes depending on the desired cutting structure. The converging portion may be referred to ~256096 1 as a projectile shape or basically a cone with a rounded end. It may be a chisel shape which is like a cone with converging flats cut on opposite sides and a rounded end.
The converging portion may be hemispherical or any of a variety of other shapes known in the art.
Such an insert is press fitted or brazed into the roller cone. Each one of the cones has a plurality of flat bottomed holes in circumférential rows on its outer surface. An exemplary hole has a diameter about 0.13 millimeters smaller than the diameter of the grip 46 of an exemplary insert. The insert is pressea into the hole in the steel cone with many thousand pounds of force. This press fit of the insert into the cone tightly secures the insert in place and prevents it from being dislodged during drilling The converging portion of the insert illustrated in FIG. 3, has an outer layer 48 for engaging rock when the insert is used in a rock bit, and an inner layer 49 between the outer layer and the main cemented tungsten carbide body of the insert. The outer layer in an exemplary embodiment comprises polycrystalline diamond (PCD) wi~h a thickness of 125 microns. The inner layer has a thickness of 380 microns and comprises a composite material of polycrystalline diamond and precemented tungsten carbide, such as disclosed in U. S. Patent No. 4,525,178. As used in this 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 1300C and an exemplary minimum pressure is about 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 .

~256al9~

1 diamond c~ystals. 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 the polycrystalline diamond including 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~ by weight or 6% 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. About 65~ of the diamond crystals are in the range of four to eight microns. The other 35% 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, high pressure process.
The diamond crystal size can range upwardly from sub-micron sizes. Preferably they range up to about twenty microns. Preferably a mix of sizes is used for dense packing.
The cobalt content can be in the range of from one to fifteen percent by volume, preferably less than about ten percent by volume. In some embodiments other catalyst metals such as iron or nickel may be used.
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.

~256096 1 Intercrystalline bonding between adjacent diamonds occurs and a unitary solid polycrystalline diamond article is formed when the milled materials are subjected to high temperature and a sufficient pressure that diamond is thermodynamically stable.
The intermediate composite layer is formed of a mixture of diamond crystals, cobalt, and precemented tungsten carbide particles. 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 ~o the desired particle size for use in making the composite material of the inner layer. In an exemplary embodiment a grit size of -325 ~. S. mesh (about 44 microns) is used in the composite material of the inner layer.
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.
In an exemplary embodiment the inner layer between the - layer of polycrystalline diamond and the cemented tungsten carbide substrate is made from a mixture of forty percent by volume of the aforementioned diamond powder (containing six percent by ~olume cobalt) and sixty percent by volume ~256096 -~3-1 precemented tungsten carbide grit. If desired, additional cobalt can be included depending on the cobalt content of the cemented tungsten carbide.
When making the composite material, it is preferred to thoroughly blend the powders. Prefera~ly the diamond crystals and cobalt powder are ball milled together as hereinabove described. After initial milling cemented 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 ball milled for an~ther one or two days. Forty-eight hours is an exemplary ball milling time.
The hlended powders for making the layers on the insert are sintered and bonded to a rock bit insert blank 51 in an asse~bly of the type illustrated in FIG. 5. The insert being formed in the embodiment illustrated in FIG. 5 has layers somewhat aifferent from those in the embodiment of FIG. 3 as hereinafter described, but the manufacturing technique is the same. The insert blank 51 comprises 2 cylindrical cemented tungsten carbide body having a converging portion at one end. The converging portion has the geometry of the completed insert, less the thickness of the layers to be formed thereon. The asse~bly is formed in a deep drawn metal cup which preferably has double walls.
There is an inner cup 52, the inside of which is formed to the desired net shape of the énd of the rock bit insert to be preformed. The inner cup is zirconium sheet having a thickness of 50 to 125 microns. The outer cup 53 is 12!~6~96 1 molybdenum with a thickness of 250 microns. A zirconium sheet 54 and molybdenum sheet 55 close the assembly at the top. The zirconium "can" thus formed protects material within i-t from the effects of nitrogen and oxygen. The molybdenum can protects the 7irconium from water which is often present during the high pressure, high temperature pressing cycle used to form the rock bit insert.
To make an assembly as illustrated in FIG. 5 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 a chisel insert is being made, the powaer can be spread with a generally conical tool.
Powder to make the outer layer is spread first, then powder to make the inner layer may be spread on the outer layer.
Finally the insert blank is p~t 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 anoth r embodiment, sufficient wax may be included with the powders to form self-supportins "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 di- 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 ~256~9~

