Heat treatment method for improving width uniformity of heat affected zone of rail welded joint

Abstract

The application discloses a heat treatment method for improving the width uniformity of a heat affected zone of a rail welded joint, and the method comprises the following steps: moving a rail through flash welding to obtain a welded rail; naturally cooling the welded joint of the welded rail to room temperature; using an electric induction coil to perform full-section induction heating on the welded joint; controlling the width of the heating area of the rail welded joint by using a cooling device during the heating process; and immediately spraying air to cool after heating to a predetermined temperature. The heat treatment method mainly aims at the technical problems that the width of the heat affected zone of the rail flash welded joint after post-weld heat treatment is not uniform, the width of the heat affected zone is too large, and the use performance and driving safety of the rail are affected. After the method is used, a pearlitic rail flash welded joint with high comprehensive performance, high uniformity of the width of the heat affected zone and small width of the heat affected zone is finally obtained.

Description

Technical Field

[0001] This invention relates to the field of rail welding technology, and specifically to a heat treatment method for improving the uniformity of the width of the heat-affected zone of rail welded joints. Background Technology

[0002] With the rapid development of the world, rail transit is also evolving towards higher speeds, heavier loads, and greater comfort and safety. Pearlitic steel rails, due to their excellent wear resistance, strength, and toughness, are widely used in railway tracks. Flash welding is currently the most widely used rail welding method. However, during flash welding, the end faces of the two rails to be welded are heated by a short circuit, causing them to melt, and the temperature gradually decreases with increasing distance from the welding end faces. Therefore, the microstructure near the weld of the flash-welded rail joint is affected by the high temperature during welding, resulting in grain coarsening and abnormal microstructure. This leads to a significant decrease in the strength, toughness, and plasticity of the welded joint, seriously affecting the rail's lifespan and traffic safety. Therefore, the control of the microstructure and properties of the welded rail joint after welding has become an urgent research topic. Post-weld heat treatment can effectively solve this problem. Post-weld heat treatment, by austenitizing the welded joint and then cooling it, can refine the grains and improve the microstructure, thereby improving the performance of the welded joint and ensuring a proper match between the welded joint and the rail base material.

[0003] There are two common post-weld normalizing heat treatment methods for rails: induction normalizing and flame normalizing. Many researchers have studied both methods. Some scholars have investigated the flame normalizing process for U75VG steel, studying the effects of normalizing flow rate, normalizing time, and cooling rate on properties such as hardness, impact, microstructure, grain size, and pearlite lamellar spacing, obtaining the optimal normalizing process parameters. Other scholars have studied induction normalizing of rail welded joints. Induction normalizing significantly improves the performance of welds, increasing plasticity by about twice and impact toughness by nearly four times compared to unnormalized welds. The results show that both flame normalizing and induction normalizing improve the microstructure of rail joints and enhance their overall performance compared to unnormalized welds. Some scholars have conducted comparative studies of the two methods. Overall, induction normalizing is more efficient, has a higher degree of automation, is less affected by operator intervention, and provides more stable quality. Joints treated with induction normalizing exhibit better microstructure and performance. Therefore, induction normalizing has become the most widely used post-weld normalizing method for rails. Induction normalizing is based on the principle of electromagnetic induction. It uses the heat generated by eddy currents in the workpiece to heat the workpiece. Through induction heating, the joint is fully austenitized and raised to a certain temperature. Then, it is cooled by air jet. By refining the grains and controlling the microstructure, the performance is improved.

[0004] However, in the induction normalizing heat treatment of rail joints, due to the "skin effect" and "corner effect" of induction heating, and the irregular cross-section of the rail, induction heating tends to concentrate on the surface. Simultaneously, the heating rate at the rail head corners and rail base is relatively fast, resulting in different heating rates at different locations on the rail. Furthermore, the varying cross-sectional thickness at different locations leads to differences in temperature conduction. Ultimately, this results in significant differences in the width of the heat-affected zone (HAZ) at different locations on the rail cross-section after heat treatment, ultimately leading to performance variations. Some scholars have optimized the normalizing process or coils to achieve a certain degree of refinement in the HAZ width. However, the control of HAZ width uniformity has not yet been studied, and there is still room for improvement in the research on refining the HAZ width. Summary of the Invention

