WC hard alloy and preparation method thereof

Abstract

The invention provides a WC hard alloy and a preparation method thereof, and belongs to the technical field of hard alloys. Raw materials of the WC hard alloy comprise first WC powder, second WC powder, Co powder, Cr3C2 powder and VC powder. The particle size of the first WC powder is 1.8 to 2.2 microns; and the particle size of the second WC powder is 0.1-0.3 [mu] m. The inherent contradiction between hardness and toughness is effectively coordinated through the double-scale structural design, the WC hard alloy with high hardness and toughness is prepared, and the feasibility of preparing the hard alloy with excellent mechanical properties by regulating and controlling the proportion of coarse and fine WC grains is verified.

Description

Technical Field

[0001] The present invention relates to the technical field of cemented carbide, in particular to a WC cemented carbide and a preparation method thereof. Background Art

[0002] Cemented carbide has the advantages of high toughness, high strength, good red hardness and wear resistance. It is widely used in the preparation of cutting tools for difficult-to-process materials, wear-resistant and corrosion-resistant parts, and mining rock drilling tools. It is known as the "teeth of industry." In the modern industrial system, cemented carbide, as a type of strategically important advanced material, has become the core basic material supporting the development of high-end manufacturing due to its excellent comprehensive performance. With the rapid progress of industries such as aerospace, precision machining, and electronic information, unprecedented stringent requirements have been placed on the processing accuracy, service life, and adaptability to complex working conditions of parts. Traditional cemented carbide has a bottleneck in which hardness and toughness are difficult to balance: coarse-grained WC can improve toughness but reduce hardness, while fine-grained WC increases hardness but sacrifices toughness. With the high-end manufacturing industry's demand for materials With increasingly stringent performance requirements, traditional cemented carbide faces significant challenges when processing difficult-to-cut materials or complex-shaped parts. To overcome this limitation, dual-scale structural cemented carbide (i.e., the coordinated distribution of coarse and fine WC grains) has become a research hotspot. By regulating the ratio of dual-scale WC, the mechanical properties of the alloy can be optimized and the "double high" (high hardness and high toughness) characteristics can be achieved, which is of great significance to promoting technological innovation in high-end manufacturing. By introducing the collaborative design of coarse and fine grain WC, twin-crystal cemented carbide has successfully achieved a "double high" breakthrough in high hardness and high toughness, providing a new approach to solving the above-mentioned industry pain points. However, current research on twin-crystal cemented carbide is still in the exploratory stage, and key scientific issues such as its grain size regulation mechanism and microstructure-performance mapping relationship have not yet been clarified. Summary of the Invention

[0003] The object of the present invention is to provide a WC cemented carbide and a preparation method thereof, so as to solve the problem in the prior art that it is difficult to achieve a balance between hardness and toughness of cemented carbide.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0005] The present invention provides a WC cemented carbide, wherein the raw materials of the WC cemented carbide include a first WC powder, a second WC powder, Co powder, Cr3C2 powder and VC powder; the particle size of the first WC powder is 1.8 to 2.2 μm; the particle size of the second WC powder is 0.1 to 0.3 μm.

[0006] Preferably, the contents of the raw materials are as follows, in percentage by mass: 39.2-59.2% of the first WC powder, 30-50% of the second WC powder, 10% of the Co powder, 0.6% of the Cr3C2 powder, and 0.2% of the VC powder.

[0007] The present invention also provides a method for preparing the above-mentioned WC cemented carbide, comprising the following steps:

[0008] (1) mixing the above raw materials, ball milling, sieving, drying and grinding to obtain a mixed powder;

[0009] (2) The mixed powder is pressed into a green compact and then pre-sintered and sintered in sequence to obtain WC cemented carbide.

[0010] Preferably, in step (1), polyethylene glycol and alcohol are added before the ball milling treatment.

[0011] Preferably, in step (1), the ball-to-material ratio of the ball milling treatment is 8 to 12:1, the rotation speed is 200 to 400 rpm, and the time is 25 to 35 hours.

[0012] Preferably, in step (1), the drying temperature is 60-100°C.

[0013] Preferably, in step (2), the pressure of the green compact is 3-20 MPa and the time is 1-2 min.

[0014] Preferably, in step (2), the specific process of the pre-sintering is:

[0015] In the first stage, the temperature was raised from 20°C to 200°C at a rate of 5-8°C / min and kept at 200°C for 60 min;

[0016] The second stage is heating from 200°C to 400°C at a heating rate of 5-8°C / min and keeping at 400°C for 60 min;

[0017] The third stage is heating from 400°C to 700°C at a heating rate of 8-12°C / min and keeping at 700°C for 60 min;

[0018] The fourth stage is to cool down from 700℃ to 20℃, with a cooling rate of 2-4℃ / min.

