An on-line heat treatment method for pressure resistance welding of a zirconium alloy tube plug structure

Abstract

The application relates to an online heat treatment method of a zirconium alloy pipe plug structure pressure resistance welding, and belongs to the technical field of nuclear power material welding. After welding, the electrode clamp is kept in a clamped state, the current of the electrode clamp is adjusted through a temperature control system, and then online heat treatment is implemented on a hot machine influencing area of a joint; a first scheme is a slow and uniform cooling scheme: according to the deviation between the actually measured temperature and the target temperature, the target temperature is cooled to 100-240 DEG C, and the temperature fluctuation amplitude is not more than + / - 5 DEG C; the target temperature is constant, and the cooling rate is 5-25 DEG C / s. A second scheme is divided into three continuous stages: a first cooling stage: from a specific temperature after welding, the temperature is cooled to 600-850 DEG C at a specified speed of 55-75 DEG C / s and is kept for 50-150 s; then the temperature is cooled to 100-240 DEG C at a speed of 3-15 DEG C / s, and then natural air cooling is carried out to room temperature. The microstructure form of the hot machine influencing area tends to be close to the base material, and the performance mutation caused by the difference in the microstructure is eliminated.

Description

Technical Field

[0001] This invention relates to the field of nuclear power material welding technology, specifically to an online heat treatment method for pressure resistance welding of zirconium alloy tube plug structures, applicable to the sealing manufacturing of pressurized water reactor nuclear fuel assemblies. Background Technology

[0002] Nuclear fuel rods are core safety components of nuclear power reactors. Their cladding tubes and end plugs are generally made of zirconium alloy, which, thanks to its low thermal neutron absorption cross-section, excellent high-temperature mechanical properties, and strong corrosion resistance, achieves a sealed enclosure of UO2 fuel pellets. Currently, the sealing connection between the zirconium alloy cladding tubes and end plugs mainly employs pressure resistance welding. This process has advantages such as high efficiency, fewer welding defects, and stable quality, making it the mainstream technology for sealing nuclear fuel rods.

[0003] However, existing zirconium alloy pressure resistance welding technology still faces key technical bottlenecks:

[0004] (1) Rapid heating and cooling during welding will cause plastic deformation and microstructure evolution in the joint area. This area is subjected to both thermal cycling and mechanical pressure during pressure resistance welding, and is called the thermomechanical affected zone. The microstructure of this area is basket-like Widmanstätten, with a large amount of second phase solid solution. The microhardness is significantly higher than that of the base metal, and the corrosion resistance is greatly reduced, while the corrosion rate is significantly increased compared to the base metal.

[0005] (2) After the nuclear fuel rods are welded, they are filled with fuel pellets and high-pressure argon gas. Traditional in-furnace heat treatment cannot be used, and there is a lack of engineering solutions for online heat treatment of zirconium alloy pressure resistance welded joints.

[0006] Historical nuclear power plant accidents have demonstrated that failure of the fuel rod cladding seal can lead to severe radioactive leaks. Therefore, addressing the performance degradation in the thermo-mechanically affected zone and the difficulty in adapting heat treatment processes in zirconium alloy pressure resistance welded joints has become a crucial issue for improving the safety of nuclear fuel rods. It is necessary to develop an integrated online heat treatment technology to achieve precise control over the microstructure and properties of the welded joint. Summary of the Invention

[0007] This invention addresses the practical technical challenges of reduced corrosion resistance in the heat-affected zone (HAZ) of current zirconium alloy tube plug pressure resistance welded joints, and the difficulty of applying traditional heat treatment processes to nuclear fuel rod structures. It proposes an integrated online heat treatment method. This method aims to make the microstructure of the HAZ approximate that of the base material while simultaneously improving its mechanical and corrosion resistance properties, thereby effectively enhancing the sealing reliability and long-term service safety of nuclear fuel rods.

