A welding heat treatment method for bainite rail

Abstract

The present invention belongs to the field of rail manufacturing technology, and specifically relates to a welding heat treatment method for bainitic rails. The method comprises: cooling the welded joint to a first temperature after welding; cooling the welded joint at the first temperature to a second temperature below the Ms point at a cooling rate greater than 10°C / s, with a predetermined temperature difference between the second temperature and the Ms point so that the amount of martensite generated is similar to that contained in the rail base material; heating the cooled welded joint to a bainite transformation temperature range for isothermal transformation; and cooling the isothermally transformed welded joint to room temperature. By precisely controlling the microstructure of the welded joint and employing a specific heat treatment process, the present invention achieves matching of the microstructure and macroscopic properties of the welded joint with the base material.

Description

Technical Field

[0001] The invention belongs to the technical field of rail manufacturing, and in particular relates to a welding heat treatment method for a bainite rail. Background Art

[0002] In rail transit systems like railways, subways, and light rail, rails are key components that carry the transportation vehicles. Their reliability and durability are crucial to the safety and efficiency of the entire transportation system. Rail fabrication and welding technologies have long been a research hotspot in materials science and engineering technology.

[0003] Currently, pearlite rails are the primary rail type used in railway transportation. As demand for rail transportation continues to grow, so too does the load capacity required. Pearlite rails can achieve very high strength levels through process and composition adjustments, but their lower toughness makes them difficult to meet the increasing load requirements of heavy-haul railways. Bainite rails, on the other hand, exhibit superior strength-toughness matching, wear resistance, and fatigue resistance after appropriate processing and microstructure control. Therefore, research on bainite rails has become a key focus in the development of high-performance rails.

[0004] In actual production, it is found that a small amount of martensite is often present in bainitic rails. Although traditional welding technology can achieve the connection of rails, due to the influence of high temperatures during the welding process, the material in the weld joint area undergoes complex thermal cycles, resulting in changes in microstructure and performance. For bainitic rails, due to the influence of high welding temperatures, the martensite content in the joint area after post-weld cooling is significantly different from that of the rail base material. This often results in significant differences in the microstructure and macroscopic properties between the weld joint and the base material. The difference in microstructure causes the deformation of the weld joint under bending stress to be inconsistent with that of the base material, which leads to the formation of cracks, affecting the strength, toughness and service life of the weld joint.

[0005] Therefore, how to achieve consistency between the microstructure and macroscopic properties of the weld joint and the base material has become an urgent problem to be solved in this field. Summary of the Invention

[0006] In view of this, the present invention provides a welding heat treatment method for a bainite rail to optimize the matching between the weld joint and the base material in terms of microstructure and macroscopic properties.

[0007] The present invention is achieved through the following technical solutions:

[0008] In one aspect, the present invention discloses a welding heat treatment method for a bainite rail, the method comprising the following steps:

[0009] Step S1, cooling the welded joint to a first temperature at a first cooling rate;

[0010] Step S2, cooling the weld joint at the first temperature to a second temperature below the Ms point at a second cooling rate higher than 10°C / s, wherein a predetermined temperature difference exists between the second temperature and the Ms point so that the amount of generated martensite is similar to the amount of martensite contained in the rail base material;

[0011] Step S3, heating the weld joint cooled in step S2 to a lower bainite transformation temperature range for isothermal transformation;

[0012] Step S4: Cooling the welded joint that has undergone the isothermal transformation to room temperature.

[0013] In some embodiments, the base material of the bainite rail includes the following components by mass percentage: C: 0.21%-0.25%, Si: 2.0%-2.5%, Mn: 0.60%-1.00%, Cr: 0.60%-0.80%, Mo: 0.40%-0.45%, Co: 1.9%-2.1%, and the balance is Fe and unavoidable impurities.

[0014] In some embodiments, in step S1 , the temperature of the welded joint after welding is above 1500° C., and the microstructure of the heat-affected zone of the bainite rail after welding is austenite.

