A method for heat treatment of hypereutectoid rail steel after flash welding

Abstract

This invention belongs to the field of metal welding technology and relates to a heat treatment method for hypereutectoid rail steel after flash welding, comprising the following steps: post-weld cooling: the welded joint is allowed to cool naturally in air after flash welding; normalizing treatment: the welded joint is heated to a normalizing temperature above the austenitizing temperature and held at that temperature; controlled cooling: the temperature of the weld heat-affected zone of the welded joint is cooled from the normalizing temperature to the isothermal transformation temperature within the pearlite phase transformation temperature range at a first cooling rate, held at that temperature until the austenite undergoes pearlite transformation, and then the welded joint is cooled from the isothermal transformation temperature to below 200°C at a second cooling rate lower than the first cooling rate. The method of this invention can effectively eliminate the network cementite structure in the flash welding heat-affected zone, while avoiding the introduction of harmful structures such as martensite or bainite, ensuring that the microstructure of the weld heat-affected zone is pearlite with fine lamellar spacing.

Description

Technical Field

[0001] This invention belongs to the field of metal material welding technology, and relates to a heat treatment method for hypereutectoid rail steel after flash welding. Background Technology

[0002] With the development of railway transportation towards high speed and heavy load, the performance requirements for rail materials are increasing. Hypereutectoid rail steel, due to its carbon content exceeding the eutectoid point (approximately 0.77% C), possesses excellent wear resistance and high strength, and is widely used in heavy-haul railway lines. Hypereutectoid rail steel with a tensile strength reaching 1380 MPa is currently a representative material for high-strength rails.

[0003] Flash welding is one of the main on-site welding processes for rails, offering advantages such as high welding efficiency, high automation, and stable joint quality. However, during flash welding of hypereutectoid rail steel, the heat-affected zone (HAZ) undergoes a complex microstructural transformation process due to the welding thermal cycle. Particularly under the peak temperature of the welding thermal cycle, austenitization occurs in the area near the weld. Because hypereutectoid steel has a high carbon content, network or semi-network cementite precipitates along the original austenite grain boundaries during subsequent cooling. The presence of this network cementite significantly reduces grain boundary bonding, becoming a preferential pathway for crack initiation and propagation, thus leading to a sharp decline in the joint's impact toughness and fatigue performance.

[0004] According to existing research, the detrimental effects of network cementite on the mechanical properties of hypereutectoid steel are mainly manifested in the following aspects: First, the continuous distribution of network cementite along grain boundaries disrupts the continuity of the matrix structure, easily leading to stress concentration under stress. Second, network cementite has high hardness and brittleness, and the interface between it and the pearlite matrix becomes a weak point for crack initiation. Third, under impact or fatigue loads, cracks tend to propagate along the grain boundary network cementite, leading to brittle fracture at the joint.

[0005] Existing techniques for eliminating network cementite in hypereutectoid steel welded joints mainly include: preheating before welding, slow cooling after welding, and overall heat treatment after welding. However, these methods have the following problems: (1) Although preheating before welding can reduce the cooling rate of the welding thermal cycle, it cannot fundamentally change the carbon distribution in hypereutectoid steel, and the network cementite still exists. (2) Slow cooling after welding can easily lead to coarse pearlite lamellar spacing, reducing the hardness and strength of the joint; (3) Although post-weld heat treatment can improve the microstructure, it is energy-intensive, inefficient, and difficult to achieve differentiated control of rails with different compositions on both sides of the weld.

[0006] Furthermore, for welding dissimilar hypereutectoid rail steel, due to the different chemical compositions of the rails on both sides of the weld, their austenitizing temperature, phase transformation temperature, and critical cooling rate for martensitic transformation are all different, making it difficult for a single heat treatment regime to simultaneously meet the performance requirements of both sides.

[0007] Therefore, there is an urgent need to develop a post-weld heat treatment process for flash welded joints of hypereutectoid rail steel that can effectively eliminate the network cementite in the heat-affected zone, while avoiding the introduction of harmful structures such as martensite or bainite, and ensuring that the welded joint has excellent comprehensive mechanical properties. Summary of the Invention

[0008] In view of this, the present invention proposes a heat treatment method for hypereutectoid rail steel after flash welding. This method can effectively eliminate the network cementite structure in the heat-affected zone of flash welding, while avoiding the introduction of harmful structures such as martensite or bainite, ensuring that the structure of the weld heat-affected zone is pearlite with fine lamellar spacing, and making the hardness of the heat-affected zone within 30mm on both sides of the weld reach more than 90% of the hardness of the corresponding rail base material.

[0009] To solve at least one of the above-mentioned technical problems, the present invention adopts the following technical solution: According to the present invention, a heat treatment method for hypereutectoid rail steel after flash welding is provided, comprising the following steps: post-weld cooling: the welded joint after flash welding is allowed to cool naturally in air; normalizing treatment: the welded joint is heated to a normalizing temperature above the austenitizing temperature and held at that temperature; controlled cooling: the temperature of the weld heat-affected zone of the welded joint is cooled from the normalizing temperature to the isothermal transformation temperature within the pearlite phase transformation temperature range at a first cooling rate, held at that temperature until the austenite undergoes a pearlite transformation, and then the welded joint is cooled from the isothermal transformation temperature to below 200°C at a second cooling rate lower than the first cooling rate.

[0010] According to one embodiment of the present invention, the hypereutectoid rail has a tensile strength of 1380 MPa or higher, and by mass percentage, the carbon content is 0.80%~0.95%, the manganese content is 0.70%~1.20%, the silicon content is 0.40%~0.90%, the chromium content is 0.20%~0.60%, the vanadium content is 0.05%~0.15%, and the remainder is iron and unavoidable impurity elements.