[0008] The core technical solution of this invention is an online heat treatment method for pressure resistance welding of zirconium alloy tube plug structures. Through coordinated control of welding and online heat treatment, and real-time temperature measurement and closed-loop controlled cooling, precise regulation of the joint microstructure and properties is achieved. Specifically, it includes the following steps:

[0009] S1. Workpiece preparation and tooling assembly:

[0010] Zirconium alloy end plugs and cladding tubes are selected as welding workpieces. The end plugs are solid cylinders with a diameter of 8-12 mm, and the cladding tubes are hollow tubes with an outer diameter of 8-12 mm and a wall thickness of 0.5-1 mm. The cylindrical rod of the end plug is clamped to the wall of the cladding tube by an electrode clamp, and the front end of the end plug is aligned and coaxially positioned with the front end of the cladding tube. A K-type thermocouple is used as a temperature sensing element to detect the temperature at the joint. The electrode clamp and the K-type thermocouple are electrically connected to the temperature control system.

[0011] S2. Pressure resistance welding process:

[0012] Current and welding pressure are applied by electrode clamps. Resistance heat is generated by the contact resistance between the end plug and the shell tube mating surface to bring the joint to the welding temperature. At the same time, pressure is applied to complete the welding. The electrode clamps are clamped on the end plug so that the current is perpendicular to the axis of the solid cylindrical end plug. The rear end of the shell tube is fixed, and the pressure is applied from the rear end of the end plug along the axis to the joint.

[0013] S3. Implementation of online heat treatment process:

[0014] After welding, the electrode clamp is kept tight without disassembling the workpiece. The current of the electrode clamp is adjusted by the temperature control system, thereby regulating the thermodynamically affected zone of the joint for online heat treatment. Two core process schemes are adopted, both of which achieve precise control of the cooling rate through real-time temperature measurement and closed-loop regulation:

[0015] The first process scheme: slow and uniform cooling scheme. Temperature data of the welded joint is collected in real time using K-type thermocouples and fed back to the temperature control system. The temperature control system, based on the deviation between the measured temperature and the target temperature (where the target temperature changes at a constant cooling rate over time), adjusts the heating current and / or uses water cooling and / or air cooling to cool the joint from the specific temperature after welding (the directly corresponding high-temperature state after welding) to 100~240℃ according to the target temperature. During the actual cooling process, the fluctuation range of the measured temperature relative to the target temperature should not exceed ±5℃ (the target temperature can be considered as a straight line changing over time, and the deviation of the changing measured temperature from this line should not exceed ±5℃), ensuring the continuous precipitation of the second phase in the thermodynamically affected zone. Then, it is air-cooled to room temperature. The constant cooling rate of the target temperature is 5-25℃ / s.

[0016] The second process scheme: staged heat preservation + slow cooling scheme: divided into three continuous stages:

[0017] First cooling stage: Cooling from a specific temperature after welding to a specific temperature (600~850℃) at a specified first cooling rate (55~75℃ / s); Holding stage: When the temperature reaches the specific temperature (600~850℃), the temperature control system switches to the holding mode (using current applied by electrode clamps for heating) and maintains the temperature for a certain period of time (50~150s); Second cooling stage: After the holding stage, cooling to 100~240℃ at a specified second cooling rate (3~15℃ / s), followed by natural air cooling to room temperature; Control method for the first and second cooling stages: By adjusting the heating current and / or using water cooling and / or air cooling airflow;

[0018] After the workpiece has cooled to room temperature, the electrode fixture is removed to obtain the zirconium alloy tube plug weld joint.

[0019] The technical effects of this invention are as follows:

[0020] By employing online heat treatment and a real-time temperature measurement-closed-loop rate-controlled cooling scheme, the microstructure of the heat-engine affected zone (HAZ) closely resembles that of the base material, eliminating abrupt performance changes caused by microstructural differences. The difference in microhardness between the HAZ and the base material is reduced to within 5-30 HV. Simultaneously, the corrosion resistance of the joint approaches that of the base material. The process parameters exhibit good repeatability and engineering operability, significantly improving the sealing reliability and service safety of nuclear fuel rods. Attached Figure Description

[0021] To clearly illustrate the technical solutions of the present invention, the accompanying drawings used in the embodiments are briefly described below. The following drawings are only some embodiments of the present invention; those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0022] Figure 1 This is a schematic diagram of welding and online heat treatment;

[0023] Figure 2 Temperature history design diagrams for the cooling stages of no heat treatment and in-weld heat treatment;

[0024] Figure 3 Micrographs of the cladding tube at different cooling rates, (a) cooling rate 5 (corresponding to Example 1), (b) cooling rate 10 (Corresponding to Example 2), (c) Cooling rate 25 (Corresponding to Example 3), (d) Natural cooling after welding (corresponding to the example). Detailed Implementation

[0025] The online heat treatment method for pressure resistance welding of zirconium alloy tube plug structures according to the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The scope of protection of the present invention is not limited thereto; all equivalent modifications or substitutions made based on the technical solution of the present invention fall within the scope of protection of the present invention.

