Low cobalt high toughness cemented carbide material and method for producing the same

Abstract

The present application relates to the technical field of hard alloy, in particular to a low-cobalt high-toughness hard alloy material and a preparation method thereof. The material disperses tungsten carbide powder in a mixed solution containing branched polyethyleneimine, sequentially adds seed cobalt solution and growth cobalt solution, adjusts the pH of the system to 7.5-7.9, and then sequentially adds ammonium oxalate solution and oxalic acid solution for deposition and aging. The precursor composite powder is obtained through solid-liquid separation, washing and drying. The hard alloy sintered body is prepared through segmented hydrogen reduction, binder addition, compression, dewaxing and gas pressure sintering. The method is conducive to improving the uniformity of cobalt phase distribution and improving the strength, toughness and wear resistance of low-cobalt hard alloy.

Description

Technical Field

[0001] This invention relates to the field of cemented carbide technology, specifically to a low-cobalt, high-toughness cemented carbide material and its preparation method. Background Technology

[0002] Tungsten carbide (WC) is the hard phase and cobalt (Co) is the binder phase. It combines high hardness, high strength, and good wear resistance, and is widely used in cutting tools, cold heading dies, fine blanking dies, and wear-resistant parts. As modern manufacturing moves towards higher efficiency, higher precision, and longer lifespan, higher demands are placed on the performance of cemented carbide: it not only needs sufficiently high hardness and wear resistance, but also excellent impact toughness and fatigue resistance to cope with alternating heavy-load conditions such as cold heading and fine blanking.

[0003] Reducing the cobalt content is a direct way to improve the hardness and wear resistance of cemented carbide. However, when the cobalt content drops to around 5 wt% or even lower, the total amount of the binder phase decreases significantly, making its continuous distribution among WC particles extremely difficult. Traditional preparation processes often involve mechanically mixing WC powder and Co powder, or using wet milling to attach cobalt salts or cobalt powder to the surface of WC particles. In the early stages of mixing, drying, forming, and liquid-phase sintering, due to the low Co content and the large size difference between WC and Co particles, Co is prone to secondary agglomeration during slurry drying due to solvent migration, or to forming local enrichment zones in the compact. During the liquid-phase sintering stage, the highly fluid molten Co further agglomerates towards these enrichment zones, resulting in the simultaneous appearance of "cobalt pools" (large regions formed by cobalt agglomeration) and numerous cobalt-poor grain boundaries in the final sintered body. This non-uniform microstructure causes the WC-WC grain boundaries, which should bear the load, to become preferential crack propagation channels due to the lack of sufficient toughening binder phase, ultimately leading to early chipping, collapse, and large lifespan dispersion failures during service. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a solution to the problem of insufficient anti-chipping ability and life stability of existing low-cobalt cemented carbide materials under alternating heavy load conditions.

[0005] Based on the above objectives, the present invention provides a low-cobalt, high-toughness cemented carbide material, which uses tungsten carbide powder as a matrix, introduces active sites on the surface of branched polyethyleneimine through surface functionalization modification, and then uses a two-step seed-growth method to successively add seed cobalt solution and growth cobalt solution, followed by a two-step precipitation process of ammonia water neutralization, ammonium oxalate aqueous solution and oxalic acid aqueous solution to form a precursor composite powder, which is then reduced by hydrogen, granulated, and finally shaped and sintered to obtain the final product.

[0006] Preferably, the tungsten carbide powder has a particle size of 2 μm and a purity of not less than 99%.

[0007] Preferably, both the seed cobalt solution and the growth cobalt solution are aqueous solutions of cobalt acetate tetrahydrate.

[0008] Preferably, the weight ratio of cobalt acetate tetrahydrate to the solution in the seed cobalt solution is 1:10.

[0009] Preferably, the weight ratio of cobalt acetate tetrahydrate to the solution in the cobalt growth solution is 1:3.

[0010] Preferably, the weight ratio of ammonium oxalate to the solution in the ammonium oxalate aqueous solution is 1:15.

[0011] Preferably, the weight ratio of ammonium oxalate to the solution in the oxalic acid aqueous solution is 1:7.

[0012] Preferably, the branched polyethyleneimine is a 50 wt% aqueous solution with a number-average molecular weight of 1200.

[0013] Preferably, based on 100 parts of total tungsten carbide powder, the amounts of branched polyethyleneimine, seed cobalt solution, growth cobalt solution, and ammonium oxalate aqueous solution added are 1.8-2.2 parts, 19.8-26.4 parts, 72-88 parts, and 40-56 parts, respectively.

[0014] Preferably, the purpose of neutralizing the ammonia water is to adjust the pH of the system to between 7.5 and 7.9.

[0015] Preferably, the purpose of the oxalic acid aqueous solution is to adjust the pH of the system to between 6.0 and 6.5.

