Heat treatment method for improving network cementite in steel rail welding heat affected zone

Abstract

The invention relates to the technical field of steel rail welding, in particular to a heat treatment method for improving network cementite in a steel rail welding heat affected zone, which comprises the following steps: A) cooling a welding joint which is formed by welding a hypereutectoid pearlite steel rail and has the surface temperature of 800-1000 DEG C; (B) the surface temperature of the welding joint is increased to 1000-1080 DEG C, and heating is stopped; c) cooling the welding joint; d) stopping heating when the surface temperature of the welding joint is increased to 930-980 DEG C; e) cooling the welding joint; the cooling rate and the final cooling temperature of a welding joint rail head heat affected zone and the cooling rate and the final cooling temperature of a welding joint rail waist heat affected zone are limited; and F) the welded joint is cooled for the fourth time to 20-30 DEG C. The method can improve the mechanical property of the steel rail, which is reduced due to welding, and eliminate a network cementite structure which possibly exists, so that the service performance of a steel rail welding joint is ensured.

Description

Technical Field

[0001] This invention relates to the field of rail welding technology, and in particular to a heat treatment method for improving the network cementite in the heat-affected zone of rail welding. Background Technology

[0002] Network cementite (carbides) is commonly found in large forgings or rolled workpieces with high carbon content. Network cementite is generally distributed in a network pattern along grain boundaries. It is brittle, reduces the mechanical properties of steel, and easily leads to quenching cracks, making it a structural defect that must be eliminated. For small and medium-sized parts, normalizing is relatively easy to destroy the cementite network structure. Heating the steel to the fully austenitic region followed by air cooling or spray cooling can suppress the precipitation of network carbides. For eutectoid and hypereutectoid steels with high carbon content, insufficient cooling can lead to the formation of thin, fine network cementite along the original austenite grain boundaries. For network cementite present 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 its formation into a continuous network.

[0003] Currently, high-speed, semi-high-speed, and mixed passenger-freight railways both domestically and internationally mostly use eutectoid pearlitic rails. These rails typically have a carbon content between 0.65% and 0.82%, a pearlitic microstructure, and exhibit good strength and toughness, along with moderate overall mechanical properties. At present, moving flash welding of rails has become the mainstream online rail welding technology on railway construction sites both domestically and internationally. The widespread use of high-carbon pearlitic rails provides a possibility to prevent the deterioration of the microstructure and properties of rail welded joints caused by network cementite. Furthermore, after the rail is subjected to welding thermal cycles, the hardened layer in the weld area disappears, and a wide low-hardness zone forms on both sides of the weld, resulting in the hardness of the weld and heat-affected zone being lower than that of the rail base material. During service on the line, the rail is prone to preferentially forming "saddle-shaped" wear on the rail head tread of the welded joint, which not only increases wheel-rail impact but also seriously affects the service life of the rail and even endangers traffic safety. Therefore, restoring the mechanical properties of rails that have been reduced due to welding and eliminating the network cementite that may exist in the heat-affected zone of rail welding have become prerequisites for the application of rails.

[0004] Patent application No. 201610909362.1 discloses a post-weld heat treatment method for welded joints of hypereutectoid rails and PG4 heat-treated hypereutectoid pearlitic rails. This method includes first cooling the welded rail joint to below 400°C, then heating the first-cooled rail joint to 860–930°C, followed by a second cooling until the tread temperature of the rail joint reaches 410–450°C. The dissimilar rail welded joints obtained using this method meet current domestic railway industry standards. However, this prior art pertains to welded joints of eutectoid pearlitic rails and is not suitable for welded joints formed from hypereutectoid pearlitic rails with higher carbon content.

[0005] Patent applications with application numbers 201810581145.3 (Invention title: Post-weld heat treatment method for welded joints of hypereutectoid steel rails and eutectoid steel rails), 201810720765.0 (Invention title: Heat treatment method for welded joints of rails), and 201810710040.3 (Invention title: Heat treatment method for welded joints of rails of dissimilar materials) describe post-weld heat treatment methods for welded joints of hypereutectoid steel rails and eutectoid steel rails. However, the microstructure of both hypereutectoid steel rails and eutectoid steel rails in the above patents is mainly pearlite. The principle of post-weld heat treatment is to use compressed air or water mist mixture as cooling medium to rapidly cool the rail joint at the austenitizing temperature, thereby refining the spacing between pearlite lamellars and improving the thermoplasticity of the rail weld area. However, this does not solve the problem of the network cementite microstructure in the weld heat-affected zone. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a heat treatment method for improving the network cementite in the heat-affected zone of rail welding, so as to improve the mechanical properties of the rail reduced by 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.

[0007] This invention provides a heat treatment method for improving the network cementite in the heat-affected zone of rail welding, comprising the following steps:

[0008] A) The welded joint of hypereutectoid pearlitic steel rail with a surface temperature of 800-1000°C is cooled for the first time to reduce the surface temperature of the welded joint to 100-200°C.

[0009] B) Perform the first full-section normalizing heating on the welded joint: stop heating when the surface temperature of the welded joint reaches 1000-1080°C;

[0010] C) Cool the welded joint after step B) a second time to reduce the surface temperature of the welded joint to 200-300°C.

[0011] D) Perform a second full-section normalizing heating on the welded joint after step C): Stop heating when the surface temperature of the welded joint after step C) reaches 930-980℃.

[0012] E) Perform a third cooling on the welded joint after step D); the initial temperature of the third cooling shall not be lower than 900°C;

[0013] During the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5℃ / s, and the final cooling temperature is 350–400℃; the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0℃ / s, and the final cooling temperature is 450–500℃.

[0014] F) Cool the welded joint after step E) for the fourth time to 20-30°C.

[0015] Preferably, in step A), the hypereutectoid pearlitic steel rail is a hypereutectoid pearlitic steel rail of the same heat treatment type;

[0016] The room temperature tensile strength of the same heat-treated hypereutectoid pearlitic steel rail is 1350–1420 MPa, the hardness is 400–430 HV, and the impact energy is 10–15 J. The chemical composition of the base material is: C mass content is 0.86%–1.10%, Si mass content is 0.50%–0.90%, Mn mass content is 0.6%–1.0%, Cr mass content is 0.10%–0.40%, and the balance is Fe and unavoidable impurities.

[0017] Preferably, in step A), the first cooling is natural cooling; the first cooling is carried out in air.

[0018] Preferably, in step B), the heating rate is 2-5°C / s;

[0019] The heating process employs a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

[0020] Preferably, in step C), the second cooling is natural cooling; the second cooling is carried out in air.

[0021] Preferably, in step D), the heating rate is 2-5°C / s;

[0022] The heating process employs a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

[0023] Preferably, in step E), during the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5 °C / s, and the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0 °C / s.

[0024] The heat-affected zone of the welded joint rail head is cooled by compressed air or a water mist mixture, with a pressure of 0.15–0.25 MPa.

[0025] The heat-affected zone of the welded joint rail web is cooled by compressed air or a water mist mixture, with a pressure of 0.40–0.50 MPa.

[0026] Preferably, in step E), the third cooling is to use a full-section air spray device to cool the heat-affected zone of the rail head and the heat-affected zone of the welded joint.