[0019] Preferably, in step (2), the specific process of sintering is:

[0020] In the first stage, the temperature was raised from 20°C to 300°C at a rate of 8-12°C / min and kept at 300°C for 60 min;

[0021] The second stage is heating from 300°C to 600°C at a heating rate of 8-12°C / min and keeping at 600°C for 60 min;

[0022] The third stage is heating from 600°C to 900°C at a heating rate of 8-12°C / min and keeping at 900°C for 60 min;

[0023] The fourth stage is heating from 900°C to 1200°C at a heating rate of 8-12°C / min and keeping at 1200°C for 60 min;

[0024] The fifth stage is heating from 1200°C to 1400°C at a heating rate of 5-8°C / min and keeping at 1400°C for 60 min;

[0025] The sixth stage is to cool down from 1400℃ to 20℃, with a cooling rate of 2-4℃ / min.

[0026] Beneficial effects of the present invention:

[0027] The present invention prepares WC cemented carbide by adjusting the proportion of fine-grained WC powder, thereby exerting the synergistic effect of coarse and fine WC, thereby improving both hardness and toughness.

[0028] The WC cemented carbide prepared by the present invention has a low average friction coefficient, indicating that it has high hardness and high toughness while having good wear resistance.

[0029] The present invention effectively coordinates the inherent contradiction between hardness and toughness through a dual-scale structural design, and prepares WC cemented carbide with both high hardness and toughness, verifying the feasibility of preparing cemented carbide with excellent mechanical properties by regulating the ratio of coarse and fine WC grains. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Density curves of WC cemented carbides prepared in Examples 1 to 3 and Comparative Example 1;

[0031] Figure 2 The line graphs are of the density and porosity of the WC cemented carbides prepared in Examples 1 to 3 and Comparative Example 1;

[0032] Figure 3 These are indentation images of WC cemented carbides prepared in Examples 1 to 3 and Comparative Example 1, where A corresponds to Comparative Example 1, B corresponds to Example 1, C corresponds to Example 2, and D corresponds to Example 3;

[0033] Figure 4 is a dot-line graph showing the Vickers hardness and toughness of the WC cemented carbides prepared in Examples 1 to 3 and Comparative Example 1;

[0034] Figure 5 The friction coefficient line graph of WC cemented carbide prepared in Examples 1 to 3 and Comparative Example 1;

[0035] Figure 6The SEM images of the WC cemented carbide samples prepared in Examples 1 to 3 and Comparative Example 1 are magnified 5000 times, wherein A corresponds to Comparative Example 1, B corresponds to Example 1, C corresponds to Example 2, and D corresponds to Example 3;

[0036] Figure 7 The SEM images of the WC cemented carbide samples prepared in Examples 1 to 3 and Comparative Example 1 are magnified 10,000 times, wherein A corresponds to Comparative Example 1, B corresponds to Example 1, C corresponds to Example 2, and D corresponds to Example 3;

[0037] Figure 8 The SEM images of the WC cemented carbide samples prepared in Examples 1 to 3 and Comparative Example 1 are magnified 20,000 times, wherein A corresponds to Comparative Example 1, B corresponds to Example 1, C corresponds to Example 2, and D corresponds to Example 3;

[0038] Figure 9 Graph showing the grain size distribution of WC cemented carbides prepared in Examples 1 to 3 and Comparative Example 1, wherein A corresponds to Comparative Example 1, B corresponds to Example 1, C corresponds to Example 2, and D corresponds to Example 3. DETAILED DESCRIPTION

[0039] The present invention provides a WC cemented carbide, wherein the raw materials of the WC cemented carbide include a first WC powder, a second WC powder, Co powder, Cr3C2 powder and VC powder; the particle size of the first WC powder is 1.8 to 2.2 μm; the particle size of the second WC powder is 0.1 to 0.3 μm.

[0040] In the present invention, the particle size of the first WC powder is preferably 1.9 to 2.1 μm, more preferably 2.0 μm.

[0041] In the present invention, the particle size of the second WC powder is preferably 0.2 μm.

[0042] In the present invention, the contents of the raw materials are as follows, by mass percentage: first WC powder 39.2-59.2%, second WC powder 30-50%, Co powder 10%, Cr3C2 powder 0.6%, VC powder 0.2%.

[0043] In the present invention, the content of each raw material is preferably, by mass percentage, 59.2% of the first WC powder, 30% of the second WC powder, 10% of Co powder, 0.6% of Cr3C2 powder, and 0.2% of VC powder; or 49.2% of the first WC powder, 40% of the second WC powder, 10% of Co powder, 0.6% of Cr3C2 powder, and 0.2% of VC powder; or 39.2% of the first WC powder, 50% of the second WC powder, 10% of Co powder, 0.6% of Cr3C2 powder, and 0.2% of VC powder.

[0044] In the present invention, the particle size of the Co powder is 1 μm; the particle size of the Cr 3 C 2 powder is 0.8 μm; and the particle size of the VC powder is 1.2 μm.

[0045] In the present invention, Cr3C2 powder and VC powder serve as inhibitors.

[0046] The present invention also provides a method for preparing the above-mentioned WC cemented carbide, comprising the following steps:

[0047] (1) mixing the above raw materials, ball milling, sieving, drying and grinding to obtain a mixed powder;

[0048] (2) The mixed powder is pressed into a green compact and then pre-sintered and sintered in sequence to obtain WC cemented carbide.

[0049] In the present invention, in step (1), polyethylene glycol and alcohol are added before the ball milling treatment.