[0016] Furthermore, the present invention also provides a method for preparing a low-cobalt, high-toughness cemented carbide material, comprising the following steps:

[0017] (1) Tungsten carbide powder is added to a dispersion system consisting of branched polyethyleneimine aqueous solution, glacial acetic acid, water and anhydrous ethanol to form a slurry; then cobalt acetate tetrahydrate is prepared into seed cobalt solution and growth cobalt solution respectively, and added to the slurry in sequence to load the cobalt source onto the surface of tungsten carbide powder. Ammonia water is then added to the obtained system to adjust the pH value. Then ammonium oxalate solution and oxalic acid aqueous solution are added to the system in sequence for deposition and aging. The obtained slurry is subjected to solid-liquid separation, washing, drying and depolymerization to obtain precursor composite powder.

[0018] (2) The precursor composite powder is reduced by staged heating under a protective atmosphere and a reducing atmosphere to obtain the reduced composite powder;

[0019] (3) The reduced composite powder is mixed with the binder solution, dried and granulated to obtain granulated powder. The granulated powder is then dewaxed and sintered after being pressed to obtain a low-cobalt high-toughness hard alloy sintered body.

[0020] Preferably, the weight ratio of tungsten carbide powder, branched polyethyleneimine aqueous solution, glacial acetic acid, water and anhydrous ethanol in step (1) is 100:1.8-2.2:0.8-1.2:55-65:18-22.

[0021] Preferably, the specific process of the segmented heating reduction in step (2) is as follows: first, the gas is replaced in an argon atmosphere at a flow rate of 90-110 mL / min for 25-35 min, then switched to high-purity hydrogen at a flow rate of 180-220 mL / min, and heated to 270-290℃ at a flow rate of 1.8-2.2℃ / min and held for 50-70 min to remove adsorbed water, crystal water and some organic cracking products; then, the gas is heated to 420-440℃ at a flow rate of 1.8-2.2℃ / min and held for 80-100 min to decompose and reduce the cobalt oxalate precursor into fine metallic cobalt; then, the gas is heated to 500-520℃ at a flow rate of 1.8-2.2℃ / min and held for 20-40 min to further remove residual organic cracking products; then, the gas is cooled to below 70-90℃ in a hydrogen atmosphere and switched back to argon.

[0022] Preferably, the adhesive solution in step (3) is a mixed solution of paraffin and n-heptane, wherein the concentration of paraffin is 5%.

[0023] Preferably, the compression molding in step (3) is performed under unidirectional compression molding at 170-190 MPa.

[0024] Preferably, the weight ratio of the binder solution and the reducing composite powder in step (3) is 1:5.

[0025] Preferably, the specific process of sintering in step (3) is as follows: under a vacuum degree not higher than 10 Pa, the temperature is increased to 190-210℃ at 0.8-1.2℃ / min and held for 25-35 min, then the temperature is increased to 470-490℃ at 0.8-1.2℃ / min and held for 50-70 min to complete dewaxing, and then the temperature is increased to 1408-1422℃ and held for 50-60 min for sintering. At the end of the holding period, 4-6 MPa argon gas is introduced and the pressure is maintained for 12-18 min.

[0026] The beneficial effects of this invention are:

[0027] This invention utilizes polyethyleneimine to perform targeted modification on the surface of tungsten carbide particles, inducing the subsequent addition of seed cobalt. The cobalt solution grows and achieves layer-by-layer enrichment of cobalt at the same site, avoiding the averaged coverage caused by a single addition. Ammonia pre-complexation provides a timing window for two-step oxalic acid precipitation. A step-by-step precipitation strategy, first using ammonium oxalate for weak nucleation and then oxalic acid for acidic completion, locks the deposition center of the cobalt oxalate precursor near the established seed site. This constructs a spatially precise precursor distribution framework at the tungsten carbide particle's load-bearing interface. After subsequent reduction and transformation, the final position of metallic cobalt is highly consistent with its precursor deposition position, effectively suppressing non-targeted migration of cobalt into the pore region during liquid-phase sintering. This ensures that the originally limited cobalt is preferentially and continuously distributed in the grain boundary region that truly bears the load, fundamentally improving the microstructure uniformity of the low-cobalt system.

[0028] This invention further employs a stepped heating reduction process, which further enhances the material's hardness and wear resistance.

[0029] In summary, this invention, through the synergistic combination of materials and processes, significantly improves the transverse fracture strength and fracture toughness of the material while maintaining the high hardness and high compressive strength of the low-cobalt system. More importantly, this uniformity of microstructure greatly reduces batch-to-batch performance fluctuations, enabling the alloy to exhibit excellent chipping resistance and life stability under high-load alternating conditions such as cold heading punches and fine blanking dies. This avoids early random failures caused by localized weak areas, meeting the stringent requirements of high-reliability applications such as precision forming of new energy vehicle components. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0031] Raw material source and specifications:

[0032] Tungsten carbide powder: Sigma-Aldrich, item number 241881, with a particle size of 2μm and a purity of not less than 99%; branched polyethyleneimine aqueous solution: Sigma-Aldrich, item number 482595, a 50wt% aqueous solution with a number average molecular weight of 1200.

