Martensite heat-resistant steel G115 large-diameter thick-wall pipe welding and heat treatment process

Abstract

The invention discloses a martensite heat-resistant steel G115 large-diameter thick-wall pipe welding and heat treatment process which comprises the following steps: step 1, performing groove processing on a to-be-welded position of a pipeline, and cleaning and preheating the to-be-welded position of the pipeline; 2, in the welding process, pulse argon arc welding is adopted for the root of the groove, flux-cored wire gas shielded welding is adopted for filling layer welding, low-hydrogen type shielded metal arc welding is adopted for capping layer welding, when filling and capping welding are conducted, segmented symmetrical synchronous welding is adopted for the welding sequence, the groove is divided into even number segments in the circumferential direction, and the number of the segments is equal to the number of the segments; two welders are used for synchronously welding at symmetrical positions; thirdly, after welding is completed, the welding position is subjected to slow cooling, high-temperature tempering and segmented cooling in sequence; through the design of dynamic heat input adjustment, segmented protection, precise temperature control heat treatment and the like, the cold crack risk is reduced, heat affected zone grain coarsening is restrained, residual stress is eliminated, and the mechanical property and service stability of a welded joint are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pipeline welding process, in particular to a martensitic heat-resistant steel G115 large-diameter thick-wall pipe welding and heat treatment process. BACKGROUND

[0002] As the core candidate material of 630℃ ultra-supercritical unit main steam pipeline, martensitic heat-resistant steel G115 (08Cr9W3Co3VNbCuBN) has excellent microstructure stability, creep strength and steam oxidation corrosion resistance, and has broad application prospects in the field of electric power engineering. However, the steel has high alloying degree (containing 9% Cr, 3% W, 3% Co and other elements), and has the technical problems of large cold cracking tendency, easy grain coarsening in heat-affected zone, easy oxidation at root, and easy uneven microstructure after post-weld heat treatment, and difficult to eliminate residual stress.

[0003] In the prior art, the welding of G115 steel usually adopts fixed heat input parameters and conventional straight-line welding sequence, and the post-weld heat treatment relies on single cooling speed and simple temperature monitoring, which has the following disadvantages: (1) the welding heat input control is extensive, and is not adjusted dynamically according to the welding layer and position, which easily leads to the appearance of coarse martensite or blocky ferrite in the heat-affected zone, reducing the plasticity and strength of the joint; (2) the single protection measure during root argon arc welding bottoming leads to prominent backside oxidation problem, affecting the mechanical properties of the joint; (3) the post-weld heat treatment temperature uniformity is poor, and lacks precise segmented cooling strategy, so that the residual stress is not completely eliminated, which easily leads to cracking risk during later service; (4) the welding material and process matching is insufficient, and some welding wires have problems such as uneven transition of W element and slag inclusion, affecting the stability of weld quality. SUMMARY

[0004] In view of the problems existing in the prior art, the present application provides a martensitic heat-resistant steel G115 large-diameter thick-wall pipe welding and heat treatment process, which reduces the cold cracking risk, inhibits the grain coarsening in the heat-affected zone, eliminates the residual stress, and improves the mechanical properties and service stability of the welded joint through dynamic heat input adjustment, segmented protection, precise temperature control heat treatment and other designs.

[0005] The technical scheme of the present application is as follows: In the first aspect of the present application, a martensitic heat-resistant steel G115 large-diameter thick-wall pipe welding and heat treatment process is provided, comprising the following steps: Step one, beveling the pipe to be welded and cleaning and preheating the pipe; Step 2: During the welding process, pulsed argon arc welding is used at the root of the bevel, flux-cored wire gas shielded welding is used for the filler layer welding, and low-hydrogen shielded metal arc welding is used for the capping layer welding. When performing filler and capping welding, the welding sequence adopts segmented symmetrical synchronous welding, which is divided into an even number of segments along the circumference of the bevel, and two welders weld synchronously in symmetrical positions. Step 3: After welding is completed, the welded area is subjected to slow cooling, high-temperature tempering and segmented cooling in sequence.

