Field of the invention

This disclosure relates generally to pick tools comprising super-hard strike tips, and a method of using the same.

Background

International patent application publication number WO 2008/105915 discloses a high impact resistant tool having a super-hard material bonded to a cemented metal carbide substrate at a non- planar interface. At the interface, the substrate has a tapered surface starting from a cylindrical rim of the substrate and ending at an elevated flatted central region formed in the substrate. The super- hard material has a pointed geometry with a sharp apex having 1.27 to 3.17 millimetres radius. The super-hard material also has a 2.54 to 12.7 millimetre thickness from the apex to the flattened central region of the substrate.

EP 2 049 769 discloses an attack tool for degrading materials, comprising a cemented carbide first segment bonded to a base segment, and a cemented carbide second segment brazed to the first segment. Super-hard material is bonded to an end of the second segment. Excess braze material may extrude to the outside of the brazed joint and brazing may result in an affected zone, which may be weakened by cracks, depressions, scrapes or other irregularities or imperfections. The affected zone is removed by grinding.

EP 3 071 791 discloses a method of making a strike construction for a pick tool, the strike construction comprising a cemented carbide strike tip joined to an end of a cemented carbide support body, sides of which depend divergently from the end. The method includes processing a precursor to the support body to increase the area of the end, and joining the strike tip and the support body. The method may include processing the strike construction to modify its surface roughness, its size dimension, the sharpness or roundedness of an edge, characterised by an osculating circle radius.

There is a need for value super-hard pick tools as a lower cost alternative to premium priced pick tools, for use in applications such as cutting, drilling or milling formations or structures comprising rock, concrete or pavement, for example. Statement of Invention

According to the invention, there is provided a pick tool for road milling or mining, the pick tool comprising a tip assembly coupled to a tool body, the tip assembly comprising a shaped cutter supported by a bolster, a first end of the bolster being connected to the tool body, the shaped cutter being disposed at a second opposing end of the bolster, the shaped cutter comprising a super-hard strike tip joined to a substrate, and a cap covering at least the super-hard strike tip, material of the cap having a melting point of at least 2000°C.

Preferable and/ or optional features of the invention are set forth in claims 2 to 13, inclusive. Brief Description of the Drawings

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which

Figure 1 is a perspective view of an exemplary pick tool;

Figure 2 is a cross-sectional view through the pick tool of Figure 1;

Figure 3 shows an exemplary strike tip in schematic cross-section; and

Figure 4 shows the strike tip of Figure 3 in perspective view, with the cap omitted for clarity.

Detailed Description

Referring to the accompanying drawings, a pick tool is indicated generally at 10. In use, such a pick tool 10 is typically driven to impact a body or formation to be degraded. In road milling and mining applications, a plurality of pick tools 10 is mounted onto a drum. The drum is coupled to and driven by a vehicle, causing the drum to rotate and for the picks to repeatedly strike asphalt or rock, for example, as the drum rotates.

As best seen in Figures 1 and 2, the pick tool 10 comprises a tip assembly 12 coupled to a tool body 14. The tip assembly 12 comprises a shaped cutter 16 supported by a bolster 18, a first end of the bolster 18 being connected to the tool body 14, the shaped cutter 16 being disposed at a second opposing end of the bolster 18. The shaped cutter 16 is connected to a portion of the bolster 18 sometimes referred to as a‘mushroom’ 20.

The bolster 18 comprises an insertion shaft 22, which is shrink fit into a bore 24 formed into the steel tool body 14. The tool body 14 has a shank 26 for mounting the tool pick onto a drum (not shown) via a coupling mechanism (also not shown). In the example arrangement shown in Fig. 2, the shank 26 is substantially not aligned with a main axis of the bolster 18.

The volume of the bolster 18 may be about 30 cm ³ and the length of the bolster 18 may be about 6.8 cm. As used herein, a shrink fit is a kind of interference fit between components achieved by a relative size change in at least one of the components (the shape may also change somewhat). This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after assembly. Shrink-fitting is understood to be contrasted with press-fitting, in which a component is forced into a bore or recess within another component, which may involve generating substantial frictional stress between the components. In some variants, the bolster 18 comprises a cemented carbide material, comprising grains of tungsten carbide having a mean size of at about 2.5 microns to about 3 microns, and at most about 10 weight per cent of metal binder material, such as cobalt (Co). Shrink fitting the bolster 18 into the tool body 14 may allow relatively stiff grades of cemented carbide to be used, which is likely to enhance support for the shaped cutter 16 and reduce the risk of fracture.

In order to reduce stresses, sharp corners at points of contact are avoided. For example, edges and corners may be radiused or chamfered, and the edge of the bore may be provided with a radius or chamfer to reduce the risk of stress-related cracks arising.

