Welding joint residual stress and welding seam corrosion resistance optimization method

Abstract

The invention provides a welding joint residual stress and weld joint corrosion resistance optimization method. An iron-based alloy, a cobalt-based alloy, a nickel-based alloy and WC reinforced high-entropy alloy powder with high crack resistance, wear resistance and corrosion resistance are used for carrying out laser cladding on a duplex stainless steel welding joint; the cladding layer is utilized to optimize the residual stress distribution of the duplex stainless steel welded joint, and the cladding layer can generate a certain degree of heat treatment effect on duplex stainless steel during cladding, so that part of residual stress of the duplex stainless steel joint is eliminated, the proportion of ferrite and austenite phases in the duplex stainless steel welded joint is improved, and the welding quality of the duplex stainless steel joint is improved. And part of ferrite is converted into austenite, the austenite content in the joint is increased, and then the corrosion resistance of the duplex stainless steel joint is improved. And meanwhile, the WC reinforced high-entropy alloy cladding layer serving as the cladding layer has good corrosion resistance, part of the welded joint is isolated from the external environment, and the corrosion resistance of the duplex stainless steel welded joint is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optimization of residual stress of welding joints and corrosion resistance of welds, and specifically relates to a method for optimizing residual stress of welding joints and corrosion resistance of welds. Background Art

[0002] Duplex stainless steel has good corrosion resistance. Under harsh media environments (such as seawater with high chloride ion content), duplex stainless steel has good resistance to pitting, crevice corrosion, stress corrosion and corrosion fatigue, and is suitable for use in harsh environments. Due to the influence of welding heat input, duplex stainless steel welded joints produce large residual stresses, change the ratio of ferrite to austenite phases in duplex stainless steel joints, and produce various welding defects during welding. The corrosion resistance of duplex stainless steel joints will be lower than that of the parent material, resulting in a decrease in the overall corrosion resistance of the components and a decrease in the performance of the material. Improving the residual stress and corrosion resistance of duplex stainless steel joints is extremely important for improving the performance and service life of duplex stainless steel and reducing material consumption.

[0003] The residual stress regulation of welded joints is mainly achieved by changing the welding process method and the design of tooling and fixtures, and the optimization of the corrosion resistance of welded joints is mainly achieved by regulating the elements and phase ratios in the joints. For duplex stainless steel, the main methods for regulating the corrosion resistance of duplex stainless steel joints are adding alloy elements to welding materials, adjusting welding process parameters to adjust heat input, and changing the ratio of ferrite and austenite in the joints to improve the corrosion resistance of the joints. However, these methods are difficult to achieve precise control of the phase composition of the joints, and there are certain difficulties in improving the corrosion resistance.

[0004] By using laser cladding and taking certain process measures, the purpose of improving the corrosion resistance of duplex stainless steel joints and optimizing the residual stress distribution of joints can be achieved at the same time. The commonly used materials for laser cladding are mainly based on rare earth elements, but rare earth elements are relatively expensive, and duplex stainless steel joints are required to have high corrosion resistance. A large part of the alloy powder cannot meet people's requirements for corrosion resistance. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] The present invention proposes a method for optimizing residual stress of a welded joint and corrosion resistance of a weld, so as to solve the technical problem of how to improve the corrosion resistance of a duplex stainless steel joint and optimize the residual stress distribution of the joint.

[0007] (II) Technical solution

[0008] In order to solve the above technical problems, the present invention proposes a method for optimizing the residual stress and corrosion resistance of a weld joint. The method for optimizing the residual stress and corrosion resistance of a weld joint uses laser cladding powder to prepare a laser cladding layer at a duplex stainless steel weld joint; wherein the laser cladding powder is an iron-based alloy, a nickel-based alloy, a cobalt-based alloy or a WC-enhanced high-entropy alloy.

[0009] Furthermore, the iron-based alloy is made of iron-chromium-silicon alloy powder, and the composition is: Cr>30%, Si>35%, and the balance is Fe.

