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

Abstract

The application discloses a heat treatment method for a welded joint of a eutectoid pearlite rail and a bainite rail, and comprises the following steps: S1, adopting a rail-shaped cooling device to perform first stage cooling on the welded joint; S2, adopting the rail-shaped cooling device to perform second stage cooling on the welded joint; S3, adopting a rail-shaped heating device to perform isothermal heat treatment on the welded joint; and S4, stopping the isothermal heat treatment and cooling to room temperature. The application controls the cooling and the isothermal heat treatment of the welded joint of the rails with different materials after welding in different temperature sections, so that the heat affected zone on both sides of the welded joint of the rail has higher strength and hardness, the wear resistance of the rail joint is ensured, the welded heat affected zone on one side of the eutectoid pearlite rail is pearlite without martensite, and the welded heat affected zone on one side of the bainite rail is bainite without obvious blocky martensite, which is helpful to ensure the safety of railway operation.

Description

Technical Field

[0001] This invention relates to the field of railway rail welding technology, and in particular to a heat treatment method for welded joints of eutectoid pearlitic rails and bainitic rails. Background Technology

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

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

[0004] For example, Chinese patent application CN201610909362.1 discloses a post-weld heat treatment method for hypereutectoid rails and PG4 heat-treated eutectoid pearlitic rail welded joints. This method includes first cooling the welded rail joint to below 400°C, then heating the first-cooled rail joint to 860–930°C, followed by a second cooling until the tread temperature of the rail joint is 410–450°C. The dissimilar rail welded joints obtained using this method meet the testing requirements for fatigue, tensile, impact, and static bending tests in the current domestic railway industry standard TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding". However, the above invention involves a post-weld normalizing heat treatment process, requiring the use of rail post-weld heat treatment equipment to locally heat the rail welded joint, which is not only complex to operate and implement but also costly.

[0005] For example, Chinese patent application number CN201410135909.8 discloses a post-weld heat treatment method for bainitic rail welded joints. It specifically discloses the heat treatment method for bainitic rail welded joints and introduces the principle of rail joint heat treatment. During the post-weld heat treatment process, the rail joint needs to be heated to above the austenitizing temperature, and compressed air is used as the cooling medium to rapidly cool the welded area to restore the mechanical properties of the rail that have been reduced due to welding. The above patent is applicable to bainitic rails of the same material. However, this patent does not describe the specific numerical value of the impact toughness of the bainitic welded joint.

[0006] For example, Chinese patent application number CN201810581145.3 discloses a post-weld heat treatment method for welded joints of hypereutectoid steel rails and eutectoid steel rails; application number CN201810720765.0 discloses a heat treatment method for welded joints of rails; and application number CN201810710040.3 discloses a heat treatment method for welded joints of rails made of dissimilar materials. The above three patents introduce post-weld heat treatment methods for welded joints of hypereutectoid steel rails and eutectoid steel rails. However, in the above patents, the microstructure of both hypereutectoid steel rails and eutectoid steel rails is mainly pearlite. The post-weld heat treatment principle is to use compressed air or water mist mixture as the cooling medium, and to rapidly cool the rail joint at the austenitizing temperature to refine the spacing between pearlite lamellars and improve the thermoplasticity of the rail weld area. The above three patents do not apply to the joints formed by welding bainitic and eutectoid pearlitic steel rails, and therefore cannot be directly applied to heat treatment of the joints formed by welding bainitic and eutectoid pearlitic steel rails.

[0007] In view of this, improvements should be made to the existing technology. Summary of the Invention

[0008] The main objective of this invention is to provide a heat treatment method for welded joints of eutectoid pearlitic and bainitic rails, as well as a split-type rail profile heating device. By implementing segmented temperature-controlled cooling and isothermal heat treatment on the welded joints of dissimilar materials after welding, the heat-affected zones on both sides of the rail joint weld maintain high strength and hardness, thereby ensuring the wear resistance of the rail joint. It ensures that the weld heat-affected zone microstructure on the heat-treated eutectoid pearlitic rail side is pearlitic and free of martensite, and on the heat-treated bainitic rail side, it ensures that the weld heat-affected zone microstructure is bainitic and free of obvious blocky martensite. Simultaneously, it enables the longitudinal hardness of the rail joint within a ±20mm radius from the weld center to reach 90-95% and 87-92% of the average hardness of the corresponding heat-treated eutectoid pearlitic and bainitic rail base materials, respectively, contributing to ensuring railway operation safety.

[0009] According to one aspect of the present invention, a heat treatment method for a welded joint between eutectoid pearlitic steel rail and bainitic steel rail is provided, comprising the following steps:

[0010] S1. The welded joints formed by welding eutectoid pearlitic steel rails and bainitic steel rails with a surface temperature of 920-990℃ are subjected to a first-stage cooling to reduce the surface temperature of the welded joints to 500-550℃. The first-stage cooling is carried out by using a rail profile cooling device, with a cooling rate of 5.0-8.0℃ / s.

[0011] S2. Perform a second stage of cooling on the welded joint to reduce the surface temperature of the welded joint to 390-440℃. The second stage of cooling is performed using a rail-following cooling device with a cooling rate of 2.0-4.0℃ / s.

[0012] S3. Perform isothermal heat treatment on the welded joint. The surface temperature of the welded joint in this stage is 340-400℃. The rail profile heating device is used to perform isothermal heat treatment on the welded joint in this stage. The heat treatment time is 0.5-1h.

[0013] S4. Stop the isothermal heat treatment and remove the rail profile heating device, allowing the welded joint to cool naturally to an ambient temperature of 20-30℃. The cooling rate during this stage is 0.2-0.9℃ / s.

[0014] According to one embodiment of the present invention, the base material of the eutectoid pearlitic rail, by weight percentage, comprises C: 0.73~0.80%, Si: 0.50~0.70%, Mn: 0.70~0.9%, Cr: 0.30~0.50%, V: 0.06~0.09%, with the balance being Fe and unavoidable impurities.

[0015] According to one embodiment of the present invention, the eutectoid pearlitic rail base material has a tensile strength of 1320~1380MPa and a hardness of 380~420HV at room temperature of 20~30℃.

