Butt welding method for boiler pipes

Abstract

The invention relates to the technical field of boiler pipe welding, in particular to a boiler pipe butt welding method which comprises the following steps: S1, pretreating the end part and the outer surface of a to-be-welded boiler pipe by adopting a dry-wet alternate cleaning method, and during operation, firstly performing dry brushing, then wiping with acetone, then polishing with abrasive paper and finally cleaning with chamois leather cloth; s2, a sawtooth-shaped composite groove is machined; s3, assembling and positioning the boiler pipes; s4, the boiler pipe is preheated through spiral flame surrounding; s5, backing welding is completed in a double-arc alternating arc-breaking welding mode during welding, and filling welding is carried out; s6, after filling welding is completed, cosmetic welding is conducted; s7, after cosmetic welding is completed, interlayer cleaning of the welding layers is completed through knocking vibration; and S8, after final welding is completed, a plurality of layers of wet towels are used for wrapping, heat preservation and slow cooling, and welding seams are checked after cooling. The method is compatible with conventional equipment, special tool investment is not needed, the small-space operation requirement of multiple rows of boiler pipes can be met, and the welding process is low in threshold, small in difficulty and suitable for rapid mastering of welders.

Description

Technical Field

[0001] The present invention relates to the technical field of boiler tube welding, and in particular to a butt welding method for boiler tubes. Background Art

[0002] As a core component of the pressure-bearing heating surface, the welding quality of boiler tubes directly determines the safety and operation reliability of boiler equipment. At present, pipeline butt welding mainly adopts two methods: manual arc welding and automatic welding equipment.

[0003] Although there are some rotary mechanical welding equipment in the prior art that can achieve circumferential welding, their applicability is extremely poor in the welding scenario of multi-row boiler tubes.

[0004] For example, the automatic welding equipment disclosed in the Chinese patent with the patent application number CN202220287111.5, although it uses a multi-axis robotic arm to control the movement of the welding torch, in the welding of multi-row boiler tubes, due to the complex welding torch transmission mechanism and rotating bracket structure of such rotary mechanical welding equipment, the required turnover space during operation is large, while multi-row boiler tubes usually adopt a side-by-side and spaced layout, and the actual tube spacing is small. It is difficult for the above-mentioned mechanical equipment to move and position flexibly among the dense tube rows, and it is impossible to effectively avoid interference between tubes. Problems such as frequent collision of the welding torch and arc deviation often occur during the welding process. Therefore, at present, manual welding is still mainly used for the butt welding of boiler tubes.

[0005] Although the manual welding method in the prior art has a certain degree of flexibility, there are often serious quality problems in many cases. When a welder performs the welding of multi-row boiler tubes, on the one hand, affected by the visual field blind area, body position and fatigue effect, it is difficult to accurately control the angle of the welding torch and the welding speed, resulting in large fluctuations in the weld penetration and reinforcement dimensions, and defects such as lack of fusion and porosity are likely to occur; in addition, the operation levels of different welders vary significantly, and the welding quality has a large discreteness, and it is difficult to balance the quality.

[0006] Therefore, it is of great practical significance to develop a control method for improving the quality of manual welding applicable to multi-row boiler tubes. Summary of the Invention

[0007] One of the technical solutions adopted by the present invention to solve the above technical problems is: a butt welding method for boiler tubes, including the following steps:

[0008] S1, pre-treat the end and outer surface of the boiler tube to be welded by using the dry-wet alternating cleaning method. During operation, first dry-brush, then wipe with acetone, then sand and finally clean with a suede cloth.

[0009] S2, machine a serrated composite groove.

[0010] S3, assemble and position the boiler tube group.

[0011] S4. After positioning, preheat the boiler tube by means of spiral flame surrounding.

[0012] S5. During welding, carry out backing welding by means of double-arc alternating interrupted arc welding, and then carry out filling welding.

[0013] S6. After filling welding is completed, carry out capping welding.

[0014] S7. After capping welding is completed, complete the cleaning between welding layers by means of knocking and vibration.

[0015] S8. After final welding is completed, wrap it with multiple layers of wet towels for heat preservation and slow cooling, and check the weld after cooling.

[0016] Based on any of the above technical solutions, the further optimization is: during the process of grinding with sandpaper, the sandpaper grinding direction is along the circumferential direction of the end of the boiler tube, so as to form uniform wire drawing patterns on the surface.

[0017] Based on any of the above technical solutions, the further optimization is: the forming method of the serrated composite groove is as follows: first, process a V-shaped groove with a groove angle of 60°-70°.

[0018] Process serrated structures on both side walls of the V-shaped groove. During operation, use a file to file along the side wall of the groove with relatively uniform force and rhythm. First, on one side wall of the groove; file out serrations in sequence according to the set spacing and depth. After completing the operation on one side, process the other side wall of the groove in the same way; during the filing process, continuously measure the spacing and depth of the serrations with a caliper.

[0019] Based on any of the above technical solutions, the further optimization is: the spacing of each serration is 2-3 mm, the depth is controlled within 0.5-1 mm, the serration shape is a right trapezoid, its hypotenuse faces the center of the groove, and the right angle side is perpendicular to the side wall of the groove.

[0020] Based on any of the above technical solutions, the further optimization is: the specific method for preheating the boiler tube by means of spiral flame surrounding is as follows: adjust the acetylene-oxygen flame to a neutral flame, ensure that the flame burns stably and the temperature is moderate, and avoid local overheating or oxidation of the boiler tube due to improper flame properties.

