The present invention relates to a WC-based cemented carbide. This application claims priority based on Japanese Patent Application No. 2023-183208 filed on October 25, 2023. The entire description in the Japanese patent applications is hereby incorporated by reference.

WC-based cemented carbides are used, for example, in the substrates of cutting tools and surface-coated cutting tools (hereinafter referred to as coated tools). Coated tools have coating layers, such as Al₂O₃ or TiCN layers, deposited on substrates and are used in cutting of carbon steel, cast iron, alloy steel, and other materials.

In cutting processes of stainless steel where the cutting edges are subject to high temperatures, the cutting edges of the coated tool may undergo plastic deformation, leading to early tool life. If the hardness of the substrate is increased to suppress this plastic deformation, the chipping resistance is reduced. Accordingly, there is a need for a substrate made of WC-based cemented carbides that satisfy compatibility of plastic deformation resistance and chipping resistance. Such cemented carbides have been proposed, for example, as described as follows:

Patent Literature 1 discloses a cemented carbide comprising a hard phase containing tungsten carbide grains and a binder phase mainly composed of iron group elements such as cobalt, wherein the relation: B/A ≤ 0.05 is satisfied where A is the number of grains of tungsten carbide and B is the number of grains of tungsten carbide having one or less contact points with other tungsten carbide grains. This WC-based cemented carbide has excellent plastic deformation resistance.

Patent Literature 2 discloses a WC-based cemented carbide that contains 10 to 13 mass% Co, 2 to 8 mass% Cr relative to Co, and 0.2 to 0.5 mass% of at least one of TaC and NbC, the balance being WC, and has a hardness of 88.6 to 89.5 HRA, where the ratio D₈₀/D₂₀ of the WC number cumulative 80% grain size D₈₀ to the number cumulative 20% grain size D₂₀ satisfies the relation: 2.0 ≤ D80/D20 ≤ 4.0, the grain size D₈₀ is 4.0 to 4.86 µm, and degree c of the WC adhesion satisfies the relation 0.36 ≤ c ≤ 0.43. This WC-based cemented carbide has excellent chipping resistance.

• PTL1: Japanese Patent No. 6256415
• PTL2: Japanese Patent No. 6774645

An object of the present invention, which has been accomplished in view of the aforementioned circumstances and proposals, is to provide a cemented carbide that exhibits high plastic deformation resistance and high chipping resistance, in use as cutting tools and coated tools.

A WC-based cemented carbide in accordance with an embodiment of the present invention comprises: 6.0 to 10.0 mass% Co, 0.08 to 0.90 mass% Cr (provided that the Cr content (mass%)/Co content (mass%) is 10% or less), 0.0 to 3.8 mass% M (where M is at least one element selected from the group consisting of V, Ta, Nb, Ti and Zr), 4.5 to 7.5 mass% C, the balance being W and inevitable impurities, wherein

• the cemented carbide comprises binder phases, hard phases, and γ phases,
• the binder phases are mainly composed of Co, the hard phases are mainly composed of W carbide, and the γ phases are mainly composed of M carbide,
• in the crystal grains constituting the hard phases, the number cumulative 99% grain size C99 is 3.30 µm or less, and the ratio C99/C50 of the number cumulative 99% grain size C99 (µm) to the number cumulative 50% grain size C50 (µm) ranges from 4.80 to 6.50, and
• the proportion (L) of the interfacial length of crystal grains constituting the hard phases in contact with crystal grains constituting the binder phases to the total interfacial length of crystal grains constituting the hard phases is 35% or more.

The WC-based cemented carbide may satisfy the condition (1):

1. (1) The Rockwell hardness (HRA) is in the range of 88.8 to 90.6.

A substrate for a cutting or coated tool made of WC-based cemented carbide of the above embodiment exhibits excellent plastic deformation resistance and chipping resistance.

The inventor has made a diligent study of cemented carbides that provide excellent plastic deformation resistance and chipping resistance when used as substrates of coated tools, and has found the following matters (1) and (2) for the WC-based cemented carbides described in Patent Literatures 1 and 2.

1. (1) The WC-based cemented carbide described in Patent Literature 1 reaches the end of tool life early when being used in intermittent cutting; and
2. (2) The WC-based cemented carbide described in Patent Literature 2 lacks plastic deformation resistance when being used in high-speed high-feed cutting.

The inventor has made further diligent investigations. As a result, the inventor has reached the following findings to derive the present invention.

