Heat insulation coating of drill rod and preparation method of heat insulation coating

Abstract

The invention belongs to the technical field of materials, and particularly relates to a heat insulation coating of a drill rod and a preparation method of the heat insulation coating. 13 to 17 parts of hollow mullite microspheres; 10 to 14 parts of magnesium titanate-silicon carbide composite powder; 6 to 10 parts of silane modified aerogel powder; 6 to 8 parts of mullite whisker; 7 to 9 parts of NiCrAlY alloy powder; the zirconium oxide-mullite composite powder and the hollow mullite microspheres form a high-temperature stable porous matrix, crystalline phase transformation is avoided, the porosity of 30%-35% is maintained, and solid heat conduction is blocked; magnesium titanate-silicon carbide composite powder synergistically scatters and absorbs infrared radiation and inhibits the high-temperature thermal radiation heat transfer proportion, in the preparation method, plasma spraying parameters are accurately controlled, coating crystal phase transformation and aerogel sintering are avoided, and it is guaranteed that the coating is stable in high-temperature structure; the porosity and the infrared shading performance of the coating are not influenced by low-temperature annealing post-treatment, so that the heat conductivity coefficient of the heat insulation coating in the interval of 500-1000 DEG C is stabilized at 0.3-0.5 W / (m.K).

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of materials, in particular to a heat insulation coating of a drill pipe and a preparation method thereof. BACKGROUND

[0002] The heat insulation drill pipe is an important equipment for developing deep oil and gas resources and geothermal resources. In the past hundred years of oil and gas resource production, the reserves of shallow oil and gas resources have been consumed, and the drilling depth of oil and gas has been deepened. The temperature of the earth's stratum increases with the increase of the depth. At present, large-scale development of 175-200℃ oil and gas reservoirs has been started internationally, and the development of ultra-deep oil and gas reservoirs is facing the problem of high temperature in the well. For geothermal resources, the stratum temperature often reaches more than 200℃, and foreign countries have already studied the technology of developing 400℃ rock stratum geothermal resources.

[0003] The development of high-temperature oil and gas and geothermal resources first needs to establish a path for the flow of matter and information from the ground to the target stratum underground, which is usually realized by drilling. The control ability of conventional drilling tools for the trajectory of the drilling wellbore is poor, and advanced directional drilling tools are difficult to withstand a high-temperature environment of more than 175℃. High-temperature drilling practice shows that the heat insulation of the drill pipe is an important method and key means to reduce the heat transfer between the inner and outer fluids of the drill pipe, one of the implementation methods of which is to attach a heat insulation layer to the inner wall of the drill pipe. However, the heat insulation coating of the drill pipe still has the following technical problems: The heat insulation performance of the existing heat insulation coating is sensitive to temperature. When the well temperature exceeds 500℃ (common temperature in deep wells and geothermal wells), the thermal conductivity of the coating will increase significantly: firstly, the proportion of heat transfer by thermal radiation in the coating increases greatly, and secondly, the crystal phase of part of the coating changes, resulting in a decrease in porosity and a shortening of the heat transfer path. Ultimately, the heat insulation effect of the heat insulation coating is greatly reduced, and the electronic components and drilling fluid inside the drill pipe cannot be effectively protected. During the deep well drilling process, the drill pipe will experience a large temperature difference cycle from the high-temperature zone of the stratum (hundreds of degrees) to the low-temperature zone of the wellhead (normal temperature). The existing heat insulation coating has poor heat insulation performance adjustment ability and cannot maintain stable heat insulation effect in different temperature ranges. For example, the aerogel composite coating which performs well in the low-temperature zone will lose its heat insulation performance in the high-temperature zone due to the sintering shrinkage of the aerogel; and the ceramic coating suitable for the high-temperature zone will easily crack due to its brittleness in the low-temperature zone.

[0004] Therefore, the present application provides a heat insulation coating of a drill pipe and a preparation method thereof to solve the above-mentioned problems. SUMMARY

[0005] The purpose of the present application is to provide a heat insulation coating of a drill pipe and a preparation method thereof to solve the problems mentioned in the background.

