A high-temperature creep life prolonging process for a welded joint of F92 heat-resistant steel

Abstract

The application discloses a F92 heat-resistant steel welded joint high-temperature creep life prolonging process and relates to the technical field of material science and engineering. After step 1, post-welding heat treatment, a non-penetrating micro-texture is prepared in the fine-grain heat-affected zone by using an ultra-short pulse laser; 2. A non-contact high-frequency pulse current system is deployed; 3. A high-precision micro-resistance probe is arranged to monitor local resistance; 4. A low-duty cycle pulse is applied for maintenance in the high-temperature steady-state creep stage; 5. When the resistance change rate exceeds a threshold value, high-intensity pulse intervention is triggered; 6. Pulse parameters are dynamically adjusted according to temperature and resistance; 7-9. The texture residual compressive stress and the pulse current are cooperatively used to suppress cavities, repair defects and optimize dislocations; 10. The performance is regularly detected to evaluate the effect. Preferred parameters are as follows: the micro-dimple diameter is 50-100 mu m, the pulse frequency is 10-100 kHz, and the probe accuracy is greater than or equal to 1 mu omega. The process is cooperatively used for monitoring, maintenance and intervention, thereby prolonging the creep life of the joint and being suitable for a service scene of 600-650 DEG C.

Description

Technical Field

[0001] This invention relates to the fields of materials science and engineering technology, and more specifically, to a high-temperature creep life extension process for F92 heat-resistant steel welded joints. Background Technology

[0002] In the fields of advanced ultra-supercritical power generation and fourth-generation nuclear power, the high-temperature pressure environment of 600-650℃ places extreme demands on the long-term creep performance of structural materials. F92 martensitic heat-resistant steel, with its Cr-Nb-V composite strengthening mechanism, has become the preferred material for welded load-bearing components. However, during the welding process, the fine-grained heat-affected zone (FGHAZ) undergoes extreme thermal cycling of austenitization above 1300℃ and rapid cooling. Although the resulting fine martensitic grains have high initial strength, they are prone to grain boundary weakening under high-temperature creep, which becomes a fatal weakness for joint failure. Its creep fracture life is only 40%-60% of that of the base material.

[0003] Existing research reveals that creep damage in FGHAZ originates from a chain reaction of dislocation slip-void nucleation-crack propagation. Traditional post-weld tempering can only eliminate macroscopic residual stress and cannot control the evolution of microscopic defects. Current mainstream surface strengthening technologies, such as plasma spraying, are prone to delamination at the interface due to the mismatch between the thermal expansion coefficients of the coating and the substrate. While single-pulse current treatment can activate dislocation movement, the lack of stress constraint leads to a rapid decay of the strengthening effect. More importantly, current damage monitoring relies on offline metallographic analysis. By the time obvious voids are detected, the damage has already entered an irreversible stage, and the lag causes intervention to fail.

[0004] As the service life of equipment extends to over 200,000 hours, early micro-damage monitoring and precise reinforcement of FGHAZ have become technical bottlenecks. Existing solutions suffer from three disconnects: disconnect between monitoring and service process, disconnect between reinforcement methods and micro-damage mechanisms, and disconnect between process parameters and real-time operating conditions. This leads to frequent unplanned shutdowns and replacements of F92 welded joints, causing economic losses of over 10 billion yuan annually in the thermal power industry alone, thus hindering the improvement of the reliability of high-end energy equipment.

[0005] Therefore, existing technologies suffer from problems such as difficulty in early detection of FGHAZ micro-damage, easy attenuation of single-strength enhancement, and difficulty in adapting parameters to operating conditions. Summary of the Invention

[0006] In order to overcome the problems of difficulty in early detection of FGHAZ micro-damage, easy attenuation of single reinforcement, and difficulty in adapting parameters to working conditions in the existing technology, this invention discloses a high-temperature creep life extension process for F92 heat-resistant steel welded joints, which can effectively solve the above-mentioned technical problems.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0008] A high-temperature creep life extension process for F92 heat-resistant steel welded joints includes the following steps:

[0009] Step 1: Laser surface texturing pretreatment: On the surface of the fine-grained heat-affected zone of the F92 steel welded joint after post-weld heat treatment, a non-penetrating microtexture is prepared using an ultra-short pulse laser. The microtexture is an array of micro-pits or micro-grooves, and the long axis of the texture is perpendicular to the weld.

[0010] Step 2: High-frequency pulsed current system integration: Deploy a non-contact electrode system in the welding joint area of ​​the equipment to be put into service. The system is connected to a high-frequency pulsed power supply, and the electrodes maintain a small gap with the joint surface.

[0011] Step 3: Deployment of in-situ damage monitoring system: Arrange high-precision micro-resistance probes in the fine-grain heat-affected zone to monitor changes in local resistance of the joint;

[0012] Step 4: Constant Field Maintenance Mode Operation: After the equipment enters the steady-state creep stage of high-temperature service, start the high-frequency pulse current system and apply a periodic pulse current with a low duty cycle.

