Dissimilar-strength hot-rolled eutectoid pearlitic rail welding joint and post-welding cooling method therefor

Abstract

A dissimilar-strength hot-rolled eutectoid pearlitic rail welding joint and a post-welding cooling method therefor. The method comprises: performing first-stage cooling on a welding joint formed by welding a low-strength hot-rolled eutectoid pearlitic rail base material and a high-strength hot-rolled eutectoid pearlitic rail base material, so that the surface temperature of the welding joint is reduced from 1000-1100°C to 400-500°C; performing second-stage cooling on the welding joint, so that the surface temperature of the welding joint is reduced from 400-500°C to 220°C or below; and naturally cooling the welding joint to an ambient temperature.

Description

Welded Joints of Hot-Rolled Eutectoid Pearlitic Steel Rails with Dissimilar Strengths and Their Post-Weld Cooling Methods

[0001] This application claims priority to Chinese Patent Application No. 202410985677.9, filed on July 22, 2024, entitled “Welded Joint of Hot-Rolled Eutectoid Pearlitic Steel Rail of Dissimilar Strength and Post-Weld Cooling Method Thereof”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of rail production technology, specifically to a welded joint of hot-rolled eutectoid pearlitic rail of dissimilar strength and its post-weld cooling method. Background Technology

[0003] Different railway line conditions place varying requirements on rail materials and performance. Based on differences in manufacturing processes, rails can be categorized into hot-rolled and heat-treated rails. For straight sections and large-radius curves on conventional railways (operating speeds below 200 km / h) and high-speed railways (operating speeds between 200 and 350 km / h), hot-rolled rails are more suitable, while heat-treated rails, with their superior wear resistance, are typically used for sections with small-radius curves. On some railways with complex track conditions, the use of high-strength heat-treated rails may lead to deeper contact fatigue crack propagation, which could actually reduce the rail's service life. While the use of hot-rolled rails, with their slightly lower wear resistance, increases natural wear, it also significantly reduces the damage caused by deeper contact fatigue crack propagation to the rail's service life. Therefore, currently, domestic conventional and high-speed railways generally use a combination of hot-rolled and heat-treated rails, which involves welding between rails of dissimilar materials / strength grades.

[0004] Currently, mobile flash welding has become the mainstream online rail welding technology at railway construction sites both domestically and internationally. For hot-rolled rails, the heat-affected zone (HAZ) is essentially subjected to heat treatment due to the welding thermal cycle. Furthermore, the presence of multiple alloys in the rail steel results in the HAZ being generally harder than the base material used for welding; that is, the HAZ of hot-rolled rails is generally harder than the base material. When the HAZ hardness of hot-rolled rails reaches more than 1.2 times the hardness of the base material, damage is more likely to occur preferentially in the base material due to its lower hardness. Therefore, for welding hot-rolled eutectoid pearlitic rails, the HAZ hardness is typically controlled to be 1.0 to 1.1 times the hardness of the base material. For heat-treated rails, the hardened layer originally belonging to the rail base material is damaged under the welding thermal cycle, resulting in coarse austenite grains and pearlite lamellar spacing in the weld heat-affected zone (HAZ). Consequently, the overall hardness of the HAZ is lower than that of the rail base material used for welding. When the hardness of the HAZ of a heat-treated rail is less than 0.9 times that of the rail base material, damage is more likely to occur preferentially in the HAZ due to its lower hardness. Furthermore, when the hardness of the HAZ of a heat-treated rail reaches more than 1.0 times (e.g., 1.1–1.2 times) of the rail base material, it is likely due to the formation of high-hardness martensite within the HAZ. Martensite exhibits high internal stress, making it prone to internal cracking under wheel-rail contact pressure, which is detrimental to the overall service safety of the rail joint. In addition, martensite can also cause overall flaking of the rail head tread. Therefore, for welding heat-treated eutectoid pearlitic steel rails, the hardness of the weld heat-affected zone is usually controlled between 0.9 and 1.0 times the hardness of the base material.

[0005] When new types of rails are laid on existing lines for replacing old rails and for trial laying, welding between the new rails and existing rails of different materials / strengths becomes necessary. For two rails with different materials and strengths, the differences between the base materials present challenges for welding. During railway service, welded rail joints are prone to "saddle-shaped" wear in the low-hardness areas of the rail head tread, which not only increases wheel-rail impact but also affects the rail's service life and may even endanger traffic safety. Therefore, restoring the strength and hardness lost during welding becomes a prerequisite for the rail's application.

[0006] In summary, when welding rails of different materials and / or strengths together, the welding of the resulting dissimilar rail joints and the subsequent post-weld heat treatment to achieve synergistic improvement in microstructure and properties are issues that need to be considered. Given that the welding quality of the rails determines whether seamless tracks can withstand the test of railway operation, and even traffic safety, there is a need in the existing technology to improve the welding and post-weld heat treatment techniques for rails of dissimilar strength grades and / or dissimilar materials. Summary of the Invention

[0007] The main objective of this invention is to provide a post-weld cooling method for hot-rolled eutectoid pearlitic steel rail welded joints of dissimilar strength, in order to solve the problem of how to perform post-weld cooling on hot-rolled eutectoid pearlitic steel rail welded joints of dissimilar strength to ensure the performance of the rail.

[0008] According to one aspect of the present invention, a post-weld cooling method for a welded joint of hot-rolled eutectoid pearlitic steel rail of dissimilar strength is provided, comprising the following steps performed sequentially:

[0009] S1, the welded joint formed by welding low-strength hot-rolled eutectoid pearlitic steel rail base material and high-strength hot-rolled eutectoid pearlitic steel rail base material is subjected to a first stage of cooling, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃, wherein the first heat-affected zone corresponding to the low-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a first cooling rate, and the second heat-affected zone corresponding to the high-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a second cooling rate less than the first cooling rate;

[0010] S2, the welded joint is cooled in the second stage, so that the surface temperature of the welded joint is reduced from 400-500℃ to below or equal to 220℃, wherein the cooling rate of the welded joint is less than or equal to 1.4℃ / s;

[0011] S3, allow the welded joint to cool naturally to ambient temperature.

