Method for improving impact toughness matching performance of welded joint of steel rail made of dissimilar materials

Abstract

The invention belongs to the field of steel rail manufacturing, and discloses a method for improving the impact toughness matching performance of a dissimilar material steel rail welding joint. The method comprises the following steps: carrying out first-stage cooling on a welding joint formed by welding the eutectoid pearlite steel rail and the bainite steel rail; carrying out full-section normalizing heating on the welded joint; the welding joint is subjected to second-stage cooling, a heat affected zone on one side of the heat treatment type eutectoid pearlite steel rail is cooled to 350-400 DEG C at the cooling rate of 4.0-7.0 DEG C / s, and a heat affected zone on one side of the hot rolling type bainite steel rail is cooled to 290-315 DEG C at the cooling rate of 2.0-3.0 DEG C / s; the welding joint is heated to a bainite transformation temperature interval for isothermal treatment; and naturally cooling the welding joint in an air environment. According to the method provided by the invention, the room-temperature impact toughness of the heat affected zone on one side of the hot-rolled bainite steel rail can be obviously improved, so that the performance of the hot-rolled bainite steel rail is better matched with that of a hot-rolled bainite steel rail base material.

Description

Technical Field

[0001] The invention relates to the technical field of rail manufacturing, and in particular to a method for improving the impact toughness matching of welded joints of rails made of different materials. Background Art

[0002] At present, the rails in service at home and abroad are mainly pearlite rails, which are composed of ferrite and cementite distributed alternately. The strength of pearlite steel can be improved by heat treatment or alloying. However, actual use shows that its toughness is low and it is prone to damage such as rolling contact fatigue. Compared with pearlite rails, low-carbon bainite rails have better toughness and strength matching, excellent resistance to rolling contact fatigue and other properties, and have attracted the attention of scholars at home and abroad. With the gradual application of bainite rails, the mixed use of bainite rails and existing pearlite rails on railway lines has undoubtedly brought huge challenges to the welding of dissimilar rails.

[0003] For welding of a single hot-rolled or heat-treated eutectoid pearlite rail, the room temperature impact toughness of the weld and the coarse grain heat affected zone near the weld (within ±10mm from the weld center) is usually 5~15J, while the room temperature impact toughness of the rail base material is usually 10~25J, and the difference between the two is small. For welding of a single hot-rolled or heat-treated bainite rail, the room temperature impact toughness of the weld and the coarse grain heat affected zone near the weld is usually 20~40J, while the room temperature impact toughness of the rail base material is usually 80~170J, and the difference between the two is large. For the dissimilar rail joint formed by welding heat-treated eutectoid pearlite rail and hot-rolled bainite rail, the room temperature impact toughness of the weld and the coarse-grained heat-affected zone near the weld in the welded state is usually 10~30J, which is equivalent to the room temperature impact toughness of the hot-rolled or heat-treated eutectoid pearlite rail base material, but significantly lower than the room temperature impact toughness of the hot-rolled or heat-treated bainite rail base material, resulting in the inconsistency between the impact toughness of the weld joint and the bainite rail base material.

[0004] Patent CN112251587A discloses a heat treatment method for a welded joint of a bainitic rail and a eutectoid pearlite rail, which belongs to the technical field of post-weld heat treatment of rails. The heat treatment method comprises: first cooling the welded joint to 720-760°C; then cooling it to 350-400°C for the second time; then cooling it to 100-180°C for the third time; then performing a tempering heat treatment; and finally cooling it naturally to room temperature. However, this method aims to improve the hardness of the welded joint and the heat-affected zones on both sides, so as to improve the problem of "saddle-shaped" wear of the welded joint of the rails caused by the low hardness of the welding area during the service of the rails on the line, but does not mention the matching of the impact toughness of the weld and the heat-affected zones on both sides of the weld.

[0005] Patent CN118773430A discloses a heat treatment method for a welded joint of a eutectic pearlite rail and a bainite rail. The method comprises the following steps: S1, cooling the welded joint formed by welding the eutectic pearlite rail and the bainite rail with a surface temperature of 920-990°C in the first stage, so that the surface temperature of the welded joint is reduced to 500-550°C, and the first stage cooling is performed by using a rail profiling cooling device for cooling, and the cooling rate is 5.0-8.0°C / s; S2, cooling the welded joint in the second stage, so that the surface temperature of the welded joint is reduced to 390-440°C, and the second stage cooling is performed by using a steel rail profiling cooling device for cooling. The rail profiling cooling device is used for cooling, and the cooling rate is 2.0~4.0℃ / s; S3, isothermal heat treatment is performed on the welded joint. The surface temperature of the welded joint in this stage is 340~400℃. In this stage, the rail profiling heating device is used for isothermal heat treatment of the welded joint, and the holding time is 0.5~1h; S4, the isothermal heat treatment is stopped and the rail profiling heating device is removed, so that the welded joint is naturally cooled to an ambient temperature of 20~30℃. The cooling rate in this stage is 0.2~0.9℃ / s. This method performs temperature-controlled cooling and isothermal heat treatment on the welded joint of dissimilar rails after welding, so that 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 on one side of the heat-treated eutectoid pearlite rail of the rail joint is pearlite without martensite, and it can ensure that the weld heat-affected zone on one side of the heat-treated bainite rail of the rail joint is bainite without obvious blocky martensite; at the same time, the longitudinal hardness of the rail joint within the ±20mm area from the center of the weld can reach 90~95% and 87~92% of the average hardness of the corresponding heat-treated eutectoid pearlite rail and heat-treated bainite rail parent materials, respectively. However, this method does not study and explain the matching of the impact toughness between the weld and the heat-affected zones on both sides of the weld and the parent material.

[0006] Therefore, how to improve the room temperature impact toughness of the area near the weld, achieve a good match between the impact toughness of the weld and the heat-affected zone on both sides of the weld and the rail parent material, and a smooth transition of the impact toughness in the weld joint area has become the key to the application of dissimilar material rail welding. Summary of the invention

[0007] In view of the above problems, the present invention provides a method for improving the impact toughness matching of welded joints of rails of different materials.

[0008] According to one aspect of the present invention, a method for improving the impact toughness matching of a welded joint of a rail of different materials is provided, the method comprising the following steps: Step S1: performing a first stage cooling on a welded joint formed by welding a heat-treated eutectoid pearlite rail and a hot-rolled bainite rail, so that the surface temperature of the welded joint is reduced to a first temperature; Step S2: performing full-section normalizing heating on the weld joint that has been cooled in the first stage to austenitize the weld joint; Step S3: After normalizing, the weld joint is cooled in the second stage, wherein the heat-affected zone on one side of the heat-treated eutectoid pearlite rail is cooled to 350-400°C at a cooling rate of 4.0-7.0°C / s, and the heat-affected zone on one side of the hot-rolled bainite rail is cooled to 290-315°C at a cooling rate of 2.0-3.0°C / s; Step S4: after the second stage cooling is completed, the welding joint is heated to the bainite transformation temperature range for isothermal treatment; Step S5: After the isothermal treatment is completed, the weld joint is naturally cooled in an air environment.

