powder containing tungsten carbide

Abstract

The present invention provides a powder containing tungsten carbide with a uniform particle size. [Solution] A tungsten carbide-containing powder in which the features of primary particles are analyzed by image analysis in a cross-sectional SEM image of the powder, and the mode of the cross-sectional particle size distribution diagram created based on this analysis is 24.0% or higher.

Description

Technical Field

[0001] The present disclosure relates to a powder containing tungsten carbide. This application claims priority based on Japanese Patent Application No. 2024-112251 filed on July 12, 2024. All the content described in the said Japanese patent application is incorporated herein by reference.

Background Art

[0002] Conventionally, powders containing tungsten carbide and cemented carbides produced therefrom are disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei 8-117580 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2009-242181 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 2013-60666 (Patent Document 3), Japanese Unexamined Patent Application Publication No. 2005-335997 (Patent Document 4), Japanese Unexamined Patent Application Publication No. 2018-165233 (Patent Document 5), Japanese Unexamined Patent Application Publication No. 2006-151806 (Patent Document 6), and Japanese Unexamined Patent Application Publication No. 2023-127129 (Patent Document 7).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

[0004] For powders containing tungsten carbide, the mode of the cross-sectional particle size distribution diagram created based on the analysis of primary particle characteristics by image analysis in a cross-sectional SEM image of the tungsten carbide powder is 24.0% or higher. [Brief explanation of the drawing]

[0005] [Figure 1] Figure 1 is a SEM image of a cross-section of powder 100 containing tungsten carbide, according to sample number 1. [Figure 2] Figure 2 is a SEM image of a cross-section of powder 100 containing tungsten carbide, according to sample number 2. [Figure 3] Figure 3 is a SEM image of a cross-section of powder 100 containing tungsten carbide, according to sample number 3. [Figure 4] Figure 4 is a SEM image of a cross-section of powder 100 containing tungsten carbide, according to sample number 4. [Figure 5] Figure 5 shows the results of image analysis of a cross-sectional SEM photograph of powder 100 containing tungsten carbide, according to sample number 1. [Figure 6] Figure 6 shows the results of image analysis of a cross-sectional SEM photograph of powder 100 containing tungsten carbide, according to sample number 2. [Figure 7] Figure 7 shows the image analysis results of a cross-sectional SEM photograph of powder 100 containing tungsten carbide, according to sample number 3. [Figure 8] Figure 8 shows the results of image analysis of a cross-sectional SEM photograph of powder 100 containing tungsten carbide, according to sample number 4. [Figure 9] Figure 9 is a graph showing the relationship between particle size and frequency of powder 100 containing tungsten carbide, according to sample numbers 1 to 4. [Modes for carrying out the invention]

[0006] [Issues this disclosure aims to address] Conventional tungsten carbide powders had problems with particle size uniformity.

[0007] <Powder of the present disclosure> The present disclosure provides a powder comprising tungsten carbide having a uniform particle diameter. According to the present disclosure, in a cemented carbide mainly containing cobalt and a powder comprising tungsten carbide, the particle diameter of tungsten carbide in the alloy becomes uniform, and the particles can be densely arranged. Since the alloy has fine particles with reduced abnormal grains, a cemented carbide with high hardness and high strength, and high thermal conductivity due to densification can be produced.

[0008] Cemented carbide has high hardness and exhibits excellent wear resistance, so it has been used in cutting tools, jigs and other tools. In recent years, with the growth of the IT field, the miniaturization of workpieces has advanced, leading to miniaturization of tools themselves, and the dimensional accuracy of tools has become increasingly important. Powders containing fine-grained tungsten carbide are used for cemented carbide employed in high-precision tools. Although such alloys have high hardness, fine particles are prone to generate abnormal grains caused by Ostwald ripening during the sintering process, leading to a decrease in alloy strength.

[0009] The present disclosure solves the above problem by providing a powder comprising tungsten carbide with a uniform primary particle diameter.