[0012] According to one embodiment of the present invention, the first cooling rate is 3.0 to 4.0 °C / s, and the second cooling rate is 1.5 to 2.5 °C / s.

[0013] According to one embodiment of the present invention, the cooling rate of the welded joint in step S2 is 0.8 to 1.4 °C / s.

[0014] According to one embodiment of the present invention, the method is carried out in a low-temperature outdoor environment with a temperature of 1 to 15°C, and a contour-following temperature control device is used in steps S1 and S2 to perform full-section heat compensation on the welded joint.

[0015] According to one embodiment of the present invention, the contour-following temperature control device includes:

[0016] The two housings are pivotally connected, each housing having a shape that matches the first heat-affected zone and the second heat-affected zone, and each housing is provided with multiple heating elements;

[0017] The control unit is configured to control the heating units of the two housings respectively.

[0018] According to one embodiment of the present invention, the heating portions of the two housings corresponding to the rail head region are smaller and more densely distributed than the heating portions corresponding to the rail web and rail bottom regions.

[0019] According to one embodiment of the present invention, the low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980-1050 MPa and a hardness of 280-310 HV at room temperature; the high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100-1180 MPa and a hardness of 320-350 HV at room temperature.

[0020] According to one embodiment of the present invention, the chemical composition of the low-strength hot-rolled eutectoid pearlitic rail base material comprises: 0.68-0.74% C, 0.50-0.80% Si, 0.70-1.0% Mn, 0.04-0.08% V, with the balance being Fe and unavoidable impurities;

[0021] The chemical composition of high-strength hot-rolled eutectoid pearlitic rail base material includes: 0.76–0.82% C, 0.50–0.80% Si, 0.70–1.0% Mn, 0.30–0.50% Cr, 0.04–0.08% V, with the balance being Fe and unavoidable impurities.

[0022] According to one embodiment of the present invention, the low-strength hot-rolled eutectoid pearlitic rail base material and the high-strength hot-rolled eutectoid pearlitic rail base material have the same rail type and the specification is 60-75 kg / m. The welded joint is formed by welding with a moving flash welding machine.

[0023] According to another aspect of the present invention, a dissimilar strength hot-rolled eutectoid pearlitic rail welded joint is provided. The welded joint is cooled by the post-weld cooling method described above. The heat-affected zone microstructure on both sides of the weld joint is pearlitic, without martensite. The longitudinal hardness of the region corresponding to the low-strength hot-rolled eutectoid pearlitic rail base material and extending from the weld center to 15 mm from the weld center reaches 106-110% of the average hardness of the low-strength hot-rolled eutectoid pearlitic rail base material. The longitudinal hardness of the region corresponding to the high-strength hot-rolled eutectoid pearlitic rail base material and extending from the weld center to 15 mm from the weld center reaches 100-105% of the average hardness of the low-strength hot-rolled eutectoid pearlitic rail base material. The hardness difference between the heat-affected zones on both sides of the weld is within 30 HV.

[0024] In the technical solution of this invention, by sequentially performing first-stage cooling, second-stage cooling, and natural cooling, and controlling the starting cooling temperature, final cooling temperature, and cooling rate of the first-stage and second-stage cooling, the following beneficial effects can be achieved: The heat-affected zones on both sides of the rail joint weld maintain high hardness, thereby ensuring the wear resistance of the rail joint; the microstructure of the heat-affected zones on both sides of the rail joint weld is guaranteed to be pearlite, without harmful martensite; the longitudinal hardness of the rail joint within ±15mm of the weld center can reach 106-110% and 100-105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlite rail and high-strength hot-rolled eutectoid pearlite rail base materials, respectively, and the hardness difference between the heat-affected zones on both sides of the rail joint weld is within 30 HV, improving the hardness difference between the heat-affected zones on both sides of the weld of dissimilar rail joints, thereby achieving good hardness matching between the heat-affected zones on both sides of the weld of dissimilar strength rails and ensuring railway operation safety. Attached Figure Description

[0025] 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 based on these drawings without creative effort.

[0026] Figure 1 shows a flowchart of a post-weld cooling method for a welded joint of hot-rolled eutectoid pearlitic steel rail of dissimilar strength according to an embodiment of the present invention;

[0027] Figure 2 shows a schematic diagram of the longitudinal hardness test location 5mm below the rail head tread of the rail joint;

[0028] Figure 3 shows a schematic diagram of the metallographic sample cutting locations in each embodiment and comparative example;

[0029] Figure 4 shows an overall schematic diagram of the split-type contour-following temperature control device according to an embodiment of the present invention;

[0030] Figure 5 shows a schematic diagram of the distribution of heating elements at the rail head of the split-type contour temperature control device according to an embodiment of the present invention. Detailed Implementation

[0031] 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.

[0032] 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.

[0033] In some countries' rail welding standards, such as the Australian rail welding standard AS1085.20-2012, it is stipulated that for certain high-strength, high-carbon, and high-alloy rails, under a 100x metallographic microscope, the percentage of martensite in the area where martensite is most severe in the rail weld joint must not exceed 5%. Otherwise, the joint will suffer premature fatigue fracture due to a large amount of hardened martensite, seriously affecting railway operation safety. The Chinese railway rail welding standard TB / T1632.2-2014 stipulates that martensite must not appear on the entire cross-section of the rail weld joint. Therefore, strictly controlling the martensite content in the rail weld structure is crucial for the stable operation of railway lines. Furthermore, as mentioned in the background section, for dissimilar strength rail weld joints, it is necessary to achieve good hardness matching between the weld seam regions and the corresponding base materials, as well as good hardness matching between the heat-affected zones on both sides of the weld. This application proposes one or more embodiments, as described below, to achieve these objectives.

