LIGHTWEIGHT CEMENTED CARBIDE

Abstract

Provided is a cemented carbide suitable for use as a material in the manufacture of a punch for metal forming and in particular for the manufacture of metal beverage cans. The cemented carbide may include a hard phase that includes WC, a binder phase and a gamma phase. The gamma phase may include metal carbides in combination with metal nitrides or metal carbonitrides. A quotient of the average grain size of WC/the average grain size of the gamma phase may be in a range of from 0.5 to 1.5.

Claims

1. A cemented carbide comprising: a hard phase comprising WC, the WC being present in an amount of from 50 to 70 wt % based on the total weight of the cemented carbide; a binder phase; and a gamma phase comprising at least one metal carbide in combination with at least one metal nitride and/or metal carbonitride, wherein a quotient of the average grain size of WC/the average grain size of the gamma phase is in a range 0.5 to 1.5.

2. The cemented carbide according to claim 1, wherein the metal carbides, metal nitrides and/or metal carbonitrides comprise one or more element selected from the group consisting of Ti, Ta, V, Nb, Zr, Hf, W, Mo and Cr.

3. The cemented carbide according to claim 1, wherein an average grain size of the WC is in a range of from 0.5 to 2 μm.

4. The cemented carbide according to claim 1, wherein an average grain size of the gamma phase is in a range of from 0.5 to 2 μm.

5. The cemented carbide according to claim 1, further comprising Mo.

6. The cemented carbide according to claim 1, further comprising Cr.

7. The cemented carbide according to claim 1, wherein the WC is present in an amount of 50-65 wt % based on the total weight of the cemented carbide.

8. The cemented carbide according to claim 1, wherein the binder phase comprises Co and Ni.

9. The cemented carbide according to claim 8, wherein Co+Ni is 10-20 wt % based on the total weight of the cemented carbide.

10. The cemented carbide according to claim 8, wherein the binder phase further comprises one or more of Fe, Cr, and Mo.

11. A tool for metal forming or metal cutting comprising a cemented carbide according to claim 1.

12. A punch for metal forming comprising a cemented carbide according to claim 1.

13. A method of making a cemented carbide comprising a hard phase including WC, a binder phase and a gamma phase, the method comprising: preparing a batch of powdered materials comprising WC in the range 50 to 70 wt %, binder phase constituents and gamma phase constituents that include at least one metal carbide in combination with at least one metal nitride and/or metal carbonitride; milling the powdered materials; pressing the milled powdered materials to form a pre-compact; and sintering the pre-compact; wherein within the sintered pre-compact, a quotient of the average grain size of WC/the average grain size of the gamma phase is in a range 0.5 to 1.5.

14. The method according to claim 13, wherein the WC is included within the powdered materials in an amount of 50-65 wt %.

15. The method according to claim 13, wherein the metal carbides, metal nitrides and/or metal carbonitrides comprise one or more element selected from the group consisting of Ti, Ta, V, Nb, Zr an Hf.

16. The method according to claim 13, wherein the gamma phase constituents within the powdered materials comprise TiC, NbC, TaC, TiN and/or TiCN.

17. The method according to claim 16, wherein the powdered batch further comprises Cr, Mo, Cr.sub.3C.sub.2 and/or Mo.sub.2C.

18. The method according to claim 17, wherein the powdered batch further comprises Co and Ni.

19. The method according to claim 13, wherein the powdered batch comprises in wt %: 55-59 WC; 10-14 TiC; 8-12 NbC; 5-13 Co; 0.1-1.0 Cr.sub.3C.sub.2; 1-9 Ni; 0.1-1.0 Mo.sub.2C; 0.5-2.5 TaC; 1-7 TiCN and/or 1-5 TiN.

