HARD METAL CEMENTED CARBIDE

Abstract

A cemented carbide suitable as a high performance hard metal material for wire drawing of high-tensile strength alloys is provided. The cemented carbide may include a relatively low binder content with additives Cr, Ta and/or Nb to provide high wear and corrosion resistance, high thermal conductivity, high hardness and a desired hardness to fracture toughness correlation.

Claims

1. A cemented carbide, comprising: WC present in an amount of at least 93 wt %: Co present in an amount of 3 to 5 wt %; Cr present in an amount of 0.1 to 0.5 wt %; Ta and/or Nb present alone or in combination in an amount of 0.05 to 0.35 wt %; and V present in an amount of 0.05 to 0.2 wt %.

2. The cemented carbide according to claim 1, wherein a wt %-quotient of Cr/Co is in a range 0.04 to 0.1.

3. The cemented carbide according to claim 1, wherein the Ta is present in an amount of from 0.05 to 0.3 wt.

4. The cemented carbide according to claim 1, wherein the Nb is present in an amount of 0.01 to 0.07 wt %.

5. The cemented carbide according to claim 1, wherein the Ta and/or Nb are present alone or in combination in an amount of.

6. The cemented carbide according to claim 2, wherein the wt %-quotient of Cr/Co is in the range 0.06 to 0.09.

7. The cemented carbide according to claim 1, wherein the Co is present in an amount of 3 to 4.5 wt %.

8. The cemented carbide according to claim 1, wherein the WC has a grain size in the range of 0.2 to 0.8 μm.

9. The cemented carbide according to claim 8, wherein the grain size in the range of 0.2 to 0.6 μm.

10. The cemented carbide according to claim 1, wherein the WC is present in an amount of at least 94 wt %.

11. The cemented carbide according to claim 3, comprising a density in a range 14.5 to 15.5 g/cm.sup.3.

12. The cemented carbide according to claim 3, comprising a Vickers hardness HV30 of 1950 to 2150.

13. The cemented carbide according to claim 3, comprising a Palmqvist fracture toughness of 8 to 9.5 MPa √m.

14. A metal wire drawing die comprising the cemented carbide according to claim 1.

15. A method of making a cemented carbide article, comprising: preparing a batch of powdered materials including WC present in an amount of at least 93 wt %, Co present in an amount of 3 to 5 wt %, Cr present in an amount of 0.1 to 0.5 wt %, Ta and/or Nb present alone or in combination in an amount of 0.05 to 0.35 wt %, and V present in an amount of 0.05 to 0.2 wt %; pressing the batch of powdered materials to form a preform; and sintering the pre-form to form the article.

16. The method according to claim 15, wherein within the batch of powdered materials a wt %-quotient of Cr/Co is in a range 0.04 to 0.1.

17. The method according to claim 15, wherein the sintering comprises vacuum or HIP processing.

18. The method according to claim 15, wherein the sintering comprises processing at a temperature in a range of 1360 to 1520° C. at a pressure in a range of 0 to 20 MPa.

19. The method according to claim 15, wherein the article is a metal wire drawing die.

20. The method according to claim 15, wherein the the Cr is provided in the form of Cr.sub.3C.sub.2 in an amount of 0.1 to 0.5 wt % and the V is provided in the form of VC powder in an amount of 0.05 to 25 wt %.

21. The method according to claim 20, wherein the VC is present in an amount of 0.1 to 0.2 wt %.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] Specific implementations of the present disclosure will now be described with reference to the various examples and accompanying drawings in which:

[0045] FIG. 1 is a graph of a hardness to toughness relationship for cemented carbide materials according to aspects of the present invention where the dotted line corresponds to a linear correlation;

[0046] FIG. 2 are micrographs of a hard metal grade A at: (a) 2000× magnifications and (b) 5000× magnifications;

[0047] FIG. 3 are micrographs of a hard metal grade B at: (a) 2000× magnifications and (b) 5000× magnifications;

[0048] FIG. 4 are micrographs of a hard metal grade C at: (a) 2000× magnifications and (b) 5000× magnifications;

[0049] FIG. 5 are micrographs of a hard metal grade D at: (a) 2000× magnifications and (b) 5000× magnifications;

[0050] FIG. 6 are micrographs of a hard metal grade E at: (a) 2000× magnifications and (b) 5000× magnifications;

[0051] FIG. 7 are micrographs of a hard metal grade F at: (a) 2000× magnifications and (b) 5000× magnifications;

[0052] FIG. 8 are SEM images of worn surfaces of various sample grades according to aspects of the present invention after sliding wear testing;

[0053] FIG. 9 is a graph of wear track width of various sample grades after testing as measured by SEM analysis;

[0054] FIG. 10 is a graph of thermal conductivity of sample grade A and a reference sample grade F.