[0034] Referring to Figure 1, this invention proposes a post-weld cooling method for welded joints of hot-rolled eutectoid pearlitic steel rails with dissimilar strength, which includes the following steps performed sequentially:

[0035] S1, the welded joint formed by welding low-strength hot-rolled eutectoid pearlitic steel rail base material and high-strength hot-rolled eutectoid pearlitic steel rail base material is subjected to a first stage of cooling, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃, wherein the first heat-affected zone corresponding to the low-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a first cooling rate, and the second heat-affected zone corresponding to the high-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a second cooling rate less than the first cooling rate;

[0036] S2, the welded joint is cooled in the second stage, so that the surface temperature of the welded joint is reduced from 400-500℃ to below or equal to 220℃, wherein the cooling rate of the welded joint is less than or equal to 1.4℃ / s;

[0037] S3, allow the welded joint to cool naturally to ambient temperature.

[0038] In an embodiment of the present invention, the welded joint is a region with a length ranging from 60 to 80 mm, including the weld and the heat-affected zone, with the weld at the center of the region. The heat-affected zone includes a first heat-affected zone corresponding to the low-strength hot-rolled eutectoid pearlitic rail base material and a second heat-affected zone corresponding to the high-strength hot-rolled eutectoid pearlitic rail base material, respectively located on both sides of the weld.

[0039] The inventors of this application recognize that the martensitic transformation initiation temperature of low-strength hot-rolled eutectoid pearlitic rail steel is 280–320°C, and the critical cooling rate for martensitic transformation is 3.0–4.0°C / s. The martensitic transformation initiation temperature of high-strength hot-rolled eutectoid pearlitic rail steel is 220–250°C, and the critical cooling rate for martensitic transformation is 1.5–2.5°C / s. To avoid the formation of abnormal structures such as martensite in the welded joints of rails, when heat-treating the welded joints of low-strength and high-strength hot-rolled eutectoid pearlitic rails, the final cooling temperature during the rapid cooling process of the heat treatment must be controlled above the relatively higher martensitic transformation initiation temperature of the low-strength hot-rolled eutectoid pearlitic rail steel. Meanwhile, the cooling rate during the post-weld heat treatment process must be limited to the critical cooling rate of martensitic transformation of high-strength hot-rolled eutectoid pearlitic rail steel with a relatively low critical cooling rate; otherwise, the joint may suffer premature fatigue fracture due to the hardened martensite structure.

[0040] In embodiments of the present invention, the initial cooling temperature for the first stage is 1000–1100°C. This higher initial cooling temperature ensures sufficient driving force for the pearlite phase transformation during subsequent cooling, resulting in sufficiently fine pearlite lamellar spacing and effectively improving the hardness of the heat-affected zones on both sides of the weld. The present invention utilizes the residual heat from rail welding to achieve the post-weld heat treatment process for the rail. Accelerated post-weld cooling is implemented for rail joints with high residual temperatures to reduce the phase transformation temperature of the rail head from austenite to pearlite, thereby increasing the hardness of the austenite recrystallization zone. The residual surface temperature of the rail joint after welding can be adjusted by modifying process parameters such as the rail welding heat input and extending or shortening the high-temperature dwell time. Based on metallurgical principles, rail joints exhibit a certain degree of dynamic undercooling under high-temperature rapid cooling conditions after welding, causing the phase transformation temperature of the austenite-to-pearlite transformation in the non-equilibrium state to shift downwards. Furthermore, as the undercooling increases, the phase transformation temperature gradually decreases. Therefore, even in the second stage cooling with a lower initial cooling temperature, the microstructure transformation from austenite to pearlite can still occur inside the joint.

[0041] This invention sets the final cooling temperature of the first stage of cooling to 400–500°C. Firstly, this final cooling temperature is controlled to be above 80°C of the higher martensitic transformation initiation temperature (i.e., 280–320°C) in both types of rail steel, to avoid the formation of harmful martensitic structures during the first stage of cooling. Secondly, considering that if the final cooling temperature is too high (e.g., 530°C), the pearlite structure obtained during cooling may not be sufficiently refined, resulting in insufficient hardness improvement, this application sets the final cooling temperature of the first stage of cooling to 400–500°C.

[0042] To achieve good hardness matching between the weld seam regions and their corresponding base materials (e.g., ensuring that the longitudinal hardness of the rail joint within ±15mm of the weld center reaches 106-110% and 100-105% of the average hardness of the base materials for low-strength hot-rolled eutectoid pearlitic rails and high-strength hot-rolled eutectoid pearlitic rails, respectively), and to achieve good hardness matching between the heat-affected zones on both sides of the weld seam (e.g., the hardness difference between the heat-affected zones on both sides of the rail joint weld seam is within 30 HV), this invention sets the cooling rate of the first heat-affected zone (corresponding to the base material for low-strength hot-rolled eutectoid pearlitic rails) to be greater than that of the second heat-affected zone (corresponding to the base material for low-strength hot-rolled eutectoid pearlitic rails) during the first-stage cooling process. This allows the hardness of the first heat-affected zone to be increased more significantly relative to the second heat-affected zone, thereby meeting the aforementioned hardness matching requirements.

[0043] In some embodiments, the first cooling rate is 3.0–4.0 °C / s, and the second cooling rate is 1.5–2.5 °C / s. By adopting an appropriate cooling rate, while maintaining high hardness in the heat-affected zone on both sides of the weld, excessively high hardness in the heat-affected zone due to excessively rapid cooling is avoided (e.g., the hardness of the heat-affected zone on both sides of the weld reaches more than 1.1 times the hardness of the hot-rolled rail base material). Since the hardness of the heat-affected zone can be improved under natural air cooling conditions after welding of hot-rolled pearlitic rails, and when the ambient temperature is too low, resulting in a low cooling rate under natural cooling conditions, the hardness of the heat-affected zone will increase significantly to more than 1.1 times the hardness of the hot-rolled rail base material, which is detrimental to the service safety of the rail joint. To improve the hardness of the heat-affected zone, the first stage of cooling uses two cooling rates corresponding to the critical cooling rates of the martensitic transformation of rail steel to cool the heat-affected zone on both sides of the rail joint weld.

