A heat treatment method and device for a chromium-molybdenum-vanadium low-alloy heat-resistant steel welded joint

Abstract

This invention discloses a heat treatment method and apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, relating to the field of heat treatment technology for welded heat-resistant steel in power plant boilers. The invention employs a flame rotating circumferentially around the welded joint at a set rate to heat it to a normalizing temperature range above the phase transformation point Ac3. The welded joint is then subjected to constant-temperature baking, maintaining a high-temperature atmosphere to stabilize the surface temperature of the weld center within ±10℃ of the normalizing temperature range. After holding at this temperature, an insulation layer is applied to slowly cool the welded joint, transforming the austenite structure into a fine, uniform ferrite + pearlite normalized structure. This invention thoroughly refines the coarse bainite structure of the welded joint through normalizing heat treatment, eliminates residual stress, reduces hardness gradient, and effectively prevents reheat cracking and cold cracking. The apparatus is compact, easy to operate, and suitable for operations in confined spaces at thermal power plant sites.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for welding heat-resistant steel in power plant boilers, specifically to a heat treatment method and apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel. Background Technology

[0002] Chromium-molybdenum-vanadium low-alloy pearlitic heat-resistant steel (such as 12Cr1MoVG steel) is widely used in the manufacture of high-temperature pressure-bearing components for thermal power units with metal wall temperatures not exceeding 580℃ due to its high creep strength, good thermal stability, and excellent oxidation resistance. It is one of the key materials for ensuring the safe operation of ultra-supercritical thermal power units. The welding quality of this type of material in the heating surface tubes, headers, and main steam pipes of power plant boilers is a core factor determining their long-term service reliability.

[0003] In actual welding processes, the coarse-grained zone of the heat-affected zone (HAZ) of chromium-molybdenum-vanadium low-alloy heat-resistant steel welded joints undergoes intense welding thermal cycles, leading to significant austenite grain growth and the formation of coarse and brittle bainite structures upon post-weld cooling. Simultaneously, the welding process inevitably generates high residual stress, creating a steep hardness gradient between the weld, HAZ, and base metal. These microstructures and stress states result in welded joints exhibiting high susceptibility to reheat embrittlement and cold cracking. Especially in recent years, with the construction of new power systems, thermal power units frequently participate in deep peak-shaving operations, experiencing drastic load changes. During long-term high-temperature service, welded joints are continuously subjected to the superposition of alternating stresses, making them highly susceptible to reheat cracking or cold cracking in the coarse-grained zone and weld toe area, ultimately leading to cracking failure and seriously threatening the safe and stable operation of the unit.

[0004] To address the aforementioned issues, the current power industry standard DL / T 869-2021 stipulates that welded joints of thin-walled, small-diameter pipes made of chromium-molybdenum-vanadium low-alloy heat-resistant steel with a wall thickness not exceeding 10 mm can be exempted from post-weld heat treatment under specific welding process conditions. However, on-site welding operations are often constrained by various factors such as ambient temperature, wind force, and working space, making welding process control quite difficult. If process control is inadequate, the welded joint will exhibit a high tendency to crack due to microstructural degradation, excessive hardness gradient, and high residual stress, posing a significant risk of service failure even without heat treatment.

[0005] Therefore, this invention proposes a heat treatment method and apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel. Summary of the Invention

[0006] The purpose of this invention is to provide a heat treatment method and apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, which is applicable to solving the problem of different types of cracking failures in the coarse-grained zone of the heat-affected zone and the weld caused by coarse bainite structure, large hardness gradient and high residual stress in welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel.

[0007] According to a first aspect of the present invention, in order to achieve the above-mentioned objective, the present invention provides the following technical solution: a heat treatment method for welded joints of chromium-molybdenum-vanadium low-alloy steel, comprising the following steps: The welding joint is heated by a flame torch, which rotates around the weld joint at a set rate to heat the joint and raise its temperature to a normalizing temperature range that is higher than the phase transformation point Ac3 of the heat-resistant steel, so that the microstructure of the weld joint undergoes complete austenitization transformation. Within the normalizing temperature range, the welded joint is baked at a constant temperature to change the heat conduction mode from contact heat conduction to radiation heat conduction. The high temperature atmosphere is maintained so that the surface temperature of the weld center is stable within ±10℃ of the normalizing temperature range. The constant temperature time is determined according to the wall thickness of the welded joint in a predetermined ratio, and the heating width covers at least 50mm on both sides of the weld. After the constant temperature insulation is completed, the welded joint is kept warm and cooled slowly by an insulation layer, so that the austenitic structure is transformed into a fine and uniform ferrite + pearlite normalized structure.