[0027] The full-section air jet device includes:

[0028] Rail head air spraying device 1; the rail head air spraying device 1 includes a left rail head air spraying device 1-1 and a right rail head air spraying device 1-2; the left rail head air spraying device 1-1 and the right rail head air spraying device 1-2 are symmetrically arranged about the center line 4 of the vertical direction of the rail; the left rail head air spraying device 1-1 includes a left rail head first nozzle 1-1-1, a left rail head second nozzle 1-1-2 and a left rail head third nozzle 1-1-3; the right rail head air spraying device 1-2 includes a right rail head first nozzle 1-2-1, a right rail head second nozzle 1-2-2 and a right rail head third nozzle 1-2-3;

[0029] Rail web spraying device 2; the rail web spraying device 2 includes a left rail web spraying device 2-1 and a right rail web spraying device 2-2; the left rail web spraying device 2-1 and the right rail web spraying device 2-2 are symmetrically arranged with respect to the center line of the vertical direction of the rail; the left rail web spraying device 2-1 includes a left rail web first nozzle 2-1-1, a left rail web second nozzle 2-1-2, a left rail web third nozzle 2-1-3 and a left rail web fourth nozzle 2-1-4; the right rail web spraying device 2-2 includes a right rail web first nozzle 2-2-1, a right rail web second nozzle 2-2-2, a right rail web third nozzle 2-2-3 and a right rail web fourth nozzle 2-2-4;

[0030] The left rail head air jet device 1-1 and the left rail waist air jet device 2-1 are connected by the first partition plate 3-1;

[0031] The right railhead air jet device 2-1 and the right rail waist air jet device 2-2 are connected by the second partition plate 3-2.

[0032] Preferably, the spacing between adjacent nozzles is 8–12 mm;

[0033] Both the first partition plate 3-1 and the second partition plate 3-2 are steel partition plates with a thickness of 0.8 to 1.2 mm.

[0034] Preferably, in step F), the fourth cooling is natural cooling.

[0035] This invention provides a heat treatment method for improving the network cementite in the heat-affected zone of rail welds, comprising the following steps: A) subjecting the welded joint of a hypereutectoid pearlitic rail, with a surface temperature of 800–1000°C, to a first cooling to reduce the surface temperature of the welded joint to 100–200°C; B) subjecting the welded joint to a first full-section normalizing: heating the welded joint to 1000–1080°C and then stopping the heating; C) subjecting the welded joint treated in step B) to a second cooling to reduce the surface temperature of the welded joint to 200–300°C; D) subjecting the welded joint treated in step C) to further heat treatment. Perform a second full-section normalizing heating: heat the surface temperature of the welded joint after step C) to 930-980℃ and then stop heating; E) perform a third cooling on the welded joint after step D); the initial temperature of the third cooling is not lower than 900℃; during the third cooling process, the cooling rate of the heat-affected zone of the rail head of the welded joint is 1.5-2.5℃ / 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 4.0-5.0℃ / s, and the final cooling temperature is 450-500℃; F) perform a fourth cooling on the welded joint after step E), cooling it to 20-30℃.

[0036] This invention employs controlled cooling of varying intensities on both sides of the heat-affected zones (HAZs) of the flash weld joints on hypereutectoid pearlitic rails after welding. This ensures that the HAZs on both sides of the rail joint are free of martensite and continuous network cementite, while maintaining high hardness in the rail head HAZ, thus guaranteeing the wear resistance of the rail joint. The longitudinal hardness of the rail joint within a ±30mm radius of the weld center can reach 91%–95% of the average hardness of the corresponding rail base material. At room temperature, the average impact energy of the rail head weld is 14–18 J, and the average impact energy of the rail web weld is 11–13 J, significantly higher than the ≥6.5 J specified in TB / T1632.2-2014. This invention helps improve the "saddle-shaped" wear caused by low hardness in the welded area during rail welding joints during railway service. The joint exhibits good impact toughness, contributing to ensuring railway operation safety. Attached Figure Description

[0037] Figure 1 A schematic diagram of a full-section air jet device provided for one embodiment of the present invention;

[0038] Figure 2 This is a diagram showing the location of the longitudinal section hardness test at a position 5mm below the rail head tread of the rail welded joint.

[0039] Figure 3 This is a schematic diagram showing the sampling location for metallographic specimens of the rail head tread surface of the welded rail joint. Detailed Implementation

[0040] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0041] This invention provides a heat treatment method for improving the network cementite in the heat-affected zone of rail welding, comprising the following steps:

[0042] A) The welded joint of hypereutectoid pearlitic steel rail with a surface temperature of 800-1000°C is cooled for the first time to reduce the surface temperature of the welded joint to 100-200°C.

[0043] B) Perform the first full-section normalizing heating on the welded joint: stop heating when the surface temperature of the welded joint reaches 1000-1080°C;

[0044] C) Cool the welded joint after step B) a second time to reduce the surface temperature of the welded joint to 200-300°C.

[0045] D) Perform a second full-section normalizing heating on the welded joint after step C): Stop heating when the surface temperature of the welded joint after step C) reaches 930-980℃.

[0046] E) Perform a third cooling on the welded joint after step D); the initial temperature of the third cooling shall not be lower than 900°C;

[0047] During the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5℃ / s, and the final cooling temperature is 350–400℃; the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0℃ / s, and the final cooling temperature is 450–500℃.

[0048] F) Cool the welded joint after step E) for the fourth time to 20-30°C.

[0049] Regarding step A):

[0050] The welded joint, formed by welding hypereutectoid pearlitic steel rails with a surface temperature of 800-1000℃, is subjected to a first cooling process to reduce the surface temperature of the welded joint to 100-200℃.

[0051] In some embodiments of the present invention, the hypereutectoid pearlitic rails are hypereutectoid pearlitic rails of the same heat treatment type. The same heat treatment type hypereutectoid pearlitic rails refer to hypereutectoid pearlitic rails with identical chemical composition, profile, and mechanical properties.

[0052] The same heat-treated hypereutectoid pearlitic steel rail has a room temperature (20-30℃) tensile strength of 1350-1420 MPa, a hardness of 400-430 HV, and an impact energy of 10-15 J; the chemical composition of the base material is: C mass content of 0.86%-1.10%, Si mass content of 0.50%-0.90%, Mn mass content of 0.6%-1.0%, Cr mass content of 0.10%-0.40%, with the balance being Fe and unavoidable impurities.

[0053] Specifically, the room temperature (20-30℃) tensile strength of the same heat-treated hypereutectoid pearlitic steel rail is 1350MPa and 1420MPa, the hardness is 400HV and 430HV, and the impact energy is 15J and 10J; the chemical composition of the base material is: C mass content is 0.86% and 0.90%, Si mass content is 0.50% and 1.0%, Mn mass content is 0.6% and 1.0%, Cr mass content is 0.10% and 0.40%, with the balance being Fe and unavoidable impurities.

[0054] The welded joint is formed by welding the same heat-treated hypereutectoid pearlitic steel rails with the same rail type and specifications of 60-75 kg / m (e.g., 60 kg / m, 75 kg / m) using a rail moving flash welding machine.

[0055] In some embodiments of the present invention, the surface temperature of the hypereutectoid pearlitic steel rail formed by welding is 800°C. This reduces the surface temperature of the welded joint to 200°C.

[0056] In some embodiments of the invention, the first cooling is natural cooling. The first cooling takes place in air.

[0057] The initial cooling rate should not be too high to prevent the formation of large areas of martensite in the weld heat-affected zone due to excessive cooling, and to prevent the formation of microcracks due to excessive internal stress. Once microcracks form within the microstructure, they cannot be eliminated by subsequent post-weld heat treatment. During natural cooling, the cooling rate varies, with faster cooling at high temperatures and relatively slower cooling at low temperatures. Specifically, after rail welding, due to convection, radiation, and heat conduction with the surrounding medium, the rail joint exhibits rapid cooling at high temperatures and a gradually decreasing cooling rate at low temperatures during natural cooling in an air environment of 20–30°C. The natural cooling rate is 3.0–2.5°C / s in the 1000–801°C temperature range, 2.5–1.2°C / s in the 800–501°C temperature range, and 1.2–0.7°C / s in the 500–301°C temperature range. Within the temperature range of 300–100℃, the natural cooling rate is 0.7–0.1℃ / s. Furthermore, without considering compositional segregation in the rail steel, natural cooling of the rail welded joint directly in air after welding, and natural cooling in air after normalizing, will not cause the formation of brittle martensite in the heat-affected zone.