1 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 pressùre 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 1300C and prefrably to 1350 to 1400C. When the assembly has been at high temperature for a sufficient period for sintering and formation of polycrystalline diamond, 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 1300C, and preferably below 200C , 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 o~ the desired size for fitting in a hole in the cone of a rock bit. The composite layer of diamond crystals and precemented carbide particles is, of course, sintered by the high temperature and pressure and is no longer in the form of discrete particles that could be separated from each other.
Two experimental rock bits formed in practice of this invention have been tested during drilling of wells in formation that is moderately difficult to drill in West Texas.
The formations drilled included salt, shale, dolomite, mixed sandstone and shale, mixed sandstone and anhydrite, and mixed sandstone and dolomite. Each of these bits was seven and seven-eights inch diameter across the gage. The shape of 1 the inserts employed was the same as the shape of conventional inserts in rock bits typically used in the same oil field formations (Smith Tool type F37).
In each of the bits the inserts in the gage row were tipped with polycrystalline diamond as hereinabove described.
Each cemented tungsten carbide insert blank was made with six micron tungsten carbide particles and 1~% by weight cobalt. The inner layer had a thickness of 380 microns and was ~ormed from 40% by volume of a diamond-cobalt mixture with 6~ by volume cobalt, and 60% by volume of -325 mesh precemented tungsten carbide grit. The outer layer was PCD
with a nominal thickness of 125 microns. There was some uneveness in forming the outer layer and in some areas it is believed that the layer was as thin as ?5 microns.
The other inserts (other than the gage inserts) in the cutting structure on the rock bit were conventional tungsten carbide înserts made with six micron tungsten carbide powder and 14% cobalt.
- One of the bits was used to drill from a depth o 1514 feet to a depth of 4094 feet or a total footage of 2580 feet.
The bit was used for forty-five and one-quarter hours for an average penetration rate of 57 feet per hour. During the run the weight on the bit was in the range of from 15, ooo to 50,000 pounds (6800 to 22,700 kilograms) and the bit was rotated at speeds in the range of from 67 to 106 RPM. The maximum WR of the bit was 425 which is a little below average ~or the field being drilled. WR refers to the weight on the bit times the rotational speed divided by the bit gage diameter.
This run may be compared with runs with conventional bits in seven nearby wells drilled with the same drilling rig at ~256096 1 roughly comparable depths. An average run lasted 54 hours and drilled a depth of 2848 feet for a rate of penetration of about 53 feet per hour. Thus the bit with diamond tipped inserts in the gage row on each cone was run at an above average penetration rate, but the run time and depth drilled were a little below average. The reason for the somewhat low total depth drilled was that the bit was run into a particularly difficult rock formation of dolomite with sandstone stringers at an excessive speed and weight, and the gage row inserts were broken. Breakage of the inserts was detected at the ground surface and drilling with that bit was ended. It is estimated that 15 to 20 feet of hole after breakage of the inserts was under gage and needed reaming. In exemplary comparable holes, the length of hole needing reaming can range rom ~ero to about l50 feet.
The second experimental bit went in the hole at 1511 feet depth and was withdrawn at 3687 feet, for a total footage drilled of 2176 feet. The drilling time was 33 hours and the average rate of penetration was 66 feet per hour. The weight on the bit ranged from 15,000 to 55,000 pounds (6800 to 25,000 kilo~ra~s) and the speed was in the range of from 78 to 120 RPM. The maximum WR of the bit was above average for the rock formation being drilled, namely 545.
~n nine offset wells drilled with the same drilling rig at roughly comparable depths in the same formations, an average run was 2680 feet drilled in about 47 hours for an average rate of penetration of 57 feet per hour.
The second experimental bit was pulled from the hole before there was any indication at the ground surface that thP

