Corrosion-resistant and high-thermal-conductivity WC-Ni hard alloy and preparation method thereof

Abstract

The invention discloses a corrosion-resistant and high-thermal-conductivity WC-Ni hard alloy and a preparation method thereof, and belongs to the technical field of hard alloy material preparation, and the corrosion-resistant and high-thermal-conductivity WC-Ni hard alloy is composed of WC, carbonyl Ni powder, Cr2C3, CrN and Mo2C. The mechanical property, the corrosion resistance and the heat-conducting property of the alloy are effectively improved by adding special components (Cr2C3 + Mo2C + CrN), combining a segmented ball milling mode and combining an isostatic pressing and low-pressure sintering process. The problems that in the traditional WC-Ni-based hard alloy preparation process, a nickel pool often occurs, and the hard alloy structure and the heat-conducting property become poor under the condition of a high-doping-amount additive are solved.

Description

Technical Field

[0001] The invention relates to the technical field of hard alloy material preparation, in particular to a corrosion-resistant, high-thermal-conductivity WC-Ni hard alloy and a preparation method thereof. Background Art

[0002] The sealing component of the nuclear main pump is a key component to prevent the leakage of nuclear reactor coolant. Any slight deviation can easily cause leakage abnormality. It is one of the most easily damaged components on the nuclear main pump. The performance and reliability of its sealing system largely determine whether the nuclear power plant can operate safely and stably for a long time. The main pump seal works for a long time (generally speaking, it needs to operate without trouble for 20,000 hours) in harsh environments such as high temperature, high pressure difference, corrosion, and radioactivity. Especially at the start-stop moment or under special working conditions, the dynamic ring and the static ring end face of the sealing part slide relative to each other, and the contact surface may also produce severe wear due to the combined effects of heat, chemistry, physics and mechanics, which can easily cause damage to the seal and thus cause failure of the main pump (according to statistics, the top three reasons for pump failure are shaft seal deterioration and seal damage, bearing wear, corrosion and rupture). It can be said that the working condition of the sealing ring determines the performance and life of the main pump seal, and the performance of the sealing ring material directly affects the working condition of the sealing ring. Therefore, it is necessary to comprehensively consider the strength, hardness, stiffness, good wear resistance, corrosion resistance, thermal conductivity and other properties of the sealing ring material.

[0003] Cemented carbide is widely used in many main pump sealing ring materials (cast iron, Al 2 O 3 Ceramic, SiC, Si 3 N 4 , CrC, etc. Considering the radiation working environment of the main pump sealing ring, the material is required to be non-radioactive under neutron radiation. Co element will produce Co60 isotope after being exposed to nuclear radiation, which is radioactive, while Ni will not. However, the toughness and wear resistance of WC-Ni cemented carbide with simple composition are lower than those of WC-Co alloy. Traditional WC-Ni cemented carbide structural parts cannot meet the requirements of strong corrosion, high temperature, high pressure, ultra-high speed, radiation protection and particle wear in corrosive media. Adding Cr to strengthen the Ni bonding 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 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. Mo, together with Cr, is an excellent grain suppression and corrosion resistance element, which can also improve the performance of WC-Ni cemented carbide.

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

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

[0006] To achieve the above object, the present invention provides a WC-Ni-based cemented carbide material with high thermal conductivity, uniform microstructure, good densification, and corrosion resistance. This material is prepared by combining a special component design with isostatic pressing + low-pressure sintering. The specific scheme is as follows:

[0007] A corrosion-resistant and high-thermal-conductivity WC-Ni cemented carbide, characterized by comprising the following raw materials: WC 87.7 wt.% - 90.7 wt.%, carbonyl Ni powder 7.0 wt. - 10.0 wt.%, Cr 2 C 3 1.4 wt.% - 1.8 wt.%, CrN 0.0 wt.% - 0.4 wt.%, Mo 2 C 0.0 wt. - 0.5 wt.%.

