Corrosion-resistant, high-thermal-conductivity WC-Ni cemented carbide and method for producing the same

Abstract

The application discloses a kind of corrosion-resistant, high-thermal-conductivity WC-Ni hard alloy and preparation method thereof, belong to hard alloy material preparation technical field, composition is WC, carbonyl Ni powder, Cr2C3, CrN and Mo2C.The application is effectively improved by special component addition (Cr2C3+Mo2C+CrN), combined with subsection ball milling mode, combined with isostatic pressing and low-pressure sintering process mechanical properties, corrosion resistance and thermal conductivity of alloy.Solve the problem of "nickel pool" and high-doped additive hard alloy organization and thermal conductivity deterioration often appeared in the preparation process of traditional WC-Ni-based hard alloy.

Description

Technical Field

[0001] This invention relates to the field of cemented carbide material preparation technology, and more specifically to a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide and its preparation method. Background Technology

[0002] The sealing components of the nuclear main pump are critical to preventing coolant leakage from the nuclear reactor. Even slight deviations can easily lead to leaks, making it one of the most vulnerable parts of the main pump. The performance and reliability of its sealing system largely determine the long-term safe and stable operation of the nuclear power plant. The main pump seal operates for extended periods (generally requiring 20,000 hours of trouble-free operation) in harsh environments such as high temperature, high pressure differentials, corrosion, and radioactivity. Especially during start-up and shutdown or under special operating conditions, the dynamic and static rings of the sealing component slide relative to each other. Their contact surfaces can also experience severe wear due to the combined effects of heat, chemicals, physical factors, and mechanical forces, easily leading to seal damage and ultimately main pump failure. (Statistics show that the top three causes of pump failure are shaft seal deterioration and seal damage, bearing wear, corrosion, and cracking.) In short, the working condition of the sealing ring determines the performance and lifespan of the main pump seal, and the performance of the sealing ring material directly affects its working condition. Therefore, it is necessary to comprehensively consider the strength, hardness, rigidity, and good wear resistance, corrosion resistance, and thermal conductivity of the sealing ring material.

[0003] Hard alloy stands out among many main pump sealing ring materials (cast iron, Al2O3 ceramic, SiC, Si3N4, CrC, etc.). Considering the radiation environment of the main pump sealing ring, the material must not produce radioactivity under neutron radiation. Co element produces Co60 isotope after nuclear radiation, which is radioactive, while Ni is not. However, the toughness and wear resistance of simple WC-Ni hard alloy are lower than those of WC-Co alloy. Traditional WC-Ni hard alloy structural parts cannot meet the requirements of complex working conditions such as strong corrosion, high temperature, high pressure, ultra-high speed, radiation protection, and particle wear in corrosive media. Adding Cr to strengthen the Ni binder phase is an effective way to effectively inhibit the creep of the alloy at high temperature, improve the mechanical properties and corrosion resistance of the alloy at room temperature, and at the same time give it better oxidation resistance. Experiments have shown that the hardness of WC-Ni-Cr alloy increases with the increase of Cr content, but when the Cr content exceeds a certain limit, it will have a negative impact on the metallographic and mechanical properties of the alloy. Mo dissolves in both the binder phase Ni and the hard phase WC, which can improve the wettability of Ni to WC. In addition, Mo, along with Cr, is an excellent grain-inhibiting and corrosion-resistant element, which can also improve the performance of WC-Ni cemented carbide.

[0004] Therefore, providing a solution to improve the oxidation resistance, mechanical properties, and corrosion resistance of WC-Ni cemented carbide is a technical problem that those skilled in the art will continue to address. Summary of the Invention

[0005] In view of this, the present invention provides a method for preparing corrosion-resistant and high thermal conductivity WC-Ni-based cemented carbide, which solves the problems of "nickel pool" and deterioration of cemented carbide microstructure and thermal conductivity under high additive content that often occur in the preparation process of traditional WC-Ni-based cemented carbide, and effectively enhances the corrosion resistance of the material.