[0010] Furthermore, the nickel-based alloy powder is F102, and the composition is: C 0.60%-1.0%, Cr 14.0%-18.0%, Si 3.5%-5.5%, B 3.0%-4.5%, Fe≤5%, and the balance is Ni.

[0011] Furthermore, the cobalt-based alloy is Co150, and the composition is: C 0.12%, Cr 28.00%, Si 1.00%, Fe21.00%, Mn≤1.00%, and the balance is Co.

[0012] Furthermore, the composition of the WC enhanced high entropy alloy is: Fe 39%, Co 7.5%, Cr 6.5%, Ni 7%, Si 5%, B 3%, WC 29.5%, Y2O3 2.5%.

[0013] Furthermore, the weld excess height and weld toe are ground before laser cladding.

[0014] Furthermore, before laser cladding, laser cladding powder is pre-placed on the treated surface of the duplex stainless steel joint; wherein, for a thin plate or medium and thick plate duplex stainless steel joint with a thickness of greater than or equal to 0.2 mm and less than 20 mm, the front cladding width of the weld is smaller than the back cladding width; for a thick plate duplex stainless steel joint with a thickness of greater than or equal to 20 mm, a double-sided V-groove is opened on the joint, and the cladding width on the side with a larger residual stress is smaller than the cladding width on the other side.

[0015] Furthermore, a ruby ​​laser or a CO2 laser is used for multi-pass laser cladding.

[0016] Furthermore, for thick plates, the side with smaller residual stress is clad first; the process parameter range of laser cladding is: laser power 1.5~2KW, scanning speed 150~300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0017] Furthermore, rosin alcohol solution or glass water is used as a binder in the pre-set powder to increase the hardness and adhesion strength of the coating.

[0018] (III) Beneficial effects

[0019] The present invention proposes a method for optimizing residual stress and corrosion resistance of welded joints, using iron-based alloy, cobalt-based alloy, nickel-based alloy and WC-enhanced high entropy alloy powder with high crack resistance, wear resistance and corrosion resistance to laser clad duplex stainless steel welded joints; the cladding layer is used to optimize the residual stress distribution of duplex stainless steel welded joints, and the cladding layer can produce a certain degree of heat treatment effect on duplex stainless steel during cladding, eliminate part of the residual stress of the duplex stainless steel joint, and improve the ratio of ferrite and austenite in the duplex stainless steel welded joint, so that part of the ferrite is transformed into austenite, and the austenite content in the joint is increased, thereby improving the corrosion resistance of the duplex stainless steel joint. At the same time, the WC-enhanced high entropy alloy cladding layer as a cladding layer itself has good corrosion resistance, isolates part of the welded joint from the external environment, and improves the corrosion resistance of the duplex stainless steel welded joint.

[0020] The beneficial effects of the present invention specifically include:

[0021] 1. Iron-based alloy, nickel-based alloy, cobalt-based alloy and WC-reinforced high-entropy alloy are used as alloy powders for laser cladding. The powder is used to laser clad the duplex stainless steel weld joint, and a cladding layer with high hardness and corrosion resistance can be obtained, thereby improving the corrosion resistance of the duplex stainless steel weld joint; and the cladding layer obtained by cladding has a high thermal expansion coefficient, which is higher than the thermal expansion coefficient of the base material duplex stainless steel. By properly adjusting the area, thickness and width of the cladding layer, the purpose of regulating the residual stress of the duplex stainless steel weld joint can be achieved, and the performance of the duplex stainless steel weld joint can be improved.

[0022] 2. The laser cladding powders used are iron-based alloys, nickel-based alloys, cobalt-based alloys and WC-enhanced high-entropy alloys. Iron-based alloys, nickel-based alloys and cobalt-based alloys are high-temperature alloys. The cladding layer obtained by laser cladding has good corrosion resistance and hardness and other properties. At the same time, the thermal expansion coefficient is higher than that of the parent material, which can play the role of regulating welding residual stress. WC-enhanced high-entropy alloy is made of cobalt, iron, chromium, nickel, silicon, boron powder, WC and Y2O3 powders with a purity higher than 99.7%. The elements and structure contained in the obtained cladding layer make it have high corrosion resistance. At the same time, the thermal expansion coefficient is also higher than that of the parent material, which can play the role of regulating welding residual stress.

