A method for improving the post-weld performance of bainitic rail welded joints

Abstract

This invention belongs to the field of post-weld heat treatment of rails and discloses a method for improving the post-weld performance of bainitic rail welded joints. The method includes: cooling the welded bainitic rail joint to 870-1000℃; rapidly cooling the welded joint to room temperature using a full-atomization process to induce a martensitic transformation in the austenite; rapidly heating the welded joint to 900-1000℃ and holding it briefly to promote the transformation of the welded joint's microstructure from martensite to austenite; continuously cooling the welded joint to the bainitic transformation temperature range; performing isothermal treatment within the bainitic transformation temperature range to complete the bainitic transformation; and air-cooling the welded joint to room temperature. This method accelerates the bainitic transformation by refining the original austenite grain size and the bainite lath size. This refinement effect helps improve the service performance of the rail weld joint and comprehensively enhances the mechanical properties of the welded joint.

Description

Technical Field

[0001] This invention relates to the field of post-weld heat treatment technology for rails, and in particular to a method for improving the post-weld performance of bainitic rail welded joints. Background Technology

[0002] Bainitic rails offer a superior strength-to-toughness ratio compared to pearlitic rails. Bainitic steel also exhibits better wear resistance and fatigue performance. Rail welding technology is crucial for ensuring the continuity and stability of railway lines. However, in traditional welding processes for bainitic rails, the grains in the weld heat-affected zone (HAZ) often become coarse. This coarse grain development leads to a significant decrease in the mechanical properties of the weld area, such as strength, toughness, and fatigue performance, thus affecting the overall service performance and lifespan of the rail. In particular, the reduced safety results in safety hazards in railway transportation and increases maintenance costs.

[0003] To improve the post-weld performance of bainitic rails, various methods are currently employed for heat treatment of the welded joints. For example, annealing or normalizing can improve the microstructure of the welded joint, thereby enhancing its mechanical properties. However, these methods do not significantly improve the performance of the weld microstructure, are time-consuming, energy-intensive, and require sophisticated equipment, increasing production costs. Other methods improve the mechanical properties of the welded joint by controlling welding parameters, such as optimizing welding parameters like current, voltage, and welding speed to control heat input and cooling rate, thus reducing grain coarsening. However, this method demands high operator skill, precise welding operation, and has limited adaptability to the welding environment. Therefore, all these methods have certain limitations and shortcomings.

[0004] In summary, while existing methods for improving the post-weld performance of bainitic rails can enhance the performance of welded joints to some extent, they still suffer from high costs, low efficiency, and complex operations. Therefore, there is still significant room for improvement in existing methods for enhancing the post-weld performance of bainitic rails. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for improving the post-weld performance of bainitic rail welded joints to enhance their strength, toughness, and fatigue properties.

[0006] According to one aspect of the present invention, a method for improving the post-weld performance of bainitic rail welded joints is provided, comprising the following steps:

[0007] Step S1: Allow the hot bainitic rail welded joint to cool naturally to 870-1000℃;

[0008] Step S2: The welded joint is rapidly cooled to room temperature in a full atomization mode to cause the austenite in the welded joint to undergo a martensitic phase transformation;

[0009] Step S3: Rapidly heat the welded joint to 900-1000℃ and hold it at that temperature briefly to promote the transformation of the microstructure of the welded joint from martensite to austenite;

[0010] Step S4: Continuously cool the welded joint to the temperature range of bainite transformation;

[0011] Step S5: Perform isothermal treatment within the temperature range of the bainitic transformation to complete the bainitic transformation of the weld joint;

[0012] Step S6: Air cool the welded joint to room temperature.

[0013] According to one embodiment of the present invention, the bainitic rail base material comprises the following components by weight percentage: C content of 0.21%-0.25%, Si content of 2.0%-2.5%, Mn content of 0.60%-1.00%, Cr content of 0.60%-0.80%, Mo content of 0.40%-0.45%, at least one of V, Nb, and Ti, wherein the V content is 0.08%-0.10% when V is present, the Ti content is 0.01%-0.03% when Ti is present, the Nb content is 0.05%-0.08% when Nb is present, and the balance is Fe and unavoidable impurities.

[0014] According to one embodiment of the present invention, in step S1, the center temperature of the heat-affected zone after welding is 1550-1650°C, and the natural cooling rate is 3-5°C / s.

[0015] According to one embodiment of the present invention, in step S2, the rapid cooling rate is 10-30°C / s.

