Post-welding cooling method for flash welded joint of dissimilar steel rail

Abstract

A post-welding cooling method for a flash welded joint of a dissimilar steel rail, comprising: when a steel rail welded joint formed by welding a hypoeutectoid steel rail and a eutectoid steel rail is welded, carrying out first-stage cooling on heat affected zones on two sides of a welded seam at different cooling speeds, wherein the cooling speed of the heat affected zone on one side of the hypoeutectoid steel rail is higher than that of the heat affected zone on one side of the eutectoid steel rail; carrying out second-stage cooling on the welded joint, wherein the heat affected zone on one side of the hypoeutectoid steel rail is cooled at a third cooling speed, the heat affected zone on one side of the eutectoid steel rail is cooled at a fourth cooling speed, and the third cooling speed is greater than the fourth cooling speed; and carrying out third-stage cooling on the welded joint in a natural cooling manner. The method can improve the hardness difference of the heat affected zones on the two sides of the welded seam of the dissimilar steel rail joint, thereby achieving the hardness matching of the heat affected zones on the two sides of the welded seam of the dissimilar strength steel rail joint.

Description

A post-weld cooling method for flash welded joints of dissimilar rails Technical Field

[0001] The invention relates to the technical field of railway rail manufacturing, in particular to a post-weld cooling method for a flash welded joint of dissimilar rails. Background Art

[0002] Currently, most railway rails used domestically and internationally have a full pearlite structure or a structure consisting of pearlite with a small amount of proeutectoid ferrite or proeutectoid cementite. The carbon content of the rails is mainly concentrated in the range of 0.7-1.1%. The rails are usually required to have a strength of no less than 880MPa and good wear resistance. However, the impact toughness of such rails cannot meet the requirements for harsh sections with special natural conditions, such as cold regions, large temperature differences throughout the year, and large temperature differences between day and night. To this end, a new type of medium-low carbon hypoeutectoid rail has been developed. The carbon content of medium-low carbon hypoeutectoid rails is between 0.50-0.65%, with moderate strength and hardness, high impact toughness, and better resistance to contact fatigue damage.

[0003] When a new type of rail is laid on an existing line for the purpose of replacing old rails and trial laying new rails, welding of the new type of rail with the existing type of rail on the line with dissimilar compositions / materials is involved. For two types of rails with different strength grades and materials, such as the already used eutectoid rail and the replacement hypoeutectoid rail, the difference in parent material properties poses a huge challenge to their welding. For the welding of dissimilar rails, due to the differences in microstructure and mechanical properties between the parent materials, the mechanical properties such as strength and hardness of the heat-affected zones on the left and right sides of the weld of the resulting rail joint are quite different, which is not conducive to the service safety of such rail joints on the railway. Therefore, how to improve or reduce the differences in microstructure and properties of the heat-affected zones on both sides of the weld of dissimilar rails through post-weld treatment has become the key to promoting the application of dissimilar rail welding.

[0004] When welding rails of different strength grades together, the welding of the resulting dissimilar rail joints and the subsequent post-weld heat treatment to achieve synergistic improvements in microstructure and performance are both issues that require consideration. The quality of rail welds determines whether a seamless line can withstand the rigors of railway operation and, ultimately, driving safety. Therefore, there is a need for improvements in the existing technology for welding and post-weld treatment of rails of dissimilar strength grades and materials.

[0005] Summary of the Invention

[0006] To address the above problems, the present invention provides a post-weld cooling method for a flash welded joint of dissimilar rails, which is used to improve the performance of a joint formed by welding a hypoeutectoid rail to a eutectoid rail.

[0007] According to one aspect of the present invention, a post-weld cooling method for a flash welded joint of dissimilar rails is provided, comprising the following steps:

[0008] Step S1: After welding, the heat-affected zones on both sides of the weld of a rail welded joint formed by welding a hypoeutectoid rail and a eutectoid rail are subjected to a first-stage cooling process at different cooling rates, wherein the heat-affected zone on the hypoeutectoid rail side is cooled at a first cooling rate, and the heat-affected zone on the eutectoid rail side is cooled at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate;

[0009] Step S2: performing a second stage cooling on the weld joint, wherein the heat-affected zone on the hypoeutectoid rail side is cooled at a third cooling rate, and the heat-affected zone on the eutectoid rail side is cooled at a fourth cooling rate, wherein the third cooling rate is greater than the fourth cooling rate;

[0010] Step S3: Cooling the welded joint in the third stage by natural cooling.

[0011] According to one embodiment of the present invention, the base material corresponding to the hypoeutectoid rail contains the following components by weight: 0.50-0.64% C, 0.40-0.70% Si, 0.55-0.8% Mn, 0.20-0.40% Cr, 0.15-0.45% Cu, 0.05-0.25% Ni, 0.02-0.08% V, and the balance is Fe and unavoidable impurities.

[0012] According to one embodiment of the present invention, the base material corresponding to the eutectoid rail contains the following components by weight: 0.73-0.82% C, 0.50-0.80% Si, 0.70-1.0% Mn, 0.30-0.50% Cr, 0.06-0.10% V, and the balance is Fe and unavoidable impurities.

[0013] According to one embodiment of the present invention, the first cooling rate is 13.2-14.2° C. / s, and the second cooling rate is 12.2-13.0° C. / s.

[0014] According to one embodiment of the present invention, the third cooling rate is 4.1-4.5° C. / s, and the fourth cooling rate is 3.6-4.0° C. / s.

[0015] According to one embodiment of the present invention, the distance between the cooling device for applying the cooling medium and the weld joint is 15 mm.

[0016] According to one embodiment of the present invention, the joint is a joint formed by welding a hypoeutectoid rail and a eutectoid rail of the same rail type and specification of 60 kg / m by a rail moving flash welder.

[0017] According to one embodiment of the present invention, the starting temperature of the first stage cooling in step S1 is 905-985°C, and the ending temperature is 485-525°C.

[0018] According to one embodiment of the present invention, the first stage cooling operation described in step S1 is to use a contoured cooling device to spray compressed air or a water mist mixture on the rail joint to cool the rail joint so that the surface temperature of the rail joint drops to a set end temperature. In the first stage cooling, the cooling air pressure used in the heat-affected zone on one side of the hypoeutectoid rail is 3.5-3.9 MPa, and the cooling air pressure used in the heat-affected zone on the other side of the eutectoid rail is 3.1-3.4 MPa.

