EP4306671A1

EP4306671A1 - Rock drill insert

Info

Publication number: EP4306671A1
Application number: EP22184187.7A
Authority: EP
Inventors: Leif Åkesson; Krystof TURBA; Ida BORGH; Mirjam LILJA
Original assignee: Sandvik Mining and Construction Tools AB
Current assignee: Sandvik Mining and Construction Tools AB
Priority date: 2022-07-11
Filing date: 2022-07-11
Publication date: 2024-01-17
Also published as: WO2024012930A1

Abstract

A rock drill insert comprising a body of cemented carbide comprising hard constituents of WC in a binder phase comprising cobalt; wherein the cemented carbide comprises 4-18 wt% Co; Cr such that the Cr/Co mass ratio in the bulk of the body is 0.04 -0.19; a balance of WC and any unavoidable impurities; wherein content of the binder phase of the cemented carbide is substantially equal throughout the rock drill insert; characterized in that: said insert has a corrected CoM / wt% Co ratio between 0.72 - 0.81 and the insert is substantially free of eta-phase; wherein said corrected CoM / wt% Co ratio is calculated according to: Corrected CoM / wt% Co = (magnetic-% Co + 1.13 <sup>∗</sup> wt% Cr) / wt% Co where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co and wt-% Cr are the weight percentage of Co and Cr in the cemented carbide, respectively.

Description

Field of invention

The present invention relates to a rock drill insert comprising chromium alloyed cemented carbide having a low carbon content.

Background

Rock drilling is a technical area in which the inserts which are used for the purpose of drilling in the rock are subjected to high stresses, repeated impacts and severe corrosive conditions due to the inherent nature of the drilling. Different drilling techniques will generate different loads on the inserts, resulting from a combination of contact stress, impacts, shear and bending. Particularly severe stress conditions are found in applications such as those in which the rock drill inserts are mounted in a rock drill bit body of a top-hammer (TH) device, a down-the-hole (DTH) drilling device or a rotary drilling device, a raise boring device or a mechanical cutting device.
Traditionally, rock drill inserts may consist of a body made of cemented carbide that comprises hard constituents such as tungsten carbide (WC) in a binder phase such as cobalt (Co). It is desirable to increase the lifetime of the inserts. WO2018/060125 discloses that by adding chromium to the cemented carbide, the performance of the drill bits is enhanced. There is however the need to further improve the performance and lifetime of the inserts, especially in hard rock drilling applications.
Therefore, the problem to be solved is how to further increase the lifetime of the drill inserts.

Definitions

By the term "bulk" is herein meant the cemented carbide of the innermost part (centre) of the rock drill insert.
By the term "eta-phase" is herein meant M₆C or M₁₂C where M = (Co, W, Cr).