[0005] The purpose of this invention is to provide a heat treatment method for improving the uniformity of the heat-affected zone (HAZ) width in rail welded joints. This method primarily addresses the technical challenge of uneven HAZ width and excessively large HAZ width in flash-welded rail joints after heat treatment, which negatively impact rail performance and traffic safety. Through the method of this invention, a pearlitic rail flash-welded joint with high overall performance, high HAZ width uniformity, and a small HAZ width is ultimately obtained. The HAZ width of the flash-welded rail joint is significantly reduced to 50mm, and the uniformity of the HAZ width is greatly improved (the maximum difference in HAZ width at different rail heights is <2mm). Other microstructure properties of the joint meet TB / T 1632 while exhibiting significant performance improvements, thus substantially enhancing the performance of the flash-welded joint.

[0006] To achieve the above-mentioned objectives, this invention provides a heat treatment method for improving the uniformity of the heat-affected zone width of rail welded joints. The rail is obtained through moving flash welding; the welded joint of the rail undergoes heat treatment via the following steps: Step (1): Allow to cool naturally to room temperature; Step (2): Use an induction coil to perform full-section induction heating on the welded joint; during the heating process, use a cooling device to cool the rail welded joint at a position 2 to 8 mm away from the outside of the induction coil, thereby controlling the temperature of the position and the area away from the welded joint to be below 200℃. Step (3): During the induction heating process, the rail head tread temperature reaches 900-1000℃ and the heating is stopped immediately. The temperature is then cooled down to below 500℃ by air cooling device and allowed to cool naturally.

[0007] In this invention, induction heating is achieved using an induction coil that mimics the profile of a steel rail. The entire cross-section of the flash-welded joint of the rail is induction heated using the principle of electromagnetic induction. A schematic diagram of the induction coil and the rail is shown below. Figure 1 As shown.

[0008] As a specific embodiment of the present invention, in the induction heating process of the present invention, a water-cooling device is added at a distance of 5mm from the outside of the induction heating coil. The structure of the water-cooling device is as follows: Figure 2 As shown, the temperature measurement locations for cooling are the rail foot edge and the rail head tread, because these two locations have a wider heat-affected zone, meaning they heat up faster. The temperature measurement and water cooling devices on both sides of the welded joint are completely identical. The water cooling devices are split-type and can be opened for easy handling of the rails.

[0009] The aforementioned water-cooling device applies water cooling to the area when the temperature at the measurement point reaches 200℃. The device uses contact heat conduction to cool the rail, and the circulating cooling water within the device also cools the device itself. By controlling the temperature at this location and in areas far from the weld joint below 200℃, phase transformation can occur, affecting the rail's performance. This reduces the width of the heat-affected zone and significantly improves its uniformity.

[0010] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of the rail according to the present invention, preferably, in step (2), the induction coil is an induction coil with a profile similar to that of the rail.

[0011] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of the rail according to the present invention, preferably, in step (2), the cooling device is a water cooling device, the water cooling device is fully contacted and wraps the rail along the cross-section of the rail, and the width of the water cooling area on both sides of the induction coil is 40-60mm.

[0012] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of the rail according to the present invention, preferably, in step (2), the detection positions of the cooling treatment temperature are the edge of the rail foot and the rail head tread.

[0013] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of the rail according to the present invention, preferably, in step (3), the air cooling device consists of an air spray box with a width of 100-200mm and an air inlet pipe, the air inlet pipe is connected to the air inlet of the air spray box, and the air spray box blows air onto the upper surface and side of the rail head.

[0014] A schematic diagram of the air-cooling device of the present invention is shown below. Figure 3 As shown.

[0015] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of the rail according to the present invention, preferably, the distance between the air spray box and the rail surface is 10-60mm and the air spray pressure is 0.05-0.5MPa.

[0016] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to the present invention, preferably, the moving flash welding includes, in sequence, a flash leveling stage, a preheating stage, a burning stage, an upsetting stage, and a spheroid pushing stage.

[0017] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint as described in this invention, preferably, the flash leveling voltage in the flash leveling stage is 360-450V; the preheating voltage in the preheating stage is 300-400V; and the burning voltage in the burning stage is 360-450V.

