Preparation method of superfine WC-Co hard alloy

Abstract

The invention discloses a preparation method of a superfine WC-Co hard alloy. 89.8-90% wt of superfine tungsten carbide, 9% wt of superfine Co powder and 1-1.2% wt of composite additive chromium carbide and vanadium carbide are adopted; wherein the weight ratio of chromium carbide to vanadium carbide is 1 / 2; ball-milling the raw materials to obtain a mixture; and the pressed blank is subjected to a specific oscillation pressure and rapid cooling sintering process, the contact form and interface state among particles are improved, the density of the hard alloy is improved, WC grain growth is inhibited, flowing of binding phase Co is promoted, the structure and mechanical properties of the alloy are remarkably improved, and the nanocrystalline WC-Co hard alloy with high strength and high density is obtained.

Description

Technical Field

[0001] The invention relates to a method for preparing ultrafine cemented carbide with high strength, high density and low defects, and belongs to the technical field of cemented carbide material preparation. Background Art

[0002] Cemented carbide has a series of excellent properties such as high hardness, wear resistance, good strength and toughness, heat resistance, and corrosion resistance, and is widely used in many industries. Ultrafine / nanocrystalline cemented carbide has become one of the best candidate materials for high-speed and precision machining due to its ultra-high hardness, strength, wear resistance and good fracture toughness. The preparation process of ultrafine / nanocrystalline cemented carbide is mainly affected by (1) raw powder and (2) sintering technology. The key is to obtain ultrafine / nanoscale raw material powder and control the growth of WC grains during the sintering process as much as possible (optimizing the sintering process and adding grain growth inhibitors). Due to the small size effect of nanopowder, high surface activity and large sintering driving force, WC grains are very easy to grow during the sintering process. In addition, relevant studies have shown that the microstructure of the material (such as WC grain size, distribution of the binder phase Co, structure of the Co phase, dislocation density, WC / WC and WC / Co interface characteristics, etc.) also has an important influence on the mechanical properties of ultrafine / nanocrystalline WC-Co cemented carbide.

[0003] Adding grain growth inhibitors is an effective method, usually represented by Cr2C3 and VC. If the inhibitor content is low, there is no obvious inhibitory effect. If the content is high, the bonding force (interface strength) between tungsten carbide and the binding phase Co will be reduced, delaying the densification process, and easily leading to component segregation, making it difficult to obtain high-density ultrafine / nanocrystalline cemented carbide, and the microstructure and performance are poor. Therefore, the appropriate additive dosage is crucial. Oscillating pressure sintering shows many advantages in enhancing densification, inhibiting grain growth, and accelerating the uniform distribution of WC and Co phases. In particular, for samples at liquid phase sintering temperature, dynamic oscillating pressure can accelerate the flow of the binding phase Co, reduce the adjacency of the hard phase WC, shorten the mean free path of the Co matrix, and improve the mechanical properties and particle size uniformity of the alloy. In response to the above problems, the present invention proposes a method for preparing an ultrafine-grained WC-Co based cemented carbide with high strength and high toughness. Summary of the Invention

[0004] Aiming at the deficiencies and difficulties existing in the prior art, the purpose of the present invention is to overcome the problems such as the increase in alloy microstructure defects (interface strength, Co phase distribution, composition segregation, porosity, etc.) and the decrease in density after the increase in the inhibitor content. By adjusting the addition amount of the composite inhibitor, the grains are refined and evenly dispersed. In particular, a method for preparing an ultrafine WC-Co-based alloy with high density, fine grain size, high hardness and high toughness is prepared by means of oscillating pressure + rapid cooling sintering (while ensuring good sintering effect to a certain extent, effectively controlling the grain growth during sintering, reducing the inhibitor for controlling grain growth introduced into the components of the nanocrystalline cemented carbide, and slowing down or avoiding the problem of reduced strength of the ultrafine cemented carbide caused by the introduction of excessive additives).

[0005] In order to achieve the above invention purpose, a method for preparing an ultrafine WC-Co cemented carbide provided by the present invention includes the following steps:

[0006] S1: Ball milling: Weigh the raw materials according to weight percentages, including 89.8 - 90%wt ultrafine tungsten carbide, 9%wt ultrafine Co powder, 1 - 1.2%wt composite additive chromium carbide + vanadium carbide; wherein, the weight ratio of chromium carbide / vanadium carbide is 1 / 2; the raw materials are ball milled to obtain a mixed material;

[0007] S2: Compression molding;

[0008] S3: Sintering; the sintering process sequentially includes the following stages:

[0009] S3-1 Dewaxing: Heat up to 550°C and keep warm for 0.5h;

[0010] S3-2 Vacuum sintering: Heat up to 1350°C and keep warm for 1h;

[0011] S3-3 Oscillating sintering: The median pressure is 20MPa, the pressure amplitude is 5MPa, the oscillation frequency is 0Hz, and the oscillation sintering holding time is 1.5h;

[0012] S3-4 Furnace cooling to obtain the WC-Co-based cemented carbide.

