Method for welding large-diameter pipe of fluidized bed boiler

Abstract

The invention discloses a method for welding a large-diameter pipe of a fluidized bed boiler. The method comprises the welding steps of groove manufacturing, welding and postweld heat treatment. The groove is cut by flame cutting, and machining allowance is reserved; the ambient temperature is measured before welding, whether the pipeline is preheated or not is selected according to the ambient temperature, during welding, tack welding is carried out firstly, then backing welding is carried out, filling welding is carried out after backing welding is finished for cosmetic filling, and welding spatter and welding slag are removed after cosmetic filling; and heat treatment is carried out in an electric heating mode after welding. The temperature difference between the inner wall and the outer wall of the pipeline is reduced through preheating, large temperature difference between the inner wall and the outer wall of the pipeline in the welding process is avoided, cold and hot cracks generated after welding of the pipeline are avoided through postweld heat treatment, and the toughness of weld joints is improved.

Description

Technical Field

[0001] The present invention relates to a welding method for large-diameter pipes of a fluidized bed boiler. Background Art

[0002] The 450t / h circulating fluidized bed boiler of model DG450 / 13.7-II1 is a single-drum, natural circulation, balanced draft, top-suspended, tightly enclosed, single-furnace, non-reheat, steam-cooled cyclone separator, tubular air preheater, and all-steel frame structure; the boiler depth is 30.8 meters, the boiler width is 27.4 meters, and the top plate height is 55.1 meters.

[0003] There are about 860 large-diameter welds in the boiler, including about 360 large-diameter welds in the boiler body and about 500 large-diameter welds in the other boiler-related auxiliary pipelines. The large-diameter pipelines have extremely high requirements for welding processes, and the heat treatment processes before and after welding have a great impact on the welding quality. For example, a welding method for large-diameter pipelines disclosed in CN110449759A realizes efficient penetration by using in-pipe laser welding in cooperation with an ultra-narrow gap single-Y groove processed at the pipe orifice, thereby improving the welding efficiency and welding quality. However, heat treatment is not performed after welding, which will cause the heat during welding to not be transferred to the inside of the pipeline, resulting in a temperature difference between the inner and outer walls of the large-diameter pipeline. The temperature difference between the inner and outer walls will cause the overall cooling deformation speed of the pipeline to be inconsistent, and the stress cannot be eliminated, affecting the weld quality. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a welding method for large-diameter pipes of a fluidized bed boiler.

[0005] The present invention is achieved through the following technical solutions.

[0006] A welding method for large-diameter pipes of a fluidized bed boiler provided by the present invention has welding steps as follows: groove making, welding, and post-weld heat treatment;

[0007] The groove is made by flame cutting and a machining allowance is left;

[0008] Before welding, the ambient temperature is measured, and whether to preheat the pipeline is selected according to the ambient temperature. During welding, tack welding is first performed, and then backing welding is carried out. After the backing welding is completed, filling welding is performed for cover filling. After cover filling, welding spatter and slag are removed;

[0009] After welding, heat treatment is performed by an electric heating method.

[0010] The groove should be kept flat and free of burrs, and there should be no defects such as cracks, interlayers, and slag inclusions.

[0011] When the ambient temperature is lower than 5°C, the pipeline is preheated:

[0012] When the wall thickness of the pipe is less than 20 mm, flame preheating is adopted before welding, and a non-contact temperature measurement is carried out with a far-infrared temperature gun; for pipes with a wall thickness ≥ 20 mm, electric heating preheating shall be adopted, and a contact temperature measurement is carried out with a K-type thermocouple; the preheating temperature is 150 - 300 °C.

[0013] If the welding is interrupted at the weld and then welded again, re-preheating is also required.

[0014] The root pass welding uses TIG welding, starting the arc from inside the groove; when finishing the arc, fill the molten pool, and high frequency must be used for arc starting and arc finishing. When welding about 3 / 4, inspect the root quality of the welding. If there are defects such as incomplete penetration or excessive weld beads, remove the weld scar and weld again.