[0006] In some embodiments of the present invention, in step one, the bevel angle is 55-65°, the blunt edge thickness is 2-3mm, and the blunt edge gap is 3-4mm.

[0007] In some embodiments of the present invention, in step one, during cleaning, anhydrous ethanol is used to clean the bevel and the surface within 20mm on both sides to remove oil and oxide scale, followed by drying at a temperature of 80-100℃ for 30-40 minutes; during preheating, flexible ceramic resistance heaters are arranged 30-50mm on each side of the bevel, and K-type thermocouples are used to monitor the preheating temperature in real time, with the thermocouple spacing not exceeding 50mm.

[0008] In some embodiments of the present invention, in step two, when pulsed argon arc welding is used, the shielding gas is Ar + 2-3% H2, the front gas flow rate is 12-15 L / min, and the back Ar gas flow rate is 8-10 L / min. The welding parameters are dynamically adjusted during the welding process: for the first layer, the current is 80-100A, the voltage is 18-20V, and the welding speed is 80-100mm / min; for the second layer, the current is 100-120A, the voltage is 20-22V, and the welding speed is 90-110mm / min; the interpass temperature is controlled at 180-220℃.

[0009] In some embodiments of the present invention, in step two, the welding parameters for gas shielded welding of flux-cored wire are: current 180-200A, voltage 24-26V, and welding speed 120-140mm / min for flat welding position; current 160-180A, voltage 22-24V, and welding speed 100-120mm / min for vertical welding position.

[0010] In some embodiments of the present invention, in step two, the welding parameters for low-hydrogen electrode arc welding are: current 150-170A, voltage 23-25V, and welding speed 110-130mm / min.

[0011] In some embodiments of the present invention, during the entire welding process of step two, the temperature field of the heat-affected zone is monitored in real time by an infrared thermal imager. When the peak temperature of the heat-affected zone exceeds 1000°C, the welding current is automatically reduced by 5-10A.

[0012] In some embodiments of the present invention, in step three, when the welding position is slowly cooled, the weld and a 100-150mm area on both sides are covered with insulating cotton, and the cooling rate is controlled by a heater: the temperature is reduced from the final welding temperature to 100-120℃ at a cooling rate of 50-60℃ / h, and the temperature is maintained for 2-2.5h.

[0013] In some embodiments of the present invention, during the high-temperature tempering in step three, multi-point temperature-controlled flexible ceramic resistance heating is used. The coverage area of ​​the heater is determined according to the pipe wall thickness, and 8-12 K-type thermocouples are arranged with a spacing of ≤40mm. The heating rate is controlled according to the formula (5500 / wall thickness)℃ / h. The constant temperature is 770℃±5℃, and the constant temperature time is calculated as wall thickness × 5.5min / mm.

[0014] In some embodiments of the present invention, during step three, when performing segmented cooling, the cooling rate is 80-100℃ / h in the temperature range of 770-400℃; and 40-50℃ / h in the temperature range of 400-100℃. During the cooling process, the residual stress of the joint is monitored in real time by a stress sensor. When the stress exceeds 300MPa, the cooling rate is reduced by 10-15℃ / h.

[0015] One or more technical solutions of the present invention have the following beneficial effects: (1) The process provided by this invention significantly reduces the risk of cold cracking by optimizing the welding sequence and heat input control. The segmented symmetrical synchronous welding is adopted, with two welders operating synchronously at symmetrical positions around the bevel, which effectively disperses local heat accumulation and avoids stress concentration. In addition, the process improves welding efficiency and quality stability. By combining welding methods (such as pulsed argon arc welding to ensure root penetration, flux-cored wire gas shielded welding to improve deposition efficiency, and low-hydrogen electrode arc welding to enhance crack resistance), the parameters of each layer are accurately matched. Finally, the segmented control of post-weld heat treatment further eliminates residual stress. Through the step-like treatment of slow cooling, high-temperature tempering and segmented cooling, combined with real-time monitoring, the residual stress is ≤250MPa, which significantly extends the pipeline life.

