Bainite and pearlite dissimilar steel rail post-welding partition temperature control heat treatment process and system

Abstract

The invention relates to the technical field of steel rail postweld heat treatment, in particular to a bainite and pearlite dissimilar steel rail postweld partition temperature control heat treatment process and system. The process comprises the steps that a welded joint is pretreated and cooled to the room temperature; the bainite steel rail heat affected zone and the pearlite steel rail heat affected zone on the two sides of the welding seam are subjected to synchronous heating and heat preservation with independent parameters through a partition induction heating device; then, independently-controlled first-stage cooling is carried out on the heat affected zones on the two sides, the pearlite side passes through a phase change zone of the pearlite side through the technology that rapid cooling is carried out firstly and then slow cooling is carried out, and the bainite side passes through a bainite phase change zone of the bainite side through the technology that rapid cooling is carried out and then isothermal is carried out; and finally, naturally cooling. According to the scheme, the refined pearlite structure and the uniform lower bainite structure are obtained on the two sides of the dissimilar joint correspondingly through partitioned precise temperature control, the hard and brittle phase is effectively avoided, the hardness uniformity, the obdurability and the fatigue life of the joint are remarkably improved, and the technical problem that the performance of the dissimilar steel rail welding joint is degraded is solved.

Description

Technical Field

[0001] This invention relates to the field of post-weld heat treatment technology for rails, specifically to a zoned temperature-controlled heat treatment process and system for dissimilar rails containing bainitic and pearlitic materials. Background Technology

[0002] As the core load-bearing component of the railway structure, the performance of the welded joints of steel rails directly determines the safety and service life of the railway. With the development of railway transportation towards high speed and heavy load, the demand for dissimilar welding of steel rails with different properties is increasing. Among them, the dissimilar welding of bainitic steel rails (which have high strength and high toughness) and pearlitic steel rails (which have good wear resistance and cost advantages) has put forward requirements for welding and post-weld heat treatment processes.

[0003] After welding dissimilar rails, the heat-affected zones on both sides of the weld have drastically different adaptability to heat treatment temperature and cooling rate due to differences in material composition. Existing post-weld heat treatment processes mostly adopt a uniform heating and cooling mode, which cannot simultaneously address the microstructure requirements of both heat-affected zones. Specifically: In terms of heating: If a uniformly high temperature is used to meet the austenite refinement requirements of bainitic rails (usually above 950℃), the pearlitic rail side will suffer from overheating, resulting in coarse grains and a significant decrease in toughness. If a uniformly lower temperature is used to accommodate pearlitic rails (usually not exceeding 940℃), the bainitic rail side will lack sufficient austenitizing temperature, making it difficult to obtain refined initial austenite grains, thus hindering the achievement of high-performance refined bainite microstructure in subsequent phase transformations. In terms of cooling: The phase transformation temperature ranges of bainite and pearlite differ significantly (pearlite approximately 650-560℃, bainite approximately 500-300℃). Using a uniform cooling rate can easily lead to insufficient microstructure transformation on one side, while the other side develops hard and brittle phases such as martensite due to the mismatch in cooling rates. This microstructure mismatch directly results in abrupt changes in the hardness gradient and insufficient impact toughness of the welded joint. Under high-speed, heavy-load cyclic loading, fatigue cracks are easily initiated, seriously threatening driving safety.

[0004] Furthermore, it's important to understand that due to the melting process and high temperatures, the austenite grains in the overheated zone of the rail weld are coarse, resulting in a significantly lower hardness in this area compared to the base material. During service, the softened rail weld joint is prone to "saddle-shaped" wear preferentially forming on the rail head tread, increasing wheel-rail impact, affecting rail service life, and even endangering traffic safety. Therefore, the current domestic railway industry standards TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" and TB / T1632.4-2014 "Rail Welding Part 4: Gas Pressure Welding" stipulate that for heat-treated rails, the average hardness of the weld area must not be less than 90% of the average hardness of the rail base material, and the microstructure of the weld and heat-affected zone should not contain harmful structures such as martensite or bainite. Both of these rail welding standards apply to pearlitic rails, while there are currently no welding standards applicable to bainitic rails, either domestically or internationally. Therefore, it is inappropriate to evaluate the mechanical properties of bainitic rail welded joints in strict accordance with the current domestic rail welding technology standards. Furthermore, excessively high longitudinal hardness of the rail head tread of the welded joint will lead to a decrease in the fatigue performance of the joint and cause early fracture.

[0005] Currently, most publicly available post-weld heat treatment technologies focus on welded joints of rails with similar or identical microstructures (both pearlitic). For example, patent CN106544933A treats welded joints of hypereutectoid steel and eutectoid pearlitic steel, refining the pearlitic microstructure through a unified heating and cooling process using specific steps. Similarly, methods disclosed in patents such as CN201810581145.3 all work by refining the pearlitic lamellar spacing and improving thermoplasticity through rapid overall cooling. The common limitation of these existing technologies is that their process designs are all based on the single or dominant phase transformation premise of "pearlitic microstructure," employing a uniform temperature field and cooling path. This makes them completely unsuitable for welded joints of dissimilar materials like bainitic and pearlitic steel, which have fundamentally different phase transformation mechanisms and kinetics.

