Mo-wc reinforced nickel-based alloy gradient coating on inner wall of steel pipe and preparation method thereof

Abstract

The application relates to the field of surface engineering and metallurgical material technology, and discloses a Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe and a preparation method.The raw material of the coating strengthening layer comprises Ni-Cr-B-Si self-fluxing alloy powder, spherical cast tungsten carbide, submicron Mo-WC pre-alloy powder, metal molybdenum powder, potassium fluoborate, light calcium carbonate, lanthanum oxide and a reactive composite binder L.In the preparation, the slurry of the base layer and the strengthening layer is layered and coated on the treated inner wall of the steel pipe, and then stepwise heat treatment is carried out, and the pre-drying, solid-phase degassing and liquid-phase remelting under high centrifugal acceleration are sequentially experienced.The scheme utilizes the molybdenum powder and the Mo-WC pre-alloy powder to inhibit the high-temperature dissolution of tungsten carbide, improves the interface metallurgical bonding strength through the gas phase purification effect of the potassium fluoborate, and eliminates the internal pore defects in combination with the solid fluorine discharge mechanism of the light calcium carbonate.The obtained coating has a dense structure, a significant radial hardness gradient and excellent high-temperature wear resistance and impact spalling resistance.

Description

Technical Field

[0001] This invention relates to the fields of surface engineering and metallurgical materials technology, specifically to a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method. Background Technology

[0002] In industries such as energy, mining, and chemicals, the transport of corrosive media containing solid particles places extremely high demands on the wear resistance and impact resistance of the inner wall of steel pipes. Currently, induction remelting or centrifugal casting techniques are common methods for preparing nickel-based tungsten carbide composite coatings on the inner wall of steel pipes to improve pipe life. However, in the actual preparation process, because the melting point of nickel-based alloys overlaps with the stable temperature range of tungsten carbide, when the molten pool temperature exceeds 1100 degrees Celsius and is maintained for a certain period of time, the liquid-phase nickel-based alloy will cause severe decarburization and erosion on the surface of the tungsten carbide particles.

[0003] This high-temperature dissolution phenomenon reduces the effective load-bearing size of the tungsten carbide hard phase and causes severe edge dissolution at the particle edges. The dissolved carbon and tungsten atoms, once they enter the matrix molten pool, readily precipitate brittle composite carbides. This not only destroys the original toughness of the coating but also makes the radial depth distribution of hardness uncontrollable. Furthermore, due to the lack of effective physical and chemical barriers, the hard phase exhibits poor chemical stability at high temperatures, leading to easy fragmentation and overall peeling of the coating during service. Traditional preparation processes often struggle to balance the physical sedimentation behavior of the hard phase with the inhibition of chemical dissolution, ultimately resulting in the coating's high-temperature wear resistance and long-term service stability failing to meet expectations. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of steel pipes and its preparation method, which solves the problems of tungsten carbide corrosion and brittle phase precipitation caused by high-temperature remelting in existing technologies, resulting in the failure of the reinforcing phase and the loss of control over the hardness gradient.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe, comprising the following raw materials in parts by weight: 45.0-52.0 parts of Ni-Cr-B-Si self-fluxing alloy powder; 30.0-35.0 parts of spherical cast tungsten carbide; Asia 5.0-8.0 parts of m-grade Mo-WC pre-alloyed powder; 3.0-5.0 parts of metallic molybdenum powder; Potassium fluoroborate 1.2-1.8 parts; Light calcium carbonate 0.5-1.0 parts; Lanthanum oxide 0.5-1.0 parts; Reactive composite adhesive L15.0-25.0 parts.

[0006] Preferably, the chemical composition of the Ni-Cr-B-Si self-fluxing alloy powder, by mass percentage, is: Cr 12.0%-18.0%, B 2.0%-3.5%, Si 3.0%-4.5%, C 0.4%-0.8%, Fe < 5.0%, with Ni as the balance; the physical properties are a particle size of 45-105 μm. m, spherical in shape; the spherical cast tungsten carbide has a carbon content of 3.8%-4.1% and an average particle size of 100-150 μm. m, microhardness 2500-3000HV.

[0007] Preferably, the sub m-grade Mo-WC pre-alloyed powder was prepared by the following method: 10.0 parts by weight of metallic molybdenum powder and 90.0 parts by weight of tungsten carbide powder were mixed and ball-milled for 12 hours, dried, and then placed under a vacuum of 1.0 × 10⁻⁶. -2 In a vacuum induction furnace with a pressure below Pa, the sintered blocks are held at 1450℃ for 2 hours, and then pulverized by airflow to a particle size of 0.8-5.0 mm. m.

[0008] Preferably, the reactive composite adhesive L is made from raw materials comprising the following parts by weight: 6.0-8.0 parts of PVA, 90.0-94.0 parts of deionized water, and a solution formed by the above two mixed with 35.0-40.0 parts of industrial-grade sodium silicate; wherein the degree of alcoholysis of the polyvinyl alcohol is 87%-89% (mol / mol), and the modulus of the industrial-grade sodium silicate is 2.8-3.2.

[0009] Preferably, the specific surface area of ​​the light calcium carbonate is 10-15 m². 2 / g, average particle size is 2-5 m, bulk density is 0.4-0.7 g / cm³ 3 The potassium fluoroborate has a purity greater than 98% and a density of 2.50 g / cm³. 3 .

