Preparation method and device of gradient structure tungsten carbide-cobalt composite grinding ball for ball mill

Abstract

The invention discloses a preparation method and device of a gradient structure tungsten carbide-cobalt composite grinding ball for a ball mill, and relates to the technical field of grinding ball preparation. Firstly, tungsten carbide powder, cobalt powder, vanadium carbide powder, chromium carbide powder and stearic acid are mixed and subjected to ball milling; carrying out layered powder filling and isostatic pressing to prepare a green body, carrying out liquid-phase sintering in a vacuum furnace, and introducing hydrogen to prevent decarburization; and then carrying out ultrasonic treatment with cerium nitrate and ammonium molybdate solutions, then carrying out hot pressing to strengthen the surface, and finally regulating and controlling stress through quenching and tempering to prepare the gradient structure grinding ball which has a hard outer layer and a tough inner layer and has high wear resistance and impact toughness. Through gradient structure design, multi-element compounding and precise process control, the outer layer of the grinding ball is wear-resistant, the inner layer is impact-resistant, the wear resistance is improved, the impact toughness is improved, the service life is prolonged, the production cost is reduced, and the grinding ball is suitable for industrial production.

Description

Technical Field

[0001] The invention relates to the technical field of grinding ball preparation, in particular to a method and device for preparing gradient structure tungsten carbide-cobalt composite grinding balls for ball mills. Background Art

[0002] In industrial fields such as ore grinding and building materials processing, ball mills are key crushing equipment, and the performance of their grinding balls directly affects production efficiency and energy consumption costs. Traditional tungsten carbide-cobalt (WC-Co) grinding balls are widely used due to their high hardness and wear resistance. However, as industrial production develops towards large-scale and refined production, existing grinding balls are gradually exposing their performance shortcomings. When working, conventional single-structure grinding balls are prone to micro-cracks on the surface due to severe impact, and fractures occur inside due to insufficient toughness, resulting in a shortened life of the grinding balls and increased replacement frequency, which in turn causes increased downtime maintenance costs and loss of production capacity. In addition, when ordinary grinding balls are grinding high-hardness materials, the surface wear is severe, which can easily lead to a decrease in the dimensional accuracy of the grinding media and affect the uniformity of the product particle size.

[0003] In the existing technology, the method of improving performance by adjusting the WC-Co composition ratio has limitations. Although increasing the cobalt content can enhance toughness, it will reduce the surface hardness and accelerate wear; conversely, increasing the proportion of tungsten carbide can increase hardness, but it will increase the brittleness of the grinding balls and weaken their impact resistance. Some studies have attempted to add hard phases such as vanadium carbide and chromium carbide, but most of them use a simple mixed sintering process, which leads to uneven distribution of the second phase and cannot form a synergistic enhancement effect. At the same time, the traditional sintering process does not provide sufficient hydrogen protection, which can easily cause decarburization or oxidation of tungsten carbide, reducing the density and mechanical properties of the material.

[0004] In terms of surface treatment, conventional quenching and tempering treatments are difficult to meet the complex working conditions of grinding balls, and lack directional optimization of the surface microstructure. Some grinding balls that use chemical coating technology are prone to peeling and failure under high-speed impact due to the low bonding strength between the coating and the substrate. In addition, the existing grinding ball preparation process mostly relies on empirical parameter control and lacks precise control of gradient structure design, multiphase composite synergy and microstructure evolution, making it difficult to achieve balanced optimization of hardness, toughness and wear resistance. Therefore, there is an urgent need to develop a new grinding ball preparation technology with a gradient structure, multiphase synergistic enhancement and suitable for industrial production. Summary of the Invention

[0005] (1) Technical problems solved In view of the deficiencies in the prior art, the present invention provides a method and device for preparing gradient-structured tungsten carbide-cobalt composite grinding balls for ball mills.