[0015] The heating and cooling rate after welding heat treatment is 6250 / wall thickness, and does not exceed 300 °C / h; after cooling to below 300 °C, slow cooling is carried out in the insulation layer.

[0016] The calculation methods for the electric heating power and the number of heaters for the heat treatment are as follows:

[0017] Pipe diameter × pipe wall thickness ÷ 625 = KVA number of heaters;

[0018] KVA number of heaters ÷ power supply voltage ÷ 1000 = total current of the equipment;

[0019] KVA number of heaters ÷ power of each heater = number of heaters.

[0020] The width of the heating is as follows:

[0021] When the ratio of the pipe diameter to the thickness D / δ ≤ 7.5, the heating width is not less than 4 times the pipe wall thickness on each side starting from the weld center;

[0022] When the ratio of the pipe diameter to the thickness 7.5 < D / δ ≤ 10, the heating width is not less than 5 times the pipe wall thickness on each side starting from the weld center;

[0023] When the ratio of the pipe diameter to the thickness 10 < D / δ ≤ 15, the heating width is not less than 6 times the pipe wall thickness on each side starting from the weld center;

[0024] When the ratio of the pipe diameter to the thickness D / δ > 15, the heating width is not less than 7 times the pipe wall thickness on each side starting from the weld center.

[0025] The width of the insulation layer is increased by 2 times the wall thickness compared to the heating width, and is not less than 150 mm.

[0026] The welding process of each weld shall be completed within 12 hours.

[0027] The beneficial effects of the present invention are as follows: By preheating, the temperature difference between the inner and outer walls of the pipeline is reduced, the formation of a large temperature difference between the inner and outer walls of the pipeline during the welding process is avoided, and post-weld heat treatment is used to prevent cold and hot cracks from occurring in the pipeline after welding, increasing the toughness of the weld. Specific embodiments

[0028] The technical solutions of the present invention will be further described below, but the scope of protection is not limited thereto.

[0029] A welding method for large-diameter pipes of a fluidized bed boiler, the welding steps being: groove preparation, welding, and post-weld heat treatment;

[0030] For groove preparation, a flame cutting is used to cut the groove, and a machining allowance is left.

[0031] Before welding, the ambient temperature is measured, and whether to preheat the pipeline is selected according to the ambient temperature. Before welding, tack welding is first carried out, and then root welding is carried out. After the root welding is completed, filling welding is carried out for capping filling. After capping, welding spatter and slag are removed.

[0032] After welding, heat treatment is carried out by means of electric heating.

[0033] Furthermore, the groove should be kept flat and free of burrs, and there should be no defects such as cracks, laminations, and slag inclusions. And a machining allowance should be left on the cut part to facilitate the removal of the hardened layer and overheated metal.

[0034] Furthermore, before the assembly of the welded parts, the oil, paint, scale, rust, etc. on the inner and outer walls of the groove surface and the nearby base metal are cleaned until a metallic luster appears. The cleaning range is: 10 - 20 mm on both the inner and outer sides of the base metal. The inner walls of the pipe ends should be flush, and the local misalignment value should not exceed 10% of the wall thickness and should not be greater than 1 mm. The deflection of the pipe ends should be checked with a straightedge, and the gap at 100 mm from the weld center should not be greater than 1 mm.

[0035] Furthermore, when the ambient temperature is lower than 5°C, the pipeline is preheated:

[0036] When the wall thickness of the pipe is less than 20 mm, flame preheating is used before welding, and a far-infrared temperature measuring gun is used for non-contact temperature measurement; for pipes with a wall thickness ≥ 20 mm, electric heating preheating should be used, and a K-type thermocouple is used for contact temperature measurement; the preheating temperature is 150 - 300°C.

[0037] The preheating width starts from the center of the butt joint. If electric heating is used, each side is not less than 4 times the thickness of the welded part; if flame preheating is used, each side is not less than 3 times the thickness of the welded part and is not less than 100 mm.