DETAILED DESCRIPTION

[0055] A high performance hard metal cemented carbide material has been developed preferentially for metal wire drawing of high-tensile strength alloys. The present material is particularly adapted with high wear and corrosion resistance, high thermal conductivity, high hardness and in particular an enhanced hardness to fracture toughness correlation. Such characteristics are achieved by the selective control of grain size, binder content and composition. In particular, the present cemented carbide comprises an ultra-fine grain size, relatively low binder content and a corresponding enhanced binder-WC bonding strength.

EXAMPLES

[0056] Conventional powder metallurgical methods including milling, pressing, shaping and sintering were used to manufacture various sample grades of a cemented carbide according to the present invention. In particular, cemented carbide grades with wt % compositions according to Table 1 and 2 (elemental) were produced using known methods. Grades A to G were prepared from powders forming the hard constituents and powders forming the binder phase. Each of the sample mixtures Grades A to F were prepared from powders forming the hard constituents and powders forming the binder. The following preparation method corresponds to Grade A of Table 1 below having starting powdered materials: WC 93.08 g, Cr3C2 0.30 g, Co 3.92 g, NbC 0.03 g, TaC 0.16 g, VC 0.14g, W 0.01g, PEG 2.25 g, Ethanol 50 ml. It will be appreciated by those skilled in the art that it is the relative amounts of the powdered materials that allow the skilled person and suitable adjustment is needed to make the powdered batch and achieve the final fully sintered composition of the cemented carbides of Table 1. Accordingly, Table 1 lists the starting materials, with the exception of cobalt, in their carbide form. As will be appreciated, the respective carbide starting materials are used for convenience and cost from standard suppliers. In particular, TaC and NbC may be added as a mixed carbide starting material with their respective wt amounts indicated in Table 1.

[0057] Each of the sample mixtures were subjected to 8 h of ball milling using ethanol as liquid media and afterwards dried in a furnace (65° C.) and sieved. The powders were uniaxially pressed at 4 Tm. Green compacts were then deppeged at 450° C. and sintered in a SinterHIP at 1450 ° C. (70 min) in argon atmosphere (50 bar). PEG was introduced in all compositions.

TABLE-US-00001 TABLE 1 Example powdered starting material compositions A to D according to aspects of the present invention and comparative grades E and F. Composition, wt % Grade WC NbC Co Cr.sub.3C.sub.2 TaC VC A 95.35 0.05 4.00 0.30 0.15 0.15 B (comparative) 94.24 0.03 5.00 0.50 0.23 — C (comparative) 96.45 0.03 3.00 0.30 0.23 — D (comparative) 95.34 0.03 4.00 0.40 0.23 — E (comparative) 96.55 — 3.30 — — 0.15 F (comparative) 92.90 — 6.20 0.30 — 0.60

TABLE-US-00002 TABLE 2 details the elemental compositions and ratios of the grades A to F. Composition, wt % (Ta + Nb)/ (Ta + Nb)/ Ta + Grade Ta Cr V Nb W Cr/Co Cr Co Nb A 0.140 0.259 0.121 0.044 89.502 0.06499 0.711 0.046 0.184 B (comparative) 0.216 0.433 — 0.027 88.461  0.086657 0.559 0.048 0.243 C (comparative) 0.216 0.260 — 0.027 90.526 0.08666 0.932 0.081 0.242 D (comparative) 0.215 0.346 — 0.027 89.493 0.08665 0.698 0.060 0.242 E(comparative) — — 0.121 — 90.629 — — — F (comparative) — 0.259 0.485 — 87.203 0.04193 — —

Characterisation

[0058] The various starting material powdered batches of Table 1 were processed to produce the final fully sintered materials. Characterisation of the sintered grades A to F was then undertaken including microstructural analysis using scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS); hardness and toughness, sliding friction and wear testing and thermal conductivity.