[0018] The flash-flattening stage of the moving flash welding of this invention is the first stage of moving flash welding of rails. Since the rails are at room temperature at this stage, a relatively high voltage needs to be applied, controlled between 350 and 450V. The two forged end faces to be welded gradually approach each other. The end faces are locally microscopically uneven. During the approach process, the locally protruding positions first make contact and short-circuit, generating a large current that melts and explodes the contact point, thus producing a flash. This flashing is continuously generated as the rails to be welded move forward and backward. This process removes impurities, stains, and local unevenness from the end faces to be welded, resulting in a good weld end face; hence the term "flash-flattening."

[0019] The preheating stage of the mobile flash welding of this invention involves repeatedly short-circuiting the two rails to be welded to heat them to a certain temperature and create a suitable temperature gradient, preparing them for subsequent stages. This stage requires a stable flash process to preheat the rails, therefore the voltage is relatively lower than in the flash leveling stage, controlled at 300–400V.

[0020] The burning stage of the moving flash welding of this invention generates a localized high-pressure protective atmosphere by producing an intense and stable flash with high voltage. Simultaneously, the splashed molten metal droplets reduce the surrounding oxygen concentration. Through the combined effect of these two aspects, oxidation of the metal end face is prevented. A high voltage of 360–450V is used to generate the intense and stable flash.

[0021] The upsetting stage of the moving flash welding of the present invention involves bringing the rail close to the contact point at a suitable speed at the end of the sintering stage and applying force to expel the liquid metal and any defects that may be generated on the end face during the flashing process, while simultaneously causing sufficient deformation at the joint to obtain a high-quality welded joint with atomic bonding.

[0022] The above-mentioned moving flash welding stage refers to the process after upsetting, where a contour cutter with the same shape as the rail moves longitudinally along the rail under the action of force to remove the weld beads ejected during the upsetting stage.

[0023] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of a welded joint of a rail according to the present invention, preferably, the rail is a pearlitic rail, wherein the mass fraction of C element in the rail is 0.61% to 0.80%, the mass fraction of Si element is 0.12% to 0.82%, the mass fraction of Mn element is 0.61% to 1.20%, the mass fraction of V element is ≤0.20%, the mass fraction of Cr element is ≤0.50%, the mass fraction of S element is ≤0.008%, the mass fraction of O element is ≤0.003%, and the balance is Fe and unavoidable impurities.

[0024] According to the heat treatment method for improving the uniformity of the width of the heat-affected zone of the welded joint of rail according to the present invention, preferably, the pearlitic rail is a 60kg / m heat-treated pearlitic rail.

[0025] The beneficial effects of this invention are: This invention discloses a heat treatment method for improving the uniformity of the heat-affected zone (HAZ) width in rail welded joints. It primarily addresses the technical challenge of uneven HAZ width and excessively large HAZ width in flash-welded rail joints after heat treatment, which negatively impact rail performance and traffic safety. Through this method, a pearlitic rail flash-welded joint with high overall performance, high HAZ width uniformity, and a small HAZ width is ultimately obtained. The HAZ width of the flash-welded joint is significantly reduced to 50mm, and the uniformity of the HAZ width is greatly improved (the maximum difference in HAZ width at different rail heights is <2mm). Other microstructure properties of the joint meet TB / T 1632 standards while exhibiting significant performance improvements, thus substantially enhancing the performance of the flash-welded joint. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or 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 only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of an induction coil and a rail according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a water-cooling device according to an embodiment of the present invention; Figure 3 This is a schematic diagram of an air-cooling device according to an embodiment of the present invention; Figure 4 This is a schematic diagram showing the sampling location of the width of the heat-affected zone according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the heat-affected zone morphology in Embodiment 1 of the present invention; Figure 6 This is the morphology of the heat-affected zone in Comparative Example 1 of the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0029] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0030] Performance testing: The welded joints obtained in the embodiments and comparative examples of this invention were subjected to microstructural property testing and analysis in accordance with standard TB / T1632. According to TB / T 1632, the width of the heat-affected zone on the surface 5 mm from the center plane was measured, as shown in the figure. Figure 4 As shown; hardness is represented by cross-sectional hardness (HRC, with a 5mm interval between two hardness points); microstructure was observed and analyzed by metallographic microscope after being etched with 4% nitric acid alcohol samples taken from standard locations.

[0031] Example 1 The rail in this embodiment is a pearlitic rail; wherein the mass fraction of C element is 0.76%, the mass fraction of Si element is 0.55%, the mass fraction of Mn element is 0.90%, the mass fraction of V element is 0.005%, the mass fraction of Cr element is 0.1%, the mass fraction of S element is 0.003%, the mass fraction of O element is 0.002%, and the balance is Fe and unavoidable impurities.

