Method for improving hardness and impact toughness of welded joint of pearlitic and bainitic rail

Abstract

The application belongs to the field of rail manufacturing, and discloses a method for improving the hardness and impact toughness of a pearlite and bainite rail welded joint. The method comprises the following steps: once cooling the welded joint formed by welding the pearlite rail and the bainite rail; heating the once-cooled welded joint to the normalizing temperature by using electric induction heating, the heating frequency of the induction heating coil on the pearlite side is 2-4 kHz, the heating frequency of the induction heating coil on the bainite side is 6-9 kHz, and austenitizing the welded joint; twice cooling the welded joint, wherein the heat-affected zone on the pearlite side is cooled at a first cooling rate, and the heat-affected zone on the bainite side is cooled at a second cooling rate; and naturally cooling the welded joint to the ambient temperature. The application is helpful to improve the "saddle-type" abrasion caused by the low hardness of the welded area in the process of the rail welded joint serving on the line, and the impact toughness of the joint is good.

Description

Technical Field

[0001] This invention relates to the field of rail manufacturing technology, and in particular to a method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic rails. Background Technology

[0002] Currently, high-speed, semi-high-speed, and mixed passenger-freight railways both domestically and internationally mostly use eutectoid pearlitic rails. These rails typically have a carbon content ranging from 0.65% to 0.82% by weight, a pearlitic microstructure, and exhibit good strength and toughness, along with moderate overall mechanical properties. The rapid development of railways has placed higher demands on the service performance of rails. Since the mechanical and weldability of traditional pearlitic rails have almost reached their limits, bainitic rails, with higher strength grades and good wear resistance and contact fatigue resistance, have emerged. These rails typically have a carbon content ranging from 0.10% to 0.30% by weight, and a multiphase microstructure consisting of bainite, a small amount of martensite (or martensite islands), and retained austenite.

[0003] With the increasing use of bainitic rails and their mixed application with existing pearlitic rails on railway lines, welding dissimilar materials has undoubtedly presented significant challenges. During welding, the hardened layer in the weld area disappears after the rails are subjected to the welding thermal cycle, resulting in a wide, low-hardness zone on both sides of the weld. The bainitic rail side experiences more severe softening after welding. Current heat treatment methods for dissimilar rail joints typically involve heating both sides of the joint to a set temperature using the same heating method, followed by controlled cooling methods such as air cooling or blower cooling to cool the heat-affected zone on both sides of the weld. While this post-weld heat treatment method can improve the hardness of both sides of the weld joint to some extent, it also significantly reduces the toughness of the heat-affected zone on the bainitic side.

[0004] For welded joints of dissimilar rails consisting of eutectoid pearlitic and bainitic steels that have undergone conventional normalizing heat treatment, the hardness and impact toughness of the weld heat-affected zone on the bainitic rail side are still generally low, which is detrimental to railway operation safety. Therefore, there is an urgent need in this field for a method to comprehensively improve the hardness and impact toughness of welded joints of pearlitic and bainitic rails. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic rails, thereby improving the tread hardness of dissimilar rails reduced by welding and simultaneously improving the impact toughness of the rail joints, thus ensuring the service performance of welded joints of dissimilar rails and the safety of railway operation.

[0006] According to one aspect of the present invention, a method for improving the hardness and impact toughness of a welded joint between pearlitic and bainitic steel rails is provided, comprising the following steps:

[0007] Step S1: Cool the welded joint formed by welding the heat-treated eutectoid pearlitic steel rail and the hot-rolled bainitic steel rail once.

[0008] Step S2: The welded joint, which has undergone the first cooling, is heated to the normalizing temperature using induction heating. During heating, the center of the weld is used as the dividing line. The heating frequency of the induction heating coil on the pearlite side is 2~4kHz, and the heating frequency of the induction heating coil on the bainite side is 6~9kHz, so as to austenitize the welded joint.

[0009] Step S3: After the normalizing heating is completed, the welded joint is subjected to secondary cooling. The heat-affected zone on the pearlite side is cooled to the first final cooling temperature at a first cooling rate, and the heat-affected zone on the bainite side is cooled to the second final cooling temperature at a second cooling rate. The first cooling rate is greater than the second cooling rate, and the first final cooling temperature is higher than the second final cooling temperature.

[0010] Step S4: After the secondary cooling is completed, the welded joint is allowed to cool naturally to the ambient temperature.

[0011] According to one embodiment of the present invention, in step S2, the heating temperature on the pearlite side is 940~990℃, and the heating temperature on the bainite side is 1000~1050℃.

[0012] According to one embodiment of the present invention, in step S3, the first cooling rate is 4.0~7.0℃ / s, and the second cooling rate is 2.4~2.8℃ / s.

[0013] According to an embodiment of the present invention, in step S3, the first final cooling temperature of the secondary cooling of the heat-affected zone on the pearlite side is 390~420°C, and the second final cooling temperature of the secondary cooling of the heat-affected zone on the bainite side is 300~330°C.

[0014] According to one embodiment of the present invention, the secondary cooling is performed by spraying compressed air or a mixture of water mist.

[0015] According to one embodiment of the present invention, the jet pressure for secondary cooling of the heat-affected zone on the pearlite side is 0.40~0.70MPa, and the jet pressure for secondary cooling of the heat-affected zone on the bainite side is 0.12~0.14MPa.

[0016] According to one embodiment of the present invention, the primary cooling is natural cooling performed in an air environment at 20~30°C.

[0017] According to one embodiment of the present invention, the final cooling temperature of the first cooling is 100~200°C.

[0018] According to one embodiment of the present invention, in step S2, the induction heating rate on the pearlite side is 7~10℃ / s, and the induction heating rate on the bainite side of the rail head portion is 15~20℃ / s.

[0019] According to one embodiment of the present invention, the base material of the heat-treated eutectoid pearlitic steel rail comprises the following chemical composition by weight percentage: C: 0.77%~0.83%, Si: 0.45%~0.90%, Mn: 0.80%~1.20%, V: 0.15%~0.45%, with the balance being Fe and unavoidable impurities. The room temperature tensile strength of the base material is 1170~1250 MPa, the hardness is 310~350 HV, and the impact energy is 18~10 J. The base material of the hot-rolled bainitic steel rail comprises the following chemical composition by weight percentage: C: 0.15%~0.19%, Si: 1.20%~1.60%, Mn: 0.70%~1.10%, Cr: 1.50%~1.90%, Mo: 0.20%~0.50%, with the balance being Fe and unavoidable impurities. The room temperature tensile strength of the base material is 1130~1200MPa, the hardness is 290~330HV, and the impact energy is 130~100J.

[0020] By adopting the above technical solutions, the method of the present invention has at least one of the following beneficial effects:

[0021] (1) The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails provided by the present invention is based on the skin effect of induction heating. Different heating depths can be obtained by adjusting the induction heating frequency. After heating, rapid cooling can be used to obtain hardened layers of different depths. The heating frequency range of the left coil of the induction heating coil (corresponding to the heat-affected zone on one side of the heat-treated eutectoid pearlitic steel rail) with the weld center as the dividing line is 2~4kHz, and the heating depth / hardened layer depth corresponds to 25~35mm. The heating frequency range of the right coil (corresponding to the heat-affected zone on one side of the hot-rolled bainitic steel rail) is 2~4kHz. The heating frequency range is 6~9kHz, and the heating depth / hardening layer depth corresponds to 5~15mm. By heating and cooling the heat-affected zones on both sides of the weld of the rail joint of dissimilar materials differently, the average hardness difference between the heat-affected zones on both sides of the rail joint weld within a range of ±20mm from the weld center is controlled within ±25HV. This ensures the wear resistance of the heat-affected zones on both sides of the rail joint weld, and also allows the heat-affected zone on one side of the bainitic rail to obtain higher surface hardness while maintaining good impact toughness in the core of the heat-affected zone. This comprehensively improves the mechanical properties of the heat-affected zone on one side of the bainitic rail.