[0033] Example 1: A method for preparing a low-cobalt, high-toughness cemented carbide material, the specific steps of which are as follows:

[0034] S1: Weigh 100g tungsten carbide powder, 2g branched polyethyleneimine aqueous solution, 1g glacial acetic acid, 60g deionized water and 20g anhydrous ethanol. First, add 2g branched polyethyleneimine aqueous solution to the mixed solvent of 60g deionized water and 20g anhydrous ethanol, then add 1g glacial acetic acid. Stir at 700r / min for 10min at 25℃, then add 100g tungsten carbide powder and continue stirring at 700r / min for 45min to obtain a uniform gray-black slurry.

[0035] S2: Weigh 2g of cobalt acetate tetrahydrate and 20g of deionized water, stir at 25℃ until completely dissolved to form a seed cobalt solution; then add the seed cobalt solution dropwise to the slurry obtained in S1 within 10min, and continue stirring at 700r / min for 20min after the addition is complete;

[0036] S3: Weigh 20g of cobalt acetate tetrahydrate and 60g of deionized water, stir at 25℃ until completely dissolved to form a cobalt growth solution; then add the cobalt growth solution dropwise to the slurry obtained in S2 within 20min, and continue stirring at 700r / min for 20min after the addition is complete;

[0037] S4: Weigh 5g of ammonia water and 15g of deionized water to prepare diluted ammonia water; add the diluted ammonia water dropwise to the slurry obtained in S3 at 25℃, and monitor the pH of the system in real time. Stop adding the ammonia water when the pH of the system stabilizes between 7.6 and 7.8 and continue stirring for 10 minutes.

[0038] S5: Weigh 3g of ammonium oxalate monohydrate and 45g of deionized water, heat and stir at 60℃ to form an ammonium oxalate solution, and add it dropwise to the slurry obtained in S4 within 10min, while maintaining the slurry temperature at 33℃; after the addition is complete, continue aging for 15min.

[0039] S6: Weigh 8g of oxalic acid dihydrate and 56g of deionized water, heat and stir at 50℃ until completely dissolved to form an oxalic acid aqueous solution, and add it dropwise to the slurry obtained in S5 within 15min, while monitoring the pH of the system in real time. Stop adding the solution when the pH of the system stabilizes between 6.0 and 6.5, and continue aging at 30℃ for 30min until the supernatant changes from light pink to nearly colorless;

[0040] S7: The slurry obtained in S6 is filtered and the filter cake is washed twice with 60g of washing liquid each time. Each washing liquid consists of 30g of anhydrous ethanol and 30g of deionized water. After washing, the mixture is dried under vacuum at 70℃ for 10h. After drying, the mixture is gently depolymerized and passed through a 100-mesh sieve to obtain the precursor composite powder.

[0041] S8: Place 100g of precursor composite powder in a conventional reduction furnace. First, purge under an argon atmosphere at 100mL / min for 30min. Then, switch to high-purity hydrogen at 200mL / min and heat to 280℃ at 2℃ / min, holding for 60min to remove adsorbed water, crystal water, and some organic pyrolysis products. Next, heat to 430℃ at 2℃ / min and hold for 90min. Then, heat to 510℃ at 2℃ / min and hold for 30min. Finally, cool to below 80℃ in a hydrogen atmosphere and switch back to argon to obtain the reduced composite powder.

[0042] S9: Weigh 1g of paraffin wax and 19g of n-heptane, and completely dissolve the paraffin wax in n-heptane at 65℃ to form a binder solution; spray the binder solution into 100g of reduced composite powder, mix for 30min, dry at 50℃ for 60min, and then pass through a 60-mesh sieve to obtain granulated powder.

[0043] S10: The obtained granulated powder is unidirectionally pressed into shape at 180MPa. The pressed blank is placed in a graphite boat. Under the condition that the vacuum degree is not higher than 10Pa, the temperature is raised to 200℃ at 1℃ / min and held for 30min. Then, the temperature is raised to 480℃ at 1℃ / min and held for 60min to complete dewaxing. After that, the temperature is raised to 1415℃ and held for 55min for sintering. Argon gas of 5MPa is introduced at the end of the holding period and pressure is maintained for 15min. Then, it is cooled to obtain a low cobalt high toughness cemented carbide sintered body.