[0021] Start heating from one end of the part of the boiler tube to be welded, and the flame moves around the tube body from the outside to the inside in a spiral trajectory, and the spiral spacing is kept at 10-15 mm.

[0022] During the heating process, the distance between the flame nozzle and the tube end is controlled within 5-10 mm to ensure that the flame heat is effectively transferred to the surface of the boiler tube.

[0023] Based on any of the above technical solutions, further optimization is: during the preheating process, the color change of the tube wall of the boiler tube is observed. When the tube wall appears dark red, it indicates that the temperature is close to or reaches the preheating target temperature of 150℃-200℃.

[0024] At the same time, during the circumferential heating process, a relatively uniform moving speed of about 1-2 cm per second is maintained to ensure uniform heating in the circumferential direction of the boiler tube.

[0025] After completing each spiral heating cycle, the heating process needs to be repeated 1-2 times to further ensure the temperature uniformity and sufficient preheating of the entire welding area, reduce welding stress, and reduce the probability of welding crack defects.

[0026] Based on any of the above technical solutions, further optimization is as follows: the specific steps of using double arc alternating arc breaking welding are as follows: Step 1, differentiated pretreatment of welding rods: select two welding rods with a diameter of 2.5 mm, marked as welding rod A and welding rod B.

[0027] Step 2, start the arc with electrodes A and B in steps: Start the arc with electrode A: Start the arc at the positioning welding point with a current of 70A, using the two-point positioning method of starting the arc first on the groove side and then on the root.

[0028] Arc striking with electrode B: After electrode A extinguishes the arc, strike an arc at the junction of the edge of the molten pool and the groove when the molten pool is in a semi-solid state. Use a 75A welding current to ignite electrode B, and quickly send the arc to the molten pool that has not been completely solidified, so that the new arc and the old molten pool are fully fused to form a molten hole again.

[0029] Step 3: During the welding process, both electrodes are moved in a composite swinging manner that combines spiral motion with zigzag swinging.

[0030] In step 4, the two electrodes perform welding operations alternately at a specified frequency.

[0031] Step 5: When approaching the end of the weld, use the current decay method to end the arc.

[0032] Based on any of the above technical solutions, the steps of filling welding are further optimized as follows: Welding preparation: After the base welding is completed, use a wire brush to clean the slag on the weld surface and a flat shovel to remove spatter; select a welding rod with a diameter of 3.2mm that matches the boiler tube; adjust the welding current to 90-100A and the arc voltage to 22-24V.

[0033] Starting welding: At one end of the weld, align the welding rod with the weld, with the tip 2-3mm away from the bottom, scratch or strike to start the arc, pull it to the bottom and stay for 0.8-1.2 seconds to form a molten pool; the welding rod is 90° to the axis of the pipe and 70°-80° to the welding direction, and maintain short arc operation.

[0034] Step - by - step swing operation: The welding rod makes a step - by - step swing, staying at both sides for 1 - 1.5 seconds to ensure fusion, moving quickly in the middle to prevent the weld bead from being too high, and moving forward 2 - 3 mm after each swing; Observe the molten pool during welding. If it collapses, reduce the current or shorten the staying time; If the fusion is poor, increase the staying time and adjust the angle of the welding rod.

[0035] Filling and finishing: Continuously fill until it is 1 - 2 mm below the surface of the base metal; When approaching the end, reduce the current in stages 3 - 5 cm in advance, reducing 5A - 10A each time, staying in the molten pool for 1 - 2 seconds more to fill the crater, and clean the slag after removing the welding rod.

[0036] Based on any of the above - mentioned technical solutions, a further optimization is as follows: The specific steps for cleaning between weld layers by knocking and vibrating are as follows: After welding is completed, wait for the weld to cool to 60 - 80 °C.

[0037] Tap the two sides of the weld with a small hammer at a frequency of 2 - 3 times per second, and the distance between the tapping points is 5 - 8 mm.

[0038] Move the tapping in a spiral shape along the weld direction, and the spiral diameter is about 20 mm.

[0039] After tapping, use a wire brush to brush off the loose slag unidirectionally along the weld direction.

[0040] After finding stubborn slag, bake it with a flame until it turns dark red and then quench it suddenly to peel it off using thermal expansion and contraction.

[0041] Based on any of the above - mentioned technical solutions, a further optimization is as follows: The specific steps for capping welding are as follows: After the fill welding and cleaning are qualified, select a 3.2 mm low - hydrogen type welding rod, and adjust the current to 85 - 95 A according to the state of the fill weld.

[0042] Arc strike on the base metal 10 - 15 mm in front of the starting end of the weld. After stabilization, pull it to the starting point. The welding rod forms an angle of 75° - 85° with the welding direction and is perpendicular to the pipe axis. Start welding with a short arc to ensure fusion.

[0043] Weld by manipulating the welding rod. Move quickly in the center of the weld, press down and pause at both sides for 0.5 - 1 second and then lift up, controlling the width of the weld to exceed the groove by 1 - 2 mm on each side, and the reinforcement height is 0 - 3 mm.

[0044] When changing the welding rod, grind a 10 - 15 mm gentle slope at the arc - ending place. Strike an arc with the new welding rod 5 - 8 mm before the joint, pull it to the joint, pause for fusion and then continue welding.

[0045] At the end, reduce the current to 70% - 80%, make a circular swing to fill the crater and then extinguish the arc.

