A method of welding a short-bead tube sheet of a steam generator

Abstract

This invention relates to the field of nuclear power welding technology, specifically disclosing a method for dissimilar steel cladding on short straight-edge tube sheets of a steam generator. The method includes the following steps: Step 1: Setting up a large-plane cladding welding station on the tube sheet; Step 2: Preheating the tube sheet; Step 3: Performing large-plane nickel-based cladding welding on region A using pulsed automatic TIG welding; Step 4: Performing stainless steel cladding welding on region C using pulsed automatic TIG welding, with a weld layer thickness of 8 mm and a slope angle of 58° to 62° below the weld layer, for a total of four layers; Step 5: Combining nickel-based cladding welding on regions B and D using pulsed automatic TIG welding, for a total of four layers, each layer being nickel-based cladding welded from bottom to top and then lap-welded with the stainless steel cladding layer; Step 6: Post-heating the tube sheet. This invention effectively increases the contact area between the stainless steel and nickel-based weld layers by setting the slope angle below the weld layer to 58° to 62°, ensuring the fusion quality of the dissimilar steels.

Description

Technical Field

[0001] This invention relates to the field of nuclear power welding technology, specifically to a method for dissimilar steel overlay welding of short straight-side tube sheets in steam generators. Background Technology

[0002] The steam generator is one of the main pieces of equipment in the primary loop of a nuclear power plant. Its primary side, due to prolonged contact with high-temperature coolants containing radioactivity and corrosive substances, requires a nickel-based weld overlay alloy or stainless steel corrosion-resistant layer on its surface. The main body of the primary side of the Hualong One ZH-65 steam generator consists of a tube sheet assembly 1 and a lower head assembly 2. The inner surface of the tube sheet is welded with a nickel-based corrosion-resistant layer, while the inner wall of the head is welded with a stainless steel corrosion-resistant layer. Therefore, there is a dissimilar steel interface 3 between the nickel-based stainless steel weld overlay layer and the stainless steel weld overlay layer on the inner wall. Welding dissimilar steels is a critical and challenging aspect of welding; improper control can lead to defects in the interface area, threatening the safety of the nuclear power system. In existing technologies, because the straight edge section of the ZH-65 steam generator tube sheet is too short, to ensure the quality of the dissimilar steel overlap area, station changes and temporary accessory welding are required during the welding process, resulting in low tube sheet manufacturing efficiency. Summary of the Invention

[0003] In order to overcome the problems existing in the prior art, the purpose of this invention is to provide a method for dissimilar steel overlay welding of short straight-side tube sheets of steam generators.

[0004] The technical solution adopted by this invention to solve its technical problem is: a method for dissimilar steel overlay welding of short straight-side tube sheets of a steam generator, comprising the following steps: Step 1: Set up the tube sheet large flat surface welding station, and place the tube sheet horizontally on the rotary platform with the large flat surface facing upwards; Step 2: Preheat the tube sheet to 160-225℃; Step 3: Large-area nickel-based overlay welding is performed on region A using pulsed automatic TIG welding process; Step 4: Stainless steel overlay welding is performed on area C using pulsed automatic TIG welding process. The thickness of the overlay layer is 8mm, and the angle of the slope below the overlay layer is 58° to 62°. A total of four layers are overlaid. Step 5: Use pulsed automatic TIG welding process to combine nickel-based overlay welding in areas B and D, with a total of four layers. Each layer is nickel-based overlay welding in a bottom-up order and is lap-welded with the stainless steel overlay layer. Step 6: Perform post-heating on the tube sheet at a temperature of 250-400℃ for at least 4 hours. Preferably, in the fourth step, the stainless steel surfacing is performed using automatic pulse TIG welding in a horizontal manner, using a 1.2mm diameter welding wire, with a single-pass surfacing layer width of 10mm and a single-layer surfacing thickness of approximately 2.5mm.

[0005] Preferably, in the fourth step, the bottom width of the weld overlay is 26mm, and the top width of the weld overlay is not less than 6mm after machining.