[0044] Example 2: A method for preparing a low-cobalt, high-toughness cemented carbide material, the specific steps of which are as follows:

[0045] S1: Weigh 100g tungsten carbide powder, 1.8g branched polyethyleneimine aqueous solution, 0.8g glacial acetic acid, 55g deionized water and 18g anhydrous ethanol. First, add the branched polyethyleneimine aqueous solution to the mixed solvent of deionized water and anhydrous ethanol, then add glacial acetic acid. Stir at 650r / min for 8min at 24℃. Then add 100g tungsten carbide powder and continue stirring at 650r / min for 40min to obtain a uniform gray-black slurry.

[0046] S2: Weigh 1.8g of cobalt acetate tetrahydrate and 18g of deionized water, stir at 24℃ until completely dissolved to form a seed cobalt solution; then add the seed cobalt solution dropwise to the slurry obtained in S1 within 8 min, and continue stirring at 650 r / min for 18 min after the addition is complete;

[0047] S3: Weigh 18g of cobalt acetate tetrahydrate and 54g of deionized water, stir at 24℃ until completely dissolved to form a cobalt growth solution; then add the cobalt growth solution dropwise to the slurry obtained in S2 within 18min, and continue stirring at 650r / min for 18min after the addition is complete.

[0048] S4: Weigh 4.5g of ammonia water and 12g of deionized water to prepare diluted ammonia water; add the diluted ammonia water dropwise to the slurry obtained in S3 at 24℃, and monitor the pH of the system in real time. Stop adding the ammonia water when the pH of the system stabilizes between 7.5 and 7.9 and continue stirring for 8 minutes.

[0049] S5: Weigh 2.5g of ammonium oxalate monohydrate and 37.5g of deionized water, heat and stir at 58℃ to form an ammonium oxalate solution, and add it dropwise to the slurry obtained in S4 within 8min, while maintaining the slurry temperature at 30℃; after the addition is complete, continue aging for 12min.

[0050] S6: Weigh 7.5g of oxalic acid dihydrate and 52.5g of deionized water, heat and stir at 48℃ until completely dissolved to form an oxalic acid aqueous solution, and add it dropwise to the slurry obtained in S5 within 12min, while monitoring the pH of the system in real time. Stop adding the solution when the pH of the system stabilizes between 6.0 and 6.5, and continue aging at 30℃ for 25min until the supernatant changes from light pink to nearly colorless;

[0051] S7: The slurry obtained in S6 is filtered and the filter cake is washed twice with 55g of washing liquid each time. Each washing liquid consists of 27g of anhydrous ethanol and 27g of deionized water. After washing, it is dried under vacuum at 68℃ for 9h. After drying, it is gently depolymerized and passed through a 100-mesh sieve to obtain the precursor composite powder.

[0052] S8: Place 100g of precursor composite powder in a conventional reduction furnace. First, purify the powder under an argon atmosphere at 90mL / min for 25min. Then, switch to high-purity hydrogen at 180mL / min, raise the temperature to 270℃ at 1.8℃ / min and hold for 50min. Next, raise the temperature to 420℃ at 1.8℃ / min and hold for 80min. Then, raise the temperature to 500℃ at 1.8℃ / min and hold for 20min. Finally, cool the powder to below 70℃ in a hydrogen atmosphere and switch back to argon to obtain the reduced composite powder.

[0053] S9: Weigh 1g of paraffin wax and 19g of n-heptane, and completely dissolve the paraffin wax in the n-heptane at 65℃ to form a binder solution; spray the binder solution into 100g of reduced composite powder, mix for 25min, dry at 45℃ for 50min, and then pass through a 60-mesh sieve to obtain granulated powder.

[0054] S10: The obtained granulated powder is unidirectionally pressed into shape at 170MPa. The pressed blank is placed in a graphite boat. Under the condition that the vacuum degree is not higher than 10Pa, the temperature is raised to 190℃ at 0.8℃ / min and held for 25min. Then, the temperature is raised to 470℃ at 0.8℃ / min and held for 50min to complete the dewaxing. After that, the temperature is raised to 1408℃ and held for 50min for sintering. Argon gas at 4MPa is introduced at the end of the holding period and the pressure is maintained for 12min. Then, the temperature is cooled to obtain a low cobalt high toughness cemented carbide sintered body.

[0055] Example 3: A method for preparing a low-cobalt, high-toughness cemented carbide material, the specific steps of which are as follows:

[0056] S1: Weigh 100g of tungsten carbide powder, 2.2g of branched polyethyleneimine aqueous solution, 1.2g of glacial acetic acid, 65g of deionized water and 22g of anhydrous ethanol. First, add the branched polyethyleneimine aqueous solution to the mixed solvent of deionized water and anhydrous ethanol, then add glacial acetic acid. Stir at 750r / min for 12min at 26℃, then add 100g of tungsten carbide powder and continue stirring at 750r / min for 50min to obtain a uniform gray-black slurry.