[0044] In an embodiment of the invention, the initial cooling temperature of the second stage is 400–500°C, the final cooling temperature is below or equal to 220°C, and the cooling rate is less than or equal to 1.4°C / s. The second cooling stage employs a lower cooling rate than the first stage to cool the rail welded joint. This cooling rate is below the critical cooling rate for the martensitic transformation of the two types of rail steel, in order to avoid the formation of harmful martensitic structures during the cooling process in the second cooling stage.

[0045] In some embodiments, the cooling rate of the welded joint in step S2 is 0.8–1.4 °C / s. When the cooling rate is below 0.8 °C / s, the cooling is too slow and takes too long, which will affect the subsequent rail welding production. When the cooling rate is above 1.4 °C / s, such as 1.5 °C / s, the critical cooling rate for martensitic transformation of the high-strength hot-rolled eutectoid pearlitic rail steel in this invention is 1.5 °C / s. A cooling rate of 1.5 °C / s may cause martensitic structure to form during the continuous cooling process after welding of the rail joint, affecting the service safety of the rail joint. Therefore, in some embodiments of this invention, the second stage cooling involves cooling the joint at a rate of 0.8–1.4 °C / s.

[0046] When the surface temperature of the welded joint drops to below or equal to 220°C, the heat treatment of the welded joint is completed. The rail joint can then be allowed to cool naturally to ambient temperature. However, the conditions for martensite formation are no longer present during the natural cooling stage.

[0047] It should be noted that, to avoid the formation of harmful martensitic structures in the heat-affected zone during rail welding and post-weld heat treatment, cooling can be artificially controlled to ensure the cooling rate during welding and post-weld heat treatment is lower than the critical cooling rate for martensitic transformation of the rail steel. Alternatively, a cooling rate higher than the critical cooling rate for martensitic transformation can be used to control the final cooling temperature of the rapid cooling stage of welding and post-weld heat treatment above the initiation temperature of the martensitic transformation of the rail steel, followed by cooling to ambient temperature at a rate lower than the critical cooling rate for martensitic transformation. During rail welding and post-weld heat treatment, to improve the hardness and toughness of the welded area, accelerated cooling methods (such as spraying compressed air or other cooling media) are typically used to obtain a pearlitic structure with fine lamellar spacing in the weld heat-affected zone.

[0048] In some embodiments, the post-weld cooling method of the present invention is carried out in a low-temperature outdoor environment with a temperature of 1–15°C. In steps S1 and S2, a contour-following temperature control device is used to perform full-section heat compensation on the welded joint. It should be noted that when rail welding is carried out at room temperature (20–30°C), the rail cools naturally after welding (air cooling), and harmful martensite structures will not form in the heat-affected zone due to excessively rapid cooling. However, when working in a low-temperature outdoor environment (1–15°C), the ambient temperature causes extremely rapid cooling during rail welding and post-weld heat treatment (cooling rate can reach 3.5–8.0°C / s), creating conditions for martensite formation. Furthermore, when the ambient temperature is too low, resulting in an excessively low cooling rate under natural cooling conditions, the hardness of the heat-affected zone will increase significantly to more than 1.1 times the hardness of the hot-rolled rail base material, which is detrimental to the service safety of the rail joint. Therefore, when welding rails and performing post-weld heat treatment in low-temperature outdoor environments, special attention must be paid to the formation of abnormal martensitic structures and excessive hardness increase, necessitating strict control of the rail welding and post-weld heat treatment processes. To achieve the cooling control at different stages as described above, the first and second stages of cooling in this invention employ a contour-following temperature control device to compensate for heat across the entire weld joint, controlling the cooling rate of the rail joint. After the second stage of cooling is completed, the contour-following temperature control device can be removed, and the rail joint can be placed in a low-temperature outdoor environment for natural cooling, allowing the joint to cool naturally to ambient temperature.

[0049] In some embodiments, the conformal temperature control device includes: two pivotally connected housings, each housing having a shape matching a first heat-affected zone and a second heat-affected zone, and each housing having multiple heating elements; and a control unit configured to control the heating elements of the two housings respectively, thereby controlling the heating capacity (e.g., heating rate, heating temperature) of the two housings. The two housings are pivotally connected so that they can be rotated relative to each other to unfold for removal from the weld joint, and can also be rotated relative to each other to close to cover the weld joint for heat compensation. Under the control of the control unit, the two housings can independently heat the first heat-affected zone and the second heat-affected zone to control their different cooling rates during the first stage of cooling.

[0050] Considering the slower heat transfer due to the greater thickness of the rail head compared to the rail web and rail base, in some embodiments of the present invention, the heating portions of the two housings corresponding to the rail head region are smaller and more densely distributed than the heating portions corresponding to the rail web and rail base regions. This eliminates the temperature difference between the rail head and the rail web and rail base regions, ensuring that the rail head, rail web, and rail base of the rail joint reach the same heating temperature at the same time.

[0051] Figure 4 shows an overall schematic diagram of a split-type contour-following temperature control device according to an embodiment of the present invention, and Figure 5 shows a schematic diagram of the distribution of heating elements at the rail head of the split-type contour-following temperature control device. A1 / A2 / B1 / B2 are terminal blocks, C is a circular track-type ceramic heating element at the rail web, D is a rotating shaft, E is a fixing ring, F is the device housing, G is an asbestos insulation layer, A3 is the rail head tread heating area, B3 is the rail head side heating area, C3 is the rail head lower jaw heating area, D3 is a circular track-type ceramic heating element, and E3 is the rail joint weld seam positioning line. As shown in Figures 4 and 5, the device housing includes a first housing and a second housing pivotally connected by a rotating shaft. The first housing and the second housing are respectively provided with a fixing ring and a corresponding mating structure, and the first housing and the second housing are fixedly connected through the cooperation of the fixing ring and the mating structure. The inner side of the device's outer casing is densely packed with small, circular, track-type ceramic heating elements that fit snugly against the rail surface, ensuring excellent heat conduction. The combined action of multiple rows of parallel circular ceramic heating elements achieves thermal compensation at the rail joint. Furthermore, a 2cm thick asbestos insulation layer is placed between the metal casing and the ceramic heating elements, providing some insulation. The split-type contour-following temperature control device is fixed with the center of the rail weld (weld positioning line) as the boundary. Terminals A1 and A2 form a circuit, providing full-section controlled heating for the left half of the device (rail weld joint); terminals B1 and B2 form a circuit, providing full-section controlled heating for the right half of the device (rail weld joint).

