Power plant valve deep hole sealing surface gradient surfacing repair method

Abstract

This invention discloses a gradient overlay welding repair method for deep-hole sealing surfaces of power plant valves. First, three-dimensional defect data is obtained through penetrant and ultrasonic testing to delineate the bevel area. A suitable U-shaped bevel is machined, with the bevel size matched to the defect depth, and the bevel is re-inspected after boring. Segmented variable-speed preheating with temperature control is performed, and insulation cotton is wrapped around the non-welding area of ​​the valve. After the welding material is dried according to specifications, a cobalt-based wear-resistant layer, a stainless steel transition buffer layer, and a stainless steel main filler layer are overlaid in layers, with limited layer thickness ratios, heat input, and interlayer temperatures. Tempering and heat preservation are differentiated according to the overlay thickness, cooling air velocity is controlled, and residual stress is detected. Finally, the sealing surface is precision machined, and the contour accuracy is checked. This invention features complete and quantitative process parameters, a three-layer gradient structure adapted to narrow deep-hole cavities with DN≥32mm, a standardized repair process, improved sealing surface forming accuracy, and reduced repair defects.

Description

Technical Field

[0001] This invention relates to the field of power plant special equipment repair technology, and more specifically to a gradient welding repair method for deep hole sealing surfaces of power plant valves. Background Technology

[0002] The deep-hole sealing surfaces of high-temperature and high-pressure valves in power plants are subjected to long-term erosion by the medium, alternating pressure, high-temperature creep, and stress concentration, making them prone to defects such as microcracks, pitting, and wear, which are common failure points in power plant equipment. Current valve sealing surface repair methods mostly employ cobalt-based alloy welding, but these methods are primarily suitable for open sealing surfaces on the outside of the valve body and cannot be adapted to the narrow, sealed internal cavity structure of deep holes with a diameter ≥ 32mm.

[0003] Existing repair processes have the following technical defects: Conventional repairs use V-shaped bevels or general standard bevels, which result in high stress concentration when applied to deep hole structures, easily causing cracks at the root of the bevel, and requiring a large amount of filler metal, leading to high cumulative heat input; Traditional welding often uses cobalt-based single-layer welding or cobalt-based and stainless steel two-layer composite welding structures, resulting in large differences in material parameters between layers and poor interface stability under the high constraint conditions of deep holes; Existing heat treatment processes use fixed heating rates and fixed holding times, without adapting to the uneven wall thickness, uneven heat dissipation, and differences in the thickness of multiple layers of welding in deep hole valve bodies, resulting in incomplete elimination of residual stress; Conventional machining and welding methods do not have specific process constraints for the narrow space of deep holes, easily leading to cutting vibration marks, excessive local heat input, high defect recurrence rate after repair, and poor repair stability. Summary of the Invention