[0013] The melting-precipitation process of WC in the Co phase is like a reversible reaction. Since W has a certain solubility in the Co phase, the solubility will increase with the increase of the sintering temperature, causing rapid grain growth. By the composite addition of Cr3C2 and VC, the grains are refined and evenly dispersed. After that, the oscillating sintering improves the simultaneous improvement of the hardness and toughness of the ultrafine cemented carbide and improves the Co phase distribution during the alloy sintering process.

[0014] A further improvement is that in S3-3, the oscillation frequency is 5Hz.

[0015] A further improvement is that in S3-4, the cooling method is forced air cooling. The rapid cooling process is adopted, with a rapid cooling rate of 20°C / min, which overcomes the grain growth caused by the too slow cooling rate and too long cooling time during the sintering process, and the tissue performance is better.

[0016] A further improvement is that the carbon content range of ultrafine tungsten carbide is 6.10% - 6.15%, and the particle size is 0.6μm; the average particle size of ultrafine Co powder is <1μm. The liquid-solid ratio of the grinding medium hexane to the material is 300 mL / Kg, the diameter of the grinding ball is φ6.8mm, the ball-to-material ratio is 10:1, the filling coefficient is 60%, and the ball milling time is 30h.

[0017] The beneficial effects of the present invention are:

[0018] (1) An effective sintering method is adopted to improve the contact morphology and interface state between particles, improve the density of the cemented carbide, inhibit the growth of WC grains, promote the flow of the binder phase Co, and significantly improve the structure and mechanical properties of the alloy.

[0019] (2) The ultrafine-grained cemented carbide prepared by this method has high strength and high toughness, the performance is significantly improved, and the service life of the material is significantly improved. Description of the Drawings

[0020] The following will clearly and completely describe the technical solution with reference to the drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments.

[0021] Figure 1 As described in Example 1, the composite additive Cr3C2 + VC is 0.0% wt, and the SEM photograph of the prepared cemented carbide.

[0022] Figure 2 As described in Example 4, the composite additive Cr3C2 + VC is 1.2% wt, and the SEM photograph of the prepared cemented carbide.

[0023] Figure 3 As described in Example 7, the SEM photograph of the prepared cemented carbide. Detailed Embodiments

[0024] The following further details the above content of the present invention in combination with the specific embodiments of the examples. However, this should not be understood that the scope of the above subject matter of the present invention is limited to the following examples. Without departing from the above technical idea of the present invention, various substitutions or changes made according to the common general knowledge and customary means in the art should be included within the scope of the present invention.

[0025] Example 1

[0026] A method for preparing ultrafine WC-Co cemented carbide includes the following steps:

[0027] 1. Raw material ratio: Weigh the raw materials according to the parts by weight in Table 1.

[0028] The raw materials include 91% wt of ultrafine tungsten carbide (carbon content range: 6.10% - 6.15%; particle size ~ 0.6 μm), 9% wt of ultrafine Co powder (average particle size < 1 μm), and the composite additive Cr3C2 + VC is 0% wt (Cr2C3 / VC = 1 / 2). The sum of the proportions of each component is 100%.

[0029] 2. Ball milling: Add the raw materials, 2% wt of paraffin wax and 0.04% of stearic acid based on the weight of the raw materials into the ball milling tank. The grinding media are alloy balls (diameter 6.8 mm), and the ball-to-material ratio is 10:1. The grinding medium is hexane, and the addition amount of hexane is 300 mL / kg. Use a drum-type ball mill for ball milling, with a rotation speed of 72 r / min and a time of 30 h. After ball milling, the slurry is sieved and then vacuum dried at 75 °C to obtain dry materials, and then granulated to obtain a mixed material.

[0030] 3. Pressing: Obtain a green compact, and the pressing pressure is 5 MPa.

