Iron-chromium-aluminum wire welding treatment method

Abstract

The invention discloses an iron-chromium-aluminum wire welding treatment method. A weldment is straightened, and a welding end face is cleaned; performing resistance butt welding on the weldment; based on a preset heat relation table, according to the diameter of the weldment, input heat and electrode spacing needed by the weldment are determined; the heat relation table comprises a plurality of weldment diameters, and input heat and electrode spacing corresponding to each weldment diameter; and according to the input heat and the electrode distance, postweld heat treatment is conducted on a welded joint on the weldment after resistance butt welding, and the welded joint obtained after heat treatment is subjected to air cooling to the room temperature. Therefore, the quality of the welding joint can be guaranteed, the problem that the iron-chromium-aluminum welding joint is prone to brittle failure is solved, the product yield is increased, the production cost is reduced, and the product quality and stability are improved.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and in particular to a method for welding iron-chromium-aluminum wire. Background Technology

[0002] Due to its high resistivity and thermal conductivity, iron-chromium-aluminum alloy is widely used in industries such as electronics manufacturing, home appliances, and industrial electric furnaces, and is an important functional material for electric heating in the field of electric heating.

[0003] The microstructure of iron-chromium-aluminum alloys is single-phase ferrite with a high degree of alloying, resulting in poor weldability. Brittle fracture frequently occurs during the actual processing and production of iron-chromium-aluminum products, especially in the welding of iron-chromium-aluminum wires, leading to low yield and high production costs.

[0004] In the actual processing and production of iron-chromium-aluminum products, a drawing process is required. When the wire is compressed and deformed to a certain extent, wire breakage often occurs. At this time, welding workers need to weld the wire so that it can participate in the drawing process again.

[0005] Resistance welding is often used for welding iron-chromium-aluminum wires. However, for iron-chromium-aluminum products with a high degree of alloying, the weld joint is prone to brittle fracture, resulting in low joint quality and seriously affecting the production and processing efficiency. Summary of the Invention

[0006] In view of the above problems, the present invention provides a welding treatment method for iron-chromium-aluminum wire, which aims to solve the problem of easy brittle fracture of the welded joint of iron-chromium-aluminum wire.

[0007] According to a first aspect of the present invention, a method for welding iron-chromium-aluminum wire is provided, comprising: Straightening of weldments and cleaning of weld end faces; Resistance welding is performed on the weldment. Based on a preset heat relationship table, the required input heat and electrode spacing for the weldment are determined according to the diameter of the weldment. The heat relationship table includes multiple weldment diameters, as well as the input heat and electrode spacing corresponding to each weldment diameter. Based on the input heat and the electrode spacing, post-weld heat treatment is performed on the welded joints on the welded parts after resistance welding. The heat-treated welded joint is cooled to room temperature by air.

[0008] Optionally, the weldment is made of iron-chromium-aluminum wire.

[0009] Optionally, the step of performing post-weld heat treatment on the weld joint of the resistance butt welded workpiece based on the input heat and the electrode spacing includes: The current and duration of heat treatment are determined based on the input heat and the electrode spacing. According to the electrode spacing, the two electrodes are placed on both sides of the welding joint; The weld joint on the weldment is subjected to electric heating treatment based on the energizing current and the energizing time.

[0010] Optionally, after determining the current and duration of heat treatment based on the input heat and the electrode spacing, the method further includes: If the actual distance between the two electrodes is greater than the electrode distance, then the energizing time and / or energizing current will be increased.

[0011] Optionally, the butt-welded components are subjected to resistance welding, including: The workpieces were subjected to resistance welding using a double forging press welding machine with a welding current of 55A.

[0012] Optionally, the temperature of the welded joint is greater than 200°C before post-weld heat treatment.

[0013] Optionally, when the diameter of the weldment is 4 mm, the required input heat of the weldment is 737 J, and the electrode spacing is 8 times the width of the weld joint.

[0014] Optionally, the current applied during the heat treatment is 80A.

