A method for improving the microstructure of a flash welded joint of a heat treated pearlitic rail

Abstract

The present application belongs to the field of rail welding and post-weld heat treatment, and particularly relates to a method for improving the structure of a flash-butt welded joint of a heat-treated pearlitic rail, which comprises the following steps: S1, performing welding treatment on the heat-treated pearlitic rail and naturally cooling; S2, performing induction normalizing heat treatment on the rail joint after natural cooling, wherein the induction normalizing heat treatment comprises a heating stage and an air blast cooling stage in sequence, and different regions divided with reference to a fusion line are respectively subjected to phased cooling in the air blast cooling stage. The method adopted by the present application can simultaneously eliminate the abnormal structure of martensite in the rail joint and reduce the width of the softened zone, thereby significantly improving the structure of the flash-butt welded joint of the heat-treated pearlitic rail.

Description

Technical Field

[0001] The present invention belongs to the field of rail welding and post-weld heat treatment, and specifically relates to a method for improving the microstructure of a flash welded joint of heat-treated pearlitic rail. Background Art

[0002] Rail transit is continuously developing in the directions of higher speed, heavy load, comfort and safety, which puts forward increasingly higher requirements for rails. Flash welding is currently the most widely used connection method for seamless rails. However, the flash welded joint of the rail after undergoing the welding thermal cycle will suffer from grain coarsening and abnormal hard and brittle martensite microstructure. Performing post-weld normalizing heat treatment on the welded joint can effectively solve the problem of grain coarsening and reduce martensite to a certain extent, but cannot completely eliminate martensite. Therefore, the stable control of the martensite microstructure in the flash welded joint of the rail has always been the focus and difficulty of rail welding research. At the same time, whether in the as-welded state or the post-weld heat treatment state, due to the gradient distribution of temperature, the longitudinal welding heat-affected zone of the joint will inevitably have a temperature range between A1 and A3, resulting in spheroidization of lamellar pearlite and forming a softened zone. This significantly reduces the wear resistance and impact resistance of the joint, and seriously affects the service life of the rail and driving safety.

[0003] Studies have shown that martensite inevitably appears in the flash welded joint of the rail, and there are many factors affecting the formation of martensite. The composition of the base material, local element segregation, welding process, post-weld heat treatment process, as well as ambient temperature and humidity will all affect the formation of martensite. In actual production, the composition, microstructure and properties of the rail base material that can leave the factory are all qualified, but local element segregation in the rail cannot be avoided. Without special intervention, martensite will inevitably appear in the joint welded from the qualified base material. At the same time, due to the temperature distribution characteristics of the welded joint, the softened zone also inevitably appears.

[0004] Chinese Patent CN202311365031.2 discloses a method for eliminating martensite microstructure in flash welded joints of hypereutectoid rails. This method is aimed at joints where martensite has already appeared, and is targeted at hypereutectoid rails. Medium-frequency induction heating is used for normalizing. During normalizing, the peak heating temperature of the joint is 20 to 30 °C higher than the normal heating temperature; and the joint is held at the peak heating temperature for 10 to 15 s, the top surface of the rail and both sides of the rail head are air-cooled, so that the air cooling is stopped after the top surface of the rail is cooled to a set temperature, and then the rail is air-cooled to room temperature, so as to eliminate the martensite microstructure in the hypereutectoid rail joint, and ensure that the hardness, grain size, strength and toughness of the joint meet the standard requirements.

[0005] Chinese patent CN202510949328 discloses a method for controlling the softening zone of bainitic rail welded joints. The method includes: welding bainitic rails using flash welding, wherein the flash welding includes a preheating stage, a continuous melting stage, and an accelerated melting stage. During the preheating stage, the thickness of the molten layer of the welded joint is controlled to be 1.5–2.5 mm; during the continuous melting stage, the temperature of the molten pool is controlled to be 1480–1530 °C; the welded high-temperature rail joint is then subjected to a three-stage gradient cooling process: first, cooling at a rate of 18–22 °C / s to near the bainitic transformation point; then, cooling at a rate of 0.08–0.12 °C / s for a predetermined time; and finally, cooling at a rate below 0.5 °C / s to room temperature. This achieves a softening zone width ≤0.5 mm, a reduction of 80% compared to the Bombardier BWR system. However, the above patent can only improve one defect in the martensite or softening zone of the joint, and cannot simultaneously eliminate martensite and significantly reduce the softening zone, thus exhibiting technical limitations.

