A heat treatment method for eutectoid pearlitic rail and bainitic rail welded joint

Abstract

This invention belongs to the field of railway rail welding technology, specifically relating to a heat treatment method for welded joints of eutectoid pearlitic and bainitic rails. The method includes: S1, subjecting the welded rail joint to a first-stage cooling process to reduce the surface temperature of the rail joint to 460-500°C; S2, subjecting the welded joint treated in S1 to a second-stage cooling process to reduce the surface temperature of the rail joint to 351-389°C; S3, subjecting the welded joint treated in S2 to isothermal heat treatment, followed by a third-stage cooling process to cool the rail joint to room temperature. This invention offers a low-cost method, and the heat-treated welded joint exhibits good hardness properties.

Description

Technical Field

[0001] This invention belongs to the field of railway rail welding technology, specifically relating to a heat treatment method for welded joints of eutectoid pearlitic steel rails and bainitic steel rails, and more particularly to a heat treatment method for welded joints of hot-rolled eutectoid pearlitic steel rails and heat-treated bainitic steel rails. Background Technology

[0002] Based on differences in manufacturing processes, rails can be subdivided into hot-rolled and heat-treated rails. Different railway line conditions have varying requirements for rail materials and performance. 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. For 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 different materials / strength grades.

[0003] 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. Combined with the effects of multiple alloys in the rail steel, the overall hardness of the weld HAZ is higher than that of the base rail material used for welding; that is, the overall hardness of the HAZ of hot-rolled rails is higher than that of the base rail material. Conversely, for heat-treated rails, the hardened layer that originally belonged to the base rail material is damaged under the welding thermal cycle, resulting in coarser austenite grains and a larger pearlite lamellar spacing in the weld HAZ than in the base rail material. Consequently, the overall hardness of the HAZ of heat-treated rails is lower than that of the base rail material used for welding. For two rails with different materials and strength grades, the differences between the base materials pose challenges to their welding. During railway line service, welded rail joints are prone to "saddle-shaped" wear, which preferentially forms in the low-hardness areas of the rail head tread. This not only increases wheel-rail impact but also affects the service life of the rail and may even endanger traffic safety. Therefore, restoring the strength and hardness lost due to welding after the rail is completed becomes a prerequisite for the rail's application.

[0004] In recent years, the rapid development of railways has placed higher demands on the strength, hardness, wear resistance, and fatigue resistance of rail base materials and welded joints. Due to their good strength and toughness and moderate overall performance, eutectoid pearlitic rails are widely used in both conventional and high-speed railways both domestically and internationally. These rails typically have a carbon content in the range of 0.72% to 0.82% by weight, and their microstructure is pearlitic. However, the mechanical and weldability of traditional eutectoid pearlitic rails have almost reached their limits. Under these circumstances, bainitic rails, which offer higher strength and good wear resistance and contact fatigue resistance, have emerged. These rails typically have a carbon content in the range of 0.20% to 0.40% by weight, and their microstructure is a multiphase structure composed of bainite, a small amount of martensite (or martensite-austenite islands), and retained austenite. Public literature and technical data rarely report on the joint welding application of hot-rolled eutectoid pearlitic rails and heat-treated bainitic rails on railway lines.

[0005] Existing technologies disclose some heat treatment techniques for rail welding. Among them, CN201610909362.1 discloses a post-weld heat treatment method for welded joints of hypereutectoid steel rails and PG4 heat-treated eutectoid pearlitic steel rails. However, the above invention is not only complex in operation and implementation, but also has high cost. CN201410135909.8 discloses a heat treatment method for welded joints of bainitic steel rails and describes the heat treatment principle of rail joints. Compressed air is used as a cooling medium to rapidly cool the welded area in order to restore the mechanical properties of the rails that have been reduced due to welding. However, this technology is applicable to bainitic steel rails with the same material. CN201810581145.3, CN201810720765.0 and CN201810710040.3 introduce post-weld heat treatment methods for welded joints of hypereutectoid steel rails and eutectoid steel rails. However, the microstructure of both hypereutectoid steel rails and eutectoid steel rails in the above patents is mainly pearlitic.

[0006] Therefore, the railway engineering field urgently needs a heat treatment method suitable for welded joints of dissimilar materials rails formed by welding heat-treated bainitic rails and hot-rolled eutectoid pearlitic rails, in order to improve the hardness of the rails reduced by welding and thus ensure the service safety of dissimilar rail welded joints. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a post-weld heat treatment method for dissimilar rail welded joints composed of hot-rolled eutectoid pearlitic steel rails and heat-treated bainitic steel rails, which has low cost and good hardness performance of the welded joint after heat treatment.

[0008] To address the aforementioned technical problems, this invention provides a heat treatment method for welded joints of eutectoid pearlitic steel rails and bainitic steel rails, the method comprising:

[0009] S1. Perform the first stage of cooling on the welded rail joint to be cooled, so that the surface temperature of the rail joint is reduced to 460~500℃.

[0010] S2. The welded joint treated in S1 is cooled in the second stage to reduce the surface temperature of the rail welded joint to 351~389℃.

[0011] S3. The welded joint after S2 treatment is subjected to isothermal heat treatment, followed by a third stage of cooling to cool the rail welded joint to room temperature.

[0012] In some embodiments, in step S1, the initial surface temperature of the rail welded joint to be cooled is 1000℃~1050℃.

[0013] In some embodiments, in step S1, the first stage of cooling uses compressed air as the cooling medium to cool the rail joint at a rate of 1.8~7.0℃ / s.

[0014] In some embodiments, in S2, the cooling rate of the second stage of cooling is 0.7~0.9℃ / s.

[0015] In some embodiments, the isothermal heat treatment temperature in step S3 is 300~350℃.

[0016] In some embodiments, the isothermal heat treatment time in step S3 is 0.8 to 1.2 hours.

[0017] In some embodiments, the first stage cooling includes artificial forced cooling using compressed air as the cooling medium, the second stage cooling includes natural cooling, and the third stage cooling is natural cooling.

[0018] In some embodiments, the base material of the eutectoid pearlitic rail, by mass percentage, comprises: 0.75-0.82% 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.

[0019] In some embodiments, the base material of the bainitic rail, by mass percentage, comprises: 0.20-0.30% C, 1.3-1.9% Si, 1.4-2.0% Mn, 0.70-1.20% Cr, 0.20-0.50% Mo, with the balance being Fe and unavoidable impurities.

[0020] In some embodiments, the eutectoid pearlitic rail is a hot-rolled eutectoid pearlitic rail, and the bainitic rail is a heat-treated bainitic rail. The welded joints of the eutectoid pearlitic rail and the bainitic rail have the same specifications, both being 60~75 kg / m.

[0021] The beneficial effects of this invention are as follows: This invention uses the residual heat from rail welding as the heat source for post-weld heat treatment of the joint. By controlling the cooling rate at different temperature ranges and controlling the phase transformation process, post-weld heat treatment of dissimilar rail joints is achieved. Through controlled cooling and isothermal heat treatment at different temperature ranges after welding, the heat-affected zones on both sides of the rail joint weld maintain high strength and hardness, thus ensuring the wear resistance of the rail joint. It ensures that the weld heat-affected zone microstructure on the side of the hot-rolled eutectoid pearlitic rail joint is pearlitic, without martensite. It also ensures that the weld heat-affected zone microstructure on the side of the heat-treated bainitic rail joint is bainitic, without obvious blocky martensite. Simultaneously, the longitudinal hardness of the rail joint within ±20mm from the weld center can reach 100-110% and 87-92% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively, contributing to ensuring railway operation safety. Attached Figure Description

[0022] To gain a more complete understanding of the embodiments of the present invention, reference should be made to the embodiments described in more detail in the accompanying drawings and by way of example below, wherein:

[0023] Figure 1 The diagram shows the longitudinal hardness test location 5mm below the rail head tread of the rail joint. Figure 2 The diagram shows the sampling location of the metallographic specimen on the rail head tread of the welded rail joint.