[0010] A method for preparing a Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe includes the following steps: (1) Mix Ni-Cr-B-Si powder, spherical cast tungsten carbide, and sub-Si powder in proportion. A mixed powder for the reinforcing layer was prepared by using m-grade Mo-WC powder, metallic molybdenum powder, potassium fluoroborate, light calcium carbonate, and lanthanum oxide. (2) Mix the reinforcing layer powder with the reactive composite adhesive L to prepare a reinforcing layer slurry, and separately mix the underlayer powder with the adhesive L to prepare an underlayer slurry; (3) First apply the base coat slurry to the inner wall of the steel pipe, and after the surface is semi-dry, apply the reinforcing layer slurry. (4) The pre-coated steel pipe is subjected to step heat treatment, which involves the pre-drying stage, the solid phase degassing stage and the liquid phase remelting stage in sequence. (5) Maintain centrifugal acceleration for atomized water spray cooling.

[0011] Preferably, in step (3), the coating thickness of the base coat slurry is 0.5-0.8 mm, and the coating thickness of the reinforcing layer slurry is 1.5-2.5 mm.

[0012] Preferably, in step (4), the technical parameters of the solid phase degassing stage are: centrifugal acceleration of 10-20g, heating to 500-650℃ at a rate of 10-15℃ / min, and holding for 10-15min.

[0013] Preferably, in step (4), the technical parameters of the liquid phase remelting stage are: centrifugal acceleration is increased to 60-80g, temperature is increased to 1050-1150℃ at a rate of 30-50℃ / min, induction frequency is set to 4-8kHz, and heat preservation is 120-180s.

[0014] Preferably, in step (4), the pre-drying stage is maintained at 120-150℃ for 90-150 minutes, and the steel pipe is rotated at 8-10 rpm; in step (5), centrifugation is stopped when the pipe temperature drops below 500-550℃.

[0015] This invention provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of steel pipes and its preparation method. It has the following beneficial effects: 1. This invention introduces submicron-sized Mo-WC pre-alloy powder and metallic molybdenum powder into the system, establishing a preferential dissolution saturation environment for molybdenum atoms during the melting process of nickel-based alloys. This effectively suppresses decarburization and edge corrosion of spherical cast tungsten carbide in the high-temperature molten pool. The enriched layer of molybdenum atoms formed on the surface of tungsten carbide particles acts as a physical barrier, hindering the dissolution and extraction of the hard phase by the base metal, ensuring the integrity of the original particle size and shape of the reinforcing phase, thereby improving the hardness stability and wear resistance of the coating under complex thermodynamic conditions.

[0016] 2. This invention utilizes the staged decomposition characteristics of potassium fluoroborate during a stepped heating process to perform in-situ chemical etching of the residual oxide film on the steel pipe substrate and the hard phase surface using the generated boron trifluoride gas. This purification mechanism, combined with the solid-phase degassing process, eliminates the physical barriers at the interface, allowing the liquid nickel-based alloy to achieve a near-zero wetting angle with the clean substrate surface during the remelting stage. This promotes deep cross-interface diffusion of alloying elements, forming a tightly bonded metallurgical transition layer and significantly enhancing the shear strength between the coating and the substrate.

[0017] 3. This invention uses lightweight calcium carbonate as an endogenous fluoride-fixing agent, combined with a high-speed centrifugal acceleration of 60g to 80g, to achieve chemical capture and rapid slag removal of reaction products. The calcium oxide produced by the decomposition of calcium carbonate can capture residual fluoride gas in situ and convert it into a low-density slag system. Under the action of centrifugal force, these slag phases migrate rapidly to the inner surface of the coating along with the bubbles and are discharged, completely solving the porosity and looseness defects caused by the accumulation of chemical reaction products in traditional processes, and ensuring that the gradient coating has extremely high relative density and structural continuity. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the method steps of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will now be clearly and completely described 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.

[0020] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0021] 45.0 to 52.0 parts of Ni-Cr-B-Si self-fluxing alloy powder, the chemical composition of which, by mass percentage, is: Cr 12.0% to 18.0%, B 2.0% to 3.5%, Si 3.0% to 4.5%, C 0.4% to 0.8%, Fe less than 5.0%, Ni balance; physical properties include a particle size of 45 to 105 μm. m is spherical.

[0022] Spherical cast tungsten carbide with a WC content of 30.0 to 35.0 parts, CAS number 12070-12-1, a carbon content of 3.8% to 4.1%, and an average particle size of 100 to 150 mm. m, microhardness 2500 to 3000 HV. 5.0 to 8.0 parts of m-grade Mo-WC pre-alloyed powder, formed by vacuum sintering of metallic molybdenum powder and tungsten carbide powder, with a Mo mass content of 5% to 10% and WC as the balance, and a particle size of 0.8 to 5.0 mm. m.

[0023] Industrial-grade metallic molybdenum powder, 3.0 to 5.0 parts, CAS No. 7439-98-7, purity greater than 99.5%, average particle size 10 to 30 mm. m.

[0024] Industrial grade potassium fluoroborate (KBF4) is 1.2 to 1.8 parts, CAS number 14075-53-7, with a purity greater than 98% and a density of 2.50 g / cm3, and is vacuum dried at 80℃.

[0025] Light calcium carbonate, 0.5 to 1.0 parts, CAS number 471-34-1, specific surface area 10 to 15 m² / g, average particle size 2 to 5 mm. m, bulk density 0.4 to 0.7 g / cm3. Lanthanum oxide (La2O3) content 0.5 to 1.0 parts, CAS number 1312-81-8, purity greater than 99.9%, average particle size 1 to 2 μm. m.