[0023] 3. The present invention utilizes a laser cladding method to have a certain heat treatment effect on the joint, and the cladding layer can isolate the weld joint from the external environment, so that the weld joint is protected; at the same time, by reasonably arranging the cladding area of ​​the cladding layer and the width and thickness of the cladding layer, the width and thickness of the laser cladding layer on the front and back sides of the duplex stainless steel weld are made inconsistent, and the tensile stress generated when the cladding layer shrinks is utilized to reasonably regulate the residual stress of the weld joint; the effect of simultaneously regulating the residual stress of the duplex stainless steel weld joint and optimizing the corrosion resistance is achieved.

[0024] 4. The present invention utilizes iron-based alloys, nickel-based alloys, cobalt-based alloys and WC-reinforced high-entropy alloys as powders for laser cladding. The laser cladding layer obtained from this part of the powder itself can form a good metallurgical bond with the duplex stainless steel base material. At the same time, the cladding layer also has good mechanical properties and corrosion resistance, which can achieve the effect of optimizing the corrosion resistance.

[0025] 5. For welding joints with different plate thicknesses, the difference in residual stress on the front and back of the joint is different. The residual stress of the thin plate itself is small after welding, and the difference in residual stress on the front and back of the joint after welding is not much. The difference in thickness of the front and back cladding layers does not need to be too large to play a role in regulating residual stress; the residual stress of medium and thick plates is slightly larger, and the cladding layer should be used to regulate the residual stress of the joint. The difference in thickness and width of the cladding layer on the front and back of the joint must be greater than the difference in parameters of the thin plate cladding layer; for thick plates, the thick plates themselves have greater rigidity, and the residual stress after welding is also greater, and the difference in residual stress on the front and back is also greater. It is difficult to directly regulate the residual stress by adjusting the thickness of the cladding layer. When welding thick plates, double-sided V-shaped grooves can be opened to reduce the difference in residual stress on both sides of the weld, and then the residual stress can be regulated by cladding the cladding layer.

[0026] 5. Through the above method, the present invention can obtain a cladding layer that is well bonded to the substrate. The cladding layer itself has high corrosion resistance, and the joint part whose corrosion resistance is changed due to welding heat input is separated from the external environment, so that the corrosion resistance of the duplex stainless steel welded joint is greatly improved; in addition, the thermal expansion coefficient of the cladding layer obtained by the present invention is higher than that of the duplex stainless steel base material itself, and the position and thickness of the cladding layer can be appropriately adjusted to achieve the effect of regulating or even eliminating the residual stress of the duplex stainless steel welded joint. The present invention can improve the corrosion resistance of the duplex stainless steel joint and optimize the welding residual stress at a low cost, thereby improving the performance and service life of the duplex stainless steel joint, saving the use of materials, and saving the cost of production and processing, and has important production significance and promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1a This is a schematic diagram of the weld before grinding. Figure 1b This is a schematic diagram of the weld after grinding;

[0028] Figure 2 Schematic diagram of powder presetting for a 2mm thick sheet;

[0029] Figure 3 This is a schematic diagram of powder presetting for a medium-thick plate with a thickness of 8mm;

[0030] Figure 4 Schematic diagram of powder presetting for 20mm thick plate. DETAILED DESCRIPTION

[0031] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below in conjunction with the drawings and examples.

[0032] The present invention proposes a method for optimizing residual stress of a weld joint and corrosion resistance of a weld. The method uses laser cladding powder to prepare a laser cladding layer on a duplex stainless steel weld joint. The laser cladding powder is an iron-based alloy, a nickel-based alloy, a cobalt-based alloy or a WC-reinforced high-entropy alloy.