[0022] (2) The method of the present invention can achieve that there is no martensite in the heat-affected zone on one side of the heat-treated eutectoid pearlitic rail joint and the volume fraction of martensite in the heat-affected zone on one side of the hot-rolled bainitic rail joint is 20%~30%, and the martensite is mostly distributed in the shallow area, which increases the surface hardness on the bainitic side of the joint and maintains the toughness of the core.

[0023] (3) The average impact energy of the full-section weld of the rail joint at room temperature is 41~48J, which is much higher than the ≥6.5J specified in TB / T 1632.2-2014. This invention helps to improve the "saddle-shaped" wear of the rail welded joint caused by the low hardness of the welded area during the service of the line. The joint has good impact toughness, which helps to ensure the safety of railway operation. It is especially suitable for the processing technology after welding dissimilar rails. Attached Figure Description

[0024] Figure 1 A flowchart of a method for improving the hardness and impact toughness of a pearlitic and bainitic rail welded joint according to an embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram of the induction coil arrangement structure at the rail head of the rail joint during the induction heating process provided in an embodiment of the present invention;

[0026] Figure 3 This is a schematic diagram of the spray panel of the rail head tread air spray device provided in an embodiment of the present invention;

[0027] Figure 4 This is a schematic diagram of the bottom nozzle structure of the rail head tread air spray device provided in an embodiment of the present invention;

[0028] Figure 5 This is a side view of the rail head tread air jet device provided in an embodiment of the present invention;

[0029] Figure 6 A schematic diagram showing the sampling location for metallographic specimens of the rail head tread surface of a welded rail joint.

[0030] Figure 7 This is a schematic diagram showing the location for testing the longitudinal section hardness of the rail head of a welded rail joint. The longitudinal section hardness test is performed 3-5 mm below the tread surface of the rail head of the rail joint.

[0031] Figure 8 This is a sampling location diagram (unit: mm) for testing the impact performance of rail welded joints obtained according to the methods in the embodiments and comparative examples of this invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] like Figure 1 As shown, the method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails provided by the present invention generally includes the following steps:

[0034] Step S1: Cool the welded joint formed by welding the heat-treated eutectoid pearlitic steel rail and the hot-rolled bainitic steel rail once.

[0035] Step S2: The welded joint that has been cooled once is heated to the normalizing temperature by induction heating. During heating, the center of the weld is used as the dividing line. The heating frequency of the induction heating coil on the pearlite side is 2~4kHz, and the heating frequency of the induction heating coil on the bainite side is 6~9kHz, so as to austenitize the welded joint.

[0036] Step S3: After the normalizing heating is completed, the welded joint is subjected to secondary cooling. The heat-affected zone on the pearlite side is cooled to the first final cooling temperature at the first cooling rate, and the heat-affected zone on the bainite side is cooled to the second final cooling temperature at the second cooling rate. The first cooling rate is greater than the second cooling rate, and the first final cooling temperature is higher than the second final cooling temperature.

[0037] Step S4: After the secondary cooling is completed, allow the welded joint to cool naturally to ambient temperature.

[0038] The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails provided by this invention is based on the skin effect of induction heating. Different heating depths can be obtained by adjusting the induction heating frequency. After heating, rapid cooling can be used to obtain hardened layers of different depths. The heating frequency range of the left coil of the induction heating coil (corresponding to the heat-affected zone on one side of the heat-treated eutectoid pearlitic steel rail) is 2~4kHz, with the weld center as the dividing line. The heating depth / hardened layer depth is 25~35mm. The heating frequency of the right coil (corresponding to the heat-affected zone on one side of the hot-rolled bainitic steel rail) is... The frequency range is 6~9kHz, and the heating depth / hardening layer depth corresponds to 5~15mm. By heating and cooling the heat-affected zones on both sides of the weld of the rail joint of dissimilar materials differently, the average hardness difference between the heat-affected zones on both sides of the rail joint weld within a range of ±20mm from the weld center is controlled within ±25HV. This ensures the wear resistance of the heat-affected zones on both sides of the rail joint weld, and also allows the heat-affected zone on one side of the bainitic rail to obtain higher surface hardness while maintaining good impact toughness in the core of the heat-affected zone. This comprehensively improves the mechanical properties of the heat-affected zone on one side of the bainitic rail.

[0039] The method of this invention can achieve a martensite-free heat-affected zone on one side of the heat-treated eutectoid pearlitic rail joint and a martensite volume fraction of 20%~30% on the heat-affected zone on the other side of the hot-rolled bainitic rail joint. The average impact energy of the full-section weld seam of the rail joint at room temperature is 41~48J, far exceeding the ≥6.5J specified in TB / T 1632.2-2014. This invention helps to improve the "saddle-shaped" wear of rail welded joints caused by low hardness in the welded area during railway service. The joint exhibits good impact toughness, contributing to railway operation safety, and is particularly suitable for post-weld processing of dissimilar rails.

[0040] The following is an exemplary description of the various operational steps of the method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails provided by the present invention.

[0041] In step S1, the weld joint formed by welding the heat-treated eutectoid pearlitic steel rail and the hot-rolled bainitic steel rail is cooled once.

[0042] In some embodiments, the base material of the heat-treated eutectoid pearlitic steel rail comprises the following chemical composition by weight percentage: C: 0.77%~0.83%, Si: 0.45%~0.90%, Mn: 0.8%~1.2%, V: 0.15%~0.45%, with the balance being Fe and unavoidable impurities. The base material has a room temperature tensile strength of 1170~1250 MPa, a hardness of 310~350 HV, and an impact energy of 18~10 J; hot-rolled The base material of the bainitic steel rail comprises the following chemical composition by weight percentage: C: 0.15%~0.19%, Si: 1.2%~1.6%, Mn: 0.7%~1.1%, Cr: 1.50%~1.90%, Mo: 0.20%~0.50%, with the balance being Fe and unavoidable impurities. The room temperature tensile strength of the base material is 1130~1200MPa, the hardness is 290~330HV, and the impact energy is 130~100J.

[0043] In some embodiments, in step S1, the welded joint is obtained by flash welding. Flash welding has many advantages, such as concentrated heat, short heating time during the welding process, and a small heat-affected zone of the welded joint. The surface temperature of the joint after welding is approximately 800~1000℃. The first stage of cooling is natural cooling in an air environment at 20~30℃, which reduces the surface temperature of the welded joint to 100~200℃.

[0044] It should be noted that the cooling rate in the first stage should not be too high to prevent the formation of large areas of martensite in the weld heat-affected zone due to excessive cooling and the formation of microcracks due to excessive internal stress. Once microcracks form within the microstructure, they cannot be eliminated by subsequent post-weld heat treatment. During natural cooling, the cooling rate varies, with faster cooling at high temperatures and relatively slower cooling at low temperatures. Specifically, after rail welding, due to convection, radiation, and heat conduction with the surrounding medium, the rail joint exhibits rapid cooling at high temperatures and a gradually decreasing cooling rate at low temperatures during natural cooling in an air environment of 20-30℃. The average cooling rate for natural cooling within the 1100-801℃ temperature range is 4.0-2.5℃ / s. The cooling rate for natural cooling within the 800-501℃ temperature range is 2.5-1.2℃ / s. The natural cooling rate is 1.2~0.7℃ / s within the temperature range of 500~301℃. Within the temperature range of 300~20℃, the natural cooling rate is 0.7~0.1℃ / s. Furthermore, without considering compositional segregation in the rail steel, natural cooling of the rail welded joint directly in air after welding, and natural cooling in air after normalizing, generally do not cause the formation of brittle martensite in the heat-affected zone.

