A method for eliminating martensite in a rail weld joint and a rail

Abstract

The application discloses a method for eliminating martensite in a rail welded joint and a rail. The method comprises the following steps: performing first-stage induction normalizing heat treatment on a welded rail joint at a first temperature; performing second-stage induction normalizing heat treatment on the rail joint subjected to the first-stage induction normalizing heat treatment at a second temperature which is lower than the first temperature; and performing air blast cooling on a tread of the rail joint. The scheme provided by the application is based on the formation principle of the martensite in the rail joint. The critical cooling speed of the martensite formation is reduced or even eliminated by first high-temperature heat treatment and slow cooling, the joint performance is improved, and the formation of the martensite is prevented by second heat treatment and appropriate air blast cooling pressure and final cooling temperature control, so that the pearlitic rail flash-butt welded joint without the martensite and meeting the standard requirements is finally obtained.

Description

Technical Field

[0001] This invention relates to the field of rail welding, and more specifically to a method for eliminating martensite in rail weld joints and the rail itself. Background Technology

[0002] Rail transit is constantly evolving with economic and technological progress, moving towards higher speeds, heavier loads, and greater comfort and safety, placing increasingly higher demands on rails. Rails are the foundation of rail transit, with pearlitic rails, represented by the U71Mn, U75V, and U78CrV series, widely used in high-speed railways, subways, and heavy-haul railways. However, after undergoing welding thermal cycles, flash welded joints of rails exhibit grain coarsening and abnormal, hard, and brittle martensitic structures, leading to a significant decline in the performance of the welded joints and severely impacting rail lifespan and train safety. Post-weld normalizing heat treatment can effectively address grain coarsening and reduce martensite to some extent, but it cannot completely eliminate martensite. Therefore, the stable control of martensitic structure in flash welded joints of rails has always been a key focus and challenge in rail welding research. Summary of the Invention

[0003] In view of this, in order to overcome at least one aspect of the above problems, embodiments of the present invention propose a method for eliminating martensite in rail weld joints, comprising the following steps: At the first temperature, the welded rail joints undergo the first stage of induction normalizing heat treatment. At a second temperature lower than the first temperature, the rail joint that has undergone the first stage of induction normalizing heat treatment is subjected to a second stage of induction normalizing heat treatment. The tread surface of the rail joint is cooled by air jet cooling.

[0004] In some embodiments, the first stage of induction normalizing heat treatment includes: The rail joint is heated to 1150~1250℃ and held at this temperature for 5~20 seconds, then heating is stopped and it is allowed to cool naturally to below 300℃.

[0005] In some embodiments, the first stage of induction normalizing heat treatment further includes: An induction coil with a profile similar to that of a steel rail is used to perform full-section induction heating on the rail joint.

[0006] In some embodiments, the second stage of induction normalizing heat treatment includes: After the first stage of induction normalizing heat treatment, the rail joint, which was below 300°C, was reheated to 900-950°C, and then heating was immediately stopped. The joint was then cooled using a jet cooling device.

[0007] In some embodiments, the second-stage induction normalizing heat treatment further includes: using an induction coil with a profile similar to that of a rail to perform full-section induction heating on the rail joint.

[0008] In some embodiments, the induction coil includes a first induction coil and a second induction coil, which are located on opposite sides of the rail joint.

[0009] In some embodiments, the tread surface of the rail joint is cooled by air jetting, which further includes: The tread surface of the rail joint is cooled by blowing air through a blower box, and the blowing is stopped after the rail joint cools down to below 510°C, allowing it to cool naturally.

[0010] In some embodiments, the distance between the air jet box and the tread of the rail joint is 10-60 mm, and the air jet pressure is 0.1-0.4 MPa.

