A method for controlling the softening zone of bainitic rail welded joints

Abstract

This invention belongs to the field of rail welding and discloses a method for controlling the softening zone of bainitic rail welded joints. The method includes: welding bainitic rails using flash welding, wherein the flash welding includes a preheating stage, a continuous melting stage, and an accelerated melting stage. During the preheating stage, the thickness of the molten layer of the welded joint is controlled to be 1.5~2.5 mm; during the continuous melting stage, the temperature of the molten pool is controlled to be 1480~1530℃; the welded high-temperature rail joint is then subjected to a three-stage gradient cooling process: first, cooling at a rate of 18~22℃ / s to near the bainitic transformation point; then, cooling at a rate of 0.08~0.12℃ / s for a predetermined time; and finally, cooling at a rate below 0.5℃ / s to room temperature. This invention significantly reduces the melting amount, avoids excessive melting of the rail base material, and reduces the expansion of the heat-affected zone, achieving a softening zone width ≤0.5 mm, which is 80% smaller than the softening zone width of the Bombardier BWR system.

Description

Technical Field

[0001] This invention relates to the field of rail welding technology, and in particular to a method for controlling the softening zone of bainitic rail welded joints. Background Technology

[0002] The rapid expansion of high-speed rail networks and the continuous increase in train speeds have provided tremendous convenience for modern society and people's travel. The rapid development and construction of high-speed rail networks have placed higher demands on the design speed of high-speed trains, and increasing operating speed is an inevitable trend for the high-quality development of railways.

[0003] In the development of high-speed railways, the dynamic loads and high-frequency impacts faced by wheel-rail systems are becoming increasingly severe. High-speed train operation places extremely high demands on the longitudinal continuity of the rail surface. When wheelsets pass through uneven areas of the rail at high speed, the impact on the rail intensifies, and the car body, affected by the reaction force, generates abnormal vibrations. These high-frequency vibrations can lead to structural fatigue fractures in both the vehicle and the track. Due to the differences in properties between the materials near the rail joint area and the rail welding materials, exhibiting non-uniformity and geometric inconsistency, a height difference will occur in a localized area of ​​the rail top under the multiple contact actions of the car body and wheelsets. This results in an uneven area along the longitudinal direction of the rail in the rail weld zone.

[0004] In recent years, bainitic rails have demonstrated outstanding performance in improving the wear resistance and toughness of rails due to their excellent strength-toughness ratio. However, the control of the softening zone in the welded joints has become a bottleneck restricting the full realization of these technological advantages. During the welding process, the heat-affected zone of the welded joints of bainitic rails is prone to microstructural degradation due to thermal cycling, forming a softening zone with a hardness significantly lower than that of the base material. Under existing processes, the softening zone of bainitic rail welded joints is generally quite wide (≥2.5mm), leading to a decrease in the load-bearing capacity and an imbalance in stress distribution in this area. Under repeated high-frequency impact loads from the wheel and rail, this easily induces rail head crushing, corrugation, and even transverse crack propagation. More seriously, an excessively wide softening zone accelerates the accumulation of microscopic damage, making the joint area the weakest link in the entire rail lifespan, forcing maintenance departments to frequently intervene in repairs or replacements, significantly increasing costs. This contradiction is particularly prominent on ultra-high-speed lines with speeds exceeding 350 km / h.

[0005] Controlling the width of the softening zone in welded joints of bainitic rails has become an urgent problem to be solved in this field. Summary of the Invention

[0006] To address the above problems, this invention provides a method for controlling the softening zone of bainitic rail welded joints.

[0007] According to one aspect of the present invention, a method for controlling the softening zone of a bainitic rail welded joint is provided, the method comprising the following steps:

[0008] Flash welding was used to weld bainitic steel rails. The flash welding process included a preheating stage, a continuous melting stage, and an accelerated melting stage. The preheating stage resulted in a weld joint molten layer thickness of 1.5~2.5 mm. The continuous melting stage controlled the molten pool temperature at 1480~1530℃.