[0006] Du Jikang et al. addressed the issue of martensite formation in the triangular region at the bottom of U75V rail flash welded joints after normalizing heat treatment. They effectively suppressed martensite formation by directly reducing the cooling rate of the triangular region by adjusting the air jetting method and decreasing the jetting pressure, or indirectly reducing the cooling rate by decreasing the jetting time. Cui Dongfang et al. conducted experimental research on the softening problem of moving flash welded joints through hardness testing, macroscopic morphology of the heat-affected zone, and microstructure observation. Their results showed that the softened zone of the welded joint corresponds to the normalized heat-affected zone, and the microstructure within the softened zone is granular pearlite. The softening of the joint is due to insufficient heating during normalizing heat treatment. However, the aforementioned literature also struggles to simultaneously suppress martensite formation and reduce the width of the softened zone.

[0007] Therefore, how to simultaneously eliminate abnormal martensitic structure in rail joints and reduce the width of the softened zone, thereby improving the microstructure of flash welded joints of heat-treated pearlitic rails, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the main objective of this invention is to provide a method for improving the microstructure of flash welded joints of heat-treated pearlitic rails. By optimizing the welding and post-weld normalizing processes and combining them with a segmented air-cooling process, abnormal martensitic microstructure of the rail joint can be eliminated simultaneously and the width of the softened zone can be reduced, thereby significantly improving the microstructure of flash welded joints of heat-treated pearlitic rails.

[0009] According to one aspect of the present invention, a method for improving the microstructure of a flash welded joint of a heat-treated pearlitic steel rail is provided, the method comprising the following steps: S1. Weld the heat-treated pearlitic steel rail and allow it to cool naturally. S2. The rail joint after natural cooling is subjected to induction normalizing heat treatment. The induction normalizing heat treatment includes a heating stage and a spray cooling stage. In the spray cooling stage, different areas divided by the reference fusion line are cooled in stages.

[0010] According to one embodiment of the present invention, in step S1, the mass fraction of each element in the heat-treated pearlitic steel rail is as follows: The mass fraction of element C is 0.50% to 0.77%; The mass fraction of silicon is 0.250% to 0.80%. The mass fraction of Mn is 0.60%–1.20%; The mass fraction of element V is ≤0.20%; The mass fraction of Cr element is ≤0.30%; The mass fraction of sulfur (S) is ≤0.008%; The mass fraction of oxygen is ≤0.004%; The balance is Fe and unavoidable impurities.

[0011] According to an embodiment of the present invention, in step S1, the welding is performed by moving flash welding, which includes four stages in sequence: flash leveling stage, preheating stage, burning stage and upsetting stage. The voltage of the flashover phase is 360–420V, and the time is 10–40s; The voltage during the preheating stage is 280V to 380V, and the time is 40 to 100 seconds. The voltage during the burning stage is 350V to 420V, and the time is 15 to 50 seconds. The upsetting amount during the upsetting stage is 9–18 mm.

[0012] According to one embodiment of the present invention, in step S2, the heating stage includes: using an induction coil to perform induction heating on the entire cross-section of the rail joint, wherein the shape of the induction coil is similar to the profile of the rail.

[0013] According to one embodiment of the present invention, the temperature of the induction heating is 900-1000°C, based on the temperature of the rail joint tread surface.

[0014] According to an embodiment of the present invention, in step S2, the area corresponding to the air jet cooling stage includes a first cooling area and a second cooling area. The first cooling area and the second cooling area are separated by a baffle. The first cooling area and the second cooling area start to cool continuously in stages simultaneously, and switch to the next stage after reaching the target temperature of each stage. The first cooling zone includes a region centered on the fusion line. The first cooling zone is continuously cooled in two stages. The first stage uses air jet cooling, and the second stage uses natural cooling. The first stage uses constant air pressure P1 to air jet cooling to temperature T1. The second cooling zone includes the areas on both sides of the first cooling zone. The second cooling zone is continuously cooled in three stages. The first and second stages both use air jet cooling, and the third stage uses natural cooling. In the first stage, air jet cooling is performed at a constant air pressure P2 to a temperature T2. In the second stage, air jet cooling is performed at a constant air pressure P3 to a temperature T3, where P2 > P3.

