Method for improving hardness and impact toughness of pearlite and bainite steel rail welded joint

Abstract

The invention belongs to the field of steel rail manufacturing, and discloses a method for improving the hardness and impact toughness of a pearlite and bainite steel rail welding joint. The method comprises the following steps: carrying out primary cooling on a welding joint formed by welding a pearlite steel rail and a bainite steel rail; the welded joint subjected to primary cooling is heated to the normalizing temperature through 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 the welded joint is austenitized; the welded joint is subjected to secondary cooling, a heat affected zone on one side of pearlite is cooled at a first cooling rate, and a heat affected zone on one side of bainite is cooled at a second cooling rate; and the welding joint is naturally cooled to the environment temperature. According to the invention, saddle-shaped abrasion of the steel rail welding joint caused by low hardness of a welding area in a line service process can be improved, and the impact toughness of the joint is good.

Description

Technical Field

[0001] The invention relates to the technical field of rail manufacturing, and in particular to a method for improving the hardness and impact toughness of a welded joint of a pearlite and bainite rail. Background Art

[0002] At present, high-speed, quasi-high-speed and mixed passenger and freight railways at home and abroad mostly use eutectoid pearlite rails. The carbon content of such rails is usually in the range of 0.65%~0.82% by weight, and the metallographic structure is pearlite, with good strength and toughness matching, and moderate comprehensive mechanical performance indicators. The rapid development of railways has put forward higher requirements on the service performance of rails. Since the mechanical properties and welding properties of traditional pearlite rails have almost reached their limits, in this case, bainite rails with higher strength grades and good wear resistance and contact fatigue resistance have come into being. The carbon content of such rails is usually in the range of 0.10%~0.30% by weight, and the metallographic structure is a complex phase structure composed of bainite + a small amount of martensite (or Mahau Island) and residual austenite.

[0003] As bainitic rails are gradually put into use, the mixed use of bainitic rails and existing pearlite rails on railway lines has undoubtedly brought huge challenges to the welding of dissimilar rails. During the welding process, after the rails are subjected to the welding heat cycle, the hardened layer in the welding area disappears and a wide low-hardness zone is formed on both sides of the weld. The hardness softening of one side of the bainitic rail is more serious after welding. The current heat treatment method for dissimilar rail joints is usually to heat both sides of the joint to a set temperature using the same heating method, and then use controlled cooling methods such as air cooling or air jet treatment to cool the heat-affected zones on both sides of the weld. Although this post-weld heat treatment method can improve the hardness of both sides of the weld joint to a certain extent, this treatment method also causes a significant decrease in the toughness of the heat-affected zone on the bainite side.

[0004] For the welded joints of dissimilar rails of eutectoid pearlite and bainite rails that have undergone conventional normalizing heat treatment, the hardness and impact toughness of the welded heat affected zone on the bainite rail side are still generally low, which is not conducive to the safety of railway operation. Therefore, a method for comprehensively improving the hardness and impact toughness of the welded joints of pearlite and bainite rails is urgently needed in this field. Summary of the invention

[0005] In view of the above problems, the present invention provides a method for improving the hardness and impact toughness of pearlite and bainite rail welding joints, so as to improve the tread hardness of dissimilar rails reduced by welding, and at the same time improve the impact toughness of the rail joints, so as to ensure the service performance of the dissimilar rail welding joints and the safety of railway operation.

[0006] According to one aspect of the present invention, there is provided a method for improving the hardness and impact toughness of a pearlite-bainite rail weld joint, comprising the following steps: Step S1: Cooling the welded joint formed by welding the heat-treated eutectoid pearlite rail and the hot-rolled bainite rail; Step S2: using electric induction heating to heat the weld joint that has been cooled once to a normalizing temperature, with the center of the weld as the dividing line, the heating frequency of the induction heating coil on the pearlite side being 2-4 kHz, and the heating frequency of the induction heating coil on the bainite side being 6-9 kHz, so as to austenitize the weld joint; Step S3: After normalizing heating is completed, the weld joint is subjected to secondary cooling, wherein the heat affected zone on the pearlite side is cooled to a first final cooling temperature using a first cooling rate, and the heat affected zone on the bainite side is cooled to a second final cooling temperature using a second cooling rate, wherein 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, the weld joint is naturally cooled to ambient temperature.

[0007] According to one embodiment of the present invention, in step S2, the heating temperature of the pearlite side is 940-990°C, and the heating temperature of the bainite side is 1000-1050°C.

[0008] According to one embodiment of the present invention, in step S3, the first cooling rate is 4.0-7.0°C / s, and the second cooling rate is 2.4-2.8°C / s.

[0009] According to one 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.

[0010] According to an embodiment of the present invention, the secondary cooling uses compressed air or water mist mixed gas for spray cooling.

[0011] According to one embodiment of the present invention, the air jet pressure for secondary cooling of the heat affected zone on the pearlite side is 0.40-0.70 MPa, and the air jet pressure for secondary cooling of the heat affected zone on the bainite side is 0.12-0.14 MPa.

[0012] According to an embodiment of the present invention, the primary cooling is natural cooling performed in an air environment at 20-30°C.

[0013] According to one embodiment of the present invention, the final cooling temperature of the primary cooling is 100-200°C.

[0014] According to one embodiment of the present invention, in step S2, the induction heating rate of the pearlite side is 7-10°C / s, and the induction heating rate of the rail head part of the bainite side is 15-20°C / s.

[0015] According to one embodiment of the present invention, the base material of the heat-treated eutectoid pearlite rail comprises the following chemical components by weight percentage: C: 0.77%~0.83%, Si: 0.45%~0.90%, Mn: 0.80%~1.20%, V: 0.15%~0.45%, the balance is Fe and unavoidable impurities, the room temperature tensile strength of the base material is 1170~1250MPa, the hardness is 310~350HV, and the impact energy is 18~10J; The base material of the hot-rolled bainite rail includes 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%, and the balance is Fe and inevitable impurities. The room temperature tensile strength of the base material is 1130-1200 MPa, the hardness is 290-330 HV, and the impact energy is 130-100 J.