[0024] Figure 3 The figure shown is a three-dimensional structural schematic diagram of the rail profile cooling device of the present invention.

[0025] Figure 4 The diagram shown is a schematic representation of the use of the rail profile cooling device of the present invention.

[0026] Figure 5 The diagram shows the distribution of electric heaters in the rail head area of ​​the split rail contour heating device used in the isothermal heat treatment of this invention.

[0027] Figure 6 The diagram shown is an overall schematic of the split-type rail contour heating device used in the isothermal heat treatment of this invention.

[0028] Figure 7 The diagram shows a flowchart of the method of the present invention.

[0029] Figure 8aThe image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Embodiment 1 of the present invention.

[0030] Figure 8b The image shows the metallographic structure of the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail weld joint in Embodiment 1 of the present invention.

[0031] Figure 9a The image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Embodiment 2 of the present invention.

[0032] Figure 9b The image shows the metallographic structure of the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail weld joint in Embodiment 2 of the present invention.

[0033] Figure 10a The image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Comparative Example 1 of the present invention.

[0034] Figure 10b The image shows the metallographic structure of the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail weld joint in Comparative Example 1 of the present invention.

[0035] Figure 11a The image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Comparative Example 2 of the present invention.

[0036] Figure 11b The image shows the metallographic structure of the heat-affected zone on the weld side of the hot-rolled eutectoid pearlite rail in Comparative Example 2 of the present invention.

[0037] Figure 12a The image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Comparative Example 5 of the present invention.

[0038] Figure 12b The image shows the metallographic structure of the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail weld joint in Comparative Example 5 of the present invention.

[0039] Figure 13a The image shows the metallographic structure of the heat-affected zone on one side of the bainitic rail weld joint in Comparative Example 7 of the present invention.

[0040] Figure 13b The image shows the metallographic structure of the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail weld joint in Comparative Example 7 of the present invention.

[0041] Figure label:

[0042] 1. Heat-treated bainitic steel rail; 2. Hot-rolled eutectoid pearlitic steel rail; 3. Weld center; 4. Rail head tread; 5. Bottom nozzle; 6. Side nozzle; 7. Compressed air channel one; 8. Compressed air channel two; 9. Rail; 10. Rail head tread heating area; 11. Rail head side heating area; 12. Rail head lower jaw heating area; 13. Tracked ceramic heater; 14. Terminal block; 15. Heater; 16. Rotating shaft; 17. Fixing shackle; 18. Device housing; 19. Asbestos insulation layer. Detailed Implementation

[0043] Embodiments of the invention are described below. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various alternative forms. The drawings are not necessarily drawn to scale; certain functions may be exaggerated or minimized to show details of specific components. Therefore, the specific structural and functional details disclosed herein should not be construed as limiting, but merely as a representative basis for teaching those skilled in the art to use the invention in various ways. As those skilled in the art will understand, various features shown and described with reference to any of the drawings may be combined with features shown in one or more other drawings to produce embodiments not explicitly shown or described. The combinations of features shown provide representative embodiments for typical applications. However, various combinations and modifications of features consistent with the teachings of the invention may be desired for certain particular applications or implementations.

[0044] Furthermore, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or action from another, and do not necessarily require or imply any actual such relationship or order between these entities or actions. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements may include not only those elements but also elements not expressly listed or inherent to such processes, methods, articles, or apparatuses.

[0045] One or more embodiments of the present invention will now be described with reference to the accompanying drawings. The flowcharts illustrate the process performed by the system according to the present invention. It is understood that the execution of the flowcharts may omit one or more steps, or may add one or more steps, and may be performed in a sequential or reverse order, or even simultaneously in some embodiments.

[0046] Domestic and international rail welding standards, such as the Chinese railway rail welding standard TB / T1632-2014, stipulate that the microstructure of the entire cross-section of pearlitic rail welded joints should be predominantly pearlitic, with a small amount of proeutectoid ferrite allowed, but martensite must not be present. In contrast, the Australian railway rail welding standard AS1085.20-2012 stipulates that for certain high-strength, high-carbon, and high-alloy rails, under a 100x metallographic microscope, the percentage of martensite in the most severely martensitic area of ​​the rail welded joint must not exceed 5%. Otherwise, the joint will have a high probability of premature fatigue fracture during railway service due to a large amount of brittle and hard martensite, affecting railway operation safety. Therefore, strictly controlling the martensite content in the rail weld microstructure is crucial for railway operation safety.

[0047] It should be noted that, compared with traditional hot-rolled high-carbon pearlitic steel rails (carbon content greater than 0.6 wt%) and traditional hot-rolled medium-low carbon bainitic steel rails (carbon content 0.2~0.4 wt%), this invention utilizes online heat treatment technology to leverage the residual heat from rail rolling, which can accelerate the phase transformation rate of supercooled austenite to pearlite or to bainite, significantly refining the pearlite lamellar spacing or bainite lath width, achieving a fine-grain strengthening effect, thereby obtaining excellent strength and toughness. Therefore, steel rails produced using online heat treatment technology will gradually become the future development trend for high-strength, high-wear-resistant steel rail production.

[0048] It should be noted that after the rail welding is completed, due to convection, radiation, and heat conduction with the surrounding medium, the rail joint exhibits a phenomenon of rapid cooling at high temperatures and a gradually slowing cooling rate at low temperatures during natural cooling in an air environment of 20-30℃. Specifically: the average cooling rate during natural cooling is 4.0-2.5℃ / s in the temperature range of 1100-801℃; 2.4-1.2℃ / s in the temperature range of 800-501℃; 1.1-0.7℃ / s in the temperature range of 500-301℃; and 0.6-0.1℃ / s in the temperature range of 300-20℃.

[0049] In this invention, the critical cooling rate for martensitic transformation of the heat-treated bainitic rail steel is 1.0~1.5℃ / s, and the martensitic transformation initiation temperature is 270~300℃; in this invention, the critical cooling rate for martensitic transformation of the hot-rolled eutectoid pearlitic rail steel is 1.8~2.5℃ / s, and the martensitic transformation initiation temperature is 180~230℃. Therefore, for the cooling process of the heat treatment of dissimilar material rail welded joints formed by welding hot-rolled eutectoid pearlitic rails and heat-treated bainitic rails, it is necessary to limit the cooling process to heat-treated bainitic rail steel with a lower critical cooling rate for martensitic transformation, and the final cooling temperature of the second stage of cooling must be controlled above 270~300℃ to avoid the formation of brittle and hard large-sized martensite structures during the heat treatment cooling process, which would affect the service safety of the rail joint.

