阅读说明：本技术 一种水轮机叶片钎涂修复方法 (Water turbine blade braze coating repairing method ) 是由 王星星 胡素梦 李帅 方乃文 高飞 陈小明 温国栋 龙伟民 彭进 倪增磊 崔大田 于 2021-08-24 设计创作，主要内容包括：本发明涉及一种水轮机叶片钎涂修复方法,包括如下步骤：S1、将低熔点钎料与高熔点硬质合金混合得到钎涂材料；S2、获得表面洁净的水轮机叶片；S3、将钎涂材料以膏状预置在水轮机叶片表面,然后将预置钎涂材料的水轮机叶片放在惰性气体保护或真空氛围中,进行加热钎焊,钎涂材料熔化在水轮机叶片表面形成复合钎涂层；S4、降低加热温度,待温度降至室温后关掉惰性气体保护或真空氛围保护炉；S5、出炉后,对覆有复合钎涂层的水轮机叶片依次进行真空热处理、打磨、清洗。本发明提供的水轮机叶片修复方法巧妙利用了钎焊技术原理,形成表面平整、结合强度高、应力小、高耐磨、耐腐蚀的复合涂层,具有工艺简单、操作方便、成本低廉、加热温度低等优点。(The invention relates to a method for repairing a turbine blade by braze welding, which comprises the following steps: s1, mixing the low-melting-point brazing filler metal with the high-melting-point hard alloy to obtain a brazing coating material; s2, obtaining the water turbine blade with clean surface; s3, pre-arranging the brazing filler metal coating material on the surface of the water turbine blade in a paste shape, then placing the water turbine blade pre-arranged with the brazing filler metal coating material in an inert gas protection or vacuum atmosphere, and heating and brazing, wherein the brazing filler metal coating material is melted on the surface of the water turbine blade to form a composite brazing filler metal coating; s4, reducing the heating temperature, and turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to the room temperature; and S5, after discharging, sequentially carrying out vacuum heat treatment, grinding and cleaning on the water turbine blade coated with the composite brazing filler metal coating. The method for repairing the water turbine blade skillfully utilizes the brazing technical principle to form the composite coating with smooth surface, high bonding strength, small stress, high wear resistance and corrosion resistance, and has the advantages of simple process, convenient operation, low cost, low heating temperature and the like.)

1. A method for repairing a turbine blade by braze welding is characterized by comprising the following steps: the method comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use;

s2, purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: presetting the brazing material obtained in the step S1 on the surface of the purified water turbine blade in a paste shape, then placing the water turbine blade with the brazing material in advance in an inert gas protection or vacuum atmosphere, heating and brazing, melting the brazing material on the surface of the water turbine blade to form a composite brazing coating, and preserving heat;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging from the furnace after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, carrying out vacuum heat treatment, polishing and cleaning on the water turbine blade coated with the composite braze coating in sequence, and thus finishing the braze coating repair of the water turbine blade.

2. The method for repairing a turbine blade by brazing according to claim 1, wherein: the temperature of the S3 heating brazing is 50-120 ℃ higher than that of the S5 vacuum heat treatment.

3. The method for repairing a turbine blade by brazing according to claim 1, wherein: the time of the vacuum heat treatment of S5 is 2.5 h.

4. The method for repairing a turbine blade by brazing according to claim 1, wherein: and S3, forming the composite braze coating on the surface of the water turbine blade into a tangential gradient coating, wherein the coating is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade.

5. The method for repairing a turbine blade by brazing according to claim 1, wherein: s3 the thickness of the composite braze coating formed on the surface of the water turbine blade is 10-1200 mu m, and the bonding strength is 295-426 MPa.

6. The method for repairing a turbine blade by brazing according to claim 1, wherein: and in the step S3, the heating welding material is heated according to the temperature in the brazing temperature time curve, and the brazing temperature time curve is designed and drawn according to the proportion of the low-melting-point composite brazing filler metal and the high-melting-point hardening material.