[0046] On the basis of any of the above technical solutions, the further optimization is as follows: The specific steps of heat preservation and slow cooling by wrapping with multiple layers of wet towels are as follows: Prepare a sufficient amount of pure cotton towels with good water absorption; Prepare warm water with the water temperature controlled at 40-50°C to ensure that the temperature is appropriate after the towels are soaked.

[0047] After the final welding is completed, soak the towels with warm water and wring them until no water drips. Starting from one end with the weld as the center, tightly wrap the first layer of towels to ensure that the weld is completely covered, and the layers of towels are closely fitted without gaps.

[0048] When the temperature of the first layer of towels drops to near the ambient temperature, quickly wrap the second layer, and repeat the operation until 3-4 layers of towels are wrapped.

[0049] Finally, wrap and seal the towels as a whole with plastic film to prevent the water from evaporating too quickly and slow down the heat dissipation rate.

[0050] During the heat preservation and slow cooling process, use an infrared thermometer to intermittently measure the weld temperature, record the temperature change, and control the weld cooling rate by increasing or decreasing the number of towel layers or adjusting the ambient temperature.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] 1. Through multi-stage pretreatment, the present invention thoroughly removes surface impurities and forms directional patterns, which can guide the uniform spreading of the deposited metal, reduce porosity and slag inclusions, and improve the fusion quality of the starting section of the weld.

[0053] 2. The present invention adopts a zigzag composite groove. By increasing the surface area and geometric diversion effect, the bonding force between the deposited metal and the base metal is enhanced, the molten pool flow is optimized, and defects such as undercut, weld bead, porosity and crack are reduced.

[0054] 3. Through spiral flame preheating, the present invention realizes uniform heating to the target temperature in the circumferential direction, reduces welding stress and hardening tendency, and reduces the risk of cold cracks.

[0055] 4. The present invention adopts double-arc alternating interrupted arc welding to promote molten pool deoxidation, refine grains, control the continuity and thermal cycle of the molten pool, improve the strength and toughness of the weld, and reduce defects such as lack of fusion and porosity.

[0056] 5. Through heat preservation and slow cooling with multiple layers of wet towels, the present invention slows down the cooling rate, promotes the transformation of austenite into ductile tissue, avoids hardened tissue and stress concentration; combined with knocking vibration and thermal and cold cycle interlayer cleaning, the slag is efficiently removed to ensure the interlayer fusion quality.

[0057] 6. The process method of the present invention is compatible with conventional equipment, does not require special tooling investment, and can meet the small-space operation requirements of multi-row boiler tubes. The welding process has a low threshold and small difficulty, which is suitable for welders to quickly master. Specific Embodiment

[0058] The embodiments of the technical solution of the present invention will be described in detail below. The following embodiments are only used to illustrate the technical solution of the present invention more clearly, so they are only examples and cannot be used to limit the protection scope of the present invention.

[0059] A butt welding method for boiler tubes disclosed by the present invention can significantly improve the welding quality of boiler tubes through overall process improvements including pretreatment, groove machining, preheating, innovative design of welding process, interlayer cleaning, and heat preservation and slow cooling.

[0060] Among them, the zigzag groove enhances the bonding force between the deposited metal and the base metal, the spiral flame preheating realizes uniform temperature rise, the double-arc alternating interrupted arc welding refines grains and optimizes the molten pool metallurgical reaction, the interlayer knocking and vibration removes slag and releases stress, and the multi-layer wet towel heat preservation and slow cooling controls the cooling rate, ultimately reducing defects such as welding cracks and pores, ensuring beautiful weld formation, and meeting the welding requirements of boiler tubes under high-pressure and high-temperature conditions.

[0061] Embodiment: A butt welding method for boiler tubes includes the following steps:

[0062] S1, pre-treat the end and outer surface of the boiler tube to be welded by the dry-wet alternating cleaning method. During operation, first dry-brush, then wipe with acetone, then polish with sandpaper, and finally clean with a suede cloth.

[0063] Based on any of the above technical solutions, further optimization is: during the process of polishing with sandpaper, the sandpaper polishing direction is along the circumferential direction of the end of the boiler tube, so as to form uniform wire drawing patterns on the surface.

[0064] Dry-brushing is used to remove large-particle impurities, acetone wiping dissolves grease, uniform wire drawing patterns are formed along the circumferential direction during sandpaper polishing to increase surface roughness, and suede cloth cleaning removes fine dust. The overall multi-stage cleaning combines mechanical cleaning and chemical cleaning to thoroughly remove oxides, oil stains and impurities at the welding part. Ensure the cleanliness of the welding part and reduce the probability of defects such as pores and slag inclusions during the welding process.

[0065] The circumferential wire drawing patterns guide the deposited metal to spread evenly along the tube circumference, actively control the flow direction of the deposited metal through the directional pattern design, and improve the fusion quality of the starting section of the weld.

[0066] S2, machine a zigzag composite groove.

[0067] Based on any of the above technical solutions, further optimization is: the forming method of the zigzag composite groove is as follows: first machine a V-shaped groove with a groove angle of 60° - 70°.

[0068] During specific operation, first form a V-groove with a groove angle of 60°-70° at the end of the boiler tube through machining to determine the welding base angle and penetration depth range; then use a file to perform secondary processing on both side walls of the V-groove, and file out a serrated structure along the side walls with uniform force.