[0009] The welded joint of the high-temperature rail was subjected to a three-stage gradient cooling process. In the first stage, the welded joint was cooled to the bainitic transformation temperature range at a cooling rate of 18~22℃ / s. In the second stage, the welded joint was cooled for a predetermined time at a cooling rate of 0.08~0.12℃ / s. In the third stage, the welded joint was cooled to room temperature at a cooling rate of less than 0.5℃ / s.

[0010] According to one embodiment of the present invention, the bainitic rail base material comprises the following components by weight percentage: C: 0.19%~0.23%, Si: 1.50%~1.60%, Mn: 2.30%~2.40%, Cr: 0.50%~0.55%, Mo: 0.14%~0.16%, V: 0.08%~0.09%, N: 0.006%~0.008%, with the balance being Fe and unavoidable impurities, and the bainitic rail has a specification of 60 kg / m.

[0011] According to one embodiment of the present invention, the voltage of the preheating stage is controlled at 380~400V, the voltage of the continuous firing stage is controlled at 360~380V, and the voltage of the accelerated firing stage is controlled at 380~400V.

[0012] According to one embodiment of the present invention, the flash speed during the preheating stage is 4.0~4.8 mm / s, and the flash speed during the continuous burning stage is 1.5~2.0 mm / s.

[0013] According to one embodiment of the present invention, upsetting in the high-temperature plastic rheological section at 900~800°C is performed with a pressure of 340~360MPa for 3~5s.

[0014] According to one embodiment of the present invention, upsetting at temperatures below 800°C uses a pressure of 400~450 MPa, and the dynamic displacement compensation is controlled to be 18~22 mm.

[0015] According to one embodiment of the present invention, the cooling of the first stage is controlled by injecting nitrogen gas.

[0016] According to one embodiment of the present invention, the cooling of the second stage is controlled by atomized water cooling, and the cooling time of the second stage is 1300~1500s.

[0017] According to one embodiment of the present invention, the cooling in the third stage is controlled by infrared radiation.

[0018] According to one embodiment of the present invention, the softened zone width of the bainitic rail welded joint is less than 0.5 mm, the hardness of the heat-affected zone is higher than 96% of the hardness of the base material, and the contact fatigue life is ≥1.1×10⁻⁶. 7 Second-rate.

[0019] Due to the adoption of the above technical solutions, the method for controlling the softening zone of bainitic rail welded joints provided by the present invention has at least one of the following beneficial effects:

[0020] (1) The method of the present invention strictly controls the heat input during the preheating stage, and controls the thickness of the molten layer to 1.5~2.5mm. Compared with the 4.2mm thick molten layer in the prior art, the amount of burning is significantly reduced, avoiding excessive melting of the rail base material. At the same time, it reduces the expansion of the heat-affected zone. During the continuous burning stage, the temperature fluctuation of the molten pool is strictly controlled to ensure the uniformity of the material microstructure, achieving a softening zone width ≤0.5mm (80% reduction compared to the Bombardier BWR system), and a contact fatigue life exceeding 1.1×10 7 The second cycle (load stress ratio R=0.1) improves upon the traditional process by 300%, meeting the ultimate load requirements of track welds for high-speed railways with a speed of 400km / h.

[0021] (2) The method of the present invention adopts a three-stage gradient controlled cooling after welding to construct a self-catalytic cycle system of bainitic phase transformation and precipitation phase. In the heat-affected zone, the overall strength and toughness of the weld softening zone are improved by the dual strengthening of fine grain strengthening and precipitation strengthening, so as to avoid the softening zone being easily worn due to its significantly lower hardness than the base material during service. Attached Figure Description

[0022] Figure 1 This is a flowchart of a method for controlling the softening zone of a bainitic rail welded joint according to an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Flash welding technology boasts high thermal efficiency, enabling the welding of large-area parts. Furthermore, it provides relatively uniform heating time across the entire end face of the weldment. Flash welding can be used to weld not only compact cross-sections but also open cross-sections. The rail's compact top cross-section, coupled with its open web and bottom cross-sections, makes it well-suited for flash welding.

[0025] During flash welding, electrodes clamp the rails, a transformer is connected, and the rail ends are gradually brought closer together until partial contact occurs. The contact point heats up, causing liquid metal to explode and generate sparks, creating continuous flashes. Once the flash heating reaches the appropriate temperature, a forging force is rapidly applied, causing the rail ends to press against each other. The current is then cut off, resulting in intense plastic deformation in the welding zone. The mating surfaces interlock and crystallize, forming a welded joint.