[0006] In summary, the existing technology lacks a post-weld heat treatment process that can perform zoned, independent, and precise temperature control based on the different phase transformation characteristics of the heat-affected zones on both sides of the welded joint of dissimilar bainitic and pearlitic rails. Summary of the Invention

[0007] In view of this, the present invention proposes a zoned temperature-controlled heat treatment process and system for post-weld bainitic and pearlitic dissimilar rails, which can at least solve the problem that the existing post-weld heat treatment process is difficult to apply to the heat-affected zones on both sides of the welded joint of bainitic and pearlitic dissimilar rails.

[0008] The first aspect of this invention proposes a post-weld zoned temperature-controlled heat treatment process for dissimilar bainitic and pearlitic rails, comprising the following steps: S1, pretreatment: performing post-weld surface treatment on the joint formed by welding bainitic and pearlitic rails, and allowing the overall temperature of the joint to naturally cool to 20-30°C. S2, zoned induction heating: using a zoned induction heating device to simultaneously but independently heat the heat-affected zones of the bainitic and pearlitic rails on both sides of the weld; wherein, the heat-affected zone of the bainitic rail is heated to 950-1050°C and held at that temperature, and the heat-affected zone of the pearlitic rail is heated to 900-940°C and held at that temperature. S3, Zoned Controlled Cooling: After heating and heat preservation, the heat-affected zone of the pearlitic rail is first cooled at a relatively fast rate to 650–560°C, and then slowly cooled at a relatively slow rate to 500–400°C; the heat-affected zone of the bainitic rail is cooled at a relatively fast rate to 320–350°C, and isothermal heat preservation treatment is performed at this temperature. S4, Natural Cooling: The joint is allowed to cool naturally to room temperature in the air.

[0009] In some embodiments, in step S2, the heating rate of the heat-affected zone of the bainitic rail is 1.5 to 5.5 °C / s, and the heating rate of the heat-affected zone of the pearlitic rail is 0.8 to 1.2 °C / s.

[0010] In some embodiments, in step S2, induction coils are used to heat the heat-affected zones of bainitic and pearlitic rails on both sides of the weld simultaneously but with independent parameters. Specifically, 2 to 4 sets of parallel induction coils are used for the heat-affected zone of the bainitic rail, with an operating frequency of 21 to 30 kHz; and 1 set of induction coils is used for the heat-affected zone of the pearlitic rail, with an operating frequency of 15 to 20 kHz.

[0011] In some embodiments, in step S2, the heat-affected zone holding time for bainitic rails and the heat-affected zone holding time for pearlitic rails are 10 to 30 seconds.

[0012] In some embodiments, in step S3: the first faster cooling rate is 5-8°C / s, the first slower cooling rate is 1-2°C / s, and / or the second faster cooling rate is 10-15°C / s.

[0013] In some embodiments, in step S3, an air-cooled nozzle is used to cool the heat-affected zone of the pearlitic rail, and the cooling medium is compressed air; the cooling channel at the bottom of the air-cooled nozzle is 25~35mm away from the surface of the welded joint, and the pressure of the compressed air is 0.15~0.25MPa.

[0014] In some embodiments, in step S3, a spray cooling system is used to spray cool the heat-affected zone of the bainitic rail, and the cooling medium is a mixture of air and water; the bottom cooling channel of the spray cooling system is 45~55mm away from the surface of the welded joint, and the pressure of the sprayed air and water mixture is 0.40~0.50MPa.

[0015] In some embodiments, in step S3, the isothermal heat preservation treatment time is 20 to 40 minutes, and the final cooling temperature is controlled at 320 to 350°C.

[0016] In some embodiments, in step S4, natural cooling is the natural cooling of the entire cross-section of the welded joint in air, with the cooling rate controlled at 0.1 to 0.8 °C / s.

[0017] A second aspect of this invention provides a zoned temperature-controlled heat treatment system for welded bainitic and pearlitic dissimilar rails, used to perform the aforementioned process. This system includes a zoned induction heating module, a zoned cooling control module, and a temperature monitoring and feedback control unit. The zoned induction heating module is configured to independently control the heating temperature and heating rate of the heat-affected zones (HAZs) of the bainitic and pearlitic rails on both sides of the weld. The zoned cooling control module is configured to independently control the cooling medium, cooling rate, and isothermal holding parameters of the HAZs on both sides of the weld. The temperature monitoring and feedback control unit is used to monitor the temperature of the HAZs on both sides of the weld in real time and provide feedback to control the zoned induction heating module and the zoned cooling control module.

[0018] The beneficial effects of this invention are as follows: By implementing zoned, precise temperature-controlled heating and cooling of the bainitic and pearlitic dissimilar rail welded joints, this invention specifically matches the phase transformation characteristics of the materials on both sides, resulting in a refined pearlitic microstructure in the heat-affected zone on the pearlitic side and a uniform lower bainitic microstructure on the bainitic side. This effectively avoids the formation of hard and brittle phases such as martensite, while ensuring the compatibility of mechanical properties between the heat-affected zones on both sides. The bainitic and pearlitic dissimilar rail welded joints treated by this invention achieve a longitudinal hardness of 92-96% of the average hardness of the corresponding rail base material within a ±30mm radius from the weld center. The average impact energy of the welded joint at room temperature is ≥42J, and the tensile strength is ≥850MPa, far exceeding the performance indicators specified in TB / T 1632.2-2014. This invention effectively alleviates the problem of abrupt hardness gradient changes in dissimilar joints, improves the wear resistance and fatigue life of the joints, avoids joint wear and cracks during railway service, and ensures the safety of high-speed and heavy-load railway operations. Attached Figure Description

[0019] 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.