[0006] To achieve the above objectives, this invention provides a WC-Ni-based cemented carbide material with high thermal conductivity, uniform microstructure, good density, and corrosion resistance. This material is prepared using a special composition design combined with isostatic pressing and low-pressure sintering. The specific method is as follows:

[0007] A corrosion-resistant, high thermal conductivity WC-Ni cemented carbide, characterized by comprising the following raw materials: WC 87.7wt.%-90.7wt.%, carbonyl Ni powder 7.0wt.-10.0wt.%, Cr2C 31.4wt.%-1.8wt.%, CrN 0.0wt.%-0.4wt.%, and Mo2C 0.0wt.-0.5wt.%.

[0008] Preferably, the raw material contains 1.4 wt.% Cr3C2, 0.4 wt.% CrN, and 0.5 wt.% Mo2C.

[0009] This invention also provides a method for preparing the above-mentioned corrosion-resistant, high thermal conductivity WC-Ni cemented carbide, comprising the following steps:

[0010] (1) Weigh each raw material according to the above proportions;

[0011] (2) Mix all raw materials except carbonyl Ni powder, add paraffin and stearic acid and ball mill to obtain mixture A;

[0012] (3) Add carbonyl Ni powder to mixture A to obtain mixture B, then add grinding media and wet grind in a drum ball mill to obtain slurry;

[0013] (4) After the slurry is sieved, it is dried under vacuum to obtain dry material. Then, according to the specifications of the sample product, cold isostatic pressing is used to obtain green sample.

[0014] (5) The compact is sintered under low pressure in a low-pressure sintering furnace to obtain a corrosion-resistant and high thermal conductivity WC-Ni hard alloy.

[0015] Furthermore, the amount of paraffin used in step (2) is 2% wt of the total mass of WC and carbonyl Ni powder;

[0016] The amount of stearic acid used is 0.06% wt of the total mass of WC and carbonyl Ni powder.

[0017] Furthermore, in step (2), the ball milling rate is 72 r / min and the ball milling time is 12-16 h.

[0018] Furthermore, the grinding medium in step (3) is hexane;

[0019] The amount of hexane added is 300 mL / kg of mixture B.

[0020] Furthermore, the ball milling rate in step (3) is 72 r / min, and the ball milling time is 20-24 h.

[0021] Furthermore, the grinding media used in steps (2) and (3) are both WC-Ni cemented carbide balls with a diameter of [missing information].

[0022] Furthermore, the method for slurry sieving in step (4) is as follows: slurry sieving through a 200-mesh screen, unloading into a tray, placing in a vacuum drying oven for drying, wiping the screen (40-mesh screen) and then performing cold isostatic pressing.

[0023] Furthermore, in step (4), the vacuum drying temperature is 70-75℃; the cold isostatic pressure is 120Mpa; and the processing time is 80s.

[0024] Furthermore, in step (5), the sintering temperature is 1460-1480℃ and the sintering pressure is 5-10 MPa.

[0025] The beneficial effects of this invention are as follows: The appropriate addition of Cr2C3+Mo2C+CrN has excellent effects on improving the dissolution and exudation process of WC and interface regulation. It can effectively improve the wettability of Ni on the hard phase of WC and increase the solid solution content of W element in the Ni binder phase. The interface regulation effect is manifested in the improvement of WC grain spheroidization rate by the addition of CrN, reducing grain boundaries and stress concentration, thereby achieving a synergistic improvement in the toughness of the binder phase and WC interface. The mechanism includes grain refinement, interface strengthening, solid solution strengthening, and dispersion strengthening, which is reflected in the effective improvement of the alloy's mechanical properties, corrosion resistance, and thermal conductivity. Therefore, this invention effectively improves the alloy's mechanical properties, corrosion resistance, and thermal conductivity by adding special components (Cr2C3+Mo2C+CrN), combined with segmented ball milling, and combined with isostatic pressing and low-pressure sintering processes.