1. (a) The plastic deformation resistance of WC-based cemented carbides depends on the grain size of the crystal grains constituting the hard phases. In detail, as the grain diameter decreases, the plastic deformation resistance of the WC-based cemented carbide improves. Thus, the plastic deformation resistance of WC-based cemented carbide improves in the case that a larger number of crystal grains with smaller grain size constituting the hard phases is present in the WC-based cemented carbide than before.
2. (b) The crystal grains constituting the hard phase are appropriately distributed in the WC-based cemented carbide to suppress contact between crystal grains in the hard phase, thereby suppressing stress concentration at contact points and thus improving the chipping resistance.

The WC-based cemented carbide in the embodiment of the present invention will now be described in detail.

Throughout the specification and the claims, a numerical range expressed as "A to B" (A and B are both numerical values) includes the upper limit (B) and the lower limit (A). In the case that units are stated only for the upper limit (B), the units for the upper limit (B) and the lower limit (A) should be the same.

The composition not expressed using a formula of the compound is not limited to stoichiometric compositions, but includes all known compositions with any atomic ratio.

The WC-based cemented carbide comprises: 6.0 to 10.0 mass% Co, 0.08 to 0.90 mass% Cr (provided that the Cr content (mass%)/Co content (mass%) is 10 percent or less), 0.0 to 3.8 mass% M (where M is at least one element selected from the group consisting of V, Ta, Nb, Ti and Zr), 4.5 to 7.5 mass% C, the balance being W and inevitable impurities.

Individual components will now be described.

In the case of a Co content of 6.0 to 10.0 mass%, the WC-based cemented carbide has excellent plastic deformation resistance when used as coated tools and substrates of coated tools.

Co is primarily present in the binder phases and is the main component of the binder phases (accounting for at least 50 atomic% of all components in the binder phases). The binder phases are composed of crystal grains with fcc and hcp structures.

The Co content is more preferably in a range of 7.0 to 9.6 mass%.

Cr is present in the form of solid solution in Co, which is the main component of the binder phase, and contributes to suppression of the growth of W carbide grains in in the hard phases, making crystal grains of W carbide finer or smaller. This function is insufficient at a Cr content of less than 0.08 mass%. A Cr content exceeding 10% of the Co content causes composite carbides of Cr and W to be precipitated and the toughness of the WC-based cemented carbide to decrease. Since the composite carbides work as the starting points of defect generation, the Cr content (mass %)/Co content (mass %) should be 10% or less. Since the upper limit of the Co content is 10.0 mass %, the upper limit of Cr content should be 0.90 mass % to allow for a margin of error.

M is an optional component (the content may be 0.0 mass%). M however is present in the form of solid solution in Co, which is the main component of the binder phases, and can increase the hardness of WC-based cemented carbide. M also forms carbide (not limited to the stoichiometric composition) and is present as the main component of the γ-phases (M accounts for more than at least 50 atomic% of all the components in the γ-phases). The upper limit of M content is set at 3.8 mass% because an M content exceeding 3.8 mass% leads to the toughness of WC-based cemented carbides to decrease and generation of the starting points of defects.

C is contained to form carbides primarily in the hard phases and γ phases. A C content of 4.5 to 7.5 mass% causes an appropriate amount of carbides to be formed in the hard phase and γ phases.

W is the main component of the hard phases. In detail, W accounts for at least 50 atom% of all the components in the hard phases. W is primarily present in the hard phases in the form of carbides (mostly WC, also including W carbides with non-stoichiometric composition).

Raw materials may contain unintended impurities, which may be incorporated during the manufacturing process. These impurities are called inevitable impurities. The content of inevitable impurities should preferably be 0.3 mass% or less in internal number when the entire WC-base cemented carbide is set as 100 mass%.

The WC-based cemented carbide ally includes binder phases, hard phases, and γ phases. The cemented carbide may unintentionally contain other phases, such as free carbon phases and η phases. The following description, however, proceeds under the assumption that only the binder phases, hard phases and γ phases exist. In the claims and the description, the binder phases, hard phases, and γ phases are composed of crystal grains and their sizes are expressed as crystal grain sizes, where the crystal grain size refers to the circle equivalent diameter.

The binder phases are composed of crystal grains that are primarily composed of Co and can contain W and C, which are components of the hard phases, M and Cr in the γ phases, and inevitable impurities. These components other than Co are considered to be present in the form of solid solution in the crystal grains constituting the binder phases. The crystal grains have fcc and hcp structures. The method of differentiating these crystalline grains constituting the binder phases will be described below.

The hard phases consist of crystal grains that are mainly composed of W carbides and may contain components of the binder phases and γ phases, and Cr and inevitable impurities. These crystal grains have an hcp structure. The method of identification of these crystal grains constituting the hard phases will be described below.