[0069] On the basis of any of the above technical solutions, the further optimization is: the spacing of each serration is 2-3 mm, the depth is controlled within 0.5-1 mm, the shape of the serration is a right trapezoid, its hypotenuse faces the center of the groove, and the right angle side is perpendicular to the side wall of the groove.

[0070] During operation, process the single-sided serrations in sequence according to the set spacing (2-3 mm) and depth (0.5-1 mm). After completion, process the other side with the same process. During the processing, use a caliper to measure in real time to ensure the dimensional accuracy. The shape of the serration is a right trapezoid, its hypotenuse faces the center of the groove, and the right angle side is perpendicular to the side wall of the groove. This geometric structure increases the unevenness of the inner wall of the groove, expands the contact area between the deposited metal and the base metal during welding. This serrated structure increases the surface area of the groove, makes the deposited metal and the base metal contact more fully, enhances the weld bonding force, and also has the following functions:

[0071] Enhance metallurgical bonding: The serrated structure significantly increases the surface area of the groove compared with the traditional V-groove. When the deposited metal is filled, the contact interface with the base metal expands, and the diffusion reaction between the liquid metal and the solid base metal is more sufficient, thereby improving the bonding strength of the weld fusion line and reducing the fracture risk of the welded joint.

[0072] Optimize the molten pool flow: The guiding effect of the hypotenuse of the right trapezoidal serration can guide the molten pool metal to converge towards the center of the groove, reduce the tendency of the liquid metal to flow down due to gravity, especially suitable for the all-position welding scenario of horizontal fixed pipe welding, improve the uniformity of the weld formation, and reduce the probability of undercut and weld bead defects. In addition, the enlarged contact area can delay the solidification speed of the molten pool metal, provide more time for the floating and discharging of the slag and gas, reduce volume-type defects such as pores and slag inclusions; at the same time, the mechanical interlocking effect formed by the serrated structure can disperse the stress concentration through the microstructure under the action of welding thermal stress, and reduce the probability of crack generation.

[0073] S3, Boiler tube alignment and positioning, the specific operation is as follows:

[0074] Setting of positioning reference: At the ends of the two boiler tubes to be welded, use a stone pen to evenly mark 4 positioning points along the circumferential direction, which are located at 0°, 90°, 180°, and 270° respectively. The marked points are 10-15 mm away from the edge of the pipe orifice, serving as the reference for alignment during assembly.

[0075] At the same time, draw a straight line along the axial direction on the outer wall of the pipeline to assist in judging the perpendicularity and coaxiality of the pipeline.

[0076] Then, use an external pipe alignment clamp for positioning. Adjust the external pipe alignment clamp at a position 50 - 80 mm away from the pipe end to preliminarily align the pipes.

[0077] Coarse alignment: Align the ports of the two boiler pipes so that the positioning points marked at the pipe ends correspond to each other in pairs. Use a steel straightedge to measure the gap between the pipe ends, adjust the pipe position to make the gap evenly distributed, and control it within the range of 2 - 2.5 mm. If there is an offset between the pipes, use a manual jack to push against the outer wall of the pipe and gradually correct the offset amount so that the offset amount does not exceed 10% of the pipe wall thickness and is not greater than 1 mm.

[0078] Coaxiality calibration.

[0079] Temporary fixation: Use spot welding to temporarily fix the assembled pipes. The spot welding positions are evenly distributed on the circumference, with the number of spots being 4 - 6. The length of each spot welding is 15 - 20 mm, and the height does not exceed 1 / 2 of the pipe wall thickness. When performing spot welding, use a small current (the welding current is 20% - 30% lower than that of formal welding) to prevent pipe deformation caused by heat concentration. After spot welding is completed, check the coaxiality and the gap between the pipe ends again. If there are any deviations, adjust them in a timely manner.

[0080] S4. After positioning, preheat the boiler pipes by means of spiral flame wrapping.

[0081] Based on any of the above technical solutions, a further optimization is as follows: The specific method for preheating the boiler pipes by means of spiral flame wrapping is as follows: Adjust the acetylene - oxygen flame to a neutral flame to ensure stable flame combustion and moderate temperature, and avoid local overheating or oxidation of the boiler pipes due to improper flame properties.

[0082] Start heating from one end of the part of the boiler pipe to be welded. The flame moves around the pipe body in a spiral trajectory from the outside to the inside, and the spiral pitch is maintained at 10 - 15 mm.

[0083] During the heating process, control the distance between the flame nozzle and the pipe end at 5 - 10 mm to ensure effective transfer of the flame heat to the surface of the boiler pipe.

[0084] Using spiral flame wrapping for heating can make the heat more evenly distributed in the circumferential and axial directions of the pipe compared with traditional linear back - and - forth heating. Controlling the spiral pitch at 10 - 15 mm can ensure appropriate overlap of adjacent heating areas, avoid the situation of local insufficient preheating, and effectively reduce the risks of welding deformation and cracks caused by uneven preheating.

[0085] S5. When welding, use the method of double - arc alternating interrupted arc welding to complete the root pass welding, and then perform filling welding.

[0086] On the basis of any of the above technical solutions, a further optimization is as follows: The specific steps of adopting the double-arc alternative interrupted arc welding method are as follows: Step 1, differential pretreatment of electrodes: Select two electrodes with a diameter of 2.5 mm, marked as electrode A and electrode B.

[0087] Electrode A is treated with a constant magnetic field of 0.3T for 30 seconds after being kept at 50°C, and electrode B is stored at room temperature.

