A welding process for main steam pipe 12Cr1MoV

Abstract

The application relates to a welding process of a main steam pipeline 12Cr1MoV, which comprises the following steps: a backing welding is carried out by argon arc welding, the welding opening is preheated in an electric heating mode, and the preheating temperature is 200-300 DEG C; after argon protection, spot welding is carried out; after the spot welding is completed, formal backing welding is carried out by two welders in a symmetrical mode: a welder welds from the 6 o'clock direction to the 3 o'clock direction, and a welder welds from the 9 o'clock direction to the 12 o'clock direction; after the first layer of backing is completed, argon arc welding is carried out for the second layer of backing according to the welding sequence of the first layer; filling and cap welding are carried out by manual electric arc welding in a multi-layer and multi-pass mode, the welding sequence is exchanged after each layer is welded; and post-welding heat treatment is carried out. The application overcomes the phenomenon that cracks and penetration are easily generated under the traditional welding method, effectively guarantees the welding joint quality, improves the physical performance of the weld, prolongs the service life of the weld under high-temperature and high-pressure working conditions, and has remarkable economic benefits.

Description

Technical Field

[0001] This invention relates to the field of alloy steel welding technology, and in particular to a welding process for 12Cr1MoV main steam pipelines. Background Technology

[0002] 12Cr1MoV high-pressure alloy pipes can be used in boilers and high-temperature, high-pressure pipelines in thermal power plants. Boilers in thermal power plants operate under extremely harsh conditions, needing to withstand high-temperature, high-pressure steam and combustion gases. 12Cr1MoV high-pressure alloy pipes possess excellent heat resistance and pressure resistance, making them ideal as boiler piping materials. The main steam pipes of thermal power boilers widely use 12Cr1MoV high-pressure alloy pipes, and these pipes are characterized by large diameters and thick walls. The diameter of the main steam pipes in large domestic thermal power boilers is generally no less than Φ426mm, and the wall thickness is no less than 20mm. Conventional argon-electric welding methods are prone to causing cracks and penetration in the weld seams of pipes subjected to high temperatures and pressures. Summary of the Invention

[0003] The purpose of this invention is to provide a welding process for 12Cr1MoV main steam pipelines, which can effectively reduce the occurrence of cracks and penetration in the weld, ensure excellent weld quality, and easily meet the relevant requirements of drawings and specifications.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] A welding process for 12Cr1MoV main steam pipeline includes the following specific steps:

[0006] 1) The root pass is welded by argon arc welding. Before assembling the weld joint, the weld joint is preheated by electric heating. The preheating area is centered on the weld and extends at least 100 mm on both sides. The preheating temperature is 200-300℃.

[0007] 2) Perform argon protection to replace the air in the protected area; then perform spot welding, with a minimum of four spot welding points, each 30±5mm in length, and the welding points should be evenly distributed in the circumferential direction.

[0008] 3) After the tack welding is completed, the formal root pass welding begins. Two welders weld symmetrically: Welder A welds from the 6 o'clock position to the 3 o'clock position using a left-right extrusion internal wire feeding method; Welder B welds from the 9 o'clock position to the 12 o'clock position using a left-right extrusion closed wire welding method. After Welder A reaches the 3 o'clock position, He continues welding towards the 12 o'clock position. Welder B stops welding after reaching the 12 o'clock position and then starts welding from the 6 o'clock position, starting from Welder A's starting point at the 6 o'clock position, towards the 9 o'clock position. The thickness of the first layer of argon arc welding root pass is 2.5-3 mm.

[0009] 4) After the first layer of the base coat is completed, both welders a and b use the left-right extrusion continuous wire feeding method to perform argon arc welding for the second layer of the base coat, following the welding sequence of the first layer. The thickness is also controlled at 2.5-3mm.

[0010] 5) Filling and Cover Welding: Manual arc welding is used, with multiple layers and multiple passes. The entire welding process is preheated at 200-300℃. During the welding process, attention should be paid to maintaining the interpass temperature. The interpass temperature should not be lower than the lower limit of the preheating temperature and should not be higher than 400℃.

