GB2247855A - Improvements in or relating to the manufacture of cutting structures for rotary drill bits - Google Patents

Abstract

In a cutting structure, for a rotary drill bit, comprising a two-layer or multi-layer element 10 brazed to a carrier 11, the brazing composition 22 has a thickness which upon solidification, is within the range of about 20 mu -150 mu across its whole area. The thicker brazing composition allows bi-metallic distortion of the cutting element to reduce during cooling, thereby reducing residual stresses in the cutting element and hence reducing the incidence of cutting element failure caused by such residual stresses. Solid separation elements, such as sheets (28), rods (32), wires or particles (35), may be incorporated in the brazing composition, and the rear surface of the substrate and/or the surface of the carrier may be formed with a recess (39) to provide the additional thickness of brazing composition. <IMAGE>

Description

"Improvements in or relating to the manufacture of cutting structures for rotary drill bits" The invention relates to the manufacture of cutting structures for rotary drill bits for use in drilling or coring holes in subsurface formations.

In particular, the invention is applicable to cutting structures for rotary drill bits of the kind comprising a bit body having a shank for connection to the drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting structures. Each cutting structure comprises a preform cutting element, often in the form of a circular disc, bonded to a carrier, the preform cutting element being a two-layer or multi-layer element including a front cutting table formed of superhard material, usually polycrystalline diamond, and a substrate of less hard material, such as cemented tungsten carbide, having a rear face bonded to a carrier. The carrier, which may also comprise cemented tungsten carbide, is in turn mounted on the bit body.In one common form of drill bit of this type, the carrier comprises a stud or post to which the preform is brazed, the stud or post being received and secured within a socket in the bit body.

Conventional two-layer or multi-layer preform cutting elements of the kind referred to above are only thermally stable up to a temperature of about 700 C- 750 C. The preforms are normally bonded to the stud or post by brazing and to enable high temperature brazing processes to be employed, without thermal degradation of the superhard layer, sophisticated cooling techniques have been developed to protect the preform from the high temperature at which brazing takes place. Such techniques are described for example in U.S. Patents Nos. 4225322, 4319707 and 4527998. One such technique is sometimes referred to as "LS Bonding".

The methods currently employed for bonding the cutting elements to their carriers normally induce considerable residual stresses in the elements. This is mainly because the two-layer or multi-layer cutting elements become distorted during the brazing process due to the so-called "bi-metallic effect" caused primarily by the differing coefficients of thermal expansion of the superhard table and the substrate. Solidification of the brazing material occurs while the cutting element is still subjected to bi-metallic distortion as a result of the brazing temperature, and the subsequent cooling induces residual stresses in the cutting element as -it is restrained by the solidified braze material from returning to its original state. It is believed that the presence of these residual stresses may have a number of deleterious effects on the cutting structure.

Thus, it may reduce the strength and/or thermal stability of the bond between the cutting element and its carrier, and may promote cracking or splitting of the element, such as circumferential or diametral cracking. It may also promote spalling of the diamond layer, in use.

For example, it has been found that cutting structures of the above kind may sometimes be subject to failure by a phenomenon which may be referred to as "diametral splitting". That is to say, after the drill bit has been in use, perhaps for only a limited period, some circular cutting elements become cracked or split along a diameter extending away from the cutting edge of the element and in a plane which is generally perpendicular to the surface of the formation being cut.

Such splitting may be accompanied by breakdown of the bond between the rear surface of the substrate of the cutting element and the surface of the carrier to one side of the split, with the result that one half of the cutting element breaks away from the carrier thus rendering the cutting structure substantially ineffective. In another mode of failure the split has been found to extend through into the main body of the carrier itself so that a portion of the carrier becomes detached, taking with it one half of the cutting element.

The present invention sets out to provide novel methods of brazing such cutting elements to their carriers which may reduce or eliminate residual stresses in the cutting elements and thus reduce the tendency for cracking, splitting or spalling to occur in use.

According to one aspect of the invention, there is provided a method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element being a two-layer or multi-layer element including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising bonding the rear face of the substrate to the carrier by means of a brazing process employing a brazing composition which has a thickness which, upon solidification of the brazing composition, is within the range of about 20ss-150p across substantially the whole of the brazing composition.In other words, although the thickness of the solidified brazing composition will normally vary across the area of the cutting element, in accordance with the present invention no part of the composition will have a thickness less than 20#. However, the maximum thickness of the composition may be less than 150cut.

