A post-weld heat treatment device and method for dissimilar steel rail welded joints

Abstract

The present application relates to the technical field of railway rail welding, in particular to a post-weld heat treatment device and method for heterogeneous flash welding joints. The post-weld heat treatment method comprises: cooling the surface of the high-temperature heterogeneous rail welding joint to rapidly reduce the surface temperature of the heat-affected zone of the heterogeneous rail welding joint; immediately performing isothermal heat treatment on the cooled heterogeneous rail welding joint; stopping the isothermal heat treatment and naturally cooling the heterogeneous rail welding joint to ambient temperature. The present application implements post-weld rapid cooling on the rail joint with high residual temperature obtained by welding to reduce the phase transition temperature from austenite to bainite in the welding heat-affected zone, thereby improving the hardness of the austenite recrystallization zone. And through the isothermal process, fine bainite structure with the best performance is generated in the heat-affected zone on both sides of the rail joint weld, thereby improving the strength and hardness of the heat-affected zone.

Description

Technical Field

[0001] This invention relates to the field of railway rail welding technology, specifically to a post-weld heat treatment device and method for dissimilar flash welded joints. Background Technology

[0002] In recent decades, the rapid development of heavy-haul railways has placed higher demands on the strength, hardness, wear resistance, and fatigue resistance of rail base materials and welded rail joints. Due to their good strength and toughness and moderate overall performance, eutectoid pearlitic rails are commonly used in heavy-haul railways both domestically and internationally. These rails typically have a carbon content ranging from 0.72% to 0.82% by weight, and their microstructure is pearlitic. However, the mechanical and weldability of traditional eutectoid pearlitic rails have almost reached their limits. Under these circumstances, bainitic rails, which offer higher strength and good wear resistance and contact fatigue resistance, have emerged. These rails typically have a carbon content ranging from 0.20% to 0.40% by weight, and their microstructure is a multiphase structure composed of bainite, a small amount of martensite (or martensite-austenite islands), and retained austenite.

[0003] The application of rails in seamless railway tracks is inseparable from welding. Currently, mobile flash welding has become the mainstream online rail welding technology on railway construction sites both domestically and internationally. For two types of rails with different strength grades and materials, the differences between the base materials present welding challenges. Furthermore, for the welding of rails undergoing online heat treatment, the hardened layer originally belonging to the rail base material disappears due to the welding thermal cycle, forming a wider low-hardness zone on both sides of the weld heat-affected zone, resulting in a lower hardness in the heat-affected zone than the rail base material. During track service, this easily leads to "saddle-shaped" wear preferentially forming on the rail head tread area of ​​the welded joint. This not only increases wheel-rail impact but also seriously affects the rail's service life and even endangers traffic safety. Therefore, restoring the mechanical properties reduced by welding after the rail is completed becomes a prerequisite for the rail's continued use. Summary of the Invention

[0004] In view of this, the present invention provides a post-weld heat treatment method for dissimilar rail welded joints composed of a low-strength heat-treatable bainitic steel rail and a high-strength heat-treatable bainitic steel rail. This method at least solves the technical problems of high cost and low hardness performance of existing post-weld heat treatment methods.

[0005] The first aspect of this invention provides a post-weld heat treatment method for dissimilar rail welded joints, wherein the dissimilar rail welded joint is formed by flash welding of a low-strength heat-treatable bainitic rail and a high-strength heat-treatable bainitic rail, and the post-weld heat treatment method for the dissimilar rail welded joint includes the following steps:

[0006] S1, cooling the welded joints of dissimilar rails with a surface temperature of 910-990℃; cooling the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail to 380-430℃ at a cooling rate of 3.0-5.0℃ / s; cooling the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail to 330-360℃ at a cooling rate of 6.0-8.0℃ / s.

[0007] S2, immediately perform isothermal heat treatment on the welded joints of dissimilar rails after cooling treatment; keep the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail between 380~430℃, and keep the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail between 330~360℃, with a holding time of 0.8~1h.

[0008] S3, stop isothermal heat treatment, and allow the dissimilar rail weld joint to cool naturally to an ambient temperature of 20-30℃ at a cooling rate of 0.2-0.9℃ / s.

[0009] In some embodiments, the low-strength heat-treated bainitic rail base material, by mass percentage, comprises C: 0.20%~0.23%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; the low-strength heat-treated bainitic rail base material has a tensile strength of 1100~1180MPa and a hardness of 270~310HV at 20~30℃.

[0010] In some embodiments, the high-strength heat-treated bainitic rail base material, by mass percentage, comprises C: 0.26%~0.30%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; the high-strength heat-treated bainitic rail base material has a tensile strength of 1200~1280MPa and a hardness of 320~340HV at 20~30℃.

[0011] In some embodiments, the low-strength heat-treated bainitic rail and the high-strength heat-treated bainitic rail that form the dissimilar rail weld joint have the same rail type and a specification of 60~75kg / m.

[0012] A second aspect of the present invention provides a post-weld heat treatment apparatus for dissimilar rail welded joints, used to implement the above-mentioned post-weld heat treatment method, including a profile cooling device for cooling the dissimilar rail welded joints. The profile cooling device includes two cooling components that are isolated and connected, and the two cooling components are respectively arranged corresponding to a low-strength heat-treated bainitic rail and a high-strength heat-treated bainitic rail. The cooling components include a spray panel located above the rail head tread of the dissimilar rail welded joint, and spray pipes connected to the spray panel and facing the two sides of the rail head. The spray panel and the spray pipes are provided with a plurality of spray holes, and the spray holes are in fluid communication with the external cooling medium.

[0013] In some embodiments, the distance between the injection hole and the weld joint of the dissimilar rail is 15~25mm.

[0014] In some embodiments, the cooling medium of the profile cooling device is a mixture of compressed air and / or water mist, wherein the gas pressure of the compressed air and / or water mist mixture ejected by the cooling component corresponding to the low-strength heat-treated bainitic steel rail is 0.15~0.25MPa, and the gas pressure of the compressed air ejected by the cooling component corresponding to the high-strength heat-treated bainitic steel rail is 0.35~0.45MPa.

[0015] In some embodiments, the post-weld heat treatment apparatus for dissimilar rail welded joints further includes a profile heating device for isothermal heat treatment of dissimilar rail welded joints, the profile heating device comprising: an apparatus housing, two tracked ceramic heaters and an insulation layer.

[0016] The device housing includes a first housing and a second housing opposite to each other, with their upper parts pivotally connected by a rotating shaft. The lower parts of the first housing and the second housing are detachably and movably connected by a connector. When the first housing is connected to the lower part of the second housing, the outer contour of the device housing matches the cross-sectional shape of the dissimilar rail weld joint.

[0017] Two tracked ceramic heaters are respectively installed corresponding to low-strength heat-treated bainitic steel rails and high-strength heat-treated bainitic steel rails. The two tracked ceramic heaters are connected in parallel and include multiple ceramic units arranged in multiple rows on the inner surface of the device housing. When the lower part of the first housing is connected to the second housing, the distance between the tracked ceramic heater and the surface of the dissimilar steel rail weld joint is 2~7mm.