[0026] This invention refines the grain size of the product through the composite addition of Cr2C3 and Mo2C, effectively improving the wettability of Ni on the WC hard phase and promoting the increase of W element solid solution in the Ni binder phase. This plays a positive role in improving the microstructure defects of WC-Ni-based cemented carbide and enhancing its mechanical properties and corrosion resistance, but significantly reduces its thermal conductivity. WC-based cemented carbide conducts heat through the free electron movement of the binder phase (Co / Ni) and the vibration of the WC lattice structure. Phonon boundary scattering at WC grain boundaries is significant; the more WC grain boundaries there are, the worse the thermal conductivity of the alloy. This invention, through the appropriate addition of CrN (N element does not exist in the alloy structure; CrN decomposes into Cr and N2 at approximately 1280℃, and N can be removed by vacuuming), found a significant improvement in the spheroidization rate of WC grains in the alloy structure, a reduction in WC grain boundaries, a decrease in phonon scattering at grain boundaries, and an enhancement of the thermal conductivity of the alloy. Furthermore, since Ni belongs to the face-centered cubic (FCC) crystal system, it is prone to plastic deformation during wet milling, forming lamellar Ni agglomerates and segregations. In addition, Ni has poor wettability to the WC hard phase (compared to Co), which makes WC-Ni cemented carbides prone to "nickel pools" and pores during sintering, forming fracture sources and seriously affecting their performance. This invention effectively reduces the porosity and defect formation of the alloy material by adding special components (Cr2C3+Mo2C+CrN), combined with segmented ball milling + isostatic pressing + low-pressure sintering process, improving the problem of deterioration of the microstructure and thermal conductivity of cemented carbides under high additive dosage, and effectively enhancing the corrosion resistance of the material. Attached Figure Description

[0027] Figure 1 The EDS elemental distribution of the cemented carbide prepared in Examples 1 and 2 of this invention (2000 times).

[0028] Figure 2 SEM images (8000x magnification) of the cemented carbide cross-sections prepared in Examples 3 and 4 of this invention; Figure 3 The image shows a 3000x SEM image and EDS elemental distribution of the cemented carbide prepared in Example 4 of this invention.

[0029] Figure 4 The polarization curves of the hard alloys prepared in embodiments 3(a) and 4(b) of this invention are compared in the corrosion performance test of the return water coolant medium in a simulated nuclear power plant. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1

[0032] A corrosion-resistant, high thermal conductivity WC-Ni cemented carbide

[0033] (1) Weigh out 93.0 wt.% WC (FSSS: 2.5 μm) and 7.0 wt.% carbonyl Ni powder (FSSS: 1.5 μm) by percentage;

[0034] (2) Mix all raw materials except carbonyl Ni powder, add 2% wt paraffin and 0.06% wt stearic acid, and ball mill at 72 r / min for 12 h to obtain mixture A; the grinding media are all WC-Ni cemented carbide balls.

[0035] (3) Carbonyl Ni powder was added to mixture A to obtain mixture B. Then, hexane was added at a rate of 300 mL / kg as the grinding medium and wet-milled in a drum ball mill at 72 r / min for 20 h to obtain a slurry. The grinding media were all WC-Ni cemented carbide balls. The ball-to-material ratio is 4:1;

[0036] (4) The slurry is passed through a 200-mesh sieve, unloaded into a tray, placed in a vacuum drying oven to dry at 75°C, wiped on a sieve (40-mesh sieve), and then, according to the specifications of the sample product, cold isostatic pressing (120 MPa, 80 s) is used to obtain the green sample.

[0037] (5) The compact is sintered at 1470℃ and 5-10 MPa in a low-pressure sintering furnace to obtain a corrosion-resistant and high thermal conductivity WC-Ni cemented carbide.

[0038] The prepared alloy samples were examined for microstructure according to GB / T 3488-2022 "Metallographic Determination of Microstructure of Cemented Carbide". The physical and mechanical properties of the alloy were measured using GB / T 3851 "Determination of Transverse Fracture Strength of Cemented Carbide" (using type B sample) and GB / T 7997 "Vickers Hardness Test Method for Cemented Carbide" (using 30 kg load). The metallographic structure of the alloy samples prepared by this method was A02B00C00, with an average WC grain size of 2.0 μm and a density of 14.90 g / cm³. 3 Rockwell hardness 87.6 HRA, Vickers hardness 1140 HV30, transverse tensile strength 2480 N / mm² 2Self-corrosion potential: -0.207V, Self-corrosion current: 5.257×10 -7 A. The results show that the microstructure of the product prepared in Example 1 contains a large number of abnormally large coarse grains.

[0039] Example 2

[0040] The scheme is basically the same as that in Example 1, except that the following raw materials are used: 7.0 wt.% carbonyl Ni powder (FSSS: 1.5 μm); 2.3 wt.% Cr3C2, with the balance being WC.