[0019] According to one embodiment of the present invention, the end temperature of the second stage cooling in step S2 is 325-365°C.

[0020] According to one embodiment of the present invention, the second stage cooling operation described in step S2 is to use a contour cooling device to spray compressed air or a water mist mixture on the weld joint to reduce the surface temperature of the rail joint to a set end temperature. In the second stage cooling, the cooling air pressure used in the heat-affected zone on one side of the hypoeutectoid rail is 1.05 to 1.20 MPa, and the cooling air pressure used in the heat-affected zone on the other side of the eutectoid rail is 0.85 to 1.00 MPa.

[0021] Due to the adoption of the above technical solution, the post-weld cooling method for a flash welded joint of dissimilar rails provided by the present invention has at least one of the following beneficial effects compared with the prior art:

[0022] (1) The present invention utilizes the residual heat of welding of the rail weld joint for heat treatment, and no further heating is required during the heat treatment process, thereby simplifying the heat treatment process and reducing costs;

[0023] (2) This method can ensure that there is no abnormal martensite structure in the heat-affected zone on both sides of the rail joint weld. At the same time, the longitudinal hardness of the rail joint in the area of ​​±15 mm from the center of the weld can reach 92% to 96% of the average hardness of the corresponding hypoeutectoid rail and eutectoid rail base material, and the difference between the average hardness of the heat-affected zone of the eutectoid rail and the average hardness of the heat-affected zone of the hypoeutectoid rail is within 25HV. At the same time, the metallographic structure of the heat-affected zone on both sides of the weld is visible pearlite, without abnormal structures such as martensite;

[0024] (3) The adoption of the present invention helps to improve the "saddle-shaped" wear of the rail weld joint caused by the low hardness of the welding area during the service of the rail, helps to ensure the safety of railway operation, and at the same time, can improve the hardness difference of the heat-affected zone on both sides of the weld of the dissimilar rail joint, thereby achieving hardness matching of the heat-affected zone on both sides of the weld of the dissimilar strength rail. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the hardness test position of the rail joint longitudinal section;

[0026] FIG2 is a schematic diagram of the cutting positions of metallographic samples in various embodiments and comparative examples;

[0027] FIG3 is a schematic diagram of a rail head profile cooling device used in the present invention;

[0028] FIG4 is a schematic diagram of the layout of nozzles on the left and right sides of the rail head profile cooling device used in the present invention;

[0029] FIG5 is a bottom view of the rail head profile cooling device used in the present invention;

[0030] FIG6 is a flow chart illustrating an example of a post-weld cooling method for a flash welded joint of dissimilar rails according to the present invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be 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 intended to limit the present invention.

[0032] The welded joint described in this invention is formed by welding hypoeutectoid and eutectoid rails of the same rail type and gauge of 60 kg / m using a rail-traveling flash welder. The welded joint encompasses an area of ​​60-80 mm in length, including the weld seam and / or heat-affected zone, with the weld seam centered. In this invention, the "ambient temperature" refers to a temperature in the range of 15-25°C.

[0033] Figure 1 is a schematic diagram of the hardness test locations for a rail joint longitudinal section. In the figure, a represents the hypoeutectoid rail on one side of the joint weld, b represents the heat-affected zone of the joint, c represents the eutectoid rail on the other side of the joint weld, d represents the rail head tread, and e represents the weld center.

[0034] Figure 2 is a schematic diagram of the metallographic sample cutting locations in various examples and comparative examples. Point e in the figure indicates the center of the weld, and point f indicates the sampling location for metallographic samples of the rail head and tread of a welded rail joint. The metallographic structure of the rail joint was examined in accordance with GB / T 13298-2015, "Methods for the Examination of Metallic Microstructures." The metallographic samples were etched with 3% nitric acid solution and observed using a German Leica MeF3 optical microscope.

[0035] Figure 3 is a schematic diagram of the rail head profile cooling device used in the method of the present invention. Figure 4 is a schematic diagram of the nozzle layout on the left and right sides of the profile cooling device. Figure 5 is a bottom view of the profile cooling device. As shown in Figures 3-5, the profile cooling device generally comprises a left cooling device and a right cooling device located on the left and right sides. The left and right cooling devices share a housing and are separated by a central partition. Media channels 1 and 2 are located on either side of the central partition. Media channel 1 transports cooling medium to the left cooling device, with nozzles 3 located on its bottom wall. Media channel 2 transports cooling medium to the right cooling device, with nozzles 4 located on its bottom wall. The profile cooling device also includes side nozzles 5 and 6 for cooling the sides of the rail head. The device can cool both the tread and the sides of the rail head. The shape and size of each nozzle can be designed, processed, and modified according to actual needs to apply different air pressures. The pressure of the medium flowing through the media channels can be monitored by a pressure detection device and adjusted according to actual needs. The left cooling device cools the heat-affected zone (HAZ) on the hypoeutectoid rail side of the rail joint, while the right cooling device cools the HZ on the eutectoid rail side of the rail joint. Depending on the pressure of the compressed air flowing into channels 1 and 2, the HZ on the left and right sides of the rail joint weld can be cooled to varying degrees.

[0036] FIG6 is a flow chart illustrating an example of a post-weld cooling method for a flash welded joint of dissimilar rails according to the present invention. The post-weld cooling method for a flash welded joint of dissimilar rails according to the present invention generally comprises the following steps:

[0037] Step S1: After welding, the heat-affected zones on both sides of the rail weld joint formed by welding a hypoeutectoid rail and a eutectoid rail are cooled in a first stage at different cooling rates, wherein the heat-affected zone on the hypoeutectoid rail side is cooled at a first cooling rate, and the heat-affected zone on the eutectoid rail side is cooled at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate;

[0038] Step S2: performing a second stage cooling on the weld joint, wherein the heat-affected zone on the hypoeutectoid rail side is cooled at a third cooling rate, and the heat-affected zone on the eutectoid rail side is cooled at a fourth cooling rate, wherein the third cooling rate is greater than the fourth cooling rate;

[0039] Step S3: Cooling the welded joint in the third stage by natural cooling.