[0020] Figure 1 A schematic diagram showing the location of a joint formed by welding bainitic and pearlitic steel rails according to an embodiment of the present invention; Figure 2 A schematic diagram of the sampling location for a metallographic specimen of the tread surface of a joint provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the full-section heating device for rail welding joints provided in an embodiment of the present invention. Detailed Implementation

[0021] 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.

[0022] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0023] The core innovation of the zonal temperature-controlled heat treatment process for dissimilar bainitic and pearlitic rails after welding, proposed in one embodiment of the first aspect of this invention, lies in breaking through the traditional uniform heating and cooling mode. It innovatively implements "zonal induction heating" and "zonal controlled cooling" on the heat-affected zones on both sides of the weld, which have drastically different materials and phase transformation characteristics (bainitic phase transformation range is 500~300℃, with the lower bainitic structure having better strength and toughness at approximately 350~320℃, and the upper bainitic structure having poorer strength and toughness at approximately 400~351℃; pearlitic phase transformation range is approximately 650~560℃). This results in lower cost and good mechanical properties of the welded joint after treatment. The process includes the following steps: S1. Pretreatment: Post-weld surface treatment is performed on the joint formed by welding bainitic and pearlitic rails. Weld ribs are removed from the joint surface, and the weld and the area within 70mm on both sides are ground to ensure a smooth surface. The joint is then allowed to cool naturally to room temperature (20-30℃). S2. Zoned Induction Heating: After pretreatment, a zoned induction heating device is used to simultaneously but independently heat the heat-affected zones (HAZs) of the bainitic and pearlitic rails on both sides of the weld. The HAZ of the bainitic rail is heated to 950-1050℃ and held to refine the austenite grains and provide sufficient nucleation sites. The HAZ of the pearlitic rail is heated to 900-940℃ and held to prevent coarse grains while ensuring sufficient austenitization. It should be noted that the HAZ is within 30-40mm of the weld, and the heated area covers the HAZ and extends 15mm beyond the base material. S3, Zoned Controlled Cooling: After heating and heat preservation, the heat-affected zones on both sides of the weld are subjected to independently controlled first-stage cooling. Specifically, the heat-affected zone of the pearlitic rail is first cooled at a relatively fast rate to 650–560°C, then slowly cooled at a relatively slow rate to 500–400°C to promote the refinement of pearlite. The heat-affected zone of the bainitic rail is cooled at a relatively fast rate to 350–320°C, and isothermal heat preservation is performed at this temperature, balancing strength and toughness. S4, Natural Cooling: The joint is allowed to cool naturally to room temperature in air. This process precisely matches the phase transformation kinetics requirements of dissimilar materials, resulting in a refined pearlite structure on the pearlite side and a uniform lower bainite structure on the bainite side, effectively avoiding hard and brittle phases such as martensite caused by parameter mismatch. Ultimately, the hardness of the joint within a 30mm radius from the weld center can recover to 92-96% of the base material, with a room temperature impact energy ≥42J and a tensile strength ≥850MPa, significantly superior to industry standards (such as TB / T1632.2-2014). This process fundamentally solves the technical challenges of decreased wear resistance, short fatigue life, and susceptibility to early wear and cracking caused by abrupt changes in hardness gradient and uneven microstructure in dissimilar rail joints. It provides key technological assurance for the safe and stable operation of high-speed and heavy-haul railways, supporting the market promotion and application of high-performance rail products.

[0024] In some embodiments, in step S2, the heating rate of the heat-affected zone of the bainitic rail is 1.5–5.5 °C / s, and the heating rate of the heat-affected zone of the pearlitic rail is 0.8–1.2 °C / s. Differential optimal heating rate windows are set to address the need for higher heating rates in bainitic rails to refine the initial austenite grains, and the need to prevent overheating and coarsening of the grains in pearlitic rails. This precise control ensures that sufficiently fine austenite grains are obtained on the bainitic side, providing ample nucleation sites for the subsequent formation of uniform and fine lower bainite; simultaneously, it avoids overheating and decreased toughness on the pearlitic side due to excessively rapid heating.

[0025] In some embodiments, in step S2, induction coils are used to simultaneously but independently heat the heat-affected zones (HAZs) of the bainitic and pearlitic rails on both sides of the weld. The bainitic rail HAZ uses 2-4 sets of parallel induction coils with an operating frequency of 21-30 kHz; the pearlitic rail HAZ uses 1 set of induction coils with an operating frequency of 15-20 kHz. Using relatively high-frequency induction coils with a larger number of sets for the bainitic side HAZ allows for more concentrated "skin effect" heating, facilitating the achievement of the required higher heating temperature (950-1050°C) at a set faster rate, and ensuring that the heating depth matches the HAZ range. Using relatively low-frequency induction coils with fewer sets for the pearlitic side provides a more suitable heating penetration depth and a gentler heating gradient, contributing to a uniform and controllable austenitizing process (900-940°C).