[0016] According to one embodiment of the present invention, the rapid cooling is performed using a rail head profile cooling device. The spray flow rates of the nozzles of the corresponding cooling devices at the rail head, rail web, and rail bottom are controlled respectively. The spray flow rates at the rail head, rail web, and rail bottom decrease sequentially. The nozzle pressure corresponding to the rail head is controlled at 1.3-1.5 MPa, the nozzle pressure corresponding to the rail web is controlled at 1.0-1.2 MPa, and the nozzle pressure corresponding to the rail bottom is controlled at 0.9-1.1 MPa.

[0017] According to one embodiment of the present invention, in step S3, the heating rate of the rapid heating is 8-10℃ / s, and the short-term heat preservation time is 15s-30s.

[0018] According to one embodiment of the present invention, in step S4, the continuous cooling includes: cooling at a cooling rate of 8-10°C / s in the range from the cooling start temperature to 801°C; cooling at a cooling rate of 4-7°C / s in the range of 800-501°C; and cooling at a cooling rate of 0.9-3°C / s in the range from 500°C to the bainite transformation temperature.

[0019] According to one embodiment of the present invention, in step S4, the temperature range of the bainitic transformation is 300-400°C.

[0020] According to one embodiment of the present invention, in step S5, the isothermal treatment time is 1-1.5 hours.

[0021] According to one embodiment of the present invention, in step S6, the cooling rate is 0.2-0.8℃ / s.

[0022] By employing the above technical solutions, the method provided by this invention improves the strength, toughness, and fatigue performance of bainitic rail welded joints. This method utilizes the residual heat temperature after welding above Ac3 for water mist quenching. During quenching, austenite undergoes a martensitic transformation. After quenching and cooling, the joint is reheated to above Ac3 for reautification. In this process, the martensitic transformation induced by quenching plays a crucial role. During the martensitic transformation, many fine subgrains can form within the original austenite grains. These subgrains can act as new nuclei during subsequent heating, promoting the formation of fine austenite grains. During reheating to the austenitization temperature, the microstructure of the martensite (such as dislocations)... Twins undergo recovery and recrystallization, forming fine austenite grains. Due to the presence of internal stress and substructure, grain growth is inhibited even during reheating, thus maintaining the fine grain size. The refinement of the original austenite grain size affects the morphology of bainite. Smaller austenite grain sizes may produce finer bainite structures, while improving the mechanical stability of the retained austenite. This can improve the strength, toughness, and fatigue performance of bainitic rails. Therefore, this method improves the service performance of welded joints and reduces production costs and operational difficulties. Attached Figure Description

[0023] Figure 1 A flowchart illustrating a method for improving the post-weld performance of bainitic rail welded joints according to an embodiment of the present invention;

[0024] Figure 2 A schematic diagram of a rail head profile cooling device used for heat treatment of welded rail joints;

[0025] Figure 3 This is a schematic diagram of a rail head contour dual-frequency induction heating device used for heat treatment of welded rail joints. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] The method of this invention utilizes the residual heat temperature after welding, above Ac3, for water mist quenching. During quenching, austenite undergoes a martensitic transformation. After quenching and cooling, it is reheated to above Ac3 for re-austenitization. In this process, the martensitic transformation induced by quenching plays a crucial role. During the martensitic transformation, many fine subgrains can form within the original austenite grains. These subgrains can act as new nuclei during subsequent heating, promoting the formation of fine austenite grains. During reheating to the austenitization temperature, the microstructures in the martensite (such as dislocations and twins) recover and recrystallize, forming fine austenite grains. Due to the presence of internal stress and substructures, grain growth is inhibited even during reheating, thus maintaining the fine grain size. The refinement of the original austenite grain size promotes bainite formation. The refined original austenite grains increase the grain boundary area, which can increase the nucleation sites for bainite transformation, thereby accelerating bainite formation. Refining the initial austenite grain size can influence the kinetics of bainite formation by increasing the difference between grain boundary activation energy and bainite autocatalytic nucleation. This typically leads to a lower critical cooling rate for bainite transformation, resulting in bainite formation over a wider range of cooling rates. Refining the initial austenite grain size also affects the bainite morphology; smaller austenite grain sizes can produce finer bainite structures while improving the mechanical stability of retained austenite. This can enhance the strength, toughness, and fatigue properties of bainitic rails.