[0016] Due to the adoption of the above technical solution, the method of the present invention has at least one of the following beneficial effects: (1) The method for improving the hardness and impact toughness of the pearlite and bainite rail weld joint provided by the present invention is based on the skin effect of induction heating. By adjusting the induction heating frequency, different heating depths can be obtained. After the heating is completed, the hardened layer of different depths can be obtained by combining rapid cooling. 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 pearlite rail) with the center of the weld 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 bainite rail) is 2~4kHz. The thermal frequency range is 6~9kHz, and the heating depth / hardened layer depth corresponds to 5~15mm. By heating and cooling the heat-affected zones on both sides of the weld of the dissimilar rail joint differently, the average hardness difference of the heat-affected zones on both sides of the rail joint weld within the area of ​​±20mm from the weld center is controlled within ±25HV, thereby ensuring the wear resistance of the heat-affected zones on both sides of the rail joint weld, and also making the heat-affected zone on one side of the bainite rail obtain a higher surface hardness while the core of the heat-affected zone still retains good impact toughness, so that the mechanical properties of the heat-affected zone on one side of the bainite rail are comprehensively improved; (2) The method of the present invention can achieve that there is no martensite in the heat-affected zone of the heat-treated eutectoid pearlite rail on one side of the dissimilar rail joint, and the volume fraction of the martensite structure in the heat-affected zone of the hot-rolled bainite rail on one side of the rail joint is 20% to 30%, and the martensite is mostly distributed in the shallow area, thereby increasing the surface hardness of the bainite side of the joint while maintaining the toughness of the core; (3) The average impact energy of the full-section weld of the rail joint at room temperature is 41-48 J, which is much higher than ≥6.5 J specified in TB / T 1632.2-2014. The present invention helps to improve the "saddle-type" wear of the rail welded joint caused by the low hardness of the welding area during the line service process. The joint has good impact toughness, which helps to ensure the safety of railway operation. It is particularly suitable for the treatment process after the welding of dissimilar rails. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A flow chart of a method for improving the hardness and impact toughness of a pearlite and bainite rail weld joint according to an embodiment of the present invention; Figure 2 A schematic diagram of the arrangement structure of the induction coils at the rail head of the rail joint during the induction heating process provided by an embodiment of the present invention; Figure 3 A schematic structural diagram of a spray panel of a rail head tread air spray device provided by an embodiment of the present invention; Figure 4 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 A schematic side structural diagram of a rail head tread air jet device provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of the sampling position of the metallographic specimen of the rail head tread of the rail welding joint; Figure 7 This is a schematic diagram of the longitudinal section hardness test position of the rail head of the rail welding joint, in which the longitudinal section hardness test is carried out at a position 3~5mm below the tread of the rail head of the rail joint; Figure 8 This is a sampling position diagram (unit: mm) for testing the impact performance of rail welded joints obtained according to the methods in the embodiments and comparative examples of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0019] like Figure 1As shown, the method for improving the hardness and impact toughness of the pearlite and bainite rail welding joint provided by the present invention generally comprises the following steps: Step S1: cooling the welded joint formed by welding the heat-treated eutectoid pearlite rail and the hot-rolled bainite rail; Step S2: using electric induction heating to heat the weld joint that has been cooled once to the normalizing temperature, with the center of the weld as the dividing line during heating, the heating frequency of the induction heating coil on the pearlite side being 2-4 kHz, and the heating frequency of the induction heating coil on the bainite side being 6-9 kHz, so as to austenitize the weld joint; Step S3: After normalizing heating is completed, the weld joint is subjected to secondary cooling, wherein the heat affected zone on the pearlite side is cooled to a first final cooling temperature using a first cooling rate, and the heat affected zone on the bainite side is cooled to a second final cooling temperature using 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, the welded joint is naturally cooled to ambient temperature.

[0020] The method for improving the hardness and impact toughness of the pearlite and bainite rail welding joint 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 the heating is completed, the hardened layer with different depths can be obtained by combining rapid cooling. The heating frequency range of the left coil of the induction heating coil with the weld center as the dividing line (corresponding to the heat-affected zone on one side of the heat-treated eutectoid pearlite rail) is 2-4 kHz, and the heating depth / hardened layer depth corresponds to 25-35 mm. The heating frequency range of the right coil (corresponding to the heat-affected zone on one side of the hot-rolled bainite rail) is 2-4 kHz. The frequency range is 6~9kHz, and the heating depth / hardened layer depth corresponds to 5~15mm. By heating and cooling the heat-affected zones on both sides of the weld of the dissimilar rail joint differently, the average hardness difference of the heat-affected zones on both sides of the rail joint weld in the area of ​​±20mm from the weld center is controlled within ±25HV, thereby ensuring the wear resistance of the heat-affected zones on both sides of the rail joint weld, and making the heat-affected zone on one side of the bainite rail obtain a higher surface hardness while the core of the heat-affected zone still retains good impact toughness, so that the mechanical properties of the heat-affected zone on one side of the bainite rail are comprehensively improved.

[0021] 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 pearlite rail of the dissimilar rail joint, and the volume fraction of martensite in the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint is 20% to 30%. The average impact energy of the full-section weld of the rail joint at room temperature is 41 to 48 J, which is much higher than the ≥6.5 J specified in TB / T 1632.2-2014. The present invention helps to improve the "saddle-type" wear of the rail welded joint caused by the low hardness of the welding area during the line service process. The joint has good impact toughness, which helps to ensure the safety of railway operation, and is particularly suitable for the treatment process after the welding of dissimilar rails.

[0022] The following is an exemplary description of the various operating steps of the method for improving the hardness and impact toughness of the pearlite and bainite rail weld joint provided by the present invention.

[0023] In step S1, a weld joint formed by welding a heat-treated eutectoid pearlite rail and a hot-rolled bainite rail is cooled once.

[0024] In some embodiments, the base material of the heat-treated eutectoid pearlite rail includes the following chemical compositions by weight percentage: C: 0.77%-0.83%, Si: 0.45%-0.90%, Mn: 0.8%-1.2%, V: 0.15%-0.45%, the balance being Fe and unavoidable impurities, the base material having 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 1900-type bainitic rail includes the following chemical components 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%, and the balance is Fe and inevitable 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.

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

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

[0027] After the first cooling is completed, in step S2, the weld joint is heated to the normalizing temperature by electric 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 that the weld joint is austenitized. The principle of electric induction heating of rail welded joints is to wrap two inductors on the outer contour of the joint. During normalizing, the two inductors are buckled and an alternating current is passed. The two symmetrical inductors form eddy currents under the action of the alternating current, and the weld and the heat-affected zones on both sides are heated by eddy currents and "skin effect" (a physical phenomenon in which an alternating current is passed through a conductor, the current density of the conductor cross section increases from the center to the surface, and 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, the depth of the hardened layer is shallow, which reduces the probability of martensite appearing in the core and improves the toughness of the core.

[0028] In some embodiments, the rail head of the heat-affected zone on the bainite rail side of the dissimilar rail joint is rapidly heated at an average heating rate of 15 to 20°C / s, and the rail head of the heat-affected zone on the eutectoid pearlite rail side of the joint is conventionally heated at an average heating rate of 7 to 10°C / s. Considering the difference in heating rates of the heat-affected zones on both sides of the weld, the rail head of the heat-affected zone on the eutectoid pearlite rail side is preferentially conventionally heated, while the time for rapid heating of the rail head of the heat-affected zone on the bainite rail side is relatively delayed. When the heating stops at the same time, the temperature of the rail head of the heat-affected zone on the bainite rail side of the joint is about 40 to 70°C higher than that of the rail head of the heat-affected zone on the eutectoid pearlite rail side. In addition, considering that the rail head plays a role in wear resistance and impact resistance, the hardness and impact toughness requirements of the rail waist and rail bottom are not as high as those of the rail head. Therefore, for the rail waist and rail bottom of the two types of rail joints, conventional heating is adopted at an average heating rate of 7 to 10°C / s.

