Cutting tool

Abstract

The invention relates to a cutting tool comprising a cemented carbide substrate. The substrate comprises, WC grains, a Co binder in an amount of between 3 and 17 wt%, fine dispersed eta phase grains in an amount between 1 and 10 vol%, and wherein the cemented carbide further comprises Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.02 and 0.17. The cutting tool has an optimized hardness / toughness combination.

Description

[0001] Cutting Tool

[0002] The present invention relates to a cutting tool where the cemented carbide substrate comprises eta phase, a Co metal binder and Ru.

[0003] Background

[0004] Cemented carbides based on tungsten carbide (WC) with a metal binder have been known in the art for more than hundred years. It has always been a strive to obtain cemented carbide tools with an optimized hardness / toughness ratio.

[0005] These properties can be optimized in different ways, but there is still a desire to further improve the properties. Especially for so called “difficult to cut” materials such as Ti alloys and Inox alloys in interrupted machining, there is still work to be done.

[0006] The impact of the carbon content on the cemented carbide structure is well known. A shortage of carbon leads to the formation of eta phase, e.g. WeCoeC, W3C03C, whereas an excess of carbon leads to precipitation of free graphite. The carbon content is usually balanced so that neither eta phase nor graphite is formed. Both eta phase and graphite are usually considered to be something to avoid. Cemented carbides that contain eta phase are known to be brittle and for that reason, eta phase is usually not desired. A low carbon content can however be beneficial since it increases the solubility of W in the binder.

[0007] Ru has been used as an additive in cemented carbides for several reasons. For example, it has shown enhanced properties in certain cutting applications. Also, Ru has been used to avoid eta phase formation in cemented carbide when a lower carbon content, and a higher solubility of W in the binder, is desired. The cost of Ru is however high, and a reduction the amount of Ru in the cemented carbide is therefore desired.

[0008] It has now been found that a combination of a fine dispersed eta phase and a certain Ru addition leads to enhanced properties, e.g. an optimized toughnesshardness combination.

[0009] It is an object of the present invention to further increase the performance of a cutting tool. Detailed description of the invention

[0010] The present invention relates to a cutting tool comprising a cemented carbide substrate comprising a cemented carbide substrate. The cemented carbide comprises:

[0011] - WC grains,

[0012] - a Co binder in an amount of between 3 and 17 wt% of the total cemented carbide,

[0013] - fine dispersed eta phase in an amount between 1 and 10 vol%, and wherein the cemented carbide further comprises Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.02 and 0.17.

[0014] By cemented carbide is herein meant that the cemented carbide comprises at least 50 wt% WC.

[0015] The cemented carbide according to the present invention comprises eta phase. By eta phase is herein meant carbides selected from Mei2C and MeeC where Me is one or more metals selected from W, the binder phase metal or metals and can also contain other elements if present in the cemented carbide.

[0016] In the present invention the cemented carbide comprises fine dispersed eta phase. By that is herein meant that the cemented carbide microstructure does not contain more than 8 clusters or eta phase grains larger than 15 pm in an area of 1 mm2in a light optical microscope image at 200 times magnification. Eta phase grains can exist as very large, brittle and unwanted grains or clusters with sizes typically above 50 pm or even larger than 100 pm and these are not part of the present invention. These unwanted grains / clusters are called dendritic eta phase. The eta phase grains of the present invention are 0.1-10 pm in average grain size and these grains are evenly distributed within the metallic binder of the cemented carbide. Figures 1-3 shows different microstructures with eta phase, fine dispersed eta (Figure 1 ), mixture of fine dispersed and larger blocks (Figure 2) and dendritic eta (Figure 3). As can be seen, it is possible to identify the fine dispersed eta phase by simply looking at the images, exact measurements are usually not necessary. The fine dispersed eta phase grains of the present invention are formed during the sintering process and carbon deficiency and equilibrium temperature needs to be controlled in the process to reach the claimed eta phase appearance and content. The difference in total carbon content between achieving the unwanted large agglomerates of eta phase, and achieving the finely dispersed eta phase, that it is aimed for, can be very small. Being close to that limit requires monitoring the microstructure to make sure that the unwanted large agglomerates are avoided. Carefully adjusting carbon contents and then monitor its result in terms of the obtained microstructure is a known working procedure to a person skilled in the art.

[0017] According to the present invention, the eta phase distribution is the same throughout the whole cemented carbide substrate. By that is herein meant that the cemented carbide does not comprise any gradients of eta phase or zones without eta phase, like e.g. in US 4,843,039.

[0018] The average grain size of the eta phase grains (minimum ferret diameter) is suitably between 0.1 and 10 pm, preferably between 0.5 and 5 pm. This can e.g. be measured by image analysis on a LOM image, using Image J according to the method for measuring the volume fraction eta phase as described in the examples with the addition that the Feret size option “exclude on edges” was additionally activated in the “Analyze particles” function. The average grain size for eta is given as the minimum Feret grain size.

