EP4720360A1

EP4720360A1 - A cutting tool

Info

Publication number: EP4720360A1
Application number: EP24730901.6A
Authority: EP
Inventors: José Luis Garcia
Original assignee: Sandvik Coromant AB
Current assignee: Sandvik Coromant AB
Priority date: 2023-06-01
Filing date: 2024-05-28
Publication date: 2026-04-08
Also published as: CN121241156A; KR20260018832A; WO2024246041A1

Abstract

The present invention relates to a cutting tool comprising a substrate of cemented carbide wherein the cemented carbide comprises WC grains, eta phase grains, Co, Cr and at least one of Ti, Ta, V and Nb.

Description

A CUTTING TOOL

TECHNICAL FIELD

The present invention relates to a cutting tool. The cutting tool comprises a substrate of cemented carbide wherein the cemented carbide comprises WC grains, eta phase grains, Co, Cr and at least one of Ti, Ta, V and Nb.

BACKGROUND

Cutting tools for metal cutting applications that comprise a substrate of cemented carbide are known in the art.

Cutting tools comprising a substrate of cemented carbide including eta phase grains and WC grains and eta phase grains have shown promising performance as cutting tools for milling applications.

It is an object of the present invention to provide a cutting tool for metal cutting applications. It is a further object to provide a cutting tool that shows high wear resistance in milling.

DESCRIPTION OF THE INVENTION

At least one of the above-mentioned objects is achieved by a cutting tool according to claim 1. Preferred embodiments are disclosed in the dependent claims.

The present invention relates to a cutting tool comprising a substrate of cemented carbide, wherein the cemented carbide comprises WC grains and eta phase grains and metallic binder, wherein the metallic binder comprises Co and Cr, the Co content in the cemented carbide is 6-18 wt%, Cr/Co weight ratio in the cemented carbide is 2% - 12%, eta phase content in the cemented carbide is 1-10 vol%, the average grain size of the eta phase grains is 0.5- 5 pm, wherein the cemented carbide further comprises one or more of Ti, Ta, Nb, V, such that the Ti/Co weight ratio is < 0.3%, and the Ta/Co weight ratio is < 1.0%, and the Nb/Co weight ratio is < 0.5%, and the V/Co weight ratio is < 5.0%, wherein the weight ratio (Ti+Ta+Nb+V)/Co is > 0.04%, preferably > 0.07%.

It was realized that the specific composition of the cemented carbide in accordance with the present invention provides a cutting tool substrate with both a sufficiently high K1C and a high hardness and a very uniform microstructure. There is a specific Cr content in the cemented carbide of the present invention. Cr has a large solubility in Co providing a further increase of the metallic binder strength when under deformation. It is believed that adding Cr causes a reduction of the stacking fault energy of Co, enhancing the work hardening of the metallic binder. However, if the content of Cr is too high unwanted Cr-carbides (such as M7C3) may form, causing embrittlement of the cemented carbide.

The cemented carbide of the present invention also includes small amount of one or more of so called “cubic carbide formers”, i.e. Ti, Ta, Nb and V. Addition of higher amount of cubic carbide formers can lead to the formation of so-called gamma-phase in cemented carbides, gamma-phase is not present in the cemented carbide of the present invention. By a cemented carbide free from gamma phase is herein meant that no gamma phase can be seen in the microstructure in a LOM image at 500x magnification.

It was surprisingly found that the combination of the addition of the specific content of Cr and the addition of cubic carbides formers (Ti, Ta, Nb, V) in the specified low levels can be used to achieve an advantageous grain size and grain size distribution of the eta phase grains in a cemented carbide comprising fine dispersed eta phase. The presence of the small amounts of one or more of said cubic carbides formers surprisingly gives the effect of higher hardness while the toughness is kept at about the same level.

The cutting tool as disclosed herein can for example be an insert or an endmill designed for milling applications.

The cemented carbide of the present invention comprises WC grains and eta phase grains embedded in a metallic binder. The metallic binder comprises Co, Cr and one or more of Ti, Ta, Nb, V. The metallic binder also comprises W that is dissolved into the metallic binder from the WC grains during the sintering of the cemented carbide.

The eta phase content in the cemented carbide of the present invention is 1-10 vol%. If the eta phase content is too high a large part of the Co content in the metallic binder is consumed into the eta phase grains and this would make the cemented carbide too brittle. If the eta phase content is too low there is an increased risk that the eta phase grains forms clusters and these are brittle instead of the well distributed eta grains.