[0013] Step 5 Damage intervention mode triggering: The resistance change rate is monitored by a resistance probe. When the resistance change rate exceeds the set threshold, the system automatically switches to damage intervention mode and triggers a high-intensity pulse current sequence.

[0014] Step 6: Dynamic adjustment of pulse parameters: Based on the equipment's service temperature (600-650℃) and real-time resistance data, dynamically adjust the pulse current frequency, peak current density, and pulse width;

[0015] Step 7: Texture-current synergistic enhancement: The residual compressive stress introduced by laser texture and the electro-migration effect of pulsed current work synergistically to suppress creep cavity nucleation;

[0016] Step 8: In-situ repair of micro-defects: Utilizing the Joule thermal peak effect of pulsed current to promote local atomic diffusion and repair early micro-defects;

[0017] Step 9: Dislocation configuration optimization: Through the dislocation activation effect of pulse current, guide the rearrangement and annihilation of dislocations, and delay crack initiation;

[0018] Step 10: Full-cycle performance verification: During service, the joint hardness, residual stress and microstructure are tested periodically to evaluate the creep life improvement effect.

[0019] Preferably, the ultrashort pulse laser in step 1 is a picosecond or femtosecond laser, and the micro-dimple array parameters are: diameter 50-100μm, depth 30-60μm, and spacing 1.5-2 times the diameter.

[0020] Preferably, the adjustment range of the high-frequency pulse power supply parameters in step 2 is: frequency 10-100kHz, peak current density ≥1000A / cm², and pulse width 10-100μs.

[0021] Preferably, the measurement accuracy of the miniature resistance probe in step 3 is not less than 1 μΩ, the sampling frequency is ≥1 kHz, and the probe placement points cover the stress concentration areas of the fine-grained heat-affected zone.

[0022] Preferably, the duty cycle of the low duty cycle periodic pulse current in step 4 is ≤10%, and the pulse interval is dynamically set according to the real-time temperature of the equipment to ensure that the average temperature rise of the connector does not exceed 5°C.

[0023] Preferably, the threshold of the resistance change rate in step 5 is determined through preliminary experiments, and the value range is 0.05%-0.1% / h; the duration of the high-intensity pulse current sequence is 1-5 min, and the peak current density is 2-3 times that of the conventional maintenance mode.

[0024] Preferably, the pulse parameter adjustment in step 6 follows the following principle: for every 50°C increase in service temperature, the pulse frequency increases by 10-20kHz, and the peak current density increases by 100-200A / cm².

[0025] Preferably, the residual compressive stress in step 7 is detected by an X-ray stress meter, and its value is controlled between 100-300 MPa, which is consistent with the direction of the electron wind generated by the pulsed current.

[0026] Preferably, the local instantaneous temperature generated by the Joule thermal peak effect in step 8 is 50-100°C higher than that of the substrate, and the action time is ≤100μs, so as to avoid adverse effects on the substrate tissue.

[0027] Preferably, the full-cycle performance verification in step 10 includes: performing a micro-area hardness test and a transmission electron microscope observation of dislocation configuration and creep void density at fixed intervals to evaluate the lifespan improvement.

[0028] Compared with existing technologies, the beneficial effects of this invention are as follows: This process addresses the difficulty in early detection of micro-damage in FGHAZ. It deploys high-precision micro-resistance probes with a measurement accuracy of no less than 1 μΩ and a sampling frequency of ≥1 kHz in the fine-grained heat-affected zone. These probes precisely cover stress concentration areas. Utilizing the direct correlation between resistance changes and micro-damage, it captures in real-time the subtle resistance fluctuations caused by void nucleation and dislocation slip during the early stages of creep. Automatic warnings are issued when the rate of change reaches a threshold of 0.05%-0.1% / h, overcoming the lag of traditional offline detection and achieving early identification of micro-damage. Addressing the issue of easy attenuation with single-strength enhancement, the process employs a synergistic enhancement mechanism of laser texturing and pulsed current. First, a non-penetrating microtexture is prepared using picosecond or femtosecond lasers, introducing 100-300 MPa residual compressive stress and locking it to the surface. Then, the electromigration effect of pulsed current is combined to form a synergistic effect of stress constraint and electron wind, suppressing void nucleation. Simultaneously, the pulsed current Joule... The thermal peak effect (local instantaneous temperature rise of 50-100℃, effect ≤100μs) promotes atomic diffusion to repair early defects and guides dislocation rearrangement and annihilation through dislocation activation, avoiding the drawbacks of unconstrained stress relaxation of single texture or unconstrained current strengthening, thus achieving long-term maintenance of the strengthening effect. To address the problem of parameters being difficult to adapt to operating conditions, the process design employs a dual-mode operation and dynamic adjustment strategy. During the steady-state creep stage, a low duty cycle pulse with a duty cycle ≤10% is applied and the interval is dynamically adjusted to control the temperature rise ≤5℃. During damage intervention, the process switches to a high-intensity sequence with 2-3 times the peak current. More importantly, a linkage rule between service temperature and parameters is established. For every 50℃ increase, the frequency is increased by 10-20kHz and the current density by 100-200A / cm². Combined with real-time resistance data, the parameters are dynamically optimized to perfectly adapt to the variable operating conditions of 600-650℃. With full-cycle performance verification, the strengthening effect is precisely controlled, ultimately improving the creep life of the joint and ensuring the safety of high-temperature service. Attached Figure Description

[0029] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary. For those skilled in the art, other embodiments can be derived from the provided drawings without creative effort.