[0057] S2: Weigh 2.4g of cobalt acetate tetrahydrate and 24g of deionized water, stir at 26℃ until completely dissolved to form a seed cobalt solution; then add the seed cobalt solution dropwise to the slurry obtained in S1 within 12min, and continue stirring at 750r / min for 22min after the addition is complete;

[0058] S3: Weigh 22g of cobalt acetate tetrahydrate and 66g of deionized water, stir at 26℃ until completely dissolved to form a cobalt growth solution; then add the cobalt growth solution dropwise to the slurry obtained in S2 within 22 min, and continue stirring at 750 r / min for 22 min after the addition is complete;

[0059] S4: Weigh 5.5g of ammonia water and 18g of deionized water to prepare diluted ammonia water; add the diluted ammonia water dropwise to the slurry obtained in S3 at 26℃, and monitor the pH of the system in real time. Stop adding the ammonia water when the pH of the system stabilizes between 7.5 and 7.9 and continue stirring for 12 minutes.

[0060] S5: Weigh 3.5g of ammonium oxalate monohydrate and 52.5g of deionized water, heat and stir at 62℃ to form an ammonium oxalate solution, and add it dropwise to the slurry obtained in S4 within 12min, while maintaining the slurry temperature at 35℃; after the addition is complete, continue aging for 18min.

[0061] S6: Weigh 8.5g of oxalic acid dihydrate and 59.5g of deionized water, heat and stir at 52℃ until completely dissolved to form an oxalic acid aqueous solution, and add it dropwise to the slurry obtained in S5 within 18min, while monitoring the pH of the system in real time. Stop adding the solution when the pH of the system stabilizes between 6.0 and 6.5, and continue aging at 30℃ for 35min until the supernatant changes from light pink to nearly colorless;

[0062] S7: The slurry obtained in S6 is filtered and the filter cake is washed twice with 65g of washing liquid each time. Each washing liquid consists of 33g of anhydrous ethanol and 33g of deionized water. After washing, it is dried under vacuum at 72℃ for 11h. After drying, it is gently depolymerized and passed through a 100-mesh sieve to obtain the precursor composite powder.

[0063] S8: Place 100g of precursor composite powder in a conventional reduction furnace. First, purify the powder under an argon atmosphere at 110mL / min for 35min. Then, switch to high-purity hydrogen at 220mL / min, raise the temperature to 290℃ at 2.2℃ / min and hold for 70min. Next, raise the temperature to 440℃ at 2.2℃ / min and hold for 100min. Then, raise the temperature to 520℃ at 2.2℃ / min and hold for 40min. Finally, cool the powder to below 90℃ in a hydrogen atmosphere and switch back to argon to obtain the reduced composite powder.

[0064] S9: Weigh 1g of paraffin wax and 19g of n-heptane, and completely dissolve the paraffin wax in the n-heptane at 65℃ to form a binder solution; spray the binder solution into 100g of reduced composite powder, mix for 35min, dry at 55℃ for 70min, and then pass through a 60-mesh sieve to obtain granulated powder.

[0065] S10: The obtained granulated powder is unidirectionally pressed into shape at 190 MPa. The pressed blank is placed in a graphite boat. Under the condition that the vacuum degree is not higher than 10 Pa, the temperature is raised to 210℃ at 1.2℃ / min and held for 35 min. Then, the temperature is raised to 490℃ at 1.2℃ / min and held for 70 min to complete the dewaxing. After that, the temperature is raised to 1422℃ and held for 60 min for sintering. Argon gas of 6 MPa is introduced at the end of the holding period and the pressure is maintained for 18 min. Then, it is cooled to obtain a low cobalt high toughness cemented carbide sintered body.

[0066] Comparative Example 1: The difference from Example 1 is that branched polyethyleneimine aqueous solution is not added in step S1, and the total liquid volume is made up with an equal amount of deionized water; the other conditions are the same as in Example 1.

[0067] Comparative Example 2: The difference from Example 1 is that in step S2, 21g of cobalt acetate tetrahydrate and 80g of deionized water are weighed, stirred at 25°C until completely dissolved to form a mixed cobalt solution, and the mixed cobalt solution is added dropwise to the slurry obtained in S1 within 30 minutes. After the addition is completed, stirring is continued at 700r / min for 20 minutes. Step S3 is omitted. The remaining conditions are the same as in Example 1.

[0068] Comparative Example 3: The difference from Example 1 is that in step S4, no ammonia water is added, only deionized water is added and stirring is continued for 10 minutes, and no ammonia complexation treatment is performed after step S4; the other conditions are the same as in Example 1.

[0069] Comparative Example 4: The difference from Example 1 is that in step S5, ammonium oxalate monohydrate and oxalate dihydrate are added together to deionized water, heated and stirred at 55°C until completely dissolved to form a mixed oxalic acid solution, which is then added dropwise to the slurry obtained in S4 within 25 minutes. After the addition is completed, the slurry is aged at 30°C for another 30 minutes. Step S6 is omitted. The remaining conditions are the same as in Example 1.