[0050] Based on the above findings, this invention discloses a heat treatment method for welded joints of eutectoid pearlitic and bainitic steel rails, such as... Figure 7 As shown, the method includes:

[0051] S1. Perform the first stage of cooling on the welded rail joint to be cooled, so that the surface temperature of the rail joint is reduced to 460~500℃.

[0052] S2. The welded joint treated in S1 is cooled in the second stage to reduce the surface temperature of the rail welded joint to 351~389℃.

[0053] S3. The welded joint after S2 treatment is subjected to isothermal heat treatment, followed by a third stage of cooling to cool the rail welded joint to room temperature.

[0054] In this invention, the rail joint obtained by welding is first cooled to a temperature of 460~500℃ (which can be 460℃, 470℃, 480℃, 490℃, or 500℃), above the martensitic transformation initiation temperature of the hot-rolled eutectoid pearlitic rail steel and the heat-treated bainitic rail steel involved. This first-stage cooling refines the pearlite lamellar spacing in the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail, thereby improving the strength, hardness, and toughness of the welded heat-affected zone of the hot-rolled eutectoid pearlitic rail. Furthermore, at the temperature of 460℃~500℃, the pearlite transformation is completed within the welded heat-affected zone on the hot-rolled eutectoid pearlitic rail side of the joint.

[0055] In some embodiments, in S1, the initial surface temperature of the rail welded joint to be cooled is 1000℃~1050℃ (which can be 1000℃, 1025℃, or 1050℃).

[0056] This invention addresses the rapid post-weld cooling of rail joints with high residual temperature (1000℃~1050℃) to reduce the phase transformation temperature of austenite to pearlite within the weld heat-affected zone, thereby increasing the hardness of the austenite recrystallization zone.

[0057] In some embodiments, in S1, the first stage of cooling uses compressed air as the cooling medium to cool the rail joint, and the cooling rate is 1.8~7.0℃ / s.

[0058] Compared to natural cooling (i.e., air cooling), the cooling rate is higher when using compressed air as the cooling medium. Above the austenitizing temperature of rail steel, when using compressed air as the cooling medium, the spacing between pearlite lamellars can be refined through a relatively large degree of supercooling (i.e., a relatively large cooling rate), thereby improving the hardness, strength, and other mechanical properties of the heat-affected zone on one side of the eutectoid pearlite rail.

[0059] In some embodiments of the present invention, in S1, the first stage of cooling is the cooling of the rail welded joint within a profile cooling device (such as...). Figure 3 and Figure 4 As shown in the figure, the cooling rate can be adjusted by controlling the pressure of the compressed air flowing into the molding device, thereby controlling the cooling rate of the first stage.

[0060] In this invention, Figure 3 and Figure 4 The rail profile cooling device in the middle only cools the rail head tread and side of the rail welded joint. Its bottom nozzle 5 and side nozzle 6 are located by... Figure 3 and Figure 4 As shown, the orifice sizes of compressed air passage 7 and compressed air passage 8 can be designed and manufactured according to actual needs to achieve cooling intensities of different strengths. The compressed air flowing through compressed air passage 7 and compressed air passage 8 has the same gas pressure, which can be monitored by a pressure gauge and adjusted according to actual needs.

[0061] In some embodiments, in S2, the second stage of cooling is natural cooling with a cooling rate of 0.7~0.9℃ / s, and the starting temperature of the second cooling stage in S2 is 460~500℃. In some embodiments, the second stage of cooling is achieved by closing the cooling channel of the rail profile cooling device after the first stage of cooling, allowing the rail welded joint to undergo natural cooling in the air environment.

[0062] In some embodiments, in S3, the isothermal heat treatment temperature is 300°C to 350°C (e.g., 300°C, 310°C, 320°C, 330°C, 340°C, or 350°C).

[0063] In this invention, the temperature is controlled at 300~350℃ during isothermal heat treatment to obtain a lath bainitic microstructure with good strength and toughness. This isothermal heat treatment temperature range can promote bainite transformation, stabilize the residual austenite in bainitic steel, reduce the overall residual stress of the welded joint, and appropriately improve the strength, hardness, and toughness of the heat-affected zone of the heat-treated bainitic rail steel.

[0064] In some embodiments of the present invention, the isothermal heat treatment process requires the use of a contour heating device (such as...). Figure 5 and Figure 6 (as shown) Figure 5 This is a schematic diagram showing the distribution of electric heaters in the rail head area of ​​the split-type rail contour heating device used in the isothermal heat treatment of this invention. The deheating area of ​​this device is as follows: Figure 5 As shown, the device includes a rail head tread heating area 10, a rail head side heating area 11, a rail head lower jaw heating area 12, and a tracked ceramic heater 13. This contour heating device is 5mm away from the surface of the rail welded joint, ensuring good heat conduction. Under the continuous heating of the tracked heating element, an isothermal heat treatment process can be achieved. The layout of the heating areas for the rail head, rail web, and rail bottom is similar. Because the rail head is thicker and heat transfer is slower, the tracked ceramic heater 13 covering the rail head has more ceramic heaters than the ceramic heaters in the rail web and rail bottom areas to ensure that the entire rail cross-section is fully heated.

[0065] Figure 6This is a schematic diagram of the split-type rail contour heating device used for isothermal heat treatment in this invention. The device includes: terminals 14, heaters 15, rotating shafts 16, fixing rings 17, a housing 18, and an asbestos insulation layer 19. There are two terminals 14 that form a circuit, allowing several parallel-arranged heaters 15 to be connected together in parallel. The heaters 15 are tracked ceramic heaters, and the housing 17 is welded from thin metal plates. This device has advantages such as compactness, flexibility, and low cost, facilitating field construction. It can be powered by a diesel generator or a 220V power supply, with a rated power of 10kW. The heaters 15 use commercial LCD tracked ceramic heaters as the heat source, and the heaters are ceramic sheets with dimensions of 10mm (length) × 10mm (width) × 6mm (thickness). Combined with insulation material and a steel structure housing, it forms a ring-shaped split heater, facilitating assembly and disassembly, and is suitable for full-section 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 according to the actual dimensions of the rail profile. It should be noted that this device can achieve isothermal (constant temperature) heat treatment of rail welded joints through program settings and continuous heat compensation during power-on. In the device design, multiple sets of tracked heaters are evenly fixed inside a device with a rail-like profile, ensuring the heaters cover and fully adhere to the rail surface to achieve good heat conduction during heating. Based on this device, isothermal heat treatment of rail welded joints can be achieved.

[0066] During the experiment, a temperature controller was used to control the heating temperature. The operating temperature range of this device is 200~600℃. This split-type device can rotate a maximum of 180° around its rotating axis.

[0067] After the isothermal process is complete, remove the device. Place the welded joint in air for natural cooling, allowing the surface temperature to drop to 20-30°C. The cooling rate during this stage is 0.1-0.6°C / s. It should be noted that after the first and second cooling stages, the residual surface temperature of the rail welded joint is low. Therefore, when the joint is allowed to cool naturally in air after the isothermal heat treatment process, martensite will not form.

[0068] It should be noted that in this invention, when the isothermal heat treatment temperature is above 350℃ but below 500℃ (for example, 351~500℃), bainitic structure can be generated. However, the mechanical properties of the bainitic structure (coarse lath bainite) generated during this temperature range are lower than those of the bainitic structure (fine lath bainite) generated when the isothermal heat treatment temperature is controlled at 300~350℃. Therefore, in this invention, the isothermal heat treatment temperature is controlled within the temperature range of 300~350℃.