[0026] Experimental materials: The end plugs are made of zirconium alloy, solid cylinders with a diameter of 8-12 mm; the end faces to be connected are polished smooth in stages, ultrasonically cleaned with acetone for 10-25 minutes, and then dried with cold air; the cladding tube is a hollow tube with an outer diameter of 8-12 mm and a wall thickness of 0.5-1 mm; the inner wall to be connected is polished smooth and then cleaned in the same way as the end plugs. The end plugs and cladding tubes are immersed in acetone solution, ultrasonically cleaned for 10-25 minutes to remove surface oil, rinsed with anhydrous ethanol, dried with cold air, and then placed in a clean, sealed sample bag for later use to avoid secondary contamination.

[0027] The core of this invention lies in the coordinated control of "integrated tooling system pressure resistance welding + online heat treatment". The following provides detailed implementation processes for two core process schemes. The following embodiments are based on the preferred parameter range determined by laboratory research.

[0028] Example 1: Slow and Uniform Cooling Scheme

[0029] S1. The end plug and the casing tube are coaxially clamped together, the thermocouple is welded to the end plug near the joint, and the vacuum chamber is evacuated.

[0030] S2. Perform pressure resistance welding (the temperature after welding is 950~1100℃).

[0031] S3. Online slow cooling is performed at a rate of 5℃ / s to cool from the post-weld temperature to 200℃, followed by air cooling, with temperature fluctuation ≤±5℃;

[0032] S4. Take joint samples, grind and polish them, and then etch them for 10~25s. Conduct microstructure observation, microhardness test (microhardness refers to ASTM E384 standard) and electrochemical corrosion test.

[0033] Example 2: Slow and Uniform Cooling Scheme

[0034] S1. Same as Example 1;

[0035] S2. Same as Example 1;

[0036] S3. The cooling rate is 10℃ / s, and the rest is the same as in Example 1;

[0037] S4. Same as Example 1.

[0038] Example 3: Slow and Uniform Cooling Scheme

[0039] S1. Same as Example 1;

[0040] S2. Same as Example 1;

[0041] S3. The cooling rate is 25°C / s, and the rest is the same as in Example 1;

[0042] S4. Same as Example 1.

[0043] Example 4: Staged heat preservation + slow cooling scheme

[0044] S1. Same as Example 1.

[0045] S2. Same as Example 1.

[0046] S3. First, allow it to cool naturally to 700℃ (natural cooling rate is about 60℃ / s), then hold it at 700℃ for 100~150s (temperature fluctuation ≤±2℃), then slowly cool it down to 200℃ at 5℃ / s and then air cool it.

[0047] S4. Sample preparation is the same as in Example 1.

[0048] Comparative Example 1: Natural cooling after welding, without heat treatment.

[0049] In Examples 1, 2, 3, and 4, the thermomechanically affected zone (TMZ) exhibited a wide lath microstructure with abundant precipitates of the second phase (Zr(Fe,Cr)2 phase and Zr(Fe,Nb)2 phase), closely resembling the microstructure of the parent material. In the comparative examples, no second phase precipitates were observed, or the amount of precipitates was minimal. The hardness of the examples was 5–30 HV lower than that of the comparative examples, and the corrosion current density and rate were significantly reduced, while the breakdown potential was increased by 0.02–0.3 V. (Microhardness was tested using a microhardness tester, with a load of 100–250 gf and a holding time of 10–25 s. For joint samples, the corrosion current density was measured perpendicular to the fusion line; for thermodynamic simulation samples, the corrosion current density was measured at uniformly distributed points across the cross-section, and the arithmetic mean was taken. Electrochemical corrosion was performed in a 3.5 wt% NaCl solution at room temperature. The open-circuit potential was first tested for 1600–1900 s until stabilization, and then the potentiodynamic polarization curve was measured. The corrosion potential, corrosion current density, breakdown potential, and corrosion rate were obtained by fitting the curves. See the table below for the data.)