Microstructure

[0059] Sintered samples were mounted in bakelite resin and polished down to 1 μm prior to further characterization. Microstructural analysis was carried out using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The polished samples were etched with Murakami etchant to reveal the microstructure and, according to the ATM 4499-1:2010, the linear intercept technique was used for measuring the WC grain size.

[0060] The linear intercept method (ISO 4499-2:2008) is a method of measurement of WC grain size. Grain-size measurements are obtained from SEM images of the microstructure. For a nominally two-phase material such as a cemented carbide (hard phase and binder phase), the linear-intercept technique gives information of the grain-size distribution. A line is drawn across a calibrated image of the microstructure of the cemented carbide. Where this line intercepts a grain of WC, the length of the line (l.sub.i) is measured using a calibrated rule (where i=1, 2, 3, . . . n for the first 1.sup.st, 2.sup.nd, 3.sup.rd, . . . , nth grain). At least 100 grains where counted for the measurements. The average WC grain size will be defined as:

[00001]$d_{\mathrm{WC}}=.Math.l_i/n$

Hardness and Toughness

[0061] Vickers indentation test was performed using 30 kgf (HV30) to assess hardness. Palmqvist fracture toughness was calculated according to:

[00002]$K1c=A\sqrt{\mathrm{HV}}\sqrt{\frac{P}{.Math.L}}$

where A is a constant of 0.0028, H is the hardness (N/mm.sup.2), P is the applied load (N) and ΣL is the sum of crack lengths (mm) of the imprints.

Sliding Friction and Wear Test

[0062] The methodology used to assess wear behavior was: [0063] Sintered samples were mounted in bakelite resin and polished down to 1 μm. [0064] Samples were afterwards dismounted from the bakelite and placed in a circular geometry holder designed for Wazau wear tester. [0065] The Wazau wear tester in the linear reciprocating module was used according to ASTM G133. Al2O3 balls of Ø10 mm were used for characterizing abrasive wear. Conditions used were: load=150N, speed=250 rpm, stroke length=10 mm, sample frequency=100 Hz (for 1 h test). Samples were immersed in lubricant while testing to simulate the real process. [0066] During each wear experiment the imposed normal contact force (FN) and the concomitant tangential friction force (FT) of pin-on-flat sliding pairs were continuously registered. The coefficient of friction (0 is calculated from the FT/FN forces ratio. [0067] After the test, the wear damage pattern was evaluated by SEM analysis and the thickness of the wear track measured.

Thermal Conductivity

[0068] The specific heat and thermal diffusivity were evaluated at five different temperatures (30, 100, 200, 300, 400 and 500° C.) by CIC Energigune technological centre. The thermal conductivity was calculated from the density and thermal diffusivity measurements according to the formula:

[00003]$\lambda \left(T\right)=\rho \left(T\right)*\mathrm{Cp}\left(T\right)*a\left(T\right)$

With:

[0069] λ—Thermal Conductivity [0070] ρ—Density (determined by picnometry) [0071] Cp—Specific Heat [0072] α—Thermal Diffusivity [0073] T—Temperature

[0074] In order to determine the specific heat (Cp), a DSC calorimeter (Differencial Scanning calorimetry) DSC Discovery 2500 equipment was used. The thermal diffusivity was measured using the NETZSCH laser flash apparatus LFA 457 MicroFlash®. The LFA 457 calculates thermal diffusivity using the “Parker Equation”

[00004]$\alpha =0.1388*\frac{L^2}{t{0.5}^2}$

With:

[0075] L=sample thickness (mm) [0076] t0.5=time at the 50% of temperature increase (s)

Results

[0077] Referring to tables 1 and 2, the present hard metal grades combine Co content between 3 wt % and 5 wt %, and optimum additions of VC, Cr.sub.3C.sub.2, NbC and TaC as grain growth inhibitors. FIG. 1 shows the HV30 to Palmqvist toughness relations for the developed grades A to D as compared to the reference grades E and F. As it can be seen, the proposed materials exhibit better hardness to toughness levels than reference grades E and F. This is probably related to the replacement of VC as GGI by higher quantities of other elements (with further benefits) such as Cr, Ta and Nb. The values of HV30 and toughness are shown in table 3.

