IE64030B1 - Improvements in or relating to cutting elements for rotary drill bits - Google Patents

Description

Improvements in or relating to cutting elements for rotary drill bits The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations and, in particular, to cutting elements for use on such bits, and methods of manufacturing such cutting elements.

Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank and a passage for supplying drilling fluid to the face of the bit. The bit body carries a plurality of so called ’'preform" cutting elements. Each cutting element comprises a thin facing table of superhard material, which defines the front cutting face of the element, bonded to a less hard substrate.

Normally the superhard table is formed of polycrystalline diamond material and the substrate is formed of cemented tungsten carbide. The preform cutting elements are usually mounted on the bit body by being bonded, for example by brazing, to a carrier which may be in the form of a stud of tungsten carbide which is received and located in a socket in the bit body.

Tungsten carbide has certain characteristics which render it particularly suitable for use in the substrate and carrier of such a cutter assembly. Thus, it exhibits high rigidity, high resistance to the erosion to which such cutter assemblies are subject in use, and hot strength. Also, the coefficient of -2expansion of tungsten carbide is sufficiently close to the coefficient of expansion of polycrystalline diamond to reduce the residual stresses which can occur when the two materials are bonded together. However, some of the other characteristics of cemented tungsten carbide have certain disadvantages. For example, cemented tungsten carbide has low toughness (i.e. it is comparatively brittle) and this can lead to failure of such cutter assemblies in use, as a result of impact forces on the assembly.

Also, when a preformed cutting element of the kind referred to is mounted on a drill bit, it is mounted with its front face facing in the direction of movement, its rear face trailing behind and part of the peripheral surface of the cutting element, between the front and rear surfaces, rubbing on the formation being drilled. As drilling proceeds this intermediate surface wears down to form a wear-flat.

Due to its two-layer construction, the cutting element is self-sharpening since the part of the less hard substrate which rubs on the formation wears away more quickly than the part of the diamond table rubbing on the formation. Since the rearward substrate wears away more quickly, it does not bear on the formation being cut to the same extent or with the same pressure as the diamond table. The compact is therefore rendered self-sharpening by this constant greater wearing away of material rearwardly of the front cutting -3face.

However, the abrasion resistance of the wearflat rubbing on the formation generates heat which is additional to the heat generated by cutting. Tungsten carbide has high abrasion resistance, and when the substrate is formed from tungsten carbide the additional heat generated by abrasion may be sufficient to cause the thermally activated deterioration of the diamond table at an increasingly rapid rate. It is generally accepted that a standard cutting element of the kind described, having a tungsten carbide substrate, generally operates efficiently only until it is about 30% worn. From thereon the cutting element may become thermally unstable and wear extremely rapidly leading quickly to failure of the cutter. When a sufficient number of cutting elements have failed in this manner the bit becomes useless for further drilling.

The combination of low toughness and high heat generation can also cause heat checking of the tungsten carbide carrier material with resultant premature failure of the bit. Heat checking is a term in the art which refers to craze cracking of the wear flat which develops on the carrier, due to abrasive heating with intermittent quenching by the drilling fluid.

The approach taken by the present invention is to overcome the above-mentioned problems by reducing the abrasion resistance of the less hard substrate of the cutting element so as to reduce the additional heat -4generated by rubbing of the wear-flat on the formation t being drilled. It is, however, desirable that abrasion ♦ resistance of the substrate should be reduced without also reducing its erosion resistance. In use the cutting elements are subjected to the substantial eroding effect of abrasive drilling fluid flowing over the cutting elements continuously during drilling. Existing cutting elements having a cemented tungsten carbide substrate have considerable resistance to such erosion and this should preferably not be significantly reduced in attempting to reduce the abrasion resistance.

According to the invention there is provided a cutting element for a rotary drill bit comprising a thin superhard table of polycrystalline diamond material, defining a front cutting face, bonded to a less hard substrate, wherein the substrate comprises a compound of cemented tungsten carbide and a softer refractory material.

Use of a substrate containing a softer refractory material in addition to the tungsten carbide may overcome the problems described above with relation to the use of cemented tungsten carbide material alone.