[0058] Regarding step B):

[0059] The welded joint is subjected to the first full-section normalizing heating: the surface temperature of the welded joint is heated to 1000-1080°C and then heating is stopped to complete the first normalizing heating process.

[0060] In some embodiments of the present invention, the heating rate is 2–5 °C / s, for example, 5 °C / s or 2 °C / s. Heating is carried out to 1000 °C or 1080 °C.

[0061] In some embodiments of the present invention, the heating is performed by using a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

[0062] The purpose of the first normalizing is to dissolve any potential network cementite. After the first normalizing heating, the weld is immediately allowed to cool naturally. It should also be noted that if the normalizing temperature is too high (e.g., above 1100℃), the grains in the heated area (especially the weld and its adjacent coarse-grained heat-affected zone) will become excessively large, thus affecting the impact toughness of the subsequent joint. Conversely, if the normalizing temperature is too low, such as heating the weld joint surface to 950℃ or below, the temperature is too low to effectively dissolve the network cementite; only partial dissolution occurs, and the normalizing process has little destructive effect on the network cementite.

[0063] In this invention, the heating rate of the medium-frequency induction heating is 2–5 °C / s. It should be noted that when the heating rate of the medium-frequency induction heating is below 2 °C / s, the production efficiency of post-weld heat treatment of the rails will be significantly slowed down. Conversely, when the heating rate of the medium-frequency induction heating is above 5 °C / s, the rapid heating rate may cause the rail head area of ​​the thicker rail joint to enter the cooling process after heating stops without being fully heated, affecting the normalizing effect. Therefore, in this invention, the heating rate of the medium-frequency induction heating in the post-weld heat treatment process of the rails is controlled at 2–5 °C / s.

[0064] Regarding step C):

[0065] The welded joint after step B) is cooled a second time to reduce the surface temperature of the welded joint to 200-300℃.

[0066] In some embodiments of the invention, the second cooling is natural cooling. The second cooling is carried out in air, reducing the surface temperature of the weld joint to 300°C.

[0067] The purpose of the second cooling is to prevent the formation of large areas of martensite due to excessively rapid cooling of the weld heat-affected zone during the cooling process, and to avoid the formation of microcracks due to excessive internal stress. Once microcracks form within the microstructure, they cannot be eliminated by subsequent heat treatment.

[0068] Regarding step D):

[0069] The welded joint after step C) is subjected to a second full-section normalizing heating: the surface temperature of the welded joint after step C) is heated to 930-980°C and then heating is stopped to complete the second normalizing heating process.

[0070] In some embodiments of the present invention, the heating rate is 2 to 5 °C / s, for example, 5 °C / s or 2 °C / s. The surface temperature of the welded joint after step C) is heated to 980 °C or 930 °C.

[0071] In some embodiments of the present invention, the heating is performed by using a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

[0072] The purpose of the second normalizing is to refine the austenite grains.

[0073] Regarding step E):

[0074] The welded joint after step D) is subjected to a third cooling; the initial temperature of the third cooling is not lower than 900°C.

[0075] The third cooling process involves using a full-section air jet device to cool the heat-affected zones of the rail head and rail web of the welded joint at varying intensities. In some embodiments of the invention, the initial temperature of the third cooling is 970°C or 920°C.

[0076] During the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5℃ / s, for example, 2.5℃ / s; the final cooling temperature is 350–400℃, for example, 350℃; the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0℃ / s, for example, 5.0℃ / s; the final cooling temperature is 450–500℃, for example, 500℃.

[0077] In some embodiments of the present invention, the heat-affected zone of the welded joint rail head is cooled by compressed air or a water mist mixture, and the pressure of the compressed air or water mist mixture is 0.15 to 0.25 MPa, for example, 0.25 MPa.

[0078] In some embodiments of the present invention, the heat-affected zone of the rail web of the welded joint is cooled by compressed air or a water mist mixture, and the pressure of the compressed air or water mist mixture is 0.40 to 0.50 MPa, for example 0.50 MPa.

[0079] Mechanical properties can be improved by applying a mixture of compressed air or water mist as a cooling medium.

[0080] In some embodiments of the present invention, the full-section air jet device includes:

[0081] Rail head air spraying device 1; the rail head air spraying device 1 includes a left rail head air spraying device 1-1 and a right rail head air spraying device 1-2; the left rail head air spraying device 1-1 and the right rail head air spraying device 1-2 are symmetrically arranged about the center line 4 of the vertical direction of the rail; the left rail head air spraying device 1-1 includes a left rail head first nozzle 1-1-1, a left rail head second nozzle 1-1-2 and a left rail head third nozzle 1-1-3; the right rail head air spraying device 1-2 includes a right rail head first nozzle 1-2-1, a right rail head second nozzle 1-2-2 and a right rail head third nozzle 1-2-3;

[0082] Rail web spraying device 2; the rail web spraying device 2 includes a left rail web spraying device 2-1 and a right rail web spraying device 2-2; the left rail web spraying device 2-1 and the right rail web spraying device 2-2 are symmetrically arranged with respect to the center line of the vertical direction of the rail; the left rail web spraying device 2-1 includes a left rail web first nozzle 2-1-1, a left rail web second nozzle 2-1-2, a left rail web third nozzle 2-1-3 and a left rail web fourth nozzle 2-1-4; the right rail web spraying device 2-2 includes a right rail web first nozzle 2-2-1, a right rail web second nozzle 2-2-2, a right rail web third nozzle 2-2-3 and a right rail web fourth nozzle 2-2-4;

[0083] The left rail head air jet device 1-1 and the left rail waist air jet device 2-1 are connected by the first partition plate 3-1;

[0084] The right railhead air jet device 2-1 and the right rail waist air jet device 2-2 are connected by the second partition plate 3-2.

[0085] Figure 1 This is a schematic diagram of a full-section air spraying device according to an embodiment of the present invention. In the diagram, 1 represents the rail head air spraying device, 1-1 is the left rail head air spraying device, 1-2 is the right rail head air spraying device, 1-1-1 is the first nozzle on the left rail head, 1-1-2 is the second nozzle on the left rail head, 1-1-3 is the third nozzle on the left rail head, 1-2-1 is the first nozzle on the right rail head, 1-2-2 is the second nozzle on the right rail head, 1-2-3 is the third nozzle on the right rail head, and 2 represents the rail waist air spraying device, 2-1 is the left rail waist air spraying device, and 2-2 is the right rail waist air spraying device. The air spray device consists of the following nozzles: 2-1-1 is the first nozzle on the left side rail web, 2-1-2 is the second nozzle on the left side rail web, 2-1-3 is the third nozzle on the left side rail web, 2-1-4 is the fourth nozzle on the left side rail web, 2-2-1 is the first nozzle on the right side rail web, 2-2-2 is the second nozzle on the right side rail web, 2-2-3 is the third nozzle on the right side rail web, 2-2-4 is the fourth nozzle on the right side rail web, 3-1 is the first partition plate, 3-2 is the second partition plate, and 4 is the centerline of the rail in the vertical direction.

[0086] In some embodiments of the present invention, the nozzles described above are all supplied with compressed air or water mist mixture at the same flow rate / pressure to cool the rail head area and rail web area of ​​the rail welded joint.

[0087] In some embodiments of the present invention, the spacing between adjacent nozzles is 8 to 12 mm, for example, 10 mm.