[0006] Preferably, in the fourth step, the arrangement of the stainless steel weld overlay is as follows: the first layer has a weld overlap of 6mm and a total of 5 weld overlays; the second layer has an interlayer offset of 4mm and a weld overlap of 5mm, and a total of 3 weld overlays; the third layer has an interlayer offset of 4mm and a weld overlap of 5mm, and a total of 2 weld overlays; the fourth layer has an interlayer offset of 4mm, a total of 2 weld overlays, and a weld overlap of 8mm.

[0007] Preferably, each stainless steel weld overlay is welded in a bottom-up order.

[0008] As a preferred option, the first layer is a weld overlay transition layer of ER309L, and the second to fourth layers are weld overlay corrosion-resistant layers of ER308L.

[0009] Preferably, in the fifth step, automatic pulse TIG welding is used for nickel-based overlay welding, and 1.2mm diameter ERNiCrFe-7A welding wire is used for horizontal welding.

[0010] Compared with the prior art, the beneficial effects of the present invention are: This invention effectively increases the contact area between the stainless steel and nickel-based weld overlays by setting the angle of the inclined surface below the weld overlay to 58° to 62°. This design allows for better filling and fusion of the molten pool in the horizontal welding position, reducing welding defects such as incomplete fusion and thus significantly improving the welding quality of dissimilar steel joints. Simultaneously, this angle ensures that the top width of the weld overlay, after machining, is not less than 6mm when the straight edge section is too short. Figure 4 The b-value ensures that after the tube sheet and end cap are assembled, the stainless steel surfacing in the circumferential seam area will not melt into the nickel-based surfacing layer, thus preventing welding defects. In traditional surfacing methods, due to the limited operating space of short straight-edge tube sheets, it is often necessary to switch between different workstations to complete the surfacing of each area. This not only increases the number of operation steps, reduces manufacturing efficiency, increases the consumption of manpower, material resources, and time, and occupies equipment resources for a long time, but also increases the quality risks during workstation changes and multiple lifting processes. However, the method proposed in this claim optimizes the surfacing layer design, enabling the tube sheet to complete the surfacing of all areas in one go in an integrated workstation without the need for workstation changes, thereby reducing the manufacturing steps from the original fifteen steps to six steps, and significantly improving the manufacturing efficiency of the tube sheet. Attached Figure Description

[0011] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1 This is a schematic diagram of the tube sheet assembly and lower head assembly of a steam generator in the prior art. Figure 2 This is a simplified cross-sectional view of the weld overlay of a tube sheet assembly in the prior art; Figure 3 This is a simplified cross-sectional view of the tube sheet assembly weld overlay in this invention; Figure 4 This is a simplified diagram of the welding overlay scheme and dimensional design for region C in this invention; Figure 5 This is a simulated finished product image of the tube sheet assembly after welding is completed in this invention.

[0013] 1. Tube sheet assembly; 2. Lower head assembly; 3. Circumferential joint between tube sheet and lower head. Detailed Implementation

[0014] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. Many specific details are set forth in the following description to provide a thorough understanding of the present invention; the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0015] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0016] In the steam generators of nuclear power plants, the primary-side equipment is in long-term contact with high-temperature coolants containing radioactivity and corrosiveness, thus requiring extremely high corrosion resistance. To meet this requirement, the inner surface of the primary side of the steam generator needs to be overlaid with a nickel-based alloy or stainless steel as a corrosion-resistant layer. Taking the Hualong One ZH-65 evaporator as an example, its primary-side main structure consists of two parts: a tube sheet assembly and a lower head assembly. Figure 1As per design requirements, a nickel-based corrosion-resistant layer is welded onto the inner surface of the tube sheet, and a stainless steel corrosion-resistant layer is welded onto the inner surface of the lower head. This results in a dissimilar steel interface between the nickel-based stainless steel weld overlay and the stainless steel weld overlay in the connection area between the tube sheet and the lower head.

[0017] Dissimilar steel welding is a technical challenge in the welding field because different types of steel differ in chemical composition, physical properties, and metallurgical behavior. Defects such as lack of fusion and cracks can easily occur at the interface during welding, seriously affecting the safe operation of nuclear power systems. To avoid dissimilar steel cladding during the final assembly stage (i.e., in the enclosed and heated high-temperature environment of the equipment), the dissimilar steel welding of nickel-based and stainless steel needs to be completed during the tube sheet component stage of manufacturing.

