A post-weld combined heat treatment method for tee branch butt joint

Abstract

This invention provides a post-weld heat treatment method for a tee branch pipe butt joint: After welding, the weld is cooled to 100°C and held at that temperature for 2 hours to allow for martensitic transformation. The main pipe and branch pipe are heated sequentially, with the main pipe reaching 650°C and the branch pipe reaching 750°C. The absolute value of the temperature difference between the branch pipe and the main pipe at the start of heating is equal to the absolute value of the temperature difference between the branch pipe and the main pipe at the end of heating. The branch pipe is held at 750°C for 9 hours. The main pipe and branch pipe are then cooled sequentially, eventually reaching 300°C simultaneously. After cooling to 300°C, the external heat source is cut off, and the pipe is kept at that temperature for slow cooling. The beneficial effects of this invention are: it ensures a controllable temperature gradient between the main pipe and branch pipe during the heating process, utilizing the higher temperature of the branch pipe to ensure sufficient stress release in its thicker sections, while preventing the induction of new thermal stress in the joint area due to excessive temperature difference.

Description

Technical Field

[0001] This invention relates to the field of tee pipe welding technology, and specifically to a post-weld heat treatment method for butt joints of tee branch pipes. Background Technology

[0002] In the energy industry, tees are key components in pipeline systems for fluid diversion and reconnection. The quality of their welded joints directly affects the safe operation and service life of the entire pipeline system. The butt joints between tee branch pipes and main pipes typically employ a full-penetration weld. The intense localized heating and subsequent rapid cooling during welding generate complex residual stress fields in the joint area. Especially for martensitic heat-resistant steels, the welding thermal cycle can induce microstructural transformation. Improper post-weld heat treatment can easily lead to cracking or fatigue failure during service, seriously threatening the long-term safe operation of the pipeline system.

[0003] Currently, the industry mainly uses two methods for post-weld heat treatment of pipe butt joints: overall heat treatment and localized heat treatment. Overall heat treatment is suitable for prefabricated pipe sections that can be completed in a heat treatment furnace, but for installed pipes or large-scale field-made tee fittings, only localized heat treatment can be used. Existing localized heat treatment technologies typically use ceramic resistance heating elements or medium-frequency induction heating coils to heat the weld area, and control the heating temperature, holding time, and cooling rate according to standard specifications. However, conventional localized heat treatment processes are mostly designed for butt joints of pipes with equal diameters. For tee branch pipe butt joints with asymmetrical structures, large differences in wall thickness, and complex heat dissipation conditions, traditional methods have significant technical limitations.

[0004] Specifically, tee branch pipe butt joints have unique geometric characteristics: the main pipe diameter is usually larger than the branch pipe diameter, and the connection between the branch pipe and the main pipe forms a spatial curved weld. The thickness, heat dissipation rate, and heat capacity of the base material on both sides differ significantly. Using a single heating method (ceramic heating elements only or medium-frequency induction only) makes it difficult to achieve a synchronous and uniform temperature field distribution on the main pipe and branch pipe, often resulting in overheating on the branch pipe side and underheating on the main pipe side, or vice versa. This temperature unevenness can induce new thermal stresses and even cause inconsistent microstructure properties in the joint area. More importantly, existing standards and specifications lack clear provisions for coordinated control strategies when there is a temperature difference between the main pipe and branch pipe. In actual construction, this often relies on the experience of operators, making it difficult to guarantee the stability of heat treatment quality.

[0005] The aforementioned problems make the tee-branch pipe joint a weak link in the entire pipeline system. Especially when dealing with thick-walled, high-alloy steel materials, the high rework rate due to improper post-weld heat treatment severely restricts construction progress and increases project costs. More importantly, some tee joints in in-service pipelines fail prematurely under high-temperature and high-pressure conditions due to poor initial heat treatment quality, becoming a potential hazard that could lead to unplanned shutdowns or even safety accidents. Therefore, developing a post-weld combined heat treatment method that adapts to the structural characteristics of tees, achieves coordinated temperature control between the main pipe and branch pipes, and ensures uniform and stable microstructure properties is of significant engineering value for improving the overall reliability of pipeline systems. Summary of the Invention