Circular cutting elements of the kind to which the invention relates may have a diameter of about 13mm or 19mm and, using prior art methods, the thickness of the brazing composition after solidification normally varies in the range of about 5 -30p. The significant increase in the thickness of the brazing compositionaccording to the present invention allows the bi metallic distortion of the cutting element to reduce during cooling (by plastic deformation of the thick brazing joint) to a level lower than that resulting when braze joints of normal thickness are used. It has been found that 75% of the bi-metallic distortion can be eliminated by this method and greater reductions may be possible.

The invention also includes within its scope various methods for achieving significant increase in the thickness of the braze joint between the cutting element and its carrier.

According to a second aspect of the invention there is provided a method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element being a two-layer or multi-laver element including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising bonding the rear face -of the substrate to the carrier by means of a brazing process employing a brazing composition comprising a fusible brazing material incorporating at least one separation element of material which remains substantially solid at the brazing temperature. The presence of the separation element or elements serves to increase the thickness of the finished braze joint, thus providing opportunity for the bi-metallic stresses in the cutting element to be dissipated during subsequent cooling.

The separation element may comprise at least one thin sheet of metallic material, which is preferably coextensive with the rear surface of the substrate, there being provided a layer of said fusible brazing material on each side of the separating element. The sheet of metallic material may have a thickness in the range of about 25p-50y and may comprise nickel or tungsten metal.

It should be pointed out that it is known in other technological fields, to incorporate thin sheets of metallic material between layers of fusible brazing material in brazed joints. Such arrangements are sometimes known as "trifoils". However, such arrangements are normally used in metallurgical joints to overcome lateral stresses (i.e. stresses in the general plane of the joint) imposed by different lateral expansion rates of the materials on the two sides of the joint. Such joints have not hitherto been used in brazing a two-layer or multi-layer cutting element to a carrier since the problem of differential lateral expansion rates does not normally arise. This is because the material of the substrate is usually the same as the material of the carrier (e.g. cemented tungsten carbide) or is a material having a similar coefficient of thermal expansion.In the present invention the trifoil is used in a new situation and to overcome the problem of transverse stress, that is to say stress in a direction transverse to the general plane of the joint resulting from transverse distortion of the cutting element due to the above-mentioned bi-metallic effect.

In an alternative arrangement according to the invention there are provided a plurality of separation elements spaced apart across the rear surface of the substrate. For example the separation elements may be in the form of spaced rods or wires, preferably arranged parallel to one another, or in the form of particles.

The rods, wires or particles may again be formed from nickel or tungsten metal.

According to a third aspect of the invention, there is provided a method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element being a two-layer or multi-layer element including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising preforming the rear surface of the substrate and/or the surface of the carrier to which it brazed with a recess, extending over at least a major part of said surface, which recess becomes filled with the brazing composition during the brazing process. For example the recess may be in the form of a concavity, such as a part-spherical cavity.

The provision of the recess serves to increase the thickness of the finished brazing joint with the advantages of stress dissipation previously referred to.

The invention includes within its scope a cutting structure when formed by any of the methods referred to above and also a drill bit incorporating one or more of such cutting structures.

The following is a more detailed description of embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic side elevation of a typical cutting structure of a kind to which the present invention relates, Figure 2 is a front view of the cutting structure of Figure 1, showing diametral splitting, Figures 3 and 4 show diagrammatically two alternative forms which diametral splitting may take, Figure 5 is a diagrammatic section through a two-layer cutting element bonded to a carrier showing, exaggerated, the results of the bi-metallic effect and, Figures 6-9 are diagrammatic sections through parts of cutting structures illustrating various methods in accordance with the present invention.

Figure 1 is a diagrammatic side elevation of a typical cutting structure mounted on a drag-type drill bit for use in drilling or coring holes in subsurface formations.

The cutting structure comprises a two-layer preform cutting element 10 mounted on a carrier 11 in the form of a stud which is located in a socket 12 in an upstanding blade 13 on the bit body. The preform cutting element is in the form of a circular tablet comprising a thin facing table 14 of polycrystalline diamond bonded to a substrate 15 of less hard material such as cemented tungsten carbide. The rear face 16 of the substrate 15 is brazed to a suitably orientated surface 17 on the stud 11, which may also be formed from tungsten carbide.