[0030] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0031] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0032] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0033] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0035] Example

[0036] This embodiment focuses on the F92 heat-resistant steel welded joint of the high-temperature superheater outlet section of a 1,000 MW advanced ultra-supercritical unit. The joint is a butt joint structure with tungsten inert gas welding (TIG) for the root pass and submerged arc welding (SAW) for the fill pass. The service environment temperature is stable at 600-650℃, and the design working pressure is 34MPa. It is a key component of the unit that withstands high temperature and high pressure and is prone to creep damage. The joint has undergone post-weld heat treatment according to specifications. The specific process is to maintain a constant temperature of 760℃ and then air cool. Preliminary basic testing confirmed that the width of the fine-grained heat-affected zone is 0.9-1.3mm, the initial Vickers hardness in the zone is 260-280HV, the residual tensile stress is 80-100MPa, and transmission electron microscopy shows that there are a small number of martensitic lath boundary dislocation entanglements, but no initial defects such as creep voids or cracks. The chemical composition of the base material meets the requirements of the F92 steel standard.

[0037] The pretreatment process aims to provide a precise foundation for subsequent processes. The procedure is as follows: The fine-grained heat-affected zone is graded and polished using 1200-grit, 1500-grit, and 2000-grit silicon carbide sandpaper. The polishing is done in a cross-grit manner. Before each sandpaper change, the surface is cleaned with anhydrous ethanol until the oxide scale, weld slag, and scratches are completely removed, and the surface roughness meets the requirements for laser processing. The entire joint is then placed in an anhydrous ethanol ultrasonic cleaning tank at 200W for 20 minutes to thoroughly remove surface oil and polishing dust. After cleaning, it is placed in a 100℃ clean drying oven for 30 minutes to prevent residual moisture from affecting the quality of laser processing. A metallographic microscope combined with EBSD electron backscatter diffraction technology is used to accurately locate the boundary of the fine-grained heat-affected zone. A clear outline is drawn using a high-temperature resistant ceramic marker pen, and based on the previous finite element stress analysis results, stress concentration points such as the middle of the fine-grained heat-affected zone on both sides of the weld are marked to provide a reference for probe placement.

[0038] Please see Figure 1 A high-temperature creep life extension process for F92 heat-resistant steel welded joints includes the following steps:

[0039] Step 1: Laser surface texturing pretreatment: On the surface of the fine-grained heat-affected zone of the F92 steel welded joint after post-weld heat treatment, a non-penetrating microtexture is prepared using an ultra-short pulse laser. The microtexture is an array of micro-pits or micro-grooves, and the long axis of the texture is perpendicular to the weld.

[0040] Step 2: High-frequency pulsed current system integration: Deploy a non-contact electrode system in the welding joint area of ​​the equipment to be put into service. The system is connected to a high-frequency pulsed power supply, and the electrodes maintain a small gap with the joint surface.

[0041] Step 3: Deployment of in-situ damage monitoring system: Arrange high-precision micro-resistance probes in the fine-grain heat-affected zone to monitor changes in local resistance of the joint;

[0042] Step 4: Constant Field Maintenance Mode Operation: After the equipment enters the steady-state creep stage of high-temperature service, start the high-frequency pulse current system and apply a periodic pulse current with a low duty cycle.

[0043] Step 5 Damage intervention mode triggering: The resistance change rate is monitored by a resistance probe. When the resistance change rate exceeds the set threshold, the system automatically switches to damage intervention mode and triggers a high-intensity pulse current sequence.

[0044] Step 6: Dynamic adjustment of pulse parameters: Based on the equipment's service temperature (600-650℃) and real-time resistance data, dynamically adjust the pulse current frequency, peak current density, and pulse width;

[0045] Step 7: Texture-current synergistic enhancement: The residual compressive stress introduced by laser texture and the electro-migration effect of pulsed current work synergistically to suppress creep cavity nucleation;

[0046] Step 8: In-situ repair of micro-defects: Utilizing the Joule thermal peak effect of pulsed current to promote local atomic diffusion and repair early micro-defects;

[0047] Step 9: Dislocation configuration optimization: Through the dislocation activation effect of pulse current, guide the rearrangement and annihilation of dislocations, and delay crack initiation;

[0048] Step 10: Full-cycle performance verification: During service, the joint hardness, residual stress and microstructure are tested periodically to evaluate the creep life improvement effect.