[0011] According to one embodiment of the present invention, the flash welding adopts a continuous flash welding or preheating flash welding process, the welding current is 300~800A, the welding voltage is 350~500V, the upsetting force is 400~800kN, the upsetting amount is 8~15mm, and the flash allowance is 20~40mm.

[0012] According to one embodiment of the present invention, in the post-weld cooling step, the welded joint is cooled to below ~540°C.

[0013] According to one embodiment of the present invention, in the normalizing process, the welded joint is heated to 950~1050°C by medium-frequency induction heating, and the holding time is 3~10 minutes. The frequency of medium-frequency induction heating is 1~10kHz, and the heating rate is 5~20°C / s.

[0014] According to one embodiment of the present invention, in the normalizing treatment step, for dissimilar steel welded joints with different compositions of hypereutectoid steel rails on both sides of the weld, the heating temperature of the heat-affected zone on both sides of the weld is controlled respectively, and the temperature difference between the two sides is controlled within the range of 0~50℃.

[0015] According to one embodiment of the present invention, in the controlled cooling step, the first cooling rate is 3.0~15.0℃ / s; the isothermal transition temperature is 550~620℃, and the isothermal time is 3~15 minutes; the second cooling rate is 0.5~2.0℃ / s.

[0016] According to one embodiment of the present invention, compressed air or a water mist mixture is used as the cooling medium to cool the temperature of the weld heat-affected zone of the welded joint from the normalizing temperature to the isothermal transformation temperature, wherein the pressure of the compressed air is 0.3~0.8MPa and the flow rate is 5~20m³. 3 / min; the water flow rate of the water mist mixture is 0.5~3.0L / min, and the gas pressure is 0.2~0.6MPa.

[0017] According to one embodiment of the present invention, during the isothermal transition, the temperature fluctuation range is controlled within ±15°C by covering the weld joint with thermal insulation material or by using auxiliary heating.

[0018] According to one embodiment of the present invention, the method further includes: post-heat treatment: after controlled cooling, a supplementary tempering treatment is performed, the tempering temperature is 400~500℃, and the holding time is 30~60 minutes.

[0019] By adopting the above technical solution, the present invention has at least one of the following advantages compared with the prior art: (1) Effectively eliminate network cementite: By heating the heat-affected zone of the weld to 950~1050℃ for austenitization, the network cementite precipitated along the grain boundaries during the flash welding process is fully dissolved. Then, by controlling the cooling, a pearlitic structure with fine lamellar spacing is obtained, which fundamentally eliminates the harmful effects of network cementite on the mechanical properties of the joint.

[0020] (2) Avoid the formation of harmful structures: By precisely controlling the cooling rate and the final cooling temperature, abnormal structures such as martensite or bainite are avoided during the cooling process. In particular, rapid cooling is stopped at 50~100℃ above the martensite transformation start temperature to ensure that austenite is transformed into pearlite.

[0021] (3) Ensure uniformity of hardness: By controlling the heat treatment process parameters reasonably, the hardness of the heat-affected zone within 30mm on both sides of the weld reaches more than 90% of the hardness of the corresponding rail base material, thus avoiding the influence of the softened zone on the mechanical properties of the joint.

[0022] (4) Adaptation to dissimilar steel welding: For dissimilar steel welding where the two sides of the weld are hypereutectoid steel rails with different compositions, the heating temperature and cooling rate of the heat-affected zones on both sides of the weld are controlled respectively to meet the difference in phase transformation characteristics of the two materials and achieve uniform performance of dissimilar steel welded joints.

[0023] (5) Improve comprehensive mechanical properties: After being processed by the process of the present invention, the welded joint has good strength, toughness and fatigue performance. Its tensile strength reaches more than 95% of the base material, its impact toughness reaches more than 90% of the base material, and its fatigue life is increased by more than 30% compared with conventional processes. Attached Figure Description

[0024] 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.

[0025] Figure 1 A flowchart of a heat treatment method for hypereutectoid rail steel after flash welding according to an embodiment of the present invention; Figure 2 This is a longitudinal section hardness test location diagram of the rail welded joint 3-5mm below the rail head tread surface, obtained according to the methods in the embodiments and comparative examples of the present invention. In this diagram, point a is the left rail, point c is the center of the weld joint, and point b is the right rail. Figure 3 This is a schematic diagram showing the sampling location of metallographic specimens on the rail head tread of a welded rail joint. Point c is the center of the weld, and point d is the sampling location of the metallographic specimens on the rail head tread. Detailed Implementation

[0026] 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.

[0027] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0028] According to the present invention, such as Figure 1 As shown, a heat treatment method for hypereutectoid rail steel after flash welding is proposed, which mainly includes the following steps: Step S10: Post-weld cooling. The welded joint after flash welding is allowed to cool naturally in the air. Step S20: Normalizing treatment, heating the welded joint to a normalizing temperature above the austenitizing temperature and holding it at that temperature; Step S30: Control the cooling. Cool the temperature of the weld heat-affected zone of the weld joint from the normalizing temperature to the isothermal transformation temperature within the pearlite phase transformation temperature range at a first cooling rate. Hold the temperature until the austenite undergoes pearlite transformation, and then cool the weld joint from the isothermal transformation temperature to below 200°C at a second cooling rate lower than the first cooling rate.