[0032] Welding is performed by moving flash welding, which is divided into five stages: flash leveling, preheating, sintering, upsetting, and stub removal. The voltage for the flash leveling stage is 405V, the voltage for the preheating stage is 355V, the voltage for the sintering stage is 390V, and the upsetting amount for the upsetting stage is 14mm.

[0033] After the above welding process is completed, and the welded joint has cooled naturally to room temperature, the following steps are taken: Figure 1 The induction coil shown, mimicking the profile of a steel rail, utilizes the principle of electromagnetic induction to induction heat the entire cross-section of the flash-welded joint of the rail. During the induction heating process, water-cooling devices are used on both sides of the induction coil, 5mm away from it. These water-cooling devices are full-contact type and wrap around the rail along its cross-section. Figure 2As shown, the width of the water-cooled area is 50mm. Cooling treatment is performed on both sides of the rail welded joint. The temperature detection positions for the cooling treatment are the edge of the rail foot and the rail head tread. When the temperature at the corresponding position reaches 200℃, water cooling is performed on that part to keep the temperature at the cooling position and the area away from the welded joint below 200℃.

[0034] Once the heating temperature reaches 950℃, immediately remove the heating device and use a 120mm wide air jet box to blow air onto the upper and side surfaces of the rail head. The air jet box should be 35mm away from the rail surface, and the air jet pressure should be 0.15MPa. Stop blowing air after the air cools to 480℃ and allow it to cool naturally.

[0035] The welded joints obtained by the above method were subjected to microstructure and property analysis in accordance with standard TB / T 1632. The sampling locations for the heat-affected zone width are as follows: Figure 4 As shown, after etching with 4% nitric acid alcohol, the hardness was represented by the cross-sectional hardness (HRC, with hardness test positions spaced 5 mm apart). The microstructure was analyzed using a metallographic microscope after etching with 4% nitric acid alcohol. Through the coordinated control of post-weld heat treatment process and heat treatment auxiliary equipment, the final obtained cross-sectional hardness of the welded joint was the average hardness H of the joint. J And the average hardness H of the base material p The ratio is 0.96, the softened zone width is <10mm, and there are no abnormal structures such as martensite or bainite. It exhibits high performance and its microstructure meets standard requirements; the morphology of the heat-affected zone is as follows: Figure 5 As shown, the interface between the heat-affected zone and the base material is basically parallel, and the width of the heat-affected zone is only 50mm, which is significantly reduced. Moreover, the maximum difference in the width of the heat-affected zone corresponding to different track heights is less than 2mm, and the uniformity of the width of the heat-affected zone is greatly improved.

[0036] Example 2 The rail in this embodiment is a pearlitic rail; wherein the mass fraction of C element is 0.76%, the mass fraction of Si element is 0.55%, the mass fraction of Mn element is 0.90%, the mass fraction of V element is 0.005%, the mass fraction of Cr element is 0.1%, the mass fraction of S element is 0.003%, the mass fraction of O element is 0.002%, and the balance is Fe and unavoidable impurities.

[0037] Welding is performed by moving flash welding, which is divided into five stages: flash leveling, preheating, sintering, upsetting, and stub removal. The voltage for the flash leveling stage is 405V, the voltage for the preheating stage is 355V, the voltage for the sintering stage is 390V, and the upsetting amount for the upsetting stage is 14mm.

[0038] After the above welding process is completed, and the welded joint has cooled naturally to room temperature, the following steps are taken: Figure 1The induction coil shown, mimicking the profile of a steel rail, utilizes the principle of electromagnetic induction to induction heat the entire cross-section of the flash-welded joint of the rail. During the induction heating process, water-cooling devices are used on both sides of the induction coil, 5mm away from it. These water-cooling devices are full-contact type and wrap around the rail along its cross-section. Figure 2 As shown, the width of the water-cooled area is 50mm. Cooling treatment is performed on both sides of the rail welded joint. The temperature detection positions for the cooling treatment are the edge of the rail foot and the rail head tread. When the temperature at the corresponding position reaches 200℃, water cooling is performed on that part to keep the temperature at the cooling position and the area away from the welded joint below 200℃.

