A heat treatment method for high-carbon pearlitic rail flash-welded joints

Abstract

The present application relates to the technical field of rail welding, and particularly relates to a heat treatment method for a high-carbon pearlite rail flash-welded joint. The heat treatment method for the high-carbon pearlite rail flash-welded joint comprises the following steps: first-stage cooling of the rail welded joint; full-face normalizing heating of the welded joint after the first-stage cooling; second-stage cooling of the welded joint after the full-face normalizing heating; and third-stage cooling of the welded joint after the second-stage cooling. In the second-stage cooling process, the rail head heat-affected zone and the rail waist heat-affected zone of the welded joint are controlled to implement different intensity cooling modes, so that the heat-affected zones on both sides of the welded joint of the rail are free of martensite and continuous network cementite, and at the same time, the rail head heat-affected zone of the welded joint has a relatively high hardness, thereby ensuring the wear resistance of the rail welded joint.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel rail welding, in particular to a heat treatment method for a high-carbon pearlite steel rail flash-welded joint. BACKGROUND

[0002] Network cementite (carbide) often appears in large-size forgings or rolled workpieces with high carbon content. Network cementite is generally distributed in a network along the grain boundary, has high brittleness, reduces the mechanical properties of the steel, and easily causes quenching cracks, and is a defect that must be eliminated. For small and medium-sized parts, it is relatively easy to destroy the network structure of cementite by normalizing, as long as the steel is heated to the complete austenite zone, and then air-cooled or spray-cooled to inhibit the precipitation of network cementite. For eutectoid steel and hypereutectoid steel with high carbon content, thin network cementite will form along the original austenite grain boundary when the cooling speed is not enough. For network cementite existing in carbon steel and alloy tool steel with a carbon content greater than 0.77%, normalizing can reduce the amount of secondary cementite and prevent it from forming a continuous network.

[0003] With the rapid development of China's railway industry, high-carbon pearlite steel rails are being used more and more widely. However, the mechanical properties of steel rail welded joints have been one of the key factors affecting the safety of railway operation. At present, hypereutectoid pearlite steel rails are used in high-speed, quasi-high-speed and passenger-freight mixed railways at home and abroad, and the carbon content is usually in the range of 0.65% to 0.82% by weight, which is relatively high, and the microstructure is pearlite, which has good strength and toughness matching, and moderate comprehensive mechanical property indicators.

[0004] At present, steel rail mobile flash welding has become the mainstream on-line welding technology for steel rails in railway construction sites at home and abroad. With the large-scale use of high-carbon pearlite steel rails, it is possible to prevent the deterioration of the microstructure and properties of steel rail welded joints caused by network cementite. During the steel rail welding process, the welding thermal cycle can cause the disappearance of the hardened layer in the welded area, and form a wide low-hardness zone on both sides of the weld, so that the hardness of the weld and the heat-affected zone is lower than that of the steel rail base material. In addition, due to the melting process and high temperature, the austenite grains in the overheated zone of the steel rail are coarse, which causes the hardness of this area to be significantly lower than that of the base material. During service, the steel rail welded joint is prone to form "saddle-type" wear at the rail head tread, which increases the wheel-rail impact, affects the service life of the steel rail, and even endangers the safety of train operation. Therefore, how to restore the mechanical properties of the steel rail reduced due to welding and eliminate the network cementite that may exist in the heat-affected zone of the steel rail welded joint has become a prerequisite for the application of the steel rail.

[0005] In order to improve the mechanical properties of the rail welded joint, the current railway industry standard TB / T1632.2-2014 "Rail Welding Part 2: Flash welding" and TB / T1632.4-2014 "Rail Welding Part 4: Gas pressure welding" stipulate that for heat treated rails, the average hardness of the welded area shall not be lower than 90% of the average hardness of the rail base material, and there shall be no harmful microstructure such as martensite or bainite in the microstructure of the weld and heat affected zone.

[0006] Currently, in order to improve the mechanical properties of the rail welded joint, the domestic rail welding is generally carried out according to the above standard after welding normalizing heat treatment. The rail welded joint is heated to a temperature above the austenitizing temperature by using intermediate frequency induction heating or oxyacetylene flame heating as the heat source, and then air cooling or air cooling process is adopted to further improve the tread hardness of the rail welded area. The patent for invention with publication number CN106544933B discloses a post-welding heat treatment method for hypereutectoid rail and PG4 heat treated eutectoid pearlite rail welded joints. This method can make the dissimilar rail welded joint meet the test requirements of the current domestic railway industry standard for fatigue, tensile, impact and static bending tests, but it is not applicable to the welded joint of the same heat treated eutectoid pearlite rail. In addition, although the three patents with application numbers CN201810581145.3, CN201810720765.0 and CN201810710040.3 introduce the post-welding heat treatment method for hypereutectoid steel rail and eutectoid steel rail welded joints, the post-welding heat treatment principle of all of them is to implement rapid cooling on the steel rail welded joint at the austenitizing temperature, so as to achieve the purpose of refining the pearlite interlamellar spacing and improving the hot plasticity of the rail welded area, but it is still difficult to effectively eliminate the martensite and continuous network cementite in the heat affected zone on both sides of the rail welded joint.

[0007] In summary, there is an urgent need in the field of railway engineering for a heat treatment method for high-carbon pearlite rail welded joints to improve the mechanical properties of the rail reduced due to welding and eliminate the possible network cementite structure, so as to ensure the service performance of the rail welded joint and the safety of railway operation. SUMMARY

[0008] Therefore, the present application provides a heat treatment method for high-carbon pearlite rail flash welded joints, which can at least solve the problems of reduced mechanical properties of the existing high-carbon pearlite rail due to welding and the network cementite structure affecting the service performance of the rail and the safety of railway operation.

[0009] The heat treatment method for high-carbon pearlite rail flash welded joints of the present application comprises the following steps:

[0010] S100, cooling the rail welded joint to a temperature of 100-200℃ in the first stage. S200, cooling the rail welded joint to a temperature of 100-200℃ in the second stage. S300, cooling the rail welded joint to a temperature of 100-200℃ in the third stage. S400, cooling the rail welded joint to a temperature of 100-200℃ in the fourth stage. S500, cooling the rail welded joint to a temperature of 100-200℃ in the fifth stage. S600, cooling the rail welded joint to a temperature of 100-200℃ in the sixth stage. S700, cooling the rail welded joint to a temperature of 100-200℃ in the seventh stage. S800, cooling the rail welded joint to a temperature of 100-200℃ in the eighth stage. S900, cooling the rail welded joint to a temperature of 100-200℃ in the ninth stage. S1000, cooling the rail welded joint to a temperature of 100-200℃ in the tenth stage. S1100, cooling the rail welded joint to a temperature of 100-200℃ in the eleventh stage. S1200, cooling the rail welded joint to a temperature of 100-200℃ in the twelfth stage. S1300, cooling the rail welded joint to a temperature of 100-200℃ in the thirteenth stage. S1400, cooling the rail welded joint to a temperature of 100-200℃ in the fourteenth stage. S1500, cooling the rail welded joint to a temperature of 100-200℃ in the fifteenth stage. S1600, cooling the rail welded joint to a temperature of 100-200℃ in the sixteenth stage. S1700, cooling the rail welded joint to a temperature of 100-200℃ in the seventeenth stage. S1800, cooling the rail welded joint to a temperature of 100-200℃ in the eighteenth stage. S1900, cooling the rail welded joint to a temperature of 100-200℃ in the nineteenth stage. S2000, cooling the rail welded joint to a temperature of 100-200℃ in the twentieth stage. S2100, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-first stage. S2200, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-second stage. S2300, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-third stage. S2400, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-fourth stage. S2500, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-fifth stage. S2600, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-sixth stage. S2700, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-seventh stage. S2800, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-eighth stage. S2900, cooling the rail welded joint to a temperature of 100-200℃ in the twenty-ninth stage. S3000, cooling the rail welded joint to a temperature of 100-200℃ in the thirtieth stage. S3100, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-first stage. S3200, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-second stage. S3300, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-third stage. S3400, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-fourth stage. S3500, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-fifth stage. S3600, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-sixth stage. S3700, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-seventh stage. S3800, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-eighth stage. S3900, cooling the rail welded joint to a temperature of 100-200℃ in the thirty-ninth stage. S4000, cooling the rail welded joint to a temperature of 100-200℃ in the fortieth stage. S4100, cooling the rail welded joint to a temperature of 100-200℃ in the forty-first stage. S4200, cooling the rail welded joint to a temperature of 100-200℃ in the forty-second stage. S4300, cooling the rail welded joint to a temperature of 100-200℃ in the forty-third stage. S4400, cooling the rail welded joint to a temperature of 100-200℃ in the forty-fourth stage. S4500, cooling the rail welded joint to a temperature of 100-200℃ in the forty-fifth stage. S4600, cooling the rail welded joint to a temperature of 100-200℃ in the forty-sixth stage. S4700, cooling the rail welded joint to a temperature of 100-200℃ in the forty-seventh stage. S4800, cooling the rail welded joint to a temperature of 100-200℃ in the forty-eighth stage. S4900, cooling the rail welded joint to a temperature of 100-200℃ in the forty-ninth stage. S5000, cooling the rail welded joint to a temperature of 100-200℃ in the fiftieth stage.