[0049] The ultrashort pulse laser mentioned in step 1 is a picosecond or femtosecond laser, and the micro-dimple array parameters are: diameter 50-100μm, depth 30-60μm, and spacing 1.5-2 times the diameter.

[0050] The high-frequency pulse power supply parameters in step 2 are adjusted within the following ranges: frequency 10-100kHz, peak current density ≥1000A / cm², and pulse width 10-100μs.

[0051] The measurement accuracy of the miniature resistance probe described in step 3 is not less than 1 μΩ, the sampling frequency is ≥1 kHz, and the probe placement points cover the stress concentration areas of the fine-grained heat-affected zone.

[0052] In step 4, the duty cycle of the low duty cycle periodic pulse current is ≤10%, and the pulse interval is dynamically set according to the real-time temperature of the equipment to ensure that the average temperature rise of the connector does not exceed 5℃.

[0053] The threshold for the resistance change rate mentioned in step 5 was determined through previous experiments, with a value range of 0.05%-0.1% / h; the duration of the high-intensity pulse current sequence is 1-5 minutes, and the peak current density is 2-3 times that of the conventional maintenance mode.

[0054] The pulse parameter adjustment in step 6 follows this principle: for every 50°C increase in service temperature, the pulse frequency increases by 10-20kHz, and the peak current density increases by 100-200A / cm².

[0055] The residual compressive stress mentioned in step 7 is detected by an X-ray stress meter, and its value is controlled between 100-300 MPa, which is consistent with the direction of the electron wind generated by the pulsed current.

[0056] The local instantaneous temperature generated by the Joule thermal peak effect in step 8 is 50-100℃ higher than that of the substrate, and the action time is ≤100μs, so as to avoid adverse effects on the substrate tissue.

[0057] The full-cycle performance verification described in step 10 includes: performing a micro-area hardness test and a transmission electron microscope observation of dislocation configuration and creep void density at fixed intervals to assess the lifespan improvement.

[0058] Laser surface texturing pretreatment: Picosecond lasers are selected as the texturing equipment. The pulse width of this equipment is stable at the picosecond level, the output power meets the requirements of microtexturing processing, and it has a high-precision motion control system, which can realize the precise forming of microstructures.

[0059] Based on the optimized parameters of the technical solution, the microtexture is determined to be a micro-pit array with the following parameters: diameter 80μm, depth 45μm, and spacing 120μm, which is 1.5 times the diameter. This parameter combination can ensure that the texture introduces sufficient residual compressive stress without significantly reducing the bearing capacity of the joint surface. The long axis of the texture is strictly perpendicular to the weld extension direction to maximize the dispersion of axial creep stress of the weld, which matches the stress characteristics of the joint.

[0060] The processing adopts a linkage mode of galvanometer scanning and motion platform. The scanning speed is set according to the laser power and the size of the micro-pit, ensuring that the required micro-pit shape can be formed in a single scan. To prevent surface oxidation of the fine grain heat-affected zone during processing, 99.99% pure argon gas is continuously introduced into the processing area for protection. The argon gas is blown vertically onto the processing surface through a customized nozzle at a flow rate of 15L / min. The distance between the nozzle and the processing area is 15mm, effectively isolating air interference.

[0061] After processing, multi-dimensional quality inspection was carried out: observation using a laser confocal microscope confirmed that the micro-pits were regularly circular, with no slag accumulation or edge turning, and the pit bottoms were flat without penetrating cracks. The dimensional deviation was controlled within ±3μm, meeting the requirements of diameter 50-100μm and depth 30-60μm. The residual stress on the surface of the fine-grained heat-affected zone was detected by an X-ray stress analyzer. The test used a Cu-Kα target, tube voltage 40kV, tube current 40mA, and scanning range 2θ=140°-160°. The results showed that the residual compressive stress introduced by the surface layer was 220-250MPa, which was completely within the control range of 100-300MPa, and the stress distribution was uniform, consistent with the direction of the electron wind generated by the subsequent pulsed current. Five micro-pits were randomly selected to prepare cross-sectional metallographic samples. After grinding, polishing, and etching, the samples were observed and confirmed that the pits were all non-penetrating structures with a depth between 42-48μm, and did not cause damage to the internal structure of the matrix.

[0062] High-frequency pulse current system integration: The high-frequency pulse power supply uses professional equipment that meets the technical solution parameter requirements. Its parameter adjustment range covers: frequency 10-100kHz, peak current density ≥1000A / cm², pulse width 10-100μs, and output waveform is a standard square wave. It has two output modes: constant current and constant voltage. It can be remotely started and stopped and parameter adjusted through the PLC control system. The equipment comes with a high-efficiency water cooling system, which can operate continuously and stably in high-temperature environments. It also has triple protection functions of overcurrent, overvoltage, and overtemperature to ensure the safety of process operation.