[0038] The distance from the flame core of the flame heating to the workpiece should be more than 10 mm. The moving speed of the nozzle should be stable, and the heating should be uniform without staying at one position for a long time. Attention should also be paid to controlling the combustion state of the flame to prevent metal oxidation or carburization. A thermometer should be used to measure the temperature to ensure that the preheating temperature and preheating width meet the process requirements.

[0039] If the welding is interrupted at the weld and then welded again, re-preheating is also required.

[0040] Furthermore, GTAW is used for the root pass welding. The arc is struck from within the groove at the start of welding; the molten pool is filled at the end of welding. High frequency must be used for striking and extinguishing the arc. When about 3 / 4 of the welding is completed, the root quality of the welding is inspected. If there are defects such as incomplete penetration or excessive weld beads, the weld scar needs to be removed and welding is redone.

[0041] GTAW welding current: Φ2.5 I = 90 - 110 (A); welding voltage: U = 10 - 14 (V).

[0042] After the root pass is completed and passes the self-inspection, filling welding should be carried out in a timely manner. It is strictly prohibited to leave the root pass overnight to prevent crack formation. The welder carefully removes the interlayer welding slag with a flat chisel and a wire brush, checks the welding quality of the filling layer. If surface defects are found, they are immediately removed by mechanical processing (angle grinder, high-speed steel saw blade). After repair welding, the welding of the next weld pass is carried out until welding is completed. The layer joints should be staggered from each other. After surfacing, the welding spatter and welding slag are removed in a timely manner.

[0043] Post-weld heat treatment is carried out by high-temperature tempering. The heating and cooling rate is 6250 / wall thickness °C / h, and does not exceed 300 °C / h; after cooling to below 300 °C, it is slowly cooled in the insulation layer. The heat treatment temperature is shown in Table 1, and the holding time after heat treatment is shown in Table 2.

[0044] Table 1 Post-weld heat treatment temperature table

[0045]

[0046] Table 2 Post-weld heat treatment holding time

[0047]

[0048]

[0049] Furthermore, the calculation methods for the electric heating power and the number of heaters for heat treatment are as follows:

[0050] Pipe diameter × pipe wall thickness ÷ 625 = number of heaters in KVA;

[0051] Number of heaters in KVA ÷ supply voltage V ÷ 1000 = total current A of the equipment;

[0052] The number of heaters (KVA) ÷ the power of each heater = the number of heaters.

[0053] The width of the heating is as follows:

[0054] When the ratio of the diameter to the thickness of the pipe D / δ ≤ 7.5, the heating width on each side starting from the weld center is not less than 4 times the wall thickness of the pipe;

[0055] When the ratio of the diameter to the thickness of the pipe 7.5 < D / δ ≤ 10, the heating width on each side starting from the weld center is not less than 5 times the wall thickness of the pipe;

[0056] When the ratio of the diameter to the thickness of the pipe 10 < D / δ ≤ 15, the heating width on each side starting from the weld center is not less than 6 times the wall thickness of the pipe;

[0057] When the ratio of the diameter to the thickness of the pipe D / δ > 15, the heating width on each side starting from the weld center is not less than 7 times the wall thickness of the pipe.

[0058] The width of the insulation layer is increased by 2 times the wall thickness compared to the heating width and is not less than 150 mm.

[0059] The welding process of each weld needs to be completed within 12 hours to avoid cracks due to too long time.

[0060] After heat treatment, acceptance shall be carried out according to Table 3.

[0061] Table 3 Quality acceptance standard table for the welding project of pipes and pipelines

[0062]

[0063] Non-destructive testing of welded joints:

[0064] (1) For pipes with a thickness not greater than 20 mm, when ultrasonic testing (PE) is used, radiographic testing (RT) shall also be used for additional testing, and the testing quantity is 20% of the PE testing quantity.

[0065] (2) For pipes with a thickness greater than 20 mm, either radiographic testing (RT) or ultrasonic testing (PE) can be selected. When PE testing is used, RT shall also be used for additional testing, and the total testing quantity is 20% of the PE testing quantity. The key points of the additional testing shall be the welds with recorded defects in the PE testing.