[0033] The iron-based alloy uses iron-chromium-silicon alloy powder, which has excellent anti-rust performance, and the composition is: Cr>30%, Si>35%, and the balance is Fe. The nickel-based alloy powder uses F102, which has excellent comprehensive performance, corrosion resistance, oxidation resistance, heat resistance, resistance to low-stress abrasive wear and good impact toughness, and the composition is: C 0.60%-1.0%, Cr14.0%-18.0%, Si 3.5%-5.5%, B 3.0%-4.5%, Fe≤5%, and the balance is Ni. The cobalt-based alloy uses Co150, which has good high-temperature corrosion resistance and high-temperature fatigue resistance, and the composition is: C 0.12%, Cr28.00%, Si 1.00%, Fe 21.00%, Mn≤1.00%, and the balance is Co. The WC reinforced high entropy alloy is composed of the following components in the following mass ratio: Fe 39%, Co 7.5%, Cr 6.5%, Ni 7%, Si 5%, B 3%, WC 29.5%, Y2O32.5%. The alloy coating has excellent corrosion resistance and is particularly suitable for surface cladding of key components in extreme service environments.

[0034] The steps of preparing the laser cladding layer of duplex stainless steel welded joints using the above powder are as follows (the steps a and c for thin plates, medium plates, and thick plates are exactly the same):

[0035] a. Grind the weld excess height and weld toe

[0036] like Figure 1a and 1b As shown, use an angle grinder to grind the 20-50mm area on both sides of the weld to remove the weld excess height. The angle grinder should be parallel to the grinded surface when working, so that the weld excess height is removed; when the weld is concave, the standard is not to damage the parent material. Technical requirements: The grinded surface must be smooth; the parent material must not be damaged, and the standard is that the amount of parent material removed does not exceed 5%; the surface texture of the grinded part must be consistent, and irregular grinding is strictly prohibited along the direction of the weld. Then use a grinder to machine the surface of the duplex stainless steel weld joint to ensure that the surface is smooth and flat; use acetone to clean and remove surface oil and dirt.

[0037] b. Pre-setting powder and laser cladding

[0038] Pre-place laser cladding powder on the treated duplex stainless steel joint surface:

[0039] (1) Thin plates (thickness range 0.2-3 mm, such as Figure 2 As shown, taking the thickness of 2mm as an example): when cladding on the front side, the preset thickness of the powder is 100μm; when cladding on the back side, the preset thickness of the powder is 80μm, and the cladding width of the front side of the weld is smaller than the cladding width of the back side;

[0040] (2) Medium and thick plates (thickness range is 3-20 mm, such as Figure 3 As shown, taking the thickness of 8mm as an example): when cladding on the front side, the preset thickness of the powder is 300μm; when cladding on the back side, the preset thickness of the powder is 800μm, and the cladding width of the front side of the weld is smaller than the cladding width of the back side;

[0041] (3) Thick plates (thickness range is 20-60 mm, such as Figure 4 As shown, taking the thickness of 20 mm as an example): in step a, a double-sided V-shaped groove is opened, and the preset powder thickness is 800 μm when the side with larger residual stress is clad, and the preset powder thickness is 1000 μm when the other side is clad, and the cladding width of the side with larger residual stress is smaller than the cladding width of the other side.

[0042] Add a binder to the pre-set powder to increase the hardness and adhesion strength of the coating. The binder is rosin alcohol solution or glass water.

[0043] The duplex stainless steel welded joint after pre-powdering is placed in a drying oven for drying, and the dried duplex stainless steel substrate and the pre-powdering layer are taken out, and multi-pass laser cladding is performed using a ruby ​​laser or a CO2 laser. For thick plates, the side with smaller residual stress is clad first. The process parameters of laser cladding are as follows: laser power 1.5-2KW, scanning speed 150-300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0044] c. Determination of residual stress and joint corrosion resistance

[0045] Residual stress determination: The indentation strain method is used, and the resistance strain gauge is used as the sensitive element for measurement. An indentation is made at the center of the strain rosette by impact loading instead of drilling. The strain increment change in the elastic area outside the indentation area is recorded by the strain gauge, so as to obtain the true elastic strain corresponding to the residual stress and calculate the residual stress.