[0063] The electrode system employs a customized non-contact ring electrode made of copper-chromium zirconium copper alloy, which combines excellent conductivity with high-temperature wear resistance. The inner diameter of the electrode is precisely matched with the outer diameter of the pipe where the welding joint is located. The inner wall is machined into an arc-shaped surface, with a fit error of ≤0.2mm with the joint surface. The gap between the electrode and the joint surface is strictly controlled within a small range to ensure stable current transmission and prevent contact wear. The electrode is fixed by three sets of evenly distributed insulating support arms made of silicon nitride ceramic, which is heat resistant above 800℃ and has a bending strength ≥300MPa, ensuring that the electrode position is stable and does not shift during high-temperature service. The inner wall of the electrode is sprayed with an insulating coating, and an annular conductive window is reserved only in the corresponding fine-grained heat-affected zone to prevent current from diffusing into untreated areas.

[0064] The power supply and electrode system are connected by a high-temperature resistant cable. The cable conductor is made of multi-strand silver-plated copper wire, and the insulation layer is a composite structure of polytetrafluoroethylene and glass fiber. The temperature resistance rating reaches 260℃, which can adapt to the high-temperature environment in which the connector is in service. In order to achieve uniform current distribution, a copper current equalization ring is installed at the connection between the electrode and the cable. The surface of the current equalization ring is silver-plated to reduce contact resistance and ensure that the current is evenly distributed in the fine-grained heat-affected zone.

[0065] After system integration, no-load commissioning and load simulation tests were conducted: Under no-load conditions, the frequency was set to 50kHz, the pulse width to 50μs, and the peak current density to 1000A / cm², and the system was run continuously for 1 hour. The electrode temperature was monitored by an infrared thermometer and found to be ≤55℃, the power supply output current fluctuation was ≤±2%, and there was no arc discharge phenomenon. The load simulation test used a stainless steel test block with a resistivity similar to that of F92 steel to simulate a high-temperature environment of 620℃. The uniformity of current distribution in the electrode coverage area was measured, and the results showed that the current density deviation was ≤5%, which met the process requirements and ensured that the system could be stably put into use.

[0066] In-situ damage monitoring system deployment: The in-situ damage monitoring system adopts a three-tier architecture of high-precision probes + high-speed data acquisition + real-time analysis. The core detection element is a miniature resistance probe that meets the technical requirements. This probe is made of a high-humidity resistant alloy and can operate stably for a long time in an environment of 600-650℃, with a measurement accuracy of not less than 1μΩ. The sampling frequency is set to 2kHz, meeting the requirement of ≥1kHz. The data acquisition instrument uses a high-precision acquisition module with a sampling rate of 10kHz and a resolution of 24 bits, which can simultaneously acquire 8 probe signals to ensure the timeliness and accuracy of data acquisition.

[0067] The probe arrangement follows the requirement in the scheme to cover the stress concentration areas in the fine-grained heat-affected zone. Based on the results of the previous stress analysis, a total of 6 monitoring points are arranged in the fine-grained heat-affected zone: 3 on each side of the weld, located in the middle of the fine-grained heat-affected zone (the area with the highest stress concentration), near the weld fusion line, and near the base material transition zone. Two redundant probes are set at the middle monitoring point to ensure the reliability of the monitoring data. The probes are fixed by laser spot welding using a YAG pulsed laser with a power of 50W. The weld diameter is 0.8mm and the depth is 0.3mm, ensuring that the probe is firmly connected to the joint surface without damaging the internal structure. The probe leads are made of 0.3mm diameter silver-plated high-temperature wires with an outer polyimide insulation layer. The wires are fixed along the outer wall of the pipe and secured every 50mm with ceramic insulating clips to prevent the probes from loosening due to wire movement.

[0068] After system deployment, baseline calibration and sensitivity verification were performed: First, resistance calibration was conducted at five temperature points: 25℃, 300℃, 500℃, 620℃, and 650℃. Resistance-temperature fitting curves for each probe were established to eliminate the interference of temperature on resistance measurement, and the calibration error was controlled within ±0.5%. Subsequently, a micro tensile testing machine was used to apply micro-stress to the fine-grained heat-affected zone to simulate microscopic damage in the early stage of creep. When a tiny void of about 50nm in size was observed through transmission electron microscopy, the monitoring system showed that the resistance change rate of the corresponding probe reached 0.03% / h, proving that the system can effectively capture early microscopic damage signals. Finally, a 24-hour continuous stability test was conducted. Under a constant temperature environment of 620℃, the resistance fluctuation of each probe was ≤0.01% / h, and there was no packet loss in data acquisition. The system stability met the standards and could be put into subsequent monitoring work.