[0011] S200, full-section normalizing heating is performed on the welded joint after the first stage cooling is completed, so that the surface of the welded joint is heated to 940-980℃;

[0012] S300, second stage cooling is performed on the welded joint after the full-section normalizing heating is completed; wherein the cooling speed of the rail head heat affected zone of the welded joint is controlled to be 3.0-5.0℃ / s, and the final cooling temperature is 350-400℃; the cooling speed of the rail waist heat affected zone of the welded joint is controlled to be 8.0-10.0℃ / s, and the final cooling temperature is 450-500℃;

[0013] S400, third stage cooling is performed on the welded joint after the second stage cooling is completed, so that the full-section temperature of the welded joint is reduced to 20-30℃.

[0014] In some embodiments, in step S300, the second stage cooling is implemented by using a cooling device, the cooling device includes a plurality of first nozzles arranged around the rail head heat affected zone of the welded joint, and a plurality of second nozzles arranged around the rail waist heat affected zone of the welded joint, and the cooling medium sprayed by the first nozzles and the second nozzles is one or both of compressed air or water mist mixed gas.

[0015] In some embodiments, the distance between the plurality of first nozzles and the rail head of the welded joint is 25-35mm, and the spraying pressure is 0.15-0.25MPa; the distance between the plurality of second nozzles and the welded joint is 45-55mm, and the spraying pressure is 0.40-0.50MPa.

[0016] In some embodiments, in step S100, the first stage cooling is natural cooling performed in an air environment of 20-30℃.

[0017] In some embodiments, in step S200, the full-section normalizing heating is implemented by using an induction profiled electric heating coil, the induction profiled electric heating coil is arranged around the circumference of the welded joint, the distance from the surface of the welded joint is 5mm-50mm, the length of the induction profiled electric heating coil in the extension direction of the steel rail is 60mm-80mm, and the temperature rise rate of the induction profiled electric heating coil is controlled to be 1.0℃ / s-20.0℃ / s during the heating process.

[0018] In some embodiments, in step S300, the open cooling temperature of the second stage cooling is not lower than 920℃.

[0019] In some embodiments, in step S400, the third stage cooling is natural cooling performed in an air environment of 20-30℃.

[0020] In some embodiments, the welded joint is a joint formed by welding two rails of the same rail type and the same specification of 60-75 kg / m by using a moving flash welding machine.

[0021] In some embodiments, the heat-treated ferrite-pearlite rail has a tensile strength of 1280-1360 MPa, a hardness of 350-400 HV, and an impact energy of 10-18 J at 20-30 DEG C. The chemical composition of the heat-treated ferrite-pearlite rail includes 0.75-0.84% of C, 0.45-0.90% of Si, 0.8-1.2% of Mn, 0.30-0.60% of Cr, and 0.03-0.12% of V, and the rest is iron and inevitable impurities.

[0022] In some embodiments, the welded joint is a region with a length of 80-120 mm, including the weld and / or the heat-affected zone on both sides of the weld.

[0023] The present application has the following advantages: the present application can make the heat-affected zone on both sides of the weld of the rail welded joint free of martensite and continuous network cementite by using different cooling methods on the heat-affected zone on both sides of the rail head and rail waist weld of the welded joint, and the heat-affected zone on the rail head of the rail welded joint has a relatively high hardness, thereby ensuring the wear resistance of the rail welded joint. The rail welded joint treated by the above heat treatment method can make the longitudinal hardness in the region of ±30 mm from the center of the weld reach 91-95% of the average hardness of the corresponding rail base material. The average impact energy of the rail head weld of the rail welded joint at room temperature is 14-17 J, and the average impact energy of the rail waist weld of the rail welded joint is 10-13 J, which is much higher than the ≥6.5 J specified in the standard TB / T 1632.2-2014. The present application can help to improve the "saddle-type" wear caused by the low hardness of the welded region of the rail welded joint during the service of the rail welded joint, and the impact toughness of the joint is good, which can help to ensure the safety of railway operation. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0025] Figure 1 A flowchart of a heat treatment method of a high-carbon pearlite rail flash-welded joint according to an embodiment of the present application is shown in the figure.

[0026] Figure 2 Figure 1 is a diagram of a hardness testing position of a longitudinal section of a rail welded joint below a rail head tread of 3-5 mm according to an embodiment of the present application;

[0027] Figure 3 Figure 2 is a structural diagram of a cooling device of a rail welded joint according to an embodiment of the present application;

[0028] Figure 4 Figure 3 is a diagram of a sampling position of a rail head tread metallographic sample of a rail welded joint. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present application clearer, the embodiments of the present application are further described in detail below with reference to the accompanying drawings.

[0030] It should be noted that all the expressions of "first" and "second" in the embodiments of the present application are used to distinguish two same-named different entities or different parameters, and the "first" and "second" are only used for the convenience of description and should not be understood as a limitation of the embodiments of the present application. The subsequent embodiments will not be described one by one.

[0031] A heat treatment method of a high-carbon pearlite rail flash-welded joint according to the present application, as shown in Figure 1, comprises the following steps: Figure 1

[0032] S100, a first-stage cooling is performed on the welded joint of the rail, so that the surface temperature of the welded joint is reduced to 100-200℃.

[0033] S200, full-face normalizing heating is performed on the welded joint after the first-stage cooling, so that the surface temperature of the welded joint is increased to 940-980℃. It should be noted that the so-called normalizing heating treatment generally refers to a heat treatment process in which a metal workpiece is heated to 30-50℃ above Ac3 (the end temperature of ferrite transformation to austenite when heated) by using a conventional method, is kept for a period of time, and then is taken out of the furnace and is naturally cooled in air, or is cooled by spraying or compressed air. The post-weld normalizing heating treatment of the rail welded joint is different from the heat treatment process usually used for small-size workpieces. Since the length of the post-weld sample of the rail can be as high as several hundred meters, it is determined that the normalizing heating treatment of the rail welded joint cannot be kept for a long time after reaching the target temperature (the temperature above the austenitizing temperature). Therefore, the rail welded joint is generally heated to the target temperature at a temperature slightly higher than the conventional normalizing temperature, and then is subjected to the heat treatment process of air cooling or air cooling, and the preferred normalizing heating temperature of the present application is 940-980℃.

[0034] ​S300, performing second stage cooling on the welded joint which has completed the full section normalizing heating; wherein the cooling speed of the toe heat affected zone of the welded joint is 3.0-5.0℃ / s, and the final cooling temperature is 350-400℃; the cooling speed of the web heat affected zone of the welded joint is 8.0-10.0℃ / s, and the final cooling temperature is 450-500℃. It should be noted that the web of the steel rail corresponds to the most serious area of the network of cementite in the steel rail steel. The inventors have found that when the cooling speed of the web heat affected zone of the welded joint and the toe heat affected zone of the welded joint are both less than 8.0℃ / s, the network of carbide in the toe heat affected zone of the welded joint disappears, but the network of carbide in the web heat affected zone of the welded joint cannot be eliminated. Based on this, the present application adopts a relatively higher cooling speed for the web heat affected zone of the welded joint than for the toe heat affected zone of the welded joint, aiming to eliminate the network of carbide in the web heat affected zone of the welded joint. In addition, in order to avoid the formation of martensite / bainite abnormal structure due to rapid cooling during the second stage cooling process, and considering that the alloy element segregation in the web area of the steel rail is higher than that in the toe area of the steel rail, the final cooling temperature of the web heat affected zone of the welded joint in the second stage cooling process is set to be 50℃ or more higher than the final cooling temperature of the toe heat affected zone of the welded joint.