[0015] In some embodiments, in step S1, the first cooling rate is controlled at 3-5°C / s, and the first temperature is 890-1000°C.

[0016] In some embodiments, in step S2, the second cooling rate is controlled at 10-30°C / s, and the predetermined temperature difference is 10-30°C.

[0017] In some embodiments, in step S3 , the heating is performed immediately after the cooling in step S2 is completed.

[0018] In some embodiments, the heating rate is 3-5°C / s.

[0019] In some embodiments, the lower bainite transformation temperature range is 350-380° C., and the isothermal transformation time is 1-1.5 h.

[0020] In some embodiments, the cooling in step S4 is natural cooling at room temperature.

[0021] In some embodiments, the base material of the bainite rail includes 2-5% by volume of martensite.

[0022] Beneficial effects of the present invention:

[0023] The welding heat treatment method of the present invention controls the martensite content in the weld joint by controlling the post-weld cooling mechanism (particularly controlling the cooling endpoint of high-temperature weld joints during rapid cooling), aligning it with the martensite content in the base material. Furthermore, the martensite expansion phase transformation that occurs during rapid cooling creates more defects in the surrounding matrix (such as dislocations and interfaces between the formed martensite and the original austenite). These defects can serve as nucleation sites for bainite during the isothermal transformation, accelerating the bainite transformation. By quenching and then heating, carbon atoms are redistributed between the martensite and retained austenite, reducing the brittleness of the martensite while increasing the stability of the austenite. Through the synergistic effect of these technical measures, the welding heat treatment method of the present invention can effectively improve the microstructure and properties of the weld joint, achieving a microstructure and macroscopic properties that match those of the base material, thereby enhancing the toughness and service life of the rail weld joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 A flow chart of a welding heat treatment method for a bainite rail provided in some embodiments of the present invention. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0027] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. Subsequent embodiments will not explain this one by one.

[0028] In order to achieve matching between the weld joint and the base material in terms of microstructure and macroscopic properties, the present invention provides a welding heat treatment method for a bainite rail.

[0029] In the method of the present invention, the term "weld joint" refers to an area with a length of 60 to 80 mm including the weld and / or the heat-affected zone, with the center of the area being the weld.

[0030] like Figure 1As shown, the welding heat treatment method for bainite rail of the present invention generally comprises the following steps:

[0031] Step S1, cooling the welded joint to a first temperature at a first cooling rate;

[0032] Step S2, cooling the weld joint at the first temperature to a second temperature below the Ms point at a second cooling rate higher than 10°C / s, wherein a predetermined temperature difference exists between the second temperature and the Ms point so that the amount of martensite generated is similar to the amount of martensite contained in the rail base material;

[0033] Step S3, heating the weld joint cooled in step S2 to a lower bainite transformation temperature range for isothermal transformation;

[0034] Step S4: Cooling the welded joint that has undergone isothermal transformation to room temperature.

[0035] The method of the present invention controls the martensite content in the weld joint by controlling the post-weld cooling mechanism (particularly controlling the cooling endpoint of high-temperature weld joints during rapid cooling), aligning it with the martensite content in the base material. Furthermore, the martensite expansion phase transformation that occurs during rapid cooling creates more defects in the surrounding matrix (such as dislocations and interfaces between the formed martensite and the original austenite). These defects serve as nucleation sites for bainite during the isothermal transformation, accelerating the bainite transformation. By quenching and then heating, carbon atoms are redistributed between the martensite and retained austenite, reducing the brittleness of the martensite while increasing the stability of the austenite. Through the synergistic effect of these technical measures, the welding heat treatment method of the present invention can effectively improve the microstructure and properties of the weld joint, achieving a microstructure and macroscopic properties that match those of the base material, thereby enhancing the strength, toughness, and service life of the rail weld joint. Furthermore, the welding heat treatment method of the present invention utilizes residual welding heat, improving treatment efficiency and saving energy.