[0040] Each step is described in detail below.

[0041] In step S1, the heat-affected zones (HAZs) on both sides of the weld, formed by welding a hypoeutectoid rail to an eutectoid rail, are cooled in a first stage after welding at different cooling rates. The HAZ on the hypoeutectoid rail side is cooled at a first cooling rate, while the HAZ on the eutectoid rail side is cooled at a second cooling rate, with the first cooling rate being greater than the second cooling rate. During this stage, the microstructure of the HAZ of the hypoeutectoid rail transforms from austenite to pearlite with a small amount of proeutectoid ferrite during cooling, while the microstructure of the HAZ of the eutectoid rail transforms from austenite to complete pearlite during cooling.

[0042] Since the hardenability of the hypoeutectoid rail steel in the present invention is significantly weaker than that of the eutectoid rail steel. In order to achieve a synergistic improvement in the strength and hardness of the heat-affected zones of the hypoeutectoid rails and the eutectoid rails on both sides of the weld, this stage uses a cooling rate that is much greater than the critical cooling rate of martensitic transformation of the hypoeutectoid and eutectoid rail steels to cool the welding heat-affected zones on both sides of the weld, and the cooling rate used in the heat-affected zone on the hypoeutectoid rail side is greater than the cooling rate used in the heat-affected zone on the eutectoid rail side. The purpose of this stage is to use a higher cooling rate above the austenitizing temperature of the rail steel to obtain a pearlite structure with finer interlamellar spacing through pearlite transformation, thereby achieving a synergistic improvement in the strength and hardness of the heat-affected zone.

[0043] In some embodiments, the base material of the hypoeutectoid rail comprises the following components by weight: 0.50-0.64% C, 0.40-0.70% Si, 0.55-0.8% Mn, 0.20-0.40% Cr, 0.15-0.45% Cu, 0.05-0.25% Ni, 0.02-0.08% V, with the balance being Fe and unavoidable impurities. In some embodiments, the base material of the eutectoid rail comprises the following components by weight: 0.73-0.82% C, 0.50-0.80% Si, 0.70-1.0% Mn, 0.30-0.50% Cr, 0.06-0.10% V, with the balance being Fe and unavoidable impurities.

[0044] In some embodiments, the first stage of cooling begins at a temperature between 905°C and 985°C, and ends at a temperature between 485°C and 525°C. This cooling phase begins when the residual temperature of the rail surface after welding drops to between 905°C and 985°C. This utilizes the residual heat from the welded rail joint for heat treatment, eliminating the need for reheating during the heat treatment process. This simplifies the heat treatment process and reduces costs. In the present invention, the residual temperature of the rail joint after welding can be adjusted by adjusting process parameters such as the rail welding heat input, thereby extending or shortening the high-temperature dwell time. The present invention utilizes the residual heat from the rail welding to perform the post-weld heat treatment process. Accelerated post-weld cooling is implemented for rail joints with high residual temperatures to lower the phase transition temperature from austenite to pearlite at the joint head, thereby increasing the hardness of the austenite recrystallization zone. Based on metallurgical principles, rail joints experience a certain degree of dynamic undercooling during rapid post-weld high-temperature cooling, which causes the phase transition temperature from austenite to pearlite to shift downward in a non-equilibrium state. The phase transition temperature gradually decreases with increasing undercooling. Therefore, even in the second stage cooling with a relatively low starting cooling temperature, the joint rail head can still undergo a microstructure transformation from austenite to pearlite.

[0045] Among them, the cooling start temperature is 905 ~ 985 ℃. During the rail welding process, the weld temperature can reach more than 1500 ℃. After the welding is completed, the three-dimensional heat transfer characteristics inside the rail cause the heat-affected zone of the rail joint welding to enter the rapid cooling stage. At the same time, after the rail is welded, a series of operations such as upsetting, pushing nodules, manually finishing the rail joint surface, and applying a rail head cooling device above the welded joint are required. These operations will consume time and are accompanied by a continuous decrease in the surface and core temperature after the rail welding is completed. Therefore, the surface temperature of the rail joint obtained is usually below 1000 ℃. After completing a series of operations such as welding, upsetting, pushing nodules, etc. in the present invention, the surface temperature of the rail joint is between 905 and 985 ℃. When the surface temperature of the rail joint is below 905 ℃, the supercooling of the pearlite transformation in the subsequent rapid cooling process will be reduced, which is not conducive to the improvement of the strength, hardness and other properties of the heat-affected zone.

[0046] It should also be noted that in the present invention, the final cooling temperature of the first stage of cooling is controlled between 485°C and 525°C. Within this temperature range, the temperature of the rail joint core is approximately 520°C to 550°C, essentially completing the pearlite transformation. When the final cooling temperature is controlled above 525°C, the rail joint core temperature is above 550°C, prematurely ending the rapid cooling of the rail joint, which will weaken the refinement of the pearlite lamellae and result in a negligible increase in joint strength and hardness. When the final cooling temperature of the first stage of cooling is controlled below 485°C, the faster cooling rate in the first stage increases the risk of martensite formation during subsequent cooling.

[0047] In some embodiments, the first stage of cooling is to use a profiled cooling device as shown in Figures 3-5 to spray compressed air or a water mist mixture on the rail joint to cool the rail joint surface, so that the rail joint surface temperature drops to a set end temperature. Using the profiled cooling device, two different cooling rates are used to cool the heat-affected zones on both sides of the rail joint weld. For example, the first cooling rate is 13.2 to 14.2°C / s, and the second cooling rate is 12.2 to 13.0°C / s. The corresponding cooling rate can be achieved by controlling the cooling air pressure on the left and right sides of the profiled cooling device. For example, the first cooling air pressure for cooling the heat-affected zone on one side of the hypoeutectoid rail is 3.5 to 3.9 MPa, and the second cooling air pressure for cooling the heat-affected zone on one side of the eutectoid rail is 3.1 to 3.4 MPa. Setting different cooling rates for the hypoeutectoid and eutectoid heat-affected zones (HAZs) allows for pearlite transformation at higher cooling rates to synergistically improve the strength and hardness of both HAZs, thereby enhancing the overall mechanical properties of the dissimilar rail weld joint. The distance between the profiled cooling device and the weld joint is 15 mm.