[0052] The heating capacity (heating rate, heating temperature) of the left and right halves of the contour-following temperature control device can be the same or different, and the specific heating temperature can be set according to the required program. The size and density of the circular, track-type electric heating elements distributed on both sides inside the split-type device can be the same. The layout of the heating areas for the rail head, rail web, and rail bottom of the split-type contour-following temperature control device is similar. Because the rail head is thicker and heat transfer is slower, the number of circular, track-type ceramic electric heating elements covering the rail head is greater, denser, and has a smaller diameter than those in the rail web and rail bottom areas, resulting in a higher heating temperature to ensure sufficient heating of the rail head area of ​​the rail joint. Conversely, the ceramic electric heating elements in the rail web and rail bottom areas of the contour-following device are larger and sparser than those in the rail head area, therefore the heating temperature in the rail web and rail bottom areas is slightly lower than that in the rail head area. By rationally arranging the size, quantity, and distribution distance of the ceramic heating elements embedded in the split-type device, the heating temperature difference between the rail head, rail web, and rail base areas is eliminated, enabling the rail head, rail web, and rail base of the rail joint to reach the same heating temperature at the same time.

[0053] As shown in Figure 4, the split-type contour-following temperature control device has advantages such as compactness, flexibility, and low cost, making it convenient for field construction. It can be powered by a diesel generator at 380V or 220V AC mains, with a rated power of 10kW. The device uses commercial LCD-shaped circular tracked ceramic heating elements as the heat source. The track head area of ​​the split-type device uses heating elements with a diameter of [missing information]. The track-type ceramic heating element is 5mm thick and circular, while the web and bottom areas of the split-type device use a diameter of [missing information]. A 5mm thick, circular, tracked ceramic heating element, combined with asbestos insulation and a steel outer shell, forms a rail-shaped, split-type heater for easy assembly and disassembly. It is suitable for localized heating of rail welded joints. The actual dimensions of the heating device, as well as the specifications and distribution of the heaters, can be adjusted and modified according to the actual profile of the rail. During the device design, multiple rows of parallel, circular, tracked ceramic heating elements are evenly fixed inside the device, which has a rail-like profile. This ensures the heating elements uniformly cover and fully adhere to the rail surface, achieving good heat conduction during heating. This device provides thermal compensation for the rail joint. During testing, a temperature controller is used to control the heating temperature. The device operates within a temperature range of 200–1000℃ and can rotate a maximum of 180° around its axis.

[0054] In embodiments of the present invention, to avoid the formation of harmful martensite structures during the cooling process after rail welding in low-temperature construction environments, a first-stage cooling process is adopted after the rail welding protrusion is completed. This process involves reducing the joint surface temperature from 1000–1100°C to 400–500°C. A split-type conformal temperature control device is used to fully cover the rail head, web, and bottom of the rail joint to slow down the post-weld cooling rate, achieving under-cooling. Simultaneously, within the split-type device, thermal compensation is used to control the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. This controlled cooling is achieved by supplementing the heat from ceramic heating elements to slow down the cooling rate of the heat-affected zones on both sides of the weld. For the left half of the device, the cooling rate is 3.0–4.0°C / s. For the right half of the device, the cooling rate is 1.5–2.5°C / s. It should be noted that the heat treatment objects in this invention are two types of hot-rolled eutectoid pearlitic steel rails. When the cooling rate in this stage is too high, the hardness of the corresponding rail weld heat-affected zone will exceed 1.1 to 1.2 times the hardness of the corresponding rail base material, which is detrimental to the service safety of the rail welded joint. For the subsequent second-stage cooling process of the rail welded joint, which reduces the temperature from 400-500℃ to below or equal to 220℃, a split-type conformal temperature control device is used to fully cover the rail head, rail web, and rail bottom of the rail joint to slow down the post-weld cooling rate of the rail joint. At the same time, in the split-type device, thermal compensation is used to control the cooling of the left half (corresponding to the weld heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic steel rail) and the right half (corresponding to the weld heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic steel rail) of the weld joint, with the weld center as the dividing line. The second-stage cooling is still slowed down by supplementing the heat of the ceramic heating element to slow down the cooling rate of the rail weld heat-affected zones on both sides of the weld. The second stage of cooling employs a rail-contouring temperature control device. The cooling rate in this stage is 0.8–1.4℃ / s, lower than the critical cooling rate for martensitic transformation of the two types of rail steel. Simultaneously, the final cooling temperature of this second stage is above the martensitic transformation initiation temperature of the two types of rail steel, thus completing the heat treatment of the welded joint. Once the surface temperature of the rail welded joint drops to below or equal to 220℃, the rail-contouring temperature control device is removed, and the rail joint is placed in a low-temperature outdoor environment for natural cooling, allowing it to cool naturally to an ambient temperature of 1–15℃. During the cooling process, an infrared thermometer can be used to collect the temperature signal of the rail head tread, the contact area between the wheel and the rail.

[0055] In some embodiments, the low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980–1050 MPa and a hardness of 280–310 HV at room temperature (20–30°C); the high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100–1180 MPa and a hardness of 320–350 HV at room temperature (20–30°C).

[0056] The low-strength hot-rolled eutectoid pearlitic rail base material and the high-strength hot-rolled eutectoid pearlitic rail base material of the present invention have different chemical composition ranges. In some embodiments, the chemical composition of the low-strength hot-rolled eutectoid pearlitic rail base material comprises: 0.68–0.74% C, 0.50–0.80% Si, 0.70–1.0% Mn, 0.04–0.08% V, with the balance being Fe and unavoidable impurities; the chemical composition of the high-strength hot-rolled eutectoid pearlitic rail base material comprises: 0.76–0.82% C, 0.50–0.80% Si, 0.70–1.0% Mn, 0.30–0.50% Cr, 0.04–0.08% V, with the balance being Fe and unavoidable impurities.