Summary of the Invention

It is an objective of the present invention to improve the lifetime of the cemented carbide inserts, this objective is achieved by providing a rock drill insert comprising a body of cemented carbide comprising hard constituents of tungsten carbide (WC) in a binder phase comprising cobalt; wherein the cemented carbide comprises 4-18 wt % Co; Cr such that the Cr/Co mass ratio in the bulk of the body is 0.04 - 0.19; a balance of WC and any unavoidable impurities; wherein content of the binder phase of the cemented carbide is substantially equal throughout the rock drill insert; said insert has a corrected CoM / wt% Co ratio between 0.72 - 0.81 and the insert is substantially free of eta-phase; wherein said corrected CoM / wt% Co ratio is calculated according to Equation 1: $Corrected CoM / wt % Co = (magnetic - % Co + 1.13 * wt % Cr) / wt % Co$
where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co and wt-% Cr are the weight percentage of Co and Cr in the cemented carbide, respectively.
Advantageously, it has surprisingly be found that when an insert having this special range of corrected CoM / wt% Co is subjected to static / low strain rate contact stress, which could be through a mechanical post-sintering treatment process such as high energy tumbling and / or through the actual drilling process, both the material's strain hardening capacity and its plasticity in compression will be further and optimally enhanced. The term enhanced plasticity is herein used to designate the material's capacity to undergo a higher degree of plastic strain before the onset of fracture. The material will at the same time exhibit enhanced ultimate compressive strength (UCS). Typically, in materials design, increasing ultimate strength results in a decrease in plasticity and vice versa. In the present invention, both of these properties are enhanced simultaneously, which leads to the reduction of the risk of premature insert breakages in the rock drilling application. Furthermore, the enhanced plasticity and strain hardening in compression allow for an optimally enhanced level of induced residual stresses in the material, which further increase the resistance of the insert to premature breakage and thus extend the insert lifetime. The strain hardening and induced residual compressive stresses are also manifested in an apparent hardness increase at and below the surface of the insert. Additionally, it has been found that if the corrected CoM / wt% Co is in this range the wear resistance of the insert is improved which also contributes to increasing the lifetime of the inserts when used in the field. Further, this range avoids the formation of brittle eta-phase.
In one embodiment the cobalt content is between 8 - 18 wt%. Advantageously, this range facilitates obtaining high fracture toughness in the material, thus making it suitable especially for toughness-focused rock drilling applications such as rotary drill bits, raise boring pilot bits, and raise boring cutters.
In another embodiment the cobalt content is between 4 - 8 wt%. Advantageously, this range makes it possible to reach particularly high wear resistance, typically required in applications such as top hammer and down the hole drilling.
In one embodiment the corrected CoM / wt% Co is between 0.73 - 0.79. Advantageously, this range results in the most optimal enhancement of the material's strain hardening capacity and its plasticity in compression.
In one embodiment the difference between an average hardness at 0.3 mm below the surface of the rock drill insert and an average hardness in the bulk of the rock drill insert is at least 30 HV3 wherein hardness is measured according to ISO EN6507. The hardness difference results from mechanically induced compressive residual stresses and strain hardening of the binder phase. Advantageously, this leads to an increased strength and apparent toughness of the rock drill insert, reducing the risk of early damage and failure of the insert and consequently increasing the insert lifetime.
In one embodiment, the difference between the hardness at any point 0.3 mm below the surface of the rock drill insert and the hardness at 1 mm below the surface of the rock drill insert is at least 20 HV3 wherein hardness is measured according to ISO EN6507. The hardness difference reflects the induced compressive residual stresses and strain hardening of the binder phase, leading to enhanced apparent toughness and strength of the insert, consequently increasing its lifetime during drilling.
In one embodiment the WC grain size mean value of the cemented carbide is above 1 µm but less than 18 µm as measured according to Jeffries method defined in the description hereinbelow. Advantageously, this grain sizes provides the optimal balance between wear resistance and toughness for rock tool applications.
In one embodiment the mean WC grain size value of the cemented carbide is above 1.5 µm but less than 10 µm. Advantageously, these grain sizes provide the optimal balance between wear resistance and toughness for rock tool applications.
In one embodiment the mass ratio Cr/Co in the cemented carbide is between 0.075-0.15. Advantageously, this provides optimum wear resistance and capacity for strain hardening.
In another embodiment the mass ratio Cr/Co in the cemented carbide is between 0.05 - 0.12. Advantageously, this provides the optimum balance between plasticity, capacity for strain hardening, wear resistance, and fracture toughness.
In one embodiment the cemented carbide has a bulk hardness of not higher than 1700 HV3. Advantageously, this means that the insert is not so brittle that it is prone to failure.
According to another aspect of the present application there is a rock drill bit body comprising one or more mounted rock drill inserts as described hereinbefore or hereinafter.

Brief description of the drawings

Figure 1 is a schematic representation of the geometry of a rock drill insert used in the wear tests.
Figure 2 shows the deformation curves in uniaxial compression for samples A and B.
Figure 3 shows the deformation curves in uniaxial compression for samples C and D.