7. The method for repairing a turbine blade by brazing according to claim 1 or 6, wherein: and S4, gradually reducing the heating temperature according to the temperature in the brazing temperature time curve.

8. The method for repairing a turbine blade by brazing according to claim 1, wherein: the weight percentage of the low-melting-point brazing filler metal and the high-melting-point hard alloy in the S1 is (1-9): 1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture of carbide and boride.

9. The method for repairing a turbine blade by brazing according to claim 1, wherein: and in the step S3, when the brazing coating is heated, acetone and deionized water are adopted to prepare the brazing coating material into a paste, and the paste is preset on the surface of the water turbine blade subjected to purification treatment in the step S2.

Technical Field

The invention relates to the technical field of surface remanufacturing, in particular to a method for repairing a turbine blade through braze coating.

Background

The blades of the water turbine are worn and cavitated by rivers and silt for a long time, so that the defects of fatigue crack and silt corrosion are generated, and the blades of the water turbine need to be repaired on site at regular intervals (about one year) to improve the working efficiency of the water turbine. The repair of the water turbine blade is usually carried out by means of surfacing, cladding, arc welding and the like. However, when the repair welding operation is carried out on the blade at present, due to the lack of a professional repair method, the repair welding is carried out on the surface defects only by the experience of operators, the working efficiency is low, the bonding strength is low, and the actual requirements of energy conservation, emission reduction and environmental protection in modern manufacturing industry are not met.

The braze coating is used as an important material surface modification technology, a low-melting-point brazing filler metal and a high-melting-point hard alloy or ceramic form a composite coating through gradient heating, the coating has the unique advantages of smooth surface, high construction precision, simple processing procedure, high bonding strength, low heating temperature, small thermal stress and the like, the surface of a base body after braze coating is high in wear resistance, corrosion resistance and high temperature resistance, and the braze coating is widely applied to the fields of water conservancy machinery, aerospace, agricultural machinery, oil drilling and the like. The Chinese patent is disclosed: an automatic surfacing device (201811650242.X) for repairing a guide vane of a water turbine improves welding efficiency and solves the problems that manual surfacing repair is high in labor intensity and difficult to guarantee surfacing quality. A method (201510549734.X) for repairing cracks of a water turbine blade repairs the cracks of the blade by means of X flaw detection and a laser beam method, but the process is complex. The online laser strengthening device (201910205181.4) for the water turbine blade overcomes the problems of large heat affected zone of a base material, low bonding strength, difficulty in thickness control, environmental pollution, large operation space, low repairing efficiency and the like in the traditional electric arc welding defect repairing process of the water turbine blade. However, the blade repairing device lacks of monitoring of repairing layer profile information and interlayer temperature, the thickness of the repairing layer cannot be accurately regulated and controlled, and the purpose of optimizing process parameters in real time cannot be achieved. The brazing coating technology is applied to the field of repairing of the blades of the water turbine, the unique advantages of smooth surface of a brazing coating, high construction precision, simple processing procedure, high bonding strength, low heating temperature, small thermal stress and the like are fully exerted, and the defects of the existing repairing technology of the blades of the water turbine are expected to be overcome.

Disclosure of Invention

The invention aims to solve the technical problems and the defects and provides a method for repairing a turbine blade by braze welding.

In order to solve the technical problems, the invention adopts the technical scheme that: a method for repairing a turbine blade by braze welding comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use;

s2, purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: presetting the brazing material obtained in the step S1 on the surface of the purified water turbine blade in a paste shape, then placing the water turbine blade with the brazing material in advance in an inert gas protection or vacuum atmosphere, heating and brazing, melting the brazing material on the surface of the water turbine blade to form a composite brazing coating, and preserving heat;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging from the furnace after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, carrying out vacuum heat treatment, polishing and cleaning on the water turbine blade coated with the composite braze coating in sequence, and thus finishing the braze coating repair of the water turbine blade.