[0028] like Figure 1 As shown, the method for improving the post-weld performance of bainitic rail welded joints provided by the present invention generally includes the following steps:

[0029] Step S1: Allow the hot bainitic rail welded joint to cool naturally to 870-1000℃;

[0030] Step S2: Rapidly cool the welded joint to room temperature in full atomization mode to cause the austenite in the welded joint to undergo a martensitic phase transformation;

[0031] Step S3: Rapidly heat the weld joint to 900-1000℃ and hold it at that temperature briefly to promote the transformation of the weld joint's microstructure from martensite to austenite.

[0032] Step S4: Continuously cool the welded joint to the temperature range of bainite transformation;

[0033] Step S5: Perform isothermal treatment within the bainitic transformation temperature range to complete the bainitic transformation of the weld joint;

[0034] Step S6: Allow the welded joint to air cool to room temperature.

[0035] In some embodiments, the bainitic rail base material of the welded joint comprises the following components by weight percentage: 0.21%-0.25% C, 2.0%-2.5% Si, 0.60%-1.00% Mn, 0.60%-0.80% Cr, 0.40%-0.45% Mo, at least one of V, Nb, and Ti, wherein the V content is 0.08%-0.10% when V is present, the Ti content is 0.01%-0.03% when Ti is present, and the Nb content is 0.05%-0.08% when Nb is present, with the balance being Fe and unavoidable impurities. The tensile strength of the rail base material is 1300-1600 MPa, and the impact toughness is 140-160 J.

[0036] In some embodiments, flash welding is used, and the center temperature of the heat-affected zone after welding is 1550-1650°C. The microstructure of the weld heat-affected zone is austenitic. In this invention, a "welded joint" is a region with a length of 80-120 mm, including the weld and the heat-affected zone, obtained after welding, with the rail weld at its center.

[0037] In step S1, the hot bainitic rail weld joint is naturally cooled to 870-1000°C. In some embodiments, for example, the weld joint can be naturally cooled at room temperature of 25-30°C to the range of 870-1000°C at a cooling rate of about 3-5°C / s.

[0038] In step S2, the welded joint is rapidly cooled to room temperature in a full atomization mode, causing the austenite in the welded joint to undergo a martensitic phase transformation. In some embodiments, for example, when the temperature of the welded joint drops to the range of 870-1000°C, a rail head profile cooling device is used to cool the welded area in a full atomization mode. The water mist spray flow rate is adjusted to regulate the cooling rate, keeping the cooling rate controlled at 10-30°C / s, causing the martensitic phase transformation in the welded joint area. During the martensitic phase transformation, many fine subgrains can form within the original austenite grains. These subgrains can act as new nuclei during subsequent heating, promoting the formation of fine austenite grains.

[0039] Figure 2A schematic diagram of a rail head profile cooling device for heat treatment of rail welded joints is shown. The cooling device consists of two parts: a left half and a right half. Each part comprises an outer shell 10 and an inner wall 20, which together form three cavities 30 distributed around the rail cross-section G. These three cavities cool the rail head, rail web, and rail base, respectively. Multiple nozzles 40 are located inside the inner wall 20, with different nozzle sizes on the nozzles of the nozzles 40 in different cavities, resulting in different spray pressures. The cavities of the rail head profile cooling device are connected to an external water source and pressure control device to adjust the water mist pressure. The rail head profile cooling device also includes a pressure sensor to detect the pressure within the cavities. The entire cooling device covers the welded joint and has a length of 80 mm.

[0040] The nozzle pressure of the railhead contour cooling device can be varied from 0.9MPa to 1.5MPa, and the cooling rate can be controlled by adjusting the jet flow rate. The jet flow rate refers to the total amount of medium ejected per unit time; the higher the flow rate, the stronger the cooling capacity.

[0041] Because the rail head (rail top), rail bottom, and rail web (rail side) have different geometries and thicknesses, their heat conduction and heat dissipation capabilities also differ, leading to inconsistent cooling rates under the same conditions. This difference in cooling rate can cause internal stress and cracks within the material. To address this, the spray flow rates of the cooling nozzles for the rail head, rail web, and rail bottom are controlled separately: the spray flow rates decrease sequentially from rail head to rail web to rail bottom. The nozzle pressure for the rail head is controlled at 1.3-1.5 MPa, for the rail web at 1.0-1.2 MPa, and for the rail bottom at 0.9-1.1 MPa, ensuring a consistent cooling rate for all three sections.

[0042] In step S3, the weld joint is rapidly heated to 900-1000°C and held at that temperature briefly to induce the microstructure of the weld joint to transform from martensite to austenite. In some embodiments, for example, the rapid heating rate is 8-10°C / s, and the brief holding time is 15-30s.