[0048] It should be noted that the rail steel with a high carbon content also has a relatively high hardenability (i.e., the ability to obtain martensite during quenching). Therefore, when the same cooling air pressure (or cooling intensity / cooling rate) is used to cool two rails with different hardenability, the strength and hardness improvement of the rail with weaker hardenability is also significantly lower than that of the rail with stronger hardenability. Based on the above-mentioned metallurgical principles, in the present invention, in view of the hardness difference of the heat-affected zone caused by the rails with different compositions (hardenability) on both sides of the rail joint weld, two different cooling air pressures are used to cool the rail weld heat-affected zones on both sides of the weld simultaneously. Specifically: a higher cooling air pressure (corresponding to a higher cooling intensity / cooling rate) is used to cool the heat-affected zone of the rail with relatively weak hardenability, while a relatively lower cooling air pressure is used for the heat-affected zone of the rail with stronger hardenability on the other side of the weld, so as to improve the hardness difference of the heat-affected zones on both sides of the weld, thereby achieving hardness matching of the heat-affected zones on both sides of the weld of rails with different strengths.

[0049] In step S2, the weld joint undergoes a second stage of cooling, wherein the heat-affected zone (HAZ) on the hypoeutectoid rail side is cooled at a third cooling rate, while the heat-affected zone (HAZ) on the eutectoid rail side is cooled at a fourth cooling rate, with the third cooling rate being greater than the fourth cooling rate. At this stage, the HAZs on both sides of the weld have completed pearlite transformation. While this stage reduces welding cooling time through rapid cooling, it also uses third and fourth cooling rates, significantly lower than the first and second cooling rates, to control cooling of the HAZ and HAZ, respectively. Furthermore, the final cooling temperature in this stage prevents the formation of brittle, hard martensitic abnormal structures in the HAZ and HAZ.

[0050] In some embodiments, a profiled cooling device as shown in Figures 3-5 is used to spray compressed air or a water mist mixture onto the rail joint to cool the rail joint surface to a set end temperature. The end temperature of the second stage of cooling can be 325-365°C. In the present invention, the critical cooling rate for martensitic transformation of the hypoeutectoid rail steel is 2.4-3.5°C / s, and the martensitic transformation start temperature is 280-320°C. The critical cooling rate for martensitic transformation of the eutectoid rail steel is 1.3-1.7°C / s, and the martensitic transformation start temperature is 210-250°C. To avoid the formation of abnormal structures such as brittle martensite in the heat-affected zone on both sides of the rail weld joint, when performing post-weld heat treatment on the welded joint between the hypoeutectoid rail and the eutectoid rail, the final cooling temperature during the rapid cooling process of the post-weld cooling treatment must be controlled above the eutectoid rail's Ms temperature. When the temperature drops to the Ms point, the cooling rate during the post-weld cooling treatment must be limited to the eutectoid rail steel with a relatively low critical cooling rate, otherwise the joint will suffer premature fatigue fracture due to the hardened martensite structure.

[0051] Using a profiled cooling device, the heat-affected zones (HAZs) on both sides of the rail joint weld are cooled at two different cooling rates. For example, the third cooling rate is 4.1-4.5°C / s, and the fourth cooling rate is 3.6-4.0°C / s. The corresponding cooling rates can be achieved by controlling the cooling air pressure on the left and right sides of the profiled cooling device. For example, the third cooling air pressure for cooling the HAZ on the hypoeutectoid rail side is 1.05-1.20 MPa, and the fourth cooling air pressure for cooling the HAZ on the eutectoid rail side is 0.85-1.00 MPa. Setting different cooling air pressures for the HAZ on the hypoeutectoid rail side and the HAZ on the eutectoid rail side during this stage not only saves welding cooling process time through cooling, but also prevents the formation of brittle, hard martensitic abnormal structures in the HAZ and HAZ due to excessively high cooling rates and low final cooling temperatures (below the martensitic transformation point of the rail steel), helping to ensure the safe operation of dissimilar rail welds on railways.

[0052] In step S3, the welded joint is cooled naturally in the third stage. For example, after the first two stages of cooling, the joint is allowed to cool naturally to an ambient temperature of 15-25°C. The cooling rate of the rail joint in this stage is approximately 0.4-0.9°C / s.

[0053] Regarding the present invention, it should be added that heat treatment technology itself is a process of controlling various factors during heating and cooling, and the various steps in heat treatment technology are interrelated and mutually influential. Based on the continuous cooling characteristics of hypoeutectoid rail and eutectoid rail steel welding, the present invention adopts a three-step cooling method (no need for post-weld normalizing heat treatment), limits the cooling rate and cooling temperature of each cooling stage, and improves the "saddle-shaped" wear of rail joints caused by low hardness in the weld area during rail service. At the same time, it avoids the damage to the service performance of the rail joints caused by the formation of martensitic structure. Therefore, the present invention represents a significant improvement over the prior art.

[0054] The present invention is further described below by way of examples, but the protection scope of the present invention is not limited thereto.

[0055] Example 1

[0056] In this embodiment, the hypoeutectoid rail base material contains the following components by weight: 0.50% C, 0.40% Si, 0.55% Mn, 0.20% Cr, 0.15% Cu, 0.05% Ni, 0.02% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 408 HV. The eutectoid rail base material contains the following components by weight: 0.73% C, 0.45% Si, 0.70% Mn, 0.35% Cr, and 0.06% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 430 HV.

[0057] After the dissimilar rail weld joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device shown in Figure 3-5 is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 905°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 485°C at cooling rates of 13.2°C / s and 12.2°C / s respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 325°C at cooling rates of 4.1°C / s and 3.6°C / s respectively. Finally, the rail joint is cooled to an ambient temperature of 15°C at a cooling rate of about 0.90°C / s along with the construction environment, thereby obtaining the dissimilar rail weld joint after post-weld cooling treatment of the present invention.