[0046] Corrosion resistance determination: Using the wire cutting method, the sample was prepared into a block sample with a size of 10mm×10mm×δmm (δ is the thickness of the joint after cladding), the sample was cleaned with acetone, and one end of a 100mm long wire was brazed together on the back side of the sample (substrate surface), the sample was sealed with dental tray powder, the top of the coating was sandpapered and polished, and placed in 3.5Wt.% NaCl and 1mol / L HCl, respectively, using a three-electrode system (working electrode, auxiliary electrode, reference electrode). Tafel test and impedance test were performed by IM6E photoelectrochemical workstation (Germany), and the test data were processed and analyzed by software.

[0047] The application effect of the present invention is further described below through specific embodiments. The joints used have all been polished in step a, and after cladding, they have all been subjected to residual stress and joint corrosion resistance testing in step c.

[0048] The laser cladding method has a certain heat treatment effect on the joint, and the cladding layer can isolate the weld joint from the external environment, so that the weld joint is protected; at the same time, by reasonably arranging the cladding area of ​​the cladding layer and the width and thickness of the cladding layer, the width and thickness of the laser cladding layer on the front and back of the duplex stainless steel weld are inconsistent, and the tensile stress generated when the cladding layer shrinks is used to reasonably regulate the residual stress of the weld joint; the effect of simultaneously regulating the residual stress of the duplex stainless steel weld joint and optimizing the corrosion resistance is achieved.

[0049] Iron-based alloys, nickel-based alloys, cobalt-based alloys or WC-reinforced high-entropy alloys are used as powders for laser cladding. The laser cladding layer obtained from the powder itself can form a good metallurgical bond with the duplex stainless steel base material. At the same time, the cladding layer also has good mechanical properties and corrosion resistance, which can achieve the effect of optimizing corrosion resistance.

[0050] Example 1

[0051] The iron-based alloy uses iron-chromium-silicon alloy powder with the composition of Cr>30%, Si>35%, and the balance Fe. The alloy powder is pre-placed on the treated duplex stainless steel joint surface. The preset thickness is 500μm for front cladding and 600μm for back cladding. The front cladding width of the weld is smaller than the back cladding width. The binder uses rosin alcohol solution or glass water; then put it into a drying oven for drying, take out the dried duplex stainless steel substrate and the pre-placed layer, and use ruby ​​laser or CO2 laser for multi-pass laser cladding. The process parameters used are in the range of: laser power 1.5~2KW, scanning speed 150~300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0052] Example 2

[0053] The nickel-based alloy powder is F102, and the composition is: C 0.60%-1.0%, Cr14.0%-18.0%, Si 3.5%-5.5%, B 3.0%-4.5%, Fe≤5%, and the balance is Ni; the alloy powder is pre-placed on the treated duplex stainless steel joint surface, the pre-placed thickness is 500μm when the front cladding is performed, and the pre-placed thickness is 600μm when the back cladding is performed, the front cladding width of the weld is smaller than the back cladding width, and the binder is rosin alcohol solution or glass water; then put it into a drying oven for drying, take out the dried duplex stainless steel substrate and the pre-placed layer, and use a ruby ​​laser or a CO2 laser for multi-pass laser cladding, and the process parameters used are in the range of: laser power 1.5-2KW, scanning speed 150-300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0054] Example 3

[0055] Co150 is selected as the cobalt-based alloy, and the composition is: C 0.12%, Cr28.00%, Si 1.00%, Fe 21.00%, Mn≤1.00%, and the balance is Co; the alloy powder is pre-placed on the treated duplex stainless steel joint surface, the pre-placed thickness is 500μm when the front cladding is performed, and the pre-placed thickness is 600μm when the back cladding is performed, the front cladding width of the weld is smaller than the back cladding width, and the binder is rosin alcohol solution or glass water; then it is placed in a drying oven for drying, and the dried duplex stainless steel substrate and the pre-placed layer are taken out, and a ruby ​​laser or a CO2 laser is used for multi-pass laser cladding, and the process parameters used are in the range of: laser power 1.5-2KW, scanning speed 150-300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0056] Example 4