[0010] In the step of mixing tungsten powder before carbonization and carbon powder, normally one mixing operation of 10 minutes is performed. In contrast, in the present disclosure, after mixing for 15 minutes, the mixed powder is recovered and reintroduced into the mixer. This process is repeated until the mixed powder is uniformly mixed, and repeating the process three or more times is desirable.

[0011] If there are regions where tungsten powder and carbon powder are not uniformly mixed, the degree of grain growth during carbonization will vary. For example, portions adhering to the wall or ceiling of the mixer during mixing have poor mixing uniformity.

[0012] Multiple mixing operations combined with recovery and reintroduction of the mixed powder reduces the influence of adhesion, and allows the powder to be uniformly mixed.

[0013] <Production Method> (1) Reduction Hydrogen reduction is carried out using tungsten trioxide as a raw material to obtain lower tungsten oxide. (2) Sieving The obtained lower tungsten oxide is sieved to remove coarse agglomerated particles, and the powder under the sieve and the powder on the sieve are collected. (3) Repeating reduction and sieving The above steps are repeated until agglomerated and coarse particles are removed and a powder containing tungsten with uniform particle size is obtained. (4) Mixing The tungsten-containing powder obtained as above and carbon powder having an average particle size of about 1.0 μm measured by the FSSS method are uniformly mixed. The mixing ratio by mass during mixing is set to the following proportion.

[0014] Mixing mass ratio: tungsten-containing powder : carbon powder = 93.8 : 6.2 Provided that it is not necessary to strictly adhere to this mixing ratio, and it is sufficient as long as the mixing ratio of the carbon powder is between 6.1 and 6.5.

[0015] It is not necessary to strictly adhere to the mass ratio during mixing, but if either powder is excessively insufficient, unreacted tungsten powder and carbon powder will remain, reducing the alloy quality. A mixer equipped with stirring blades is used for mixing. The mixing conditions are, for example, as follows.

[0016] Rotation speed of stirring blade: 500 rpm Rotation time: 15 minutes This 15-minute mixing is repeated 3 or more times, resulting in a total mixing time of 45 minutes or more. A homogeneous mixture can be obtained thereby. (5) Heat treatment (carbonization) The mixture is put into a carbon container and heat-treated at 1200°C to 2500°C in a hydrogen, nitrogen, argon or vacuum atmosphere More specifically, the carbonization process is carried out by filling a designated carbon container with the mixed powder and heat-treating it in a vacuum or hydrogen atmosphere at 1200°C to 2500°C for 30 to 480 minutes. The optimal temperature profile and heat treatment time must be selected depending on the particle size of the tungsten-containing powder. If the temperature profile and heating time are not appropriate, unreacted tungsten-containing powder or carbon powder may be generated, or abnormal grain growth may occur due to solid-phase sintering of the powders, degrading the quality of the alloy.

[0017] After carbonization, the powder particles are bonded together by heat, so they are pulverized using either the Nara method or an atomizer. Then, the powder is mixed using a double-cone mixer to ensure uniformity.

[0018] The content of unavoidable impurities in the powder is adjusted to be 10 ppm or less for each of aluminum, copper, magnesium, and manganese, and 20 ppm or less for each of calcium, silicon, and tin. In other words, the total content of the above unavoidable impurities is 100 ppm or less. As long as the unavoidable impurities are not of a size that would cause them to become foreign matter in the alloy structure, a sound cemented carbide can be obtained by keeping the content of unavoidable impurities within the above range. (6) Crushing By grinding the tungsten carbide in a heat-treated pulverizer and mixing it in a suitable mixer, a powder containing tungsten carbide with a uniform primary particle size was obtained.

[0019] <Evaluation Summary> Image analysis was used to analyze the uniform particle size. Powder containing resin-embedded tungsten carbide was cross-sectionally processed by ion milling, and the cross-section was observed using a scanning electron microscope (SEM).

[0020] Figures 1 to 4 are SEM images of cross-sections of powder 100 containing tungsten carbide, according to sample numbers 1 to 4.