[0016] According to one embodiment of the present invention, the base material of the bainitic rail, by weight percentage, comprises C: 0.20~0.30%, Si: 0.9~1.6%, Mn: 1.4~2.0%, Cr: 0.70~1.50%, Mo: 0.20~0.50%, with the balance being Fe and unavoidable impurities.

[0017] According to one embodiment of the present invention, the tensile strength of the bainitic rail base material is 1250~1290MPa and the hardness is 330~370HV at room temperature of 20~30℃.

[0018] According to one embodiment of the present invention, in steps S1 and S2, cooling using a rail profile cooling device includes spraying a mixture of compressed air and / or water mist onto the weld joint.

[0019] According to one embodiment of the present invention, in step S1, the rail profile cooling device is 10-30mm away from the surface of the welded joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.50-0.80MPa; in step S2, the rail profile cooling device is 10-30mm away from the surface of the welded joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.20-0.40MPa.

[0020] According to one embodiment of the present invention, the welded joint is formed by welding heat-treated eutectoid pearlitic steel rails and heat-treated bainitic steel rails with the same rail type and specifications of 60-75 kg / m using a rail moving flash welding machine.

[0021] According to one embodiment of the present invention, in step S3, the isothermal heat treatment includes using a rail profile heating device to perform full-section isothermal heat treatment on the welded joint, wherein the distance between the rail profile heating device and the surface of the welded joint is 3~10mm.

[0022] According to one embodiment of the present invention, the rail profile heating device used in step S3 is a split-type rail profile heating device, which includes:

[0023] The device housing is fitted onto the outside of the rail and is I-shaped. The device housing includes a first housing and a second housing that are opposite each other. The upper parts of the first housing and the second housing are pivotally connected by a rotating shaft.

[0024] Tracked ceramic heaters, multiple tracked ceramic heaters are arranged in parallel on the inner surface of the device housing, used to provide a heat source for isothermal heat treatment;

[0025] Terminal blocks are located diagonally on the upper surface of the device housing; and an insulation layer is located inside the device housing. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some implementation examples of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 A process flow diagram of a heat treatment method for a welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to an exemplary embodiment of the present invention is shown.

[0028] Figure 2 A schematic diagram of the longitudinal hardness test position 5 mm below the rail head tread of a rail joint according to an exemplary embodiment of the present invention is shown.

[0029] Figure 3 A schematic diagram of the sampling location of the metallographic specimen of the rail head tread of the rail welded joint according to an exemplary embodiment of the present invention is shown.

[0030] Figure 4 A three-dimensional structural schematic diagram of a rail contour cooling device according to an exemplary embodiment of the present invention is shown.

[0031] Figure 5 A three-dimensional structural schematic diagram of a rail contour cooling device according to an exemplary embodiment of the present invention is shown.

[0032] Figure 6 A schematic diagram of a split-type rail profile heating device according to an exemplary embodiment of the present invention is shown.

[0033] Figure 7 A schematic diagram showing the distribution of electric heaters in the rail head area of ​​a split rail profile heating device according to an exemplary embodiment of the present invention is provided.

[0034] The reference numerals in the figure are explained as follows:

[0035] 1. Device housing; 2. Tracked ceramic heater; 3. Terminal block; 4. Rotating shaft; 5. Insulation layer; 6. Fixing ring. Detailed Implementation

[0036] The following detailed description of the embodiments is intended to exemplify the principles of the present invention, but should not be construed as limiting the scope of the invention. The present invention can be implemented in many different forms and is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

[0037] These embodiments are provided to make this disclosure thorough and complete, and to fully express the scope of the invention to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​set forth in these embodiments should be interpreted as merely exemplary and not as limiting.

[0038] It should be noted that, in the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicating orientation or positional relationships, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0039] It should also be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention depending on the specific circumstances. When a specific device is described as being located between a first device and a second device, an intermediary device may or may not be present between the specific device and the first or second device.

[0040] All terms used in this invention have the same meaning as understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in general dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and not as idealized or highly formalized, unless expressly defined herein.

[0041] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0042] 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.

[0043] 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%), online heat treatment technology utilizes the residual heat from rail rolling to accelerate the phase transformation rate of supercooled austenite to pearlite or to bainite, effectively refining the pearlite lamellar spacing or the 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.

[0044] In this invention, the critical cooling rate for martensitic transformation of the heat-treated bainitic rail steel is 1.0~1.4℃ / s, and the martensitic transformation initiation temperature is 290~335℃; in this invention, the critical cooling rate for martensitic transformation of the heat-treated eutectoid pearlitic rail steel is 1.5~2.0℃ / 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 heat-treated 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 290~335℃ 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.

[0045] Based on the above understanding, such as Figure 1 As shown, the present invention provides a heat treatment method for a welded joint of eutectoid pearlitic steel rail and bainitic steel rail, which includes the following steps:

[0046] S1. The welded joints formed by welding eutectoid pearlitic steel rails and bainitic steel rails with a surface temperature of 920-990℃ are subjected to a first-stage cooling to reduce the surface temperature of the welded joints to 500-550℃. The first-stage cooling is carried out by using a rail profile cooling device, with a cooling rate of 5.0-8.0℃ / s.

[0047] S2. Perform a second stage of cooling on the welded joint to reduce the surface temperature of the welded joint to 390-440℃. The second stage of cooling is performed using a rail-following cooling device with a cooling rate of 2.0-4.0℃ / s.

[0048] S3. Perform isothermal heat treatment on the welded joint. The surface temperature of the welded joint in this stage is 340-400℃. The rail profile heating device is used to perform isothermal heat treatment on the welded joint in this stage. The heat treatment time is 0.5-1h.

[0049] S4. Stop the isothermal heat treatment and remove the rail profile heating device, allowing the welded joint to cool naturally to an ambient temperature of 20-30℃. The cooling rate during this stage is 0.2-0.9℃ / s.