[0069] It should be noted that during the removal of the contour cooling device and the installation of the contour heating device, the time consumed due to factors such as the operator's skill level and proficiency will inevitably lead to a temperature drop of 30-50°C on the surface of the rail joint.

[0070] In some embodiments, the isothermal treatment time in S3 is 0.8~1.2h.

[0071] In this invention, when the holding time of the isothermal process exceeds 1.2 hours, the joint strength, hardness, and toughness remain essentially unchanged, and the long holding time will severely reduce the production efficiency of rail joint heat treatment. Therefore, this invention preferably controls the holding time of the isothermal heat treatment to 0.8~1.2 hours, and in some embodiments, the holding time of the isothermal heat treatment is controlled to 1 hour.

[0072] In some embodiments, in S1, the first stage of cooling uses compressed air as the cooling medium to cool the rail joint, and the cooling rate is 1.8~7.0℃ / s.

[0073] Based on the principles of metallurgy, rail joints exhibit a certain degree of dynamic undercooling under high temperature and rapid cooling conditions after welding. This causes the phase transformation temperature of austenite to pearlite to shift downward in the non-equilibrium state, and the phase transformation temperature gradually decreases as the undercooling increases.

[0074] In some embodiments of the present invention, the cooling rate can be adjusted by controlling the magnitude of the compressed air pressure, thereby achieving control of the first-stage cooling rate. After the rail is welded, the rail joint, whose surface residual temperature is still above the austenitizing temperature, is subjected to the first-stage cooling. By using a relatively high cooling rate (higher than the critical cooling rate for the martensitic transformation of the rail steel), the pearlite lamellar spacing is refined, thereby improving the strength, hardness, and toughness of the heat-affected zone.

[0075] In some embodiments, rail joints with an initial welding temperature of 1000-1050°C are cooled in a first stage to a temperature of 460-500°C, which is above the martensitic transformation initiation temperature of the hot-rolled eutectoid pearlitic rail steel and the heat-treated bainitic rail steel involved. This stage refines the pearlite lamellar spacing in the heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail by employing a relatively large cooling rate (i.e., 1.8-7.0°C / s), thereby improving the strength, hardness, and toughness of the welded heat-affected zone of the hot-rolled eutectoid pearlitic rail. Furthermore, at the 460-500°C temperature, the pearlite transformation is completed within the welded heat-affected zone on the hot-rolled eutectoid pearlitic rail side of the joint. To ensure a cooling rate of 1.8-7.0°C / s in the first cooling stage, the following method can be used: the profile cooling device is 20 mm away from the surface of the rail weld joint; the pressure of the compressed air ejected by the cooling device is 0.18-0.70 MPa.

[0076] In some embodiments, the first stage cooling includes artificial forced cooling using compressed air as the cooling medium, the second stage cooling includes natural cooling, and S3 cooling is natural cooling.

[0077] In some embodiments, the second stage of cooling is the natural cooling of the rail welded joint in the air environment by closing the cooling channel of the rail profile cooling device after the first stage of cooling is completed.

[0078] In some embodiments, the base material of the eutectoid pearlitic rail, by mass percentage, comprises: 0.75-0.82% 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.

[0079] In some embodiments, the base material of the bainitic rail, by mass percentage, comprises: 0.20-0.30% C, 1.3-1.9% Si, 1.4-2.0% Mn, 0.70-1.20% Cr, 0.20-0.50% Mo, with the balance being Fe and unavoidable impurities.

[0080] In some embodiments, the eutectoid pearlitic rail is a hot-rolled eutectoid pearlitic rail, and the bainitic rail is a heat-treated bainitic rail. The specifications of the welded joints of the eutectoid pearlitic rail and the bainitic rail are the same, both being 60~75kg / m.

[0081] In some embodiments, the rail welded joint is welded using a rail moving flash welding machine.

[0082] like Figure 1 As shown, in some embodiments, the rail welded joint of the present invention is a joint formed by welding a heat-treated bainitic steel rail 1 with the same rail type and a specification of 60~75kg / m to a hot-rolled eutectoid pearlitic steel rail 2 using a rail moving flash welding machine. The welded joint includes a region with a length of 60~80mm, including the weld and / or heat-affected zone, and the weld center 3 of this region is as follows: Figure 1 As shown. In this invention, "room temperature" refers to a temperature in the range of 20~30℃.

[0083] The heat-treated bainitic rail described in this invention is a heat-treated rail produced based on online heat treatment technology. Compared with traditional hot-rolled bainitic rails, the strong grain refinement strengthening effect introduced by online heat treatment technology can further improve the strength, hardness, and toughness of the rail on the basis of the mechanical properties of traditional hot-rolled rails.

[0084] Regarding this invention, it should be added that heat treatment technology itself is a process of controlling various factors during heating and cooling. The steps in heat treatment technology are interconnected and influence each other. This invention 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 of each invention patent differ (including rail mechanical properties and temperature distribution), and the cooling devices and implementation processes used are also different, resulting in fundamental differences that prevent simple application of these processes.

[0085] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the embodiments described herein.

[0086] In this invention, "rail head" can be simply referred to as "rail head," which refers to the head of the rail welded joint, including the rail head tread and the side of the rail head, which is in contact with the wheel.

[0087] The sampling locations of the metallographic specimens on the rail head tread of the welded rail joints in Examples 1-3 and Comparative Examples 1-8 are shown in the diagram below. Figure 2 As shown in the diagram. 3 represents the weld center, and 4 represents the rail head tread.

[0088] In this invention, the longitudinal hardness of the heat-affected zone on both sides of the rail welded joint within ±20mm of the weld center meets the usage standard, mainly referring to the following indicators:

[0089] The longitudinal hardness of the heat-affected zone on both sides of the rail weld joint within ±20mm from the weld center should reach more than 100% and 87% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. The average hardness of the heat-affected zone of the hot-rolled eutectoid pearlitic rail weld should preferably be 100-110% of the hardness of the corresponding rail base material, while the average hardness of the heat-affected zone of the bainitic rail weld should preferably be 87-92% of the hardness of the corresponding rail base material.

[0090] Example 1

[0091] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1180 MPa and a hardness of 320 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.30% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1150 MPa and a hardness of 310 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.75% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities.

[0092] After the upsetting and push-off processes of the moving flash welding process, the welded joint of the 60kg / m rail undergoes heat treatment. First, the rail joint, with a residual temperature of 1000℃, is subjected to a first-stage cooling at a first cooling rate of 1.8℃ / s to reduce the surface temperature of the rail head to 460℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 351°C. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a temperature drop of 51°C on the rail joint surface, with a cooling rate of 0.9°C / s. This means the surface temperature of the rail joint during isothermal heat treatment was 300°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 300°C for 1 hour. After the isothermal process, the rail profile heating device (shown) was removed. Figure 5 , Figure 6 As shown, the rail joint is naturally cooled to an ambient temperature of 20°C in the air at a cooling rate of 0.6°C / s, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0093] During post-weld heat treatment, the first cooling stage involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first cooling stage, the compressed air pressure sprayed by the cooling device is 0.18MPa. The second stage of cooling is achieved after the first stage of cooling is completed and the rail head profile cooling device is turned off. Figure 3 , Figure 4The cooling channel (shown) allows for natural cooling of the rail welded joint in the air environment. An infrared thermometer is used to monitor the rail head tread temperature. When the surface temperature of the rail welded joint drops to 300℃, isothermal heat treatment is performed. During isothermal heat treatment, the track-type ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0094] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0095] The rail welded joint obtained in this embodiment shows that the longitudinal hardness of the heat-affected zone within a 20mm radius from the weld center reaches 100% and 89% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Metallographic examination results are shown in […]. Figure 8a and Figure 8b As shown in the metallographic microscope, the weld heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail joint is composed of pearlite, with no martensite. Conversely, the HAZ on the other side of the heat-treated bainitic rail joint is composed of bainite, with no blocky martensite. The dissimilar rail welded joints obtained using this process help ensure railway operational safety.