[0015] Optionally, the power-on time is 20 seconds.

[0016] Optionally, the width of the welded joint is 6 mm.

[0017] The above-described one or more technical solutions in the embodiments of this specification have at least the following technical effects: This specification provides a method for welding iron-chromium-aluminum wire, which involves straightening the workpiece and cleaning the welding end face; performing resistance welding on the workpiece; determining the required input heat and electrode spacing based on a preset heat relationship table and the workpiece diameter; the heat relationship table includes multiple workpiece diameters and the corresponding input heat and electrode spacing for each diameter; and performing post-weld heat treatment on the weld joint after resistance welding according to the input heat and electrode spacing, followed by air cooling of the heat-treated weld joint to room temperature. This method ensures the quality of the weld joint, solves the problem of brittle fracture in iron-chromium-aluminum weld joints, improves product yield, reduces production costs, and enhances product quality and stability.

[0018] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0019] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 A flowchart of a welding method for iron-chromium-aluminum wire according to an embodiment of the present invention is shown.

[0020] Figure 2 A schematic diagram of a resistance wire stress-strain curve in an embodiment of the present invention is shown.

[0021] Figure 3 The metallographic morphology of a welded joint in an embodiment of the present invention is shown. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0023] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0024] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0025] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0026] Due to its high resistivity and thermal conductivity, iron-chromium-aluminum alloy is widely used in industries such as electronics manufacturing, home appliances, and industrial electric furnaces, and is an important functional material for electric heating in the field of electric heating.

[0027] The microstructure of iron-chromium-aluminum alloys is single-phase ferrite with a high degree of alloying, resulting in poor weldability. Brittle fracture frequently occurs during the actual processing and production of iron-chromium-aluminum products, especially in the welding of iron-chromium-aluminum wires, leading to low yield and high production costs.

[0028] In the actual processing and production of iron-chromium-aluminum products, a drawing process is required. When the wire is compressed and deformed to a certain extent, wire breakage often occurs. At this time, welding workers need to weld the wire so that it can participate in the drawing process again.

[0029] Resistance welding is often used for welding iron-chromium-aluminum wires. However, for iron-chromium-aluminum products with a high degree of alloying, the weld joint is prone to brittle fracture, resulting in low joint quality and seriously affecting the production and processing efficiency.

[0030] Existing technologies for welding iron-chromium-aluminum resistance wires involve steps such as impurity cleaning, assembly and positioning, preheating, welding, and slag removal, which improve welding quality and efficiency. However, due to the lap joint welding method, it is difficult to perform the wire drawing process after welding. While the tensile strength of the welded joint meets the requirements, elongation results are not shown, and the critical brittle fracture problem in iron-chromium-aluminum welding remains unresolved.

[0031] Based on the above situation, combined with Figure 1 As shown, the present invention provides a method for welding iron-chromium-aluminum wire, which includes steps 101 to 105: Step 101: Straighten the weldment and clean the weld end face; In this embodiment, the weldment is an iron-chromium-aluminum wire. Before welding, an iron-chromium-aluminum wire of the same material is selected as the weldment, and the weldment is straightened to ensure that the weldment axis is consistent, ensuring accurate welding and avoiding stress concentration due to welding deviation. At the same time, the welding end face of the weldment is thoroughly cleaned to remove surface oil, oxide scale, dust and other impurities to avoid impurities affecting the bonding strength of the weld joint.

[0032] Step 102: Perform resistance welding on the workpieces; In this embodiment, resistance welding is used to butt weld the straightened and cleaned workpieces to form an initial weld joint between the two wires. The quality of the weld joint is then improved through heat treatment.

[0033] For example, a double-forging pressure welding machine can be used for resistance welding. This type of welding machine has a forging function, which can apply a certain pressure to the weld joint during the welding process, promote the plastic deformation of the joint metal, and reduce defects such as pores and cracks inside the joint. At the same time, the welding current is set to 55A. This current value is the optimal current for welding 4mm diameter iron-chromium-aluminum wire after multiple experiments. It can ensure that the wire is fully melted to form a good joint, and avoid the problems of overheating and coarse grains caused by excessive current, or incomplete penetration caused by insufficient current.