[0088] In this invention, the geometric height and position of the nozzle and the separator can be designed and adjusted according to the height difference of the rails being processed. In some embodiments of this invention, the first separator 3-1 and the second separator 3-2 are both steel separators with a thickness of 0.8 to 1.2 mm (e.g., 1 mm), which are inserted into the device through side openings. When the separator is inserted into contact with the lower jaw of the rail head at the welded joint of the rail, it can isolate the rail head and the rail web. 4 is the vertical rail centerline, that is, the two sides of the rail are symmetrical about this centerline.

[0089] In this invention, the actual cooling rate can be adjusted by controlling the pressure of the cooling medium flowing into the full-section air jet device, thereby achieving control of different cooling rates / intensities for the rail head and rail web areas of the rail welded joint.

[0090] In some embodiments of the present invention, the cooling channel at the bottom of the rail head air jet device of the full-section air jet device is 13 to 17 mm away from the surface of the welded joint, for example, 15 mm.

[0091] In some embodiments of the present invention, the cooling channel at the bottom of the rail waist air jet device of the full-section air jet device is 23-27 mm away from the surface of the welded joint, for example, 25 mm.

[0092] In this invention, rapid cooling of the heat-affected zone (HAZ) of hypereutectoid pearlitic rail welds above the austenitizing temperature by injecting compressed air or a water mist mixture can refine the pearlite lamellar spacing and improve the mechanical properties of the welded joint. Specifically, the gas pressure injected by the cooling assembly can be adjusted by regulating the number and diameter of the injection holes to achieve different cooling intensities for the rail head and rail web HAZ of the welded rail joint. In this invention, the compressed air and / or water mist mixture injected by the cooling assembly have the same cooling rate / cooling capacity.

[0093] The purpose of the third cooling is to obtain a pearlitic structure with finer interlamellar spacing and to suppress the precipitation of network carbides. At the same time, it avoids the formation of martensite due to excessively high cooling rate or low final cooling temperature in the rapid cooling stage, as well as the severe deterioration of the mechanical properties of the heat-affected zone due to excessively low cooling rate or excessively high final cooling temperature in the rapid cooling stage.

[0094] It should be noted that the rail web corresponds to the region in which the network of cementite in the rail steel is most severe. When both the heat-affected zone (HAZ) of the welded joint rail web and the heat-affected zone of the welded joint rail head are cooled at a rate lower than 4.0℃ / s, the network carbides in the heat-affected zone of the welded joint rail head disappear, but the network carbides in the heat-affected zone of the welded joint rail web cannot be eliminated. Based on this, the present invention uses a relatively higher cooling rate for the heat-affected zone of the welded joint rail web than for the heat-affected zone of the welded joint rail head, aiming to eliminate the network carbides in the heat-affected zone of the welded joint rail web. In addition, to avoid the formation of abnormal martensite / bainite structures due to rapid cooling during the third cooling process, and considering that the segregation of alloying elements in the rail web region is higher than that in the rail head region, the final cooling temperature of the heat-affected zone of the welded joint rail web during the third cooling process is set above the final cooling temperature of the heat-affected zone of the welded joint rail head, which is 50℃ or higher.

[0095] Regarding step F):

[0096] The welded joint after step E) is cooled for the fourth time to 20-30°C.

[0097] In some embodiments of the present invention, the fourth cooling is natural cooling.

[0098] After the third cooling is completed, the full-section air spray device is removed, allowing the heat-affected zone of the welded joint to undergo a fourth cooling in the air environment, thereby completing the post-weld heat treatment process of the rail involved in this invention.

[0099] The purpose of the fourth cooling step is to prevent the formation of large areas of martensite due to excessively rapid cooling in the weld heat-affected zone, and to avoid the formation of microcracks due to excessive internal stress. Once microcracks form within the microstructure, they cannot be eliminated by subsequent heat treatment.

[0100] In this invention, the critical cooling rate for martensitic transformation during the continuous cooling process of the heat-treated hypereutectoid pearlitic rail weld is 1.5–2.0 °C / s, and the Ms temperature (the initial temperature of martensite formation) is 220–260 °C. To avoid the formation of abnormal structures such as martensite in the heat-affected zone of the rail weld joint, the final cooling temperature during the rapid cooling process of the post-weld heat-treated pearlitic rail weld joint must be controlled above the Ms temperature of the rail steel. When the final cooling temperature is slightly higher than the Ms temperature, to avoid the formation of abnormal structures such as martensite in the heat-affected zone of the rail weld joint, the cooling rate at this stage must be lower than the critical cooling rate for martensitic transformation during the continuous cooling process of the rail weld; otherwise, the joint will suffer premature fatigue fracture due to a large amount of hardened martensite.

[0101] This invention utilizes normalizing treatment to adjust the microstructure of steel, making it more uniform and thus improving its hardness, strength, and toughness. Simultaneously, multiple normalizing treatments effectively eliminate network cementite, further enhancing the overall performance of the steel. For hypereutectoid steel, normalizing effectively eliminates network cementite. Specifically, the first normalizing is performed in the Accm+ (100–150°C) temperature range to dissolve the network cementite; the second normalizing is performed in the Accm+ (30–50°C) temperature range to refine the austenite grains. Furthermore, normalizing can suppress the precipitation of network carbides, resulting in a completely fine pearlite microstructure, or only a small amount of strip-shaped carbides precipitating along grain boundaries. According to metallurgical principles, the Accm temperature of steel refers to the final temperature at which all secondary cementite dissolves into austenite during heating, or it can be understood as the temperature at which all cementite dissolves into austenite when hypereutectoid steel is heated; it is an important parameter in the metal heat treatment process. In this invention, the Accm temperature of the hypereutectoid pearlitic rail steel involved is 770–800°C. Furthermore, for the normalizing heat treatment of steel, in addition to strictly controlling the temperature, the following points should also be noted: ensure uniform heating to avoid localized overheating or incomplete heating; maintain sufficient holding time during the holding stage to ensure full transformation of the internal structure of the steel; and avoid excessively fast or slow cooling rates during the cooling process to prevent affecting the normalizing effect.

[0102] Considering the large cross-sectional dimensions of the rail specimens, especially the rail head area, and the fact that the rail head bears locomotive loads and is prone to fatigue wear, and that there is almost no conventional heat preservation process for the normalizing treatment of rail welded joints, the first and second normalizing heating temperatures used in this invention are relatively high to ensure uniform heating of the rail head cross-section and sufficient normalizing process. Specifically, the preferred heating temperature for the first normalizing heating is controlled at 1000–1080℃, while the preferred heating temperature for the second normalizing heating is controlled at 930–980℃.

[0103] In this invention, the first stage cooling, the second cooling, and the fourth cooling are natural cooling of the entire cross-section of the welded joint in air before and after normalizing. Since the cooling rate during the martensite formation initiation stage of the rail steel (220–260°C) is less than 1.5–2.0°C / s, no martensite is generated during the first stage cooling, the second cooling, and the fourth cooling. Because a second normalizing heating is performed after the first normalizing heating and cooling, the mechanical properties after the first normalizing heating and cooling will not affect the mechanical properties after the subsequent second normalizing heating and cooling, provided that macroscopic defects such as cracks are not generated. That is, whether the cooling stage after the first normalizing heating uses natural cooling (air cooling) or artificial rapid cooling with compressed air or a water mist mixture will not affect the mechanical properties after the subsequent second normalizing heating and cooling. Therefore, this invention preferably uses natural cooling (air cooling) after the first normalizing heating and cooling of the rail welded joint to avoid the generation of macroscopic defects such as cracks. The starting temperature of the third cooling stage is not lower than 900℃, aiming to provide sufficient phase transformation driving force and provide the thermodynamic conditions for pearlitic phase transformation to improve the mechanical properties of the welded joint during subsequent cooling. In this invention, after the third cooling stage, the final cooling temperatures on both sides of the weld joint rail head and rail web are significantly higher than the martensitic transformation initiation temperature (Ms temperature) of the hypereutectoid pearlitic rail steel, aiming to avoid the formation of brittle and hard martensite in the weld heat-affected zone due to improper cooling process. At the same time, the cooling rate of the heat-affected zone of the heat-treated hypereutectoid pearlitic rail welded joint rail head and rail web is controlled above 2.0℃ / s during the third cooling stage, aiming to refine the pearlite lamellar spacing by rapidly cooling the supercooled austenite and improve the mechanical properties of the heat-affected zone. For the fourth stage of cooling, in order to avoid the formation of hardened martensite in the heat-affected zone of the joint during the cooling process, this invention preferably adopts natural cooling, selecting a natural cooling method with a rate lower than the critical cooling rate of martensitic transformation of the two types of rail steel, so that the surface temperature of the rail welded joint is reduced to 20-30℃.