[0050] The first and second stages of cooling involve cooling the welded rail joint within a profile cooling device. The cooling rate can be adjusted by controlling the pressure of the compressed air flowing into the device, thus controlling the cooling rate of the first and second stages. After the rail is welded using a flash welding machine, the rail joint, while still at a temperature above its austenitizing temperature, undergoes the first stage of cooling. This stage employs a relatively high cooling rate (higher than the critical cooling rate for martensitic transformation of the rail steel) to refine the pearlite lamellar spacing, thereby improving the strength, hardness, and toughness of the heat-affected zone (HAZ). The second stage of cooling aims to prevent the formation of brittle martensite in the HAZ of the two types of rail steel during the cooling process by using a relatively low cooling rate. The isothermal heat treatment following the second stage of cooling aims to achieve sufficient bainitic transformation in the HAZ on the heat-affected side of the heat-treated bainitic rail joint, thereby improving the strength, hardness, and toughness of the HAZ.

[0051] This invention addresses the issue of rapid post-weld cooling in rail joints with high residual heat after welding. This reduces the phase transformation temperature of the austenite-to-pearlite transformation within the heat-affected zone, thereby increasing the hardness of the austenite recrystallization zone. Based on metallurgical principles, rail joints exhibit dynamic undercooling under high-temperature rapid cooling conditions after welding. This causes the phase transformation temperature of the austenite-to-pearlite transformation in a non-equilibrium state to shift downwards, and the phase transformation temperature gradually decreases with increasing undercooling. For the isothermal heat treatment process, the ceramic heating element embedded in the contour heating device, under continuous power supply and pre-programmed control, can achieve continuous heat compensation within the device, thus realizing the isothermal heat treatment process. In this invention, the temperature is controlled between 340 and 400°C during the isothermal heat treatment process to obtain a lath bainite structure with good strength and toughness.

[0052] In this invention, rail joints with an initial welding temperature of 920–990°C are cooled to 500–550°C in the first stage. This temperature is above the martensitic transformation initiation temperature of the heat-treated eutectoid pearlitic rail steel and the heat-treated bainitic rail steel involved. This stage employs a relatively large cooling rate (i.e., 5.0–8.0°C / s) to refine the pearlite lamellar spacing in the heat-affected zone on one side of the heat-treated eutectoid pearlitic rail, thereby improving the strength, hardness, and toughness of the welded heat-affected zone. Furthermore, at the 500–550°C temperature, the pearlite transformation within the welded heat-affected zone on the heat-treated eutectoid pearlitic rail side of the joint is essentially complete. To ensure a cooling rate of 5.0–8.0°C / s in the first stage of cooling, 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 0.50–0.80 MPa.

[0053] In this invention, the initial cooling temperature of the second stage is 550–500°C, and the final cooling temperature of the second stage is controlled to be higher than the martensitic transformation start temperature of heat-treated bainitic rail steel and heat-treated eutectoid pearlitic rail steel. Simultaneously, the final cooling temperature of the second stage is 390–440°C. During the second stage cooling, to ensure a cooling rate of 2.0–4.0°C / s, the following method can be used: the profile cooling device is 20 mm away from the weld joint; the pressure of the compressed air or water mist mixture sprayed by the cooling device is 0.20–0.40 MPa.

[0054] It should be noted that, in this invention, due to the limited cooling capacity of the rail profile cooling device, the maximum cooling rate in the first stage will not exceed 8.0℃ / s. In this invention, the cooling rate of the welded joint in the second stage of cooling is controlled between 2.0 and 4.0℃ / s. When the cooling rate in the second stage is lower than 2.0℃ / s, the cooling time will be significantly prolonged, thus affecting the overall production efficiency of the rail joint heat treatment. For the isothermal heat treatment after the second stage of cooling, if the holding time of the isothermal process exceeds 1 hour, the joint strength, hardness, and toughness remain essentially unchanged, and the long holding time will severely reduce the production efficiency of the rail joint heat treatment. Therefore, this invention controls the holding time of the isothermal heat treatment to 1 hour.

[0055] Furthermore, it should be noted that both the heat-treated eutectoid pearlitic rail and the heat-treated bainitic rail described in this invention are heat-treated rails produced using online heat treatment technology. Compared to traditional hot-rolled eutectoid pearlitic rails and 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 rails on top of the mechanical properties of traditional hot-rolled rails. Therefore, at present, heat-treated rails with better overall performance are gradually replacing hot-rolled rails with relatively lower overall performance.

[0056] According to the principles of metallurgy, bainitic microstructure can be obtained through isothermal transformation. In this invention, a split-type profile heating device is used to perform isothermal heat treatment on the rail welded joint at 340–400°C for 1 hour. This isothermal heat treatment promotes bainitic transformation, stabilizes residual austenite in bainitic steel, reduces the overall residual stress of the welded joint, and appropriately improves the strength, hardness, and toughness of the heat-affected zone of the heat-treated bainitic rail steel. The profile device is 5 mm away from the surface of the rail welded joint, ensuring good heat conduction. The isothermal heat treatment process can be achieved under the continuous heating of the tracked electric heating element. After the isothermal process is completed, the device is removed. The welded joint is placed in an air environment for natural cooling, reducing the surface temperature of the welded joint to the ambient temperature of 20–30°C. The cooling rate during this stage is 0.2–0.9°C / s. It should be noted that due to the cooling in the first and second stages, the residual temperature on the surface of the rail joint is low. Therefore, when the joint is allowed to cool naturally in the air after the isothermal heat treatment process is completed, martensitic structure will not form.

[0057] It should be noted that in this invention, bainitic structure can be generated when the isothermal heat treatment temperature is between 401 and 500°C. 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 between 340 and 400°C. Therefore, in this invention, the isothermal heat treatment temperature is controlled within the temperature range of 340 to 400°C.

[0058] It should be noted that 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 will inevitably lead to a temperature drop of 30-50°C on the surface of the rail joint.

[0059] The rail welded joint described in this invention is formed by welding a heat-treated eutectoid pearlitic steel rail and a heat-treated bainitic steel rail of the same type and specifications of 60-75 kg / m using a rail moving flash welding machine. The welded joint includes a region of 60-80 mm in length, including the weld and / or heat-affected zone, with the weld at the center of this region.