[0011] In some embodiments, the rail is a pearlitic rail, wherein the mass fraction of C is 0.60% to 0.80%, the mass fraction of Si is 0.30% to 0.80%, the mass fraction of Mn is 0.50% to 1.20%, the mass fraction of V is ≤0.20%, the mass fraction of Cr is ≤0.35%, the mass fraction of S is ≤0.006%, the mass fraction of O is no more than 0.003%, and the remainder is Fe.

[0012] Based on the same inventive concept, according to another aspect of the present invention, embodiments of the present invention also provide a rail, said rail being prepared by the method described in any of the above embodiments.

[0013] The present invention has one of the following beneficial technical effects: The solution proposed in this invention is based on the formation principle of martensite in rail joints. First, the critical cooling rate for martensite formation is increased by reducing or even eliminating component segregation through a first high-temperature heat treatment and slow cooling. Through a second heat treatment and appropriate air jet cooling pressure and final cooling temperature control, the joint performance is improved while preventing the formation of martensite. Finally, a pearlitic rail flash welded joint without martensite and whose microstructure and properties meet the standard requirements are obtained. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0015] Figure 1 A schematic flowchart illustrating a method for eliminating martensite in rail welded joints provided in an embodiment of the present invention; Figure 2 A schematic diagram of the front view of the heating coil during the normalizing heat treatment of the welded joint provided in an embodiment of the present invention; Figure 3 A schematic side view of the heating coil during the normalizing heat treatment of the welded joint provided in an embodiment of the present invention; Figure 4 A schematic diagram of the welding head cooling process provided for an embodiment of the present invention; Figure 5 This is a schematic diagram of full-section sample sampling provided for an embodiment of the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0017] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0018] According to one aspect of the present invention, embodiments of the present invention provide a method for eliminating martensite in rail weld joints, such as... Figure 1 As shown, it may include the following steps: S1, At the first temperature, the welded rail joint is subjected to the first stage of induction normalizing heat treatment. S2, at a second temperature lower than the first temperature, the rail joint that has undergone the first stage induction normalizing heat treatment is subjected to a second stage induction normalizing heat treatment. S3, the tread surface of the rail joint is cooled by air jet.

[0019] The proposed solution is based on the formation principle of martensite in rail joints. First, the critical cooling rate for martensite formation is increased by reducing or even eliminating component segregation through a first high-temperature heat treatment and slow cooling. Then, the joint performance is improved by a second heat treatment and appropriate air jet cooling pressure and final cooling temperature control, while preventing the formation of martensite. Finally, a pearlitic rail flash welded joint without martensite and whose microstructure and properties meet the standard requirements are obtained.

[0020] In some embodiments, the first stage of induction normalizing heat treatment includes: The rail joint is heated to 1150~1250℃ and held at this temperature for 5~20 seconds, then heating is stopped and it is allowed to cool naturally to below 300℃.

[0021] In some embodiments, the first stage of induction normalizing heat treatment further includes: An induction coil with a profile similar to that of a steel rail is used to perform full-section induction heating on the rail joint.

[0022] In some embodiments, the second stage of induction normalizing heat treatment includes: After the first stage of induction normalizing heat treatment, the rail joint, which was below 300°C, was reheated to 900-950°C, and then heating was immediately stopped. The joint was then cooled using a jet cooling device.

[0023] In some embodiments, the second-stage induction normalizing heat treatment further includes: using an induction coil with a profile similar to that of a rail to perform full-section induction heating on the rail joint.

[0024] Specifically, induction normalizing heat treatment uses an induction coil with a profile similar to that of a steel rail to induction heat the entire cross-section of the flash welded joint of the rail. A schematic diagram of induction heating is shown below. Figure 2 As shown. Induction normalizing heat treatment is divided into two stages: the first stage is high-temperature normalizing heat treatment, and the second stage is medium-temperature induction normalizing heat treatment.

[0025] The first stage of high-temperature induction normalizing heat treatment involves heating the rail to 1150–1250°C, holding it at that temperature for 5–20 seconds after reaching the set temperature, disconnecting the power supply to stop heating, and allowing it to cool slowly and naturally. Once the joint has cooled to 300°C, the second stage of induction normalizing heat treatment begins.