The cemented carbide according to the present invention has a low carbon content so that eta phase grains are formed. This will result in a cemented carbide having both a W content in the binder and eta phase grains. By eta phase is herein meant carbides selected from Me^C and MeeC where Me is one or more metals selected from W and the binder phase metal or metals.

In the present invention the cemented carbide comprises fine dispersed eta phase grains. By fine dispersed 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 mm² in a light optical microscope image at 200 times magnification. Eta phase grains can exist as very large, brittle and unwanted form with grain sizes typically above 50 pm or even larger than 100 pm and these are not part of the present invention. The eta phase grains of the present invention are 0.5-5 pm in average grain size and these grains are evenly distributed within the metallic binder of the cemented carbide. 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 substoichiometric carbon content between achieving the unwanted large agglomerates of eta phase, and achieving the finely distributed 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.

In one embodiment of the present invention the cemented carbide comprises 70-90 vol% WC, preferably 75-85 vol% WC.

In one embodiment of the present invention the cemented carbide consists of WC grains and eta phase grains in a metallic binder of Co, Cr and Ti and wherein some W is dissolved in the metallic binder. W will inevitably be dissolved in the metallic binder during sintering and the exact amount depends on several things, such as the overall composition of the cemented carbide, the exact carbon content etc.

In one embodiment of the present invention the Co content in the cemented carbide is 10- 15 wt%.

In one embodiment of the present invention the Cr/Co weight ratio in the cemented carbide is 2% - 10%.

In one embodiment of the present invention the eta phase content is 1-9 vol%, preferably 1-7 vol%. In one embodiment of the present invention the average grain size of the eta phase grains is 1-3 pm, preferably 1-2 pm.

In one embodiment of the present invention the WC grain size is 0.3-3 pm, preferably 0.5 - 1.5 pm.

In one embodiment of the present invention the cemented carbide consists of WC grains and eta phase grains and metallic binder and wherein the cemented carbide consists of the elements W, C, Co, Cr, Ta and Nb and wherein the Ta/Co ratio is 0.20-0.40% and wherein the Nb/Co weight ratio is 0.10-0.20%.

In one embodiment of the present invention the cemented carbide consists of WC grains and eta phase grains and metallic binder and wherein the cemented carbide consists of the elements W, C, Co, Cr and Ti and wherein the Ti/Co weight ratio is 0.04-0.20%, preferably 0.05-0.16% or 0.10-0.20%.

In one embodiment of the present invention the cemented carbide is free from gamma phase.

In one embodiment of the present invention the content of eta phase in a portion of the substrate adjacent to the surface of the substrate corresponds to the content of eta phase in the innermost portion of the substrate. The eta phase distribution is thus 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.

In one embodiment of the present invention the cemented carbide consists of WC grains and eta phase grains in a metallic binder of Co, Cr and one or more of Ti and/or Ta and/or Nb and/or V, and wherein some W is dissolved in the metallic binder. W will inevitably be dissolved in the metallic binder during sintering and the exact amount depends on several things, such as the overall composition of the cemented carbide, the exact carbon content etc.

In one embodiment of the present invention the cemented carbide consists of 75-85 vol% WC grains and 1-10 vol% eta phase grains and balance vol% metallic binder. The area fraction in the image is herein considered to correspond to represent the volume fraction. METHODS

The cemented carbide of the present invention is made in accordance with normal cemented carbide manufacturing methods. The steps of milling, drying, pressing and sintering are used.

The carbon content needs to be adjusted during the cemented carbide manufacturing to achieve the correct eta phase content.

The formation of evenly or finely distributed eta phase grains in accordance with the present invention is achieved by controlling the carbon content carefully during the cemented carbide manufacturing. The cemented carbide in the present invention has a substoichiometric carbon content within a certain range. Substoichiometric carbon is a measure of the carbon content in relation to the stoichiometric carbon content.

Suitably the carbon content is between -0.40 and -0.16 wt% substoichiometric carbon, preferably between -0.35 and -0.17 wt% substoichiometric carbon.

The stoichiometric carbon content may be calculated by assuming that the carbides in the cemented carbide are completely stoichiometric, for example the atomic ratio W:C is assumed to be 1 :1. Since other carbide forming elements are present in the cemented carbide, such as Cr, the corresponding carbide Cr3C₂, is also assumed to be stoichiometric.

This means that the term substoichiometric carbon, as used herein, is the total carbon content in the cemented carbide determined by chemical analysis minus the calculated stoichiometric carbon content based on WC and other carbides present in the cemented carbide. In the sintered cemented carbide:

[Carbon content] = [stoichiometric carbon content] + [subsoichiometric carbon content]

As an example, if the stoichiometric carbon content for a particular cemented carbide is 5.60 wt%, and this cemented carbide would be made with a carbon content of 5.30 wt%, the substoichiometric carbon content would be -0.30 wt%.