[0019] In one embodiment of the present invention, the volume fraction of the eta phase is suitably between 1 and 10 vol%, preferably between 1 and 7 vol% and more preferably between 2 and 5 vol%.

[0020] If the eta phase content is too high in the sintered cemented carbide the cemented carbide will be brittle. On the other hand, if the eta phase content is too low, the formed eta phase will be unevenly distributed like in large clusters (dendritic) leading to a decrease in toughness of the cemented carbide.

[0021] According to the present invention, the cemented carbide comprises Ru in an amount so that the weight ratio Ru / (Ru+Co) is between 0.02 and 0.17, preferably between 0.03 and 0.12, more preferably between 0.04 and 0.09. Ru is added to enhance the mechanical properties (HV - K1c combination) and also wear resistance. If the Ru content is too high, the cemented carbide will have a reduced hardness (HV) and fracture toughness (K1c) combination. Also, the binder phase becomes less homogeneous due to the appearance of a second binder phase (duplex binder), and if the Ru content is too low, no effect of the Ru addition can be seen.

[0022] The ratio between the face centered cubic phase (fee) and the hexagonal closed-packed phase (hep) of the binder is found to influence the measured hardness / toughness properties. Face centered cubic phase (fee) is a metastable phase in the binder after sintering which is beneficial since it will enhance the hardness / toughness properties (HV - K1 c combination). The amount of fee in the binder should therefore be as high as possible. The amount of fee should be above 50%, preferably above 70%, more preferably above 80%. The amount of fee is herein measured by XRD on a mechanically unaffected area with majority of WC removed by electrolytic etching and using corresponding peak heights as a reference for fee and hep phases amounts.

[0023] The amount of fee phase is estimated based on peak height from diffractograms, a more detailed description of how this has been calculated is found in the examples.

[0024] The average WC grain size in the cemented carbide is chosen based on the specific cutting application in mind. Suitably, the average WC grain size is between 0.1 and 12 pm, preferably between 0.4 and 9 pm. The average WC grain size can be measured by e.g. image analysis.

[0025] In one embodiment of the present invention the cemented carbide comprises 83-97 wt% WC, preferably 87-96 wt% WC.

[0026] According to the present invention, the metal binder is Co. The Co metal binder is present in an amount of between 3 and 17 wt% of the cemented carbide, preferably between 4 and 13 wt%.

[0027] The Co metal binder will also contain other elements that are inevitably dissolved in the binder during sintering. Examples of such elements are e.g. W from the WC but depending on other elements added, the binder can also contain Ru, Cr and also other elements.

[0028] The cemented carbide can also comprise other constituents common in the art of cemented carbides, e.g. carbides, carbonitrides or nitrides of one or more of Ti, Ta, Nb, Cr or V. In one embodiment of the present invention, the cemented carbide comprises Cr. Cr is a common addition in cemented carbides but the amount of Cr that can be added is usually limited by the solubility of Cr in the metal binder. If the amount of Cr exceeds the solubility in the binder, a brittle carbide, CryCs, is precipitated in the microstructure and the mechanical properties of the cemented carbide will deteriorate.

[0029] The upper limit for the Cr addition according to the present invention, i.e. formation of the CryCs, can however be higher than for a conventional cemented carbide without eta due to the presence of eta phase in the microstructure. Cr will be part of the eta phase and a higher amount of Cr can thus be added to the cemented carbide without formation of the brittle CryCs carbides.

[0030] Cr can be added up to the limit where CryCs is formed, however, for practical reasons it is not suitable to be too close to that limit. Suitably, Cr is added so that the weight ratio Cr / (Cr+Co) in the cemented carbide is between 0.01 and 0.1 , preferably 0.01 and 0.07, more preferably between 0.01 and 0.035.

[0031] Adding Cr to the cemented carbide in these amounts is beneficial since it contributes to microstructure refinement. A higher Cr content also leads to solid solution strengthening in the binder which will lead to a higher hardness without decrease in toughness. If the amount of Cr is below the set range, the hardness and other mechanical properties will not be enough.

[0032] By cutting tool is herein meant an insert, drill or endmill.

[0033] In one embodiment of the present invention, the cemented carbide substrate is provided with a coating, preferably deposited by PVD or CVD.

[0034] In one embodiment of the present invention, the cemented carbide comprises 1 and 10 vol% fine dispersed eta phase, 3 to 17 wt% Co, Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.02 and 0.17 and Cr in an amount so that the weight fraction of Cr / (Cr+Co) is between 0.01 and 0.1 where the balance is WC.