[0008] Preferably, in the raw materials, Cr 3 C 2 is 1.4 wt.%, CrN is 0.4 wt.%, and Mo 2 C is 0.5 wt.%.

[0009] The present invention also provides a method for preparing the above-mentioned corrosion-resistant and high-thermal-conductivity WC-Ni cemented carbide, which includes the following steps:

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

[0011] (2) Mix all raw materials except carbonyl Ni powder, add paraffin wax and stearic acid, and perform ball milling to obtain a mixed material A;

[0012] (3) Add carbonyl Ni powder to the mixed material A to obtain a mixed material B, and then add a grinding medium and perform wet milling in a drum ball mill to obtain a slurry;

[0013] (4) After passing the slurry through a sieve and vacuum drying, obtain a dry material, and then, according to the requirements of the sample product specifications, use cold isostatic pressing to form a green body sample;

[0014] (5) Use a low-pressure sintering furnace to perform low-pressure sintering on the compact to obtain a corrosion-resistant and high-thermal-conductivity WC-Ni cemented carbide.

[0015] Furthermore, the dosage of the paraffin wax described in step (2) is 2% wt of the total mass of WC and carbonyl nickel powder;

[0016] The dosage of the stearic acid is 0.06% wt of the total mass of WC and carbonyl nickel 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 described in step (3) is hexane;

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

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

[0021] Furthermore, the grinding bodies used in step (2) and step (3) are both WC - Ni cemented carbide balls, and their diameter is

[0022] Furthermore, the method for the slurry to pass through the sieve in step (4) is as follows: the slurry passes through a 200 - mesh sieve, is discharged into a material tray, is placed in a vacuum drying oven for drying, and after sieving (40 - mesh sieve), cold isostatic pressing is carried out.

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

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

[0025] The beneficial effects of the present invention are as follows: The appropriate addition of Cr 2 C 3 +Mo 2 C + CrN in the present invention has excellent effects on the WC dissolution and precipitation process and interface regulation and improvement, can effectively improve the wettability of Ni to the WC hard phase, and the increase in the solid solution amount of W element in the Ni binder phase. The manifestation of the interface regulation effect is reflected in the improvement of the WC grain spheroidization rate by the addition of CrN, reducing the grain boundary and stress concentration, thereby realizing the synergistic improvement of the toughness and strength of the interface between the binder phase and WC. The mechanisms include grain refinement, interface strengthening, solid solution strengthening, and dispersion strengthening, which are reflected in the effective improvement of the mechanical properties, corrosion resistance, and thermal conductivity of the alloy. Therefore, through the addition of special components (Cr 2 C 3 +Mo 2(C + CrN), combined with the segmented ball milling method, and isostatic pressing and low-pressure sintering processes effectively improve the mechanical properties, corrosion resistance, and thermal conductivity of the alloy.

[0026] In the present invention, through 2 C 3 + Mo 2 by the compound addition method, the product grains are refined, effectively improving the wettability of Ni to the WC hard phase, promoting the increase in the solid solution amount of W element in the Ni bonding phase, and playing a positive role in improving the tissue defects of WC-Ni-based cemented carbide and enhancing its mechanical properties and corrosion resistance, but the thermal conductivity decreases significantly. WC-based cemented carbide conducts heat through the free electron movement of the bonding phase (Co / Ni) and the vibration of the WC lattice structure. The phonon boundary scattering at the WC grain boundaries is very large. The more WC grain boundaries, the worse the thermal conductivity of the alloy material. In the present invention, through the appropriate addition of CrN (N element does not exist in the alloy tissue, and CrN decomposes into Cr and N at about 1280 °C 2 , and by evacuating, N can be excluded), it is found that the spheroidization rate of WC grains in the alloy tissue is significantly improved, the WC grain boundaries are reduced, the phonon scattering at the grain boundaries is reduced, and the thermal conductivity of the alloy material is enhanced. In addition, since Ni belongs to the face-centered cubic (F.C.C) crystal system, it is prone to plastic deformation during wet milling, forming flaky Ni aggregation segregation, and the wettability of Ni to the WC hard phase is poor (compared with Co), resulting in the easy appearance of "nickel pools" and pores during the sintering process of WC-Ni-based cemented carbide, forming fracture sources, seriously affecting its performance. In the present invention, through the addition of special components (Cr 2 C 3 + Mo 2 C + CrN), combined with the segmented ball milling method + isostatic pressing + low-pressure sintering process, effectively reduces the porosity and defect generation of the alloy material, improves the problem of the deterioration of the WC-based cemented carbide tissue and thermal conductivity under high-dosage additives, and effectively enhances the improvement of the corrosion resistance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the EDS element distribution (2000 times) of the cemented carbide prepared in Example 1 and Example 2 of the present invention;