[0009] According to one embodiment of the present invention, the base material of the heat-treated eutectoid pearlite rail comprises the following chemical components by weight percentage: C: 0.77%-0.83%, Si: 0.45%-0.90%, Mn: 0.8%-1.2%, V: 0.1%-0.5%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 10-18 J; the base material of the hot-rolled bainite rail comprises the following chemical components by weight percentage: C: 0.20%-0.35%, Si: 1.0%-1.8%, Mn: 1.2%-1.9%, Cr: 0.80%-1.60%, Mo: 0.20%-0.50%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 140-170 J.

[0010] According to an embodiment of the present invention, in step S1 , the first stage cooling is natural cooling in an air environment.

[0011] According to an embodiment of the present invention, in step S1, the first temperature is 100-200°C.

[0012] According to one embodiment of the present invention, in step S2, normalizing heating is performed to 990-1060°C.

[0013] According to one embodiment of the present invention, in step S2, a medium frequency induction profile electric heating coil and / or an oxygen-acetylene flame profile heater are used to perform full-section normalizing heating on the rail joint.

[0014] According to one embodiment of the present invention, in step S3, the second stage cooling is implemented by using a rail head and tread air jet device, using compressed air or a water mist mixture as a cooling medium to differentially control cooling of both sides of the weld of the weld joint.

[0015] According to one embodiment of the present invention, the distance between the bottom cooling channel of the rail head tread air jet device and the surface of the welded joint is controlled to be 25-30 mm, the air jet pressure in the heat-affected zone on one side of the heat-treated eutectoid pearlite rail is 0.40-0.70 MPa, and the air jet pressure in the heat-affected zone on one side of the hot-rolled bainite rail is 0.20-0.30 MPa.

[0016] According to one embodiment of the present invention, the bainite transformation temperature range is 325-355° C., and the isothermal treatment time is 0.5-1 h.

[0017] According to one embodiment of the present invention, in step S5, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0018] Due to the adoption of the above technical solution, the method of the present invention has at least one of the following beneficial effects: (1) The method for improving the impact toughness matching of the welded joint of dissimilar rail provided by the present invention is to adopt different cooling intensities (cooling rates) for cooling the heat-affected zones on both sides of the weld after the normalizing heating of the dissimilar rail joint is completely austenitized. For the pearlite side, the pearlite lamellar spacing in the heat-affected zone of the heat-treated eutectoid pearlite rail normalizing is refined by rapid cooling, the toughness of the heat-affected zone on this side is improved, and the strength and hardness of the zone are also improved. The cooling end point is controlled to avoid the formation of brittle martensitic structure due to the final cooling temperature being lower than the martensitic transformation starting temperature (Ms point) of the rail steel during the cooling process at this stage. For the bainite side, the temperature is reduced to 5-10°C below the martensitic transformation starting temperature (Ms point) of the bainitic rail steel by rapid cooling, aiming to introduce a small amount of martensite as the nucleation site of the bainitic structure during the subsequent isothermal heat treatment process by rapid cooling. The joint is then heated to the bainite transformation temperature range for isothermal treatment to produce a refined bainite structure and improve the strength and toughness of the heat-affected zone on this side. At the same time, by controlling the cooling end point, a large amount of brittle martensite structure is avoided to be formed due to the low final cooling temperature during the cooling process at this stage. During the martensite phase transformation, many fine subgrains can be formed inside the original austenite grains. These subgrains can serve as new nucleation points in the subsequent heating process, promoting the refinement of the bainite structure and improving its strength and toughness. (2) The joint weld and the heat-affected zone within ±10 mm from the center of the weld treated by the method of the present invention maintain high room temperature impact toughness, thereby ensuring the impact fracture resistance of the rail joint. It can be achieved that there is no martensite in the welding heat-affected zone on the side of the heat-treated eutectoid pearlite rail of the dissimilar rail welding joint, and there is no obvious block martensite in the welding heat-affected zone on the side of the hot-rolled bainite rail of the rail welding joint. At room temperature, the average impact toughness of the full-section weld of the joint is 40~55J, which is much higher than TB / T 1632.2-2014 stipulates ≥6.5J. At the same time, the average room temperature impact toughness of the heat-affected zone of the heat-treated eutectoid pearlite rail on one side of the weld center within the area 10mm away from the weld center is 9~17J, and the average room temperature impact toughness of the heat-affected zone of the hot-rolled bainite rail on one side of the weld center is 60~80J. Compared with the previous heat treatment method, the room temperature impact toughness of the heat-affected zone of the hot-rolled bainite rail on one side has been significantly improved, which is more matched with the performance of the hot-rolled bainite rail base material. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A flow chart of a method for improving the impact toughness matching of a welded joint of a rail of different materials according to an embodiment of the present invention; Figure 2 A schematic structural diagram of a spray panel of a rail head tread air spray device provided by an embodiment of the present invention; Figure 3 A schematic diagram of the bottom nozzle structure of the rail head tread air spray device provided in an embodiment of the present invention; Figure 4 A schematic side structural diagram of a rail head tread air jet device provided in an embodiment of the present invention; Figure 5 An overall schematic diagram of a split rail profile heating device provided by an embodiment of the present invention; Figure 6 A schematic diagram of the distribution of electric heaters in the rail head area corresponding to the split rail profiling heating device provided in an embodiment of the present invention; Figure 7 The sampling position diagram of the impact performance test of the rail welded joint obtained according to the methods in the embodiments and comparative examples of the present invention (unit: mm); Figure 8 Schematic diagram of the sampling position of metallographic specimens of rail head and tread of rail welded joint. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] like Figure 1 As shown, the method for improving the impact toughness matching of the welded joint of rails of different materials provided by the present invention generally comprises the following steps: Step S1: performing a first stage cooling on a welded joint formed by welding a heat-treated eutectoid pearlite rail and a hot-rolled bainite rail, so that the surface temperature of the welded joint is reduced to a first temperature; Step S2: normalizing the weld joint after the first stage of cooling to austenitize the weld joint; Step S3: After normalizing, the weld joint is cooled in the second stage, wherein the heat-affected zone on one side of the heat-treated eutectoid pearlite rail is cooled to 350-400°C at a cooling rate of 4.0-7.0°C / s, and the heat-affected zone on one side of the hot-rolled bainite rail is cooled to 290-315°C at a cooling rate of 2.0-3.0°C / s; Step S4: After the second stage cooling is completed, the welding joint is heated to the bainite transformation temperature range for isothermal treatment; Step S5: After the isothermal treatment is completed, the welded joint is naturally cooled in an air environment.