[0066] Spectral analysis of weld metal

[0067] (1) For the welds of the heating surface pipes, not less than 10% shall be inspected. If the material does not conform, 100% re-inspection shall be carried out for this batch of welds and pursued.

[0068] (2) For the Class I welds of other pipes and pipelines, 100%.

[0069] (3) For Class II and Class III welds of other pipes and pipelines, the proportion of welds to be subjected to spectral analysis shall be not less than 10%. If the material is found to be inconsistent, 100% re-inspection shall be carried out on this batch of welds.

Claims

1. A method for welding large-diameter pipes of a fluidized bed boiler, the welding steps comprising: groove preparation, welding, and post-weld heat treatment; The groove is made by flame cutting and a machining allowance is left; Before welding, measure the ambient temperature and choose whether to preheat the pipeline according to the ambient temperature. When welding, do spot welding first, then do base welding, and then do filling welding to cover the surface after base welding. After covering the surface, clean the welding spatter and slag. After welding, heat treatment is performed using electric heating.

2. The method for welding large-diameter pipes of fluidized bed boilers according to claim 1, characterized in that: The groove should be kept smooth and free of burrs, and must not have defects such as cracks, interlayers, and slag inclusions.

3. The method for welding large-diameter pipes of fluidized bed boilers according to claim 1, characterized in that: When the ambient temperature is below 5°C, preheat the pipeline: When the pipe wall thickness is less than 20mm, flame preheating is used before welding, and non-contact temperature measurement is performed using a far-infrared temperature measuring gun; pipes with a wall thickness ≥20mm should be preheated using electric heating, and contact temperature measurement should be performed using a K-type thermocouple; the preheating temperature should be between 150 and 300°C.

4. The method for welding large-diameter pipes of fluidized bed boilers according to claim 3, characterized in that: If welding is interrupted at the weld, it must be preheated again when it is restarted.

5. The method for welding large-diameter pipes of fluidized bed boilers according to claim 1, characterized in that: The base welding adopts argon arc welding, and the arc is started from the groove; the molten pool is filled when the arc is closed. High frequency must be used for arc starting and arc closing. When the welding is about 3 / 4, the root quality of the welding is checked. If there are defects such as incomplete welding or excessive weld nodules, the weld scars need to be removed and re-welded.

6. The method for welding large-diameter pipes of fluidized bed boilers according to claim 1, characterized in that: The post-weld heat treatment heating and cooling speed is 6250 / wall thickness (°C / h) and does not exceed 300°C / h; after cooling to below 300°C, it is slowly cooled in the insulation layer.

7. The method for welding large-diameter pipes of fluidized bed boilers according to claim 6, characterized in that: The electric heating power and the number of heaters for the heat treatment are calculated as follows: Pipe diameter × pipe wall thickness ÷ 625 = heater KVA; Heater KVA number ÷ power supply voltage (V) ÷ 1000 = total current of the equipment (A); Heater KVA number ÷ power of each heater = number of heaters.

8. The method for welding large-diameter pipes of fluidized bed boilers according to claim 7, characterized in that: The heating width is: When the ratio of pipe diameter to thickness D / δ≤7.5, the heating width on each side from the weld center shall not be less than 4 times the pipe wall thickness; When the ratio of pipe diameter to thickness is 7.5<D / δ≤10, the heating width on each side from the weld center shall not be less than 5 times the pipe wall thickness; When the ratio of pipe diameter to thickness is 10<D / δ≤15, the heating width on each side from the weld center shall not be less than 6 times the pipe wall thickness; When the diameter-to-thickness ratio of the pipe D / δ>15, the heating width on each side from the weld center shall not be less than 7 times the pipe wall thickness.

9. The method for welding large-diameter pipes of fluidized bed boilers according to claim 6, characterized in that: The width of the insulation layer is twice the wall thickness of the heating width and is not less than 150mm.

10. The method for welding large-diameter pipes of fluidized bed boilers according to claim 1, characterized in that: The welding process of each weld must be completed within 12 hours.

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