[0026] The reactive composite adhesive L liquid of the present invention comprises the following raw materials by mass: 6.0 to 8.0 parts of polyvinyl alcohol (PVA), model PVA-1788, CAS number 9002-89-5, average degree of polymerization 1700, degree of alcoholysis 87% to 89% (mol / mol), and secondary transformation point in water 75 to 85°C.

[0027] Industrial grade sodium silicate (water glass), 35.0 to 40.0 parts, CAS No. 1344-09-8, modulus 2.8 to 3.2, density 1.35 to 1.40 g / cm3, solid content 35% to 40%, is a colorless, transparent, viscous liquid.

[0028] 90.0 to 94.0 parts of deionized water.

[0029] The 20# steel or L360NS pipeline steel pipe used as the base material needs to undergo degreasing and sandblasting pretreatment on its inner wall. Preparation Example 1: Preparation of Standard Reactive Composite Binder L1 Liquid This preparation example provides a method for preparing a standard reactive composite adhesive L1 liquid, including the following steps: Step 1: Add 6.0 parts by weight of polyvinyl alcohol powder to 94.0 parts by weight of deionized water, and stir at 200 rpm for 2.5 hours at 92 to 95°C to completely dissolve it, thus obtaining a polyvinyl alcohol solution.

[0030] Step 2: Cool the obtained polyvinyl alcohol solution to below 40°C.

[0031] Step 3: Mix the polyvinyl alcohol solution obtained in Step 2 with 38.0 parts of industrial-grade sodium silicate solution according to the specified ratio, and stir at 500 rpm for 45 minutes.

[0032] Step 4: Add 0.2% of the total mass of the mixed solution of silicone defoamer, continue stirring for 15 minutes, and let stand for 24 hours to age. The resulting liquid is the standard reactive composite adhesive L1 liquid.

[0033] Preparation Example 2: Preparation of High-Viscosity Reactive Composite Binder L2 Liquid This preparation example provides a method for preparing a high-viscosity reactive composite binder L2 liquid, including the following steps: Step 1: Add 8.0 parts by weight of polyvinyl alcohol powder to 92.0 parts by weight of deionized water, and stir at 95°C for 3 hours to obtain a high-concentration polyvinyl alcohol solution.

[0034] Step 2: Cool the solution to 35°C.

[0035] Step 3: Mix the polyvinyl alcohol solution obtained in Step 2 with 40.0 parts of industrial-grade sodium silicate solution according to the specified ratio, and stir at 600 rpm for 60 minutes.

[0036] Step 4: Add 0.3% of the total mass of silicone defoamer, let stand and age for 24 hours, and the resulting liquid is the high-viscosity reactive composite adhesive L2 liquid.

[0037] Preparation Example 3: Sub Preparation of m-grade Mo-WC pre-alloyed powder This preparation example provides sub The preparation method of m-grade Mo-WC pre-alloyed powder includes the following steps: Step 1: By weight, place 10.0 parts of industrial-grade molybdenum powder and 90.0 parts of tungsten carbide powder in a ball mill jar, and ball mill at 300 rpm for 12 hours using anhydrous ethanol as the medium.

[0038] Step 2: Vacuum dry the slurry at 80°C for 4 hours.

[0039] Step 3: Place the powder in a graphite crucible and heat it to 1450°C in a vacuum induction furnace with a vacuum degree of less than 1.0 x 10^-2 Pa, and hold it at that temperature for 2 hours.

[0040] Step 4: After crushing, the sintered blocks are fed into an air jet mill for further pulverization to obtain particles with a size between 0.8 and 5.0 mm. sub-m m-grade Mo-WC pre-alloyed powder.

[0041] Example 1 This embodiment provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method. (See attached document.) Figure 1 This includes the following steps: Step 1, Preparation of Reinforcing Layer B Powder: Weigh out 45.0 parts by weight of nickel-chromium-boron-silicon alloy powder, 30.0 parts by weight of spherical cast tungsten carbide, and 5.0 parts by weight of... M-grade molybdenum-tungsten carbide pre-alloyed powder, 3.0 parts of metallic molybdenum powder, 1.2 parts of potassium fluoroborate, 0.5 parts of light calcium carbonate, and 0.5 parts of lanthanum oxide were placed in a three-dimensional mixer and dry-mixed at 40 rpm for 2 hours to obtain a reinforced layer mixed powder.

[0042] Step 2, Slurry preparation: By mass, take 100.0 parts of the reinforcing layer mixed powder obtained in Step 1 and 15.0 parts of the standard reactive composite binder L1 liquid obtained in Preparation Example 1 and mix them. Stir at 150 rpm for 40 minutes in a planetary mixer to prepare the reinforcing layer slurry. Separately, take 100.0 parts of the underlayment A powder and 15.0 parts of L1 liquid and prepare the underlayment slurry using the same process.

[0043] Step 3, Pre-coating: The inner wall of the No. 20 steel pipe to be processed is sandblasted to remove rust. Using a spiral spray gun, a 0.5mm thick base coat is evenly applied to the inner wall and allowed to dry at room temperature for 40 minutes until the surface is semi-dry. Then, a 1.5mm thick reinforcing layer is applied over the base coat.