[0029] Figure 2 A schematic diagram of the arrangement structure of the induction coils at the rail head of the rail joint during the induction heating process provided in an embodiment of the present invention. As shown in the figure, the heat-affected zones on the left and right sides of the weld can be coated with two independent sets of induction heating coils, each set of induction heating coils is wrapped around the outer contour of the corresponding heat-affected zone, and each set of induction heating coils includes two sub-coils (i.e., coils a1, b1 and coils a2, b2). The currents of the two sets of induction heating coils can be changed 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 induction heating coil on the bainite side is 6~9kHz. In order to avoid the heat treatment area staying at high temperature for too long, thereby causing the coarsening of the austenite grains, the heating termination time on the left and right sides of the weld can be synchronized by setting a suitable computer program. For example, compared with the heating on the pearlite side, the heating frequency on the bainite side is higher, and the heating speed on the bainite side is slower when the number of coil turns is the same. The heating speed on the bainite side can be increased by increasing the number of turns of the heating coil. Figure 2 As shown, a group of heating coils can be set on the pearlite side, and three groups of heating coils can be set on the rail head part on the bainite side, so as to improve the heating speed of the rail head part of the bainite heat affected zone. Optionally, the start heating time of both can also be adjusted, and finally both sides of the weld can reach their respective heating target temperatures at the same time.

[0030] In some embodiments, the heating temperature of the rail head of the heat affected zone on the eutectoid pearlite rail side of the rail joint is controlled to be 940~990℃, and the heating temperature of the rail head of the heat affected zone on the bainite rail side of the rail joint is controlled to be 1000~1050℃. It should also be noted that when the normalizing temperature is too high (such as higher than 1100℃), the austenite grains in the heated area (especially the coarse-grained heat affected zone in the weld and its vicinity) will be too large, thereby affecting the impact toughness of the subsequent joints. When the normalizing heating temperature is low, such as heating the surface temperature of the weld joint to 900℃ and below, due to the low driving force of the pearlite / bainite phase transformation during the cooling process, the microstructure refinement effect during the cooling process will be unsatisfactory, which is manifested as the improvement of the hardness and impact toughness of the rail weld heat affected zone after heat treatment is not obvious. When the normalizing heating temperature is lower than 880℃, the austenitization process of the heat-affected zone of the bainitic rail welded joint is incomplete, which will lead to insufficient phase transformation during the subsequent cooling process, and thus the improvement of the hardness and impact toughness of the heat-affected zone of the rail welded joint after heat treatment is not ideal. Therefore, the normalizing temperature of the heat-affected zone on the bainitic rail side of the rail joint is set higher.

[0031] In step S3, secondary cooling is performed immediately after normalizing heating is completed, wherein the heat affected zone on the pearlite side is cooled to a first final cooling temperature using a first cooling rate, and the heat affected zone on the bainite side is cooled to a second final cooling temperature using 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.

[0032] 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 air jet device can be used to quickly cool the heat affected zones on both sides of the rail joint weld using compressed air or water mist mixture as a cooling medium. The cooling intensity / cooling rate of the heat affected zone on the eutectoid pearlite rail side of the rail joint corresponding to the left side of the device is higher than the cooling intensity / cooling rate of the heat affected zone on the bainite rail side of the rail joint corresponding to the right side of the device.

[0033] Figure 3 A schematic diagram of the structure of the spray panel of the rail head tread air spray device provided by an embodiment of the present invention, Figure 4 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 A side structural schematic diagram of a rail head tread air jet device provided in an embodiment of the present invention.

[0034] As shown in the figure, the two cooling assemblies are separated by a central partition plate located in the middle of the device. The cooling assembly on the left is arranged corresponding to the heat-affected zone of the heat-treated eutectoid pearlite rail according to the present invention, and the cooling assembly on the right is arranged corresponding to the heat-affected zone of the hot-rolled bainite rail according to the present invention. Each cooling assembly includes a spray panel located above the tread of the rail head of the dissimilar rail welded joint (see Figure 3 The left side of the center partition corresponds to the bottom injection hole 3 and the right side corresponds to the bottom injection hole 4), and the injection pipes connected to the injection panel and facing the two sides of the rail head (see Figure 4 The left side of the central partition plate 7 corresponds to the side injection hole 5 and the right side corresponds to the side injection hole 6). Multiple injection holes are arranged on the injection panel and the injection pipe. The injection holes are connected to the external cooling medium fluid. Specifically, the injection hole 3 on the cooling component on the left side of the device is connected to the external cooling medium through the channel 1; the injection hole 4 on the cooling component on the right side of the device is connected to the external cooling medium through the channel 2. The central partition plate 7 protrudes to a position closer to the top surface of the rail head than the injection holes 3 and 4. The contact position between the central partition plate and the center of the weld on the surface of the rail head of the rail joint is provided with a flexible asbestos gasket, which can achieve full fit between the partition plate and the center of the weld on the rail head of the rail joint, avoiding the influence of the airflow difference on both sides of the weld on the cooling process.

[0035] In the present invention, when the rail head and tread air jet device is used to cool the rail joint, the rail joint is under the air jet device (such as Figure 5 As shown in the figure, the actual cooling rate can be adjusted by controlling the pressure of the cooling medium flowing into the device, thereby achieving controlled cooling of the left and right halves of the rail weld joint with different cooling rates / cooling intensities.

[0036] In some embodiments, the distance between the bottom cooling channel of the rail head and tread air jet device and the surface of the welded joint is controlled to be 25-30 mm. The cooling medium of the rail head and tread air jet device is a compressed air and / or water mist mixture, wherein the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling assembly corresponding to the heat-affected zone of the heat-treated eutectoid pearlite rail on the left side of the rail joint weld is 0.40-0.70 MPa, and the gas pressure of the compressed air and / or water mist mixture sprayed by the cooling assembly corresponding to the heat-affected zone of the hot-rolled bainite rail on the right side of the rail joint weld is 0.12-0.14 MPa.

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

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

[0039] By injecting compressed air and / or water mist mixture to quickly cool the heat affected zone of heat treated eutectoid pearlite rail and the heat affected zone of hot rolled bainite rail above the austenitizing temperature, the pearlite lamella spacing in the heat affected zone of the heat treated eutectoid pearlite rail can be refined and thermodynamic conditions can be provided for obtaining a small amount of martensite structure in the shallow surface layer of the heat affected zone of the hot rolled bainite rail through cooling.

[0040] In the present embodiment, the critical cooling rate of martensite transformation during the continuous cooling process of the heat-treated eutectoid pearlite rail steel is 2.0-2.5°C / s, and the Ms temperature of the rail steel (the starting temperature of martensite formation) is 250-280°C; the critical cooling rate of martensite transformation during the continuous cooling process of the hot-rolled bainite rail steel is 1.8-2.2°C / s, and the Ms temperature of the rail steel (the starting temperature of martensite formation) is 300-330°C. When the cooling rate of the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint reaches 2.4-2.8°C / s during the continuous cooling process and the final cooling temperature of the cooling stage is 300-330°C, in addition to the bainite structure, the surface of the heat-affected zone on this side will also generate a martensite structure with a volume fraction of 20%-30%, which can improve the surface hardness of the heat-affected zone on this side, and the refined microstructure can also improve the impact toughness of the heat-affected zone on this side to a certain extent. When the cooling rate of the heat affected zone on one side of the hot-rolled bainitic rail of the rail joint reaches 3.0~4.0℃ / s during the continuous cooling process and the final cooling temperature of the cooling stage is 300~330℃, in addition to the bainite structure, the heat affected zone on this side will also generate a martensite structure with a volume fraction of 32%~40%, which will lead to excessive hardness and deterioration of impact toughness in the heat affected zone on this side. Furthermore, if the cooling rate of the heat affected zone on one side of the hot-rolled bainitic rail of the rail joint is kept higher than the martensitic transformation cooling rate of the rail steel during the rapid cooling stage, and the final cooling temperature of the cooling stage is further reduced to 300~330℃ below the Ms temperature (starting temperature of martensite formation) of the rail steel (such as below 280℃), it will lead to a significant increase in the martensitic structure content and hardness in the heat affected zone on this side and further deterioration of impact toughness. Therefore, it is necessary to reasonably control the cooling rate and final cooling temperature in this operation.