[0026] The second stage of induction normalizing heat treatment involves reheating the joint, which was below 300°C after the first stage of induction normalizing heat treatment, until the temperature reaches 900-950°C. Heating is then stopped immediately, and the joint is cooled using a jet cooling device.

[0027] In some embodiments, the induction coil includes a first induction coil and a second induction coil, which are located on opposite sides of the rail joint.

[0028] Specifically, such as Figure 3 As shown, the induction coil may include a first induction coil and a second induction coil, and are symmetrically arranged on both sides of the welded joint. This allows for simultaneous heating of both sides of the rail joint, effectively avoiding the problem of large temperature gradients on both sides of the joint and inside the cross-section caused by unilateral heating or uneven heating.

[0029] In some embodiments, the tread surface of the rail joint is cooled by air jetting, which further includes: The tread surface of the rail joint is cooled by blowing air through a blower box, and the blowing is stopped after the rail joint cools down to below 510°C, allowing it to cool naturally.

[0030] In some embodiments, the distance between the air jet box and the tread of the rail joint is 10-60 mm, and the air jet pressure is 0.1-0.4 MPa.

[0031] Specifically, such as Figure 4 As shown, the length and width of the air spray box can be 210mm and 71mm respectively. The air inlet of the air spray box is connected to an air inlet pipe. This air spray box only sprays air onto the rail head tread surface, and the cooling air is compressed air at room temperature. During the air spraying process, the air spray box is 10-60mm away from the rail surface, and the air spraying pressure is 0.1-0.4MPa. After the joint is cooled to below 510℃, the air spraying is stopped, and the rail is allowed to cool naturally at room temperature.

[0032] In some embodiments, the rail is a pearlitic rail, wherein the mass fraction of C is 0.60% to 0.80%, the mass fraction of Si is 0.30% to 0.80%, the mass fraction of Mn is 0.50% to 1.20%, the mass fraction of V is ≤0.20%, the mass fraction of Cr is ≤0.35%, the mass fraction of S is ≤0.006%, the mass fraction of O is no more than 0.003%, and the remainder is Fe.

[0033] In some embodiments, the microstructure and properties of the welded joint obtained by the above method are analyzed, and the microstructure and property testing and analysis are performed with reference to standard TB / T 1632. In addition to detecting martensite at the locations specified in standard TB / T 1632, martensite testing is also performed on the entire cross-section of the welded joint. A large plate is taken from a position 5 mm from the center plane of the rail, and then a metallographic sample with a width of 20 mm is taken from the large plate, centered on the weld, from the rail head to the rail bottom. A sampling diagram is shown below. Figure 5 As shown.

[0034] The proposed solution is based on the formation principle of martensite in rail joints. First, a high-temperature heat treatment and slow cooling are used to reduce or even eliminate compositional segregation, thereby increasing the critical cooling rate for martensite formation. Second, a secondary heat treatment and appropriate air-blast cooling pressure and final cooling temperature control improve joint performance while preventing martensite formation, ultimately resulting in a pearlitic rail flash welded joint without martensite and meeting standard performance requirements. If local segregation is severe, a high cooling rate cannot be used, and the performance may not meet requirements; even air cooling may result in martensite formation. However, if the cooling rate and final cooling temperature are not properly controlled, martensite may appear even if the rail composition is completely homogeneous. Therefore, both post-weld heat treatments and subsequent air-blast cooling are indispensable.