[0058] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using compressed air as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.5 MPa and 3.1 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.05 MPa and 0.85 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0059] The rail joint obtained in this embodiment after post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0060] In the present invention, the average hardness of the hypoeutectoid rail base metal is 408 HV, and the average hardness of the eutectoid rail base metal is 430 HV. For the rail welded joint treated by the present invention, the longitudinal hardness of the rail joint within the area ±15 mm from the weld center meets 92% and 93% of the corresponding average hardness of the hypoeutectoid and eutectoid rail base metals, respectively. The difference between the average hardness of the eutectoid rail heat-affected zone on one side and the average hardness of the hypoeutectoid rail heat-affected zone on the other side is 25 HV. At the same time, the metallographic structure of the heat-affected zones of the rails on both sides of the weld is visible pearlite, without abnormal structures such as martensite.

[0061] Example 2

[0062] In this embodiment, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, with the balance being Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV. The eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, with the balance being Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device shown in Figure 3-5 is used to perform the first stage cooling on the rail welded joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 365°C at cooling rates of 4.5°C / s and 4.0°C / s respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s along with the construction environment, thereby obtaining the dissimilar rail welded joint after post-weld cooling treatment of the present invention.

[0063] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the gas pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the gas pressures injected by the cooling device in the HAZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.00 MPa, respectively. The third stage of cooling involves natural cooling in air. The temperature of the rail head tread is monitored using an infrared thermometer.

[0064] The rail joint obtained in this embodiment after post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0065] In the present invention, the average hardness of the hypoeutectoid rail base material is 418 HV, and the average hardness of the eutectoid rail base material is 441 HV. For the rail welded joints treated by the present invention, the longitudinal hardness of the rail joint within the area ±15 mm from the weld center meets 95% and 94% of the corresponding average hardness of the hypoeutectoid rail and eutectoid rail base materials, respectively. The difference between the average hardness of the eutectoid rail heat-affected zone on one side and the average hardness of the hypoeutectoid rail heat-affected zone on the other side is within 18 HV. At the same time, the metallographic structure of the heat-affected zones of the rails on both sides of the weld is visible pearlite, without abnormal structures such as martensite.

[0066] Example 3

[0067] In this embodiment, the hypoeutectoid rail base material contains the following components by weight: 0.50% C, 0.40% Si, 0.55% Mn, 0.20% Cr, 0.15% Cu, 0.05% Ni, 0.02% V, with the balance being Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 408 HV. The eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, with the balance being Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device shown in Figure 3-5 is used to perform the first stage cooling on the rail welded joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 365°C at cooling rates of 4.5°C / s and 4.0°C / s respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s along with the construction environment, thereby obtaining the dissimilar rail welded joint after post-weld cooling treatment of the present invention.

[0068] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device with compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.00 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0069] The rail joint obtained in this embodiment after post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0070] In the present invention, the average hardness of the hypoeutectoid rail base metal is 408 HV, and the average hardness of the eutectoid rail base metal is 441 HV. For rail weld joints treated with the present invention, the longitudinal hardness of the rail joint within a ±15 mm zone from the weld center meets 92% and 94% of the corresponding average hardness of the hypoeutectoid and eutectoid rail base metals, respectively. Furthermore, the difference between the average hardness of the eutectoid rail heat-affected zone (HAZ) on one side and the average hardness of the hypoeutectoid rail heat-affected zone on the other side is within 14 HV. Furthermore, the metallographic structure of the HAZ on both sides of the weld is visible pearlite, with no abnormal structures such as martensite.

[0071] Example 4

[0072] In this embodiment, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV. The eutectoid rail base material contains the following components by weight: 0.73% C, 0.55% Si, 0.70% Mn, 0.35% Cr, and 0.06% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 430 HV.

[0073] After the dissimilar rail weld joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device shown in Figure 3-5 is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 905°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 485°C at cooling rates of 13.2°C / s and 12.2°C / s respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 325°C at cooling rates of 4.1°C / s and 3.6°C / s respectively. Finally, the rail joint is cooled to an ambient temperature of 15°C at a cooling rate of 0.90°C / s along with the construction environment, thereby obtaining the dissimilar rail weld joint after post-weld cooling treatment of the present invention.

[0074] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.5 MPa and 3.1 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.05 MPa and 0.85 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0075] The rail joint obtained in this embodiment after post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0076] In the present invention, the average hardness of the hypoeutectoid rail base metal is 418 HV, and the average hardness of the eutectoid rail base metal is 430 HV. For rail weld joints treated with the present invention, the longitudinal hardness of the rail joint within the ±15 mm zone from the weld center meets 94% and 93% of the corresponding average hardness of the hypoeutectoid and eutectoid rail base metals, respectively. The difference between the average hardness of the eutectoid rail heat-affected zone (HAZ) on one side and the average hardness of the hypoeutectoid rail heat-affected zone on the other side is 7 HV. Furthermore, the metallographic structure of the HAZ on both sides of the weld is visible pearlite, with no abnormal structures such as martensite.

[0077] Example 5

[0078] In this embodiment, the hypoeutectoid rail base material contains the following components by weight: 0.60% C, 0.60% Si, 0.70% Mn, 0.30% Cr, 0.35% Cu, 0.20% Ni, 0.05% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 413 HV. The eutectoid rail base material contains the following components by weight: 0.76% C, 0.65% Si, 0.90% Mn, 0.35% Cr, 0.07% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 420 HV.

[0079] After the dissimilar rail weld joint completes the upsetting and nodule pushing in the mobile flash welding process, a rail head cooling device is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 905°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 485°C at cooling rates of 13.2°C / s and 12.2°C / s, respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 325°C at cooling rates of 4.1°C / s and 3.6°C / s, respectively. Finally, the rail joint is cooled to an ambient temperature of 15°C at a cooling rate of 0.90°C / s according to the construction environment, thereby obtaining the dissimilar rail weld joint after post-weld cooling treatment of the present invention.