[0041] The heat-affected zone on one side of the hot-rolled bainite rail of the dissimilar rail joint is rapidly cooled to generate a certain amount of martensite structure (volume fraction 20-30%), thereby improving the surface hardness of the heat-affected zone on this side, while not generating a large amount of martensite (volume fraction above 30%), which would lead to excessive hardness and deterioration of impact toughness of the heat-affected zone on this side. For the heat-affected zone on one side of the eutectoid pearlite rail of the rail joint, a relatively large cooling rate is adopted to generate pearlite structure with finer interlamellar spacing, and the final cooling temperature of the rapid cooling stage is controlled above the martensite transformation temperature of the rail steel at 50°C, so as to avoid the formation of brittle martensite in the pearlite structure, which would endanger the service safety of the heat-affected zone of the pearlite rail welding.

[0042] In step S4, after the secondary cooling is completed, the welded joint is naturally cooled to the ambient temperature. For example, the joint can be naturally cooled to an ambient temperature of 20-30°C in an air environment, and the cooling rate of the process is about 0.1-0.9°C / s.

[0043] Optionally, in some embodiments, the dissimilar rail welding joint is a joint formed by welding a heat-treated eutectoid pearlite rail a and a hot-rolled bainite rail b of the same rail type and specification of 60-75 kg / m by a rail moving flash welder.

[0044] Compared with conventional weld joint heat treatment, the method of the embodiment of the present invention will produce obvious phase transformation hysteresis during the rapid heat treatment of bainitic steel. That is, under rapid heating conditions, it is necessary to appropriately increase the heating temperature (i.e., austenitizing temperature) to ensure that the bainitic steel can be completely austenitized. The present invention implements different heating for the heat-affected zones on both sides of the rail weld joint with the center of the weld as the dividing line. For the heat-affected zone on one side of the eutectoid pearlite rail of the dissimilar rail joint, the heating of the heat-affected zone on that side is achieved by conventional medium-frequency induction heating coils and medium-frequency induction heating power. For the heat-affected zone on one side of the bainite rail of the dissimilar rail joint, rapid heating of the heat-affected zone on that side is achieved by increasing the number of induction heating coils and adjusting the induction heating power.

[0045] Therefore, for the cooling of the heat affected zone on one side of the bainitic rail of the rail joint, in order to generate a certain amount of martensite structure to improve the surface hardness and impact toughness of the heat affected zone on this side, while not generating a large amount of martensite which will cause the hardness of the heat affected zone on this side to be too high and the impact toughness to deteriorate, the cooling rate of the heat affected zone on one side of the hot-rolled bainitic rail of the rail joint during the continuous cooling process should be controlled at 2.4~2.8℃ / s and the final cooling temperature of the cooling stage should be controlled at 300~330℃ at the same time, which can ensure that in addition to the bainite structure, the martensite structure with a volume fraction of 20%~30% can be generated in the heat affected zone on this side, so as to take into account the hardness and impact toughness of the heat affected zone on this side and achieve a good match of the hardness of the heat affected zones on both sides of the weld within a certain range. For the heat affected zone on one side of the eutectoid pearlite rail of the rail joint, a relatively large cooling rate is adopted to generate a pearlite structure with finer interlamellar spacing. At the same time, the final cooling temperature of the rapid cooling stage is controlled above the martensitic transformation temperature of the rail steel by 50°C to avoid the formation of brittle martensite in the pearlite structure, which would endanger the service safety of the heat affected zone of the pearlite rail welding.

[0046] When testing the joints after heat treatment, follow the Figure 6 The sampling positions shown in the figure are: c is the center of the weld, and d is the sampling position of the metallographic sample of the rail head tread. The metallographic structure of the joint was inspected according to GB / T13298-2015 "Methods for the inspection of metal microstructures". The metallographic sample was etched with 3% nitric acid alcohol solution, and the metallographic structure was observed with a Leica MeF3 optical microscope. The rail joint was processed into a longitudinal hardness test specimen in accordance with TB / T 1632.2-2014 "Rail welding part 2: flash welding" standard. According to GB / T 230.1-2009, according to Figure 7 The schematic diagram shows that the joint is tested for longitudinal Vickers HV hardness at a position 5mm below the tread. The test points are arranged symmetrically on both sides of the weld with a spacing of 2mm. Figure 8Schematic diagram of processing rail joint impact test specimens. Among them, the impact toughness of the weld is the average value of 14 impact specimens of the full section of the rail joint, and the U-shaped notch is processed at the weld. For the area 10mm, 20mm, and 30mm below the surface of the rail head of the rail joint, the impact toughness of the heat-affected zone on the side of the heat-treated pearlite rail at -15mm from the center of the weld is the impact toughness value obtained by the impact test of the impact specimen with the U-shaped notch processed in the heat-affected zone -15mm from the center of the weld, and the test result is the average value of 4 impact specimens in the horizontal direction. The impact toughness of the heat-affected zone on the side of the hot-rolled bainite rail at +15mm from the center of the weld is the impact toughness value obtained by the impact test of the impact specimen with the U-shaped notch processed in the heat-affected zone +15mm from the center of the weld, and the test result is the average value of 4 impact specimens in the horizontal direction.

[0047] The heat treatment method of the present invention can achieve the following effects: (1) The depth of the hardened layer of the rail head is the area parallel to the rail head surface and downward with a hardness not less than 80% of the rail head surface hardness. The depth of the hardened layer of the heat affected zone on the pearlite side is about 25~35mm, and the depth of the hardened layer of the heat affected zone on the bainite side is about 5~15mm; the average hardness difference of the heat affected zone on both sides of the rail joint weld in the area ±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; (2) For the area 10 mm below the rail head surface of the rail joint, the impact toughness at the center of the weld is 47~58 J. The hardness of the heat-affected zone on the heat-treated pearlite rail side at -15 mm from the weld center is 295~337 HV, and the impact toughness is 19~16 J. The hardness of the heat-affected zone on the hot-rolled bainite rail side at +15 mm from the weld center is 281~322 HV, and the impact toughness is 85~80 J; for the area 20 mm below the rail head surface of the rail joint, the impact toughness at the center of the weld is 40~45 J. The hardness of the heat-affected zone on the heat-treated pearlite rail side at -15 mm from the weld center is 291~334 HV, and the impact toughness is 17~15 J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the weld center is 277~318HV, and the impact toughness is 82~75J; for the 30mm area below the rail head surface of the rail joint, the impact toughness at the weld center is 31~39J. The hardness of the heat-affected zone on the heat-treated pearlite rail at -15mm from the weld center is 285~327HV, and the impact toughness is 14~12J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the weld center is 263~282HV, and the impact toughness is 70~65J.