[0050] In the present invention, the amount of polyethylene glycol added is 1-3% of the mass of all raw materials, preferably 2%; the amount of alcohol added is sufficient to cover all the materials in the ball mill.

[0051] In the present invention, in step (1), the ball-to-material ratio of the ball milling treatment is 8 to 12:1, preferably 9 to 10:1; the rotation speed is 200 to 400 rpm, preferably 250 to 350 rpm, and more preferably 300 rpm; and the time is 25 to 35 h, preferably 28 to 32 h, and more preferably 30 h.

[0052] In the present invention, in step (1), the drying temperature is 60 to 100°C, preferably 70 to 90°C, and more preferably 80°C.

[0053] In the present invention, in step (2), the pressure of the green compaction molding is 3 to 20 MPa, preferably 3 MPa or 20 MPa, and the time is 1 to 2 minutes.

[0054] In the present invention, in step (2), the specific process of the pre-sintering is:

[0055] In the first stage, the temperature was raised from 20°C to 200°C at a rate of 5-8°C / min and kept at 200°C for 60 min;

[0056] The second stage is heating from 200°C to 400°C at a heating rate of 5-8°C / min and keeping at 400°C for 60 min;

[0057] The third stage is heating from 400°C to 700°C at a heating rate of 8-12°C / min and keeping at 700°C for 60 min;

[0058] The fourth stage is to cool down from 700℃ to 20℃, with a cooling rate of 2-4℃ / min.

[0059] In the present invention, the temperature in the first stage of pre-sintering is raised from 20° C. to 200° C., so that the gas adsorbed on the surface of the powder particles during compaction is continuously released because the heat causes the gas to separate from the surface of the particles.

[0060] In the present invention, in step (2), the specific process of sintering is:

[0061] In the first stage, the temperature was raised from 20°C to 300°C at a rate of 8-12°C / min and kept at 300°C for 60 min;

[0062] The second stage is heating from 300°C to 600°C at a heating rate of 8-12°C / min and keeping at 600°C for 60 min;

[0063] The third stage is heating from 600°C to 900°C at a heating rate of 8-12°C / min and keeping at 900°C for 60 min;

[0064] The fourth stage is heating from 900°C to 1200°C at a heating rate of 8-12°C / min and keeping at 1200°C for 60 min;

[0065] The fifth stage is heating from 1200°C to 1400°C at a heating rate of 5-8°C / min and keeping at 1400°C for 60 min;

[0066] The sixth stage is to cool down from 1400℃ to 20℃, with a cooling rate of 2-4℃ / min.

[0067] In this invention, the sintering process is a critical stage in the densification of cemented carbide. Liquid-phase sintering allows the Co phase to flow and diffuse evenly throughout the hard phase, WC, ultimately imparting toughness to the sample. The melting point of Co is 1380°C, and the maximum sintering temperature is 1400°C to ensure sufficient sintering. During the heating process, a period of dwell time is maintained at each stage.

[0068] Raw material information in the examples and comparative examples of the present invention:

[0069] First WC powder: purity 99.95%, supplier Xiamen Jinlu;

[0070] The second WC powder: purity 99.90%, supplier Shanghai Shuitian;

[0071] Co powder: purity 99.90%, supplier Shanghai Shuitian;

[0072] Cr3C2 powder: purity 99.90%, supplier Shanghai Shuitian;

[0073] VC powder: purity 99.90%, supplier Shanghai Shuitian.

[0074] Instrument and equipment information:

[0075] Electronic balance, model FA2104J;

[0076] All-round planetary ball mill, model QM-QX-4;

[0077] Hot air circulation oven, model HBL240A;

[0078] Electric powder tablet press, model DY-30;

[0079] Tubular atmosphere furnace, model ZSK-7-12;

[0080] High vacuum and high temperature brazing furnace, model TYQH-10;

[0081] Ultrasonic cleaning machine, model JP-010T;

[0082] Metallographic sample polishing machine, model MP-2;

[0083] Metallographic specimen mounting machine, model XQ-2B;

[0084] Digital Vickers hardness tester, model HVS-30Z;

[0085] Inverted metallographic microscope, model MDS400;

[0086] Scanning electron microscope, model Hitachi SU-70;

[0087] Material surface properties comprehensive tester, model CFT-I.

[0088] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0089] Example 1

[0090] Weigh 32.56g of WC powder with a particle size of 2μm, 16.5g of WC powder with a particle size of 0.2μm, 5.5g of Co powder with a particle size of 1μm, 0.33g of Cr3C2 powder with a particle size of 1.2μm, and 0.11g of VC powder with a particle size of 0.8μm. The total mass of the raw materials is 55g, of which the mass content of WC powder with a particle size of 2μm is 59.2%, the mass content of WC powder with a particle size of 0.2μm is 30%, the content of Co powder is 10%, the content of Cr3C2 powder is 0.6%, and the content of VC powder is 0.2%.