[0039] Once the heating temperature reaches 920℃, immediately remove the heating device and use a 120mm wide air jet box to blow air onto the upper and side surfaces of the rail head. The air jet box should be 35mm away from the rail surface, and the air jet pressure should be 0.15MPa. Stop blowing air after the air cools to 480℃ and allow it to cool naturally.

[0040] The welded joints obtained by the above method were subjected to microstructure and property analysis in accordance with standard TB / T 1632. The sampling locations for the heat-affected zone width are as follows: Figure 4 As shown, after etching with 4% nitric acid alcohol, the hardness was represented by the cross-sectional hardness (HRC, with hardness test positions spaced 5 mm apart). The microstructure was analyzed using a metallographic microscope after etching with 4% nitric acid alcohol. Through the coordinated control of post-weld heat treatment process and heat treatment auxiliary equipment, the final obtained cross-sectional hardness of the welded joint was the average hardness H of the joint. J And the average hardness H of the base material p The ratio is 0.94, the width of the softened zone is 11.7mm, and there are no abnormal structures such as martensite and bainite. The heat-affected zone is basically parallel to the interface of the base material, and the width of the heat-affected zone is only 50mm, which is greatly reduced. Moreover, the maximum difference in the width of the heat-affected zone corresponding to different track positions is <2mm, and the uniformity of the width of the heat-affected zone is greatly improved.

[0041] Example 3 In this embodiment, the rail used is a pearlitic rail; wherein the mass fraction of C element is 0.76%, the mass fraction of Si element is 0.55%, the mass fraction of Mn element is 0.90%, the mass fraction of V element is 0.005%, the mass fraction of Cr element is 0.1%, the mass fraction of S element is 0.003%, the mass fraction of O element is 0.002%, and the balance is Fe and unavoidable impurities.

[0042] Welding is performed by moving flash welding, which is divided into five stages: flash leveling, preheating, sintering, upsetting, and stub removal. The voltage for the flash leveling stage is 405V, the voltage for the preheating stage is 355V, the voltage for the sintering stage is 390V, and the upsetting amount for the upsetting stage is 14mm.

[0043] After the above welding process is completed, and the welded joint has cooled naturally to room temperature, the following steps are taken: Figure 1 The induction coil shown, mimicking the profile of a steel rail, utilizes the principle of electromagnetic induction to induction heat the entire cross-section of the flash-welded joint of the rail. During the induction heating process, water-cooling devices are used on both sides of the induction coil, 5mm away from it. These water-cooling devices are full-contact type and wrap around the rail along its cross-section. Figure 2 As shown, the width of the water-cooled area is 50mm. Cooling treatment is performed on both sides of the rail welded joint. The temperature detection positions for the cooling treatment are the edge of the rail foot and the rail head tread. When the temperature at the corresponding position reaches 200℃, water cooling is performed on that part to keep the temperature at the cooling position and the area away from the welded joint below 200℃.

[0044] Once the heating temperature reaches 950℃, immediately remove the heating device and use a 120mm wide air jet box to blow air onto the upper and side surfaces of the rail head. The air jet box should be 40mm away from the rail surface, and the air jet pressure should be 0.20MPa. Stop blowing air after the air cools to 480℃ and allow it to cool naturally.

[0045] The welded joints obtained by the above method were subjected to microstructure and property analysis in accordance with standard TB / T 1632. The sampling locations for the heat-affected zone width are as follows: Figure 4 As shown, after etching with 4% nitric acid alcohol, the hardness was represented by the cross-sectional hardness (HRC, with hardness test positions spaced 5 mm apart). The microstructure was analyzed using a metallographic microscope after etching with 4% nitric acid alcohol. Through the coordinated control of post-weld heat treatment process and heat treatment auxiliary equipment, the final obtained cross-sectional hardness of the welded joint was the average hardness H of the joint. J And the average hardness H of the base material p The ratio is 0.96, the width of the softened zone is <10mm, and there are no abnormal structures such as martensite and bainite; the heat-affected zone is basically parallel to the interface of the base material, the width of the heat-affected zone is only 50mm, which is greatly reduced, and the difference in the width of the heat-affected zone corresponding to different track heights is <2mm, which greatly improves the uniformity of the width of the heat-affected zone.