[0088] Place electrode A in a constant-temperature drying oven, set the temperature to 50°C, and keep it warm for 1 hour to fully evaporate the moisture inside the electrode and enhance its stability; after the insulation is completed, place electrode A in a constant magnetic field device with a magnetic field strength of 0.3T, and the treatment time lasts for 30 seconds; the magnetic field treatment changes the distribution of alloy elements in the electrode coating, promoting deoxidation of the molten pool and refining the weld grains during welding.

[0089] Electrode B is kept in a room-temperature storage state and used as a comparison reference to form a differential effect with electrode A during the welding process.

[0090] During welding, grain refinement and deoxidation effects: After being treated by the magnetic field, electrode A can promote deoxidation of the molten pool during the welding process, effectively reduce the oxygen content in the weld, and reduce the generation probability of oxide inclusion defects. At the same time, the magnetic field action promotes the refinement of weld grains. Grain refinement can significantly improve the strength, toughness, and fatigue resistance of the weld metal, enhance the comprehensive mechanical properties of the weld, and meet the requirements for high-quality welds in boiler tube welding.

[0091] In addition, it also has a defect suppression effect: The pretreatment process of electrode A reduces the moisture content, reduces the source of hydrogen from the source, and inhibits the generation of hydrogen pores during welding. During the double-arc alternative interrupted arc welding process, the alternating use of different electrodes can adjust the solidification speed of the molten pool and the metallurgical reaction process, helping to prevent welding defects such as lack of fusion and lack of penetration, and further improving the weld quality.

[0092] Step 2, step-by-step arc ignition of electrode A and electrode B: Arc ignition of electrode A: Ignite the arc at the positioning weld with a current of 70A, and use the two-point positioning method of first the groove side and then the root to ignite the arc.

[0093] During specific operation, use a welding current of 70A to ignite electrode A at the positioning weld.

[0094] Adopt the two-point positioning method to ignite the arc. First, gently touch the tip of the electrode to the edge of one side of the groove, and quickly scratch to ignite the arc.

[0095] Then quickly move the arc to the root of the groove, pause for 1-2 seconds, and use the high temperature of the arc to penetrate the root of the groove to form a molten hole.

[0096] During the arc ignition process, keep the short-arc operation, and strictly control the arc length not to exceed the diameter of the electrode to ensure stable arc and concentrated heat.

[0097] Arc ignition of electrode B: After the arc of electrode A goes out, when the molten pool is in a semi-solid state, arc ignition is carried out at the junction of the molten pool edge and the groove. Use a welding current of 75A to ignite electrode B, and quickly send the arc to the incompletely solidified molten pool, so that the new arc is fully fused with the old molten pool, and a molten hole is formed again.

[0098] The reference for arc ignition at the junction of the molten pool edge and the groove is as follows: Visual judgment, this position presents a transitional state between metallic luster and dark red molten pool. The metallic luster part shows the characteristics of being solidified but still having good weldability, while the dark red molten pool part maintains a certain degree of liquid fluidity; Tactile judgment: Gently touch this position with the electrode, there will be a slight sense of resistance, neither as if the electrode easily sinks into the completely liquid molten pool nor as if there is no reaction like solid metal.

[0099] This step-by-step arc ignition method of electrode A and electrode B for the two-point positioning arc ignition of electrode A can effectively control the formation position and size of the initial molten pool. Arc ignition on the groove side first can quickly heat the groove edge by the arc, reducing the difficulty of subsequent root penetration; then move the arc to the root and pause to form a molten hole, ensuring complete penetration of the root and avoiding the defect of incomplete penetration. Keep the short arc operation, so that the arc heat is highly concentrated in the welding area, reducing the range of the heat affected zone, increasing the temperature gradient of the molten pool, promoting the rapid solidification of the molten pool, and being beneficial to refining the weld grains.

[0100] During the relay arc ignition of electrode B, after the arc of electrode A goes out, electrode B ignites an arc at the junction of the molten pool edge and the groove when the molten pool is in a semi-solid state, and uses the residual heat of the molten pool to achieve rapid fusion of the new and old arcs. The new arc is fully fused with the old molten pool, ensuring the continuous existence of the molten hole and maintaining the continuity of the molten pool during the welding process, providing a stable foundation for subsequent welding.

[0101] Electrode A uses a current of 70A. The lower current can avoid the risks of molten pool spatter and burn-through caused by excessive current at the initial stage of arc ignition, facilitating the welder to accurately control the arc and the molten pool; Electrode B uses a current of 75A. The slightly higher current helps to quickly melt the electrode and the base metal, ensure the full fusion of the new molten pool and the old molten pool, and at the same time maintain the stable combustion of the arc. This differential current setting adapts to the requirements of arc energy for different stages of welding, improving the stability and controllability of the welding process.

[0102] Both arc ignitions of the two electrodes emphasize short arc operation, which can effectively reduce arc swing, reduce the probability of air intrusion into the molten pool, and prevent pores from forming in the weld.

[0103] Step 3, during the welding process, both electrodes adopt a combined swing method of spiral movement and zigzag swing for displacement.

[0104] During the helical movement, it makes a circular motion with the electrode axis as the center and a helical diameter of 2 mm. For each complete helical movement cycle, the electrode advances 1 - 1.5 mm along the welding direction.

[0105] The helical movement can cause the molten pool to be stirred in three-dimensional directions, promoting the discharge of molten slag and gas.