To achieve the correct carbon content in the final sintered cemented carbide manufacturing, W and/or W₂C is added in such amounts that the desired substoichiometric carbon is achieved. The cemented carbide has such a low carbon content that eta phase is formed. The eta phase formed is, however, not present as large grains or agglomerates but is of a fine grain size and well distributed. The desired form of eta phase is provided by controlling the carbon balance carefully during manufacturing.

If the carbon content in the sintered cemented carbide is too low, i.e. lower than -0.40 wt% substoichiometric carbon content, the amount of eta phase becomes too large and the particle size increases considerably so the cemented carbide will be brittle. On the other hand, if the carbon content is higher than -0.16 wt% substoichiometric carbon content but still in the eta phase forming region, the formed eta phase will be unevenly distributed in the form of large agglomerates leading to a decrease in toughness of the cemented carbide.

The carbon content is to be measured on the sintered cemented carbide, since some of the carbon will be lost during sintering due to the formation of e.g. CO2. The exact amount of carbon lost depends on the specific sintering furnace and sintering process. The powder will thus have a small excess of carbon compared to what is aimed for in the sintered cemented carbide. Typically the substoichiometric carbon value in the sintered material is about 5-25% lower than the substoichiometric carbon value in the powder composition. For example, a substoichiometric carbon value in the powder composition of -0.20 wt% may give a substoichiometric carbon value in the sintered material of from about -0.21 wt% to about -0.25 wt%. Carbon content in the sintered cemented carbide or the powder can be measured by the LECO CS844 instrument. It is up to the person skilled in the art to adjust the W and/or W2C additions so that the aimed eta content in the cemented carbide is achieved.

The amount of eta phase in the cemented carbide was determined by image analysis of LOM (light optical microscope). The magnifications of the images is suitably 2000X. The area fraction in the image is considered to correspond to the volume fraction in the cemented carbide. The volume fraction of WC can be determined in the corresponding way as the eta phase volume fraction.

The average grain size of the WC grains is herein measured using the Saltykov method on an image of a cross section of the substrate. The test area is a square with a size such that at least 50 grains but no more than 100 grains are included At least 700 grains should be counted. The formula used for determining Na (number of grains per square millimeter) is wherein M is the magnification used and A is the area of the test square. Nintercepted is the number of grains intercepted by the test squares’ sides. A grain intercepted at the corner of the test square counts as one fourth of a grain. Ninside is the number of whole grains inside the test square. The average grain size of WC, dwc, can then be determined by

The average grain size of the eta phase grains is herein defined as the average value of the maximum Feret diameter of the eta phase grains. This value can be determined by, e.g., image analysis on a light optical microscope (LOM) image.

The chemical composition of the cemented carbide can be measured by chemical analysis with XRF (X-ray fluorescence) using for example a Panalytical Axios Max Advanced instrument.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will be described with reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional light optical microscope picture of sample Reference 2

FIG. 2 is a cross-sectional light optical microscope picture of sample Invention 2A

EXAMPLES

Exemplifying embodiments of the present invention will now be disclosed in more detail and compared to reference embodiments. Coated cutting tools (inserts) were manufactured and analysed and evaluated in cutting tests.

Cemented carbide substrates of 6 different compositions were manufactured.

The cemented carbides were made from raw material powders in accordance with Table 1. The average WC grain size in the initial powder was 1.37 pm. Table 1. Powder composition

The powders were milled in a ball mill together with a milling liquid (water/ethanol with a ratio of 9/91) and an organic binder, 2 wt % PEG. The amount of PEG is not included in the dry powder weight presented in Table 1. After the milling the slurry was dried. The dried agglomerates were then pressed into a green body. The green body was sintered in 40 mbar in Ar and CO at 1410°C.

The average content of the eta phase was determined by counting its area in LOM (light optical microscope) images taken at x2000 using the software Image J and “Analyze particles” function with “include holes” and “0- 1 nfinity” filter settings. Prior to measurements, images were converted into 8-bit black and white images using Automatic threshold setup. The area fraction was assumed to correspond to the volume fraction in the sample. An average value from 10 images is shown in Table 2.