[0035] In one embodiment of the present invention, the cemented carbide comprises 2 and 5 vol% fine dispersed eta phase, 3 to 17 wt% Co, Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.05 and 0.09 and Cr in an amount so that the weight fraction of Cr / (Cr+Co) is between 0.012 and 0.08 where the balance is WC.

[0036] Drawings

[0037] Figure 1 shows a LOM image (polished surface etched by Murakami solution) of cemented carbide microstructure having a fine dispersed eta phase.

[0038] Figure 2 shows a LOM image (polished surface etched by Murakami solution) of a cemented carbide microstructure having a mixture of fine dispersed eta phase and larger, unwanted, blocks of eta phase.

[0039] Figure 3 shows a LOM image (polished surface etched by Murakami solution) of a cemented carbide microstructure having dendritic eta phase, i.e. uneven clusters.

[0040] Figure 4 shows a graph of the effect on the Delta value (combined effect of hardness and toughness) for different Ru additions.

[0041] Figure 5 shows a graph of the effect of Ru addition on the fee content.

[0042] Figure 6 shows a graph of the effect on the Delta value (combined effect of hardness and toughness) of different Cr additions.

[0043] Figure 7 - shows an example of a diffractogram with alternation between 111 and 200 fee reflections for a sample according to the present invention.

[0044] Figure 8 - shows an example of a diffractogram with reflections corresponding fee and hep phase for a comparative example.

[0045] Example 1

[0046] Samples were prepared to see the impact of different Ru additions. The raw material powders according to Table 1 , where the balance was WC with a grain size (FSSS) of 1 .2-1 .4 pm, were milled in a ball mill for 10 h together with an organic binder (2 wt% PEG based on total powder weight) and a milling liquid (water / ethanol) to form a slurry which was dried and milled in agate mortar to obtain a powder blend. The powder was pressed into green bodies. The green bodies were sintered in a vacuum furnace where maximum sintering temperature was 1410°C and sintering time was 1 h at 40 mbar vacuum sintering. The average cooling speed was

[0047] 4.5 °C / min from 1410 to 1100°C and 21 .5 °C / min from 1100 to 100°C.

[0048] Table 1

[0049] The toughness (K1 C) and the hardness (HV30) were measured on the sintered bodies after grinding and polishing. The HV30 has been measured according to ISO 6507:2018. The fracture toughness, K1 C, has been measured according to ISO 28079. The results are shown in Table 2b.

[0050] The volume fraction of eta phase was determined by image analysis using the software Image J using the “Analyze particles” function with “include holes” and the “0-I nfinity” filter settings. Prior to the measurements, color LOM images were converted into 8-bit black and white images using Automatic threshold setup. The images used for the analysis was LOM images with a magnification of 1000X, between 10 and 12 images were processed and the values in Table 2a are an average value of these. When studying the LOM images of the samples, all samples according to the invention disclosed fine dispersed eta phase. In Table 2a, the calculated total carbon content as well as the calculated substoichiometric carbon content is shown. The total carbon content in the cemented carbide calculated is from the carbon in the raw materials.

[0051] The substoichiometric carbon content is the stoichiometric carbon value minus the total carbon value, i.e. a measure of how large the carbon deficit is. The stoichiometric carbon content is calculated by assuming that the WC is completely stoichiometric, i.e. that the atomic ratio W:C is 1 :1. If other carbides are present, also those are assumed to be stoichiometric.

[0052] All values relating to carbon in Table 2a are calculated values. In a sintered cemented carbide, the actual total carbon and substoichiometric carbon (subst. C content), if analyzed, will differ since, during sintering, some carbon is always lost due that some part of the carbon reacts with oxygen, which is an impurity in the raw materials, which outgas as CO or CO2 during sintering, and reduces the total final C content of the alloy. As a rule of thumb, the substoichiometric carbon in the sintered cemented carbide is about 0.1 wt% lower than the calculated value.

[0053] The Coercivity (He) and the Com values are measured using a Foerster Koerzimat 1.097 MS instrument using the standard DIN IEC 60404-7. The magnetic properties of cemented carbides are determined by the ferromagnetic properties of the Co binder phase, whereas the hard phases (WC etc.) are non-ferromagnetic. The contribution of Co in the binder phase to the measured magnetic moment is always only a percentage of the (theoretical) magnetic moment of 100% pure Co. This can, for example, be due to that some metals in the cemented carbide composition, such as W and Cr, can dissolve in the Co binder phase during sintering and reduce the ferromagnetic properties of the Co binder phase compared to pure Co. The Com value (pTm3 / kg) has then been divided by the Co content in each sample to be able to make a comparison. The results are shown in Table 2b.

[0054] Table 2a Table 2b

[0055] The data from Table 2 is also be displayed in Figure 4, where the toughness and hardness values have been recalculated into “delta” (y-axis). For cemented carbides having Co as binder, it is possible to obtain a reference line from literature data where the hardness is plotted against the toughness (K1 C). “delta” is a calculated value of the distance from that reference line to the experimentally measured values of HV and KIC.

