Preparation method of hard alloy with low thermal expansion coefficient

Abstract

The invention discloses a preparation method of a hard alloy with a low thermal expansion coefficient, which comprises the following steps: carrying out ball milling on mixed powder of tungsten carbide powder and cobalt powder and silicon nitride powder in a ball milling tank filled with nitrogen to obtain composite powder, and adding the silicon nitride powder in an amount which is 2-7 percent of the mass of the composite powder; drying the composite powder in a nitrogen protective atmosphere; the dried composite powder is put into a graphite female die, a hydraulic machine is used for prepressing, and the prepressing pressure is 9-12 MPa; the assembled graphite mold is placed in a spark plasma sintering system, the initial axial pressure ranges from 10 MPa to 50 MPa, vacuumizing is conducted to be below 10 Pa, and electrified sintering is started; the heating rate is 30-50 DEG C / min, the sintering temperature is 1680-1800 DEG C, and sintering is finished after heat preservation is conducted for 2-10 min; cooling is conducted, the cooling rate is 30-50 DEG C / min, the temperature is reduced to 800-1000 DEG C, and furnace cooling is conducted; taking out the sample by using a hydraulic machine, and performing sand blasting treatment on graphite on the sample surface to obtain the hard alloy material.

Description

Technical Field

[0001] The invention belongs to the technical field of powder metallurgy, and in particular relates to a method for preparing a hard alloy with a low thermal expansion coefficient. Background Art

[0002] WC-based ceramic materials have excellent properties such as high hardness, high strength, high wear resistance, good corrosion resistance, high surface finish and good electrical processing. They are widely used in wear-resistant fields such as upper and lower molds, protective sleeves and water jet sand tubes for high-precision glass lenses pressed at high temperatures. When used in high temperature and hot and cold alternating working conditions, the thermal expansion coefficient is as low as possible, which can effectively reduce the thermal stress between workpieces and maintain the dimensional accuracy of the workpieces.

[0003] At present, the linear thermal expansion coefficient of WC-based ceramic materials manufactured abroad can reach as low as 4.5×10 -6 / K (room temperature to 873K). In China, on the one hand, the stable manufacturing of WC-based ceramic materials for high-end molds is still in the industrial research and development stage, and most of them are wear-resistant materials with relatively low material requirements, such as materials for water jet sand pipes; on the other hand, the linear thermal expansion coefficient of domestically manufactured products is currently higher than 4.8×10 -6 / K (room temperature to 873K). Therefore, China not only needs to solve the problem of stable production and manufacturing of high-end WC-based ceramic materials, but also needs to develop materials with lower thermal expansion coefficients. Summary of the invention

[0004] In view of the above technical problems, the present invention provides a method for preparing a cemented carbide with a low thermal expansion coefficient. The prepared cemented carbide material has both sufficient hardness and fracture toughness and a low thermal expansion coefficient.

[0005] The present invention provides a method for preparing a cemented carbide with a low thermal expansion coefficient. Step 1: mixing and drying raw material powders: ball milling a mixed powder of tungsten carbide powder (WC powder) and cobalt powder (Co powder) with silicon nitride powder (Si3N4 powder) in a ball mill filled with nitrogen to obtain a composite powder, wherein the amount of Si3N4 powder added is 2-7% of the mass of the composite powder, and the composite powder is placed in a nitrogen protective atmosphere for drying;

[0006] Step 2, mold filling: put the dried composite powder into a graphite female mold and pre-press it using a hydraulic press with a pre-pressing pressure of 9-12 MPa;

[0007] Step 3, sintering: placing the assembled graphite mold in a spark plasma sintering system, with an initial axial pressure of 10-50 MPa, and the axial pressure range is controlled at 10-80 MPa during the sintering process, evacuating to below 10 Pa, and starting the sintering with power on; the heating rate is 30-100°C / min, and the pressure increases with the increase of temperature, the sintering temperature is 1680-1800°C, and the sintering is terminated after 2-10 minutes of heat preservation, and the furnace is cooled;

[0008] Step 4, sampling: taking out the sample by using a hydraulic press, and sandblasting the graphite on the surface of the sample to obtain a cemented carbide material.

[0009] Furthermore, the Co powder accounts for 0 to 1% of the mass of the mixed powder of the WC powder and the Co powder.

[0010] Furthermore, the particle size of Si3N4 powder is ≤1um; the purity of WC powder is ≥99.95%, the average particle size is ≤1μm, and it contains an inhibitor within a mass fraction of 1%, the inhibitor is vanadium carbide VC or chromium carbide Cr3C2.

[0011] Furthermore, the obtained cemented carbide is placed in a ball mill for ball milling, the ball milling speed is 130-150 r / min, the ball-to-material ratio is 3.5:1, and the ball milling time is 8-24 h.

[0012] Furthermore, the drying temperature of the composite powder in the nitrogen protective atmosphere is 60-80°C.

[0013] The present invention also protects a hard alloy prepared by adopting the above-mentioned method for preparing the hard alloy with low thermal expansion coefficient.