[0026] In some embodiments, in step S2, the heat-affected zone of the bainitic rail is kept warm for 10 to 30 seconds, which makes the austenitization process of the heat-affected zone on both sides sufficient and uniform, effectively preventing excessive growth of austenite grains, especially on the pearlite side which is sensitive to overheating.

[0027] In some embodiments, in step S3: the first faster cooling rate is 5-8°C / s, the first slower cooling rate is 1-2°C / s; and / or, the second faster cooling rate is 10-15°C / s. The pearlite side employs a two-stage cooling path of "faster first, slower second": first, a faster cooling rate of 5-8°C / s ensures rapid passage through the high-temperature zone, suppressing the precipitation of proeutectoid ferrite and allowing entry into the ideal pearlite phase transformation zone; then, a slower cooling rate of 1-2°C / s provides sufficient diffusion time within this phase transformation zone, promoting the formation of a pearlite structure with fine lamellar spacing and excellent strength and toughness. For the pearlite rail in this invention, its martensitic transformation initiation temperature is 300-350°C, and the critical martensitic transformation cooling rate is 1.5-2.0°C / s. To avoid the formation of hardened martensite structures detrimental to the service performance of the pearlite rail steel during the post-weld heat treatment cooling process, the temperature range of the first slower cooling stage is set at 500-400°C. The relatively fast cooling rate of 10~15℃ / s on the bainite side is intended to suppress the formation of high-temperature transformation products (such as ferrite or pearlite) and rapidly cool it to the lower bainite phase transformation region, thus creating conditions for obtaining a high-strength and high-toughness uniform lower bainite structure through subsequent isothermal treatment.

[0028] In some embodiments, in step S3, an air-cooled nozzle is used to cool the heat-affected zone of the pearlitic rail, and the cooling medium is compressed air. The cooling channel at the bottom of the air-cooled nozzle is 25-35 mm away from the surface of the welded joint, and the pressure of the compressed air is 0.15-0.25 MPa. By specifying the use of compressed air for air cooling and the specific nozzle distance and pressure range, the implementation of the cooling rate (5-8℃ / s and 1-2℃ / s) is specified and made operable. This parameter combination ensures that the cooling intensity is sufficiently uniform and controllable within the pearlitic phase transformation temperature range, achieving the required cooling rate while avoiding structural hardness or performance fluctuations caused by local overcooling or uneven cooling.

[0029] In some embodiments, in step S3, a spray cooling system is used to spray cool the heat-affected zone of the bainitic rail. The cooling medium is a mixture of air and water, which provides stronger cooling capacity than simple air cooling, and the cooling intensity can be finely adjusted by the water-to-air ratio. The bottom cooling channel of the spray cooling system is 45-55 mm away from the surface of the welded joint, and the pressure of the sprayed air-to-water mixture is 0.40-0.50 MPa, ensuring that the spray can uniformly cover the target area, achieving the purpose of quickly and uniformly cooling the joint to the lower bainitic isothermal temperature range (350-320°C), while avoiding quenching stress or martensite formation due to excessively intense cooling.

[0030] In some embodiments, in step S3, the isothermal holding time is 20–40 minutes, and the final cooling temperature is controlled at 350–320°C. The 20–40 minute isothermal time provides sufficient time for the bainitic phase transformation to complete, ensuring a fully transformed, uniform lower bainitic structure and maximizing its strength and toughness potential. Specific control of the final cooling temperature at 350–320°C (the lower limit of the 500–300°C bainitic phase transformation range) further locks in the temperature point at which the isothermal treatment ends, preventing excessive temperature reduction that could lead to the dangerous range of martensite formation.

[0031] In some embodiments, in step S4, natural cooling refers to the natural cooling of the entire cross-section of the welded joint in air at a rate controlled between 0.1 and 0.8 °C / s. This slow and uniform natural cooling phase is crucial after the completion of the critical partitioned phase transformation. This controlled slow cooling rate of 0.1 to 0.8 °C / s allows the stress in the entire welded joint (including the weld metal) to be fully released and homogenized at a lower temperature, effectively reducing residual stress and avoiding additional thermal and structural stresses caused by excessively rapid cooling.

[0032] The second aspect of the present invention provides a zonal temperature-controlled heat treatment system for welded dissimilar bainitic and pearlitic rails, used to perform the above-mentioned zonal temperature-controlled heat treatment process for welded dissimilar bainitic and pearlitic rails. The zonal temperature-controlled heat treatment system includes: a zonal induction heating module, a zonal cooling control module, and a temperature monitoring and feedback control unit.