20. The method according to claim 13, wherein the gamma phase constituents comprises pre-alloyed metal carbides and metal nitrides and/or metal carbonitrides.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0030] A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0031] FIG. 1 is a graph of average grain size (μm) of the gamma phase and WC phase of samples A to G according to specific aspects of the present invention;

[0032] FIG. 2 are micrographs at 2000× magnification of: (a) sample C (without TiN and/or TiCN in its composition) and (b) sample D (TiN and/or TiCN included);

[0033] FIG. 3 is micrographs at 5000× magnification of: (a) sample C (without TiN and/or TiCN in its composition) and (b) sample D (TiN and/or TiCN included);

[0034] FIG. 4 is micrographs at 2000× magnification of: sample A (without pre-alloyed gamma-phase) and sample B (with pre-alloyed gamma-phase);

[0035] FIG. 5 is micrographs at 5000× magnification of: sample A (without pre-alloyed gamma-phase) and sample B (with pre-alloyed gamma-phase);

[0036] FIG. 6 is micrographs at 2000× magnification of: (a) sample E (without pre-alloyed gamma-phase) and sample F (with pre-alloyed gamma-phase);

[0037] FIG. 7 is micrographs at 5000× magnification of: (a) sample E (without pre-alloyed gamma-phase) and sample H (with pre-alloyed gamma-phase);

[0038] FIG. 8 is magnified images of crosshatching simulation in: (a) sample E and (b) sample I.

[0039] FIG. 9 is magnified images of the worn surfaces after sliding wear test of: (a) sample E and (b) sample I;

[0040] FIG. 10 is a micrograph at 5000× magnification of a worn surface of sample F after sliding wear test;

[0041] FIG. 11 is SEM images of adhesive wear response of: (a) sample E and (b) sample I.

DETAILED DESCRIPTION

[0042] The inventors have identified a cemented carbide material having improved toughness for alike hardness levels of existing materials for example as described in EP 2439294 A1 with a corresponding low density so as to provide a lightweight component. When utilised as a punch for metal forming and in particular as a punch for the manufacture of beverage cans, the present material exhibits lower wear rates during linear reciprocation against Al.sub.2O.sub.3, lower adhesion of aluminium during linear reciprocating wear tests, improved surface characteristics to enable surface roughening in addition to moderate to high corrosion resistance.

[0043] The desired physical and mechanical characteristics are achieved, at least in part, by controlling the average grain size of the gamma phase with regard to the hard phase WC in combination with selecting appropriate constituents of the gamma phase being formed from metal carbides, metal nitrides and/or metal carbonitrides. In particular, the present material grade achieves selective refinement of the gamma phase only. Such refinement is achieved by the combination of cubic metal carbides with cubic metal nitrides and/or cubic metal carbonitrides. Additionally, the present composition may utilise pre-alloyed gamma phase materials within the initial powdered batch.

Examples

[0044] 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 the compositions in wt % according to Table 1 were produced according to known methods. Grades A to I were prepared from powders forming the hard constituents, powders forming the binder and powders forming the gamma phase. Each of the sample mixtures Grades A to I were prepared from powders forming the hard constituents and powders forming the binder. The following preparation method corresponds to Grade G of Table 1 below having starting powdered materials: WC 44.36 g, Cr.sub.3C.sub.2 0.37 g, Co 5.98 g, Ni 2.99 g, NbC 11.91 g, Mo.sub.2C 0.37 g, TiC 5.59 g, TaC 1.12 g, TiN 0.19 g, 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.

[0045] Each of the sample mixtures were subjected to 8h 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 1410° C. (70 min) in argon atmosphere (50 bar). PEG was introduced in all compositions. Some other sintering trials at higher temperatures were carried out, but no significant differences in terms of final grain coarsening were observed.

TABLE-US-00001 TABLE 1 Example grade material compositions A to I according to the present invention Composition % Pre alloyed γ- Grade WC TiC NbC Co Cr.sub.3C.sub.2 Ni Mo Mo.sub.2C TaC TiN phase A 62.30 12.00 12.00 7.00 1.00 2.50 0.20 3.00 No B 62.30 12.00 12.00 7.00 1.00 2.50 0.20 3.00 Yes C 59.30 15.00 15.00 6.50 0.50 3.50 0.20 No D 59.30 10.00 15.00 6.50 0.50 3.50 0.20 5.00 No E 57.00 12.00 10.50 9.00 0.50 5.00 0.50 1.50 4.00 Yes F 56.50 10.50 10.50 9.00 0.50 5.00 0.50 2.00 5.50 No G 59.50 7.50 16.00 8.00 0.50 4.00 0.50 1.50 2.50 No H 58.00 12.00 10.50 8.00 0.50 5.00 0.50 1.50 4.00 No I 63.20 20.80 8.20 2.00 5.60 0.20 No

[0046] The average grain size of the WC powders and gamma phase constituent powders was varied for grades A to I as detailed in FIG. 1. Medium coarse grain WC powder was used to assist reduction of differences in the grain size with the gamma phase.