[0008] Furthermore, the normalizing temperature range is 80°C to 120°C above the phase transformation point Ac3 temperature of the heat-resistant steel.

[0009] Furthermore, the predetermined ratio of the constant temperature time is calculated as 1-1.5 min / mm.

[0010] Furthermore, during the flame heating process, the distance between the flame core and the workpiece surface is controlled to be above 10mm, the flame gun rotation speed is 3-6mm / s, and the rotation mode is half-circular reciprocating motion.

[0011] Furthermore, the insulation layer has a thickness of no more than 30 mm and a width that covers at least 50 mm on each side of the weld.

[0012] According to a second aspect of the present invention, the present invention provides a heat treatment apparatus for a chromium-molybdenum-vanadium low-alloy heat-resistant steel welded joint, for implementing a heat treatment method for a chromium-molybdenum-vanadium low-alloy heat-resistant steel welded joint described in the first aspect, comprising at least two sets of positioning sleeves, the top and bottom of the two sets of positioning sleeves being fixedly connected to a sealing cover by bolts, the diameter of the sealing cover at the fixed end of the positioning sleeve being larger than the diameter at the end away from the fixed sleeve, and the two sets of positioning sleeves being fixedly spliced ​​together into a complete cylinder by clamps; The positioning sleeve is slidably connected to a flame heating component, and a driving component is installed on the outer side of the flame heating component. The driving component is used to drive the flame heating component to rotate and move around the welding joint. The inner wall of the positioning sleeve is provided with a heat insulation layer.

[0013] Furthermore, the flame heating assembly includes an arc-shaped plate slidably mounted on the inner wall of the positioning sleeve, and a rack is fixedly mounted on the bottom of the outer wall of the arc-shaped plate, with the rack protruding a certain distance from the outer wall of the arc-shaped plate to form a stepped surface; The inner wall of the positioning sleeve is provided with a guide groove that matches the arc plate. When the arc plate is installed in the guide groove, the inner wall of the arc plate is flush with the inner wall of the positioning sleeve. Multiple flame guns are evenly installed at equal intervals on the inner wall of the arc-shaped plate, and the nozzles of the flame guns are set in a conical shape.

[0014] Furthermore, an inner cavity is provided in the middle of the outer wall of the arc-shaped plate, and the two ends of the inner cavity are closed. When the outer wall of the arc-shaped plate is in contact with the side wall of the guide groove, the inner cavity is sealed. The positioning sleeve is equipped with a gas pipeline at the position corresponding to the inner cavity, which is used to deliver combustion gas into the inner cavity.

[0015] Furthermore, the drive assembly includes a motor fixedly mounted on the outer wall of the positioning sleeve, and a gear is fixedly mounted on the output end of the motor, with the gear penetrating the positioning sleeve and meshing with a rack.

[0016] Furthermore, the insulation layer is made of aluminum silicate refractory fiber, which is fixed to the inner wall of the module by high-temperature resistant glass fiber fixing straps.

[0017] This invention has at least the following beneficial effects: 1. This invention breaks through the traditional post-weld tempering heat treatment method that only involves tempering below the phase transformation point temperature. It innovatively employs flame heating to raise the weld joint to a normalizing temperature range above the material's phase transformation point Ac3, causing complete austenitization. Subsequent slow cooling at this temperature yields a fine and uniform ferrite-pearlite normalized microstructure. This microstructure is consistent with the original microstructure of the base material, with a high degree of grain refinement and dispersed carbides. This fundamentally eliminates the original coarse, brittle, and hard bainite microstructure in the coarse-grained region of the weld heat-affected zone, completely solving the embrittlement problem caused by microstructure coarsening and significantly improving the weld joint's resistance to reheat cracking and cold cracking.

[0018] 2. This invention enables thorough microstructural reconstruction within the welded joint during high-temperature normalizing, effectively releasing existing residual welding stress during austenitization. Simultaneously, after normalizing, the microstructure of the weld zone, heat-affected zone, and base metal zone becomes more uniform, resulting in more consistent hardness values ​​across the three regions and a significantly reduced hardness difference, creating a smooth hardness transition. This uniform hardness distribution avoids the stress concentration and crack initiation risks induced by excessive hardness gradients in traditional weld conditions or after simple tempering, significantly improving the stability of the welded joint under high-temperature service and alternating loads.