[0069] High-temperature service simulation and process operation

[0070] Constant field maintenance mode operation: The processed welded joint sample is installed in a high-temperature creep testing machine to simulate the actual service environment for loading and temperature control. The heating process adopts a stepped heating strategy: from room temperature to 300℃, hold for 30 min; then to 500℃, hold for 40 min; and finally to 620℃, hold for 1 h, to ensure uniform temperature of the joint, with a temperature deviation of ≤±2℃, which fully conforms to the service temperature range of 600-650℃. The loading method is axial stress loading. According to the creep limit of F92 steel at 620℃, the loading stress is set to 80% of the creep limit. The loading process adopts stress control mode with a loading rate of 0.5MPa / s to avoid initial damage caused by impact loads.

[0071] When the testing machine displays that the joint has entered the steady-state creep stage (creep rate stabilizes at 1×10⁻⁶), -7 After a period of 2 hours without significant fluctuations (below / h), the high-frequency pulse current system is activated and set to constant field maintenance mode. The mode parameters are strictly determined according to the optimized values ​​of the technical solution: duty cycle 8%, meeting the requirement of ≤10%; frequency 50kHz, pulse width 50μs, peak current density 1000A / cm², meeting the parameter range of frequency 10-100kHz, peak current density ≥1000A / cm², and pulse width 10-100μs. To control the average temperature rise of the joint to not exceed 5℃, three sets of K-type thermocouples are arranged near the electrodes to monitor the joint surface temperature in real time. When the temperature rises above 3℃, the system automatically adjusts the pulse interval from the initial 10s to 12s, controlling the temperature rise by reducing the pulse energy input per unit time. In actual operation, the average temperature rise of the joint is stable between 3.5-4.5℃, which fully meets the process requirements.

[0072] During constant-field maintenance mode operation, the in-situ monitoring system continuously collects resistance data and generates a resistance change rate report every hour. Simultaneously, the high-frequency pulse power supply records output current, voltage, and other parameters in real time, forming a process operation database. During the first 1000 hours of operation, the resistance change rate of each probe remained stable between 0.02% and 0.04% / h, not reaching the damage intervention threshold. The system continued to operate in constant-field maintenance mode, and the joint creep rate remained stable at 8 × 10⁻⁶. -8 The speed was around 100 km / h, with no obvious acceleration, indicating that the process maintenance effect was initially apparent.

[0073] Damage intervention mode triggering and parameter adjustment: When the process runs for 1200 hours, the probe located in the middle of the fine grain heat-affected zone on the right side of the weld detected a resistance change rate of 0.08% / h, exceeding the threshold of 0.07% / h determined through 20 sets of previous tests. This threshold is within the set range of 0.05%-0.1% / h. The system automatically triggers the damage intervention mode through the PLC, issuing an audible and visual alarm signal while rapidly adjusting the high-frequency pulse current system parameters to the intervention mode: the peak current density is increased to 2500A / cm², which is 2.5 times that of the maintenance mode, meeting the requirement of 2-3 times; the frequency is 60kHz, the pulse width is 80μs, and the set duration is 3min, which is within the range of 1-5min.

[0074] During the intervention, the system monitored the joint temperature and resistance changes in real time. After 1.5 minutes of operation, the corresponding probe resistance change rate dropped to 0.05% / h. After 3 minutes of operation, the resistance change rate stabilized below 0.03% / h, and the system automatically stopped the intervention and resumed the constant field maintenance mode. During the intervention, the highest instantaneous temperature rise of the joint was 95℃, which was 50-100℃ higher than that of the substrate, and the duration of the temperature rise was ≤80μs. The requirement of ≤100μs was met by metallographic observation, which confirmed that no overheating damage was caused to the substrate. The intervention effect was good and safe and controllable.

[0075] During subsequent operation up to 2000 hours, due to adjustments in the test plan, the service temperature was increased to 650℃. According to the parameter adjustment rules in the technical plan, for every 50℃ increase, the frequency increased by 10-20kHz and the peak current density increased by 100-200A / cm². The system automatically adjusted the constant field maintenance mode parameters to: frequency 70kHz, 20kHz higher than at 620℃; peak current density 1160A / cm², 160A / cm² higher than at 620℃; and pulse width remained unchanged at 50μs. After adjustment, temperature monitoring confirmed that the average temperature rise of the connector was still controlled within 5℃, and the resistance change rate was stable between 0.03%-0.05% / h. The parameters and operating conditions were well adapted, proving that the dynamic adjustment mechanism was effective.

[0076] Microstructure evolution analysis: In accordance with the technical requirements, transmission electron microscopy (TEM) samples were prepared from the fine-grained heat-affected zone of the joint at four time points: before process implementation (0h), 1200h of constant field maintenance (before intervention), 100h after intervention, and 3000h of operation. The microstructure evolution was observed using TEM to verify the effects of texture-current synergistic enhancement, in-situ repair of micro-defects, and dislocation configuration optimization.

[0077] 0h (initial state): The microstructure is a typical tempered martensitic lath structure with lath width of about 0.2-0.3 μm. There are a small number of dislocation entanglements at the lath boundaries, no obvious creep cavities, and the precipitates are mainly V(C,N) carbonitrides, which are uniformly distributed in the grain and grain boundaries.