[0035] S400, performing third stage cooling on the welded joint which has completed the second stage cooling, so that the temperature of the welded joint is reduced to 20-30℃. Thus, the post-weld heat treatment process of the steel rail involved in the present application is completed. In this stage, the cooling rate of the welded joint is 0.8-0.2℃ / s.

[0036] By implementing different cooling methods in the heat affected zones on both sides of the weld seam of the toe and the web of the welded joint, the present application can make the heat affected zones on both sides of the weld seam of the steel rail welded joint free of martensite and continuous network of cementite, while the toe heat affected zone of the steel rail welded joint retains a relatively high hardness, thereby ensuring the wear resistance of the steel rail welded joint. The welded joint treated by the above-mentioned heat treatment method can make the longitudinal hardness in the area of ±30mm from the center of the weld seam reach 91%-95% of the average hardness of the corresponding steel rail base material. The average impact energy of the toe weld seam of the welded joint at room temperature is 14-17J, and the average impact energy of the web weld seam of the welded joint is 10-13J, which is much higher than the ≥6.5J specified in the standard TB / T 1632.2-2014. The present application helps to improve the "saddle-type" wear caused by the low hardness of the welded area during the service of the steel rail welded joint in the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0037] In some embodiments, the welded joint is a region with a length of 80-120mm, including the weld seam and / or the heat affected zone, the heat affected zone being on both sides of the weld seam, and further including the base material region in the heat affected zone.

[0038] In some embodiments, in step S300, the second stage cooling is performed by a cooling device, which includes a plurality of first nozzles arranged around the toe heat-affected zone of the welded joint and a plurality of second nozzles arranged around the waist heat-affected zone of the welded joint. The cooling medium sprayed by the first nozzles and the second nozzles is one or both of compressed air or water mist mixture. The actual cooling speed can be adjusted by controlling the pressure of the cooling medium flowing into the cooling device, thereby achieving controlled cooling of the toe and waist areas of the welded joint of the steel rail at different cooling speeds / cooling intensities. Figure 3 As shown in FIG. 1, the plurality of first nozzles and the plurality of second nozzles are arranged around the outer periphery of the toe heat-affected zone and the waist heat-affected zone of the welded joint by a fixed frame arranged around the toe and the waist of the steel rail. A horizontal partition plate is arranged on the left and right sides of the fixed frame, respectively, for separating the first nozzles and the second nozzles. The spray port of each first nozzle and each second nozzle faces the welded joint, and the other end is in communication with the cooling medium pipeline outside. Specifically, the plurality of first nozzles and the plurality of second nozzles corresponding to the toe and the waist are divided by the partition plates B1 and B1'. The area above the partition plates B1 and B1' is arranged corresponding to the toe tread and the toe side of the welded joint of the steel rail, and the area below the partition plates B1 and B1' is arranged corresponding to the waist area of the welded joint of the steel rail. In the figure, the cooling nozzles A1 and A1', A2 and A2', A3 and A3' are a plurality of first nozzles applied to the toe side and the toe tread of the welded joint of the steel rail, and these first nozzles are symmetrical about the center line D of the vertical direction of the steel rail. In the present application, the same flow / pressure of compressed air is introduced into these first nozzles to cool the toe area of the welded joint of the steel rail, so as to achieve the same cooling effect on the toe heat-affected zone of the steel rail. The cooling nozzles C1 and C1', C2 and C2', C3 and C3', C4 and C4' are second nozzles applied to the waist of the welded joint of the steel rail, which are longitudinally arranged at an interval of 10 mm, and these second nozzles are symmetrical about the center line D of the vertical direction of the steel rail. In the present application, the same flow / pressure of water mist mixture is introduced into these second nozzles to cool the waist area of the welded joint of the steel rail, so as to achieve the same cooling effect on the waist heat-affected zone of the steel rail. It should be added that the geometric height, number and position of the first nozzles, the second nozzles and the partition plates in the figure can be designed and adjusted according to the height difference of the steel rail to be processed. B1 and B1' are steel partition plates with a thickness of 1 mm, and positioning holes are arranged on the side surface of the partition plate for inserting the partition plate. When the partition plate is inserted into contact with the lower jaw of the toe of the welded joint of the steel rail, it can play a role in isolating the toe and the waist. Wherein D is the center line of the vertical direction of the steel rail, i.e. the steel rail is symmetrical about the center line on both sides.

[0039] In some embodiments, the plurality of first nozzles are 25-35 mm away from the rail head of the welded joint, and the jetting pressure is 0.15-0.25 MPa; the plurality of second nozzles are 45-55 mm away from the welded joint, and the jetting pressure is 0.40-0.50 MPa. This satisfies the requirement that the rail head heat-affected zone in the second stage cooling is cooled by compressed air or water mist mixed gas with a relatively slow cooling rate, and the rail waist heat-affected zone is cooled by compressed air or water mist mixed gas with a relatively fast cooling rate.

[0040] In some embodiments, in step S100, the first stage cooling is natural cooling in an air environment of 20-30°C. It should be noted that the cooling speed of the first stage cannot be too high to prevent the formation of a large area of martensite in the welded heat-affected zone due to the cooling speed being too fast and the formation of micro-cracks due to excessive internal stress of the structure. When micro-cracks form inside the structure, the micro-cracks cannot be eliminated by subsequent post-weld heat treatment. During the natural cooling process, there is a situation that the cooling speed is fast at a high temperature stage and relatively slow at a low temperature stage, and the cooling speed at each temperature stage is different. Specifically, after the completion of the rail welding, due to the effects of convection, radiation, and heat conduction with the surrounding medium, the rail welded joint is naturally cooled in an air environment of 20-30°C, which has the phenomenon that the cooling speed is fast at a high temperature stage and gradually slows down at a low temperature stage. The average cooling rate of natural cooling in the temperature range of 1000-801°C is 3.0-2.5°C / s. The cooling rate of natural cooling in the temperature range of 800-501°C is 2.5-1.2°C / s. The cooling rate of natural cooling in the temperature range of 500-301°C is 1.2-0.7°C / s. In the temperature range of 300-20°C, the cooling rate of natural cooling is 0.7-0.1°C / s. At the same time, without considering the composition segregation of the rail steel, the natural cooling of the rail welded joint after the completion of the post-welding in the air and the natural cooling after the completion of the normalizing heating in the air usually does not cause the formation of brittle and hard martensite in the heat-affected zone. The first stage cooling is air cooling, but the temperature drop speed is still fast, and the reason is that the temperature difference between the welded joint and the air is large.

[0041] In some embodiments, in step S200, the full-section normalizing heating is implemented by using an induction profiled electric heating coil, which is arranged around the circumference of the welded joint at a distance of 5mm-50mm from the surface of the welded joint, and has a length of 60mm-80mm along the extension direction of the rail. In the process of induction heating, the weld of the rail joint is located at the center of the induction heating coil, and the induction profiled electric heating coil controls the temperature rising rate of the full section of the welded joint to be 1.0℃ / s-20.0℃ / s. By using the induction profiled electric heating coil for heating, the uniform heating of the full section of the welded joint can be ensured, and the problems of local overheating or uneven heating that may occur in the traditional heating mode can be avoided. The temperature rising rate of 1.0℃ / s-20.0℃ / s in the heating process helps to reduce the growth of austenite grains, maintain the fine-grained structure of the material, and thus improve the toughness and strength of the welded joint.