[0019] 3. The flame heat treatment device designed in this invention adopts a modular split structure, and each functional module can be quickly assembled and disassembled. The overall size is small and the weight is light. It can be flexibly applied to the working environment of dense pipe rows and narrow space in thermal power units. The positioning sleeve can be adapted and replaced according to different pipe diameters to ensure that the device is reliably fixed on the heated surface pipe. The flame heating component and multiple equally spaced conical flame nozzles, together with the gear and rack transmission structure driven by an external motor, realize automatic, uniform and controllable flame scanning heating of the welded joint in the circumference. No manual hand operation is required, which reduces labor intensity while ensuring the consistency and reproducibility of heating.

[0020] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic flowchart of the heat treatment method described in this invention; Figure 2 This is a schematic diagram of the overall structure of the heat treatment apparatus described in this invention; Figure 3 This is a schematic diagram of the internal structure of the positioning sleeve of the present invention; Figure 4 This is a three-dimensional schematic diagram of the arc-shaped plate structure of the present invention; Figure 5 This is a schematic diagram of the tube sample analysis results of the heat treatment method described in this invention; Figure 6 This is a schematic diagram of the hardness verification results of the heat treatment method described in this invention; Figure 7 This is a schematic diagram of the analysis results of the tube sample in Comparative Example 1 of the present invention; Figure 8 This is a schematic diagram of the hardness verification results in Comparative Example 1 of the present invention; Figure 9 This is a schematic diagram of the analysis results of the tube sample in Comparative Example 2 of the present invention; Figure 10 This is a schematic diagram of the hardness verification results in Comparative Example 2 of the present invention.

[0023] Figure label: 1. Positioning sleeve; 2. Sealing cover; 3. Clamp; 4. Arc plate; 5. Rack; 6. Guide groove; 7. Flame gun; 8. Inner cavity; 9. Gas pipeline; 10. Motor; 11. Gear; 12. Temperature monitoring module. Detailed Implementation

[0024] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] Example 1: Please see Figure 1 This invention provides a technical solution: a heat treatment method for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, comprising the following steps: S1. Heating step: Using flame heating, the flame gun rotates around the weld joint at a set rate to heat the weld joint, raising its temperature to the normalizing temperature range above the phase transformation point Ac3 of the heat-resistant steel, so that the microstructure of the weld joint undergoes complete austenitization transformation. S2. The welded joint is baked at a constant temperature within the normalizing temperature range, so that the heat conduction mode changes from contact heat conduction to radiation heat conduction. The high temperature atmosphere is maintained so that the surface temperature of the weld center is stable within ±10℃ of the normalizing temperature range. The constant temperature time is determined according to the wall thickness of the welded joint according to a predetermined ratio, and the heating width covers at least 50mm on both sides of the weld. S3. After the constant temperature heat preservation is completed, the welded joint is kept warm and cooled slowly by the heat preservation layer, so that the austenitic structure is transformed into a fine and uniform ferrite + pearlite normalized structure.

[0026] For the technical solution of this embodiment, the normalizing temperature range is 80°C to 120°C above the phase transformation point Ac3 temperature of the heat-resistant steel. In this embodiment, the chromium-molybdenum-vanadium low-alloy heat-resistant steel is 12Cr1MoVG, whose Ac3 ≈ 885°C, so the heating temperature is about 980°C, which ensures that the microstructure of the pearlitic heat-resistant steel welded joint undergoes complete austenitization transformation.

[0027] For the technical solution of this embodiment, the predetermined ratio of the constant temperature time is calculated as 1-1.5 min / mm.