Detailed description

Figure 1 shows a rock drill insert 2 comprising a body of cemented carbide comprising hard constituents of WC in a binder phase comprising cobalt; wherein the cemented carbide comprises 4-18 wt % Co; Cr such that the Cr/Co mass ratio in the bulk of the body is 0.04 -0.19; a balance of WC and any unavoidable impurities; wherein content of the binder phase of the cemented carbide is substantially equal throughout the rock drill insert; wherein said insert has a corrected CoM / wt% Co ratio between 0.72 - 0.81 and the insert is substantially free of eta-phase; wherein said corrected CoM / wt% Co ratio is calculated according to equation 1. $Corrected CoM / wt % Co = (magnetic - % Co + 1.13 * wt % Cr) / wt % Co$
Where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co and wt-% Cr are the weight percentage of Co and Cr in the cemented carbide, respectively.
This specific range of corrected CoM / wt% Co is achieved by careful control of the carbon content. The corrected CoM / wt% Co of a sintered sample is measured and calculated by using commercially available Foerster Koerzimat CS 1.096 equipment. The sample is weighed and then put into the magnetic coil as described in the Koerzimat CS 1.096 V3.09 manual. The magnetic moment is measured and from that the weight-specific saturation magnetization, σs, is calculated from the ratio of magnetic moment to weight of the sample. Then the proportion of magnetic material in % (known as magnetic-% Co) is calculated by dividing σs with the material constant for Co, which is 2010 10^-7 Tm³/kg. For chromium containing materials a correction factor of 1.13 ^∗ wt% Cr is used (the 1.13 factor is derived from the ratio of the atomic weights of cobalt and chromium), as in Equation. 1: $Corrected CoM / wt % Co = (magnetic - % Co + 1.13 * wt % Cr) / wt % Co$
where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co and wt-% Cr are the weight percentage of Co and Cr in the cemented carbide, respectively.
The rock drill insert 2 of the present invention is produced by means of a process in which a powder comprising the elements of the cemented carbide is milled and compacted into a compact which is then sintered. A grinding step to obtain the precise dimension of the drill insert is generally made. A drill insert of the present invention generally has a cylindrical base part and a rounded top which may be hemispherical, conical or asymmetric. It should be understood that the rock drill insert could have alternative geometries to that shown in figure 1. Typically, the curved surface of the cylindrical base part is ground to obtain the precise diameter wanted, while the surfaces of the top part and the circular base part are kept in their as sintered state. The drill insert is then subjected to mechanical posttreatment which introduces high levels of compressive stresses in the insert, such as high energy tumbling.
The binder phase content of the cemented carbide is substantially equal throughout the rock drill insert, i.e., no substantial gradient of Co content is present when going from the surface of the rock drill insert to its interior.
In one embodiment the cobalt content is preferably between 5- 16 wt%.
In another embodiment the cobalt content is between 8 - 18 wt%, preferably between 10 - 16 wt%.
In another embodiment the cobalt content is between 4 - 10 wt%, preferably between 4 - 8 wt%.
In one embodiment the corrected CoM / wt% Co is between 0.72 - 0.81, preferably between 0.73 - 0.79, more preferably between 0.74 - 0.78.
In one embodiment the difference between an average hardness at 0.3 mm below the surface of the rock drill insert and an average hardness in the bulk of the rock drill insert is at least 30 HV3, preferably at least 35 HV3, more preferably at least 40 HV3, even more preferably at least 40 HV3, even more preferably at least 50 HV3, even more preferably at least 60 HV3, wherein hardness is measured according to ISO EN6507.
In one embodiment, the difference between the hardness at any point 0.3 mm below the surface of the rock drill insert and the hardness at 1 mm below the surface of the rock drill insert is at least 20 HV3, preferably at least 35 HV3, more preferably at least 40 HV3, more preferably at least 45 HV3 wherein hardness is measured according to ISO EN6507.
The HV3 measurements were carried out in the following way, using the KB30S programmable hardness tester by KB Prüftechnik GmbH:

Sectioning of insert sample along its longitudinal axis.
Grinding and polishing of sectioned surface using progressively finer grit and polishing suspensions.
Scanning the edge of the sample.
Programming the hardness tester to make series of indentations at defined distances to the edge.
Programming the distances between the individual indentations at each distance from edge to 0.3 mm or more.
Indentation with 3 kg load at all programmed coordinates.
Computer moves stage to each coordinate with indentation and runs auto adjust light and auto focus, followed by automatic measurement of the size of each indentation.
User inspects all photos of the indentations for possible focus errors and other effects which may lead to an invalid result and manually re-evaluates the selected invalid ones in each series (if any are present).