[0033] The core function of the zoned induction heating module is to apply independent and precise heating to areas of different materials on both sides of the weld. This module consists of two independently controlled bainite-side heating units and a pearlite-side heating unit, such as... Figure 3 As shown, side a1 represents the rail head of a pearlitic rail, and side b1 represents the rail web of a pearlitic rail; side a2 represents the rail head of a bainitic rail, and side b2 represents the rail web of a bainitic rail. To address the requirement for rapid heating of the heat-affected zone of the bainitic rail to a relatively high temperature (950–1050℃) and for refining the austenite grains, the bainitic side heating unit is equipped with 2 to 4 sets (e.g., 3 sets) of parallel-arranged induction coils. These induction coils are arranged longitudinally along the rail, with a coil spacing of 5–10 mm, and operate in the mid-frequency range of 21–30 kHz. The arrangement of multiple coil sets increases the heating coverage area and uniformity, and the higher frequency facilitates a more concentrated heating effect to meet the required heating rate (1.5–5.5℃ / s). To prevent overheating and grain coarsening in the heat-affected zone of pearlitic rails and to ensure relatively low heating temperatures (900–940℃), the pearlitic side heating unit is equipped with a set of induction coils operating in the mid-frequency range of 15–20 kHz. The relatively low frequency helps to achieve a gentler heating gradient, ensuring a uniform and controllable austenitization process, matching its slower heating rate (0.8–1.2℃ / s). The induction coils of the two heating units are locked together using several locking bolts to ensure that the distance between the inner surface of the coil and the rail tread and sides is maintained at 3–5 mm, achieving efficient coupling and precise temperature control.

[0034] The core function of the zoned cooling control module is to enable independent, programmed cooling of both heat-affected zones immediately after heating. This module includes two independently controlled bainitic side cooling units and a pearlitic side cooling unit. To meet the requirement that the heat-affected zone of bainitic rails needs to enter the isothermal zone at a relatively fast cooling rate (10–15°C / s), the bainitic side cooling unit employs a spray cooling system. This system includes a nozzle array surrounding the heat-affected zone, an independent water-air supply and mixing device, and precise pressure and flow control valves. The nozzles spray a mixture of air and atomized water, and the cooling intensity is precisely controlled by adjusting the water-air ratio and total pressure (e.g., 0.40–0.50 MPa). The distance between the bottom of the nozzle array and the rail surface is typically set at 45–55 mm to ensure uniform spray coverage and meet cooling intensity requirements. To implement the complex cooling path of "rapid cooling (5-8℃ / s) followed by slow cooling (1-2℃ / s)" in the heat-affected zone of pearlitic rails, the pearlitic side cooling unit employs an air-cooled array consisting of several air-cooled nozzles. These nozzles are connected to an adjustable-pressure compressed air source (e.g., 0.15-0.25 MPa), and the airflow is controlled by independent valves or variable-frequency fans. The arrangement of the nozzle array also considers the coverage area, with the distance between its bottom and the rail surface typically set at 25-35 mm to provide suitable and adjustable air-cooling intensity. This unit usually has segmented control capabilities to switch between different cooling rates in different temperature ranges.

[0035] The temperature monitoring and feedback control unit ensures the process is executed precisely according to set parameters. It includes a temperature monitoring section and a feedback control section. For the temperature monitoring section, high-response, high-precision non-contact infrared thermometers or embedded thermocouples are installed at key locations in the heat-affected zone on both sides of the weld (e.g., at a specific distance from the weld center) to monitor temperature changes in real-time and continuously during heating and cooling, with an accuracy of ±5℃. For the feedback control section, based on the real-time acquired temperature data, the PLC or industrial computer within the control unit compares it with a preset process curve and dynamically generates control commands. These commands adjust the output power of the induction coil power supply in real-time (to control the heating rate and holding temperature) and regulate the pressure and flow rate of the cooling unit medium (to control the cooling rate and isothermal temperature), forming a closed-loop automatic control system that ensures a high degree of consistency between the actual process parameters and the target values.

[0036] In some embodiments, the post-weld zoned temperature-controlled heat treatment system further includes a positioning and fixing device for clamping the rail joint in a fixed position to ensure the stability of the joint position during heating and cooling. Specifically, a typical configuration of the positioning and fixing device employs a rigid modular structure integrating multi-directional clamping and precision positioning functions. Its core includes a stable base with a precision positioning guide rail and multiple sets of independent clamps driven by hydraulics or pneumatics and equipped with high-temperature resistant pads. These clamps work together in both vertical (acting on the rail head and rail base) and horizontal (acting on the rail head side and rail web) directions to firmly clamp the welded joint in a preset position, effectively suppressing thermal deformation and displacement during heat treatment.

[0037] To more clearly illustrate the technical solution of the present invention, the implementation process and technical effects of the present invention will be described in detail below in conjunction with specific embodiments, comparative examples and industry standards.

[0038] To objectively evaluate the effect of post-weld heat treatment, this invention prepares longitudinal hardness test specimens for rail joints according to TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. Hardness testing is performed according to GB / T 230.1-2009, at a depth of 5mm below the rail head tread (e.g., ...). Figure 1 As shown in the figure, Vickers (HV) hardness tests were performed with points symmetrically arranged 2 mm apart from the weld seam on both sides. Metallographic examination was conducted according to GB / T13298-2015 "Metallic Microstructure Examination Methods". Figure 2 Samples were taken from the locations shown. After sample preparation, the samples were etched with a 3% nitric acid-alcohol solution and observed and graded using a Leica MeF3 optical microscope. Figure 1 In the diagram, point a represents a pearlitic rail, point b represents a bainitic rail, and point c represents the center of the joint weld. Figure 2 In the diagram, point c is the center of the joint weld, and point d is the sampling location.