[0034] During implementation, SGHYZ iron-chromium-aluminum resistance wire with a diameter of 4mm can be selected. A double forging and welding machine can be used, and the welding current can be adjusted to 55A. After the straightened and cleaned ends of the two wires are joined together, the welding machine can be started to perform resistance welding. During the welding process, the welding machine automatically applies forging pressure to fully combine the molten metal and form an initial weld joint.

[0035] Step 103: Based on a preset heat relationship table, determine the required input heat and electrode spacing for the weldment according to the diameter of the weldment; the heat relationship table includes multiple weldment diameters, and the input heat and electrode spacing corresponding to each weldment diameter; In this embodiment, a heat relationship table is pre-constructed using extensive experimental data. This table contains multiple common weldment diameter specifications, along with the optimal input heat and electrode spacing corresponding to each diameter specification. During actual welding, the corresponding input heat and electrode spacing can be directly retrieved from the heat relationship table based on the specific diameter of the weldment to be processed, eliminating the need for repeated experiments and improving processing efficiency.

[0036] Step 104: Based on the input heat and the electrode spacing, perform post-weld heat treatment on the weld joint on the resistive butt welded parts; In this embodiment, after resistance welding is completed, the welded joint is subjected to targeted heat treatment according to the input heat and electrode spacing obtained from the query. By precisely controlling the heat input and electrode position, the microstructure and properties of the welded joint are optimized.

[0037] It should be noted that the temperature of the welded joint is greater than 200°C before post-weld heat treatment.

[0038] Specifically, after resistance welding is completed, the welded joint must be heat-treated immediately, and the temperature of the welded joint must be no lower than 200°C before the heat treatment begins. This is because after welding, the joint is in a warm state. If it is allowed to cool down before heat treatment, residual stress will cause irreversible changes in the internal structure of the joint, making it difficult for subsequent heat treatment to effectively eliminate defects. However, heat treatment when the joint temperature is no lower than 200°C can utilize the residual heat of the joint, combined with external heat input, to more efficiently optimize the structure, eliminate stress, and reduce defects.

[0039] For example, a double forging press welding machine is used to perform resistance welding on 4mm diameter iron-chromium-aluminum wire (welding current 55A). After welding, the temperature of the weld joint is measured immediately to ensure that it is between 200℃ and 300℃. Then, the electrodes are quickly arranged according to the preset parameters and the heat treatment process is started to prevent the joint temperature from dropping below 200℃.

[0040] By limiting the joint temperature before heat treatment, the timeliness and effectiveness of the heat treatment are ensured, avoiding structural defects caused by stress generated during joint cooling. Utilizing the residual heat of the joint for heat treatment not only improves the efficiency of stress relief and microstructure optimization but also reduces energy consumption from external heat input, achieving the dual benefits of energy saving and quality improvement.

[0041] Step 105: Cool the heat-treated welded joint to room temperature.

[0042] In this embodiment, after the heat treatment is completed, the welded joint is placed in the air to allow it to cool slowly to room temperature, thus avoiding the generation of new residual stress due to rapid cooling.

[0043] This embodiment achieves standardization and precision in the welding process through standardized pre-welding preparation, precise parameter matching, targeted post-weld heat treatment, and natural air cooling. It effectively eliminates residual stress in the weld joint, optimizes the joint microstructure, significantly improves the tensile strength and elongation of the weld joint, and avoids brittle fracture. Simultaneously, the use of a pre-set heat relationship table simplifies the parameter determination process, improves production efficiency, and reduces the risk of increased production costs due to joint quality issues.

[0044] In an optional embodiment, the post-weld heat treatment of the weld joint on the resistive butt welded workpiece based on the input heat and the electrode spacing includes: The current and duration of heat treatment are determined based on the input heat and the electrode spacing. According to the electrode spacing, the two electrodes are placed on both sides of the welding joint; The weld joint on the weldment is subjected to electric heating treatment based on the energizing current and the energizing time.