[0043] A railhead-modeling dual-frequency induction heating coil can be used to heat the weld joint within ±40mm of the weld center, controlling the heating rate at 8-10℃ / s to rapidly heat the weld joint to 870-1000℃. Holding this temperature for 15-30 seconds promotes the recombination of the weld microstructure from martensite to austenite. During reheating to the austenitizing temperature, the microstructure in the martensite (such as dislocations and twins) recovers and recrystallizes, forming fine austenite grains. Due to the presence of internal stress and substructure, grain growth is inhibited even during reheating, thus maintaining a fine grain size.

[0044] The rail head contouring dual-frequency induction heating coil has a width of 80mm, covering the welded joint. Figure 3 A schematic diagram of the heating device is shown. The device mainly consists of an induction heating coil C and a power controller P. The induction heating coil C is distributed around the rail section G, with a 10mm gap between its inner wall and the rail R. The induction coil C can be a dual-frequency induction heating coil.

[0045] In step S4, the welded joint is continuously cooled to the temperature range of the bainitic transformation. In some embodiments, for example, continuous cooling includes: cooling at a cooling rate of 8-10°C / s in the range from the cooling start temperature to 801°C; cooling at a cooling rate of 4-7°C / s in the range of 800-501°C; and cooling at a cooling rate of 0.9-3°C / s in the range from 500°C to the bainitic transformation temperature. The temperature range of the bainitic transformation is 300-400°C.

[0046] In step S5, isothermal treatment is performed within the temperature range of bainitic transformation to complete the bainitic transformation of the welded joint. After continuous cooling, when the temperature of the welded joint drops to the range of 300-400℃, isothermal treatment is performed for 1-1.5 hours using a rail head profile dual-frequency induction heating device, completing the bainitic transformation, meaning that austenite no longer transforms into new bainite. The refinement of the original austenite grain size accelerates bainitic kinetics, promotes bainite formation, and generates refined bainite laths. The refined original austenite grains increase the grain boundary area, which can increase the nucleation sites for bainitic transformation, thereby accelerating bainite formation. The refined original austenite grain size can affect the bainite formation kinetics by increasing the difference between the grain boundary activation energy and the autocatalytic nucleation of bainite, which usually leads to a lower critical cooling rate for bainitic transformation, thus forming bainite over a wider cooling rate range. Refining the original austenite grain size affects the morphology of bainite. Smaller austenite grain size can produce finer bainite structure and improve the mechanical stability of retained austenite, which can improve the strength, toughness and fatigue performance of bainitic rails.

[0047] In step S6, the welded joint is air-cooled to room temperature. In some embodiments, for example, the cooling rate is 0.2-0.8°C / s.

[0048] The present invention will be further illustrated by the following examples, but the scope of protection of the present invention is not limited thereto.

[0049] Example 1

[0050] The rails are made of 75 kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1500 MPa and an elongation of 15%. The room temperature U-shaped impact energy of the rail head is 155 J. The chemical composition by weight percentage is: C 0.21%, Si 2.0%, Mn 0.60%, Cr 0.60%, Mo 0.40%, V 0.08%, Ti 0.01%, Nb 0.05%, with the balance being Fe and unavoidable impurities.

[0051] A mobile flash welding machine was used to perform flash welding of the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 3 °C / s until the surface temperature of the rail head reached 900 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the weld joint to room temperature at a cooling rate of 30 °C / s. The nozzle pressure corresponding to the rail head was controlled at 1.5 MPa, the nozzle pressure corresponding to the rail web was controlled at 1.2 MPa, and the nozzle pressure corresponding to the rail base was controlled at 1.1 MPa to ensure that the cooling rates of the rail head, rail web, and rail base were consistent. A rail head contour dual-frequency induction heating device was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 8 °C / s, and the weld joint was quickly heated to 900 °C before heating was stopped, allowing the microstructure of the weld joint to re-austenite. After heating is stopped, continuous cooling is carried out at room temperature (25-30℃). The cooling rate is 10℃ / s in the range of 900℃-801℃; 6℃ / s in the range of 800℃-501℃; and 2℃ / s in the range of 500℃-350℃. When the temperature drops to 350℃, isothermal heat treatment is performed for 1 hour using a rail-head contour dual-frequency induction heating device. After the isothermal treatment, air cooling is carried out at room temperature at a cooling rate of 0.8℃ / s until room temperature is reached.