Characterisation

[0047] Characterisation of the sample grades was undertaken including magnetic properties; microstructure, density, hardness and toughness and sliding wear performance.

Magnetic Properties

[0048] Coercivity force, Hc, and magnetic saturation of Co, Com, were measured in all sintered samples to study if eta-phase or graphite were present in the microstructure.

Microstructure, Density, Hardness and Toughness

[0049] The density of the sintered alloys was measured by Archimedes method as well as theoretically calculated.

[0050] Sintered samples were then 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). Vickers indentation test was performed using 30 kgf (HV30) to assess hardness. Palmqvist fracture toughness was calculated according to

[00001]$K1c=A\sqrt{\mathrm{HV}}\sqrt{\frac{P}{\Sigma L}}$

where A is a constant of 0.0028, H is the hardness (N/mm2), P is the applied load (N) and ΣL is the sum of crack lengths (mm) of the imprints.

[0051] 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:

d.sub.wc=Σl.sub.i/n

Sliding Wear Test

[0052] Can tooling is one of the main applications in which the use of lightweight grades would be an improvement in the metal forming process when used for the carbide punches. Replicating can tooling conditions implies testing wear damage in samples which have been previously texturized in similar way to the ones used in the field (crosshatching). This operation leaves a rough surface finish that facilitates the mechanical bonding of aluminum. The methodology used to assess wear behaviour is described below: [0053] Sintered samples, were grinded to achieve Ra˜0.15-0.25 μm. [0054] The samples were then mounted in a bakelite and to simulate the crosshatching procedure, were texturized using a diamond pad RED 3M in a polishing machine (30N, 50 rpm and contra-rotation in lubricated conditions). The intention was to achieve Ra˜0.25-0.35 μm in the end product. [0055] The samples were afterwards dismounted from the bakelite and placed in a circular geometry holder designed for Wazau wear tester. [0056] The Wazau wear tester in a linear reciprocating module was used according to ASTM G133 “Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear”. Al.sub.2O.sub.3 balls of Ø10 mm were used for characterizing abrasive wear. Galling or adhesive wear damage was tested utilizing Al balls of the same size. 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. [0057] During each wear experiment the imposed normal contact force (F.sub.N) and the concomitant tangential friction force (F.sub.T) of pin-on-flat sliding pairs were continuously registered. The coefficient of friction (μ) is calculated from the F.sub.T/F.sub.N forces ratio.

[0058] After the test, the wear damage pattern was evaluated by SEM and confocal interferometry. Wear resistance was evaluated from measuring the depth of the wear tracks from the 2D profiles.

Results

Material Characterization

[0059] With a view to achieving a hard metal grade with low density (i.e. bellow 10.30 g/cm.sup.3) but with optimal mechanical properties and wear resistance, partial or total replacement of WC by lighter carbides, such as TiC and/or NbC was considered since WC is a carbide with a high density (over 15 g/cm.sup.3). If those carbides are added in quantities over the solubility limit of the binder, they precipitate and form an additional phase i.e., the cubic carbides or gamma phase. Usually, cubic carbides contain all or some of the elements: Ti, Ta, Nb, W, Hf and Zr, and have a core-rim structure.

[0060] Despite the beneficial effect in decreasing the density, the addition of the above carbides in large quantities can decrease some of the desired mechanical properties in particular wear resistance. The properties that are more adversely affected by the introduction of cubic carbides are toughness, strength and thermal conductivity. Also, for similar hardness values higher wear rates can be found for those materials when tested in sliding friction conditions, partially related to a lower interfacial strength between the cubic carbides and the binder. On the other hand, some properties might be improved through the addition of cubic carbides, such as hot hardness and resistance to plastic deformation. Also, the gamma phase might contribute to reduce friction forces and act as an anti-galling agent.