[0015] According to one embodiment of the present invention, in the first cooling zone, P1 is 0.1 to 0.45 MPa and T1 is 400 to 490 °C.

[0016] According to one embodiment of the present invention, in the second cooling zone, P2 is 0.4 to 0.6 MPa, and T2 is 590 to 610°C; The P3 is 0.1 to 0.4 MPa, and the T3 is 400 to 490 °C.

[0017] According to one embodiment of the present invention, for a rail with a specification of 60 kg / m, the width of the first cooling zone is 40 to 70 mm; The width of the second cooling zone is 60-100 mm.

[0018] According to one embodiment of the present invention, in the first cooling zone and the second cooling zone, the vertical distance between the air jet surface of the air jet box and the tread surface of the rail joint is 10-60 mm.

[0019] By adopting the above technical solution, the present invention has the following advantages compared with the prior art: This invention optimizes the welding and post-weld normalizing processes and combines them with a segmented air-cooling process to simultaneously eliminate abnormal martensitic structures in rail joints and reduce the width of the softened zone (the width of the softened zone is controlled within 10mm), thereby significantly improving the microstructure of flash welded joints of heat-treated pearlitic rails. Attached Figure Description

[0020] 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 drawings can be obtained from these drawings without creative effort.

[0021] Figure 1This is a flowchart illustrating a method for improving the microstructure of a flash welded joint of a heat-treated pearlitic steel rail according to an embodiment of the present invention; Figure 2 This is a front view of a rail joint during the heating stage according to an embodiment of the present invention; Figure 3 This is a left view of a rail joint during the heating stage according to an embodiment of the present invention; Figure 4 This is a front view of a rail joint in the air-cooling stage according to an embodiment of the present invention; Figure 5 This is a left view of a rail joint in the air-cooling stage according to an embodiment of the present invention.

[0022] In the diagram, 1-rail joint, 2-induction coil, 3-first cooling zone, 4-fusion line, 5-second cooling zone, 6-baffle, 7-air spray box. Detailed Implementation

[0023] 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.

[0024] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0025] like Figure 1 As shown, an embodiment of the present invention provides a method for improving the microstructure of a flash welded joint of a heat-treated pearlitic steel rail, comprising the following steps: S1, welding the heat-treated pearlitic steel rail and allowing it to cool naturally; S2, performing induction normalizing heat treatment on the naturally cooled rail joint 1, wherein the induction normalizing heat treatment includes a heating stage and a spray cooling stage, wherein, in the spray cooling stage, different regions divided by the reference fusion line 4 are cooled in stages.

[0026] In step S1, the mass fraction of each element in the heat-treated pearlitic steel rail is as follows: the mass fraction of C is 0.50% to 0.77%; the mass fraction of Si is 0.250% to 0.80%; the mass fraction of Mn is 0.60% to 1.20%; the mass fraction of V is ≤0.20%; the mass fraction of Cr is ≤0.30%; the mass fraction of S is ≤0.008%; the mass fraction of O is ≤0.004%; and the balance is Fe and unavoidable impurities. This composition ratio can balance the hardenability and plasticity of the steel rail to a certain extent, avoid excessive segregation of primary carbon and manganese in the base material, and reduce the risk of martensite precipitation after welding.

[0027] In step S1, the welding adopts moving flash welding, which includes four stages in sequence: flash leveling stage, preheating stage, sintering stage, and upsetting stage. The voltage of the flash leveling stage is 360-420V and the time is 10-40s; the voltage of the preheating stage is 280V-380V and the time is 40-100s; the voltage of the sintering stage is 350V-420V and the time is 15-50s; and the upsetting amount in the upsetting stage is 9-18mm.

[0028] like Figure 2 , Figure 3 As shown, in step S2, the heating stage includes: using an induction coil 2 to induction heat the entire cross section of the rail joint 1, and the shape of the induction coil 2 is similar to the profile of the rail.

[0029] Based on the temperature of the tread surface of rail joint 1, the induction heating temperature is 900-1000℃. This temperature can completely austenitize rail joint 1 while avoiding overheating that would cause grain coarsening. The induction heating temperature is selected from any value or any range between 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 970℃, 980℃, 985℃, and 1000℃.