[0029] In embodiments of the present invention, the tensile strength of the hypereutectoid steel rail forming the welded joint can be above 1380 MPa. By mass percentage, the carbon content is 0.80%~0.95%, the manganese content is 0.70%~1.20%, the silicon content is 0.40%~0.90%, the chromium content is 0.20%~0.60%, the vanadium content is 0.05%~0.15%, and the remainder is iron and unavoidable impurities. Flash welding can be performed using continuous flash welding or preheating flash welding processes. Preferably, the welding current is controlled at 300~800 A, the welding voltage at 350~500 V, the upsetting force at 400~800 kN, the upsetting amount at 8~15 mm, and the flash allowance at 20~40 mm to ensure stable welded joint quality, controllable heat-affected zone width, provide a microstructure basis for subsequent heat treatment, and avoid the impact of welding defects on the heat treatment effect.

[0030] In the post-weld cooling step S10, after flash welding, the welded joint can be naturally cooled in air to below 540°C. The pearlite transformation temperature of hypereutectoid rail steel is between 550 and 620°C. Cooling the welded joint to below 540°C ensures that the pearlite transformation has been completed in the heat-affected zones on both sides of the weld. Furthermore, the critical cooling rate for martensitic transformation in hypereutectoid rail steel is 1.0 to 2.0°C / s. The cooling rate with ambient temperature is typically lower than the lower limit of the critical cooling rate for martensitic transformation in rail steel (1.0°C / s), preventing the martensitic transformation from being triggered by an excessively rapid cooling rate. If the ambient temperature is low, a large temperature difference between the welded joint and the environment can lead to excessively rapid cooling. Preferably, the welded joint can be naturally cooled in air to 330 to 540°C, meaning that rapid cooling is stopped above 330°C. The martensitic transformation start temperature of hypereutectoid rail steel is 180~230℃. Controlling the final cooling temperature during the cooling stage to above 100℃ of the martensitic transformation start temperature can effectively avoid the formation of martensitic structures that are harmful to toughness and plasticity due to excessively fast cooling rate or excessively low final cooling temperature.

[0031] In the normalizing treatment step S20, a medium-frequency induction coil can be used to heat the weld joint. The heating temperature is controlled at 950~1050℃, and the holding time is 3~10 minutes. The purpose of the heating temperature is to induce austenitization in the heat-affected zone and fully dissolve the network cementite precipitated along the grain boundaries, while simultaneously obtaining refined austenite grains, providing more nucleation sites for subsequent pearlite transformation. Preferably, the frequency of the medium-frequency induction heating is 1~10kHz, and the heating rate is 5~20℃ / s. In an embodiment of the present invention, for dissimilar steel weld joints with hypereutectoid steel rails of different compositions on both sides of the weld, the heating temperature of the heat-affected zone on both sides of the weld is separately controlled by controlling the power output of the medium-frequency induction coils on both sides of the weld, with the temperature difference between the two sides controlled within the range of 0~50℃.

[0032] After the normalizing heating and holding are completed, the welded joint is controlled to cool in step S30. The cooling process is divided into the following three stages: Phase 1: Rapid Cooling Phase Compressed air or a water mist mixture is used as the cooling medium to rapidly cool the temperature of the weld heat-affected zone from the normalizing temperature to the pearlite phase transformation temperature range of 550~620℃, with a cooling rate of 3.0~15.0℃ / s.

[0033] The compressed air pressure is 0.3~0.8MPa, and the flow rate is 5~20m³ / h. 3 / min; the water flow rate of the water mist mixture is 0.5~3.0L / min, and the gas pressure is 0.2~0.6MPa.

[0034] Second stage: Isothermal transition stage The austenite is held at a temperature range of 550~620℃ for 3~15 minutes to induce a pearlite transformation.

[0035] During the isothermal transition stage, the temperature can be controlled by covering the welded joint with insulation material or by using auxiliary heating, with the temperature fluctuation range controlled within ±15℃.

[0036] Phase Three: Slow Cooling Phase After the pearlite transformation is complete, the weld joint is cooled from the isothermal transformation temperature to below 200℃ at a cooling rate of 0.5~2.0℃ / s.

[0037] It should be noted that the critical cooling rate for martensitic transformation in hypereutectoid rail steel is 1.0~2.0℃ / s, and the martensitic transformation initiation temperature is 180~230℃. During the rapid cooling stage, to avoid the formation of brittle martensite, the final cooling temperature must be controlled to be 50~100℃ above the martensitic transformation initiation temperature, i.e., rapid cooling must cease above 280~330℃, and a slow cooling stage must commence.

[0038] Depending on the performance requirements of the welded joint, supplementary tempering can be performed after step S30. In a preferred embodiment of the present invention, the post-weld heat treatment method further includes: Step S40: Post-heat treatment, tempering temperature 400~500℃, holding time 30~60 minutes. Post-heat treatment can further eliminate residual stress and improve joint toughness.

[0039] The following are specific embodiments of the heat treatment method for hypereutectoid rail steel after flash welding according to the present invention.

[0040] Example 1 This embodiment describes the heat treatment process following flash welding of the same type of hypereutectoid rail.

[0041] Material Information: Both sides of the weld are made of the same hypereutectoid rail steel, with the following chemical composition (mass fraction %): C 0.85%, Mn 0.92%, Si 0.58%, Cr 0.35%, V 0.08%, P 0.015%, S 0.010%, and the balance being Fe. The rail has a tensile strength of 1320 MPa and a hardness of 380 HV.

[0042] Welding process: Continuous flash welding process is adopted, with a welding current of 550A, a welding voltage of 420V, an upsetting force of 600kN, an upsetting amount of 12mm, and a flash allowance of 30mm.