[0018] The insulation layer is placed between the outer shell of the device and the tracked ceramic heater.

[0019] In some embodiments, the number of ceramic units covering the rail head of the dissimilar rail welded head in the tracked ceramic heater is greater than the number of ceramic units covering the rail web and rail bottom of the dissimilar rail welded head.

[0020] In some embodiments, the number of ceramic units in the tracked ceramic heater corresponding to the low-strength heat-treated bainitic steel rail is greater than the number of ceramic units in the tracked ceramic heater corresponding to the high-strength heat-treated bainitic steel rail.

[0021] The beneficial effects of this invention are as follows: This invention implements rapid post-weld cooling for rail joints with high residual heat after welding, thereby reducing the phase transformation temperature of austenite to bainite within the weld heat-affected zone, and thus increasing the hardness of the austenite recrystallization zone. During the cooling stage, a relatively high cooling rate (higher than the critical cooling rate for martensitic transformation in rail steel) is used to allow the rail weld heat-affected zone to enter the bainitic transformation phase region. An isothermal process is then used to generate fine, optimally performing bainitic structures on both sides of the rail joint weld, thereby improving the strength and hardness of the heat-affected zone. The isothermal heat treatment process employs heat compensation to achieve isothermal heat treatment, obtaining fine lath bainitic structures with good strength and toughness, thus improving the strength, hardness, and toughness of the rail. Attached Figure Description

[0022] 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.

[0023] Figure 1 A flowchart illustrating a post-weld heat treatment method for dissimilar rail welded joints, provided as an embodiment of the present invention;

[0024] Figure 2 This is a structural cross-sectional view of the dissimilar rail welding joint provided by the present invention;

[0025] Figure 3 A schematic diagram showing the sampling location for metallographic specimens of the rail head tread surface of a welded rail joint.

[0026] Figure 4 This is a schematic diagram of the spray panel of a contour cooling device provided in one embodiment of the present invention;

[0027] Figure 5 This is a schematic diagram of the structure of a contour cooling device provided in one embodiment of the present invention;

[0028] Figure 6 This is a schematic diagram of the usage state of a contour cooling device provided in one embodiment of the present invention;

[0029] Figure 7 This is a schematic diagram showing the distribution of electric heaters in the rail head area of ​​a contour heating device provided in one embodiment of the present invention.

[0030] Figure 8 This is a schematic diagram of the structure of a contour heating device provided in one embodiment of the present invention;

[0031] Explanation of reference numerals in the attached figures:

[0032] 1. Spray panel; 2. Spray hole; 3. First channel; 4. Second channel; 5. Spray pipe; 6. First housing; 7. Second housing; 8. Rotating shaft; 9. Connecting ring; 10. Hook; 11. Insulation layer; 12. Partition; 13. Tracked ceramic heater; 14. Ceramic unit. Detailed Implementation

[0033] 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.

[0034] 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.

[0035] Domestic and international rail welding standards, such as the Chinese railway rail welding standard TB / T1632-2014, stipulate that the microstructure of bainitic rail welded joints should be predominantly bainite, with a small amount of proeutectoid ferrite allowed, but martensite must not be present. In contrast, the Australian railway rail welding standard AS1085.20-2012 stipulates that for certain high-strength, high-carbon, and high-alloy rails, under a 100x metallographic microscope, the percentage of martensite in the most severely martensitic area of ​​the rail welded joint must not exceed 5%. Otherwise, the joint will have a high probability of premature fatigue fracture during railway service due to a large amount of brittle and hard martensite, affecting railway operation safety. Therefore, strictly controlling the martensite content in rail weld microstructure is crucial for railway operation safety.

[0036] Compared to traditional hot-rolled high-carbon pearlitic rails (carbon content greater than 0.6 wt%) and traditional hot-rolled medium-low carbon bainitic rails (carbon content 0.2 wt%~0.4 wt%), online heat treatment technology utilizes the residual heat from rail rolling to accelerate the phase transformation rate of supercooled austenite to pearlite or bainite, significantly refining the pearlite lamellar spacing or bainite lath width, achieving a fine-grain strengthening effect, thereby obtaining excellent strength and toughness. Therefore, rails produced using online heat treatment technology will gradually become the future development trend for high-strength, high-wear-resistant rail production.

[0037] In this invention, the dissimilar rail welding joint is as follows: Figure 2 and Figure 3 As shown, a low-strength heat-treated bainitic steel rail a and a high-strength heat-treated bainitic steel rail b are formed by flash welding. c is the weld center and d is the tread sampling position.

[0038] In this invention, low-strength heat-treatable bainitic rails achieve optimal fine bainitic microstructure when subjected to isothermal heat treatment at 380~430℃, corresponding to a critical cooling rate of 1.6~2.0℃ / s for martensitic transformation and a martensitic transformation onset temperature of 330~375℃. High-strength heat-treatable bainitic rails achieve optimal fine bainitic microstructure when subjected to isothermal heat treatment at 330~360℃, corresponding to a critical cooling rate of 1.1~1.5℃ / s for martensitic transformation and a martensitic transformation onset temperature of 300~325℃.

[0039] Therefore, for the heat treatment cooling process of rail welded joints formed by welding a low-strength heat-treated bainitic rail and a high-strength heat-treated bainitic rail, the final cooling temperature is controlled within the bainitic transformation temperature range of the two rail steels during the cooling process to provide conditions for subsequent isothermal heat treatment. After the isothermal heat treatment process, the rail joint is allowed to cool naturally to a temperature of 20-30°C. Because the cooling rate of natural cooling is relatively low, below the critical cooling rate of martensitic transformation of the two rail steels, and most of the supercooled austenite has already completed its transformation, the subsequent rail welded joint will not form a brittle and hard martensite structure under natural cooling conditions.

[0040] Based on the above findings, the first aspect of the present invention provides a post-weld heat treatment method for dissimilar rail welded joints, such as... Figure 1 As shown, it includes the following steps:

[0041] S1 involves cooling the welded joints of dissimilar rails with surface temperatures ranging from 910 to 990°C; reducing the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail to 380–430°C at a cooling rate of 3.0–5.0°C / s; and reducing the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail to 330–360°C at a cooling rate of 6.0–8.0°C / s. This provides conditions for subsequent isothermal heat treatment.

[0042] S2, immediately perform isothermal heat treatment on the welded joints of dissimilar rails after cooling treatment; keep the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail between 380~430℃, and keep the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail between 330~360℃, with a holding time of 0.8~1h.

[0043] S3, stop the isothermal heat treatment and allow the dissimilar rail weld joint to cool naturally to the ambient temperature at a cooling rate of 0.2 to 0.9℃ / s. In this invention, the ambient temperature is 20 to 30℃.