[0052] The final microstructure of the welded joint consists of bainite and a small amount of retained austenite. After treatment using the method of the present invention to improve the post-weld performance of bainitic rail welded joints, the average size of the original austenite grains in the welded joint microstructure, as determined by GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals", is 11 μm. The average tensile strength of the welded joint reaches 1435 MPa, and the average impact energy is 155 J. The strength and impact toughness of the full-section welded joint are at the same level as those of the rail base material.

[0053] Example 2

[0054] The rails are made of 60kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1450MPa and an elongation of 16%. The room temperature U-shaped impact energy of the rail head is 160J. The chemical composition by weight percentage is: C 0.25%, Si 2.5%, Mn 1.00%, Cr 0.80%, Mo 0.45%, V 0.10%, Ti 0.03%, Nb 0.08%, with the balance being Fe and unavoidable impurities.

[0055] A mobile flash welding machine was used to perform flash welding of the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 5 °C / s until the surface temperature of the rail head reached 1000 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the weld joint to room temperature at a cooling rate of 10 °C / s. The nozzle pressure corresponding to the rail head was controlled at 1.3 MPa, the nozzle pressure corresponding to the rail web was controlled at 1.0 MPa, and the nozzle pressure corresponding to the rail base was controlled at 0.9 MPa to ensure that the cooling rates of the rail head, rail web, and rail base were consistent. A rail head contour dual-frequency induction heating device was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 10 °C / s, and the weld joint was quickly heated to 1000 °C before heating was stopped, allowing the microstructure of the weld joint to re-austenite. After heating is stopped, continuous cooling is carried out at room temperature (25-30℃). The cooling rate is 8℃ / s in the range of 1000℃-801℃; 5℃ / s in the range of 800℃-501℃; and 1℃ / s in the range of 500℃-400℃. When the temperature drops to 400℃, isothermal heat treatment is performed for 1.2 hours using a rail-head contour dual-frequency induction heating device. After the isothermal treatment, air cooling is carried out at room temperature at a cooling rate of 0.5℃ / s until room temperature is reached.

[0056] The final microstructure of the welded joint consists of bainite and a small amount of retained austenite. After treatment using the method of the present invention to improve the post-weld performance of bainitic rail welded joints, the average size of the original austenite grains in the welded joint microstructure, as determined by GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals", is 15 μm. The average tensile strength of the welded joint reaches 1395 MPa, and the average impact energy is 158 J. The strength and impact toughness of the full-section welded joint are at the same level as those of the rail base material.

[0057] Example 3

[0058] The rails are made of 75kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1550MPa and an elongation of 13%. The room temperature U-shaped impact energy of the rail head is 150J. The chemical composition by weight percentage is: C 0.23%, Si 2.2%, Mn 0.80%, Cr 0.70%, Mo 0.43%, V 0.09%, Ti 0.02%, Nb 0.06%, with the balance being Fe and unavoidable impurities.

[0059] A mobile flash welding machine was used to perform flash welding of the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 4 °C / s until the surface temperature of the rail head reached 870 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the weld joint to room temperature at a cooling rate of 20 °C / s. The nozzle pressure corresponding to the rail head was controlled at 1.4 MPa, the nozzle pressure corresponding to the rail web was controlled at 1.1 MPa, and the nozzle pressure corresponding to the rail base was controlled at 1.0 MPa to ensure that the cooling rates of the rail head, rail web, and rail base were consistent. A rail head contour dual-frequency induction heating device was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 9 °C / s, and the weld joint was quickly heated to 950 °C before heating was stopped, allowing the microstructure of the weld joint to re-austenite. After heating is stopped, continuous cooling is carried out at room temperature (25-30℃). The cooling rate is 9℃ / s in the range of 870℃-801℃; 6℃ / s in the range of 800℃-501℃; and 2℃ / s in the range of 500℃-300℃. When the temperature drops to 300℃, isothermal heat treatment is performed for 1.5 hours using a rail-head contour dual-frequency induction heating device. After the isothermal treatment, air cooling is carried out at room temperature at a cooling rate of 0.2℃ / s until room temperature is reached.

[0060] The final microstructure of the welded joint consists of bainite and a small amount of retained austenite. After treatment using the method of the present invention to improve the post-weld performance of bainitic rail welded joints, the average size of the original austenite grains in the welded joint microstructure, as determined by GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals", is 13 μm. The average tensile strength of the welded joint reaches 1535 MPa, and the average impact energy is 144 J. The strength and impact toughness of the full-section welded joint are at the same level as those of the rail base material.

