Preparation method of binding-phase-free nano hard alloy

Abstract

The invention discloses a preparation method of a binding-phase-free nano hard alloy. The preparation method comprises the following steps: carrying out airflow classification on nano WC powder; the graded nano WC powder and the Cr-containing inhibitor are subjected to dynamic oxidation calcination, and W-Cr composite oxide is obtained; mixing the W-Cr composite oxide with carbon black, and performing one-step carbonization in a hydrogen atmosphere to obtain WC-Cr composite powder; the WC-Cr composite powder is subjected to airflow crushing and grading; the material is obtained through multi-stage spark plasma sintering. The prepared product is compact and uniform in structure and excellent in performance, the use cost is reduced, and the problems that existing non-binding-phase nanometer hard alloy grains grow abnormally and are poor in compactness are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of powder metallurgy materials and relates to a preparation method of a binderless nanocrystalline cemented carbide. Background Art

[0002] Cemented carbides with a binder phase content lower than 0.5 wt.% are called binderless cemented carbides, which have excellent wear resistance, corrosion resistance, good polishability, high thermal conductivity, low expansion coefficient and other excellent mechanical properties, and are widely used in fields such as finishing tools, precision molds, highly wear-resistant seals, precision electronic packaging materials, etc.

[0003] For binderless nanocrystalline cemented carbides to exhibit the above properties, it is necessary to achieve a uniform microstructure and high density. To achieve such a microstructure, on the one hand, it is necessary to prepare a nano tungsten carbide composite powder with fine enough particle size and uniform enough composition, and on the other hand, it is necessary to ensure high density of the alloy without abnormal grain growth. In the actual production process, there are often cases where the tungsten carbide powder grains are too coarse or the average grain size is small but there are abnormally grown powders; even if there is fine and uniform tungsten carbide powder, it is difficult to prepare a dense cemented carbide with a uniform structure. Insufficient temperature and pressure make it difficult to alloy and result in low porosity, while too high temperature and pressure lead to abnormal grain growth and large equipment loss, resulting in a low alloy qualification rate. Under these factors, the performance advantages of binderless nanocrystalline cemented carbides are not fully demonstrated, making it difficult for this material to be widely used. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a preparation method of a binderless nanocrystalline cemented carbide, which produces a product with a dense and uniform structure, excellent performance, reduces the use cost, and solves the problems of abnormal grain growth and poor density of existing binderless nanocrystalline cemented carbides.

[0005] The technical solution adopted by the present invention is a preparation method of a binderless nanocrystalline cemented carbide, which includes the following steps: S1, subjecting the nano WC powder to air classification; S2, dynamically oxidizing and calcining the classified nano WC powder and a Cr-containing inhibitor to obtain a W-Cr composite oxide, wherein the molar ratio of W:Cr in the classified nano WC powder to the Cr-containing inhibitor is 1:(0.015 - 0.05); S3, mixing the W-Cr composite oxide obtained in S2 with carbon black and performing one-step carbonization in a hydrogen atmosphere, and treating it in a way of segmented heating; wherein the weight ratio of the W-Cr composite oxide to carbon black is 1:(0.185 - 0.198) to obtain a WC-Cr composite powder; S4, subjecting the WC-Cr composite powder to air crushing and classification; S5. The classified WC-Cr composite powder is sintered by multi-stage spark plasma sintering at a sintering pressure of 70-90 MPa and a sintering temperature of 290-1750 °C. After 36-55 min, it is cooled to room temperature with the furnace. The obtained binderless nanocomposite cemented carbide has a uniform structure, an average grain size of tungsten carbide <500 nm, and a relative density >99.70%.

[0006] Further, in S1, the specific surface area of the nano-WC powder is 4.50-5.20 m 2 / g.

[0007] Further, in S1, the rotation speed of the classification wheel is 4250-4500 r / min, and the air pressure of the classification machine is 0.35-0.42 MPa.

[0008] Further, in S2, the Cr-containing inhibitor is one or more of chromium, chromium oxide, chromium salts or chromates in any proportion.

[0009] Further, in S2, the calcination temperature is 500-600 °C, and the time is 0.5-2 h.

[0010] Further, in S2, the dynamic oxidation calcination is carried out in a rotary kiln. The rotation speed of the rotary kiln is 10-15 r / min, and the flow rate of compressed air is 25-35 L / min.