[0014] The present invention adds Si3N4 powder to prepare the hard alloy, which has high hardness and high wear resistance and a relatively lower thermal expansion coefficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a thermal expansion curve of the cemented carbide material prepared in Example 1. DETAILED DESCRIPTION

[0016] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention are provided for the purpose of illustration and description, and are not intended to be exhaustive or to limit the present invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention, and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

[0017] First, cut the graphite paper according to the size of the graphite negative mold and place it on the inner wall of the negative mold, and make it fit the inner wall perfectly; then put the lower pressure head into the graphite negative mold, and put in two layers of graphite paper with the same diameter as the pressure head to make it fit the pressure head; then put the composite powder mixed and dried in proportion into the graphite negative mold; finally, put in two layers of graphite paper and the upper pressure head in turn, and use a hydraulic press to apply pre-pressure and maintain pressure; after the above work is completed, use high-temperature graphite felt to wrap the mold. The size of the graphite mold and pressure head is determined by the size of the sample to be prepared.

[0018] The spark plasma sintering system used in the following examples is from SINTER LAND Co., Ltd. of Japan, and the models are LABOX-350 and LABOX-6020. It should be noted that the preparation of cemented carbide blocks using the method of the present invention and equipment of other manufacturers and models is also within the scope of protection of the present invention. Spark plasma sintering includes pulsed power-on pressure sintering, electric field assisted sintering, pulsed current rapid sintering, etc.

[0019] The density results in the following examples were measured using the Archimedes drainage method; the hardness test was obtained using a Vickers hardness tester under a load of 30 kg and maintained for 15 seconds; the fracture toughness test was performed in accordance with the international standard ISO 28079-2009

[0020] 《Hardmetals-Palmqvist toughness test》, measure and calculate the total length of the four cracks at the indentation tip, and calculate the fracture toughness KIC based on the Vickers hardness value of the sample under the indentation.

[0021] Example 1

[0022] preparation The cemented carbide block material, wherein the mass ratio of the two components of the mixed powder a is WC:Co=99:1, and the mass ratio of the final composite powder is a:Si3N4:Cr3C2=92:7:1.

[0023] 1. Powder mixing: 300g of the powder weighed according to the above mass ratio is placed in a carbide ball milling jar for ball milling, wherein the abrasive is carbide ball, the ball-to-material ratio is 3.5:1, the ball milling speed is 150r / min, and the ball milling time is 24h. The powder after ball milling is dried at high temperature in vacuum, the drying temperature is 80℃, and the drying time is 48h.

[0024] 2. Mold loading: Place the dried composite powder into a graphite female mold and pre-press it using a hydraulic press with a pre-pressing pressure of 10 MPa.

[0025] 3. Sintering: Place the assembled graphite mold in the spark plasma sintering system, set the initial axial pressure to 30MPa, evacuate to below 10Pa, and start sintering with power on; set the heating rate to 30℃ / min, and when the temperature reaches 1400℃~1500℃, increase the pressure to 40~60MPa, and then continue to heat up, and the sintering temperature is 1800℃; end the sintering after keeping warm for 2min, cool down, the cooling rate is 30~50℃ / min, down to 1000℃, and cool with the furnace.

[0026] 4. Sampling: Take out the sample using a hydraulic press and use sandblasting to remove graphite on the sample surface to obtain cemented carbide material. After fine processing, there are no defects such as white spots and edge collapse on the surface. For specific performance parameters, refer to Table 1.

[0027] also, Figure 1 The thermal expansion curves of the cemented carbide material prepared in this example at different temperatures are given.

[0028] Example 2

[0029] preparation The cemented carbide block material, wherein the mass ratio of the two components of the mixed powder a is WC:Co=99.5:0.5, and the final composite powder mass ratio is a:Si3N4:Cr3C2=95:4:1.

[0030] 1. Powder mixing: 300g of the powder weighed according to the above weight ratio is placed in a carbide ball milling jar for ball milling, wherein the abrasive is carbide ball, the ball-to-material ratio is 3.5:1, the ball milling speed is 140r / min, and the ball milling time is 24h. The powder after ball milling is dried at high temperature and vacuum, the drying temperature is 80℃, and the drying time is 48h.

[0031] 2. Mold loading: Place the composite powder obtained after drying into a graphite female mold and pre-press it using a hydraulic press with a pre-pressing pressure of 12 MPa.

[0032] 3. Sintering: Place the assembled graphite mold in the spark plasma sintering system, set the initial axial pressure to 20MPa, evacuate to below 10Pa, and start sintering with power on; set the heating rate to 40℃ / min, and when the temperature reaches 1400℃~1500℃, increase the pressure to 30~50MPa, and then continue to heat up, the sintering temperature is 1690℃; end sintering after keeping warm for 8min; cool down at a rate of 30~50℃ / min to 800℃, and cool with the furnace.

[0033] 4. Sampling: Take out the sample using a hydraulic press and use sandblasting to remove graphite on the sample surface to obtain cemented carbide material. After fine processing, there are no defects such as white spots and edge collapse on the surface. For specific performance parameters, refer to Table 1.