[0057] In some embodiments, the low-strength hot-rolled eutectoid pearlitic rail base material and the high-strength hot-rolled eutectoid pearlitic rail base material have the same rail type and a specification of 60-75 kg / m. The welded joint is formed by welding with a mobile flash welding machine.

[0058] It should be noted that heat treatment technology itself involves 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, simple data application and comparison are not possible. The chemical composition and heat treatment processes of rails developed in various countries inevitably overlap. Influenced by factors such as smelting capacity, heat treatment equipment, and operator skill levels, the applicable objects (including rail mechanical properties and temperature distribution) of each invention patent differ, as do the cooling devices and implementation processes used, resulting in fundamental differences that prevent simple application of these processes. Furthermore, based on the difference in hardness change in the heat-affected zone during the continuous cooling process of hot-rolled eutectoid pearlitic rail steel and heat-treated eutectoid pearlitic rail steel, this application adopts a cooling method with different intensities for the heat-affected zones on both sides of the rail joint weld, limiting the cooling rate and temperature at each cooling stage. This improves the "saddle-shaped" wear of the rail joint caused by the low hardness of the welded area during rail service, while avoiding the damage to the service performance of the rail joint caused by the formation of martensitic structure. Therefore, this application has significant progress compared with other patent applications.

[0059] Through the above technical solution, the present invention can achieve the following beneficial effects:

[0060] 1. This invention achieves high hardness in both heat-affected zones on both sides of the weld of the flash welded joint of dissimilar hot-rolled eutectoid pearlitic steel rail by implementing controlled cooling of different intensities on the weld seams after welding, thereby ensuring the wear resistance of the rail joint.

[0061] 2. It can ensure that the microstructure of the heat-affected zone on both sides of the rail joint weld is pearlite, without harmful martensite;

[0062] 3. It can make the longitudinal hardness of the rail joint within ±15mm of the weld center reach 106-110% and 100-105% of the average hardness of the base material of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively. The hardness difference between the heat-affected zones on both sides of the rail joint weld is within 30HV, which improves the hardness difference between the heat-affected zones on both sides of the weld of dissimilar rail joints, thereby achieving good hardness matching between the heat-affected zones on both sides of the weld of dissimilar strength rails and ensuring railway operation safety.

[0063] According to another aspect of the present invention, a dissimilar strength hot-rolled eutectoid pearlitic rail welded joint is provided. The welded joint is cooled by the post-weld cooling method described above. The heat-affected zone microstructure on both sides of the weld joint is pearlitic, without martensite. The longitudinal hardness of the region corresponding to the low-strength hot-rolled eutectoid pearlitic rail base material and extending from the weld center to 15 mm from the weld center reaches 106-110% of the average hardness of the low-strength hot-rolled eutectoid pearlitic rail base material. The longitudinal hardness of the region corresponding to the high-strength hot-rolled eutectoid pearlitic rail base material and extending from the weld center to 15 mm from the weld center reaches 100-105% of the average hardness of the low-strength hot-rolled eutectoid pearlitic rail base material. The hardness difference between the heat-affected zones on both sides of the weld is within 30 HV.

[0064] The following description is based on specific embodiments and comparative examples. In the following embodiments and comparative examples, both the low-strength hot-rolled eutectoid pearlitic steel rail and the high-strength hot-rolled eutectoid pearlitic steel rail were produced by Panzhihua Iron and Steel Group. Figure 2 is a schematic diagram of the longitudinal hardness test position 5mm below the rail head tread of the rail joint, where a is the low-strength hot-rolled eutectoid pearlitic steel rail, b is the welded joint, c is the high-strength hot-rolled eutectoid pearlitic steel rail, d is the rail head tread of the rail joint, and e is the weld center. Figure 3 is a schematic diagram of the metallographic sample cutting position in each embodiment and comparative example, where e is the weld center and f is the metallographic sample sampling position.

[0065] Example 1

[0066] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0067] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0068] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0069] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint was observed using a Leica MeF3 optical microscope.

[0070] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 106% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 23HV higher than the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0071] Example 2

[0072] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.08% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1180 MPa and a hardness of 350 HV at room temperature (20–30°C).

[0073] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0074] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0075] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint was observed using a Leica MeF3 optical microscope.

[0076] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 110% and 105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 27HV higher than the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0077] Example 3

[0078] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0079] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0080] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0081] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint was observed using a Leica MeF3 optical microscope.

[0082] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 110% and 105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is 5HV higher than the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0083] Example 4

[0084] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0085] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0086] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0087] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint was observed using a Leica MeF3 optical microscope.

[0088] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 106% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is 9HV higher than the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0089] Example 5

[0090] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0091] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0092] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0093] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint was observed using a Leica MeF3 optical microscope.

[0094] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 107% and 103% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 28HV higher than the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0095] Comparative Example 1

[0096] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0097] After the upsetting and push-off processes of the moving flash welding process, the dissimilar rail welded joint with a specification of 60kg / m is directly air-cooled (naturally cooled) to an ambient temperature of 1℃ from the residual temperature of 1100℃, thus obtaining the dissimilar rail welded joint obtained in this comparative example.

[0098] The rail joint obtained in this comparative example was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0099] For the rail welded joint in this comparative example, compared with the base material of the rail on both sides of the weld, the entire weld heat-affected zone shows an increasing hardness trend. Within a range of ±15mm from the weld center, the longitudinal hardness of the rail joint meets 118% and 123% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 63 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, due to the lack of cooling mitigation measures during the joint cooling process, coupled with an excessively low ambient temperature of 1℃ and a rapid cooling rate, a significant amount of martensite, in addition to pearlite, appears in the metallographic structure of the weld heat-affected zone on both sides of the rail joint weld. Under this process, the excessive hardness difference between the weld heat-affected zones on both sides of the rail joint weld, and the presence of a significant amount of martensite in both weld heat-affected zones, are detrimental to railway operation safety.