[0006] (2) Technical solution A method for preparing a gradient structure tungsten carbide-cobalt composite grinding ball for a ball mill comprises: S1 Composite Powder Preparation: Mix 60-75 parts by weight of tungsten carbide powder with an average particle size of 0.5-1.5 μm, 20-30 parts by weight of cobalt powder, 3-8 parts by weight of vanadium carbide powder, and 2-5 parts by weight of chromium carbide powder. Add 1-3% of stearic acid as a dispersant to the total weight of the powder. Mill the mixture in a planetary ball mill at a speed of 300-500 rpm for 8-12 hours to obtain a uniform mixed powder. S2 gradient green body preparation: The mixed powder is filled into the mold in layers, with the outer layer filled with powder containing 18-22% cobalt and the inner layer filled with powder containing 28-32% cobalt. The green body with gradient structure is obtained by isostatic pressing at a pressure of 150-200 MPa for 10-15 minutes. S3 liquid phase sintering: Place the green body in a vacuum furnace and heat it to 1350-1450℃ at 5-10℃ / min. Keep it at this temperature for 2-3 hours to make the cobalt phase form a continuous liquid phase and promote the rearrangement of tungsten carbide particles. During this period, hydrogen is introduced at a flow rate of 50-100mL / min. The reaction formula is WC+H2→W+CH4. S4 surface strengthening treatment: immerse the sintered grinding balls in an aqueous solution containing 5-10% cerium nitrate and 3-5% ammonium molybdate, ultrasonically treat at 50-60°C for 30-60 minutes, and then hot-press at 180-200°C for 1-2 hours to form a rare earth-molybdenum composite strengthening layer on the surface; S5 heat treatment control: heat the grinding balls to 800-900℃ at 3-5℃ / min, keep them at this temperature for 1-2 hours, then water quench them, and then temper them at 400-500℃ for 2-3 hours to adjust the internal stress distribution.

[0007] Furthermore, the carbon content of the tungsten carbide powder in S1 is controlled at 6.1-6.3%, and the oxygen content of the cobalt powder is ≤0.1%.

[0008] Furthermore, during the isostatic pressing in S2, the mold is made of rubber material, and the pressure fluctuation range during the pressure holding process is ≤5 MPa.

[0009] Furthermore, the vacuum degree of the vacuum furnace in S3 is maintained at 1×10 -3 -5×10 -3 Pa, the density of the grinding balls after sintering reaches 98-99.5% of the theoretical density.

[0010] Furthermore, the frequency of the ultrasonic treatment in S4 is 20-40 kHz, and the power density is 0.5-1.0 W / cm 2 , the hot pressing pressure is 30-50MPa.

[0011] Furthermore, the hardness of the outer layer of the grinding ball is HRA88-92, and the hardness of the inner layer is HRA82-86, forming a gradient mechanical property from the outside to the inside.

[0012] A device for preparing gradient structure tungsten carbide-cobalt composite grinding balls for the ball mill, comprising: Planetary ball mill mixing unit, including a ball mill with adjustable speed, a dispersing stirring paddle and a temperature control system; The isostatic pressing unit is equipped with a hydraulic system, a rubber mold bin and a pressure sensor, with a pressure control accuracy of ±1MPa; Vacuum sintering furnace unit, equipped with vacuum adjustment device, hydrogen inlet pipeline and three-stage temperature control system; Surface treatment unit, including ultrasonic treatment tank, hot pressing mold and temperature and pressure dual control device; The heat treatment unit consists of a quenching furnace, a tempering furnace and an automatic transmission track, and the heating rate control range is 1-5℃ / min.

[0013] Furthermore, the inner lining of the ball mill jar of the planetary ball mill mixing unit is made of zirconium oxide, and the ball-to-material ratio is 8-12:1.

[0014] Furthermore, the vacuum sintering furnace unit is equipped with an infrared thermometer and a gas flow regulating valve to monitor and adjust the hydrogen flow and temperature in real time.

[0015] Furthermore, the hot pressing mold of the surface treatment unit is made of graphite, the surface treatment tank is provided with a circulating cooling system, and the temperature fluctuation range is ≤±2°C.

[0016] (3) Beneficial technical effects Compared with the existing technology, the beneficial effects of the present invention are: The gradient structural design imparts high hardness to the outer layer of the grinding balls, effectively resisting wear during grinding, while maintaining high toughness within the inner layer to prevent fracture from impact. This "hard outside, tough inside" characteristic more than doubles the service life of the grinding balls under complex operating conditions. The combined addition of vanadium carbide, chromium carbide, and rare earth elements forms a dispersion-strengthened phase during sintering, refining the grain size and optimizing microstructure distribution, further enhancing the material's wear resistance and fatigue resistance.

[0017] During the manufacturing process, precisely controlled isostatic pressing and vacuum liquid-phase sintering ensure a dense and uniform internal structure of the grinding balls, achieving a density exceeding 98% of the theoretical value and eliminating defects such as pores and cracks found in traditional processes. Surface strengthening treatment, through the introduction of a rare earth-molybdenum composite layer, not only improves surface hardness and oxidation resistance but also enhances the bond between the surface and the substrate, effectively inhibiting the initiation and propagation of microcracks. Precise temperature and stress control during the heat treatment process creates residual compressive stress within the grinding balls, significantly enhancing their impact resistance.