[0022] The method for improving the impact toughness matching of the welded joint of the dissimilar rail provided by the present invention is to adopt different cooling intensities (cooling rates) for cooling the heat-affected zones on both sides of the weld after the normalizing heating of the dissimilar rail joint is completely austenitized. For the pearlite side, the pearlite lamellar spacing in the heat-affected zone of the heat-treated eutectoid pearlite rail normalizing is refined by rapid cooling, the toughness of the heat-affected zone on this side is improved, and the strength and hardness of the zone are also improved. The cooling end point is controlled to avoid the formation of a brittle martensitic structure during the cooling process of this stage due to the final cooling temperature being lower than the martensitic transformation starting temperature (Ms point) of the rail steel. For the bainite side, the temperature is reduced to 5-10°C below the martensitic transformation starting temperature (Ms point) of the bainitic rail steel by rapid cooling, aiming to introduce a small amount of martensite as the nucleation site of the bainitic structure during the subsequent isothermal heat treatment process by rapid cooling. The joint is then heated to the bainite transformation temperature range for isothermal treatment to produce a refined bainite structure and improve the strength and toughness of the heat-affected zone on this side. At the same time, by controlling the cooling end point, it is possible to avoid the formation of a large amount of brittle martensite structure due to the low final cooling temperature during the cooling process at this stage. During the martensitic transformation, many fine subgrains can be formed inside the original austenite grains. These subgrains can serve as new nucleation points during the subsequent heating process, promoting the refinement of the bainite structure and improving its strength and toughness.

[0023] The joint weld and the heat-affected zone within the area of ​​±10 mm from the center of the weld treated by the method of the present invention maintain high room temperature impact toughness, thereby ensuring the impact fracture resistance of the rail joint, and achieving the goal of no martensite in the welding heat-affected zone on one side of the heat-treated eutectoid pearlite rail of the dissimilar rail welding joint, and no obvious blocky martensite in the welding heat-affected zone on one side of the hot-rolled bainite rail of the rail welding joint. At room temperature, the average impact toughness of the full-section weld of the joint is 40-55 J, which is much higher than ≥6.5 J specified in TB / T 1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on one side of the heat-treated eutectoid pearlite rail within the area of ​​10 mm from the center of the weld is 9-17 J, and the average room temperature impact toughness of the heat-affected zone on one side of the hot-rolled bainite rail within the area of ​​10 mm from the center of the weld is 60-80 J, achieving a smooth transition of the impact toughness of the welding joint area and a good match with the rail base material.

[0024] In some embodiments, the base material of the heat-treated eutectoid pearlite rail includes the following chemical components by weight: C: 0.77%-0.83%, Si: 0.45%-0.90%, Mn: 0.8%-1.2%, V: 0.1%-0.5%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 10-18 J. The base material of the hot-rolled bainite rail includes the following chemical components by weight: C: 0.20%-0.35%, Si: 1.0%-1.8%, Mn: 1.2%-1.9%, Cr: 0.80%-1.60%, Mo: 0.20%-0.50%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 140-170 J.

[0025] In some embodiments, in step S1, the welded joint is obtained by flash welding. Flash welding has many advantages such as heat concentration, short heating time during welding, and small heat-affected zone of the welded joint. The surface temperature of the joint after welding is about 800-1000°C. The first stage cooling is natural cooling in an air environment of 20-30°C, so that the surface temperature of the welded joint is reduced to 100-200°C.

[0026] It should be noted that the cooling rate in the first stage should not be too high to prevent the formation of large-area martensite in the heat affected zone of welding due to too fast cooling rate and microcracks due to excessive internal stress of the organization. When microcracks are formed inside the organization, they cannot be eliminated by subsequent post-weld heat treatment. In the process of natural cooling, there is a situation where the cooling rate is faster in the high temperature stage and relatively slower in the low temperature stage, and the cooling rate of each temperature section is different. Specifically: after the rail welding is completed, due to the convection, radiation and heat conduction with the surrounding medium, the rail joint is naturally cooled in an air environment of 20~30℃, and there is a phenomenon of rapid cooling in the high temperature stage and gradual slowing of the cooling rate in the low temperature stage. The average cooling rate of natural cooling in the temperature range of 1100~801℃ is 4.0~2.5℃ / s. The cooling rate of natural cooling in the temperature range of 800~501℃ is 2.5~1.2℃ / s. The cooling rate of natural cooling in the temperature range of 500~301℃ is 1.2~0.7℃ / s. In the temperature range of 300~20℃, the cooling rate of natural cooling is 0.7~0.1℃ / s. At the same time, without considering the component segregation of rail steel, the natural cooling of rail welded joints directly in the air after welding and the natural cooling in the air after normalizing heating usually do not cause the formation of brittle martensite in the heat affected zone.

[0027] In some embodiments, in step S2, the weld joint that has been cooled in the first stage is subjected to full-section normalizing heating to austenitize the weld joint. In this step, a medium-frequency induction profile electric heating coil and / or an oxygen-acetylene flame profile heater are used to perform full-section normalizing heating on the rail joint, and heating is stopped when the surface temperature of the weld joint is heated to 990-1060°C, completing the normalizing heating process.

[0028] After the normalizing heating is completed, the natural cooling and possible artificial transfer of the welded joint process will inevitably lead to a temperature drop of 30~50℃. In addition, it should be noted that when the normalizing temperature is too high (such as higher than 1100℃), the austenite grains in the heated area (especially the weld and its critical coarse-grained heat-affected zone) will be too large, which will affect the impact toughness of the subsequent joint. When the normalizing heating temperature is low, such as heating the surface temperature of the welded joint to 950℃ and below, due to the low phase transformation driving force during the cooling process, the microstructure refinement effect during the cooling process will not be obvious, which is manifested as the improvement of the hardness and impact toughness of the heat-affected zone of the rail welding after heat treatment is not ideal. When the normalizing heating temperature is lower than 880℃, due to the incomplete austenitization process of the heat-affected zone of the bainitic rail welding of the welded joint, the phase transformation in the subsequent cooling process will be insufficient, which will lead to the improvement of the hardness and impact toughness of the heat-affected zone of the rail welding after heat treatment.

[0029] In step S3, after normalizing, the weld joint is cooled in the second stage, wherein the heat affected zone on one side of the heat treated eutectoid pearlite rail is cooled to 350-400°C at a cooling rate of 4.0-7.0°C / s, and the heat affected zone on one side of the hot rolled bainite rail is cooled to 290-315°C at a cooling rate of 2.0-3.0°C / s.