[0044] According to the principles of metallurgy, while keeping the content of alloys other than carbon in steel essentially constant, as the carbon content increases, the bainite transformation C-curve (including the bainite transformation start and end curves) shifts to the right, corresponding to a decrease in the bainite transformation start and end temperatures. Simultaneously, the martensitic transformation start temperature in the steel also decreases with increasing carbon content. Therefore, for bainitic rails with low carbon content (i.e., the low-strength heat-treated bainitic rail of this invention), a higher isothermal temperature range is required to generate fine bainite structure during the cooling process after welding to improve the mechanical properties of the weld heat-affected zone. Conversely, for bainitic rails with high carbon content (i.e., the high-strength heat-treated bainitic rail of this invention), a relatively lower isothermal temperature range is required to generate fine bainite structure during the cooling process after welding to improve the mechanical properties of the weld heat-affected zone.

[0045] Based on the above principles, in this invention, the cooling stage employs a relatively high cooling rate (higher than the critical cooling rate for martensitic transformation of the rail steel) to allow the heat-affected zone of the rail weld to enter the bainitic transformation phase region. An isothermal process is then used to generate fine, optimally performing bainitic microstructure in the heat-affected zones on both sides of the rail joint weld, thereby improving the strength and hardness of the heat-affected zone. The isothermal heat treatment process utilizes heat compensation to achieve the desired effect. In this invention, the surface temperature of the heat-affected zone of low-strength heat-treated bainitic rails is maintained between 380 and 430°C, while the surface temperature of the heat-affected zone of high-strength heat-treated bainitic rails is maintained between 330 and 360°C. The aim is to obtain a fine lath bainitic microstructure with good strength and toughness. For the heat-affected zones on both sides of the rail weld joint, the differences in the composition of the rail base material result in differences in the required isothermal temperature range for obtaining a fine bainitic microstructure through the isothermal process.

[0046] This invention addresses the issue of rapid post-weld cooling in rail joints with high residual heat after welding. This reduces the phase transformation temperature of the austenite-to-bainite transformation within the weld heat-affected zone, thereby increasing the hardness of the austenite recrystallization zone. Based on metallurgical principles, rail joints exhibit a certain degree of dynamic undercooling under high-temperature rapid cooling conditions after welding. This causes the phase transformation temperature of the austenite-to-bainite transformation in a non-equilibrium state to shift downwards, and the phase transformation temperature gradually decreases with increasing undercooling.

[0047] In this invention, the cooling rates during the cooling process are 3.0–5.0 °C / s and 6.0–8.0 °C / s, respectively, which are limited by the cooling capacity of the cooling device. During the cooling process, when combined with low-strength heat-treated bainitic steel rails (… Figure 2 The cooling device corresponding to the left half of the middle part and the low-strength heat-treated bainitic steel rail ( Figure 2 When the maximum cooling rates of the cooling devices corresponding to the right half of the diagram are lower than 3.0℃ / s and 6.0℃ / s respectively, the cooling effect is poor and the cooling time is significantly prolonged, thus affecting the overall production efficiency of rail joint heat treatment. For isothermal heat treatment after cooling, when the holding time of the isothermal process exceeds 1 hour, the strength and hardness of the heat-affected zone of the joint remain essentially unchanged, and the longer holding time will reduce the production efficiency of rail joint heat treatment. Therefore, this invention controls the holding time of isothermal heat treatment to 0.8~1 hour, and to ensure sufficient martensite transformation, this invention controls the holding time of isothermal heat treatment to 1 hour.

[0048] Furthermore, it should be noted that both the low-strength heat-treated bainitic rail and the high-strength heat-treated bainitic rail described in this invention are heat-treated rails produced using online heat treatment technology. Compared to traditional hot-rolled bainitic rails, the strong grain refinement effect introduced by online heat treatment technology can further improve the strength, hardness, and toughness of the rail based on the mechanical properties of traditional hot-rolled rails.

[0049] In some embodiments, the chemical composition of the rail steel that yields the microstructure and mechanical properties must meet the following conditions: by mass percentage, the low-strength heat-treated bainitic rail base material comprises C: 0.20%~0.23%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; in order to control the tensile and impact properties of the heat-treated bainitic rail base material, the low-strength heat-treated bainitic rail base material has a tensile strength of 1100~1180MPa and a hardness of 270~310HV at 20~30℃.

[0050] In some embodiments, the chemical composition of the rail steel that yields the microstructure and mechanical properties must meet the following conditions: by mass percentage, the high-strength heat-treated bainitic rail base material comprises C: 0.26%~0.30%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; in order to control the tensile and impact properties of the heat-treated bainitic rail base material, the high-strength heat-treated bainitic rail base material has a tensile strength of 1200~1280MPa and a hardness of 320~340HV at 20~30℃.

[0051] In some embodiments, the low-strength heat-treated bainitic rail and the high-strength heat-treated bainitic rail forming the dissimilar rail weld joint have the same rail type and a specification of 60~75 kg / m. Specifically, the weld joint includes a region of 60~80 mm in length, including the weld and / or heat-affected zone, with the weld at the center of this region.

[0052] A second aspect of the present invention provides a post-weld heat treatment apparatus for welded joints of dissimilar rails, used to implement the above-mentioned post-weld heat treatment method, such as... Figures 4 to 6 As shown, a profile cooling device is included for cooling dissimilar rail weld joints. The profile cooling device comprises two isolated cooling components, such as... Figure 4 and Figure 5 As shown, the two cooling components are divided by the middle partition 12. The cooling component on the left is set to correspond to the low-strength heat-treated bainitic steel rail, and the cooling component on the right is set to correspond to the high-strength heat-treated bainitic steel rail. Each cooling component includes a spray panel 1 located above the rail head tread of the dissimilar rail welding joint, and a spray pipe 5 connected to the spray panel 1 and facing the two sides of the rail head. The spray panel 1 and the spray pipe 5 are provided with multiple spray holes 2. The spray holes 2 are in fluid communication with the external cooling medium. Specifically, the spray holes 2 on the left cooling component are in communication with the external cooling medium through the first channel 3, and the spray holes 2 on the right cooling component are in communication with the external cooling medium through the second channel 4.

[0053] In this invention, when the profile cooling device is used to cool the rail weld joint, the rail weld joint is located inside the profile cooling device (e.g., Figure 6 As shown, the actual cooling rate can be adjusted by controlling the pressure of the cooling medium flowing into the molding device, thereby achieving controlled cooling of the left and right halves of the rail welded joint with different cooling rates / intensities.

[0054] In some embodiments, the distance between the spray hole 2 and the weld joint of dissimilar rails is 15~25mm. Preferably, in order to ensure that the longitudinal hardness of the heat-affected zone of the rail welding within ±20mm from the weld center reaches the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, the rail profile cooling device is 20mm away from the surface of the weld joint.