[0057] WC reinforced high entropy alloy, the composition is: Fe 39%, Co 7.5%, Cr 6.5%, Ni 7%, Si 5%, B 3%, WC 29.5%, Y2O32.5%; the alloy powder is pre-placed on the treated duplex stainless steel joint surface, the pre-placed thickness is 500μm during front cladding, and the pre-placed thickness is 600μm during back cladding, the front cladding width of the weld is smaller than the back cladding width, and the binder is rosin alcohol solution or glass water; then put it into a drying oven for drying, take out the dried duplex stainless steel substrate and the pre-placed layer, use a ruby ​​laser or a CO2 laser for multi-pass laser cladding, the process parameters used are in the range of: laser power 1.5-2KW, scanning speed 150-300mm / min, and inert gas Ar or He gas protection is used during cladding.

[0058] By using different alloy powders to laser clad duplex stainless steel joints, duplex stainless steel welded joints with good mechanical properties and corrosion resistance can be obtained. The residual stress distribution of duplex stainless steel butt joints is analyzed. The thickness, width and distribution unevenness of the cladding layer on the front and back of the duplex stainless steel welded joint are used to make the thickness and width of the cladding layer on the back of the duplex stainless steel joint greater than the front cladding layer. The thermal expansion coefficient of the cladding layer is greater than that of the parent material and the effect of thermal expansion and contraction is used. After cooling, the cladding layer generates a stress on the parent material and the weld, which can offset the original stress, thereby reducing or even eliminating the residual stress of the duplex stainless steel joint. The cladding layer has good corrosion resistance, and the joint with difficult-to-control corrosion resistance is isolated from the external environment, thereby improving the corrosion resistance of the duplex stainless steel.

[0059] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for optimizing residual stress of welded joints and corrosion resistance of welds, characterized in that: The method for optimizing residual stress of welded joints and corrosion resistance of welds is to use laser cladding powder to prepare a laser cladding layer at a duplex stainless steel welded joint; wherein the laser cladding powder is an iron-based alloy, a nickel-based alloy, a cobalt-based alloy or a WC-enhanced high-entropy alloy.

2. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 1, characterized in that: The iron-based alloy uses iron-chromium-silicon alloy powder with the composition of Cr>30%, Si>35%, and the balance Fe.

3. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 1, characterized in that: The nickel-based alloy powder is F102, and the composition is: C 0.60%-1.0%, Cr 14.0%-18.0%, Si 3.5%-5.5%, B 3.0%-4.5%, Fe≤5%, and the balance is Ni.

4. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 1, characterized in that: The cobalt-based alloy is Co150, and the composition is: C 0.12%, Cr 28.00%, Si 1.00%, Fe 21.00%, Mn≤1.00%, and the balance is Co.

5. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 1, characterized in that: The composition of WC reinforced high entropy alloy is: Fe 39%, Co 7.5%, Cr 6.5%, Ni 7%, Si 5%, B 3%, WC 29.5%, Y2O32.5%.

6. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to any one of claims 1 to 5, characterized in that: Before laser cladding, grind the weld reinforcement and weld toe.

7. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 6, characterized in that: Before laser cladding, laser cladding powder is pre-placed on the treated surface of the duplex stainless steel joint; for thin plate or medium and thick plate duplex stainless steel joints with a thickness of greater than or equal to 0.2 mm and less than 20 mm, the front cladding width of the weld is smaller than the back cladding width; for thick plate duplex stainless steel joints with a thickness of greater than or equal to 20 mm, a double-sided V-groove is opened on the joint, and the cladding width on the side with greater residual stress is smaller than the cladding width on the other side.

8. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 7, characterized in that: Multi-pass laser cladding is performed using a ruby ​​laser or a CO2 laser.

9. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 8, characterized in that: For thick plates, the side with smaller residual stress is clad first; the process parameter range of laser cladding is: laser power 1.5~2KW, scanning speed 150~300mm / min, and inert gas Ar or He gas protection is used during cladding.

10. The method for optimizing residual stress of welded joints and corrosion resistance of welds according to claim 7, characterized in that: Rosin alcohol solution or glass water is used as a binder in the pre-set powder to increase the hardness and adhesion strength of the coating.

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