[0044] Step 4, Stepped Heat Treatment: The pre-coated steel pipe is loaded into a horizontal centrifugal induction composite equipment. First, it enters the pre-drying stage, where the steel pipe is rotated at 8 rpm and held at 120°C for 150 minutes. Then, it enters the solid-phase degassing stage, where the medium-frequency induction power supply is turned on, the centrifugal acceleration is adjusted to 10g, and the temperature is raised to 500°C at a rate of 10°C per minute and held for 15 minutes. Finally, it enters the liquid-phase remelting stage, where the centrifugal acceleration is increased to 60g, and the temperature is rapidly raised to 1050°C at a rate of 30°C per minute. The induction frequency is set to 4kHz, and the temperature is held for 180 seconds.

[0045] Step 5, Cooling and Shaping: While maintaining a centrifugal acceleration of 60g, the outer wall of the steel pipe is cooled by atomized water spray. When the pipe temperature drops below 550℃, centrifugation is stopped, the steel pipe is removed, and the gradient coating preparation is completed.

[0046] Example 2 This embodiment provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method, including the following steps: Step 1, Preparation of Reinforcing Layer B Powder: Weigh out 50.0 parts by weight of nickel-chromium-boron-silicon alloy powder, 32.5 parts by weight of spherical cast tungsten carbide, and 6.5 parts by weight of... The reinforced layer mixed powder is prepared by dry mixing m-grade molybdenum-tungsten carbide pre-alloy powder, 4.0 parts of metallic molybdenum powder, 1.5 parts of potassium fluoroborate, 0.8 parts of light calcium carbonate, and 0.7 parts of lanthanum oxide in a three-dimensional mixer for 2.5 hours.

[0047] Step 2, Slurry preparation: By weight, take 100.0 parts of reinforcing layer mixed powder and 20.0 parts of standard reactive composite binder L1 liquid obtained in Preparation Example 1 and mix them. Stir at 180 rpm for 45 minutes in a planetary mixer to prepare the reinforcing layer slurry; prepare the bottom layer A powder in the same proportion.

[0048] Step 3, Pre-coating: Apply a 0.6mm thick base coat to the inner wall of the sandblasted L360NS pipeline steel pipe, and after 50 minutes, apply a 2.0mm thick reinforcing layer coat.

[0049] Step 4, stepped heat treatment: The steel pipe is pre-dried at 135℃ for 120 minutes with a rotation speed of 10 rpm. Solid-phase degassing stage: Centrifugal acceleration of 15g, heating to 650℃ at 12℃ per minute and holding for 12 minutes. Liquid-phase remelting stage: Centrifugal acceleration of 75g, heating to 1100℃ at 40℃ per minute, induction frequency set to 6kHz, holding for 150 seconds.

[0050] Step 5, cooling and shaping: Maintain a centrifugal acceleration of 75g, cool with atomized water spray until the temperature drops below 500℃, then stop centrifugation and remove the steel pipe.

[0051] Example 3 This embodiment provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method, including the following steps: Step 1, Preparation of Reinforcing Layer B Powder: Weigh out 52.0 parts by weight of nickel-chromium-boron-silicon alloy powder, 35.0 parts by weight of spherical cast tungsten carbide, and 8.0 parts by weight of... M-grade molybdenum-tungsten carbide pre-alloy powder, 5.0 parts of metallic molybdenum powder, 1.8 parts of potassium fluoroborate, 1.0 parts of light calcium carbonate, and 1.0 parts of lanthanum oxide were placed in a three-dimensional mixer and dry-mixed for 3 hours to obtain a reinforced layer mixed powder.

[0052] Step 2, Slurry preparation: By weight, take 100.0 parts of reinforcing layer mixed powder and 25.0 parts of high viscosity reactive composite binder L2 liquid obtained in Preparation Example 2 and mix them. Stir at 200 rpm for 50 minutes in a planetary mixer to prepare the reinforcing layer slurry; prepare the bottom layer A powder in the same proportion.

[0053] Step 3, Pre-coating: First apply a 0.8mm thick base coat to the inner wall of the steel pipe, and after 60 minutes, apply a 2.5mm thick reinforcing layer coat.

[0054] Step 4, stepped heat treatment: The steel pipe is pre-dried at 150℃ for 90 minutes with a rotation speed of 10 rpm. Solid-phase degassing stage: Centrifugal acceleration of 20g, heating to 850℃ at 15℃ per minute and holding for 10 minutes. Liquid-phase remelting stage: Centrifugal acceleration of 80g, heating to 1150℃ at 50℃ per minute, induction frequency set to 8kHz, holding for 120 seconds.

[0055] Step 5, cooling and shaping: Maintain a centrifugal acceleration of 80g, cool with atomized water spray to below 550℃, then stop centrifugation and remove the steel pipe.

[0056] Example 4 This embodiment provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method. Based on Example 2, this embodiment focuses on the influence of the residence time during the solid phase degassing stage on the coating quality, including the following steps: Step 1, Preparation of reinforced layer B powder: The types and mass ratios of each component are completely consistent with those in Example 2.

[0057] Step 2, Slurry preparation: By weight, take 100.0 parts of the reinforcing layer mixed powder and 20.0 parts of the L1 liquid obtained in Preparation Example 1 to prepare the reinforcing layer slurry.

[0058] Step 3, Pre-coating: Apply a 0.6mm thick base coat and a 2.0mm thick reinforcing layer to the inner wall of the steel pipe.