[0046] Comparative Example 1: In this comparative example, the rails used are pearlitic rails; the mass fraction of C is 0.76%, the mass fraction of Si is 0.55%, the mass fraction of Mn is 0.90%, the mass fraction of V is 0.005%, the mass fraction of Cr is 0.1%, the mass fraction of S is 0.003%, the mass fraction of O is 0.002%, and the balance is Fe and unavoidable impurities.

[0047] Welding is performed by moving flash welding, which is divided into five stages: flash leveling, preheating, sintering, upsetting, and stub removal. The voltage for the flash leveling stage is 405V, the voltage for the preheating stage is 355V, the voltage for the sintering stage is 390V, and the upsetting amount for the upsetting stage is 14mm.

[0048] After the welding process is completed and the welded joint has cooled naturally to room temperature, the following steps are taken: Figure 1 The induction coil shown is shaped like a steel rail and uses the principle of electromagnetic induction to induction heat the entire cross-section of the flash welded joint of the rail. Once the heating temperature reaches 950℃, the heating device is immediately removed, and a 120mm wide air jet box is used to blow air onto the upper surface and sides of the rail head. The air jet box is 35mm away from the rail surface, and the air pressure is 0.15MPa. The air jet is stopped after the air cools down to 480℃ and is allowed to cool naturally.

[0049] The welded joints obtained by the above method were subjected to microstructure and property analysis in accordance with standard TB / T 1632. The sampling locations for the heat-affected zone width are as follows: Figure 4 As shown, after etching with 4% nitric acid alcohol, the hardness was represented by the cross-sectional hardness (HRC, with hardness test positions spaced 5 mm apart). The microstructure was analyzed using a metallographic microscope after etching with 4% nitric acid alcohol. Through the coordinated control of post-weld heat treatment process and heat treatment auxiliary equipment, the final obtained cross-sectional hardness of the welded joint was the average hardness H of the joint. J And the average hardness H of the base material p The ratio is 0.96, the softened zone width is <10mm, and there are no abnormal structures such as martensite or bainite; the morphology of the heat-affected zone is as follows. Figure 6 As shown, the width of the heat-affected zone is irregular from the rail head to the rail bottom, decreasing first and then increasing, with the widest width at the rail bottom. The minimum width of the heat-affected zone is 68mm, and the maximum width is 112mm. The width of the heat-affected zone increases significantly, and the maximum difference in the width of the heat-affected zone corresponding to different rail heights is 44mm, indicating that the width of the heat-affected zone is very uneven.

[0050] Comparative Example 2: In this comparative example, the rails used are pearlitic rails; the mass fraction of C is 0.76%, the mass fraction of Si is 0.55%, the mass fraction of Mn is 0.90%, the mass fraction of V is 0.005%, the mass fraction of Cr is 0.1%, the mass fraction of S is 0.003%, the mass fraction of O is 0.002%, and the balance is Fe and unavoidable impurities.

[0051] Welding is performed by moving flash welding, which is divided into five stages: flash leveling, preheating, sintering, upsetting, and stub removal. The voltage for the flash leveling stage is 405V, the voltage for the preheating stage is 355V, the voltage for the sintering stage is 390V, and the upsetting amount for the upsetting stage is 14mm.

[0052] After the welding process is completed and the welded joint has cooled naturally to room temperature, the following steps are taken: Figure 1 The induction coil shown, mimicking the profile of a steel rail, utilizes the principle of electromagnetic induction to induction heat the entire cross-section of the flash-welded joint of the rail. During the heating process, a set of [unclear - possibly a device or device] is added 5mm away from both sides of the induction heating coil. Figure 2 The temperature measurement and water cooling device shown is used to water-cool the corresponding part when the temperature reaches 200℃, ensuring that the temperature at that location and the area away from the weld joint is controlled below 200℃. Once the heating temperature reaches 950℃, the heating device is immediately removed, and a 120mm wide air jet box is used to blow air onto the upper surface and sides of the rail head. The air jet box is 20mm away from the rail surface, and the air pressure is 0.6MPa. Air jetting is stopped after the air cools to 480℃, allowing it to cool naturally.