It will be appreciated that Figure 1 illustrates only one example of the many possible variations of the type of cutting structure to which the invention is applicable and many other arrangements will be known to those skilled in the art.

Figure 2 is a diagrammatic front elevation of the cutting structure of Figure 1 and illustrates one kind of failure mode, herein referred to as "diametral splitting", to which such cutting structures may -be subject. As will be seen from Figure 2, a crack or split 18 appears in the cutting element 14 and extends generally along a diameter of the circular cutting element from a point on the cutting edge 19 where the cutting element engages the formation 20. Figure 2 illustrates the situation after the bit has been in use and a wear flat 21 has developed.

Figures 3 and 4 illustrate diagrammatically two alternative failure modes which can occur as a result of diametral splitting. As explained below, it is believed that the crack 18 originates in the substrate 15 and it may not always extend into the diamond table 14. However the crack does not remain in the substrate and two primary forms of crack propagation are seen to occur. These are shown in Figures 3 and 4 respectively. Figure 3 shows one type in which the crack traverses the brazed joint 22 between the substrate and the carrier 11 and continues through the carrier 11, as indicated at 23, resulting in the loss of half of the cutting element and of a portion of the carrier.

Figure 4 shows a second type in which the crack 18 turns through 90O and propagates along the brazed joint 22, again resulting in the loss of half the cutting element. Other secondary failure modes are also observed which lead to total bond failure and post fracture.

As previously mentioned, diametral splitting and other modes of failure are believed to be caused by the residual stresses which remain in the cutting element as a result of the bi-metallic effect which occurs when the cutting element is heated as it is brazed to its carrier. This will be explained in greater detail with reference to Figure 5.

As previously mentioned, a typical two-layer cutting element comprises a table 14 of polycrystalline diamond bonded to a substrate 15 of cemented tungsten carbide. These materials have significantly different mechanical and physical properties, but for simplicity only thermal expansion will be considered. The coefficient of thermal expansion of the tungsten carbide layer 15 is approximately double the coefficient for polycrystalline diamond. Consequently when the cutting element 10 is heated, for example in the process for brazing it to the carrier 11, the cutting element distorts, as shown exaggerated in Figure 5, with the front diamond surface 24 becoming concave and the rear tungsten carbide surface 16 becoming convex.

During the cooling stage of the brazing process, as the bonding composition 22 cools and solidifies, it attains sufficient strength at some temperature to prevent the distorted cutting element from returning to its initial cold condition. The cutting element is therefore forcibly held in an elevated temperature state, as it cools. Residual stress is therefore developed in the cutting element and in the braze material 22. The residual stress in the cutting element is shown simplified in Figure 5. The arrows 25 indicate that the diamond layer 14 is in compression and the arrows 26 indicate that the tungsten carbide substrate 15 is in tension, the tension being greater further from the diamond layer 14. A neutral plane lies in the substrate near the diamond table 14 and is indicated diagrammatically 17.The degree of tension in the substrate increases from zero at the neutral plane to a maximum at the rear surface 16.

It is believed that diametral splitting is the result of this residual tension in the brittle tungsten carbide, the tension being relieved by cracking when the cutting element is subjected to impact or drilling loads. It is believed that compression in the diamond table 14 may also contribute to spalling of the diamond table under conditions of edge loading.

Since the distorted cutting element is attached to the carrier 11 by the braze joint 22, some stresses are also developed across the braze joint.

Since the cutting element is trying to revert to a more flat condition, the outer peripheral regions of the joint 22 will be in compression transversely of the joint whereas the central region of the joint will be in tenSion.

In a prior art braze joint formed by LS Bonding, the thickness of the joint will be in the range of 5p- 30 and it is believed that this thickness is insufficient to permit relief of the stress in the cutting element by plastic deformation of the material of the joint. According to the present invention such relief of stress is permitted by increase in thickness of the brazed joint, and/or by design of the joint so that it may better absorb such transverse stress. In the simplest embodiment of the invention, sufficient braze alloy is used for the joint 22 to give a resultant solidified joint of a thickness in the range of 20#-150# instead of the more usual range of 5#-30j.