[0104] Beneficial effects:

[0105] 1. This invention implements controlled cooling of different intensities on both sides of the heat-affected zone of the flash weld joint of hypereutectoid pearlitic rail after welding, so that the heat-affected zone on both sides of the rail joint weld is free of martensite and continuous network cementite, while the heat-affected zone of the rail head of the rail joint retains high hardness, thereby ensuring the wear resistance of the rail joint.

[0106] 2. This invention enables the longitudinal hardness of the rail joint within a 30mm radius of the weld center to reach 91%–95% of the average hardness of the corresponding rail base material. At room temperature, the average impact energy of the rail head weld is 14–18 J, and the average impact energy of the rail web weld is 11–13 J, significantly higher than the ≥6.5 J specified in TB / T 1632.2-2014. This invention helps improve the "saddle-shaped" wear of rail welded joints caused by low hardness in the welded area during railway service. The joint exhibits good impact toughness, contributing to ensuring railway operation safety.

[0107] The present invention does not impose any special restrictions on the source of the raw materials used above, and they can be commercially available.

[0108] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a heat treatment method for improving the network cementite in the heat-affected zone of rail welding provided by the present invention, but it should not be construed as a limitation on the scope of protection of the present invention.

[0109] In the embodiments, Figure 1 The full-section air jet device shown includes:

[0110] Rail head air spraying device 1; the rail head air spraying device 1 includes a left rail head air spraying device 1-1 and a right rail head air spraying device 1-2; the left rail head air spraying device 1-1 and the right rail head air spraying device 1-2 are symmetrically arranged about the center line 4 of the vertical direction of the rail; the left rail head air spraying device 1-1 includes a left rail head first nozzle 1-1-1, a left rail head second nozzle 1-1-2 and a left rail head third nozzle 1-1-3; the right rail head air spraying device 1-2 includes a right rail head first nozzle 1-2-1, a right rail head second nozzle 1-2-2 and a right rail head third nozzle 1-2-3;

[0111] Rail web spraying device 2; the rail web spraying device 2 includes a left rail web spraying device 2-1 and a right rail web spraying device 2-2; the left rail web spraying device 2-1 and the right rail web spraying device 2-2 are symmetrically arranged with respect to the center line of the vertical direction of the rail; the left rail web spraying device 2-1 includes a left rail web first nozzle 2-1-1, a left rail web second nozzle 2-1-2, a left rail web third nozzle 2-1-3 and a left rail web fourth nozzle 2-1-4; the right rail web spraying device 2-2 includes a right rail web first nozzle 2-2-1, a right rail web second nozzle 2-2-2, a right rail web third nozzle 2-2-3 and a right rail web fourth nozzle 2-2-4;

[0112] The left rail head air jet device 1-1 and the left rail waist air jet device 2-1 are connected by the first partition plate 3-1;

[0113] The right railhead air jet device 2-1 and the right rail waist air jet device 2-2 are connected by the second partition plate 3-2.

[0114] All nozzles described above are supplied with compressed air at the same pressure to cool the rail head and web areas of the rail welded joint. The spacing between adjacent nozzles is 10 mm.

[0115] The first partition plate 3-1 and the second partition plate 3-2 are both 1mm thick steel partition plates, inserted into the device through side openings. When the partition plate is inserted into contact with the lower jaw of the rail head at the welded joint of the rail, it can isolate the rail head and the rail web. 4 is the vertical rail centerline, that is, the two sides of the rail are symmetrical about this centerline.

[0116] The cooling channel at the bottom of the railhead air jet device of the full-section air jet device is 15mm away from the surface of the welded joint.

[0117] The cooling channel at the bottom of the rail waist air jet device of the full-section air jet device is 25mm away from the surface of the welded joint.

[0118] Example 1

[0119] The room temperature (20-30℃) tensile, impact, and hardness properties of a heat-treated hypereutectoid pearlitic rail base material were controlled. The rail base material had a tensile strength of 1350 MPa, a hardness of 400 HV, and an impact energy of 15 J. The chemical composition of the rail base material was as follows: C content of 0.86%, Si content of 0.50%, Mn content of 0.6%, Cr content of 0.10%, with the balance being Fe and unavoidable impurities.

[0120] After upsetting and slugging during the moving flash welding process, the welded joint of the 60kg / m heat-treatable hypereutectoid pearlitic steel rail undergoes post-weld heat treatment:

[0121] First, the welded rail joint, with a surface temperature of 800°C, is allowed to cool naturally in air to reduce the surface temperature of the rail head to 200°C. Then, a medium-frequency induction heating coil is used to perform full-section normalizing heating on the welded rail joint area. Heating is stopped when the surface temperature of the welded rail joint reaches 1000°C (heating rate of 5°C / s), completing the first normalizing heating process. Immediately after the first normalizing heating, a second cooling process is performed, which is natural cooling in air, with a final cooling temperature of 300°C. After the second cooling, the entire cross-section of the welded rail joint is subjected to a second full-section normalizing heating, with the surface temperature of the welded joint reaching 980°C (heating rate of 5°C / s) before heating is stopped, completing the second normalizing heating process. Immediately after the second normalizing heating, a third cooling process is performed, with an initial cooling temperature of 970°C. The process is carried out using... Figure 1 The illustrated full-section cooling device uses compressed air as the cooling medium to cool the heat-affected zones (HAZs) of the welded rail joint at different intensities. Compressed air is used to cool the HAZ of the rail head at a pressure of 0.25 MPa, and compressed air is used to cool the HAZ of the rail web at a pressure of 0.50 MPa. The cooling rate of the HAZ of the rail head is 2.5 °C / s, and the final cooling temperature is 350 °C. The cooling rate of the HAZ of the rail web is 5.0 °C / s, and the final cooling temperature is 500 °C. After the third cooling step, the full-section cooling device is removed, allowing the HAZ of the welded joint to undergo a fourth cooling step in air, cooling to an ambient temperature of 20–30 °C, thus completing the post-weld heat treatment process of the rail involved in this invention.

[0122] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm. Figure 2 This diagram shows the longitudinal section hardness test location 5mm below the rail head tread of the rail welded joint. In the diagram, 5-1 and 5-2 are heat-treated hypereutectoid pearlitic rails, 6 is the weld center of the welded joint, and 7 is the location 5mm below the rail head tread of the rail welded joint.

[0123] Figure 3 This diagram illustrates the sampling locations for metallographic specimens from the rail head tread of a welded rail joint. In the diagram, 6 represents the center of the weld joint, and 8 represents the sampling location for the metallographic specimens from the rail head tread. Figure 3The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0124] The rail welded joint obtained in this embodiment exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 94% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 17J, and the average impact energy of the rail web weld is 12J, significantly higher than the ≥6.5J specified in TB / T 1632.2-2014. Furthermore, metallographic microscopy reveals no martensite or continuous network cementite in the HAZ on either side of the rail joint weld. This invention helps improve the "saddle-shaped" wear caused by low hardness in the welded area during railway service, and the joint exhibits good impact toughness, contributing to railway operational safety.