[0035] Example 1 In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. First, the rail is heated to 1150°C and held at this temperature for 20 seconds. After the holding time, the power is disconnected and heating is stopped. The rail is allowed to cool slowly and naturally in its original position. After the joint cools to 300°C, it is reheated by the induction coil. Heating is stopped immediately after the temperature rises to 950°C. Immediately, air was sprayed onto the rail head tread using an air-jet box with a length of 210mm and a width of 71mm. The cooling air was compressed air at room temperature. The air-jet box was 40mm away from the rail surface, and the air pressure was 0.25MPa. After cooling the joint to below 510℃, the air-jet was stopped, and the joint was allowed to cool naturally to room temperature in its original position. Inspection revealed no martensite structure at the standard location or along the entire cross-section of the joint. The martensite in the flash weld joint of the pearlitic rail was completely eliminated, and the hardness and other microstructures of the joint met the standard requirements.

[0036] Example 2 In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. First, the rail is heated to 1200°C and held at this temperature for 15 seconds. After the holding time, the power is disconnected and heating is stopped. The rail is allowed to cool slowly and naturally in its original position. After the joint cools to 300°C, it is reheated by the induction coil. Heating is stopped immediately after the temperature rises to 950°C. Immediately, air was sprayed onto the rail head tread using an air-jet box with a length of 210mm and a width of 71mm. The cooling air was compressed air at room temperature. The air-jet box was 40mm away from the rail surface, and the air pressure was 0.25MPa. After cooling the joint to below 510℃, the air-jet was stopped, and the joint was allowed to cool naturally to room temperature in its original position. Inspection revealed no martensite structure at the standard location or along the entire cross-section of the joint. The martensite in the flash weld joint of the pearlitic rail was completely eliminated, and the hardness and other microstructures of the joint met the standard requirements.

[0037] Example 3 In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. First, the rail is heated to 1250°C and held at this temperature for 5 seconds. After the holding time, the power is disconnected and heating is stopped. The rail is allowed to cool slowly and naturally in its original position. After the joint cools to 300°C, it is reheated by the induction coil. Heating is stopped immediately after the temperature rises to 950°C. Immediately, air was sprayed onto the rail head tread using an air-jet box with a length of 210mm and a width of 71mm. The cooling air was compressed air at room temperature. The air-jet box was 40mm away from the rail surface, and the air pressure was 0.25MPa. After cooling the joint to below 510℃, the air-jet was stopped, and the joint was allowed to cool naturally to room temperature in its original position. Inspection revealed no martensite structure at the standard location or along the entire cross-section of the joint. The martensite in the flash weld joint of the pearlitic rail was completely eliminated, and the hardness and other microstructures of the joint met the standard requirements.

[0038] Example 4 In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. First, the rail is heated to 1200°C and held at this temperature for 15 seconds. After the holding time, the power is disconnected and heating is stopped. The rail is allowed to cool slowly and naturally in its original position. After the joint cools to 300°C, it is reheated by the induction coil. Heating is stopped immediately after the temperature rises to 930°C. Immediately, air was sprayed onto the rail head tread using an air-jet box with a length of 210mm and a width of 71mm. The cooling air was compressed air at room temperature. The air-jet box was 50mm away from the rail surface, and the air pressure was 0.32MPa. After cooling the joint to below 510℃, the air-jet was stopped, and the joint was allowed to cool naturally to room temperature in its original position. Inspection revealed no martensite structure at the standard location or along the entire cross-section of the joint. The martensite in the flash weld joint of the pearlitic rail was completely eliminated, and the hardness and other microstructures of the joint met the standard requirements.

[0039] Comparative Example 1: In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. Heating is stopped immediately after the joint is heated to 950°C, and air is immediately sprayed onto the rail head tread using an air jet box with a length of 210mm and a width of 71mm. The cooling air is compressed air at room temperature. The air jet box is 40mm away from the rail surface, and the air jet pressure is 0.25MPa. Air jetting is stopped after the joint is cooled to below 510°C, and the joint is allowed to cool naturally to room temperature in its original position. Inspection revealed martensitic structure in the joint, which does not meet the standard requirements.