[0059] Step 4, stepped heat treatment: The pre-drying process is the same as in Example 2. When entering the solid-phase degassing stage, the centrifugal acceleration is set to 15g, and the temperature is increased to 650℃ at a rate of 12℃ per minute, but the holding time is shortened to 5 minutes. Subsequently, the liquid-phase remelting stage begins, with the remaining process parameters consistent with Example 2.

[0060] Step 5, Cooling and Shaping: Keep the pipe under centrifugal conditions and spray atomized water to cool it to below 500°C, then remove the steel pipe.

[0061] Example 5 This embodiment provides a Mo-WC reinforced nickel-based alloy gradient coating for the inner wall of a steel pipe and its preparation method. Based on Example 2, this embodiment focuses on the influence of induction frequency on the uniformity of the coating structure, including the following steps: Step 1, Preparation of reinforced layer B powder: The types and mass ratios of each component are completely consistent with those in Example 2.

[0062] Step 2, Slurry preparation: The slurry preparation ratio and adhesive type are consistent with those in Example 2.

[0063] Step 3, Pre-coating: Apply a 0.6mm thick base coat and a 2.0mm thick reinforcing layer to the inner wall of the steel pipe.

[0064] Step 4, stepped heat treatment: The parameters for the pre-drying and solid-phase degassing sections are the same as in Example 2. When entering the liquid-phase remelting stage, the centrifugal acceleration is set to 75g, the temperature is raised to 1100℃ and held for 150s, but the frequency of the induction power supply is adjusted to 10kHz high-frequency mode.

[0065] Step 5, Cooling and Shaping: Keep the pipe under centrifugal conditions and spray atomized water to cool it to below 500°C, then remove the steel pipe.

[0066] Comparative Example 1: Compared with Example 2, the difference is that no nitrite is added when preparing the reinforcing layer B powder. The missing mass fraction of the m-grade Mo-WC pre-alloyed powder and metallic Mo powder is made up by Ni-Cr-B-Si self-fluxing alloy powder, and the rest are the same.

[0067] Comparative Example 2: Compared with Example 2, the difference is that potassium fluoroborate is not added when preparing the reinforcing layer B powder, but all other aspects are the same.

[0068] Comparative Example 3: Compared with Example 2, the difference is that light calcium carbonate is not added when the reinforcing layer B powder is prepared, but all other aspects are the same.

[0069] Comparative Example 4: Compared with Example 2, the difference is that the solid phase degassing stage of 500 to 650°C is not set in the heat treatment process of step 4. Instead, after the pre-drying is completed, the temperature is directly raised to 1100°C at a rate of 40°C per minute for liquid phase remelting. All other aspects are the same.

[0070] Each component is weighed and prepared according to the above-mentioned mass proportions.

[0071] Test Example 1: Analysis of Coating Density and Residual Gas Content From the steel pipes prepared in Example 2, Comparative Example 3 and Comparative Example 4, samples with dimensions of 10mm*10mm*thickness were cut from the middle region of the coating using wire cutting. Three samples were taken from each group.

[0072] The sample was placed in an ultrasonic cleaner and cleaned with anhydrous ethanol for 30 minutes to remove surface oil and cutting residue, and then dried in an 80°C oven for 60 minutes.

[0073] The mass of the sample in air was measured using an electronic analytical balance, and its buoyancy in distilled water was then measured using the displacement method. The apparent density of the sample was calculated based on Archimedes' principle. Combined with the theoretical density calculated from the powder ratio, the relative density of each group of samples was obtained.

[0074] The sample after density measurement is crushed into particles smaller than 1 mm. 0.5 g of sample gas is weighed and placed in the graphite crucible of the oxygen, nitrogen and hydrogen analyzer. Melt extraction is performed under high temperature inert gas protection to determine the residual oxygen content inside the coating.

[0075] The sample particles were treated by alkaline fusion precipitation separation method, and the mass fraction of residual fluorine in the coating was detected by ion chromatography.

[0076] The average value of the test data for each group of 3 samples was taken, and the experimental data were recorded and summarized.

[0077] Table 1. Results of Coating Density and Residual Impurity Elemental Analysis ; According to the data analysis in Table 1, Example 2 is significantly better than Comparative Example 3 and Comparative Example 4 in terms of density performance, with its relative density remaining stable at around 99.0%, while the residual oxygen and fluorine contents are at a low level.

[0078] Comparing the data from Example 2 and Comparative Example 4, it can be concluded that the stepped heat treatment process has a decisive impact on the coating quality. Comparative Example 4 skipped the solid-phase degassing stage from 500 to 650°C and directly proceeded to high-temperature remelting, resulting in the boron trifluoride gas released from the decomposition of potassium fluoroborate not being fully discharged before the alloy powder melted. Due to the surface tension of the liquid alloy, these gases that could not escape in time were trapped in the molten pool. As the temperature increased, although the pores migrated under the action of centrifugal force, they were difficult to completely eliminate, ultimately forming a large number of micropores in the coating. The residual fluorine content of Comparative Example 4, which was as high as 1000 ppm and the density was less than 96%, demonstrates the necessity of the preheating degassing stage for removing reaction products and improving the density of the coating.

[0079] Comparing the data from Example 2 and Comparative Example 3 reveals that light calcium carbonate plays a crucial role in the endogenous fluorine fixation of the system. Although Comparative Example 3 employed a stepped heating process, the lack of calcium oxide produced from the decomposition of calcium carbonate meant that the residual fluoride gas generated could only diffuse naturally through the physical gaps in the porous layer. In the inner walls of large-diameter, long pipes used in industrial production, the long and difficult path for complete gas diffusion easily leads to fluorine residues within the coating.