[0061] Comparative Example 1

[0062] The rails are made of 75kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1550MPa, an elongation of 13%, and a room temperature U-shaped impact energy of 150J. The chemical composition by weight percentage is: C 0.23%, Si 2.2%, Mn 0.80%, Cr 0.70%, Mo 0.43%, V 0.09%, Ti 0.02%, Nb 0.06%, with the balance being Fe and unavoidable impurities.

[0063] Using a mobile flash welding machine for rails, flash welding of rails was carried out with a heat input of 8MJ. After welding, the rails were cooled in air at 25℃. The cooling rates were as follows: 15℃ / s in the range of 1500℃-1001℃; 9℃ / s in the range of 1000℃-801℃; 6℃ / s in the range of 800℃-501℃; 2℃ / s in the range of 500℃-350℃; and 0.2℃ / s in the range of 350℃-25℃.

[0064] The final microstructure of the welded joint consists of bainite, martensite, and a small amount of retained austenite. The average size of the original austenite grains in the welded joint microstructure, determined using GB / T 6394-2017 (Method for Determination of Average Grain Size of Metals), is 150 μm. The average tensile strength of the welded joint is approximately 750 MPa, and the average impact energy is 35 J. The strength and impact toughness of the full-section welded joint differ significantly from those of the base material.

[0065] Comparative Example 2

[0066] The rails are made of 75kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1550MPa, an elongation of 13%, and a room temperature U-shaped impact energy of 150J. The chemical composition by weight percentage is: C 0.23%, Si 2.2%, Mn 0.80%, Cr 0.70%, Mo 0.43%, V 0.09%, Ti 0.02%, Nb 0.06%, with the balance being Fe and unavoidable impurities.

[0067] A mobile flash welding machine was used to perform flash welding on the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 5 °C / s until the surface temperature of the rail head reached 950 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the rails to room temperature at a cooling rate of 20 °C / s. The nozzle pressure corresponding to the rail head was controlled at 1.4 MPa, the nozzle pressure corresponding to the rail web was controlled at 1.1 MPa, and the nozzle pressure corresponding to the rail base was controlled at 1.0 MPa to ensure that the cooling rates of the rail head, rail web, and rail base were consistent.

[0068] The final microstructure of the welded joint is martensite. The average austenite grain size of the welded joint microstructure, determined using GB / T 6394-2017 (Method for Determination of Average Grain Size of Metals), is 40 μm, consistent with the austenite grain size of the rail base material. The average tensile strength of the welded joint reaches 1635 MPa, and the average impact energy is 24 J. While the strength of the full-section welded joint is higher than that of the base material, its impact toughness is significantly lower.

[0069] Comparative Example 3

[0070] The rails are made of 75 kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1500 MPa and an elongation of 15%. The room temperature U-shaped impact energy of the rail head is 155 J. The chemical composition by weight percentage is: C 0.21%, Si 2.0%, Mn 0.60%, Cr 0.60%, Mo 0.40%, V 0.08%, Ti 0.01%, Nb 0.05%, with the balance being Fe and unavoidable impurities.

[0071] A mobile flash welding machine was used to perform flash welding on the rails with a heat input of 8 MJ. After flash welding, the rails were allowed to cool naturally in air at 25°C. The cooling rates were as follows: 15°C / s in the range of 1500°C-1001°C; 9°C / s in the range of 1000°C-801°C; 6°C / s in the range of 800°C-501°C; 2°C / s in the range of 500°C-350°C; and 0.2°C / s in the range of 350°C-25°C. A rail-head contour dual-frequency induction heating coil was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 8°C / s, and the weld joint was rapidly heated to 900°C before heating was stopped, resulting in the re-austenitization of the weld joint microstructure. After heating is stopped, continuous cooling in air is carried out at room temperature of 25-30℃. The cooling rate is 10℃ / s in the range of 900℃-801℃; 6℃ / s in the range of 800℃-501℃; and 2℃ / s in the range of 500℃-350℃. When the temperature drops to 350℃, isothermal heat treatment is performed for 1 hour using a rail-head profile dual-frequency induction heating coil. After the isothermal treatment, continuous cooling is carried out at room temperature of 25℃-30℃ at a cooling rate of 0.8℃ / s to room temperature.