[0039] Examples 1 to 6 below all use the same type of rail with a specification of 60kg / m. The bainitic and pearlitic rails are welded using a rail-moving flash welding machine. The bainitic rail has a room temperature (20~30℃) tensile strength of 951~1000MPa, a hardness of 310~330HV, and an impact energy ≥150J; the pearlitic rail has a room temperature (20~30℃) tensile strength of 900~950MPa, a hardness of 280~300HV, and an impact energy ≥35J. The chemical composition of the bainitic rail base material is: C 0.20-0.25 wt%, Si 0.70-1.00 wt%, Mn 1.80-2.20 wt%, Cr 0.30-0.50 wt%, Mo 0.20-0.40 wt%, with the balance being Fe and unavoidable impurities. The chemical composition of the pearlitic steel base material is: C 0.61-0.65 wt%, Si 0.50-0.80 wt%, Mn 1.00-1.30 wt%, Cr 0.20-0.40 wt%, with the balance being Fe and unavoidable impurities. All embodiments strictly follow the aforementioned process steps and core parameter ranges of this invention. The pretreatment in step S1 and the natural cooling in step S4 are performed in the same manner. The following embodiments only describe the zoned induction heating in step S2 and the zoned controlled cooling in step S3.

[0040] Example 1 Induction heating stage: Three sets of induction coils are used on the bainite side, with a heating temperature of 950℃, a heating rate of 1.5℃ / s, and a holding time of 30s; one set of induction coils is used on the pearlite side, with a heating temperature of 900℃, a heating rate of 1.0℃ / s, and a holding time of 30s.

[0041] The cooling process was controlled in zones: the pearlite side was first cooled to 650℃ at 5℃ / s, and then slowly cooled to 400℃ at 1℃ / s; the bainite side was cooled to 330℃ at 10℃ / s and kept at an isothermal temperature for 30 minutes.

[0042] Performance results: The heat-affected zone on the pearlite side exhibited a refined pearlite microstructure with a grain size of grade 7; the bainite side exhibited a uniform lower bainite microstructure with a grain size of grade 8. The joint hardness was: pearlite side 285±8 HV, bainite side 310±10 HV, and weld 295±9 HV. The Charpy U-impact energy of the weld was ≥42 J, and the tensile strength of the joint across its entire cross-section was ≥850 MPa.

[0043] Example 2 Induction heating stage: Two sets of induction coils are used on the bainite side, with a heating temperature of 1000℃, a heating rate of 2.0℃ / s, and a holding time of 20s; one set of induction coils is used on the pearlite side, with a heating temperature of 920℃, a heating rate of 1.0℃ / s, and a holding time of 10s.

[0044] The cooling process was controlled in zones: the pearlite side was first cooled to 620°C at a rate of 6°C / s, and then slowly cooled to 450°C at a rate of 1.5°C / s; the bainite side was cooled to 320°C at a rate of 12°C / s and kept isothermal for 25 minutes.

[0045] Performance results: Pearlite grain size grade 7, bainite grain size grade 8. Joint hardness: pearlite side 290±7HV, bainite side 315±9HV, weld 300±8HV. Impact energy ≥45J, tensile strength ≥860MPa.

[0046] Example 3 Induction heating stage: Four sets of induction coils are used on the bainite side, with a heating temperature of 1050℃, a heating rate of 2.5℃ / s, and a holding time of 20s; one set of induction coils is used on the pearlite side, with a heating temperature of 940℃, a heating rate of 1.2℃ / s, and a holding time of 20s.

[0047] The cooling process was controlled in zones: the pearlite side was first cooled to 580℃ at 8℃ / s, and then slowly cooled to 500℃ at 2℃ / s; the bainite side was cooled to 350℃ at 15℃ / s and kept isothermal for 35 minutes.

[0048] Performance results: Pearlite grain size grade 7, bainite grain size grade 9. Joint hardness: Pearlite side 295±8HV, bainite side 320±10HV, weld 305±9HV. Impact energy ≥43J, tensile strength ≥870MPa.

[0049] Example 4 Induction heating stage: Three sets of induction coils are used on the bainite side, with a heating temperature of 980℃, a heating rate of 2.2℃ / s, and a holding time of 30s; one set of induction coils is used on the pearlite side, with a heating temperature of 910℃, a heating rate of 0.9℃ / s, and a holding time of 10s.

[0050] The cooling process was controlled in zones: the pearlite side was first cooled to 630℃ at 5℃ / s, and then slowly cooled to 480℃ at 1.2℃ / s; the bainite side was cooled to 320℃ at 11℃ / s and kept isothermal for 32 minutes.

[0051] Performance results: Pearlite grain size grade 7, bainite grain size grade 8. Joint hardness: Pearlite side 288±7HV, bainite side 312±8HV, weld 298±8HV. Impact energy ≥44J, tensile strength ≥855MPa.

[0052] Example 5 Induction heating stage: Two sets of induction coils are used on the bainite side, with a heating temperature of 960℃, a heating rate of 1.8℃ / s, and a holding time of 30s; one set of induction coils is used on the pearlite side, with a heating temperature of 930℃, a heating rate of 1.1℃ / s, and a holding time of 10s.