[0004] The purpose of this invention is to overcome the technical defects of existing deep hole sealing surface repair processes for power plant valves, such as coarse parameters, weak adaptability to working conditions, low repair accuracy, and insufficient elimination of residual stress. This invention provides a gradient welding repair method for deep hole sealing surfaces of power plant valves. By using a beveling process adapted to the deep hole structure, a three-layer gradient welding structure, a segmented preheating process, a thickness-linked tempering process, and a dedicated deep hole welding control method, this invention achieves standardized, high-precision, and low-defect repair of deep hole sealing surfaces of power plant valves.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: a method for gradient overlay welding repair of deep hole sealing surfaces of power plant valves, characterized by comprising the following steps: S1. Defect pre-location: Three-dimensional quantitative location data of defects in the deep hole sealing surface of power plant valves are obtained by combining penetrant testing and ultrasonic testing to clarify the depth, length and spatial orientation range of the defects and delineate the beveling processing area to be repaired. S2. Preparation of U-shaped bevel for deep hole adaptation: For the defect area of ​​the sealing surface of valve deep hole with DN≥32mm, a U-shaped bevel is machined. The root of the bevel is a rounded transition structure with a radius of 3-5mm. The inclination angle of the bevel sidewall is 8°-12°, the total depth of the bevel is 1.5-3mm, and the upper edge of the bevel is rounded with a radius of 2mm. The cutting is performed using an extended rod internal boring tool with a single cutting feed rate ≤0.15mm. S3. Segmented preheating treatment of valves: The valves are placed in the furnace with an initial temperature of 250℃, heated to 350℃ at a rate of 60℃ / h, and then heated to 450℃~500℃ at a rate of 30℃ / h, and held for 2 hours; Welding operations are started within 3 minutes after the valves are taken out of the furnace, and the non-welding areas of the valve body are wrapped with insulation cotton throughout the welding process. S4. Three-layer gradient layered controllable surfacing: A cobalt-based wear-resistant layer, a stainless steel transition buffer layer, and a stainless steel main filler layer are formed by sequential surfacing; the cobalt-based wear-resistant layer is surfacing using D802 cobalt-based alloy welding rods, with a single layer thickness of 0.6-1.0 mm and a single-pass welding heat input controlled at 8-12 kJ / cm; an austenitic stainless steel transition buffer layer with a thickness of 0.8-1.2 mm is surfacing on the cobalt-based wear-resistant layer; the stainless steel main filler layer is surfacing in multiple passes using austenitic stainless steel welding rods, with a single pass filler thickness ≤1.5 mm; a narrow-diameter extended curved-handle welding torch is used throughout the surfacing process, with a torch oscillation amplitude ≤8 mm, an interpass temperature not lower than 250℃, and an interpass dwell time ≤90 s; S5. Thickness-linked precision tempering heat treatment: After the welding is completed, the valve is put into the furnace at 450℃ and tempered at a constant rate of 150℃ / h to 720±20℃; when the total welding thickness is <4mm, it is held for 2h; when the total welding thickness is 4~6mm, it is held for 2.5h; when the total welding thickness is >6mm, it is held for 3h; during the cooling stage, when the furnace temperature is above 250℃, the cooling rate is ≤50℃ / h; after the furnace temperature drops below 250℃, it is taken out of the furnace and naturally air-cooled. S6. Post-repair performance verification and sealing surface finishing: Inspect the defects in the repaired area and the hardness of the sealing surface; after passing the inspection, perform finishing on the sealing surface, and the surface roughness Ra of the sealing surface after finishing is ≤1.6μm.

[0006] This invention completes the entire repair process sequentially, including defect location, dedicated beveling, segmented preheating, three-layer gradient welding, thickness-linked tempering, and precision machining and inspection. It is equipped with complete and quantified dimensional, temperature, operation, and inspection parameters, forming a standardized repair process specifically for the sealing surface of deep-hole valves with DN≥32mm, and unifying the complete repair operation specifications under confined internal cavity conditions.

[0007] Preferably, in step S1, penetrant testing is used to identify open-type defects on the sealing surface, and ultrasonic testing is used to detect buried defects inside the sealing surface, as well as the defect depth and spatial orientation. The two sets of test data are fused to generate three-dimensional quantitative location data of the defects.

[0008] This invention distinguishes between the detection objects and data types of penetrant testing and ultrasonic testing, and integrates the two types of detection data to generate three-dimensional quantitative location information of defects. It can completely distinguish between surface opening defects and internal buried defects, accurately delineate the beveling processing boundary, and avoid the beveling processing range being too large or too small.

[0009] As a preferred option, in step S2, when the defect burial depth is ≤1.5mm, the radius of the bevel root is 3mm and the inclination angle of the bevel sidewall is 8°; when the defect burial depth is >1.5mm, the radius of the bevel root is 5mm and the inclination angle of the bevel sidewall is 12°; after the bevel processing is completed, the bottom of the bevel is inspected using a dye penetrant testing method.