[0006] To achieve the above object, the present application provides the following technical solutions: a heat insulation coating of a drill pipe, comprising 35-45 parts of zirconia-mullite composite powder; 13-17 parts of hollow mullite microspheres; 10-14 parts of magnesium titanate-silicon carbide composite powder; 6-10 parts of silane modified aerogel powder; 6-8 parts of mullite whiskers; and 7-9 parts of NiCrAlY alloy powder. The zirconia-mullite composite powder, the hollow mullite microspheres, the magnesium titanate-silicon carbide composite powder, the silane modified aerogel powder, the mullite whiskers and the NiCrAlY alloy powder are mixed according to the above proportions.

[0007] Preferably, the mass ratio of zirconia to mullite in the zirconia-mullite composite powder is 7:3.

[0008] Preferably, the particle size of the hollow mullite microspheres is 50-100 um.

[0009] Preferably, the mass ratio of magnesium titanate to silicon carbide in the magnesium titanate-silicon carbide composite powder is 8:2.

[0010] Preferably, the silane modified aerogel powder is selected from methyltrimethoxysilane. Preferably, the surface of the methyltrimethoxysilane is grafted.

[0011] Preferably, the diameter of the mullite whiskers is 1-2 um, and the length is 20-50 um.

[0012] Preferably, the particle size of the NiCrAlY alloy powder is 10-15 um.

[0013] A preparation method of a heat insulation coating of a drill pipe, and the specific steps of the preparation method are as follows: Step one, raw material pretreatment: the composite powder is pre-mixed, and the raw materials are dried and sieved; Step two, drill pipe substrate pretreatment: the surface of the drill pipe is degreased and derusted, sandblasting roughening treatment is performed, and the drill pipe is preheated; Step three, preparation of the heat insulation coating by plasma spraying: spraying is performed by using atmospheric plasma spraying equipment, the spraying parameters are accurately controlled to ensure that the coating has no crystal phase transition at high temperature, maintains stable porosity, and realizes the functions of infrared light shielding, toughening and thermal shock resistance; Step four, low-temperature annealing post-treatment: the sprayed drill pipe is placed in a vacuum annealing furnace for low-temperature annealing treatment.

[0014] Preferably, the specific steps of step one are as follows: all the raw materials are weighed according to the proportions, put into a horizontal ball mill for dry mixing, alumina balls are selected as the ball milling medium, the ball milling speed is controlled at 200-250 r / min, the ball milling time is 2-3 h, the mixed raw materials are put into a vacuum drying box and dried at 80-100℃ for 1-2 h to remove the adsorbed water in the raw materials, and after drying, the materials are sieved through a 100-mesh standard sieve to remove agglomerated particles, so that the sprayed powder with good fluidity is obtained. The specific content of the second step is as follows: the drill pipe is placed into an ultrasonic cleaning machine, cleaned with acetone or anhydrous ethanol for 10-15 min to remove surface oil stains; then soaked in dilute hydrochloric acid for 5-8 min to remove surface oxide skin and rust, and finally cleaned with clean water and dried for standby; the surface of the drill pipe is treated by sand blasting with brown corundum sand, the sand blasting pressure is 0.4-0.6 MPa, the sand blasting distance is 100-150 mm, the sand blasting angle is 45° to 60°, and the surface roughness of the treated drill pipe reaches Ra 5-8 um; the sand-blasted drill pipe is placed in a resistance furnace and preheated at 150-200℃ for 30-40 min with dry nitrogen protection.

[0015] Preferably, the specific content of the third step is as follows: the pre-processed spraying powder is added to the powder feeder, the plasma spraying equipment is started, and the drill pipe surface is sprayed according to the above parameters; the drill pipe is kept rotating at a uniform speed during the spraying process to ensure uniform coating thickness; In the third step, the parameters of the spraying equipment are set as follows: arc power: 30-35 kW; plasma gas: argon and hydrogen; powder feeding rate: 20-25 g / min; spraying distance: 120-150 mm; gun moving speed: 300-400 mm / s; coating thickness: 300-500 um; In the fourth step, the specific parameters in the annealing process are as follows: annealing temperature: 200-250℃; holding time: 2-3h; cooling rate: 5-10℃ / min; protective atmosphere: dry nitrogen.