In the hard phases, it is preferable that the ratio C99/C50 of the number cumulative 99% grain size C99 (µm) to the number cumulative 50% grain size C50 (µm) be within a range from 4.80 to 6.50, for the following reasons: A ratio C99/C50 less than 4.80 leads to an insufficient number of fine crystal grains constituting the hard phases and a decrease in plastic deformation resistance, whereas a ratio C99/C50 exceeding 6.50 leads to a large difference in grain size (difference in grain diameter) between the coarse crystal grains and fine crystal grains constituting the hard phases, and thus formation of large crystal grains constituting the hard phases, resulting in a reduction in plastic deformation resistance of WC-based cemented carbide. The ratio C99/C50 is more preferably in the range of 5.30 to 6.30.

The grain size C50 should preferably be 0.68 µm or less, more preferably be 0.60 µm or less. The lower limit of the number cumulative 50% grain size C50 (µm) may have any value within the range of a ratio C99/C50 between 4.80 and 6.50 µm. In the case that the product is manufactured according to one example of the methods described below, the lower limit is about 0.38 µm. In other words, the number cumulative 50% grain size C50 (µm) is preferably in the range of 0.38 to 0.68 µm, more preferably 0.38 to 0.60 µm.

The number cumulative 99% grain size C99 (µm) should preferably be 3.30 µm or less; a grain size exceeding 3.30 µm leads to a reduction in plastic deformation resistance of the WC-based cemented carbide. The number cumulative 99% grain size C99 (µm) should more preferably be 3.20 µm or less. The number cumulative 99% grain size C99 (µm) may have any lower limit within the range of a ratio C99/C50 between 4.80 µm and 6.50 µm. In the case that the product is manufactured according to one example of the methods described below, the lower limit of the number cumulative 99% grain size C99 (µm) is about 2.50 µm. In other words, the number cumulative 99% grain size C99 (µm) is preferably in the range of 2.50 to 3.30 µm, more preferably 2.50 to 3.20 µm.

The crystal grains constituting the hard phases are in contact with the crystal grains constituting the binder phases, the γ-phases, and other hard phases. It is preferred that the proportion L of the interfacial length in contact with the crystal grains constituting the binder phases in the total interfacial length of the crystal grains constituting the hard phases be 35% or more. A proportion of less than 35% leads to an increase in opportunity of contact between crystal grains constituting the hard phases and thus ready stress concentration, which results in a decrease in the chipping resistance of the WC-based cemented carbide. The proportion L is more preferably 40% or more. The proportion L may have any upper limit. In the case that the product is manufactured in accordance with one example of the methods described below, the upper limit of the proportion L is about 55%. In other words, the proportion L is in the range of preferably 35 to 55%, more preferably 40 to 55%.

There are no restrictions on the proportion of the crystal grains constituting the hard phases in contact with the crystal grains constituting the γ phases and the crystal grains constituting the phases, such as free carbon phases and η phases, unintentionally generated in the manufacturing process, with the proviso that the proportion of the interfacial length in contact with the crystal grains constituting the binder phases in the total interfacial length of the crystal grains constituting the hard phases is 35% or more.

The γ-phases consist mainly of M carbides (not limited to stoichiometric composition) and may contain the components of the binder phases, the components of the hard phases, Cr and inevitable impurities. The γ-phases are composed of crystal grains with an fcc structure. The method of differentiating crystal grains constituting the γ-phases will be described below.

.

The WC-based cemented carbide should preferably have a Rockwell hardness (HRA) in the range of 88.8 to 90.4. In a Rockwell hardness within this range, the cemented carbide exhibits more reliable improvements in plastic deformation resistance and chipping resistance. The Rockwell hardness (HRA) is measured in accordance with ISO 3738-1:1982 and ISO 3738-2:1988.

The binder phases, hard phases, and γ phases, and the crystal grains constituting these phases are differentiated and the grain size and the cumulative number of grains are measured as follows.