[0055] In some embodiments, the cooling medium of the profile cooling device is a mixture of compressed air and / or water mist. The gas pressure of the compressed air and / or water mist mixture sprayed by the cooling component corresponding to low-strength heat-treated bainitic steel rails is 0.15~0.25 MPa, and the gas pressure of the compressed air sprayed by the cooling component corresponding to high-strength heat-treated bainitic steel rails is 0.35~0.45 MPa. Rapid cooling by spraying compressed air can effectively eliminate abnormal microstructures such as martensite that may occur due to excessively rapid local cooling rates or micro-regional component segregation, thus improving the microstructure of the welded joint. Specifically, the gas pressure sprayed by the cooling component can be adjusted by the number and diameter of the spray holes 2 to meet the cooling effect for dissimilar steel rail welded joints. Figure 6 As shown, the rail profile cooling device only cools the rail head tread and side of the rail welded joint. The orifice size of its air jet channel can be designed and manufactured according to actual needs to achieve cooling intensities. The compressed air flowing through the first channel 3 and the second channel 4 has the same gas pressure, which can be monitored by a pressure gauge and adjusted as needed. The orifice diameter of the jet hole 2 of the cooling component on the left is larger than that of the jet hole 2 of the cooling device on the right, ensuring that the gas pressure ejected from the cooling component on the left is lower than that ejected from the cooling component on the right.

[0056] In some embodiments, the post-weld heat treatment apparatus for dissimilar rail welded joints further includes a profile heating device for isothermal heat treatment of the dissimilar rail welded joints, such as... Figure 7 and Figure 8 As shown, the contour heating device includes:

[0057] The device housing includes a first housing 6 and a second housing 7 that are opposite each other. The first housing 6 and the second housing 7 are welded from thin metal plates. The upper parts of the first housing 6 and the second housing 7 are pivotally connected by a rotating shaft 8. The lower parts of the first housing 6 and the second housing 7 are detachably connected by a connector. For example, a connecting ring 9 and a hook 10 are respectively provided at the lower end of the first housing 6 and the second housing 7. When the hook 10 is hooked in the connecting ring 9, the lower part of the first housing 6 and the second housing 7 are connected. The outer contour of the device housing matches the cross-sectional shape of the dissimilar rail weld joint so as to completely enclose the rail weld joint.

[0058] Two tracked ceramic heaters 13 are respectively installed corresponding to low-strength heat-treated bainitic steel rails and high-strength heat-treated bainitic steel rails. The two tracked ceramic heaters 13 are connected in parallel and include multiple ceramic units 14 arranged in parallel rows on the inner surface of the device housing, used to provide a heat source for isothermal heat treatment. When the lower part of the first housing 6 is connected to the second housing 7, the distance between the tracked ceramic heaters 13 and the welded joint surface of the dissimilar steel rails is 2~7mm, preferably 5mm, which can achieve good heat conduction.

[0059] The insulation layer 11 is disposed between the outer shell of the device and the tracked ceramic heater 13. Specifically, the insulation layer 11 is an asbestos insulation layer 11.

[0060] like Figure 7 and Figure 8 As shown, the contour heating device is a split-type structure that can rotate around the rotation axis 8. Among them, Figure 7 In the diagram, A represents the rail head tread heating area; B represents the rail head side heating area; C represents the rail head lower jaw heating area; D is part of a tracked ceramic heater; and E is the boundary line between two tracked ceramic heaters 13. A1, B1, A2, and B2 are terminals; terminals A1 and B1 form a circuit to heat the right half of the rail welded joint, while terminals A2 and B2 form a circuit to heat the left half of the rail welded joint. Multiple rows of parallel ceramic units 14 are connected in parallel to collectively provide heat to the welded joint.

[0061] The contour heating device of this invention has advantages such as compactness, flexibility, and low cost, facilitating on-site construction. It can be powered by a diesel generator or a 220V power supply, with a rated power of 10kW. This contour heating device uses a commercial LCD tracked ceramic heater 13 as the heat source, such as a ceramic plate with dimensions of 10mm (length) × 10mm (width) × 6mm (thickness). Equipped with insulation material and a steel structure shell, it is made into a ring-shaped, split heater, easy to assemble and disassemble, suitable for full-section heating of rail welded joints. The actual dimensions of the contour heating device and the specifications and distribution of the tracked ceramic heater 13 can be adjusted according to the actual dimensions of the rail profile. It should be noted that this contour heating device can achieve isothermal (constant temperature) heat treatment of rail welded joints through program settings and continuous heat compensation during power-on. In the design process of the contour heating device, two tracked heaters are fixed inside the outer shell of the device, which has a similar profile to a rail. The two tracked ceramic heaters 13 are covered with the surface of the rail and fully attached to the surface of the rail to achieve good heat conduction during the heating process. Based on this contour heating device, the isothermal heat treatment process of the rail welded joint can be realized.

[0062] During the experiment, a temperature controller can be used to control the heating temperature. The operating temperature range of this contour heating device is 200~600℃, and it can rotate a maximum of 180° around the rotating axis.

[0063] In this invention, a split-type contour heating device is used to perform isothermal heat treatments at temperatures of 380~430℃ and 330~360℃ on low-strength heat-treated bainitic steel rails (left half) and high-strength heat-treated bainitic steel rails (right half) at welded joints, respectively, with holding times of 0.8~1h. This promotes bainitic transformation, stabilizes residual austenite in the bainitic steel, and improves the strength, hardness, and toughness of the heat-affected zone of the heat-treated bainitic steel rail. The distance between the tracked ceramic heater 13 and the surface of the dissimilar rail weld joint is preferably set to 5mm to achieve good heat conduction. Under the continuous heating action of the tracked ceramic heater 13, the isothermal heat treatment process can be achieved. After the isothermal process is completed, the contour heating device is removed. The welded joint is then placed in an air environment for natural cooling, reducing the surface temperature of the welded joint to an ambient temperature of 20~30℃. It should be noted that, due to the rapid cooling process that had already been carried out, the surface temperature of the rail joint was low. Therefore, when the rail welded joint was allowed to cool naturally in the air after the isothermal process was completed, martensitic structure would not form.

[0064] In some embodiments, the number of ceramic units 14 covering the rail head of the dissimilar rail weld joint in the tracked ceramic heater 13 is greater than the number of ceramic units 14 covering the rail web and rail base of the dissimilar rail weld joint. Because the rail head is thicker and heat transfer is slower, the number of ceramic heaters covering the rail head is greater than the number of ceramic heaters in the rail web and rail base areas to ensure that the entire cross-section of the rail is fully heated.