[0010] This invention matches the bevel arc and sidewall inclination angle according to the depth of defect burial. After bevel processing, a color penetration re-inspection process is added to achieve adaptive adjustment of the bevel forming size under defects of different depths, ensuring that the bottom of the bevel is formed completely without processing micro-defects.

[0011] Preferably, in step S4, the ratio of the weld thickness of the stainless steel transition buffer layer to the weld thickness of the cobalt-based wear-resistant layer is 1.2:1 to 1.5:1. After the stainless steel transition buffer layer is welded, the interlayer temperature is checked. When the interlayer temperature is ≥250℃, the stainless steel main filler layer is welded.

[0012] This invention defines a fixed thickness ratio between the stainless steel transition buffer layer and the cobalt-based wear-resistant layer, and sets operating conditions that allow filling only after the interlayer temperature reaches the standard, standardizing the thickness matching relationship between the two dissimilar weld overlay layers, and ensuring that the interlayer welding temperature is uniform and controllable.

[0013] Preferably, in step S4, the D802 cobalt-based alloy welding rod is dried at 300℃~350℃ for 1 hour before use, and the austenitic stainless steel welding rod is dried at 250℃~300℃ for 1 hour before use; the dried welding rod is placed in an insulation cylinder, and the temperature inside the insulation cylinder is maintained at 150℃~200℃.

[0014] This invention defines differentiated drying temperatures, durations, and storage temperatures in heat-insulating cylinders for two types of welding materials, standardizes the pretreatment criteria for welding materials before surfacing, reduces the amount of moisture carried into the surfacing layer by the welding materials, and lowers the probability of porosity formation inside the surfacing layer.

[0015] As a preferred option, in step S3, the furnace temperature fluctuation during the heat preservation process is controlled within ±10℃; the thickness of the insulation cotton covering the non-welding area of ​​the valve body is ≥20mm.

[0016] This invention controls the range of furnace temperature fluctuation during the preheating and heat preservation stage, and limits the minimum thickness of insulation cotton in the non-welding area of ​​the valve body, thereby stabilizing the overall preheating temperature of the valve body and reducing the temperature difference between the inner and outer walls of the valve body.

[0017] As a preferred option, in step S5, the air velocity in the air-cooled environment is ≤2m / s; after the valve is cooled to room temperature, the residual stress on the surface of the repair area is detected by the blind hole method, and the peak value of the residual stress is ≤150MPa.

[0018] This invention limits the air velocity in the air-cooled environment after tempering and sets quantitative judgment indicators for residual stress, unifies the requirements for tempering cooling environment and the acceptance standard for residual stress after repair, and controls the residual stress level of valve body after weld overlay repair.

[0019] As a preferred option, in step S6, after the sealing surface is finished, the contour accuracy of the sealing surface is tested using the replication putty method; at least 8 test points are evenly distributed along the circumference of the sealing surface, and the detected contour deviation is ≤ ±0.05mm.

[0020] This invention employs a copy putty method and sets the number of circumferential detection points and contour deviation thresholds for the sealing surface, standardizing the contour accuracy detection method after the sealing surface is finely machined, and unifying the acceptance standard for the forming dimensions of the sealing surface.

[0021] Beneficial effects: (1) According to the defect depth, the present invention matches the corresponding bevel arc parameters and side wall angle, and with the limited micro-cutting feed parameters, it adapts to the narrow and closed processing space of deep holes, standardizes the bevel forming accuracy of the deep hole defect area, and unifies the deep hole sealing surface repair processing standard.

[0022] (2) The present invention sets up a three-layer gradient welding structure consisting of a cobalt-based wear-resistant layer, a stainless steel transition buffer layer, and a stainless steel main filling layer, and limits the thickness ratio between the transition buffer layer and the wear-resistant layer, standardizes the interlayer structural dimensions of multi-layer dissimilar material welding, and determines the overall structural matching degree and forming consistency standard of the welding layer.