1. (1) Any surface or cross-section of the cemented carbide is smoothed by milling to remove minute irregularities that interfere with the EBSD measurement. To measure the size of the crystal grains in each phase, several fields of view (e.g., two fields of view) are determined on the smoothed surface, each field of view being 90 µm (length) by 120 µm (width) in size. Each field of view is observed with a field emission scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and an electron backscatter diffractometer (EBSD (e.g., OIM Data Collection by AMETEK)) at an acceleration voltage of 15kV with a measuring point interval of 0.1 µm. The EBSD pattern and EDS data are simultaneously captured.
   The milling of the surface or cross-section is performed with, for example, a focused ion beam (FIB) system.
2. (2) The observed results of EBSD measurement are analyzed with software, e.g., OIM Analysis ver. 7.3.1 by EDAX/TSL. The EDS count values corresponding to elements captured from all the measuring points inside individual crystal grains are averaged into an observed EDS value of each element in each crystal grain. The composition of each crystal grain is then determined from the observed value.
3. (3) The phase to which each crystal grain belongs is identified in accordance with the definition of crystal grains, as described above. In detail, the hard phases are each defined by an aggregate of crystal grains identified from the EBSD pattern as having a hcp crystal structure and in which W carbides account for at least 50 atomic%. The binder phases are each identified as an aggregate of crystal grains with a hcp or fcc crystal structure. The γ phases are also each identified as an aggregate of crystal grains with an fcc crystal structure; hence, EBSD measurements alone cannot differentiate between the binder phases and the γ phases. Thus, the aggregates of crystal grains having EDS count values of M (V, Ti, Nb, Ta, or Zr) observed by EDS measurement are determined to be γ phases, and the remaining aggregates of crystal grains identified as aggregates of crystal grains with an fcc crystal structure are determined to be binder phases.
4. (4) In the case that adjacent measuring points are determined to be in the same phase, the boundary (midpoint) between those two measuring points is defined as the interface of the crystal grains if the difference in orientation observed from the measuring points is at least 5 degrees.
5. (5) The crystal grains identified as those constituting the hard phases, the γ phases, and the binder phases in paragraph (3) are again subjected to EDS measurement to confirm that the crystal grains identified as constituting the hard phases contain W carbides in at least 50 atomic%; that the crystal grains identified as constituting the γ phases contain M carbides in at least 50 atomic%; and that the crystal grains identified constituting the binder phases contain at least 50 atomic% Co.
6. (6) Based on the result of paragraph (5), a graph is created where the size of the crystal grains constituting the hard phases is taken on the horizontal axis, and the proportions of the cumulative number of crystal grains constituting the hard phases corresponding to each grain size on the vertical axis (cumulative number of crystal grains constituting the hard phases up to each grain size)/(total number of crystal grains constituting the hard phase) × 100) is taken on the vertical axis.

The number cumulative 50% grain size C50 (µm), at which the number of cumulative grains has reached 50%, and the number cumulative 99% grain size C99 (µm), at which the cumulative count percentage has reached 99%, are determined for each observation field, and the ratio C99/C50 is calculated by arithmetic averaging of the results.

The binder, hard, and γ phases and their constituent crystal grains are differentiated, and the interfacial lengths of the crystal grains constituting each phase are measured as follows:

1. (1) Any surface or cross-section of the cemented carbide is smoothed by milling to remove minute irregularities that interfere with EBSD measurement. In measurement of the size of the crystal grains in each phase, several fields of view (e.g., 4 or 5 fields of view) are determined on the smoothed surface, each field of view being, for example, 37 µm (length) by 47 µm (width) in size. Each field of view is observed with a field emission scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and an electron backscatter diffractometer (EBSD (e.g., OIM Data Collection by AMETEK)) at an acceleration voltage of 15kV with a measuring point interval of 0.1 µm. The EBSD pattern and EDS data are simultaneously captured.
2. (2) The observed results of the EBSD measurement are analyzed with software, e.g., OIM Analysis ver. 7.3.1 by EDAX/TSL. The EDS count values corresponding to elements in individual crystal grains, obtained from all the measuring points inside these crystal grains are averaged into an observed EDS value of each element in each crystal grain. The composition of each crystal grain is then derived from the observed value.
3. (3) The phase to which each crystal grain belongs is identified in accordance with the definition of crystal grains, as described above. In detail, the hard phases are each defined by an aggregate of crystal grains identified from the EBSD pattern as having a hcp crystal structure and in which W carbides account for at least 50 atomic%. The binder phases are each identified as an aggregate of crystal grains with an hcp or fcc crystal structure. The γ phases are also each identified as an aggregate of crystal grains with an fcc crystal structure; hence, EBSD measurements alone cannot differentiate between the binder phases and the γ phases. Thus, the aggregates of crystal grains having EDS count values of M (V, Ti, Nb, Ta, or Zr) observed by EDS measurement are determined to be γ phases, and the remaining aggregates of crystal grains identified as aggregates of crystal grains with an fcc crystal structure are determined to be binder phases.
4. (4) In the case that adjacent measuring points are determined to be in the same phase, the boundary (midpoint) between those two measuring points is defined as the interface of the crystal grains if the difference in orientation observed from the measuring points is at least 5 degrees.
5. (5) Based on the results of paragraph (3), the total interfacial length of the crystal grains constituting the hard phases and the interfacial length where the crystal grains constituting the hard phases are in contact with the crystal grains constituting the binder phases are calculated in each of the several views, as described above. The interfacial length is calculated with the aforementioned software, such as OIM Analysis ver. 7.3.1 from EDAX/TSL, for analysis of the results of the EBSD measurement.