The average hardness at a certain depth from the surface is defined as the average of at least 50 measured hardness values at that depth evenly distributed around the insert.
In one embodiment the mean value of the cemented carbide grain size is above 1 µm but less than 18 µm as measured according to Jeffries method defined in the description.
The WC grain size is chosen to suit the desired end properties of the cemented carbide in terms of, for example, toughness, strength, wear resistance and thermal conductivity. According to one embodiment the WC mean grain size is above 1 µm, or above 1.25 µm, or above 1.5 µm, or above 1.75 µm, or above 2.0 µm. If the WC grain size is too large, the material becomes difficult to sinter. Therefore, it is preferred that the WC mean grain size is less than 18 µm, or less than 15 µm, or less than 10 µm, or less than 6 µm.
The micrographs for WC grain size evaluation were obtained using a scanning electron microscope (SEM) in backscatter electron (BSE) contrast. Prior to the imaging, the material samples were polished using standard procedures and etched with Murakami solution to generate contrast at grain boundaries. The mean WC grain size was then evaluated using the Jeffries method described below, from at least two different micrographs for each material. An average value was then calculated from the mean grain size values obtained from the individual micrographs (for each material respectively). The procedure for the mean grain size evaluation using a modified Jeffries method was the following:
A rectangular frame of suitable size is selected within the SEM micrograph so as to contain a minimum of 300 WC grains. The grains inside the frame and those intersected by the frame are manually counted, and the mean grain size is obtained from equations (2-4): $M = \frac{L_{scale mm} \times 10^{- 3}}{L_{scale micro} \times 10^{- 6}}$
$νol % WC = 100 \times (- 1.308823529 \times \frac{(\frac{wt % Co}{100} - 1)}{(\frac{wt % Co}{100} + 1.308823529)})$
$d = \frac{1500}{M} \times \sqrt{\frac{L_{1} \times L_{2} \times vol % WC}{(n)}}$
Where:

d = mean WC grain size (µm)
Li, L₂ = length of sides of the frame (mm)
M = magnification
L_{scale mm} = measured length of scale bar on micrograph in mm
L_{scale micro} = actual length of scale bar with respect to magnification (µm)
n₁=no. of grains fully within the frame
n₂=no. of grains intersected by frame boundary
wt % Co = known cobalt content in weight %.

Equation 3 is used to estimate the WC fraction based on the known Co content in the material. Equation 4 then yields the mean WC grain size from the ratio of the total WC area in the frame to the number of grains contained in it. Equation 4 also contains a correction factor compensating for the fact that in a random 2D section, not all grains will be sectioned through their maximum diameter.
In one embodiment the mass ratio Cr/Co in the cemented carbide is between 0.075 - 0.15, more preferably between 0.85 - 0.12.
In another embodiment the mass ratio Cr/Co in the cemented carbide is between 0.05 - 0.12, preferably between 0.05 - 0.10.
According to yet another embodiment, the M₇C₃ phase is present in the cemented carbide, where M designates a combination of Cr, Co and W, i.e. (Cr,Co,W)₇C₃. The Co solubility can reach as high as 38 at. % of the metallic content in the M₇C₃ carbide. The balance of Cr:Co:W is influenced by the overall carbon content in the cemented carbide.
In another embodiment cemented carbide has a bulk hardness of not higher than 1700 HV3, preferably not higher than 1650 HV3, more preferably not higher than 1600 HV3. The cemented carbide of the rock drill insert has suitably a hardness of the bulk of at least 800 HV3, or at least 900 HV3, or at least 1000 HV3.
According to one embodiment, rock drill inserts 2 according to the invention are mounted in a rock drill bit body of a top-hammer (TH) device or a down-the-hole (DTH) drilling device or a rotary drilling device or a raise boring pilot bit device or a raise boring cutter device or a push boring (blind boring) device or a mechanical cutting device or a horizontal directional drilling (HDD) device. The rotary drilling device may be an oil and gas rotary cutter device.