[0016] Compared with the prior art, the beneficial effects of the present application are: 1) The present application forms a high-temperature stable porous matrix by zirconia-mullite composite powder and hollow mullite microspheres, there is no crystal phase transition, the porosity is maintained at 30%-35%, and the solid-state heat conduction is blocked; the magnesium titanate-silicon carbide composite powder cooperates to scatter and absorb infrared radiation, and the proportion of high-temperature thermal radiation heat transfer is inhibited; in the preparation method, the plasma spraying parameters are precisely controlled to avoid crystal phase transition and aerogel sintering of the coating, and the high-temperature structural stability of the coating is ensured; the low-temperature annealing after treatment does not affect the porosity and infrared light shielding performance of the coating, so that the thermal conductivity of the heat insulation coating in the range of 500-1000℃ is stable at 0.3-0.5 W / (m·K); 2) The present application uses silane modified SiO2 aerogel powder to consider low thermal conductivity in low temperature area and structural stability in high temperature area; mullite whiskers improve the fracture toughness of the coating; NiCrAlY alloy powder matches the thermal expansion coefficient of the drill pipe matrix to relieve thermal stress during temperature difference cycles; the matrix is roughened by sand blasting and preheated to improve the adhesion of the coating; low-temperature annealing after treatment eliminates residual stress to avoid low-temperature brittle cracking of the coating, so that the coating has no cracking and peeling phenomenon under a large temperature difference cycle from room temperature to 1000℃, and the heat insulation performance is stable. BRIEF DESCRIPTION OF DRAWINGS

[0017] Fig. 1 A raw material composition diagram for a thermal insulation coating layer; Fig. 2 A preparation method flow chart for a thermal insulation coating layer. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0019] Embodiment: Please refer to Figs. 1-2 The present application provides a technical solution: A thermal insulation coating layer of a drill pipe, comprising 35-45 parts of zirconia-mullite composite powder; 13-17 parts of hollow mullite microspheres; 10-14 parts of magnesium titanate-silicon carbide composite powder; 6-10 parts of silane-modified aerogel powder; 6-8 parts of mullite whiskers; and 7-9 parts of NiCrAlY alloy powder.

[0020] In the zirconia-mullite composite powder, the mass ratio of zirconia to mullite is 7:3.

[0021] The particle size of the hollow mullite microspheres is 50-100 um.

[0022] In the magnesium titanate-silicon carbide composite powder, the mass ratio of magnesium titanate to silicon carbide is 8:2.

[0023] The silane-modified The surface of the aerogel powder is grafted with methyltrimethoxysilane.

[0024] The diameter of the mullite whiskers is 1-2 um, and the length is 20-50 um.

[0025] The particle size of the NiCrAlY alloy powder is 10-15 um.

[0026] A preparation method of a thermal insulation coating layer of a drill pipe, the specific steps of the preparation method being as follows: Step one, raw material pretreatment: the composite powder is pre-mixed, and the raw materials are dried and sieved; Step two, drill pipe substrate pretreatment: the surface of the drill body is degreased and derusted, sandblasting roughening treatment is performed, and the drill pipe is preheated; Step three, plasma spraying preparation of thermal barrier coating: using atmospheric plasma spraying equipment for spraying, by precise control of spraying parameters, ensure that the coating at high temperature no crystal phase transition, maintain stable porosity, while realizing the function of infrared light shielding, toughening and thermal shock resistance; Step four, low temperature annealing after treatment: the sprayed drill rod is put into a vacuum annealing furnace for low temperature annealing treatment.