[0065] In some embodiments, the number of ceramic units 14 in the tracked ceramic heater 13 corresponding to low-strength heat-treated bainitic rails is greater than the number of ceramic units 14 in the tracked ceramic heater 13 corresponding to high-strength heat-treated bainitic rails. This optimizes heating efficiency and temperature distribution to meet the heat treatment requirements of different types of rails. For low-strength heat-treated bainitic rails, the heat treatment process requires more uniform temperature and more precise control to achieve optimal strength-toughness matching and processing performance. A relatively larger number of smaller ceramic units 14 can provide a more uniform heat distribution, ensuring uniform temperature of the rail during heat treatment and avoiding local overheating or failure to reach the expected temperature. For low-strength heat-treated bainitic rails, due to the relatively low carbon content in the steel, the bainite transformation C-curve (including the bainite transformation start and end curves) shifts to the left, corresponding to an increase in the bainite transformation start and end temperatures. Therefore, a relatively high isothermal temperature is required during the isothermal heat treatment of the heat-affected zone on one side of the low-strength heat-treated bainitic rail weld joint. Similarly, the isothermal heat treatment process for the heat-affected zone on one side of the high-strength heat-treated bainitic steel rail weld joint requires a relatively low isothermal temperature. Therefore, the number of ceramic units 14 in the track-type ceramic heater 13 corresponding to the high-strength heat-treated bainitic steel rail is less than the number of ceramic units 14 in the track-type ceramic heater 13 corresponding to the low-strength heat-treated bainitic steel rail.

[0066] Regarding this invention, it should be added that heat treatment technology itself is a process of controlling various factors during heating and cooling. The steps in heat treatment technology are interconnected and influence each other. This application may inevitably have overlapping and intersecting process parameters with other patent documents, but the applicable objects and heat treatment equipment differ between patents, therefore, data cannot be simply applied or compared. The chemical composition and heat treatment processes of steel rails developed in various countries inevitably overlap. Influenced by factors such as smelting capacity, heat treatment equipment, and operator skill levels, the applicable objects of each invention patent differ (including the mechanical properties and temperature distribution of steel rails), and the cooling devices and implementation processes used are also different, resulting in fundamental differences that prevent simple application of these processes.

[0067] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the embodiments described herein.

[0068] Example 1

[0069] The room temperature (20-30℃) tensile properties and hardness of a low-strength heat-treatable bainitic rail base material were controlled, wherein the tensile strength of the rail base material was 1100 MPa and the hardness was 270 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.20% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile properties and hardness of a high-strength heat-treatable bainitic rail base material were also controlled, wherein the tensile strength of the rail base material was 1200 MPa and the hardness was 320 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.26% C, 1.20% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities.

[0070] After upsetting and slugging during the moving flash welding process, the welded joint of a 60kg / m rail undergoes heat treatment. First, the rail joint, with an residual temperature of 910℃, is cooled using a profile cooling device. This device simultaneously cools the left half (corresponding to the heat-affected zone on the side of the low-strength heat-treated bainitic rail) and the right half (corresponding to the heat-affected zone on the side of the high-strength heat-treated bainitic rail), with the weld center as the dividing line. The cooling rate for the left half is 3.0℃ / s, and for the right half, it is 6.0℃ / s. This reduces the surface temperatures of the left and right halves of the rail joint to 380℃ and 330℃, respectively.

[0071] After completing the cooling steps described above, the rail welded joint is immediately subjected to isothermal heat treatment. During this stage, the surface temperatures of the left and right halves of the rail welded joint are 380°C and 330°C, respectively. This stage involves simultaneously performing isothermal heat treatment on the left and right halves of the rail welded joint using a profile heating device, with a holding time of 1 hour. Then, the isothermal heat treatment of the welded joint is stopped, and the profile heating device is removed. The rail welded joint is then allowed to cool naturally in air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the post-weld heat-treated rail welded joint of the present invention.

[0072] During the cooling process, the contour cooling device uses compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The contour cooling device is 20mm away from the rail head tread. The compressed air pressure ejected by the cooling component on the left is 0.15MPa, and the compressed air pressure ejected by the cooling component on the right is 0.35MPa. An infrared thermometer is used to monitor the temperature of the rail head tread. During isothermal heat treatment, the tracked ceramic heater in the contour heating device is 5mm away from the surface of the rail weld joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0073] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0074] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 90% and 85% of the average hardness of the base materials for the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail, respectively. Furthermore, the longitudinal hardness difference within this 20mm radius is controlled within 30 HV. Metallographic microscopy reveals that the HAZ microstructure on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side is predominantly bainite, with no obvious large-sized blocky martensite. This process contributes to ensuring railway operational safety.

[0075] Example 2

[0076] The room temperature (20-30℃) tensile properties and hardness of a low-strength heat-treatable bainitic rail base material were controlled, wherein the tensile strength of the rail base material was 1180 MPa and the hardness was 310 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.23% C, 1.60% Si, 1.80% Mn, 1.10% Cr, 0.40% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile properties and hardness of a high-strength heat-treatable bainitic rail base material were also controlled, wherein the tensile strength of the rail base material was 1280 MPa and the hardness was 340 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.30% C, 1.60% Si, 1.80% Mn, 1.10% Cr, 0.40% Mo, with the balance being Fe and unavoidable impurities.

[0077] After upsetting and slugging during the moving flash welding process, the welded joint of a 75kg / m rail undergoes heat treatment. First, the rail joint, with an residual temperature of 990℃, is cooled using a profile cooling device. This device simultaneously cools the left half (corresponding to the heat-affected zone on the side of the low-strength heat-treated bainitic rail) and the right half (corresponding to the heat-affected zone on the side of the high-strength heat-treated bainitic rail), with the weld center as the dividing line. The cooling rate for the left half is 5.0℃ / s, and for the right half, it is 8.0℃ / s. This reduces the surface temperatures of the left and right halves of the rail joint to 430℃ and 360℃, respectively.

[0078] After completing the cooling steps described above, the rail welded joint is immediately subjected to isothermal heat treatment. During this stage, the surface temperatures of the left and right halves of the rail welded joint are 430°C and 360°C, respectively. This stage involves simultaneously performing isothermal heat treatment on the left and right halves of the rail welded joint using a profile heating device, with a holding time of 1 hour. Then, the isothermal heat treatment of the welded joint is stopped, and the profile heating device is removed. The rail welded joint is then allowed to cool naturally in air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the post-weld heat-treated rail welded joint of the present invention.

[0079] During the cooling process, the contour cooling device uses compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The contour cooling device is 20mm away from the rail head tread. The compressed air pressure ejected by the cooling component on the left is 0.25MPa, and the compressed air pressure ejected by the cooling component on the right is 0.45MPa. An infrared thermometer is used to monitor the temperature of the rail head tread. During isothermal heat treatment, the tracked ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0080] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0081] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 93% and 89% of the average hardness of the base materials for the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail, respectively. Furthermore, the longitudinal hardness difference within this 20mm radius is controlled within 15 HV. Metallographic microscopy reveals that the HAZ microstructure on both the low-strength heat-treated bainitic rail and high-strength heat-treated rail sides is predominantly bainitic, with no obvious large-sized blocky martensite. This process contributes to ensuring railway operational safety.