[0060] In this invention, "room temperature" refers to a temperature in the range of 20 to 30°C.

[0061] Regarding this invention, it should be added that heat treatment technology itself is a process of controlling various factors during heating and cooling. The steps in heat treatment technology are interconnected and influence each other. This application may inevitably have overlapping and intersecting process parameters with other patent documents, but the applicable objects and heat treatment equipment differ between patents, therefore, simple data application and comparison are not possible. The chemical composition and heat treatment processes of steel rails developed in various countries inevitably overlap. Influenced by factors such as smelting capacity, heat treatment equipment, and operator skill levels, the applicable objects of each invention patent differ (including the mechanical properties and temperature distribution of steel rails), and the cooling devices and implementation processes used are also different, resulting in fundamental differences that prevent simple application of these processes.

[0062] In the heat treatment method for the welded joint of eutectoid pearlitic rail and bainitic rail according to an embodiment of the present invention, by implementing temperature-segmented controlled cooling and isothermal heat treatment on the welded joint of dissimilar materials after welding, the heat-affected zones on both sides of the rail joint weld maintain high strength and hardness, thereby ensuring the wear resistance of the rail joint; it can ensure that the weld heat-affected zone microstructure on the side of the heat-treated eutectoid pearlitic rail is pearlitic and without martensite, and it can also ensure that the weld heat-affected zone microstructure on the side of the heat-treated bainitic rail is bainitic and without obvious blocky martensite; at the same time, the longitudinal hardness of the rail joint in the area ±20mm from the weld center can reach 90-95% and 87-92% of the average hardness of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively, which helps to ensure railway operation safety.

[0063] like Figure 2 The diagram shows the longitudinal hardness test location 5mm below the rail head tread of the rail joint. In the diagram, a represents a heat-treated bainitic rail, c represents the weld center, and b represents a heat-treated eutectoid pearlitic rail.

[0064] like Figure 3 The diagram shows the sampling location for metallographic specimens on the rail head tread of a welded rail joint. Where c represents the weld center and d represents the rail head tread.

[0065] In some specific embodiments, in order to control the tensile and impact properties of heat-treated eutectoid pearlitic rail base material at room temperature of 20~30℃, the tensile strength of the eutectoid pearlitic rail base material is 1320~1380MPa and the hardness is 380~420HV.

[0066] In some specific embodiments, the chemical composition of the rail steel that yields the microstructure and mechanical properties must meet the following conditions: by weight percentage, the base material of the eutectoid pearlitic rail includes C: 0.73~0.80%, Si: 0.50~0.70%, Mn: 0.70~0.9%, Cr: 0.30~0.50%, V: 0.06~0.09%, with the balance being Fe and unavoidable impurities.

[0067] Based on the above embodiments, in order to control the tensile and impact properties of heat-treated bainitic rail base material at room temperature of 20~30℃, the tensile strength of the bainitic rail base material is 1250~1290MPa and the hardness is 330~370HV.

[0068] In some specific embodiments, the chemical composition of the rail steel that yields the microstructure and mechanical properties must meet the following conditions: by weight percentage, the base material of the bainitic rail includes C: 0.20~0.30%, Si: 0.9~1.6%, Mn: 1.4~2.0%, Cr: 0.70~1.50%, Mo: 0.20~0.50%, with the balance being Fe and unavoidable impurities.

[0069] like Figure 4 and 5 The diagram shows a 3D structural schematic and a usage schematic of the rail profile cooling device. It cools only the rail head tread and sides of the welded rail joint. The orifice size of its air jet channels can be designed and manufactured according to actual needs to achieve cooling intensities. The compressed air flowing through channels one and two has the same pressure, which can be monitored by a pressure gauge and adjusted as needed.

[0070] Based on the above embodiments, in steps S1 and S2, cooling using a rail-following cooling device includes spraying compressed air and / or a water mist mixture onto the weld joint. During rail welding, the grains in the near-weld zone become coarse due to high temperatures, leading to a significant reduction in the toughness and plasticity of the weld joint. Cooling by spraying compressed air or a water mist mixture can rapidly lower the temperature, thereby significantly refining the grains and improving the toughness and overall mechanical properties of the weld joint.

[0071] Based on the above embodiments, in step S1, the rail-shaped cooling device is 10-30mm away from the surface of the weld joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.50-0.80MPa; in step S2, the rail-shaped cooling device is 10-30mm away from the surface of the weld joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.20-0.40MPa. Rapid cooling by spraying compressed air can effectively eliminate abnormal microstructures such as martensite that may occur due to excessively rapid local cooling or micro-regional component segregation, thus improving the microstructure of the weld joint. Preferably, to ensure that the longitudinal hardness of the rail weld heat-affected zone within ±20mm of the weld center reaches the average hardness of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail base materials, the rail-shaped cooling device is 20mm away from the surface of the weld joint.

[0072] Based on the above embodiments, the welded joint is formed by welding heat-treated eutectoid pearlitic steel rails and heat-treated bainitic steel rails with the same rail type and specifications of 60-75 kg / m using a moving flash welding machine. Moving flash welding features high automation and stable process, effectively ensuring the stability and reliability of weld quality. During the welding process, impurities and iron oxide at the joint can be effectively removed, thereby improving the quality of the welded joint. Due to its fast welding speed and stable process, the heat-affected zone is relatively narrow. This helps reduce the impact of welding on the properties of the rail base material and maintains the overall performance stability of the rail.

[0073] Based on the above embodiments, in step S3, the isothermal heat treatment includes performing full-section isothermal heat treatment on the welded joint using a rail-shaped heating device. The distance between the rail-shaped heating device and the surface of the welded joint is 3-10 mm. During the welding process, due to the unevenness of thermal expansion and cooling contraction, residual stress will be generated in the weld and its surrounding area. If these residual stresses are not eliminated, they may lead to deformation and cracking of the weld, or even breakage during use, affecting the safety of the entire structure. Through full-section isothermal heat treatment, the residual stress in the welded joint can be effectively reduced and uniformly released, improving the stability and reliability of the joint. Preferably, the distance between the rail-shaped heating device and the surface of the welded joint is 5 mm.