[0106] During the zigzag swing, on the basis of the helical movement, the electrode simultaneously makes a small zigzag swing, and the swing amplitude is controlled within 3 - 5 mm. On both sides of the groove, the electrode stays for 0.8 seconds and makes a tiny tremor action with a frequency of 2 Hz.

[0107] The staying and tremor help the molten pool and the groove wall to be fully fused, ensuring the forming quality of the weld edge.

[0108] Through the continuous agitation of the molten pool by the helical movement, the contact area and time between the liquid metal in the molten pool, the decomposition products of the electrode coating, and the base metal are increased, promoting more complete metallurgical reactions such as deoxidation and desulfurization, effectively reducing the content of harmful impurities in the weld, and improving the purity and mechanical properties of the weld metal.

[0109] Step 4, the two electrodes perform welding operations alternately according to the specified frequency.

[0110] During the entire process of double-arc alternating interrupted arc welding, the number of times of mutual arc ignition and arc extinction between electrode A and electrode B within one minute remains in the range of 45 to 55 times.

[0111] Each time the electrodes are alternated, the heat input of the new arc and the rapid cooling of the old molten pool alternate, which helps to refine the weld grains; the rapid cooling and the periodic change of heat input can break the growth trend of coarse grains, forming a finer and denser weld structure, and enhancing the strength, toughness, and fatigue resistance of the weld.

[0112] In addition, high-frequency alternating welding enables the molten pool to undergo metallurgical reactions such as deoxidation and desulfurization multiple times in a short period, which is conducive to the full discharge of harmful gases and impurities. At the same time, the state of the molten pool is continuously adjusted during the alternation process, which can avoid problems such as element segregation and slag inclusion caused by the action of a single heat source for a long time, and reduce the probability of defects such as pores and cracks.

[0113] Step 5, when approaching the end of the weld, the arc is extinguished by means of current decay. The progressive current decay method avoids the rapid solidification of the molten pool caused by the sudden reduction of heat during instantaneous arc interruption, preventing defects such as too deep arc pits and shrinkage cavities. The current decay process keeps the molten pool in a slow cooling state, which helps the weld metal to form a more uniform and dense organizational structure during solidification.

[0114] During specific operation, the welding current is reduced to 80% of the normal welding current and welding is continued for 2 - 3 seconds.

[0115] Continue to reduce the current to 60% of the normal welding current and weld for 1 - 2 seconds.

[0116] Reduce the current to 40% of the normal welding current, make three circular fusions at the crater, with each fusion lasting 0.5 seconds. Then quickly extinguish the arc, remove the electrode, and cover the weld with heat-insulating cotton for at least 5 minutes to prevent crater cracks.

[0117] The circular fusion operation can remelt the metal at the crater and distribute it evenly, eliminating the local stress concentration caused by the rapid reduction of the current. Through multiple fusions, it can further promote the discharge of gas and impurities in the molten pool, reduce the risk of remaining defects such as pores and slag inclusions at the crater, and ensure the structural integrity of the weld end.

[0118] Based on any of the above technical solutions, the further optimization is as follows: The steps of filling welding are as follows: Welding preparation: After the root pass welding is completed, clean the molten slag on the weld surface with a wire brush and remove the spatter with a flat chisel; select an electrode with a diameter of 3.2 mm that matches the boiler tube; adjust the welding current to 90 - 100 A and the arc voltage to 22 - 24 V.

[0119] Initial welding: At one end of the weld, align the electrode with the weld, with the tip 2 - 3 mm from the bottom, strike or scratch to strike an arc, pull it to the bottom and stay for 0.8 - 1.2 seconds to form a molten pool; the electrode forms a 90° angle with the tube axis and a 70° - 80° angle with the welding direction, and maintain a short arc operation.

[0120] Step - type oscillation operation: The electrode makes a step - type oscillation, staying on both sides for 1 - 1.5 seconds to ensure fusion, and moving quickly in the middle to prevent the weld from being too high. Move forward 2 - 3 mm after each oscillation; observe the molten pool during welding. If it collapses, reduce the current or shorten the staying time; if the fusion is poor, increase the staying time and adjust the electrode angle.

[0121] When the electrode makes a step - type oscillation, staying on both sides for 1 - 1.5 seconds can allow enough filler metal to accumulate on both sides of the groove, ensure full fusion of the weld and the base metal, and avoid undercut defects; moving quickly in the middle can prevent excessive accumulation of weld metal, effectively control the weld reinforcement, and make the weld surface transition smoothly.

[0122] Filling and finishing: Continuously fill until it is 1 - 2 mm below the surface of the base metal; when approaching the end, reduce the current in stages 3 - 5 cm in advance, reducing 5 - 10 A each time, stay in the molten pool for 1 - 2 seconds more to fill the crater, and clean the welding slag after removing the electrode.

[0123] S6, after the filling welding is completed, carry out capping welding.

[0124] Based on any of the above technical solutions, the further optimization is as follows: The specific steps of capping welding are as follows: After the filling welding and cleaning are qualified, select a 3.2mm low-hydrogen electrode, and adjust the current to 85 - 95A according to the state of the filling weld.

[0125] Arc strike on the base metal 10 - 15mm in front of the starting end of the weld. After stabilization, pull it to the starting point. The electrode forms an angle of 75° - 85° with the welding direction and is perpendicular to the pipe axis. Start welding with a short arc to ensure fusion.