[0030] After the rail joint is normalized and heated to the complete austenitization temperature, the heat-affected zone on both sides of the weld is cooled with different cooling intensities (cooling rates). For the left side of the weld (pearlite side), rapid cooling is used to refine the interlamellar spacing of the pearlite structure in the heat-affected zone of the heat-treated eutectoid pearlite rail, and improve the impact toughness, strength and hardness of the heat-affected zone on this side. At the same time, it should also be avoided that the cooling process is improperly controlled during this stage, such as the formation of brittle martensitic structure due to excessive cooling rate and final cooling temperature lower than the martensitic transformation starting temperature (Ms point) of the rail steel. For the right side of the weld (bainite side), the temperature of the heat-affected zone of the hot-rolled bainitic rail is reduced to 5~10℃ below the martensitic transformation starting temperature (Ms point) of the rail steel through rapid cooling, aiming to introduce a small amount of martensite as the nucleation site of the bainite structure during the subsequent isothermal heat treatment. Subsequently, the joint is heated to the bainite phase transformation temperature range through the profiling heating device to generate refined bainite structure, improve the impact toughness, strength and hardness of the heat-affected zone on this side, and at the same time, the formation of a large amount of brittle and hard martensite structure due to the final cooling temperature being significantly lower than the martensite phase transformation starting temperature (Ms point) of the bainite rail steel during the cooling process at this stage should be avoided. If a large amount of brittle and hard martensite structure is formed in the heat-affected zone of the hot-rolled bainite rail due to improper control of the cooling process at this stage, it cannot be eliminated through the subsequent isothermal treatment process, which will deteriorate the impact toughness, strength and hardness of the heat-affected zone on this side.

[0031] In some embodiments, the second stage cooling is implemented using a rail head and tread air jet device, using compressed air or a water mist mixture as a cooling medium to differentially control the cooling of both sides of the weld of the weld joint.

[0032] Figure 2 A schematic diagram of the structure of the spray panel of the rail head tread air spray device provided by an embodiment of the present invention, Figure 3 A schematic diagram of the bottom nozzle structure of the rail head tread air spray device provided in an embodiment of the present invention, Figure 4 A side structural schematic diagram of a rail head tread air jet device provided in an embodiment of the present invention.

[0033] As shown in the figure, the two cooling assemblies are separated by a central partition plate located in the middle of the device. The cooling assembly on the left is arranged corresponding to the heat-affected zone of the heat-treated eutectoid pearlite rail according to the present invention, and the cooling assembly on the right is arranged corresponding to the heat-affected zone of the hot-rolled bainite rail according to the present invention. Each cooling assembly includes a spray panel located above the tread of the rail head of the dissimilar rail welded joint (see Figure 2 The left side of the center partition corresponds to the bottom injection hole 3 and the right side corresponds to the bottom injection hole 4), and the injection pipes connected to the injection panel and facing the two sides of the rail head (see Figure 3 The left side of the central partition plate corresponds to the side injection hole 5 and the right side corresponds to the side injection hole 6). Multiple injection holes are arranged on the injection panel and the injection pipe, and the injection holes are connected with the external cooling medium fluid. Specifically, the injection hole 3 on the cooling component on the left side of the device is connected with the external cooling medium through the channel 1; the injection hole 4 on the cooling component on the right side of the device is connected with the external cooling medium through the channel 2.

[0034] In the present invention, when the rail head and tread air jet device is used to cool the rail joint, the rail joint is under the air jet device (such as Figure 4 As shown in the figure, the actual cooling rate can be adjusted by controlling the pressure of the cooling medium flowing into the device, thereby achieving controlled cooling of the left and right halves of the rail weld joint with different cooling rates / cooling intensities.

[0035] In some embodiments, the distance between the bottom cooling channel of the rail head and tread air jet device and the surface of the welded joint is controlled to be 25-30 mm. The cooling medium of the rail head and tread air jet device is a compressed air and / or water mist mixture, wherein the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling assembly corresponding to the heat-affected zone of the heat-treated eutectoid pearlite rail on the left side of the rail joint weld is 0.40-0.70 MPa, and the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling assembly corresponding to the heat-affected zone of the hot-rolled bainite rail on the right side of the rail joint weld is 0.20-0.30 MPa.

[0036] By injecting compressed air and / or water mist mixture to quickly cool the heat affected zone of heat treated eutectoid pearlite rail and the heat affected zone of hot rolled bainite rail above the austenitizing temperature, the pearlite lamella spacing in the heat affected zone of the heat treated eutectoid pearlite rail can be refined and thermodynamic conditions can be provided for the heat affected zone of the hot rolled bainite rail to obtain a small amount of martensite structure through cooling.

[0037] Specifically, the pressure of the gas medium sprayed by the cooling assembly can be adjusted by the number and aperture size of the spray holes 3 and 4 to meet the cooling effect of the heat-affected zone on the left side of the weld of the heat-treated eutectoid pearlite rail and the heat-affected zone on the right side of the weld of the dissimilar rail welding joint.

[0038] like Figure 4 As shown, the rail head tread air jet device cools the rail head tread and the side of the rail head on the left and right sides of the weld of the rail welding joint. The aperture size of the jet channel can be designed and processed according to actual needs to achieve cooling of different intensities. The gas pressure of the compressed air flowing through channel 1 and channel 2 can be monitored by a pressure gauge and specifically adjusted according to actual needs. In the present invention, the compressed air and / or water mist mixed gas ejected by the cooling component has the same cooling rate / cooling capacity.

[0039] In the present invention, the critical cooling rate of martensite transformation during the continuous cooling process of heat-treated eutectoid pearlite rail steel is 2.0~2.5℃ / s, and the Ms temperature of rail steel (the starting temperature of martensite formation) is 250~280℃. The critical cooling rate of martensite transformation during the continuous cooling process of hot-rolled bainite rail steel is 1.3~1.6℃ / s, and the Ms temperature of rail steel (the starting temperature of martensite formation) is 300~320℃. In order to avoid the appearance of abnormal structures such as martensite in the heat-affected zone of the rail welding joint on the pearlite side, when the welded joint is subjected to post-weld heat treatment, the final cooling temperature of the heat-affected zone on the pearlite side during the rapid cooling process of the post-weld heat treatment needs to be controlled above the Ms temperature. In the present invention, the final cooling temperature is controlled to be 350~400℃. For the bainite side, a small amount of martensite is generated in the heat-affected zone, and a small amount of martensite is introduced by rapid cooling as a nucleation site for bainite structure during the subsequent isothermal heat treatment. Subsequently, the entire joint is heated to the bainite phase transformation temperature range through a profiling heating device to generate a refined bainite structure and improve the impact toughness of the heat-affected zone on this side. To this end, the temperature of the heat-affected zone of the bainite rail is reduced to 5~10℃ below the martensitic phase transformation starting temperature (Ms point) of the rail steel.