[0074] like Figure 6As shown, the present invention also proposes a split-type rail profile heating device, comprising: a device housing 1, which is fitted onto the outside of the rail and is I-shaped; the device housing 1 includes a first shell and a second shell that are symmetrical to each other; the upper parts of the first shell and the second shell are pivotally connected by a rotating shaft 4, so that the device housing 1 can be split open from the middle; the first shell and the second shell can be welded from thin metal plates; the rotating shaft 4 can be arranged in the middle of the upper surface of the device housing 1 along the extension direction of the device housing 1; a tracked ceramic heater 2, multiple tracked ceramic heaters 2 are arranged parallel to each other on the inner surface of the device housing 1 and connected in parallel to provide a heat source for isothermal heat treatment; two terminals 3, two terminals 3 are arranged diagonally on the upper surface of the device housing 1, and a circuit is formed between the two terminals 3; and a heat insulation layer 5, the heat insulation layer 5 is arranged on the inner side of the device housing 1, specifically, an asbestos insulation layer 5. A fixing ring 6 for opening or closing the device housing 1 is also provided at the connection between the bottom of the first shell and the second shell.

[0075] This device boasts advantages such as compactness, flexibility, and low cost, facilitating on-site construction. It can be powered by a diesel generator or a 220V power supply, with a rated power of 10kW. The device uses a commercial LCD tracked ceramic heater as its heat source, with the heater consisting of ceramic discs measuring 10mm (length) × 10mm (width) × 6mm (thickness). Equipped with insulation material and a steel outer shell, it forms a ring-shaped, split heater for easy assembly and disassembly, 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, the device can achieve isothermal heat treatment of rail welded joints.

[0076] 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.

[0077] The distribution of electric heaters in the rail head area corresponding to the split-type rail profile heating device used in the isothermal heat treatment of this invention is as follows: Figure 7As shown in the diagram, A represents the rail head tread heating area; B represents the rail head side heating area; C represents the rail head lower jaw heating area; and D represents the track-type ceramic heater. It should be noted that the layout of the heating areas for the rail head, rail web, and rail base is similar in this heating device. Because the rail head is thicker and heat transfer is slower, the number of ceramic heaters covering the rail head is greater than the number of ceramic heaters in the rail web and rail base areas to ensure that the entire rail cross-section is fully heated.

[0078] The solution of this application will be described below through specific embodiments and comparative examples.

[0079] Example 1

[0080] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1250 MPa and a hardness of 330 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.20% C, 0.90% 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 heat-treated eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1320 MPa and a hardness of 380 HV. The chemical composition of rail steel that yields the desired microstructure and mechanical properties must meet the following conditions: 0.73% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.06% V, with the balance being Fe and unavoidable impurities.

[0081] After the upsetting and slugging processes of the mobile flash welding of 60kg / m steel rails, the welded joint undergoes heat treatment. First, the rail joint, with a residual temperature of 920℃, is cooled at a first-stage cooling rate of 5.0℃ / s to reduce the surface temperature of the rail head to 501℃. Then, it is cooled at a second-stage cooling rate of 2.0℃ / s to reduce the surface temperature of the rail head to 440℃. During the removal of the profile cooling device and the installation of the profile heating device, time consumption due to factors such as operator skill level and proficiency resulted in a 40℃ drop in the surface temperature of the rail joint, meaning the surface temperature of the rail joint during isothermal heat treatment was 400℃. The rail welded joint with a surface temperature of 400℃ was then subjected to isothermal heat treatment using a profile heating device for 1 hour. After the isothermal process is completed, the conformal heating device is removed, and the rail joint is allowed to cool naturally in the air at a cooling rate of 0.9℃ / s to an ambient temperature of 20℃, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0082] During post-weld heat treatment, the first and second stages of cooling are performed 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 is 0.50MPa; during the second stage, the compressed air pressure is 0.20MPa. An infrared thermometer is used to monitor the rail head tread temperature. During isothermal heat treatment, the tracked ceramic heater in the profile heating device is 5mm away from the surface of the rail weld joint. This profile heating device is equipped with a temperature control system for real-time monitoring of the heating temperature.

[0083] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0084] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius of the weld center, reaching 90% and 87% of the average hardness of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Simultaneously, under a 100x metallographic microscope, the HAZ on the heat-treated eutectoid pearlitic rail side of the joint shows pearlite microstructure without martensite, while the HAZ on the heat-treated bainitic rail side shows bainite microstructure without blocky martensite. This process helps ensure railway operation safety.

[0085] Example 2

[0086] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1290 MPa and a hardness of 370 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.30% C, 1.60% Si, 2.00% Mn, 1.50% Cr, 0.50% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of heat-treated eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1380 MPa and a hardness of 420 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.80% C, 0.70% Si, 0.90% Mn, 0.50% Cr, 0.09% V, with the balance being Fe and unavoidable impurities.

[0087] After the upsetting and slugging processes of the mobile flash welding of 75kg / m rails, the welded joint undergoes heat treatment. First, the rail joint, with a residual temperature of 990℃, is cooled at a first-stage cooling rate of 8.0℃ / s to reduce the surface temperature of the rail head to 550℃. Then, it is cooled at a second-stage cooling rate of 4.0℃ / s to reduce the surface temperature of the rail head to 390℃. During the removal of the profile cooling device and the installation of the profile heating device, time consumption due to factors such as operator skill level and proficiency results in a 50℃ temperature drop on the surface of the rail joint, meaning the surface temperature of the rail joint during isothermal heat treatment is 340℃. The rail welded joint with a surface temperature of 340℃ is then subjected to isothermal heat treatment using the profile heating device for 1 hour. After the isothermal process is completed, the conformal heating device is removed, and the rail joint is allowed to cool naturally in the air at a cooling rate of 0.2℃ / s to an ambient temperature of 30℃, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0088] During post-weld heat treatment, the first and second stages of cooling were performed 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 was 20mm away from the rail head tread. During the first stage of cooling, the compressed air pressure was 0.80MPa; during the second stage, the compressed air pressure was 0.40MPa. An infrared thermometer was used to monitor the rail head tread temperature. During isothermal heat treatment, the tracked ceramic heater in the profile heating device was 5mm away from the surface of the welded rail joint. This profile heating device was equipped with a temperature control system for real-time monitoring of the heating temperature.