[0043] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0044] (2) Medium frequency induction heating normalizing treatment: The welded joint is heated by a medium frequency induction coil. The heating temperature is 1000℃, the holding time is 6 minutes, the induction heating frequency is 3kHz, and the heating rate is 12℃ / s.

[0045] (3) Controlled cooling: The first stage of rapid cooling uses compressed air as the cooling medium, with a pressure of 0.5 MPa and a flow rate of 12 m³ / s. 3 The temperature of the weld heat-affected zone is rapidly cooled from 1000℃ to 580℃ at a rate of 8.0℃ / s.

[0046] The second stage isothermal transition: maintain isothermal temperature at 580℃ for 8 minutes, with temperature fluctuations controlled within ±10℃.

[0047] The third stage is slow cooling: the welded joint is cooled from 580°C to room temperature at a cooling rate of 1.0°C / s.

[0048] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite at the original austenite grain boundaries has been eliminated, and the microstructure is pearlite with fine lamellar spacing. The average pearlite lamellar spacing is 0.18 μm. No abnormal microstructures such as martensite or bainite were found.

[0049] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 365HV, which reaches 96.1% of the hardness of the base material.

[0050] Mechanical property test results: Tensile strength: 1295 MPa (98.1% of the base material) Elongation after fracture: 11.5% Impact toughness (room temperature): 28 J / cm 2 Fatigue life (stress amplitude 400MPa): 2.8×10 6 Second loop Example 2 This embodiment describes the same heat treatment process after flash welding of hypereutectoid rails. The difference between this embodiment and Embodiment 1 lies in the heating temperature and holding time.

[0051] Material information: Same as in Example 1.

[0052] Welding process: Same as in Example 1.

[0053] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0054] (2) Medium frequency induction heating normalizing treatment: heating temperature is 980℃, holding time is 8 minutes, induction heating frequency is 4kHz, and heating rate is 10℃ / s.

[0055] (3) Controlled cooling: The first stage of rapid cooling uses a water mist mixture as the cooling medium, with a water flow rate of 1.5 L / min and a gas pressure of 0.4 MPa, to rapidly cool the temperature of the weld heat-affected zone from 980℃ to 600℃ at a cooling rate of 6.0℃ / s.

[0056] The second stage isothermal transition: maintain isothermal temperature at 600℃ for 6 minutes, with temperature fluctuations controlled within ±12℃.

[0057] The third stage is slow cooling: the welded joint is cooled from 600°C to room temperature at a cooling rate of 0.8°C / s.

[0058] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated and the microstructure is pearlite with fine lamellar spacing. The average lamellar spacing of the pearlite is 0.20 μm. No abnormal microstructures such as martensite or bainite were found.

[0059] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 360HV, which reaches 94.7% of the hardness of the base material.

[0060] Mechanical property test results: Tensile strength: 1288MPa (97.5% of the base material) Elongation after fracture: 12.0% Impact toughness (room temperature): 30 J / cm 2 Fatigue life (stress amplitude 400 MPa): 3.1 × 10⁻⁶ 6 Second loop Example 3 This embodiment describes the same heat treatment process after flash welding of hypereutectoid rails, but differs from Embodiment 1 in the cooling method.

[0061] Material Information: Both sides of the weld are made of the same hypereutectoid rail steel, with the following chemical composition (mass fraction %): C 0.90%, Mn 1.05%, Si 0.72%, Cr 0.42%, V 0.10%, P 0.012%, S 0.008%, and the balance being Fe. The rail has a tensile strength of 1350 MPa and a hardness of 395 HV.

[0062] Welding process: Preheating flash welding process is adopted, with a preheating temperature of 400℃, 5 preheating cycles, a welding current of 600A, a welding voltage of 400V, an upsetting force of 650kN, an upsetting amount of 10mm, and a flash allowance of 35mm.

[0063] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is cooled to 500°C in the air.

[0064] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1020℃, holding time is 5 minutes, induction heating frequency is 5kHz, and heating rate is 15℃ / s.

[0065] (3) Controlled cooling: The first stage of rapid cooling uses compressed air as the cooling medium, with a pressure of 0.6 MPa and a flow rate of 15 m³ / h. 3 The temperature of the weld heat-affected zone is rapidly cooled from 1020℃ to 560℃ at a rate of 10.0℃ / s.

[0066] The second stage isothermal transition: maintain isothermal temperature at 560℃ for 10 minutes, with temperature fluctuations controlled within ±10℃.

[0067] The third stage is slow cooling: the welded joint is cooled from 560°C to room temperature at a cooling rate of 1.2°C / s.

[0068] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated and the microstructure is pearlite with fine lamellar spacing. The average lamellar spacing of the pearlite is 0.15μm. No abnormal microstructures such as martensite or bainite were found.

[0069] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 380HV, which reaches 96.2% of the hardness of the base material.

[0070] Mechanical property test results: Tensile strength: 1320MPa (97.8% of the base material) Elongation after fracture: 10.8% Impact toughness (room temperature): 26 J / cm 2 Fatigue life (stress amplitude 400MPa): 2.5×10 6 Second loop Example 4 This embodiment describes the heat treatment process after flash welding of heteroeutectoid rails.