[0018] Based on the welding characteristics of stainless steel and nickel-based materials, to ensure the quality of the weld overlay, only nickel-based weld overlays with high Cr-Ni content are allowed to overlap onto stainless steel weld overlays with low Cr-Ni content; stainless steel weld overlays are not allowed to overlap onto nickel-based weld overlays, otherwise numerous welding defects will occur. Therefore, in the manufacturing of the tube sheet for the ZH-65 steam generator, the welding sequence for dissimilar steels was determined to be: first, weld the stainless steel on the upper part of the straight edge section of the tube sheet, and then weld the nickel-based material on the lower part.

[0019] However, in actual welding operation experience, regarding Figure 2 As shown in the diagram, the weld overlay position on the straight edge of the tube sheet is a horizontal weld. In this position, the lower weld overlay is usually welded first, and the upper weld overlay is then used to overlap the lower weld overlay to better ensure operational accessibility and fusion quality. However, in this case, based on the background description above, the properties of stainless steel and nickel-based materials require that the upper stainless steel layer be welded first. Figure 2 Area c), then weld the lower nickel base ( Figure 2 In area B), the lower nickel-based weld overlay is used to overlap the upper stainless steel weld overlay. Furthermore, the straight edge section of the tube sheet of the ZH-65 steam generator is designed to be too short (less than 40mm). The design requirements do not allow the stainless steel weld overlay to extend too far into the arc section of the tube sheet, resulting in insufficient space for overlapping dissimilar steels.

[0020] This led to Figure 2 In the working position, the dissimilar steels are first welded in area C and then in area B. The welding operation space and angle are severely restricted. At the same time, due to the short straight section and the three-phase interface of nickel-based, stainless steel and low alloy steel base materials, the welding quality of the dissimilar steel joint is difficult to guarantee.

[0021] To address this issue, existing technologies employ a workstation switching solution, that is, in Figure 2 After the large flat surface is welded at the workstation, the tube sheet is hoisted onto the positioner to make it vertical (i.e., Figure 2(The workstation shown is rotated 90° to ensure that area C + area D are in a flat welding position for welding, thus solving the problem of limited operating space. The specific process flow is as follows:) Step 1: Preheating. Preheat the tube sheet (welding area of ​​the connecting plate) to reduce thermal stress during the welding process.

[0022] Step 2: Welding connecting plates. Weld 8 connecting plates around the tube sheet to connect with the positioner during subsequent station changes.

[0023] Step 3: Post-heat treatment, performing post-heat treatment on the tube sheet (welding area of ​​the connecting plate). Step 4: Inspection of the weld seams of the connecting plate. Perform flaw detection on the weld seams of the connecting plate to ensure welding quality.

[0024] Step 5: Set up the welding station. Place the tube sheet horizontally on the rotary platform with the large flat surface facing up, ready for welding.

[0025] Step 6: Preheating, heating the entire tube sheet. Step 7: Large-plane nickel-based overlay welding, using automated TIG welding process on the large-plane surface of the tube sheet ( Figure 2 (Area A) Nickel-based corrosion-resistant layer is welded on.

[0026] Step 8: Nickel-based overlay welding on the arc segment: Electrode welding (EMF) is used on the arc segment of the tube sheet (…). Figure 2 (D region) surfacing a nickel-based corrosion-resistant layer.

[0027] Step 9: Post-heating, perform overall post-heating on the tube sheet. Step 10: Workstation conversion: The tube sheet is hoisted onto the positioner and positioned in a vertical position for subsequent welding.

[0028] Step 11: Preheating: Heating the straight edge section of the tube sheet. Step 12: Welding stainless steel onto the straight edge section: On the straight edge section of the tube sheet ( Figure 2 In the C area, a stainless steel corrosion-resistant layer is deposited using shielded metal arc welding (SMAW).

[0029] Step 13: Nickel-based overlay welding in the transition area: in the transition area between the arc section and the straight edge section of the tube sheet ( Figure 2 (Central B area) Manual welding of nickel-based corrosion-resistant layer using welding rods.