[0011] 6) During the first layer of arc welding filler, welders A and B exchange welding sequences. Welder B starts welding from 6 o'clock to 9 o'clock, while welder A welds symmetrically from 3 o'clock to 12 o'clock. Welder B continues welding towards 12 o'clock after reaching 9 o'clock, and welder A stops welding after reaching 12 o'clock. Welder A then starts welding from 6 o'clock, taking over from welder B's starting point at 6 o'clock, and continues welding towards 3 o'clock. After the first layer of arc welding filler is completed, welders A and B exchange welding sequences. Welder A welds from 6 o'clock to 3 o'clock, while welder B welds from 9 o'clock to 12 o'clock. Welder A continues welding towards 12 o'clock after reaching 3 o'clock, and welder B stops welding after reaching 12 o'clock. Welder B then starts welding from 6 o'clock, taking over from welder A's starting point at 6 o'clock, and continues welding towards 9 o'clock. The welding sequence is exchanged after each layer of filler and cover weld is completed.

[0012] 7) Post-weld heat treatment: The post-weld heat treatment of the weld is high-temperature tempering with a heating and cooling rate of ≤150℃, a heating temperature of 720±10℃, a holding time of not less than 1.5 hours, and when the temperature drops to below 300℃, the heat treatment equipment is powered off and cooled to room temperature in the insulation layer.

[0013] A V-shaped bevel with blunt edge is adopted, with a bevel angle of 60±3° and a pairing distance of 3~4mm.

[0014] The welding machine selected is the ZX7-400 inverter welding machine. For argon arc welding, DC positive polarity is used, and for electric arc welding, DC reverse polarity is used.

[0015] In the aforementioned filling and cover welding process, the first two layers of filling use φ3.2 welding rods with a welding current of 90-120A, and the third layer of filling up to the cover welding use φ4.0 welding rods with a welding current of 100-160A.

[0016] The welding rod is R317 and has been dried at 350-400℃ and kept warm for more than 2 hours. When using it, it should be placed in a heat-insulating container at 80-120℃ and taken out as needed.

[0017] The argon arc welding uses R31, φ2.5mm welding wire, 2.4mm cerium tungsten electrode, welding current of 100-140A, 300A air-cooled welding torch, argon purity of 99.99%, and argon flow rate of 10-15L / min.

[0018] Compared with existing technologies, the beneficial effects of this invention are:

[0019] This invention overcomes the problems of cracking and penetration that easily occur in traditional welding methods, effectively ensuring the quality of welded joints. It improves the physical properties of the weld, extends its service life under high temperature and high pressure conditions, and yields significant economic benefits. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the bevel of the present invention.

[0021] Figure 2 This is a schematic diagram of the circumferential points of the welding sequence of the present invention. Detailed Implementation

[0022] The present invention will now be described in detail, but the scope of the present invention is not limited to the embodiments described below.

[0023] The example provided uses the welding of the main steam pipe in a power generation boiler project. The pipe specifications are Φ580×30mm, and the material is 12Cr1MoV.

[0024] A welding process for 12Cr1MoV main steam pipes includes the following pre-welding preparations: Inspect the pipe wall for scale, cracks, and surface defects such as scratches and pits. Verify the machined bevel edges for cracks, burrs, and missing edges. Only certified and qualified pipes may be used; otherwise, they are strictly prohibited. Welders must be qualified personnel holding the relevant certifications for the same material.

[0025] The welding machine selected is the ZX7-400 inverter welding machine. For argon arc welding, DC positive polarity is used, and for electric arc welding, DC reverse polarity is used.