[0034] Example 3

[0035] preparation A cemented carbide block material, wherein the mass ratio of the two components in the mixed powder a is WC:Co=99:1, and the mass ratio of the final composite powder is a:Si3N4:VC=97:2:1.

[0036] 1. Powder mixing: 300g of the powder weighed according to the above weight ratio is placed in a carbide ball milling jar for ball milling, wherein the abrasive is carbide ball, the ball-to-material ratio is 3.5:1, the ball milling speed is 140r / min, and the ball milling time is 8h. The powder after ball milling is dried at high temperature and vacuum, the drying temperature is 80℃, and the drying time is 48h.

[0037] 2. Mold loading: Place the composite powder obtained after drying into a graphite female mold and pre-press it using a hydraulic press with a pre-pressing pressure of 9 MPa.

[0038] 3. Sintering: Place the assembled graphite mold in the spark plasma sintering system, set the initial axial pressure to 50MPa, evacuate to below 10Pa, and start sintering with power on; set the heating rate to 50℃ / min, and when the temperature reaches 1400℃~1500℃, increase the pressure to 50~80MPa, and then continue to heat up, the sintering temperature is 1680℃; end sintering after keeping warm for 10min; cool down at a cooling rate of 30~50℃ / min to 1000℃, and cool with the furnace.

[0039] 4. Sampling: Take out the sample using a hydraulic press and use sandblasting to remove graphite on the sample surface to obtain cemented carbide material. After fine processing, there are no defects such as white spots and edge collapse on the surface. For specific performance parameters, refer to Table 1.

[0040] Comparative Example 1

[0041] preparation Pure A powder cemented carbide bulk material.

[0042] Different from Examples 1-3, the powder is pure a powder, no silicon nitride powder is added, the mixed powder of tungsten carbide powder and cobalt powder is directly subjected to high-temperature vacuum drying, and the other steps are the same as Example 3.

[0043] Table 1

[0044] Sintering temperature(℃) Density(g / cm3) Thermal expansion coefficient (873K) Hardness (Hv30) KIC(Mpa·m1 / 2) Example 1 1800 13.04 <![CDATA[4.22×10 -6 / K]]> 1855.2 6.86 Example 2 1690 13.95 <![CDATA[4.38×10 -6 / K]]> 1984.2 6.85 Example 3 1680 14.77 <![CDATA[4.46×10 -6 / K]]> 1951.7 6.75 Comparative Example 1 1680 15.56 <![CDATA[4.49×10 -6 / K]]> 2567.9 7.41

[0045] It can be seen from Table 1 that the cemented carbide with Si3N4 powder added has high hardness and high wear resistance, as well as a relatively lower thermal expansion coefficient.

[0046] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field and related fields without creative work should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a cemented carbide with a low thermal expansion coefficient, characterized in that: The following steps are involved: Step 1, mixing and drying raw material powders: ball milling a mixed powder of tungsten carbide powder and cobalt powder with silicon nitride powder in a ball mill filled with nitrogen to obtain a composite powder, wherein the amount of silicon nitride powder added is 2-7% of the mass of the composite powder, and the composite powder is placed in a nitrogen protective atmosphere for drying; Step 2, mold filling: put the dried composite powder into a graphite female mold and pre-press it using a hydraulic press with a pre-pressing pressure of 9 to 12 MPa; Step 3, sintering: placing the assembled graphite mold in a spark plasma sintering system, with an initial axial pressure of 10-50 MPa, evacuating to below 10 Pa, starting power-on sintering, and controlling the axial pressure range in the sintering process to be 10-80 MPa; heating rate of 30-50 ° C / min, sintering temperature of 1680-1800 ° C, and sintering after 2-10 minutes of heat preservation; cooling, cooling rate of 30-50 ° C / min, down to 800-1000 ° C, and cooling with the furnace; Step 4, sampling: taking out the sample by using a hydraulic press, and sandblasting the graphite on the surface of the sample to obtain a cemented carbide material.

2. The method for preparing a cemented carbide with a low thermal expansion coefficient according to claim 1, characterized in that: The cobalt powder accounts for 0 to 1% of the mass of the mixed powder of the tungsten carbide powder and the cobalt powder.

3. The method for preparing a cemented carbide with a low thermal expansion coefficient according to claim 1, characterized in that: Silicon nitride powder particle size ≤1um.

4. The method for preparing a cemented carbide with a low thermal expansion coefficient according to claim 1, characterized in that: The purity of tungsten carbide powder is ≥99.95%, the average particle size is ≤1μm, and it contains inhibitors within 1% by mass, such as vanadium carbide or chromium carbide.

5. The method for preparing a cemented carbide with a low thermal expansion coefficient according to claim 1, characterized in that: The ball mill speed is 130-150 r / min, the ball-to-material ratio is 3.5:1, and the ball milling time is 8-24 h.

6. The method for preparing a cemented carbide with a low thermal expansion coefficient according to claim 1, characterized in that: The drying temperature of the composite powder in a nitrogen protective atmosphere is 60-80°C.

7. A cemented carbide prepared by the method for preparing a cemented carbide with a low thermal expansion coefficient according to any one of claims 1 to 6.

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