Example 1 - Samples

The samples shown in table 1 were prepared by milling WC having a dm_FSSS=16.5-18.0 µm with Co and Cr₃C₂ with 2% polyethylene glycol (PG8000) in milling liquid containing 92% alcohol in a ball mill. The powder was then spray dried in a N₂ atmosphere. Inserts were produced by uniaxially pressing the powder to form green bodies and then sintering the green bodies in a Sinter HIP furnace for 60 minutes at 1410°C and with a 55 bar Ar pressure during the last 20 minutes. The properties of the inserts were measured using the methods as described hereinabove.

Table 1: Sample summary

Sample	Co content (wt%)	Cr content (wt%)	Cr / Co mass ratio	HV20 in bulk	Average grain size (µm)	Corrected CoM / wt% Co
A (comparison)	11.0	1.1	0.10	1099	4.92	0.88
B (invention)	11.0	1.1	0.10	1092	4.31	0.78
C (comparison)	13.5	1.35	0.10	1095	3.74	0.87
D (invention)	13.5	1.35	0.10	1059	3.76	0.74

Example 2 - Uniaxial compression test / plastic deformation

Samples A-D were strained at room temperature in uniaxial compression until fracture using an Instron 5989 test frame, at a constant rate of crosshead displacement equal to 0.6 mm / min, while recording load-displacement curves. The test fixture, the hardness and parallelism of the counter surfaces, as well as the sample geometry were in accordance with the ISO 4506:2017 E standard "Hardmetals - Compression test". A compliance curve obtained by loading the test fixture without a sample, accounting for elastic deformation of the test rig and load string, was subtracted from the load-displacement curve measured on the samples directly during each measurement. Engineering stress was calculated from the load values by dividing the load with the initial minimum cross-sectional area, obtained from the minimum diameter measured on each individual test sample prior to testing. Elastic deformation of the samples was subtracted from the stress - displacement curves during test data post-processing using linear regression, in order to isolate only the plastic deformation of the materials. This isolation of the plastic deformation from the stress - displacement curves was carried out as follows:

Linear regression was applied to the part of the data set corresponding to the initial section of the stress - displacement curve which was visually estimated to be linear.
The displacement range of this partial data set used for the linear regression was then varied so as to maximize the R² value of the fit.
The first derivative of the resulting regression equation (slope of the fitted line) was used to calculate the plastic deformation from the total measured displacement using the relationship in equation 5 below: $e_{plast} = e - σ / a$
where "e_plast" designates plastic deformation, "e" the measured displacement after the subtraction of the compliance curve, "σ" the engineering stress, and "a" the coefficient corresponding to the slope of the line fitted by linear regression to the elastic part of the stress - displacement curve, as described above.

Figure 2 compares the deformation curves in uniaxial compression for samples A and B, i.e. the samples having 11 wt% Co. Sample A (comparative sample) is illustrated with a solid line and sample B (inventive sample) is illustrated from a dashed line.
Figure 3 compares the deformation curves in uniaxial compression for samples C and D, i.e. the samples having 13.5 wt% Co. Sample C (comparative sample) is illustrated with a solid line and sample D (inventive sample) is illustrated from a dashed line.
The deformation curves in figures 2 and 3 were plotted from 0.001 mm of plastic deformation.
The inventive samples both show a more pronounced strain hardening, i.e. a steeper deformation curve, throughout most of the deformation until failure; higher ultimate compressive strength (UCS) and substantially greater plasticity (plastic deformation to failure) as compared to the comparative sample. Figure 3 shows that this effect is present also when the samples have equal mean tungsten carbide grain size, in addition to having equal binder phase content.