[0011] Further, the specific multi-stage temperature rise in S3 is as follows: the temperature of the first stage is 540-580 °C, the temperature of the second stage is 610-630 °C, the temperature of the third stage is 710-830 °C, the temperature of the fourth stage is 890-940 °C, and the temperature of the fifth stage is 1010-1070 °C. The carbonization time corresponding to each stage temperature is 16-32 min; the rotation speed of the rotary kiln is 3-4 r / min; Further, in S4, the rotation speed of the classification wheel is 4050-4150 r / min, and the air pressure of the classification machine is 0.30-0.32 MPa.

[0012] Further, in S4, the specific surface area of the classified WC-Cr composite powder >5.50 m 2 / g, and the particle size distribution width <1.10.

[0013] Further, in S5, the multi-stage spark plasma sintering is specifically as follows: heating up to 290-310 °C at 20 °C / min, holding for 20-30 min, heating up to 1185-1215 °C at 10-15 °C / min, holding for 10-15 min, heating up to 1650-1750 °C at 9-11 °C / min, and holding for 6-10 min.

[0014] The beneficial effects of the present invention are: 1. The present invention uses nano tungsten carbide powder for classification to ensure the uniformity of powder particle size. A Cr-containing inhibitor is introduced into the classified nano WC powder. The staged addition of the inhibitor optimizes the dispersion of Cr, making it more evenly embedded in the composite oxide.

[0015] 2. The present invention further ensures the uniformity of components through the rotary oxidation and carbonization of tungsten carbide and chromium. The Cr-containing inhibitor is specifically one or two or a mixture of more of chromium, chromium oxide, chromium salts or chromates, with a wide source and controllable price. The used Cr inhibitor undergoes multiple crystallization reaction processes of solid-phase mixing - oxidation - reduction - carbonization, which is beneficial to the mutual diffusion between tungsten and chromium and improves the uniformity of their distribution, and the product performance is stable and controllable.

[0016] 3. The present invention coordinates the composite powder and the sintering process to ensure that the prepared binderless nano cemented carbide has a uniform structure and high density, which is conducive to industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0018] Figure 1 It is the morphology of the WC-Cr composite powder in Embodiment 1 of the present invention.

[0019] Figure 2 It is the microstructure of the binderless nano cemented carbide prepared in Embodiment 1 of the present invention.

[0020] Figure 3 It is the binderless nano cemented carbide prepared in Comparative Example 4 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0022] Embodiment 1, A method for preparing a binderless nano cemented carbide, comprising the following steps: S1, subject WC (nano WC powder) to air classification, wherein the specific surface area of WC (BET: the total area possessed by a unit mass of material) is 4.8 m 2 / g, the rotational speed of the classification wheel is 4400 r / min, the air pressure in the classifier is 0.39 MPa, and the nano-aggregated particles are evenly dispersed; S2, subject the classified nano-WC powder and the Cr inhibitor (elemental chromium) to dynamic oxidative calcination in a rotary furnace at a calcination temperature of 550 °C for 1 h; a diffusion reaction occurs with the nano-WC powder during the in-situ rotary oxidation process to form a W-Cr composite oxide with uniformly distributed Cr; the molar ratio of W:Cr in the classified nano-WC powder to the Cr inhibitor is 1:0.035; the rotational speed of the rotary furnace is 12 r / min, and the compressed air flow rate is 29 L / min.

[0023] S3, use a plowshare mixer to mix the W-Cr composite oxide obtained in S2 with carbon black, and place it in a rotary furnace for one-step carbonization under a hydrogen atmosphere. In this step, a stepwise heating method is used for treatment; the weight ratio of the W-Cr composite oxide to carbon black is 1:0.193, the temperature of the first stage of the rotary furnace is 560 °C, the temperature of the second stage is 620 °C, the temperature of the third stage is 780 °C, the temperature of the fourth stage is 895 °C, the temperature of the fifth stage is 1050 °C, the carbonization time corresponding to each stage temperature is 24 min, and the rotational speed is 3 r / min; WC-Cr composite powder is prepared.