[0071] Material Information: One side of the weld is made of hypereutectoid rail steel A, with the following chemical composition (mass fraction %): C 0.82%, Mn 0.85%, Si 0.55%, Cr 0.30%, V 0.06%, P 0.018%, S 0.012%, balance Fe. This rail has a tensile strength of 1310 MPa and a hardness of 375 HV. The other side of the weld is made of hypereutectoid rail steel B, with the following chemical composition (mass fraction %): C 0.88%, Mn 1.10%, Si 0.68%, Cr 0.45%, V 0.12%, P 0.010%, S 0.008%, balance Fe. This rail has a tensile strength of 1340 MPa and a hardness of 390 HV.

[0072] Welding process: Continuous flash welding process is adopted, with a welding current of 580A, a welding voltage of 410V, an upsetting force of 620kN, an upsetting amount of 11mm, and a flash allowance of 32mm.

[0073] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0074] (2) Medium frequency induction heating normalizing treatment: The welded joint is heated by a zoned control medium frequency induction coil. The heating temperature on the A side of the rail is 990℃, the heating temperature on the B side of the rail is 1010℃, the temperature difference between the two sides is 20℃, the heat preservation time is 7 minutes, the induction heating frequency is 3.5kHz, and the heating rate is 11℃ / s.

[0075] (3) Controlled cooling: The first stage of rapid cooling: Compressed air is used as the cooling medium at a pressure of 0.45 MPa and a flow rate of 10 m3 / min to rapidly cool the temperature of the weld heat-affected zone from the normalizing temperature to 590℃ at a cooling rate of 7.5℃ / s.

[0076] The second stage isothermal transition: maintain isothermal temperature at 590℃ for 7 minutes, with temperature fluctuations controlled within ±12℃.

[0077] The third stage is slow cooling: the welded joint is cooled from 590°C to room temperature at a cooling rate of 0.9°C / s.

[0078] Microstructure and performance testing results: The microstructure testing results of the weld heat-affected zone show that the network cementite in both heat-affected zones has been eliminated, and the microstructure consists entirely of pearlite with fine lamellar spacing. The average lamellar spacing of pearlite in the heat-affected zone on rail A side is 0.19 μm, and the average lamellar spacing of pearlite in the heat-affected zone on rail B side is 0.17 μm. No abnormal microstructures such as martensite or bainite were found in either case.

[0079] Hardness test results: The average hardness of the heat-affected zone on side A of the rail is 358 HV, reaching 95.5% of the hardness of the base material. The average hardness of the heat-affected zone on side B of the rail was 372 HV, reaching 95.4% of the hardness of the base material. Mechanical property test results: Tensile strength: 1285MPa (joint fracture occurred in the heat-affected zone on side A of the rail). Elongation at break: 11.2% Impact toughness (room temperature): 27 J / cm 2 Fatigue life (stress amplitude 400MPa): 2.6×10 6 Second loop Example 5 This embodiment describes the heat treatment process after flash welding of heteroeutectoid rails. The difference between this embodiment and Embodiment 4 is that the heating temperatures on both sides are the same.

[0080] Material information: Same as in Example 4.

[0081] Welding process: Same as in Example 4.

[0082] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0083] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1000℃, holding time is 6 minutes, induction heating frequency is 4kHz, and heating rate is 12℃ / s.

[0084] (3) Controlled cooling: The first stage of rapid cooling uses a water mist mixture as the cooling medium, with a water flow rate of 2.0 L / min and a gas pressure of 0.35 MPa, to rapidly cool the temperature of the weld heat-affected zone from 1000℃ to 610℃ at a cooling rate of 5.5℃ / s.

[0085] The second stage isothermal transition: maintain isothermal temperature at 610℃ for 5 minutes, with temperature fluctuations controlled within ±15℃.

[0086] The third stage is slow cooling: the welded joint is cooled from 610°C to room temperature at a cooling rate of 0.7°C / s.

[0087] (4) Post-heat treatment: Perform supplementary tempering treatment at a temperature of 450°C and a holding time of 45 minutes.

[0088] Microstructure and performance testing results: The microstructure testing results of the weld heat-affected zone show that the network cementite in both heat-affected zones has been eliminated, and the microstructure consists entirely of pearlite with fine lamellar spacing. The average lamellar spacing of pearlite in the heat-affected zone on rail A side is 0.21 μm, and the average lamellar spacing of pearlite in the heat-affected zone on rail B side is 0.18 μm. No abnormal microstructures such as martensite or bainite were found in either case.

[0089] Hardness test results: The average hardness of the heat-affected zone on side A of the rail was 355 HV, reaching 94.7% of the hardness of the base material; the average hardness of the heat-affected zone on side B of the rail was 368 HV, reaching 94.4% of the hardness of the base material. Mechanical property test results: Tensile strength: 1278MPa Elongation after fracture: 11.8% Impact toughness (room temperature): 29 J / cm 2 Fatigue life (stress amplitude 400MPa): 2.9×10 6 Second loop Example 6 This embodiment describes a heat treatment process following flash welding of hypereutectoid rails, employing a higher heating temperature and a longer holding time.

[0090] Material Information: Both sides of the weld are made of the same hypereutectoid rail steel, with the following chemical composition (mass fraction %): C 0.92%, Mn 1.15%, Si 0.75%, Cr 0.50%, V 0.12%, P 0.010%, S 0.006%, balance Fe. The rail has a tensile strength of 1380 MPa and a hardness of 405 HV.

[0091] Welding process: Continuous flash welding process is adopted, with a welding current of 620A, a welding voltage of 430V, an upsetting force of 680kN, an upsetting amount of 13mm, and a flash allowance of 38mm.

[0092] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0093] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1040℃, holding time is 4 minutes, induction heating frequency is 6kHz, and heating rate is 18℃ / s.

