Cutting tool

Abstract

The invention relates to a cutting tool comprising a cemented carbide substrate wherein the substrate comprises WC, eta phase in an amount of 1 to 10 vol% and between 3 and 20 wt% of a Co-based metal binder. The metal binder further comprises Al, Ni and possibly Cr so that: the weight ratio Ni / (Ni+Co) in the cemented carbide is between 0.1 and 0.5, the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0 and 0.1 and the weight ratio Al / (Al+Cr+Ni+Co) in the cemented carbide is between 0.01 and 0.07. The composition gives the cemented carbide improved properties also when coated at higher temperatures.

Description

[0001] Cutting tool

[0002] The present invention relates to a cutting tool where the cemented carbide substrate comprises eta phase, a Co-based metal binder where the composition of the cemented carbide has been optimized for coatings deposited at higher temperatures like e.g. CVD coatings.

[0003] Background

[0004] Cemented carbide substrates for cutting tools based on tungsten carbide (WC) with a metal binder have been known in the art for more than hundred years. It is an ongoing strive to continuously improve the cutting performance.

[0005] 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 have been 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. However, in recent years, it has been discovered that eta phase in a cemented carbide can be beneficial in certain cutting applications. Heat treating a cemented carbide with a Co binder and eta phase at approximately 500-600°C increased the coercivity of the cemented carbide and the performance, e.g. wear resistance, crack resistance etc., is further enhanced. However, if the cemented carbide is subjected to an even higher temperature this effect is not achieved. This has led to that the benefits from the eta phase in the cemented carbide cannot fully be achieved when the cemented carbide is provided with a coating deposited using CVD technigue which usually is deposited at temperatures above 800°C.

[0006] It is an object of the present invention to provide a cemented carbide comprising eta phase that shows improved properties also when provided with coatings deposited at temperatures above 800°C.

[0007] Detailed description

[0008] The invention relates to a cutting tool comprising a cemented carbide substrate the substrate comprises WC, eta phase in an amount of 1 to 10 vol% and between 3 and 20 wt% of a Co-based metal binder. The metal binder further comprises Al, Ni and possibly Cr so that: the weight ratio Ni / (Ni+Co) in the cemented carbide is between 0.1 and 0.5, the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0 and 0.1 and the weight ratio AI / (AI+Cr+Ni+Co) in the cemented carbide is between 0.01 and 0.07.

[0009] By cemented carbide is herein meant that the cemented carbide comprises at least 50 wt% WC.

[0010] By Co-based binder is herein meant that the metal binder contains at least 50 wt% Co. The amount of metal binder phase is herein given by the total amount of possible binder elements, if present, where the possible binder elements are selected from Ni, Co, Cr, Mn, Al and Cu. It is however well known in the art of cemented carbide manufacturing that other elements, e.g. W and C from the WC will be present in the metal binder since they are inevitably dissolved in the metal binder during sintering. The amounts of possible binder elements are either given by the amount of raw material used or measured using conventional chemical analysis of the cemented carbide.

[0011] In the present invention the metal binder is present in an amount of between 3 and 20 wt% of the sintered cemented carbide, preferably between 5 and 15 wt% of the sintered cemented carbide. The metal binder comprises at least 50 wt% Co, preferably at least 60 wt% Co.

[0012] In one embodiment of the present invention the binder metals are selected from Co, Ni, Cr and Al.

[0013] According to the invention, the cemented carbide comprises Ni in an amount so that Ni / (Ni+Co) is between 0.1 and 0.5, preferably between 0.2 and 0.4.

[0014] Adding Ni to the cemented carbide in these amounts is beneficial since it contributes to a better chemical resistance and high temperature strength of the binder. A too high Ni content will lead to a decrease in combined HV and K1 c properties and limitation in coatability by CVD coatings.

[0015] According to an embodiment of the 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, CT7C3, is precipitated in the microstructure and the mechanical properties of the cemented carbide will deteriorate.

[0016] The upper limit for the Cr addition according to the present invention, i.e. formation of the CT7C3, 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 CT7C3 carbides.

[0017] Cr can be added up to the limit where CT7C3 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+Ni) in the cemented carbide is between 0 and 0.1 , preferably 0 and 0.06, more preferably between 0.01 and 0.03.

[0018] 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. Adding Cr to the cemented carbide can be beneficial since it contributes to an increased hardness as well as improving other mechanical properties and also reduces abnormal grain growth.

[0019] According to the invention, the cemented carbide comprises Al in an amount so that AI / (AI+Cr+Ni+Co) is between 0.01 and 0.07, preferably between 0.03 and 0.05.

[0020] Adding Al to the cemented carbide in these amounts is beneficial since it contributes to a better chemical resistance and high temperature strength of the binder. A higher Al content also leads to the uncontrolled formation of AI2O3 particles inside the material during sintering which can be detrimental.

[0021] 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.

[0022] 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 mm2is analysed in 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 fine grained and are evenly distributed within the metallic binder of the cemented carbide. Typically, the presence of fine dispersed eta phase is identified simply by looking at the microstructure in e.g. a LOM or SEM image, exact measurements are usually not necessary.

[0023] 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.

[0024] 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 4,843,039A.

[0025] 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 SEM 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.

[0026] 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 1 and 5 vol%. 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.