[0053] The cooling process was controlled in zones: the pearlite side was first cooled to 600℃ at 7℃ / s, and then slowly cooled to 500℃ at 1.8℃ / s; the bainite side was cooled to 330℃ at 13℃ / s and kept isothermal for 28 minutes.

[0054] Performance results: Pearlite grain size grade 7, bainite grain size grade 8. Joint hardness: pearlite side 292±8HV, bainite side 314±9HV, weld 302±8HV. Impact energy ≥43J, tensile strength ≥865MPa.

[0055] Example 6 Induction heating stage: Four sets of induction coils are used on the bainite side, with a heating temperature of 1020℃, a heating rate of 2.3℃ / s, and a holding time of 15s; one set of induction coils is used on the pearlite side, with a heating temperature of 925℃, a heating rate of 1.05℃ / s, and a holding time of 15s.

[0056] The cooling process was controlled in zones: the pearlite side was first cooled to 580℃ at 7℃ / s, and then slowly cooled to 420℃ at 1.6℃ / s; the bainite side was cooled to 330℃ at 14℃ / s and kept isothermal for 38 minutes.

[0057] Performance results: Pearlite side grain size grade 7, bainite side grain size grade 9. Joint hardness: pearlite side 294±7HV, bainite side 318±10HV, weld 303±9HV. Impact energy ≥46J, tensile strength ≥875MPa. Overall performance is the best.

[0058] To highlight the superiority of the technical solution of this invention, the following comparative examples are provided. The comparative examples use the same bainitic steel rails, pearlitic steel rails, and welding methods as the examples, only changing the post-weld heat treatment parameters; the performance testing standards are the same as those in the examples.

[0059] Comparative Example 1 (Unzoned heating) Both sides use a set of induction coils, which are uniformly heated to 920℃, held for 10 seconds, and then uniformly air-cooled.

[0060] Performance results: Insufficient austenitization on the bainite side, with coarse grains (level 5), forming upper bainite; uneven microstructure on the pearlite side. Low and uneven hardness (270±12HV on the bainite side, 280±10HV on the pearlite side). Impact energy is only 28J, tensile strength is 720MPa, and microcracks exist at the joints.

[0061] Comparative Example 2 (Uniform High-Temperature Heating) Both sides use 3 sets of induction coils, which are uniformly heated to 1000℃, held for 10 minutes, and then uniformly air-cooled.

[0062] Performance results: Severe overheating on the pearlite side, coarse grains (level 4), Widmanstätten structure, and significantly reduced toughness. Low hardness (260±15HV on the pearlite side), impact energy 22J, tensile strength 700MPa, and prone to quenching cracks.

[0063] Comparative Example 3 (zoned heating but uniform cooling) The heating parameters are the same as in Example 2 (zoned temperature control), but the cooling is done by uniform air cooling on both sides (cooling rate 8°C / s).

[0064] Performance results: Due to excessively rapid cooling, a hard and brittle martensite phase appeared on the pearlite side; the cooling rate on the bainite side was mismatched, resulting in a mixed structure. The hardness fluctuated greatly (260-350HV), the impact energy was 25J, the tensile strength was 710MPa, and the risk of brittle fracture at the joint was high.

[0065] Comparative Example 4 (Insufficient heating temperature on the bainite side) The bainite side is heated to 900°C (below the lower limit of this invention), the pearlite side parameters are the same as in Example 2, and the cooling is the same as in Example 2.

[0066] Performance results: Insufficient austenite grain refinement on the bainite side (level 6), low lower bainite content, and no strength recovery. Hardness 265±10HV, impact energy 32J, tensile strength 750MPa, poor performance.

[0067] Comparative Example 5 (Heating temperature on the pearlite side is too high) One set of coils on the pearlite side is heated to 980°C (higher than the upper limit of this invention), and the parameters on the bainite side are the same as in Example 2. The cooling is the same as in Example 2.

[0068] Performance results: The pearlite grains are severely coarse (level 4), and the toughness is significantly deteriorated. The impact energy is 26J and the tensile strength is 730MPa, which are below the acceptable levels.

[0069] Comparative Example 6 (without post-weld heat treatment) After welding, the material is air-cooled directly without any heating or cooling control.

[0070] Performance results: The heat-affected zone exhibits disordered microstructure, with a mixture of martensite and upper bainite on the bainite side and coarse lamellar pearlite on the pearlite side. The hardness gradient shows a drastic abrupt change (250-360 HV), the impact energy is only 20 J, and the tensile strength is 680 MPa. These properties are completely unacceptable for high-speed, heavy-load applications.

[0071] By comparing the embodiments and comparative examples, it can be seen that the technical solution of the present invention has the following significant beneficial effects: Organizational optimization: Through the synergistic effect of "zoned induction heating" and "zoned controlled cooling", the phase transformation characteristics of dissimilar materials such as bainite and pearlite are precisely matched, so that the heat-affected zone on the pearlite side obtains a refined pearlite structure and the bainite side obtains a uniform lower bainite structure, effectively avoiding the generation of hard and brittle harmful phases such as martensite.