[0078] 1200h (before intervention): The martensitic laths show slight coarsening, the dislocation density at the lath boundaries increases significantly, forming a dislocation wall structure, and tiny voids of 50-100nm in size appear at the grain boundaries with a void density of about 6×10³ / cm². The precipitated phase does not show obvious growth. At this time, early creep damage has already occurred.

[0079] 100 hours after intervention: the number of grain boundary voids was significantly reduced, the void density dropped to 1.2×10³ / cm², some micro voids were healed by atomic diffusion, the dislocation wall structure disintegrated, the dislocations were uniformly distributed, and the martensite lath morphology remained basically intact, proving that the in-situ repair of micro defects and the optimization of dislocation configuration were effective.

[0080] 3000h: The void density is still controlled within 2×10³ / cm², the maximum void size is ≤200nm, there is no void penetration, the dislocation distribution is uniform, the size of the precipitated phase has increased slightly but still remains diffusely distributed, which fully demonstrates that the residual compressive stress introduced by laser texture and the electro-migration effect of pulsed current work together to effectively suppress creep void nucleation and growth.

[0081] Mechanical properties and stress testing: In accordance with the full-cycle performance verification requirements in the technical solution, micro-area hardness tests were conducted at fixed intervals using a Vickers hardness tester with a load of 100g and a holding pressure of 15s. At each time point, 10 test points were evenly selected in the fine-grained heat-affected zone, and the average value was taken as the result: the hardness was 270HV at 0h; 262HV at 600h, a slight decrease which is normal creep softening; at 1200h (before intervention), it dropped to 240HV, which meets the damage warning characteristics; 100h after intervention, it rebounded to 258HV, proving that the damage repair was effective; at 3000h, the hardness remained above 250HV, which is significantly higher than the 210HV of the untreated control group at the same time, and the hardness stability was significantly enhanced.

[0082] Residual stress was detected using an X-ray stress analyzer, with the same detection method as post-texturing stress detection to ensure data consistency: the residual compressive stress was 240 MPa at 0 h; it decreased to 180 MPa at 1200 h, with a stress relaxation rate of approximately 25%; it rebounded to 205 MPa 100 h after intervention; and it remained at 175 MPa at 3000 h, which was significantly higher than the residual compressive stress of 80 MPa in the untreated control group at the same time, demonstrating that the texture-current synergistic strengthening mechanism effectively delayed residual stress relaxation and continued to play an anti-creep role.

[0083] Full-cycle performance verification and life assessment: When the process reaches 3000 hours, the high-temperature service simulation test is stopped, and a comprehensive full-cycle performance assessment of the welded joint is conducted. Simultaneously, a control group joint (with the same materials, welding process, and service simulation conditions) is set up for comparative analysis to fully verify the process effectiveness.

[0084] Macroscopic performance evaluation: Ultrasonic and penetrant testing were used to perform non-destructive testing on the two groups of joints. The joints treated with this process showed no macroscopic cracks, deformation, or oxidation spalling. The ultrasonic reflection signals in the weld and fine-grained heat-affected zone were uniform, with no abnormal defect signals. In contrast, the joints in the control group showed microscopic cracks of about 2 mm in length in the fine-grained heat-affected zone, and obvious oxidation discoloration on the surface. Tensile test results showed that the tensile strength of the joints treated with this process was 690 MPa, only 0.7% lower than the initial state of 700 MPa. The tensile strength of the joints in the control group decreased to 620 MPa, a decrease of 11.4%. The process has a significant effect on maintaining macroscopic strength.

[0085] Microscopic performance evaluation: In addition to the aforementioned transmission electron microscopy observation, scanning electron microscopy was used to observe the fracture morphology of the joint. The fracture surface of the treated group was a typical ductile fracture with a large number of dimples of uniform size. The fracture surface of the control group was a brittle fracture with obvious intergranular cracking characteristics. The creep void density was statistically analyzed using image analysis software. The density of the treated group was 2.5×10³ / cm², while that of the control group was 18×10³ / cm², with a void density reduction of 86.1%, which fully demonstrates the effectiveness of the process in suppressing microscopic damage.

[0086] Life assessment: Based on creep rate data and fracture mechanics model, the creep fracture life of the joint treated by this process is predicted to reach 12,500 hours under 620°C and set stress conditions; the measured creep fracture life of the control group joint is 6,800 hours. This process improves the creep life of the joint by 83.8%, far exceeding the expected target. At the same time, no process failures occurred during the entire cycle operation. The high-frequency pulse current system and the in-situ monitoring system maintained stable operation, and the reliability and stability of the process were fully verified.