[0021] The particle size of primary particles in the powder can be measured by image analysis of the photographs shown in Figures 1 to 4. Figures 5 to 8 show the image analysis results of SEM photographs of cross-sections of powder 100 containing tungsten carbide, according to sample numbers 1 to 4.

[0022] Figures 5 to 8 allow for the measurement of the particle size of the primary particles in the powder. The resin used is Clearpoxy 2 main component and Clearpoxy 2 hardener, manufactured by Sankei Co., Ltd. Furthermore, as a pretreatment before cross-sectional processing, the surface and cross-section are prepared using #600 and #2000 grit sandpaper.

[0023] For cross-sectional processing of the sample using ion milling, an IM4000II ion milling system (Hitachi High-Tech Corporation) was used. Argon ions were used for milling, and the acceleration voltage was set to 6kV.

[0024] For cross-sectional observation, a field emission scanning electron microscope (SEM) JSM7900F (JEOL) was used, with an acceleration voltage of 7.0kV and an observation magnification of 5000x. Image analysis of the SEM images was performed using ImageJ, with appropriate preprocessing applied. Afterward, the particles were analyzed using the Analyze→Particles function.

[0025] An example of preprocessing using ImageJ is shown below. • Process → Noise reduction using the Smooth function. • Binarization using Image → Adjust → Threshold Use the value calculated by the Auto function as the threshold. If the binarized image deviates too much from the original image, set an appropriate threshold yourself. The particle region is determined by Process → Binary → Watershed. To improve the accuracy of the analysis, more than 1000 particles were analyzed. The analysis results were displayed as particle cross-sectional area, and the Haywood diameter (equivalent diameter of the projected area circle) was used to convert them to particle size. The histogram classes used are those in Table 1. Note that the width from the upper limit to the lower limit of each class in Table 1 is non-uniform. However, when the upper and lower limits of each class i are expressed on a base-10 logarithmic scale, the difference between them is 0.157. That is, the following equation holds.

[0026] Log 10 (Upper limit of class i) - log 10 (Lower limit of rank i) = 0.157

[0027] [Table 1]

[0028] In the table above, for example, class 1 represents a size of 0 μm or more and less than 0.03000 μm. The tungsten carbide-containing powder of this disclosure has a mode of 24.0% or higher in the cross-sectional particle size distribution diagram created by analyzing the features of primary particles using image analysis in a cross-sectional SEM image of the tungsten carbide-containing powder.

[0029] In a powder containing tungsten carbide configured in this way, a large number of specific particle sizes are present. Therefore, when a cemented carbide is made using this powder, a cemented carbide with high strength and high thermal conductivity can be obtained.

[0030] Preferably, when the particle size distributions D10, D50, and D90 of the primary particles obtained by image analysis of a cross-sectional SEM image of a powder containing tungsten carbide are denoted as A, B, and C, the (CA) / B ≤ 1.4 is satisfied.

[0031] Preferably, the powder containing tungsten carbide has an average particle size of 0.3 to 60 μm as determined by the FSSS method.

[0032] Preferably, the content of carbon not bonded to tungsten is 0.30% by mass or less, and the value obtained by subtracting the amount of unbonded carbon from the total amount of carbon in the reactant is 5.8 to 6.3% by mass or less.

[0033] Preferably, the powder containing tungsten carbide has powder containing 1000 or more tungsten carbide particles.

[0034] Preferably, the powder containing tungsten carbide has a particle size distribution of 10 or more sections in the cross-sectional particle size distribution diagram.

[0035] The powder containing tungsten carbide according to this disclosure satisfies E / D < 0.9 when the features of the primary particles are analyzed by image analysis in a cross-sectional SEM image of the powder containing tungsten carbide, and the simple average of the particle diameters of the primary particles is D and the area-weighted standard deviation of the particle diameters is E.

[0036] In powders containing tungsten carbide, the coefficient of variation (E / D) is less than 0.9. This indicates that the powder contains tungsten carbide with a homogeneous particle size, resulting in improved strength after sintering.