[0091] Grinding balls were added at a ball-to-material ratio of 10:1. Polyethylene glycol (2% by weight) was melted and poured into the mill. Alcohol was then added to cover the balls. The milling process was performed at a speed of 300 rpm for 30 hours. After the milling process, the balls were separated using a sieve. The inner wall of the mill and the grinding balls were repeatedly cleaned with anhydrous ethanol. The milled material was then dried in a hot air circulating oven at 80°C.

[0092] Then weigh 30g and 3g (three portions) of the above-mentioned dried raw materials respectively, and press them into a large sample with a size of Φ30 (mm) and three small samples with a size of Φ10 (mm), respectively. Place the above-mentioned weighed powders in a mold respectively, and then press them with an electric powder tablet press. The large sample of Φ30 (mm) is pressed with a pressure of 20MPa, and the small sample of Φ10 (mm) is pressed with a pressure of 3MPa. Maintain the pressure for 2 minutes and demold to obtain the sample.

[0093] The above sample was placed in a graphite crucible, a layer of quartz sand was spread on the sample, and then transferred to a tubular atmosphere furnace and evacuated. Heating was carried out only after the vacuum degree reached 1.0E-1MPa. The temperature was increased from 20°C to 200°C at a heating rate of 6°C / min, and kept at 200°C for 60 minutes. Then the temperature was increased from 200°C to 400°C at a heating rate of 6.67°C / min, and kept at 400°C for 60 minutes. Then the temperature was increased from 400°C to 700°C at a heating rate of 10°C / min, and finally the temperature was reduced to 20°C at a cooling rate of 3.78°C / min. After pre-sintering, the samples were transferred to a high vacuum and high temperature brazing furnace for sintering. The specific sintering process was as follows: heating from 20°C to 300°C at a heating rate of 9.33°C / min, keeping at 300°C for 60 min, then heating from 300°C to 600°C at a heating rate of 10°C / min, keeping at 600°C for 60 min, then heating from 600°C to 900°C at a heating rate of 10°C / min, keeping at 900°C for 60 min, then heating from 900°C to 1200°C at a heating rate of 10°C / min, keeping at 1200°C for 60 min, heating from 1200°C to 1400°C at a heating rate of 6.67°C / min, keeping at 1400°C for 60 min, and finally cooling to 20°C at a cooling rate of 2.3°C / min to obtain WC cemented carbide, which was marked as group B.

[0094] Example 2

[0095] The difference from Example 1 is that the mass of WC powder with a particle size of 2 μm is 27.06 g, the mass of WC powder with a particle size of 0.2 μm is 22 g, the total mass of the raw materials is 55 g, the mass content of WC powder with a particle size of 2 μm is 49.2%, and the mass content of WC powder with a particle size of 0.2 μm is 40%. Other conditions are the same, and WC cemented carbide is obtained, which is marked as Group C.

[0096] Example 3

[0097] The difference from Example 1 is that the mass of WC powder with a particle size of 2 μm is 21.56 g, the mass of WC powder with a particle size of 0.2 μm is 27.5 g, the total mass of the raw materials is 55 g, the mass content of WC powder with a particle size of 2 μm is 39.2%, and the mass content of WC powder with a particle size of 0.2 μm is 50%. Other conditions are the same, and WC cemented carbide is obtained, which is marked as group D.

[0098] Comparative Example 1

[0099] The difference from Example 1 is that the mass of WC powder with a particle size of 2 μm is 49.06 g, and WC powder with a particle size of 0.2 μm is not added. The total mass of the raw materials is 55 g, of which the mass content of WC powder with a particle size of 2 μm is 89.2%. Other conditions are the same, and WC cemented carbide is obtained, which is marked as Group A.

[0100] Before the WC cemented carbide samples prepared in Examples 1 to 3 and Comparative Example 1 were subjected to hardness, friction and wear tests and SEM image capture, the samples needed to be pre-treated. The pre-treatment process of the samples was as follows:

[0101] The first step is to mount the specimen: Use a metallographic specimen mounting machine to mount the specimen. First, remove the upper die from the mounting machine and clean both the upper die and the mounting machine with alcohol and a dust-free cloth. Turn the handwheel to parallel the lower die to the platform, then place the specimen in the center of the lower die. Turn the handwheel to lower the lower die to a certain height, then add 5 spoonfuls of mounting powder. Then, place the upper die in and allow it to fall naturally. Make sure the upper die is lower than the platform. Then, tighten the lid and adjust the time. Once the temperature reaches 140°C, turn the handwheel to press the specimen firmly. Keep the temperature for 15 minutes. After mounting, wait for the mold to cool before removing the specimen.

[0102] The second step is grinding and polishing: Use a metallographic specimen grinding and polishing machine to grind and polish the specimen. During the grinding and polishing process, water needs to be continuously used for cooling. First, use a 1500-mesh diamond grinding wheel to grind the specimen until the surface of the specimen and the scratches on the surface are evenly ground off; then change to a 3000-mesh diamond grinding wheel to grind the specimen until the scratches on the surface of the specimen are invisible to the naked eye; then use a 0.25μm polishing agent to polish the specimen until the scratches are invisible under a microscope, and the polishing is completed.