TABLE-US-00003 TABLE 3 Composition, wt % HV30 KIc (MPa × m.sup.0.5) A 2074 8.6 B comparative 1975 9.3 C comparative 2073 8.4 D comparative 2008 8.8 E comparative 1923 8.6 F comparative 2042 8.2 Hardness and toughness values for present grade A and comparatives B to F

[0078] The microstructures of the reference and developed hard metal grades are shown at 2000× and 5000× from FIG. 2 to FIG. 7. FIG. 2 are micrographs of hard metal grade A at: (a) 2000× magnifications and (b) 5000× magnifications. FIG. 3 are micrographs of hard metal comparative grade B at: (a) 2000× magnifications and (b) 5000× magnifications. FIG. 4 are micrographs of hard metal comparative grade C at: (a) 2000× magnifications and (b) 5000× magnifications. FIG. 5 are micrographs of hard metal comparative grade D at: (a) 2000× magnifications and (b) 5000× magnifications. FIG. 6 are micrographs of hard metal comparative grade E at: (a) 2000× magnifications and (b) 5000× magnifications. FIG. 7 are micrographs of hard metal comparative grade F at: (a) 2000× magnifications and (b) 5000× magnifications.

Wear Response

[0079] The wear damage in terms of abrasion was evaluated by using Al.sub.2O.sub.3 balls. As it can be seen in FIG. 8, the wear tracks revealed that all samples underwent the same wear mechanism based on grain pull out due to abrasive effect of the hard counterpart. Despite these similarities in the mechanism, reference sample E suffered more wear than the rest due to its lower hardness. In addition, sample E does not contain any Ta, Nb and Cr, but only VC as a grain refiner, which was found to embrittle the material. These observations are in full agreement with wear track width measurements shown in FIG. 9.

Thermal Conductivity

[0080] The thermal conductivity of standard WC/Co hard metals is about twice as high as that of high-speed steel. Both, thermal conductivity and thermal expansion can be tailored by changing the volume fraction of binder phase and the grain size of hard carbide phase. High thermal conductivity is a key property in wire drawing applications to dissipate heat along the tool and avoid premature failure due to properties degradation at high temperatures and thermal damage. FIG. 10 compares thermal conductivity of sample A to the reference sample F from room temperature up to 500° C. As it can be seen from the FIG. 10, since this property is very sensitive to grain size, F presents lower values of thermal conductivity. The presence of VC (a powerful grain refiner) in a larger amount as compared to grade A, renders this material less thermally conductive due to its finer grain size. In addition to this, the Co content in grade F is larger than in grade A, a fact that further contributes to its lower thermal conductivity.

[0081] Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently described subject matter pertains.

[0082] Unless otherwise indicated, any reference to “wt %” refers to the mass fraction of the component relative to the total mass of the cemented carbide.

[0083] Where a range of values is provided, for example, concentration ranges, percentage range or ratio ranges, it is understood that each intervening value, to the tenth of the unit of the lower limit, unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the described subject matter. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and such embodiments are also encompassed within the described subject matter, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the described subject matter.

[0084] It should be understood that the terms “a” and “an” as used above and elsewhere herein refer to “one or more” of the enumerated components. It will be clear to one of ordinary skill in the art that the use of the singular includes the plural unless specifically stated otherwise. Therefore, the terms “a”, “an” and “at least one” are used interchangeably in this application.

[0085] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as size, weight, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present subject matter. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0086] Throughout the application, descriptions of various embodiments use “comprising” language; however, it will be understood by one of skill in the art that, in some instances, an embodiment can alternatively be described using the language “consisting essentially of” or “consisting of”.

[0087] The present subject matter being thus described, it will be apparent that the same may be modified or varied in many ways. Such modifications and variations are not to be regarded as a departure from the spirit and scope of the present subject matter, and all such modifications and variations are intended to be included within the scope of the following claims.