[0028] Figure 2 is the cross-sectional SEM photo (8000 times) of the cemented carbide prepared in Example 3 and Example 4 of the present invention; Figure 3 is the SEM photo (3000 times) and EDS element distribution of the cemented carbide prepared in Example 4 of the present invention;

[0029] Figure 4 is the comparison of the polarization curve graphs of the corrosion performance tests of the cemented carbides prepared in Case 3(a) and 4(b) of the present invention in the simulated primary coolant medium of a nuclear power plant. Specific Embodiment

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] Embodiment 1

[0032] A corrosion-resistant and highly thermally conductive WC-Ni cemented carbide

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

[0034] (2) Mix all raw materials except nickel carbonyl powder, add 2% wt paraffin wax 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) Add nickel carbonyl powder to mixture A to obtain mixture B, then add hexane as the grinding medium at a rate of 300 mL / kg and wet mill in a drum ball mill at 72 r / min for 20 h to obtain a slurry; the grinding media are all WC-Ni cemented carbide balls The ball-to-material ratio is 4:1;

[0036] (4) Pass the slurry through a 200-mesh sieve, discharge it into a tray, place it in a vacuum drying oven at 75 °C, wipe the sieve (40-mesh sieve), and then, according to the requirements of the specimen product specifications, use cold isostatic pressing (120 Mpa, 80 s) to obtain a green body specimen;

[0037] (5) Use a low-pressure sintering furnace to sinter the compact at 1470 °C and 5-10 Mpa under low pressure to obtain a corrosion-resistant and highly thermally conductive WC-Ni cemented carbide.

[0038] The prepared alloy specimens were subjected to microstructure inspection according to GB / T 3488-2022 "Metallographic Determination of Microstructure of Cemented Carbides", and the physical and mechanical properties of the alloy were measured using GB / T 3851 "Method for Determining Transverse Rupture Strength of Cemented Carbides" (selecting Type B specimens) and GB / T 7997 "Vickers Hardness Test Method for Cemented Carbides" (selecting a 30 kg load). The metallography of the alloy samples prepared by this method is A02B00C00, the average grain size of WC is 2.0 μm, and the density is 14.90 g / cm 3, Rockwell hardness 87.6 HRA, Vickers hardness 1140 HV30, transverse rupture strength 2480 N / mm 2 , Self-corrosion potential: -0.207 V, self-corrosion current: 5.257×10 -7 A. The results show that there are a large number of abnormally grown coarse grains in the material structure of the product prepared in Example 1.

[0039] Example 2

[0040] The scheme is basically the same as that of Example 1, the difference is that the following raw materials are used: carbonyl Ni powder 7.0 wt.% (FSSS: 1.5 μm); Cr 3 C 2 2.3 wt.%, and the balance is WC.

[0041] According to the scheme of Example 1, through the characterization of metallography and physical properties, the metallography of the alloy sample prepared by this method is A06B00C00, the average grain size of WC is 1.0 μm, and the density is 14.39 g / cm 3 , Rockwell hardness 90.2 HRA, Vickers hardness 1390 HV30, transverse rupture strength 2650 N / mm 2 .

