A method for efficiently improving the performance of welded joints of 75N390HB grade rails

Abstract

The application discloses a method for efficiently improving the performance of a 75N 390HB grade rail welded joint, which comprises the following steps: adopting a fixed flash welding machine to weld the 75N 390HB grade rail joint without needing to perform a heat treatment after welding, and controlling the upsetting force in the welding process to be between 550-570 kN, the welding time to be between 160-170 s, and the melting end speed to be between 2.20-2.30 mm / s, so that the average hardness H J of the joint after welding is greater than the average hardness H P of the base material, the ratio H J / H P is greater than or equal to 0.94, the average hardness H J1 of the soft point of the joint is greater than the average hardness H P of the base material, the ratio H J1 / H P is greater than or equal to 0.85, and the softening area W is less than or equal to 16 mm.

Description

Technical Field

[0001] This invention belongs to the field of rail welding technology, specifically relating to a method for efficiently improving the performance of welded joints of 75N 390HB grade rails. Background Technology

[0002] The research and development of pearlitic steel rails in my country is characterized by "high carbon, low silicon, and medium manganese" as the main elements, with chromium and vanadium added as alloying strengthening elements according to different strength and hardness requirements. Due to the high strength and high hardness of high-hardness pearlitic steel rails used in heavy-haul railways, the carbon content of these rails can reach about 1%, and their alloy content is also relatively high, resulting in very poor weldability. This is mainly manifested in the following ways: ① Numerous defects in the overheated weld zone. In flash welding, carbon and manganese segregation along the austenite grain boundaries occurs in the overheated zone, causing abnormal microstructures such as bainite or martensite to appear in the joint under undercooled conditions. Secondary cementite is also prone to appear near the heat-affected zone, leading to increased joint brittleness. ② Due to the cyclic heating in the heat-affected zone, the grain size increases significantly, and the unbalanced process of rapid heating and cooling of the joint reduces the toughness of the flash weld joint. The impact strength of the joint is only about 50% of that of the base material, increasing the risk of rail breakage at the joint during rail line service. ③ Due to the burning loss of carbon and alloys during welding, the mechanical properties of the joint are significantly reduced, especially the joint hardness. The performance matching between the joint and the base material is poor, resulting in saddle-shaped wear and joint peeling during track service, which seriously affects the smoothness of seamless tracks. The poor welding performance of high-hardness rails severely restricts the widespread application of rails in seamless, heavy-load tracks.

[0003] The applicant has disclosed a method for fixing flash welding and joint heat treatment of high-hardness 75N 390HB grade steel rails in patent application No. 2023108878831 (hereinafter referred to as Document 1). This method first performs fixed flash welding on the joint of the high-hardness 75N 390HB grade steel rail, followed by heat treatment, which can reduce the average hardness H of the high-hardness 75N 390HB grade steel rail joint. J Compared with the average hardness H of the base material P The ratio of H satisfies: J / H P ≥0.90, average hardness H of joint soft spots J1 Compared with the average hardness H of the base material P The ratio of H satisfies: J1 / H P The strength is ≥0.83, and the softened area W≤20mm, meeting the standard requirements. However, the method disclosed in Document 1 requires a heat treatment operation after flash welding, which is time-consuming and inefficient. Summary of the Invention

[0004] To address one or more problems existing in the prior art, the present invention provides a method for efficiently improving the performance of welded joints of 75N 390HB grade rails. This method involves performing fixed flash welding on the joints of the 75N 390HB grade rails, without further heat treatment. The fixed flash welding method includes the following steps:

[0005] 1) Grind and remove rust from the end face of the 75N 390HB grade steel rail and the electrode clamping end;

[0006] 2) Use a fixed flash welding machine for rail joint welding; control the oil temperature of the welding machine at 38-42℃, the water temperature at 18-22℃, and the mains voltage at 416-425V.