[0087] This embodiment focuses on the F92 welded joint of a high-temperature superheater in an ultra-supercritical unit. Strictly adhering to the digital parameters in the technical solution, it fully implements a complete life-extending process encompassing laser texture pretreatment, pulsed current system integration, in-situ monitoring, dynamic control, and performance verification. Through precise equipment selection, parameter setting, and process control, it effectively solves the core problems of existing technologies, such as difficulty in early detection of micro-damage in the fine-grained heat-affected zone, easy attenuation of single-strength reinforcement, and difficulty in adapting parameters to operating conditions. After implementation, the creep life of the joint is improved, and its performance remains stable during 3000 hours of high-temperature service at 600-650℃. All test indicators meet the requirements of the technical solution, fully demonstrating the feasibility and effectiveness of the process. This provides direct technical reference and implementation basis for the engineering application of F92 heat-resistant steel welded joints in high-temperature pressure equipment, and possesses significant engineering promotion value.

[0088] The same or similar labels correspond to the same or similar parts;

[0089] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0090] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all implementation methods here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A high-temperature creep life extension process for F92 heat-resistant steel welded joints, characterized in that, Includes the following steps: Step 1: Laser surface texturing pretreatment: On the surface of the fine-grained heat-affected zone of the F92 steel welded joint after post-weld heat treatment, a non-penetrating microtexture is prepared using an ultra-short pulse laser. The microtexture is an array of micro-pits or micro-grooves, and the long axis of the texture is perpendicular to the weld. Step 2: High-frequency pulsed current system integration: Deploy a non-contact electrode system in the welding joint area of ​​the equipment to be put into service. The system is connected to a high-frequency pulsed power supply, and the electrodes maintain a small gap with the joint surface. Step 3: Deployment of in-situ damage monitoring system: Arrange high-precision micro-resistance probes in the fine-grain heat-affected zone to monitor changes in local resistance of the joint; Step 4: Constant Field Maintenance Mode Operation: After the equipment enters the steady-state creep stage of high-temperature service, start the high-frequency pulse current system and apply a periodic pulse current with a low duty cycle. Step 5 Damage intervention mode triggering: The resistance change rate is monitored by a resistance probe. When the resistance change rate exceeds the set threshold, the system automatically switches to damage intervention mode and triggers a high-intensity pulse current sequence. Step 6: Dynamic adjustment of pulse parameters: Based on the equipment's service temperature and real-time resistance data, dynamically adjust the pulse current frequency, peak current density, and pulse width; Step 7: Texture-current synergistic enhancement: The residual compressive stress introduced by laser texture and the electro-migration effect of pulsed current work synergistically to suppress creep cavity nucleation; Step 8: In-situ repair of micro-defects: Utilizing the Joule thermal peak effect of pulsed current to promote local atomic diffusion and repair early micro-defects; Step 9: Dislocation configuration optimization: Through the dislocation activation effect of pulse current, guide the rearrangement and annihilation of dislocations, and delay crack initiation; Step 10: Full-cycle performance verification: During service, the joint hardness, residual stress and microstructure are tested periodically to evaluate the creep life improvement effect.

2. The process according to claim 1, characterized in that, The ultrashort pulse laser mentioned in step 1 is a picosecond or femtosecond laser, and the micro-dimple array parameters are: diameter 50-100μm, depth 30-60μm, and spacing 1.5-2 times the diameter.

3. The process according to claim 1, characterized in that, The high-frequency pulse power supply parameters in step 2 are adjusted within the following ranges: frequency 10-100kHz, peak current density ≥1000A / cm², and pulse width 10-100μs.

4. The process according to claim 1, characterized in that, The measurement accuracy of the miniature resistance probe described in step 3 is not less than 1 μΩ, the sampling frequency is ≥1 kHz, and the probe placement points cover the stress concentration areas of the fine-grained heat-affected zone.

5. The process according to claim 1, characterized in that, In step 4, the duty cycle of the low duty cycle periodic pulse current is ≤10%, and the pulse interval is dynamically set according to the real-time temperature of the equipment to ensure that the average temperature rise of the connector does not exceed 5℃.

6. The process according to claim 1, characterized in that, The threshold for the resistance change rate mentioned in step 5 was determined through previous experiments, with a value range of 0.05%-0.1% / h; the duration of the high-intensity pulse current sequence is 1-5 minutes, and the peak current density is 2-3 times that of the conventional maintenance mode.

7. The process according to claim 1, characterized in that, The pulse parameter adjustment in step 6 follows this principle: for every 50°C increase in service temperature, the pulse frequency increases by 10-20kHz, and the peak current density increases by 100-200A / cm².

8. The process according to claim 1, characterized in that, The residual compressive stress mentioned in step 7 is detected by an X-ray stress meter, and its value is controlled between 100-300 MPa, which is consistent with the direction of the electron wind generated by the pulsed current.

9. The process according to claim 1, characterized in that, The local instantaneous temperature generated by the Joule thermal peak effect in step 8 is 50-100°C higher than that of the substrate, and the action time is ≤100μs, so as to avoid adverse effects on the substrate tissue.

10. The process according to claim 1, characterized in that, The full-cycle performance verification described in step 10 includes: performing a micro-area hardness test and a transmission electron microscope observation of dislocation configuration and creep void density at fixed intervals to assess the lifespan improvement.

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