[0027] In one embodiment of the present invention the substrate comprises WC, eta phase in an amount of 1 to 10 vol% and between 3 and 20 wt% of a Co-based metal binder wherein the binder comprises Co, Ni, Cr and Al so that the weight ratio Ni / (Ni+Co) in the cemented carbide is between 0.1 and 0.5, the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0 and 0.03; and the weight ratio AI / (AI+Cr+Ni+Co) in the cemented carbide is between 0.02 and 0.05.

[0028] In one embodiment of the present invention the substrate comprises WC, eta phase in an amount of 1 to 10 vol% and between 3 and 20 wt% of a Co-based metal binder wherein the binder comprises Co, Ni, Cr and Al so that the weight ratio Ni / (Ni+Co) in the cemented carbide is between 0.2 and 0.4, the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0.03 and 0.06; and the weight ratio AI / (AI+Cr+Ni+Co) in the cemented carbide is between 0.02 and 0.05.

[0029] By cutting tool is herein meant an insert, drill or endmill for metal cutting operations.

[0030] The cutting tool of the present invention is suitably provided with a coating. Any coating suitable for cutting tools can be deposited but preferably a coating that is deposited at a temperature above 800°C. A typical coating deposited at such a temperature would be a CVD coating, for example comprising one or more layers of TiN, Ti(C,N) and AI2O3. Typically, these coatings are deposited at a temperature between 800 and 1100°C.

[0031] Example 1

[0032] Samples were prepared from raw material powders according to Table 1 where the balance was WC with a grain size (FSSS) of 1 .3-1 .4 pm were milled in a ball mill for 8 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 the maximum sintering temperature was 1430°C under a protective atmosphere 2 m Bar Argon for 45 minutes.

[0033] Table 1 The cemented carbide samples all contained a fine dispersed eta phase throughout the microstructure.

[0034] The samples were subjected to either a CVD coating process TiCN +AI2O3 coating deposited at 850°C +1050°C (total >10h) or heat treatment at either 500°C or 1200°C for 10h. The coercivity was measured before and after coating / heat treatment using a

[0035] Foerster Koerzimat CS 1.096 from Foerster Instruments Inc. using the standard DIN IEC 60404-7 norm. The composition of the cemented carbide samples and the difference in Coercivity, AHc (kA / m), between the untreated sample compared to the coated / heat treated is displayed in Table 2.

[0036] Table 2 It can be seen from the results in Table 2, that the difference in coercivity before and after coating / heat treatment depends on the amount of Al in the cemented carbide and that the largest increase of coercivity can be seen at the highest Al content for the sample subjected to a CVD coating process. For the corresponding sample subjected to either lower temperature (500°C) or higher temperature (1200°C) no such change can be seen.

[0037] Example 2

[0038] Different cemented carbide samples were prepared according to Example 1 . The raw materials used and the amounts are displayed in Table 3.

[0039] Table 3

[0040] The cemented carbide samples all contained a fine dispersed eta phase throughout the microstructure. The volume fraction of eta phase which is assumed to be the same as the area fraction, was determined for some of the samples by image analysis of LOM images using the software Image J using the “Analyze particles” function with “include holes” and the “O-Infinity” 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 4 are an average value of these.

[0041] Table 4

[0042] The coercivity was measured before and after coating / heat treatment using the method disclosed in Example 1. The composition of the cemented carbide samples and the different in Coercivity, AHc, between the untreated sample compared to the coated / heat treated is displayed in Table 5. The weight ratio AI / (AI+Cr+Ni+Co) was approx, the same for all samples, i.e. between 0.27 and 0.29.

[0043] Table 5 It can be seen from the results in Table 5, that a significant increase in coercivity is observed for most of the CVD coated samples whereas for the samples subjected to either lower temperature (500°C) or higher temperature (1200°C) no such change can be seen. For samples subjected to lower temperatures (500°C) very small changes in coercivity is observed. For samples subjected to higher temperatures (1200°C), there is significant decrease in coercivity.

Claims

Claims1 . A cutting tool comprising a cemented carbide substrate wherein the substrate comprises WC, eta phase in an amount of 1 to 10 vol% and between 3 and 20 wt% of a Co-based metal binder wherein the binder further comprises Al, Ni and and possibly Cr so that:- the weight ratio Ni / (Ni+Co) in the cemented carbide is between 0.1 and 0.5;- the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0 and 0.1 ; and- the weight ratio AI / (AI+Cr+Ni+Co) in the cemented carbide is between 0.01 and 0.07.

2. A cutting tool according to claim 1 wherein the weight ratio in the cemented carbide Ni / (Ni+Co) is between 0.2 and 0.4.

3. A cutting tool according to any of the preceding claims wherein the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0 and 0.05.

4. A cutting tool according to any of the preceding claims wherein the weight ratio Cr / (Cr+Ni+Co) in the cemented carbide is between 0.01 and 0.03.

5. A cutting tool according to any of the preceding claims wherein the weight ratio AI / (AI+Cr+Ni+Co) in the cemented carbide is between 0.03 and 0.05.

6. A cutting tool according to any of the preceding claims wherein the amount of eta phase is between 1 and 7 vol %.

7. A cutting tool according to any of the preceding claims wherein the amount of metal binder is between 5 and 15 wt%.

8. A cutting tool according to any of the preceding claims wherein the substrate is provided with a coating.

9. A cutting tool according to claim 8 wherein the coating is a CVD coating.

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