[0016] (2) By using precise preheating (interpass temperature of 180-220℃), low-hydrogen welding materials and slow cooling process, the cold cracking rate of the welded joint is reduced to less than 0.5% (the cold cracking rate of the prior art is about 3-5%).

[0017] (3) By controlling the dynamic heat input and staggering the welding sequence, the grain size of the heat-affected zone is refined to 5-8μm (approximately 10-15μm in the prior art), the tensile strength of the weld is ≥660MPa (matching the base material), and the impact absorption energy is ≥45J (higher than 41J in the prior art).

[0018] (4) Compared with the existing process, the welding efficiency of the present invention is increased by 15-20% (dynamic parameters reduce rework) and the protective gas consumption is reduced by 10-12% (precise flow control). Attached Figure Description

[0019] Figure 1 This is a flowchart of the welding and heat treatment process of the present invention. Detailed Implementation

[0020] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0021] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, unless otherwise expressly indicated by the invention, the singular form is intended to include the plural form as well. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0022] In a typical embodiment of the present invention, a welding and heat treatment process for large-diameter thick-walled tubes made of martensitic heat-resistant steel G115 is proposed, comprising the following steps: Step 1: Beveling the pipe section to be welded, followed by cleaning and preheating; The bevel angle is 55-65°, the blunt edge thickness is 2-3mm, and the blunt edge gap is 3-4mm. By limiting the bevel angle to 55-65°, the arc is ensured to fully cover the root of the bevel, reducing the risk of incomplete fusion. If the angle is too small (e.g., <55°), it can easily lead to welding torch interference and insufficient penetration; if the angle is too large (e.g., >65°), it increases filler material and heat input, causing deformation. Simultaneously, the 2-3mm blunt edge thickness provides stable root support, preventing burn-through. Existing technologies often have unclear blunt edge thicknesses, frequently resulting in burn-through due to being too thin (<2mm) or incomplete fusion due to being too thick (>3mm). This parameter, through precise control, ensures uniform root penetration and improves joint density. The key role of the 3-4mm blunt edge gap is to promote shielding gas flow and molten pool penetration. Existing technologies often have inaccurate gap control (e.g., <3mm results in insufficient gas coverage on the back side, >4mm results in excessive heat input), leading to root oxidation and slag inclusions.

[0023] During cleaning, anhydrous ethanol is used to clean the bevel and the surface within 20mm on both sides to remove oil and oxide scale. This is followed by drying at 80-100℃ for 30-40 minutes. Anhydrous ethanol cleaning and drying ensure the cleanliness of the bevel surface. G115 steel is sensitive to impurities; if existing cleaning techniques are incomplete, oil and oxide scale can form inclusions during welding, reducing joint strength. By limiting the cleaning range (20mm on both sides) and the drying parameters (80-100℃, 30-40 minutes), the surface contaminant residue rate is reduced to <0.1%, avoiding hydrogen-induced cracking and porosity defects. In particular, the drying temperature of 80-100℃ effectively removes adsorbed moisture, reducing the hydrogen source and lowering the risk of cold cracking by 60% compared to existing technologies.

[0024] During preheating, flexible ceramic resistance heaters are placed 30-50mm on each side of the bevel, and K-type thermocouples are used to monitor the preheating temperature in real time, with the thermocouple spacing not exceeding 50mm. Because uneven preheating in existing technologies easily leads to grain coarsening in the heat-affected zone (size reaching 10-15μm), a dense arrangement of thermocouples with a spacing ≤50mm ensures a temperature gradient ≤5℃, refining the grain size to 5-8μm. This not only improves the joint toughness but also reduces the difficulty of post-weld heat treatment.

[0025] In summary, surface treatment and preheating optimization provide a preliminary guarantee for welding quality and improve process stability.

[0026] Step 2: During the welding process, pulsed argon arc welding is used at the root of the bevel, gas shielded welding with flux-cored wire is used for the filler layer welding, and low-hydrogen shielded metal arc welding is used for the capping layer welding. When performing filler and capping welding, the welding sequence adopts segmented symmetrical synchronous welding, which is divided into an even number of segments along the circumference of the bevel, and two welders weld synchronously in symmetrical positions.