As a further optimization of the braze-coating repairing method for the water turbine blade, the temperature of the heating braze-welding in the S3 is 50-120 ℃ higher than that of the vacuum heat treatment in the S5.

As a further optimization of the braze-coating repairing method of the water turbine blade, the vacuum heat treatment time of S5 is 2.5 h.

As a further optimization of the method for repairing the turbine blade by braze coating, in S3, the composite braze coating formed on the surface of the turbine blade is a tangentially graded coating, that is, the coating is perpendicular to the normal direction along the inner surface or the outer surface of the turbine blade.

As a further optimization of the method for repairing the turbine blade by braze welding, S3 shows that the thickness of the composite braze welding layer formed on the surface of the turbine blade is 10-1200 mu m, and the bonding strength is 295-426 MPa.

As a further optimization of the braze welding repair method for the water turbine blade, the heating welding material in S3 is heated according to the temperature in the braze welding temperature-time curve, and the braze welding temperature-time curve is designed and drawn according to the proportion of the low-melting-point composite brazing filler metal and the high-melting-point hardening material.

As a further optimization of the braze welding repair method for the turbine blade of the present invention, the step-wise decreasing of the heating temperature in S4 is a step-wise decreasing of the temperature according to the temperature in the braze welding temperature time curve.

As a further optimization of the braze welding repair method for the water turbine blade, the weight percentage of the low-melting-point brazing filler metal and the high-melting-point hard alloy in S1 is (1-9): 1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture of carbide and boride.

As a further optimization of the water turbine blade braze-coating repairing method, in S3, when braze-coating is heated, the braze-coating material is prepared into a paste by using acetone and deionized water and is preset on the surface of the water turbine blade subjected to purification treatment in S2.

The invention has the following beneficial effects:

the method for repairing the water turbine blade skillfully utilizes the brazing technical principle, organically combines the low-melting-point brazing filler metal (iron base, copper base and nickel base) and the high-melting-point hard alloy (carbide, boride and diamond) to form the composite coating with smooth surface, high bonding strength, small stress, high wear resistance and corrosion resistance, has the advantages of simple process, convenience in operation, low cost, low heating temperature and the like, and overcomes the defects of complex process, high labor intensity, large influence area, low bonding strength, difficulty in controlling thickness, environmental pollution, large operation space, low repairing efficiency and the like of the traditional method.

Secondly, in the repair method of the water turbine blade, the prepared composite coating is a tangential gradient coating, namely a brazing coating formed by low-melting-point brazing filler metal and high-melting-point hard alloy, and is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade, and the repair method is different from the coating repaired by the traditional method. In the method for repairing the water turbine blade, the abrasion resistance of the prepared composite coating is improved by 1.65-7.35 times compared with that of the matrix, and the corrosion resistance is improved by 2-3.5 orders of magnitude.

In the repair method of the water turbine blade, the iron-based brazing filler metal in the low-melting-point brazing filler metal is introduced into the coating on the surface of the water turbine blade, so that the material cost is low, and meanwhile, the iron-based alloy brazing filler metal and the common steel substrate blade have good compatibility and compact texture, and the uniformity (thickness uniformity) of the coating in the tangential gradient direction can be effectively improved; the nickel-based brazing filler metal is introduced into the composite coating, so that the surface hardness of the water turbine blade is improved, and the wear resistance of the coating is obviously improved; the copper-based brazing filler metal contains elements such as Ni, Mn, Pr or Y yttrium, and the like, so that the coating can be quickly wetted and spread on the surface of the base blade, and the corrosion resistance and the wear resistance of the brazing coating are improved. The composite coating prepared by the repairing method contains high-melting-point carbide and boride, the two substances are high in hardness, easy to wet, small in brittleness and low in cost, the bonding strength and the wear resistance of the composite coating can be further improved, and particularly, a small amount of diamond particles are contained in the boride, so that the composite coating is high in strength and high in wear resistance, and the service life of the repaired water turbine blade is remarkably prolonged.