. The presence of the softer refractory material in the substrate has the effect of reducing the abrasion resistance of the substrate, and thereby reducing the heat generated by rubbing of the wear flat of the substrate on the formation being drilled.

In addition, the material containing the -5softer refractory material may be even stronger than cemented tungsten carbide in cantilever bending and shear forces to which cutter assemblies may be subject in use.

The softer refractory material may be tungsten metal, or may be selected from boron, hafnium, iridium, molybdenum, niobium, osmium, rhenium, rhodium, tantalum and zirconium.

The refractory material and tungsten carbide together may constitute at least about 50% by weight, and preferably at least about 80% by weight, of the material from which the substrate is formed.

The substrate may comprise a metal matrix composite including refractory material particles and tungsten carbide particles in a metal binder phase. The metal matrix composite may be formed by sintering, by an infiltration process, or by hot pressing a mixture of powdered tungsten carbide and refractory material with a catalyst.

The metal binder phase of the composite may comprise a material selected from: Cu, Co, Ni + Cu, Ni + Fe, Ni + Fe + Mo, Co + Ni.

The invention also provides a method of forming a cutting element for a rotary drill bit comprising placing in a high pressure/high temperature press a layer of polycrystalline diamond particles, said layer having a front face and a rear face, and, in contact with said rear face, a further layer comprising -6a mixture including powdered tungsten carbide, a powdered softer refractory material and a catalyst, and subjecting said layers to high pressure and temperature so as to form, on cooling, a solid composite compact comprising a superhard table of polycrystalline diamond material bonded to a less hard substrate comprising a compound including cemented tungsten carbide and said softer refractory material.

The softer refractory material may be tungsten metal, or may be selected from boron, hafnium, iridium, molybdenum, niobium, osmium, rhenium, rhodium, tantalum and zirconium. The catalyst may be cobalt.

The invention includes within its scope a cutter assembly for a rotary drill bit comprising a cutting element according to the invention mounted, for example by brazing, on a carrier which may be in the form of a stud or post to be received in a socket in the bit body.

The invention also includes within its scope a rotary drill bit of the kind first referred to including cutting elements or cutter assemblies according to the invention.

The following is a more detailed description of an embodiment of the invention, reference being made to the accompanying drawing which is a side elevation of a typical cutter assembly to which the present invention may be applied.

The cutter assembly comprises a generally -7cylindrical stud 10, which is formed adjacent one end thereof with an inclined plane surface 11 which is disposed at an angle to the longitudinal axis of the stud 10. The stud 10 may be formed from cemented tungsten carbide, but is preferably formed from a material containing at least a proportion of tungsten metal in accordance with British Patent Specification No. 2216577.

Mounted on the inclined surface 11 of the stud 10 is a preform cutting element 12 comprising a superhard facing table 13 of polycrystalline diamond bonded to a thicker and less hard substrate 14. The cutting element is in the form of a circular disc.

The rear surface 15 of the cutting element is 15 flat and is brazed to the inclined plane surface 11 of the stud 10. The cutting element may be brazed to the stud by the process known as LS bonding in which a heat sink is applied to the polycrystalline diamond table of the cutting element to prevent thermal degradation of the table during the bonding process.

A plurality of such cutter assemblies are mounted in the body of a rotary drill bit of the kind first referred to. The bit body is formed over the surface thereof with a plurality of cylindrical sockets of circular cross-section and received in each socket is the stud 10 of a cutter assembly. The cutter assembly is usually shrink-fitted or brazed into its socket. The general construction of such drill bits is well known -8and will not therefore be described in further detail.

Hitherto, prior art two-layer cutting elements of the kind shown in the drawing have comprised a substrate 14 of cemented tungsten carbide. The front diamond table 13 and the substrate 14 are formed simultaneously by placing layers of polycrystalline diamond particles and tungsten carbide in a high pressure/high temperature press, a catalyst, such as cobalt, normally also being present in the layer of tungsten carbide.

According to the present invention, the material to form the substrate comprises tungsten carbide and some other softer refractory material.

The material of the substrate may comprise a metal matrix composite including tungsten metal particles and tungsten carbide particles in a metal binder phase.