[0026] Argon arc welding uses R31, φ2.5mm welding wire, 2.4mm cerium tungsten electrode, and a welding current of 100-140A. A 300A air-cooled welding torch is used, with argon purity of 99.99% and an argon flow rate of 10-15L / min. An argon gauge with stable pressure and flexible adjustment is selected. The argon delivery pipe should be intact and clamped with pipe clamps to prevent leakage at the joints, which would affect the quality of shielding.

[0027] The welding process includes the following specific steps:

[0028] 1) Use a V-shaped bevel with a blunt edge, the bevel angle is 60±3°, and the assembly distance is 3~4mm. The bevel is machined, and light tools such as angle grinders can be used for finishing. Clean and polish the bevel and the inner and outer walls within 20~30mm to remove paint stains, oxide scale and other contaminants until the metal luster is exposed.

[0029] 2) TIG welding is used for the root pass. To prevent oxidation of the root weld metal and its impact on weld quality, local argon purging is employed during the root pass. Before assembling the weld joint, a fireproof plug with a wire is placed 500mm from both sides of the weld joint, one end connected to a gas pipe and the other end sealed with a vent hole to prevent excessive gas pressure from affecting weld quality. After the argon protection is in place, the weld joint is preheated using electric heating. The preheating area is centered on the weld, extending at least 100mm to both sides, at a temperature of 200–300℃. Simultaneously with preheating, the welding process begins... Figure 1 The weld seam shown is in a free state during the assembly process, and no external force should be applied to prevent cracks from forming during welding.

[0030] 3) When the preheating temperature, assembly gap and misalignment all meet the welding requirements, argon protection is performed to replace the air in the protection area; a lit lighter is placed near the weld gap, and when the flame goes out, it indicates that the internal air has been purged. At this time, tack welding can be performed. The tack welding process is the same as the formal welding process. There are four tack welding points, each 30mm long, located at the 2, 4, 8 and 10 o'clock positions in the circumferential direction.

[0031] 4) After tack welding is completed, the formal root pass welding begins, performed symmetrically by two welders with welding certificates for this material. Figure 2 As shown: Welder a welds from the 6 o'clock position to the 3 o'clock position, using a left-right squeezing internal wire feeding method. This involves lightly placing the ceramic nozzle of the argon arc welding torch against the center of the weld seam and swinging the torch left and right, squeezing the nozzle against the bevel wall at a uniform speed. The welding wire is inserted into the pipe from the side of the weld seam and adheres to one side of the bevel to ensure wire feeding stability. During the welding process, continuous wire feeding is maintained throughout, allowing the tungsten electrode to burn at the root of the bevel. The tungsten electrode drives the molten iron to form a molten pool on the base material. The molten pool melts the welding wire, with a slightly faster feed in the middle and a pause on both sides to completely dissipate the heat to both sides of the bevel, preventing the molten pool from sagging and forming an inward concavity. Welder b starts welding from 9 o'clock to 12 o'clock, using the left-right extrusion blind wire welding method. The welding torch tip is lightly placed in the middle of the weld, and the welding torch is swung left and right. The tip presses against the bevel wall from side to side and moves forward at a constant speed. The welding wire is kept close to one side of the bevel to maintain stable wire feeding. The root of the welding wire is always melting in the molten pool. The arc is also slightly faster in the middle and pauses on both sides to prevent the molten pool from falling and forming weld spatter.

[0032] When welding to the tack weld, check for defects. If there are no defects, the tack weld can be fused and welding can continue; if there are defects, they can be cut off with an angle grinder before welding. Welder a can continue welding towards the 12 o'clock direction after reaching the 3 o'clock position; welder b can stop welding after reaching the 12 o'clock position and then start welding from the 6 o'clock position (where welder a started) towards the 9 o'clock position. The first layer of TIG welding should be 3mm thick. After completing the first layer of TIG welding, inspect the weld quality. If there are defects, repair them immediately; if there are no defects, immediately proceed with the second layer of TIG welding.