[0007] 3) Flash welding process for joint fixing: The weld seam of the rail is controlled between 10-12mm, the camber is controlled between 0.35-0.55mm, the number of flash welding preheating times is controlled between 10-12, the secondary preheating current is controlled between 58-60kA, the flash welding upsetting amount is controlled between 10-13mm, the upsetting force is controlled between 550-570kN, the welding time is controlled between 160-170s, and the final melting speed is controlled between 2.20~2.30mm / s;

[0008] 4) Air cool the high-temperature welded joint to below 200°C to ensure that each part of the joint undergoes a complete phase transformation. When the joint temperature is below 300°C, the cooling can be accelerated appropriately.

[0009] The mechanical properties of the 75N 390HB grade rail joints welded by the aforementioned fixed flash welding method meet the following requirements: tensile strength Rm≥1185MPa, joint elongation A≥10.5%, joint impact KU2≥19.2J, and average joint hardness H. J Compared with the average hardness H of the base material P The ratio of H satisfies: J / H P ≥0.94, average hardness H of joint soft spots J1 Compared with the average hardness H of the base material P The ratio of: H J1 / H P ≥0.85, softened area W≤16mm.

[0010] In some embodiments, the chemical composition of the 75N 390HB grade rail is as follows (by weight percentage): C: 0.78–0.86%, Si: 0.50–0.80%, Mn: 0.80–1.20%, Cr: 0.20–0.55%, Nb: 0.01–0.03%, RE: 0.0005–0.0020%, P ≤ 0.010%, S ≤ 0.005%, Al ≤ 0.004%, with the remainder being Fe and unavoidable impurities.

[0011] In some embodiments, the fixed flash welder is a GAAS80 / 580 fixed flash welder.

[0012] In some embodiments, the 75N 390HB grade steel rail is produced through a process of smelting, continuous casting, slow cooling of the cast billet, and rolling, wherein the size of the cast billet is controlled to be 280mm × 380mm.

[0013] The method for improving the performance of 75N 390HB grade rail welded joints, based on the above technical solutions, optimizes and controls the fixed flash welding process parameters, thereby improving the weldability of the rail joint and achieving a hardness similar to that of the base material. Furthermore, this welding method eliminates the need for post-weld heat treatment, effectively improving the mechanical properties of the rail joint. Therefore, it is time-efficient and highly effective, meeting the following requirements: tensile strength Rm ≥ 1185 MPa, joint elongation A ≥ 10.5%, and joint impact KU2 ≥ 19.2 J. Moreover, it yields excellent joints with properties matching those of the base material, with an average hardness H of the welded joint. J Compared with the average hardness H of the base material P The ratio of H satisfies: J / H P ≥0.94, average hardness H of joint soft spots J1 Compared with the average hardness H of the base material P The ratio of: H J1 / H P ≥0.85, softened area W≤16mm, reaching or even exceeding the weld joint performance of the above-mentioned literature 1. Detailed Implementation

[0014] The present invention aims to provide a method for efficiently improving the performance of welded joints of 75N 390HB grade rails.