[0006] The purpose of this invention is to provide a post-weld heat treatment method for the butt joint of a tee branch pipe, in order to solve the problem that the existing technology lacks clear provisions on the coordinated control strategy when there is a temperature difference between the main pipe and the branch pipe of the tee pipe, and in actual construction, it often relies on the experience of operators, making it difficult to guarantee the stability of heat treatment quality.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A post-weld heat treatment method for a butt joint of a tee branch pipe includes the following steps: S1. After the welding of the tee branch pipe butt joint is completed, the weld is cooled to 100℃ and kept at a constant temperature for 2 hours to carry out martensitic transformation; S2. The main pipe and branch pipe start heating one after the other. The main pipe is heated to 650℃ and the branch pipe is heated to 750℃. The absolute value of the temperature difference between the branch pipe and the main pipe when the branch pipe starts heating is equal to the absolute value of the temperature difference between the branch pipe and the main pipe when the branch pipe finishes heating. S3. The branch pipe is kept at a constant temperature of 750℃ for 9 hours, and the main pipe and branch pipe begin to cool down successively. They eventually reach 300℃ simultaneously, and the cooling rate of both is no greater than 6250 divided by the wall thickness; S4. After the main pipe and branch pipes are cooled to 300℃, cut off the external heat source and keep them warm and cool slowly.

[0008] As a preferred embodiment, the branch pipe is heated by a medium-frequency induction heater, and the heating ropes are symmetrically arranged on both sides of the weld seam of the tee branch pipe, with the maximum width extending to the shoulder of the tee. The position of the straight pipe weld seam is 5G.

[0009] As a preferred embodiment, the temperature control thermocouple of the medium-frequency induction heater is arranged with one above and one below the center of the weld, and the detection thermocouple is arranged at a position no more than 100mm away from the weld.

[0010] As a preferred embodiment, the main pipe is heated by a ceramic resistance heating element, and a temperature-controlled thermocouple is arranged on each side shoulder of the three-way branch pipe. The heating width of the ceramic resistance heating element is 600mm outward from the shoulder.

[0011] The beneficial effects of this invention are as follows: 1. This invention employs differentiated target temperatures for the main pipe and branch pipe during the heating phase and initiates heating sequentially according to a specific time sequence. By controlling the dynamic balance of the temperature difference between the two, the absolute value of the temperature difference between the branch pipe and the main pipe when heating is completed is equal to the temperature difference value at the start of heating. This ensures that the main pipe and branch pipe maintain a controllable temperature gradient throughout the heating process. This utilizes the higher temperature of the branch pipe to ensure that the stress in its thicker parts is fully released, while preventing the induction of new thermal stress in the joint area due to excessive temperature difference.

[0012] 2. This invention achieves a uniform decrease in temperature across the entire joint area by requiring both the main pipe and branch pipes to simultaneously reach 300°C during cooling, and strictly controlling their cooling rates to not exceed a limit determined by the wall thickness. This synchronous cooling mechanism fundamentally eliminates the thermal stress caused by the difference in heat dissipation rates between the main pipe and branch pipes, allowing the originally complex irregular joint to complete the stress relaxation process like a homogeneous component. Finally, cutting off the heat source at 300°C and maintaining a slow cooling temperature further solidifies the low-stress state. Attached Figure Description

[0013] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a temperature line graph according to an embodiment of the present invention.

[0014] Figure 2 This is a schematic diagram of the heat source distribution according to an embodiment of the present invention. Detailed Implementation

[0015] To illustrate the features of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0016] Example: Please see Figures 1 to 2 This embodiment provides a post-weld heat treatment method for a butt joint of a tee branch pipe, the specific implementation process of which is as follows: The tee branch pipe butt joint to be processed is made of P92 heat-resistant steel. The main pipe specifications are φ711×150mm, material P92, and the branch pipe specifications are φ381×100mm, with a branch pipe length of 220mm, also made of P92. The weld position is 5G (horizontally fixed). Post-weld heat treatment is performed according to the following steps: S1. Martensitic transformation treatment After all welding work on the tee branch pipe butt joint is completed, the weld area should be kept warm and cooled slowly. When the temperature reading at the weld temperature measurement point drops to 100℃, start timing and maintain this temperature for 2 hours to allow the austenite formed during welding to fully transform into martensite and avoid hardening cracks.

[0017] S2. Differentiated heating After the martensitic transformation is completed, heating begins on the main pipe and branch pipes. The main pipe is heated using ceramic resistance heating elements, with a heating width extending 600mm outwards from the tee shoulder. A temperature-controlled thermocouple is placed on each shoulder of the branch pipe. The branch pipes are heated using medium-frequency induction heaters, with heating ropes symmetrically arranged on both sides of the branch pipe weld, extending to the tee shoulder at maximum heating width. A temperature-controlled thermocouple is placed 50mm above and below the weld centerline, and a detection thermocouple is placed 80mm from the weld edge.