[0030] like Figure 4 , Figure 5As shown, in step S2, the area corresponding to the air-jet cooling stage includes the first cooling area 3 and the second cooling area 5. The first cooling area 3 and the second cooling area 5 are separated by a baffle 6. The first cooling area 3 and the second cooling area 5 start continuous cooling in stages simultaneously, and switch to the next stage after reaching the target temperature of each stage. The first cooling area 3 includes the area centered on the fusion line 4. The first cooling area 3 is continuously cooled in two stages. The first stage uses air-jet cooling, and the second stage uses natural cooling. The first stage uses air-jet cooling with a constant air pressure P1 to cool to temperature T1. The second cooling area 5 includes the areas on both sides of the first cooling area 3. The second cooling area 5 is continuously cooled in three stages. The first and second stages both use air-jet cooling, and the third stage uses natural cooling. The first stage uses air-jet cooling with a constant air pressure P2 to cool to temperature T2, and the second stage uses air-jet cooling with a constant air pressure P3 to cool to temperature T3, where P2 > P3.

[0031] In the first cooling zone 3, P1 is 0.1 to 0.45 MPa. This constant air pressure ensures a stable cooling rate and effectively suppresses martensite formation. When T1 reaches 400 to 490°C, natural cooling is then employed.

[0032] In the second cooling zone 5, P2 is 0.4-0.6 MPa and T2 is 590-610℃; P3 is 0.1-0.4 MPa and T3 is 400-490℃. The high-pressure section is used first, followed by the low-pressure section, with obvious pressure switching. The differential cooling rate, which is fast at the beginning and slow at the end, is used to suppress pearlite spheroidization and narrow the softening zone. Finally, natural cooling is used.

[0033] For a 60kg / m rail, the first cooling zone 3 covers the fusion line 4 and the element segregation area nearby, with a width of 40-70mm; the second cooling zone 5 covers the spheroidization area on both sides, with a width of 60-100mm.

[0034] In the first cooling zone 3 and the second cooling zone 5, the cooling air is compressed air at room temperature. The vertical distance between the air jet surface of the air jet box 7 and the tread surface of the rail joint 1 is 10-60mm. This distance range can stably control the heat exchange intensity, taking into account both the cooling control accuracy and the equipment installation space.

[0035] Based on the formation principle of martensite and softened zone in rail joint 1, embodiments of the present invention provide a process that optimizes welding and post-weld normalizing processes and combines them with segmented air-jet cooling. First, a good joint is obtained through appropriate welding and post-weld heat treatment processes. Then, different cooling methods are used for different areas of the joint after normalizing heat treatment. On the one hand, a smaller cooling rate controls the abnormal martensitic structure of the joint near the fusion line 4; on the other hand, a larger cooling rate reduces the spheroidization rate of lamellar pearlite in the spheroidized zone while refining the lamellar pearlite structure in this region (pearlite spheroidization easily occurs in the spheroidized zones on both sides of the joint, forming a wide softened zone). Ultimately, this eliminates the abnormal martensitic structure of the joint while reducing the width of the softened zone, significantly improving the microstructure of the heat-treated pearlitic rail flash weld joint.

[0036] The following are specific embodiments and process parameters of the method for improving the microstructure of flash welded joints of heat-treated pearlitic steel rails according to the present invention. The microstructure and performance analysis of the welded joints obtained in each embodiment are based on standard TB / T1632.

[0037] Example 1 In this embodiment, the heat-treated pearlitic steel rail has a C element mass fraction of 0.75%, a Si element mass fraction of 0.54%, a Mn element mass fraction of 0.90%, a V element mass fraction of 0.002%, a Cr element mass fraction of 0.09%, a S element mass fraction of 0.005%, and an O element mass fraction of 0.001%, with the balance being Fe and unavoidable impurities. Welding is performed using a moving flash welding method, with a flash leveling voltage of 374V and a time of 32s, a preheating stage voltage of 330V and a total time of 85s, a burn-in stage voltage of 365V and a time of 20s, and an upsetting amount of 14mm. After welding, the rail is allowed to cool naturally to room temperature in air before induction normalizing heat treatment begins. The rail head tread temperature is used as the reference for temperature measurement; heating is immediately stopped once the temperature reaches 900℃, and segmented, staged air-blast cooling begins immediately. The first cooling zone 3 is 40mm wide, with a spray distance of 40mm and a spray pressure of 0.2MPa. Spraying in this zone is stopped after it cools to 450℃, allowing it to cool naturally. The second cooling zone 5 is an 80mm wide area outside the first cooling zone on both sides of the joint. It is first cooled to 600℃ with a spray distance of 40mm and a spray pressure of 0.6MPa, then cooled to 450℃ with a spray distance of 40mm and a spray pressure of 0.2MPa, and finally allowed to cool naturally at room temperature. Analysis of the microstructure of the welded joint obtained using the above method revealed no martensitic structure, and the softened zone was significantly reduced to 6mm, indicating a significant improvement in microstructure.