[0041] Following the method described in Example 1, the metallographic and physical properties of the alloy sample prepared by this method were characterized. The metallographic structure was A06B00C00, the average grain size of WC was 1.0 μm, and the density was 14.39 g / cm³. 3 Rockwell hardness 90.2 HRA, Vickers hardness 1390 HV30, transverse tensile strength 2650 N / mm² 2 .

[0042] The grains were significantly refined and the hardness was significantly improved, but the alloy microstructure showed porosity defects (A06). Possible reasons include: ① The large addition of Cr3C2 additives increased the viscosity of the Ni binder phase, making it difficult to fill the pores due to its dissolution in the Ni binder phase; ② Cr3C2 was distributed at the WC / Ni interface and formed segregation (metallographic and EDS elemental distribution showed poor Cr dispersion and obvious Cr aggregation in the microstructure), which hindered the flow of the Ni binder phase and caused porosity in the alloy; ③ The high Cr3C2 addition increased the sintering liquid phase temperature, requiring a higher sintering temperature process. However, increasing the sintering temperature easily caused abnormal growth of WC grains, which was particularly noticeable for ultrafine particle cemented carbides.

[0043] Example 3

[0044] The scheme is basically the same as that in Example 2, except that the following raw materials are used: 7.0 wt.% carbonyl Ni powder (FSSS: 1.5 μm), 1.8 wt.% Cr3C2, 0.5 wt.% Mo2C, and the balance is WC.

[0045] Following the method described in Example 2, the metallographic and physical properties of the alloy sample prepared by this method were characterized. The metallographic structure was A02B00C00, the average grain size of WC was 1.0 μm, and the density was 14.68 g / cm³. 3 Rockwell hardness 90.4 HRA, Vickers hardness 1400 HV30, fracture toughness 9.1 MPa·m 1 / 2 Transverse tensile strength 2830 N / mm 2 Thermal conductivity: 58.087 W / mK (100℃), self-corrosion potential: -0.118 V, self-corrosion current: 4.889 × 10⁻⁶-7 A.

[0046] In Example 3, with the addition of Cr3C2+Mo2C composite and a low-pressure sintering process at 1470℃ and 5MPa, the wettability of Ni on the WC hard phase was improved, the alloy microstructure had normal porosity, the dispersion of Cr was improved, and the distribution of Mo was uniform. According to the phase calculation of the electrochemical polarization curve, the self-corrosion potential of the material was higher and the self-corrosion current was lower than that of Example 1 without the addition of Cr / Mo elements. This indicates that the material in this example is more difficult to corrode and has a lower corrosion rate, thus improving the corrosion resistance of the material.

[0047] Example 4

[0048] The scheme is basically the same as that in Example 3, except that the following raw materials are used: 7.0 wt.% carbonyl Ni powder (FSSS: 1.5 μm), 1.4 wt.% Cr3C2, 0.4 wt.% CrN, 0.5 wt.% Mo2C, with the balance being WC. In this example, CrN needs to undergo nitrogen removal treatment during sintering (CrN decomposes into Cr and N2 at 1280℃; N can be removed by vacuuming. Experiments show that under the method of this invention, when the CrN addition is >1.0 wt.%, N is difficult to remove effectively, remaining in the alloy structure and forming pores, while a small amount of addition does not have this problem).

[0049] Following the method described in Example 3, the metallographic and physical properties of the alloy sample prepared by this method were characterized. The metallographic structure was A02B00C00, the average grain size of WC was 1.0 μm, and the density was 14.60 g / cm³. 3 Rockwell hardness 90.7 HRA, Vickers hardness 1440 HV30, fracture toughness 9.7 MPa·m 1 / 2 Transverse tensile strength 3020 N / mm 2 Thermal conductivity: 69.026 W / mK (100℃), self-corrosion potential: -0.086 V, self-corrosion current: 3.153 × 10⁻⁶ -7 A.

[0050] In Example 4, more passivation of WC grains was clearly observed in the alloy microstructure, the grain spheroidization was significantly improved, the number of WC grain boundaries was reduced, the thermal conductivity was improved by nearly 11 W / mK (100℃), and the transverse fracture strength was improved by about 20 N / mm. 2 The fracture toughness increased by 0.6 MPa·m 1 / 2 The hardness increased by 0.3 HRA. According to the phase calculation based on the electrochemical polarization curve, the self-corrosion potential of this material is higher and the self-corrosion current is lower than that of the material in Case 3. This indicates that the material in this case is more difficult to corrode and has a lower corrosion rate, thus improving the corrosion resistance of the material.