[0030] Step 14: Post-heating Step 15: Grind and remove the connecting plate. Perform post-heat treatment on the tube sheet after welding, grind and remove the connecting plate, and finally inspect and detect defects.

[0031] The existing process described above requires station changes and welding of temporary accessories (connecting plates), increasing the workload of welding material baking, preheating, tube sheet hoisting, connecting plate welding, inspection, and removal. This leads to reduced tube sheet manufacturing efficiency, long-term resource occupation of the positioner, and increased quality risks during station changes and tube sheet hoisting. Furthermore, the cumbersome operation steps increase the risk of errors and manufacturing costs.

[0032] Example 1 Therefore, this embodiment discloses a dissimilar steel overlay welding method for short straight-side tube sheets of steam generators, which does not require changing work stations, such as... Figures 3-5 As shown, the detailed steps are as follows: Step 1: Set up the tube sheet large-plane welding station. Place the tube sheet horizontally and stably on the rotary platform with the large-plane side facing up, ensuring the tube sheet remains stable during the welding process and avoiding any impact on weld quality due to movement. Simultaneously, adjust the rotation speed and direction of the rotary platform to meet the requirements of the subsequent pulsed automatic TIG welding process.

[0033] Step 2: Preheat the entire tube sheet. Use heating equipment to preheat the entire tube sheet, strictly controlling the preheating temperature between 160-225℃. The purpose of preheating is to reduce thermal stress generated during welding, prevent cracking, and improve the plasticity and toughness of the weld joint. Uniform heating is required during preheating to avoid localized overheating or insufficient heating.

[0034] Step 3: Perform large-plane nickel-based cladding on region A (region A is a large flat surface) using pulsed automatic TIG welding process. Select a suitable nickel-based welding wire (such as ERNiCrFe-7A), and set the welding parameters of the automatic TIG welding machine, including peak current, base current, welding speed, and wire feed speed. These welding parameters can be adjusted by those skilled in the art based on their experience, and do not require further explanation.

[0035] Step 4: Stainless steel surfacing is performed on region C (region C is the straight edge section) using pulsed automatic TIG welding. Before surfacing, based on design requirements and numerous welding test results, a weld bead layout is designed to ensure the bottom width of the weld bead layer is 26mm. Figure 4 In section a of d, the design requirement is that it should not exceed 26mm, and the top of the weld overlay should be no less than 6mm after machining. Figure 4 segment b in d, Figure 4 The dashed line in d represents the machining line. The angle of the slope below the weld overlay is designed to be 58° to 62°. Figure 3The mid-angle α is used to ensure the lap quality of subsequent nickel-based weld overlays. A suitable stainless steel welding wire is selected, and an automatic pulse TIG welding process is employed for weld overlay at the horizontal welding position. Using a 1.2mm diameter welding wire, four layers are welded by precisely controlling the width (10mm) and thickness (approximately 2.5mm) of each weld overlay, resulting in a total weld overlay thickness of 8mm after machining (design requirement). During the weld overlay process, the interlayer offset and weld overlap are precisely controlled to ensure that the dimensions of the stainless steel weld overlay meet the above requirements, thereby ensuring good fusion between weld overlays and between the weld overlay and the base material, without defects such as incomplete fusion or cracks.

[0036] Step 5: Apply pulsed automatic TIG welding to both regions B (the transition section) and D (the arc section) for nickel-based cladding. This process is also performed using automatic pulsed TIG welding at the horizontal welding position. The final simulated part cross-section after all regions have been clad is shown below. Figure 5 As shown.

[0037] The same nickel-based welding wire (ERNiCrFe-7A) as that used in area A was selected, and four layers were welded in total. Each layer was welded in a bottom-up order and then lapped with the stainless steel layer that had already been welded.

[0038] During the welding process, welding parameters and welding sequence are strictly controlled to ensure good fusion between the nickel-based weld overlay and the stainless steel weld overlay, as well as between the layers of the nickel-based weld overlay, to form a high-quality dissimilar steel joint.