[0018] The main pipe begins heating at a rate of 41℃ / h (6250 / 150≈41℃ / h), with a target temperature of 650℃. When the main pipe temperature reaches 258.26℃, the branch pipe begins heating, increasing its temperature to 750℃ at a rate of 80℃ / h (8000 / 100=80℃ / h). Through precise control, the absolute value of the temperature difference between the branch pipe at the start of heating (main pipe temperature 258.26℃) and the main pipe (158.26℃) is approximately equal to the absolute value of the temperature difference between the branch pipe at the end of heating (branch pipe 750℃, main pipe 591.39℃) and the main pipe (158.61℃). After the branch pipe reaches 750℃, it immediately begins to maintain a constant temperature.

[0019] S3. Constant Temperature The branch tubes were kept at a constant temperature of 750℃ for 9 hours, and the main tubes were also kept at a constant temperature of 650℃ to ensure tissue homogenization.

[0020] S4. Synchronous Cooling After the constant temperature period ended, the main pipe and the branch pipe began to cool down one after the other. When the branch pipe reached a constant temperature of 7.66 hours, the ceramic resistance of the main pipe began to cool down at a rate of 41℃ / h. When the branch pipe reached a constant temperature of 9 hours, it began to cool down at a rate of 62.5℃ / h. Both eventually reached 300℃ at the same time.

[0021] S5. Insulates and slows down cooling. When both the main pipe and branch pipe temperature measuring points show a temperature drop to 300℃, immediately disconnect the power supply to the medium-frequency induction heater and ceramic resistance heating element, and stop external heating. Continue to keep the insulation cotton wrapped around the joint, allowing it to cool naturally to room temperature in still air. Do not force ventilation or pour water to accelerate cooling during the cooling process.

[0022] After heat treatment, hardness tests were performed on the weld and heat-affected zone. The results showed that the hardness values ​​of the base material were distributed between 200-210 HB, and the hardness values ​​of the weld were between 240-250 HB, which met the technical requirements of P92 steel base material and weld. The metallographic structure of the weld was tempered martensite. No recordable defects were found in the non-destructive testing (ultrasound + magnetic particle), indicating that the combined heat treatment method achieved the expected results.

[0023] The above embodiments and accompanying drawings are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. The present invention has been described in detail with reference to preferred embodiments. Those skilled in the art should understand that any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the present invention do not depart from the spirit of the present invention and should also fall within the protection scope of the claims of the present invention. Other related technical structures not disclosed in detail in the present invention are existing technologies in the art.

Claims

1. A method of postweld combined heat treatment of a tee branch butt joint, characterized in that, Includes the following steps: S1. After the welding of the tee branch pipe butt joint is completed, the weld is cooled to 100℃ and kept at a constant temperature for 2 hours to carry out martensitic transformation; S2. The main pipe and branch pipe start heating one after the other. The branch pipe heats up at a rate of 8000°C / wall thickness, and the main pipe heats up at a rate of 6250°C / wall thickness. The main pipe is heated to 650°C, and the branch pipe is heated to 750°C. The absolute value of the temperature difference between the branch pipe and the main pipe when the branch pipe starts heating is equal to the absolute value of the temperature difference between the branch pipe and the main pipe when the branch pipe finishes heating. S3. The branch pipe is kept at a constant temperature of 750℃ for 9 hours. The main pipe and the branch pipe start to cool down one after the other, and eventually reach 300℃ at the same time. The cooling rate of both is 6250 / wall thickness. S4. After the main pipe and branch pipes are cooled to 300℃, cut off the external heat source and keep them warm and cool slowly.

2. The post-weld heat treatment method for the butt joint of a tee branch pipe according to claim 1, characterized in that: The branch pipe is heated by a medium-frequency induction heater. The heating ropes are symmetrically arranged on both sides of the weld seam of the tee branch pipe, with the maximum width reaching the shoulder of the tee. The position of the straight pipe weld seam is 5G.

3. The post-weld heat treatment method for the butt joint of a tee branch pipe according to claim 2, characterized in that: The temperature control thermocouples of the medium-frequency induction heater are arranged one above and one below the center of the weld, and the detection thermocouples are arranged at a distance of no more than 100mm from the weld.

4. The post-weld heat treatment method for the butt joint of a tee branch pipe according to claim 1, characterized in that: The main pipe is heated by a ceramic resistance heating element. A temperature-controlled thermocouple is arranged on each side of the shoulder of the three-way branch pipe. The heating width of the ceramic resistance heating element is 600mm outward from the shoulder.

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