[0045] After one cooling cycle, in step S2, induction heating is used to heat the weld joint to the normalizing temperature. During heating, the center of the weld is used as the dividing line. The heating frequency of the induction heating coil on the pearlite side is 2~4kHz, and the heating frequency of the induction heating coil on the bainite side is 6~9kHz, which austenitizes the weld joint. The principle of induction heating for rail weld joints is to cover the outer contour of the joint with two inductors. During normalizing, the two inductors are engaged, and an alternating current is passed through. The two symmetrical inductors form eddy currents under the action of the alternating current. The eddy currents and the "skin effect" are used to heat the weld and the heat-affected zones on both sides (a physical phenomenon in which the current density in the cross-section of a conductor increases from the center to the surface, while most of the current flows along the surface of the conductor). The induction heating frequency on the bainite side is higher, and its electromagnetic induction "skin effect" is more significant. After rapid cooling, its hardened layer depth is shallower, reducing the probability of martensite formation in the core and improving the toughness of the core.

[0046] In some embodiments, the rail head portion of the heat-affected zone on the bainitic rail side of the dissimilar rail joint is rapidly heated at an average heating rate of 15-20°C / s, while the rail head portion of the heat-affected zone on the eutectoid pearlitic rail side is conventionally heated at an average heating rate of 7-10°C / s. Considering the difference in heating rates between the heat-affected zones on both sides of the weld, the rail head portion of the heat-affected zone on the eutectoid pearlitic rail side is prioritized for conventional heating, while the rapid heating of the rail head portion on the bainitic rail side is relatively delayed. When heating stops simultaneously, the temperature of the rail head portion of the heat-affected zone on the bainitic rail side is approximately 40-70°C higher than that on the eutectoid pearlitic rail side. Furthermore, considering that the rail head portion plays a role in wear resistance and impact resistance, while the requirements for hardness and impact toughness of the rail web and rail base are not as high as those for the rail head, conventional heating at an average heating rate of 7-10°C / s is applied to both the rail web and rail base of the rail joint with both materials.

[0047] Figure 2 This is a schematic diagram of the induction coil arrangement structure at the rail head of the rail joint during induction heating, provided in an embodiment of the present invention. As shown, the heat-affected zones on both sides of the weld can utilize two independent sets of induction heating coils. Each set of coils surrounds the outer contour of the corresponding heat-affected zone, and each set includes two sub-coils (i.e., coils a1, b1 and coils a2, b2). The current of the two sets of coils can vary independently to achieve their respective heating operations. The heating frequency of the induction heating coil on the pearlite side is 2~4kHz, and the heating frequency of the coil on the bainite side is 6~9kHz. To avoid prolonged high-temperature residence time in the heat-treated area, which could lead to austenite grain coarsening, a suitable computer program can be used to synchronize the heating termination times on both sides of the weld. For example, compared to the heating on the pearlite side, the heating frequency on the bainite side is higher. With the same number of coil turns, the heating speed on the bainite side is slower. The heating speed on the bainite side can be increased by increasing the number of coil turns. Figure 2 As shown, one set of heating coils can be installed on the pearlite side and three sets of heating coils can be installed on the bainite side of the rail head section, thereby increasing the heating speed of the bainite heat-affected zone rail head section. Optionally, the start heating time of both can be adjusted to ultimately achieve simultaneous heating of both sides of the weld to their respective target heating temperatures.

[0048] In some embodiments, the heating temperature of the rail head portion of the heat-affected zone on the eutectoid pearlitic rail side of the rail joint is controlled at 940~990℃, and the heating temperature of the rail head portion of the heat-affected zone on the bainitic rail side of the rail joint is controlled at 1000~1050℃. It should also be noted that when the normalizing temperature is too high (e.g., above 1100℃), the austenite grains in the heated area (especially the weld and the coarse-grained heat-affected zone nearby) will be too large, thus affecting the impact toughness of the subsequent joint. Conversely, when the normalizing heating temperature is too low, such as heating the surface temperature of the welded joint to 900℃ or below, the driving force for the pearlite / bainite phase transformation during cooling is low, resulting in an unsatisfactory microstructure refinement effect during cooling, manifested as an insignificant improvement in the hardness and impact toughness of the rail weld heat-affected zone after heat treatment. When the normalizing temperature is below 880℃, the austenitization process in the heat-affected zone of the bainitic rail weld joint is incomplete, leading to insufficient phase transformation during subsequent cooling. This results in an unsatisfactory improvement in the hardness and impact toughness of the heat-affected zone after heat treatment. Therefore, the normalizing temperature for the heat-affected zone on the bainitic rail side of the rail joint is set relatively high.

[0049] In step S3, after the normalizing heating is completed, a second cooling is performed. The heat-affected zone on the pearlite side is cooled to the first final cooling temperature at a first cooling rate, and the heat-affected zone on the bainite side is cooled to the second final cooling temperature at a second cooling rate. The first cooling rate is greater than the second cooling rate, and the first final cooling temperature is higher than the second final cooling temperature.

[0050] In some embodiments, the heat-affected zone on the pearlite side is cooled to a final cooling temperature of 390-420°C at a cooling rate of 4.0-7.0°C / s, and the heat-affected zone on the bainite side is cooled to a final cooling temperature of 300-330°C at a cooling rate of 2.4-2.8°C / s. For example, a rail head tread spray device can be used to rapidly cool the heat-affected zones on both sides of the rail joint weld using compressed air or a water mist mixture as the cooling medium. The cooling intensity / cooling rate of the heat-affected zone on the eutectoid pearlite side of the rail joint corresponding to the left side of the device is higher than that of the heat-affected zone on the bainite side of the rail joint corresponding to the right side of the device.

[0051] Figure 3 This is a schematic diagram of the spray panel of the rail head tread air spray device provided in an embodiment of the present invention. Figure 4 This is 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 5 This is a side view of the rail head tread air jet device provided in an embodiment of the present invention.

[0052] As shown in the figure, the two cooling components are divided by a central partition plate located in the middle of the device. The cooling component on the left corresponds to the heat-affected zone of the heat-treated eutectoid pearlitic rail involved in this invention, and the cooling component on the right corresponds to the heat-affected zone of the hot-rolled bainitic rail involved in this invention. Each cooling component includes a spray panel located above the rail head tread of the dissimilar rail weld joint (see figure). Figure 3 The left side of the central partition plate 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 4 The central partition plate 7 has a left-side corresponding side spray hole 5 and a right-side corresponding side spray hole 6. Multiple spray holes are provided on the spray panel and spray pipes, and these spray holes are in fluid communication with the external cooling medium. Specifically, the spray hole 3 on the cooling component on the left side of the device is connected to the external cooling medium through channel 1; the spray hole 4 on the cooling component on the right side of the device is connected to the external cooling medium through channel 2. The central partition plate 7 protrudes to a position closer to the top surface of the rail head than the spray holes 3 and 4. A flexible asbestos gasket is provided at the contact point between the central partition plate and the center of the weld seam on the rail head surface of the rail joint, ensuring full fit between the partition plate and the center of the weld seam and preventing the airflow difference on both sides of the weld seam from affecting the cooling process.

[0053] In this invention, when using a rail head tread air spray device to cool the rail joint, the rail joint is positioned under the air spray device (e.g., Figure 5 As shown, 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 welded joint with different cooling rates / intensities.

[0054] In some embodiments, the distance between the bottom cooling channel of the rail head tread air spray device and the surface of the welded joint is controlled to be 25-30 mm. The cooling medium of the rail head tread air spray device is a mixture of compressed air and / or water mist, wherein the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling component corresponding to the heat-affected zone of the heat-treated eutectoid pearlitic 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 component corresponding to the heat-affected zone of the hot-rolled bainitic rail on the right side of the rail joint weld is 0.12-0.14 MPa.