[0103] Performance testing:

[0104] 1. Density test

[0105] 1.1 Actual density

[0106] The density of cemented carbide samples is measured based on the Archimedes displacement principle, which states that the buoyancy of a sample in water is equal to the weight of the same volume of water it displaces. One sample is selected from each group for density testing. The specific steps are as follows:

[0107] (1) Cleaning and drying the sample: Before weighing, place the sample in a beaker and submerge it in alcohol for ultrasonic cleaning. After cleaning, place it in an electric constant temperature blast drying oven to dry thoroughly.

[0108] (2) Weighing dry weight: Use an electronic balance to weigh the weight of the sample in air (M1). Weigh each sample five times and take the average value.

[0109] (3) Weighing the floating weight: Install a long strip, a small tray, a beaker filled with distilled water and a hanging net on the electronic balance, and weigh the weight of the sample in water (M2). Weigh each sample five times and take the average value.

[0110] (4) Calculate the actual density of the cemented carbide according to the solid density formula (1).

[0111]

[0112] In the formula:

[0113] ρ 实 is the actual density of the sample (g / cm 3 );

[0114] ρ 水 The density of the sample immersed in pure water at room temperature (take 1.0g / cm at room temperature) 3 );

[0115] M1 is the mass of the sample in air (g);

[0116] M2 is the mass of the sample immersed in pure water (g);

[0117] The actual density of the cemented carbide specimens is shown in Table 1:

[0118] Table 1 Actual density of cemented carbide

[0119]

[0120] 1.2 Theoretical density

[0121] Theoretical density refers to the density of the solid phase in a porous material, that is, the density of the same material in a non-porous state, or the density of a pure substance. The theoretical density of cemented carbide is the density of the hard alloy when it is not affected by other substances, such as pores and impurities. The theoretical density of a mixture is calculated based on the mass percentage of each component and the theoretical density. Theoretical density can be calculated using formula (2).

[0122]

[0123] In the formula:

[0124] ρ is the theoretical density of the sample (g / cm 3 );

[0125] p is the theoretical density of each component in the sample (g / cm 3 );

[0126] n is the content of each component in the sample (%);

[0127] The theoretical density of each component of the raw material powder is shown in Table 2.

[0128] Table 2 Theoretical density of each component

[0129]

[0130] According to the calculation formula of theoretical density in formula (2) and the theoretical density of each component in Table 2, the theoretical density of the sample can be calculated as: 14.372 g / cm 3 The variables of this experiment are the different ratios of coarse and fine WC. The total ratio of coarse and fine WC is 89.2%, so the theoretical density of the sample is only one.

[0131] 1.3 Relative density

[0132] Relative density refers to the ratio of a substance's actual density to its theoretical density and is one of the important indicators for measuring alloy performance. The relative density of cemented carbide is the ratio of its actual density to its theoretical density and can be calculated using formula (3).

[0133]

[0134] In the formula:

[0135] ρ 相 Indicates the relative density of the sample (%);

[0136] ρ 实 Indicates the actual density of the sample (g / cm 3 );

[0137] ρ 理 Indicates the theoretical density of the sample (g / cm 3);

[0138] The calculated results of the relative density of each sample are shown in Table 3.

[0139] Table 3 Relative density of cemented carbide

[0140]

[0141] Figure 1 It is a line graph of actual density, theoretical density and relative density. Figure 1 It can be seen that the actual density of the four WC cemented carbides is smaller than the theoretical density, that is, the relative density does not reach 100%. This is because in actual conditions, the gaps between the grains of the powder are not completely filled during the compaction process, the forming agent (PEG) will evaporate during the sintering process, and there is contamination and impurities in the experimental operation process, which leads to the formation of pores and the theoretical density cannot be reached.

[0142] Group A is all coarse-grained WC. Although the gaps between coarse-grained particles are large, the overall distribution is uniform. The uniform distribution of coarse grains and the smaller number of grain boundaries are conducive to the sufficient flow of the liquid phase during sintering, thereby obtaining a higher density. Group B begins to add fine-grained WC, and the fine-grained ratio is low. Local uneven particles are prone to occur during mixing. During sintering, local fine-grained particles preferentially adsorb the liquid phase, hindering the flow of the liquid phase and inhibiting the densification efficiency during sintering, resulting in a lower density than Group A. The fine-grained WC ratio in Group C further increases, and the synergistic effect of coarse and fine particles gradually emerges, namely the promoting effect of coarse-grained particles and the filling effect of fine-grained particles. The number of grain boundaries of coarse-grained particles is small, which promotes the flow of liquid phase to fill pores during sintering. Fine-grained particles can be embedded in the coarse-grained gaps, reducing the pore volume and increasing the density. The density is higher than that of Group B. The fine-grained ratio in Group D is too high, and the larger number of grain boundaries makes it difficult for the liquid phase to flow evenly during sintering, hindering the grain rearrangement in the subsequent densification stage. It may also be due to process pollution caused by a large number of impurities, resulting in a significant drop in density.