Example 3 - Abrasion wear test

Rock drill bit inserts with a 10 mm outer diameter and a hemispherical top geometry were produced out of all four materials (A,B,C,D) and in their as ground state subjected to wear testing using a rotating granite log counter surface with continual water flow aimed at the insert / rock contact. During the test, the insert / rock contact was maintained by applying a constant force of 10 kgf (98 N). Since the inserts were ground only on their cylindrical section, the part of the insert in contact with the rock surface was in all cases in the as sintered state. While the granite log was rotating, the insert was moved along it with a constant feed rate of 0.9 mm / s, resulting in a total sliding distance between 432 and 446 m. All inserts were carefully weighed prior to and after the testing. Volume wear per unit of sliding distance was then calculated for each material from measured mass loss and density. Three inserts were tested for each of the four materials (A,B,C,D) and average values for the volume wear per unit of sliding distance were calculated from the three tests. The results are shown in table 2 below: Table 2: Wear rate (volume loss per meter of sliding distance) in abrasion wear test against granite counter surface.

Sample A B C D

Volume wear (mm³/m) 1.53E-03 8.95E-04 1.37E-03 1.06E-03

Standard deviation 1.05E-04 6.84E-05 7.59E-05 1.04E-04
Lower volume wear (higher wear resistance) was recorded for each of the inventive samples. In particular, material D was found to exhibit lower volume wear than material C, despite having the same nominal binder content, identical mean WC grain size, and a lower room temperature hardness.

Claims

A rock drill insert (2) comprising a body of cemented carbide comprising hard constituents of WC in a binder phase comprising cobalt;
wherein the cemented carbide comprises 4-18 wt % Co;

Cr such that the Cr/Co mass ratio in the bulk of the body is 0.04 -0.19;

a balance of WC and any unavoidable impurities;

wherein content of the binder phase of the cemented carbide is substantially equal throughout the rock drill insert;
characterized in that:
said insert has a corrected CoM / wt% Co ratio between 0.72 - 0.81 and the insert is substantially free of eta-phase; wherein said corrected CoM / wt% Co ratio is calculated according to: $Corrected CoM / wt % Co = (magnetic - % Co + 1.13 * wt % Cr) / wt % Co$
where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co and wt-% Cr are the weight percentage of Co and Cr in the cemented carbide, respectively.
The rock drill insert (2) according to claim 1 wherein the cobalt content is between 8 - 18 wt%.
The rock drill insert (2) according to claim 1 wherein the cobalt content is between 4 - 8 wt%.
The rock drill insert (2) according to any of the previous claims wherein the corrected CoM / wt% Co is between 0.73 - 0.79.
The rock drill insert (2) according to any of the previous claims wherein the difference between an average hardness at 0.3 mm below the surface of the rock drill insert and an average hardness in the bulk of the rock drill insert is at least 30 HV3 wherein hardness is measured according to ISO EN6507.
The rock drill insert (2) according to any of the previous claims, wherein the difference between the hardness at any point 0.3 mm below the surface of the rock drill insert and the hardness at 1 mm below the surface of the rock drill insert is at least 20 HV3 wherein hardness is measured according to ISO EN6507.
The rock drill insert (2) according to any of the previous clams, wherein a WC grain size mean value of the cemented carbide is above 1 µm but less than 18 µm as measured according to Jeffries method defined in the description.
The rock drill insert (2) according to any of the previous claims, wherein a WC grain size mean value of the cemented carbide is above 1.5 µm but less than 10 µm.
The rock drill insert (2) according to any of the previous claims, wherein the mass ratio Cr/Co in the cemented carbide is between 0.075-0.15.
The rock drill insert (2) according to any of claims 1-8 wherein the mass ratio Cr/Co in the cemented carbide is between 0.05 - 0.12.
The rock drill insert (2) according to any of the previous claims, wherein said cemented carbide has a bulk hardness of not higher than 1700 HV3.
A rock drill bit body comprising one or more mounted rock drill inserts (2) according to any of the previous claims.