[0028] Regarding the technical solution of this embodiment, during the flame heating process, the distance between the flame core and the workpiece surface is controlled to be above 10mm. Maintaining a certain distance between the flame core and the workpiece surface primarily prevents the hottest flame core from directly impacting the weld joint surface, thereby avoiding defects such as overheating, melting, or even carburization or carbonization of the pipe surface due to excessive local heat flux density. This distance utilizes the physical characteristics of the flame, transforming the heat transfer method from concentrated direct impact to more extensive radiation and convection heat transfer, thus expanding the area of ​​the heating spot and allowing heat to be distributed more gently and evenly in the heating area. This helps to form a gentler temperature gradient in the circumferential and axial directions of the entire weld joint, significantly reducing the huge thermal stress caused by localized rapid heating, creating favorable prerequisites for the subsequent uniform austenitization of the microstructure, and also protecting the integrity of the original pipe surface and the stability of its chemical composition. The flame torch rotates at a speed of 3-6 mm / s, in a semi-circular reciprocating motion. This rotational speed range is crucial for achieving uniform circumferential temperature in the weld joint. At this speed, the movement of the flame torch and the input and conduction of heat reach a dynamic balance. If the speed is too fast, the heat received per unit length of weld will be insufficient, making it difficult for the temperature on the back side of the heated tube to reach the required austenitizing temperature, resulting in incomplete heating or incomplete microstructure transformation. If the speed is too slow, the flame will remain in a certain local area for too long, which is equivalent to reducing the effective distance between the flame core and the workpiece, easily causing local overheating. Maintaining this speed range ensures that the flame energy continuously and uniformly scans and covers the surface of the tube.

[0029] Regarding the technical solution of this embodiment, the thickness of the insulation layer is no more than 30mm and the width covers at least 50mm on both sides of the weld. The thickness of the insulation layer module is no more than 30mm to ensure that the cooling rate is not too slow and to reduce the time in the reheat crack sensitive area. The length is not shorter than the heating width to ensure that the insulation layer completely covers the heating constant temperature area.

[0030] The heat treatment process parameters for this embodiment are shown in Table 1 below: Precautions during operation include: (1) During the heating process, the distance between the flame core and the workpiece surface should be controlled at more than 10mm, and the spray gun should be rotated around the pipe. It is strictly forbidden to heat at a fixed point for a long time to ensure that the temperature of the heating area is uniform.

[0031] (2) Monitor the heating temperature of the welded joint in real time to ensure that the heating temperature is controlled within the range required by the process.

[0032] (3) After the surface temperature of the center of the weld reaches the set constant temperature, reduce the flame energy. The heat conduction changes from contact heat conduction to radiation heat conduction, forming a high temperature atmosphere, which can maintain the temperature at a constant temperature until the heat preservation ends.

[0033] (4) After the constant temperature is completed, the prefabricated insulation layer is used to cover the heating area and slow down the cooling. The width of the insulation layer should cover the heating area and the thickness should not exceed 30 mm, so as to reduce the time the joint stays in the reheat crack sensitive temperature range and reduce the risk of crack initiation.

[0034] Example 2: like Figure 2 As shown, this embodiment provides a heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, used to implement a heat treatment method for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel described in Embodiment 1, including: At least two sets of positioning sleeves 1, the top and bottom of the two sets of positioning sleeves 1 are fixedly connected to a sealing cover 2 by bolts, the diameter of the sealing cover 2 at the fixed end of the positioning sleeve 1 is larger than the diameter at the end away from the fixed sleeve, and the two sets of positioning sleeves 1 are fixedly spliced ​​into a complete cylinder by clamps 3. The positioning sleeve 1 has a flame heating component that is slidably connected inside. A drive component is installed on the outer side of the flame heating component. The drive component is used to drive the flame heating component to rotate and move around the welding joint. The inner wall of the positioning sleeve 1 is provided with a heat insulation layer.

[0035] Regarding the technical solution of this embodiment, such as Figure 2 and Figure 3 As shown, the flame heating assembly includes an arc-shaped plate 4 that is slidably installed on the inner wall of the positioning sleeve 1. A rack 5 is fixedly installed on the bottom of the outer wall of the arc-shaped plate 4. The rack 5 protrudes from the outer wall of the arc-shaped plate 4 by a certain distance to form a stepped surface. The inner wall of the positioning sleeve 1 is provided with a guide groove 6 that is adapted to the arc plate 4. When the arc plate 4 is installed in the guide groove 6, the inner wall of the arc plate 4 is flush with the inner wall of the positioning sleeve 1. Specifically, the arc-shaped plate 4 slides on the inner wall of the sleeve via the guide groove 6, allowing precise control of its circumferential movement trajectory and ensuring smooth and reliable motion. The rack 5 protruding from the bottom of the outer wall of the arc-shaped plate 4 meshes with the drive gear 11, converting rotational power into circumferential reciprocating or circular motion of the arc-shaped plate 4, achieving circumferential flame scanning heating of the welded joint. The stepped surface formed by the protrusion of the rack 5 engages with the corresponding stepped surface in the guide groove 6 during arc-shaped plate 4 installation, providing axial and radial bidirectional limiting to prevent the arc-shaped plate 4 from detaching or swaying during movement. Simultaneously, the arc-shaped plate... After installation, the inner wall of plate 4 is flush with the inner wall of positioning sleeve 1. This flush structure ensures that the inner ring surface of the entire heating device is smooth and continuous, without any protrusions or depressions. The advantages are: it can avoid interference or jamming between the arc plate 4 and the inner wall of the sleeve during the sliding process, and it can also ensure that the flame gun 7 or heating element will not be hindered by the uneven inner wall when rotating in the circumferential direction, thus allowing the flame gun 7 to be close to the surface of the pipe for efficient heating with a very small gap; in addition, the smooth inner wall will not scratch the surface of the pipe or the insulation layer when the device is disassembled and assembled, and it is also convenient to quickly clean and replace the arc plate 4.