Turning now to Figures 3 and 4, the shaped cutter 16 comprises a super-hard strike tip 28 joined to a substrate 30 at an interface 32, and a cap 34 wholly covering the super-hard strike tip, extending past the interface 32 and wholly covering the substrate 30. The cap 34 may alternatively only partially cover the substrate 30. Regardless of the extent to which the substrate 30 is covered, the cap 34 always completely covers the strike tip 28, for reasons which will become clear. The cap 34 is contiguous the strike tip 28 in that it is adjacent to and touching the strike tip 28. A peripheral profile of the cap 34 matches that of the strike tip 28, or in other words, the shape of the cap 34 matches that of the strike tip 28. This is a direct consequence of the manufacturing process used to produce the shaped cutter 16. Further detail on the manufacturing process is provided below.

In Figures 3 and 4, the cap 34 is generally cup-shaped, having a planar apex area 36 and a side wall 38 depending from the planar apex area 36 toward the substrate 30. Alternatively, the cap 34 may be generally conical in shape, and having a pointed apex. In this embodiment, the apex area 36 is substantially circular in plan view, the effect of which can be seen on the strike tip 28 in Figure 4. The cap 34 itself has rotational symmetry about a main central axis 40. In Figures 3 and 4, the side wall 38 is curved but it could alternatively be planar. The shape of the side wall 38 is not significant to the invention, but, as mentioned above, the shape of the cap 34 must match that of at least the strike tip 28, and optionally the substrate 30 too.

In the example of Figures 3 and 4, the substrate 30 has a diameter of 15 mm, and the planar apex area 36 has a diameter of 2 mm.

The shaped cutter 16 comprises super-hard material. Specifically, material of the strike tip 28 is super-hard. Synthetic and natural diamond, polycrystalline diamond (PCD), cubic boron nitride (cBN) and polycrystalline cBN (PCBN) material are examples of super-hard materials. As used herein, synthetic diamond, which is also called man-made diamond, is diamond material that has been manufactured. As used herein, polycrystalline diamond (PCD) material comprises an aggregation of a plurality of diamond grains, a substantial portion of which are directly inter- bonded with each other and in which the content of diamond is at least about 80 volume per cent of the material. Interstices between the diamond grains may be at least partly filled with a filler material that may comprise catalyst material for synthetic diamond, or they may be substantially empty. As used herein, a catalyst material for synthetic diamond is capable of promoting the growth of synthetic diamond grains and or the direct inter-growth of synthetic or natural diamond grains at a temperature and pressure at which synthetic or natural diamond is thermodynamically stable. Examples of catalyst materials for diamond are Fe, Ni, Co and Mn, and certain alloys including these elements. Bodies comprising PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains. As used herein, a PCD grade is a variant of PCD material characterised in terms of the volume content and or size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. Different PCD grades may have different microstructure and different mechanical properties, such as elastic (or Young’s) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K1C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

The substrate 30 is manufactured from a cemented tungsten carbide material. The material of the substrate is not important to the invention and other materials could be used instead. The cap 34 material is refractory and has a melting point of at least 2000°C. It is malleable in the sense that it is important that the starting material of the cap 34 is pre-formable into a specific shape or profile, for example, using a drawing process, in order to form the cap. The internal profile will ultimately impart shape to the finished strike tip 28 (and optionally substrate 30). The cap 34 material must be able to comfortably withstand the conventional high pressure and high temperatures found in ultra- high pressure presses (more details below). Finally, the cap 34 material should be relatively unreactive with material of the strike tip 28. The cap 34 material is however permitted to be carbide forming with material of the strike tip (and optionally, the substrate 30 too). The cap 34 material preferably comprises niobium but tantalum or molybdenum may be used instead. Alternatively, the cap 34 may comprise steel.

An example method of making a strike tip 28 comprising a PCD structure formed joined to a substrate will now be described.

In general, a shaped cutter 16 is made by placing an aggregation comprising a plurality of diamond grains onto a cemented carbide substrate 30 in the presence of a catalyst material for diamond, thus providing a pre-sinter assembly. This pre-sinter assembly is then be subjected to an ultra-high pressure and high temperature at which diamond is more thermodynamically stable than graphite, to sinter together the diamond grains and form a PCD structure joined to the substrate 30. Binder material within the cemented carbide substrate 30 provides a source of catalyst material, such as cobalt, iron or nickel, or mixtures or alloys including any of these elements. A source of catalyst material may also be provided within the aggregation of diamond grains, in the form of admixed powder or deposits on the diamond grains, for example. A source of catalyst material may additionally or alternatively be provided proximate a boundary of the aggregation other than the boundary between the aggregation and the substrate 30, for example, adjacent a boundary of the aggregation that will correspond to the strike tip 28 of the sintered PCD structure.