[0048] The present invention is further described below by means of specific examples, but the protection scope of the present invention is not limited thereto.

[0049] Example 1 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.15%, and the balance is Fe and inevitable 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 bainite rail base material includes the following components by weight percentage: C: 0.15%, Si: 1.20%, Mn: 0.70%, Cr: 1.50%, Mo: 0.20%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1130 MPa, a hardness of 290 HV, and an impact energy of 130 J.

[0050] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 60kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the rail joints with a surface temperature of 800°C obtained by welding are placed in the air to cool naturally until the surface temperature of the rail head drops to 200°C, and then the induction profiling electric heating coil is used to heat the entire section of the rail welding joint area. The induction heating frequency on the pearlite side is 2~4kHz, the heating rate is 7℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 940℃. The induction heating frequency of the rail head part on the bainite side is 6~9kHz, the heating rate is 15℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 1000℃. After normalizing heating is completed, cooling is immediately carried out, and compressed air (or water mist mixed gas) is used as the cooling medium. The gas pressures of the compressed air or water mist mixed gas ejected from the left and right sides of the rail head tread air jet device are 0.40MPa and 0.12MPa respectively, and the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 4.0℃ / s and 2.4℃ / s respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the joint weld are 420℃ and 330℃ respectively at this stage. The rail head tread air jet device is removed, and the welded joint is cooled in an air environment (i.e., naturally cooled) to an ambient temperature of 20~30℃, and the cooling rate is 0.1~0.9℃ / s, thereby obtaining the heat-treated rail welded joint of this embodiment.

[0051] The rail joint obtained in this embodiment has an average hardness of 285 HV on the heat-affected zone of the eutectoid pearlite rail side of the rail joint in the area of ​​±20 mm from the center of the weld, and an average hardness of 265 HV on the heat-affected zone of the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones 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 48 J, which is much higher than ≥6.5 J specified in TB / T 1632.2-2014. For the area 10mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 58J, the hardness of the heat-affected zone on the heat-treated pearlite rail side at -15mm from the center of the weld is 295HV, and the impact toughness is 19J, and the hardness of the heat-affected zone on the hot-rolled bainite rail side at +15mm from the center of the weld is 281HV, and the impact toughness is 85J; for the area 20mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 45J, and the hardness of the heat-affected zone on the heat-treated pearlite rail side at -15mm from the center of the weld is 291HV, and the impact toughness is 17J. The hardness of the heat-affected zone on one side of the hot-rolled bainite rail at +15mm from the center of the weld is 277HV, and the impact toughness is 82J; for the 30mm area below the rail head surface of the rail joint, the impact toughness of the weld center is 39J, and the hardness of the heat-affected zone on one side of the heat-treated pearlite rail at -15mm from the center of the weld is 285HV, and the impact toughness is 14J. The hardness of the heat-affected zone on one side of the hot-rolled bainite rail at +15mm from the center of the weld is 263HV, and the impact toughness is 70J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on one side of the heat-treated eutectoid pearlite rail of the rail joint, and the volume fraction of martensite structure in the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint is 20%, and the martensite is concentrated in the rail head surface area within 5mm from the tread. The present invention helps to improve the "saddle-type" wear caused by the low hardness of the welding area of ​​the rail welded joint during the line service process, and the joint has good impact toughness, which helps to ensure the safety of railway operation.

[0052] Example 2 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.77%, Si: 0.45%, Mn: 0.80%, V: 0.15%, and the balance is Fe and inevitable 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 bainite rail base material includes the following components by weight percentage: C: 0.15%, Si: 1.20%, Mn: 0.70%, Cr: 1.50%, Mo: 0.20%, and the balance is Fe and inevitable impurities. The rail base material has a tensile strength of 1130 MPa, a hardness of 290 HV, and an impact energy of 130 J.

[0053] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 60kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joint with a surface temperature of 800°C is placed in the air to cool naturally until the surface temperature of the rail head drops to 200°C, and then the induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. The induction heating frequency on the pearlite side is 2~4kHz, the heating rate is 7℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 990℃. The induction heating frequency of the rail head part on the bainite side is 6~9kHz, the heating rate is 15℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 1050℃. After normalizing heating is completed, cooling is immediately carried out, and compressed air (or water mist mixed gas) is used as the cooling medium. The gas pressures of the compressed air or water mist mixed gas ejected from the left and right sides of the rail head tread air jet device are 0.70MPa and 0.14MPa respectively, and the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 7.0℃ / s and 2.8℃ / s respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the joint weld are 390℃ and 300℃ respectively at this stage. The rail head tread air jet device is removed, and the welded joint is cooled in an air environment (i.e., naturally cooled) to an ambient temperature of 20-30℃, and the cooling rate is 0.1-0.9℃ / s, thereby obtaining the heat-treated rail welded joint of this embodiment.

[0054] The rail joint obtained in this embodiment has an average hardness of 300HV on the surface of the heat-affected zone on the eutectoid pearlite rail side of the rail joint in the area of ​​±20mm from the center of the weld, and an average hardness of 281HV on the surface of the heat-affected zone on the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones on both sides of the weld is controlled within ±20HV. The average impact energy of the full-section weld of the rail joint at room temperature is 44J, which is much higher than the ≥6.5J specified in TB / T 1632.2-2014. For the area 10mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 53J, and the hardness of the heat-affected zone on the side of the heat-treated pearlite rail at -15mm from the center of the weld is 312HV, and the impact toughness is 17J. The hardness of the heat-affected zone of the hot-rolled bainite rail at +15mm from the center of the weld is 294HV, and the impact toughness is 83J; for the area 20mm below the surface of the rail head of the rail joint, the impact toughness of the center of the weld is 43J, and the hardness of the heat-affected zone of the heat-treated pearlite rail at -15mm from the center of the weld is 310HV, and the impact toughness is 16J. The hardness of the heat-affected zone of the hot-rolled bainite rail at +15mm from the center of the weld is 292HV, and the impact toughness is 78J; for the area 30mm below the surface of the rail head of the rail joint, the impact toughness of the center of the weld is 36J, and the hardness of the heat-affected zone of the heat-treated pearlite rail at -15mm from the center of the weld is 307HV, and the impact toughness is 13J. The hardness of the heat-affected zone of the hot-rolled bainite rail at +15mm from the center of the weld is 272HV, and the impact toughness is 67J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone of the heat-treated eutectoid pearlite rail on one side of the rail joint, and the volume fraction of martensite in the heat-affected zone of the hot-rolled bainite rail on one side of the rail joint is 30%, and the martensite is concentrated in the rail head surface area within 5mm from the tread. The present invention helps to improve the "saddle-type" wear of the rail welded joint caused by the low hardness of the welding area during the line service process, and the joint has good impact toughness, which helps to ensure the safety of railway operation.

[0055] Example 3 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.83%, Si: 0.90%, Mn: 1.20%, V: 0.45%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1250 MPa, a hardness of 350 HV, and an impact energy of 10 J; the hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.19%, Si: 1.60%, Mn: 1.10%, Cr: 1.90%, Mo: 0.50%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1200 MPa, a hardness of 330 HV, and an impact energy of 100 J.