[0053] The welded joints obtained by the above method were subjected to microstructure and property analysis in accordance with standard TB / T 1632. The sampling locations for the heat-affected zone width are as follows: Figure 4 As shown, after etching with 4% nitric acid alcohol, the hardness was represented by the cross-sectional hardness (HRC, with hardness test positions spaced 5 mm apart). The microstructure was analyzed using a metallographic microscope after etching with 4% nitric acid alcohol. Through the coordinated control of post-weld heat treatment process and heat treatment auxiliary equipment, the final obtained cross-sectional hardness of the welded joint was the average hardness H of the joint. J And the average hardness H of the base material p The ratio is 0.97, and the heat-affected zone is basically parallel to the interface with the base material. The width of the heat-affected zone is only 50mm, and the maximum difference in the width of the heat-affected zone corresponding to different rail heights is <2mm. However, a large number of abnormal martensitic structures appear in the joint, which does not meet the requirements of standard TB / T 1632 and will seriously affect the service performance and safety of the joint.

[0054] Therefore, by using the auxiliary cooling process of the induction normalizing heat treatment developed in this invention, combined with a suitable post-weld heat treatment process, the two can be synergistically controlled to ultimately obtain a pearlitic rail flash welded joint with high comprehensive performance, high uniformity of heat-affected zone width, and small heat-affected zone width.

[0055] It should be noted that the components or steps in the above embodiments can be interchanged, substituted, added, or deleted. Therefore, the combinations formed by these reasonable permutations and transformations should also fall within the protection scope of this invention, and the protection scope of this invention should not be limited to the above embodiments.

[0056] The above are exemplary embodiments disclosed in this invention. The order of the disclosed embodiments is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. However, it should be noted that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of this invention (including the claims) is limited to these examples. Various changes and modifications can be made without departing from the scope defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular.

[0057] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A heat treatment method for improving the uniformity of the width of the heat-affected zone in rail welded joints, characterized in that, The rails are obtained by moving flash welding; the welded joints of the welded rails are heat-treated through the following steps: Step (1): Allow to cool naturally to room temperature; Step (2): Use an induction coil to perform full-section induction heating on the welded joint; during the heating process, use a cooling device to cool the rail welded joint at a position 2 to 8 mm away from the outside of the induction coil, thereby controlling the temperature of the position and the area away from the welded joint to be below 200℃. Step (3): During the induction heating process, the rail head tread surface is heated to 900-1000℃ and then the heating is stopped immediately. The air is then cooled to below 500℃ by a blower and allowed to cool naturally.

2. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a welded rail joint according to claim 1, characterized in that, In step (2), the induction coil is an induction coil with a profile similar to a steel rail.

3. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a welded rail joint according to claim 1, characterized in that, In step (2), the cooling device is a water cooling device. The water cooling device fully contacts and wraps around the rail along the cross-section of the rail. The width of the water cooling area on both sides of the induction coil is 40-60 mm.

4. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 1, characterized in that, In step (2), the temperature of the cooling process is detected at the edge of the rail foot and the tread of the rail head.

5. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 1, characterized in that, In step (3), the air-cooling device consists of an air-spray box with a width of 100-200mm and an air inlet pipe. The air inlet pipe is connected to the air inlet of the air-spray box, and the air-spray box blows air onto the upper surface and sides of the rail head.

6. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 5, characterized in that, The distance between the air jet box and the surface of the rail is 10-60mm, and the air jet pressure is 0.05-0.5MPa.

7. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 1, characterized in that, The moving flash welding process includes, in sequence, the flash leveling stage, the preheating stage, the burning stage, the upsetting stage, and the slag pushing stage.

8. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 7, characterized in that, The flash leveling voltage during the flash leveling stage is 350–450V; the preheating voltage during the preheating stage is 300–400V; and the burning voltage during the burning stage is 360–450V.

9. A heat treatment method for improving the uniformity of the width of the heat-affected zone of a welded rail joint according to any one of claims 1-8, characterized in that, The rail is a pearlitic steel rail, wherein the mass fraction of C is 0.61% to 0.80%, the mass fraction of Si is 0.12% to 0.82%, the mass fraction of Mn is 0.61% to 1.20%, the mass fraction of V is ≤0.20%, the mass fraction of Cr is ≤0.5%, the mass fraction of S is ≤0.008%, the mass fraction of O is ≤0.003%, and the balance is Fe and unavoidable impurities.

10. The heat treatment method for improving the uniformity of the width of the heat-affected zone of a rail welded joint according to claim 9, characterized in that, The pearlitic steel rail is a heat-treated pearlitic steel rail with a strength of 60 kg / m.

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