[0094] (3) Controlled cooling: The first stage of rapid cooling uses compressed air as the cooling medium, with a pressure of 0.7 MPa and a flow rate of 18 m³ / h. 3The temperature of the weld heat-affected zone was rapidly cooled from 1040℃ to 550℃ at a rate of 12.0℃ / s.

[0095] The second stage isothermal transition: maintain isothermal temperature at 550℃ for 12 minutes, with temperature fluctuations controlled within ±10℃.

[0096] The third stage is slow cooling: the welded joint is cooled from 550°C to room temperature at a cooling rate of 1.5°C / s.

[0097] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated and the microstructure is pearlite with fine lamellar spacing. The average lamellar spacing of the pearlite is 0.14 μm. No abnormal microstructures such as martensite or bainite were found.

[0098] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 390HV, which reaches 96.3% of the hardness of the base material.

[0099] Mechanical property test results: Tensile strength: 1345 MPa (97.5% of the base material strength) Elongation after fracture: 10.2% Impact toughness (room temperature): 24 J / cm 2 Fatigue life (stress amplitude 400 MPa): 2.3 × 10⁻⁶ 6 Second loop Comparative Example 1 This comparative example does not employ the post-weld heat treatment process of the present invention, but only performs natural cooling after flash welding.

[0100] Material information: Same as in Example 1.

[0101] Welding process: Same as in Example 1.

[0102] Post-weld treatment: After flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air without any heat treatment.

[0103] Microstructure and performance testing results: Microstructure testing of the weld heat-affected zone showed that there was obvious network cementite at the original austenite grain boundaries, with a network level of 3 (evaluated according to GB / T 13298 standard). The microstructure consisted of pearlite + network cementite, with an average pearlite lamellar spacing of 0.32 μm. A small amount of martensite was found in the heat-affected zone near the weld, with a martensite content of approximately 8%.

[0104] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 335HV, which is only 88.2% of the hardness of the base material.

[0105] Mechanical property test results: Tensile strength: 1185 MPa (89.8% of the base material) Elongation after fracture: 8.5% Impact toughness (room temperature): 12 J / cm 2 Fatigue life (stress amplitude 400MPa): 6.5×10 5 Second loop Analysis: Without post-weld heat treatment, the weld heat-affected zone retained a large amount of network cementite and contained harmful structures such as martensite, which severely reduced the overall mechanical properties of the joint. In particular, the impact toughness was only 42.9% of that in Example 1 of this invention, and the fatigue life was only 23.2% of that in Example 1 of this invention.

[0106] Comparative Example 2 This comparative example uses the traditional normalizing process, but without controlled cooling.

[0107] Material information: Same as in Example 1.

[0108] Welding process: Same as in Example 1.

[0109] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0110] (2) Normalizing treatment: The heating temperature is 850℃ and the holding time is 30 minutes.

[0111] (3) Air cooling: After the normalizing and heat preservation is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0112] Microstructure and performance testing results: The microstructure testing results of the weld heat-affected zone showed that the network cementite was partially eliminated, but discontinuous network cementite still existed, with a network level of 1.5. The microstructure consisted of pearlite + discontinuous network cementite, with an average pearlite lamellar spacing of 0.45 μm. No martensite was found, but a small amount of bainite was found in local areas of the heat-affected zone.

[0113] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 320HV, which is only 84.2% of the hardness of the base material.

[0114] Mechanical property test results: Tensile strength: 1150MPa (87.1% of the base material) Elongation after fracture: 9.2% Impact toughness (room temperature): 18 J / cm 2 Fatigue life (stress amplitude 400MPa): 1.2×106 Second loop Analysis: The normalizing temperature was too low (850℃), which failed to fully dissolve the network cementite. Furthermore, the lack of controlled cooling resulted in large interlamellar spacing of the pearlite and a significant decrease in the hardness of the heat-affected zone. Consequently, the overall mechanical properties were inferior to those of the embodiments of the present invention.

[0115] Comparative Example 3 This comparative example uses a high normalizing temperature, but the cooling rate is too fast, resulting in the formation of martensite.

[0116] Material information: Same as in Example 1.

[0117] Welding process: Same as in Example 1.

[0118] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0119] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1000℃, and holding time is 6 minutes.

[0120] (3) Rapid cooling: The welded joint is rapidly cooled to room temperature by water quenching, with a cooling rate of about 50℃ / s.

[0121] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated, but the microstructure is mainly coarse martensite + retained austenite, with a martensite content of about 85% and a retained austenite content of about 15%. The martensite microstructure is coarse and has a distinct lath morphology.

[0122] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 520HV, which is much higher than the hardness of the base material (136.8%), and the hardness distribution is extremely uneven.

[0123] Mechanical property test results: Tensile strength: 1250MPa (94.7% of the base material) Elongation after fracture: 3.5% Impact toughness (room temperature): 6 J / cm 2 Fatigue life (stress amplitude 400MPa): 1.8×10 5 Second loop Analysis: The cooling rate was too fast, far exceeding the critical cooling rate for martensitic transformation in hypereutectoid steel (1.0~2.0℃ / s), resulting in the formation of a large amount of coarse martensite. Although the hardness was significantly improved, the impact toughness and fatigue life decreased sharply, the elongation after fracture was extremely low, and the joint was obviously brittle.

[0124] Comparative Example 4 This comparative example uses the normalizing temperature of the present invention, but the isothermal transformation temperature is too low.

[0125] Material information: Same as in Example 1.

[0126] Welding process: Same as in Example 1.

[0127] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0128] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1000℃, and holding time is 6 minutes.