[0080] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.5 MPa and 3.1 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.05 MPa and 0.85 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0081] The rail joint obtained in this embodiment after post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0082] In the present invention, the average hardness of the hypoeutectoid rail base metal is 413 HV, and the average hardness of the eutectoid rail base metal is 420 HV. For the rail welded joint treated by the present invention, the longitudinal hardness of the rail joint within the area ±15 mm from the weld center meets 93% and 93% of the corresponding average hardness of the hypoeutectoid rail and eutectoid rail base metal, respectively. The difference between the average hardness of the eutectoid rail heat-affected zone on one side and the average hardness of the hypoeutectoid rail heat-affected zone on the other side is 7 HV. At the same time, the metallographic structure of the heat-affected zone of the rails on both sides of the weld is visible pearlite, without abnormal structures such as martensite.

[0083] Comparative Example 1

[0084] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV; the eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV.

[0085] After completing the upsetting and nodule pushing process during the mobile flash welding of a 60kg / m dissimilar rail weld joint, the rail joint with a residual temperature of 985°C is directly air-cooled to ambient temperature (25°C), thereby obtaining a rail weld joint under air-cooling (natural cooling) conditions.

[0086] The rail joint obtained in this comparative example was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, Model HBV-30A) was used to perform a longitudinal Vickers hardness test on the hardness specimen at a position 5 mm below the rail head tread, with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T 4340.1-2009 "Metal Vickers hardness test Part 1: Test method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal microstructure examination method". The rail joint metallographic specimen was etched with 3% nitric acid alcohol solution and the metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0087] For rail welded joints that have not been treated with the post-weld cooling method provided by the present invention, the entire weld heat affected zone is in a softened state compared to the hardness of the rail parent material on both sides of the weld. The average hardness of the hypoeutectoid rail parent material is 418HV, and the average hardness of the eutectoid rail parent material is 442HV. For rail welded joints treated in this comparative example, the longitudinal hardness of the rail joint within the area of ​​±15mm from the center of the weld meets 84% ​​and 87% of the corresponding average hardness of the hypoeutectoid rail and the eutectoid rail parent material, respectively, and the average hardness of the eutectoid rail heat affected zone on one side is 33HV different from the average hardness of the hypoeutectoid rail heat affected zone on the other side, which exceeds 25HV. The overall hardness of the joint is low. At the same time, the metallographic structure of the rail weld heat affected zone on both sides of the weld is visible pearlite, without abnormal structures such as martensite. Under this process, the rail weld heat affected zone on both sides of the rail joint weld is significantly softened, which can easily lead to "saddle-shaped" wear in the heat affected zone of the rail joint during subsequent service, which is not conducive to railway operation safety.

[0088] Comparative Example 2

[0089] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV; the eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s, respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 205°C at cooling rates of 4.5°C / s and 4.0°C / s, respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s along with the construction environment, thereby obtaining a dissimilar rail weld joint that has undergone post-weld cooling treatment.

[0090] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.00 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0091] The rail joints obtained in this comparative example, which had undergone post-weld heat treatment, were machined into longitudinal hardness specimens. A Brinell hardness tester (Model HBV-30A, Laizhou Testing Machine Factory, Shandong Province) was used to perform longitudinal Vickers hardness testing on the hardness specimens at a position 5 mm below the rail head tread, with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimens was inspected in accordance with GB / T13298-2015 "Methods for the Examination of Metal Microstructures". The rail joint metallographic specimens were etched with a 3% nitric acid alcohol solution and the metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0092] For rail weld joints not treated with the post-weld cooling method provided by the present invention, the final cooling temperature in the second cooling stage was below the Ms temperature of both rail steels, and the cooling rate exceeded the critical martensitic transformation rate of both rail steels. This resulted in varying degrees of martensite formation in both the hypoeutectoid and eutectoid heat-affected zones (HAZs). The martensite content in the HAZ of the hypoeutectoid rail steel was higher than that in the eutectoid rail steel. Hardness tests showed that the average hardness of the hypoeutectoid rail base metal was 418 HV, while the average hardness of the eutectoid rail base metal was 441 HV.

[0093] For the rail welded joint treated in this comparative example, the longitudinal hardness of the rail joint within a ±15mm zone from the weld center met 97% and 95% of the average hardness of the corresponding hypoeutectoid rail and eutectoid rail base metal, respectively. Furthermore, the difference between the average hardness of the eutectoid rail heat-affected zone and the average hardness of the hypoeutectoid rail heat-affected zone was within 14V. However, the presence of varying degrees of brittle martensite in the heat-affected zones of the rails on both sides of the weld compromised the safety of the rail joint in railway service.

[0094] Comparative Example 3

[0095] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV; the eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s, respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 300°C at cooling rates of 4.5°C / s and 4.0°C / s, respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s according to the construction environment, thereby obtaining the dissimilar rail weld joint after post-weld cooling treatment of the present invention.

[0096] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device using compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.00 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0097] The rail joint obtained in this comparative example that had undergone post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0098] For rail weld joints not treated with the post-weld cooling method provided by the present invention, the final cooling temperature during the second cooling stage is within the hypoeutectoid rail steel's Ms temperature range and higher than the eutectoid rail steel's Ms temperature range. Furthermore, the cooling rate during the second cooling stage exceeds the hypoeutectoid rail steel's critical martensitic transformation rate. This results in a small amount of martensite forming on the hypoeutectoid rail weld heat-affected zone (HAZ), while no martensite forms on the eutectoid rail weld heat-affected zone (HAZ). Hardness tests show that the average hardness of the hypoeutectoid rail base material is 418 HV, while the average hardness of the eutectoid rail base material is 441 HV.

[0099] For the rail welded joint treated in this comparative example, the longitudinal hardness of the rail joint within the ±15mm zone from the weld center met 96% and 93% of the average hardness of the corresponding hypoeutectoid rail and eutectoid rail base metal, respectively. Furthermore, the difference between the average hardness of the eutectoid rail's heat-affected zone and the average hardness of the hypoeutectoid rail's heat-affected zone was within 9V. The presence of a small amount of martensite in the heat-affected zone on the hypoeutectoid rail side of the rail joint compromised the safety of the rail joint in railway service.

[0100] Comparative Example 4

[0101] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV; the eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device is used to perform the first stage cooling on the rail weld joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s, respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 225°C at cooling rates of 4.5°C / s and 4.0°C / s, respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s along with the construction environment, thereby obtaining a dissimilar rail weld joint that has undergone post-weld cooling treatment.