Finally, the proportion of the interfacial length between the crystal grains constituting the hard phases and the crystal grains constituting the binder phases to the total interfacial length of the crystal grains constituting the hard phases is calculated by arithmetically averaging the observed values obtained for each field of measurement.

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The contents of W, Co, Cr, M (V, Ta, Nb, Ti, Zr), and C are measured with an electron beam microanalyzer. Any surface or cross section of the WC-based cemented carbide is mirror-finished to remove minute irregularities that interfere with measurements by an electron beam microanalyzer. The sample is then placed into the electron beam microanalyzer and irradiated with electron beams. For example, three observation fields of view with a size of 100 µm (length) by 100 µm (width) are selected, and the components and their contents are determined from the characteristic X-rays emitted from each observation field, and the results are averaged.

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An exemplary method of production will now be described.

The method involves, preparing predetermined raw powders, and then (1) mixing step, (2) molding step, (3) sintering step, (4) finishing step, and optional (5) coating step, in sequence. These processes will be described below.

In order to produce a WC-based cemented carbide of the embodiment it is desirable to add WC powder that has been ground and processed into fine particles, and to sinter the WC particles at relatively low sintering temperature, which suppresses growth of WC particles, accelerates distribution of Co around WC particles, and thus well disperse WC particles. Accordingly, it is recommended to perform the following two-stage mixing:

The primary mixing stage involves pre-grinding a portion of the raw WC powder into finely ground WC powder. In the primary mixing stage, 20 to 50 mass% of the WC powder to be used is agitated in a known device, such as an atomizer or ball mill. For example, mixing conditions using an atomizer preferably includes a rotation speed of 40 to 90 min^-1, a total load of superhard balls of 100 to 300 kg, and a mixing time of 70 to 240 min.

To yield a WC-based cemented carbide having a more preferred Rockwell hardness in the range of 88.8 to 90.4, the mixing time should be adjusted within the range of 120 to 240 min.

In the secondary mixing stage, the remaining WC powder, powder for binder phases, and powder for γ-phases are added to the ground powder from the primary mixing stage to produce the composition of the WC-based cemented carbide to be manufactured. These are mixed in a known device, such as an atomizer or ball mill. Preferred mixing conditions using an atomizer includes a rotation speed of 40 to 90 min^-1, a total load of cemented carbide balls of 200 to 500 kg, and a mixing time of 70 to 420 min.

To yield a WC-based cemented carbide having a more preferred Rockwell hardness in the range of 88.8 to 90.4, the mixing time should be adjusted within the range of 240 to 420 min.

The raw powder for WC-based cemented carbide that has undergone these two-stage mixing steps contains a large amount of fine-grained WC powder, which contributes to improved resistance to plastic deformation. As a result, for the crystal grains constituting the hard phase, the number cumulative 50% grain size C50 decreases, and the ratio C99/C50 of the number cumulative 99% grain size C99 to the number cumulative 50% grain size C50 can be controlled to be within a range of 4.80 to 6.50.

The two-stage mixing step promotes formation of fresh surfaces on the WC particles, concentration of Co around the surfaces, and an improvement in sintering ability. Despite the upper limit of the sintering temperature described later is set to a relatively low temperature of 1430°C, which suppresses the growth of the hard phases, Co readily diffuses around the hard phases, forming a WC-based cemented carbide with dispersed crystal grains constituting the hard phases. For these crystal grains, the number cumulative 99% grain size C99 can be controlled to 3.30 µm or less and the length L to 35% or more.

The raw powder material that has undergone the mixing step is shaped into a substrate with a predetermined shape (e.g., an insert shape conforming to specification CNMG120408). Molding can be performed, for example, by press molding under a pressure of 100 MPa.

This process involves sintering the formed raw material. To prevent excess growth of the crystal grains constituting the hard phases, the maximum holding temperature is set to 1350 to 1430°C, and the material is held for 50 to 120 minutes under a vacuum atmosphere of 10^-1 Pa or less. While any cooling rate is available, more preferred is cooling from the maximum holding temperature to 1100°C to 1200°C at a rate of 35°C/min. A cooling rate slower than this rate may accelerate growth of the crystal grains forming the hard phases during the cooling process, potentially leading to coarsening of grains. Thus, adequate cooling reduces the occurrence of coarse-grained WC caused by abnormal grain growth of the crystal grains.