Drawings

FIG. 1 is a brazing temperature time profile for an example of the present invention.

Detailed Description

The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

Example 1

A method for repairing a turbine blade by braze welding comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use; the weight percentage of the low-melting-point brazing filler metal to the high-melting-point hard alloy is (1-9): 1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture S2 of carbide and boride and is used for purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: preparing the brazing coating material obtained in the step S1 into paste by adopting acetone and deionized water, and presetting the paste on the surface of the water turbine blade subjected to purification treatment in the step S2, then placing the water turbine blade with the brazing coating material in advance in an inert gas protection or vacuum atmosphere, and carrying out heating brazing according to the temperature in a brazing temperature-time curve, wherein the heating temperature is 1105 ℃, and the brazing temperature-time curve is drawn according to the proportion design of the low-melting-point composite brazing filler metal and the high-melting-point hardening material, as shown in figure 1, the brazing coating material is melted on the surface of the water turbine blade to form a composite brazing coating, and the heat is preserved;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature according to the temperature in the brazing temperature time curve, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, sequentially carrying out vacuum heat treatment on the water turbine blade coated with the composite braze coating, wherein the heat treatment temperature is 1055 ℃, the time is 2.5 hours, and finishing the braze coating repair of the water turbine blade by polishing metallographic abrasive paper and cleaning tap water.

And S3, forming the composite braze coating on the surface of the water turbine blade into a tangential gradient coating, wherein the coating is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade.

The iron-based brazing filler metal is a mixture of 55% of iron powder, 25% of ferrotitanium powder, 5% of molybdenum powder, 5% of chromium powder and 10% of iron carbide according to weight percentage, and the melting temperature is 1030-1155 ℃; the copper-based brazing filler metal is a mixed brazing filler metal containing 79% of copper, 12% of nickel, 6% of manganese and 3% of rare earth praseodymium, and the melting temperature is 955-1045 ℃; the nickel-based brazing filler metal is a powdery brazing filler metal mixed by 81.5% of nickel, 9.5% of chromium, 3% of boron, 1% of yttrium and 5% of iron, and the melting temperature is 965-1130 ℃;

the carbide is a mixture of a carbonization composition and diamond in a weight ratio of 9.5:1, and the carbonization composition is a mixture of 60% of tungsten carbide, 25% of chromium carbide and 15% of titanium carbide; the boride is a mixture of a boronized composition and diamond in a weight ratio of 8:1, the boronized composition is a mixture of 70% of chromium boride and 30% of tungsten boride, and the weight percentage of the carbide to the boride is 1.2: 1. the granularity of the boride is-60-100 meshes, the granularity of the carbide is-50-200 meshes, and the granularity of the diamond is-100-450 meshes.

In this embodiment S3, the composite braze coating formed on the surface of the turbine blade has a thickness of 1200 μm and a bonding strength of 420 MPa.

Example 2

A method for repairing a turbine blade by braze welding comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use; the weight percentage of the low-melting-point brazing filler metal to the high-melting-point hard alloy is 1: 1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture S2 of carbide and boride and is used for purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: preparing the brazing coating material obtained in the step S1 into paste by adopting acetone and deionized water, presetting the paste on the surface of the water turbine blade subjected to purification treatment in the step S2, then placing the water turbine blade with the brazing coating material in the inert gas protection or vacuum atmosphere, and carrying out heating brazing according to the temperature in a brazing temperature-time curve, wherein the heating temperature is 1195 ℃, the brazing temperature-time curve is drawn according to the proportion design of the low-melting-point composite brazing filler metal and the high-melting-point hardening material, and the brazing coating material is melted on the surface of the water turbine blade to form a composite brazing coating and is subjected to heat preservation;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature according to the temperature in the brazing temperature time curve, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, sequentially carrying out vacuum heat treatment on the water turbine blade coated with the composite braze coating, wherein the heat treatment temperature is 1075 ℃, the time is 2.5 hours, and finishing the braze coating repair of the water turbine blade by grinding metallographic abrasive paper and cleaning tap water.