[0038] Example 2 In this embodiment, the heat-treated pearlitic steel rail has a C element mass fraction of 0.75%, a Si element mass fraction of 0.54%, a Mn element mass fraction of 0.90%, a V element mass fraction of 0.002%, a Cr element mass fraction of 0.09%, a S element mass fraction of 0.005%, and an O element mass fraction of 0.001%, with the balance being Fe and unavoidable impurities. Welding is performed using a moving flash welding method, with a flash leveling voltage of 374V and a time of 32s, a preheating stage voltage of 330V and a total time of 85s, and a burn-in stage voltage of 365V and a time of 20s, with an upsetting amount of 14mm. After welding, the rail is allowed to cool naturally to room temperature in air before induction normalizing heat treatment begins. The rail head tread temperature is used as the reference for temperature measurement; heating is immediately stopped once the temperature reaches 950℃, and segmented, staged air-blast cooling begins immediately. The first cooling zone 3 is 50 mm wide, with a spray distance of 40 mm and a spray pressure of 0.2 MPa. Spraying in this zone is stopped after it cools to 450°C, allowing it to cool naturally. The second cooling zone 5 is an 80 mm wide area outside the first cooling zone on both sides of the joint. It is first cooled to 600°C with a spray distance of 40 mm and a spray pressure of 0.6 MPa, then cooled to 450°C with a spray distance of 40 mm and a spray pressure of 0.2 MPa, and finally allowed to cool naturally at room temperature. Analysis of the microstructure of the welded joint obtained using the above method revealed no martensitic structure, and the softened zone was significantly reduced to 2 mm, indicating a significant improvement in microstructure.

[0039] Example 3 In this embodiment, the heat-treated pearlitic steel rail has a C element mass fraction of 0.75%, a Si element mass fraction of 0.54%, a Mn element mass fraction of 0.90%, a V element mass fraction of 0.002%, a Cr element mass fraction of 0.09%, a S element mass fraction of 0.005%, and an O element mass fraction of 0.001%, with the balance being Fe and unavoidable impurities. Welding is performed using a moving flash welding method, with a flash leveling voltage of 374V and a time of 32s, a preheating stage voltage of 330V and a total time of 85s, and a burn-in stage voltage of 365V and a time of 20s, with an upsetting amount of 14mm. After welding, the rail is allowed to cool naturally to room temperature in air before induction normalizing heat treatment begins. The rail head tread temperature is used as the reference for temperature measurement; heating is immediately stopped once the temperature reaches 950℃, and segmented, staged air-blast cooling begins immediately. The first cooling zone 3 is 50 mm wide, with a spray distance of 30 mm and a spray pressure of 0.18 MPa. Spraying in this zone is stopped after it cools to 450°C, allowing it to cool naturally. The second cooling zone 5 is an 80 mm wide area outside the first cooling zone on both sides of the joint. It is first cooled to 600°C with a spray distance of 30 mm and a spray pressure of 0.5 MPa, then cooled to 450°C with a spray distance of 30 mm and a spray pressure of 0.18 MPa, and finally allowed to cool naturally at room temperature. Analysis of the microstructure of the welded joint obtained using the above method revealed no martensitic structure, and the softened zone was significantly reduced to 3 mm, indicating a significant improvement in microstructure.