[0024] S4, perform air jet milling classification on the WC-Cr composite powder prepared in S3. Among them: the rotational speed of the classification wheel is 4090 r / min, the air pressure in the classifier is 0.30 MPa, and the specific surface area (BET) of the classified WC-Cr composite powder is 5.70 m 2 / g, the particle size distribution width (Span) is 1.05. The larger the Span value, the wider the particle size distribution; the morphology of the classified WC-Cr composite powder is as Figure 1 shown.

[0025] S5, perform rapid sintering on the WC-Cr composite powder classified in S4 through a spark plasma sintering device: the sintering pressure is 80 MPa, heat up to 300 °C at 20 °C / min, hold for 25 min, heat up to 1200 °C at 12 °C / min, hold for 13 min, heat up to 1690 °C at 10 °C / min, hold for 9 min, and cool with the furnace to room temperature (18 - 22 °C) to prepare a binderless nano-cemented carbide.

[0026] The binderless nano-cemented carbide prepared in Example 1 has a uniform structure, the average grain size of tungsten carbide is 450 nm, and the relative density is 99.75%. The microstructure of the binderless nano-cemented carbide is as Figure 2 shown.

[0027] Example 2, A method for preparing a binderless nano-cemented carbide, comprising the following steps: S1. Classify WC by air classification, where the specific surface area of WC is 4.50 m 2 / g, the rotational speed of the classification wheel is 4500 r / min, and the air pressure of the classifier is 0.42 MPa.

[0028] S2. Dynamically oxidize and calcine the classified nano WC powder and the Cr-containing inhibitor (chromium oxide) in a rotary kiln at a calcination temperature of 600 °C for 0.5 h to prepare a W-Cr composite oxide, where the molar ratio of W:Cr in the classified nano WC powder to the Cr-containing inhibitor is 1:0.05; the rotational speed of the rotary kiln is 10 r / min, and the compressed air flow rate is 25 L / min.

[0029] S3. Use a plowshare mixer to mix the W-Cr composite oxide obtained in S2 with carbon black and place it in a rotary kiln for one-step carbonization under a hydrogen atmosphere, where the weight ratio of the W-Cr composite oxide to carbon black is 1:0.198, the temperature of the first section of the rotary kiln is 580 °C, the temperature of the second section is 630 °C, the temperature of the third section is 830 °C, the temperature of the fourth section is 940 °C, the temperature of the fifth section is 1070 °C, the carbonization time corresponding to each section temperature is 16 min, and the rotational speed is 4 r / min; prepare a WC-Cr composite powder.

[0030] S4. Perform air crushing and classification on the WC-Cr composite powder prepared in S3, where: the rotational speed of the classification wheel is 4050 r / min, and the air pressure of the classifier is 0.30 MPa; the specific surface area of the classified WC-Cr composite powder is 5.75 m 2 / g, and the particle size distribution width (Span) is 1.02.

[0031] S5. Rapidly sinter the WC-Cr composite powder classified in S4 through a spark plasma sintering device: the sintering pressure is 70 MPa, heat up to 310 °C at 20 °C / min, hold for 30 min, heat up to 1215 °C at 15 °C / min, hold for 10 min, heat up to 1750 °C at 11 °C / min, hold for 6 min, and cool with the furnace to room temperature (18 - 22 °C); prepare a binderless nano cemented carbide.

[0032] The binderless nano cemented carbide prepared in Example 2 has a uniform structure, an average grain size of tungsten carbide of 475 nm, and a relative density of 99.79%.

[0033] Example 3, A method for preparing a binderless nano cemented carbide, comprising the following steps: S1. Classify WC by air classification, where the specific surface area of WC is 5.20 m 2 / g, the rotational speed of the classification wheel is 4250 r / min, and the air pressure in the classifier is 0.35 MPa, effectively ensuring the uniformity of the particle size of the nano WC powder.

[0034] S2, Dynamically oxidize and calcine the classified nano WC powder and the Cr-containing inhibitor (chromium chloride) in a rotary furnace at a calcination temperature of 500 °C for 2 h to obtain a W-Cr composite oxide, where the molar ratio of W:Cr in the classified nano WC powder to the Cr-containing inhibitor is 1:0.015; the rotational speed of the rotary furnace is 15 r / min, and the compressed air flow rate is 35 L / min.