[0026] The bainitic rail flash welding method used in this invention is mainly divided into five stages: flash leveling, sintering, upsetting, and pressure holding. Each stage can be further divided into multiple sub-stages. The preheating stage, continuous sintering stage, and accelerated sintering stage in this invention mainly involve the sintering stage. Bainitic flash welding uses the current in the main welding circuit as the control reference data to control the speed and displacement of the moving frame, realizing a heating method that combines short-time short circuit and flash welding. Its characteristic is that, under the premise of meeting sufficient heat input, it increases the proportion of heating by contact resistance, which is beneficial to improving the temperature uniformity of the weld joint end face.

[0027] like Figure 1 As shown, the method for controlling the softening zone of bainitic rail welded joints provided by the present invention generally includes the following steps:

[0028] Step S1: Weld the bainitic steel rail using flash welding. The flash welding process includes a preheating stage, a continuous burning stage, and an accelerated burning stage. During the preheating stage, the thickness of the molten layer of the weld joint is 1.5~2.5mm. During the continuous burning stage, the temperature of the molten pool is controlled at 1480~1530℃.

[0029] Step S2: The welded high-temperature rail joint is subjected to three-stage gradient cooling. In the first stage, the welded joint is cooled to near the bainitic transformation point at a cooling rate of 18~22℃ / s. In the second stage, the welded joint is cooled for a predetermined time at a cooling rate of 0.08~0.12℃ / s. In the third stage, the welded joint is cooled to room temperature at a cooling rate of less than 0.5℃ / s.

[0030] The method of this invention strictly controls the heat input during the preheating stage, limiting the molten layer thickness to 1.5~2.5mm. Compared to the 4.2mm thick molten layer in existing technologies, this significantly reduces the amount of material burned, preventing excessive melting of the rail base material and minimizing the expansion of the heat-affected zone. If the molten layer thickness is less than 1.5mm, insufficient penetration of the molten pool during fine burning will affect the material's density. During the continuous burning stage, strict control of the molten pool temperature fluctuations ensures the uniformity of the material's microstructure, achieving a softened zone width ≤0.5mm (80% reduction compared to the Bombardier BWR system) and a contact fatigue life exceeding 1.1×10⁻⁶. 7 The second cycle (load stress ratio R=0.1) improves upon traditional processes by 300%, meeting the ultimate load requirements of track welds for high-speed railways with a speed of 400km / h.

[0031] Traditional flash welding processes employ single-stage cooling, resulting in coarsened weld joint microstructure (original austenite grains ≥35μm) and a tensile strength loss of 8%~12% (the joint only reaches 1230-1290MPa when the base metal is 1400MPa). The method of this invention employs a three-stage gradient controlled cooling post-weld process, matching the cooling rate with the bainitic phase transformation kinetics to reduce fluctuations in the nucleation rate of precipitated phases. During the bainitic transformation, a self-catalytic cycle system of bainitic phase transformation and precipitates is constructed: bainitic nucleation removes carbon from austenite, inducing the preferential precipitation of (V, Mo) (C, N) composite phases at the bainite / austenite interface, enhancing the pinning force of the precipitates on the grain boundaries, while simultaneously restricting bainitic lath growth and refining the lath structure; the carbon removal effect during the bainitic phase transformation accelerates the formation of precipitates, which reduces the local carbon concentration, thereby promoting bainitic nucleation. This cycle forms a positive feedback self-catalytic strengthening mechanism, achieving dual strengthening of grain refinement and precipitation in the heat-affected zone, thus improving the overall strength and toughness of the weld softened zone and preventing the softened zone from being easily worn due to its significantly lower hardness compared to the base material during service.

[0032] In some embodiments of the present invention, the bainitic rail base material comprises the following components by weight percentage: C: 0.19%~0.23%, Si: 1.50%~1.60%, Mn: 2.30%~2.40%, Cr: 0.50%~0.55%, Mo: 0.14%~0.16%, V: 0.08%~0.09%, N: 0.006%~0.008%, with the balance being Fe and unavoidable impurities. The V and Mo elements in the bainitic rail, along with C and N elements, form carbonitride precipitates during post-weld cooling. These precipitates, in conjunction with dislocations and grain boundaries, enhance the strength of the joint area. The bainitic rail has a specification of 60 kg / m.