[0033] 5) After the first layer of the base coat is completed, both welders a and b use the left and right extrusion wire welding method to perform argon arc welding for the second layer of the base coat, following the welding sequence of the first layer. The thickness is also controlled at 3mm. They also check the weld for defects. If there are no defects, they remove the argon protection device and perform manual arc welding for filling and covering.

[0034] 6) Filling and Cover Welding: Manual arc welding is used, with multiple layers and multiple passes. The entire welding process is preheated at 200-300℃. Two welders use the same current, and multiple layers and multiple passes are welded. The lateral swing width of the welding rod should not exceed 12mm. During the welding process, attention should be paid to maintaining the interpass temperature. The temperature is measured with an infrared thermometer throughout the process. The interpass temperature should not be lower than the lower limit of the preheating temperature and should not be higher than 400℃.

[0035] 7) During the first layer of arc welding filler, welders A and B exchange welding sequences. Welder B starts welding from 6 o'clock towards 9 o'clock, while welder A welds symmetrically from 3 o'clock towards 12 o'clock. After welder B reaches 9 o'clock, welder A continues welding towards 12 o'clock. After reaching 12 o'clock, welder A stops welding and then starts welding from the 6 o'clock starting point of welder B towards 3 o'clock. After the first layer of arc welding filler is completed, welders A and B exchange welding sequences again. Welder A starts welding from 6 o'clock... Welding begins at point 3. Welder B welds from point 9 to point 12. Welder A welds to point 3 and then continues welding towards point 12. Welder B stops welding at point 12 and then starts welding from point 6, where welder A began, towards point 9. Welders A and B must not finish their welds at the same point. The joints of each layer must be staggered by 20±5mm. Welds should be as smooth as possible at the joints and when finishing the weld to facilitate slag removal and avoid dead corners and arc crater cracks.

[0036] After each layer and each weld is completed, use an angle grinder to clean up the slag, spatter, etc., and only after the weld quality is inspected and found to be qualified can welding continue.

[0037] The third layer of filler and the cover weld are performed by two welders using the same current in multiple layers and multiple passes, with the welding sequence being changed after each layer is completed.

[0038] After each layer and each weld is completed, welding can continue only after the self-inspection is qualified. The lateral swing width should not exceed 12mm until the welding of the entire weld is completed.

[0039] After the overall welding is completed, use an angle grinder to clean the weld slag and spatter from the weld surface. After passing the self-inspection, mark the welder number and proceed with post-weld heat treatment.

[0040] The first two layers of filler use φ3.2 welding rods with a welding current of 90-120A. The third layer of filler up to the cover layer uses φ4.0 welding rods with a welding current of 100-160A. The welding rods are R317 and have been dried at 350-400℃ and kept at that temperature for more than 2 hours. When using them, they should be kept in an insulated container at 80-120℃ and taken out as needed.

[0041] 8) Post-weld heat treatment: The post-weld heat treatment of the weld shall adopt high-temperature tempering, with a heating and cooling rate ≤150℃, a heating temperature of 720±10℃, and a holding time of not less than 1.5 hours. When the temperature drops to below 300℃, the insulation layer of the heat treatment equipment shall not be removed, the heat treatment equipment shall be powered off, and the residual heat of the heating belt shall be used to slowly cool the weld to room temperature within the insulation layer.

[0042] 9) Weld quality inspection: After the weld is heat treated, 100% RT and UT inspections should be performed (to be completed by dedicated personnel).

[0043] 10) Weld Repair: If any weld fails inspection, the same weld joint and the same location should generally not be repaired more than twice; otherwise, it should be cut off and re-welded. Preheating is mandatory during repair, and heating should be maintained throughout the entire process. After repair, heat treatment and non-destructive testing should be performed again.