[0031] 4. Sintering: Sinter the green compact. The specific process is divided into four stages:

[0032] 4-1 The first stage is dewaxing. First, raise the temperature from room temperature to 250 °C at a rate of 4 °C / min, then to 320 °C at a rate of 1.2 °C / min, then to 370 °C at a rate of 0.3 °C / min and hold for 3 h, then to 450 °C at a rate of 2.6 °C / min and hold for 30 min, and then to 550 °C at a rate of 3.3 °C / min and hold for 30 min to complete dewaxing.

[0033] 4-2 The second stage is vacuum sintering. First, raise the temperature to 650 °C at a rate of 3.3 °C / min, then to 1100 °C at a rate of 4.5 °C / min and hold for 1 h. After holding, introduce argon to adjust the vacuum degree in the furnace (the vacuum degree is 3 kPa), and then continue to raise the temperature to 1350 °C at a rate of 4 °C / min and hold for 1 h.

[0034] 4-3 The third stage is low-pressure sintering. Continue to raise the temperature to 1380 °C at a rate of 1 °C / min, then close the vacuum, and charge argon to make the pressure in the furnace reach 0.9 MPa, and then continue to hold and sinter for 90 min.

[0035] 4-4 The fourth stage is cooling. Cool down to room temperature with the furnace, and the cooling rate is about 2 °C / min. Obtain WC-Co-based cemented carbide.

[0036] The prepared WC-Co-based cemented carbide was surface polished and then its surface morphology and physical properties were characterized. The metallographic structure of the alloy sample prepared by this method was A02B00C00, the average grain size of WC was 630 nm, the density was 14.61, the coercive force was 20.9 kA / m, the Rockwell hardness was 91.4 HRA, the Vickers hardness HV30 was 1530, and the fracture toughness was 11.9 MPa·m 1 / 2 .

[0037] Example 2

[0038] A method for preparing ultrafine WC-Co cemented carbide, the process steps are the same as those in Example 1.

[0039] The difference from Example 1 lies in the different addition amounts of inhibitors, that is, the composite additive Cr3C2 + VC is 0.6% wt (Cr2C3 / VC = 1 / 2), and the ultrafine WC is 90.40% wt.

[0040] The metallographic structure of the alloy sample prepared in this example was A02B00C00, the average grain size of WC was 490 nm, the density was 14.54, the coercive force was 26.9 kA / m, the Rockwell hardness was 92.9 HRA, the Vickers hardness HV30 was 1900, and the fracture toughness was 9.4 MPa·m 1 / 2 .

[0041] Example 3

[0042] A method for preparing ultrafine WC-Co cemented carbide, the process steps are the same as those in Example 1.

[0043] The difference lies in the different addition amounts of inhibitors, that is, Cr3C2 + VC is 1.0% wt (Cr2C3 / VC = 1 / 2), and the ultrafine WC is 90.00% wt.

[0044] The metallographic structure of the alloy sample prepared in this example was A02B00C00, the average grain size of WC was 360 nm, the density was 14.45, the coercive force was 31.0 kA / m, the Rockwell hardness was 93.3 HRA, the Vickers hardness HV30 was 1940, and the fracture toughness was 9.3 MPa·m 1 / 2 .

[0045] Example 4

[0046] A method for preparing ultrafine WC-Co cemented carbide, the process steps are the same as those in Example 1.

[0047] The difference lies in the different addition amounts of inhibitors, that is, Cr3C2 + VC is 1.2% wt (Cr2C3 / VC = 1 / 2), and the ultrafine WC is 89.80% wt.

[0048] The metallography of the alloy sample prepared in this example is A02B00C00, the average grain size of WC is 340 nm, the density is 14.43, the coercive force is 32.6 kA / m, the Rockwell hardness is 93.6 HRA, the Vickers hardness HV30 is 1960, and the fracture toughness is 9.2 MPa·m 1 / 2 。

[0049] The adjacent WC grains are reduced, the average particle size is significantly refined, and the structure and particle size distribution are also more uniform, but the small pores are also increasing.

[0050] Example 5

[0051] A method for preparing ultrafine WC-Co cemented carbide, except for the following process steps, the rest are the same as in Example 4.

[0052] 4. Sintering:

[0053] 4-3 The third stage is vibration sintering, the median pressure is 20 MPa, the pressure amplitude is 5 MPa, the vibration frequency is 0 Hz, and the vibration sintering holding time is 90 min;

[0054] The difference is that the vibration sintering process is used in the third stage of the sintering process. The dewaxing stage is the same as in Example 1. The difference is that after heating to 1350 °C and holding for 1 h to complete the vacuum sintering, the vibration sintering process is used, the designed median pressure is 20 MPa, the pressure amplitude is 5 MPa, the vibration frequency is 0 Hz, and the vibration sintering holding time is 90 min; after the vibration sintering is completed, the sintering stops, and it is cooled to room temperature with the furnace to obtain WC-Co based cemented carbide.