[0096] Example 2

[0097] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1180 MPa and a hardness of 320 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.30% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1150 MPa and a hardness of 310 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.75% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities.

[0098] After the upsetting and push-off processes of the moving flash welding process, the welded joint of the 60kg / m rail is heat-treated. First, the rail joint, with a residual temperature of 1050℃, is subjected to a first-stage cooling at a first cooling rate of 1.8℃ / s to reduce the surface temperature of the rail head to 460℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 351°C. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a temperature drop of 0.9°C / s on the surface of the rail joint from 51°C, meaning the surface temperature of the rail joint during isothermal heat treatment was 300°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 300°C for 0.8 hours. After the isothermal process, the rail profile heating device (shown) was removed. Figure 5 , Figure 6 As shown, the rail joint is naturally cooled to an ambient temperature of 20°C in the air at a cooling rate of 0.6°C / s, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0099] During post-weld heat treatment, the first cooling stage involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first cooling stage, the compressed air pressure sprayed by the cooling device is 0.18MPa. The second stage of cooling is achieved after the first stage of cooling is completed and the rail head profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows for natural cooling of the rail welded joint in the air environment. An infrared thermometer is used to monitor the rail head tread temperature. When the surface temperature of the rail welded joint drops to 300℃, isothermal heat treatment is performed. During isothermal heat treatment, the track-type ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0100] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0101] The rail welded joint obtained in this embodiment shows that the longitudinal hardness of the heat-affected zone within a 20mm radius from the weld center reaches 100% and 90% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Metallographic examination results are shown in […]. Figure 9a and Figure 9b As shown in the metallographic microscope, the weld heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail joint is composed of pearlite, with no martensite. Conversely, the HAZ on the other side of the heat-treated bainitic rail joint is composed of bainite, with no blocky martensite. The dissimilar rail welded joints obtained using this process help ensure railway operational safety.

[0102] Example 3

[0103] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1180 MPa and a hardness of 320 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.30% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1150 MPa and a hardness of 310 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.75% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities.

[0104] After the upsetting and push-off processes of the moving flash welding process, the welded joint of the 60kg / m rail undergoes heat treatment. First, the rail joint, with a residual temperature of 1000℃, is subjected to a first-stage cooling at a first cooling rate of 1.8℃ / s to reduce the surface temperature of the rail head to 460℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4The cooling channel (shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 351°C. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a temperature drop of 51°C on the rail joint surface, with a cooling rate of 0.9°C / s. This means the surface temperature of the rail joint during isothermal heat treatment was 300°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 300°C for 0.98 hours. After the isothermal process, the rail profile heating device (shown) was removed. Figure 5 , Figure 6 As shown, the rail joint is naturally cooled to an ambient temperature of 20°C in the air at a cooling rate of 0.6°C / s, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0105] During post-weld heat treatment, the first cooling stage involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first cooling stage, the compressed air pressure sprayed by the cooling device is 0.18MPa. The second stage of cooling is achieved after the first stage of cooling is completed and the rail head profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows for natural cooling of the rail welded joint in the air environment. An infrared thermometer is used to monitor the rail head tread temperature. When the surface temperature of the rail welded joint drops to 300℃, isothermal heat treatment is performed. During isothermal heat treatment, the track-type ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0106] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0107] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone within a 20mm radius from the weld center, reaching 100% and 92% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. The metallographic morphology and type of the weld on both sides of the rail welded joint in this embodiment are the same as those in Example 1. Figure 8aand Figure 8b and in Example 2 Figure 9a and Figure 9b The metallographic structures on both sides of the weld seam of the rail welded joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. Conversely, the HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. The dissimilar rail welded joints obtained using this process help ensure railway operational safety.

[0108] Example 4

[0109] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1180 MPa and a hardness of 320 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 1.30% Si, 1.40% Mn, 0.70% Cr, 0.20% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1150 MPa and a hardness of 310 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.75% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities.

[0110] After the upsetting and push-off processes of the moving flash welding process, the welded joint of the 60kg / m rail undergoes heat treatment. First, the rail joint, with a residual temperature of 1000℃, is subjected to a first-stage cooling at a first cooling rate of 1.8℃ / s to reduce the surface temperature of the rail head to 460℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 351°C. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a temperature drop of 51°C on the rail joint surface, with a cooling rate of 0.9°C / s. This means the surface temperature of the rail joint during isothermal heat treatment was 300°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 300°C for 1.2 hours. After the isothermal process, the rail profile heating device (shown) was removed. Figure 5 , Figure 6As shown, the rail joint is naturally cooled to an ambient temperature of 20°C in the air at a cooling rate of 0.6°C / s, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0111] During post-weld heat treatment, the first cooling stage involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first cooling stage, the compressed air pressure sprayed by the cooling device is 0.18MPa. The second stage of cooling is achieved after the first stage of cooling is completed and the rail head profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows for natural cooling of the rail welded joint in the air environment. An infrared thermometer is used to monitor the rail head tread temperature. When the surface temperature of the rail welded joint drops to 300℃, isothermal heat treatment is performed. During isothermal heat treatment, the track-type ceramic heater in the contour heating device is 5mm away from the surface of the rail welded joint. This contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0112] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0113] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone within a 20mm radius from the weld center, reaching 100% and 87% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. The metallographic morphology and type of the weld on both sides of the rail welded joint in this embodiment are the same as those in Example 1. Figure 8a and Figure 8b and in Example 2 Figure 9a and Figure 9bThe metallographic structures on both sides of the weld seam of the rail welded joint shown are basically consistent. In this embodiment, because the isothermal treatment time is slightly longer than 0.8 h in Example 2 and 0.98 h in Example 3, the hardness of the heat-affected zone on the side of the heat-treated bainitic rail joint is slightly lower than that on the side of the heat-affected zone of the heat-treated bainitic rail joint in Examples 2 and 3. Under a metallographic microscope, the weld heat-affected zone on the side of the hot-rolled eutectoid pearlitic rail joint is pearlitic with no martensite. In contrast, the weld heat-affected zone on the side of the heat-treated bainitic rail joint is bainitic with no blocky martensite. The dissimilar rail welded joint obtained under this process helps ensure railway operation safety.

[0114] Example 5

[0115] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1280 MPa and a hardness of 360 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.30% C, 1.90% Si, 2.00% Mn, 1.20% Cr, 0.50% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1250 MPa and a hardness of 350 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.82% C, 0.80% Si, 1.00% Mn, 0.50% Cr, 0.08% V, with the balance being Fe and unavoidable impurities.