[0061] One of the main wear mechanisms for sintered pieces containing high cubic carbide contents that are subjected to wear tests is the pull-out of individual or clusters of carbide grains. This preferential pull-out is mainly related to a poor interfacial strength between the carbide and the binder, and it accelerates wear rates due to two main reasons. Firstly, wear rates increase because full carbide grains are easily de-attached from the surface. Secondly, the detached grains tend to sit between the hard metal piece and the workpiece material. Since they have high hardness levels, they act as abrasive media, promoting abrasive wear mechanisms. In order to decrease grain pull-out and minimize their effects, it was one aim to develop grades with a refined gamma phase grain size and an improved interfacial strength.

Refinement of the Gamma Phase

[0062] In order to decrease the grain size of the gamma phase two strategies were applied: (1) the addition of TiN or TiCN, and (2) the use of pre-alloyed gamma phase powder.

[0063] On the one hand, TiC is a low-density carbide (i.e. density around 4.9 g/cm.sup.3) and therefore, its addition to the composition contributes to a decrease the overall density of the material. Accordingly, the developed grades may have relatively high TiC content, i.e., between 7.5% wt to 15% wt i.e., corresponding to a volume content between 15% to 30%, as can be seen in Table 1. On the other hand, it is known that TiN and TiCN are used to refine grain size and improve the strength in TiC-based cermets. Consequently, since TiC may be one of the main gamma phase elements, it was of interest to evaluate the effect of TiN and/or TiCN in reducing the grain size of the gamma phase. In doing so, the microstructure of materials with similar composition both with and without the addition of TiN was evaluated.

[0064] FIG. 2 are micrographs at 2000× magnification of: (a) material C (without TiN and/or TiCN in its composition) and (b) material D (TiN and/or TiCN included). FIG. 3 are micrographs at 5000× magnification of: (a) material C (without TiN and/or TiCN in its composition) and (b) material D (TiN and/or TiCN included). As will be noted from the microstructures of FIG. 2 and FIG. 3, the use of TiCN significantly reduces the mean grain size of the gamma-phase in the sintered material. Importantly, the mean WC grainsize, in light grey, was also reduced but to a lower degree.

Pre-Alloyed Gamma Phase

[0065] The influence of a pre-alloyed gamma phase (i.e. (W Ti Ta)C) as a gamma phase grain growth inhibitor was evaluated. It was observed that the use of pre-alloyed gamma phase significantly reduces the mean grain size of the gamma phase in the sintered material. However, it also reduces the mean WC grain size. A clear example is shown when comparing the microstructures of samples A and B. In particular, FIG. 4 is micrographs at 2000× magnification of: sample A (without pre-alloyed gamma-phase) and sample B (with pre-alloyed gamma-phase) and FIG. 5 is micrographs at 5000× magnification of: sample A (without pre-alloyed gamma-phase) and sample B (with pre-alloyed gamma-phase). The use of pre-alloyed gamma phase significantly reduces the mean grain size of the gamma-phase in the sintered material. It will be noted the mean WC grain size, in light grey, is also reduced as seen at 2000× (FIG. 4) and 5000× (FIG. 5).

Combination of TiN/TiCN and Pre-Alloyed Gamma Phase

[0066] Both strategies can be combined if the grain size of the gamma phase is to be further reduced. An example is shown in FIG. 6 and FIG. 7, at 2000× and 5000×, for materials E and H where FIG. 6 is micrographs at 2000× magnification of: (a) sample E (without pre-alloyed gamma-phase) and sample F (with pre-alloyed gamma-phase) and FIG. 7 is micrographs at 5000× magnification of: (a) sample E (without pre-alloyed gamma-phase) and sample F (with pre-alloyed gamma-phase). Both sample materials E and F have similar compositions, but material E combines TiN and pre-alloyed gamma phase, whereas material F has the same amount of TiN as material E, but does not contain pre-alloyed (W,Ti,Ta)C gamma phase The use of pre-alloyed gamma phase, in addition to TiN, reduces slightly more the gamma-phase mean grain size as compared with the material with only TiN. It was noted that at this stage the additional grain refinement obtained was limited.