[0042] The grains are significantly refined and the hardness value is significantly improved, but there are pore A06 defects in the alloy structure. The possible reasons are: ① The large addition of Cr 3 C 2 additive makes it dissolve in the binder phase Ni, increasing its viscosity and making it difficult to flow smoothly to fill the pores. ② Cr 3 C 2 is distributed at the WC / Ni interface to form segregation (metallography and EDS element distribution show that Cr in the alloy has poor dispersion and obvious aggregation in the structure), hindering the flow of the binder phase Ni and causing pores in the alloy. ③ The high addition of Cr 3 C 2 increases the sintering liquid phase temperature, and a sintering process at a higher temperature is required. However, increasing the sintering temperature is likely to cause abnormal growth of WC grains, which is particularly obvious for ultrafine-grained cemented carbides.

[0043] Example 3

[0044] The scheme is basically the same as that of Example 2, the difference is that the following raw materials are used: carbonyl Ni powder 7.0 wt.% (FSSS: 1.5 μm), Cr 3 C 2 1.8 wt.%, Mo 2 C 0.5 wt.%, and the balance is WC.

[0045] Measured by characterizing the metallography and physical properties according to the solution of Example 2, the metallography of the alloy sample prepared by this method is A02B00C00, the average grain size of WC is 1.0 μm, and the density is 14.68 g / cm 3 , Rockwell hardness 90.4 HRA, Vickers hardness 1400 HV30, fracture toughness 9.1 MPa·m 1 / 2 , transverse rupture strength 2830 N / mm 2 , thermal conductivity 58.087 W / mK (100 °C), self-corrosion potential: -0.118 V, self-corrosion current: 4.889×10 -7 A.

[0046] In Example 3, Cr 3 C 2 +Mo 2 Composite addition, at 1470 °C, under the low-pressure sintering process of 5 Mpa, the wettability of Ni to the WC hard phase is improved, the pores in the alloy structure are normal, the dispersion of Cr elements is improved, and the Mo elements are evenly distributed. Through the calculation of the electrochemical polarization curve, the self-corrosion potential of the material is higher and the self-corrosion current is smaller compared with Case 1 without adding Cr / Mo elements. It shows that the tendency of the material in this example to corrode is more difficult and the corrosion rate is lower, and the corrosion resistance of the material is improved.

[0047] Example 4

[0048] It is basically the same as the solution of Example 3, the difference is that the following raw materials are used: carbonyl Ni powder 7.0 wt.% (FSSS: 1.5 μm), Cr 3 C 2 1.4 wt.%, CrN 0.4 wt.%, Mo 2 C 0.5 wt.%, and the balance is WC. In this example, during sintering, CrN needs to be subjected to denitrification treatment (CrN decomposes into Cr and N at 1280 °C 2 , and by evacuating, N can be excluded. Experiments show that under the method of the present invention, when the addition of CrN > 1.0 wt.%, N is difficult to be effectively removed and stays in the alloy structure to form pores, while a small amount of addition has no such problem)

[0049] Measured by characterizing the metallography and physical properties according to the solution of Example 3, the metallography of the alloy sample prepared by this method is A02B00C00, the average grain size of WC is 1.0 μm, and the density is 14.60 g / cm 3 , Rockwell hardness 90.7 HRA, Vickers hardness 1440 HV30, fracture toughness 9.7 MPa·m 1 / 2 , transverse rupture strength 3020 N / mm 2, Thermal conductivity: 69.026 W / mK (100 °C), Self-corrosion potential: -0.086 V, Self-corrosion current: 3.153×10 -7 A.

[0050] In Example 4, more passivation phenomena of WC grains were significantly observed in the alloy structure, the spheroidization rate of the grains was significantly improved, the WC grain boundaries were reduced, the thermal conductivity was increased by nearly 11 W / mK (100 °C), and the transverse rupture strength was increased by about 20 N / mm 2 , and the fracture toughness was increased by 0.6 MPa·m 1 / 2 , the hardness was increased by 0.3 HRA. Through the calculation of the electrochemical polarization curve, the material of this example has a higher self-corrosion potential and a smaller self-corrosion current (decreased) compared with the material of Case 3, indicating that the material of this example is more difficult to corrode and has a lower corrosion rate, and the corrosion resistance of the material is improved.