[0072] The final microstructure of the welded joint consisted of bainite and a small amount of retained austenite. The average size of the original austenite grains in the welded joint microstructure, determined using GB / T 6394-2017 (Method for Determination of Average Grain Size of Metals), was 45 μm. The average tensile strength of the welded joint reached 1385 MPa, and the average impact energy was 125 J. While the strength of the full-section welded joint was higher than that of the base metal, its impact toughness was significantly lower.

[0073] Comparative Example 4

[0074] Bainitic steel rails with a specification of 75 kg / m were used. The rail base material had a room temperature (20-25℃) tensile strength of 1350 MPa, an elongation of 11%, and a room temperature U-shaped impact energy of 130 J. However, the chemical composition was not within the range of the steel rail chemical composition involved in this invention. In this comparative example, the rail base material included the following components by weight percentage: C content 0.15%, Si content 2.0%, Mn content 0.60%, Cr content 0.60%, Mo content 0.40%, V content 0.08%, Ti content 0.01%, Nb content 0.05%, with the balance being Fe and unavoidable impurities.

[0075] A mobile flash welding machine was used to perform flash welding of the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 3 °C / s until the surface temperature of the rail head reached 900 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the rails to room temperature at a cooling rate of 30 °C / s. The nozzle pressure for the rail head was controlled at 1.5 MPa, the nozzle pressure for the rail web at 1.2 MPa, and the nozzle pressure for the rail base at 1.1 MPa to ensure consistent cooling rates for the rail head, rail web, and rail base. A rail head contour dual-frequency induction heating coil was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 8 °C / s, and the weld joint was rapidly heated to 900 °C before heating was stopped, allowing the weld joint microstructure to re-austenite. After heating is stopped, continuous cooling is performed at room temperature (25℃-30℃). The cooling rate is 10℃ / s in the range of 900℃-801℃; 6℃ / s in the range of 800℃-501℃; and 2℃ / s in the range of 500℃-350℃. When the temperature drops to 350℃, isothermal heat treatment is performed for 1 hour using a rail-head contour dual-frequency induction heating device. After the isothermal treatment, continuous cooling in air is performed at room temperature (25℃-30℃) at a cooling rate of 0.8℃ / s to room temperature.

[0076] The final microstructure of the welded joint consisted of bainite and a small amount of retained austenite. The average size of the original austenite grains in the welded joint microstructure, measured according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals," was 11 μm. The reduced carbon content led to a decrease in the mechanical properties of the rail base material, resulting in a lower microstructure and performance of the welded joint. The average tensile strength of the welded joint reached 1235 MPa, and the average impact energy was 125 J. However, the strength and impact toughness of the full-section welded joint were lower than the required strength and toughness.

[0077] Comparative Example 5

[0078] The rails are made of 75kg / m bainitic steel. The rail base material has a room temperature (20-25℃) tensile strength of 1550MPa, an elongation of 13%, and a room temperature U-shaped impact energy of 150J. The chemical composition by weight percentage is: C 0.23%, Si 2.2%, Mn 0.80%, Cr 0.70%, Mo 0.43%, V 0.09%, Ti 0.02%, Nb 0.06%, with the balance being Fe and unavoidable impurities.

[0079] A mobile flash welding machine was used to perform flash welding of the rails with a heat input of 8 MJ. After flash welding, the rails were cooled at a rate of 5 °C / s until the surface temperature of the rail head reached 910 °C. A rail head contour cooling mist device was used, with cooling water as the cooling medium, to quench the rails to room temperature at a cooling rate of 5 °C / s. The nozzle pressure corresponding to the rail head was controlled at 0.4 MPa, the nozzle pressure corresponding to the rail web at 0.2 MPa, and the nozzle pressure corresponding to the rail base at 0.1 MPa to ensure that the cooling rates of the rail head, rail web, and rail base were consistent. A rail head contour dual-frequency induction heating device was used to heat the weld joint within ±40 mm of the weld center. The heating rate was controlled at 9 °C / s, and the weld joint was quickly heated to 950 °C before heating was stopped, allowing the microstructure of the weld joint to re-austenite. After heating is stopped, cooling is carried out at room temperature of 25℃-30℃. The cooling rate is 9℃ / s in the range of 950℃-801℃; 6℃ / s in the range of 800℃-501℃; and 2℃ / s in the range of 500℃-350℃. When the temperature drops to 350℃, isothermal heat treatment is performed for 1 hour using a rail head contouring dual-frequency induction heating device. After the isothermal treatment, cooling is carried out at room temperature of 25℃-30℃ at a cooling rate of 0.2℃ / s to room temperature.