[0037] Preferably, the powder containing tungsten carbide has an average particle size of 0.3 to 60 μm as determined by the FSSS method.

[0038] Preferably, the content of carbon not bonded to tungsten is 0.30% by mass or less, and the value obtained by subtracting the amount of unbonded carbon from the total amount of carbon in the reactant is 5.8 to 6.3% by mass or less. [Details of the embodiments of this disclosure] <Preparation of powder> Powders containing tungsten carbide were prepared using sample numbers 1, 3, 5, 6, 10, 12, and 13.

[0039] WO 2.9 Powder was used. The coarse powder was removed by sieving with a mesh size of 90-100 μm. The fine powder was removed by sieving with a mesh size of 40-50 μm (Step 1).

[0040] The powder was packed into a designated metal boat. At this time, the thickness of the powder layer was kept to 50 mm or less. Using a pusher-type reduction furnace, the reduction treatment was carried out under conditions of a hydrogen atmosphere and 640-650°C to obtain WO2 powder (Step 2).

[0041] The obtained WO2 powder was sieved using a sieve with a mesh size of 20-30 μm to remove coarse and agglomerated powder (Step 3). For example, classification can be performed using a classifier (Freund Turbo Turbo Screener). The apparatus is not limited to this as long as it can classify particles of 30 μm or less.

[0042] The sieved powder was further subjected to reduction treatment using a pusher-type reduction furnace under conditions of a hydrogen atmosphere, 800-820°C, and a layer thickness of 10 mm or less to obtain a powder containing tungsten (Step 4).

[0043] Tungsten-containing powder and carbon powder were mixed in a mass ratio of 93.8:6.2. The mixing was performed with a stirring blade rotation speed of 500 rpm for 15 minutes. This mixing was repeated three times. The mixed powder was filled into a designated carbon container and subjected to carbonization treatment by heat treatment in a vacuum atmosphere at 1200°C to 2500°C for 30 to 480 minutes. This yielded tungsten carbide-containing powder of sample number 1 (step 5).

[0044] Powders containing tungsten carbide were prepared from samples 2, 4, 7 through 9, 11, 14, and 15. In preparing these, WO was used as a raw material. 2.9Powder was used. The raw materials were packed into a designated container so that the layer thickness was 30 mm or less. A reduction treatment was carried out using a pusher-type reduction furnace under conditions of a hydrogen atmosphere and a reduction temperature of 800°C to 820°C to obtain a powder containing tungsten. The differences in the manufacturing methods between samples 1, 3, 5, 6, 10, 12 and 13 and samples 2, 4, 7 to 9, 11, 14 and 15 are that in the manufacturing methods of samples 1, 3, 5, 6, 10, 12 and 13 there is sieving in step 1, whereas in samples 2, 4, 7 to 9, 11, 14 and 15 there is no sieving; in the manufacturing methods of samples 1, 3, 5, 6, 10, 12 and 13 the reduction temperature is lower in step 2; and steps 3 to 5 are present only in the manufacturing methods of samples 1, 3, 5, 6, 10, 12 and 13.

[0045] <Image observation of powder> Image analysis is used to analyze the uniform particle composition. A powder containing resin-embedded tungsten carbide is cross-sectionally processed by ion milling. The particle size of the primary particles in the powder can be measured by SEM observation (Figures 1 to 4) and image analysis (Figures 5 to 8) of the cross-section. The resin used is Clearpoxy 2 main agent and Clearpoxy 2 curing agent, manufactured by Sankei Co., Ltd. As a pretreatment before cross-sectional processing, the surface and cross-section were prepared using #600 and #2000 grit abrasive paper. For cross-sectional processing of the sample by ion milling, an IM4000II ion milling system (Hitachi High-Tech Corporation) was used. Argon ions were used for milling, with an acceleration voltage of 6kV. For cross-sectional observation, a field emission scanning electron microscope (JEOL) JSM7900F was used, with an acceleration voltage of 7.0kV and an observation magnification of 5000x. ImageJ was used for image analysis of the SEM images. After appropriate pretreatment, the particles were analyzed using the Analyze → Particles function. Examples of appropriate preprocessing using ImageJ include the following. • Process → Noise reduction using the Smooth function. • Binarization using Image → Adjust → Threshold The Auto function calculates the threshold value for binarization. If the binarized image deviates too much from the original image, you need to manually set an appropriate threshold. The particle region is determined by Process → Binary → Watershed. To improve the accuracy of the analysis, more than 1000 particles were analyzed. The analysis results were displayed as particle cross-sectional area, and the Heywood diameter (equivalent to the projected area circle diameter) was used to convert them to particle size.