[0056] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 75kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the welded rail joint with a surface temperature of 800°C is placed in the air to cool naturally until the surface temperature of the rail head drops to 200°C, and then the induction profiling electric heating coil is used to heat the entire section of the rail welded joint area. The induction heating frequency on the pearlite side is 2~4kHz, the heating rate is 8℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 990℃. The induction heating frequency of the rail head part on the bainite side is 6~9kHz, the heating rate is 18℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 1050℃. After normalizing heating is completed, cooling is immediately carried out, and compressed air (or water mist mixed gas) is used as the cooling medium. The gas pressures of the compressed air or water mist mixed gas ejected from the left and right sides of the rail head tread air jet device are 0.70MPa and 0.14MPa respectively, and the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 7.0℃ / s and 2.8℃ / s respectively. The final cooling temperatures of the heat-affected zones on the left and right sides of the joint weld are 390℃ and 300℃ respectively at this stage. The rail head tread air jet device is removed, and the welded joint is cooled in an air environment (i.e., naturally cooled) to an ambient temperature of 20-30℃, and the cooling rate is 0.1-0.9℃ / s, thereby obtaining the heat-treated rail welded joint of this embodiment.

[0057] The rail joint obtained in this embodiment has an average hardness of 330 HV on the surface of the heat-affected zone on the eutectoid pearlite rail side of the rail joint in the area of ​​±20 mm from the center of the weld, and an average hardness of 305 HV on the surface of the heat-affected zone on the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones on both 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, which is much higher than the ≥6.5 J specified in TB / T 1632.2-2014. For the area 10 mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 47 J. The hardness of the heat-affected zone on the side of the heat-treated pearlite rail at -15 mm from the center of the weld is 337 HV, and the impact toughness is 16 J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 322HV, and the impact toughness is 80J; for the area 20mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 40J. The hardness of the heat-affected zone on the heat-treated pearlite rail at -15mm from the center of the weld is 334HV, and the impact toughness is 15J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 318HV, and the impact toughness is 75J; for the area 30mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 31J, and the hardness of the heat-affected zone on the heat-treated pearlite rail at -15mm from the center of the weld is 327HV, and the impact toughness is 12J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 282HV, and the impact toughness is 65J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone of the heat-treated eutectoid pearlite rail on one side of the rail joint, and the volume fraction of martensite in the heat-affected zone of the hot-rolled bainite rail on one side of the rail joint is 30%, and the martensite is concentrated in the rail head surface area within 5mm from the tread. The present invention helps to improve the "saddle-type" wear of the rail welded joint caused by the low hardness of the welding area during the line service process, and the joint has good impact toughness, which helps to ensure the safety of railway operation.

[0058] Example 4 In this embodiment, the heat-treated eutectoid pearlite rail base material includes the following components by weight percentage: C: 0.80%, Si: 0.68%, Mn: 1.0%, V: 0.30%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1200 MPa, a hardness of 330 HV, and an impact energy of 14 J; the hot-rolled bainite rail base material includes the following components by weight percentage: C: 0.17%, Si: 1.40%, Mn: 1.0%, Cr: 1.70%, Mo: 0.40%, the balance is Fe and inevitable impurities, the rail base material has a tensile strength of 1160 MPa, a hardness of 310 HV, and an impact energy of 115 J.

[0059] After the upsetting and pushing of the nodules in the mobile flash welding process of the dissimilar rails with a specification of 68kg / m are completed, the welded joints are subjected to post-weld heat treatment. First, the rail joints with a surface temperature of 800°C obtained by welding are placed in the air to cool naturally until the surface temperature of the rail head drops to 200°C, and then the induction profiling electric heating coil is used to heat the entire section of the rail welding joint area. The induction heating frequency on the pearlite side is 2~4kHz, the heating rate is 10℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 960℃. The induction heating frequency of the rail head part on the bainite side is 6~9kHz, the heating rate is 20℃ / s, and the heating is stopped when the surface temperature of the rail head reaches 1020℃. After normalizing heating is completed, cooling is immediately carried out, and compressed air (or water mist mixed gas) is used as the cooling medium. The gas pressures of the compressed air or water mist mixed gas ejected from the left and right sides of the rail head tread air jet device are 0.60MPa and 0.13MPa respectively, and the heat-affected zones on the left side (heat-treated eutectoid pearlite side) and the right side (hot-rolled bainite side) of the joint weld are rapidly cooled at cooling rates of 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 joint weld are 405℃ and 315℃ respectively at this stage. The rail head tread air jet device is removed, and the welded joint is cooled in an air environment (i.e., naturally cooled) to an ambient temperature of 20-30℃, and the cooling rate is 0.1-0.9℃ / s, thereby obtaining the heat-treated rail welded joint of this embodiment.

[0060] The rail joint obtained in this embodiment has an average hardness of 310HV on the surface of the heat-affected zone on the eutectoid pearlite rail side of the rail joint in the area of ​​±20mm from the center of the weld, and an average hardness of 295HV on the surface of the heat-affected zone on the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones on both sides of the weld is controlled within ±20HV. The average impact energy of the full-section weld of the rail joint at room temperature is 46J, which is much higher than the ≥6.5J specified in TB / T 1632.2-2014. For the area 10mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 55J. The hardness of the heat-affected zone on the side of the heat-treated pearlite rail at -15mm from the center of the weld is 318HV, and the impact toughness is 18J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 300HV, and the impact toughness is 83J; for the area 20mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 44J. The hardness of the heat-affected zone on the heat-treated pearlite rail at -15mm from the center of the weld is 313HV, and the impact toughness is 16J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 295HV, and the impact toughness is 79J; for the area 30mm below the surface of the rail head of the rail joint, the impact toughness at the center of the weld is 35J, and the hardness of the heat-affected zone on the heat-treated pearlite rail at -15mm from the center of the weld is 302HV, and the impact toughness is 13J. The hardness of the heat-affected zone on the hot-rolled bainite rail at +15mm from the center of the weld is 288HV, and the impact toughness is 68J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone of the heat-treated eutectoid pearlite rail on one side of the rail joint, and the volume fraction of martensite in the heat-affected zone of the hot-rolled bainite rail on one side of the rail joint is 26%, and the martensite is concentrated in the surface area of ​​the rail head within 5mm from the tread. The present invention helps to improve the "saddle-type" wear of the rail welded joint caused by the low hardness of the welding area during the line service process, and the joint has good impact toughness, which helps to ensure the safety of railway operation.

[0061] Comparative Example 1 The process conditions involved in the selection of rail materials, mechanical properties of rail base materials, welding and post-weld normalizing heat treatment cooling process in Comparative Example 1 and Example 1 are all the same, except that the heating temperature of the rail head portion of the heat affected zone on the eutectoid pearlite rail side of the rail joint in this comparative example is 900°C, and the heating temperature of the rail head portion of the heat affected zone on the bainite rail side of the rail joint is 950°C.