[0036] In some embodiments, the base material of the bainitic rail forming the welded joint comprises the following components, by mass percentage: C: 0.21%-0.25%, Si: 2.0%-2.5%, Mn: 0.60%-1.00%, Cr: 0.60%-0.80%, Mo: 0.40%-0.45%, Co: 1.9%-2.1%, with the remainder being Fe and unavoidable impurities. The addition of cobalt promotes the nucleation and growth of bainite, contributing to the formation of fine and uniform bainite laths or lamellae. This microstructure provides more grain boundaries, thereby hindering dislocation movement and increasing the yield strength and hardness of the weld structure.

[0037] In some embodiments, the base material of the bainitic rail contains 2-5% by volume of martensite, and the martensitic transformation start temperature (i.e., "Ms point") of the rail with this composition is approximately 230-280°C.

[0038] In some embodiments, in step S1, the temperature of the weld joint after welding is above 1500°C, and the microstructure of the heat-affected zone of the bainitic rail after welding is austenite. For example, during welding, the heat input is controlled so that the center temperature of the heat-affected zone is between 1550°C and 1650°C, ensuring that the microstructure of the heat-affected zone is austenite. After welding is completed, the temperature of the weld joint is maintained above 1500°C due to residual heat.

[0039] In some embodiments, the cooling rate in step S1 is controlled at 3-5°C / s, and the first temperature is 890-1000°C. For example, a welded joint with a temperature above 1500°C after welding can be naturally cooled at room temperature of 25-30°C, and then cooled to 890-1000°C at a first cooling rate of 3-5°C / s, thereby laying the foundation for subsequent quenching and bainite transformation.

[0040] In some embodiments, in step S2, the weld joint at the first temperature is rapidly cooled to a second temperature below the Ms point at a second cooling rate exceeding 10°C / s. A predetermined temperature difference exists between the second temperature and the Ms point so that the amount of martensite generated is similar to that contained in the rail base material. Rapid cooling corresponds to a cooling rate of 10-30°C / s, and the temperature difference between the second temperature and the Ms point can be controlled to be within a range of 10-30°C. For example, a rail head profile cooling device can be used to perform full atomization cooling, with the spray flow rate adjusted to achieve a second cooling rate of 10-30°C / s, thereby rapidly cooling the weld joint to the second temperature below the Ms point.

[0041] When the temperature of the weld joint drops to the Ms point, the weld joint is cooled at a cooling rate higher than the critical cooling rate of martensite transformation, and martensite begins to appear. However, it is necessary to focus on controlling the amount of martensite generated. Since the martensite content in the base material is relatively small, if the amount of martensite generated in the weld joint area is too large, on the one hand, it will lead to a mismatch between the microstructure of the weld joint and the base material. On the other hand, the martensite phase is a source-inducing factor of fatigue cracks, which will reduce the toughness of the rail and is not conducive to improving the macroscopic mechanical properties of the rail. In the method of the present invention, the amount of martensite structure generated is controlled by controlling the cooling end temperature of this step to be below the Ms point and with a small temperature difference from the Ms point.

[0042] The transformation of martensite into an expansion phase creates numerous defects in the surrounding matrix during its formation, such as dislocations and interface defects between the newly formed martensite and the original austenite. These defects can serve as nucleation sites for bainite during the isothermal transformation, accelerating the subsequent bainite transformation. Furthermore, quenching is performed before the isothermal transformation. During this process, carbon can transfer from the supersaturated martensite to the untransformed austenite. After quenching, carbon-rich austenite and carbon-poor martensite are obtained. The retained austenite is carbon-rich and stable, which helps improve the toughness of the rail.