[0100] Comparative Example 2

[0101] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0102] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 300℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0103] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0104] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0105] For the welded joints of the rails in this comparative example, the final cooling temperature of 300℃ in the first cooling stage of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic steel rail is within the martensitic transformation initiation temperature range of 280–320℃ for low-strength hot-rolled eutectoid pearlitic steel rails, while this temperature is above 220–250℃ for high-strength hot-rolled eutectoid pearlitic steel rails. Simultaneously, the cooling rate of the HAZ on the low-strength hot-rolled eutectoid pearlitic steel rail side in the first cooling stage is 4.0℃ / s, while the cooling rate of the HAZ on the high-strength hot-rolled eutectoid pearlitic steel rail side in the first cooling stage is 2.5℃ / s. Therefore, a small amount of martensite forms in the HAZ on the low-strength hot-rolled eutectoid pearlitic steel rail side, while no martensite forms in the HAZ on the high-strength hot-rolled eutectoid pearlitic steel rail side. Hardness tests showed that the longitudinal hardness of the rail joint within a ±15mm radius of the weld center met 125% and 112% of the average hardness of the base metal for the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side was 8 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. The formation of a small amount of brittle martensite in the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is detrimental to the service safety of the rail joint in railway operations.

[0106] Comparative Example 3

[0107] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0108] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 245℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0109] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0110] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0111] For the welded joints of the rails in this comparative example, the final cooling temperature of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic rail side (245℃) during the first cooling stage is below the martensitic transformation initiation temperature range of 280–320℃ for low-strength hot-rolled eutectoid pearlitic rails, while this temperature is between 220–250℃ for high-strength hot-rolled eutectoid pearlitic rails. Simultaneously, the cooling rate of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side during the first cooling stage is 4.0℃ / s, while the cooling rate of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side during the first cooling stage is 2.5℃ / s. Therefore, a small amount of martensite is formed in both the HAZ on the low-strength and high-strength hot-rolled eutectoid pearlitic rail sides. Hardness tests show that the longitudinal hardness of the rail joint within a 15mm radius from the weld center meets 130% and 117% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail base material and high-strength hot-rolled eutectoid pearlitic rail base material, respectively. Furthermore, the average hardness of the heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 10 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Because a small amount of brittle martensite structure forms in the heat-affected zones on both sides of the weld joint, it is detrimental to the service safety of the rail joint in railway operations.

[0112] Comparative Example 4

[0113] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, and 0.08% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1180 MPa and a hardness of 350 HV at room temperature (20–30°C).

[0114] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at a first cooling rate of 2.5℃ / s. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s respectively, and the heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0115] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 2.5℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0116] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0117] For the dissimilar rail welded joints in this comparative example, the cooling rate of the heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is relatively low, resulting in insufficient hardness increase during cooling. Within a ±15mm radius of the weld center, the longitudinal hardness of the rail joint meets 105% of the average hardness of the base materials for both the low-strength and high-strength hot-rolled eutectoid pearlitic rails. Furthermore, the average hardness of the heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 42 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the heat-affected zones on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. The difference in average hardness between the weld heat-affected zone on one side of high-strength hot-rolled eutectoid pearlitic steel rail and the weld heat-affected zone on the other side of low-strength hot-rolled eutectoid pearlitic steel rail is significant. The average hardness of the weld heat-affected zone on the other side of low-strength hot-rolled eutectoid pearlitic steel rail is relatively low, which is detrimental to the service safety of the rail joint in the railway.

[0118] Comparative Example 5

[0119] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0120] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 990℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0121] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0122] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0123] For the rail welded joint in this comparative example, the surface temperature of the rail welded joint was slightly lower before the start of the first stage of cooling, resulting in insufficient driving force for pearlite phase transformation during subsequent cooling, insufficient refinement of the pearlite lamellar spacing, and insufficient hardness improvement in the heat-affected zones on both sides of the weld. Hardness tests showed that the longitudinal hardness of the rail joint within ±15mm from the weld center met 104% and 99% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlite rail and high-strength hot-rolled eutectoid pearlite rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone on the low-strength hot-rolled eutectoid pearlite rail side was 6 HV higher than that on the high-strength hot-rolled eutectoid pearlite rail side. At the same time, the metallographic structure of the heat-affected zones on both sides of the weld was visible pearlite, without any abnormal structures such as martensite. Because the hardness of the heat-affected zone on both sides of the weld joint obtained in this comparative example does not meet the requirement that the longitudinal hardness of the rail joint reaches 106-110% and 100-105% of the average hardness of the base material of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively, this comparative example is not conducive to the service safety of the rail joint in railways.

[0124] Comparative Example 6

[0125] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0126] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.2℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.2℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0127] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.2℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at rates of 1.4℃ / s and 1.2℃ / s for the heat-affected zones on both sides of the weld, respectively. Heat treatment was completed when the surface temperature of the weld joint dropped to 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. Infrared thermometers are used to monitor the temperature of the rail head tread.

[0128] The rail joints in this comparative example were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0129] For the dissimilar rail welded joints in this comparative example, the longitudinal hardness of the rail joint within a ±15mm radius of the weld center meets 106% and 94% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic rail side is 28 HV higher than that on the high-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. However, since the hardness of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 94% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 100-105% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joints in railways.

[0130] Comparative Example 7

[0131] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0132] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 530℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0133] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0134] The rail joints in this comparative example were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0135] For the dissimilar rail welded joints in this comparative example, the high final cooling temperature in the first stage resulted in insufficient refinement of the pearlitic structure during cooling, leading to a less significant increase in hardness. Hardness tests showed that the longitudinal hardness of the rail joint within a ±15mm radius of the weld center met 102% and 94% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side was 13 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the weld heat-affected zones on both sides of the rails was visible pearlite, without any abnormal structures such as martensite. Because the hardness of the heat-affected zone on both sides of the weld joint obtained in this comparative example does not meet the requirement that the longitudinal hardness of the obtained rail joint reaches 106-110% and 100-105% of the average hardness of the base material of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively, this comparative example is not conducive to the service safety of the rail joint in railways.