[0089] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0090] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 95% and 92% of the average hardness of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Simultaneously, under a 100x metallographic microscope, the HAZ on the heat-treated eutectoid pearlitic rail side of the joint shows pearlite microstructure without martensite, while the HAZ on the heat-treated bainitic rail side shows bainite microstructure without blocky martensite. This process helps ensure railway operation safety.

[0091] Example 3

[0092] The room temperature (20-30℃) tensile and impact properties of heat-treated bainitic rail base material were controlled, with a tensile strength of 1270 MPa and a hardness of 350 HV. The chemical composition of the rail steel to obtain this microstructure and mechanical properties must meet the following conditions: 0.25% C, 1.30% Si, 1.70% Mn, 1.20% Cr, 0.35% Mo, with the balance being Fe and unavoidable impurities. The room temperature (20-30℃) tensile and impact properties of heat-treated eutectoid pearlitic rail base material were also controlled, with a tensile strength of 1350 MPa and a hardness of 400 HV. The chemical composition of rail steel that yields this microstructure and mechanical properties must meet the following conditions: 0.77% C, 0.60% Si, 0.80% Mn, 0.40% Cr, 0.07% V, with the balance being Fe and unavoidable impurities.

[0093] After the upsetting and slugging processes of the mobile flash welding of 68kg / m rails, the welded joint undergoes heat treatment. First, the rail joint, with a residual temperature of 960℃, is cooled at a first-stage cooling rate of 6.5℃ / s to reduce the surface temperature of the rail head to 530℃. Then, it is cooled at a second-stage cooling rate of 3.0℃ / s to reduce the surface temperature of the rail head to 420℃. During the removal of the profile cooling device and the installation of the profile heating device, time consumption due to factors such as operator skill level and proficiency resulted in a 40℃ temperature drop on the surface of the rail joint, meaning the surface temperature of the rail joint during isothermal heat treatment was 380℃. The rail welded joint with a surface temperature of 380℃ was then subjected to isothermal heat treatment using the profile heating device for 1 hour. After the isothermal process is completed, the conformal heating device is removed, and the rail joint is allowed to cool naturally in the air at a cooling rate of 0.5℃ / s to an ambient temperature of 25℃, thereby obtaining the rail welded joint of the present invention after post-weld heat treatment.

[0094] During post-weld heat treatment, the first and second stages of cooling were performed 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 was 20mm away from the rail head tread. During the first stage of cooling, the compressed air pressure was 0.65MPa; during the second stage, the compressed air pressure was 0.30MPa. An infrared thermometer was used to monitor the rail head tread temperature. During isothermal heat treatment, the tracked ceramic heater in the profile heating device was 5mm away from the surface of the welded rail joint. This profile heating device was equipped with a temperature control system for real-time monitoring of the heating temperature.

[0095] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0096] The rail welded joint obtained in this embodiment exhibits longitudinal hardness in the heat-affected zone (HAZ) within a 20mm radius from the weld center, reaching 93% and 90% of the average hardness of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail base materials, respectively. Simultaneously, under a 100x metallographic microscope, the HAZ on the heat-treated eutectoid pearlitic rail side of the joint shows pearlite microstructure without martensite, while the HAZ on the heat-treated bainitic rail side shows bainite microstructure without blocky martensite. This process helps ensure railway operation safety.

[0097] Comparative Example 1

[0098] The process conditions involved in the rail welding and post-weld cooling in this comparative example are the same as those in Example 1. The only difference is that after the rail welded joint completes the second stage of cooling to the surface temperature of the rail head of 440°C, this comparative example does not perform isothermal heat treatment on the rail welded joint. Instead, the joint is allowed to cool naturally in the air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0099] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0100] The welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, show a pearlitic microstructure (HID) on the heat-treated eutectoid pearlitic rail side with no martensite. The HID on the heat-treated bainitic rail side also shows bainite with no blocky martensite. However, hardness testing reveals that the longitudinal hardness of the HID within ±20mm of the weld center reaches 90% and 75% of the average hardness of the corresponding base materials for the heat-treated eutectoid pearlitic and bainitic rails, respectively. For this rail welded joint that did not undergo isothermal heat treatment, the HID on the heat-treated eutectoid pearlitic rail side exhibits good hardness, but the HID on the heat-treated bainitic rail side is relatively low. This makes it prone to saddle-shaped wear during track service, leading to HID collapse. Therefore, the rail joints obtained using this process are detrimental to railway operation safety.

[0101] Comparative Example 2

[0102] The process conditions involved in the rail welding and post-weld cooling in this comparative example are consistent with those in Example 1. The only difference is that after the second stage of cooling of the rail welded joint to a rail head surface temperature of 440°C is completed, this comparative example does not perform isothermal heat treatment on the rail welded joint. Instead, the joint is allowed to continue cooling in a rail head profile cooling device at a cooling rate of 2.0°C / s until the rail head surface temperature reaches 170°C. Subsequently, the profile cooling device applied above the rail head of the rail joint is removed, allowing the joint to be naturally cooled in the air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0103] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 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.

[0104] The welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, showed that the microstructure of the heat-affected zone (HAZ) on one side of the heat-treated eutectoid pearlitic rail was predominantly pearlite, with a small amount of blocky martensite. The HAZ on the other side of the heat-treated bainitic rail was also predominantly bainitic, with a small amount of blocky martensite. This is because the cooling rate used during the continuous cooling process from the second cooling stage until the rail head surface temperature reached 170°C exceeded the critical cooling rate for the martensitic transformation of both eutectoid pearlitic and heat-treated bainitic rail steels. Furthermore, the final cooling temperature of 170°C in this stage was below the martensitic transformation initiation temperature for both eutectoid pearlitic and heat-treated bainitic rail steels, creating conditions for the formation of brittle and hard martensite. Hardness testing showed that the longitudinal hardness of the HAZ within ±20mm of the weld center reached 92% and 83% of the average hardness of the corresponding heat-treated eutectoid pearlitic and heat-treated bainitic rail base materials, respectively. For the rail welded joint obtained in this comparative example, considering the presence of brittle and hard martensite in the heat-affected zones on both sides of the weld and the lower hardness of the heat-affected zone on the side of the heat-treated bainitic rail, the rail joint obtained under this process will be detrimental to railway operation safety.