[0036] Multiple flame guns 7 are evenly installed at equal intervals on the inner wall of the arc plate 4. The nozzles of the flame guns 7 are set in a conical shape. The multiple flame guns 7 are evenly arranged at equal intervals on the inner wall of the arc plate 4, which can form multiple independent and evenly distributed heat source points in the circumferential direction. When the arc plate 4 moves back and forth in the circumferential direction or swings in a small range along the positioning sleeve 1, these flame guns will heat the entire circumferential surface of the weld joint at multiple points simultaneously, thereby greatly improving the circumferential uniformity of the heat field and avoiding local overheating or underheating caused by fluctuations or pauses in the movement speed of a single flame gun. The spacing between each flame gun is equal, so that the heat-affected zones of adjacent flames overlap and compensate for each other, forming a continuous and stable annular heating zone, effectively eliminating the heating blind zone and ensuring that the temperature of the weld and the heat-affected zones on both sides is consistent in the circumferential direction. The nozzle of the flame gun 7 is set in a conical shape, which can produce a flame focusing effect, making the flame energy more concentrated and reducing the heat loss to the outside, thereby improving the directionality and heating efficiency of the heat source.

[0037] Regarding the technical solution of this embodiment, an inner cavity 8 is provided in the middle of the outer side wall of the arc plate 4. The two ends of the inner cavity 8 are closed. When the outer side wall of the arc plate 4 is in contact with the side wall of the guide groove 6, the inner cavity 8 is sealed. A gas pipeline 9 is installed on the positioning sleeve 1 at the position corresponding to the inner cavity 8, for supplying combustion gas to the inner cavity 8; Specifically, this design cleverly integrates the gas delivery channel into the sliding interface between the arc-shaped plate 4 and the positioning sleeve 1. When the arc-shaped plate 4 is installed in the guide groove 6, its outer wall fits tightly against the side wall of the guide groove 6, forming a closed annular or arc-shaped inner cavity 8. This inner cavity 8 acts as a gas distribution main pipe. The fixed gas pipeline 9 on the positioning sleeve 1 directly supplies gas to the inner cavity 8. After entering the sealed inner cavity 8, the gas is evenly distributed to each flame gun 7 installed on the arc-shaped plate 4. This solution eliminates the complex gas supply method that requires connecting flexible hoses or rotary joints to movable parts in traditional systems, avoiding... To mitigate the risks of pipe entanglement, wear, or leakage caused by the reciprocating motion of the arc plate 4, the inner cavity 8 moves together with the arc plate 4, while the gas supply interface is located on the stationary positioning sleeve 1. The dynamic seal between the two is achieved solely through the natural contact of the mating surfaces, resulting in a simple and reliable structure. At the same time, the closed inner cavity 8 at both ends ensures that the gas does not leak from the ends of the arc plate 4, ensuring stable gas supply pressure and operational safety. This design significantly reduces the number of moving pipes, lowers the failure rate of the device, and improves the convenience of disassembly, assembly, and maintenance. It is especially suitable for working environments requiring frequent movement and operation in confined spaces.

[0038] It should be further explained that the dynamic seal between the arc plate 4 and the guide groove 6 is not an absolute seal in the strict sense, because there will be a certain degree of leakage during the movement of the arc plate 4, but this does not affect the overall performance of the device or cause any adverse effects.