The average particle size of the eta phase grains is herein calculated in the same software as being the average maximum Feret diameter of the eta particle grains. When measuring the maximum Feret diameter the filters “0.01 -I nfinity” and “exclude on edges” were additionally activated in “Analyze particles” function and only images with 2000 magnification were used. Prior to measurements, images were converted into 8-bit black and white images using Automatic threshold setup. An average value from 6 images is shown in Table 2.

The substoichiometric carbon content in the sintered cemented carbide was calculated by first measuring the total carbon content in the sintered cemented carbide by using a LEGO CS 844 instrument, for this analysis, the sample was crushed prior to the analysis. The accuracy of the values is ±0.01 wt%. The W, Co and Cr content is measured with XRF (X- ray fluorescence) using a Panalytical Axios Max Advanced instrument. By subtracting the cobalt, the chromium and carbon amounts from the total weight of the sample, the W content is achieved which is used to calculate the stoichiometric carbon content, assuming the WC has a 1 :1 ratio. By subtracting the stoichiometric carbon content from the total carbon as measured by the LECO CS 844 instrument, the substoichiometric carbon value is achieved. The substoichiometric carbon value in the sintered material differs from that in the powder. This is due to that some of the carbon reacts with oxygen and outgas as CO or CO2 during sintering, and this reduces the total final C content of the cemented carbide.

The carbon in the powder was adjusted to achieve the desired microstructure in the sintered cemented carbides.

The eta phase content and the average grain size of the eta phase grains were measured in accordance with the methods herein disclosed, see Table 2. The eta phase grains in the samples containing eta phase were evenly distributed through the whole substrate body, no gradient in eta phase content was observed in the samples. No gamma phase grains, no very large eta phase grains and no graphite was found in the cemented carbide.

The area fraction of the WC, the eta phase grains and the metallic binder of the cemented carbides were studied in LOM and in SEM. Details of the sintered cemented carbide are presented in Table 2.

Table 2 Cemented carbide details Hardness and toughness were measured in accordance with the method disclosed above and the achieved values are presented in Table 3.

Table 3 Hardness and toughness of cemented carbides The inventive samples showed higher hardness compared to the reference samples. The K1C levels of the inventive samples are lower than the reference samples, but all samples have high K1C levels. Studies of SEM images of all the samples showed a more uniform distribution of the eta phase grains in the inventive samples as compared to the reference samples.

Claims

1 . A cutting tool comprising a substrate of cemented carbide, wherein the cemented carbide comprises WC grains and eta phase grains and metallic binder, wherein the metallic binder comprises Co and Cr, the Co content in the cemented carbide is 6- 18 wt%, the Cr/Co weight ratio in the cemented carbide is 2% - 12%, the eta phase content in the cemented carbide is 1-10 vol%, the average grain size of the eta phase grains is 0.5 - 5 pm, wherein the cemented carbide further comprises one or more of Ti, Ta, Nb, V, such that the Ti/Co weight ratio is < 0.3%, and the Ta/Co weight ratio is < 1.0%, and the Nb/Co weight ratio is < 0.5%, and the V/Co weight ratio is < 5.0%, wherein the weight ratio (Ti+Ta+Nb+V)/Co is > 0.04% .

2. The cutting tool in accordance with claim 1 , wherein the Co content in the cemented carbide is 10 - 15 wt%.

3. The cutting tool in accordance with any of the preceding claims, wherein the Cr/Co weight ratio in the cemented carbide is 2% - 10%.

4. The cutting tool in accordance with any of the preceding claims, wherein the eta phase content is 1 - 9 vol%.

5. The cutting tool in accordance with any of the preceding claims, wherein the average grain size of the eta phase grains is 1 - 3 pm.

6. The cutting tool in accordance with any of the preceding claims, wherein the WC grain size is 0.3 - 3 pm.

7. The cutting tool in accordance with any of the preceding claims, wherein the cemented carbide consists of WC grains and eta phase grains and metallic binder and wherein the cemented carbide consists of the elements W, C, Co, Cr, Ta and Nb and wherein the Ta/Co ratio is 0.20 - 0.40% and wherein the Nb/Co weight ratio is 0.10 - 0.20%.

8. The cutting tool in accordance with any of the preceding claims, wherein the cemented carbide consists of WC grains and eta phase grains and metallic binder and wherein the cemented carbide consists of the elements W, C, Co, Gr and Ti and wherein the Ti/Co weight ratio is 0.10 - 0.20%.

9. The cutting tool in accordance with any of the preceding claims, wherein the cemented carbide is free from gamma phase.

10. The cutting tool in accordance with any of the preceding claims, wherein the cemented carbide consists of 75-85 vol% WC grains and 1-10 vol% eta phase grains and balance vol% metallic binder.