[0027] When using pulsed argon arc welding, the shielding gas is Ar + 2-3% H2, the gas flow rate on the front side is 12-15 L / min, and the Ar gas flow rate on the back side is 8-10 L / min. The welding parameters are dynamically adjusted during the welding process: for the first layer, the current is 80-100A, the voltage is 18-20V, and the welding speed is 80-100 mm / min; for the second layer, the current is 100-120A, the voltage is 20-22V, and the welding speed is 90-110 mm / min; the interpass temperature is controlled at 180-220℃.

[0028] When using pure Ar in existing technologies, severe oxidation occurs on the back side, leading to a decrease in the mechanical properties of the joint. The addition of H2 (2-3%) improves the gas reducing properties, resulting in a back side oxide scale thickness of ≤5μm, which is 50% thinner than existing technologies. At the same time, the coordinated control of the front flow rate of 12-15L / min and the back flow rate of 8-10L / min ensures arc stability and molten pool fluidity, avoiding concave or incomplete fusion defects.

[0029] Furthermore, dynamic parameter adjustments further optimize heat input. The design of 80-100A current and 18-20V for the first layer avoids overheating or incomplete melting at the root, while the increasing current of 100-120A for the second layer matches the filling requirements. The control of the interlayer temperature at 180-220℃ (lower than the existing technology's upper limit of 250℃) reduces heat accumulation, ensuring that the peak temperature of the heat-affected zone is ≤1000℃, thus reducing the risk of grain coarsening.

[0030] The welding parameters for flux-cored wire gas shielded welding are as follows: for flat welding, the current is 180-200A, the voltage is 24-26V, and the welding speed is 120-140mm / min; for vertical welding, the current is 160-180A, the voltage is 22-24V, and the welding speed is 100-120mm / min.

[0031] In existing technologies using fixed parameters, molten metal tends to flow downwards at vertical welding positions, resulting in poor weld formation. In this embodiment, the vertical welding current is lower (160-180A for vertical welding, 180-200A for horizontal welding), the voltage is lower (22-24V for vertical welding, 24-26V for horizontal welding), and combined with a slower welding speed (100-120mm / min), balancing the surface tension of the molten pool with gravity and preventing collapse or undercut. Simultaneously, the gas shielding characteristics of the flux-cored wire reduce spatter, achieving a deposition efficiency of ≥95%. This dynamic adjustment also optimizes the microstructure of the heat-affected zone. Through the set position-specific parameters, heat input is evenly distributed, grain size deviation is ≤1μm, and the formation of blocky ferrite is avoided. Under these parameters, the weld tensile strength is ≥660MPa, and the performance consistency across all positions is improved, reducing the risk of localized weakening.

[0032] The welding parameters for low-hydrogen shielded metal arc welding are: current 150-170A, voltage 23-25V, and welding speed 110-130mm / min. Because G115 steel is sensitive to hydrogen, existing technologies using ordinary welding electrodes can easily lead to cold cracking due to hydrogen diffusion. By matching the current to 150-170A and the voltage to 23-25V, the arc energy is concentrated, resulting in a hydrogen residue of ≤2mL / 100g. Simultaneously, the welding speed of 110-130mm / min ensures smooth weld formation and avoids stress concentration caused by excessive weld reinforcement. Furthermore, these welding parameters improve the mechanical properties of the cap coat; the slower welding speed (110-130mm / min) allows for sufficient tempering, ensuring the cap coat hardness matches the filler coat and reducing the softened zone.

[0033] Throughout the entire welding process, the temperature field of the heat-affected zone is monitored in real time using an infrared thermal imager. When the peak temperature of the heat-affected zone exceeds 1000℃, the welding current is automatically reduced by 5-10A. This prevents grain coarsening and microstructure deterioration, and solves the problem of grain coarsening caused by heat input overload.

[0034] Step 3: After welding is completed, the welded area is subjected to slow cooling, high-temperature tempering and segmented cooling in sequence.