[0143] 2. Porosity and density analysis

[0144] Porosity refers to the percentage of the volume of pores in cemented carbide materials in their natural state to the total volume, while density refers to the percentage of the volume of pores excluding the total volume. Density of cemented carbide is an important physical property of the material. Density directly affects the mechanical properties, corrosion resistance and service life of cemented carbide, and is a key parameter for evaluating its process quality. The steps for measuring porosity and density are as follows:

[0145] When measuring the actual density of the sample, the dry weight (M1) and floating weight (M2) of the sample have been weighed, which has made preliminary preparations for the porosity and density test. Next, prepare four sets of beakers, wipe them clean, and put four sets of samples in them. Pour distilled water into them to cover the samples and soak them for 48 hours. After soaking, take out the samples, wipe off the surface moisture, and weigh the wet weight (M3) of the samples on an electronic balance.

[0146] The calculation formula of porosity is as follows (4):

[0147]

[0148] The calculation formula of density is as follows (5):

[0149]

[0150] Where:

[0151] P represents the porosity of cemented carbide (%);

[0152] M1 represents the mass of cemented carbide in air, in g;

[0153] M2 represents the mass of cemented carbide in water, unit is g;

[0154] M3 represents the mass of cemented carbide after being immersed in distilled water for 48 hours, in g;

[0155] The calculation results are shown in Table 4.

[0156] Table 4 Porosity and density of cemented carbide

[0157] serial number A B C D Porosity (%) 0.797 0.047 0.243 0.328 Density (%) 99.203 99.953 99.757 99.672

[0158] Figure 2 It is a line graph of density and porosity of WC cemented carbide. Figure 6 It can be seen that the density of the samples shows a trend of first increasing and then decreasing. The density of group A (0% fine-grained WC powder) is the lowest, indicating that the pores between the coarse WC particles are relatively large, and the particle arrangement structure is relatively loose. The density of groups B, C, and D has been greatly improved with the addition of fine-grained WC. The density of group B (30% fine crystals) is the best, indicating that the distribution structure of coarse and fine-grained WC particles in this ratio is tight, and the fine crystals effectively fill the gaps between the coarse crystals, reducing the generation of pores. The proportion of fine crystals in group C (40% fine crystals) and group D (50% fine crystals) is further increased, and the density shows a downward trend, indicating that too high a proportion of fine-grained WC may lead to uneven particle distribution, local agglomeration, hinder liquid phase diffusion, and lead to reduced density. The density of the four groups of samples is above 99%, indicating that the samples are worth testing.

[0159] 3. Volume shrinkage

[0160] During the sintering process, the forming agent (PEG) will evaporate, and the liquid phase will flow to fill the gaps in the hard phase, reducing the porosity of the sample and increasing its density, resulting in volume shrinkage. To calculate the volume shrinkage rate, it is necessary to measure the volume of the sample before sintering after the sample is pressed and then measure and calculate the volume after sintering. The shrinkage rate can be calculated using formula (6):

[0161]

[0162] Where:

[0163] η represents the shrinkage of cemented carbide (%);

[0164] R 前 Indicates the mass or volume of cemented carbide before sintering, in g or mm 3 ;

[0165] R 后 Indicates the mass or volume of cemented carbide before and after sintering, in g or mm 3 .

[0166] The volume shrinkage of each group of samples is shown in Table 5:

[0167] Table 5 Volume shrinkage of cemented carbide

[0168]

[0169] As can be seen from Table 5, the volume shrinkage of the samples is between 17.2% and 19.9%. The volume shrinkage of the samples is mainly affected by ball milling and sintering. Ball milling will gradually refine and homogenize the raw material powder, and sintering will densify the sample. Therefore, the sample will shrink in volume after sintering.

[0170] 4. Vickers hardness and toughness test

[0171] 4.1 Hardness test

[0172] The specific operation is as follows: First, clean the sample with an ultrasonic cleaner, and wait for the sample to dry after cleaning. Place the sample on an automatic turret digital display Vickers hardness tester for point measurement. The test force is 30 kgf and the pressure is maintained for 15 seconds. After the point measurement is completed, the clarity can be adjusted by controlling the height of the workbench. Take one sample from each group and measure the hardness at 5 points. The hardness value can be calculated by observing and measuring the length of the two diagonals of the indentation under a metallographic microscope. Finally, the hardness value is averaged. The calculation of Vickers hardness is shown in formula (7):

[0173]

[0174] In the formula:

[0175] HV represents the Vickers hardness of cemented carbide (kgf / mm 2 );

[0176] P represents the applied load (unit: kg, 30 kg was used in this experiment); d represents the average diagonal length of the indentation (mm);

[0177] The average Vickers hardness of the cemented carbide specimens is shown in Table 6.

[0178] Table 6 Vickers hardness test results of cemented carbide

[0179]

[0180] 4.2 Toughness calculation

[0181] The toughness of cemented carbide is an inherent mechanical property index of the material, which reflects the ability of cemented carbide to resist brittle fracture in the presence of cracks. The toughness is related to the composition, internal structure, grain size and distribution of the cemented carbide. Under the condition of the same liquid phase and unchanged sintering process, the fracture toughness of cemented carbide will increase with the increase of WC particle size. The experiment uses the indentation method to measure the toughness of the sample. The toughness measurement is carried out simultaneously with the hardness measurement, that is, when the diagonal of the indentation is measured with a metallographic microscope, the crack lengths L1, L2, L3 and L4 at the four corners of the indentation are measured at the same time. The cemented carbide indentation is as follows: Figure 3 shown.