[0027] The specific steps of step one are as follows: all raw materials are weighed according to the proportion, put into a horizontal ball mill for dry mixing, alumina balls (diameter 5-10mm, ball to material ratio 2:1) are selected as the ball milling medium, the ball milling speed is controlled at 200-250r / min, the ball milling time is 2-3h, the mixed raw materials are put into a vacuum drying box and dried at 80-100℃ for 1-2h to remove the adsorbed water in the raw materials, and then sieved through a 100 mesh standard sieve to remove agglomerated particles, obtaining a well-flowing sprayed powder; The dry mixing of the horizontal ball mill with alumina ball medium can fully disperse and mix the six components of zirconia-mullite composite powder, hollow mullite microspheres, magnesium titanate-silicon carbide composite powder, etc., avoiding local enrichment or lack of components. This uniformity is the basis for the subsequent synergistic functions of the coating, such as porous thermal insulation, infrared light shielding, toughening and thermal shock resistance. For example, the uniform distribution of magnesium titanate-silicon carbide composite powder can ensure that the infrared radiation is fully scattered / absorbed at high temperatures, and the uniform arrangement of hollow mullite microspheres can form a continuous porous thermal insulation structure, avoiding the local thermal conductivity coefficient from rising due to uneven composition, ensuring uniform composition of the sprayed powder and realizing the synergistic function of multiple components; Vacuum drying is used to remove the adsorbed water in the raw materials to avoid the generation of bubbles, pinholes and other coating defects due to water evaporation during spraying; sieving through a 100 mesh sieve removes agglomerated particles to ensure good powder flowability. If the powder is agglomerated or contains moisture, it will cause poor powder feeding, uneven coating thickness, and even unmelted particles, which will destroy the continuity of the thermal insulation structure, affect the high-temperature thermal insulation stability, and improve the powder flowability to meet the requirements of plasma spraying process; The specific content of step two is as follows: the drill rod is put into an ultrasonic cleaning machine and cleaned with acetone or anhydrous ethanol for 10-15min to remove surface oil stains; then it is soaked in dilute hydrochloric acid (mass fraction 5%-10%) for 5-8min to remove surface oxide scale and rust, and finally it is rinsed with clean water and dried for use; brown corundum sand (particle size 0.5-1.0mm) is used for sandblasting treatment of the drill rod surface, the sandblasting pressure is 0.4-0.6MPa, the sandblasting distance is 100-150mm, and the sandblasting angle is 45° to 60°, the surface roughness of the treated drill rod reaches Ra 5-8um, and the sandblasted drill rod is put into a resistance furnace and preheated at 150-200℃ for 30-40min with dry nitrogen protection.

[0028] Ultrasonic cleaning and dilute hydrochloric acid soaking can completely remove oil, scale and rust on the surface of the drill pipe, and eliminate the impurity isolation layer. If there are contaminants on the surface of the substrate, it will cause the coating and the substrate to form a weakly bonded interface, and the thermal stress will cause peeling in the deep well temperature difference cycle. The purified surface can make the sprayed particles and the substrate form a more intimate physical / chemical bond, purify the substrate surface, improve the coating adhesion, and avoid peeling at high temperature.

[0029] Brown corundum sand blasting makes the surface of the drill pipe form a rough interface with concave and convex surfaces. During spraying, the molten powder particles will be embedded in the concave pits, forming a mechanical anchoring force similar to anchoring, which significantly improves the bonding strength of the coating and the substrate. This structure can effectively alleviate the thermal stress generated by temperature difference cycle, avoid the problem of existing coating cracking at low temperature and peeling at high temperature due to insufficient bonding strength, and construct mechanical interlocking force by sand blasting and roughening to strengthen the thermal shock resistance. 150-200℃ preheating and nitrogen protection can reduce the temperature difference between the drill pipe substrate and the sprayed powder, avoid the rapid cooling of the molten particles when they come into contact with the cold substrate, and cause a huge thermal stress, which leads to the instantaneous cracking of the coating. At the same time, nitrogen protection can prevent the substrate surface from being re-oxidized during preheating, maintain surface activity, and further improve the coating adhesion. The preheated substrate can also prolong the wetting time of the sprayed particles on the surface, promote the densification of the coating, reduce the thermal shock of the preheating treatment, and protect the initial structure of the coating.

[0030] The specific content of step three is as follows: the pretreated sprayed powder is added to the powder feeder, the plasma spraying equipment is started, and the drill pipe surface is sprayed according to the above parameters; during the spraying process, the drill pipe is kept rotating at a uniform speed (rotation speed 10-15 r / min) to ensure the uniform thickness of the coating. Optimized spraying parameters can achieve moderate flattening but not excessive densification of molten particles: on the one hand, zirconia-mullite composite powder and hollow mullite microspheres maintain their original crystal phase at high temperature, forming a stable porous structure that blocks solid-state heat conduction; on the other hand, it avoids the sintering and shrinkage of silane modified aerogel powder due to high spraying temperature, ensuring that it still plays a low thermal conductivity advantage at low temperature, accurately controls the coating structure, and solves the problem of attenuation of thermal insulation performance at high temperature.