[0125] Example 2

[0126] The room temperature (20-30℃) tensile, impact, and hardness properties of a heat-treated hypereutectoid pearlitic rail base material were controlled. The rail base material had a tensile strength of 1350 MPa, a hardness of 400 HV, and an impact energy of 15 J. The chemical composition of the rail base material was as follows: C content of 0.86%, Si content of 0.50%, Mn content of 0.6%, Cr content of 0.10%, with the balance being Fe and unavoidable impurities.

[0127] After upsetting and slugging during the moving flash welding process, the welded joint of the 60kg / m heat-treatable hypereutectoid pearlitic steel rail undergoes post-weld heat treatment:

[0128] First, the welded rail joint, with a surface temperature of 800°C, is allowed to cool naturally in air to reduce the surface temperature of the rail head to 200°C. Then, a medium-frequency induction heating coil is used to perform full-section normalizing heating on the welded rail joint area. Heating is stopped when the surface temperature of the welded rail joint reaches 1080°C (heating rate of 2°C / s), completing the first normalizing heating process. Immediately after the first normalizing heating, a second cooling process is performed, which is natural cooling in air, with a final cooling temperature of 300°C. After the second cooling, the entire cross-section of the welded rail joint is subjected to a second full-section normalizing heating, with the surface temperature of the welded joint reaching 930°C (heating rate of 2°C / s) before heating is stopped, completing the second normalizing heating process. Immediately after the second normalizing heating, a third cooling process is performed, with an initial cooling temperature of 920°C. The process is carried out using... Figure 1 The illustrated full-section cooling device uses compressed air as the cooling medium to cool the heat-affected zones (HAZs) of the welded rail joint at different intensities. Compressed air is used to cool the HAZ of the rail head at a pressure of 0.25 MPa, and compressed air is used to cool the HAZ of the rail web at a pressure of 0.50 MPa. The cooling rate of the HAZ of the rail head is 2.5 °C / s, and the final cooling temperature is 350 °C. The cooling rate of the HAZ of the rail web is 5.0 °C / s, and the final cooling temperature is 500 °C. After the third cooling step, the full-section cooling device is removed, allowing the HAZ of the welded joint to undergo a fourth cooling step in air, cooling to an ambient temperature of 20–30 °C, thus completing the post-weld heat treatment process of the rail involved in this invention.

[0129] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0130] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0131] The rail welded joint obtained in this embodiment exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 95% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 18J, and the average impact energy of the rail web weld is 13J, significantly higher than the ≥6.5J specified in TB / T 1632.2-2014. Furthermore, metallographic microscopy reveals no martensite or continuous network cementite in the HAZ on either side of the rail joint weld. This invention helps improve the "saddle-shaped" wear caused by low hardness in the welded area during railway operation, and the joint exhibits good impact toughness, contributing to safe railway operation.

[0132] Example 3

[0133] The room temperature (20-30℃) tensile, impact, and hardness properties of a heat-treated hypereutectoid pearlitic rail base material were controlled. The rail base material had a tensile strength of 1420 MPa, a hardness of 430 HV, and an impact energy of 10 J. The chemical composition of the rail base material was as follows: C content of 1.10%, Si content of 0.90%, Mn content of 1.0%, Cr content of 0.40%, with the balance being Fe and unavoidable impurities.

[0134] After upsetting and slugging during the moving flash welding process, the welded joint of the 75kg / m heat-treatable hypereutectoid pearlitic steel rail undergoes post-weld heat treatment:

[0135] First, the welded rail joint, with a surface temperature of 800°C, is allowed to cool naturally in air to reduce the surface temperature of the rail head to 200°C. Then, a medium-frequency induction heating coil is used to perform full-section normalizing heating on the welded rail joint area. Heating is stopped when the surface temperature of the welded rail joint reaches 1000°C (heating rate of 2°C / s), completing the first normalizing heating process. Immediately after the first normalizing heating, a second cooling process is performed, which is natural cooling in air, with a final cooling temperature of 300°C. After the second cooling, the entire cross-section of the welded rail joint is subjected to a second full-section normalizing heating, with the surface temperature of the welded joint reaching 980°C (heating rate of 2°C / s) before heating is stopped, completing the second normalizing heating process. Immediately after the second normalizing heating, a third cooling process is performed, with an initial cooling temperature of 970°C. The process is carried out using... Figure 1The illustrated full-section cooling device uses compressed air as the cooling medium to cool the heat-affected zones (HAZs) of the welded rail joint at different intensities. Compressed air is used to cool the HAZ of the rail head at a pressure of 0.25 MPa, and compressed air is used to cool the HAZ of the rail web at a pressure of 0.50 MPa. The cooling rate of the HAZ of the rail head is 2.5 °C / s, and the final cooling temperature is 350 °C. The cooling rate of the HAZ of the rail web is 5.0 °C / s, and the final cooling temperature is 500 °C. After the third cooling step, the full-section cooling device is removed, allowing the HAZ of the welded joint to undergo a fourth cooling step in air, cooling to an ambient temperature of 20–30 °C, thus completing the post-weld heat treatment process of the rail involved in this invention.

[0136] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0137] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0138] The rail welded joint obtained in this embodiment exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 91% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 14J, and the average impact energy of the rail web weld is 11J, significantly higher than the ≥6.5J specified in TB / T 1632.2-2014. Furthermore, metallographic microscopy reveals no martensite or continuous network cementite in the HAZ on either side of the rail joint weld. This invention helps improve the "saddle-shaped" wear caused by low hardness in the welded area during railway service, and the joint exhibits good impact toughness, contributing to railway operational safety.

[0139] Example 4

[0140] The room temperature (20-30℃) tensile, impact, and hardness properties of a heat-treated hypereutectoid pearlitic rail base material were controlled. The rail base material had a tensile strength of 1390 MPa, a hardness of 415 HV, and an impact energy of 13 J. The chemical composition of the rail base material was as follows: C content of 0.98%, Si content of 0.70%, Mn content of 0.8%, Cr content of 0.25%, with the balance being Fe and unavoidable impurities.

[0141] After upsetting and slugging during the moving flash welding process, the welded joint of the heat-treated hypereutectoid pearlitic steel rail with a specification of 68kg / m undergoes post-weld heat treatment:

[0142] First, the welded rail joint, with a surface temperature of 800°C, is allowed to cool naturally in air to reduce the surface temperature of the rail head to 200°C. Then, a medium-frequency induction heating coil is used to perform full-section normalizing heating on the welded rail joint area. Heating is stopped when the surface temperature of the welded rail joint reaches 1040°C (heating rate of 5°C / s), completing the first normalizing heating process. Immediately after the first normalizing heating, a second cooling process is performed, which is natural cooling in air, with a final cooling temperature of 200°C. After the second cooling, the entire cross-section of the welded rail joint is subjected to a second full-section normalizing heating, with the surface temperature of the welded joint reaching 950°C (heating rate of 5°C / s) before heating is stopped, completing the second normalizing heating process. Immediately after the second normalizing heating, a third cooling process is performed, with an initial cooling temperature of 940°C. The process is carried out using... Figure 1 The illustrated full-section cooling device uses compressed air as the cooling medium to cool the heat-affected zones (HAZs) of the welded rail joint at different intensities. Compressed air is used to cool the HAZ of the rail head at a pressure of 0.25 MPa, and compressed air is used to cool the HAZ of the rail web at a pressure of 0.50 MPa. The cooling rate of the HAZ of the rail head is 2.5 °C / s, and the final cooling temperature is 350 °C. The cooling rate of the HAZ of the rail web is 5.0 °C / s, and the final cooling temperature is 500 °C. After the third cooling step, the full-section cooling device is removed, allowing the HAZ of the welded joint to undergo a fourth cooling step in air, cooling to an ambient temperature of 20–30 °C, thus completing the post-weld heat treatment process of the rail involved in this invention.