[0042] In some embodiments, in step S300, the open cooling temperature of the second stage cooling is not lower than 920℃, which aims to provide sufficient phase transformation driving force and provide the thermodynamic conditions for the phase transformation of pearlite to improve the mechanical properties of the welded joint in the subsequent third cooling stage. Specifically, after the full-section normalizing heating is completed, the subsequent natural cooling and the process of possible artificial transfer of the welded joint will inevitably cause a temperature drop of 10-20℃. In addition, it should be noted that when the normalizing temperature is too high (e.g., higher than 1100℃), the grains in the heated area (especially the weld and the adjacent coarse-grained heat-affected zone) will be too large, which will affect the impact toughness of the subsequent joint. When the normalizing heating temperature is low, such as heating the surface temperature of the welded joint to 900℃ or below, the phase transformation driving force during the cooling process will be low, which will result in an unobvious microstructure refinement effect during the cooling process, and the hardness and impact toughness of the rail welded heat-affected zone after heat treatment will not be improved ideally. When the normalizing heating temperature is lower than 850℃, the austenitizing process in the heat-affected zone of the welded joint will not be complete, which will result in insufficient phase transformation during the subsequent cooling process, and thus the hardness and impact toughness of the rail welded heat-affected zone after heat treatment will not be improved ideally. Based on this, the open cooling temperature of the second stage cooling is not lower than 920℃, which can make the entire welded joint reach the austenitizing temperature. This process can redissolve the network of cementite and other undesirable structures in the weld, and homogenize the structure of the weld and the base material, thereby improving the overall performance of the welded joint.

[0043] In some embodiments, in step S400, the third stage cooling is natural cooling in an air environment of 20-30℃. The entire welded joint is ensured to reach room temperature, and the microstructure morphology is fixed, so that the treated welded joint has good comprehensive mechanical properties.

[0044] In some embodiments, as shown in FIG. 2, the method comprises the following steps: Figure 2As shown, the welded joint is formed by welding two same heat-treated type of ferrite-pearlite steel rails a with the same rail type and the specification of 60-75 kg / m by using the moving flash welding machine, and the weld c is formed between the two same heat-treated type of ferrite-pearlite steel rails.

[0045] In some embodiments, the heat-treated type of ferrite-pearlite steel rail has the tensile strength of 1280-1360 MPa, the hardness of 350-400 HV and the impact energy of 10-18 J at 20-30 ℃, and the chemical composition of the heat-treated type of ferrite-pearlite steel rail contains the C element with the mass fraction of 0.75%-0.84%, the Si element with the mass fraction of 0.45%-0.90%, the Mn element with the mass fraction of 0.8%-1.2%, the Cr element with the mass fraction of 0.30%-0.60% and the V element with the mass fraction of 0.03%-0.12%, and the rest is iron and inevitable impurities.

[0046] Specifically, in the present application, the critical cooling rate of the martensite transformation in the continuous cooling process of the heat-treated type of ferrite-pearlite steel rail is 2.3-2.8 ℃ / s, and the Ms temperature (the start temperature of the martensite formation) of the steel rail is 270-320 ℃. In order to avoid the abnormal structure such as the martensite in the heat-affected zone of the steel rail welded joint, when the post-weld heat treatment is performed on the heat-treated type of ferrite-pearlite steel rail welded joint, the final cooling temperature in the rapid cooling process of the post-weld heat treatment needs to be controlled to be higher than the Ms temperature of the steel rail. When the final cooling temperature is slightly higher than the Ms temperature, in order to avoid the abnormal structure such as the martensite in the heat-affected zone of the steel rail welded joint, the cooling rate at this stage needs to be lower than the critical cooling rate of the martensite transformation in the continuous cooling process of the steel rail, otherwise the joint will be prone to premature fatigue fracture due to a large amount of quenched martensite. In some national steel rail welding standards, such as the Australian steel rail welding standard AS1085.20-2012, for some high-strength grades, high-carbon content and high-alloy content steel rails, under the observation magnification of 100x of the metallographic microscope, the percentage content of the martensite structure in the most serious area of the martensite in the steel rail welded joint should not be higher than 5%, otherwise the joint will be prone to premature fatigue fracture due to a large amount of quenched martensite structure, which seriously affects the safety of railway operation. Therefore, strictly controlling the content of the martensite in the steel rail welded structure is crucial for the stable operation of the railway line.

[0047] In the application, the final cooling temperature on both sides of the weld seam of the rail head and rail waist of the welded joint after the second cooling stage is significantly higher than the martensite transformation start temperature (Ms temperature) of the eutectoid pearlitic rail steel, aiming to avoid the formation of brittle and hard martensite in the welding heat affected zone due to improper operation in the cooling process. At the same time, the cooling rate of the heat affected zone of the rail head and rail waist of the welded joint of the second cooling stage heat treatment type eutectoid pearlitic rail is controlled to be above 3.0℃ / s, aiming to refine the pearlite interlamellar spacing by rapid cooling of the supercooled austenite, and improve the mechanical properties of the heat affected zone. For the third stage cooling, in order to avoid the generation of quenched martensite structure in the joint heat affected zone during the cooling process, the application preferably adopts the natural cooling mode, and the surface temperature of the rail welded joint is reduced to 20~30℃ by selecting the natural cooling mode below the critical cooling rate of the martensite transformation of the two kinds of rail steels.

[0048] In the application, the eutectoid pearlitic rail welding heat affected zone above the austenitizing temperature is rapidly cooled by spraying compressed air or water mist mixed gas, which can refine the pearlite interlamellar spacing and improve the mechanical properties of the welded joint. Specifically, the gas pressure sprayed by the cooling assembly can be adjusted by adjusting the number and aperture size of the nozzles to meet the cooling effect of different cooling intensities on the heat affected zone of the rail head and rail waist of the rail welded joint. In the application, the compressed air and / or water mist mixed gas sprayed by the cooling assembly has the same cooling speed / cooling capacity.

[0049] The high-carbon pearlitic rail flash-welded joint heat treatment method provided by the application processes the rail welded joint into a longitudinal hardness sample according to the TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. Figure 2 The longitudinal Vickers HV hardness of the joint is detected at the position 5mm below the tread, and the measuring points are symmetrically arranged to the left and right sides with the weld seam as the center, and the interval is 2mm. According to the sampling position d shown in the figure, the metallographic structure of the joint is tested according to the GB / T13298-2015 "Metal Microstructure Test Method". 3% nitric acid alcohol solution is used for etching the metallographic sample, and Leica MeF3 optical microscope is used for metallographic structure observation. Figure 4

[0050] The high-carbon pearlitic rail flash-welded joint heat treatment method provided by the application will be further described below in combination with specific examples:

[0051] Example 1

[0052] ​The application controls the tensile, impact performance and hardness of a heat-treated proeutectoid ferrite pearlite rail base at room temperature (20-30℃), wherein the tensile strength of the rail base is 1280MPa, the hardness is 350HV, and the impact energy is 18J. The chemical composition of the rail steel with the mechanical properties needs to meet the following conditions: 0.75% content of C, 0.45% content of Si, 0.80% content of Mn, 0.30% content of Cr, 0.03% content of V, and the balance of Fe and inevitable impurities.

[0053] After the top forging and the push of the rail head of the heat-treated proeutectoid ferrite pearlite rail with the specification of 60kg / m in the process of the moving flash welding, the post-weld heat treatment of the welded joint is carried out. First, the surface temperature of the welded joint of the rail at 800℃ is placed in the air to carry out the first stage cooling (natural cooling) to reduce the surface temperature of the rail head of the welded joint to 200℃, and then the full section of the rail welded joint area is heated by using the intermediate frequency induction profiled electric heating coil. When the surface temperature of the rail welded joint reaches 980℃, the heating is stopped, and the subsequent natural cooling and the process of artificially transferring the welded joint will inevitably cause the temperature to drop by 10-20℃. When the surface temperature of the rail welded joint drops to 960℃, the cooling device is used to cool the rail head and the rail waist heat affected zone of the rail welded joint at different intensities by using compressed air (or water mist mixed gas) as the cooling medium. Among them, by adjusting the pressure and the distance to the rail of the multiple first nozzles and the multiple second nozzles, the cooling speed of the rail head heat affected zone of the welded joint is controlled to be 5.0℃ / s, and the final cooling temperature is 350℃. The cooling speed of the rail waist heat affected zone of the welded joint is 10.0℃ / s, and the final cooling temperature is 450℃. After the second stage cooling is completed, the cooling device is removed, and the full section of the heat affected zone of the welded joint is cooled in the air environment to the ambient temperature of 20-30℃ in the third stage cooling (natural cooling), so as to complete the post-weld heat treatment process of the rail involved in the application. In this stage, the cooling rate of the welded joint is 0.8-0.2℃ / s.