In some production methods, the aggregation may comprise substantially loose diamond grains, or diamond grains held together by a binder material. The aggregations may be in the form of granules, discs, wafers or sheets, and may contain catalyst material for diamond and or additives for reducing abnormal diamond grain growth, for example, or the aggregation may be substantially free of catalyst material or additives. In some production methods, aggregations in the form of sheets comprising a plurality of diamond grains held together by a binder material may be provided. The sheets may be made by a method such as extrusion or tape casting, in which slurries comprising diamond grains having respective size distributions suitable for making the desired respective PCD grades, and a binder material is spread onto a surface and allowed to dry. Other methods for making diamond-containing sheets may also be used. Alternative methods for depositing diamond-bearing layers include spraying methods, such as thermal spraying. The binder material may comprise a water-based organic binder such as methyl cellulose or polyethylene glycol (PEG) and different sheets comprising diamond grains having different size distributions, diamond content and or additives may be provided. For example, sheets comprising diamond grains having a mean size in the range from about 15 microns to about 80 microns may be provided. Discs may be cut from the sheet or the sheet may be fragmented. The sheets may also contain catalyst material for diamond, such as cobalt, and or precursor material for the catalyst material, and or additives for inhibiting abnormal growth of the diamond grains or enhancing the properties of the PCD material. For example, the sheets may contain about 0.5 weight per cent to about 5 weight per cent of vanadium carbide, chromium carbide or tungsten carbide.

In some production methods, the aggregation of diamond grains may include precursor material for catalyst material. For example, the aggregation may include metal carbonate precursor material, in particular metal carbonate crystals, and the method may include converting the binder precursor material to the corresponding metal oxide (for example, by pyrolysis or decomposition), admixing the metal oxide based binder precursor material with a mass of diamond particles, and milling the mixture to produce metal oxide precursor material dispersed over the surfaces of the diamond particles. The metal carbonate crystals may be selected from cobalt carbonate, nickel carbonate, copper carbonate and the like, in particular cobalt carbonate. The catalyst precursor material may be milled until the mean particle size of the metal oxide is in the range from about 5 nm to about 200 nm. The metal oxide may be reduced to a metal dispersion, for example in a vacuum in the presence of carbon and/ or by hydrogen reduction. The controlled pyrolysis of a metal carbonate, such as cobalt carbonate crystals provides a method for producing the corresponding metal oxide, for example cobalt oxide (Co304), which can be reduced to form cobalt metal dispersions. The reduction of the oxide may be carried out in a vacuum in the presence of carbon and/or by hydrogen reduction. The substrate 30 may have a non-planar or a substantially planar interface surface on which the PCD structure is formed. For example, the interface surface 42 may be configured to reduce or at least modify residual stress within the PCD.

Extending away from the interface surface, the substrate 30 may be flared outwardly. In other words, the substrate may be frusto-conical in shape, with the shortest parallel surface being adjacent the interface surface. Alternatively, the substrate may be flared inwardly. In other words, the substrate may be frusto-conical in shape, with the longest parallel surface being adjacent the interface surface. Optionally, the substrate may be flared outwardly initially, and then flared inwardly. Of course, the substrate need not have a circular base but it could instead have a square base, thus making the substrate frusto-pyramidal in the various configurations above. Usually though, the substrate has straight side walls and is generally cylindrical in shape.

A cup having a pre-shaped internal surface is provided for use in assembling the diamond aggregation, which may be in the form of an assembly of diamond-containing sheets, onto the substrate 30. The aggregation is placed into the cup and arranged to fit substantially conformally against the internal surface. The substrate 30 is then inserted into the cup with the interface surface going in first and pushed against the aggregation of diamond grains. The substrate 30 is firmly held against the aggregation by means of a second cup placed over it and inter- engaging or joining with the first cup to form a pre-sinter assembly. Post-sintering, the first cup is to become the cap 34 of the invention.

The pre-sinter assembly is placed into a capsule designed for an ultra-high pressure press and subjected to an ultra-high pressure of at least about 5.5 GPa and a temperature of at least about 1 ,300 degrees centigrade, in order to sinter the diamond grains and form a construction comprising a PCD structure sintered onto the substrate. In one version of the method, when the pre-sinter assembly is treated at the ultra-high pressure and high temperature, the binder material within the substrate melts and infiltrates the aggregation of diamond grains. The presence of the molten catalyst material from the substrate 30 and or from a source provided within the aggregation will promote the sintering of the diamond grains by intergrowth with each other to form a PCD structure.