Interfacial Strength

[0067] As mentioned, one objective of the present invention is to increase the interfacial strength between the gamma phase and the binder to reduce grain pull-out during wear. In order to do so, the addition of several additives such as Mo.sub.2C, TaC and Cr.sub.2C.sub.3, as well as the use of pre-alloyed gamma phase, was evaluated. However, it is extremely difficult to measure interface strength and therefore, indirect techniques were used. In doing so, interfacial strength was evaluated by studying the response of the materials to crosshatching and wear.

Hardness, Palmqvist Toughness and Density

[0068] The hardness, Palmqvist toughness and density of the studied materials are shown in Table 2. Please note that all material samples A to I have similar densities of between 9.99 and 10.72 g/cm.sup.3. In addition, no significant changes in HV to KIc relation were found except for grades A and F, with A the grade with the poorest HV to KIc relation, and F the grade with the best. It was noted that, as expected, the materials with finer gamma phase grain size (i.e grades B and D), had higher hardness levels than their respective counter grades (i.e grades A and C).

TABLE-US-00002 TABLE 2 Density, hardness and toughness of studied grades K1C Density ISO28079 Grade (g/cm3) HV30 (MPa/√m) A 10.59 1461 8.77 B 10.72 1595 8.30 C 9.99 1464 9.49 D 10.09 1483 9.05 E 10.01 1349 10.25 F 10.00 1411 10.50 G 10.60 1355 10.50 H 10.06 1359 10.40 I 9.90 1400 9.50

Material Performance

Crosshatching Resistance

[0069] Samples were texturized to simulate crosshatching process carried out by can makers.

[0070] Interfacial strength between the binder and the hard particles was evaluated by SEM inspection after crosshatching simulation, as well as the wear damage produced by the process itself in the surfaces of the samples.

[0071] FIG. 8 are magnified images of crosshatching simulation in: (a) sample E and (b) sample I. As it can be seen in FIG. 8, WC grain fragmentation and debonding are observed in both samples due to the high stresses of the diamond abrasive grains during the process. Nevertheless, sample I showed slightly more surface damage and more grains pull-out. Accordingly, it is suspected that adhesion wear mechanisms during in-service performance (Al or steel galling) would be enhanced potential leading to early tool failure.

Sliding Wear Response

[0072] The wear damage (abrasion) was evaluated using a Al.sub.2O.sub.3 ball. FIG. 9 is magnified images of the worn surfaces after sliding wear test of: (a) sample E and (b) sample I. The wear track depths for grade E and I were 2.20±0.18 μm and 2.76±0.08 μm respectively, indicating that sample I suffers larger wear damage. In particular, as can be seen in FIG. 9, the worn regions that correspond to the respective wear tracks are quite similar, showing a smooth surface with initial asperities from the crosshatching having ploughed away. Also grain pull out due to the abrasive effect of the hard counterpart was observed. Despite these similarities, sample I has larger amount of TiC which is hard but brittle, therefore being able to promote further abrasive effect if it is chipped or detached. This confirms the measurement of deeper wear tracks in sample I. The presence of refined gamma phase is also determinant in that the interfaces are better adhered, presenting better resistance to grain pull out.

[0073] FIG. 10 is a micrograph at 5000× magnification of a worn surface of sample F after sliding wear test. As can be seen, some WC gains appear to be chipped and some pitting is preferentially observed, indicating the sample is susceptible to tribocorrosion damage (abrasive+lubricant effect).

[0074] The adhesive wear was analyzed by using an Al ball. FIG. 11 is SEM images of adhesive wear response of: (a) Sample E and (b) Sample I. From FIG. 11b it can be seen that sample I exhibits a larger amount of galling (Al adhesion), both at the scratches and at the grain pull outs left by crosshatching, whereas sample E mainly shows galling within the regions of grain pull out as can be seen from FIG. 11a. As commented, sample I shows poorest performance under crosshatching, leaving further grain pull out and cracking providing more regions to which the Al may adhere. Also, the higher amount of binder in sample I allows for more welding. The local galling at all these regions would promote full grain detachment.

CONCLUSIONS

[0075] A wear resistant lightweight cemented carbide grade with density <10.0 g/cm.sup.3, hardness HV30 1300-1500 and fracture toughness 10-11 MPa m was successfully developed. 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.

[0076] Unless otherwise indicated, any reference to “wt %” refers to the mass fraction of the component relative to the total mass of the cemented carbide.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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”.

[0081] 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.