[0143] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0144] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0145] The rail welded joint obtained in this embodiment exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 93% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 16J, and the average impact energy of the rail web weld is 12J, significantly higher than the ≥6.5J specified in TB / T 1632.2-2014. Furthermore, metallographic microscopy reveals no martensite or continuous network cementite in the HAZ on either side of the rail joint weld. This invention helps improve the "saddle-shaped" wear caused by low hardness in the welded area during railway service, and the joint exhibits good impact toughness, contributing to railway operational safety.

[0146] Comparative Example 1

[0147] The selection of rail materials, mechanical properties of the rail base material, and process conditions involved in welding and post-weld normalizing heat treatment cooling are all the same in this comparative example and Example 1. The only difference is that the heating temperature of the first and second normalizing of the rail welded joint in this comparative example is 900°C, which is lower than the post-weld normalizing process range of the present invention.

[0148] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0149] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0150] The rail welded joint obtained in this comparative example, due to the heating temperature of 900℃ for both the first and second normalizing processes, which is lower than that of the process method of this invention, suffers from insufficient supercooling during the cooling process after normalizing and insufficient driving force for pearlite phase transformation during subsequent cooling. Consequently, the mechanical properties of the heat-affected zones on both sides of the weld joint are lower than those obtained using the process method of this invention. Furthermore, the network cementite in the rail web region of the rail welded joint is not completely eliminated. Hardness testing shows that the longitudinal hardness of the rail weld heat-affected zone within ±30mm from the weld center only reaches 89% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material, lower than the 91%–95% hardness achieved by this invention. Additionally, at room temperature, the average impact energy of the rail head weld is 12J, and the average impact energy of the rail web weld is 9J, lower than the average impact energy of 14–18J for the rail head weld and 11–13J for the rail web weld described in this invention. The rail web region of the welded joint obtained in this comparative example has obvious network cementite, and the hardness and impact energy of the heat-affected zone are lower than the beneficial effects obtained by using the present invention, which is not conducive to railway operation safety.

[0151] Comparative Example 2

[0152] The comparative example is consistent with Example 1 in terms of rail material selection, mechanical properties of rail base material, welding cooling process, heating temperature of post-weld normalizing heat treatment, and cooling rate of post-weld normalizing cooling stage. The difference is that in this comparative example, the final cooling temperature of the heat-affected zone of the rail head of the welded joint in the third stage of cooling after normalizing heating is 250°C, which is lower than the final cooling temperature control period of the heat-affected zone of the rail head of the welded joint in the third stage of cooling in this invention.

[0153] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0154] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0155] The rail welded joint obtained in this comparative example, due to the final cooling temperature of the heat-affected zone (HAZ) of the rail head during the third cooling stage after post-weld normalizing heating being 250°C, falls within the Ms temperature range (the starting temperature for martensite formation) of the heat-treated hypereutectoid pearlitic rail steel involved in this invention. This results in the formation of a small amount of brittle and hard martensite in the HAZ of the rail head. However, due to the higher final cooling temperature of the HAZ of the rail web, martensite failed to form. In this comparative example, brittle and hard martensite structure was formed in the HAZ of the rail head of the rail welded joint. Furthermore, the longitudinal hardness of the rail welded HAZ within a 30mm radius from the weld center reached 96% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. The average impact energy of the rail head weld at room temperature was 16J, and the average impact energy of the rail web weld was 11J. Because the process method of this invention was not used, brittle and hard martensite appeared in the heat-affected zone of the rail head of the rail welded joint obtained in this comparative example, and the beneficial effects obtained by using the process method of this invention were not achieved, which is detrimental to railway operation safety.

[0156] Comparative Example 3

[0157] The comparative example is consistent with Example 3 in terms of rail material selection, mechanical properties of the rail base material, rail welding process conditions, heating temperature during post-weld normalizing, cooling method and steps during post-weld normalizing, final cooling temperature of the second cooling stage, cooling method and steps of the fourth cooling stage, and cooling rate. The difference is that in this comparative example, the cooling rate of the compressed air sprayed by the cooling device for the rail web area of ​​the rail welded joint during the third cooling process after the completion of normalizing heating is 3.0℃ / s, which is lower than the cooling rate control range of the cooling device for the rail web area of ​​the rail welded joint during the third cooling process after the second normalizing heating of the rail welded joint in this invention.

[0158] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0159] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0160] The rail welded joint obtained in this comparative example has a cooling rate of 3.0℃ / s corresponding to the water mist mixture sprayed by the cooling device in the rail web area of ​​the rail welded joint. This rate is lower than the cooling rate control range of the cooling device in the rail web area of ​​the rail welded joint during the third cooling process after the second normalizing heating following rail welding in this invention. Consequently, the network cementite in the heat-affected zone of the rail web of the rail welded joint is not completely eliminated, resulting in a lower effect than that obtained by the process method of this invention. In this comparative example, the longitudinal hardness of the rail welded joint in the heat-affected zone within a 30mm radius from the weld center reaches 91% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 14J, and the average impact energy of the rail web weld is 9J. Because the network cementite in the heat-affected zone of the rail web of the rail welded joint is not completely eliminated, the microstructure and properties of the joint obtained in this comparative example are lower than those obtained by the process method of this invention, which is detrimental to railway operation safety.

[0161] Comparative Example 4

[0162] The selection of rail materials, mechanical properties of the rail base material, and flash welding conditions are the same in this comparative example as in Example 4. The difference is that there is no subsequent normalizing heating and cooling process after the rail welding is completed in this comparative example. That is, the rail welded joint obtained by flash welding is directly cooled naturally in the air to an ambient temperature of 30°C, thus obtaining the rail welded joint under this comparative example.

[0163] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0164] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0165] The rail welded joint obtained in this comparative example exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 87% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 11 J, and the average impact energy of the rail web weld is 9 J. Metallographic microscopy reveals a significant amount of continuous network cementite within the heat-affected zone of the rail web, but no martensite is observed. Because the process method of this invention was not employed, the performance of the rail welded joint is lower, and a significant amount of network cementite is present in the rail web HAZ. The microstructure and properties of the joint obtained in this comparative example are lower than those obtained using the process method of this invention, which is detrimental to railway operation safety.

[0166] Comparative Example 5

[0167] The comparative example is consistent with Example 1 in terms of rail material selection, mechanical properties of rail base material, flash welding conditions, cooling process and cooling rate during the normalizing heat treatment after rail welding. The difference is that in this comparative example, the final cooling temperature of the heat-affected zone of the rail head and rail web of the rail joint during the third cooling process after the second normalizing heating is 350°C. This final cooling temperature is lower than the final cooling temperature process control range of this invention.

[0168] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0169] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0170] The rail welded joint obtained in this comparative example exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 95% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 13 J, and the average impact energy of the rail web weld is 8 J. Due to segregation in the rail web region, a certain number of point-like martensite structures appear in the HAZ of the rail web welded joint during the heat treatment cooling process, resulting in a decrease in impact performance. Because the process method of this invention was not used, the impact performance of the rail web region of the rail welded joint is lower, and a certain number of point-like martensite structures appear in the HAZ. The microstructure and properties of the joint obtained in this comparative example are lower than the beneficial effects obtained by using the process method of this invention, which is detrimental to railway operation safety.