[0015] The technical solution adopted by this invention to solve its technical problem is to optimize and control the fixed flash welding process parameters, specifically including the following steps: 1) 75N 390HB grade steel rails (chemical composition by weight percentage: C: 0.78~0.86%, Si: 0.50~0.80%, Mn: 0.80~1.20%, Cr: 0.20~0.55%, Nb: 0.01~0.03%, RE: 0.0005~0.0020%, P≤0.010%, S≤0.005%, Al≤0.004%, the remainder being Fe and unavoidable impurities) are smelted and continuously welded. The steel rails are produced using a casting, slow cooling, and rolling process, with the billet dimensions controlled at 280mm × 380mm. The end faces are ground and rust-removed, and the electrode clamping ends are also ground and rust-removed. 2) Rail welding is performed using a GAAS80 / 580 fixed flash welding machine. The welding machine oil temperature is controlled at 38-42℃ (e.g., 40℃), the water temperature at 18-22℃ (e.g., 20℃), and the mains voltage between 416-425V (e.g., 420V). 3) Fixed flash welding of the joints. Process: The weld seam allowance for rails is controlled between 10-12mm, the camber is controlled between 0.35-0.55mm, the number of preheating cycles for flash welding is controlled between 10-12, the secondary preheating current is controlled between 58-60kA, the upsetting amount for flash welding is controlled between 10-13mm, the upsetting force is controlled between 550-570kN, the welding time is controlled between 160-170s, and the final melting velocity is controlled between 2.20 and 2.30. Between mm / s; and 4) air cool the high-temperature welded joint to below 200℃ to ensure that each position of the joint undergoes a complete phase transformation. When the joint temperature is less than 300℃, the cooling can be accelerated appropriately. After the joint is cooled, grind the weld bead of the joint flat, that is, grind the push-out allowance at the rail head and rail bottom (the maximum allowable push-out allowance for the rail head is 2.0mm, the maximum allowable push-out allowance for the lower jaw of the rail head and the rail web is 2.0mm, and the maximum allowable push-out allowance for the rail bottom is 1.5mm).

[0016] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0017] Example:

[0018] In this embodiment, the high-hardness 75N 390HB grade steel rail of this invention is first produced by rolling a 280mm×380mm billet to produce 75kg / m steel rails according to the existing method (including smelting, continuous casting, slow cooling of the billet, and rolling process). Table 1 below shows the chemical composition and percentage content of the billets of the rolled steel rails 1-3, and also shows the hardness of the 75N 390HB grade steel rails of steel rails 1-3. It can be seen that the chemical composition and content of steel rails 1-3 can ensure that the hardness of the resulting steel rails reaches above 390HB.

[0019] Table 1: Chemical composition and content (by weight percentage) of the billets for rails 1-3, and hardness of each actual rail.

[0020] 1 0.78 0.50 0.80 0.20 0.01 0.001 0.009 0.005 0.003 391 2 0.81 0.72 0.98 0.37 0.02 0.001 0.008 0.005 0.004 394 3 0.86 0.80 1.20 0.55 0.03 0.002 0.008 0.004 0.003 396

[0021] Fixed flash welding was performed on the cross-sections of rails 1-3. By adjusting parameters such as preheating current, upsetting force, welding time, and final melting speed of the fixed flash welding process, and by comparing and analyzing the joint mechanical properties, the fixed flash welding process for high-hardness 75N390HB grade rails was determined. Table 2 below lists the main adjustment parameters for the welding process of rails 1-3 (in each example, the high-temperature welded joints were air-cooled to approximately 180°C). Table 3 below shows the mechanical property indices of the rail welded joints for each example.

[0022] Table 2: Main Adjustment Parameters for Welding Process of Rails 1-3

[0023] 1 1 58 160 550 2.20 2 2 59 165 560 2.25 3 3 60 170 570 2.30 4 1 63 140 600 2.50

[0024] Table 3: Mechanical properties of rail welded joints in Examples 1-4 of Table 2

[0025]

[0026] As shown in Tables 2 and 3 above, the welding processes for the cross-sections of rails 1-3 in Examples 1-3 all ensure that the welded joints of the rails have good mechanical properties, satisfying the following requirements: tensile strength Rm ≥ 1185 MPa, joint elongation A ≥ 10.5%, and joint impact KU2 ≥ 19.2 J. However, when the welding process of Example 4 differs from that of Example 1, specifically in the preheating current, upsetting force, welding time, and final melting speed parameters, the tensile strength, elongation, and impact KU2 of the rail welded joints all decrease to varying degrees. Therefore, this invention determines the welding processes of Examples 1-3 as the fixed flash welding process for 75N 390HB grade rails, which can significantly improve the mechanical properties of the welded joints of 75N 390HB grade rails.