[0055] The pressure of the gas medium ejected by the cooling component can be adjusted by the number and diameter of the injection holes 3 and 4 to achieve the desired cooling effect on the heat-affected zone on the left side of the weld of the dissimilar rail weld joint, which is a heat-treated eutectoid pearlitic rail, and on the right side of the weld, which is a hot-rolled bainitic rail.

[0056] The rail head tread air jet device cools the rail head tread and sides of the rail on the left and right sides of the weld joint. The orifice size of its jet channel can be designed and manufactured according to actual needs to achieve cooling intensities. The gas pressure of the compressed air flowing through channel 1 and channel 2 can be monitored by a pressure gauge and adjusted according to actual needs. In this invention, the compressed air and / or water mist mixture ejected by the cooling component has the same cooling rate / cooling capacity.

[0057] Rapid cooling of the heat-affected zone (HAZ) of heat-treated eutectoid pearlitic rails and hot-rolled bainitic rails above the austenitizing temperature by spraying compressed air and / or a water mist mixture can refine the pearlite lamellar spacing in the HAZ of heat-treated eutectoid pearlitic rails and provide thermodynamic conditions for obtaining a small amount of martensite in the shallow layer of the HAZ of hot-rolled bainitic rails through cooling.

[0058] In this embodiment, the critical cooling rate for martensitic transformation during the continuous cooling process of heat-treated eutectoid pearlitic rail steel is 2.0~2.5℃ / s, and the Ms temperature (the starting temperature for martensite formation) of the rail steel is 250~280℃. For hot-rolled bainitic rail steel, the critical cooling rate for martensitic transformation during continuous cooling is 1.8~2.2℃ / s, and the Ms temperature (the starting temperature for martensite formation) of the rail steel is 300~330℃. When the cooling rate of the heat-affected zone on one side of the hot-rolled bainitic rail joint reaches 2.4~2.8℃ / s during continuous cooling, and the final cooling temperature of this cooling stage is simultaneously 300~330℃, in addition to the formation of bainitic microstructure, a martensite microstructure with a volume fraction of 20%~30% will also be formed on the surface of this heat-affected zone. This can improve the surface hardness of the heat-affected zone, and the refined microstructure can also improve the impact toughness of the heat-affected zone to a certain extent. When the cooling rate of the heat-affected zone (HAZ) on one side of the hot-rolled bainitic rail joint reaches 3.0~4.0℃ / s during the continuous cooling process, and the final cooling temperature of this stage is simultaneously 300~330℃, in addition to bainitic microstructure, martensite with a volume fraction of 32%~40% will also be formed in this HAZ. This will lead to excessively high hardness and deterioration of impact toughness in this HAZ. Furthermore, if the cooling rate of the HAZ on one side of the hot-rolled bainitic rail joint during the rapid cooling stage is kept higher than the martensitic transformation cooling rate of the rail steel, and the final cooling temperature of this stage is further reduced to 300~330℃ below the Ms temperature (the starting temperature of martensite formation) of the rail steel (e.g., below 280℃), it will lead to a significant increase in the martensite content and hardness in this HAZ, as well as further deterioration of impact toughness. Therefore, it is necessary to reasonably control the cooling rate and final cooling temperature during this operation.

[0059] By rapidly cooling one side of the heat-affected zone (HAZ) of the hot-rolled bainitic rail in the dissimilar rail joint, a certain amount of martensite (20-30% by volume) is generated to improve the surface hardness of the HAZ on that side, while avoiding the formation of excessive martensite (above 30% by volume), which would lead to excessively high hardness and deterioration of impact toughness in the HAZ. For the HAZ on the side of the eutectoid pearlitic rail in the rail joint, a relatively high cooling rate is used to generate pearlitic structure with fine lamellar spacing. At the same time, the final cooling temperature of this rapid cooling stage is controlled above 50°C, the martensitic transformation temperature of the rail steel, to avoid the formation of brittle and hard martensite in the pearlitic structure, which would compromise the service safety of the welded HAZ of the pearlitic rail.

[0060] In step S4, after the secondary cooling is completed, the welded joint is allowed to cool naturally to ambient temperature. For example, the joint can be allowed to cool naturally in air to an ambient temperature of 20~30℃, and the cooling rate of this process is approximately 0.1~0.9℃ / s.

[0061] Optionally, in some embodiments, the dissimilar rail welded joint is a joint formed by welding a heat-treated eutectoid pearlitic rail a and a hot-rolled bainitic rail b with the same rail type and specifications of 60~75kg / m using a rail moving flash welding machine.

[0062] Compared to conventional heat treatment of welded joints, the method of this invention exhibits a significant phase transformation hysteresis during the rapid heat treatment of bainitic steel. That is, under rapid heating conditions, the heating temperature (i.e., the austenitizing temperature) needs to be appropriately increased to ensure complete austenitization of the bainitic steel. This invention implements different heating methods for the heat-affected zones on both sides of the rail welded joint, with the weld center as the dividing line. For the heat-affected zone on the eutectoid pearlitic rail side of the dissimilar rail joint, heating is achieved using conventional medium-frequency induction heating coils and medium-frequency induction heating power. For the heat-affected zone on the bainitic rail side of the dissimilar rail joint, rapid heating is achieved by increasing the number of induction heating coils and adjusting the induction heating power.

[0063] Therefore, for the cooling of the heat-affected zone (HAZ) on one side of the bainitic rail in the rail joint, in order to generate a certain amount of martensite to improve the surface hardness and impact toughness of the HAZ on that side, while avoiding the generation of a large amount of martensite which would lead to excessively high hardness and deterioration of impact toughness in the HAZ, the cooling rate of the HAZ on one side of the hot-rolled bainitic rail in the rail joint during the continuous cooling process needs to be controlled at 2.4~2.8℃ / s, and the final cooling temperature of this cooling stage needs to be controlled at 300~330℃. This ensures that in addition to generating bainite, a volume fraction of 20%~30% martensite can also be generated in the HAZ on that side, so as to balance the hardness and impact toughness of the HAZ on that side and achieve a good match of hardness on both sides of the weld within a certain range. For the heat-affected zone on one side of the eutectoid pearlitic rail joint, a relatively large cooling rate is adopted to generate a pearlitic structure with a fine lamellar spacing. At the same time, the final cooling temperature of this rapid cooling stage is controlled above 50°C above the martensitic transformation temperature of the rail steel to avoid the formation of brittle and hard martensite in the pearlitic structure, which would endanger the service safety of the welded heat-affected zone of the pearlitic rail.

[0064] When inspecting the joint after heat treatment, according to Figure 6 The sampling locations shown in the figure are: c is the weld center, and d is the sampling location for the metallographic specimens of the rail head tread. The metallographic structure of the joint was examined according to GB / T13298-2015 "Metallic Materials - Microstructural Testing Methods". The metallographic specimens were etched with a 3% nitric acid alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope. The rail joint was processed into longitudinal hardness specimens according to TB / T 1632.2-2014 "Rail Welding - Part 2: Flash Welding". According to GB / T 230.1-2009,... Figure 7 The schematic diagram shows the longitudinal Vickers HV hardness test performed on the joint at a position 5mm below the tread surface. The test points are symmetrically arranged to the left and right sides centered on the weld, with a spacing of 2mm. This is performed in accordance with TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. Figure 8The diagram illustrates the fabrication of impact test specimens for rail joints. The weld impact toughness is the average of 14 impact specimens across the entire cross-section of the rail joint, with the U-shaped notch machined at the weld. For the regions 10mm, 20mm, and 30mm below the rail head surface of the rail joint, the impact toughness of the heat-affected zone (HAZ) on one side of the heat-treated pearlitic rail at -15mm from the weld center is the impact toughness value obtained from impact tests on impact specimens with the U-shaped notch machined in the HAZ at -15mm from the weld center; the test result is the average of 4 impact specimens in the horizontal direction. The impact toughness of the HAZ on one side of the hot-rolled bainitic rail at +15mm from the weld center is the impact toughness value obtained from impact tests on impact specimens with the U-shaped notch machined in the HAZ at +15mm from the weld center; the test result is the average of 4 impact specimens in the horizontal direction.