[0182] Calculate toughness according to formula (8):

[0183]

[0184] Where:

[0185] K IC Indicates the toughness of cemented carbide, unit: MPa·m 1 / 2 ;

[0186] HV represents the Vickers hardness of the hardness alloy, unit is kg / mm 2 ;

[0187] L represents the total length of the crack, that is, L=L1+L2+L3+L4, in mm.

[0188] The calculation results of toughness of cemented carbide specimens are shown in Table 7:

[0189] Table 7 Cemented carbide toughness test results

[0190]

[0191]

[0192] Figure 4The Vickers hardness and fracture toughness are analyzed by Figure 4 It can be seen that with the increase of the proportion of fine-grained WC, the hardness of the sample first increases and then decreases. The hardness of group A (0% fine-grained WC) is the lowest, and the hardness of group C (40% fine-grained WC) is the highest, reaching 1806.783 kgf / mm 2 This is because fine-grained WC hinders dislocation movement by increasing the grain boundary area, significantly improving the hardness. Therefore, the addition of fine-grained WC can significantly increase the hardness of the sample, but too much fine-grained WC will lead to a decrease in hardness.

[0193] Toughness analysis: Coarse-grained WC delays crack propagation by crack bridging and deflection, thereby improving toughness. Figure 3 It can be seen that the toughness of the samples shows a trend of decrease-increase-decrease. The fracture toughness of group A (0% fine-grained WC) is higher, but the lack of fine-grained support has limited crack propagation resistance. The fracture toughness of group C (40% fine-grained WC) is the highest, reaching 11.2218 MPa·m 1 / 2 , indicating that the distribution of coarse grains and fine grains reaches a better ratio, crack propagation needs to bypass the coarse-fine grain interface, energy dissipation increases, and thus toughness is improved.

[0194] Comprehensive results of the hardness and toughness of cemented carbide showed that group C (40% fine-grained WC) achieved a balance between hardness and toughness, showing mechanical properties of high hardness and toughness, verifying the effectiveness of the dual-scale structural design.

[0195] 5. Friction loss test

[0196] 5.1 Friction loss

[0197] The friction and wear test used the scratch method. The CFT-I type material surface performance comprehensive tester was used to perform reciprocating friction on the surface of the polished cemented carbide sample with a load of 20N, a reciprocating speed of 500rpm, a reciprocating length of 5mm, and a time of 30min. The specific experimental steps are as follows:

[0198] (1) Turn on the computer to enter the instrument program interface, open the control box and preheat for 15 minutes.

[0199] (2) Install the experimental components and specimens, and select a spring replacement that can cover the load required for the test (the load required for this experiment is 20N); when clamping the steel ball, choose the side with a smooth surface; select a suitable fixture to install the specimen, and use four screws to fix the specimen on the platform.

[0200] (3) Parameter setting: Select the reciprocating friction mode in the program, load 20N, speed 500rpm, reciprocating distance 5mm, and time 30min.

[0201] (4) Adjust the rotary lifting knob to move the steel ball to a position about 2-3 mm above the sample, move the slider to observe the friction track, and ensure that the friction track is in the sample.

[0202] (5) No-load zeroing: Adjust the zeroing knob on the control box panel and observe the program to make the values ​​of load 1, load 2, friction 1, and friction 2 equal to 0.

[0203] (6) Click the start button of the program to start the friction and wear test.

[0204] 5.2 Friction coefficient analysis

[0205] The friction coefficient is usually a measure of the wear resistance of a material. Generally speaking, the better the wear resistance of an alloy, the smaller the friction coefficient. There are many factors that affect the friction and wear performance of cemented carbide, such as sample hardness, density, WC grain size and distribution, and surface microstructure. The average friction coefficient of each group of samples is shown in Table 8:

[0206] Table 8 Average friction coefficient of cemented carbide

[0207] serial number A B C D Average friction coefficient 0.710 0.661 0.449 0.498

[0208] Figure 5 The friction coefficient of cemented carbide is shown in Table 8. Figure 5 It can be seen that as the proportion of fine grains increases, the average friction coefficient of the sample first decreases and then increases. The average friction coefficient of the sample has an opposite trend to the change of the sample hardness, that is, the higher the hardness of the sample, the lower its friction coefficient. This is because generally high-hardness materials have better resistance to deformation, thereby reducing the friction coefficient, but high-hardness and brittle materials will also increase the friction coefficient. The average friction coefficient of group C (40% fine-grained WC) is the lowest, that is, its wear resistance is relatively the best. Compared with the average friction coefficient of the control group A (0% fine-grained WC), it is reduced by 37%. Its characteristics of both high hardness and high toughness can effectively improve its wear resistance. Therefore, the appropriate addition of fine grains to dual-scale cemented carbide can effectively improve its wear resistance, but too many fine grains will lead to a decrease in wear resistance.