[0040] Comparative Example 1 In this comparative example, the heat-treated pearlitic steel rail used had a carbon content of 0.75%, a silicon content of 0.54%, a manganese content of 0.90%, a v content of 0.002%, a chromium content of 0.09%, a sulfur content of 0.005%, and an oxygen content of 0.001%, with the balance being fe and unavoidable impurities. Welding was performed using a moving flash welding method: flash leveling voltage 374V for 32 seconds, preheating voltage 330V for 85 seconds, and burn-in stage voltage 365V for 20 seconds, with an upsetting amount of 14 mm. After welding, the rail was allowed to cool naturally to room temperature before induction normalizing heat treatment. The rail head tread temperature was used as the reference for temperature measurement; heating was stopped immediately after reaching 900℃, and the rail was allowed to cool naturally at room temperature. Analysis of the microstructure of the welded joint obtained by the above method revealed no martensitic structure, but the softened zone was significantly increased, with a width of 35 mm, which does not meet the standard requirements.

[0041] Comparative Example 2 In this comparative example, the heat-treated pearlitic steel rail used had a carbon content of 0.75%, a silicon content of 0.54%, a manganese content of 0.90%, a v content of 0.002%, a chromium content of 0.09%, a sulfur content of 0.005%, and an oxygen content of 0.001%, with the balance being fe and unavoidable impurities. Welding was performed using a moving flash welding method: flash leveling voltage 374V for 32 seconds, preheating voltage 330V for 85 seconds, and burn-in stage voltage 365V for 20 seconds, with an upsetting amount of 14 mm. After welding, the rail was allowed to cool naturally to room temperature before induction normalizing heat treatment began. The rail head tread temperature was used as the reference for temperature measurement; heating was stopped immediately after reaching 900℃, and uniform air cooling was then applied to the entire joint. The cooling zone width was 200 mm, the air spray distance was 40 mm, and the air spray pressure was 0.5 MPa. Air spraying was stopped after cooling to 450 °C, and the joint was allowed to cool naturally. Analysis of the microstructure of the welded joint obtained using this method revealed martensitic structure, and the softened zone increased to 18 mm, which does not meet the standard requirements.

[0042] Comparative Example 3 In this comparative example, the heat-treated pearlitic steel rail used had a carbon content of 0.75%, a silicon content of 0.54%, a manganese content of 0.90%, a v content of 0.002%, a chromium content of 0.09%, a sulfur content of 0.005%, and an oxygen content of 0.001%, with the balance being fe and unavoidable impurities. Welding was performed using a moving flash welding method: flash leveling voltage 374V for 32 seconds, preheating voltage 330V for 85 seconds, and burn-in stage voltage 365V for 20 seconds, with an upsetting amount of 14 mm. After welding, the rail was allowed to cool naturally to room temperature before induction normalizing heat treatment began. The rail head tread temperature was used as the reference for temperature measurement; heating was immediately stopped once the temperature reached 900℃, and segmented, staged air-blast cooling was immediately initiated. The first cooling zone is 40mm wide, with a spray distance of 40mm and a spray pressure of 0.6MPa. Spraying is stopped after this zone cools to 450℃, allowing it to cool naturally. The second cooling zone is an 80mm wide area outside the first cooling zone on both sides of the joint. This area is cooled to 450℃ with a spray distance of 40mm and a spray pressure of 0.6MPa. Analysis of the microstructure of the welded joint obtained using this method shows a significant reduction in the width of the softened zone, but the joint exhibits a large amount of martensite, failing to meet the standard requirements.

[0043] As can be seen from Examples 1-3 and Comparative Examples 1-3, after optimizing the welding and post-weld normalizing process of 60kg / m heat-treated pearlitic steel rails according to the method of the present invention and combining it with regional and segmented air-blast cooling treatment, no abnormal martensite structure is generated inside the welded joint, and the width of the softened zone can be controlled within 10mm, with a minimum of only 2mm (Example 2). However, Comparative Examples 1-3, which do not use the method of the present invention, all have obvious defects: Comparative Example 1 has no air-blast cooling and only air cooling, and the softened zone of the joint is as wide as 35mm; Comparative Example 2 has uniform air-blast cooling throughout the entire area, which will generate martensite due to the excessively fast cooling rate of the fusion line, and the softened zone also reaches 18mm; Comparative Example 3 uses high-pressure air-blast in the first cooling area, and a large amount of martensite is precipitated by rapid cooling in the element segregation zone near the fusion line. This demonstrates that the present invention, through a combination of constant pressure slow cooling in the fusion line region and segmented gradient pressure cooling in the heat-affected zones on both sides, can simultaneously eliminate the martensitic structure of the joint and significantly reduce the width of the softened zone, thereby significantly optimizing the microstructure of the flash welded joint of the heat-treated pearlitic rail and improving the overall mechanical properties of the joint.