[0035] When slow cooling the welded area, insulation cotton is used to cover the weld and a 100-150mm area on both sides. The cooling rate is controlled by a heater: the temperature is reduced from the final welding temperature to 100-120℃ at a rate of 50-60℃ / h, and held for 2-2.5h. By cooling the welded area, the cooling rate of the weld is matched with the phase transformation kinetics, achieving a martensite transformation rate ≥98%. Simultaneously, the 2-2.5h holding time provides sufficient time for hydrogen diffusion, further reducing the risk of cold cracking. Slow cooling (50-60℃ / h) avoids thermal stress concentration, laying the foundation for subsequent high-temperature tempering.

[0036] During high-temperature tempering, multi-point temperature-controlled flexible ceramic resistance heating is employed. The heater coverage area is determined based on the pipe wall thickness, and 8-12 K-type thermocouples are arranged with a spacing ≤40mm. The heating rate is controlled using the formula (5500 / wall thickness)℃ / h. The constant temperature is 770℃±5℃, and the constant temperature time is calculated as wall thickness × 5.5min / mm. Using the formula (5500 / wall thickness)℃ / h ensures that the temperature rise is inversely proportional to the wall thickness. For example, for a 40mm wall thickness pipe, the heating rate is 137.5℃ / h, reducing thermal stress by 20%. Combined with dense thermocouple placement (spacing ≤40mm), the temperature uniformity deviation is ≤3℃, avoiding localized over-tempering. The constant temperature range of 770℃±5℃ allows for uniform precipitation of M23C6 carbides in G115 steel, improving microstructural stability. The constant temperature time of wall thickness × 5.5min / mm (e.g., 220min for a 40mm wall thickness) ensures sufficient stress relaxation.

[0037] During segmented cooling, the cooling rate is 80-100℃ / h within the temperature range of 770-400℃; and 40-50℃ / h within the temperature range of 400-100℃. Residual stress in the joint is monitored in real time using stress sensors. When the stress exceeds 300MPa, the cooling rate is reduced by 10-15℃ / h. Rapid cooling (80-100℃ / h) in the high-temperature stage (770-400℃) suppresses the precipitation of brittle phases, while slow cooling (40-50℃ / h) in the low-temperature stage (400-100℃) allows for stress relaxation.

[0038] Example 1: Welding of Φ325×40mm G115 steel pipe Pretreatment: Bevel angle 60°, blunt edge 2.5mm, gap 3.5mm; cleaned with anhydrous ethanol and dried at 80℃ for 35min.

[0039] Welding materials: G115 special flux-cored welding wire (Φ1.4mm) dried at 280℃ for 1.2h, CHH767 welding rod (Φ3.2mm) dried at 380℃ for 2.2h.

[0040] Welding process: Root TIG welding: Ar + 2.5% H2 (14L / min for front side, 9L / min for back side), current 90A, voltage 19V, speed 90mm / min.

[0041] Filler layer (flat welding): current 190A, voltage 25V, speed 130mm / min; vertical welding current 170A, voltage 23V, speed 110mm / min.

[0042] Cover layer: Current 160A, voltage 24V, speed 120mm / min; segmented symmetrical synchronous welding (40mm segment).

[0043] Post-weld heat treatment: Slow cooling: Reduce temperature from 300℃ to 110℃ (55℃ / h), and maintain the temperature for 2.2 hours; Tempering: heating rate 5500 / 40=137.5℃ / h, constant temperature 770℃±5℃, constant temperature time 6h; Cooling: 770-400℃ (90℃ / h), 400-100℃ (45℃ / h); Performance testing: tensile strength 675MPa, impact energy absorbed 48J, residual stress 230MPa, no cracks.

[0044] Example 2: Welding of Φ426×50mm G115 steel header Pretreatment: Bevel angle 65°, blunt edge 3mm, gap 4mm; dry at 100℃ for 30min; Welding process: Root argon arc welding current 100A, voltage 20V, speed 85mm / min; filler layer vertical welding current 180A, voltage 24V, speed 105mm / min; Heat treatment: Heating rate 5500 / 50=110℃ / h, constant temperature 770℃±5℃, constant temperature time 7h; cooling rate 770-400℃ (85℃ / h), 400-100℃ (48℃ / h); Performance testing: Tensile strength 682MPa, impact absorbed energy 51J, residual stress 225MPa, meeting the requirements.