[0039] In this embodiment, the driving component includes a motor 10 fixedly mounted on the outer wall of the positioning sleeve 1. A gear 11 is fixedly mounted on the output end of the motor 10, and the gear 11 penetrates the positioning sleeve 1 and meshes with the rack 5. This driving scheme places the power source externally in the positioning sleeve 1, achieving spatial separation between the driving component and the heating cavity. The motor 10 is mounted on the outer wall of the sleeve, avoiding the influence of high-temperature radiation and the gas atmosphere in the heating area, significantly improving the operational reliability and service life of the motor 10. The gear 11 meshes directly with the internal rack 5 through a through hole, eliminating the need for intermediate transmission components. The transmission chain is simple and compact, and the power transmission is direct and efficient, avoiding backlash, wear, or energy loss caused by adding intermediate links such as couplings and transmission shafts. This structure makes the meshing position of the gear 11 and rack 5 fixed and easy to observe, facilitating adjustment of the meshing clearance and lubrication maintenance during device assembly. Because the motor 10 is externally mounted, the vibration and heat generated during its movement are not directly conducted to the heating area, which helps maintain the temperature stability of the flame heating. Furthermore, if the motor 10 or gear 11 malfunctions, it can be quickly replaced or repaired from the outside without disassembling the entire heating device, significantly improving the maintainability of the device under on-site construction conditions. The through-hole, after proper sealing treatment (such as adding a high-temperature resistant sealing ring), will not cause gas leakage, while ensuring that gear 11 remains stably engaged with rack 5 during the reciprocating motion of the arc plate 4, driving the arc plate 4 to move smoothly and accurately along the preset trajectory, achieving uniform scorching heating of the entire circumference of the welded joint.

[0040] In this embodiment, the insulation layer is made of aluminosilicate refractory fiber, which is fixed to the inner wall of the module using high-temperature resistant fiberglass straps. Aluminosilicate refractory fiber itself has extremely low thermal conductivity and good thermal shock resistance, effectively slowing down the cooling rate after the welded joint has completed its constant-temperature insulation process, achieving the required slow cooling. This material is lightweight and elastic, allowing it to tightly conform to the irregular gaps between the outer wall of the pipe and the inner wall of the device, forming a complete and uniform insulation covering. This prevents rapid heat loss from the gaps, ensuring temperature uniformity throughout the heating zone during the slow cooling phase. Furthermore, aluminosilicate refractory fiber is chemically stable and will not pulverize or release corrosive gases under long-term high-temperature use, thus preventing contamination or damage to the pipe surface.

[0041] In this embodiment, a temperature monitoring module 12 is also installed on the positioning sleeve 1. It is arranged at a 90° angle to the flame gun 7 and is located between the two guns. The temperature monitoring module 12 includes an infrared thermometer and a temperature display terminal. The detection probe of the infrared thermometer is at the same horizontal position as the gun, ensuring that it can be directly facing the center area of ​​the weld joint. It collects the temperature data of the heating surface in real time and transmits it to the temperature display terminal. The temperature display terminal can display the temperature change curve in real time, realizing accurate monitoring of the heating temperature with a temperature measurement accuracy of ±1℃.

[0042] The technical solution of this invention will be further elaborated below with specific examples: Based on the pipe specification Ф63.5×4mm, select the appropriate positioning sleeve 1, assemble the positioning sleeve 1 into two half-circle modules, and select a prefabricated insulation layer made of aluminum silicate refractory fiber material with a thickness of 25mm and a width of 150mm (covering 50mm on each side of the weld seam), and use high-temperature resistant fiberglass fixing straps to fix it inside the module.

[0043] Place the pipe welding joint inside the positioning sleeve 1, adjust the height of the positioning sleeve 1 to make the center of the weld horizontal and centered, and ensure that the temperature measuring point and the center point of the spray gun are located on the weld. Tighten the bolts and clamps 3 to fix the positioning sleeve 1 to the pipe.

[0044] Point the infrared thermometer probe at the center of the weld, connect it to the temperature display terminal, calibrate the thermometer to ensure a temperature measurement accuracy of ±1℃, and start the temperature monitoring system; connect the oxygen-acetylene spray gun to the gas supply pipelines of the oxygen and acetylene cylinders, start the automatic ignition, and adjust the flow regulating valve to a neutral flame.