[0070] Comparative Example 5: The difference from Example 1 is that in step S5, oxalic acid dihydrate and deionized water are weighed, heated and stirred at 50°C until completely dissolved to form an oxalic acid aqueous solution, and added dropwise to the slurry obtained in S4 within 15 minutes. After the addition is completed, aging continues for 15 minutes. In step S6, ammonium oxalate monohydrate and deionized water are weighed, heated and stirred at 60°C until an ammonium oxalate solution is formed, and added dropwise to the slurry treated in step S5 within 10 minutes. After the addition is completed, aging continues at 30°C for 30 minutes. The remaining conditions are the same as in Example 1.

[0071] Comparative Example 6: The difference from Example 1 is that the seed cobalt solution, growth cobalt solution, ammonium oxalate solution and oxalic acid aqueous solution are mixed together and then added to the uniform gray-black slurry obtained in S1. S2, S3, S4, S5 and S6 are omitted; the other conditions are the same as in Example 1.

[0072] Comparative Example 7: The difference from Example 1 is that in step S8, the gas is first purged in an argon atmosphere at a flow rate of 100 mL / min for 30 min, then switched to high-purity hydrogen at a flow rate of 200 mL / min, directly heated to 510°C at a flow rate of 2°C / min and held for 180 min, and then cooled to below 80°C in a hydrogen atmosphere and switched back to argon. The other conditions are the same as in Example 1.

[0073] Performance testing

[0074] The sintered bodies obtained in Examples 1-3 and Comparative Examples 1-7 were processed into test specimens using the same slow wire EDM machine and the same diamond wheel surface grinder. The specimens used for density, Rockwell hardness, fracture toughness, magnetic properties and metallographic structure testing were processed into 10mm×10mm×5mm block specimens; the specimens used for transverse fracture strength testing were processed into 35mm×5mm×5mm rectangular specimens; and the specimens used for wear resistance testing were processed into 20mm×20mm×5mm plate specimens. After processing, all sintered body specimens were successively ground smooth with 800-grit, 1200-grit and 2000-grit diamond sandpaper, and then polished to a mirror finish with 1μm diamond suspension. The edges of the specimens were blunted to 0.1mm. Before testing, the specimens were ultrasonically cleaned in anhydrous ethanol for 10 minutes and dried at 50°C for 30 minutes.

[0075] Grain size: Refer to GB / T 3488.1-2024. Tungsten carbide grain size refer to GB / T 3488.2-2018. Take the average value as the result of this sample, and then take the average value of 5 samples as the result of this group of samples.

[0076] Density of sintered body: Refer to GB / T 3850-2015. Take 5 10mm×10mm×5mm samples for each group of samples. First, dry at 105℃ for 1h. After cooling to room temperature, weigh the mass in air. Then, immerse in boiling deionized water for 30min. After taking it out, let it stand in deionized water at 25℃ for 20min. Weigh the mass in the immersion solution and the mass in the air after saturation. Calculate the density according to the standard formula. Each sample is measured twice. Take the average value as the result of a single sample.

[0077] Rockwell hardness: The test was conducted in accordance with GB / T 3849.1-2015. Five 10mm×10mm×5mm mirror-polished specimens were taken for each group of samples. The test was conducted on the Rockwell hardness tester using the A scale. The total test force was 588.4N. Five different test points were selected for each specimen. The distance between the centers of the indentation was not less than 3mm, and the distance between the center of the indentation and the edge of the specimen was not less than 2mm. After removing the highest and lowest values, the average of the remaining three values ​​was taken as the result of a single specimen. The average of the five specimens was then taken as the Rockwell hardness result of the group of samples.

[0078] Transverse fracture strength: Refer to GB / T 3851-2015. Take 5 rectangular specimens of 35mm×5mm×5mm for each group of samples. The surface of the specimen is finely ground with diamond wheel to a surface roughness Ra of no more than 0.4μm. The three-point bending method is used for testing. The span of the support point is set to 30mm. The loading head is located in the middle of the specimen span. The loading speed is controlled at 0.5mm / min. Record the maximum load when the specimen breaks and calculate the transverse fracture strength according to the standard formula.

[0079] Fracture toughness: Refer to JB / T 12616-2016. For each sample group, take 5 mirror-polished specimens of 10mm×10mm×5mm. Use a Vickers indenter on a micro Vickers hardness tester to apply a load of 98N to the polished surface of the specimen and hold for 15s to prepare indentations. Prepare 5 effective indentations for each specimen. The distance between indentations should not be less than 3 times the crack length, and the distance from the center of the indentation to the edge should not be less than 2 times the crack length. Measure the diagonal length of the indentation and the length from the four vertices to the crack tip using a metallographic microscope. Remove indentations with incomplete cracks or those that interfere with the edge. Calculate the fracture toughness according to the standard formula. Take the average value of the 5 effective indentations as the result for a single specimen, and then take the average value of the 5 specimens as the result for the sample group.