[0045] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A welding and heat treatment process for large-diameter thick-walled pipes made of martensitic heat-resistant steel G115, characterized in that, Includes the following steps: Step 1: Beveling the pipe section to be welded, followed by cleaning and preheating; Step 2: During the welding process, pulsed argon arc welding is used at the root of the bevel, flux-cored wire gas shielded welding is used for the filler layer welding, and low-hydrogen shielded metal arc welding is used for the capping layer welding. When performing filler and capping welding, the welding sequence adopts segmented symmetrical synchronous welding, which is divided into an even number of segments along the circumference of the bevel, and two welders weld synchronously in symmetrical positions. Step 3: After welding is completed, the welded area is subjected to slow cooling, high-temperature tempering and segmented cooling in sequence.

2. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step one, the bevel angle is 55-65°, the blunt edge thickness is 2-3mm, and the blunt edge gap is 3-4mm.

3. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step one, during cleaning, anhydrous ethanol is used to clean the bevel and the surface within 20mm on both sides to remove oil and oxide scale. Then, drying is performed at a temperature of 80-100℃ for 30-40 minutes. During preheating, flexible ceramic resistance heaters are placed 30-50mm on each side of the bevel, and K-type thermocouples are used to monitor the preheating temperature in real time. The thermocouple spacing does not exceed 50mm.

4. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step two, when pulsed argon arc welding is used, the shielding gas is Ar + 2-3% H2, the gas flow rate on the front side is 12-15 L / min, and the Ar gas flow rate on the back side is 8-10 L / min. The welding parameters are dynamically adjusted during the welding process: for the first layer, the current is 80-100A, the voltage is 18-20V, and the welding speed is 80-100mm / min; for the second layer, the current is 100-120A, the voltage is 20-22V, and the welding speed is 90-110mm / min; the interpass temperature is controlled at 180-220℃.

5. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step two, the welding parameters for gas shielded welding with flux-cored wire are as follows: for flat welding position, current 180-200A, voltage 24-26V, welding speed 120-140mm / min; for vertical welding position, current 160-180A, voltage 22-24V, welding speed 100-120mm / min.

6. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step two, the welding parameters for low-hydrogen shielded metal arc welding are: current 150-170A, voltage 23-25V, and welding speed 110-130mm / min.

7. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, Throughout the entire welding process in step two, the temperature field of the heat-affected zone is monitored in real time using an infrared thermal imager. When the peak temperature of the heat-affected zone exceeds 1000℃, the welding current is automatically reduced by 5-10A.

8. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step three, when the welding position is slowly cooled, the weld and the 100-150mm area on both sides are covered with insulating cotton. The cooling rate is controlled by a heater: the temperature is reduced from the final welding temperature to 100-120℃ at a rate of 50-60℃ / h, and the temperature is maintained for 2-2.5h.

9. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step three, during high-temperature tempering, multi-point temperature-controlled flexible ceramic resistance heating is used. The heater coverage area is determined according to the pipe wall thickness. 8-12 K-type thermocouples are arranged with a spacing of ≤40mm. The heating rate is controlled according to the formula (5500 / wall thickness)℃ / h. The constant temperature is 770℃±5℃, and the constant temperature time is calculated as wall thickness × 5.5min / mm.

10. The welding and heat treatment process for large-diameter thick-walled martensitic heat-resistant steel G115 pipes as described in claim 1, characterized in that, In step three, during segmented cooling, the cooling rate is 80-100℃ / h in the temperature range of 770-400℃ and 40-50℃ / h in the temperature range of 400-100℃. During the cooling process, the residual stress of the joint is monitored in real time by a stress sensor. When the stress exceeds 300MPa, the cooling rate is reduced by 10-15℃ / h.

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