[0136] Comparative Example 8

[0137] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0138] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 2.5℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0139] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 2.5℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 10℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0140] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0141] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 104% and 103% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 37 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the metallographic structure of the heat-affected zones on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 104% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0142] Comparative Example 9

[0143] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.75% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1048 MPa and a hardness of 307 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, and 0.08% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1180 MPa and a hardness of 350 HV at room temperature (20–30°C).

[0144] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0145] During post-weld cooling, the first stage of cooling involved a rail-contouring temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contouring temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contouring temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0146] The rail joints obtained from the comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0147] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 113% and 105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the high-strength hot-rolled eutectoid pearlitic rail side is 21 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the metallographic structure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 113% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0148] Comparative Example 10

[0149] In this comparative example, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.67% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 977 MPa and a hardness of 278 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0150] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 15°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0151] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to a minimum ambient temperature of 15℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0152] The rail joints used in the comparative example were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness test - Part 1: Test method", using the HV scale. Referring to the sampling method shown in Figure 3, the metallographic structure of the rail joint specimens was examined according to GB / T13298-2015 "Metallic materials - Microstructure test method". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure of the rail joints was observed using a Leica MeF3 optical microscope.

[0153] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 104% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the high-strength hot-rolled eutectoid pearlitic rail side is 31 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 104% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0154] By comparing the longitudinal hardness of the rail head tread and the metallographic structure of the welded joint obtained from various embodiments and comparative examples, it can be seen that: The post-weld cooling treatment method provided by this invention, applied to welded joints of a low-strength hot-rolled eutectoid pearlitic rail and a high-strength hot-rolled eutectoid pearlitic rail, ensures that the heat-affected zone (HAZ) on both sides of the rail joint weld is composed solely of pearlite, without martensite or other abnormal structures. Simultaneously, the longitudinal hardness of the rail joint within ±15mm of the weld center can reach 106–110% and 100–105% of the average hardness of the corresponding low-strength and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. The hardness difference between the HAZ on both sides of the rail joint weld is within 30 HV, improving the hardness difference between the HAZ on both sides of the weld of dissimilar rail joints, thereby achieving good hardness matching between the HAZ on both sides of the weld of dissimilar strength rails and ensuring railway operation safety.

[0155] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A postweld cooling method for a heterogeneous strength hot-rolled ferrite-pearlite steel rail welded joint, characterized by, The method comprises the following steps in sequence: S1, a first stage cooling is performed on a welded joint formed by welding a low-strength hot-rolled proeutectoid pearlite rail base material and a high-strength hot-rolled proeutectoid pearlite rail base material, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃, wherein a first heat affected zone corresponding to the low-strength hot-rolled proeutectoid pearlite rail base material is cooled at a first cooling speed, and a second heat affected zone corresponding to the high-strength hot-rolled proeutectoid pearlite rail base material is cooled at a second cooling speed smaller than the first cooling speed; S2, a second stage cooling is performed on the welded joint, so that the surface temperature of the welded joint is reduced from 400-500℃ to lower than or equal to 220℃, wherein the cooling speed of the welded joint is smaller than or equal to 1.4℃ / s; S3, the welded joint is naturally cooled to ambient temperature.

2. The method of claim 1, wherein, The first cooling speed is 3.0-4.0℃ / s, and the second cooling speed is 1.5-2.5℃ / s.

3. The method of claim 1, wherein, The cooling speed of the welded joint in step S2 is 0.8-1.4℃ / s.

4. The method of claim 1, wherein, The method is performed in a field low-temperature environment with a temperature of 1-15℃, and a profiling temperature control device is used to perform full-section heat compensation on the welded joint in steps S1 and S2.

5. The method of claim 4, wherein, The profiling temperature control device comprises: two pivotally connected housings, each having a shape matching the first heat affected zone and the second heat affected zone, and each being provided with a plurality of heating portions; a control portion configured to control the heating portions of the two housings respectively.

6. The method of claim 5, wherein, The heating portions of the two housings corresponding to the regions of the rail head are smaller in size and more densely distributed than the heating portions corresponding to the regions of the rail waist and rail bottom.

7. The method of claim 1, wherein, The low-strength hot-rolled proeutectoid pearlite rail base material has a tensile strength of 980-1050MPa and a hardness of 280-310HV at room temperature; and the high-strength hot-rolled proeutectoid pearlite rail base material has a tensile strength of 1100-1180MPa and a hardness of 320-350HV at room temperature.

8. The method of claim 1, wherein, The chemical composition of the low-strength hot-rolled proeutectoid pearlite rail base material comprises: 0.68-0.74% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.04-0.08% of V, and the balance of Fe and inevitable impurities; The chemical composition of the high-strength hot-rolled proeutectoid pearlite rail base material comprises: 0.76-0.82% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.30-0.50% of Cr, 0.04-0.08% of V, and the balance of Fe and inevitable impurities.

9. The method of claim 1, wherein, The low-strength hot-rolled proeutectoid pearlite rail base material and the high-strength hot-rolled proeutectoid pearlite rail base material are of the same rail type and have a gauge of 60-75kg / m, and the welded joint is formed by moving flash butt welding.

10. A dissimilar strength hot-rolled ferrite-pearlite steel rail welded joint, characterized in that, The welded joint is cooled by the post-weld cooling method according to any one of claims 1-9; the heat-affected zone on both sides of the weld of the welded joint is pearlite without martensite; the longitudinal hardness of the region corresponding to the low-strength hot-rolled type of the parent material of the proeutectoid pearlite rail and from the center of the weld to 15 mm away from the center of the weld reaches 106-110% of the average hardness of the low-strength hot-rolled type of the parent material of the proeutectoid pearlite rail; the longitudinal hardness of the region corresponding to the high-strength hot-rolled type of the parent material of the proeutectoid pearlite rail and from the center of the weld to 15 mm away from the center of the weld reaches 100-105% of the average hardness of the low-strength hot-rolled type of the parent material of the proeutectoid pearlite rail; and the hardness difference of the heat-affected zone on both sides of the weld is within 30 HV.

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