[0040] In step S4, after the second stage cooling is completed, the temperature of the weld joint is raised to the bainite transformation temperature range for isothermal treatment. In some embodiments, the bainite transformation temperature range is 325-355°C, and the isothermal treatment time is 0.5-1h.

[0041] For the isothermal heat treatment after the second stage cooling, when the holding time of the isothermal process is less than 0.5h, the mechanical properties such as the impact toughness of the joint are not significantly improved. When the holding time of the isothermal process exceeds 1h, the mechanical properties such as the impact toughness of the joint are not significantly different from those when the holding time of the isothermal process is 1h, and the longer holding time will seriously reduce the production efficiency of the heat treatment of the rail joint. Therefore, the present invention controls the holding time of the isothermal heat treatment to 0.5~1h. According to the principle of metallurgy, bainite structure can be obtained by isothermal transformation. In the present invention, the rail joint is subjected to isothermal heat treatment at a temperature range of 325~355℃ by a split-type profiling heating device, and the holding time is 0.5~1h. The isothermal heat treatment can promote bainite transformation, stabilize the residual austenite in bainitic steel, and appropriately improve the mechanical properties such as the impact toughness of the heat-affected zone of the heat-treated bainitic rail steel. On the other hand, during the welding process, residual stress will be generated in the weld and its surrounding areas due to the unevenness of thermal expansion and cooling contraction. If these residual stresses are not eliminated, they may cause 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 weld joint can be effectively reduced and evenly released, improving the stability and reliability of the joint.

[0042] In some embodiments, the isothermal heat treatment includes isothermal heat treatment of the entire cross section of the joint using a rail profile heating device, and the distance between the rail profile heating device and the surface of the welded joint is 3 to 10 mm. Preferably, the distance between the rail profile heating device and the surface of the welded joint is 5 mm.

[0043] Figure 5 The invention shows a split rail profile heating device for isothermal treatment, which comprises: a device shell F1, which is sleeved on the outer side of the rail and is in an I-shape, and comprises a first shell and a second shell symmetrical to each other, and the upper parts of the first shell and the second shell are pivotally connected by a rotating shaft D1, so that the device shell F1 can be split from the middle, and the first shell and the second shell can be welded by metal thin plates; the rotating shaft D1 can be arranged in the middle of the upper surface of the device shell F1 along the extension direction of the device shell F1; a tracked ceramic heater C1, a plurality of tracked ceramic heaters C1 are arranged in parallel on the inner surface of the device shell F1, and are connected in parallel to provide a heat source for isothermal heat treatment; two terminals A1 and B1, the two terminals A1 and B1 are arranged on the diagonals of the upper surface of the device shell F1, and a loop is formed between the two terminals; and a thermal insulation layer G1, which is arranged on the inner side of the device shell F1, specifically, it is an asbestos insulation layer G1. A fixing clamp ring E1 for opening or closing the device housing F1 is also provided at the connection between the first housing and the bottom of the second housing.

[0044] The device has the advantages of being small, flexible, and low-cost, and is convenient for field construction. The device can be powered by a diesel generator or a 220V power supply, with a rated power of 10kW. The device uses a commercial LCD crawler ceramic heater as a heat source, and the heater is a ceramic sheet with a size of 10mm (length) × 10mm (width) × 6mm (thickness). Equipped with insulation materials and a steel structure shell, it is made into a ring-shaped split heater, which is easy to assemble and disassemble and is suitable for full-section heating of rail welded joints. The actual size of the heating device and the specifications and distribution of the heater can be adjusted according to the actual size of the rail profile. It should be pointed out that the device can realize the isothermal (constant temperature) heat treatment process of the rail welded joint through program setting and continuous heat compensation during power-on. During the design of the device, multiple groups of crawler heaters are evenly fixed on the inner side of the device with a similar rail profile, so that the heater is coated on the surface of the rail and fully fits the surface of the rail to achieve good heat conduction during the heating process. Based on this device, the isothermal heat treatment process of the rail welded joint can be realized.

[0045] During the test, a temperature controller is used to control the heating temperature. The operating temperature range of the device is 200~600℃. The split device can rotate up to 180° around the rotation axis.

[0046] The distribution of electric heaters in the rail head area corresponding to the split rail profile heating device is as follows: Figure 6 As shown. Among them, A is the rail head tread heating area; B is the rail head side heating area; C is the rail head jaw heating area; C1 is a crawler ceramic electric heater. It should be noted that the layout of the rail head, rail waist and rail bottom heating areas in this heating device is similar. Because the rail head is thicker and the heat transfer is slower, the number of ceramic heaters covering the rail head is more than the number of ceramic heaters in the rail waist and rail bottom areas to ensure that the entire section of the rail is fully heated.

[0047] In step S5, after the isothermal treatment is completed, the welded joint is naturally cooled in an air environment. When the isothermal process is completed, the split rail profiling heating device can be removed. The welded joint is placed in an air environment for natural cooling to reduce the surface temperature of the welded joint to an ambient temperature of 20 to 30°C. The cooling rate in this stage is 0.1 to 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 relatively low. Therefore, after the isothermal heat treatment process is completed, the martensitic structure will not be formed when the joint is naturally cooled in an air environment.

[0048] When testing the heat-treated joints, refer to the TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. Figure 7The schematic diagram shows that the rail joint is processed into 14 full-section impact specimens. The impact toughness of the weld and the impact toughness of the heat-affected zone 10mm away from the center of the weld are the average values ​​of the 14 full-section impact specimens of the rail joint. The impact toughness of the weld is the impact toughness value obtained by the impact test of the impact specimen with a U-shaped notch processed at the weld. The impact toughness of the heat-affected zone 10mm away from the center of the weld is the impact toughness value obtained by the impact test of the impact specimen with a U-shaped notch processed at the heat-affected zone 10mm away from the center of the weld. According to Figure 8 The sampling positions shown in the figure are: c is the center of the weld, and d is the sampling position of the rail head tread metallographic sample. The metallographic structure of the joint was inspected according to GB / T13298-2015 "Methods for Metal Microstructure Inspection". The metallographic sample was etched with 3% nitric acid alcohol solution, and the metallographic structure was observed with a Leica MeF3 optical microscope.

[0049] The present invention is further described below by means of specific examples, but the protection scope of the present invention is not limited thereto.

[0050] Example 1 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.10%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1150 MPa, a hardness of 310 HV, and an impact energy of 18 J. The hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.20%, Si: 1.00%, Mn: 1.20%, Cr: 0.80%, Mo: 0.20%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1280 MPa, a hardness of 370 HV, and an impact energy of 170 J.