[0082] Example 3

[0083] The room temperature (20-30℃) tensile properties and hardness of a low-strength heat-treatable bainitic rail base material were controlled, wherein the tensile strength of the rail base material was 1140 MPa and the hardness was 290 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.21% C, 1.4% Si, 1.6% Mn, 0.90% Cr, 0.30% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile properties and hardness of a high-strength heat-treatable bainitic rail base material were also controlled, wherein the tensile strength of the rail base material was 1240 MPa and the hardness was 330 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.28% C, 1.4% Si, 1.6% Mn, 0.90% Cr, 0.30% Mo, with the balance being Fe and unavoidable impurities.

[0084] After upsetting and slugging during the moving flash welding process, the welded joint of a 68kg / m rail undergoes heat treatment. First, the rail joint, with an residual temperature of 950℃, is cooled using a profile cooling device. This device simultaneously cools the left half (corresponding to the heat-affected zone on the low-strength heat-treated bainitic rail side) and the right half (corresponding to the heat-affected zone on the high-strength heat-treated bainitic rail side), with the weld center as the dividing line. The cooling rate for the left half is 4.0℃ / s, and for the right half, it is 7.0℃ / s. This reduces the surface temperatures of the left and right halves of the rail joint to 405℃ and 345℃, respectively.

[0085] After completing the cooling steps described above, the rail welded joint is immediately subjected to isothermal heat treatment. During this stage, the surface temperatures of the left and right halves of the rail welded joint are 405°C and 345°C, respectively. This stage involves simultaneously performing isothermal heat treatment on the left and right halves of the rail welded joint using a profile heating device, with a holding time of 1 hour. Then, the isothermal heat treatment of the welded joint is stopped, and the profile heating device is removed. The rail welded joint is then allowed to cool naturally in air at a cooling rate of 0.5°C / s to an ambient temperature of 20°C, thereby obtaining the post-weld heat-treated rail welded joint of the present invention.

[0086] During the cooling process, the contour cooling device uses compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The contour cooling device is 20mm away from the rail head tread. The compressed air pressure ejected by the cooling component on the left is 0.20MPa, and the compressed air pressure ejected by the cooling component on the right is 0.40MPa. An infrared thermometer is used to monitor the temperature of the rail head tread. During isothermal heat treatment, the tracked ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0087] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0088] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 91% and 87% of the average hardness of the base materials for the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail, respectively. Furthermore, the longitudinal hardness difference within this 20mm radius is controlled within 24 HV. Metallographic microscopy reveals that the HAZ microstructure on both the low-strength heat-treated bainitic rail and high-strength heat-treated rail sides is predominantly bainitic, with no obvious large-sized blocky martensite. This process contributes to ensuring railway operational safety.

[0089] Example 4

[0090] The room temperature (20-30℃) tensile properties and hardness of a low-strength heat-treatable bainitic rail base material were controlled, wherein the tensile strength of the rail base material was 1100 MPa and the hardness was 270 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.20% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile properties and hardness of a high-strength heat-treatable bainitic rail base material were also controlled, wherein the tensile strength of the rail base material was 1240 MPa and the hardness was 330 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.28% C, 1.4% Si, 1.6% Mn, 0.90% Cr, 0.30% Mo, with the balance being Fe and unavoidable impurities.

[0091] After upsetting and slugging during the moving flash welding process, the welded joint of a 68kg / m rail undergoes heat treatment. First, the rail joint, with an residual temperature of 950℃, is cooled using a profile cooling device. This device simultaneously cools the left half (corresponding to the heat-affected zone on the low-strength heat-treated bainitic rail side) and the right half (corresponding to the heat-affected zone on the high-strength heat-treated bainitic rail side), with the weld center as the dividing line. The cooling rate for the left half is 4.0℃ / s, and for the right half, it is 7.0℃ / s. This reduces the surface temperatures of the left and right halves of the rail joint to 405℃ and 345℃, respectively.

[0092] After completing the cooling steps described above, the rail welded joint is immediately subjected to isothermal heat treatment. During this stage, the surface temperatures of the left and right halves of the rail welded joint are 405°C and 345°C, respectively. This stage involves simultaneously performing isothermal heat treatment on the left and right halves of the rail welded joint using a profile heating device, with a holding time of 1 hour. Then, the isothermal heat treatment of the welded joint is stopped, and the profile heating device is removed. The rail welded joint is then allowed to cool naturally in air at a cooling rate of 0.5°C / s to an ambient temperature of 20°C, thereby obtaining the post-weld heat-treated rail welded joint of the present invention.

[0093] During the cooling process, the contour cooling device uses compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The contour cooling device is 20mm away from the rail head tread. The compressed air pressure ejected by the cooling component on the left is 0.20MPa, and the compressed air pressure ejected by the cooling component on the right is 0.40MPa. An infrared thermometer is used to monitor the temperature of the rail head tread. During isothermal heat treatment, the tracked ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0094] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0095] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 92% and 87% of the average hardness of the base materials for the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail, respectively. Furthermore, the longitudinal hardness difference within this 20mm radius is controlled within 39 HV. Metallographic microscopy reveals that the HAZ microstructure on both the low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail sides is predominantly bainite, with no obvious large-sized blocky martensite. This process contributes to ensuring railway operational safety.

[0096] Comparative Example 1

[0097] The process conditions involved in the rail base material, rail welding, and cooling process in this comparative example are largely the same as those in Example 1. The difference is that after the rail welded joint has cooled to the surface temperatures of the left and right halves of the rail welded joint at 380°C and 330°C respectively, this comparative example does not perform isothermal heat treatment on the rail welded joint. Instead, the joint is allowed to cool naturally in the air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0098] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0099] Under a metallographic microscope, the welded rail joints obtained in this comparative example showed that the microstructure of the weld heat-affected zone (HAZ) on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side was predominantly bainitic, with no obvious large-sized blocky martensite. However, hardness testing revealed that the longitudinal hardness of the weld HAZ within ±20mm of the weld center reached 89% and 84% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively. The longitudinal hardness of the rail joint within ±20mm of the weld center was controlled below 29 HV.

[0100] For the rail welded joint that did not undergo isothermal heat treatment, the longitudinal hardness of the heat-affected zone (HAZ) within ±20mm of the weld center is not within the range of 90%–93% and 85%–89% of the average hardness of the base material for low-strength heat-treated bainitic rails and high-strength heat-treated bainitic rails, respectively. The overall joint hardness is too low, which easily leads to saddle-shaped wear in the HAZ during track service, resulting in HAZ collapse. Therefore, the rail joint obtained under this process will be detrimental to railway operation safety.

[0101] Comparative Example 2

[0102] The process conditions involved in the rail base material, welding, and post-weld cooling in this comparative example are largely the same as those in Example 1. The difference is that after the rail welded joint has cooled to the surface temperatures of the left and right halves of the rail welded joint at 380°C and 330°C respectively, this comparative example continues to cool the left and right halves of the rail welded joint at cooling rates of 3.0°C / s and 6.0°C / s respectively until the ambient temperature reaches 20°C, thus obtaining the rail joint of this comparative example.