The upper limit for the cooling rate was determined based on the constraints of the equipment available at the time of filing.

The sintered compact is ground into the specified shape, for example, the CNMG120408 insert shape defined in ISO Standard 1832:2017. It is noted that the sintered compact can be ground into any other shape for drills, milling cutters, or end mills.

A predetermined coating layer is formed by any known method.

The WC-based cemented carbide of this embodiment can be machined into a substrate with a predetermined shape and used as a cutting tool as it is. Alternately, the cemented carbide may be coated with a hard layer by any known method to serve as a surface-coated cutting tool.
The above description is summarized in the following Appendices:

• (Appendix 1)
  A WC-based cemented carbide comprising: 6.0 to 10.0 mass% Co, 0.08 to 0.90 mass% Cr (provided that the Cr content (mass%)/Co content (mass%) is 10% or less), 0.0 to 3.8 mass% M (where M is at least one element selected from the group consisting of V, Ta, Nb, Ti and Zr), 4.5 to 7.5 mass% C, the balance being W and inevitable impurities, wherein
  • the cemented carbide comprises binder phases, hard phases, and γ phases,
  • the binder phases are mainly composed of Co, the hard phases are mainly composed of W carbide, and the γ phases are mainly composed of M carbide,
  • in the crystal grains constituting the hard phases, the number cumulative 99% grain size C99 is 3.30 µm or less, and the ratio C99/C50 of the number cumulative 99% grain size C99 (µm) to the number cumulative 50% grain size C50 (µm) ranges from 4.80 to 6.50, and
  • the proportion (L) of the interfacial length of crystal grains constituting the hard phases in contact with crystal grains constituting the binder phases to the total interfacial length of crystal grains constituting the hard phases is 35% or more.
• (Appendix 2)
  The WC-based cemented carbide described in Appendix 1, having a Rockwell hardness (HRA) in a range of 88.8 and 90.6.
• (Appendix 3)
  The WC-based cemented carbide described in Appendix 1 or 2, wherein the crystal grains constituting the hard phases have a number cumulative 50% grain size C50 (µm) of 0.68 or less.
• (Appendix 4)
  The WC-based cemented carbide described in any one of Appendices 1 to 3, wherein the proportion (L) of the interfacial length of crystal grains constituting the hard phases in contact with crystal grains constituting the binder phases to the total interfacial length of crystal grains constituting the hard phases is 40% or more.

The present invention will now be described in more detail with reference to examples where the cemented carbide of the present invention is used as a cutting tool substrate; however, the examples should not be construed to limit the present invention.

The following raw material powders were prepared:

• WC powder with an average particle size as listed in Table 1,
• Co powder with an average particle size of 1.0 µm,
• Cr₃C₂ powder with an average particle size of 1.6 µm,
• TiC powder with an average particle size of 1.0 µm,
• TaC powder with an average particle size of 1.9 µm,
• NbC powder with an average particle size of 1.1 µm,
• ZrC powder with an average particle size of 1.2 µm, and
• VC powder with an average particle size of 1.7 µm.
• These powders were blended according to the formulation A to G and A' to E' shown in Table 1.

The average particle size of each raw material powder is the Fischer size measured by an air permeation method with a Fischer sub-sieve classifier.

As described above, the mixing step, molding step, sintering step, and finishing step were performed in sequence.

In the primary mixing stage, the WC powder was mixed and ground according to the formulation (mass %) and mixing times shown in Tables 2 and 3, based on the average particle size of the WC powder listed in Table 1 to yield finely ground WC powder. In the secondary mixing stage the WC powder after the primary mixing stage was mixed with the remaining WC powder not processed in the primary mixing stage (100 - the mass% of WC powder consumed in the primary mixing stage), raw material powder for the hard phase, and raw material powder for the binder phase for the mixing time specified in Tables 2 and 3. The raw powder after the secondary mixing stage was press-molded into green compacts with a shape of CNMG120408 insert under a pressure of 100 MPa. In the subsequent sintering step, the green compacts were sintered at holding temperatures and times shown in Tables 2 and 3 (some of the examples were cooled at cooling rates shown in Table 3). In the finishing step, the sintered compacts were ground to produce inserts with the specified CNMG120408 shape. The inserts were analyzed by the method described above. The results are shown in Table 4.

For comparison, raw powders were prepared according to the formulations A to E, G, and B' shown in Table 1. The mixtures were then subjected to the mixing step, molding step, sintering step, and finishing step in sequence.