[0171] Comparative Example 6

[0172] This comparative example is consistent with Example 3 in terms of rail material selection, mechanical properties of the rail base material, flash welding conditions, and cooling procedures during the post-weld normalizing heat treatment. The difference lies in the cooling rate of the heat-affected zone at the rail head during the third cooling process after normalizing heating. The final cooling temperature is 350°C. The cooling rate of the heat-affected zone at the rail web is 2.0°C / s, with a final cooling temperature of 450°C. In this comparative example, the cooling rate of the rail head and web regions during the third cooling stage is significantly lower than the process control range of this invention.

[0173] The welded joints of the rails were processed into longitudinal hardness test specimens. The joints were processed into longitudinal hardness test specimens according to TB / T 1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. According to GB / T 230.1-2009, [the process was carried out as follows]. Figure 2 The joint was subjected to longitudinal Vickers HV hardness test at a position 5mm below the tread surface. The test points were symmetrically arranged to the left and right sides with the weld as the center, with a spacing of 2mm.

[0174] according to Figure 3 The sampling locations shown were used to examine the metallographic structure of the welded joints according to GB / T13298-2015 "Metallic Microstructure Examination Method". The metallographic samples were etched using a 3% nitric acid-alcohol solution, and the metallographic structure was observed using a Leica MeF3 optical microscope.

[0175] The rail welded joint obtained in this comparative example exhibits a longitudinal hardness in the heat-affected zone (HAZ) within a 30mm radius from the weld center that reaches 88% of the average hardness of the corresponding heat-treated hypereutectoid pearlitic rail base material. At room temperature, the average impact energy of the rail head weld is 10J, and the average impact energy of the rail web weld is 8J. Metallographic microscopy reveals a significant amount of continuous network cementite within the HAZ of the rail web, but no martensite is observed. Because the process method of this invention was not employed, the performance of the rail welded joint is lower, and a significant amount of network cementite is present in the rail web HAZ. The microstructure and properties of the joint obtained in this comparative example are lower than those obtained using the process method of this invention, which is detrimental to railway operation safety.

[0176] By comparing the embodiments and comparative examples of this invention, it can be seen that: 1. This invention, through controlled cooling of different intensities on both sides of the heat-affected zone of the flash weld joint of hypereutectoid pearlitic rails after welding, ensures that the heat-affected zone on both sides of the rail joint weld is free of martensite and continuous network cementite, while maintaining high hardness in the heat-affected zone of the rail head, thus guaranteeing the wear resistance of the rail joint; 2. The longitudinal hardness of the rail joint within ±30mm from the weld center can reach 91% to 95% of the average hardness of the corresponding rail base material. The average impact energy of the rail head weld joint at room temperature is 14 to 18 J, and the average impact energy of the rail web weld joint is 11 to 13 J, far exceeding the ≥6.5 J specified in TB / T 1632.2-2014. This invention helps to improve the "saddle-shaped" wear caused by the low hardness of the welded area during the service of rail welded joints on the railway line, and the joint has good impact toughness, which helps to ensure the safety of railway operation.

[0177] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A heat treatment method for improving the network cementite in the heat-affected zone of rail welding, comprising the following steps: A) The welded joint of hypereutectoid pearlitic steel rail with a surface temperature of 800-1000°C is cooled for the first time to reduce the surface temperature of the welded joint to 100-200°C. B) Perform the first full-section normalizing heating on the welded joint: stop heating when the surface temperature of the welded joint reaches 1000-1080°C; C) Cool the welded joint after step B) a second time to reduce the surface temperature of the welded joint to 200-300°C. D) Perform a second full-section normalizing heating on the welded joint after step C): Stop heating when the surface temperature of the welded joint after step C) reaches 930-980℃. E) Perform a third cooling on the welded joint after step D); the initial temperature of the third cooling shall not be lower than 900°C; During the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5℃ / s, and the final cooling temperature is 350–400℃; the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0℃ / s, and the final cooling temperature is 450–500℃. F) Cool the welded joint after step E) for the fourth time to 20-30°C.

2. The heat treatment method according to claim 1, characterized in that, In step A), the hypereutectoid pearlitic steel rails are all hypereutectoid pearlitic steel rails of the same heat treatment type; The room temperature tensile strength of the same heat-treated hypereutectoid pearlitic steel rail is 1350–1420 MPa, the hardness is 400–430 HV, and the impact energy is 10–15 J. The chemical composition of the base material is: C mass content is 0.86%–1.10%, Si mass content is 0.50%–0.90%, Mn mass content is 0.6%–1.0%, Cr mass content is 0.10%–0.40%, and the balance is Fe and unavoidable impurities.

3. The heat treatment method according to claim 1, characterized in that, In step A), the first cooling is natural cooling; the first cooling is carried out in the air.

4. The heat treatment method according to claim 1, characterized in that, In step B), the heating rate is 2-5°C / s; The heating process employs a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

5. The heat treatment method according to claim 1, characterized in that, In step C), the second cooling is natural cooling; the second cooling is carried out in air.

6. The heat treatment method according to claim 1, characterized in that, In step D), the heating rate is 2-5°C / s; The heating process employs a medium-frequency induction molding electric heating coil to perform full-section normalizing heating on the welded joint.

7. The heat treatment method according to claim 1, characterized in that, In step E), during the third cooling process, the cooling rate of the heat-affected zone of the welded joint rail head is 1.5–2.5 °C / s, and the cooling rate of the heat-affected zone of the welded joint rail web is 4.0–5.0 °C / s. The heat-affected zone of the welded joint rail head is cooled by compressed air or a water mist mixture, with a pressure of 0.15–0.25 MPa. The heat-affected zone of the welded joint rail web is cooled by compressed air or a water mist mixture, with a pressure of 0.40–0.50 MPa.

8. The heat treatment method according to claim 1, characterized in that, In step E), the third cooling process involves using a full-section air jet device to cool the heat-affected zone of the rail head and the heat-affected zone of the welded joint. The full-section air jet device includes: Rail head air spraying device 1; the rail head air spraying device 1 includes a left rail head air spraying device 1-1 and a right rail head air spraying device 1-2; the left rail head air spraying device 1-1 and the right rail head air spraying device 1-2 are symmetrically arranged about the center line 4 of the vertical direction of the rail; the left rail head air spraying device 1-1 includes a left rail head first nozzle 1-1-1, a left rail head second nozzle 1-1-2 and a left rail head third nozzle 1-1-3; the right rail head air spraying device 1-2 includes a right rail head first nozzle 1-2-1, a right rail head second nozzle 1-2-2 and a right rail head third nozzle 1-2-3; Rail web spraying device 2; the rail web spraying device 2 includes a left rail web spraying device 2-1 and a right rail web spraying device 2-2; the left rail web spraying device 2-1 and the right rail web spraying device 2-2 are symmetrically arranged with respect to the center line of the vertical direction of the rail; the left rail web spraying device 2-1 includes a left rail web first nozzle 2-1-1, a left rail web second nozzle 2-1-2, a left rail web third nozzle 2-1-3 and a left rail web fourth nozzle 2-1-4; the right rail web spraying device 2-2 includes a right rail web first nozzle 2-2-1, a right rail web second nozzle 2-2-2, a right rail web third nozzle 2-2-3 and a right rail web fourth nozzle 2-2-4; The left rail head air jet device 1-1 and the left rail waist air jet device 2-1 are connected by the first partition plate 3-1; The right railhead air jet device 2-1 and the right rail waist air jet device 2-2 are connected by the second partition plate 3-2.

9. The heat treatment method according to claim 8, characterized in that, The spacing between adjacent nozzles is 8–12 mm; Both the first partition plate 3-1 and the second partition plate 3-2 are steel partition plates with a thickness of 0.8 to 1.2 mm.

10. The heat treatment method according to claim 1, characterized in that, In step F), the fourth cooling is natural cooling.

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