[0045] Specifically, the input heat is directly related to the electrical work, calculated using the formula W=I²Rt, where W is the electrical work, I is the current, R is the resistance of the workpiece, and t is the energizing time. Based on the obtained input heat and electrode spacing, combined with the resistivity and length of the workpiece (determined by the electrode spacing), the required energizing current and energizing time for heat treatment can be accurately calculated, ensuring that the input heat meets the preset requirements. It should be noted that the input heat and electrode spacing determined above are the optimal input heat and electrode spacing recommended for a specific workpiece diameter. In actual implementation, the actual electrode spacing may change. If the actual electrode spacing changes, the energizing current and energizing time need to be adjusted accordingly. Generally, when the actual electrode spacing is larger than the recommended electrode spacing, the energizing current and energizing time will be increased.

[0046] Regarding the electrode arrangement, in this embodiment, two electrodes are fixed on both sides of the welding joint, so that the positional relationship between the electrodes and the welding joint meets the preset electrode spacing requirements, ensuring that the heat can be concentrated on the welding joint area during heat treatment, while uniformly covering the heat-affected zone.

[0047] According to the calculated current and time, the power supply is turned on to heat the welded joint. Through the effect of resistance heating, the microstructure of the welded joint is optimized and residual stress is eliminated.

[0048] For example, taking a 4mm diameter iron-chromium-aluminum wire as an example, the input heat is found to be 737J, and the electrode spacing is 48mm (24mm between the electrode on one side and the weld center). The resistivity of this wire is known to be approximately 1.45μΩ. According to the formula for electrical work, when the current is 80A, the required energizing time is 20s, at which time 737J of heat can be accurately input; place the two electrodes on both sides of the welding joint, with a distance of 24mm between each side and the center of the weld, and heat with 80A current for 20s to complete the heat treatment.

[0049] This embodiment achieves precise control of the input heat by accurately calculating the energizing current and energizing time; the reasonable arrangement of electrode positions ensures the effective range of heat application. The combination of these two factors allows the welded joint to receive uniform and sufficient heat during heat treatment, further optimizing the joint microstructure, improving its mechanical properties, and effectively avoiding joint quality problems caused by improper heat input or electrode position deviations.

[0050] It is worth noting that if the actual distance between the two electrodes is greater than the electrode spacing, the energizing time and / or energizing current will be increased.

[0051] In other words, after determining the energizing current and energizing time for heat treatment, if, during actual operation, factors such as equipment limitations or workpiece size deviations cause the actual distance between the two electrodes to be greater than the preset electrode distance, the length of the workpiece through which the current passes will increase, leading to increased resistance and insufficient heat input for the same current and time. In this case, it is necessary to supplement the heat input by increasing the energizing time and / or energizing current to ensure that the actual heat input is sufficient to guarantee the heat treatment effect.

[0052] For example, taking a 4mm diameter iron-chromium-aluminum wire as an example, the preset electrode spacing is 48mm, corresponding to an energizing current of 80A and an energizing time of 20s. If the electrode spacing is adjusted to 60mm (greater than 48mm) in actual operation, the energizing time needs to be extended to 30s, or the energizing time can be kept at 20s while increasing the current to 90A, or a combination of increasing the current to 85A and extending the time to 25s can be used to ensure that the heat input is greater than 737J to guarantee the heat treatment effect.

[0053] This embodiment provides a flexible solution for parameter adjustment in actual production, effectively addressing potential deviations in electrode spacing. By dynamically adjusting the energizing time and / or current, the stability of the heat input is ensured, preventing a decrease in heat treatment effect due to actual spacing deviations, and guaranteeing the consistency and reliability of the welded joint quality.

[0054] In an optional embodiment, when the weldment diameter is 4 mm, the required heat input is 737 J, and the electrode spacing is 8 times the weld joint width. The energizing current during heat treatment is 80 A. The energizing time is 20 s. The weld joint width is 6 mm.