[0043] In some embodiments, in step S3, heating is performed immediately after the rapid cooling is completed, and the weld joint is heated to the bainite transformation temperature range for isothermal transformation. Immediate heating is performed to avoid the formation of excessive martensite. For example, the weld joint that has just completed quenching can be immediately heated using the rail head imitation dual-frequency induction heating coil of the rail. The heating rate is controlled at 3~5℃ / s, and the weld joint is heated to the lower bainite transformation temperature range, for example, 350~380℃. Isothermal transformation is performed within this temperature range, and the transformation time is, for example, 1~1.5h. By controlling the isothermal transformation temperature at 350~380℃, lower bainite can be obtained, which will split the original austenite grains, refine the structure, and increase the yield strength and hardness of the weld structure.

[0044] In some embodiments, in step S4, the isothermal transformed welded joint is cooled to room temperature. For example, after the isothermal transformation is completed, the welded joint can be naturally cooled to room temperature in an environment of 25-30°C at a cooling rate of 0.2-0.8°C / s.

[0045] The microstructure of the rail weld joint processed by the method of the present invention is composed of bainite as the main phase, and also includes martensite accounting for about 2% to 5% by volume and retained austenite accounting for 10% to 15% by volume.

[0046] To further illustrate the application and effect of the welding heat treatment method of the present invention, several specific examples are provided below. The joint structure and performance testing methods in the following examples are all conventional methods in the art.

[0047] Example 1

[0048] The bainitic rail base material in this embodiment comprises the following components, by mass percentage: C: 0.21%, Si: 2.0%, Mn: 0.60%, Cr: 0.60%, Mo: 0.40%, Co: 1.9%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 2% by volume, and retained austenite accounts for approximately 10% by volume. The rail base material has a yield strength of 1300 MPa, a tensile strength of 1450 MPa, an elongation at break of 15%, and a room-temperature impact energy of 145 J.

[0049] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 890°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled so that the center temperature of the heat-affected zone is around 1550°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding to utilize the residual heat for heat treatment.

[0050] A rail head profile cooling device is used for full atomization mode cooling quenching. The injection flow rate of the cooling medium is controlled to achieve a cooling rate of 10°C / s. The weld joint is quickly cooled to a temperature 10°C lower than the Ms point (specifically, approximately 240°C), thereby forming a martensite phase in the weld joint with a content similar to that of the base material.

[0051] The welded joint, having just been quenched, is immediately heated using a dual-frequency induction heating coil shaped like the rail head. The heating rate is controlled at 3°C / s to a lower bainite transformation temperature range of approximately 350-380°C. The isothermal transformation takes 1.5 hours to complete.

[0052] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.5°C / s.

[0053] Test results show that the welded joint microstructure is primarily composed of fine, uniform lower bainite, with retained austenite (approximately 10.5% by volume) and a small amount of martensite (approximately 1.8% by volume) dispersed within the bainite. The welded joint achieves a yield strength of 1280 MPa, a tensile strength of 1400 MPa, an elongation at break of 17%, and a room-temperature impact energy of 150 J. The mechanical properties of the welded joint are similar to those of the rail base material.

[0054] Example 2

[0055] The bainitic rail base material in this embodiment comprises the following components, by mass percentage: C: 0.23%, Si: 2.2%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, Co: 2.0%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 3% by volume, and retained austenite accounts for approximately 12% by volume. The rail base material has a yield strength of 1350 MPa, a tensile strength of 1460 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J.

[0056] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 950°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled to keep the center temperature of the heat-affected zone at 1600°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding, utilizing the residual heat from welding for heat treatment.

[0057] A rail head profile cooling device is used for full atomization mode cooling quenching. The injection flow rate of the cooling medium is controlled to achieve a cooling rate of 20°C / s. The weld joint is quickly cooled to a temperature 20°C lower than the Ms point (specifically, approximately 230°C), thereby forming a martensite phase in the weld joint with a content similar to that of the base material.

[0058] The welded joint, having just been quenched, was immediately heated using a dual-frequency induction heating coil shaped like the rail head. Heating was controlled at a rate of 4°C / s to a lower bainite transformation temperature range of 350-380°C. The isothermal transformation took 1.3 hours to complete.