[0062] The rail welded joint obtained from this comparative example has a low heating temperature during the normalizing heating process, which leads to insufficient supercooling during the cooling process after normalizing heating and insufficient driving force for pearlite phase transformation and bainite phase transformation during the subsequent cooling process, which in turn leads to low mechanical properties of the heat-affected zones on both sides of the weld of the rail welded joint. The average hardness of the heat-affected zone on the side of the eutectoid pearlite rail of the rail joint obtained in this comparative example within the area of ​​±20mm from the center of the weld is 275HV, and the average hardness of the heat-affected zone on the side of the bainite rail of the rail joint is 245HV, and the average hardness difference of the heat-affected zones on both sides of the weld is controlled outside ±25HV. The average impact energy of the full-section weld of the rail joint at room temperature is 37J. The hardness and impact energy of the rail welded joint obtained in this comparative example are lower than the beneficial effects obtained by adopting the present invention.

[0063] Comparative Example 2 The rail material selection, rail base material mechanical properties, welding cooling process, post-weld normalizing heat treatment heating temperature and cooling rate in the post-weld normalizing cooling stage in this comparative example are consistent with those in Example 1. The difference is that in this comparative example, the final cooling temperature of the heat affected zone of the hot-rolled bainite rail side of the rail welded 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 of the hot-rolled bainite rail side of the rail joint during the secondary cooling process after the post-weld normalizing heating of the rail joint of the present invention.

[0064] The rail welded joint obtained in this comparative example has a martensite structure with a volume fraction of about 40% formed in the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint during the secondary cooling process after the normalizing heating of the rail joint is completed, which results in the formation of a martensite structure with a volume fraction of about 40% in the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint. The average hardness of the heat-affected zone on one side of the eutectoid pearlite rail of the rail joint in the area of ​​±20mm from the center of the weld is 285HV, and the average hardness of the heat-affected zone on one side of the bainite rail of the rail joint is 278HV, and the average hardness difference of the heat-affected zone on both sides of the weld is controlled within ±10HV. The average impact energy of the full-section weld of the rail joint at room temperature is 20J. Due to the obvious amount of martensite in the heat-affected zone of the hot-rolled bainite rail of the rail welded joint obtained in this comparative example, the hardness of the heat-affected zone on this side is too high, and the average impact energy of the weld is low, which is not conducive to the safety of railway operation.

[0065] Comparative Example 3 The rail material selection, rail base material mechanical properties, welding cooling process, post-weld normalizing heat treatment heating temperature and cooling rate in the post-weld normalizing cooling stage in this comparative example are consistent with those in Example 1. The difference is that in this comparative example, the final cooling temperature of the heat-affected zones on both sides of the weld in the secondary cooling process after normalizing heating of the rail welded joint is 270°C.

[0066] The rail joint obtained in this comparative example has a final cooling temperature of the heat affected zones on both sides of the weld in the secondary cooling after the normalizing heating of the joint after welding is completed within the Ms temperature range (the starting temperature of martensite formation) of the heat-treated eutectoid pearlite rail steel involved in the present invention, and 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 the present invention. At the same time, since the cooling rate of the secondary cooling is higher than the critical cooling rate of martensite transformation of the above two rail steels, a significant amount of martensitic structure is formed in the heat affected zones on both sides of the weld of the rail joint.

[0067] The rail welded joint obtained from this comparative example has an average hardness of 297HV on the heat-affected zone of the eutectoid pearlite rail side of the rail joint in the area of ​​±20mm from the center of the weld, and an average hardness of 277HV on the heat-affected zone of the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones on both sides of the weld is controlled within ±20HV. The average impact energy of the full-section weld of the rail joint at room temperature is 12J. In addition, under metallographic microscope observation, the volume fraction of martensite structure in the heat-affected zone of the heat-treated eutectoid pearlite rail side of the rail joint is 10%, and the volume fraction of martensite structure in the heat-affected zone of the hot-rolled bainite rail side of the rail joint is 32%. Since the process method of the present invention is not adopted, a significant amount of martensite is formed in the heat-affected zones on both sides of the weld of the rail joint obtained in this comparative example, and the average impact energy of the weld is lower than the beneficial effect obtained by adopting the process method of the present invention, which is not conducive to the safety of railway operation.

[0068] Comparative Example 4 The rail material selection, rail base material mechanical properties, rail welding process conditions, heating temperature during post-weld normalizing, cooling method during post-weld normalizing and final cooling temperature of secondary cooling, cooling method and steps in the final cooling stage, cooling rate, etc. in this comparative example are consistent with those in Example 3. The difference is that the cooling rate of the heat affected zone on one side of the eutectoid pearlite rail in the secondary cooling process after normalizing heating in the rail joint in this comparative example is 3.0°C / s, which is lower than the cooling rate control range of 4.0~7.0°C / s of the heat affected zone on one side of the eutectoid pearlite rail in the second cooling process after post-weld normalizing heating of the rail of the present invention, thereby obtaining the rail welded joint under this comparative example.

[0069] The rail joint obtained in this comparative example has a cooling rate of 3.0°C / s in the heat affected zone on one side of the eutectoid pearlite rail during the second cooling process after normalizing heating, which is lower than the preferred cooling rate process range of 4.0-7.0°C / s of the present invention. As a result, the pearlite structure refinement effect in the heat affected zone on one side of the eutectoid pearlite rail during the second cooling process after normalizing heating of the rail welded joint is lower than the beneficial effect obtained by the process method of the present invention, which in turn causes the mechanical properties of the heat affected zone on one side of the eutectoid pearlite rail of the rail joint to be lower than the beneficial effect obtained by the process method of the present invention.

[0070] The average hardness of the heat affected zone on the side of the eutectoid pearlite rail of the rail joint in the area of ​​±20mm from the center of the weld is 308HV, and the average hardness of the heat affected zone on the side of the bainite rail of the rail joint is 305HV. The difference in the average hardness of the heat affected zones on both sides of the weld is controlled within ±10HV. The average impact energy of the full-section weld of the rail joint at room temperature is 20J. In addition, under metallographic microscope observation, there is no martensite in the heat affected zone on the side of the heat-treated eutectoid pearlite rail of the rail joint, and the volume fraction of martensite structure in the heat affected zone on the side of the hot-rolled bainite rail of the rail joint is 30%. The hardness of the heat affected zone on the side of the eutectoid pearlite rail of the rail joint obtained in this comparative example and the full-section weld impact energy are lower than the beneficial effects obtained by adopting the present invention.

[0071] Comparative Example 5 The rail material selection, rail base material mechanical properties, and rail flash welding conditions in this comparative example are consistent with those in Example 4. The difference is that in this comparative example, there is no subsequent normalizing heating and cooling process after the rail welding is completed, that is, the rail welded joint obtained by flash welding is directly naturally cooled in the air to an ambient temperature of 30° C., thereby obtaining the rail welded joint in this comparative example.

[0072] The rail welded joint obtained from this comparative example has an average hardness of 280HV on the heat-affected zone of the eutectoid pearlite rail side of the rail joint in the area of ​​±20mm from the center of the weld, and an average hardness of 263HV on the heat-affected zone of the bainite rail side of the rail joint. The difference in average hardness of the heat-affected zones on both sides of the weld is within ±20HV. The average impact energy of the full-section weld of the rail joint at room temperature is 15J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the heat-treated eutectoid pearlite rail side of the rail joint, and there is no martensite structure in the heat-affected zone on the hot-rolled bainite rail side of the rail joint. The hardness of the heat-affected zones on both sides of the weld of the rail joint obtained in this comparative example is low, and the average impact energy of the weld is also significantly low, which is not conducive to the safety of railway operation.