And S3, forming the composite braze coating on the surface of the water turbine blade into a tangential gradient coating, wherein the coating is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade.

The iron-based brazing filler metal is a mixture of 55% of iron powder, 25% of iron carbide, 5% of molybdenum boride, 5% of chromium powder and 10% of iron boride according to weight percentage, and the melting temperature is 1030-1145 ℃; the copper-based brazing filler metal is a mixed brazing filler metal containing 79% of copper, 12% of nickel, 6% of manganese and 3% of rare earth yttrium, and the melting temperature is 955-1040 ℃; the nickel-based brazing filler metal is a powdery brazing filler metal mixed by 81.5% of nickel, 9.5% of chromium, 3% of boron, 1% of yttrium and 5% of iron, and the melting temperature is 965-1130 ℃;

the carbide is a mixture of a carbonization composition and diamond in a weight ratio of 9:1, and the carbonization composition is a mixture of 60% of tungsten carbide, 25% of chromium carbide and 15% of boron carbide; the boride is a mixture of a boronized composition and diamond in a weight ratio of 9.5:1, the boronized composition is a mixture of 70% of chromium boride and 30% of tungsten boride, and the weight percentage of the carbide to the boride is 1: 1.

in this embodiment S3, the composite braze coating formed on the surface of the turbine blade has a thickness of 10 μm and a bonding strength of 295 MPa.

Example 3

A method for repairing a turbine blade by braze welding comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use; the weight percentage of the low-melting-point brazing filler metal to the high-melting-point hard alloy is 5:1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture S2 of carbide and boride and is used for purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: preparing the brazing coating material obtained in the step S1 into paste by adopting acetone and deionized water, presetting the paste on the surface of the water turbine blade subjected to purification treatment in the step S2, then placing the water turbine blade with the brazing coating material in the inert gas protection or vacuum atmosphere, and carrying out heating brazing according to the temperature in a brazing temperature-time curve, wherein the heating temperature is 1131 ℃, the brazing temperature-time curve is drawn according to the proportion design of the low-melting-point composite brazing filler metal and the high-melting-point hardening material, and the brazing coating material is melted on the surface of the water turbine blade to form a composite brazing coating and is subjected to heat preservation;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature according to the temperature in the brazing temperature time curve, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, sequentially carrying out vacuum heat treatment on the water turbine blade coated with the composite braze coating, wherein the heat treatment temperature is 1046 ℃, the time is 2.5 hours, and the braze coating repair of the water turbine blade is completed by grinding metallographic abrasive paper and cleaning tap water.

And S3, forming the composite braze coating on the surface of the water turbine blade into a tangential gradient coating, wherein the coating is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade.

The iron-based brazing filler metal comprises 55% of iron powder, 25% of ferrotitanium powder, 5% of molybdenum powder, 5% of chromium powder and 10% of iron carbide according to weight percentage, and the melting temperature is 1030-1155 ℃; the copper-based brazing filler metal is a mixed brazing filler metal containing 79% of copper, 12% of nickel, 6% of manganese and 3% of rare earth praseodymium, and the melting temperature is 955-1045 ℃; the nickel-based brazing filler metal is a powdery brazing filler metal mixed by 81.5% of nickel, 9.5% of chromium, 3% of boron, 1% of yttrium and 5% of iron, and the melting temperature is 965-1130 ℃;

the carbide is a mixture of a carbonization composition and diamond in a weight ratio of 9:1, and the carbonization composition is a mixture of 60% of tungsten carbide, 25% of boron carbide and 15% of titanium carbide; the boride is a mixture of a boronized composition and diamond in a weight ratio of 8.85:1, the boronized composition is a mixture of 70% of chromium boride and 30% of tungsten boride, and the weight percentage of the carbide to the boride is 1.1: 1.

in this embodiment S3, the composite braze coating formed on the surface of the turbine blade has a thickness of 605 μm and a bonding strength of 357.5 MPa.