[0102] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device with compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.00 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0103] The rail joint obtained in this comparative example that had undergone post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0104] For rail weld joints not treated with the post-weld cooling method provided by the present invention, the final cooling temperature in the second cooling stage was below the hypoeutectoid rail steel's Ms temperature and within the eutectoid rail steel's Ms temperature range. Furthermore, because the cooling rate in the second cooling stage exceeded the critical cooling rate for martensitic transformation of both rail steels, varying degrees of martensite appeared in both the hypoeutectoid rail heat-affected zone (HAZ) and the eutectoid rail heat-affected zone (HAZ). Furthermore, the martensite content in the HAZ on the hypoeutectoid rail side was higher than that in the HAZ on the eutectoid rail side. Hardness tests showed that the average hardness of the hypoeutectoid rail base metal was 418 HV, while the average hardness of the eutectoid rail base metal was 441 HV.

[0105] For the rail weld joints treated in the comparative example, the longitudinal hardness of the rail joint within the ±15mm zone from the weld center met 96% and 95% of the average hardness of the corresponding hypoeutectoid rail and eutectoid rail base metal, respectively. Furthermore, the difference between the average hardness of the eutectoid rail heat-affected zone and the average hardness of the hypoeutectoid rail heat-affected zone was within 18V. However, the presence of varying degrees of martensite in the heat-affected zones of the rails on both sides of the weld compromised the safety of the rail joint in railway service.

[0106] Comparative Example 5

[0107] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.64% C, 0.70% Si, 0.80% Mn, 0.40% Cr, 0.45% Cu, 0.25% Ni, 0.08% V, and the balance is Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 418 HV; the eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, and the balance is Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing during the mobile flash welding process, the rail head cooling device is used to perform the first stage cooling of the rail welded joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 300°C at cooling rates of 14.2°C / s and 13.0°C / s, respectively. Then, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s along with the construction environment, thereby obtaining the dissimilar rail welded joint obtained in this comparative example.

[0108] During the post-weld cooling process, the first stage involves cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device with compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During this first stage, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid and eutectoid rail sides of the rail joint are 3.9 MPa and 3.4 MPa, respectively. The second stage involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0109] The rail joint obtained in this comparative example that had undergone post-weld heat treatment was machined into a longitudinal hardness specimen. A Brinell hardness tester (Shandong Laizhou Testing Machine Factory, model HBV-30A) was used to perform longitudinal Vickers hardness testing on the hardness specimen at a position 5 mm below the rail head tread with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Metal Microstructure Test Method", and the metallographic specimen of the rail joint was etched with a 3% nitric acid alcohol solution. The metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0110] In this comparative example, the average hardness of the hypoeutectoid rail base material was 418 HV, and the average hardness of the eutectoid rail base material was 441 HV. For the rail welded joints treated by the present invention, the longitudinal hardness of the rail joint within the ±15 mm zone from the weld center met 97% and 96% of the corresponding average hardnesses of the hypoeutectoid rail and eutectoid rail base materials, respectively. Furthermore, the difference between the average hardness of the eutectoid rail heat-affected zone (HAZ) and the average hardness of the hypoeutectoid rail heat-affected zone (HAZ) was within 18 HV. For rail welded joints treated using this post-weld cooling method, a small amount of martensitic structure appeared in the HZ of the hypoeutectoid rail weld because the final cooling temperature of the first cooling stage was within the temperature range of the hypoeutectoid rail steel, and because the cooling rate of the HZ on the hypoeutectoid side was higher than the critical cooling rate for martensitic transformation of the rail steel. However, because the final cooling temperature of the eutectoid rail side in the second cooling stage was above the rail steel temperature, no abnormal martensitic structure formed in the HZ on the eutectoid rail side. Since the brittle and hard martensite structure appears in the weld heat affected zone on one side of the hypoeutectoid rail, it is not conducive to the service safety of the rail joint and the railway.

[0111] Comparative Example 6

[0112] In this comparative example, the hypoeutectoid rail base material contains the following components by weight: 0.50% C, 0.40% Si, 0.55% Mn, 0.20% Cr, 0.15% Cu, 0.05% Ni, 0.02% V, with the balance being Fe and inevitable impurities. The average hardness of the hypoeutectoid rail base material is 408 HV. The eutectoid rail base material contains the following components by weight: 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.10% V, with the balance being Fe and inevitable impurities. The average hardness of the eutectoid rail base material is 441 HV. After the dissimilar rail joint completes the upsetting and nodule pushing in the mobile flash welding process, the rail head cooling device is used to perform the first stage cooling on the rail welded joint with a surface residual temperature of 985°C formed by welding. During the cooling process, the surface temperature of the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to 525°C at cooling rates of 14.2°C / s and 13.0°C / s, respectively. Then, the heat-affected zone on the hypoeutectoid rail side and the eutectoid rail side of the rail joint is cooled to a surface temperature of 365°C at cooling rates of 4.5°C / s and 4.7°C / s, respectively. Finally, the rail joint is cooled to an ambient temperature of 25°C at a cooling rate of 0.40°C / s according to the construction environment, thereby obtaining the dissimilar rail welded joint after post-weld cooling treatment of the present invention.

[0113] During the post-weld cooling process, the first and second stages of cooling involve cooling the rail head tread and side surfaces of the rail joint using a rail head cooling device with compressed air or a water mist mixture as the cooling medium. The cooling device is located 15 mm from the rail head tread. During the first stage of cooling, the compressed air pressures injected by the cooling device in the heat-affected zones (HAZs) on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 3.9 MPa and 3.4 MPa, respectively. During the second stage of cooling, the compressed air pressures injected by the cooling device in the HZs on the hypoeutectoid rail side and the eutectoid rail side of the rail joint are 1.20 MPa and 1.30 MPa, respectively. The third stage of cooling involves natural cooling in air. The rail head tread temperature is monitored using an infrared thermometer.