[0055] The metallography of the alloy sample prepared in this example is A02B00C00, the average grain size of WC is 340 nm, the density is 14.46, the coercive force is 32.8 kA / m, the Rockwell hardness is 93.5 HRA, the Vickers hardness HV30 is 1960, and the fracture toughness is 9.5 MPa·m 1 / 2 。

[0056] Example 6

[0057] A method for preparing ultrafine WC-Co cemented carbide, except for the following process steps, the rest are the same as in Example 5.

[0058] The difference is that the vibration frequency is 5 Hz.

[0059] The metallography of the alloy sample prepared in this example is A02B00C00, the average grain size of WC is 330 nm, the density is 14.55, the coercive force is 33.5 kA / m, the Rockwell hardness is 93.7 HRA, the Vickers hardness HV30 is 2010, and the fracture toughness is 9.8 MPa·m 1 / 2 。

[0060] The oscillating pressure can crush the powder particle aggregates in the initial stage of sintering, intensify the particle sliding and rearrangement. In the middle and late stages of sintering, it has a cyclic loading - partial unloading - reloading effect on the powder particles, causing stress softening, promoting the plastic deformation of the particles, and can provide a higher diffusion driving force, promoting grain boundary diffusion and lattice diffusion, and promoting the densification process of this cemented carbide.

[0061] Example 7

[0062] A method for preparing ultrafine WC-Co cemented carbide, except for the following process steps, the rest is the same as Example 6.

[0063] 4-4 Cooling, forced air cooling is used for cooling, and the cooling rate is about 20 °C / min.

[0064] The metallography of the alloy sample prepared in this example is A02B00C00, the average grain size of WC is 310 nm, the density is 14.56, the coercive force is 34.2 kA / m, the Rockwell hardness is 93.9 HRA, the Vickers hardness HV30 is 2030, and the fracture toughness is 10.1 MPa·m 1 / 2 。

[0065]

[0066] Table 1

[0067] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modification or equivalent change made to the above embodiments based on the technical essence of the present invention falls within the protection scope of the present invention. Common knowledge such as the specific structure and characteristics known in the present invention scheme is not described in detail here. Those of ordinary skill in the art know all the common technical knowledge in the technical field to which the invention belongs before the application date or priority date, can know all the existing technologies in this field, and have the ability to apply the conventional experimental means before this date. Those of ordinary skill in the art can, under the inspiration given in this application, combine their own abilities to complete and implement this scheme. Some typical known structures or known methods should not become an obstacle for those of ordinary skill in the art to implement this application.

Claims

1. A preparation method of ultrafine WC-Co cemented carbide, characterized in that, It includes the following steps: S1: Ball milling: Weigh the raw materials according to the weight percentage, including 89.8 - 90%wt of ultrafine tungsten carbide, 9%wt of ultrafine Co powder, and 1 - 1.2%wt of composite additive chromium carbide + vanadium carbide; among them, the weight ratio of chromium carbide / vanadium carbide is 1 / 2; after ball milling the raw materials, a mixture is obtained; S2: Compression molding; S3: Sintering; The sintering process sequentially includes the following stages: S3 - 1 Dewaxing: Heat up to 550°C and keep warm for 0.5 h; S3 - 2 Vacuum sintering: Heat up to 1350°C and keep warm for 1 h; S3 - 3 Oscillatory sintering: The median pressure is 20 MPa, the pressure amplitude is 5 MPa, the oscillation frequency is 0 Hz, and the holding time for oscillatory sintering is 1.5 h; S3 - 4 Furnace cooling to obtain ultrafine WC - Co cemented carbide.

2. The method according to claim 1, wherein, In step S3 - 3, the oscillation frequency is 5 Hz.

3. The method according to claim 2, wherein in step S3 - 4, the cooling method is forced air cooling.

4. The method according to claim 1, wherein in step S1, the carbon content range of the ultrafine tungsten carbide is 6.10% - 6.15%, the particle size is 0.6 μm; the average particle size of the ultrafine Co powder is <1 μm; the liquid - solid ratio of the grinding medium hexane to the material is 300 mL / Kg, the diameter of the grinding balls is φ6.8 mm, the ball - to - material ratio is 10:1, the filling coefficient is 60%, and the ball milling time is 30 h.

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