[0059] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.6°C / s.

[0060] Test results show that the welded joint microstructure is primarily composed of fine, uniform lower bainite, with retained austenite (approximately 12.5% ​​by volume) and a small amount of martensite (approximately 3.2% by volume) dispersed within the bainite. The welded joint achieves a yield strength of 1310 MPa, a tensile strength of 1420 MPa, an elongation at break of 20%, and a room-temperature impact energy of 180 J. The mechanical properties of the welded joint are similar to those of the rail base material.

[0061] Example 3

[0062] The bainitic rail base material in this embodiment comprises the following components, by mass percentage: C: 0.25%, Si: 2.5%, Mn: 1.00%, Cr: 0.80%, Mo: 0.45%, Co: 2.1%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 4.8% by volume, and retained austenite accounts for approximately 15% by volume. The rail base material has a yield strength of 1380 MPa, a tensile strength of 1480 MPa, an elongation at break of 19%, and a room-temperature impact energy of 140 J.

[0063] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 1000°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled to keep the center temperature of the heat-affected zone at 1650°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding, utilizing the residual heat from welding for heat treatment.

[0064] A rail head profile cooling device is used for full atomization mode cooling quenching. The injection flow rate of the cooling medium is controlled to achieve a cooling rate of 30°C / s. The weld joint is quickly cooled to a temperature 30°C lower than the Ms point (specifically, about 200°C), thereby forming a martensite phase in the weld joint with a content similar to that of the base material.

[0065] The welded joint, having just been quenched, is immediately heated using a dual-frequency induction heating coil, similar to the rail head profile. The heating rate is controlled at 5°C / s to a lower bainite transformation temperature range of 360-380°C. The isothermal transformation takes 1 hour to complete the bainite transformation.

[0066] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.3°C / s.

[0067] Test results show that the welded joint microstructure is primarily composed of fine, uniform lower bainite, with retained austenite (approximately 14.5% by volume) and a small amount of martensite (approximately 4.5% by volume) dispersed within the bainite. The welded joint achieves a yield strength of 1350 MPa, a tensile strength of 1470 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J. The mechanical properties of the welded joint are similar to those of the rail base material.

[0068] Comparative Example 1

[0069] The bainitic rail base material in this comparative example comprises the following components, by mass percentage: C: 0.23%, Si: 2.2%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 3.5% by volume, and retained austenite accounts for approximately 11% by volume. The rail base material exhibits a yield strength of 1350 MPa, a tensile strength of 1460 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J.

[0070] The high-temperature welded joint obtained by the flash welding technology was naturally cooled to about 950℃ in a room temperature environment of 25~30℃.

[0071] A rail head profile cooling device was used for full atomization cooling quenching. The spray flow rate of the cooling medium was controlled to achieve a cooling rate of 20°C / s. The weld joint was quickly cooled to the lower bainite transformation temperature range of 350~380°C, and the bainite transformation was completed in 1.3h of isothermal transformation.

[0072] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.6°C / s.

[0073] Test results show that the welded joint microstructure consists primarily of bainite laths, with retained austenite (approximately 25% by volume) dispersed within the bainite. The welded joint exhibits a yield strength of only 1100 MPa, a tensile strength of 1300 MPa, an elongation at break of 18%, and a room-temperature impact energy of 150 J. The lack of cobalt in the base metal of this welded joint results in incomplete bainite transformation. Furthermore, the resulting microstructure lacks martensite, resulting in significant differences in mechanical properties (particularly yield strength and tensile strength) compared to those of the rail base metal.

[0074] Comparative Example 2

[0075] The bainitic rail base material in this comparative example comprises the following components, by mass percentage: C: 0.23%, Si: 2.2%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, Co: 2.0%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 3% by volume, and retained austenite accounts for approximately 12% by volume. The rail base material exhibits a yield strength of 1350 MPa, a tensile strength of 1460 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J.