[0114] The rail joint obtained in this comparative example, which had undergone post-weld heat treatment, was machined into a longitudinal hardness specimen. A Brinell hardness tester (Model HBV-30A, Laizhou Testing Machine Factory, Shandong Province) was used to perform a longitudinal Vickers hardness test on the hardness specimen at a position 5 mm below the rail head tread, with a measuring point spacing of 2 mm. The measuring points were symmetrically arranged on the left and right sides with the weld as the center. The Vickers hardness test method was carried out in accordance with GB / T 4340.1-2009 "Metal Vickers Hardness Test Part 1: Test Method" and the HV scale was used. Referring to the sampling method shown in Figure 2, the metallographic structure of the rail joint metallographic specimen was inspected in accordance with GB / T13298-2015 "Methods for the Examination of Metal Microstructures". The rail joint metallographic specimen was etched with a 3% nitric acid alcohol solution and the metallographic structure of the rail joint was observed using a German Leica MeF3 optical microscope.

[0115] In this comparative example, the average hardness of the hypoeutectoid rail base material is 408HV, and the average hardness of the eutectoid rail base material is 441HV. For the rail welded joints processed in this comparative example, the longitudinal hardness of the rail joint within the area ±15mm from the weld center meets 92% and 96% of the corresponding average hardness of the hypoeutectoid rail and eutectoid rail base materials, respectively, and the difference between the average hardness of the heat-affected zone of the eutectoid rail and the average hardness of the heat-affected zone of the hypoeutectoid rail is 48HV. Due to a certain degree of composition segregation during the alloying process of the eutectoid rail steel, a small amount of martensite is generated in the heat-affected zone on the eutectoid rail side of the rail joint, while no martensite is observed in the heat-affected zone on the hypoeutectoid rail side of the rail joint. The generation of martensite is detrimental to the subsequent service safety of the rail joint on the railway.

[0116] By comparing the longitudinal hardness of the rail head tread and the metallographic structure of the welded joint obtained in each embodiment and comparative example, it can be seen that the post-weld cooling treatment method provided by the present invention is used to perform post-weld cooling treatment on the hypoeutectoid rail and eutectoid rail welded joint. This method can ensure that there is no abnormal martensite structure in the heat-affected zone on both sides of the rail joint weld. At the same time, the longitudinal hardness of the rail joint in the area ±15mm from the center of the weld can reach 92-96% of the average hardness of the corresponding hypoeutectoid rail and eutectoid rail base material, and the difference between the average hardness of the heat-affected zone of the eutectoid rail and the average hardness of the heat-affected zone of the hypoeutectoid rail is within 25HV. At the same time, the metallographic structure of the heat-affected zone on both sides of the weld is visible pearlite, without abnormal structures such as martensite. The use of the present invention helps to improve the "saddle-shaped" wear of the rail welded joint caused by the low hardness of the welding area during the service of the rail line, and helps to ensure the safety of railway operation. At the same time, the hardness difference of the heat-affected zones on both sides of the weld of the dissimilar rail joint can be improved, thereby achieving hardness matching of the heat-affected zones on both sides of the weld of the dissimilar strength rails, which is beneficial to ensuring railway driving safety.

[0117] The above-described embodiments merely illustrate the implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, and all such modifications and improvements fall within the scope of protection of the present invention.

Claims

1. A post-weld cooling method for flash welded joints of dissimilar rails, characterized in that: The following steps are involved: Step S1: performing a first stage cooling of the heat affected zones on both sides of the weld at different cooling rates after welding of a rail welded joint formed by welding a hypoeutectoid rail and a eutectoid rail, wherein the heat affected zone on one side of the hypoeutectoid rail is cooled at a first cooling rate, and the heat affected zone on one side of the eutectoid rail is cooled at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate; Step S2: performing a second stage cooling on the weld joint, wherein the heat affected zone on one side of the hypoeutectoid rail is cooled at a third cooling rate, and the heat affected zone on one side of the eutectoid rail is cooled at a fourth cooling rate, wherein the third cooling rate is greater than the fourth cooling rate; Step S3: Cooling the welded joint in the third stage by natural cooling.

2. The method according to claim 1, characterized in that The base material corresponding to the hypoeutectoid rail contains the following components by weight percentage: 0.50-0.64% C, 0.40-0.70% Si, 0.55-0.8% Mn, 0.20-0.40% Cr, 0.15-0.45% Cu, 0.05-0.25% Ni, 0.02-0.08% V, and the balance is Fe and unavoidable impurities.

3. The method according to claim 1, characterized in that The base material corresponding to the eutectoid rail comprises the following components by weight percentage: 0.73-0.82% C, 0.50-0.80% Si, 0.70-1.0% Mn, 0.30-0.50% Cr, 0.06-0.10% V, and the balance is Fe and inevitable impurities.

4. The method according to claim 1, characterized in that: The first cooling rate is 13.2-14.2°C / s, and the second cooling rate is 12.2-13.0°C / s.

5. The method according to claim 1, characterized in that The third cooling rate is 4.1-4.5°C / s, and the fourth cooling rate is 3.6-4.0°C / s.

6. The method according to claim 4 or 5, characterized in that: The joint is a joint formed by welding a hypoeutectoid rail and a eutectoid rail of the same rail type and specification of 60kg / m through a rail moving flash welder.

7. The method according to claim 1, characterized in that The first stage cooling in step S1 has a starting temperature of 905-985°C and an ending temperature of 485-525°C.

8. The method according to claim 7, characterized in that The first stage cooling operation described in step S1 is to use a profiled cooling device to spray compressed air or a water mist mixture to cool the rail joint, so that the surface temperature of the rail joint drops to a set end temperature. In the first stage cooling, the cooling air pressure used in the heat affected zone on one side of the hypoeutectoid rail is 3.5-3.9 MPa, and the cooling air pressure used in the heat affected zone on the one side of the eutectoid rail is 3.1-3.4 MPa.

9. The method according to claim 1, characterized in that: The end temperature of the second stage cooling described in step S2 is 325-365°C.

10. The method according to claim 9, characterized in that The second stage cooling operation described in step S2 is to use a profiled cooling device to spray compressed air or a water mist mixture on the welded joint to reduce the surface temperature of the rail joint to a set end temperature. In the second stage cooling, the cooling air pressure used in the heat affected zone on one side of the hypoeutectoid rail is 1.05-1.20 MPa, and the cooling air pressure used in the heat affected zone on the one side of the eutectoid rail is 0.85-1.00 MPa.

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