[0018] This preparation method has good industrial adaptability. Through standardized equipment and parameter control, it can achieve mass production of grinding balls. Practical application verification shows that this patented grinding ball reduces wear rate by over 60% and improves impact toughness by 90% when processing high-hardness ores. It also reduces downtime caused by grinding ball failure, lowering production costs for enterprises, and providing a high-performance, long-life grinding media solution for the ball milling industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic flow chart of a method for preparing gradient structure tungsten carbide-cobalt composite grinding balls for ball mills; Figure 2 This is a schematic diagram of a device for preparing gradient structure tungsten carbide-cobalt composite grinding balls for ball mills. DETAILED DESCRIPTION

[0020] refer to Figures 1 to 2 , the specific implementation of the present invention is as follows: Example 1 Composite powder preparation Weigh 70 kg of tungsten carbide powder (6.2% carbon content) with an average particle size of 1.0 μm, 25 kg of cobalt powder (0.08% oxygen content), 5 kg of vanadium carbide powder, and 3 kg of chromium carbide powder. Add 1.5 kg of stearic acid dispersant. Place these raw materials into the zirconia-lined milling jar of a planetary ball mill with a ball-to-material ratio of 10:1. Mill at 400 rpm for 10 hours. During the milling process, pause stirring every two hours to check the powder mix for uniformity and ensure there are no noticeable agglomerates. After milling, sieve the mixture through a vibrating screen (200 mesh) to remove any large impurities and obtain a uniformly mixed powder.

[0021] Gradient body preparation The mixed powder is divided into two groups, one group is adjusted to contain 20% cobalt as the outer layer powder, and the other group is adjusted to contain 30% cobalt as the inner layer powder. A cylindrical rubber mold with an inner diameter of 50mm and a height of 120mm is selected. First, fill the outer layer powder into the bottom of the mold with a thickness of 20mm, then fill the inner layer powder into the middle with a thickness of 80mm, and finally cover it with a 20mm thick outer layer of powder. Place the mold in an isostatic pressing device and maintain the pressure at 180MPa for 12 minutes. During the pressure maintenance process, the pressure is monitored in real time by a pressure sensor to ensure that the fluctuation range does not exceed 5MPa. After the pressure maintenance is completed, slowly release the pressure to remove the blank and conduct an appearance inspection to ensure that there are no cracks or dents on the surface.

[0022] Liquid Phase Sintering Place the blank in a vacuum furnace, close the furnace door and evacuate until the vacuum reaches 2×10 -3The furnace was heated to 1400°C at a rate of 8°C / min while introducing hydrogen at a flow rate of 80 mL / min. During the heating process, the furnace temperature was monitored in real time using an infrared thermometer. After reaching the target temperature, the furnace was held for 2.5 hours to fully liquefy the cobalt phase and promote the rearrangement and densification of the tungsten carbide particles. After the holding period, the hydrogen flow was stopped, the heating power was turned off, and the furnace was cooled to room temperature. The grinding ball blanks were removed and measured to have a density of 99.2% of the theoretical density.

[0023] Surface strengthening treatment Prepare 50L of an aqueous solution containing 8% cerium nitrate and 4% ammonium molybdate, immerse the sintered grinding balls in the solution, and place them in an ultrasonic treatment tank. Set the ultrasonic frequency to 30kHz and the power density to 0.8W / cm 2 The temperature was controlled at 55°C for 45 minutes. During the treatment, the solution was stirred every 15 minutes to ensure sufficient contact between the grinding ball surfaces. After the ultrasonic treatment, the grinding balls were removed, rinsed three times with deionized water, and placed in a graphite hot-pressing mold. Hot-pressing was performed at 190°C and 40 MPa for 1.5 hours to form a rare earth-molybdenum composite strengthening layer on the grinding ball surfaces.

[0024] Heat treatment control The grinding balls were placed in a quenching furnace and heated to 850°C at a rate of 4°C / min. After holding at this temperature for 1.5 hours, they were quickly quenched in 25°C water. After quenching, the grinding balls were transferred to a tempering furnace and heated to 450°C at a rate of 3°C / min for 2.5 hours. After tempering, the balls were cooled to room temperature in the furnace, completing the preparation. Testing revealed that the outer layer of the grinding balls had a hardness of HRA90, while the inner layer had a hardness of HRA84.