[0027] According to the requirements of TB / T 1632.2-2014 standard, for heat-treated rails: the ratio of the average hardness HJ of the joint to the average hardness HP of the base material, and the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base material should meet the following requirements: Hj≥0.90Hp, Hj1≥0.8Hp, and the softened area W≤20mm). The Rockwell hardness of the rail joint 5mm below the rail head tread on the longitudinal section obtained in Example 1-3 was tested using an HR-150A Rockwell hardness tester. The test results are shown in Table 4-6 below.

[0028] Table 4: Rockwell hardness test results 5mm below the rail head tread on the longitudinal section of the rail joint obtained in Example 1

[0029]

[0030] Table 5: Rockwell hardness test results 5mm below the rail head tread on the longitudinal section of the rail joint obtained in Example 2

[0031]

[0032] Table 6: Rockwell hardness test results 5mm below the rail head tread on the longitudinal section of the rail joint obtained in Example 3

[0033]

[0034] Based on the results in Tables 4-6 above, it can be seen that the joints in Examples 1-3 treated by the fixed flash welding process provided by this invention all have good welding performance, and the ratio of the average hardness HJ of the joint to the average hardness HP of the base material satisfies H J / H P ≥0.94, average hardness H of joint soft spots J1 Compared with the average hardness H of the base material P The ratio of: H J1 / H P The softened area W ≤ 16mm is ≥0.85, meeting the standard requirements and achieving or even exceeding the performance of the welded joint in Document 1. However, compared to the method disclosed in Document 1, the method provided by this invention does not require heat treatment after flash welding, thus shortening the processing time and improving efficiency. Therefore, the method of this invention has good prospects for widespread application and can be promoted and used in domestic rail welding bases or online welding construction units.

[0035] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for efficiently improving the performance of welded joints of 75N 390HB grade steel rails, characterized in that, After the joint of the 75N390HB grade steel rail is fixed by flash welding, no further heat treatment is required. The fixed flash welding method includes the following steps: 1) Grind and remove rust from the end face of the 75N 390HB grade steel rail and the electrode clamping end; 2) Use a GAAS80 / 580 fixed flash welding machine for rail joint welding; control the oil temperature of the welding machine at 38-42℃, the water temperature at 18-22℃, and the mains voltage at 416-425V. 3) Joint fixing flash welding process: The weld seam of the rail is controlled between 10-12 mm, the camber is controlled between 0.35-0.55 mm, the number of flash welding preheating times is controlled between 10-12, the secondary preheating current is controlled between 58-60 kA, the flash welding upsetting amount is controlled between 10-13 mm, the upsetting force is controlled between 550-570 kN, the welding time is controlled between 160-170 s, and the final melting speed is controlled between 2.20-2.30 mm / s; 4) Air cool the high-temperature welded joint to below 200°C to ensure that each part of the joint undergoes a complete phase transformation. When the joint temperature is below 300°C, accelerate the cooling appropriately. The mechanical properties of the 75N 390HB grade rail joints welded by the aforementioned fixed flash welding method meet the following requirements: tensile strength Rm≥1185MPa, joint elongation A≥10.5%, joint impact KU2≥19.2 J, and average joint hardness H. J Compared with the average hardness H of the base material P The ratio of H satisfies: J / H P ≥0.94, average hardness H of joint soft spots J1 Compared with the average hardness H of the base material P The ratio of: H J1 / H P ≥0.85, softened area W≤16 mm; The chemical composition of the 75N 390HB grade steel rail, by weight percentage, is C: 0.78–0.86%, Si: 0.50–0.80%, Mn: 0.80–1.20%, Cr: 0.20–0.55%, Nb: 0.01–0.03%, RE: 0.0005–0.0020%, P≤0.010%, S≤0.005%, Al≤0.004%, with the remainder being Fe and unavoidable impurities. The 75N 390HB grade steel rail is produced through smelting, continuous casting, slow cooling of the cast billet, and rolling processes, wherein the size of the cast billet is controlled to be 280mm×380mm.

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