, 560g6 1 bit was dull, damaged or worn. Inspection of the bit showed that there was no perceptlble wear on the diamond tipped gage row inserts. A few gage row inserts had minor chipping of the diamond layer. Under these same drilling conditions, appreciable wear would he observed on standard ce~ented tungsten carbide gage row inserts. It also appears that there was less wear on the cemented tungsten carbide - inserts inwardly from the gage row than would be observed in a standard bit having cemented tungsten carbide inserts in the gage row. Almost no wear was seen on the carbide inserts of the test bit. In most bits the inner inserts are so badly worn that the rate of penetration decreases markedly. It is believed that the lack of wear on the gage row inserts not only maintains the full gage of the hole but also affords some protection for tungsten carbide inserts inwardly from the gage row. ~his may occur since the weight on the bit is mcre uniformly distributed on the inserts in the absence of gage row wear and the other inserts are not overloaded.
Wear resistant gage row inserts may have an additional ZO benefit by protecting the bearings used to mount the cones on the rock bit body. When the bit wears and cuts under gage, or when reaming an under gage hole, there may be an undue load on ~he cone bearings. Such "pinching" of the bit may contribute to premature bearing failure. Thus, an embodiment Z5 of rock bit with gage row inserts tipped with PCD and other inserts of cemented metal carbide is suitable for practice of this invention.
Since the diamond tipped inserts show negligible wear, it is believed that the principal mode of failure is likely -to be chipping or breakage of inserts. It is therefore desirable to operate such a bit with higher rotary speed and ~s~

--19-- .
1 lower weight than conventional roller cone rock bits. Good circulation is also desirable for removing chips r~pidly for minimizing cone wear.
FIG. 4 illustrates another embodiment of rock bit insert tipped with polycrystalline diamond as provided in practice of this invention. This insert has a conventional cemented tungsten carbide blank 57 with a cylindrical grip 58 and a converging end on the portion of the insert that protrudes beyond the surface of the cone in which it is pressed. There is an inner layer 59 that is 380 microns thick adjacent to the cemented tungsten carbide blank at the converging end. The inner layer is overlain by an intermediate layer 60 that is 250 microns thicX. This in turn is overlain by an outer layer 61 that is 125 microns thick.
The outer layer 61 is PCD as hereinabove described.
The intermediate layer is formed of a mixture of 60~ by volume PCD (including 6% by volume cobalt) and 40~ by volume precemented tungsten carbide. The inner layer is formed of a mixture of 40~ by volume PCD and 60~ by volume precemented tungsten carbide. After sintering at high pressure and high temperature to form composite materials in the layers as hereinabove described, the volume percentages may differ slightly from the proportions in the original mixture due to interactions. For example, sume of the diamond may dissolve in the cobalt phase, thereby slightly reducing the volume proportion of diamond in the PCD and composite.
By having a layer of PCD on the outer surface of the converging portion of the ~nsertr wear of the insert as it is used against a rock formation can be negligible. By ~L2~6t~