[0065] The heat treatment method of the present invention can achieve the following effects:

[0066] (1) The depth of the hardened layer of the rail head is the area parallel to the surface of the rail head and downward with a hardness not less than 80% of the surface hardness of the rail head. The hardened layer depth of the heat-affected zone on the pearlite side is about 25~35mm, and the hardened layer depth of the heat-affected zone on the bainite side is about 5~15mm. The average hardness difference between the heat-affected zones on both sides of the rail joint weld within ±20mm from the weld center is controlled within ±25HV. The average impact energy of the full-section weld of the rail joint at room temperature is 41~48J.

[0067] (2) For the area 10mm below the rail head surface of the rail joint, the impact toughness at the weld center is 47~58J. The hardness of the heat-affected zone on one side of the heat-treated pearlitic rail at -15mm from the weld center is 295~337HV, and the impact toughness is 19~16J. The hardness of the heat-affected zone on one side of the hot-rolled bainitic rail at +15mm from the weld center is 281~322HV, and the impact toughness is 85~80J; For the area 20mm below the rail head surface of the rail joint, the impact toughness at the weld center is 40~45J. The hardness of the heat-affected zone on one side of the heat-treated pearlitic rail at -15mm from the weld center is 291~334HV, and the impact toughness is 17~15J. The hardness of the heat-affected zone (HAZ) on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 277–318 HV, and the impact toughness is 82–75 J. For the area 30 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 31–39 J. The hardness of the HAZ on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 285–327 HV, and the impact toughness is 14–12 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 263–282 HV, and the impact toughness is 70–65 J.

[0068] The present invention will be further illustrated by specific embodiments below, but the scope of protection of the present invention is not limited thereto.

[0069] Example 1

[0070] In this embodiment, the heat-treated eutectoid pearlitic rail base material comprises the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.15%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1170 MPa, a hardness of 310 HV, and an impact energy of 18 J. The hot-rolled bainitic rail base material comprises the following components by weight percentage: C: 0.15%, Si: 1.20%, Mn: 0.70%, Cr: 1.50%, Mo: 0.20%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1130 MPa, a hardness of 290 HV, and an impact energy of 130 J.

[0071] After upsetting and stub removal during the moving flash welding process, the welded joint of a 60kg / m dissimilar rail undergoes post-weld heat treatment. First, the rail joint, with a surface temperature of 800℃, is allowed to cool naturally in air until the rail head surface temperature drops to 200℃. Then, an induction heating coil is used to heat the entire welded joint area. The induction heating frequency for the pearlitic side is 2~4kHz, with a heating rate of 7℃ / s, and heating is stopped when the rail head surface temperature reaches 940℃. For the bainitic side of the rail head, the induction heating frequency is 6~9kHz, with a heating rate of 15℃ / s, and heating is stopped when the rail head surface temperature reaches 1000℃. After normalizing heating, cooling is immediately performed using compressed air (or a water mist mixture) as the cooling medium. The compressed air or water mist mixture injected from the left and right sides of the rail head tread air spray device has a gas pressure of 0.40 MPa and 0.12 MPa, respectively, and rapidly cools the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the weld joint at cooling rates of 4.0℃ / s and 2.4℃ / s, respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the weld joint during this stage are 420℃ and 330℃, respectively. The rail head tread air spray device is then removed, allowing the welded joint to cool in the air environment (i.e., natural cooling) to an ambient temperature of 20~30℃ at a cooling rate of 0.1~0.9℃ / s, thus obtaining the heat-treated rail welded joint of this embodiment.

[0072] The rail joint obtained in this embodiment has an average surface hardness of 285 HV on the heat-affected zone (HAZ) side of the rail joint on the eutectoid pearlitic side and 265 HV on the bainitic side within a 20 mm radius from the weld center. The average hardness difference between the HAZ sides of the weld is controlled within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 48 J, significantly higher than the ≥6.5 J specified in TB / T 1632.2-2014. For the area 10mm below the rail head surface of the rail joint, the impact toughness at the weld center is 58J; the hardness of the heat-affected zone on the side of the heat-treated pearlitic rail at -15mm from the weld center is 295HV, and the impact toughness is 19J; the hardness of the heat-affected zone on the side of the hot-rolled bainitic rail at +15mm from the weld center is 281HV, and the impact toughness is 85J. For the area 20mm below the rail head surface of the rail joint, the impact toughness at the weld center is 45J; the hardness of the heat-affected zone on the side of the heat-treated pearlitic rail at -15mm from the weld center is 291HV, and the impact toughness is 17J. The hardness of the heat-affected zone (HAZ) on the side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 277 HV, and the impact toughness is 82 J. For the area 30 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 39 J. The hardness of the HAZ on the side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 285 HV, and the impact toughness is 14 J. The hardness of the HAZ on the side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 263 HV, and the impact toughness is 70 J. Furthermore, under metallographic microscopy, no martensite is found in the HAZ on the side of the heat-treated eutectoid pearlitic rail joint, while the volume fraction of martensite in the HAZ on the side of the hot-rolled bainitic rail joint is 20%, with the martensite concentrated within 5 mm of the rail head surface. This invention helps to improve the "saddle-shaped" wear caused by low hardness in the welded area during rail welding joint service on the railway line. The joint exhibits good impact toughness, which helps ensure railway operation safety.

[0073] Example 2

[0074] In this embodiment, the heat-treated eutectoid pearlitic rail base material comprises the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.15%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1170 MPa, a hardness of 310 HV, and an impact energy of 18 J. The hot-rolled bainitic rail base material comprises the following components by weight percentage: C: 0.15%, Si: 1.20%, Mn: 0.70%, Cr: 1.50%, Mo: 0.20%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1130 MPa, a hardness of 290 HV, and an impact energy of 130 J.

[0075] After upsetting and stub removal during the moving flash welding process, the welded joint of a 60kg / m dissimilar rail undergoes post-weld heat treatment. First, the rail joint, with a surface temperature of 800℃, is allowed to cool naturally in air until the rail head surface temperature drops to 200℃. Then, an induction heating coil is used to heat the entire welded joint area. The induction heating frequency for the pearlitic side is 2~4kHz, with a heating rate of 7℃ / s, and heating is stopped when the rail head surface temperature reaches 990℃. For the bainitic side of the rail head, the induction heating frequency is 6~9kHz, with a heating rate of 15℃ / s, and heating is stopped when the rail head surface temperature reaches 1050℃. After normalizing heating, cooling is immediately performed using compressed air (or a water mist mixture) as the cooling medium. The compressed air or water mist mixture injected from the left and right sides of the rail head tread air spray device has a gas pressure of 0.70 MPa and 0.14 MPa, respectively, and rapidly cools the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the weld joint at cooling rates of 7.0℃ / s and 2.8℃ / s, respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the weld joint during this stage are 390℃ and 300℃, respectively. The rail head tread air spray device is then removed, allowing the welded joint to cool in the air environment (i.e., natural cooling) to an ambient temperature of 20~30℃ at a cooling rate of 0.1~0.9℃ / s, thus obtaining the heat-treated rail welded joint of this embodiment.