[0129] (3) Controlled cooling: The first stage of rapid cooling: Compressed air is used as the cooling medium to rapidly cool the temperature of the weld heat-affected zone from 1000℃ to 480℃, with a cooling rate of 8.0℃ / s.

[0130] The second stage isothermal transition: hold the temperature isothermally at 480℃ for 8 minutes.

[0131] The third stage is slow cooling: the welded joint is cooled from 480°C to room temperature at a cooling rate of 1.0°C / s.

[0132] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated, but the microstructure is a mixture of pearlite and bainite, with a bainite content of about 40% and an average pearlite lamellar spacing of 0.22 μm.

[0133] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 425HV, which is higher than the hardness of the base material (111.8%).

[0134] Mechanical property test results: Tensile strength: 1320MPa (100% of the base material) Elongation after fracture: 7.5% Impact toughness (room temperature): 15 J / cm 2 Fatigue life (stress amplitude 400MPa): 8.5×10 5 Second loop Analysis: The isothermal transformation temperature is too low (480℃), below the pearlite phase transformation temperature range (550~620℃), resulting in the formation of a large amount of bainite. Although the tensile strength and hardness are high, the toughness and fatigue life are significantly reduced, and the overall mechanical properties are inferior to those of the embodiment of this invention.

[0135] Comparative Example 5 This comparative example uses the normalizing temperature of the present invention, but the rapid cooling final cooling temperature is too low, entering the martensitic transformation range.

[0136] Material information: Same as in Example 1.

[0137] Welding process: Same as in Example 1.

[0138] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0139] (2) Medium frequency induction heating normalizing treatment: heating temperature is 1000℃, and holding time is 6 minutes.

[0140] (3) Controlled cooling: The first stage of rapid cooling: Compressed air is used as the cooling medium to rapidly cool the temperature of the weld heat-affected zone from 1000℃ to 300℃, with a cooling rate of 8.0℃ / s.

[0141] Second stage slow cooling: The welded joint is cooled from 300°C to room temperature at a cooling rate of 1.0°C / s.

[0142] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite has been eliminated, but the microstructure is a mixture of pearlite and martensite, with a martensite content of about 25% and an average pearlite lamellar spacing of 0.25 μm.

[0143] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 455HV, which is higher than the hardness of the base material (119.7%), and the hardness distribution is uneven.

[0144] Mechanical property test results: Tensile strength: 1350MPa (102.3% of the base material) Elongation after fracture: 5.2% Impact toughness (room temperature): 10 J / cm 2 Fatigue life (stress amplitude 400MPa): 4.2×10 5 Second loop Analysis: The rapid cooling final cooling temperature was too low (300℃), close to the martensitic transformation initiation temperature (180~230℃). During the subsequent cooling process, some austenite transformed into martensite. Although the tensile strength was slightly improved, the toughness and fatigue life were severely reduced.

[0145] Comparative Example 6 The comparative example used a relatively low heating temperature, and the network cementite was not fully dissolved.

[0146] Material information: Same as in Example 1.

[0147] Welding process: Same as in Example 1.

[0148] Post-weld heat treatment process: (1) Post-weld cooling: After the flash welding is completed, the welded joint is allowed to cool naturally to room temperature in the air.

[0149] (2) Medium frequency induction heating normalizing treatment: heating temperature is 880℃, and holding time is 6 minutes.

[0150] (3) Controlled cooling: The first stage of rapid cooling: Compressed air is used as the cooling medium to rapidly cool the temperature of the weld heat-affected zone from 880℃ to 580℃, with a cooling rate of 8.0℃ / s.

[0151] The second stage isothermal transition: hold the temperature isothermally at 580℃ for 8 minutes.

[0152] The third stage is slow cooling: the welded joint is cooled from 580°C to room temperature at a cooling rate of 1.0°C / s.

[0153] Microstructure and performance test results: The microstructure test results of the weld heat-affected zone show that the network cementite is partially dissolved, but there is still obvious discontinuous network cementite. The network level is grade 2. The microstructure is pearlite + discontinuous network cementite. The average spacing between pearlite lamellars is 0.28 μm.

[0154] Hardness test results: The average hardness of the heat-affected zone within 30mm on both sides of the weld is 345HV, which reaches 90.8% of the hardness of the base material.

[0155] Mechanical property test results: Tensile strength: 1225MPa (92.8% of the base material) Elongation at break: 9.8% Impact toughness (room temperature): 19 J / cm 2 Fatigue life (stress amplitude 400 MPa): 1.1 × 10⁻⁶ 6 Second loop Analysis: The heating temperature was too low (880℃), which did not reach the temperature required to fully dissolve the network cementite (950~1050℃). As a result, the network cementite was not eliminated, and it still has a certain impact on the impact toughness and fatigue performance of the joint.

[0156] The performance test results of the six embodiments and six comparative examples of the present invention are summarized in Table 1 below: Table 1 Weld performance test results

[0157] Performance comparison analysis: (1) Elimination effect of network cementite: All six embodiments of the present invention achieved the elimination of network cementite (level 0), while Comparative Example 1 (without heat treatment) still retained level 3 network cementite, Comparative Example 2 (low normalizing temperature) was level 1.5, and Comparative Example 6 (insufficient heating temperature) was level 2. This proves that the heating temperature range (950~1050℃) of the present invention is a necessary condition for eliminating network cementite.