[0080] In Example 2, calcium carbonate was introduced, and the calcium oxide generated in situ captured residual boron fluoride, transforming it into a chemically stable and low-density calcium fluoride slag system. In a high-speed centrifugal flow field of 60g or more, these high-melting-point slag phases underwent density separation from the liquid alloy, rapidly migrating to the inner surface of the coating and being discharged, thereby reducing the residual fluorine content of Example 2 by nearly an order of magnitude compared to Comparative Example 3.

[0081] Test Example 2: Interface Bond Strength and Purification Effect Test Samples were cut from the coated areas of the steel pipes prepared in Example 2 and Comparative Example 2 using a wire cutting device. The sample size was 20 mm x 20 mm x the full thickness of the pipe wall.

[0082] To ensure the perpendicularity of the load during the shear test, the end faces of the substrate side and the coating side of the specimen were mechanically ground to control the parallelism deviation within 0.05 mm.

[0083] The treated sample is installed in a dedicated shear strength test fixture, ensuring that the shear surface is precisely aligned with the interface between the coating and the steel substrate.

[0084] The universal testing machine was used for loading, the displacement control mode was set, the loading rate was set to 0.5 mm per minute, and the shear load was continuously applied until the interface completely fractured.

[0085] Record the maximum load value at the moment of fracture and measure the stress area of ​​the actual shear surface. Calculate the shear bond strength according to the formula.

[0086] Five samples were tested for each group. Data with excessively large deviations were removed, and the arithmetic mean of the remaining data was taken as the final test result for that group.

[0087] Table 2. Test data on shear bond strength between coating and substrate. ; According to the data in Table 2, the average shear bond strength of Example 2 remained stable above 400 MPa. The fracture locations of some samples occurred on the substrate side or inside the coating, rather than at the strict interface, indicating that a high-strength metallurgical bond had formed between the coating and the substrate. In contrast, Comparative Example 2, lacking the potassium fluoroborate additive, showed a significant drop in shear strength to below 200 MPa, and its fracture characteristics all exhibited typical straight interface peeling.

[0088] Based on the mechanism analysis of this scheme, in the solid-phase degassing stage of Example 2, the boron trifluoride gas generated by the thermal decomposition of potassium fluoroborate plays a key chemical purification role. Since a very thin oxide film remains on the surface of the steel pipe substrate and the WC hard phase after pretreatment, these oxides significantly increase the wetting angle of the liquid nickel-based alloy during conventional smelting, hindering the cross-interface diffusion of metal atoms.

[0089] In Example 2, boron trifluoride gas etched these oxide films in situ, converting them into gaseous fluorine oxides that were discharged through the exhaust system, thereby exposing a clean metal surface. In the subsequent liquid-phase remelting stage, the clean interface caused the wetting angle between the liquid alloy and the substrate surface to approach zero, promoting the deep penetration of elements such as nickel and chromium into the matrix through grain boundaries and lattice vacancies, forming a uniformly thick diffusion transition layer.

[0090] Comparative Example 2 data confirms the negative consequences of lacking chemical purification methods. Without potassium fluoroborate, even with increased remelting temperature or extended centrifugation time, liquid metal struggles to completely penetrate the residual oxide film. This physical contact results in numerous microscopic unbonded areas at the interface, where shear strength depends entirely on mechanical interlocking force. Consequently, under shear loads, cracks rapidly propagate along the oxide inclusion interface where stress is most concentrated, leading to low overall load-bearing capacity.

[0091] The test results objectively prove that the potassium fluoroborate-mediated interface purification mechanism is the core of the process to achieve coatings that do not peel off or flake off under heavy load conditions.

[0092] Test Example 3: Cross-sectional Radial Hardness Gradient Distribution Test A 20mm x 15mm cross-sectional sample was cut from the composite pipes prepared in Example 2 and Comparative Example 1 using wire cutting technology to ensure that the cross-section covers the entire area from the inner surface of the coating to the steel substrate.

[0093] The sample was placed in a mounting machine for thermosetting resin mounting, and then wet-polished on a metallographic sample pre-grinding machine by changing from coarse to fine sandpaper to 400 grit, 800 grit, and 1500 grit to remove the heat-affected layer generated during cutting.

[0094] Use 2.5 m and 0.5 The sample surface was finely polished with a diamond polishing solution of m until no obvious grinding marks were observed on the surface and the phase interface was clear under a metallographic microscope.

[0095] A digital Rockwell hardness tester was used for testing, with a load set at 150 kg and a loading time of 10 s.

[0096] Starting from the innermost surface of the coating, measurements are taken radially toward the steel substrate, with a test level set every 0.5 mm.

[0097] Select three parallel test points on each horizontal plane, with a spacing of no less than 2 mm between the points to avoid deformation strengthening interference, and record the hardness values ​​of each point.

[0098] Continuous measurements were taken until the metallurgical interface was penetrated to a depth of 3 mm into the steel substrate, and all data were collected.

[0099] Table 3 shows the Rockwell hardness distribution of the coating along the radial depth. ; Based on the data analysis in Table 3, Example 2 exhibits a significant hardness gradient distribution. With increasing radial depth, the hardness value shows a steady upward trend, reaching a peak in the bottom region of the reinforcing layer near the substrate, with an average hardness of approximately 66.5 HRC. In contrast, while Comparative Example 1 also shows a certain gradient trend, its overall hardness level is significantly lower, and in the core reinforcing region near the bonding interface, its hardness value fails to exceed 60 HRC.