[0054] For the cooling involved in the above post-weld heat treatment process of the rail, the first stage cooling is the natural cooling in the air. The second stage cooling is the cooling of the rail head and the rail waist heat affected zone of the rail welded joint at different intensities by using the cooling device with compressed air (or water mist mixed gas) as the cooling medium. In the second stage cooling process, the distance between the spray port of the first nozzle and the surface of the welded joint is 30mm, the gas pressure of the sprayed compressed air or water mist mixed gas is 0.25MPa, the distance between the spray port of the second nozzle and the surface of the welded joint is 50mm, and the pressure of the sprayed compressed air or water mist mixed gas is 0.50MPa. The third stage cooling is the natural cooling of the full section of the heat affected zone of the welded joint in the air. In the whole post-weld heat treatment process, the infrared temperature detector is used to monitor the temperature of the rail head tread of the rail welded joint.

[0055] The rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard TB / T 1632.2-2014 "Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the hardness of the joint is measured according to the standard GB / T 230.1-2009 "Metallic Materials - Hardness Test - Part 1: Vickers, Knoop and Michell Hardness Test". Figure 2 The longitudinal Vickers HV hardness of the joint is measured at a position 5mm below the tread, and the measuring points are symmetrically arranged on both sides of the weld with a spacing of 2mm. According to the standard GB / T 13298-2015 "Metallic Microstructure Test Method", the metallographic structure of the joint is tested according to the sampling position d shown in the figure. The metallographic sample is etched with 3% nitric acid alcohol solution, and the metallographic structure is observed with a Leica MeF3 optical microscope. Figure 4 The longitudinal Vickers HV hardness of the joint is measured at a position 5mm below the tread, and the measuring points are symmetrically arranged on both sides of the weld with a spacing of 2mm. According to the standard GB / T 13298-2015 "Metallic Microstructure Test Method", the metallographic structure of the joint is tested according to the sampling position d shown in the figure. The metallographic sample is etched with 3% nitric acid alcohol solution, and the metallographic structure is observed with a Leica MeF3 optical microscope.

[0056] The steel rail welded joint obtained by the present embodiment has a longitudinal hardness of the rail welded heat-affected zone within the range of ±30mm from the center of the weld, which is 95% of the average hardness of the corresponding heat-treated eutectoid pearlite rail base material. At room temperature, the average impact energy of the rail head weld of the welded joint is 17J, and the average impact energy of the rail waist weld of the welded joint is 13J, which is much higher than the requirement of ≥6.5J specified in TB / T 1632.2-2014. In addition, under the observation of the metallographic microscope, there is no martensite and continuous network of cementite in the heat-affected zone on both sides of the weld of the rail welded joint. The present application helps to improve the "saddle-type" wear caused by the low hardness of the welded area during the service of the rail welded joint on the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0057] Example 2

[0058] The tensile, impact performance and hardness of a heat-treated eutectoid pearlite rail base material at room temperature (20-30℃) are controlled, wherein the tensile strength of the rail base material is 1280MPa, the hardness is 350HV, and the impact energy is 18J. The chemical composition of the rail steel with the mechanical properties needs to meet the following conditions: 0.75% content of C, 0.45% content of Si, 0.80% content of Mn, 0.30% content of Cr, 0.03% content of V, and the balance of Fe and unavoidable impurities.

[0059] After the upsetting and the push of the rail head of the heat treated eutectoid pearlite rail with the gauge of 60kg / m in the process of the moving flash butt welding, the post-weld heat treatment of the welded joint is carried out. Firstly, the welded joint with the surface temperature of 800℃ is placed in the air to carry out the first stage cooling (natural cooling) to reduce the surface temperature of the rail head of the welded joint to 200℃, and then the full section of the welded joint area is heated by using the intermediate frequency induction profiled electric heating coil. When the surface temperature of the welded joint reaches 940℃, the heating is stopped, and the subsequent natural cooling and the manual transfer of the welded joint will inevitably cause the temperature drop of 10~20℃. When the surface temperature of the welded joint drops to 920℃, the cooling device is used to cool the rail head and the rail waist heat affected zone of the welded joint by using the compressed air (or the water mist mixed gas) as the cooling medium. The cooling speed of the rail head heat affected zone of the welded joint is controlled to be 3.0℃ / s, and the final cooling temperature is 400℃. The cooling speed of the rail waist heat affected zone of the welded joint is 8.0℃ / s, and the final cooling temperature is 500℃. After the second stage cooling is completed, the cooling device is removed, and the full section of the heat affected zone of the welded joint is cooled in the air environment to the ambient temperature of 20~30℃, so that the post-weld heat treatment process of the rail involved in the present application is completed. In this stage, the cooling rate of the welded joint is 0.8~0.2℃ / s.

[0060] For the cooling involved in the above-mentioned post-weld heat treatment process of the rail, the first stage cooling is the natural cooling in the air. The second stage cooling is the cooling of the rail head and the rail waist heat affected zone of the welded joint by using the cooling device with the compressed air (or the water mist mixed gas) as the cooling medium. In the second stage cooling process, the distance between the spray port of the first nozzle and the surface of the welded joint is 30mm, and the gas pressure of the sprayed compressed air or water mist mixed gas is 0.15MPa. The distance between the spray port of the second nozzle and the surface of the welded joint is 50mm, and the pressure of the sprayed compressed air or water mist mixed gas is 0.40MPa. The third stage cooling is the natural cooling of the full section of the heat affected zone of the welded joint in the air. In the whole post-weld heat treatment process, the infrared temperature detector is used to monitor the temperature of the rail head tread of the welded joint.

[0061] The rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the hardness of the joint is measured according to the standard of GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at the position of 5mm below the tread, and the measuring points are symmetrically arranged to the left and right sides of the weld as the center with the interval of 2mm. According to the standard of GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature", the hardness of the joint is measured according to the standard of GB / T 230.1-2009 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 4The sampling position d shown is used for metallographic examination of the joint according to GB / T13298-2015 "Metal Microstructure Test Method". A 3% nitric acid alcohol solution is used for etching of the metallographic sample, and a Leica MeF3 optical microscope is used for metallographic observation.

[0062] The steel rail welded joint obtained by the present embodiment has a longitudinal hardness of the rail welded heat-affected zone within a region of ±30 mm from the center of the weld reaching 93% of the average hardness of the heat-treated proeutectoid ferrite steel rail base material. At room temperature, the average impact energy of the rail head weld of the welded joint is 16 J, and the average impact energy of the rail waist weld of the welded joint is 11 J, which is much higher than the ≥6.5 J specified in TB / T 1632.2-2014. In addition, under the observation of a metallographic microscope, there is no martensite and continuous network of cementite in the heat-affected zone on both sides of the weld of the steel rail welded joint. The present application helps to improve the "saddle-type" wear caused by the low hardness of the welded area during the service of the rail welded joint on the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0063] Example 3

[0064] The tensile, impact performance and hardness of a heat-treated proeutectoid ferrite steel rail base material at room temperature (20-30℃) are controlled, wherein the tensile strength of the rail base material is 1360 MPa, the hardness is 400 HV, and the impact energy is 10 J. The chemical composition of the steel rail steel with the mechanical properties needs to meet the following conditions: 0.84% content of C, 0.90% content of Si, 1.20% content of Mn, 0.60% content of Cr, 0.12% content of V, and the balance of Fe and unavoidable impurities.

[0065] The heat-treated eutectoid pearlite rail with a gauge of 68 kg / m is subjected to post-weld heat treatment after the top forging and the push of the protrusion during the moving flash welding process. First, the rail welded joint with a surface temperature of 800℃ is placed in the air to perform the first stage cooling (natural cooling) to reduce the surface temperature of the rail welded joint to 200℃, and then the full section of the rail welded joint area is heated by using the intermediate frequency induction profiled electric heating coil. When the surface temperature of the rail welded joint reaches 980℃, the heating is stopped, and then the natural cooling and the artificial transfer of the welded joint will inevitably cause a temperature drop of 10-20℃. When the surface temperature of the rail welded joint drops to 960℃, the cooling device is used to cool the rail head and the rail waist heat affected zone of the rail welded joint with different intensities by using compressed air (or water mist mixed gas) as the cooling medium. Among them, by adjusting the pressure and the distance to the rail of the multiple first nozzles and the multiple second nozzles, the cooling speed of the rail head heat affected zone of the welded joint is controlled to be 5.0℃ / s, and the final cooling temperature is 350℃. The cooling speed of the rail waist heat affected zone of the welded joint is 10.0℃ / s, and the final cooling temperature is 450℃. After the second stage cooling is completed, the cooling device is removed, and the full section of the heat affected zone of the welded joint is subjected to the third stage cooling (natural cooling) in the air environment to the ambient temperature of 20-30℃, so as to complete the rail post-weld heat treatment process involved in the present application. In this stage, the cooling rate of the welded joint is 0.8-0.2℃ / s.