[0103] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0104] For the rail welded joint obtained in this comparative example, because the cooling rates on both sides of the weld are consistently higher than the critical cooling rates for martensitic transformation of the two corresponding rail steels, and the final cooling temperatures are also below the critical temperatures for martensitic transformation of the two rail steels, a significant number of large-sized blocky martensite structures are formed in the heat-affected zones on both sides of the weld. Hardness tests show that the longitudinal hardness of the rail weld heat-affected zone within ±20mm of the weld center reaches 96% and 92% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively, and the longitudinal hardness difference of the rail joint within ±20mm of the weld center is controlled within 36 HV. Considering the significant number of large-sized blocky martensite structures appearing in the heat-affected zones on both sides of the weld in this comparative example, the rail joint obtained under this process will be detrimental to railway operation safety.

[0105] Comparative Example 3

[0106] The comparative example uses the same rail base material and welding process as Example 2. The difference is that, in this comparative example, no treatment is applied to the joint after rail welding; instead, the joint is allowed to cool naturally in the air to an ambient temperature of 30°C, thus obtaining the rail joint of this comparative example.

[0107] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0108] For the rail welded joints in this comparative example, metallographic microscopy revealed that the microstructure of the weld heat-affected zone (HAZ) on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side was predominantly bainitic, with no obvious large-sized blocky martensite. However, hardness testing showed that the longitudinal hardness of the HAZ within ±20mm of the weld center reached 86% and 82% of the average hardness of the corresponding low-strength and high-strength heat-treated bainitic rail base materials, respectively. For the rail joints in this comparative example, the overall hardness of the HAZ on both sides of the weld was relatively low. The hardness of the rail joint obtained using this process is not within the range of 90%–93% and 85%–89% of the average hardness of the corresponding low-strength and high-strength heat-treated bainitic rail base materials obtained using this invention, indicating an overall low joint hardness. Therefore, the rail joints obtained using this process will be detrimental to railway operation safety.

[0109] Comparative Example 4

[0110] The rail base material and welding process conditions in this comparative example are the same as those in Example 2. The difference is that in this comparative example, the rail joint is cooled after welding when the surface temperature of the rail head drops to 890°C. Subsequently, the process parameters and conditions involved in the isothermal heat treatment and the subsequent cooling process are also the same as in Example 2, thus obtaining the rail joint of this comparative example.

[0111] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0112] Under metallographic microscopy, the welded rail joints obtained in this comparative example show that the microstructure of the welded heat-affected zone on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side is mainly bainitic, with no obvious large-sized blocky martensite.

[0113] However, hardness tests show that the longitudinal hardness of the heat-affected zone (HAZ) within ±20mm of the weld center reaches 88% and 83% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively. For the rail joint in this comparative example, the overall hardness of the HAZ on both sides of the weld is relatively low. The hardness of the rail joint obtained using this process is not within the range of 90%–93% and 85%–89% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively, as obtained using this invention. Therefore, the overall hardness of the joint is low. Consequently, the rail joint obtained using this process will be detrimental to railway operation safety.

[0114] Comparative Example 5

[0115] The rail base material, welding, and post-weld cooling process conditions in this comparative example are largely the same as those in Example 3. The difference lies in the fact that in this comparative example, the rail joint is cooled using a profile cooling device until the surface temperatures of the left and right halves of the welded rail joint drop to 385°C and 315°C respectively, followed by isothermal heat treatment for 1 hour. Subsequently, the cooling conditions are the same as in Example 3, thus obtaining the rail joint of this comparative example.

[0116] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0117] The welded joint obtained in this comparative example, under a metallographic microscope, shows that the heat-affected zone (HAZ) on the low-strength heat-treated bainitic rail side of the joint is predominantly bainitic, with no obvious large-sized blocky martensite. However, because the cooling rate of the HAZ on the high-strength heat-treated bainitic rail side exceeds the critical cooling rate for the martensitic transformation of the rail steel, and the final cooling temperature of the first cooling stage is within the martensitic transformation temperature range of the rail steel, the HAZ on this side, in addition to being predominantly bainitic, also exhibits a significant amount of blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm of the weld center reaches 93% and 92% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively. Subsequent isothermal heat treatment cannot eliminate the formed martensite. For the rail joint obtained in this comparative example, the hardness of the HAZ on the low-strength heat-treated bainitic rail side is good. Considering the significant amount of brittle and hard martensite formed in the weld heat-affected zone on one side of the high-strength heat-treated bainitic rail, the rail joints obtained under this process will be detrimental to railway operation safety.

[0118] Comparative Example 6

[0119] This comparative example shares most of the same rail base material, welding, and post-weld cooling process conditions as Example 1. The difference lies in that, in this comparative example, the rail joint is cooled using a profile cooling device until the surface temperature of the heat-affected zone on both sides of the weld reaches 450°C, at which point cooling is stopped. Immediately afterwards, isothermal heat treatment is performed, with both sides of the weld treated at 450°C. The subsequent cooling process conditions and parameters are consistent with those in Example 1, thus obtaining the rail joint of this comparative example.

[0120] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0121] Under a metallographic microscope, the welded rail joints obtained in this comparative example show that the microstructure of the weld heat-affected zone (HAZ) on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side is predominantly bainitic, with no obvious large-sized blocky martensite. However, hardness testing shows that the longitudinal hardness of the weld HAZ within ±20mm of the weld center reaches 87% and 83% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively. This is because the isothermal heat treatment temperature of 450℃ is relatively high, resulting in insufficient refinement of the bainitic microstructure at this stage, thus leading to a less significant increase in hardness in the HAZ on both sides of the weld joint.

[0122] For the rail joints in this comparative example, the hardness of the weld heat-affected zone on one side of the low-strength heat-treated bainitic rail and the weld heat-affected zone on the other side of the high-strength heat-treated bainitic rail are both outside the range of 90%–93% and 85%–89% of the average hardness of the base materials of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail, respectively, within the ±20mm area from the weld center obtained using this invention. The overall hardness of the joint is too low. Therefore, the rail joints obtained under this process will be detrimental to railway operation safety.

[0123] Comparative Example 7

[0124] The comparative example follows the same process conditions as Example 1 in terms of rail base material, welding, and post-weld cooling. The difference lies in the fact that in this comparative example, after the surface temperatures of the left and right halves of the rail welded joint have decreased to 380°C and 330°C respectively, the isothermal heat treatment of the rail welded joint is performed for only 0.5 hours. After the isothermal process, the conformal heating device is removed, and the rail joint is allowed to cool naturally in the air at a rate of 0.2°C / s to an ambient temperature of 20°C, thus obtaining the rail joint of this comparative example.