[0116] After the upsetting and push-off processes of the moving flash welding process, the welded joint of the 75kg / m rail is heat-treated. First, the rail joint, with a residual temperature of 1050℃, is subjected to a first-stage cooling at a first cooling rate of 7.0℃ / s to reduce the surface temperature of the rail head to 500℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (shown) allows the rail welded joint to cool naturally in air to a surface temperature of 389°C at a cooling rate of 0.7°C / s. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency caused the surface temperature of the rail joint to drop from 39°C, meaning the surface temperature of the rail joint during isothermal heat treatment was 350°C. The profile heating device was used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 350°C for 0.9 hours. After the isothermal process, the rail profile heating device (shown) was removed. Figure 5 , Figure 6As shown in the figure, the rail joint is allowed to cool naturally in the air to an ambient temperature of 30°C, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0117] During post-weld heat treatment, the first stage of cooling involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first cooling process, the compressed air sprayed by the cooling device has a pressure of 0.70MPa. The rail head profile cooling device is then turned off. Figure 3 , Figure 4 The cooling channel (as shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 389°C. During the removal of the profile cooling device and the installation of the profile heating device, time consumption due to factors such as the operator's skill level and proficiency caused the rail joint surface temperature to drop from 39°C, meaning the surface temperature of the rail joint during isothermal heat treatment was 350°C. The profile heating device was used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 350°C for 0.9 hours. After the isothermal process, the rail profile heating device (as shown) was removed. Figure 5 , Figure 6 As shown in the figure, the rail joint is naturally cooled to an ambient temperature of 30°C in air to obtain the rail welded joint of the present invention after post-weld heat treatment. During isothermal heat treatment, the distance between the track-type ceramic heater in the contour heating device and the surface of the rail welded joint is 5mm. The contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0118] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0119] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone within a 20mm radius from the weld center, reaching 110% and 91% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. The metallographic morphology and type of the weld on both sides of the rail welded joint in this embodiment are similar to those in Example 1. Figure 8a and Figure 8b and in Example 2 Figure 9a and Figure 9bThe metallographic structures on both sides of the weld seam of the rail welded joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. Conversely, the HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. The dissimilar rail welded joints obtained using this process help ensure railway operational safety.

[0120] Example 6

[0121] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1230 MPa and a hardness of 340 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.25% C, 1.60% Si, 1.70% Mn, 1.00% Cr, 0.30% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of hot-rolled eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1200 MPa and a hardness of 330 HV. The chemical composition of rail steel that yields the desired microstructure and mechanical properties must meet the following conditions: 0.79% C, 0.65% Si, 0.85% Mn, 0.40% Cr, 0.06% V, with the balance being Fe and unavoidable impurities.

[0122] After the upsetting and push-off processes of the moving flash welding of 68kg / m rails, the welded joint is heat-treated. First, the rail joint, with a residual temperature of 1050℃, is subjected to a first-stage cooling at a first cooling rate of 4.0℃ / s to reduce the surface temperature of the rail head to 480℃. Then, the rail profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (as shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 370°C at a cooling rate of 0.7°C / s. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a 30°C drop in the surface temperature of the rail joint, meaning the surface temperature of the rail joint during isothermal heat treatment was 340°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 340°C for 1 hour. After the isothermal process, the rail profile heating device (as shown) was removed. Figure 5 , Figure 6 As shown, the rail joint is allowed to cool naturally in the air to an ambient temperature of 25°C, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0123] During post-weld heat treatment, the first stage of cooling involves using a rail head profile cooling device with compressed air as the cooling medium to cool the rail head tread and sides of the rail joint. The cooling device is 20mm away from the rail head tread. During the first stage of cooling, the compressed air pressure sprayed by the cooling device is 0.40MPa; then the rail head profile cooling device is turned off. Figure 3 , Figure 4 The cooling channel (as shown) allows the rail welded joint to cool naturally in the air to a surface temperature of 370°C. During the removal of the profile cooling device and the installation of the profile heating device, the time consumed due to factors such as the operator's skill level and proficiency resulted in a 30°C drop in the surface temperature of the rail joint, meaning the surface temperature of the rail joint during isothermal heat treatment was 340°C. The profile heating device was then used to perform isothermal heat treatment on the rail welded joint at a surface temperature of 340°C for 1 hour. After the isothermal process, the rail profile heating device (as shown) was removed. Figure 5 , Figure 6 As shown in the figure, the rail joint is naturally cooled to an ambient temperature of 30°C in air to obtain the rail welded joint of the present invention after post-weld heat treatment. During isothermal heat treatment, the distance between the track-type ceramic heater in the contour heating device and the surface of the rail welded joint is 5mm. The contour heating device is equipped with a temperature control system to monitor the heating temperature in real time.

[0124] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0125] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone within a 20mm radius from the weld center, reaching 105% and 89% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. The metallographic morphology and type of the weld on both sides of the rail welded joint in this embodiment are similar to those in Example 1. Figure 8a and Figure 8b and in Example 2 Figure 9a and Figure 9bThe metallographic structures on both sides of the weld seam of the rail welded joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. Conversely, the HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. The dissimilar rail welded joints obtained using this process help ensure railway operational safety.

[0126] Comparative Example 1

[0127] The process conditions involved in the rail welding and post-weld cooling in this comparative example are basically the same as those in Example 1. The difference is that in this comparative example, the isothermal temperature for isothermal heat treatment after the second stage of cooling of the rail welded joint is 370°C. Then, the rail welded joint is allowed to cool naturally in the air to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0128] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0129] The welded rail joints obtained in this comparative example, under a metallographic microscope, show that the heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail is composed of pearlite, with no martensite. The HAZ on one side of the heat-treated bainitic rail is composed of bainite, with no blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 100% and 86% of the average hardness of the corresponding base metals for the hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail, respectively. Metallographic examination results are shown below. Figure 10a and Figure 10b As shown, for the rail welded joint obtained in this comparative example, the pearlite microstructure transformation in the weld heat-affected zone on the hot-rolled eutectoid pearlitic rail side is complete, and the hardness is good. However, due to the slightly higher isothermal temperature, the bainite laths formed in the weld heat-affected zone on the heat-treated bainitic rail side are wider and have lower hardness, which easily causes saddle-shaped wear in the weld heat-affected zone on this side during line service, leading to low collapse of the weld heat-affected zone. Therefore, the rail joint obtained under this process will be detrimental to railway operation safety.

[0130] Comparative Example 2

[0131] The process conditions involved in the rail welding and post-weld cooling process in this comparative example are the same as those in Example 1. The difference is that after the second stage of cooling is completed, when the surface temperature of the rail head drops to 300°C, the rail joint in this comparative example is not subjected to isothermal heat treatment. Instead, the joint is allowed to cool naturally in the air to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0132] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0133] The welded rail joints obtained in this comparative example, under a metallographic microscope, show that the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. The HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 100% and 84% of the average hardness of the corresponding base metals for the hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail, respectively. Metallographic examination results are shown below. Figure 11a and Figure 11b As shown, for the rail welded joint obtained in this comparative example, the pearlite microstructure transformation in the weld heat-affected zone on the hot-rolled eutectoid pearlitic rail side is complete, and the hardness is good. However, due to the lack of isothermal heat treatment, the bainite laths formed in the weld heat-affected zone on the heat-treated bainitic rail side of the rail welded joint are wider and have lower hardness. This makes it prone to saddle-shaped wear in the weld heat-affected zone during line service, leading to low-density collapse of the weld heat-affected zone. Therefore, the rail joint obtained under this process will be detrimental to railway operation safety.