[0025] Example 2 Composite powder preparation Take 65 kg of tungsten carbide powder with an average particle size of 0.8 μm, 28 kg of cobalt powder, 6 kg of vanadium carbide powder, and 4 kg of chromium carbide powder, and add 1.8 kg of stearic acid. Mill the mixture in a planetary ball mill at 350 rpm for 11 hours, using a ball-to-powder ratio of 9:1. After milling, sieve the mixture through a 180-mesh vibrating screen to obtain a mixed powder.

[0026] Gradient body preparation The mixed powder was adjusted to have an outer layer cobalt content of 19% and an inner layer cobalt content of 31%. A rubber mold with an inner diameter of 45 mm was used, and the same layered filling method as in Example 1 was adopted. The green body was prepared at a pressure of 170 MPa for 13 minutes.

[0027] Liquid Phase Sintering The green body was placed in a vacuum furnace and vacuumed to 1.8×10 -3Pa, heated to 1380°C at 7°C / min, introduced hydrogen at 70 mL / min, and kept at this temperature for 2.8 hours. After cooling, the ball density reached 99% of the theoretical density.

[0028] Surface strengthening treatment An aqueous solution containing 7% cerium nitrate and 3.5% ammonium molybdate was prepared, and the ultrasonic treatment parameters were set to a frequency of 25 kHz and a power density of 0.7 W / cm 2 , temperature 52 ° C, treatment for 50 minutes. Then hot pressing at 185 ° C, 35 MPa for 2 hours.

[0029] Heat treatment control The ball was heated to 830°C at a rate of 3.5°C / min, quenched, and then tempered at 430°C for 2.8 hours. The outer layer hardness of the grinding ball was HRA89, and the inner layer hardness was HRA83.

[0030] Example 3 Composite powder preparation Weigh 72 kg of tungsten carbide powder with an average particle size of 1.2 μm, 24 kg of cobalt powder, 7 kg of vanadium carbide powder, and 3 kg of chromium carbide powder, add 2 kg of stearic acid, and ball mill them in a ball mill at a speed of 450 r / min for 9 hours, with a ball-to-material ratio of 11:1. After sieving, a mixed powder is obtained.

[0031] Gradient body preparation The powder was adjusted to have an outer layer containing 21% cobalt and an inner layer containing 29% cobalt, and was molded using a mold with an inner diameter of 55 mm and a pressure of 190 MPa for 11 minutes.

[0032] Liquid Phase Sintering The vacuum degree of the vacuum furnace reaches 2.2×10 -3 Pa, heated to 1420℃ at 9℃ / min, passed hydrogen at 90mL / min, kept warm for 2.2 hours, and the ball density reached 99.3% of the theoretical density.

[0033] Surface strengthening treatment Prepare a solution containing 9% cerium nitrate and 4.5% ammonium molybdate, with an ultrasonic frequency of 35 kHz and a power density of 0.9 W / cm 2 , temperature 58℃, treatment for 40 minutes, and then hot pressing at 195℃ and 45MPa for 1.2 hours.

[0034] Heat treatment control The grinding balls were heated to 870℃ at a rate of 4.5℃ / min and then quenched. They were then tempered at 470℃ for 2.2 hours. The outer layer hardness of the grinding balls was HRA91 and the inner layer hardness was HRA85.

[0035] Comparative Example Powder mixing 75 kg of tungsten carbide powder and 25 kg of cobalt powder were taken, 1 kg of stearic acid was added, and the mixture was ball milled in a planetary ball mill at a speed of 300 r / min for 6 hours. No vanadium carbide and chromium carbide were added.

[0036] Body preparation The mixed powder was directly filled into the mold and formed under a pressure of 80 MPa using unidirectional pressing, and no gradient structure was formed.

[0037] Sintering treatment The sintering was carried out in an ordinary resistance furnace at 1400°C in air atmosphere for 2 hours without the introduction of hydrogen.

[0038] Post-processing Only simple grinding and polishing were performed without surface strengthening or heat treatment. Testing showed that the grinding balls had a uniform hardness of HRA86 and a density of 95% of the theoretical density.

[0039] Performance test results:

[0040] The test results show that the grinding balls of Examples 1-3 significantly outperformed the comparative example in terms of hardness gradient, density, wear resistance, and impact toughness. In particular, Example 3, through optimized component ratios and process parameters, achieved the best performance across all performance indicators, demonstrating that the preparation method of the present invention can effectively improve the overall performance of grinding balls.