[0044] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications or equivalent substitutions made to the present invention without departing from the spirit and scope thereof should be covered within the protection scope of the claims of the present invention.

[0045] 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 disclosed embodiments of the present invention is limited to these examples; within the framework of the embodiments of the present invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of the present invention should be included within the protection scope of the embodiments of the present invention.

Claims

1. A method for improving the microstructure of flash welded joints of heat-treated pearlitic steel rails, characterized in that, The method includes the following steps: S1. Weld the heat-treated pearlitic steel rail and allow it to cool naturally. S2. The rail joint (1) after natural cooling is subjected to induction normalizing heat treatment. The induction normalizing heat treatment includes a heating stage and a spray cooling stage. In the spray cooling stage, different areas divided by the reference fusion line (4) are cooled in stages.

2. The method according to claim 1, characterized in that, In step S1, the mass fraction of each element in the heat-treated pearlitic steel rail is as follows: The mass fraction of element C is 0.50% to 0.77%; The mass fraction of silicon is 0.250% to 0.80%. The mass fraction of Mn is 0.60%–1.20%; The mass fraction of element V is ≤0.20%; The mass fraction of Cr element is ≤0.30%; The mass fraction of sulfur (S) is ≤0.008%; The mass fraction of oxygen is ≤0.004%; The balance is Fe and unavoidable impurities.

3. The method according to claim 1, characterized in that, In step S1, the welding is performed using moving flash welding, which includes four stages in sequence: flash leveling stage, preheating stage, sintering stage, and upsetting stage. The voltage of the flashover phase is 360–420V, and the time is 10–40s; The voltage during the preheating stage is 280V to 380V, and the time is 40 to 100 seconds. The voltage during the burning stage is 350V to 420V, and the time is 15 to 50 seconds. The upsetting amount during the upsetting stage is 9–18 mm.

4. The method according to claim 1, characterized in that, In step S2, the heating stage includes: using an induction coil (2) to perform induction heating on the entire cross section of the rail joint (1), wherein the shape of the induction coil (2) is similar to the profile of the rail.

5. The method according to claim 4, characterized in that, Based on the temperature of the tread surface of the rail joint (1), the temperature of the induction heating is 900-1000℃.

6. The method according to claim 1, characterized in that, In step S2, the area corresponding to the air-jet cooling stage includes a first cooling area (3) and a second cooling area (5). The first cooling area (3) and the second cooling area (5) are separated by a baffle (6). The first cooling area (3) and the second cooling area (5) start to cool continuously in stages simultaneously. After reaching the target temperature of each stage, they switch to the next stage. The first cooling zone (3) includes a region centered on the fusion line (4). The first cooling zone (3) is continuously cooled in two stages. The first stage uses air jet cooling, and the second stage uses natural cooling. The first stage uses constant air pressure P1 to air jet cooling to temperature T1. The second cooling zone (5) includes the areas on both sides of the first cooling zone (3). The second cooling zone (5) is continuously cooled in three stages. The first and second stages both use air spray cooling, and the third stage uses natural cooling. In the first stage, air spray cooling is carried out at a constant air pressure P2 to a temperature T2. In the second stage, air spray cooling is carried out at a constant air pressure P3 to a temperature T3. P2 > P3.

7. The method according to claim 6, characterized in that, In the first cooling zone (3). The P1 is 0.1 to 0.45 MPa, and the T1 is 400 to 490 °C.

8. The method according to claim 6, characterized in that, In the second cooling zone (5), P2 is 0.4 to 0.6 MPa, and T2 is 590 to 610 °C; The P3 is 0.1 to 0.4 MPa, and the T3 is 400 to 490 °C.

9. The method according to claim 6, characterized in that, For a rail with a specification of 60kg / m, the width of the first cooling zone (3) is 40-70mm; The width of the second cooling zone (5) is 60-100 mm.

10. The method according to claim 6, characterized in that, In the first cooling zone (3) and the second cooling zone (5), the vertical distance between the air jet surface of the air jet box (7) and the tread surface of the rail joint (1) is 10-60 mm.

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