[0033] In some embodiments of the present invention, the voltage during the preheating stage is controlled at 380~400V, the duration is approximately 10~20s, and the flash speed during the preheating stage is 4.0~4.8mm / s. This stage initially softens the end face, establishes conductive contact points, and allows the end face temperature to rise uniformly to 1200~1300℃. The use of a higher voltage and a higher moving speed during the preheating stage shortens the thermal action time, avoids excessive melting of the base material, and balances efficiency and heat input. This stage aims to form a uniform initial molten layer, providing a foundation for subsequent continuous firing, while reducing the expansion of the heat-affected zone.

[0034] In some embodiments of the present invention, the voltage during the continuous burning stage is controlled at 360~380V, and the flash speed is 1.5~2.0mm / s. This stage employs a lower voltage and a lower moving speed to ensure that the molten pool has sufficient time to reach the target temperature and achieve temperature homogenization, thereby enabling precise temperature control of the molten pool, reducing the amount of base material burned, and suppressing the expansion of the heat-affected zone.

[0035] In some embodiments of the present invention, the voltage during the accelerated burning stage is controlled at 380~400V, the duration is approximately 2~4s, and the burning speed is above 3.0mm / s. The accelerated burning stage forces the removal of oxides to prevent ash spots.

[0036] In some embodiments of the present invention, during the upsetting process, upsetting is performed at a pressure of 340-360 MPa for 3-5 seconds during the high-temperature plastic rheological stage at 900-800°C. Afterwards, upsetting is performed at a pressure of 400-450 MPa, with the dynamic displacement compensation controlled at 18-22 mm. This dynamic upsetting, using a lower pressure at high temperatures, allows the material to soften sufficiently, promoting grain boundary slip and dislocation movement, forming a uniform initial plastic rheology. This stage avoids localized stress concentration caused by high pressure, reducing the risk of folding and crack initiation. Subsequently, the pressure is increased to 400-450 MPa, utilizing the material's low rheological stress characteristics at high temperatures to accelerate the densification process. Dynamic pressure adjustment can match the material's real-time deformation resistance, avoiding flow stagnation due to strain hardening and preventing defect formation.

[0037] In some embodiments of the present invention, the first stage of cooling is controlled by nitrogen injection at a flow rate of approximately 20-30 m / s, achieving a cooling rate of 18-22 °C / s to suppress the formation of proeutectoid ferrite and pearlite. The second stage of cooling is controlled by atomized water cooling at a rate of 0.08-0.12 °C / s, precisely matching the phase deformation nucleation rate. The second stage cooling time is 1300-1500 s, ensuring a bainite transformation rate ≥98%. This combination of nitrogen injection and atomized water cooling allows for real-time adjustment of the cooling intensity deviation to ≤5%, avoiding performance fluctuations caused by reliance on experience in traditional processes. The third stage of cooling is controlled by infrared radiation to suppress the decomposition of retained austenite (volume fraction ≤2.5%).

[0038] In some embodiments of the present invention, the softened zone width of the bainitic rail welded joint obtained by the method of the present invention is less than 0.5 mm, the hardness of the heat-affected zone is higher than 96% of the hardness of the base material, and the contact fatigue life is ≥1.1×10⁻⁶. 7 Tensile strength ≥1400MPa, elongation after fracture ≥12%, impact energy at -40℃ ≥110J, HAZ hardness ≥96% of the parent material, hardness matching degree ≥96%.

[0039] The present invention will be further illustrated by the following embodiments, but the scope of protection of the present invention is not limited thereto. In the following embodiments, the width of the softened region of the welded joint was detected by microhardness profiling, and the test conditions for contact fatigue life were load-stress ratio R=0.1 and Hertz contact stress 1500MPa.