[0035] S3, Use a plow blade mixer to mix the W-Cr composite oxide obtained in S2 with carbon black and place it in a rotary furnace for one-step carbonization under a hydrogen atmosphere, where the weight ratio of the W-Cr composite oxide to carbon black is 1:0.185, the temperature of the first section of the rotary furnace is 540 °C, the temperature of the second section is 610 °C, the temperature of the third section is 710 °C, the temperature of the fourth section is 890 °C, and the temperature of the fifth section is 1010 °C. The carbonization time corresponding to each section temperature is 32 min, and the rotational speed is 3 r / min; WC-Cr composite powder is prepared.

[0036] S4, Perform air flow crushing and classification on the WC-Cr composite powder prepared in S3, where: the rotational speed of the classification wheel is 4150 r / min, the air pressure in the classifier is 0.32 MPa, and the specific surface area of the classified WC-Cr composite powder is 5.75 m 2 / g, and the particle size distribution width (Span) is 1.02.

[0037] S5, Rapidly sinter the WC-Cr composite powder classified in S4 through a spark plasma sintering device: the sintering pressure is 90 MPa, heat up to 290 °C at 20 °C / min, hold for 20 min, heat up to 1185 °C at 10 °C / min, hold for 15 min, heat up to 1650 °C at 9 °C / min, hold for 10 min, and cool to room temperature (18 - 22 °C) with the furnace; Prepare a binderless nano cemented carbide.

[0038] The binderless nano cemented carbide prepared in Example 3 has a uniform structure, the average grain size of tungsten carbide is 485 nm, and the relative density is 99.63%.

[0039] Comparative Example 1, Do not go through S1, and the others are the same as in Example 1.

[0040] The prepared binderless nano cemented carbide has abnormal growth of tungsten carbide grains, the average grain size of tungsten carbide is 920 nm, and the relative density is 99.73%.

[0041] Comparative Example 2, Among them, S2 is statically oxidized and calcined in a muffle furnace, and the others are the same as in Example 1.

[0042] For the prepared binderless nanocomposite cemented carbide, abnormal growth of tungsten carbide grains occurs, the average grain size of tungsten carbide is 1.2 μm, and the relative density is 98.50%.

[0043] Comparative Example 3 In S2, the molar ratio of W:Cr is 1:0.01, and the others are the same as in Example 1.

[0044] For the prepared binderless nanocomposite cemented carbide, obvious abnormal growth of tungsten carbide grains occurs, the average grain size of tungsten carbide cannot be statistically analyzed, and the relative density is 95.40%.

[0045] Comparative Example 4 In S3, the weight ratio of the W-Cr composite oxide to carbon black is 1:0.35, and the others are the same as in Example 1.

[0046] For the prepared binderless nanocomposite cemented carbide, a large amount of free carbon exists, the average grain size of tungsten carbide cannot be statistically analyzed (1.2 μm), and the relative density is 92.50%. As Figure 3 shown.

[0047] Comparative Example 5 Without going through S4, the others are the same as in Example 1.

[0048] For the prepared binderless nanocomposite cemented carbide, the structure is uniform, the average grain size of tungsten carbide is 920 nm, and the relative density is 97.60%.

[0049] Comparative Example 6 In S5, the sintering pressure is 45 MPa, and the others are the same as in Example 1.

[0050] For the prepared binderless nanocomposite cemented carbide, a large number of pores exist, the average grain size of tungsten carbide is 470 μm, and the relative density is 86.90%.

[0051] In S2 of the embodiment of the present invention, during the rotary calcination process, the nano-WC contacts with the Cr-containing inhibitor. In an environment with a controllable oxygen partial pressure, the nano-WC first undergoes a diffusion reaction with the Cr-containing inhibitor, and during the continuous rotation process, the dispersion of Cr is optimized, making it more uniformly embedded in the composite oxide, so as to more effectively inhibit grain growth during subsequent carbonization. The Cr-containing inhibitor is one or more of chromium, chromium oxide, chromium salts or chromates in any proportion; the chromium salt can be chromium trichloride, chromium sulfate, chromium nitrate or chromium acetate; the chromate can be potassium chromate, which has a wide source and controllable price. The multi-stage temperature design of S3 is mainly to match the phases in the reaction process of the W-Cr composite oxide.