1 pro-~iding a transition between the PCD layer and the - cemented carbide subs-trate, a transition in modulus of elasticity and coefficient of thermal expansion helps pro~ect the PCD layer. The intermediate layer 60 near the PCD layer has a relatively higher p~opor~ion of PCD
- and lower proportion of cemented carbide than the inner layer 59, thereby providing a transition between the outer layer and cemented carbide blank in two steps instead o~ the single step in the embodiment hereinabove described and illustrated in FIG. 3.
FIG. 5 illustrates still another embodiment of insert as provided in practice of this inventionv In this embodiment the converging portion of the insert blank 51 has an outer layer 63 of polycrystalline diamond. This outer PCD layer is separated from the blank by an inner layer 64 having a gradual transition of properties between the PCD layer and the cemented carbide blank. Such a transition layer has a high proportion of PCD te.g., 80% or more by volume) and low proportion of cemented tungsten carbide adjacent to the outer PCD layer 63. There is a gradual diminution in the diamond con~ent of the transition layer 64 with concomitant increase in the proportion of precemented tungsten carbide toward the cemented carbide blank. For example, the polycrystalline diamond content adjacent to the blank may be 20% by volume and the precemented tungsten carbide content may be 80~ by volume.
A gradual transition in proportion of diamond in the transi~lon layer provides, in effect, a very large number of steps in mechanical properties between the outer PCD
layer and the cemented tungsten carbide substrate.

1.256Q9~;

l The transition layer provides a transition in a variety of important properties of the PCD layer and carbide substrate. It compensates for differences in coefficient o~ thermal expansion and modulus of elasticity.
It has a sonic velocity intermediate between the PCD and carbide which means that a stress wave travelling through the insert has less stress concentration at the inter~ace.
A transition layer of diamond crystals and precemented tungsten carbide particles is significantly different from a mere mixture of diamond, cobalt and carbide powders.
It is believed that the precemented carbide particles act as a plurality of "mini-anvils" that provide a pressure distribution in the layer very different from the pressure distribution in a mix of powders, resulting in better compaction. The precemented carbide has less shrinkage than a powder mixture, improving the rheology of pressing.
Further, the distribution of carbide in the composite is rather different from a powder; an analogy could be use of steel reinforcing bar in concrete as compared with powdered steel. Significantly different proper-ties can result from the difference in distribution.
~ t will be apparent that many modifications and variations can be made in the rock bits provided in practice of this invention. One example of a three cone rock bit has been des ribed and illustrated herein with conventional mounting of the cones. Diamond tipped inserts may be used on cones for two cone rock bits or for a variety of other rock bit variations. PCD tipped inserts may be used for all of the cutting structure of the rock bit and not just the gage row inserts.
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~Z56~96 1 Further, variations in the inserts themselves will be apparent. For example, in the embodiments herein described the outer PCD layer has a thickness of about 125 microns and thicker layers can be employed iE desired~
The PCD layer can be in the range of 75 to 600 microns thick. Surprisingly, however, since the PCD is extremely resistant to erosion by rock formations being drilled, a layer only 125 microns thick is adequate for a long life roc~ bit. The thickness of the transition layer, or layers, can range from 100 microns to 3 millimeters or more. Other proportions of PCD and precemented carbide particles in the transition layers may also be used as desiredA For example, these layers can range from 5 to 95% by volume polycrystalline diamond and from 95 to 5%
precemented tungsten carbide. Depending on the binder content of the cemented tungsten carbide, additional cobalt may be included in the mixture for good sintering of the transition layer. Other metal carbides such as tantalum carbide or titanium carbide may be suitable in some embodiments.
It will therefore be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

WHAT IS CLAIMED IS:

1. A rock bit comprising:
a steel body;
means at one end of the body for connecting the bit to a drill string;
means at the opposite end of the body for mounting at least one roller cone on the body for rotation around an axis transverse to the axis of the bit;
at least one roller cone so mounted on the body for rolling on the bottom of a bore hole being drilled;
a plurality of inserts in such a cone for crushing rock at the bottom of such a bore hole, at least a portion of such inserts comprising:
a cemented tungsten carbide body having a grip length embedded in the cone and a converging end portion protruding from the surface of the cone;
a polycrystalline diamond layer on the converging end of the carbide body; and at least one transition layer between the polycrystalline diamond layer and the carbide body, the transition layer comprising a composite containing diamond crystals and precemented tungsten carbide particles.

2. A rock bit as recited in Claim 1 wherein the transition layer comprises 40% by volume polycrystalline diamond and 60% by volume precemented tungsten carbide particles.