[0080] Wear resistance: Tests were conducted according to GB / T 34501-2017. Three 20mm×20mm×5mm plate-shaped specimens were taken for each sample group. The surfaces were polished to a Ra value not exceeding 0.2μm. The initial mass was measured using an analytical balance. The test was performed using a rubber wheel abrasive wear method. The rubber wheel had an outer diameter of 200mm, a wheel width of 25mm, a Shore A60 hardness, a rotation speed of 200r / min, a normal load of 130N, and used 180-mesh silicon carbide abrasive grains dried at 105℃ for 2h. The abrasive feed rate was 300g / min, and the single test duration was 30min. After the test, the specimens were ultrasonically cleaned in anhydrous ethanol for 5min, dried at 50℃ for 20min, and then weighed again to calculate the mass loss. The volumetric wear was calculated by combining the density results. The average value of three parallel specimens for each sample group was taken as the result. All test results are shown in Table 1.

[0081] Table 1 Performance Test Results

[0082] Tungsten carbide average grain size / um <![CDATA[Density / (g / cm 3 )]]> Rockwell hardness HRA Transverse fracture strength / MPa <![CDATA[Fracture toughness / (MPa·m 1 / 2 )]]> Abrasion loss / mg <![CDATA[Wear-resistant volume loss / (mm 3 )]]> Example 1 1.54 14.93 91.5 3920 11.6 33.4 2.237 Example 2 1.48 14.90 91.6 3850 11.1 32.8 2.201 Example 3 1.62 14.95 91.3 4010 11.8 34.2 2.288 Comparative Example 1 1.86 14.87 90.9 3420 10.2 38.5 2.589 Comparative Example 2 1.79 14.89 91.0 3520 10.5 37.6 2.525 Comparative Example 3 1.74 14.90 91.1 3600 10.7 36.8 2.470 Comparative Example 4 1.91 14.88 90.8 3450 10.0 39.8 2.675 Comparative Example 5 1.97 14.86 90.7 3380 9.9 41.2 2.773 Comparative Example 6 2.05 14.84 90.6 3290 9.6 44.8 3.019 Comparative Example 7 1.83 14.89 91.0 3490 10.3 38.9 2.612

[0083] Data Analysis: As can be seen from the data of the embodiments in Table 1, the low-cobalt high-toughness cemented carbide prepared by the present invention exhibits a good balance between density, Rockwell hardness, transverse fracture strength, fracture toughness, and wear resistance. With the establishment of the sequence of branched polyethyleneimine aqueous solution site adsorption, seed cobalt pre-occupation, cobalt growth followed by enrichment, ammonia pre-complexation, and two-step oxalic acid deposition, a continuous but not too thick bonded phase film is more easily formed at the load-bearing grain boundaries after sintering. The crack propagation path is more tortuous, and local stress is more easily passivated. Therefore, it not only maintains the high hardness and wear resistance required by the low-cobalt system, but also takes into account the anti-chipping ability and life stability that are more important for cold heading dies, fine blanking dies, and interrupted cutting tools.

[0084] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, omitting the branched polyethyleneimine aqueous solution significantly reduces the overall performance of the material, particularly manifested in the simultaneous deterioration of transverse fracture strength, fracture toughness, and wear resistance. The main reason for this is that the branched polyethyleneimine aqueous solution does not simply disperse the material, but rather provides an irreplaceable spatial starting point for subsequent steps.

[0085] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, even with the addition of all cobalt acetate tetrahydrate in a single step, and the retention of branched polyethyleneimine aqueous solution, ammonia, and two-step oxalic acid deposition, the overall performance still falls short of the level of Example 1. The main reason is that this single addition disrupts the concentration gradient established by the initial occupancy of a small amount of seed cobalt followed by the enrichment of the remaining grown cobalt. This prevents the gradual amplification of the adsorption sites formed in the early stages, resulting in subsequent deposition exhibiting more of an average coverage rather than localized enrichment.

[0086] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, even with the retention of branched polyethyleneimine aqueous solution and the two-step cobalt acetate tetrahydrate addition, the material still exhibits a decrease in strength and toughness after the ammonia pre-complexation is removed. The main reason for this is that the free cobalt ions in the liquid phase lack a transitional stable phase, preventing the previously established seed sites and local enrichment advantages from being fully realized.

[0087] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 4 and 5, even when both ammonium oxalate monohydrate and oxalate dihydrate are introduced, the overall performance of the material will significantly decrease if they are added together at once or the order of addition is reversed. The main reason for this is that the effects of the two oxalate sources are not simply additive, but rather exhibit a significant temporal synergistic effect.