[0052] The traditional methods for improving the uniformity of inhibitor distribution mainly start from liquid-phase doping or optimizing the mixing process, but there are generally problems such as poor stability and difficulties in industrial production.

[0053] In the embodiments of the present invention, high-activity nano-WC powder with optimized BET is used as the raw material. Subsequently, the uniformity of the particle size of the nano-powder is effectively ensured through classification. First, high-activity nano-tungsten carbide reacts with a Cr-containing inhibitor during oxidative calcination to form a W-Cr composite oxide with uniform Cr distribution. Then, through the step-by-step operation of carbonization with carbon black, the interfacial bonding between Cr and WC is enhanced by the uniformly distributed Cr, improving the effect of the Cr inhibitor during the carbonization process to obtain a nano-WC-Cr composite powder with a high specific surface area. Through the multi-crystallization reaction process of reduction-carbonization, the mutual diffusion between tungsten and chromium is promoted and their distribution uniformity is improved. In addition, by coordinating the composite powder and the sintering process parameters, it is ensured that the prepared binderless nano-cemented carbide has a uniform structure and high density. The problems of insufficiently fine and non-uniform powder particle size, abnormal grain growth and poor density of the binderless nano-cemented carbide are effectively solved.

[0054] The above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A preparation method of a binderless nano-cemented carbide, characterized in that, It includes the following steps: S1. Classify the nano WC powder by air classification; S2. Dynamically oxidize and calcine the classified nano WC powder and the Cr-containing inhibitor to obtain a W-Cr composite oxide, wherein the molar ratio of W:Cr in the classified nano WC powder to the Cr-containing inhibitor is 1:(0.015 - 0.05); S3. Mix the W-Cr composite oxide obtained in S2 with carbon black and perform one-step carbonization under a hydrogen atmosphere, and process it in a way of segmented heating; wherein the weight ratio of the W-Cr composite oxide to carbon black is 1:(0.185 - 0.198) to obtain a WC-Cr composite powder; S4. Crush and classify the WC-Cr composite powder by air; S5. Sinter the classified WC-Cr composite powder by multi-stage spark plasma sintering, with a sintering pressure of 70 - 90 MPa and a sintering temperature of 290 - 1750 °C, and then cool it to room temperature with the furnace after 36 - 55 min to obtain the product.

2. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S1, the specific surface area of the nano WC powder is 4.50 to 5.20 m 2 / g.

3. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S1, the rotation speed of the classification wheel is 4250 - 4500 r / min, and the carrier gas pressure of the classifier is 0.35 - 0.42 MPa.

4. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S2, the Cr-containing inhibitor is one or more of chromium, chromium oxide, chromium salts or chromates in any proportion.

5. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, wherein In S2, the calcination temperature is 500 - 600 °C and the time is 0.5 - 2 h.

6. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, wherein In S2, the dynamic oxidation calcination is carried out in a rotary furnace, the rotation speed of the rotary furnace is 10 - 15 r / min, and the compressed air flow rate is 25 - 35 L / min.

7. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, wherein, The specific segmented heating in S3 is as follows: the temperature of the first stage is 540 - 580 °C, the temperature of the second stage is 610 - 630 °C, the temperature of the third stage is 710 - 830 °C, the temperature of the fourth stage is 890 - 940 °C, and the temperature of the fifth stage is 1010 - 1070 °C. The carbonization time corresponding to each stage temperature is 16 - 32 min; the rotation speed of the rotary furnace is 3 - 4 r / min.

8. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S4, the rotation speed of the classification wheel is 4050 - 4150 r / min, and the carrier gas pressure of the classifier is 0.30 - 0.32 MPa.

9. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S4, the specific surface area of the classified WC-Cr composite powder > 5.50 m 2 / g, and the particle size distribution width < 1.

10.

10. The preparation method of a non-bonded phase nano-cemented carbide according to claim 1, characterized in that, In S5, the multi-stage spark plasma sintering is specifically as follows: heat up to 290 - 310 °C at a rate of 20 °C / min, hold for 20 - 30 min, heat up to 1185 - 1215 °C at a rate of 10 - 15 °C / min, hold for 10 - 15 min, and heat up to 1650 - 1750 °C at a rate of 9 - 11 °C / min, and hold for 6 - 10 min.

Images