[0066] For the cooling involved in the above rail post-weld heat treatment process, the first stage cooling is the natural cooling in the air. The second stage cooling is the cooling of the rail head and the rail waist heat affected zone of the rail welded joint with different intensities by using the cooling device with compressed air (or water mist mixed gas) as the cooling medium. In the second stage cooling process, the distance between the spray port of the first nozzle and the surface of the welded joint is 30mm, the gas pressure of the sprayed compressed air or water mist mixed gas is 0.25MPa, the distance between the spray port of the second nozzle and the surface of the welded joint is 50mm, and the pressure of the sprayed compressed air or water mist mixed gas is 0.50MPa. The third stage cooling is the natural cooling of the full section of the heat affected zone of the welded joint in the air. During the whole post-weld heat treatment process, the infrared temperature detector is used to monitor the temperature of the rail head tread of the welded joint.

[0067] The rail welded joint is processed into a longitudinal hardness specimen. The joint is processed into a longitudinal hardness specimen according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding". According to GB / T 230.1-2009, the hardness of the rail welded joint is measured according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at the position of 5mm below the tread according to the schematic diagram, and the measuring points are symmetrically arranged to the left and right sides with the weld as the center, and the interval is 2mm. According to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding", the hardness of the rail welded joint is measured according to the standard of GB / T 230.1-2009. Figure 4The sampling position d shown is used for metallographic examination of the joint according to GB / T13298-2015 "Metal Microstructure Test Method". A 3% nitric acid alcohol solution is used for etching of the metallographic sample, and a Leica MeF3 optical microscope is used for metallographic observation.

[0068] The steel rail welded joint obtained by the present embodiment has a longitudinal hardness of the rail welded heat-affected zone within a region of ±30 mm from the center of the weld that is 91% of the average hardness of the heat-treated proeutectoid ferrite steel rail base material. At room temperature, the average impact energy of the rail head weld of the welded joint is 14 J, and the average impact energy of the rail waist weld of the welded joint is 10 J, which is much higher than the ≥6.5 J specified in TB / T 1632.2-2014. In addition, under observation by a metallographic microscope, there is no martensite and continuous network of cementite in the heat-affected zone on both sides of the weld of the steel rail welded joint. The present application helps to improve the "saddle-type" wear caused by low hardness in the welded area during the service of the rail welded joint on the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0069] Example 4

[0070] The tensile, impact properties and hardness of a heat-treated proeutectoid ferrite steel rail base material at room temperature (20-30℃) are controlled, wherein the tensile strength of the rail base material is 1320 MPa, the hardness is 375 HV, and the impact energy is 14 J. The chemical composition of the steel rail steel with the mechanical properties needs to meet the following conditions: 0.80% content of C, 0.68% content of Si, 1.00% content of Mn, 0.45% content of Cr, 0.05% content of V, and the balance of Fe and unavoidable impurities.

[0071] After the upsetting and the push-out of the rail head of the heat treated eutectoid pearlite rail with the gauge of 68kg / m during the moving flash butt welding process, the post-weld heat treatment of the welded joint is carried out. Firstly, the welded joint with the surface temperature of 800℃ is placed in the air to carry out the first stage cooling (natural cooling) to reduce the surface temperature of the rail head of the welded joint to 200℃, and then the full section of the welded joint area is heated by using the intermediate frequency induction profiled electric heating coil. When the surface temperature of the welded joint reaches 960℃, the heating is stopped, and then the natural cooling and the manual transfer of the welded joint will inevitably cause the temperature drop of 10-20℃. When the surface temperature of the welded joint drops to 940℃, the cooling device is used to cool the rail head and the rail waist heat affected zone of the welded joint by using compressed air (or water mist mixed gas) as the cooling medium. The cooling speed of the rail head heat affected zone of the welded joint is controlled to be 5.0℃ / s, and the final cooling temperature is 350℃. The cooling speed of the rail waist heat affected zone of the welded joint is 10.0℃ / s, and the final cooling temperature is 450℃. After the second stage cooling is completed, the cooling device is removed, and the full section of the heat affected zone of the welded joint is cooled in the air environment to the ambient temperature of 20-30℃, so that the post-weld heat treatment process of the rail involved in the application is completed. In this stage, the cooling rate of the welded joint is 0.8-0.2℃ / s.

[0072] For the cooling involved in the above-mentioned post-weld heat treatment process of the rail, the first stage cooling is the natural cooling in the air. The second stage cooling is the cooling of the rail head and the rail waist heat affected zone of the welded joint by using the cooling device with compressed air (or water mist mixed gas) as the cooling medium. In the second stage cooling process, the distance between the jet port of the first nozzle and the surface of the welded joint is 30mm, the gas pressure of the compressed air or the water mist mixed gas jetted by the first nozzle is 0.25MPa, the distance between the jet port of the second nozzle and the surface of the welded joint is 50mm, and the pressure of the compressed air or the water mist mixed gas jetted by the second nozzle is 0.50MPa. The third stage cooling is the natural cooling of the full section of the heat affected zone of the welded joint in the air. During the whole post-weld heat treatment process, the infrared temperature detector is used to monitor the temperature of the rail head tread of the welded joint.

[0073] The rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the hardness of the joint is measured according to the standard of GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at the position of 5mm below the tread, and the measuring points are symmetrically arranged to the left and right sides of the weld as the center with the interval of 2mm. According to the standard of GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature", the hardness of the joint is measured according to the standard of GB / T 230.1-2009 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 4The sampling position d shown is used for metallographic examination of the joint according to GB / T13298-2015 "Metal Microstructure Test Method". A 3% nitric acid alcohol solution is used for etching of the metallographic sample, and a Leica MeF3 optical microscope is used for metallographic observation.

[0074] The steel rail welded joint obtained by the present embodiment has a rail welded heat-affected zone in the region of ±30mm from the center of the weld, and the longitudinal hardness reaches 93% of the average hardness of the corresponding heat-treated proeutectoid pearlite rail base material. At room temperature, the average impact energy of the rail head weld of the welded joint is 16J, and the average impact energy of the rail waist weld of the welded joint is 12J, which is much higher than the ≥6.5J specified in TB / T 1632.2-2014. In addition, under the observation of a metallographic microscope, there is no martensite and continuous network of cementite in the heat-affected zone on both sides of the weld of the steel rail welded joint. The present application helps to improve the "saddle-type" wear caused by the low hardness of the welded area during the service of the rail welded joint on the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0075] Comparative Example 1

[0076] The steel rail material selection, mechanical properties of the rail base material, and process conditions involved in the heating and cooling process of the post-weld normalizing of the steel rail welded joint in the present comparative example are consistent with those in Example 1, the only difference being that the normalizing temperature of the steel rail welded joint in the present comparative example is 900℃, which is lower than the steel rail post-weld full-section normalizing heating process range of the present application.

[0077] The steel rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to TB / T 1632.2-2014 "Steel Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the longitudinal hardness of the joint is measured according to the following method: Figure 2 The schematic diagram shows the longitudinal Vickers HV hardness detection of the joint at a position 5mm below the tread. The measuring points are symmetrically arranged on both sides of the weld center with a spacing of 2mm. According to GB / T 230.1-2009, the longitudinal hardness of the joint is measured according to the following method: Figure 4 The sampling position d shown is used for metallographic examination of the joint according to GB / T13298-2015 "Metal Microstructure Test Method". A 3% nitric acid alcohol solution is used for etching of the metallographic sample, and a Leica MeF3 optical microscope is used for metallographic observation.