[0050]

[0051] This invention employs an integrated tooling system for pressure resistance welding combined with online heat treatment (slow cooling or isothermal treatment), which allows the joint microstructure to more closely resemble the base material, achieves uniform hardness, and improves corrosion resistance. The slow cooling process is simple to operate and suitable for mass production, while the isothermal process yields superior performance, meeting high corrosion resistance requirements. Both processes overcome the limitations of traditional furnace heat treatment, offer good repeatability, and are well-suited to the practical engineering needs of nuclear fuel rod sealing welding.

Claims

1. An online heat treatment method for pressure resistance welding of zirconium alloy tube plug structures, characterized in that, By coordinating welding and online heat treatment control, and using real-time temperature measurement and closed-loop controlled cooling, precise regulation of joint microstructure and properties can be achieved. This includes the following steps: S1. Workpiece preparation and tooling assembly: Zirconium alloy end plugs and cladding tubes are selected as welding workpieces. The cylindrical rod of the end plug is clamped to the wall of the cladding tube by an electrode clamp, and the front end of the end plug is aligned and coaxially positioned with the front end of the cladding tube. A type K thermocouple is used as a temperature sensing element to detect the temperature at the joint. The electrode clamp and the type K thermocouple are electrically connected to the temperature control system. S2. Pressure resistance welding process: Current and welding pressure are applied by electrode clamps. Resistance heat is generated by the contact resistance between the end plug and the shell tube mating surface to bring the joint to the welding temperature. At the same time, pressure is applied to complete the welding. The electrode clamps are clamped on the end plug so that the current is perpendicular to the axis of the solid cylindrical end plug. The rear end of the shell tube is fixed, and the pressure is applied from the rear end of the end plug along the axis to the joint. S3. Implementation of online heat treatment process: After welding, the electrode clamp is kept tight without disassembling the workpiece. The current of the electrode clamp is adjusted by the temperature control system to perform online heat treatment on the thermomechanically affected zone of the joint. Two core process schemes are adopted, both of which achieve precise control of the cooling rate through real-time temperature measurement and closed-loop regulation: The first process scheme: slow and uniform cooling scheme: real-time temperature data of the welded joint is collected by a K-type thermocouple and fed back to the temperature control system; the temperature control system, based on the deviation between the measured temperature and the target temperature, where the target temperature changes with time at a constant cooling rate, adjusts the heating current and / or uses water cooling and / or air cooling airflow to cool the joint from a specific temperature after welding to 100~240℃ according to the target temperature. In actual cooling, the fluctuation range of the measured temperature relative to the target temperature does not exceed ±5℃, ensuring the continuous precipitation of the second phase in the thermodynamically affected zone; then air cooling to room temperature; the constant cooling rate of the target temperature is 5-25℃ / s. The second process scheme: staged heat preservation + slow cooling scheme: divided into three continuous stages: First cooling stage: Cooling from a specific temperature after welding to a specific temperature (600~850℃) at a specified first cooling rate (55~75℃ / s); Holding stage: When the temperature reaches the specific temperature (600~850℃), the temperature control system switches to the holding mode and maintains the temperature for a certain period of time (50~150s); Second cooling stage: After the holding stage ends, cooling to 100~240℃ at a specified second cooling rate (3~15℃ / s), followed by natural air cooling (approximately 60℃ / s) to room temperature; Control method for the first and second cooling stages: By adjusting the heating current and / or using water cooling and / or air cooling airflow; After the workpiece has cooled to room temperature, the electrode fixture is removed to obtain the zirconium alloy tube plug weld joint.

2. The method according to claim 1, characterized in that, The end plug is a solid cylinder with a diameter of 8-12mm, and the outer casing is a hollow tube with an outer diameter of 8-12mm and a wall thickness of 0.5-1mm.

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