[0073] Comparative Example 6 The process conditions involved in the selection of rail materials, mechanical properties of rail base materials, welding and post-weld normalizing heat treatment heating process in this comparative example are consistent with those in Example 4. The difference is that the cooling rate of the heat affected zone on the bainite rail side of the rail joint in the secondary cooling process of the rail joint in this comparative example is 1.6°C / s, and the final cooling temperature in this stage is 350°C, thereby obtaining the rail welded joint in this comparative example.

[0074] The cooling rate of the heat-affected zone on the side of the hot-rolled bainite rail in the secondary cooling process of the rail joint obtained in this comparative example is low, the bainite refinement effect is not ideal, and the mechanical properties are not improved enough. For the rail welded joint obtained in this comparative example, the average hardness of the heat-affected zone on the side of the eutectoid pearlite rail of the rail joint in the area ±20mm from the center of the weld is 310HV, the average hardness of the heat-affected zone on the side of the hot-rolled bainite rail of the rail joint is 280HV, and the average hardness difference of the heat-affected zones on both sides of the weld is not within ±20HV. The average impact energy of the full-section weld of the rail joint at room temperature is 32J. In addition, under metallographic microscope observation, there is no martensite in the heat-affected zone on the side of the heat-treated eutectoid pearlite rail of the rail joint, and there is no martensite structure in the heat-affected zone on the side of the hot-rolled bainite rail of the rail joint. Since the process method of the present invention is not adopted, the hardness of the heat-affected zone on the side of the hot-rolled bainite rail of the rail joint obtained in this comparative example is low.

[0075] Comparative Example 7 The process conditions involved in the selection of rail materials, mechanical properties of rail base materials, welding and post-weld normalizing heat treatment heating process in this comparative example are consistent with those in Example 4. The difference is that in this comparative example, the heating frequency on both sides of the weld of the rail joint during normalizing is 2-4 kHz, and the heating temperature is 960° C., thereby obtaining the rail welded joint in this comparative example.

[0076] The heating frequency of the heat-affected zone on one side of the hot-rolled bainitic rail is low during the normalizing process of the rail joint obtained by this comparative example, and the core is heated more. The heat dissipation of the core is slow during the rapid cooling process, resulting in coarsening of the core structure, unsatisfactory bainite refinement effect, and insufficient improvement in mechanical properties. For the rail welded joint obtained by this comparative example, the average hardness of the heat-affected zone on the side of the eutectoid pearlite rail of the rail joint in the area of ​​±20mm from the center of the weld is 310HV, and the average hardness of the heat-affected zone on the side of the hot-rolled bainitic rail of the rail joint is 275HV. The average impact energy of the full-section weld of the rail joint at room temperature is 35J. Since the process method of the present invention is not adopted, the hardness of the heat-affected zone on the side of the hot-rolled bainitic rail of the rail joint obtained in this comparative example is low, and the average impact energy of the weld is also significantly low, which is not conducive to the safety of railway operation.

[0077] By comparing the embodiments of the present invention with the comparative examples, it can be seen that: The present invention implements post-weld normalizing heating at different heating temperatures and controlled cooling of different intensities on the heat-affected zones on both sides of the weld of the flash welded joint of the dissimilar heat-treated eutectoid pearlite rail and the hot-rolled bainite rail, so that the average hardness difference of the heat-affected zones on both sides of the weld of the rail joint in the area of ​​±20 mm from the weld center is controlled to be ±25 HV, thereby ensuring the wear resistance of the heat-affected zones on both sides of the weld of the rail joint; it can be achieved that there is no martensite in the heat-affected zone on one side of the heat-treated eutectoid pearlite rail of the dissimilar rail joint, and the volume fraction of the martensite structure in the heat-affected zone on one side of the hot-rolled bainite rail of the rail joint is 20%-30%; the average impact energy of the full-section weld of the rail joint at room temperature is 41-48 J, which is much higher than ≥6.5 J specified in TB / T 1632.2-2014.

[0078] The present invention helps to improve the "saddle-shaped" wear of rail welded joints caused by low hardness in the welding area during the service of the line, and the joints have good impact toughness, which helps to ensure the safety of railway operation. The present invention is particularly suitable for the treatment process after welding of dissimilar rails. The technical advantages of the present invention are very obvious, and the market promotion prospects are very broad.

[0079] The above-mentioned embodiments only express the implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for improving the hardness and impact toughness of pearlite and bainite rail weld joints, characterized in that: The following steps are involved: Step S1: cooling the welded joint formed by welding the heat-treated eutectoid pearlite rail and the hot-rolled bainite rail; Step S2: using electric induction heating to heat the weld joint that has been cooled once to a normalizing temperature, with the center of the weld as the dividing line, the heating frequency of the induction heating coil on the pearlite side being 2-4 kHz, and the heating frequency of the induction heating coil on the bainite side being 6-9 kHz, so as to austenitize the weld joint; Step S3: After normalizing heating is completed, the weld joint is subjected to secondary cooling, wherein the heat affected zone on the pearlite side is cooled to a first final cooling temperature using a first cooling rate, and the heat affected zone on the bainite side is cooled to a second final cooling temperature using a second cooling rate, wherein 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, the weld joint is naturally cooled to ambient temperature.

2. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 1, characterized in that: In step S2, the heating temperature on the pearlite side is 940-990°C, and the heating temperature on the bainite side is 1000-1050°C.

3. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 1, characterized in that: In step S3, the first cooling rate is 4.0-7.0°C / s, and the second cooling rate is 2.4-2.8°C / s.

4. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 3, characterized in that: 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.

5. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 3, characterized in that: The secondary cooling adopts compressed air or water mist mixed gas to perform air jet cooling.

6. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 5, characterized in that: The air jet pressure for secondary cooling of the heat affected zone on the pearlite side is 0.40~0.70MPa, and the air jet pressure for secondary cooling of the heat affected zone on the bainite side is 0.12~0.14MPa.

7. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 1, characterized in that: The primary cooling is natural cooling performed in an air environment at 20-30°C.

8. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 7, characterized in that: The final cooling temperature of the primary cooling is 100-200°C.

9. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 1, characterized in that: In step S2, the induction heating rate on the pearlite side is 7-10°C / s, and the induction heating rate on the rail head part on the bainite side is 15-20°C / s.

10. The method for improving the hardness and impact toughness of pearlite and bainite rail weld joints according to claim 1, characterized in that: The base material of the heat-treated eutectoid pearlite rail includes the following chemical components by weight percentage: C: 0.77%-0.83%, Si: 0.45%-0.90%, Mn: 0.80%-1.20%, V: 0.15%-0.45%, the balance is Fe and unavoidable impurities, the room temperature tensile strength of the base material is 1170-1250MPa, the hardness is 310-350HV, and the impact energy is 18-10J; the hot-rolled Bainite rail The base material of the body rail includes 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%, and the balance is Fe and inevitable 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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