[0076] The rail joint obtained in this embodiment has an average surface hardness of 300 HV on the heat-affected zone (HAZ) side of the eutectoid pearlitic rail within a 20 mm radius from the weld center, and an average surface hardness of 281 HV on the HAZ side of the bainitic rail. The average hardness difference between the HAZ sides of the weld is controlled within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 44 J, significantly higher than the ≥6.5 J specified in TB / T 1632.2-2014. For the area 10 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 53 J, and the hardness of the HAZ on the heat-treated pearlitic rail side at -15 mm from the weld center is 312 HV, with an impact toughness of 17 J. The hardness of the heat-affected zone (HAZ) on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 294 HV, and the impact toughness is 83 J. For the area 20 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 43 J. For the heat-affected zone on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, the hardness is 310 HV, and the impact toughness is 16 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 292 HV, and the impact toughness is 78 J. For the area 30 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 36 J. For the heat-affected zone on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, the hardness is 307 HV, and the impact toughness is 13 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 272 HV, and the impact toughness is 67 J. Furthermore, under metallographic microscopy, the heat-affected zone (HAZ) on one side of the rail joint in the heat-treated eutectoid pearlitic rail joint showed no martensite, while the volume fraction of martensite in the HAZ on the other side of the rail joint in the hot-rolled bainitic rail joint was 30%, with the martensite concentrated on the rail head surface within 5 mm of the tread. This invention helps to improve the "saddle-shaped" wear of rail welded joints during railway service caused by the low hardness of the welded area. The joint exhibits good impact toughness, contributing to ensuring railway operation safety.

[0077] Example 3

[0078] In this embodiment, the heat-treated eutectoid pearlitic rail base material comprises the following components by weight percentage: C: 0.83%, Si: 0.90%, Mn: 1.20%, V: 0.45%, with the balance being Fe and unavoidable 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 bainitic rail base material comprises the following components by weight percentage: C: 0.19%, Si: 1.60%, Mn: 1.10%, Cr: 1.90%, Mo: 0.50%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1200 MPa, a hardness of 330 HV, and an impact energy of 100 J.

[0079] After upsetting and slugging during the moving flash welding process, the welded joint of a 75kg / m dissimilar rail undergoes post-weld heat treatment. First, the rail joint, with a surface temperature of 800℃, is allowed to cool naturally in air until the rail head surface temperature drops to 200℃. Then, an induction heating coil is used to heat the entire welded joint area. The induction heating frequency for the pearlite side is 2~4kHz, with a heating rate of 8℃ / s, and heating is stopped when the rail head surface temperature reaches 990℃. For the bainite side of the rail head, the induction heating frequency is 6~9kHz, with a heating rate of 18℃ / s, and heating is stopped when the rail head surface temperature reaches 1050℃. After normalizing heating, cooling is immediately performed using compressed air (or a water mist mixture) as the cooling medium. The compressed air or water mist mixture injected from the left and right sides of the rail head tread air spray device has a gas pressure of 0.70 MPa and 0.14 MPa, respectively, and rapidly cools the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the weld joint at cooling rates of 7.0℃ / s and 2.8℃ / s, respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the weld joint during this stage are 390℃ and 300℃, respectively. The rail head tread air spray device is then removed, allowing the welded joint to cool in the air environment (i.e., natural cooling) to an ambient temperature of 20~30℃ at a cooling rate of 0.1~0.9℃ / s, thus obtaining the heat-treated rail welded joint of this embodiment.

[0080] The rail joint obtained in this embodiment has an average surface hardness of 330 HV on the heat-affected zone (HAZ) side of the eutectoid pearlitic rail within a 20 mm radius from the weld center, and an average surface hardness of 305 HV on the HAZ side of the bainitic rail. The average hardness difference between the HAZ sides of the weld is controlled within ±25 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 41 J, significantly higher than the ≥6.5 J specified in TB / T 1632.2-2014. For the area 10 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 47 J. The hardness of the HAZ on the heat-treated pearlitic rail side at -15 mm from the weld center is 337 HV, and the impact toughness is 16 J. The hardness of the heat-affected zone (HAZ) on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 322 HV, and the impact toughness is 80 J. For the area 20 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 40 J. The hardness of the HAZ on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 334 HV, and the impact toughness is 15 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 318 HV, and the impact toughness is 75 J. For the area 30 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 31 J. The hardness of the HAZ on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 327 HV, and the impact toughness is 12 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 282 HV, and the impact toughness is 65 J. Furthermore, under metallographic microscopy, the heat-affected zone (HAZ) on one side of the rail joint in the heat-treated eutectoid pearlitic rail joint showed no martensite, while the volume fraction of martensite in the HAZ on the other side of the rail joint in the hot-rolled bainitic rail joint was 30%, with the martensite concentrated on the rail head surface within 5 mm of the tread. This invention helps to improve the "saddle-shaped" wear of rail welded joints during railway service caused by the low hardness of the welded area. The joint exhibits good impact toughness, contributing to ensuring railway operation safety.

[0081] Example 4

[0082] In this embodiment, the heat-treated eutectoid pearlitic rail base material comprises the following components by weight percentage: C: 0.80%, Si: 0.68%, Mn: 1.0%, V: 0.30%, with the balance being Fe and unavoidable 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 bainitic rail base material comprises the following components by weight percentage: C: 0.17%, Si: 1.40%, Mn: 1.0%, Cr: 1.70%, Mo: 0.40%, with the balance being Fe and unavoidable impurities. The rail base material has a tensile strength of 1160 MPa, a hardness of 310 HV, and an impact energy of 115 J.

[0083] After upsetting and stub removal during the moving flash welding process, the welded joint of a 68kg / m dissimilar rail undergoes post-weld heat treatment. First, the rail joint, with a surface temperature of 800℃, is allowed to cool naturally in air until the rail head surface temperature drops to 200℃. Then, an induction heating coil is used to heat the entire welded joint area. The induction heating frequency for the pearlite side is 2~4kHz, with a heating rate of 10℃ / s, and heating is stopped when the rail head surface temperature reaches 960℃. For the bainite side of the rail head, the induction heating frequency is 6~9kHz, with a heating rate of 20℃ / s, and heating is stopped when the rail head surface temperature reaches 1020℃. After normalizing heating, cooling is immediately performed using compressed air (or a water mist mixture) as the cooling medium. The compressed air or water mist mixture injected from the left and right sides of the rail head tread air spray device has a gas pressure of 0.60 MPa and 0.13 MPa, respectively, and rapidly cools the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the weld joint at cooling rates of 6.0℃ / s and 2.6℃ / s, respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the weld joint during this stage are 405℃ and 315℃, respectively. The rail head tread air spray device is then removed, allowing the welded joint to cool in the air (i.e., natural cooling) to an ambient temperature of 20~30℃ at a cooling rate of 0.1~0.9℃ / s, thus obtaining the heat-treated rail welded joint of this embodiment.

[0084] The rail joint obtained in this embodiment has an average surface hardness of 310 HV on the heat-affected zone (HAZ) side of the eutectoid pearlitic rail within a 20 mm radius from the weld center, and an average surface hardness of 295 HV on the HAZ side of the bainitic rail. The average hardness difference between the HAZ sides of the weld is controlled within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 46 J, significantly higher than the ≥6.5 J specified in TB / T 1632.2-2014. For the area 10 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 55 J. The hardness of the HAZ on the heat-treated pearlitic rail side at -15 mm from the weld center is 318 HV, and the impact toughness is 18 J. The hardness of the heat-affected zone (HAZ) on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 300 HV, and the impact toughness is 83 J. For the area 20 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 44 J. The hardness of the HAZ on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 313 HV, and the impact toughness is 16 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 295 HV, and the impact toughness is 79 J. For the area 30 mm below the rail head surface of the rail joint, the impact toughness at the weld center is 35 J. The hardness of the HAZ on one side of the heat-treated pearlitic rail, located 15 mm from the weld center, is 302 HV, and the impact toughness is 13 J. The hardness of the HAZ on one side of the hot-rolled bainitic rail, located 15 mm from the weld center, is 288 HV, and the impact toughness is 68 J. Furthermore, under metallographic microscopy, the heat-affected zone (HAZ) on one side of the rail joint in the heat-treated eutectoid pearlitic rail joint showed no martensite, while the volume fraction of martensite in the HAZ on the other side of the rail joint in the hot-rolled bainitic rail joint was 26%, with the martensite concentrated on the rail head surface within 5 mm of the tread. This invention helps to improve the "saddle-shaped" wear of rail welded joints during railway service caused by the low hardness of the welded area. The joint exhibits good impact toughness, contributing to ensuring railway operation safety.