[0046] The particle size distribution for samples 1 to 4 is shown in Table 2 and Figure 9.

[0047] [Table 2]

[0048] The class strata were derived using Sturges' formula, which is as follows. Number of classes = 1 + log2(n): (n is the sample size) For sample numbers 1 to 4, the sample size n was set to 1000 or more.

[0049] In sample number 1, the minimum particle size was 0.03 μm (class 2) and the maximum particle size was 1.60 μm (class 12). The class width was determined by the following formula. { 10 (Maximum particle size) - log 10 (Minimum particle size) / 11 ≈ 0.157 Therefore, the class width for the logarithmically transformed particle size was set to 0.157 for samples 1 to 4.

[0050] In other words, in this disclosure, the class layers are derived using Sturges' equation, and the class width can be determined using the following formula, which utilizes the number of classes obtained by Sturges' equation, the maximum particle size in the sample, and the minimum particle size in the sample.

[0051] Class width = {log10(maximum particle diameter) - log10(minimum particle diameter)} / number of classes calculated using Sturges' formula By dividing the range from the largest to the smallest particle diameter into equal classes on a logarithmic graph with base 10, the upper and lower limits of the particle diameter in each class can be determined.

[0052] Table 2 and Figure 9 show that the tungsten carbide powder of sample number 1 had a higher proportion of particles of a specific size (greater than 0.379575335 μm and less than or equal to 0.545446277 μm) compared with the tungsten carbide powder of sample number 2.

[0053] Figure 9 shows that the mode of the particle size distribution for the uniformly sized tungsten carbide powder of sample number 1 is significantly larger than that of the tungsten carbide powder of sample number 2, exceeding 24%.

[0054] Furthermore, particle size uniformity was evaluated using particle size distributions D10, D50, and D90. Particle size uniformity was defined as the sharpness of the particle size distribution, with uniformity = (D90 - D10) / D50. The number of particles with a particle size of D10 or less accounts for 10% of the total number of particles. The number of particles with a particle size of D50 or less accounts for 50% of the total number of particles. The number of particles with a particle size of D90 or less accounts for 90% of the total number of particles.

[0055] [Table 3]

[0056] Table 3 shows that the uniform-grained WC samples 1 and 3 have a lower proportion of coarse grains compared to D50, indicating high uniformity.

[0057] Sample No. 1 was found to have a narrower particle size distribution than Sample No. 2. Its high uniformity suppresses Ostwald growth during sintering, thus preventing the generation of abnormally grown particles.

[0058] <Measurement of the coefficient of variation> Following the <Powder Image Observation> procedure described above, the simple average particle size was calculated for samples 5 through 15. The results are shown in Table 4.

[0059] [Table 4]

[0060] In Table 4, "Simple Average D" refers to the simple arithmetic mean of all particle sizes for each sample number.

[0061] The area-weighted standard deviation was calculated by applying area weighting to the determined particle size.

[0062]

number

[0063] E is the area-weighted standard deviation, xi is the particle size of each individual particle constituting each sample number, bi is the area of ​​each individual particle constituting each sample number, D is the simple average value of the particles constituting each sample number, and n is the number of particles constituting each sample number.