[0031] The high-temperature environment of plasma spraying makes part of the NiCrAlY alloy powder melt and form a metallurgical bond with the drill pipe substrate, while also being uniformly dispersed in the coating, matching the thermal expansion coefficient of the substrate and relieving the thermal stress of temperature difference cycle; mullite whiskers are arranged directionally during spraying, forming a fiber reinforced network to improve the fracture toughness of the coating; magnesium titanate-silicon carbide composite powder is uniformly distributed after melting, and cooperates to play the roles of infrared scattering (magnesium titanate) and absorption (silicon carbide), inhibiting the proportion of thermal radiation heat at high temperature, achieving multifunctional synergy, and giving consideration to high-temperature thermal insulation and thermal shock resistance.

[0032] The drill pipe is rotated at a uniform speed and the spray gun is moved at a uniform speed during spraying, so that the coating thickness can be controlled within 300-500 um and the uniformity error is ≤±5%. The uniform coating thickness can avoid heat penetration caused by local thin spots, ensure consistent overall heat insulation effect of the drill pipe, effectively protect internal electronic components and drilling fluid, solve the problem of local heat insulation failure caused by uneven thickness of the existing coating, ensure uniform coating thickness, and improve overall heat insulation reliability. In step three, the parameters of the spraying equipment are set as follows: arc power: 30-35 kW; plasma gas: argon (flow rate 40-45 L / min) and hydrogen (flow rate 5-8 L / min); powder feeding rate: 20-25 g / min; spraying distance: 120-150 mm; spray gun moving speed: 300-400 mm / s; coating thickness: 300-500 um (which can be adjusted according to actual heat insulation requirements). In step four, the specific parameters during the annealing process are as follows: annealing temperature: 200-250℃; holding time: 2-3h; cooling rate: 5-10℃ / min (cooling to room temperature in the furnace); protective atmosphere: dry nitrogen (purity ≥ 99.99%).

[0033] During plasma spraying, rapid cooling of the coating can generate internal residual stress (tensile stress), which will gradually accumulate in deep well high temperature-ambient temperature repeated cycles, leading to coating cracking. Low-temperature annealing at 200-250℃ can release residual stress gently, without causing phase transition or porosity reduction of the coating (avoiding affecting high-temperature heat insulation performance); a slow cooling rate of 5-10℃ / min further reduces thermal stress, ensuring that the coating does not crack or peel off under temperature cycling from room temperature to 1000℃, eliminating residual stress from spraying and avoiding cracking under temperature cycling.

[0034] The annealing environment of vacuum and dry nitrogen protection can prevent the coating and substrate from oxidizing during annealing, avoiding the formation of an oxide layer that affects adhesion; low-temperature annealing can also promote slight interfacial diffusion within the coating, further strengthening the bonding strength between the coating and the substrate, while making the internal component distribution of the coating more uniform, reducing micro defects, prolonging the service life of the coating under deep well high temperature and vibration conditions, and improving the structural stability and service life of the coating.

[0035] Low-temperature annealing does not cause sintering of silane-modified aerogel powder or phase transition of zirconia-mullite composite powder, ensuring that the coating still maintains a low thermal conductivity in the low-temperature zone (room temperature) and maintains stable porosity and infrared light shielding performance in the high-temperature zone (1000℃), solving the pain points of existing coatings, such as brittle cracking in the low-temperature zone and heat insulation failure in the high-temperature zone, achieving stable heat insulation performance in the full temperature range, balancing high-temperature performance and low-temperature stability, and achieving adaptation in the full temperature range.

[0036] The foregoing illustrates and describes embodiments of the present application. However, it is to be understood that the application is not limited to the above-described embodiments, and that various changes and modifications can be suggested to one skilled in the art. It is intended that the present application encompass any and all such changes and modifications without departing from the spirit or scope of the underlying principles of the application. In addition, it should be understood that the figures are not necessarily drawn to scale and that, where appropriate, reference numerals have been maintained from one figure to another and from one embodiment to another in order to illustrate similar constructions.

[0037] While the application has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, since the scope of the application is indicated by the appended claims rather than by the foregoing description, and all changes and modifications that come within the meaning of the claims are desired to be embraced therein.