[0039] Step 6: Perform post-heat treatment on the entire tube sheet. Immediately after the weld overlay is completed, perform post-heat treatment on the tube sheet to eliminate residual stress generated during welding and improve the weld microstructure. The post-heat treatment temperature should be controlled between 250-400℃, and the duration should be no less than 4 hours. During the post-heat treatment, the temperature should be kept uniformly rising and falling to avoid damage to the tube sheet caused by rapid heating and cooling. After the post-heat treatment, allow the tube sheet to cool naturally to room temperature, and then perform necessary inspections and tests to ensure that the weld overlay quality meets design requirements.

[0040] By setting the angle of the slope below the weld overlay to 58° to 62°, the contact area between the stainless steel weld overlay and the nickel-based weld overlay is effectively increased. This design allows the molten pool to fill and fuse better in the horizontal welding position, reducing welding defects such as incomplete fusion, thereby significantly improving the welding quality of dissimilar steel joints.

[0041] In some optional embodiments, in the fourth step, automatic pulsed TIG welding is used for horizontal welding of the stainless steel, using a 1.2mm diameter welding wire, and the pulsed TIG welding parameters control the width of a single weld layer to 10mm. Figure 4(Section a, c) The single-layer weld thickness is approximately 2.5mm, ensuring the uniformity and quality of the weld layer. Controlling the single-layer thickness at 2.5mm using pulsed TIG welding parameters is the optimal choice derived from extensive practice and research, facilitating comprehensive weld quality control. When the single-layer thickness is too small, increasing the number of weld layers is necessary to achieve the required total weld thickness. This not only significantly reduces welding efficiency, increases production costs and cycle time, but also increases the workload and difficulty of interlayer processing and quality control due to the excessive number of layers. Conversely, when the single-layer thickness is large, the increased volume of the molten pool at the weld overlap slows down the cooling rate, easily leading to incomplete fusion of the molten pool metal, resulting in incomplete fusion defects that severely affect the mechanical properties and corrosion resistance of the weld. Therefore, controlling the single-layer thickness at 2.5mm effectively avoids incomplete fusion defects while ensuring welding efficiency, ensuring the weld quality meets requirements. The welding parameters are shown in the table below.

[0042]

[0043] In some optional embodiments, in the fourth step, the bottom width of the weld overlay is set to 26 mm. Figure 4 Section A), the top of the weld overlay layer after machining should be no less than 6mm ( Figure 4 (Section b). The wider the bottom of the weld overlay, the better it is for the arrangement of the stainless steel weld overlay and the dimensional control after the weld is completed. This can ensure the uniformity and consistency of the weld overlay, thus providing a good foundation for the subsequent overlapping of the nickel-based weld overlay and ensuring the quality of the overlap. However, the maximum width of the stainless steel bottom allowed by the design is 26mm. Therefore, this invention aims to maximize the bottom width of the weld overlay while meeting the design requirements and improving the welding quality.

[0044] In some optional embodiments, in step four, after the weld overlay thickness is machined to 8mm, the top width of the weld overlay is at least 6mm. This is because during subsequent assembly, the area reserved at the top of the straight section of the tube sheet needs to be manually welded for stainless steel weld overlay. During manual welding, due to the influence of human operation, direct contact with the nickel-based weld overlay will generate a large number of PT defects, affecting the weld quality. Therefore, reserving a width of at least 6mm for the top stainless steel weld overlay provides a safe operating space for manual welding, avoids direct contact with the nickel-based weld overlay, and thus reduces the generation of PT defects.

[0045] In some alternative embodiments, in the fourth step, Figure 3The angle α directly affects the welding quality of dissimilar steel areas during subsequent nickel-based surfacing. Multiple process tests have shown that when α is less than 40°, nickel-based surfacing lap joints are impossible because the molten pool cannot fill and fuse properly. When α is between 40° and 58°, although surfacing lap joints are possible, incomplete fusion defects will occur at the interface between the nickel-based and stainless steel dissimilar steels due to poor fusion of the molten pool, compromising weld quality. When α is greater than 62°, the width of the top surfacing layer will be less than 6mm, failing to meet the requirements for stainless steel surfacing operations after thick circumferential butt welds. Therefore, considering all these factors, the bottom bevel angle α is designed to be between 58° and 62° to ensure welding quality and ease of operation.