[0209] 6. Microstructure Observation

[0210] 6.1SEM surface morphology analysis

[0211] The samples were ultrasonically cleaned before taking SEM images, and then stored in sealed bags after being fully dried. SEM electron microscope images of four groups of cemented carbide samples were taken at magnifications of 5000 times, 10000 times, and 20000 times to observe their micromorphology. The results are as follows: Figure 6 、 Figure 7 and Figure 8 shown.

[0212] Depend on Figure 6 It can be seen that the gaps between the grains of group A (0% fine-grained WC) are significantly more and larger than those of group B (30% fine-grained WC), group C (40% fine-grained WC), and group D (50% fine-grained WC). Therefore, the density of group A (0% fine-grained WC) is lower than that of the other three groups. Figure 7 and Figure 8 It can be seen that: Group B (30% fine-grained WC), Group C (40% fine-grained WC), and Group D (50% fine-grained WC) clearly show both coarse and fine WC grains. The gaps between the coarse WC grains can be well filled by fine-grained WC, thereby improving the density. The two WC grain sizes of Group B (30% fine-grained WC) and Group C (40% fine-grained WC) are distributed relatively evenly, and Group B (30% fine-grained WC) has the best liquid phase flow filling effect. Group D (50% fine-grained WC) has an excessive proportion of fine grains, resulting in local concentration. Therefore, Group B (30% fine-grained WC) has the best density, followed by Group C (40% fine-grained WC).

[0213] by Figure 7 Taking the SEM image of C as an example, we can clearly see both coarse and fine WC grains and the bonding phase Co. The gray-white particles are WC grains, most of which are trapezoidal and triangular. The dark gray area is the bonding phase Co, which can be seen to fill the gaps between the grains very well. This is because during high-temperature sintering, Co will form a liquid phase that flows and fills the gaps between the grains.

[0214] The SEM images of each group were imported into the software ImageJ, the grain size of each group was measured and the grain size distribution diagram was drawn by software Origin. Figure 9 As shown. Figure 9 It can be seen that the grain size distribution of group A presents a peak value. This is because group A contains only one type of grain size (coarse-grained WC), and the grain size is generally larger. Groups B, C, and D are dual-scale cemented carbide specimens with two types of WC added, so their grain size distribution presents two peak values, and the distribution percentage of fine-grained WC gradually increases.

[0215] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A WC cemented carbide, characterized in that: The raw materials of the WC cemented carbide include first WC powder, second WC powder, Co powder, Cr3C2 powder and VC powder; the particle size of the first WC powder is 1.8-2.2 μm; the particle size of the second WC powder is 0.1-0.3 μm.

2. The WC cemented carbide according to claim 1, characterized in that Calculated by mass percentage, the content of each raw material is: first WC powder 39.2-59.2%, second WC powder 30-50%, Co powder 10%, Cr3C2 powder 0.6%, VC powder 0.2%.

3. The method for preparing WC cemented carbide according to claim 1 or 2, characterized in that: The steps include: (1) mixing the above raw materials, ball milling, sieving, drying and grinding to obtain a mixed powder; (2) The mixed powder is pressed into a green compact and then pre-sintered and sintered in sequence to obtain WC cemented carbide.

4. The method for preparing WC cemented carbide according to claim 3, wherein: In step (1), polyethylene glycol and alcohol are added before the ball milling treatment.

5. The method for preparing WC cemented carbide according to claim 3 or 4, characterized in that: In step (1), the ball-to-material ratio of the ball milling treatment is 8 to 12:1, the rotation speed is 200 to 400 rpm, and the time is 25 to 35 hours.

6. The method for preparing WC cemented carbide according to claim 5, characterized in that: In step (1), the drying temperature is 60-100°C.

7. The method for preparing WC cemented carbide according to claim 4 or 6, characterized in that: In step (2), the pressure of the green compact is 3-20 MPa and the time is 1-2 min.

8. The method for preparing WC cemented carbide according to claim 7, characterized in that: In step (2), the specific process of the pre-sintering is: In the first stage, the temperature was raised from 20°C to 200°C at a rate of 5-8°C / min and kept at 200°C for 60 min; The second stage is heating from 200°C to 400°C at a heating rate of 5-8°C / min and keeping at 400°C for 60 min; The third stage is heating from 400°C to 700°C at a heating rate of 8-12°C / min and keeping at 700°C for 60 min; The fourth stage is to cool down from 700℃ to 20℃, with a cooling rate of 2-4℃ / min.

9. The method for preparing WC cemented carbide according to claim 6 or 8, characterized in that: In step (2), the specific process of sintering is: In the first stage, the temperature was raised from 20°C to 300°C at a rate of 8-12°C / min and kept at 300°C for 60 min; The second stage is heating from 300°C to 600°C at a heating rate of 8-12°C / min and keeping at 600°C for 60 min; The third stage is heating from 600°C to 900°C at a heating rate of 8-12°C / min and keeping at 900°C for 60 min; The fourth stage is heating from 900°C to 1200°C at a heating rate of 8-12°C / min and keeping at 1200°C for 60 min; The fifth stage is heating from 1200°C to 1400°C at a heating rate of 5-8°C / min and keeping at 1400°C for 60 min; The sixth stage is to cool down from 1400℃ to 20℃, with a cooling rate of 2-4℃ / min.

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