Cemented carbide composition

Abstract

The invention relates to a cutting tool comprising a substrate of a cemented carbide comprising WC grains, eta phase grains and Cr and at least one metallic binder selected from Co, Ni, and Fe, said metallic binder being present in an amount ranging from 6 to 22 wt%; wherein the weight ratio of Cr to said at least one metallic binder ranges from 0.13 to 0.30, and wherein the eta phase content ranges from 1 to 10 vol%.

Description

[0001] Cemented carbide composition

[0002] The present invention relates to a cemented carbide composition.

[0003] Background of the invention

[0004] Cemented carbide compositions are generally known for use as substrates for metal machining applications such as turning, milling, and drilling. To provide a long tool life, a cutting tool should have high resistance against different types of wear. While the toughness generally decreases with increasing hardness, it would be desirable to provide an insert with both high hardness and high toughness.

[0005] Attempts to provide cemented carbide-based substrates benefiting from high hardness and toughness have been made. However, further improvements would be desirable. The present invention intends to provide cemented carbide-based cutting tools with improved hardness and toughness or at least substantially maintained toughness.

[0006] The invention

[0007] The invention relates to a cutting tool comprising a substrate of a cemented carbide comprising WC grains, eta phase grains and Cr and at least one metallic binder selected from Co, Ni, and Fe, said metallic binder being present in an amount ranging from 6 to 22 wt%, preferably 6 to 18 wt%, more preferably 8 to 14 wt%; wherein the weight ratio of Cr to said at least one metallic binder ranges from 0.13 to 0.30, and wherein the eta phase content ranges from 1 to 10 vol%, preferably from 1 .5 to 9 vol%, more preferably from 2 to 9 vol% or 2.5 to 8 vol% or 2.5 to 6.5 vol%.

[0008] According to one embodiment, the weight ratio of Cr to said at least one metallic binder ranges from 0.13 to 0.25.

[0009] According to one embodiment, the weight ratio of Cr to said at least one metallic binder ranges from 0.14 to 0.2.

[0010] According to one embodiment, the average grain size of the eta phase grains ranges from 0.1 to 10 pm.

[0011] According to one embodiment, the average WC grain size ranges from 0.1 to 3 pm.

[0012] According to one embodiment, said at least one metallic binder is Co.

[0013] According to one embodiment, the cemented carbide further comprises and / or Zr. According to one embodiment, the cemented carbide further comprises in an amount ranging from 0.01 to 0.9, preferably from 0.015 to 0.4 wt%.

[0014] According to one embodiment, the cemented carbide further comprises Zr in an amount ranging from 0.01 to 0.5, preferably from 0.02 to 0.1 wt%.

[0015] According to one embodiment, the substrate is coated.

[0016] The cutting tool suitably is an insert or an end mill for machining metals including machining operations such as milling, turning or drilling.

[0017] Cr acts as a grain growth inhibitor. Without sufficient Cr, tungsten carbide (WC) grains can grow excessively during sintering leading to a coarser microstructure. This can reduce the hardness and wear resistance of the substrate. An insufficient amount of Cr can thus negatively impact the overall mechanical properties, such as toughness and strength. The material may become less durable and more prone to failure under stress. If the Cr content is too high, the Cr will have a strong affinity for carbon resulting in the formation of brittle Cr rich carbides. This can adversely affect the hardness and toughness of the cemented carbides. High concentrations of Cr can also decrease the fracture toughness making the material prone to cracking under stress.

[0018] A too high metallic binder content of e.g. Co content generally decreases the hardness of the substrate. This may render the cemented carbide less effective in applications requiring high wear resistance. A too high Co content may also reduce the abrasive wear resistance of the cemented carbide making it less durable in demanding environments.

[0019] If the Co content is too low, the Co will act as a binder that provides toughness and ductility to cemented carbides. Also, a too low Co content can make the material more brittle and prone to cracking under stress and may result in poor densification, higher porosity and lower mechanical strength.

[0020] By “eta phase” is herein meant carbides selected from Mei2C and Me6C where Me is one or more metals selected from W and the binder phase metal or metals.

[0021] According to one embodiment, the cemented carbide comprises from 65 to 95 vol% WC, for example from 70 to 90 vol% WC, preferably from 75 to 85 vol% WC.