[0105] Comparative Example 3

[0106] The process conditions involved in the rail welding process in this comparative example are the same as those in Example 2. The difference is that, in this comparative example, no heat treatment is applied to 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.

[0107] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0108] The welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, showed that the heat-affected zone (HAZ) on the heat-treated eutectoid pearlitic rail side consisted of pearlite, with no martensite. The HAZ on the heat-treated bainitic rail side consisted of bainite, with no blocky martensite. However, hardness testing showed that the longitudinal hardness of the HAZ within ±20mm of the weld center reached 87% and 79% of the average hardness of the corresponding base metals for the heat-treated eutectoid pearlitic and bainitic rails, respectively. For the rail joints obtained in this comparative example, the hardness of the HAZ on the heat-treated eutectoid pearlitic rail side was acceptable, while the hardness of the HAZ on the heat-treated bainitic rail side was too low. Under this process, the hardness of the rail joint is not within the range of 90-95% and 87-92% of the average hardness of the base material of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail, respectively, which are achieved within the ±20mm range from the weld center obtained by the present 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.

[0109] Comparative Example 4

[0110] The rail welding process conditions in this comparative example are the same as those in Example 2. 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 910°C. Subsequently, the process parameters and conditions involved in the second stage cooling and isothermal heat treatment are also the same as in Example 2, thus obtaining the rail joint of this comparative example.

[0111] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0112] The welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, showed that the heat-affected zone (HAZ) on the heat-treated eutectoid pearlitic rail side of the joint was pearlitic with no martensite. The HAZ on the heat-treated bainitic rail side of the joint was bainitic with no blocky martensite. However, hardness testing showed that the longitudinal hardness of the HAZ within ±20mm from the weld center reached 89% and 86% of the average hardness of the corresponding base metals for the heat-treated eutectoid pearlitic and bainitic rails, respectively. For the rail joints obtained in this comparative example, the hardness of the HAZ on the heat-treated eutectoid pearlitic rail side was acceptable, but the hardness of the HAZ on the heat-treated bainitic rail side was relatively low. Under this process, the hardness of the rail joint is not within the range of 90-95% and 87-92% of the average hardness of the base material of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail, respectively, which are achieved within the ±20mm range from the weld center obtained by the present 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.

[0113] Comparative Example 5

[0114] The rail welding and post-weld cooling process conditions in this comparative example are largely the same as those in Example 3. The difference is that in this comparative example, after the rail joint undergoes a second stage of cooling to 330°C, it is subjected to isothermal heat treatment, with a holding time of 1 hour. Subsequently, the cooling conditions are also consistent with those in Example 3, thus obtaining the rail joint of this comparative example.

[0115] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0116] The welded joints of the rails obtained in this comparative example, observed under a 100x metallographic microscope, show a pearlitic microstructure (HHA) on one side of the heat-treated eutectoid pearlitic rail, with no martensite. However, the HHA on the other side of the heat-treated bainitic rail is predominantly bainitic, with a significant amount of blocky martensite. Hardness testing shows that the longitudinal hardness of the HHA within ±20mm of the weld center reaches 93% and 91% of the average hardness of the corresponding base metals of the heat-treated eutectoid pearlitic and bainitic rails, respectively. This is because the cooling rate of the second stage is 3.0℃ / s, which is above the critical cooling rate for martensitic transformation in both the heat-treated bainitic and eutectoid pearlitic rails. Furthermore, the final cooling temperature of 330℃ in the second stage is within the martensitic transformation initiation temperature range of the heat-treated bainitic rail steel, but higher than the martensitic transformation initiation temperature range of the heat-treated eutectoid pearlitic rail steel. Therefore, martensite forms in the heat-affected zone (HAZ) of heat-treated bainitic rails after the second-stage cooling process, while no martensite forms in the HAZ of heat-treated eutectoid pearlitic rails. Subsequent isothermal heat treatment cannot eliminate the already formed martensite. For the rail joints obtained in this comparative example, both the heat-affected zone side of the heat-treated eutectoid pearlitic rail and the heat-treated bainitic rail side exhibit good HAZ hardness. Considering the significant amount of brittle and hard martensite formed in the HAZ of the heat-treated bainitic rail, the rail joints obtained under this process will be detrimental to railway operation safety.

[0117] Comparative Example 6

[0118] 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 570°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.

[0119] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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.

[0120] The welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, show a pearlitic microstructure (HAM) on one side of the heat-treated eutectoid pearlitic rail, with no martensite. The HAM on the other side of the heat-treated bainitic rail also shows bainite, without blocky martensite. However, hardness testing indicates that the longitudinal hardness of the HAM within ±20mm of the weld center reaches 86% and 87% of the average hardness of the corresponding base metals for the heat-treated eutectoid pearlitic and bainitic rails, respectively. This may be due to the relatively high final cooling temperature of 570℃ in the first stage, resulting in insufficient refinement of the pearlitic lamellar structure at this stage, thus leading to a less significant increase in hardness in the HAM on the heat-treated eutectoid pearlitic rail side.

[0121] For the rail joints obtained in this comparative example, the hardness of the weld heat-affected zone on one side of the heat-treated eutectoid pearlitic rail does not reach 90-95% of the average hardness of the rail base material within the ±20mm area from the weld center obtained using this invention. However, the hardness of the weld heat-affected zone on one side of the heat-treated bainitic rail is within the range of 87-92% of the average hardness of the rail base material, indicating that the overall hardness of the joint is too low. Therefore, the rail joints obtained under this process will be detrimental to railway operation safety.