[0051] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 60kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joints with a surface temperature of 800℃ are placed in the air to cool naturally until the surface temperature of the rail head drops to 200℃, and then the medium-frequency induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. When the surface temperature of the rail welded joint reaches 990℃, the heating is stopped, and the natural cooling and manual transfer of the welded joint process will inevitably lead to a temperature drop of 30~50℃. When the surface temperature of the rail welding joint drops to 950℃, compressed air (or water mist mixture) is used as the cooling medium. The gas pressures of compressed air or water mist mixture sprayed from the left and right sides of the rail head tread air spray device are 0.40MPa and 0.20MPa respectively. The heat affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 4.0℃ / s and 2.0℃ / s respectively. At this stage, the final cooling temperatures of the heat affected zones on the left and right sides of the joint weld are 350℃ and 315℃ respectively.

[0052] After the second stage of cooling is completed, the joint is immediately heated to 325°C by a profiling isothermal device for isothermal treatment, and the isothermal time is 1 hour. After the isothermal treatment is completed, the profiling isothermal device is removed, and the heat-affected zones on both sides of the joint weld are naturally cooled to an ambient temperature of 20-30°C in an air environment, thereby obtaining the rail welded joint after post-weld heat treatment of this embodiment. During the final natural cooling process, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0053] The average impact toughness of the full-section weld of the rail joint obtained in this embodiment is 50J, which is much higher than ≥6.5J specified in TB / T1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on the side of the heat-treated eutectoid pearlite rail in the area 10mm away from the center of the weld is 15J, and the average room temperature impact toughness of the heat-affected zone on the side of the hot-rolled bainite rail in the area 10mm away from the center of the weld is 72J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the side of the heat-treated eutectoid pearlite rail of the joint, and there is no obvious block martensite in the heat-affected zone on the side of the hot-rolled bainite rail of the joint.

[0054] Example 2 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.10%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1150 MPa, a hardness of 310 HV, and an impact energy of 18 J. The hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.20%, Si: 1.00%, Mn: 1.20%, Cr: 0.80%, Mo: 0.20%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1280 MPa, a hardness of 370 HV, and an impact energy of 170 J.

[0055] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 68kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joints with a surface temperature of 800℃ are placed in the air to cool naturally until the surface temperature of the rail head drops to 200℃, and then the medium-frequency induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. When the surface temperature of the rail welded joint reaches 1060℃, the heating is stopped, and the natural cooling and manual transfer of the welded joint will inevitably lead to a temperature drop of 30~50℃. When the surface temperature of the rail welding joint drops to 1020℃, compressed air (or water mist mixture) is used as the cooling medium. The gas pressures of compressed air or water mist mixture sprayed from the left and right sides of the rail head tread air spray device are 0.40MPa and 0.20MPa respectively. The heat affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 4.0℃ / s and 2.0℃ / s respectively. The final cooling temperatures of the heat affected zones on the left and right sides of the joint weld are 350℃ and 315℃ respectively.

[0056] After the second stage of cooling is completed, the joint is immediately heated to 355°C by a profiling isothermal device for isothermal treatment, and the isothermal time is 1 hour. After the isothermal treatment is completed, the profiling isothermal device is removed, and the heat-affected zones on both sides of the joint weld are naturally cooled to an ambient temperature of 20-30°C in an air environment, thereby obtaining the rail welded joint after post-weld heat treatment of this embodiment. During the final natural cooling process, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0057] The average impact toughness of the full-section weld of the rail joint obtained in this embodiment is 57J, which is much higher than ≥6.5J specified in TB / T1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on the heat-treated eutectoid pearlite rail side in the area 10mm away from the center of the weld is 16J, and the average room temperature impact toughness of the heat-affected zone on the hot-rolled bainite rail side in the area 10mm away from the center of the weld is 77J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the heat-treated eutectoid pearlite rail side of the joint, and there is no obvious block martensite in the heat-affected zone on the hot-rolled bainite rail side of the joint.

[0058] Example 3 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.10%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1150 MPa, a hardness of 310 HV, and an impact energy of 18 J. The hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.20%, Si: 1.00%, Mn: 1.20%, Cr: 0.80%, Mo: 0.20%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1280 MPa, a hardness of 370 HV, and an impact energy of 170 J.

[0059] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 75kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joints with a surface temperature of 800℃ are placed in the air to cool naturally until the surface temperature of the rail head drops to 200℃, and then the medium-frequency induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. When the surface temperature of the rail welded joint reaches 1060℃, the heating is stopped, and the natural cooling and manual transfer of the welded joint will inevitably lead to a temperature drop of 30~50℃. When the surface temperature of the rail welding joint drops to 1020℃, compressed air (or water mist mixture) is used as the cooling medium. The gas pressures of the compressed air or water mist mixture sprayed from the left and right sides of the rail head tread air spray device are 0.70MPa and 0.30MPa respectively. The heat affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 7.0℃ / s and 3.0℃ / s respectively. At this stage, the final cooling temperatures of the heat affected zones on the left and right sides of the joint weld are 400℃ and 290℃ respectively.

[0060] After the second stage of cooling is completed, the joint is immediately heated to 355°C by a profiling isothermal device for isothermal treatment, and the isothermal time is 1 hour. After the isothermal treatment is completed, the profiling isothermal device is removed, and the heat-affected zones on both sides of the joint weld are naturally cooled to an ambient temperature of 20-30°C in an air environment, thereby obtaining the rail welded joint after post-weld heat treatment of this embodiment. During the final natural cooling process, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0061] The average impact toughness of the full-section weld of the rail joint obtained in this embodiment is 55J, which is much higher than ≥6.5J specified in TB / T1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on the side of the heat-treated eutectoid pearlite rail in the area 10mm away from the center of the weld is 17J, and the average room temperature impact toughness of the heat-affected zone on the side of the hot-rolled bainite rail in the area 10mm away from the center of the weld is 80J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the side of the heat-treated eutectoid pearlite rail of the joint, and there is no obvious block martensite in the heat-affected zone on the side of the hot-rolled bainite rail of the joint.

[0062] Example 4 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.83%, Si: 0.90%, Mn: 1.20%, V: 0.50%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1250 MPa, a hardness of 350 HV, and an impact energy of 10 J. The hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.35%, Si: 1.80%, Mn: 1.80%, Cr: 1.40%, Mo: 0.50%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1350 MPa, a hardness of 400 HV, and an impact energy of 155 J.

[0063] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 60kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joints with a surface temperature of 800℃ are placed in the air to cool naturally until the surface temperature of the rail head drops to 200℃, and then the medium-frequency induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. When the surface temperature of the rail welded joint reaches 1030℃, the heating is stopped, and the natural cooling and manual transfer of the welded joint process will inevitably lead to a temperature drop of 30~50℃. When the surface temperature of the rail welding joint drops to 1000℃, compressed air (or water mist mixture) is used as the cooling medium. The gas pressures of compressed air or water mist mixture sprayed from the left and right sides of the rail head tread air spray device are 0.50MPa and 0.25MPa respectively. The heat affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 5.0℃ / s and 2.5℃ / s respectively. At this stage, the final cooling temperatures of the heat affected zones on the left and right sides of the joint weld are 370℃ and 300℃ respectively.