[0022] According to one embodiment, the average WC grain size in the cemented carbide is 0.1 to 3 pm or 0.1 to 2 pm, preferably 0.1 to 1.0 pm. The particle size ofthe WC powder used is selected to provide a desired WC grain size in the final cemented carbide taking into consideration the effects of all components in the raw material powder mixture and the milling procedure used. The particle size (FSSS) of the WC powder prior to milling ranges from 0.5 to 7 pm such as from 0.65 to 7 pm or from 0.65 to 1 pm.

[0023] According to one embodiment, the average size of the eta phase grains is 0.1 to 10 pm, preferably 0.5 to 5 pm, more preferably 0.5 to 3 pm or 0.8 to 2 pm or 1 to 1 .7 pm.

[0024] To obtain the desired carbon content in the sintered cemented carbide, W and / or W2C are preferably admixed prior to sintering.

[0025] If the carbon content in the sintered cemented carbide is too low, the amount of eta phase becomes too large whereby the amount of eta phase grains increases considerably resulting in brittle cemented carbide.

[0026] The carbon content in the sintered cemented carbide is measured since some of the carbon will be lost during the sintering process due to the formation of e.g. CO2. The exact amount of carbon lost depends on the specific sintering furnace and the sintering process. The admixed powder shall thus have a small excess of carbon compared to what is aimed for in the sintered cemented carbide.

[0027] Preferably, the eta phase is finely dispersed, i.e. 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.

[0028] The cemented carbide 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 metal binder and in the eta phase grains.

[0029] According to one embodiment, the cemented carbide comprises a gamma phase which may comprise and / or Zr.

[0030] Detailed description of the figures

[0031] Figure 1 shows very fine-dispersed eta phase (invention) while figures 2-4 show phases with the presence of some distinct clusters. Definitions and Methods

[0032] Eta phase content

[0033] The amount of eta phase in the cemented carbide was determined by image analysis of LOM (light optical microscope) using the software Image J using the “Analyze particles” function with “exclude on edges” and the “O-Infinity” filter settings. Prior to the measurements, colour LOM images were converted into 8-bit black and white images using Automatic threshold setup. The magnifications of the images were 1000X. At least five measurements were done for each magnification and the values in table 2 represents an average value thereof. The value presented in table 2 is thus an average from a total of at least five image analyses performed on at least five images with one measurement on each image. The area fraction in the image is assumed to correspond to the volume fraction in the cemented carbide. The volume fraction of the WC can be determined in the same way as the eta phase volume fraction.

[0034] Eta phase grain size

[0035] 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 was determined herein by image analysis on a light optical microscope (LOM) image using the software Image J using the “Analyze particles and the “0- Infinity” filter settings. The Feret size option “exclude on edges” was activated in the “Analyze particles” function. Prior to the measurements, colour LOM images were converted into 8-bit black and white images using Automatic threshold setup. The images used for the analysis were LOM images with a magnification of 1000X and at least 5 images were processed and maximum Feret diameters were obtained for each image and an overall average value of the maximum Feret diameter was calculated.

[0036] WC grain size determination

[0037] The average grain size of the WC, d, is herein determined from the value of the magnetic coercivity. The relationship between coercivity and grain size of WC is described, e.g., in Roebuck et al., Measurement Good Practice No. 20, National Physical Laboratory, ISSN 1368-6550, November 1999, Revised February 2009, Section 3.4.3, pages 19-20. For the purposes of this application the grain size of the WC, d, is determined according to formula

[0038] (8) on page 20 in the above-mentioned reference:

[0039] K=(Ci+diWCo)+ (c2+d2Wco) / d. Re-arranging the formula: d = (c2+d2WCo) / (K-(ci+diWCo)), wherein d= WC grain size of the cemented carbide body, K= coercivity of the cemented carbide body in kA / m, herein measured according to standard DIN IEC 60404-7, WCo= wt% Co in the cemented carbide body, Ci = 1.44, c2= 12.47, di = 0.04, and d2= -0.37.

[0040] S-value

[0041] The dissolved amount (wt%) of tungsten (W) in the binder phase may be expressed as the S-value. S= o16.1 x 100 [%], where o is the measured magnetic moment (MM) of the binder phase in pT m3kg-1. The S-value depends on the content of W in the binder phase and increases with a decreasing tungsten content. The dissolution of W in the binder phase, i.e. the S-value, depends on the carbon content in the carbides and the degree of carbon saturation in the powder.