[0085] Comparative Example 1

[0086] The selection of rail materials, mechanical properties of the rail base material, and process conditions involved in welding and post-weld normalizing heat treatment cooling are the same in Comparative Example 1 and Example 1. The difference is that the heating temperature of the rail head part of the heat-affected zone on the eutectoid pearlitic rail side of the rail joint in this comparative example is 900°C, and the heating temperature of the rail head part of the heat-affected zone on the bainitic rail side of the rail joint is 950°C.

[0087] The rail welded joint obtained in this comparative example has low mechanical properties due to the relatively low heating temperature during normalizing. This results in insufficient supercooling during the cooling process after normalizing and insufficient driving force for the pearlitic and bainitic phase transformations during subsequent cooling. In contrast, the average hardness of the heat-affected zone (HAZ) on the eutectoid pearlitic side of the rail joint obtained in this comparative example is 275 HV, while the average hardness of the bainitic side is 245 HV. The average hardness difference between the HAZ on both sides of the weld is controlled to be outside ±25 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 37 J. The hardness and impact energy of the rail welded joint obtained in this comparative example are lower than the beneficial effects obtained by using this invention.

[0088] Comparative Example 2

[0089] The comparative example is consistent with Example 1 in terms of rail material selection, mechanical properties of the rail base material, welding cooling process, heating temperature of post-weld normalizing heat treatment, and cooling rate of the post-weld normalizing cooling stage. The difference is that in this comparative example, the final cooling temperature of the heat-affected zone on the hot-rolled bainitic rail side of the rail joint during the secondary cooling process after normalizing heating is 290°C, which is lower than the final cooling temperature control range of the heat-affected zone on the hot-rolled bainitic rail side of the rail joint during the secondary cooling process after normalizing heating of the rail joint in this invention.

[0090] The rail welded joint obtained in this comparative example exhibits a martensite structure with a volume fraction of approximately 40% in the heat-affected zone (HAZ) on the hot-rolled bainitic rail side. This is because the final cooling temperature of the HAZ on the hot-rolled bainitic rail side of the rail joint during the secondary cooling process after normalizing is within the Ms temperature range (the starting temperature for martensite formation) of the hot-rolled bainitic rail steel involved in this invention. Within a range of ±20 mm from the weld center, the average hardness of the HAZ on the eutectoid pearlitic rail side of the rail joint is 285 HV, and the average hardness of the HAZ on the bainitic rail side is 278 HV. The average hardness difference between the two HAZ sides is controlled within ±10 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 20 J. The presence of a significant amount of martensite in the HAZ of the hot-rolled bainitic rail in this comparative example results in excessively high hardness in that side of the HAZ, while the average weld impact energy is low, which is detrimental to railway operation safety.

[0091] Comparative Example 3

[0092] The selection of rail material, mechanical properties of the rail base material, welding cooling process, heating temperature of post-weld normalizing heat treatment, and cooling rate of post-weld normalizing cooling stage are all the same in this comparative example and Example 1. The difference is that the final cooling temperature of the heat-affected zone on both sides of the weld joint in this comparative example is 270°C during the secondary cooling process after normalizing heating.

[0093] The rail joint obtained in this comparative example has a significant amount of martensite structure formed in both heat-affected zones of the weld after the joint is heated by normalizing after welding. This is because the final cooling temperature of the heat-affected zones on both sides of the weld is within the Ms temperature range (the starting temperature of martensite formation) of the heat-treated eutectoid pearlitic rail steel involved in this invention, while the final cooling temperature is below the Ms temperature range (the starting temperature of martensite formation) of the hot-rolled bainitic rail steel involved in this invention. Furthermore, the cooling rate of the secondary cooling is higher than the critical cooling rate of martensite transformation of the two types of rail steel mentioned above.

[0094] The rail welded joint obtained in this comparative example exhibits an average hardness of 297 HV in the heat-affected zone (HAZ) on the rail side of the eutectoid pearlitic steel joint within a 20 mm radius from the weld center, and an average hardness of 277 HV in the HAZ on the rail side of the bainitic steel joint. The average hardness difference between the HAZ on both sides of the weld is controlled within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 12 J. Furthermore, under a metallographic microscope, the volume fraction of martensite in the HAZ on the rail side of the heat-treated eutectoid pearlitic steel joint is 10%, and the volume fraction of martensite in the HAZ on the rail side of the hot-rolled bainitic steel joint is 32%. Because the process method of this invention was not adopted, a significant amount of martensite was formed in the HAZ on both sides of the weld of the rail joint obtained in this comparative example. Simultaneously, the average weld impact energy is lower than the beneficial effect obtained by using the process method of this invention, which is detrimental to railway operation safety.

[0095] Comparative Example 4

[0096] The comparative example is consistent with Example 3 in terms of rail material selection, mechanical properties of the rail base material, rail welding process conditions, heating temperature during post-weld normalizing, cooling method and final cooling temperature during secondary cooling, cooling method and steps in the final cooling stage, and cooling rate. The difference is that in this comparative example, the cooling rate of the heat-affected zone on the eutectoid pearlite side of the rail joint during the secondary cooling process after normalizing is 3.0℃ / s, which is lower than the cooling rate control range of 4.0~7.0℃ / s for the heat-affected zone on the eutectoid pearlite side of the rail during the second cooling process after post-weld normalizing in this invention. Thus, the rail welded joint under this comparative example is obtained.

[0097] The rail joint obtained in this comparative example has a lower cooling rate of 3.0℃ / s in the heat-affected zone on the rail side of the eutectoid pearlite steel during the second cooling process after normalizing heating. This rate is lower than the preferred cooling rate range of 4.0~7.0℃ / s in this invention. Consequently, the pearlite structure refinement effect in the heat-affected zone on the rail side of the eutectoid pearlite steel during the second cooling process after normalizing heating is lower than the beneficial effect obtained by the process method of this invention. As a result, the mechanical properties of the heat-affected zone on the rail side of the eutectoid pearlite steel are lower than the beneficial effect obtained by the process method of this invention.

[0098] Within a 20mm radius from the weld center, the average hardness of the heat-affected zone (HAZ) on the rail side of the eutectoid pearlitic steel joint was 308 HV, and the average hardness of the HAZ on the rail side of the bainitic steel joint was 305 HV. The average hardness difference between the HAZs on both sides of the weld was controlled within ±10 HV. The average impact energy of the full-section weld of the rail joint at room temperature was 20 J. Furthermore, under a metallographic microscope, no martensite was found in the HAZ on the rail side of the heat-treated eutectoid pearlitic steel joint, while the volume fraction of martensite in the HAZ on the rail side of the hot-rolled bainitic steel joint was 30%. The hardness of the HAZ on the rail side and the full-section weld impact energy obtained in this comparative example are lower than the beneficial effects obtained by using this invention.

[0099] Comparative Example 5

[0100] The selection of rail materials, mechanical properties of the rail base material, and flash welding conditions are the same in this comparative example as in Example 4. The difference is that there is no subsequent normalizing heating and cooling process after the rail welding is completed in this comparative example. That is, the rail welded joint obtained by flash welding is directly cooled naturally in the air to an ambient temperature of 30°C, thus obtaining the rail welded joint under this comparative example.