[0051] Example 5

[0052] The scheme is basically the same as that in Example 4, except that the following raw materials are used: 10.0 wt.% carbonyl Ni powder (FSSS: 1.5 μm), 1.4 wt.% Cr3C2, 0.4 wt.% CrN, 0.5 wt.% Mo2C, with the balance being WC.

[0053] Following the scheme of Example 4, the metallographic and physical properties of the alloy sample prepared by this method were characterized. The metallographic structure was A02B00C00, the average grain size of WC was 1.0 μm, and the density was 14.41 g / cm³. 3 Rockwell hardness 88.3 HRA, Vickers hardness 1200 HV30, fracture toughness 10.1 MPa·m 1 / 2 Transverse tensile strength 3460 N / mm 2 .

[0054] Example 6

[0055] The scheme is basically the same as that in Example 5, except that the WC particle size (FSSS: 0.5μm) is reduced, and the ball milling time in step (2) is changed to 16h and the ball milling time in step (3) is changed to 24h.

[0056] Following the scheme of Example 5, the metallographic and physical properties of the alloy sample prepared by this method were characterized. The metallographic structure was A02B00C00, the average grain size of WC was 0.4 μm, and the density was 14.33 g / cm³. 3 It has a Rockwell hardness of 92.6 HRA, a Vickers hardness of HV30 of 1790, and a fracture toughness of 9.6 MPa·m. 1 / 2 Transverse tensile strength 3280 N / mm 2 Thermal conductivity: 59.724 W / mK (100℃), self-corrosion potential: -0.165 V, self-corrosion current: 4.345 × 10⁻⁶ -7 A.

[0057] In this example, the material binder phase increased, the average grain size decreased, the hardness and strength properties were further improved, the thermal conductivity decreased slightly, and the self-corrosion potential increased by 0.079V compared to Case 4, indicating that the corrosion tendency has increased. However, the corrosion rate of the self-corrosion current reaction showed little change.

[0058] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for producing corrosion-resistant, high thermal conductivity WC-Ni cemented carbide, characterized in that The method comprises the following steps: (1) each raw material is weighed according to the following proportion: WC 87.7wt.%-90.7wt.%, carbonyl Ni powder 7.0wt.-10.0wt.%, Cr2C3 1.4wt.%-1.8wt.%, 0.0wt.%<CrN≤0.4wt.%, 0.0wt.%<Mo2C≤0.5wt.%; (2) all the raw materials except carbonyl Ni powder are mixed, paraffin and stearic acid are added for ball milling to obtain a mixture A; the amount of the paraffin is 2.0wt.% of the total mass of WC and carbonyl Ni powder; the amount of the stearic acid is 0.06wt.% of the total mass of WC and carbonyl Ni powder; (3) carbonyl Ni powder is added to the mixture A to obtain a mixture B, then grinding medium is added for wet milling in a drum-type ball mill to obtain a slurry; the grinding medium is hexane; the added amount of the hexane is 300 mL / kg of the mixture B; (4) the slurry is sieved and then vacuum dried to obtain a dry material, then a green sample is obtained by cold isostatic pressing according to the product specification requirements; the vacuum drying temperature is 70-75℃; the cold isostatic pressing pressure is 120MPa, and the processing time is 80s; (5) the green sample is sintered in a low-pressure sintering furnace to obtain a WC-Ni hard alloy with corrosion resistance and high thermal conductivity; the sintering temperature is 1460-1480℃, and the sintering pressure is 5-10MPa.

2. The method of claim 1, wherein the WC-Ni cemented carbide has a corrosion resistance and a high thermal conductivity, and is characterized by, The ball milling rate in step (2) is 72r / min, and the ball milling time is 12-16h.

3. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 1, characterized in that, The ball milling rate in step (3) is 72r / min, and the ball milling time is 20-24h.

4. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 1, characterized in that, The grinding bodies used in steps (2) and (3) are WC-Ni hard alloy balls with a diameter of φ6.8 mm.

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