[0064] After the second stage of cooling is completed, the joint is immediately heated to 340°C by a profiling isothermal device for isothermal treatment, and the isothermal time is 1 hour. After the isothermal treatment is completed, the profiling isothermal device is removed, and the heat-affected zones on both sides of the joint weld are naturally cooled to an ambient temperature of 20-30°C in an air environment, thereby obtaining the rail welded joint after post-weld heat treatment of this embodiment. During the final natural cooling process, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0065] The average impact toughness of the full-section weld of the rail joint obtained in this embodiment is 52J, which is much higher than ≥6.5J specified in TB / T1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on the heat-treated eutectoid pearlite rail side in the area 10mm away from the center of the weld is 9J, and the average room temperature impact toughness of the heat-affected zone on the hot-rolled bainite rail side in the area 10mm away from the center of the weld is 70J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the heat-treated eutectoid pearlite rail side of the joint, and there is no obvious block martensite in the heat-affected zone on the hot-rolled bainite rail side of the joint.

[0066] Example 5 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.80%, Si: 0.68%, Mn: 1.0%, V: 0.30%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1200 MPa, a hardness of 330 HV, and an impact energy of 14 J; the hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.35%, Si: 1.80%, Mn: 1.80%, Cr: 1.40%, Mo: 0.50%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1350 MPa, a hardness of 400 HV, and an impact energy of 155 J.

[0067] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 60kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joints with a surface temperature of 800℃ are placed in the air to cool naturally until the surface temperature of the rail head drops to 200℃, and then the medium-frequency induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. When the surface temperature of the rail welded joint reaches 1030℃, the heating is stopped, and the natural cooling and manual transfer of the welded joint process will inevitably lead to a temperature drop of 30~50℃. When the surface temperature of the rail welding joint drops to 1000℃, compressed air (or water mist mixture) is used as the cooling medium. The gas pressures of compressed air or water mist mixture sprayed from the left and right sides of the rail head tread air spray device are 0.50MPa and 0.25MPa respectively. The heat affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 5.0℃ / s and 2.5℃ / s respectively. At this stage, the final cooling temperatures of the heat affected zones on the left and right sides of the joint weld are 370℃ and 300℃ respectively.

[0068] After the second stage of cooling is completed, the joint is immediately heated to 340°C by a profiling isothermal device for isothermal treatment, and the isothermal time is 1 hour. After the isothermal treatment is completed, the profiling isothermal device is removed, and the heat-affected zones on both sides of the joint weld are naturally cooled to an ambient temperature of 20-30°C in an air environment, thereby obtaining the rail welded joint after post-weld heat treatment of this embodiment. During the final natural cooling process, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

[0069] The average impact toughness of the full-section weld of the rail joint obtained in this embodiment is 58J, which is much higher than ≥6.5J specified in TB / T1632.2-2014. At the same time, the average room temperature impact toughness of the heat-affected zone on the side of the heat-treated eutectoid pearlite rail in the area 10mm away from the center of the weld is 13J, and the average room temperature impact toughness of the heat-affected zone on the side of the hot-rolled bainite rail in the area 10mm away from the center of the weld is 78J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the side of the heat-treated eutectoid pearlite rail of the joint, and there is no obvious block martensite in the heat-affected zone on the side of the hot-rolled bainite rail of the joint.

[0070] Comparative Example 1 The process conditions involved in the rail material selection, rail base material mechanical properties, welding and post-weld normalizing heat treatment cooling process, and isothermal heat treatment process in this comparative example are consistent with those in Example 1. The only difference is that the normalizing heating temperature of the rail welded joint in this comparative example is 950° C., which is lower than the post-weld normalizing process range of the rail in the present invention.

[0071] The rail weld joint obtained by this comparative example has a normalizing heating temperature of 950°C, which is lower than the normalizing temperature set in the process method of the present invention, resulting in insufficient supercooling during the cooling process of the rail weld joint after normalizing heating and insufficient driving force for pearlite phase transformation and bainite phase transformation during the subsequent cooling process. The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 30J. At the same time, the average room temperature impact toughness of the heat-affected zone on one side of the heat-treated eutectoid pearlite rail within the area 10mm away from the center of the weld is 12J, and the average room temperature impact toughness of the heat-affected zone on one side of the hot-rolled bainite rail within the area 10mm away from the center of the weld is 35J, which is much different from the toughness of the bainite rail base material.

[0072] Comparative Example 2 The rail material selection, rail base material mechanical properties, welding cooling process, post-weld normalizing heat treatment heating temperature, post-weld normalizing cooling stage cooling rate, and isothermal heat treatment process in this comparative example are consistent with those in Example 1. The difference is that in this comparative example, the final cooling temperature of the left and right sides of the weld in the second stage cooling process after normalizing heating is 270°C.

[0073] In the rail joint obtained in this comparative example, since the final cooling temperature of the left side of the weld in the second stage cooling process after the normalizing heating of the joint after welding is completed is within the Ms temperature range of the heat-treated eutectoid pearlite rail steel (the starting temperature of martensite formation), and the final cooling temperature of the right side of the weld is below the Ms temperature of the hot-rolled bainite rail steel (the starting temperature of martensite formation), brittle and hard martensite is formed in the heat-affected zones on both sides of the weld, and the percentage of martensite structure in the heat-affected zone of the hot-rolled bainite rail is higher than the percentage of martensite structure in the heat-affected zone of the heat-treated eutectoid pearlite rail.

[0074] The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 20 J. At the same time, the average room temperature impact toughness of the heat-affected zone on one side of the heat-treated eutectoid pearlite rail within the area 10 mm from the center of the weld is 7 J, and the average room temperature impact toughness of the heat-affected zone on one side of the hot-rolled bainite rail within the area 10 mm from the center of the weld is 30 J. Because too much brittle and hard martensite is formed in the heat-affected zones on both sides of the weld, and the martensite is not completely eliminated after isothermal treatment, the mechanical properties (especially room temperature impact toughness) of the heat-affected zones on both sides of the rail joint weld are poor, which is not conducive to the safety of railway operation.

[0075] Comparative Example 3 The rail material selection, rail base material mechanical properties, rail welding process conditions, heating temperature during post-weld normalizing, cooling method and steps during post-weld normalizing, final cooling temperature of the second cooling stage, cooling method and steps of the third cooling stage, cooling rate, isothermal treatment process, etc. in this comparative example are consistent with those in Example 1. The difference is that the cooling rate of the left side of the weld in the second cooling process after normalizing heating of the rail welded joint in this comparative example is 3.0°C / s, which is lower than the cooling rate process range of the left side of the weld in the second cooling process after normalizing heating of the rail welded joint in the present invention.