[0042] Examples

[0043] A cemented carbide composition was prepared by mixing raw material powders containing tungsten carbide (WC), cobalt (Co) powder, chromium carbide (Cr3C2), vanadium carbide (VC) and zirconium carbide (ZrC) powder and tungsten powder (W) according to table 1 .

[0044] Table 1

[0045] Ctot is the total carbon content in the powder mixture (weighed quantity in the powders). The WC powder had a Fischer method (FSSS) particle size of 0.82 pm. Csis the total carbon content in the sintered cemented carbide composition as measured with a LECO CS844 instrument. The powders were milled in a ball mill together with a milling liquid (with 87 wt% ethanol) and an organic binder (2 wt% PEG). The amount of PEG is not included in the dry powder weight presented in Table 1 . After milling, the slurry formed was dried in a spray dryer and thereafter pressed to inserts in a pressing operation at about 172 MPa. All powder batches were milled in 1 kg or 4 kg batches and spray dried in a lab spray. The green body was sintered in vacuum for 60 min at a temperature of 1430°C.

[0046] The average content of the eta phase was determined by the method as described herein. An average value from 8 images is shown in Table 2.

[0047] The average grain size of the eta grains was determined by the method as described herein. An average value from 8 images is shown in Table 2.

[0048] The eta phase content and the average grain size of the eta phase grains as set out in table 2 were measured in accordance with the methods disclosed herein. The eta phase grains in the samples containing eta phase were evenly distributed throughout the whole substrate. No gradient in the eta phase content was observed in the samples. No large eta phase grains or graphite were found in the cemented carbide.

[0049] 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 (Scanning Electron Microscope). Details of the sintered cemented carbide are presented in Table 2 including the density, the coercivity and the degree of magnetic moment (S) which, as can be noted, are at about the same level for all samples.

[0050] Table 2 Table 3 shows Hv10 and Hv30 hardness and K1C toughness of the invention and references 1 and 2. It can be noted that the hardness of the invention samples were at a higher level than Ref.2 while the toughness was on a similar level or somewhat lower level as the references. Table 3

Claims

8Claims1. Cutting tool comprising a substrate of a cemented carbide comprising WC grains, eta phase grains and Cr and at least one metallic binder selected from Co, Ni, and Fe, said metallic binder being present in an amount ranging from 6 to 22 wt%; wherein the weight ratio of Cr to said at least one metallic binder ranges from 0.13 to 0.30, and wherein the eta phase content ranges from 1 to 10 vol%.

2. Cutting tool according to claim 1 , wherein the weight ratio of Cr to said at least one metallic binder ranges from 0.13 to 0.25.

3. Cutting tool according to claim 1 or 2, wherein the weight ratio of Cr to said at least one metallic binder ranges from 0.14 to 0.2.

4. Cutting tool according to any one of claims 1 to 3, wherein the metallic binder is present in an amount ranging from 6 to 18 wt%.

5. Cutting tool according to any one of claims 1 to 4, wherein the metallic binder is present in an amount ranging from 8 to 14 wt%.

6. Cutting tool according to any one of claims 1 to 5, wherein the eta phase content ranges from 1 .5 to 9 vol%.

7. Cutting tool according to any one of claims 1 to 6, wherein the average grain size of the eta phase grains ranges from 0.1 to 10 pm.

8. Cutting tool according to any one of claims 1 to 7, wherein the average WC grain size ranges from 0.1 to 3 pm.

9. Cutting tool according to any one of claims 1 to 8, wherein said at least one metallic binder is Co.

10. Cutting tool according to any one of claims 1 to 9, wherein the cemented carbide further comprises and / or Zr.11 . Cutting tool according to any one of claims 1 to 10, wherein the cemented carbide further comprises in an amount ranging from 0.01 to 0.9 wt%.

12. Cutting tool according to any one of claims 1 to 10, wherein the cemented carbide further comprises in an amount ranging from 0.015 to 0.4 wt%.

13. Cutting tool according to any one of claims 1 to 12, wherein the cemented carbide further comprises Zr in an amount ranging from 0.01 to 0.5 wt%.

14. Cutting tool according to any one of claims 1 to 12, wherein the cemented carbide further comprises Zr in an amount ranging from 0.02 to 0.1 wt%.

15. Cutting tool according to any one of claims 1 to 14, wherein the substrate is coated.

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