[0055] Specifically, the width of the welded joint is approximately equal to the width of the heat-affected zone plus the weld width. This combination of parameters precisely matches the characteristics of 4mm diameter iron-chromium-aluminum wire, achieving optimal heat treatment results.

[0056] To facilitate understanding and implementation by those skilled in the art, combined with Figure 2 , Figure 3 As shown in the example, this embodiment is illustrated below: Experimental materials SGHYZ resistance wire (iron-chromium-aluminum material) produced by a certain materials company was selected as the experimental welding part, with a diameter of 4.0 mm and a length of 110 mm.

[0057] Experimental equipment: double forging and welding machine, heat treatment power supply equipment, precision measuring tools, tensile testing machine.

[0058] Experimental steps: Pre-welding preparation: Take several SGHYZ resistance wires with a diameter of 4.0mm and straighten them to ensure that the wire axes are aligned; then clean the welding end face of each wire to remove surface oil, oxide scale and dust.

[0059] Resistance welding: A double forging and pressing welding machine is used to butt-weld the straightened and cleaned wires. The welding current is set to 55A, and the welding machine is started to complete the resistance welding and obtain the initial welded joint.

[0060] Electrode spacing setting: Measure the width of the weld joint, which is approximately 6mm. Set the electrode spacing to 8 times the width of the weld joint, i.e., 48mm (24mm between the single-sided electrode and the center of the weld).

[0061] Post-weld heat treatment: After welding, immediately measure the temperature of the weld joint to ensure it is not lower than 200℃; place two electrodes on both sides of the weld joint, with a distance of 24mm between each side and the center of the weld; set the current to 80A and the energizing time to 20s, and start the heat treatment equipment to heat the weld joint with an input heat of approximately 737J.

[0062] Air cooling treatment: After heat treatment, place the weldment in the air to cool to room temperature.

[0063] Performance testing: The cooled weldment was installed on a tensile testing machine, and the tensile rate was set to 45 mm / min. Tensile tests were conducted to test the tensile strength and total elongation of the welded joint.

[0064] Comparative experimental design Blank control group: Unwelded original SGHYZ resistance wires were selected and subjected to tensile tests to measure their tensile strength and total elongation.

[0065] Untreated group: The welded parts are not subjected to post-weld heat treatment, but are directly air-cooled to room temperature, and then subjected to tensile tests.

[0066] Experimental groups with different electrode spacings: The distance D between the single-sided electrode and the center of the weld was set to 10mm, 15mm, 20mm, 25mm, 30mm, 40mm, and 50mm (corresponding to electrode spacings of 20mm, 30mm, 40mm, 50mm, 60mm, 80mm, and 100mm). Each group of samples was tested 3 times. All samples were heat-treated with an 80A current and a 20s energizing time. After cooling, tensile tests were performed.

[0067] Extended time test group: Select samples with a single electrode and weld center distance D=50mm (electrode distance 100mm), use 80A current, extend the energizing time to 30s, perform heat treatment and then cool, and then perform tensile test.

[0068] Experimental results Blank control group: The tensile strength of the original unwelded sample was approximately 600 MPa, and the total elongation was approximately 27%.

[0069] Untreated group: The tensile strength of the samples that were not heat-treated after welding dropped to about 530 MPa, and the total elongation dropped significantly to about 5%.

[0070] Experimental groups with different electrode spacings: When the distance D between the single-sided electrode and the center of the weld is between 10-25mm, the tensile properties of the welded joint are gradually optimized. When D=25mm (electrode spacing 50mm), it is close to the optimal state, and the tensile strength and total elongation are close to the level of the original sample. When D is higher than 25mm, the tensile strength and elongation generally show a downward trend.

[0071] In the extended-time experimental group, the tensile strength and elongation of the sample with D=50mm and energized for 30s increased again, approaching the optimal value and second only to the sample with D=25mm.