[0023] (3) The present invention adopts a segmented variable speed heating preheating method, combined with furnace temperature fluctuation control and valve full-process heat preservation constraint, which is adapted to the structural characteristics of uneven wall thickness of deep hole valve body, and provides valve preheating temperature uniformity conditions, providing a stable welding temperature environment for subsequent welding operations.

[0024] (4) The present invention matches the corresponding tempering and heat preservation time according to the weld thickness, and limits the cooling rate and air cooling environment conditions to adapt to weld valves with different repair thicknesses, thereby eliminating the residual stress generated by multi-layer gradient welds and making the peak value of residual stress in the repair area ≤150MPa.

[0025] (5) This invention defines the specific welding parameters for deep holes, the specifications for drying and heat preservation of welding materials, and the standards for precision machining and contour detection of sealing surfaces. It unifies the welding accuracy and repair forming accuracy of deep holes in narrow spaces, and improves the machining accuracy and service stability of valve deep hole sealing surfaces after repair. Attached Figure Description

[0026] Figure 1This is a cross-sectional schematic diagram of the gradient welding repair of the deep hole sealing surface of a power plant valve according to the present invention.

[0027] Wherein: 1-Valve; 2-Cobalt-based wear-resistant layer; 3-Stainless steel transition buffer layer; 4-Stainless steel main body filler layer. Detailed Implementation

[0028] To make the technical means, creative features and objectives of the present invention easier to understand, the present invention will be further described below in conjunction with specific embodiments.

[0029] Example: A gradient welding repair method for deep hole sealing surfaces of power plant valves, targeting pitting and microcrack defects on the deep hole sealing surfaces of a DN50 high-temperature and high-pressure gate valve in a power plant. The gradient welding repair method of this invention is adopted, and the steps are as follows: S1. Defect pre-location: Penetrant testing is used to identify open defects on the sealing surface, and ultrasonic testing is used to detect internal buried defects and their depth and spatial orientation. The two sets of data are fused to generate three-dimensional quantitative location data of the defects. The maximum depth of the defects is measured to be 1.2 mm and the length is 3.5 mm. Based on this, the beveling area to be repaired is delineated.

[0030] S2. Preparation of U-shaped bevel for deep hole adaptation: For the defect area of ​​the deep hole sealing surface of DN50 valve, a U-shaped bevel is machined using an extended rod inner hole boring tool, with a single cutting feed controlled at 0.12mm; since the defect burial depth is ≤1.5mm, the radius of the bevel root is 3mm, the sidewall inclination angle is 8°, the total bevel depth is 2mm, and the upper edge of the bevel is rounded with a radius of 2mm; after machining, the bottom of the bevel is inspected using a dye penetrant test to confirm that there are no machining cracks.

[0031] S3. Segmented preheating treatment of valve 1: Valve 1 is placed in the furnace with an initial temperature of 250℃, heated to 350℃ at a rate of 60℃ / h, and then heated to 480℃ at a rate of 30℃ / h. The temperature is held for 2 hours, and the furnace temperature fluctuation is controlled within ±10℃. Welding operation begins within 2 minutes after valve 1 is taken out of the furnace. During the entire welding process, the non-welding area of ​​the valve body is wrapped with 20mm thick insulation cotton.

[0032] S4. Three-layer gradient layered controllable overlay welding: welding material pretreatment, D802 cobalt-based alloy welding rod is dried at 320℃ for 1 hour, austenitic stainless steel welding rod is dried at 280℃ for 1 hour, and then placed in a heat preservation cylinder at 180℃.

[0033] Three layers of structure are welded together in sequence: Cobalt-based wear-resistant layer 2: It is built up using D802 cobalt-based alloy welding rods, with a single layer thickness of 0.8 mm and a single-pass welding heat input control of 10 kJ / cm.