Alternative forms of brazed joint are illustrated diagrammatically in Figures 6-9 where, for simplicity, the bi-metallic distortion of the cutting element, which in any case will be very small in practice, is not shown.

Referring to Figure 6, the brazed joint 27 includes a thin circular foil 28 on either side of which are layers 29, 30 of braze alloy.

The braze alloy 29, 30 may be that sold under the trade name "COCUMAN" in which case the foil 28 may be a nickel foil of a thickness of 25#. Alternatively, the braze alloy may be that sold under the trade name "GAMMA" in which case the foil may be a tungsten metal foil of 25# thickness. The thickness of each layer 29 or 30 may be in the range of 5ss-50 giving an overall thickness for the whole joint in the range of 60p-150p.

However, the invention is not limited to these dimensions and ranges and the overall thickness of the joint will be selected to be sufficient to absorb sufficient of the residual stress in the cutting element to reduce or minimise diametral splitting.

In the arrangement of Figure 7, the joint 31 comprises wires or rods 32 of nickel or tungsten embedded in a conventional brazing alloy 33. In the arrangement of Figure 8 the joint 34 comprises powder particles 35 of nickel or tungsten embedded in a conventional brazing alloy 36. In either of these arrangements the overall thickness of the joint is preferably within the ranges specified above.

Figure 9 illustrates diagrammatically an alternative structure for achieving a thicker braze joint. In this case the substrate 37 of the cutting element is formed with a part-spherical concavity 39 so as to define a chamber 40 which becomes filled with the brazing alloy 41. This permits a thicker layer of brazing alloy than is the case with nominally flat surfaces being brazed together and thus provides a greater body of braze material for absorbing the residual stresses in the cutting element.

Alternatively, or additionally, a similar concavity may be formed in the surface 42 of the carrier 43.

Claims (16)

1. A method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element comprising at least two layers including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising bonding the rear face of the substrate to the carrier by means of a brazing process employing a brazing composition which has a thickness which, upon solidification of the brazing composition, is within the range of about 20p-150p across substantially the whole of the brazing composition.

2. A method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element comprising at least two layers including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising bonding the rear face of the substrate to the carrier by means of a brazing process employing a brazing composition comprising a fusible brazing material incorporating at least one separation element of material which remains substantially solid at the brazing temperature.

3. A method according to Claim 2, wherein the separation element comprises at least one thin sheet of metallic material, there being provided a layer of said fusible brazing material on each side of the separating element.

4. A method according to Claim 3, wherein the sheet of metallic material has a thickness in the range of about 25#-50#.

5. A method according to Claim 3 or Claim 4, wherein the metallic material is selected from nickel or tungsten metal.

6. A method according to any of Claims 3 to 5, where said thin sheet of metallic material is substantially coextensive with the rear surface of the substrate.

7. A method according to any of Claims 2 to 5, wherein there are provided a plurality of separation elements Spaced apart across the rear surface of the substrate.

8. A method according to Claim 7, wherein the separation elements are selected from rods, wires or particles.

9. A method according to Claim 8, wherein the separation elements are formed from a metal selected from nickel or tungsten.

10. A method according to Claim 8 or Claim 9, wherein the separation elements are selected from rods or wires, and said elements are arranged parallel to one another.

11. A method of forming a cutting structure, for a rotary drill bit, of the kind comprising a preform cutting element bonded to a carrier, the preform cutting element comprising at least two layers including a front cutting table formed of superhard material and a substrate formed of less hard material and having a rear face bonded to the carrier, the method comprising preforming at least one of the rear surface of the substrate and the surface of the carrier to which it is brazed with a recess, extending over at least a major part of said surface, which recess becomes filled with the brazing composition during the brazing process.

12. A method according to Claim 11, wherein said recess is in the form of a concavity.

13. A method according to Claim 12, wherein said cavity is part-spherical.

14. A method of forming a cutting structure, fcr a rotary drill bit, substantially as hereinbefore described with reference to the accompanying drawings.

15. A cutting structure when formed by a method according to any of the preceding claims.

16. A rotary drill bit incorporating a number of cutting structures formed by a method according to any of the preceding Claims 1 to 14.