[0072] Superior performance: The treated welded joint exhibits excellent performance. Within a 30mm radius from the weld center, the longitudinal hardness can reach 92% to 96% of the average hardness of the corresponding rail base material; the average impact energy of the welded joint at room temperature is ≥42J, and the tensile strength is ≥850MPa. All indicators are far superior to the requirements of current industry standards (such as TB / T 1632.2-2014).

[0073] Improved reliability: This process significantly alleviates the problem of sudden changes in hardness gradient in welded joints of dissimilar rails, greatly improves the wear resistance and fatigue life of the joints, and fundamentally avoids safety hazards such as abnormal wear, crack initiation and propagation of joints that may occur during line service, providing reliable technical support for the development of high-speed and heavy-haul railways.

[0074] This invention has significant technical advantages and broad market application prospects.

[0075] The above embodiments are only used to illustrate the implementation of the present invention, and are not intended to limit the scope of protection of the present invention. For those skilled in the art, any modifications and improvements made to the present invention without departing from its principles should also be considered to fall within the scope of protection of the present invention.

Claims

1. A zoned temperature-controlled heat treatment process for dissimilar bainitic and pearlitic steel rails after welding, characterized in that, Includes the following steps: S1, Pretreatment: Perform post-weld surface treatment on the joint formed by welding bainitic and pearlitic steel rails, and allow the overall temperature of the joint to cool naturally to 20-30℃. S2, Zoned Induction Heating: A zoned induction heating device is used to simultaneously but independently heat the heat-affected zones of the bainitic rail and the pearlitic rail on both sides of the weld; wherein the heat-affected zone of the bainitic rail is heated to 950-1050℃ and held at that temperature, and the heat-affected zone of the pearlitic rail is heated to 900-940℃ and held at that temperature. S3, zoned controlled cooling: After the heating and heat preservation are completed, the heat-affected zone of the pearlitic rail is first cooled to 650-560°C at a first relatively fast cooling rate, and then slowly cooled to 500-400°C at a first relatively slow cooling rate; the heat-affected zone of the bainitic rail is cooled to 320-350°C at a second relatively fast cooling rate, and isothermal heat preservation treatment is carried out at this temperature. S4, Natural Cooling: Allow the connector to cool naturally to room temperature in the air.

2. The process according to claim 1, characterized in that, In step S2, the heating rate of the heat-affected zone of the bainitic rail is 1.5 to 5.5 °C / s, and the heating rate of the heat-affected zone of the pearlitic rail is 0.8 to 1.2 °C / s.

3. The process according to claim 1, characterized in that, In step S2, induction coils are used to heat the heat-affected zones of the bainitic rail and the pearlitic rail on both sides of the weld simultaneously but with independent parameters. Specifically, 2 to 4 sets of parallel induction coils are used for the heat-affected zone of the bainitic rail, with an operating frequency of 21 to 30 kHz; and 1 set of induction coils is used for the heat-affected zone of the pearlitic rail, with an operating frequency of 15 to 20 kHz.

4. The process according to claim 1, characterized in that, In step S2, the heat-affected zone of the bainitic rail is kept warm for 10 to 30 seconds, and the heat-affected zone of the pearlitic rail is kept warm for 10 to 30 seconds.

5. The process according to claim 1, characterized in that, In step S3: The first faster cooling rate is 5-8℃ / s, and the first slower cooling rate is 1-2℃ / s; And / or, the second faster cooling rate is 10-15°C / s.

6. The process according to claim 1, characterized in that, In step S3, an air-cooled nozzle is used to cool the heat-affected zone of the pearlitic rail, and the cooling medium is compressed air; the cooling channel at the bottom of the air-cooled nozzle is 25~35mm away from the surface of the welded joint, and the pressure of the compressed air is 0.15~0.25MPa.

7. The process according to claim 1, characterized in that, In step S3, a spray cooling system is used to spray cool the heat-affected zone of the bainitic rail. The cooling medium is a mixture of air and water. The bottom cooling channel of the spray cooling system is 45-55 mm away from the surface of the welded joint, and the pressure of the sprayed air-water mixture is 0.40-0.50 MPa.

8. The process according to claim 1, characterized in that, In step S3, the isothermal heat preservation treatment lasts for 20 to 40 minutes, and the final cooling temperature is controlled at 320 to 350°C.

9. The process according to claim 1, characterized in that, In step S4, the natural cooling is the natural cooling of the entire cross section of the welded joint in the air, and the cooling rate is controlled at 0.1 to 0.8℃ / s.

10. A zoned temperature-controlled heat treatment system for dissimilar bainitic and pearlitic steel rails after welding, characterized in that, For performing the process according to any one of claims 1 to 9, comprising: The zoned induction heating module is configured to independently control the heating temperature and heating rate of the heat-affected zones of the bainitic and pearlitic rails on both sides of the weld. The zoned cooling control module is configured to independently control the cooling medium, cooling rate, and isothermal insulation parameters of the heat-affected zones on both sides of the weld. The temperature monitoring and feedback control unit is used to monitor the temperature of the heat-affected zones on both sides of the weld in real time and provide feedback control to the partitioned induction heating module and the partitioned cooling control module.

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