[0100] This difference in hardness distribution verifies the synergistic effect of the dynamic physical field induction and the molybdenum-reinforced phase protection mechanism in this scheme. During the preparation process of Example 2, a high-speed centrifugal force field of 60g to 80g caused spherical cast tungsten carbide particles with a density of approximately 16.0 g / cm³ to overcome the viscous resistance of the melt and rapidly settle towards the tube wall during their residence in the liquid alloy. Because the tungsten carbide particles achieved a high volume fraction accumulation near the matrix interface, forming a robust anti-wear skeleton, a significant hardness peak was generated near a depth of 2.5 mm.

[0101] The data also reflect the protective effect of molybdenum on the hard phase. Comparing the data of Example 2 and Comparative Example 1, Comparative Example 1, due to the absence of molybdenum... The peak hardness of m-grade molybdenum-tungsten carbide pre-alloyed powder and metallic molybdenum powder is significantly weakened. This is because, in a high-temperature molten pool above 1100℃, if no molybdenum atoms preferentially dissolve to reach saturation, the nickel-based liquid will erode the carbon atoms on the surface of the tungsten carbide, causing edge dissolution or decomposition of the tungsten carbide. This not only reduces the actual effective load-bearing size of the hard phase particles but also leads to the precipitation of brittle composite carbides in the matrix, interfering with the linear distribution of the hardness gradient.

[0102] In Example 2, molybdenum atoms preferentially diffuse to the surface of tungsten carbide particles during the initial melting stage. The resulting molybdenum-rich solid solution layer acts as a barrier, inhibiting the dissolution and loss of tungsten carbide into the nickel-based matrix. Furthermore, due to the sub-... The extremely small particle size of the m-sized molybdenum tungsten carbide particles results in a slow settling velocity in a centrifugal field, allowing them to be partially dispersed in the upper and middle regions of the coating. This phenomenon explains why the hardness of Example 2 remained between 52 and 59 HRC in the 0.5 mm to 1.5 mm depth range, significantly higher than that of Comparative Example 1. This microstructural characteristic, combining macroscopic bottom-layer deposition with microscopic upper-middle dispersion, demonstrates that this process can control the physical sedimentation and chemical stability of the hard phase, achieving a balance between the coating's wear resistance and impact resistance.

[0103] Test Example 4: High-Temperature Abrasive Wear Performance Test From the finished pipes prepared in Example 2, Comparative Example 1 and Comparative Example 2, block samples with dimensions of 15mm x 15mm x coating thickness were cut using an electric discharge wire cutting machine, and three parallel samples were prepared for each group.

[0104] The sample was ground to a uniform height to ensure a flat test surface. Then, it was ultrasonically cleaned with alcohol and acetone in sequence to remove residual processing fluid and impurities from the surface.

[0105] Weigh the original mass of the sample using an analytical balance with an accuracy of 0.1 mg and record the data.

[0106] The sample is mounted on the fixture of the high-temperature wear testing machine, using a pin-disc contact method, and silicon carbide abrasive with high hardness (80 mesh) is selected as the abrasive material.

[0107] Turn on the heating device to raise the ambient temperature to 300℃ and keep it at that temperature for 20 minutes to ensure uniform temperature inside and outside the sample.

[0108] Experimental parameters were set as follows: normal load was 100 N, rotation speed was 200 revolutions per minute, and wear time lasted for 60 minutes.

[0109] After the test, the sample was allowed to cool naturally to room temperature before being ultrasonically cleaned and dried again.

[0110] Weigh the worn sample and calculate the average mass loss and volumetric wear rate of each group of samples.

[0111] Table 4. Abrasive wear test results for each group of samples under 300℃ operating conditions. ; Based on the data analysis in Table 4, under high-temperature abrasive wear conditions at 300℃, the wear resistance of Example 2 is significantly better than that of Comparative Example 1 and Comparative Example 2. The average weight loss of Example 2 is only 15.4 mg, while the weight loss of Comparative Example 1 is about 2.2 times that of its counterpart, and the weight loss of Comparative Example 2 is as high as about 4.5 times.

[0112] Experimental data directly demonstrate the core role of molybdenum in improving the high-temperature stability of hard phases. Comparing Example 2 with Comparative Example 1 reveals that, without the addition of molybdenum... In the case of m-grade molybdenum tungsten carbide pre-alloyed powder and metallic molybdenum powder, the high-temperature wear resistance of the coating decreases significantly.

[0113] The mechanism lies in the fact that during the high-temperature melting process, molybdenum atoms preferentially accumulate on the surface of tungsten carbide particles and form a solid solution protective layer, effectively preventing nickel atoms in the matrix from decarburizing and eroding the tungsten carbide. In Comparative Example 1, due to the lack of this protective mechanism, the tungsten carbide particles experienced severe dissolution during the remelting stage, resulting in weakening of the hard phase edges and the formation of a brittle phase between the hard phase and the matrix.

[0114] During high-temperature wear, the matrix softens due to heat, weakening its support for the hard phase. The dissolved tungsten carbide particles are more prone to breakage and detachment under the cutting and impact of the abrasive, resulting in increased weight loss.