[0056] The delta value should be above zero, the higher the better. Delta is:

[0057] Where AHV and AKIC are differences between measured (HV, KIC) and calculated (HVcaic, KICcaic) values and the coefficient I gets the value -1 when AHV and AKIC are negative numbers and +1 when AHV and AKIC are positive values. AHV = HV - HVcaic, (2)

[0058] AKIC = KIC - KICcaic, (3) Where a, b and c are constants determined by fitting experimentally measured HV and KIC values to the equation 4: a = 2670000, b = 4590, c = 1240.

[0059] In Figure 4 it can clearly be seen that the addition of Ru affects the delta value, the higher delta value the better toughness / hardness combination.

[0060] The amount of fee phase was measured using XRD. To enhance the signal from the binder phase, WC was removed from sintered cemented carbides samples by electrolytic etching of a small area in saturated Na2COs water solution at room temperature with a 3V applied voltage and 25-30 min dissolution time. The etched zone changed color and a sponge-like binder structure without WC can be observed using SEM. The XRD signal is recorded on the etched zone where WC was removed when scanning from 37 to 52 20 with Cu-a reference.

[0061] For the fee phase amount evaluation, 2 main reflections corresponding to 111 or 200 indexes are used together as they found to alternate in diffractograms from spot to spot even within the same sample, probably due to texturing in the binder structure. For that reason, the sum of height of fcc_111 and fcc_200 reflections have been used as indicator of fee phase amount:

[0062] % fee = (h(fcc_111 ) + h(fcc_200) / (h(fcc_111 ) + h(fcc_200) + h(hcp_main))

[0063] Where hxyz - heights in pixels of corresponding peaks above baseline in a diffractogram image. The peak positions change slightly depending on Ru content in material: hcp_main is around 46.0 - 46.4, fcc_111 is around 43.4 - 43.8 and fcc_200 is around 50.6 - 51 .0. A plot of the fee content with varying Ru content can be seen in Figure 5. As can be seen, there is a clear drop in fee content when the Ru content increases over a certain limit.

[0064] A diffractogram of a cemented carbide according to the present invention, Invention 1 , with identified peaks are shown in Figure 7. For comparison, a diffractogram of a comparative sample, Comparative 2, is shown in Figure 8. Example 2

[0065] Another set of cemented carbide samples were prepared where the composition was the same as Invention 2 with the difference that the Cr content was varied. The samples were made using the same methods and raw materials as in Example 1 . The amounts of the raw material powders are given in Table 3.

[0066] Table 3

[0067] In Tables 4a and 4b, the sintered samples have been analyzed in the same way as in Example 1 regarding amount of eta, fee, carbon content as shown in Table 4a. The hardness, toughness (K1 C), He and Com / wt%Co was also measured in the same way as in Example 1 and the results are shown in Table 4b.

[0068] Table 4a Table 4b

[0069] The “delta value" was calculated in the same way from the hardness and toughness values as in Example 1 . In Figure 6, the delta value has been plotted against the Cr content.

Claims

Claims1 . A cutting tool comprising a cemented carbide substrate comprising:- WC grains,- a Co binder in an amount of between 3 and 17 wt% of the total cemented carbide,- fine dispersed eta phase in an amount between 1 and 10 vol%, and wherein the cemented carbide further comprises Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.02 and 0.17.

2. A cutting tool according to claim 1 wherein the cemented carbide comprises Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.03 and 0.12.

3. A cutting tool according to any of the preceding claims wherein the cemented carbide comprises Ru in an amount so that the weight fraction of Ru / (Ru+Co) is between 0.04 and 0.09.

4. A cutting tool according to any of the preceding claims wherein the cemented carbide comprises fine dispersed eta phase in an amount between 1 and 7 vol%.

5. A cutting tool according to any of the preceding claims wherein the cemented carbide comprises fine dispersed eta phase in an amount between 2 and 5 vol%.

6. A cutting tool according to any of the preceding claims wherein Cr is present in the cemented carbide in an amount so that the weight fraction of Cr / (Cr+ Co) is between 0.01 and 0.10.

7. A cutting tool according to any of the preceding claims wherein Cr is present in the cemented carbide in an amount so that the weight fraction of Cr / (Cr+Co) is between 0.01 and 0.07.

8. A cutting tool according to any of the preceding claims wherein the Co binder is present in an amount of between 4 and 13 wt%.

9. A cutting tool according to any of the preceding claims wherein the amount of fee in the binder is more than 50%.

10. A cutting tool according to any of the preceding claims wherein the amount of fee in the binder is more than 70%.11 . A cutting tool according to any of the preceding claims wherein the cutting tool is provided with a CVD or PVD coating.

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