[0045] Start the drive motor 10 and adjust the speed to 2 mm / s. After moving half a revolution, it will automatically turn the direction of motion and reciprocate. Monitor the temperature rise of the welding joint in real time. When the temperature rises to 980℃, adjust the flow valve to maintain the temperature of the welding joint. The constant temperature time is 4 minutes. After the constant temperature is completed, turn off the flame and monitor the temperature drop. Remove the device after it cools to room temperature.

[0046] like Figure 5 As shown, the tube samples after heat treatment were cut and sampled for analysis. Metallographic analysis and hardness test were carried out. The test results are as follows: Metallographic structure: There is no obvious overheated coarse grain section in the heat-affected zone of the weld joint. The structure of the heat-affected zone and the weld is fine and uniform ferrite + pearlite. The pearlite has a complete morphology. Carbides are distributed in a dotted manner in the grain. There is no carbide aggregation at the grain boundaries. The grain size is about 7. The structure is uniform.

[0047] Hardness testing: The average hardness of the weld is 220 HV0.2, the average hardness of the heat-affected zone is 201 HV0.2, and the average hardness of the base metal is 182 HV0.2. The overall hardness distribution is uniform, with a peak hardness of 237 HV0.2, meeting the requirements of GB / T 5310 and DL / T 869 standards. The maximum hardness difference between the weld, heat-affected zone, and base metal is 43 HV0.2, indicating a significantly reduced hardness gradient. Figure 6 As shown.

[0048] Service verification: After the above heat treatment, the welded joint was installed and operated for 30,000 hours without any reheat cracking failure in the heat-affected zone, and the operation was good.

[0049] Comparative Example 1: Analysis of tube samples without heat treatment A comparative analysis was conducted on 12Cr1MoVG pipe samples that were not subjected to post-weld heat treatment.

[0050] The coarse-grained microstructure of the heat-affected zone is mainly acicular bainite with relatively large grains, accompanied by a small amount of intergranular carbide precipitation and ferrite. The weld zone microstructure is coarse bainite. The base metal microstructure is ferrite + pearlite, with pearlite lamellars locally dispersed and grain boundary carbides aggregated in fine particles, such as... Figure 7 As shown; Hardness testing: The average hardness of the weld zone was 284 HV0.2, and the average hardness of the base metal was 188 HV0.2, both conforming to the relevant provisions of GB / T5310 and DL / T 869 for 12Cr1MoVG pipes (base metal hardness range: 140-205 HV, weld hardness not exceeding the base metal hardness by 100 HBW). However, the hardness of the coarse-grained zone in the heat-affected zone was generally higher, with an average hardness of 278 HV0.2, approximately 90 HV0.2 higher than the average hardness of the base metal. Notably, the maximum local hardness in this area reached 298 HV0.2, exceeding the hardness of the weld zone. A large hardness gradient was observed. Figure 8 As shown.

[0051] Comparative Example 2: Implementation of Post-Weld Tempering Heat Treatment Method Based on DL / T 869 Standard Requirements Workpiece pretreatment: Clean the slag on the surface of the 12Cr1MoVG welded joint to ensure that the surface is clean and free of impurities.

[0052] The pipe butt weld was constructed by wrapping a resistance heating rope around the weld seam, insulating it, and then heating it with electricity. The heating process is shown in Table 2 below: Once the temperature drops to 400°C, the heating device is cooled to room temperature.

[0053] Cut tube samples for testing: The coarse-grained microstructure of the heat-affected zone is mainly acicular bainite with relatively large grains, accompanied by a small amount of intergranular carbide precipitation and ferrite. The weld zone microstructure is coarse bainite. The base metal microstructure is ferrite + pearlite, with pearlite lamellae locally dispersed and grain boundary carbides aggregated in fine particles. The microstructure is basically consistent with that before post-weld heat treatment and has not changed. Figure 9 As shown.

[0054] Hardness testing: The average hardness of the weld was 226 HV0.2, the average hardness of the heat-affected zone was 183 HV0.2, the average hardness of the base metal was 187 HV0.2, and the peak hardness was 228 HV0.2, meeting the requirements of GB / T 5310 and DL / T 869 standards. The maximum hardness difference between the weld, heat-affected zone, and base metal was 54 HV0.2, indicating a significant improvement in hardness uniformity. However, due to the lack of improvement in microstructure, the tendency to crack remains high. Figure 10 As shown.