[0040] Comparative Example 2: In this embodiment, the pearlitic steel rail used has the following element mass fractions: C 0.76%, Si 0.55%, Mn 0.89%, V 0.002%, Cr 0.08%, S 0.006%, and O 0.002%. After the joint obtained by moving flash welding is cooled to room temperature, an induction coil with a profile similar to the rail is used to perform induction normalizing heat treatment on the entire cross-section of the flash welded rail joint. First, the rail is heated to 1150°C and held at this temperature for 20 seconds. After the holding time, the power is disconnected and heating is stopped. The rail is allowed to cool slowly and naturally in its original position. After the joint cools to 300°C, it is reheated by the induction coil. Heating is stopped immediately after the temperature rises to 950°C. Immediately, air was sprayed onto the rail head tread using an air jet box with a length of 210mm and a width of 71mm. The cooling air was compressed air at room temperature. The air jet box was 40mm away from the rail surface, and the air pressure was 0.6MPa. After cooling the joint to below 510℃, the air jetting was stopped, and the joint was allowed to cool naturally to room temperature in its original position. Inspection revealed a significant amount of martensite at the standard position of the rail head and throughout the entire cross-section of the joint, failing to meet the standard requirements.

[0041] Based on the same inventive concept, according to another aspect of the present invention, embodiments of the present invention also provide a rail, which is prepared by the steps described in any of the above embodiments.

[0042] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0043] It should be understood that, as used herein, the singular form “a” is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, “and / or” refers to any and all possible combinations of one or more of the associated listed items.

[0044] The embodiment numbers disclosed in the above embodiments of the present invention are merely for description and do not represent the superiority or inferiority of the embodiments.

[0045] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for eliminating martensite in rail weld joints, characterized in that, Includes the following steps: At the first temperature, the welded rail joints undergo the first stage of induction normalizing heat treatment. At a second temperature lower than the first temperature, the rail joint that has undergone the first stage of induction normalizing heat treatment is subjected to a second stage of induction normalizing heat treatment. The tread surface of the rail joint is cooled by air jet cooling.

2. The method as described in claim 1, characterized in that, The first stage of induction normalizing heat treatment includes: The rail joint is heated to 1150~1250℃ and held at this temperature for 5~20 seconds, then heating is stopped and it is allowed to cool naturally to below 300℃.

3. The method as described in claim 2, characterized in that, The first stage of induction normalizing heat treatment also includes: An induction coil with a profile similar to that of a steel rail is used to perform full-section induction heating on the rail joint.

4. The method as described in claim 1, characterized in that, The second stage of induction normalizing heat treatment includes: After the first stage of induction normalizing heat treatment, the rail joint, which was below 300°C, was reheated to 900-950°C, and then heating was immediately stopped. The joint was then cooled using a jet cooling device.

5. The method as described in claim 4, characterized in that, The second stage of induction normalizing heat treatment also includes: using an induction coil with a profile similar to that of a rail to perform full-section induction heating on the rail joint.

6. The method as described in claim 2 or 4, characterized in that, The induction coil includes a first induction coil and a second induction coil, which are located on both sides of the rail joint, respectively.

7. The method as described in claim 1, characterized in that, The tread surface of the rail joint is cooled by air jetting, further comprising: The tread surface of the rail joint is cooled by blowing air through a blower box, and the blowing is stopped after the rail joint cools down to below 510°C, allowing it to cool naturally.

8. The method as described in claim 7, characterized in that, The distance between the air jet box and the tread of the rail joint is 10-60mm, and the air jet pressure is 0.1-0.4MPa.

9. The method as described in claim 1, characterized in that, The rail is a pearlitic steel rail, wherein the mass fraction of C is 0.60% to 0.80%, the mass fraction of Si is 0.30% to 0.80%, the mass fraction of Mn is 0.50% to 1.20%, the mass fraction of V is ≤0.20%, the mass fraction of Cr is ≤0.35%, the mass fraction of S is ≤0.006%, the mass fraction of O is no more than 0.003%, and the remainder is Fe.

10. A type of steel rail, characterized in that, The rail is prepared using the method described in any one of claims 1-9.

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