[0040] Example 1

[0041] The rail base material of this embodiment comprises the following components by weight percentage: C: 0.19%, Si: 1.50%, Mn: 2.30%, Cr: 0.50%, Mo: 0.14%, V: 0.08%, N: 0.006%, with the balance being Fe and unavoidable impurities. The tensile strength of the base material is 1435 MPa, and the hardness is 390 HB.

[0042] Flash welding was used to weld bainitic steel rails. During the preheating stage, the voltage was controlled at 380V±0.5V, the flash speed at 4.0mm / s, the weld joint formed a molten layer thickness of 1.6mm, and the duration was 20s. During the continuous sintering stage, the voltage was 360V±0.5V, the flash speed at 1.5mm / s, and the molten pool temperature was 1480℃ (measured with an infrared thermometer, fluctuation ±18℃). During the accelerated sintering stage, the voltage was 380V±0.5V, the sintering speed was 3.0mm / s, and the duration was 4s. During the upsetting operation, the pressure during the high-temperature plastic rheological stage (900~800℃) was 340MPa for 5 seconds (pressure sensor error ≤±5MPa); below 800℃, 400MPa pressure upsetting was used, with a dynamic displacement compensation of 18mm (closed-loop control using a laser displacement sensor).

[0043] The welded high-temperature rail joint was subjected to a three-stage gradient cooling process. In the first stage, nitrogen gas was sprayed at a cooling rate of 18℃ / s to cool the joint to 470℃. In the second stage, atomized water cooling was used at a controlled cooling rate of 0.08℃ / s for 1500s. In the third stage, infrared slow cooling was used to cool the welded joint to room temperature at a cooling rate of approximately 0.4℃ / s.

[0044] Performance test results: The softened zone width of the welded joint is approximately 0.48 mm, the tensile strength of the heat-affected zone is 1420 MPa, the hardness is 380 HB, the elongation after fracture is 13.5%, and the contact fatigue life is 1.15 × 10⁻⁶. 7 The loop continues.

[0045] Example 2

[0046] The rail base material of this embodiment comprises the following components by weight percentage: C: 0.21%, Si: 1.55%, Mn: 2.35%, Cr: 0.52%, Mo: 0.15%, V: 0.085%, N: 0.007%, with the balance being Fe and unavoidable impurities. The tensile strength of the base material is 1450 MPa, and the hardness is 395 HB.

[0047] Flash welding was used to weld bainitic steel rails. During the preheating stage, the voltage was controlled at 390V±0.5V, the flash speed at 4.4mm / s, the weld joint formed a molten layer thickness of 2.0mm, and the duration was 15s. During the continuous sintering stage, the voltage was 370V±0.5V, the sintering speed was 1.8mm / s, and the molten pool temperature was 1500℃ (measured with an infrared thermometer, fluctuation ±15℃). During the accelerated sintering stage, the voltage was 390V±0.5V, the sintering speed was 3.0mm / s, and the duration was 3s. During the upsetting operation, the pressure during the high-temperature plastic rheological stage (900~800℃) was 350MPa for 4 seconds (pressure sensor error ≤±5MPa); below 800℃, 430MPa pressure upsetting was used, with a dynamic displacement compensation of 20mm (closed-loop control using a laser displacement sensor).

[0048] The welded high-temperature rail joint was subjected to a three-stage gradient cooling process. The first stage involved nitrogen injection at a cooling rate of 20°C / s to cool the joint to 480°C. The second stage used atomized water cooling at a controlled cooling rate of 0.10°C / s for 1400s. The third stage involved infrared slow cooling at a cooling rate of approximately 0.5°C / s to cool the welded joint to room temperature.

[0049] Performance test results: The softened zone width of the welded joint is approximately 0.46 mm, the tensile strength of the heat-affected zone is 1435 MPa, the hardness is 382 HB, the elongation after fracture is 14%, and the contact fatigue life is 1.18 × 10⁻⁶ mm. 7 The loop continues.

[0050] Example 3

[0051] The rail base material of this embodiment comprises the following components by weight percentage: C: 0.23%, Si: 1.58%, Mn: 2.39%, Cr: 0.55%, Mo: 0.16%, V: 0.09%, N: 0.008%, with the balance being Fe and unavoidable impurities. The tensile strength of the base material is 1465 MPa, and the hardness is 405 HB.