[0055] In the description of this specification, the references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0056] The foregoing has shown and described the basic principles, main features, and advantages of this application. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of the claims of this application.

Claims

1. A heat treatment method for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, characterized in that, Includes the following steps: The flame heating method is adopted, and the flame gun (7) rotates around the weld joint at a set rate to heat the weld joint, so that its temperature rises to the normalizing temperature range higher than the phase transformation point Ac3 temperature of the heat-resistant steel, so that the microstructure of the weld joint undergoes complete austenitization transformation. Within the normalizing temperature range, the welded joint is baked at a constant temperature to change the heat conduction mode from contact heat conduction to radiation heat conduction. The high temperature atmosphere is maintained so that the surface temperature of the weld center is stable within ±10℃ of the normalizing temperature range. The constant temperature time is determined according to the wall thickness of the welded joint in a predetermined ratio, and the heating width covers at least 50mm on both sides of the weld. After the constant temperature insulation is completed, the welded joint is kept warm and cooled slowly by an insulation layer, so that the austenitic structure is transformed into a fine and uniform ferrite + pearlite normalized structure.

2. The heat treatment method for a welded joint of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 1, characterized in that: The normalizing temperature range is 80°C to 120°C above the phase transformation point Ac3 temperature of the heat-resistant steel.

3. The heat treatment method for a welded joint of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 1, characterized in that: The predetermined ratio of the constant temperature time is calculated as 1-1.5 min / mm.

4. The heat treatment method for a welded joint of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 1, characterized in that: During the flame heating process, the distance between the flame core and the workpiece surface is controlled to be above 10mm, and the rotation speed of the flame gun (7) is 3-6mm / s, and the rotation mode is half-cycle reciprocating motion.

5. The heat treatment method for a welded joint of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 1, characterized in that: The insulation layer is no more than 30mm thick and its width covers at least 50mm on both sides of the weld.

6. A heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel, used to implement the heat treatment method for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to any one of claims 1 to 5, characterized in that, It includes at least two sets of positioning sleeves (1), and the top and bottom of the two sets of positioning sleeves (1) are fixedly connected to a sealing cover (2) by bolts. The diameter of the sealing cover (2) at one end fixed to the positioning sleeve (1) is greater than the diameter at the end away from the fixed sleeve. The two sets of positioning sleeves (1) are fixedly spliced ​​into a complete cylinder by clamps (3). The positioning sleeve (1) is internally slidably connected to a flame heating component, and a driving component is installed on the outer side of the flame heating component. The driving component is used to drive the flame heating component to rotate and move around the welding joint. The inner wall of the positioning sleeve (1) is provided with a heat insulation layer.

7. The heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 6, characterized in that: The flame heating assembly includes an arc-shaped plate (4) that is slidably installed on the inner wall of the positioning sleeve (1). A rack (5) is fixedly installed on the bottom of the outer wall of the arc-shaped plate (4). The rack (5) protrudes from the outer wall of the arc-shaped plate (4) by a certain distance to form a stepped surface. The inner wall of the positioning sleeve (1) is provided with a guide groove (6) that is compatible with the arc plate (4). When the arc plate (4) is installed in the guide groove (6), the inner wall of the arc plate (4) is flush with the inner wall of the positioning sleeve (1). Multiple flame guns (7) are evenly installed at equal intervals on the inner wall of the arc plate (4), and the nozzle of the flame gun (7) is set in a conical shape.

8. The heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 7, characterized in that: An inner cavity (8) is provided in the middle of the outer wall of the arc plate (4). The two ends of the inner cavity (8) are closed. When the outer wall of the arc plate (4) is in contact with the side wall of the guide groove (6), the inner cavity (8) is sealed. The positioning sleeve (1) is equipped with a gas pipeline (9) at the position corresponding to the inner cavity (8) for supplying combustion gas to the inner cavity (8).

9. The heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy steel according to claim 8, characterized in that: The drive assembly includes a motor (10) fixedly installed on the outer wall of the positioning sleeve (1). A gear (11) is fixedly installed at the output end of the motor (10), and the gear (11) passes through the positioning sleeve (1) and meshes with the rack (5).

10. The heat treatment apparatus for welded joints of chromium-molybdenum-vanadium low-alloy heat-resistant steel according to claim 1, characterized in that: The insulation layer is made of aluminum silicate refractory fiber and is fixed to the inner wall of the module by high-temperature resistant fiberglass fixing straps.

Images