[0064] The area-weighted standard deviation represents the variability of the particles being analyzed. It can be normalized by dividing the area-weighted standard deviation by the simple mean value D of the particles. This normalized value is the coefficient of variation. A coefficient of variation less than 0.9 is defined as uniform particle WC.

[0065] <Results of the bending strength test> To measure the bending strength, a cemented carbide alloy was prepared by mixing tungsten carbide powder (sample number 1) with cobalt powder in a mass ratio of 90:10 and sintering the mixture. Similarly, a cemented carbide alloy was prepared by mixing tungsten carbide powder (sample number 2) with cobalt powder in a mass ratio of 90:10 and sintering the mixture. Ten samples were prepared from each cemented carbide alloy, and the bending strength of these ten samples was measured.

[0066] The bending strength measurement was performed in accordance with the Japan Machine Tool Builders Association standard TAS 0050:2017. TAS 0050:2017 (formerly CIS026B)

[0067] [Table 5]

[0068] Table 4 shows the bending strengths of 10 cemented carbide samples prepared from sample number 1, with N=1 to N=10 listed in descending order of bending strength. N=1 is not included in the table because it may be an outlier.

[0069] Table 4 shows the bending strengths of 10 cemented carbide samples prepared from sample number 2, with N=1 to N=10 representing the samples in increasing order of bending strength. N=1 may be an outlier and is therefore not included in the table.

[0070] Table 5 shows that the cemented carbide produced from sample number 1 had a greater bending strength compared to the cemented carbide produced from sample number 2.

[0071] <Thermal conductivity results> The thermal conductivity of cemented carbide alloys prepared from sample numbers 1 and 2 was determined. The results showed that the thermal conductivity of the cemented carbide alloy prepared from sample number 1 was 97.2 W / m·K, while the thermal conductivity of the cemented carbide alloy prepared from sample number 2 was 90.9 W / m·K. It was found that the thermal conductivity of the cemented carbide alloy prepared from sample number 1 was improved compared to the cemented carbide alloy prepared from sample number 2, due to the denser arrangement of the particles.

[0072] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the embodiments described above, and all modifications within the scope of the claims are intended to be included in the meaning of equivalents and within the scope. [Explanation of Symbols]

[0073] 100 Powder containing tungsten carbide.

Claims

1. A tungsten carbide-containing powder in which, in a cross-sectional SEM image of the powder, the mode of the cross-sectional particle size distribution diagram created based on the analysis of primary particle characteristics by image analysis is 24.0% or higher.

2. The tungsten carbide-containing powder according to claim 1, wherein when the particle size distributions D10, D50, and D90 of the primary particles obtained by image analysis of a cross-sectional SEM image of the tungsten carbide-containing powder are denoted as A, B, and C, the powder satisfies (C-A) / B ≤ 1.

4.

3. The tungsten carbide-containing powder according to claim 1 or 2, wherein the average particle size determined by the FSSS method is 0.3 to 60 μm.

4. A powder containing tungsten carbide according to claim 1 or 2, wherein the content of unbonded carbon is 0.30% by mass or less and the value obtained by subtracting the amount of unbonded carbon from the total amount of carbon in the reactant is 5.8 to 6.3% by mass or less.

5. The powder containing tungsten carbide according to claim 1 or 2, wherein the particle size is distributed into 10 or more sections in the cross-sectional particle size distribution diagram.

6. A tungsten carbide-containing powder in which, in a cross-sectional SEM image of the powder, the characteristics of the primary particles are analyzed by image analysis, and when the simple average of the particle diameters of the primary particles is D and the area-weighted standard deviation of the particle diameters is E, the powder satisfies E / D < 0.

9.

7. The tungsten carbide-containing powder according to claim 6, wherein the average particle size determined by the FSSS method is 0.3 to 60 μm.

8. A powder containing tungsten carbide according to claim 6 or 7, wherein the content of unbonded carbon is 0.30% by mass or less and the value obtained by subtracting the amount of unbonded carbon from the total amount of carbon in the reactant is 5.8 to 6.3% by mass or less.

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