[0088] As can be seen from the data in Table 1 for Example 1 and Comparative Example 6, when the seed cobalt solution, growth cobalt solution, ammonium oxalate solution, and oxalic acid aqueous solution are simultaneously added to the slurry obtained in step S1, and all timing controls from steps S2 to S6 are omitted, the overall performance of the material suffers the greatest decline. The main reason is that when the five steps of anchoring, seed occupancy, local enrichment, weak nucleation, and acidic completion are compressed into a single mixing process, the previous step cannot provide reaction sites or enhancement for the subsequent step, ultimately resulting in a structure with both random deposition and aggregation. This indicates that the present invention is not a mechanical superposition of several known methods, but rather exhibits a typical synergistic effect.

[0089] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A low-cobalt, high-toughness cemented carbide material, characterized in that, The cemented carbide material is based on tungsten carbide powder. Active sites are introduced on the surface of branched polyethyleneimine through surface functionalization modification. Then, a seed cobalt solution and a growth cobalt solution are added dropwise in a two-step seed-growth method. The precursor composite powder is formed by ammonia neutralization and two-step precipitation of ammonium oxalate aqueous solution and oxalic acid aqueous solution. The powder is then reduced with hydrogen, granulated, and finally shaped and sintered. Both the seed cobalt solution and the growth cobalt solution are aqueous solutions of cobalt acetate tetrahydrate. Based on a total weight of 100 parts of tungsten carbide powder, the amounts of branched polyethyleneimine, seed cobalt solution, growth cobalt solution, and ammonium oxalate aqueous solution added are 1.8-2.2 parts, 19.8-26.4 parts, 72-88 parts, and 40-56 parts, respectively.

2. The low-cobalt, high-toughness cemented carbide material according to claim 1, characterized in that, The weight ratio of cobalt acetate tetrahydrate to the solution in the seed cobalt solution is 1:10; the weight ratio of cobalt acetate tetrahydrate to the solution in the growth cobalt solution is 1:3; the weight ratio of ammonium oxalate to the solution in the ammonium oxalate aqueous solution is 1:15; and the weight ratio of oxalic acid to the solution in the oxalic acid aqueous solution is 1:

7.

3. The low-cobalt, high-toughness cemented carbide material according to claim 1, characterized in that, The purpose of neutralizing the ammonia water is to adjust the pH of the system to between 7.5 and 7.9; the purpose of the oxalic acid aqueous solution is to adjust the pH of the system to between 6.0 and 6.

5.

4. A method for preparing a low-cobalt, high-toughness cemented carbide material according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Tungsten carbide powder is added to a dispersion system consisting of branched polyethyleneimine aqueous solution, glacial acetic acid, water and anhydrous ethanol and mixed to form a slurry; Subsequently, cobalt acetate tetrahydrate was prepared into seed cobalt solution and growth cobalt solution and added to the slurry in sequence. Then, ammonia water was added to the system to adjust the pH. Then, ammonium oxalate solution and oxalic acid aqueous solution were added to the system in sequence for deposition and aging. The resulting slurry was subjected to solid-liquid separation, washing, drying and depolymerization to obtain precursor composite powder. (2) The precursor composite powder is reduced by staged heating under a protective atmosphere and a reducing atmosphere to obtain the reduced composite powder; (3) The reduced composite powder is mixed with the binder solution, dried and granulated to obtain granulated powder, and the granulated powder is pressed and sintered to obtain a low cobalt high toughness hard alloy sintered body.

5. The preparation method according to claim 4, characterized in that, The specific process of the segmented heating reduction in step (2) is as follows: first, purify the gas in an argon atmosphere at a rate of 90-110 mL / min for 25-35 min, then switch to high-purity hydrogen at a rate of 180-220 mL / min, heat the gas to 270-290℃ at a rate of 1.8-2.2℃ / min and hold for 50-70 min; then heat the gas to 420-440℃ at a rate of 1.8-2.2℃ / min and hold for 80-100 min; then heat the gas to 500-520℃ at a rate of 1.8-2.2℃ / min and hold for 20-40 min, and finally cool the gas to below 70-90℃ in a hydrogen atmosphere and switch back to argon.

6. The preparation method according to claim 4, characterized in that, The adhesive solution in step (3) is a mixed solution of paraffin and n-heptane, wherein the weight ratio of paraffin to n-heptane is 1:

20.

7. The preparation method according to claim 4, characterized in that, The compression molding in step (3) is carried out under unidirectional compression molding at 170-190MPa.

8. The preparation method according to claim 4, characterized in that, The specific sintering process in step (3) is as follows: under a vacuum degree not higher than 10 Pa, the temperature is increased to 190-210℃ at 0.8-1.2℃ / min and held for 25-35 min, then the temperature is increased to 470-490℃ at 0.8-1.2℃ / min and held for 50-70 min to complete dewaxing, then the temperature is increased to 1408-1422℃ and held for 50-60 min for sintering, and 4-6 MPa argon gas is introduced at the end of the holding period and the pressure is maintained for 12-18 min.