[0076] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 950°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled to keep the center temperature of the heat-affected zone at 1600°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding, utilizing the residual heat from welding for heat treatment.

[0077] A rail head profile cooling device is used for full atomization mode cooling quenching. The injection flow rate of the cooling medium is controlled to achieve a cooling rate of 20°C / s, and the weld joint is quickly cooled to a temperature 50°C lower than the Ms point (specifically, approximately 200°C).

[0078] The welded joint, having just been quenched, was immediately heated using a dual-frequency induction heating coil shaped like the rail head. Heating was controlled at a rate of 4°C / s to a lower bainite transformation temperature range of 350-380°C. The isothermal transformation took 1.3 hours to complete.

[0079] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.6°C / s.

[0080] Test results show that the weld joint microstructure is primarily composed of bainite laths, with retained austenite (approximately 11.5% by volume) and a significant amount of martensite (approximately 10% by volume) dispersed within the bainite. The weld joint exhibits a yield strength of 1410 MPa, a tensile strength of 1490 MPa, an elongation at break of 10%, and a room-temperature impact energy of only 110 J. Due to the excessively low final quenching temperature, the weld joint produces excessive martensite, significantly reducing its toughness and mismatching the mechanical properties of the parent material.

[0081] Comparative Example 3

[0082] The bainitic rail base material in this comparative example comprises the following components, by mass percentage: C: 0.23%, Si: 2.2%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, Co: 2.0%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 3% by volume, and retained austenite accounts for approximately 12% by volume. The rail base material exhibits a yield strength of 1350 MPa, a tensile strength of 1460 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J.

[0083] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 950°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled to keep the center temperature of the heat-affected zone at 1600°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding, utilizing the residual heat from welding for heat treatment.

[0084] A rail head profile cooling device is used for full atomization mode cooling quenching. The injection flow rate of the cooling medium is controlled to achieve a cooling rate of 20°C / s, and the weld joint is quickly cooled to a temperature 20°C lower than the Ms point (specifically, approximately 230°C).

[0085] The welded joint, having just been quenched, was immediately heated using a dual-frequency induction heating coil, similar to the rail head profile. Heating was controlled at a rate of 4°C / s to a temperature range of 400-450°C for the upper bainite transformation. The isothermal transformation took 1.3 hours to complete.

[0086] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.6°C / s.

[0087] Test results show that the welded joint microstructure consists primarily of feathery upper bainite laths with large spacing between the laths. Retained austenite (approximately 12.3% by volume) and martensite (approximately 3.2% by volume) are dispersed within the bainite. The welded joint exhibits a yield strength of 1150 MPa, a tensile strength of 1250 MPa, an elongation at break of 10%, and a room-temperature impact strength of only 130 J. Due to the high isothermal transformation temperature, the welded joint produces an upper bainite microstructure, failing to meet expected performance.

[0088] Comparative Example 4

[0089] The bainitic rail base material in this comparative example comprises the following components, by mass percentage: C: 0.23%, Si: 2.2%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, Co: 2.0%, with the balance being Fe and unavoidable impurities. The base material microstructure is martensite + retained austenite + lower bainite. Martensite accounts for approximately 3% by volume, and retained austenite accounts for approximately 12% by volume. The rail base material exhibits a yield strength of 1350 MPa, a tensile strength of 1460 MPa, an elongation at break of 18%, and a room-temperature impact energy of 160 J.

[0090] The high-temperature welded joint obtained using flash welding technology is naturally cooled to approximately 950°C in a room temperature environment of 25-30°C. During welding, the heat input is controlled to keep the center temperature of the heat-affected zone at 1600°C to ensure the formation of austenite in the heat-affected zone. Furthermore, the joint temperature is maintained above 1500°C after welding, utilizing the residual heat from welding for heat treatment.