[0046] In some optional embodiments, in the fourth step, the layout of the stainless steel weld overlay is designed as follows: during the first layer of weld overlay, the weld overlap is 6mm ( Figure 4 (Section e in section a), a total of 5 weld layers; during the second weld layer, the interlayer offset is 4mm ( Figure 4 (Section d in b), the weld overlap is 5mm, and a total of 3 welds are made; the third layer has an interlayer offset of 4mm, a weld overlap of 5mm, and a total of 2 welds; the fourth layer has an interlayer offset of 4mm, a total of 2 welds, and a weld overlap of 8mm. (Note:) Figure 4 In the middle, 'e' represents the weld overlap, which is only applied when... Figure 4 The annotation in 'a' (first layer) is for illustrative purposes only; subsequent layers are not annotated again. 'd' represents the inter-layer offset, and this value is only used in... Figure 4 (b (second layer) is an illustration; subsequent layers are not labeled again.) Among them, such as Figure 4 As shown, the first layer ( Figure 4 a) The overlap is set at 6mm to ensure that the total width after the first layer of welding reaches 26mm. This width design is beneficial to the size control of the subsequent stainless steel welding layer and the overlap quality of the nickel-based welding layer. By welding 5 layers with an overlap of 6mm between each layer, the entire welding area can be evenly covered, ensuring the uniformity of the width and thickness of the bottom layer.

[0047] Second floor ( Figure 4 b) The interlayer offset is 4mm, the weld overlap is 5mm, and a total of 3 welds are made, resulting in a total second layer width of 20mm: For the second layer, the interlayer offset is set to 4mm to control the slope angle below the weld layer to approximately 60°, ensuring the quality of the weld layer after the subsequent nickel-based overlap. The weld overlap is set to 5mm, with a total of 3 welds, resulting in a total second layer width of 20mm. This width design facilitates the welding of subsequent layers and ensures a good transition with the first weld layer. Additionally, since 308 stainless steel cannot be directly welded to the base material, a distance of at least 2mm must be maintained between the 309 stainless steel and its edge. Figure 4(Section b in section f) This requirement can be met by welding three times with a 5mm overlap.

[0048] The third floor ( Figure 4 c) The interlayer offset is 4mm, the weld overlap is 5mm, and two passes are welded, resulting in a total width of 15mm for the third layer. During the third layer weld, the interlayer offset remains at 4mm to control the angle of the slope below the weld layer. The weld overlap is still 5mm, but two passes are welded, making the total width of the third layer 15mm. This width design is based on the first two layers of weld and also considers the ease of operation for the subsequent fourth layer weld.

[0049] Fourth floor ( Figure 4 d) 4mm interlayer offset, two weld passes, 8mm overlap: For the fourth layer, the interlayer offset remains 4mm, with two weld passes. The overlap is increased to 8mm because the third layer is only 15mm wide, making the welding operation more difficult. After the first weld pass, the weld thickness may meet the requirements, but because the molten pool is drooping at the horizontal weld position, the pool is not full enough. Therefore, an additional weld pass is needed with an 8mm overlap to ensure that the weld width after machining reaches the minimum requirement of 6mm. This design ensures that the final dimensions and quality of the weld layer meet the design requirements.

[0050] In some optional embodiments, during the surfacing operation at the horizontal welding position, the molten pool will sag due to gravity. If a top-down surfacing sequence is used, incomplete fusion defects are easily generated at the overlap during the interpassing process due to the molten pool sag, severely affecting the welding quality. To avoid this problem, the present invention designs the surfacing sequence of each region to be from bottom to top. This sequence allows each surfacing pass to complete the overlap with the next pass before the molten pool sags, effectively reducing the generation of incomplete fusion defects. At the same time, considering the characteristic of the molten pool sags at the horizontal welding position, pulsed TIG welding has significant advantages over constant current TIG welding. Pulsed TIG welding can more precisely control the shape and temperature of the molten pool by adjusting the pulse parameters, thereby better dealing with the molten pool sag problem and ensuring the stability of the welding process.