Claims

1. A thermal barrier coating for a drill pipe, characterized by, The composite powder of zirconia-mullite 35-45 parts; hollow mullite microspheres 13-17 parts; magnesium titanate-silicon carbide composite powder 10-14 parts; silane modified Aerogel powder 6-10 parts; mullite whisker 6-8 parts; NiCrAlY alloy powder 7-9 parts.

2. A thermal barrier coating for a drill pipe as defined in claim 1, wherein: The mass ratio of zirconia to mullite in the zirconia-mullite composite powder is 7:

3.

3. A thermal barrier coating for a drill pipe as defined in claim 1, wherein: The particle size of the hollow mullite microspheres is 50-100 um.

4. A thermal barrier coating for a drill pipe as defined in claim 1, wherein: The mass ratio of magnesium titanate to silicon carbide in the magnesium titanate-silicon carbide composite powder is 8:

2.

5. A thermal barrier coating for a drill pipe as defined in claim 1, wherein: The silane modification The aerogel powder is selected to have surface grafting of methyltrimethoxysilane.

6. A thermal barrier coating for a drill pipe as defined in claim 1, wherein: The diameter of the mullite whisker is 1-2 um, and the length is 20-50 um.

7. A thermal barrier coating for a drill pipe as defined in claim 1 wherein: The particle size of the NiCrAlY alloy powder is 10-15 um.

8. A method of producing a thermal barrier coating for a drill pipe according to any one of claims 1-7, characterized by: The specific steps of the preparation method are as follows: Step one, raw material pretreatment: the composite powder is premixed, and the raw materials are dried and sieved; Step two, drill body pretreatment: the surface of the drill body is degreased and derusted, sandblasting roughening treatment is performed, and the drill rod is preheated; Step three, plasma spraying to prepare a thermal insulation coating: the spraying is performed by using an atmospheric plasma spraying device, the spraying parameters are accurately controlled to ensure that the coating has no crystal phase transition at high temperature, maintains stable porosity, and realizes the functions of infrared light shielding, toughening and thermal shock resistance; Step four, low-temperature annealing post-treatment: the sprayed drill rod is placed in a vacuum annealing furnace for low-temperature annealing treatment.

9. A thermal barrier coating for a drill pipe and a method of making the same according to claim 8, characterized in that: The specific steps of step one are as follows: all raw materials are weighed according to the proportion, put into a horizontal ball mill for dry mixing, alumina balls are selected as the ball milling medium, the ball milling speed is controlled at 200-250 r / min, the ball milling time is 2-3 h, the mixed raw materials are put into a vacuum drying box and dried at 80-100℃ for 1-2 h to remove the adsorbed water in the raw materials, and then sieved through a 100-mesh standard sieve to remove agglomerated particles, thereby obtaining a spraying powder with good fluidity; The specific content of step two is as follows: the drill rod is placed in an ultrasonic cleaner and cleaned with acetone or anhydrous ethanol for 10-15 min to remove surface oil stains; then it is soaked in dilute hydrochloric acid for 5-8 min to remove surface oxide skin and rust, and finally it is rinsed with clean water and dried for standby, the surface of the drill rod is sandblasted with brown corundum sand, the sandblasting pressure is 0.4-0.6 MPa, the sandblasting distance is 100-150 mm, the sandblasting angle is 45° to 60°, and the surface roughness of the treated drill rod reaches Ra 5-8 um, the sandblasted drill rod is placed in an electric resistance furnace and preheated at 150-200℃ for 30-40 min while dry nitrogen is introduced for protection.

10. A thermal barrier coating for a drill pipe and method of making the same according to claim 8, wherein: The specific content of step three is as follows: the pretreated spraying powder is added to the powder feeder, the plasma spraying device is started, and the drill rod surface is sprayed according to the above parameters, and the drill rod is rotated at a uniform speed during the spraying process to ensure uniform coating thickness; In step three, the spraying device parameters are set as follows: arc power: 30-35 kW; plasma gas: argon and hydrogen; powder feeding rate: 20-25 g / min; spraying distance: 120-150 mm; gun moving speed: 300-400 mm / s; coating thickness: 300-500 um; In step four, the specific parameters during the annealing treatment are as follows: annealing temperature: 200-250℃; holding time: 2-3 h; cooling rate: 5-10℃ / min; protective atmosphere: dry nitrogen.

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