[0041] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a gradient structure tungsten carbide-cobalt composite grinding ball for a ball mill, characterized in that: include: S1 Composite Powder Preparation: Mix 60-75 parts by weight of tungsten carbide powder with an average particle size of 0.5-1.5 μm, 20-30 parts by weight of cobalt powder, 3-8 parts by weight of vanadium carbide powder, and 2-5 parts by weight of chromium carbide powder. Add 1-3% of stearic acid as a dispersant to the total weight of the powder. Mill the mixture in a planetary ball mill at a speed of 300-500 rpm for 8-12 hours to obtain a uniform mixed powder. S2 gradient green body preparation: The mixed powder is filled into the mold in layers, with the outer layer filled with powder containing 18-22% cobalt and the inner layer filled with powder containing 28-32% cobalt. The green body with gradient structure is obtained by isostatic pressing at a pressure of 150-200 MPa for 10-15 minutes. S3 liquid phase sintering: Place the green body in a vacuum furnace and heat it to 1350-1450°C at 5-10°C / min, then keep it at that temperature for 2-3 hours to make the cobalt phase form a continuous liquid phase and promote the rearrangement of tungsten carbide particles; During this period, hydrogen is introduced at a flow rate of 50-100 mL / min, and the reaction formula is WC+H2→W+CH4; S4 surface strengthening treatment: immerse the sintered grinding balls in an aqueous solution containing 5-10% cerium nitrate and 3-5% ammonium molybdate, ultrasonically treat at 50-60°C for 30-60 minutes, and then hot-press at 180-200°C for 1-2 hours to form a rare earth-molybdenum composite strengthening layer on the surface; S5 heat treatment control: heat the grinding balls to 800-900℃ at 3-5℃ / min, keep them at this temperature for 1-2 hours, then water quench them, and then temper them at 400-500℃ for 2-3 hours to adjust the internal stress distribution.

2. The preparation method according to claim 1, characterized in that The carbon content of the tungsten carbide powder in S1 is controlled at 6.1-6.3%, and the oxygen content of the cobalt powder is ≤0.1%.

3. The preparation method according to claim 1, characterized in that During the isostatic pressing in S2, the mold is made of rubber, and the pressure fluctuation range during the pressure holding process is ≤5MPa.

4. The preparation method according to claim 1, characterized in that The vacuum degree of the vacuum furnace in S3 is maintained at 1×10 -3 -5×10 -3 Pa, the density of the grinding balls after sintering reaches 98-99.5% of the theoretical density.

5. The preparation method according to claim 1, characterized in that The frequency of the ultrasonic treatment in S4 is 20-40 kHz, and the power density is 0.5-1.0 W / cm 2 , the hot pressing pressure is 30-50MPa.

6. The preparation method according to claim 1, characterized in that The hardness of the outer layer of the grinding ball is HRA88-92, and the hardness of the inner layer is HRA82-86, forming a gradient mechanical property from the outside to the inside.

7. A device for implementing the method for preparing the gradient structure tungsten carbide-cobalt composite grinding balls for ball mills according to any one of claims 1 to 6, characterized in that: include: Planetary ball mill mixing unit, including a ball mill with adjustable speed, a dispersing stirring paddle and a temperature control system; Isostatic pressing unit is equipped with hydraulic system, rubber mold chamber and pressure sensor, with pressure control accuracy of ±1MPa; Vacuum sintering furnace unit, equipped with vacuum adjustment device, hydrogen inlet pipeline and three-stage temperature control system; Surface treatment unit, including ultrasonic treatment tank, hot pressing mold and temperature and pressure dual control device; The heat treatment unit consists of a quenching furnace, a tempering furnace and an automatic transmission track, and the heating rate control range is 1-5℃ / min.

8. The device according to claim 7, characterized in that The ball mill jar of the planetary ball mill mixing unit is lined with zirconium oxide, and the ball-to-material ratio is 8-12:

1.

9. The device according to claim 7, characterized in that The vacuum sintering furnace unit is equipped with an infrared thermometer and a gas flow regulating valve to monitor and adjust the hydrogen flow and temperature in real time.

10. The device according to claim 7, characterized in that The hot pressing mold of the surface treatment unit is made of graphite, and the surface treatment tank is equipped with a circulating cooling system, with a temperature fluctuation range of ≤±2°C.

Images