[0051] Example 5

[0052] The scheme is basically the same as that of Example 4, and the difference is that the following raw materials are used: 10.0 wt.% of carbonyl Ni powder (FSSS: 1.5 μm), Cr 3 C 2 1.4 wt.%, CrN 0.4 wt.%, Mo 2 C 0.5 wt.%, and the balance is WC.

[0053] According to the scheme of Example 4, through the characterization of metallography and physical properties, the metallography of the alloy sample prepared by this method is A02B00C00, the average grain size of WC is 1.0 μm, and the density is 14.41 g / cm 3 , Rockwell hardness 88.3 HRA, Vickers hardness 1200 HV30, fracture toughness 10.1 MPa·m 1 / 2 , transverse rupture strength 3460 N / mm 2 .

[0054] Example 6

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

[0056] According to the scheme of Example 5, through the characterization of metallography and physical properties, the metallography of the alloy sample prepared by this method is A02B00C00, the average grain size of WC is 0.4 μm, and the density is 14.33 g / cm 3 , Rockwell hardness 92.6 HRA, Vickers hardness HV30 is 1790, fracture toughness 9.6 MPa·m 1 / 2 , transverse rupture strength 3280 N / mm 2, Thermal conductivity: 59.724 W / mK (100 °C), Self-corrosion potential: -0.165 V, Self-corrosion current: 4.345×10 -7 A.

[0057] In this example, the material bonding phase increases, the average grain size decreases, the hardness and strength properties are further improved, the thermal conductivity decreases slightly, the self-corrosion potential increases by 0.079 V compared with Case 4, indicating that the corrosion tendency increases somewhat. In terms of the corrosion rate reflected by the self-corrosion current, the change is small.

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

Claims

1. A corrosion-resistant, high thermal conductivity WC-Ni hard alloy, characterized in that: The invention comprises 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.%, Mo2C 0.0wt.%-0.5wt.%.

2. A method for preparing corrosion-resistant, high thermal conductivity WC-Ni cemented carbide, characterized in that: The following steps are involved: (1) Weighing the raw materials according to the ratio described in claim 1; (2) mixing all raw materials except carbonyl Ni powder, adding paraffin wax and stearic acid, and ball milling to obtain a mixture A; (3) adding carbonyl Ni powder to mixed material A to obtain mixed material B, and then adding grinding media to perform wet grinding in a drum ball mill to obtain a slurry; (4) the slurry is passed through a sieve and then vacuum dried (70-75° C.) to obtain a dry material, and then cold isostatic pressing is performed to obtain a green sample according to the sample product specification requirements; (5) The pressed green body is sintered at low pressure in a low pressure sintering furnace to obtain corrosion-resistant and high thermal conductivity WC-Ni cemented carbide.

3. A method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: The amount of paraffin wax used in step (2) is 2%wt of the total mass of WC and carbonyl Ni powder; The amount of stearic acid used is 0.06%wt of the total mass of WC and carbonyl Ni powder.

4. A method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: In step (2), the ball milling rate is 72 r / min and the ball milling time is 12-16 h.

5. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: The grinding medium in step (3) is hexane; The amount of hexane added is 300 mL / kg of mixed material B.

6. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: The ball milling rate in step (3) is 72 r / min, and the ball milling time is 20-24 h.

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

8. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: The vacuum drying temperature in step (4) is 70-75°C; The cold isostatic pressing pressure is 120 MPa and the processing time is 80 s.

9. The method for preparing a corrosion-resistant, high thermal conductivity WC-Ni cemented carbide according to claim 2, characterized in that: In step (5), the sintering temperature is 1460-1480° C. and the sintering pressure is 5-10 MPa.

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