[0051] In some optional embodiments, the first layer is a 309L transition layer, and the second to fourth layers are 308L corrosion-resistant layers. The choice of a 309L transition layer for the second layer is based on the base material. The base material is low-alloy steel, and its chemical composition differs significantly from stainless steel. If 308L is directly welded onto the base material, the base material's dilution effect on the Cr-Ni content in the 308L welding material would make it difficult to achieve the required Cr-Ni content for the corrosion-resistant layer, thus affecting the corrosion resistance of the weld layer. 309L welding material has a higher Cr-Ni content; welding a 309L layer first as a transition layer effectively reduces the dilution of the base material on the subsequent 308L welding layer, ensuring good corrosion resistance. 308L and 309L welding materials differ in physical properties; 308L has greater fluidity than 309L. During the surfacing process, in order to ensure the quality of the weld bead formation and maintain the same width and thickness as the 308 weld bead, a lower current is required compared to when surfacing 309. A lower current reduces the welding heat input, slows down the melting rate of the 308 welding material, and thus allows for better control of the shape and size of the molten pool, resulting in a more aesthetically pleasing weld bead while ensuring the dimensional accuracy of the weld bead.

[0052] In some optional embodiments, in step five, automatic pulsed TIG welding is used for nickel-based surfacing, employing 1.2mm diameter ERNiCrFe-7A welding wire for horizontal welding. Again, automatic pulsed TIG welding is used, with pulsed current controlling the molten pool to improve welding quality in the horizontal welding position. Using 1.2mm diameter ERNiCrFe-7A welding wire is suitable for the surfacing requirements of nickel-based alloys, ensuring good fusion between the nickel-based and stainless steel surfacing layers. The surfacing parameters are as follows (table parameters are provided for nickel-based surfacing, but not for stainless steel surfacing; consistency is recommended).

[0053]

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for dissimilar steel overlay welding of a short straight-side tube sheet for a steam generator, characterized in that, Includes the following steps: Step 1: Set up the tube sheet large flat surface welding station, and place the tube sheet horizontally on the rotary platform with the large flat surface facing upwards; Step 2: Preheat the tube sheet to 160-225℃; Step 3: Large-area nickel-based overlay welding is performed on region A using pulsed automatic TIG welding process; Step 4: Stainless steel overlay welding is performed on area C using pulsed automatic TIG welding process. The thickness of the overlay layer after machining is not less than 6mm, and the angle of the slope below the overlay layer is 58° to 62°. A total of four layers are overlaid. Step 5: Use pulsed automatic TIG welding process to combine nickel-based overlay welding in areas B and D, with a total of four layers. Each layer is nickel-based overlay welding in a bottom-up order and is lap-welded with the stainless steel overlay layer. Step 6: Perform post-heating on the tube sheet at a temperature of 250-400℃ for at least 4 hours.

2. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 1, characterized in that, In the fourth step, the stainless steel surfacing is performed using automatic pulse TIG welding in a horizontal manner, with a 1.2mm diameter welding wire, a single pass surfacing layer width of 10mm, and a single layer surfacing thickness of approximately 2.5mm.

3. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 1, characterized in that, In the fourth step, the bottom width of the weld overlay is 26mm, and the top width of the weld overlay after machining is not less than 6mm.

4. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 1, characterized in that, In the fourth step, the arrangement of the stainless steel weld overlay is as follows: four layers are welded in total. When welding the first layer, the weld overlap is 6mm, and a total of 5 welds are welded. When welding the second layer, the interlayer offset is 4mm, the weld overlap is 5mm, and a total of 3 welds are welded. When welding the third layer, the interlayer offset is 4mm, the weld overlap is 5mm, and a total of 2 welds are welded. When welding the fourth layer, the interlayer offset is 4mm, a total of 2 welds are welded, and the weld overlap is 8mm.

5. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 4, characterized in that, Each stainless steel weld overlay is welded in a bottom-up sequence.

6. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 4, characterized in that, The first layer is a weld overlay transition layer of ER309L, and the second to fourth layers are weld overlay corrosion-resistant layers of ER308L.

7. The dissimilar steel overlay welding method for short straight-side tube sheets of a steam generator according to claim 1, characterized in that, In the fifth step, automatic pulse TIG welding is used for nickel-based overlay welding, and 1.2mm diameter ERNiCrFe-7A welding wire is used for horizontal welding.

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