[0034] Stainless steel transition buffer layer 3: A 1.0 mm thick austenitic stainless steel transition buffer layer is deposited on the cobalt-based wear-resistant layer. The thickness ratio of this layer to the cobalt-based wear-resistant layer is 1.25:1. After the stainless steel transition buffer layer is deposited, the interlayer temperature is detected using a surface thermometer. Once the interlayer temperature is confirmed to be ≥250℃, the stainless steel main filler layer is deposited.

[0035] Stainless steel main body filler layer 4: Austenitic stainless steel welding rods are used for multi-pass layered welding, with a single filler thickness of 1.2mm and a total weld thickness of about 4mm.

[0036] The entire welding process uses a narrow-diameter, extended-handle welding torch with a torch oscillation amplitude of ≤6mm, the interpass temperature is maintained at 260℃~280℃, and the interpass dwell time is ≤70s.

[0037] S5. Thickness-linked precision tempering heat treatment: After the welding is completed, valve 1 is put into the furnace at 450℃ and tempered at a constant rate of 150℃ / h to 720℃. Since the total thickness of the welding is 4mm, the holding time is 2.5h. During the cooling stage, when the furnace temperature is above 250℃, the cooling rate is controlled at 40℃ / h. After the furnace temperature drops below 250℃, it is taken out of the furnace and naturally air-cooled in an environment with a wind speed ≤2m / s. After valve 1 cools to room temperature, the residual stress on the surface of the repair area is detected by the blind hole method. The peak value of the residual stress is measured to be 120MPa, which meets the requirement of ≤150MPa.

[0038] S6. Post-repair performance verification and sealing surface finishing: Ultrasonic testing is used to detect defects in the repaired area, and Rockwell hardness tester is used to test the hardness of the sealing surface. After passing the test, the sealing surface is finished. The surface roughness Ra of the sealing surface after finishing is Ra≤1.6μm. After finishing, the sealing surface contour accuracy is tested using the replication putty method. Eight test points are evenly distributed along the circumference of the sealing surface. The measured contour deviation is ≤±0.03mm, which meets the accuracy requirements.

Claims

1. A method for repairing the deep hole sealing surface of a power plant valve using gradient welding, characterized in that... Includes the following steps: S1. Defect pre-location: Three-dimensional quantitative location data of defects in the deep hole sealing surface of power plant valves are obtained by combining penetrant testing and ultrasonic testing to clarify the depth, length and spatial orientation range of the defects and delineate the beveling processing area to be repaired. S2. Preparation of U-shaped bevel for deep hole adaptation: For the defect area of ​​the sealing surface of valve deep hole with DN≥32mm, a U-shaped bevel is machined. The root of the bevel is a rounded transition structure with a radius of 3-5mm. The inclination angle of the bevel sidewall is 8°-12°, the total depth of the bevel is 1.5-3mm, and the upper edge of the bevel is rounded with a radius of 2mm. The cutting is performed using an extended rod internal boring tool with a single cutting feed rate ≤0.15mm. S3. Segmented preheating treatment of valves: The valves are placed in the furnace with an initial temperature of 250℃, heated to 350℃ at a rate of 60℃ / h, and then heated to 450℃~500℃ at a rate of 30℃ / h, and held for 2 hours; Welding operations are started within 3 minutes after the valves are taken out of the furnace, and the non-welding areas of the valve body are wrapped with insulation cotton throughout the welding process. S4. Three-layer gradient layered controllable surfacing: A cobalt-based wear-resistant layer, a stainless steel transition buffer layer, and a stainless steel main filler layer are formed by sequential surfacing; the cobalt-based wear-resistant layer is surfacing using D802 cobalt-based alloy welding rods, with a single layer thickness of 0.6-1.0 mm and a single-pass welding heat input controlled at 8-12 kJ / cm; an austenitic stainless steel transition buffer layer with a thickness of 0.8-1.2 mm is surfacing on the cobalt-based wear-resistant layer; the stainless steel main filler layer is surfacing in multiple passes using austenitic stainless steel welding rods, with a single pass filler thickness ≤1.5 mm; a narrow-diameter extended curved-handle welding torch is used throughout the surfacing process, with a torch oscillation amplitude ≤8 mm, an interpass temperature not lower than 250℃, and an interpass dwell time ≤90 s; S5. Thickness-linked precision tempering heat treatment: After the welding is completed, the valve is put into the furnace at 450℃ and tempered at a constant rate of 150℃ / h to 720±20℃; when the total welding thickness is <4mm, it is held for 2h; when the total welding thickness is 4~6mm, it is held for 2.5h; when the total welding thickness is >6mm, it is held for 3h; during the cooling stage, when the furnace temperature is above 250℃, the cooling rate is ≤50℃ / h; after the furnace temperature drops below 250℃, it is taken out of the furnace and naturally air-cooled. S6. Post-repair performance verification and sealing surface finishing: Inspect the defects in the repaired area and the hardness of the sealing surface; after passing the inspection, perform finishing on the sealing surface, and the surface roughness Ra of the sealing surface after finishing is ≤1.6μm.