[0101] The rail welded joint obtained in this comparative example exhibits an average hardness of 280 HV in the heat-affected zone (HAZ) on the eutectoid pearlitic rail side within a ±20 mm radius from the weld center, and an average hardness of 263 HV in the HAZ on the bainitic rail side. The average hardness difference between the HAZs on both sides of the weld is within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 15 J. Furthermore, metallographic microscopy reveals no martensite in either the heat-treated eutectoid pearlitic rail side or the hot-rolled bainitic rail side of the rail joint. The low hardness of the HAZs on both sides of the weld in this comparative example, along with the significantly lower average weld impact energy, is detrimental to railway operation safety.

[0102] Comparative Example 6

[0103] The selection of rail material, mechanical properties of the rail base material, welding and post-weld normalizing heat treatment processes in this comparative example are the same as those in Example 4. The difference is that in this comparative example, the cooling rate of the heat-affected zone on the bainitic rail side of the rail joint during the secondary cooling process is 1.6℃ / s, and the final cooling temperature at this stage is 350℃, thus obtaining the rail welded joint under this comparative example.

[0104] The rail joint obtained in this comparative example exhibits a lower cooling rate in the heat-affected zone (HAZ) on the hot-rolled bainitic rail side during secondary cooling, resulting in unsatisfactory bainite refinement and insufficient improvement in mechanical properties. In the welded rail joint obtained in this comparative example, the average hardness of the HAZ on the eutectoid pearlitic rail side is 310 HV within ±20 mm of the weld center, while the average hardness on the hot-rolled bainitic rail side is 280 HV. The average hardness difference between the HAZ on both sides of the weld is not within ±20 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 32 J. Furthermore, metallographic microscopy reveals no martensite in either the heat-treated eutectoid pearlitic rail side or the hot-rolled bainitic rail side of the rail joint. The lower hardness of the HAZ on the hot-rolled bainitic rail side is due to the absence of the process method described in this invention.

[0105] Comparative Example 7

[0106] The selection of rail material, mechanical properties of the rail base material, and process conditions involved in welding and post-weld normalizing heat treatment are the same as those in Example 4. The difference is that in this comparative example, the heating frequency on both sides of the weld during normalizing is 2~4kHz, and the heating temperature is 960℃, thus obtaining the rail welded joint under this comparative example.

[0107] The rail joint obtained in this comparative example exhibits a low heating frequency in the heat-affected zone (HAZ) of the hot-rolled bainitic rail during normalizing, resulting in excessive heating of the core. Slow heat dissipation from the core during rapid cooling leads to coarsening of the core microstructure and unsatisfactory bainite refinement, resulting in insufficient improvement in mechanical properties. In the rail welded joint obtained in this comparative example, the average hardness of the HAZ on the eutectoid pearlitic rail side within a ±20mm radius from the weld center is 310 HV, while the average hardness of the HAZ on the hot-rolled bainitic rail side is 275 HV. The average impact energy of the full-section weld of the rail joint at room temperature is 35 J. Because the process method of this invention was not adopted, the hardness of the HAZ on the hot-rolled bainitic rail side of the rail joint obtained in this comparative example is lower, and the average weld impact energy is also significantly lower, which is detrimental to railway operation safety.

[0108] By comparing the embodiments and comparative examples in this invention, it can be seen that:

[0109] This invention achieves post-weld normalizing heating at different temperatures and controlled cooling at different intensities in the heat-affected zones (HAZs) on both sides of the weld joint of dissimilar heat-treated eutectoid pearlitic rails and hot-rolled bainitic rails. This results in an average hardness difference of ±25 HV between the HZs on both sides of the rail joint within a ±20 mm radius from the weld center, thus ensuring the wear resistance of the HZs on both sides of the rail joint weld joint. It also achieves the following: no martensite in the HZ of the heat-treated eutectoid pearlitic rail side of the dissimilar rail joint, and a martensite volume fraction of 20%~30% in the HZ of the hot-rolled bainitic rail side of the rail joint. The average impact energy of the full-section weld of the rail joint at room temperature is 41~48 J, far exceeding the ≥6.5 J specified in TB / T 1632.2-2014.

[0110] This invention helps to improve the "saddle-shaped" wear of rail welded joints caused by low hardness in the welded area during railway service. The joint exhibits good impact toughness, contributing to safe railway operation. This invention is particularly applicable to the post-weld processing of dissimilar rails. The technical advantages of this invention are significant, and its market prospects are very broad.

[0111] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails, characterized in that, Includes the following steps: Step S1: Cool the welded joint formed by welding the heat-treated eutectoid pearlitic steel rail and the hot-rolled bainitic steel rail once. Step S2: The welded joint, which has undergone the first cooling, is heated to the normalizing temperature using induction heating. During heating, the center of the weld is used as the dividing line. The heating frequency of the induction heating coil on the pearlite side is 2~4kHz, and the heating frequency of the induction heating coil on the bainite side is 6~9kHz, so as to austenitize the welded joint. Step S3: After the normalizing heating is completed, the welded joint is subjected to secondary cooling. The heat-affected zone on the pearlite side is cooled to the first final cooling temperature at a first cooling rate, and the heat-affected zone on the bainite side is cooled to the second final cooling temperature at a second cooling rate. The first cooling rate is greater than the second cooling rate, and the first final cooling temperature is higher than the second final cooling temperature. Step S4: After the secondary cooling is completed, allow the welded joint to cool naturally to ambient temperature; In step S2, the heating temperature on the pearlite side is 940~990℃, and the heating temperature on the bainite side is 1000~1050℃. In step S3, the first cooling rate is 4.0~7.0℃ / s, the second cooling rate is 2.4~2.8℃ / s, the first final cooling temperature of the secondary cooling of the heat-affected zone on the pearlite side is 390~420℃, and the second final cooling temperature of the secondary cooling of the heat-affected zone on the bainite side is 300~330℃.

2. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 1, characterized in that, The secondary cooling is achieved by spraying compressed air or a mixture of water mist.

3. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 2, characterized in that, The jet pressure for secondary cooling of the heat-affected zone on the pearlite side is 0.40~0.70MPa, and the jet pressure for secondary cooling of the heat-affected zone on the bainite side is 0.12~0.14MPa.

4. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 1, characterized in that, The first cooling is a natural cooling process carried out in an air environment at 20~30°C.

5. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 4, characterized in that, The final cooling temperature of the first cooling is 100~200℃.

6. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 1, characterized in that, In step S2, the induction heating rate on the pearlite side is 7~10℃ / s, and the induction heating rate on the bainite side of the rail head is 15~20℃ / s.

7. The method for improving the hardness and impact toughness of welded joints of pearlitic and bainitic steel rails according to claim 1, characterized in that, The base material of the heat-treated eutectoid pearlitic steel rail comprises the following chemical composition by weight percentage: C: 0.77%~0.83%, Si: 0.45%~0.90%, Mn: 0.80%~1.20%, V: 0.15%~0.45%, with the balance being Fe and unavoidable impurities. The room temperature tensile strength of the base material is 1170~1250 MPa, the hardness is 310~350 HV, and the impact energy is 18~10 J. The hot-rolled bainitic... The base material of the rail comprises the following chemical components by weight percentage: C: 0.15%~0.19%, Si: 1.20%~1.60%, Mn: 0.70%~1.10%, Cr: 1.50%~1.90%, Mo: 0.20%~0.50%, with the balance being Fe and unavoidable impurities. The room temperature tensile strength of the base material is 1130~1200MPa, the hardness is 290~330HV, and the impact energy is 130~100J.

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