In the mixing step, WC powder at the proportion shown in Tables 2 and 3 was mixed in the primary mixing stage (in Table 2, Comparative Examples where the primary mixing stage is indicated as "-" had no primary mixing stage and only a secondary mixing stage). After the mixing step, the raw powder was press-molded into green compacts with an insert shape of CNMG120408 under a pressure of 100 MPa. The green compacts were then sintered at holding temperatures and time shown in Tables 2 and 3 (some of the Comparative Examples were cooled at cooling rates shown in Table 3). In the finishing step, the sintered compacts were grounded to produce inserts with a specified shape of CNMG120408. The inserts were analyzed by the method described above. The results are shown in Table 5.

On the surface of each insert of Examples and Comparative Examples, a TiN layer (average thickness 0.2 µm), a TiCN layer (average thickness 3.5 µm), an Al₂O₃ layer (average thickness 2.5 µm), and a TiN (average thickness 0.2 µm) were formed in sequence by chemical vapor deposition into a coating layer. Row powder Formulation (mass%) WC mean grain size (µm) Co Cr₃C₂ VC TaC NbC TiC ZrC WC A 9.2 0.4 - 1.4 - - 0.1 Balance 2.5 B 8.4 0.6 - - 0.9 0.4 - Balance 2.5 C 7.6 0.2 0.2 - 0.7 - - Balance 2.5 D 6.9 0.4 - 0.6 - 0.1 - Balance 2.5 E 9.8 0.5 - 1.2 0.8 - - Balance 2.5 F 9.3 0.3 0.3 1.1 0.7 0.6 0.1 Balance 2.5 G 9.0 - - 1.3 - - 0.1 Balance 2.5 A' 9.2 0.4 - 1.4 - - 0.1 Balance 4.5 B' 8.4 0.6 - - 0.9 0.4 - Balance 4.5 C' 7.6 0.2 0.2 - 0.7 - - Balance 4.5 D' 6.9 0.4 - 0.6 - 0.1 - Balance 4.5 E' 9.8 0.5 - 1.2 0.8 - - Balance 4.5

In Table 1, the symbol "-" indicates that the component was not included in the formulation.

In Table 2, the symbol "-" indicates that the primary mixing stage was not performed. Step Raw powder Primary mixing stage Secondary mixing stage Sintering conditions (Main sintering) WC powder content (mass%) Mixing time (min) Mixing time (min) Holding temp. (°C) Holding time (min) Cooling rate (°C/min) Step of Example 11 F 35 120 120 1380 80 40 12 A' 40 180 360 1380 60 50 13 B' 35 150 270 1410 75 40 14 C' 30 240 360 1410 90 45 15 D' 25 120 240 1430 60 35 16 E' 45 150 300 1360 90 30 Step of Comparative Example 9' B' 10 120 240 1420 90 30 Example Raw Powder Formulation (mass%) Cr content/Co content (%) C50 (µm) C99 (µm) C99/C50 L (%) Rockwell hardness (HRA) Co Cr V Ta Nb Ti Zr C W 1 A 9.2 0.35 - 1.3 - - 0.1 4.9 Balance 4 0.49 2.79 5.69 48.3 89.5 2 B 8.4 0.52 - - 0.8 0.3 - 5.4 Balance 6 0.51 2.76 5.41 44.2 89.7 3 C 7.6 0.17 0.2 - 0.6 - - 5.7 Balance 2 0.5 2.72 5.44 41.9 89.9 4 D 6.8 0.35 - 0.6 0.6 0.1 - 6.7 Balance 5 0.47 2.67 5.68 38.2 90.3 5 E 9.8 0.43 - 1.1 0.7 - - 4.7 Balance 4 0.58 3.04 5.24 51.8 88.9 6 A 9.2 0.35 - 1.3 - - 0.1 4.8 Balance 4 0.58 2.82 4.86 47.8 89.4 7 B 8.4 0.52 - - 0.8 0.3 - 5.5 Balance 6 0.54 2.73 5.06 42.9 89.8 8 C 7.6 0.17 0.2 - 0.6 - - 5.7 Balance 2 0.52 2.74 5.27 43.8 89.8 9 D 6.8 0.35 - 0.6 0.6 0.1 - 6.6 Balance 5 0.47 2.62 5.57 35.9 90.9 10 E 9.8 0.43 - 1.1 0.7 - - 4.8 Balance 4 0.58 3.16 5.45 51.6 88.5 11 G 9.3 0.29 0.2 1.0 0.7 0.5 0.1 5.6 Balance 3 0.54 3.21 5.94 52.4 89.6 12 A' 9.2 0.35 - 1.3 - - 0.1 4.9 Balance 4 0.5 2.96 5.92 49.4 89.5 13 B' 8.4 0.52 - - 0.8 0.3 - 5.4 Balance 6 0.48 3.08 6.42 48.6 89.7 14 C' 7.6 0.17 0.2 - 0.6 - - 5.7 Balance 2 0.47 2.82 6.00 44.2 90.1 15 D' 6.8 0.35 - 0.6 0.6 0.1 - 6.7 Balance 5 0.51 3.14 6.16 42.1 90.4 16 E' 9.8 0.43 - 1.1 0.7 - - 4.7 Balance 4 0.53 3.02 5.70 49.8 89.3