[0126] Weld by moving the electrode. Move quickly in the center of the weld, press down on both sides and pause for 0.5 - 1 second before lifting. Control the width of the weld to exceed the groove on each side by 1 - 2mm, and the reinforcement height is 0 - 3mm.

[0127] When changing the electrode, grind a 10 - 15mm gentle slope at the arc extinguishing place. Strike an arc with the new electrode 5 - 8mm in front of the joint, pull it to the joint, pause for fusion, and then continue welding.

[0128] At the end, reduce the current to 70% - 80%, swing the electrode in a circle to fill the crater and then extinguish the arc.

[0129] Arc strike on the base metal 10 - 15mm in front of the starting end of the weld. Wait until the arc is stable and then pull it to the starting point, which can avoid defects such as pores and lack of fusion caused by unstable arc strike at the starting end.

[0130] S7. After the capping welding is completed, complete the cleaning between weld layers by knocking and vibrating.

[0131] Based on any of the above technical solutions, the further optimization is as follows: The specific steps of completing the cleaning between weld layers by knocking and vibrating are as follows: After welding is completed, wait until the weld cools to 60 - 80°C.

[0132] Tap the two sides of the weld with a small hammer at a frequency of 2 - 3 times per second, and the distance between the tapping points is 5 - 8mm.

[0133] Move the tapping in a spiral shape along the weld direction, and the spiral diameter is about 20mm.

[0134] After tapping, use a wire brush to brush off the loose slag in one direction along the weld direction.

[0135] When stubborn slag is found, bake it with a flame until it turns dark red and then quench it suddenly to peel it off using thermal expansion and contraction.

[0136] For slag that is difficult to remove by conventional cleaning, this method can effectively remove it, ensure no impurities between layers, ensure tight fusion between multi-layer welds, and improve the overall quality of the weld.

[0137] S8. After the final welding is completed, wrap it with multiple layers of wet towels for heat preservation and slow cooling, and check the weld after cooling.

[0138] Based on any of the above technical solutions, further optimization is that the specific steps of wrapping with multiple layers of wet towels to keep warm and slow down cooling are as follows: prepare sufficient pure cotton towels with good water absorption; prepare warm water, and control the water temperature at 40-50℃ to ensure that the temperature of the towel is appropriate after being soaked.

[0139] After the final welding is completed, soak the towel with warm water, wring it until it stops dripping, and wrap the first layer of towel tightly from one end with the weld as the center to ensure that the weld is completely covered and each layer of towel fits tightly without leaving any gaps.

[0140] When the temperature of the first layer of towels drops to close to the ambient temperature, quickly wrap the second layer, and repeat the process until 3-4 layers of towels are wrapped.

[0141] Finally, wrap the towel completely with plastic film to prevent water from evaporating too quickly and slow down the rate of heat loss.

[0142] Multi-layer wrapping and adding layer by layer according to temperature changes, combined with plastic film sealing, can effectively slow down the rate of heat loss and avoid excessive temperature gradients in welds due to rapid cooling.

[0143] During the insulation and slow cooling process, use an infrared thermometer to intermittently measure the weld temperature and record the temperature changes. Control the weld cooling rate by increasing or decreasing the number of towel layers or adjusting the ambient temperature.

[0144] Based on any of the above technical solutions, further optimization is: during the preheating process, the color change of the tube wall of the boiler tube is observed. When the tube wall appears dark red, it indicates that the temperature is close to or reaches the preheating target temperature of 150℃-200℃.

[0145] At the same time, during the circumferential heating process, a relatively uniform moving speed of about 1-2 cm per second is maintained to ensure uniform heating in the circumferential direction of the boiler tube.

[0146] After completing each spiral heating cycle, the heating process needs to be repeated 1-2 times to further ensure the temperature uniformity and sufficient preheating of the entire welding area, reduce welding stress, and reduce the probability of welding crack defects.

[0147] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments can still be modified, or some or all of the technical features can be replaced by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. For those skilled in the art, any replacement improvements or changes made to the implementation methods of the present invention fall within the protection scope of the present invention.

[0148] Matters not described in detail in this invention are well-known techniques to those skilled in the art.

Claims

1. A butt welding method for boiler tubes, characterized in that, It includes the following steps: S1, Pre-treat the end and outer surface of the boiler tube to be welded by the dry-wet alternate cleaning method. During operation, first dry-brush, then wipe with acetone, then sand and finally clean with a suede cloth; S2, Process the serrated composite groove; S3, Align and position the boiler tubes; S4, After positioning, preheat the boiler tube by spiral flame circumfluence; S5, During welding, use the double-arc alternate interrupted arc welding method to complete the root welding, and then carry out the filling welding; S6, After the filling welding is completed, carry out the capping welding; S7, After the capping welding is completed, complete the cleaning between welding layers by knocking and vibrating; S8, After the final welding is completed, wrap it with multiple layers of wet towels for heat preservation and slow cooling, and check the weld after cooling.

2. The butt welding method for boiler tubes according to claim 1, characterized in that: During the process of sanding with sandpaper, the sanding direction is along the circumferential direction of the end of the boiler tube to form uniform wire drawing patterns on the surface.

3. The butt welding method for boiler tubes according to claim 2, characterized in that The forming method of the serrated composite groove is as follows: First, process a V-shaped groove with a groove angle of 60°-70°; Process serrated structures on both side walls of the V-shaped groove. During operation, use a file to file along the side wall of the groove with relatively uniform force and rhythm. First, on one side wall of the groove, file out serrations in sequence according to the set spacing and depth. After completing the operation on one side, process the other side wall of the groove in the same way; during the filing process, continuously measure the spacing and depth of the serrations with a caliper.