[0115] Meanwhile, the data from Comparative Example 2 and Comparative Example 2 further reveal the contribution of interface purification to anti-wear performance. In Comparative Example 2, due to the absence of potassium fluoroborate, residual oxide film inclusions existed between the coating and the hard phase particles. This prevented the formation of a true metallurgical bond between the hard phase and the matrix, relying solely on mechanical interlocking forces. Under high-temperature conditions, these microscopic interface defects became stress concentration points. When the abrasive passed over the coating surface, the tungsten carbide particles, lacking effective wetting and bonding, underwent integral pull-out and peeling, leaving numerous wear pits. This large-area peeling wear was the main reason for the abnormally high weight loss in Comparative Example 2.

[0116] In summary, Example 2 utilizes potassium fluoroborate for interface purification and molybdenum for phase protection of tungsten carbide, ensuring extremely high interfacial bonding strength and high-temperature stability between the hard phase and the substrate. This synergistic effect enables the coating to maintain the integrity of the hard phase framework under harsh high-temperature abrasive wear environments, exhibiting excellent long service life.

[0117] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe, characterized in that: Made from the following ingredients in parts by weight: 45.0-52.0 parts of Ni-Cr-B-Si self-fluxing alloy powder; 30.0-35.0 parts of spherical cast tungsten carbide; Asia 5.0-8.0 parts of m-grade Mo-WC pre-alloyed powder; 3.0-5.0 parts of metallic molybdenum powder; Potassium fluoroborate 1.2-1.8 parts; Light calcium carbonate 0.5-1.0 parts; Lanthanum oxide 0.5-1.0 parts; Reactive composite adhesive L15.0-25.0 parts.

2. The Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of the steel pipe according to claim 1, characterized in that: The chemical composition of the Ni-Cr-B-Si self-fluxing alloy powder, by mass percentage, is: Cr 12.0%-18.0%, B 2.0%-3.5%, Si 3.0%-4.5%, C 0.4%-0.8%, Fe < 5.0%, with Ni as the balance; the physical properties are a particle size of 45-105 μm. m, spherical in shape; the spherical cast tungsten carbide has a carbon content of 3.8%-4.1% and an average particle size of 100-150 μm. m, microhardness 2500-3000HV.

3. The Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of the steel pipe according to claim 1, characterized in that: The sub m-grade Mo-WC pre-alloyed powder was prepared by the following method: 10.0 parts by weight of metallic molybdenum powder and 90.0 parts by weight of tungsten carbide powder were mixed and ball-milled for 12 hours, dried, and then placed under a vacuum of 1.0 × 10⁻⁶. -2 In a vacuum induction furnace with a pressure below Pa, the sintered blocks are held at 1450℃ for 2 hours, and then pulverized by airflow to a particle size of 0.8-5.0 mm. m.

4. The Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of the steel pipe according to claim 1, characterized in that: The reactive composite adhesive L is made from raw materials comprising the following parts by weight: 6.0-8.0 parts of PVA, 90.0-94.0 parts of deionized water, and a solution formed by the above two mixed with 35.0-40.0 parts of industrial-grade sodium silicate; wherein the degree of alcoholysis of the polyvinyl alcohol is 87%-89% (mol / mol), and the modulus of the industrial-grade sodium silicate is 2.8-3.

2.

5. The Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of the steel pipe according to claim 1, characterized in that: The specific surface area of ​​the light calcium carbonate is 10-15 m². 2 / g, average particle size is 2-5 m, bulk density is 0.4-0.7 g / cm³ 3 The potassium fluoroborate has a purity greater than 98% and a density of 2.50 g / cm³. 3 .

6. A method for preparing a Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe, as described in any one of claims 1-5, characterized in that: Includes the following steps: (1) Mix Ni-Cr-B-Si powder, spherical cast tungsten carbide, and sub-Si powder in proportion. A mixed powder for the reinforcing layer was prepared by using m-grade Mo-WC powder, metallic molybdenum powder, potassium fluoroborate, light calcium carbonate, and lanthanum oxide. (2) Mix the reinforcing layer powder with the reactive composite adhesive L to prepare a reinforcing layer slurry, and separately mix the underlayer powder with the adhesive L to prepare an underlayer slurry; (3) First apply the base coat slurry to the inner wall of the steel pipe, and after the surface is semi-dry, apply the reinforcing layer slurry. (4) The pre-coated steel pipe is subjected to a step heat treatment, which involves the pre-drying stage, the solid phase degassing stage and the liquid phase remelting stage in sequence. (5) Maintain centrifugal acceleration for atomized water spray cooling.

7. The method for preparing the Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe according to claim 6, characterized in that: In step (3), the coating thickness of the base layer slurry is 0.5-0.8 mm, and the coating thickness of the reinforcing layer slurry is 1.5-2.5 mm.

8. The method for preparing the Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe according to claim 6, characterized in that: In step (4), the technical parameters of the solid phase degassing stage are: centrifugal acceleration of 10-20g, heating to 500-650℃ at a rate of 10-15℃ / min, and holding for 10-15min.

9. The method for preparing the Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe according to claim 6, characterized in that: In step (4), the technical parameters of the liquid phase remelting stage are: centrifugal acceleration is increased to 60-80g, temperature is increased to 1050-1150℃ at a rate of 30-50℃ / min, induction frequency is set to 4-8kHz, and heat is maintained for 120-180s.

10. The method for preparing the Mo-WC reinforced nickel-based alloy gradient coating on the inner wall of a steel pipe according to claim 6, characterized in that: In step (4), the pre-drying stage is maintained at 120-150℃ for 90-150 minutes, and the steel pipe is rotated at 8-10 rpm. In step (5), centrifugation is stopped when the pipe temperature drops below 500-550℃.

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