The tungsten metal may be replaced by another refractory material which is softer than the tungsten carbide. Such refractory materials include boron, hafnium, iridium, molybdenum, niobium, osmium, rhenium, rhodium, tantalum and zirconium.

The tungsten metal (or other softer refractory material) and tungsten carbide together preferably constitute at least about 50% by weight, and more preferably 80%, of the material from which the carrier is formed. The substrate may be formed by sintering or hot-pressing. Such methods are well known in the art - 9and will not therefore be described in detail.

As previously explained, the presence of the tungsten metal or other softer refractory material in the substrate 14 serves to reduce the abrasion resistance of the substrate. The reduction in abrasion resistance results in less generation of heat when the cutting element bears against the formation being drilled, and thus delays the onset of thermal degradation of the diamond table.

As previously mentioned, although the stud 10 may be formed from cemented tungsten carbide, it might also be formed from similar material to the substrate 14, and may comprise, for example, tungsten carbide including a softer refractory material such as tungsten metal, as described in our British Patent Application No 2216577. This may be advantageous, since after a certain degree of wear has occurred the wear-flat will also extend across part of the end of the stud 10, and reduction of the abrasion resistance of the stud will in that case assist in reducing the heat generated during drilling.

The arrangements described above are also described in our Application No 522/90, the main claim of which relates to a cutting element for a rotary drill bit comprising a thin superhard table of polycrystalline diamond material, defining a front cutting face, bonded to a less hard substrate, wherein the substrate is formed of a metal matrix composite containing tungsten metal particles in a metal binder phase.

Claims (13)

1. A cutting element for a rotary drill bit comprising a thin superhard table of polycrystalline diamond material, defining a front cutting face, bonded 5 to a less hard substrate, wherein the substrate comprises a compound of cemented tungsten carbide and a softer refractory material.

2. A cutting element according to Claim 1, wherein the softer refractory material is tungsten 10 metal.

3. A cutting element according to Claim 1, wherein the softer refractory material is selected from boron, hafnium, iridium, molybdenum, niobium, osmium, rhenium, rhodium, tantalum and zirconium. 15

4. A cutting element according to any of Claims 1 to 3, wherein the refractory material and tungsten carbide together constitute at least about 50% by weight of the material from which the substrate is formed.

5. A cutting element according to any of Claims 1 20 to 3, wherein the refractory material and tungsten carbide together constitute at least about 80% by weight of the material from which the substrate is formed.

6. A cutting element according to any of Claims 1 to 5, wherein the substrate is a metal matrix composite 25 including refractory material particles and tungsten carbide particles in a metal binder phase.

7. A cutting element according to Claim 6, wherein the metal matrix composite is formed by -11sintering, by an infiltration process, or by hot •i pressing a mixture of powdered tungsten carbide and refractory material with a catalyst. fc

8. A cutting element according to Claim 6 or Claim 7, wherein the metal binder phase of the composite comprises a material selected from: Cu, Co, Ni + Cu, Ni + Fe, Ni + Fe + Mo, Co + Ni.

9. A method of forming a cutting element for a rotary drill bit comprising placing in a high pressure/high temperature press a layer of polycrystalline diamond particles, said layer having a front face and a rear face, and, in contact with said rear face, a further layer comprising a mixture including powdered tungsten carbide, a powdered softer refractory material and a catalyst, and subjecting said layers to high pressure and temperature so as to form, on cooling, a solid composite compact comprising a superhard table of polycrystalline diamond material bonded to a less hard substrate comprising a compound including cemented tungsten carbide and said softer refractory material.

10. A method according to Claim 9, wherein the softer refractory material is tungsten metal. t

11. A method according to Claim 9, wherein the softer refractory material is selected from boron, hafnium, iridium, molybdenum, niobium, osmium, rhenium, rhodium, tantalum and zirconium.

12. A method according to any of Claims 9 to 11, -12wherein the catalyst is cobalt. *

13. A cutter assembly for a rotary drill bit ? comprising a cutting element according to any of Claims 1 to 8 mounted on a carrier in the form of a stud or 5 post to be received in a socket in the bit body.