[0122] Comparative Example 7

[0123] The process conditions involved in the rail welding and post-weld cooling in this comparative example are the same as those in Example 1. The only difference is that in this comparative example, after the second stage of cooling of the rail welded joint to a surface temperature of 440°C, the isothermal heat treatment of the rail welded joint is carried out for 0.5 hours. After the isothermal process, the conformal heating device is removed, and the rail joint is allowed to cool naturally in the air at a cooling rate of 0.9°C / s to an ambient temperature of 20°C, thereby obtaining the rail joint of this comparative example.

[0124] The rail joints are machined into longitudinal hardness test specimens. According to GB / T 230.1-2009, [the process is as follows]. Figure 2 Longitudinal Vickers HV hardness test was performed on the joint at a position 5mm below the tread surface. The test points were arranged symmetrically to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 3 The sampling 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 welded rail joints obtained in this comparative example, observed under a 100x metallographic microscope, show a pearlitic microstructure (HAM) on one side of the heat-treated eutectoid pearlitic rail, with no martensite. The HAM on the other side of the heat-treated bainitic rail also shows bainite, without blocky martensite. However, hardness testing indicates that the longitudinal hardness of the HAM within ±20mm of the weld center reaches 90% and 86% of the average hardness of the corresponding heat-treated eutectoid pearlitic and bainitic rail base materials, respectively. For the rail joints obtained in this comparative example, the hardness of the HAM on the heat-treated eutectoid pearlitic rail side reaches 90-95% of the average hardness of the rail base material within ±20mm of the weld center obtained using this invention. However, due to the short isothermal time of 0.5 hours, the bainitic lath structure is not fully 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.

[0126] After implementation, this invention has at least the following beneficial effects:

[0127] (1) By implementing staged controlled cooling and isothermal heat treatment on the welded joints of dissimilar materials after welding, the heat-affected zones on both sides of the joint weld can maintain high strength and hardness, thereby ensuring the wear resistance of the rail joint.

[0128] (2) It can ensure that there is no martensite in the weld heat-affected zone on one side of the heat-treated eutectoid pearlitic rail joint and no obvious blocky martensite in the weld heat-affected zone on one side of the heat-treated bainitic rail joint.

[0129] (3) The longitudinal hardness of the rail joint within a ±20mm radius from the weld center can reach 90-95% and 87-92% of the average hardness of the base material of the corresponding heat-treated eutectoid pearlitic rail and heat-treated bainitic rail, respectively, which helps to ensure railway operation safety. The technical advantages of this invention are very obvious, and the market promotion prospects are broad.

[0130] The above are exemplary embodiments disclosed in this invention. The order of the disclosed embodiments is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. However, it should be noted that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of this invention (including the claims) is limited to these examples. Various changes and modifications can be made without departing from the scope defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular.

[0131] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A heat treatment method for a welded joint between eutectoid pearlitic steel rail and bainitic steel rail, characterized in that, Includes the following steps: S1. The welded joint formed by welding eutectoid pearlitic steel rail and bainitic steel rail with a surface temperature of 920-990°C is subjected to a first stage of cooling to reduce the surface temperature of the welded joint to 500-550°C. The first stage of cooling is carried out by using a rail profile cooling device, and the cooling rate is 5.0-8.0°C / s. S2. Perform a second stage of cooling on the welded joint to reduce the surface temperature of the welded joint to 390-440°C. The second stage of cooling is performed using a rail-following cooling device at a cooling rate of 2.0-4.0°C / s. S3. The welded joint is subjected to isothermal heat treatment. The surface temperature of the welded joint in this stage is 340-400℃. The welded joint is subjected to isothermal heat treatment using a rail profile heating device, and the heat treatment time is 1 hour. S4. Stop the isothermal heat treatment and remove the rail profile heating device, allowing the welded joint to cool naturally to an ambient temperature of 20-30°C. The cooling rate during this stage is 0.2-0.9°C / s.

2. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, By weight percentage, the base material of the eutectoid pearlitic steel rail comprises C: 0.73~0.80%, Si: 0.50~0.70%, Mn: 0.70~0.9%, Cr: 0.30~0.50%, V: 0.06~0.09%, with the balance being Fe and unavoidable impurities.

3. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, The eutectoid pearlitic rail base material has a tensile strength of 1320~1380MPa and a hardness of 380~420HV at room temperature of 20~30℃.

4. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, By weight percentage, the base material of the bainitic rail comprises C: 0.20~0.30%, Si: 0.9~1.6%, Mn: 1.4~2.0%, Cr: 0.70~1.50%, Mo: 0.20~0.50%, with the balance being Fe and unavoidable impurities.

5. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, The bainitic rail base material has a tensile strength of 1250~1290MPa and a hardness of 330~370HV at room temperature of 20~30℃.

6. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, In steps S1 and S2, the cooling using the rail profile cooling device includes spraying a mixture of compressed air and / or water mist onto the welded joint.

7. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, In step S1, the rail profile cooling device is 10-30mm away from the surface of the welded joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.50-0.80MPa; in step S2, the rail profile cooling device is 10-30mm away from the surface of the welded joint, and the pressure of the compressed air and / or water mist mixture sprayed is 0.20-0.40MPa.

8. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, The welded joint is formed by welding heat-treated eutectoid pearlitic steel rails and heat-treated bainitic steel rails with the same rail type and specifications of 60-75 kg / m using a rail moving flash welding machine.

9. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, In step S3, the isothermal heat treatment includes using a rail profile heating device to perform full-section isothermal heat treatment on the welded joint, wherein the distance between the rail profile heating device and the surface of the welded joint is 3~10mm.

10. The heat treatment method for the welded joint of eutectoid pearlitic steel rail and bainitic steel rail according to claim 1, characterized in that, The rail profile heating device used in step S3 is a split-type rail profile heating device, which includes: The device housing is fitted onto the outside of the rail and is in the shape of an I-beam. The device housing includes a first housing and a second housing that are opposite to each other. The upper parts of the first housing and the second housing are pivotally connected by a rotating shaft. Tracked ceramic heaters, a plurality of said tracked ceramic heaters are arranged in parallel on the inner surface of the device housing, for providing a heat source for isothermal heat treatment; Two terminals are located diagonally on the upper surface of the device housing; and an insulation layer is located inside the device housing.

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