[0125] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling locations shown were used to examine the metallographic structure of the joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0126] Under a metallographic microscope, the welded rail joints obtained in this comparative example show that the microstructure of the weld heat-affected zone (HAZ) on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side is predominantly bainitic, with no obvious large-sized blocky martensite. However, hardness testing shows that the longitudinal hardness of the weld HAZ within ±20mm of the weld center reaches 89% and 84% of the average hardness of the corresponding low-strength heat-treated bainitic rail and high-strength heat-treated bainitic rail base materials, respectively. For the rail joint in this comparative example, the short isothermal time of 0.5 hours resulted in incomplete refinement of the bainitic lath structure, leading to minimal improvement in strength and hardness. Consequently, the hardness of the weld heat-affected zone on both the low-strength and high-strength heat-treated bainitic rail sides was not within the range of 90%–93% and 85%–89% of the average hardness of the base materials of the corresponding low-strength and high-strength heat-treated bainitic rails, respectively, within the ±20mm radius from the weld center obtained using this invention. Therefore, the overall hardness of the joint was low. Consequently, the rail joint obtained under this process is detrimental to railway operation safety.

[0127] As can be seen from the embodiments and comparative examples of this invention, this invention, by implementing segmented controlled cooling and isothermal heat treatment on the welded joints of dissimilar heat-treated bainitic rails after welding, maintains high hardness in the heat-affected zones on both sides of the weld, thereby ensuring the wear resistance of the rail welded joint. It ensures that the weld heat-affected zone on both the low-strength heat-treated bainitic rail side and the high-strength heat-treated bainitic rail side of the rail welded joint is bainitic, without obvious blocky martensite. Simultaneously, it allows the longitudinal hardness of the rail joint within ±20mm of the weld center to reach 90%–93% and 85%–89% of the average hardness of the corresponding low-strength and high-strength heat-treated bainitic rail base materials, respectively. This helps control the longitudinal hardness of the rail joint within ±20mm of the weld center to within 40 HV, ensuring the safe service of the rail joint. This invention has significant technical advantages and broad market prospects.

[0128] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0129] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A post-weld heat treatment method for welded joints of dissimilar steel rails, characterized in that, The dissimilar rail weld joint is formed by flash welding of a low-strength heat-treated bainitic rail and a high-strength heat-treated bainitic rail. The method includes the following steps: S1, cooling the dissimilar rail weld joints with surface temperatures of 910-990℃, so that the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail is reduced to 380-430℃ at a cooling rate of 3.0-5.0℃ / s, and the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail is reduced to 330-360℃ at a cooling rate of 6.0-8.0℃ / s. S2, immediately perform isothermal heat treatment on the welded joints of dissimilar rails after cooling treatment, so that the surface temperature of the heat-affected zone of the low-strength heat-treated bainitic rail is maintained between 380~430℃, and the surface temperature of the heat-affected zone of the high-strength heat-treated bainitic rail is maintained between 330~360℃, with a holding time of 0.8~1h. S3, stop the isothermal heat treatment and allow the dissimilar rail weld joint to cool naturally to an ambient temperature of 20-30℃ at a cooling rate of 0.2-0.9℃ / s.

2. The post-weld heat treatment method for dissimilar rail welded joints according to claim 1, characterized in that, By mass percentage, the low-strength heat-treated bainitic rail base material comprises C: 0.20%~0.23%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; the low-strength heat-treated bainitic rail base material has a tensile strength of 1100~1180MPa and a hardness of 270~310HV at 20~30℃.

3. The post-weld heat treatment method for dissimilar rail welded joints according to claim 1, characterized in that, By mass percentage, the high-strength heat-treated bainitic rail base material comprises C: 0.26%~0.30%, Si: 1.2%~1.6%, Mn: 1.4%~1.8%, Cr: 0.70%~1.10%, Mo: 0.20%~0.40%, with the balance being Fe and unavoidable impurities; the high-strength heat-treated bainitic rail base material has a tensile strength of 1200~1280MPa and a hardness of 320~340HV at 20~30℃.

4. The post-weld heat treatment method for dissimilar rail welded joints according to claim 1, characterized in that, The low-strength heat-treated bainitic steel rail and the high-strength heat-treated bainitic steel rail that form the dissimilar rail weld joint have the same rail type and a specification of 60~75kg / m.

5. A post-weld heat treatment apparatus for welded joints of dissimilar steel rails, characterized in that, The method for implementing the post-weld heat treatment method according to any one of claims 1 to 4 is characterized by comprising a profile cooling device for cooling the welded joint of dissimilar rails. The profile cooling device comprises two isolated cooling components, each corresponding to a low-strength heat-treated bainitic rail and a high-strength heat-treated bainitic rail, respectively. Each cooling component includes a spray panel (1) located above the rail head tread of the dissimilar rail welded joint, and spray pipes (5) connected to the spray panel (1) and facing two sides of the rail head. The spray panel (1) and the spray pipes... (5) Multiple spray holes (2) are provided on the upper part, and the spray holes (2) are in fluid communication with the external cooling medium; the distance between the spray holes (2) and the welded joint of the dissimilar steel rail is 15~25mm; the cooling medium of the profile cooling device is compressed air and / or water mist mixture, wherein the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling component corresponding to the low-strength heat-treated bainitic steel rail is 0.15~0.25MPa, and the gas pressure of the compressed air sprayed by the cooling component corresponding to the high-strength heat-treated bainitic steel rail is 0.35~0.45MPa.

6. The post-weld heat treatment apparatus for dissimilar rail welded joints according to claim 5, characterized in that, It also includes a profile heating device for isothermal heat treatment of welded joints of dissimilar rails, the profile heating device comprising: The device housing includes a first housing (6) and a second housing (7) opposite to each other. The upper parts of the first housing (6) and the second housing (7) are pivotally connected by a rotating shaft (8). The lower parts of the first housing (6) and the second housing (7) are detachably connected by a connector. When the first housing (6) is connected to the lower part of the second housing (7), the outer contour of the device housing matches the cross-sectional shape of the dissimilar rail welded joint. Two tracked ceramic heaters (13) are respectively provided with the low-strength heat-treated bainitic steel rail and the high-strength heat-treated bainitic steel rail. The two tracked ceramic heaters (13) are connected in parallel and include multiple ceramic units (14) arranged in multiple rows in parallel on the inner surface of the device housing. When the lower part of the first housing (6) is connected to the second housing (7), the distance between the tracked ceramic heater (13) and the surface of the dissimilar steel rail weld joint is 2~7mm. The insulation layer (11) is disposed between the outer shell of the device and the tracked ceramic heater (13).

7. The post-weld heat treatment apparatus for dissimilar rail welded joints according to claim 6, characterized in that, The number of ceramic units (14) covering the rail head of the dissimilar rail welded head in the tracked ceramic heater (13) is greater than the number of ceramic units (14) covering the rail web and rail bottom of the dissimilar rail welded head.

8. The post-weld heat treatment apparatus for dissimilar rail welded joints according to claim 6, characterized in that, The number of ceramic units (14) of the track-type ceramic heater (13) corresponding to the low-strength heat-treated bainitic steel rail is greater than the number of ceramic units (14) of the track-type ceramic heater (13) corresponding to the high-strength heat-treated bainitic steel rail.

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