[0158] (2) Control of abnormal structures: No abnormal structures such as martensite or bainite appeared in any of the six embodiments of the present invention. By precisely controlling the cooling rate and final cooling temperature, the transformation of austenite into fine lamellar pearlite was ensured. Comparative Example 3 (water quenching) formed 85% martensite, Comparative Example 4 (isothermal temperature too low) formed 40% bainite, and Comparative Example 5 (final cooling temperature too low) formed 25% martensite. These abnormal structures seriously impaired the toughness of the joint.

[0159] (3) Hardness distribution: The hardness of the heat-affected zone in all six embodiments of the present invention reached more than 94% of the hardness of the base material, meeting the requirement of not less than 90%. The hardness of Comparative Examples 1 and 2 was significantly lower (84.2%~88.2%), indicating that improper heat treatment or lack of proper heat treatment would lead to softening of the heat-affected zone. The hardness of Comparative Examples 3, 4, and 5 was higher (111.8%~136.8%), mainly due to martensitic or bainitic structures, resulting in uneven hardness distribution.

[0160] (4) Comprehensive mechanical properties: The tensile strength of all six embodiments of the present invention reached more than 97% of that of the parent material, and the impact toughness reached 24~30 J / cm. 2 The fatigue life reaches 2.3 × 10⁻⁶. 6 After several cycles, the overall mechanical properties of all comparative examples showed a significant decrease. In particular, comparative examples 3, 4, and 5, although possessing high tensile strength, exhibited extremely poor toughness (impact toughness of only 6-15 J / cm). 2 Its fatigue life is extremely low (0.18×10⁻⁶). 6 ~0.85×10 6 (repeated cycles).

[0161] (5) Sensitivity to process parameters: Comparative analysis shows that the heat treatment process of this invention is quite sensitive to parameters such as heating temperature, isothermal transformation temperature, and rapid cooling final cooling temperature. Insufficient heating temperature (Comparative Example 6) will lead to incomplete elimination of network cementite; excessively fast cooling rate or excessively low final cooling temperature (Comparative Examples 3 and 5) will introduce martensite structure; excessively low isothermal transformation temperature (Comparative Example 4) will introduce bainite structure. Only by strictly following the process parameter range of this invention can ideal microstructure and properties be obtained.

[0162] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Any modifications or equivalent substitutions to the present invention without departing from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

[0163] 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 disclosed embodiments of the present invention is limited to these examples; within the framework of the embodiments of the present invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of the present invention should be included within the protection scope of the embodiments of the present invention.

Claims

1. A method for heat treatment of hypereutectoid rail steel after flash welding, characterized in that, Includes the following steps: Post-weld cooling: The welded joint is allowed to cool naturally in the air after flash welding is completed; Normalizing treatment: The welded joint is heated to a normalizing temperature above the austenitizing temperature and held at that temperature; Controlled cooling: The temperature of the weld heat-affected zone of the weld joint is cooled from the normalizing temperature to the isothermal transformation temperature within the pearlite phase transformation temperature range at a first cooling rate, and then held at the temperature until the austenite undergoes a pearlite transformation. After that, the weld joint is cooled from the isothermal transformation temperature to below 200°C at a second cooling rate lower than the first cooling rate.

2. The heat treatment method according to claim 1, characterized in that, The hypereutectoid rail has a tensile strength of 1380 MPa or higher, and by mass percentage, it contains 0.80% to 0.95% carbon, 0.70% to 1.20% manganese, 0.40% to 0.90% silicon, 0.20% to 0.60% chromium, and 0.05% to 0.15% vanadium, with the remainder being iron and unavoidable impurities.

3. The heat treatment method according to claim 2, characterized in that, The flash welding adopts continuous flash welding or preheating flash welding process, with a welding current of 300~800A, a welding voltage of 350~500V, an upsetting force of 400~800kN, an upsetting amount of 8~15mm, and a flash allowance of 20~40mm.

4. The heat treatment method according to claim 3, characterized in that, In the post-weld cooling step, the welded joint is cooled to below 540°C.

5. The heat treatment method according to claim 3, characterized in that, In the normalizing process, the welded joint is heated to 950~1050℃ using medium-frequency induction heating, and the holding time is 3~10 minutes. The frequency of medium-frequency induction heating is 1~10kHz, and the heating rate is 5~20℃ / s.

6. The heat treatment method according to claim 5, characterized in that, In the normalizing process, for dissimilar steel welded joints with hypereutectoid steel rails of different compositions on both sides of the weld, the heating temperature of the heat-affected zone on both sides of the weld is controlled separately, and the temperature difference between the two sides is controlled within the range of 0~50℃.

7. The heat treatment method according to claim 3, characterized in that, In the controlled cooling step The first cooling rate is 3.0~15.0℃ / s; The isothermal transition temperature is 550~620℃, and the isothermal time is 3~15 minutes; The second cooling rate is 0.5~2.0℃ / s.

8. The heat treatment method according to claim 7, characterized in that, Compressed air or a water mist mixture is used as the cooling medium to cool the temperature of the weld heat-affected zone of the weld joint from the normalizing temperature to the isothermal transformation temperature, wherein the pressure of the compressed air is 0.3~0.8MPa and the flow rate is 5~20m³ / h. 3 / min; the water flow rate of the water mist mixture is 0.5~3.0L / min, and the gas pressure is 0.2~0.6MPa.

9. The heat treatment method according to claim 7, characterized in that, During the isothermal transition, the temperature fluctuation range is controlled within ±15℃ by covering the welded joint with insulation material or by using auxiliary heating.

10. The heat treatment method according to claim 1, characterized in that, The method further includes: Post-heat treatment: After controlled cooling, a supplementary tempering treatment is performed at a temperature of 400~500℃ and a holding time of 30~60 minutes.

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