4. The butt welding method for boiler tubes according to claim 3, characterized in that: The spacing of each serration is 2-3mm, the depth is controlled within 0.5-1mm, the shape of the serration is a right trapezoid, its hypotenuse faces the center of the groove, and the right angle side is perpendicular to the side wall of the groove.

5. The butt welding method for boiler tubes according to claim 4, characterized in that, The specific method of preheating the boiler tube by spiral flame circumfluence is as follows: Adjust the acetylene-oxygen flame to a neutral flame to ensure stable flame combustion and moderate temperature, and avoid local overheating or oxidation of the boiler tube due to improper flame properties; Start heating from one end of the part of the boiler tube to be welded, and the flame moves around the tube body from the outside to the inside in a spiral trajectory, and the spiral spacing is kept at 10-15mm; During the heating process, the distance between the flame nozzle and the tube end is controlled within 5-10mm to ensure effective transfer of the flame heat to the surface of the boiler tube.

6. The butt welding method for boiler tubes according to claim 5, characterized in that, The specific steps of using the double-arc alternate interrupted arc welding method are as follows: Step 1, Differentiated pre-treatment of electrodes: Select two electrodes with a diameter of 2.5mm, marked as electrode A and electrode B; Step 2, Step-by-step arc ignition of electrode A and electrode B: Arc ignition of electrode A: Ignite the arc at the positioning weld with a current of 70A, and use the two-point positioning method of first the groove side and then the root to ignite the arc; Arc ignition of electrode B: After electrode A goes out, when the molten pool is in a semi-solid state, ignite the arc at the junction of the molten pool edge and the groove, use a welding current of 75A to ignite electrode B, quickly send the arc to the incompletely solidified molten pool, and make the new arc fully fuse with the old molten pool to form a new molten hole again; Step 3, During the welding process, both electrodes use a composite swing method of spiral movement combined with zigzag swing to shift; Step 4, The two electrodes alternately carry out welding operations according to the specified frequency; Step 5, When approaching the end of the weld, use the current decay method to stop the arc.

7. The butt welding method for boiler tubes according to claim 6, characterized in that, The steps of the filling welding are as follows: Welding preparation: After the root pass welding is completed, clean the molten slag on the surface of the weld with a wire brush and chisel the spatter; Select an electrode with a diameter of 3.2 mm that matches the boiler tube; Adjust the welding current to 90 - 100 A and the arc voltage to 22 - 24 V; Initial welding: At one end of the weld, align the electrode with the weld, with the tip 2 - 3 mm away from the bottom, strike or scratch to initiate the arc, pull it to the bottom and stay for 0.8 - 1.2 seconds to form a molten pool; The electrode forms a 90° angle with the tube axis and a 70° - 80° angle with the welding direction, and keep the short arc operation; Step - type swing operation: The electrode makes a step - type swing, staying on both sides for 1 - 1.5 seconds to ensure fusion, moving quickly in the middle to prevent the weld from being too high, and moving forward 2 - 3 mm after each swing; Observe the molten pool during welding. If it collapses, reduce the current or shorten the staying time; If the fusion is poor, increase the staying time and adjust the electrode angle; Filling and finishing: Continuously fill until it is 1 - 2 mm below the surface of the base metal; When approaching the end, reduce the current in stages 3 - 5 cm in advance, reducing 5 A - 10 A each time, stay in the molten pool for 1 - 2 seconds more to fill the crater, and clean the welding slag after removing the electrode.

8. The butt welding method for boiler tubes according to claim 7, characterized in that, The specific steps of knocking and vibrating to complete the inter - layer cleaning of the weld layer are as follows: After welding is completed, wait for the weld to cool to 60 - 80 °C; Tap the two sides of the weld with a small hammer at a frequency of 2 - 3 times per second, and the distance between the tapping points is 5 - 8 mm; Move the tapping in a spiral shape along the weld direction, with a spiral diameter of about 20 mm; After tapping, use a wire brush to brush the loose molten slag unidirectionally along the weld direction; After finding stubborn molten slag, bake it with a flame until it turns dark red and then quench it suddenly to peel it off by thermal expansion and contraction.

9. The butt welding method for boiler tubes according to claim 8, characterized in that, The specific steps of multi - layer wet towel wrapping for heat preservation and slow cooling are as follows: Prepare a sufficient amount of pure cotton towels with good water absorption; Prepare warm water with the water temperature controlled at 40 - 50 °C to ensure that the temperature of the towel is appropriate after being soaked; After the final pass welding is completed, soak the towel with warm water and wring it until it stops dripping. Starting from one end, tightly wrap the first layer of towel centered on the weld, ensuring that the weld is completely covered, and the layers of towels are closely fitted without gaps; When the temperature of the first layer of towel drops to near the ambient temperature, quickly wrap the second layer, and repeat the operation until 3 - 4 layers of towels are wrapped; Finally, wrap the whole towel with a plastic film to seal it, prevent the water from evaporating too quickly, and slow down the heat dissipation rate; During the heat preservation and slow cooling process, use an infrared thermometer to intermittently measure the temperature of the weld, record the temperature change, and control the cooling rate of the weld by increasing or decreasing the number of towel layers or adjusting the ambient temperature.