[0078] The rail welded joint obtained from the present comparative example has a normalizing heating temperature of 900 °C, which is lower than the process method of the present application. This results in insufficient driving force for the pearlite phase transformation during the supercooling process and the subsequent cooling process after the normalizing heating of the rail welded joint, and further results in the mechanical properties of the heat-affected zone on both sides of the weld of the rail welded joint being lower than the beneficial effect obtained by using the process method of the present application. At the same time, the network cementite in the web region of the rail welded joint cannot be completely eliminated. The hardness test shows that the longitudinal hardness of the rail welded joint heat-affected zone in the region of ± 30 mm from the center of the weld is only 89% of the average hardness of the corresponding heat-treated proeutectoid pearlite rail base material, which is lower than the hardness of 91% to 95% of the average hardness of the corresponding rail base material obtained by the present application. At the same time, at room temperature, the average impact energy of the weld of the rail head of the welded joint is 15 J, and the average impact energy of the weld of the rail web of the welded joint is 10 J. The rail welded joint obtained in the present comparative example has obvious network cementite in the web region and the hardness of the heat-affected zone is lower than the beneficial effect obtained by using the present application, which is not conducive to the safety of railway operation.

[0079] Comparative Example 2

[0080] The steel rail material selection, mechanical properties of the rail base material, welding cooling process, normalizing heating temperature after welding and cooling speed during the normalizing cooling stage in the present comparative example are all consistent with those in Example 1. The difference is that the final cooling temperature of the rail head heat-affected zone of the rail welded joint during the second stage cooling process after normalizing heating in the present comparative example is 260 °C, which is lower than the control period of the final cooling temperature of the rail head heat-affected zone of the rail welded joint during the second stage cooling process according to the present application.

[0081] The rail welded joint was processed into a longitudinal hardness specimen. The joint was processed into a longitudinal hardness specimen according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the hardness of the joint was tested according to the standard of GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 2 The longitudinal Vickers HV hardness of the joint was tested at a position 5 mm below the tread, and the measuring points were symmetrically arranged on both sides of the weld center with a spacing of 2 mm. According to the sampling position d shown in the drawing, the metallographic structure of the joint was tested according to the standard of GB / T 13298-2015 "Metallic Material Microstructure Test Method". The metallographic specimen was etched with 3% nitric acid alcohol solution, and the metallographic structure was observed by Leica MeF3 optical microscope. Figure 4

[0082] ​The rail welded joint obtained from the present comparative example, because the final cooling temperature of the rail head heat affected zone in the second cooling stage after the post-weld normalizing heating of the welded joint is 260℃, which is lower than the Ms temperature (the start temperature of martensite formation) of the heat-treated ferrite-pearlite steel rail steel involved in the present application, resulting in the formation of brittle and hard martensite in the rail head heat affected zone of the rail welded joint. And because the final cooling temperature of the rail waist heat affected zone is too high, martensite is not formed. In the present comparative example, brittle and hard martensite structure is formed in the rail head heat affected zone of the rail welded joint. In addition, the longitudinal hardness of the rail welded heat affected zone within the range of ±30mm from the center of the weld reaches 96% of the average hardness of the heat-treated ferrite-pearlite steel rail base material, the average impact energy of the rail head weld of the welded joint at room temperature is 12J, and the average impact energy of the rail waist weld of the welded joint is also 12J. Because the process method of the present application is not used, brittle and hard martensite appears in the rail head heat affected zone of the rail welded joint obtained from the present comparative example, which is lower than the beneficial effect obtained by using the process method of the present application, and is not conducive to the safety of railway operation.

[0083] Comparative Example 3

[0084] The rail material selection, mechanical properties of the rail base material, rail welding process conditions, heating temperature during post-weld normalizing heating, cooling method and steps of the first stage cooling and final cooling temperature of the second cooling stage, cooling method and steps of the third cooling stage, cooling speed, etc. in the present comparative example are consistent with those in Example 3, except that the cooling speed corresponding to the water mist mixed gas sprayed by the cooling device in the rail waist area of the rail welded joint during the second stage cooling process after the normalizing heating of the rail welded joint in the present comparative example is 7.0℃ / s, which is lower than the cooling speed control range of the cooling device in the rail waist area of the rail welded joint during the second stage cooling process after the post-weld normalizing heating of the rail welded joint in the present application.

[0085] The rail welded joint was processed into a longitudinal hardness specimen. The joint was processed into a longitudinal hardness specimen according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Butt Welding". According to GB / T 230.1-2009, the hardness of the joint was measured according to the standard of GB / T 230.1-2009 "Metallic Materials - Brinell Hardness Test - Part 1: Test Method". Figure 2 The longitudinal Vickers HV hardness of the joint was detected at a position 5mm below the tread, and the measuring points were symmetrically arranged on both sides of the weld center with a spacing of 2mm. According to the sampling position d shown in the drawing, the metallographic structure of the joint was tested according to the standard of GB / T13298-2015 "Metallic Microstructure Test Method". 3% nitric acid alcohol solution was used for etching of the metallographic specimen, and Leica MeF3 optical microscope was used for metallographic structure observation. Figure 4

[0086] ​The rail welded joint obtained from the present comparative example has a corresponding cooling speed of 7.0°C / s of the water mist mixed gas sprayed by the rail waist area cooling device of the rail welded joint, which is lower than the cooling speed control interval of the rail waist area cooling device of the rail welded joint in the second stage cooling process after the post-weld normalizing heating of the present application, resulting in that the net-like cementite in the rail waist heat affected zone of the rail welded joint cannot be completely eliminated, which is lower than the beneficial effect obtained by using the process method of the present application. In the present comparative example, the longitudinal hardness of the rail welded heat affected zone in the area of ±30mm from the center of the weld is 91% of the average hardness of the corresponding heat-treated proeutectoid ferrite rail base material. At room temperature, the average impact energy of the weld seam of the rail head of the welded joint is 14J, and the average impact energy of the weld seam of the rail waist of the welded joint is 8J. Since the net-like cementite in the rail waist heat affected zone of the rail welded joint cannot be completely eliminated, the microstructure and properties of the welded joint obtained in the present comparative example are lower than the beneficial effect obtained by using the process method of the present application, which is not conducive to the safety of railway operation.

[0087] Comparative Example 4

[0088] The rail material selection, rail base material mechanical properties and rail flash welding conditions in the present comparative example and example 4 are consistent, except that no subsequent normalizing heating and cooling process is carried out after the rail welding in the present comparative example, i.e. the rail welded joint obtained by flash welding is directly naturally cooled to an ambient temperature of 30°C in air, so as to obtain the rail welded joint in the present comparative example.

[0089] The rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding". According to GB / T 230.1-2009, the longitudinal hardness of the joint is measured according to the standard of TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at a position 5mm below the tread, and the measuring points are symmetrically arranged to the left and right sides with the weld as the center, and the interval is 2mm. According to the sampling position d shown in the drawing, the metallographic structure of the joint is tested according to the standard of GB / T 13298-2015 "Metal Microstructure Test Method". 3% nitric acid alcohol solution is used for etching of the metallographic sample, and Leica MeF3 optical microscope is used for metallographic structure observation. Figure 4

[0090] ​The longitudinal hardness of the rail welded heat-affected zone of the rail welded joint obtained from the present comparative example in the area of ±30 mm from the center of the weld reaches 87% of the average hardness of the heat-treated proeutectoid pearlite rail base material. The average impact energy of the rail head weld of the rail welded joint at room temperature is 11 J, and the average impact energy of the rail waist weld of the rail welded joint is 9 J. Under the observation of a metallographic microscope, a significant amount of continuous network cementite appears in the rail waist heat-affected zone of the rail welded joint, but no martensite appears. Due to the failure to use the process method of the present application, the performance of the rail welded joint is low, and a significant amount of network cementite appears in the rail waist heat-affected zone. The joint obtained in the present comparative example has lower performance than the beneficial effect obtained by using the process method of the present application, which is not conducive to the safety of railway operation.

[0091] Comparative Example 5

[0092] The rail material selection, mechanical properties of the rail base material, rail flash welding conditions, cooling process and cooling speed during the normalizing heating treatment and cooling process of the rail in the present comparative example are consistent with those in Example 1. The difference is that in the present comparative example, the final cooling temperature of the rail head and rail waist heat-affected zone during the second stage cooling process after normalizing heating of the rail welded joint is 350°C, which is lower than the final cooling temperature process control range of the present application.