2. The method for gradient welding repair of deep hole sealing surfaces of power plant valves according to claim 1, characterized in that, In step S1, penetrant testing is used to identify open-type defects on the sealing surface, and ultrasonic testing is used to detect buried defects inside the sealing surface, as well as the depth and spatial orientation of the defects. The two sets of test data are fused to generate three-dimensional quantitative location data of the defects.

3. The method for gradient welding repair of deep hole sealing surfaces of power plant valves according to claim 1, characterized in that, In step S2, when the defect burial depth is ≤1.5mm, the radius of the bevel root is 3mm and the inclination angle of the bevel sidewall is 8°; when the defect burial depth is >1.5mm, the radius of the bevel root is 5mm and the inclination angle of the bevel sidewall is 12°; after the bevel processing is completed, the bottom of the bevel is inspected using the dye penetrant testing method.

4. The method for gradient welding repair of deep hole sealing surfaces of power plant valves according to claim 1, characterized in that, In step S4, the ratio of the weld thickness of the stainless steel transition buffer layer to the weld thickness of the cobalt-based wear-resistant layer is 1.2:1 to 1.5:

1. After the stainless steel transition buffer layer is welded, the interlayer temperature is checked. When the interlayer temperature is ≥250℃, the stainless steel main filler layer is welded.

5. The gradient welding repair method for deep hole sealing surfaces of power plant valves according to claim 1 or 4, characterized in that, In step S4, D802 cobalt-based alloy welding rods are dried at 300℃~350℃ for 1 hour before use, and austenitic stainless steel welding rods are dried at 250℃~300℃ for 1 hour before use; the dried welding rods are placed in an insulation cylinder, and the temperature inside the insulation cylinder is maintained at 150℃~200℃.

6. The method for gradient welding repair of deep hole sealing surfaces of power plant valves according to claim 5, characterized in that, In step S3, the furnace temperature fluctuation during the heat preservation process is controlled within ±10℃; the thickness of the insulation cotton covering the non-welding area of ​​the valve body is ≥20mm.

7. The gradient welding repair method for deep hole sealing surfaces of power plant valves according to claim 6, characterized in that, In step S5, the air velocity in the air-cooled environment is ≤2m / s; after the valve is cooled to room temperature, the residual stress on the surface of the repair area is detected by the blind hole method, and the peak value of the residual stress is ≤150MPa.

8. The method for gradient welding repair of deep hole sealing surfaces of power plant valves according to claim 1, characterized in that, In step S6, after the sealing surface is finished, the contour accuracy of the sealing surface is checked by the replication putty method; at least 8 detection points are evenly distributed along the circumference of the sealing surface, and the detected contour deviation is ≤ ±0.05mm.

Images