[0080] The final microstructure of the welded joint consisted of bainite and a small amount of retained austenite. Because the quenching process did not involve high cooling rates below the Ms point, martensitic transformation did not occur, preventing the use of micron-sized martensite laths to refine the austenite grains, thus hindering the refinement of the original austenite grain size. The average size of the original austenite grains in the welded joint microstructure, determined using GB / T 6394-2017 (Method for Determination of Average Grain Size of Metals), was 86 μm. The average tensile strength of the welded joint reached 1035 MPa, and the average impact energy was 75 J. However, the strength and impact toughness of the full-section welded joint were lower than the required strength and toughness.

[0081] Comparative analysis of the above embodiments and comparative examples shows that, using the method of the present invention, the original austenite grain size is significantly refined from 80-160 μm to 10-20 μm. This refinement accelerates the bainite transformation during continuous cooling, resulting in finer bainite lath sizes. This refinement helps improve the service performance of the welded joint of the rail. Furthermore, the refinement of the original austenite grains improves the uniformity of the bainite microstructure during continuous cooling, enhancing the mechanical properties of the welded joint. With conventional welding methods, the tensile strength of the welded joint is 900-1100 MPa, and the impact energy is 30-50 J. However, after refining the original austenite grain size, the tensile strength of the welded joint increases to 1350-1550 MPa, and the impact energy increases to 140-150 J. The method of the present invention is applicable to bainitic rails, is simple to operate, easy to control, and can be easily integrated into existing welding and heat treatment production lines without large-scale equipment modifications, thus having broad application prospects.

[0082] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for improving the post-weld performance of bainitic rail welded joints, characterized in that, It includes the following steps: Step S1: Naturally cool the welded hot bainitic rail welded joint to 870 - 1000 °C, the center temperature of the post-weld heat-affected zone is 1550 - 1650 °C, and the cooling rate of the natural cooling is 3 - 5 °C / s; Step S2: Rapidly cool the welded joint to room temperature in a full atomization mode, causing the austenite of the welded joint to undergo martensitic transformation, and the cooling rate of the rapid cooling is 10 - 30 °C / s; Step S3: Rapidly heat the welded joint to 900 - 1000 °C and hold it briefly, the heating rate of the rapid heating is 8 - 10 °C / s, and the holding time of the brief holding is 15 s - 30 s, promoting the structure of the welded joint to transform from martensite to austenite; Step S4: Continuously cool the welded joint to the temperature range of bainite transformation, and the continuous cooling includes: cooling at a cooling rate of 8 - 10 °C / s within the range from the starting cooling temperature to 801 °C; Cooling at a cooling rate of 4 - 7 °C / s within the range of 800 - 501 °C; and cooling at a cooling rate of 0.9 - 3 °C / s within the range from 500 °C to the bainite transformation temperature; Step S5: Perform isothermal treatment within the temperature range of bainite transformation to complete the bainite transformation of the welded joint; Step S6: Air-cool the welded joint to room temperature.

2. The method according to claim 1, characterized in that, The bainitic rail base material includes the following components by weight percentage: C content is 0.21% - 0.25%, Si content is 2.0% - 2.5%, Mn content is 0.60% - 1.00%, Cr content is 0.60% - 0.80%, Mo content is 0.40% - 0.45%, at least one of V, Nb, and Ti, when containing V, the V content is 0.08% - 0.10%, when containing Ti, the Ti content is 0.01% - 0.03%, when containing Nb, the Nb content is 0.05% - 0.08%, and the balance is Fe and inevitable impurities.

3. The method according to claim 1, characterized in that, The rapid cooling is carried out by using a rail head profiling cooling device, and the injection flow rates of the nozzles of the corresponding cooling devices for the rail head, rail waist, and rail bottom are respectively controlled. The injection flow rates of the rail head, rail waist, and rail bottom decrease in sequence. The nozzle pressure corresponding to the rail head is controlled at 1.3 - 1.5 MPa, the nozzle pressure corresponding to the rail waist is controlled at 1.0 - 1.2 MPa, and the nozzle pressure corresponding to the rail bottom is controlled at 0.9 - 1.1 MPa.

4. The method according to claim 2, characterized in that, In step S4, the temperature range of bainite transformation is 300 - 400 °C.

5. The method according to claim 2, characterized in that, In step S5, the time of the isothermal treatment is 1 - 1.5 h.

6. The method according to claim 2, characterized in that, In step S6, the cooling rate is 0.2 - 0.8 °C / s.

Images