[0091] A rail head profile cooling device was used for full atomization cooling quenching. The spray flow rate of the cooling medium was controlled to achieve a cooling rate of 8°C / s, and the weld joint was cooled to a temperature 20°C lower than the Ms point (specifically, approximately 230°C).

[0092] The welded joint, having just been quenched, was immediately heated using a dual-frequency induction heating coil shaped like the rail head. Heating was controlled at a rate of 4°C / s to a lower bainite transformation temperature range of 350-380°C. The isothermal transformation took 1.3 hours to complete.

[0093] The welded joint that has completed the isothermal transformation is naturally cooled to room temperature in an environment of 25-30°C, with a cooling rate of about 0.6°C / s.

[0094] Test results show that the welded joint microstructure is primarily composed of coarse lower bainite laths, with retained austenite (approximately 13.5% by volume) and martensite (approximately 2.5% by volume) dispersed within the bainite. The welded joint exhibits a yield strength of 1050 MPa, a tensile strength of 1150 MPa, an elongation at break of 12%, and a room-temperature impact strength of only 120 J. The welded joint exhibits a coarse bainite structure due to the low cooling rate during quenching, significantly reducing its mechanical properties.

[0095] The above examples and comparative examples clearly demonstrate the deficiencies in the microstructure and macroscopic properties of welded joints when the weld heat treatment method of the present invention is not employed. These deficiencies result in key performance indicators such as yield strength, tensile strength, elongation at break, and impact energy failing to meet the standards specified by the present invention. The weld heat treatment method of the present invention effectively modulates the microstructure of welded joints, achieving high strength and excellent impact toughness.

[0096] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0097] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A welding heat treatment method for bainite rails, characterized in that: The steps include: Step S1, cooling the welded joint to a first temperature at a first cooling rate, wherein the first temperature is 890-1000° C.; Step S2, cooling the weld joint at the first temperature to a second temperature below the Ms point at a second cooling rate higher than 10°C / s, wherein a predetermined temperature difference exists between the second temperature and the Ms point so that the amount of generated martensite is similar to the amount of martensite contained in the rail base material; Step S3, heating the weld joint cooled in step S2 to a lower bainite transformation temperature range for isothermal transformation; Step S4: Cooling the welded joint that has undergone the isothermal transformation to room temperature.

2. The welding heat treatment method for bainite rail according to claim 1, characterized in that: The base material of the bainite rail includes the following components by mass percentage: C: 0.21%-0.25%, Si: 2.0%-2.5%, Mn: 0.60%-1.00%, Cr: 0.60%-0.80%, Mo: 0.40%-0.45%, Co: 1.9%-2.1%, and the balance is Fe and unavoidable impurities.

3. The welding heat treatment method for bainite rail according to claim 2, characterized in that: In step S1 , the temperature of the welded joint after welding is above 1500° C., and the microstructure of the heat-affected zone of the bainite rail after welding is austenite.

4. The welding heat treatment method for bainite rail according to claim 2, characterized in that: In step S1, the first cooling rate is controlled at 3-5°C / s.

5. The welding heat treatment method for bainite rail according to claim 2, characterized in that: In step S2, the second cooling rate is controlled at 10-30°C / s, and the predetermined temperature difference is 10-30°C.

6. The welding heat treatment method for bainite rail according to claim 2, characterized in that: In step S3 , the heating is performed immediately after the cooling in step S2 is completed.

7. The welding heat treatment method for bainite rail according to claim 6, characterized in that: The heating rate is 3-5°C / s.

8. The welding heat treatment method for bainite rail according to claim 2, characterized in that: The lower bainite transformation temperature range is 350-380° C., and the isothermal transformation time is 1-1.5 h.

9. The welding heat treatment method for bainite rail according to claim 2, characterized in that: The cooling in step S4 is natural cooling at room temperature.

10. The welding heat treatment method for bainite rail according to claim 5, characterized in that: The base material of the bainite rail includes martensite accounting for 2-5% by volume.

Images