[0044] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A welding process for 12Cr1MoV main steam pipe, characterized in that, The specific steps include the following: 1) The root pass is welded by argon arc welding. Before assembling the weld joint, the weld joint is preheated by electric heating. The preheating area is centered on the weld and extends at least 100 mm on both sides. The preheating temperature is 200-300℃. 2) Perform argon protection to replace the air in the protected area; then perform spot welding, with a minimum of four spot welding points, each 30±5mm in length, and the welding points should be evenly distributed in the circumferential direction. 3) After the tack welding is completed, the formal root pass welding begins, with two welders welding symmetrically: Welder a welds from the 6 o'clock position to the 3 o'clock position using a left-right extrusion internal wire feeding method; Welder b welds from the 9 o'clock position to the 12 o'clock position using a left-right extrusion closed wire welding method. After welder a reaches the 3 o'clock position, welder b continues welding towards the 12 o'clock position. After welder b reaches the 12 o'clock position, welder b stops welding and then starts welding from the 6 o'clock position, starting from welder a's starting point at the 6 o'clock position, towards the 9 o'clock position. The thickness of the first layer of argon arc welding root pass is 2.5-3 mm. 4) After the first layer of the base coat is completed, both welders a and b use the left and right extrusion wire welding method to perform argon arc welding for the second layer of the base coat, following the welding sequence of the first layer. The thickness is also controlled at 2.5-3mm. 5) Filling and Cover Welding: Manual arc welding is used, with multiple layers and multiple passes. The entire welding process is preheated at 200-300℃. During the welding process, attention should be paid to maintaining the interpass temperature. The interpass temperature should not be lower than the lower limit of the preheating temperature and should not be higher than 400℃. 6) During the first layer of arc welding filler, welders A and B exchange welding sequences. Welder B starts welding from 6 o'clock to 9 o'clock, while welder A welds symmetrically from 3 o'clock to 12 o'clock. Welder B continues welding towards 12 o'clock after reaching 9 o'clock, and welder A stops welding at 12 o'clock and then starts welding from the point where welder B started at 6 o'clock towards 3 o'clock. After the first layer of arc welding filler is completed, welders A and B exchange welding sequences again. Welder A welds from 6 o'clock to 3 o'clock, while welder B welds from 9 o'clock to 12 o'clock. Welder A continues welding towards 12 o'clock after reaching 3 o'clock, and welder B stops welding at 12 o'clock and then starts welding from the point where welder A started at 6 o'clock towards 9 o'clock. The welding sequence is changed after each layer of filler and cover weld is completed. 7) Post-weld heat treatment: The post-weld heat treatment of the weld is high-temperature tempering with a heating and cooling rate of ≤150℃, a heating temperature of 720±10℃, a holding time of not less than 1.5 hours, and when the temperature drops to below 300℃, the heat treatment equipment is powered off and cooled to room temperature in the insulation layer.

2. The welding process for a 12Cr1MoV main steam pipeline according to claim 1, characterized in that, A V-shaped bevel with blunt edge is adopted, with a bevel angle of 60±3° and a pairing distance of 3~4mm.

3. The welding process for a 12Cr1MoV main steam pipeline according to claim 1, characterized in that, The welding machine selected is the ZX7-400 inverter welding machine. For argon arc welding, DC positive polarity is used, and for electric arc welding, DC reverse polarity is used.

4. The welding process for a 12Cr1MoV main steam pipeline according to claim 1, characterized in that, In the aforementioned filling and cover welding process, the first two layers of filling use φ3.2 welding rods with a welding current of 90-120A, and the third layer of filling up to the cover welding use φ4.0 welding rods with a welding current of 100-160A.

5. The welding process for a 12Cr1MoV main steam pipeline according to claim 4, characterized in that, The welding rod is R317 and has been dried at 350-400℃ and kept warm for more than 2 hours. When using it, it should be placed in a heat-insulating container at 80-120℃ and taken out as needed.

6. The welding process for a 12Cr1MoV main steam pipeline according to claim 1, characterized in that, The argon arc welding uses R31, φ2.5mm welding wire, 2.4mm cerium tungsten electrode, welding current of 100-140A, 300A air-cooled welding torch, argon purity of 99.99%, and argon flow rate of 10-15L / min.

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