[0076] The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 38. At the same time, the average room temperature impact toughness of the heat-affected zone on the heat-treated eutectoid pearlite rail side in the area 10 mm away from the center of the weld is 9J, and the average room temperature impact toughness of the heat-affected zone on the hot-rolled bainite rail side in the area 10 mm away from the center of the weld is 72J.

[0077] The rail weld joint obtained in this comparative example has a weaker pearlite structure refinement effect in the heat-affected zone on the left side of the weld due to the lower cooling rate on the left side of the weld during the second cooling process after normalizing heating.

[0078] Comparative Example 4 The rail material selection, rail base material mechanical properties, and rail flash welding conditions in this comparative example are consistent with those in Example 4. The difference is that in this comparative example, there is no subsequent normalizing heating and cooling process after the rail welding is completed, that is, the rail welded joint obtained by flash welding is directly naturally cooled in the air to an ambient temperature of 30° C., thereby obtaining the rail welded joint in this comparative example.

[0079] The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 22 J. At the same time, the average room temperature impact toughness of the heat-affected zone of the heat-treated eutectoid pearlite rail on one side in the area 10 mm from the center of the weld is 7 J, and the average room temperature impact toughness of the heat-affected zone of the hot-rolled bainite rail on one side in the area 10 mm from the center of the weld is 31 J. Due to the lack of post-weld heat treatment, the impact toughness of the rail joint obtained in this comparative example is poor and has a large gap with the parent material.

[0080] Comparative Example 5 The rail material selection, rail base material mechanical properties, rail flash welding conditions, cooling process and cooling rate during the normalizing heat treatment cooling process after rail welding in this comparative example are consistent with those in Example 5. The difference is that in this comparative example, the final cooling temperature of the heat affected zone on the left side of the weld during the second cooling process after normalizing heating of the rail joint is 540°C, while the right side of the weld still adopts the same process as in Example 5, and the subsequent isothermal heat treatment process is also consistent with that in Example 5.

[0081] The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 30 J. At the same time, the average room temperature impact toughness of the heat-affected zone on the heat-treated eutectoid pearlite rail side in the area 10 mm away from the center of the weld is 8 J, and the average room temperature impact toughness of the heat-affected zone on the hot-rolled bainite rail side in the area 10 mm away from the center of the weld is 78 J.

[0082] Since 540℃ is in the pearlite transformation range of the eutectoid pearlite rail steel on the left side of the rail joint weld, and after the second cooling, the heat affected zone on the left side of the eutectoid pearlite rail steel is isothermally treated. This results in a weak pearlite layer refinement effect during the normalizing cooling process of the eutectoid pearlite rail steel on the left side of the rail joint weld, which in turn leads to low mechanical properties of the heat affected zone on the left side of the rail weld joint weld.

[0083] Comparative Example 6 The rail material selection, rail base material mechanical properties, rail flash welding conditions, cooling process and cooling rate during the normalizing heat treatment after rail welding in this comparative example are consistent with those in Example 5. The difference is that in this comparative example, the final cooling temperature of the heat affected zone on the right side of the weld during the second cooling process after normalizing heating of the rail welded joint is 330°C, and the isothermal treatment is carried out by using a contoured isothermal device to keep warm at 330°C instead of heating, and the holding time is 1h.

[0084] The average impact toughness of the full-section weld of the rail joint obtained in this comparative example is 32 J. At the same time, the average room temperature impact toughness of the heat-affected zone of the heat-treated eutectoid pearlite rail on one side in the area 10 mm from the center of the weld is 12 J, and the average room temperature impact toughness of the heat-affected zone of the hot-rolled bainite rail on one side in the area 10 mm from the center of the weld is 48 J. The room temperature impact toughness of the hot-rolled bainite rail on one side of the comparative example has been improved, but there is still a large gap with the impact toughness of the hot-rolled bainite rail base material.

[0085] The above-mentioned embodiments only express the implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for improving the impact toughness matching of dissimilar rail welded joints, characterized in that: The following steps are involved: Step S1: performing a first stage cooling on a welded joint formed by welding a heat-treated eutectoid pearlite rail and a hot-rolled bainite rail, so that the surface temperature of the welded joint is reduced to a first temperature; Step S2: performing full-section normalizing heating on the weld joint that has been cooled in the first stage to austenitize the weld joint; Step S3: After normalizing, the weld joint is cooled in the second stage, wherein the heat-affected zone on one side of the heat-treated eutectoid pearlite rail is cooled to 350-400°C at a cooling rate of 4.0-7.0°C / s, and the heat-affected zone on one side of the hot-rolled bainite rail is cooled to 290-315°C at a cooling rate of 2.0-3.0°C / s; Step S4: after the second stage cooling is completed, the welding joint is heated to the bainite transformation temperature range for isothermal treatment; Step S5: After the isothermal treatment is completed, the weld joint is naturally cooled in an air environment.

2. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: The base material of the heat-treated eutectoid pearlite rail includes the following chemical components by weight percentage: C: 0.77%-0.83%, Si: 0.45%-0.90%, Mn: 0.8%-1.2%, V: 0.1%-0.5%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 10-18 J; the base material of the hot-rolled bainite rail includes the following chemical components by weight percentage: C: 0.20%-0.35%, Si: 1.0%-1.8%, Mn: 1.2%-1.9%, Cr: 0.80%-1.60%, Mo: 0.20%-0.50%, the balance is Fe and inevitable impurities, and the room temperature impact energy of the base material is 140-170 J.

3. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: In step S1 , the first stage cooling is natural cooling in an air environment.

4. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: In step S1, the first temperature is 100-200°C.

5. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: In step S2, normalizing heating is performed to 990-1060°C.

6. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 5, characterized in that: In step S2, the rail joint is fully normalized and heated using a medium frequency induction profile electric heating coil and / or an oxygen-acetylene flame profile heater.

7. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: In step S3, the second stage cooling is implemented by using a rail head and tread air jet device, using compressed air or a water mist mixture as a cooling medium to differentially control the cooling of both sides of the weld of the welded joint.

8. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 7, characterized in that: The distance between the bottom cooling channel of the rail head tread air jet device and the surface of the welded joint is controlled to be 25-30 mm, the air jet pressure in the heat-affected zone on one side of the heat-treated eutectoid pearlite rail is 0.40-0.70 MPa, and the air jet pressure in the heat-affected zone on one side of the hot-rolled bainite rail is 0.20-0.30 MPa.

9. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: The bainite transformation temperature range is 325-355° C., and the isothermal treatment time is 0.5-1 h.

10. The method for improving the impact toughness matching of dissimilar rail weld joints according to claim 1, characterized in that: In step S5, the cooling rate of the heat-affected zones on both sides of the joint weld is 0.8-0.1°C / s.

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