Example 4

A method for repairing a turbine blade by braze welding comprises the following steps:

s1, preparing brazing coating materials: mixing the low-melting-point brazing filler metal and the high-melting-point hard alloy to obtain a brazing coating material for later use; the weight percentage of the low-melting-point brazing filler metal to the high-melting-point hard alloy is 3.5: 1, the low-melting-point brazing filler metal is a mixture of any two or three of iron-based brazing filler metal, copper-based brazing filler metal and nickel-based brazing filler metal, and the high-melting-point hard alloy is a mixture S2 of carbide and boride and is used for purifying the surface of the water turbine blade: firstly, polishing the surface of a water turbine blade eroded by silt, and then cleaning the surface by adopting alcohol or acetone to obtain the water turbine blade with a clean surface;

s3, heating and braze coating: preparing the brazing coating material obtained in the step S1 into paste by adopting acetone and deionized water, presetting the paste on the surface of the water turbine blade subjected to purification treatment in the step S2, then placing the water turbine blade with the brazing coating material in the inert gas protection or vacuum atmosphere, and carrying out heating brazing according to the temperature in a brazing temperature-time curve, wherein the heating temperature is 1147 ℃, the brazing temperature-time curve is drawn according to the proportion design of the low-melting-point composite brazing filler metal and the high-melting-point hardening material, and the brazing coating material is melted on the surface of the water turbine blade to form a composite brazing coating and is subjected to heat preservation;

s4, furnace cooling: after the brazing heat preservation is finished, gradually reducing the heating temperature according to the temperature in the brazing temperature time curve, turning off the inert gas protection or vacuum atmosphere protection furnace after the temperature is reduced to room temperature, and discharging after 8 hours;

s5, post-processing: and after the water turbine blade is taken out of the furnace, sequentially carrying out vacuum heat treatment on the water turbine blade coated with the composite braze coating, wherein the heat treatment temperature is 1077 ℃, the time is 2.5 hours, and finishing the braze coating repair of the water turbine blade by grinding metallographic abrasive paper and cleaning tap water.

And S3, forming the composite braze coating on the surface of the water turbine blade into a tangential gradient coating, wherein the coating is perpendicular to the normal direction along the inner surface or the outer surface of the water turbine blade.

The iron-based brazing filler metal comprises 55% of iron powder, 25% of ferrotitanium powder, 5% of molybdenum powder, 5% of chromium powder and 10% of iron carbide according to weight percentage, and the melting temperature is 1030-1155 ℃; the copper-based brazing filler metal is a mixed brazing filler metal containing 79% of copper, 12% of nickel, 6% of manganese and 3% of rare earth praseodymium, and the melting temperature is 955-1045 ℃; the nickel-based brazing filler metal is a powdery brazing filler metal mixed by 81.5% of nickel, 9.5% of chromium, 3% of boron, 1% of yttrium and 5% of iron, and the melting temperature is 965-1130 ℃;

the carbide is a mixture of a carbonization composition and diamond in a weight ratio of 9:1, and the carbonization composition is a mixture of 60% of tungsten carbide, 25% of boron carbide and 15% of titanium carbide; the boride is a mixture of a boronized composition and diamond in a weight ratio of 8.85:1, the boronized composition is a mixture of 70% of chromium boride and 30% of tungsten boride, and the weight percentage of the carbide to the boride is 1.1: 1.

in this embodiment S3, the composite braze coating formed on the surface of the turbine blade has a thickness of 125 μm and a bonding strength of 390 MPa.

The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.