The shaped cutter 16 is thus formed. Hitherto, the first and second cups were then removed from the shaped cutter, for example using leaching, machining, sandblasting and/or grinding. By contrast, in this invention, the first cup remains and only the second cup is removed. The number of process steps required to produce the finished pick tool 10 has been reduced, notably due to the absence of the leaching and sandblasting sub-processes.

The benefit of not removing what was the first cup in the pre-sinter assembly is that significant process costs associated with the first cup removal are avoided. Such costs otherwise account for a large proportion of the tip assembly production.

Furthermore, micro cracking caused by the leaching process and specifically due to thermal shock and the difference in Coefficient of Thermal Expansion of cobalt and diamond, is prevented. Leaching is typically used to remove the (niobium) first cup.

Similarly, by not subjecting the shaped cutter to sandblasting, micro surface cracking imparted by the sandblasting process, is avoided. Sandblasting is typically used to remove the (niobium) carbide layer under the (niobium) first cup, exposed by the leaching process.

Above all, surface micro cracking can inadvertently affect the production yield of a plurality of tip assemblies. If a significant number of tip assemblies fail to meet the necessary quality standards, and the yield is correspondingly low, the unit cost of each tip assembly and thus of a pick tool 10 incorporating said tip assembly, crucially and undesirably increases.

Ultimately, the pick tool 10 is assembled, and the shaped cutter 16 including cap 34, supported by the bolster 18, is coupled to the tool body 14 and supplied to the end user, e.g. the manufacturers of road milling equipment. It is the end user who then removes the cap 34 during the very early stages of the operation of the road milling equipment. The cap 34 is simply and incidentally worn away during routine operation of the (road milling) equipment.

Beneficially, the cost savings can be passed on by the manufacturers of the shaped cutter to the end users of the pick tool. Retaining the cap 34 (i.e. what was the‘first cup’) on the strike tip 28, and optionally also on the substrate 30, facilitates a new‘value’ pick tool in the product range, which is quite apart from the existing‘premium’ product pick tools. The quality of existing premium pick tools can be assured by way of surface inspection, visual or otherwise, during which any surface defects can be located, identified and categorised, and the quality of the shaped cutter (or resultant pick tool) graded accordingly. However, with the shaped cutter 16 of the invention, it is not possible to inspect for any defects due to the presence of the cap covering the strike tip (and optionally the substrate). Therefore, the quality of the resultant pick tool 10 is less certain. For some customers of the shaped cutter, this is totally unacceptable. For other customers though, feedback suggests that this would be an acceptable commercial risk and certainly worth the benefit of a reduced price pick tool.

Furthermore, the inventors have also found during testing of the pick tool 10 that the cap 34 appears to prolong the life of the tool 10. As the usual wear scar reaches the peripheral edge of the tip assembly, the cap 34 prevents chipping of the trailing edge (as opposed to the leading edge which comes into direct contact with the formation to be struck), thereby postponing the maintenance trigger for pick tool 10 replacement.

While the examples above refer to PCD strike tips, the super-hard material may include certain composite materials comprising diamond or cBN grains held together by a matrix comprising ceramic material, such as silicon carbide (SiC), or cemented carbide material, such as Co-bonded WC material. For example, certain SiC-bonded diamond materials may comprise at least about 30 volume per cent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC).

In examples where strike tips 28 are used to break up bodies comprising hard structures, such as stones, dispersed within a softer matrix structure, the configuration of the tip assembly in general and the apex area in particular may be selected according to the composition of the body. For example, picks 10 according to this invention may be used to break up road or pavement bodies comprising asphalt, which may comprise grains of stones dispersed with in a tar-based matrix. The shaped cutter 16 may be selected to have a strike surface configured according to the statistical distributions of the sizes of the grains and the distances between the stones, such that the effect of digging out the stones may be enhanced. For example, the apex area, its edge and the surrounding surfaces of the strike tip may be configured to increase the likelihood of the apex area fitting between the stones and to increase the cutting of the matrix on impact.

Where the weight or volume per cent content of a constituent of a polycrystalline or composite material is measured, it is understood that the volume of the material within which the content is measured is to be sufficiently large that the measurement is substantially representative of the bulk characteristics of the material. For example, if PCD material comprises inter-grown diamond grains and cobalt filler material disposed in interstices between the diamond grains, the content of the filler material in terms of volume or weight per cent of the PCD material should be measured over a volume of the PCD material that is at least several times the volume of the diamond grains so that the mean ratio of filler material to diamond material is a substantially true representation of that within a bulk sample of the PCD material (of the same grade).

While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.