Compartive Example Raw powder Formulation (mass%) Cr content/Co content (%) C50 (µm) C99 (µm) C99/C50 L (%) Rockwell hardness (HRA) Co Cr v Ta Nb Ti Zr C W 1' A 9.2 0.35 - 1.3 - - 0.1 5.6 Balance 4 0.84 3.21 3.82 39.2 88.9 2' B 8.4 0.52 - - 0.8 0.3 - 5.8 Balance 6 0.73 3.08 4.22 38.1 89.3 3' C 7.6 0.17 0.2 - 0.6 - - 5.7 Balance 2 0.69 2.92 4.23 34.6 89.7 4' D 6.8 0.35 - 0.6 0.6 0.1 - 5.8 Balance 5 0.62 2.62 4.23 31.8 90.2 5' E 9.8 0.43 - 1.1 0.7 - - 5.6 Balance 4 0.86 2.93 3.41 37.2 88.7 6' G 9.1 - - 1.2 0.5 - 0.1 5.6 Balance 0 0.53 2.62 4.94 30.2 89.5 7' A 9.2 0.35 - 1.3 - - 0.1 5.6 Balance 4 0.47 2.38 5.06 32.9 90.7 8' B 8.4 0.52 - - 0.8 0.3 - 5.8 Balance 6 0.59 2.79 4.73 37.1 88.9 9' B' 8.4 0.50 - - 0.8 0.4 - 5.9 Balance 6 0.64 3.48 5.44 42.5 88.7

In Tables 4 and 5, the symbol "-" indicates that the component was not formulated. The contents of inevitable impurities in all Examples and Comparative Examples were 0.3 mass% or less, calculated as a percentage in the entire WC-based cemented carbide (100 mass%).

Cutting tests were then conducted for 15 minutes each on Examples 1 to 16 and Comparative Examples 1' to 9' under the cutting conditions 1 to 4 below. Every minute after the start of the cutting test, each sample was visually observed for checking of chipping and detachment due to plastic deformation and for measurement of the width of the flank wear. For samples where the width of the flank wear exceeded 0.2 mm, the elapsed time since the start of testing (measuring time) corresponding to the wear width exceeding 0.2 mm was determined by fitting a straight line between that wear width and the wear width measured at the immediately preceding measuring time. This yielded the elapsed time since the start of testing (time to end of service life) corresponding to a 0.2 mm width of the flank wear. The results are shown in Tables 6 to 9.

• Workpiece Material: stainless steel SUS304 round bar, outer diameter 100 mm
• Cutting Speed: 150 m/s
• Depth of Cut: 1.5 mm
• Feed Per Revolution: 0.3 mm
• Wet Cutting

• Workpiece Material: stainless steel SUS316 round bar, outer diameter 100 mm
• Cutting Speed: 150 m/s
• Depth of Cut: 2.0 mm
• Feed Per Revolution: 0.2 mm
• Wet Cutting

• Workpiece Material: stainless steel SUS630 round bar, outer diameter 100 mm
• Cutting Speed: 120 m/s
• Depth of Cut: 2.0 mm
• Feed Per Revolution: 0.15 mm
• Wet Cutting

• Workpiece Material: stainless steel SUS304 hexagonal prism-shaped timber with an outer diameter of 80 mm
• Cutting Speed: 100 m/s
• Depth of Cut: 2.0 mm
• Feed Per Revolution: 0.3 mm

Tables 6 to 9 evidentially demonstrate that all Examples exhibit low flank wear, no chipping or detachment due to plastic deformation, and thus excellent cutting performance even during interrupted cutting or high-speed, high-feed machining. In contrast, all Comparative Examples exhibit either chipping or detachment due to plastic deformation before the end of the cutting test time (15 minutes) and a width of the flank wear of 0.2 mm, resulting in the end of the service life.

The above-disclosed embodiments are merely illustrative in all respects and are not restrictive. The scope of the present invention is defined by the claims, not by the above-disclosed embodiments, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims.