Tungsten carbide-chromium carbide high-temperature composite powder and preparation method thereof

Abstract

The invention discloses tungsten carbide-chromium carbide high-temperature composite powder and a preparation method thereof, and relates to the field of thermal spraying coating materials. The tungsten carbide-chromium carbide high-temperature composite powder is prepared from the following components in percentage by mass: 18 to 27 percent of tungsten carbide, 2 to 5 percent of cobalt, 15 to 25 percent of nickel-chromium alloy and 45 to 60 percent of chromium carbide. The preparation method of the tungsten carbide-chromium carbide high-temperature composite powder comprises the following steps that S100, tungsten carbide powder and cobalt powder are mixed and then sintered for the first time, and pre-alloyed cobalt-tungsten carbide hard phase powder is obtained; and S200, after the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder and chromium carbide powder are mixed, secondary sintering is conducted, and the high-wear-resistance tungsten carbide-chromium carbide high-temperature composite powder is obtained. The microhardness of a coating formed after thermal spraying of the prepared composite powder can be improved to 1180 HV or above, the wear resistance reaches 95% or above of that of a traditional high-quality WC-12Co coating, and the coating is far better than a similar system coating prepared through a traditional one-time sintering method.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermal spraying coating materials, in particular to a tungsten carbide-chromium carbide high-temperature composite powder and a preparation method thereof. BACKGROUND

[0002] Tungsten carbide (WC) -based hard alloy coating is widely used in the surface strengthening and repair of key mechanical components, such as aircraft engine blades, oil drilling tools, and molds, due to its extremely high hardness and excellent wear resistance. In traditional WC-Co coatings, such as WC-12Co, cobalt (Co) as a binder phase has good wettability and bonding strength with tungsten carbide, effectively transferring the load and making the coating exhibit excellent comprehensive performance.

[0003] In recent years, the cost of WC-Co coating has increased significantly. Therefore, the industry has begun to seek alternative material systems with lower costs. Chromium carbide (Cr3C2) has excellent high-temperature oxidation resistance and corrosion resistance, and nickel-chromium (NiCr) alloy as a binder phase has a significantly lower cost than cobalt. Therefore, the WC-Cr3C2-NiCr system is considered as a potential alternative. Currently, the powder of this system is usually prepared by mixing all raw materials such as WC, Cr3C2, and NiCr at one time and then sintering (one-step sintering).

[0004] However, this traditional one-step sintering process has significant defects. The interfacial bonding strength between the NiCr binder phase and the WC hard phase is much lower than the bonding strength between Co and WC. This leads to the easy detachment of WC particles from the NiCr matrix during coating preparation and use, resulting in a macroscopic hardness (usually only about 1000 HV) and wear resistance of the coating that is far inferior to that of WC-Co coating, which cannot meet the needs of high-performance working conditions and limits its application as an effective alternative.

[0005] Therefore, developing a new material and preparation process that can significantly reduce costs and make the hardness and wear resistance of WC-Cr3C2-NiCr system coating close to or even reach the level of traditional WC-Co coating has become a technical problem that needs to be solved in the field. SUMMARY

[0006] The purpose of the present application is to provide a tungsten carbide-chromium carbide high-temperature composite powder and a preparation method thereof. By using a two-step process of pre-alloying the core hard phase and then overall composite sintering, a strong Co-WC unit is constructed in the microstructure, thereby significantly improving the overall bonding strength of the hard phase and the binder phase in the coating, and achieving a breakthrough in the hardness and wear resistance of the coating under the premise of low cost.

[0007] To solve the above technical problems, the present application adopts the following technical solutions:

[0008] A tungsten carbide-chromium carbide high-temperature composite powder comprises the following components by mass percentage:

[0009] Tungsten carbide 18~27%, cobalt 2~5%, nickel-chromium alloy 15~25%, chromium carbide 45~60%.

[0010] The component design of the tungsten carbide-chromium carbide high-temperature composite powder of the present application breaks the traditional high-cobalt bonding system thinking mode, establishes chromium carbide as the main hard phase of the system, and uses nickel-chromium alloy as the main bonding phase, thereby constructing a brand-new material matrix. Under this framework, the trace amount of cobalt of 2~5% in the formula is not used as the main bonding phase, but is endowed with the key interface modification function, uses the excellent wettability between cobalt and tungsten carbide to construct a strong and tough interface bridging layer between the tungsten carbide particles and the nickel-chromium matrix, thereby solving the core technical problem of the weak intrinsic bonding force between nickel-chromium and tungsten carbide. The system fully utilizes the high-temperature hardness, oxidation resistance of Cr3C2 and the corrosion resistance and toughness of NiCr. Through the interface strengthening of the trace amount of cobalt, the hardness and wear resistance of the coating prepared from the composite powder in a high-temperature environment can approach the level of the traditional WC-12Co coating, and at the same time, the coating has better high-temperature oxidation resistance.

[0011] A preparation method of the tungsten carbide-chromium carbide high-temperature composite powder according to any one of claims 1, comprising the following steps:

[0012] S100, mixing tungsten carbide powder and cobalt powder, and then performing first sintering to obtain pre-alloyed cobalt-tungsten carbide hard phase powder;

[0013] S200, mixing the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder and chromium carbide powder, and then performing second sintering to obtain the high-wear-resistant tungsten carbide-chromium carbide high-temperature composite powder.

[0014] In view of the problems of high cost of traditional tungsten carbide-cobalt coating and insufficient performance of tungsten carbide-chromium carbide-nickel-chromium system coating due to weak interface bonding, the application provides a brand new solution. The core is to actively build a high-performance interface transition zone in the composite material through step-by-step sintering and interface regulation strategy. First, tungsten carbide powder and cobalt powder are mixed, and the first sintering is carried out by programmed temperature rising. In the process, the Co phase is melted, the rapid densification is completed, the thermodynamic driving force and time window for Ostwald ripening (coarsening) of WC grains through dissolution-precipitation mechanism are greatly inhibited, and a pre-alloyed cobalt-tungsten carbide hard phase core with fine grain strengthening structure and excellent Co / WC interface bonding strength is successfully prepared. Then, the hard phase core is uniformly mixed with nickel-chromium alloy powder and chromium carbide powder, and the second sintering is carried out. The second sintering is completed under a controllable argon-hydrogen mixed reducing atmosphere and a specific temperature rising program. Instead of directly bonding with the surface of tungsten carbide, the nickel-chromium binder phase realizes firm metallurgical interdiffusion connection with the cobalt-rich phase at the outer edge of the pre-alloyed hard phase core through atomic diffusion. The cobalt phase plays a crucial role in interface wetting and metallurgical bridging, fundamentally overcoming the inherent defects of poor intrinsic wettability and weak interface bonding between nickel-chromium alloy and tungsten carbide.

[0015] The coating formed by the prepared composite powder after thermal spraying is equivalent to or even surpasses the hardness and wear resistance of traditional high-performance coatings. The microhardness can be increased to more than 1180HV, and the wear resistance can reach more than 95% of that of traditional high-quality WC-12Co coating. At the same time, the use amount of high-priced tungsten carbide and cobalt is significantly reduced.

[0016] Further, in step S100, the first sintering is carried out under vacuum conditions, and the temperature rising program includes: first stage: temperature rising to 500℃ at a rate of 5-10℃ / min, and holding for 30-60 minutes; second stage: temperature rising to 800℃ at a rate of 3-5℃ / min and holding for 60-120 minutes;

[0017] Third stage: temperature rising to a sintering temperature of 1230-1270℃ at a rate of 10-15℃ / min, and holding for 30-90 minutes;

[0018] After sintering, the furnace is cooled.

[0019] The present application discards the traditional single rate to high temperature and long time holding of extensive mode, through the heat process design of stepwise heating and segmented holding, the physical and chemical reactions of sintering each stage are actively guided and optimized. The first stage realizes the physical and chemical purification of the powder system by slow heating and holding. The purpose is to gently and thoroughly release the moisture and gas adsorbed on the surface of the raw material powder, avoid the destruction of the uniformity of the powder bed caused by the sharp release of these volatile components in the subsequent high temperature stage, and eliminate the internal stress of the particles to create a clean and stable thermodynamic environment for atomic diffusion.

[0020] The second stage is committed to completing the key interface pre-alloying in the solid state. In this medium temperature range, cobalt atoms (Co) obtain sufficient diffusion energy and can fully diffuse and diffuse in the grain boundary of tungsten carbide (WC) particle surface. This process is not a simple contact, but a preliminary and firm Co / WC atomic level metallurgical bonding interface is formed. At the same time, the thermal driving force of this stage can further reduce the surface trace oxides, so as to pre-build a clean and strong bonding interface before entering the high temperature, which lays an irreplaceable foundation for the subsequent liquid phase wetting.

[0021] The third stage (rapidly rising to 1230-1270℃ and short time holding) is the key to inhibit grain coarsening and obtain high-performance structure. Rapid heating makes cobalt melt, which can quickly wet and wrap WC particles with excellent bonding foundation, realizing efficient densification. The high temperature holding time is extremely shortened to 30-90 minutes, which directly and effectively limits the time and thermodynamic driving force of WC grain Ostwald ripening (coarsening) through dissolution-precipitation mechanism, thereby successfully limiting the fine WC grain structure in the final product.

[0022] The temperature rising scheme of the program realizes the two goals of interface bonding strengthening and grain structure fine and dense which are usually difficult to achieve simultaneously on traditional equipment through the synergistic effect of stage purification-solid state pre-bonding-short time liquid phase densification.

[0023] Further, in step S200, the second sintering includes: first vacuumizing to ≤1×10⁻²Pa, then filling with flowing argon-hydrogen mixed gas, programmed heating to 1150-1200℃, switching to pure argon gas, holding for 2-4 hours, and cooling with the furnace.

[0024] The step S200 provides a set of atmosphere processes of purification first, reduction second and protection third. First, high vacuum (≤1*10-2Pa) is drawn to thoroughly remove moisture and volatile impurities in the furnace; then, precisely proportioned argon-hydrogen mixed gas is introduced in the key temperature rising stage, and the reducing property of hydrogen is used to chemically remove trace oxides on the surface of the powder, especially the nickel-chromium and chromium carbide phases, to create an extremely clean powder surface state, which provides a decisive prerequisite for the subsequent realization of firm metallurgical bonding between different phases and improves the overall integrity of the composite powder and the coating bonding strength.

[0025] Further, in the argon-hydrogen mixed gas, the volume ratio of argon to hydrogen is 18-22:1.

[0026] Controlling the hydrogen ratio to about 5% (volume ratio 18-22:1) can fully exert its reducing effect, avoid the risk of forming an explosive atmosphere or causing hydrogen embrittlement, and ensure the thoroughness and uniformity of interface purification.

[0027] Further, the programmed temperature rising includes: rising to 800℃ at 10℃ / min and keeping for 30 minutes, and then rising to 1180-1230℃ at 5℃ / min.

[0028] The staged programmed temperature rising allows the reduction reaction and stress release to be fully carried out at the low temperature stage, so that the components can diffuse smoothly and fully, and finally form a strong and uniform composite structure.

[0029] Further, in the step S100, the average particle size of the tungsten carbide powder is 1-5μm, and the average particle size of the cobalt powder is 1-3μm.

[0030] Selecting sub-micron to micron-sized fine particles significantly increases the contact area and reaction interface of the two phases, ensuring that in the process of rapid sintering, the cobalt phase can uniformly and quickly wrap and penetrate the tungsten carbide particles, achieve full and uniform alloying, avoid local composition unevenness or unalloyed core caused by mismatched raw material particle size, and thus ensure the consistency of the performance of the pre-alloyed hard phase.

[0031] Further, in the step S200, the mass ratio of nickel to chromium in the nickel-chromium alloy powder is 3-5:1, and the average particle size of the nickel-chromium alloy powder is 1-4μm; the average particle size of the chromium carbide powder is 1-5μm.

[0032] To ensure the coordination of each component in space distribution and chemical reaction during the second step of composite sintering, the particle sizes of the three components are controlled in a similar coarse range, which is beneficial to the uniformity of mixing and the particle rearrangement during sintering; the ratio of nickel to chromium (3-5:1) in the nickel-chromium alloy is limited, which determines the basic components of the binder phase with good high-temperature strength and oxidation resistance; and the hard phase particles in the final composite powder can be uniformly wrapped and bonded by a sufficient amount of binder phase with optimized performance, so that the ideal comprehensive mechanical properties and spraying process are obtained.

[0033] Further, the crushing treatment is performed after the sintering in step S100, and the particle size range is screened to 5-20 μm; the crushing treatment is performed after the sintering in step S200, and the particle size range is screened to 15-43 μm.

[0034] The particle size range can make the prepared composite powder have excellent flowability, transport stability and melting characteristics in the flame flow, so that a dense, uniform and high-bonding-strength high-performance coating can be deposited to realize the efficient performance transfer from the powder to the coating.

[0035] Further, in step S100, the first sintering includes:

[0036] Under a vacuum degree of ≤5×10 -2 Pa, the spheroidization treatment is performed by using radio frequency plasma, wherein the plasma power is 40-60 kW, the powder feeding rate is 30-50 g / min, the carrier gas is argon, and the residence time of the powder in the plasma high-temperature zone is 5-20 milliseconds.

[0037] The step S100 of the present application can also use a vacuum plasma spheroidization technology to replace the traditional sintering. The process utilizes a high-temperature plasma of more than 10,000 degrees to melt and alloy the mixture of tungsten carbide and cobalt powder in an instant, and the molten droplets directly form spherical particles under the action of surface tension and rapidly solidify. The tungsten carbide grain coarsening caused by long-time high-temperature sintering is completely avoided, and a pre-alloyed structure with fine grains is obtained; spherical powder is directly obtained, which greatly improves the powder flowability and is beneficial to the subsequent process; and the alloying and spheroidization are completed in one step, and the process is simple and efficient.

[0038] Compared with the prior art, the present application has the following beneficial effects:

[0039] The coating formed by the prepared composite powder after thermal spraying has a microhardness of more than 1180 HV, which is higher than about 1010 HV of the conventional one-time sintering product. Under the same test conditions, the wear resistance of the coating reaches more than 95% of that of the traditional high-quality WC-12Co coating, which is much better than that of the traditional one-time sintering method. At the same time, the formula of the present application greatly reduces the amount of high-priced metals tungsten and cobalt, and the high-temperature oxidation resistance is still significantly better than that of the commercial WC-12Co. Detailed Implementation

[0040] The present invention will now be further described.

[0041] Example 1

[0042] A tungsten carbide-chromium carbide high-temperature composite powder, comprising the following components by mass percentage:

[0043] Tungsten carbide 22%, cobalt 3%, nickel-chromium alloy 20%, chromium carbide 55%.

[0044] A method for preparing the aforementioned tungsten carbide-chromium carbide high-temperature composite powder includes the following steps:

[0045] S100. After uniformly mixing tungsten carbide powder and cobalt powder, a first sintering is performed to obtain pre-alloyed cobalt-tungsten carbide hard phase powder. The first sintering is carried out under vacuum conditions, and the heating program includes: First stage: heating to 500℃ at a rate of 8℃ / min and holding for 50 minutes; Second stage: heating to 800℃ at a rate of 4℃ / min and holding for 90 minutes; Third stage: heating to a sintering temperature of 1250℃ at a rate of 12℃ / min and holding for 60 minutes.

[0046] After sintering, the furnace is cooled.

[0047] The tungsten carbide powder has an average particle size of 4 μm, and the cobalt powder has an average particle size of 2 μm.

[0048] S200. The pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder, and chromium carbide powder are mixed and placed in a vacuum sintering furnace. The furnace is first evacuated to a vacuum level of 5 × 10⁻⁶. -3 Pa, then a flowing argon-hydrogen mixture is introduced, with an argon to hydrogen volume ratio of 20:1. Sintering is performed according to the following procedure: the temperature is increased to 800℃ at 10℃ / min and held for 30 minutes, then increased to 1180℃ at 5℃ / min, switching to pure argon gas, and held for 3 hours, followed by furnace cooling; the high wear-resistant tungsten carbide-chromium carbide high-temperature composite powder is obtained.

[0049] The nickel-chromium alloy powder has a nickel to chromium mass ratio of 4:1, and the average particle size of the nickel-chromium alloy powder is 2 μm; the average particle size of the chromium carbide powder is 3 μm.

[0050] After sintering in step S100, the particles are crushed and sieved to a particle size range of 12μm; after sintering in step S200, the particles are crushed and sieved to a particle size of 35μm.

[0051] Example 2

[0052] A tungsten carbide-chromium carbide high-temperature composite powder, comprising the following components by mass percentage:

[0053] Tungsten carbide 18%, cobalt 2%, nickel-chromium alloy 25%, chromium carbide 55%.

[0054] A preparation method of the tungsten carbide-chromium carbide high-temperature composite powder comprises the following steps:

[0055] S100, after the tungsten carbide powder and the cobalt powder are uniformly mixed, first sintering is performed to obtain a pre-alloyed cobalt-tungsten carbide hard phase powder; the first sintering is performed under vacuum conditions, and a temperature rising program comprises: a first stage: the temperature is raised to 500 DEG C at a rate of 10 DEG C / min, and the temperature is kept for 30 min; a second stage: the temperature is raised to 800 DEG C at a rate of 5 DEG C / min and kept for 60 min; a third stage: the temperature is raised to a sintering temperature of 1230 DEG C at a rate of 15 DEG C / min, and the temperature is kept for 30 min;

[0056] After the sintering is completed, the furnace is cooled. The average particle size of the tungsten carbide powder is 1 mu m, and the average particle size of the cobalt powder is 1 mu m.

[0057] S200, after the pre-alloyed cobalt-tungsten carbide hard phase powder, the nickel-chromium alloy powder and the chromium carbide powder are mixed, the mixture is placed in a vacuum sintering furnace, vacuum is first extracted to 1*10-2 Pa, then flowing argon-hydrogen mixed gas is filled, and sintering is performed according to the following program: the temperature is raised to 800 DEG C at a rate of 10 DEG C / min and kept for 30 min, then the temperature is raised to 1150 DEG C at a rate of 5 DEG C / min, pure argon gas is switched, the temperature is kept for 2 hours, and the furnace is cooled; the tungsten carbide-chromium carbide high-temperature composite powder is obtained.

[0058] The mass ratio of nickel to chromium in the nickel-chromium alloy powder is 3:1, and the average particle size of the nickel-chromium alloy powder is 1 mu m; the average particle size of the chromium carbide powder is 1 mu m.

[0059] After sintering in step S100, crushing treatment is performed, and screening is performed to a particle size range of 5 mu m; after sintering in step S200, crushing treatment is performed, and screening is performed to a particle size of 15 mu m.

[0060] Example 3

[0061] A tungsten carbide-chromium carbide high-temperature composite powder comprises the following components in percentage by mass:

[0062] Tungsten carbide 20%, cobalt 5%, nickel-chromium alloy 15%, chromium carbide 60%.

[0063] A preparation method of the tungsten carbide-chromium carbide high-temperature composite powder comprises the following steps:

[0064] S100, after mixing tungsten carbide powder and cobalt powder uniformly, first sintering is performed to obtain pre-alloyed cobalt-tungsten carbide hard phase powder; the first sintering is performed under vacuum conditions, and the temperature rising procedure comprises: first stage: temperature rising to 500 DEG C at a rate of 5 DEG C / min, and holding for 60 min; second stage: temperature rising to 800 DEG C at a rate of 3 DEG C / min and holding for 120 min; third stage: temperature rising to a sintering temperature of 1270 DEG C at a rate of 10 DEG C / min, and holding for 90 min;

[0065] After sintering is completed, furnace cooling is performed. The average particle size of the tungsten carbide powder is 5 mu m, and the average particle size of the cobalt powder is 3 mu m.

[0066] S200, after mixing the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder and chromium carbide powder, the mixture is placed in a vacuum sintering furnace, vacuum is first extracted to 1*10 3 Pa, flowing argon-hydrogen mixed gas is filled, and the following procedure is used for sintering: temperature rising to 800 DEG C at a rate of 10 DEG C / min and holding for 30 min, then temperature rising to 1200 DEG C at a rate of 5 DEG C / min, switching to pure argon gas, holding for 4 hours, and furnace cooling; the high wear-resistant tungsten carbide-chromium carbide high-temperature composite powder is obtained.

[0067] The mass ratio of nickel to chromium in the nickel-chromium alloy powder is 5:1, and the average particle size of the nickel-chromium alloy powder is 4 mu m; the average particle size of the chromium carbide powder is 5 mu m.

[0068] After sintering in step S100, crushing treatment is performed, and screening is performed to a particle size range of 20 mu m; after sintering in step S200, crushing treatment is performed, and screening is performed to a particle size of 43 mu m.

[0069] Example 4

[0070] A tungsten carbide-chromium carbide high-temperature composite powder comprises the following components in percentage by mass:

[0071] Tungsten carbide 27%, cobalt 4%, nickel-chromium alloy 24%, and chromium carbide 45%.

[0072] A preparation method of the tungsten carbide-chromium carbide high-temperature composite powder comprises the following steps:

[0073] S100, after mixing tungsten carbide powder and cobalt powder uniformly, first sintering is performed to obtain pre-alloyed cobalt-tungsten carbide hard phase powder; the first sintering is performed under vacuum conditions, and the temperature rising procedure comprises: first stage: temperature rising to 500 DEG C at a rate of 9 DEG C / min, and holding for 45 min; second stage: temperature rising to 800 DEG C at a rate of 4 DEG C / min and holding for 80 min; third stage: temperature rising to a sintering temperature of 1240 DEG C at a rate of 14 DEG C / min, and holding for 70 min;

[0074] After sintering, furnace cooling is performed.

[0075] The average particle size of the tungsten carbide powder is 4 μm, and the average particle size of the cobalt powder is 2 μm.

[0076] S200, the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder and chromium carbide powder are mixed, and then are placed in a vacuum sintering furnace, vacuumized to 8×10 -3 Pa, and then flowing argon-hydrogen mixed gas is filled, and sintering is performed according to the following procedure: heating to 800 ℃ at 10 ℃ / min and keeping for 30 min, heating to 1190 ℃ at 5 ℃ / min, switching to pure argon gas, keeping for 3 hours, and furnace cooling; the high wear-resistant tungsten carbide-chromium carbide high-temperature composite powder is obtained.

[0077] The mass ratio of nickel to chromium in the nickel-chromium alloy powder is 3.5:1, and the average particle size of the nickel-chromium alloy powder is 3 μm; the average particle size of the chromium carbide powder is 2 μm.

[0078] After sintering in step S100, crushing treatment is performed, and screening is performed to a particle size range of 10 μm; after sintering in step S200, crushing treatment is performed, and screening is performed to a particle size range of 40 μm.

[0079] Example 5

[0080] The formula and step S200 are the same as those in Example 1.

[0081] In step S100, the tungsten carbide powder and the cobalt powder are uniformly mixed, and then the mixed powder is sent into a vacuum plasma spheroidization device, and under the protection of a vacuum degree of 1×10 -2 Pa or high-purity argon gas, spheroidization treatment is performed by using radio frequency plasma, wherein the plasma power is 50 kW, and the residence time of the powder in the high-temperature zone of the plasma is 10 milliseconds; after cooling, the pre-alloyed WC-Co phase powder with a sphericity of 92.3% is obtained. After sintering in step S200, crushing treatment is performed, and screening is performed to a particle size of 35 μm.

[0082] Comparative Example 1

[0083] The formula is the same as that in Example 1.

[0084] The tungsten carbide powder, the cobalt powder, the nickel-chromium alloy powder and the chromium carbide powder are directly mixed uniformly, and then are sintered at 1200 ℃ for 3 hours under the protection of argon gas, and then are crushed and screened to 35 μm to obtain a comparative composite powder.

[0085] Comparative Example 2

[0086] The formula is the same as that in Example 1.

[0087] In the preparation method, step S100 is performed in a common vacuum furnace at 1250 ℃ for 2 hours.

[0088] Other parameters and step S200 are the same as in Example 1.

[0089] Comparative Example 3

[0090] The formulation is the same as in Example 1.

[0091] The preparation method is the same as in Example 1, but pure argon is used throughout step S200, and no high vacuum extraction or argon-hydrogen mixed gas operation is performed.

[0092] Other parameters and step S100 are the same as in Example 1.

[0093] Comparative Example 4

[0094] Commercial WC-12Co spraying powder.

[0095] Comparative Example 5

[0096] The same formulation and two-step process as in Example 1 are used.

[0097] In step S200, the low-temperature platform of "ramp to 800°C at 10°C / min and hold for 30 minutes" is removed, and instead, the temperature is directly ramped to 1180°C at 10°C / min from room temperature, then the atmosphere is switched and held for 3 hours.

[0098] Comparative Example 6

[0099] The formulation is the same as in Example 1.

[0100] The preparation method includes the following steps:

[0101] S100, under vacuum conditions, mix tungsten carbide powder and cobalt powder, then perform first sintering at a sintering temperature of 1250°C for 1-3 hours to obtain pre-alloyed cobalt-tungsten carbide hard phase powder;

[0102] S200, under vacuum conditions, mix the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder, and chromium carbide powder, then perform second sintering at a sintering temperature of 1180°C for 2-4 hours to obtain the high-wear-resistant tungsten carbide-chromium carbide composite powder.

[0103] Coating preparation: The powders obtained in Examples 1-4 and Comparative Examples 1-6 were used to prepare coatings by the same HVOF system and process. The specific parameters were as follows: a JP-5000 HVOF system was used, kerosene was used as fuel, the oxygen flow rate was set to 800 L / min, the kerosene flow rate was 22 L / h, the spraying distance was fixed at 330 mm, the spray gun was perpendicular to the substrate surface and moved at a speed of 600 mm / s, the powder feeding rate was controlled at about 60 g / min, and the powder feeding gas was nitrogen (flow rate 10 L / min). Before spraying, the 45# steel substrate was sandblasted to a roughness Ra of about 4.0 μm and preheated to 100°C. Through multi-layer spraying, the final thickness of all the coatings was controlled to be 300±20 μm.

[0104] Microhardness test: A Vickers hardness tester was used, the load was 300 g, the pressure was maintained for 15 s, and 10 points of each sample were tested and averaged.

[0105] Abrasion resistance test: A rubber wheel abrasive wear tester (ASTM G65) was used, 50-70 mesh quartz sand was used as the abrasive, the load was 13 N, the wear distance was 1000 m, the weight loss of the coating was measured, and the abrasion resistance percentage relative to (WC-12Co coating) was calculated (the abrasion resistance of the WC-12Co coating was set to 100%).

[0106] High-temperature oxidation weight gain test: The coating sample was exposed to static air at 800°C for 50 hours, and the mass increase per unit area was measured. The less the weight gain, the better the oxidation resistance.

[0107] The results of the coating microhardness and abrasion resistance tests are shown in Table 1.

[0108] Table 1 Results of the coating microhardness and abrasion resistance tests after spraying the powders obtained in Examples 1-5 and Comparative Examples 1-6

[0109]

[0110] Examples 1-5 had a microhardness of 1180-1230 HV and a relative abrasion resistance of 93±3%-105±3%, which were close to or even better than the commercial WC-12Co powder of Comparative Example 4. The data of Examples 1-4 collectively proved that the "two-step" process combined with the specific formulation of the present application could obtain similar or even better comprehensive performance to the traditional high-end WC-12Co coating (Comparative Example 4) under the premise of reducing the cost by about 50%. Examples 1-4 were rich in chromium carbide (Cr3C2) and nickel-chromium (NiCr), which could form a dense oxide film at high temperature, so the data (1.8-2.5) were much better than WC-12Co (12.5). Comparative Examples 1-3 and 5-6 had uneven microstructures or the protective effect of chromium phase was not fully played due to process defects, but the high-temperature oxidation resistance was still significantly better than the commercial WC-12Co of Comparative Example 4.

[0111] The vacuum plasma spheroidization process adopted in Example 5, by achieving instant high temperature with extremely high power, makes tungsten carbide and cobalt powder complete melting and alloying in extremely short residence time and solidify rapidly. This ultra-fast process greatly suppresses the tendency of tungsten carbide grains to grow at high temperature, so that the initial fine grains are retained, thus forming a more fine-grained structure inside the material. This is the fundamental reason for the significant improvement of the coating microhardness (1230 HV), that is, the fine-grained strengthening effect is more fully played.

[0112] At the same time, the process directly produces pre-alloyed powder with high sphericity. The excellent flowability of spherical powder first guarantees its high uniformity with subsequent nickel-chromium, chromium carbide components, so that the composition of the final composite powder is uniform. When sprayed by high-velocity oxygen flame, this spherical and uniform powder has more consistent flight and melting behavior, which is conducive to the formation of a more dense and less defective coating. This better coating microstructure is the direct reason for its outstanding performance in wear resistance (105%) and high-temperature oxidation resistance (1.9 mg / cm²).

[0113] The data of Comparative Example 1 forms the most direct and strongest contrast with Example 1. Both have exactly the same composition, but due to the use of traditional one-step sintering method, its performance has dropped dramatically, with wear resistance only about 55% of Example 1. This proves that the process structure of the two-step method itself is the most fundamental reason for the qualitative change in performance, rather than simply adjusting the composition.

[0114] The coating performance of Comparative Example 2 is significantly lower than Example 1, which uses the complete temperature rising program. The fundamental reason is that it lacks the precise heat process control of the "three-step temperature rising" designed by the present application in the S100 step. Comparative Example 2 simplifies the S100 step to the conventional operation of directly rising to 1250°C and holding for 2 hours, which leads to three key defects. First, it lacks systematic degassing and activation preparation. It skips the 500°C low-temperature degassing holding period, and the water and gas adsorbed in the powder may be released sharply in the subsequent high-temperature stage, interfering with the densification process and possibly introducing micro defects. Second, it interrupts the key step of solid-state pre-alloying. It does not go through the long-term medium-temperature holding at 800°C, which makes the cobalt atoms (Co) unable to complete sufficient solid-state pre-diffusion on the surface of tungsten carbide (WC) particles. Therefore, when entering the high-temperature liquid phase sintering, there is a lack of pre-constructed strong and tough atomic-level bonding interface between Co and WC particles as a foundation. Third, it exacerbates the adverse process of grain coarsening. Directly entering the high-temperature holding for 2 hours provides sufficient time and thermal driving force for WC grains to grow through the dissolution-precipitation mechanism, resulting in a significantly larger grain size in the core of the pre-alloyed hard phase than in Example 1, which controls the high temperature for a short time.

[0115] The data of Comparative Example 3 highlights the necessity of reducing atmosphere in the second sintering step. The lack of hydrogen's purifying effect on the powder surface leads to a decrease in the quality of interfacial bonding. Its performance is better than that of Comparative Example 1 but significantly lower than that of Example 1, proving that a reducing atmosphere is a key guarantee for achieving strong metallurgical bonding and fully releasing the potential of the two-step method.

[0116] Comparative Example 5 cancels the low-temperature holding platform at 800°C and adopts a direct temperature rising procedure, which leads to two key technical defects. First, the reduction reaction of hydrogen is insufficient due to the lack of a suitable low-temperature kinetic window, and the trace amount of oxides on the surface of the powder particles cannot be effectively removed. Second, due to the difference in the thermal expansion coefficients between the components (especially the nickel-chromium binder phase and the hard phase), a large thermal stress is generated during the rapid temperature rising process. The former directly deteriorates the interfacial cleanliness, and the latter introduces microscopic defects and hinders the uniform diffusion between atoms. The combined effect of the two leads to a decrease in the interfacial bonding quality and poor uniformity of the composite powder prepared.

[0117] The process of Comparative Example 6 is essentially a solid-phase sintering that completely relies on long-time thermal diffusion. Under high temperature for several hours, tungsten carbide (WC) grains will significantly grow through the dissolution-precipitation mechanism. Coarse grains can provide a certain hardness, but will reduce the toughness and interfacial bonding strength of the material, limiting the upper limit of hardness. Traditional sintering relies on atomic thermal diffusion to form bonding, and its driving force and efficiency are limited. Although the two-step method structure allows more opportunities for cobalt (Co) to form a bridge between WC and NiCr, due to the lack of field activation and atmosphere purification, the interface may have residual impurities or areas that are not fully alloyed, resulting in an improvement in bonding strength but not optimal. The performance of Comparative Example 6 is much better than that of Comparative Example 1 using the same composition, which proves that the process sequence of the present invention, i.e., alloying WC-Co first and then compounding, has a fundamental advantage.

Claims

1. A tungsten carbide-chromium carbide high-temperature composite powder, characterized in that, By mass percentage, it includes the following components: Tungsten carbide 18~27%, cobalt 2~5%, nickel-chromium alloy 15~25%, chromium carbide 45~60%.

2. A method for preparing tungsten carbide-chromium carbide high-temperature composite powder as described in any one of claims 1, characterized in that, Includes the following steps: S100. After mixing tungsten carbide powder and cobalt powder, a first sintering is performed to obtain pre-alloyed cobalt-tungsten carbide hard phase powder. S200: After mixing the pre-alloyed cobalt-tungsten carbide hard phase powder, nickel-chromium alloy powder, and chromium carbide powder, a second sintering is performed to obtain the high wear-resistant tungsten carbide-chromium carbide high-temperature composite powder.

3. The preparation method according to claim 2, characterized in that, In step S100, the first sintering is carried out under vacuum conditions, and the heating program includes: heating to 500°C at a rate of 5~10°C / min and holding for 30~60 minutes; heating to 800°C at a rate of 3~5°C / min and holding for 60~120 minutes; heating to a sintering temperature of 1230~1270°C at a rate of 10~15°C / min and holding for 30~90 minutes. After sintering, the furnace is cooled.

4. The preparation method according to claim 2, characterized in that, In step S200, the second sintering includes: first, evacuating to ≤1×10⁻²Pa, then filling with a flowing argon-hydrogen mixture, heating to 1150~1200℃, switching to pure argon, holding at that temperature for 2~4 hours, and then cooling with the furnace.

5. The preparation method according to claim 4, characterized in that, In the argon-hydrogen mixture, the volume ratio of argon to hydrogen is 18~22:

1.

6. The preparation method according to claim 4, characterized in that, The programmed temperature rise includes: increasing the temperature to 800°C at 10°C / min and holding it for 30 minutes, then increasing the temperature to 1180~1230°C at 5°C / min.

7. The preparation method according to claim 3 or 4, characterized in that, In step S100, the average particle size of the tungsten carbide powder is 1~5μm, and the average particle size of the cobalt powder is 1~3μm.

8. The preparation method according to claim 2, characterized in that, In step S200, the mass ratio of nickel to chromium in the nickel-chromium alloy powder is 3~5:1, the average particle size of the nickel-chromium alloy powder is 1~4μm, and the average particle size of the chromium carbide powder is 1~5μm.

9. The preparation method according to claim 7, characterized in that, After sintering in step S100, the particles are crushed and sieved to a particle size range of 5~20μm; after sintering in step S200, the particles are crushed and sieved to a particle size range of 15~43μm.

10. The preparation method according to claim 2, characterized in that, In step S100, the first sintering includes: Vacuum degree ≤ 5 × 10 -2 At Pa, spheroidization is performed using radio frequency plasma, with a plasma power of 40~60kW, a powder feeding rate of 30~50g / min, argon as the carrier gas, and a residence time of the powder in the high-temperature plasma region of 5~20 milliseconds.

Images