[0134] Comparative Example 3

[0135] The process conditions involved in the rail welding process in this comparative example are the same as those in Example 5. The difference is that this comparative example does not perform any heat treatment on the joint after the rail welding is completed. Instead, the joint is allowed to cool naturally in the air to an ambient temperature of 30°C, thereby obtaining the rail joint of this comparative example.

[0136] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0137] The metallographic morphology and type of the weld seam on both sides of the rail welded joint obtained in this comparative example are the same as those in Comparative Example 1. Figure 10a and Figure 10b And in Comparative Example 2 Figure 11a and Figure 11b The metallographic structures on both sides of the weld seam of the rail weld joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. The HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. Hardness tests show that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 97% and 80% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. For the rail joint obtained in this comparative example, the hardness of the HAZ on the hot-rolled eutectoid pearlitic rail side is acceptable, while the hardness of the HAZ on the heat-treated bainitic rail side is too low. Under this process, the hardness of the rail joint is not within the range of 100-110% and 87-92% of the average hardness of the base material for hot-rolled eutectoid pearlitic rails and heat-treated bainitic rails, respectively, which are achieved within the ±20mm range from the weld center obtained using this invention. The overall hardness of the joint is too low. Therefore, the rail joint obtained under this process will be detrimental to railway operation safety.

[0138] Comparative Example 4

[0139] The rail welding process conditions in this comparative example are the same as those in Example 5. The difference is that in this comparative example, the first stage of cooling is performed after the rail welding is completed and the surface temperature of the rail head joint drops to 990°C. Subsequently, the process parameters and conditions involved in the second stage cooling and isothermal heat treatment are also the same as in Example 5, thus obtaining the rail joint of this comparative example.

[0140] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0141] The metallographic morphology and type of the weld seam on both sides of the rail welded joint obtained in this comparative example are the same as those in Comparative Example 1. Figure 10a Figure 10b And in Comparative Example 2 Figure 11a and Figure 11b The metallographic structures on both sides of the weld seam of the rail joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is pearlitic with no martensite. The HAZ on the heat-treated bainitic rail side of the joint is bainitic with no blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 100% and 86% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. For the rail joint obtained in this comparative example, the hardness of the HAZ on the hot-rolled eutectoid pearlitic rail side is acceptable. However, due to the slightly lower surface temperature of the rail head, the driving force for the bainitic phase transformation in the HAZ on the heat-treated bainitic rail side of the joint is insufficient during subsequent cooling, resulting in insufficient refinement of the bainitic structure and consequently, a slightly lower hardness on that side. Under this process, the hardness of the heat-affected zone (HAZ) on the side of the hot-rolled eutectoid pearlitic rail within ±20mm of the weld center reaches 100% of the hardness of the corresponding rail base material, while the hardness of the HAZ on the side of the heat-treated bainitic rail is only 86% of the hardness of the corresponding rail base material. Because the hardness of this HAZ is lower than 87% of the hardness of the corresponding rail base material, the HAZ on the side of the heat-treated bainitic rail is more prone to collapse due to wear during railway line service, resulting in changes in the rail joint profile and thus being detrimental to railway operation safety.

[0142] Comparative Example 5

[0143] The rail welding and post-weld cooling process conditions in this comparative example are mostly the same as those in Example 6. The difference is that in this comparative example, after the rail joint undergoes the first stage of cooling to 280°C, it is immediately subjected to isothermal heat treatment, with a holding time of 1 hour. Subsequently, the cooling conditions used are also consistent with those in Example 6, thus obtaining the rail joint of this comparative example.

[0144] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0145] The metallographic examination results of the rail welded joints obtained in this comparative example are shown below. Figure 12a and Figure 12b As shown in the figure, under a metallographic microscope, the weld heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail joint is composed of pearlite, with no martensite. In contrast, the HAZ on the other side of the heat-treated bainitic rail joint is predominantly bainite, with a significant amount of blocky martensite. Hardness testing indicates that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 107% and 95% of the average hardness of the corresponding base materials for the hot-rolled eutectoid pearlitic and heat-treated bainitic rails, respectively. This is because the cooling rate in the first stage is 4.0℃ / s, which is above the critical cooling rate for martensitic transformation in both the heat-treated bainitic rail and the hot-rolled eutectoid pearlitic rail. Simultaneously, the final cooling temperature of 280℃ in this stage is within the martensitic transformation initiation temperature range of the heat-treated bainitic rail, but higher than the martensitic transformation initiation temperature range of the hot-rolled eutectoid pearlitic rail. Therefore, martensite forms in the heat-affected zone (HAZ) of heat-treated bainitic rails after the first stage of cooling, while no martensite forms in the HAZ of hot-rolled eutectoid pearlitic rails. Subsequent isothermal heat treatment cannot eliminate the already formed martensite. For the rail joints in this comparative example, both the hot-rolled eutectoid pearlitic rail side and the heat-treated bainitic rail side exhibit good HAZ hardness. Considering the significant amount of brittle and hard martensite already formed in the HAZ of the heat-treated bainitic rail and the high internal stress of the martensite, the joint is more prone to fatigue cracking due to the martensite structure during railway service. Therefore, rail joints obtained under this process will be detrimental to railway operation safety.

[0146] Comparative Example 6

[0147] The rail welding and post-weld cooling process conditions in this comparative example are mostly the same as those in Example 1. The difference is that in this comparative example, the first stage of cooling is stopped when the rail joint reaches 560°C, and then the second stage of cooling is performed. The process conditions and parameters used for the second stage cooling, isothermal heat treatment, and subsequent cooling are consistent with those in Example 1, thus obtaining the rail joint of this comparative example.

[0148] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0149] The metallographic morphology and type of the weld seam on both sides of the rail welded joint obtained in this comparative example are the same as those in Comparative Example 1. Figure 10a and Figure 10b And in Comparative Example 2 Figure 11a and Figure 11b The metallographic structures on both sides of the weld joint of the rail shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail joint is composed of pearlite, with no martensite. The HAZ on one side of the heat-treated bainitic rail joint is composed of bainite, with no blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 92% and 87% of the average hardness of the corresponding base metals of the hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail, respectively. This is because the final cooling temperature of 560℃ in the first stage is relatively high, resulting in insufficient refinement of the pearlite structure in this stage, thus leading to a less significant increase in hardness in the HAZ on the hot-rolled eutectoid pearlitic rail side of the joint.

[0150] For the rail joint in this comparative example, the hardness of the weld heat-affected zone on one side of the heat-treated bainitic rail is within 87-92% of the average hardness of the corresponding rail base material. However, the hardness of the weld heat-affected zone on one side of the hot-rolled eutectoid pearlitic rail is not within the range of 100-110% of the average hardness of the rail base material obtained by using this invention within ±20mm from the weld center. The overall hardness of the joint is relatively low. The weld heat-affected zone on the hot-rolled eutectoid pearlitic rail side of the joint is more prone to collapse due to wear during railway line service, resulting in changes in the rail joint profile, which is detrimental to railway operation safety.

[0151] Comparative Example 7

[0152] The rail welding and post-weld cooling process conditions in this comparative example are mostly the same as those in Example 6. The difference is that in this comparative example, after the rail joint undergoes the first stage of cooling to 210°C, it is immediately subjected to isothermal heat treatment, with a holding time of 1 hour. Subsequently, the cooling conditions used are also consistent with those in Example 6, thus obtaining the rail joint of this comparative example.