3. A rock bit as recited in Claim 2 wherein the polycrystalline diamond layer includes from 1 to 10% by volume cobalt.

4. A rock bit as recited in Claim 1 wherein the polycrystalline diamond layer has a thickness in the range of from 75 to 600 microns.

5. A rock bit as recited in Claim 4 wherein the transition layer has a thickness in the range of from 100 to 600 microns.

6. A rock bit as recited in Claim 5 wherein the transition layer comprises from 5 to 95% by volume diamond and from 95 to 5% by volume precemented tungsten carbide.

7. A rock bit as recited in Claim 1 comprising a second transition layer between the first transition layer and the cemented tungsten carbide body, the second transition layer comprising a composite containing a lower proportion of diamond crystals and a higher proportion of precemented tungsten carbide than the first transition layer.

8. A rock bit as recited in Claim 1 wherein the transition layer has a gradual transition between a relatively higher diamond content and relatively lower precemented tungsten carbide content adjacent to the polycrystalline diamond layer, and a relatively lower diamond content and relatively higher precemented tungsten carbide content adjacent to the tungsten carbide body.

9. A rock bit wherein inserts as recited in Claim 1 are in a gage row of inserts in such cone.

10. A rock bit as recited in Claim 9 wherein the inserts inwardly from the gage row inserts comprise cemented tungsten carbide without a layer of polycrystalline diamond.

11. An insert for a roller cone rock bit comprising:
a cemented metal carbide body having a grip length for insertion in the cone of a roller cone rock bit and a converging portion at one end for protruding beyond the face of the cone and crushing rock.
a polycrystalline diamond layer on at least a major portion of the protruding converging end of the body;
and a transition layer between the body and the polycrystalline diamond layer comprising a mixture of precemented metal carbide particles and diamond crystals, at least a portion of the diamond crystals being bonded directly to other diamond crystals.

12. An insert as recited in Claim 11 wherein the body is formed of cobalt cemented tungsten carbide and the transition layer comprises tungsten carbide precemented with cobalt.

13. An insert as recited in Claim 12 wherein the transition layer comprises 40% by volume polycrystalline diamond and 60% by volume precemented tungsten carbide particles.

14. An insert as recited in Claim 12 wherein the polycrystalline diamond layer includes from 1 to 10%
by volume cobalt.

15. An insert as recited in Claim 12 comprising a second transition layer between the first transition layer and the cemented tungsten carbide body, the second transition layer comprising a composite containing a lower proportion of diamond crystals and a higher proportion of precemented tungsten carbide than the first transition layer.

16. An insert as recited in Claim 12 wherein the transition layer has a gradual transition between a relatively higher diamond content and relatively lower precemented tungsten carbide content adjacent to the polycrystalline diamond layer, and a relatively lower diamond content and relatively higher precemented tungsten carbide content adjacent to the tungsten carbide body.

17. An insert as recited in Claim 12 wherein the transition layer comprises from 5 to 95% by volume diamond and from 95 to 5% by volume precemented tungsten carbide particles.

18. An insert as recited in Claim 11 wherein the polycrystalline diamond layer has a thickness in the range of from 75 to 600 microns.

19. An insert as recited in Claim 11 wherein the transition layer has a thickness in the range of from 100 to 600 microns.

20. A rock bit as recited in Claim 1 wherein such an insert has a converging end portion selected from the group consisting of a projectile shape, a cone with a rounded end, a chisel shape, and hemispherical.

21. A rock bit as recited in Claim 1 wherein substantially the entire converging portion of such an insert has a polycrystalline diamond layer and a transition layer comprising diamond crystals and precemented tungsten carbide between the diamond layer and the cemented tungsten carbide body.

22. An insert as recited in Claim 11 wherein the insert has a converging end portion selected from the group consisting of a projectile shape, a cone with a rounded end, a chisel shape, and hemispherical.

23. An insert as recited in Claim 11 wherein sub-stantially the entire converging portion has a polycrystalline diamond layer and a transition layer comprising diamond crystals and precemented tungsten carbide between the diamond layer and the cemented tungsten carbide body.