[0072] Experimental conclusions Experimental verification shows that when the diameter of the weldment is 4mm, the combination of parameters set by this invention, such as welding current of 55A, electrode spacing of 48mm, heat treatment current of 80A, energizing time of 20s, and input heat of 737J, can effectively improve the tensile strength and elongation of the welded joint of iron-chromium-aluminum wire, and avoid brittle fracture of the joint. If the electrode spacing is more than 8 times the width of the welded joint, the heat input can be increased by extending the energizing time or increasing the current, which can also ensure the quality of the joint.

[0073] By standardizing pre-welding preparation and precisely setting welding and heat treatment parameters, the technical challenge of brittle fracture in iron-chromium-aluminum wire welded joints is effectively solved. Post-weld heat treatment can significantly eliminate residual stress in the welded joint, optimize the joint microstructure, and improve the tensile strength and elongation of the joint, bringing the quality of the welded joint close to that of the original base material. The preset heat relationship table and clear parameter combinations simplify the operation process, improve production efficiency, and reduce production costs. The method has good flexibility, and the energizing time and / or current can be adjusted according to the actual electrode spacing deviation to ensure stable heat treatment results, making it suitable for large-scale production scenarios.

[0074] In summary, the iron-chromium-aluminum wire welding method provided in this specification involves straightening the workpiece and cleaning the welding end face; performing resistance welding on the workpiece; determining the required input heat and electrode spacing based on a preset heat relationship table and the workpiece diameter; the heat relationship table includes multiple workpiece diameters and the corresponding input heat and electrode spacing for each diameter; and performing post-weld heat treatment on the weld joint after resistance welding according to the input heat and electrode spacing, followed by air cooling of the heat-treated weld joint to room temperature. This method ensures weld joint quality, solves the problem of brittle fracture in iron-chromium-aluminum weld joints, improves product yield, reduces production costs, and enhances product quality and stability.

[0075] The above are merely various embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for welding iron-chromium-aluminum wire, characterized in that, include: Straightening of weldments and cleaning of weld end faces; Resistance welding is performed on the weldment. Based on a preset heat relationship table, the required input heat and electrode spacing for the weldment are determined according to the diameter of the weldment. The heat relationship table includes multiple weldment diameters, as well as the input heat and electrode spacing corresponding to each weldment diameter. Based on the input heat and the electrode spacing, post-weld heat treatment is performed on the welded joints on the welded parts after resistance welding; The heat-treated welded joint is then air-cooled to room temperature.

2. The method according to claim 1, characterized in that, The weldment is made of iron-chromium-aluminum wire.

3. The method according to claim 1, characterized in that, The step of performing post-weld heat treatment on the weld joint of the resistive butt welded workpiece according to the input heat and the electrode spacing includes: The current and duration of heat treatment are determined based on the input heat and the electrode spacing. According to the electrode spacing, the two electrodes are placed on both sides of the welding joint; The weld joint on the weldment is subjected to electric heating treatment based on the energizing current and the energizing time.

4. The method according to claim 3, characterized in that, After determining the energizing current and energizing time during heat treatment based on the input heat and the electrode spacing, the method further includes: If the actual distance between the two electrodes is greater than the electrode distance, then the energizing time and / or energizing current will be increased.

5. The method according to claim 1, characterized in that, The resistance welding of the weldment includes: The workpieces were subjected to resistance welding using a double forging press welding machine with a welding current of 55A.

6. The method according to claim 1, characterized in that, Before post-weld heat treatment, the temperature of the welded joint is greater than 200°C.

7. The method according to claim 1, characterized in that, When the diameter of the weldment is 4 mm, the required heat input for the weldment is 737 J, and the electrode spacing is 8 times the width of the weld joint.

8. The method according to claim 7, characterized in that, The current applied during the heat treatment is 80A.

9. The method according to claim 8, characterized in that, The power-on time is 20 seconds.

10. The method according to claim 7, characterized in that, The width of the welded joint is 6mm.

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