[0153] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0154] The metallographic examination results of the rail welded joints obtained in this comparative example are shown below. Figure 13a and Figure 13b As shown in the figure, under a metallographic microscope, the microstructure of the weld heat-affected zone (HAZ) on one side of the hot-rolled eutectoid pearlitic rail joint is dominated by pearlite, with a significant amount of blocky martensite. Similarly, the HAZ on the other side of the heat-treated bainitic rail joint is dominated by bainite, also with a significant amount of blocky martensite. Hardness testing shows that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 115% and 97% of the average hardness of the corresponding base materials for the hot-rolled eutectoid pearlitic and heat-treated bainitic rails, respectively. This is because the cooling rate of the first stage is 4.0℃ / s, which is above the critical cooling rate for martensitic transformation in both the heat-treated bainitic and hot-rolled eutectoid pearlitic rails. Furthermore, the final cooling temperature of 210℃ in this stage is below the martensitic transformation initiation temperature of the heat-treated bainitic rail, but within the range of the martensitic transformation initiation temperature for the hot-rolled eutectoid pearlitic rail. Therefore, a significant amount of martensite appears in both the heat-affected zone (HAZ) of the heat-treated bainitic rail and the heat-affected zone of the hot-rolled eutectoid pearlitic rail after the first stage of cooling. The subsequent isothermal heat treatment process cannot eliminate the already formed martensite. For the rail joint obtained in this comparative example, the hardness of the HAZ on both the hot-rolled eutectoid pearlitic rail and the heat-treated bainitic rail side is higher than that obtained using this invention, where the longitudinal hardness of the rail joint within a ±20mm radius from the weld center reaches 100-110% and 87-92% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Considering the significant amount of brittle and hard martensite already formed in the HAZ on both sides of the rail joint weld and the high internal stress of the martensite, the joint is more prone to fatigue cracking due to the martensite structure during railway service. Therefore, the rail joint obtained under this process will be detrimental to railway operation safety.

[0155] Comparative Example 8

[0156] The process conditions involved in the rail welding and post-weld cooling in this comparative example are basically the same as those in Example 1. The difference is that in this comparative example, after the second stage of cooling of the rail welded joint is completed, the isothermal heat treatment is carried out for 0.5 hours after the surface temperature of the rail welded joint drops to 300°C. After the isothermal process is completed, the rail profile heating device is removed. Figure 5 , Figure 6As shown in the figure, the rail joint is allowed to cool naturally in the air to an ambient temperature of 20°C, thus obtaining the rail joint of this comparative example.

[0157] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 1 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 The sampling method shown is used to examine the metallographic structure of the joint according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic sample was etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0158] The metallographic morphology and type of the weld seam on both sides of the rail welded joint obtained in this comparative example are the same as those in Comparative Example 1. Figure 10a and Figure 10b And in Comparative Example 2 Figure 11a and Figure 11b The metallographic structures on both sides of the weld seam of the rail weld joint shown are consistent. Under a metallographic microscope, the heat-affected zone (HAZ) on the hot-rolled eutectoid pearlitic rail side of the joint is composed of pearlite, without martensite. The HAZ on the heat-treated bainitic rail side of the joint is composed of bainite, without blocky martensite. Hardness tests show that the longitudinal hardness of the HAZ within ±20mm from the weld center reaches 100% and 85% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. For the rail joint obtained in this comparative example, the hardness of the HAZ on the hot-rolled eutectoid pearlitic rail side is within 100-110% of the average hardness of the rail base material within ±20mm from the weld center obtained using this invention. However, due to the short isothermal time of 0.5 hours, the bainitic lath structure is not sufficiently refined, resulting in minimal improvement in strength and hardness. Consequently, the hardness of the weld heat-affected zone on one side of the heat-treated bainitic rail is outside the 87-92% range of the average hardness of the rail base material, leading to an overall low hardness of the joint. Therefore, rail joints obtained under this process will be detrimental to railway operation safety.

[0159] As can be seen from the embodiments and comparative examples of this invention, by implementing staged controlled cooling and isothermal heat treatment on the welded joints of dissimilar materials after welding, this invention can maintain high strength and hardness in the heat-affected zones on both sides of the weld, thereby ensuring the wear resistance of the rail joint. It can also ensure that there is no martensite in the weld heat-affected zone on the side of the hot-rolled eutectoid pearlitic rail joint and no obvious blocky martensite in the weld heat-treated bainitic rail joint. Simultaneously, the longitudinal hardness of the rail joint within ±20mm from the weld center can reach 100-110% and 87-92% of the average hardness of the corresponding hot-rolled eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively, contributing to ensuring railway operation safety. This invention has significant technical advantages and broad market prospects.

[0160] This invention document is intended to illustrate how to use the disclosed techniques and various embodiments, and is not intended to limit its true scope and equivalent spirit. Furthermore, the foregoing description is not exhaustive of all possibilities or to limit the scope of protection to the precise forms disclosed. Changes and variations are possible in accordance with the foregoing teachings. The selected and illustrated embodiments provide the best illustration of the principles of the technology and its practical application, and enable those skilled in the art to use the disclosed technology for various conceivable specific applications with various modifications. Therefore, various changes and modifications made to the above embodiments without substantially departing from the spirit and principles of the technology described herein are intended to be included within the scope of this invention.

Claims

1. A heat treatment method for welded joints of eutectoid pearlitic steel rails and bainitic steel rails, characterized in that, The method includes: S1. The welded rail joint to be cooled is subjected to the first stage of cooling to reduce the surface temperature of the rail joint to 460~500℃. The initial surface temperature of the rail joint to be cooled is 1000℃~1050℃. The first stage of cooling uses compressed air as the cooling medium to cool the rail joint, and the cooling rate is 1.8~7.0℃ / s. S2. The welded joint treated in S1 is subjected to a second stage of cooling to reduce the surface temperature of the rail welded joint to 351~389℃. The cooling rate of the second stage of cooling is 0.7~0.9℃ / s. S3. The welded joint after S2 treatment is subjected to isothermal heat treatment, followed by a third stage of cooling to cool the rail welded joint to room temperature. The isothermal heat treatment temperature is 300~350℃ and the isothermal heat treatment time is 0.8~1.2h.

2. The method according to claim 1, characterized in that, The first stage of cooling includes artificial forced cooling using compressed air as the cooling medium, the second stage of cooling includes natural cooling, and the third stage of cooling is natural cooling.

3. The method according to claim 1, characterized in that, The base material of the eutectoid pearlitic rail, by mass percentage, comprises: 0.75-0.82% 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.

4. The method according to claim 1, characterized in that, The base material of the bainitic rail, by mass percentage, comprises: 0.20-0.30% C, 1.3-1.9% Si, 1.4-2.0% Mn, 0.70-1.20% Cr, 0.20-0.50% Mo, with the balance being Fe and unavoidable impurities.

5. The method according to claim 1, characterized in that, The eutectoid pearlitic steel rail is a hot-rolled eutectoid pearlitic steel rail, and the bainitic steel rail is a heat-treated bainitic steel rail. The welded joints of the eutectoid pearlitic steel rail and the bainitic steel rail have the same specifications, both being 60~75kg / m.

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