[0052] Flash welding was used to weld bainitic steel rails. During the preheating stage, the voltage was controlled at 398V±0.5V, the flash speed at 4.8mm / s, the weld joint formed a molten layer thickness of 2.5mm, and the duration was 10s. During the continuous sintering stage, the voltage was 360V±0.5V, the sintering speed was 2.0mm / s, and the molten pool temperature was 1530℃ (measured with an infrared thermometer, fluctuation ±13℃). During the accelerated sintering stage, the voltage was 400V±0.5V, the sintering speed was 3.0mm / s, and the duration was 2s. In the upsetting operation, the pressure during the high-temperature plastic rheological stage (900~800℃) was 360MPa for 3 seconds (pressure sensor error ≤±5MPa); below 800℃, 450MPa pressure upsetting was used, with a dynamic displacement compensation of 22mm (closed-loop control using a laser displacement sensor).

[0053] The welded high-temperature rail joint was subjected to a three-stage gradient cooling process. In the first stage, nitrogen gas was sprayed at a cooling rate of 22℃ / s to cool the joint to 490℃. In the second stage, atomized water cooling was used at a controlled cooling rate of 0.12℃ / s for 1300s. In the third stage, infrared slow cooling was used to cool the welded joint to room temperature at a cooling rate of approximately 0.5℃ / s.

[0054] Performance test results: The softened zone width of the welded joint is approximately 0.42 mm, the tensile strength of the heat-affected zone is 1440 MPa, the hardness is 392 HB, the elongation after fracture is 13.8%, and the contact fatigue life is 1.16 × 10⁻⁶. 7 The loop continues.

[0055] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for controlling the softening zone of a bainitic rail welded joint, characterized in that, Includes the following steps: Flash welding was used to weld bainitic steel rails. The flash welding process included a preheating stage, a continuous melting stage, and an accelerated melting stage, followed by an upsetting stage. The preheating stage resulted in a weld joint molten layer thickness of 1.5~2.5mm. The continuous melting stage controlled the molten pool temperature at 1480~1530℃. The welded high-temperature rail joint was subjected to a three-stage gradient cooling process. In the first stage, the welded joint was cooled to the bainitic transformation temperature range at a cooling rate of 18~22℃ / s. In the second stage, the welded joint was cooled for 1300~1500s at a cooling rate of 0.08~0.12℃ / s. In the third stage, the welded joint was cooled to room temperature at a cooling rate of less than 0.5℃ / s. The base material of the bainitic rail comprises the following components by weight percentage: C: 0.19%~0.23%, Si: 1.50%~1.60%, Mn: 2.30%~2.40%, Cr: 0.50%~0.55%, Mo: 0.14%~0.16%, V: 0.08%~0.09%, N: 0.006%~0.008%, with the balance being Fe and unavoidable impurities. The specification of the bainitic rail is 60 kg / m.

2. The method according to claim 1, characterized in that, The voltage control during the preheating stage is 380~400V, the voltage control during the continuous firing stage is 360~380V, and the voltage control during the accelerated firing stage is 380~400V.

3. The method according to claim 1, characterized in that, The flash velocity during the preheating stage is 4.0~4.8 mm / s, and the flash velocity during the continuous burning stage is 1.5~2.0 mm / s.

4. The method according to claim 1, characterized in that, Upsetting at 900~800℃ with a pressure of 340~360MPa for 3~5s.

5. The method according to claim 1, characterized in that, Upsetting below 800℃ uses a pressure of 400~450Mpa, and the dynamic displacement compensation is controlled at 18~22mm.

6. The method according to claim 1, characterized in that, The cooling in the first stage is controlled by injecting nitrogen gas.

7. The method according to claim 1, characterized in that, The second stage of cooling is controlled by atomized water cooling.

8. The method according to claim 1, characterized in that, The cooling in the third stage is controlled by infrared radiation.

9. The method according to claim 1, characterized in that, The softened zone width of the bainitic rail welded joint is less than 0.5 mm, the hardness of the heat-affected zone is higher than 96% of the hardness of the base metal, and the contact fatigue life is ≥1.1×10⁻⁶. 7 Second-rate.

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