[0093] The rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding". According to GB / T 230.1-2009, the hardness of the rail welded joint is measured according to the standard GB / T 230.1-2009 "Metallic Materials - Brinell Hardness Test - Part 1: Test Method". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at a position 5 mm below the tread, and the measuring points are symmetrically arranged to the left and right of the weld as the center with a spacing of 2 mm. According to the sampling position d shown in the drawing, the metallographic structure of the joint is tested according to GB / T 13298-2015 "Metallic Microstructure Test Method". A 3% nitric acid alcohol solution is used for etching the metallographic sample, and a Leica MeF3 optical microscope is used for metallographic structure observation. Figure 4

[0094] ​The longitudinal hardness of the rail welded heat-affected zone of the steel rail welded joint obtained from the present comparative example in the area of ± 30 mm from the center of the weld reaches 95% of the average hardness of the heat-treated proeutectoid pearlite rail base material. At room temperature, the average impact energy of the rail head weld of the welded joint is 17 J, and the average impact energy of the rail waist weld of the welded joint is 8 J. Due to the segregation in the rail waist area, a certain amount of point-like martensite structure appears in the rail waist heat-affected zone of the steel rail welded joint during the heat treatment cooling process, and the impact performance decreases. Due to the failure to adopt the process method of the present application, the impact performance of the rail waist area of the steel rail welded joint is low, and a certain amount of point-like martensite appears in the rail waist heat-affected zone. The joint obtained in the present comparative example has lower performance than the beneficial effect obtained by adopting the process method of the present application, which is not conducive to the safety of railway operation.

[0095] Comparative Example 6

[0096] The steel rail material selection, mechanical properties of the steel rail base material, steel rail flash welding conditions, post-weld normalizing heating treatment and cooling process of the steel rail in the present comparative example are consistent with those in Example 3. The difference is that in the present comparative example, the cooling speed of the rail head heat-affected zone of the steel rail welded joint during the second stage cooling process after normalizing heating is 2.0 ℃ / s, and the final cooling temperature is 350 ℃. The cooling speed of the rail waist heat-affected zone of the welded joint is 7.0 ℃ / s, and the final cooling temperature is 450 ℃. In the present comparative example, the cooling speed of the rail head and rail waist area of the steel rail welded joint during the second cooling stage is much lower than the process control interval of the present application.

[0097] The steel rail welded joint is processed into a longitudinal hardness sample. The joint is processed into a longitudinal hardness sample according to the standard TB / T 1632.2-2014 "Steel Rail Welding Part 2: Flash Welding". According to GB / T 230.1-2009, the hardness of the joint is measured according to the standard GB / T 228.1-2010 "Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature". Figure 2 The longitudinal Vickers HV hardness of the joint is detected at a position 5 mm below the tread, and the measuring points are symmetrically arranged to the left and right of the weld center with a spacing of 2 mm. According to the sampling position d shown in the drawing, the metallographic structure of the joint is tested according to the standard GB / T 13298-2015 "Metallic Microstructure Test Method". 3% nitric acid alcohol solution is used for etching of the metallographic sample, and Leica MeF3 optical microscope is used for metallographic structure observation. Figure 4 Figure 2 Figure 4 Figure 2 Figure 4

[0098] ​The longitudinal hardness of the rail welded joint obtained from the present comparative example in the region of ±30 mm from the center of the weld reaches 88% of the average hardness of the heat-affected zone of the parent material of the heat-treated eutectoid pearlite rail. The average impact energy of the rail head weld of the welded joint is 10 J, and the average impact energy of the rail waist weld of the welded joint is 8 J at room temperature. A large number of continuous network cementite appear in the heat-affected zone of the rail waist of the rail welded joint under the observation of the metallographic microscope, but no martensite appears. Due to the fact that the process method of the present application is not used, the performance of the rail welded joint is low, and a large number of network cementite appear in the heat-affected zone of the rail waist, the joint structure and performance obtained in the present comparative example are lower than the beneficial effects obtained by using the process method of the present application, which is not conducive to the safety of railway operation.

[0099] It can be known from the comparison between the examples and the comparative examples in the present application that: 1. By implementing controlled cooling of different intensities on the heat-affected zones on both sides of the rail head and rail waist weld of the eutectoid pearlite rail flash butt welded joint after welding, the heat-affected zones on both sides of the weld of the rail welded joint are free of martensite and continuous network cementite, and the heat-affected zone of the rail head of the rail welded joint retains a relatively high hardness, thereby ensuring the wear resistance of the rail welded joint; 2. The longitudinal hardness of the rail welded joint in the region of ±30 mm from the center of the weld reaches 91%-95% of the average hardness of the parent material of the rail. The average impact energy of the rail head weld of the welded joint is 14-17 J, and the average impact energy of the rail waist weld of the welded joint is 10-13 J at room temperature, which is much higher than the ≥6.5 J specified in TB / T 1632.2-2014. The present application helps to improve the "saddle-type" wear caused by the low hardness of the welded area of the rail welded joint during the service of the rail welded joint on the line, and the impact toughness of the joint is good, which helps to ensure the safety of railway operation.

[0100] The present application has obvious technical advantages and broad market prospects.

[0101] The technical features of the above examples can be combined arbitrarily. In order to make the description simple, all possible combinations of the technical features in the above examples are not described, but as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the present application.

[0102] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A heat treatment method for a flash-welded joint of a high-carbon pearlitic steel rail, characterized in that, Includes the following steps: S100 is used to perform the first stage of cooling on the welded joint of the rail, so that the surface temperature of the welded joint is reduced to 100-200℃. S200, after the first stage of cooling is completed, the welded joint is subjected to full-section normalizing heating, so that the surface temperature of the welded joint is raised to 940-980°C; S300, the welded joint that has completed full-section normalizing heating is subjected to a second stage of cooling; wherein, the cooling rate of the heat-affected zone of the rail head of the welded joint is controlled at 3.0~5.0℃ / s, and the final cooling temperature is 350~400℃; the cooling rate of the heat-affected zone of the rail web of the welded joint is controlled at 8.0~10.0℃ / s, and the final cooling temperature is 450~500℃; S400, the welded joint that has completed the second stage of cooling is subjected to a third stage of cooling, so that the temperature of the entire cross section of the welded joint is reduced to 20~30℃.

2. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 1, characterized in that, In step S300, the second stage of cooling is implemented using a cooling device, which includes a plurality of first nozzles arranged around the heat-affected zone of the rail head of the welded joint and a plurality of second nozzles arranged around the heat-affected zone of the rail web of the welded joint. The cooling medium sprayed by the first nozzles and the second nozzles is one or both of compressed air or water mist mixture.

3. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 2, characterized in that, The distance between the plurality of first nozzles and the rail head of the welded joint is 25~35mm, and the spraying pressure is 0.15~0.25MPa; the distance between the plurality of second nozzles and the welded joint is 45~55mm, and the spraying pressure is 0.40~0.50MPa.

4. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 1, characterized in that, In step S100, the first stage of cooling is natural cooling carried out in an air environment at 20~30°C.

5. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 1, characterized in that, In step S200, the full-section normalizing heating is carried out using an induction-based electric heating coil. The induction-based electric heating coil is arranged around the circumference of the welded joint, and the distance from the surface of the welded joint is 5mm to 50mm. The length of the induction-based electric heating coil along the extension direction of the rail is 60mm to 80mm. During the heating process, the induction-based electric heating coil controls the temperature rise rate of the full section of the welded joint to be 1.0℃ / s to 20.0℃ / s.

6. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 1, characterized in that, In step S300, the starting temperature of the second stage cooling is not lower than 920°C.

7. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 1, characterized in that, In step S400, the third stage of cooling is natural cooling carried out in an air environment at 20~30°C.

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

9. The heat treatment method for the flash-welded joint of high-carbon pearlitic steel rail according to claim 8, characterized in that, The heat-treated eutectoid pearlitic steel rail has a tensile strength of 1280~1360MPa, a hardness of 350~400HV, and an impact energy of 10~18J at 20~30℃. The chemical composition of the heat-treated eutectoid pearlitic steel rail contains 0.75%~0.84% ​​C, 0.45%~0.90% Si, 0.8%~1.2% Mn, 0.30%~0.60% Cr, and 0.03%~0.12% V, with the remainder being iron and unavoidable impurities.

10. The heat treatment method for the post-flash weld joint of high-carbon pearlitic steel rail according to any one of claims 1 to 8, characterized in that, The welded joint is a region with a length of 80 to 120 mm, including the weld and / or heat-affected zone, with the heat-affected zone located on both sides of the weld.

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