Method for controlling softening area of bainite steel rail welded joint

Abstract

The invention belongs to the field of steel rail welding, and discloses a bainite steel rail welding joint softening area control method. The method comprises the steps that the bainite steel rail is welded through flash welding, the flash welding comprises a preheating stage, a continuous burning stage and an accelerated burning stage, the thickness of a fusion layer of a welding joint is controlled to range from 1.5 mm to 2.5 mm in the preheating stage, and the temperature of a molten pool is controlled to range from 1480 DEG C to 1530 DEG C in the continuous burning stage; and the welded high-temperature steel rail welding joint is subjected to three-stage gradient cooling, cooling is conducted at the cooling speed of 18-22 DEG C / s till the temperature is close to the bainite phase transformation point, then cooling is conducted at the cooling speed of 0.08-0.12 DEG C / s for a preset duration, and finally cooling is conducted at the cooling speed lower than 0.5 DEG C / s till the room temperature is achieved. According to the method, the burning quantity is remarkably reduced, excessive melting of steel rail base metal is avoided, meanwhile, expansion of a heat affected zone is reduced, and the width of the softening zone is smaller than or equal to 0.5 mm and is reduced by 80% compared with the width of the softening zone of a Bombadi BWR system.

Description

Technical Field

[0001] The present invention relates to the technical field of rail welding, and in particular to a method for controlling a softening zone of a bainite rail welded joint. Background Art

[0002] The rapid expansion of the high-speed rail network and the continued increase in train speeds have provided tremendous convenience for the development of today's society and people's travel. The rapid development and construction of the high-speed rail network has placed higher requirements on the design speed of EMUs. Increasing operating speeds is an inevitable trend in the high-quality development of railways.

[0003] With the development of high-speed railways, the dynamic load and high-frequency impact problems faced by the wheel-rail system are becoming increasingly severe. High-speed train operation places very high demands on the longitudinal continuity of the rail surface. When the wheelset passes through an uneven area of ​​the rail at high speed, the impact on the rail is aggravated, and the vehicle body is affected by the reaction force, which will produce abnormal vibrations. High-frequency vibrations can cause structural fatigue fractures of the vehicle and track. Due to the differences in properties between the materials near the rail joint area and the rail welding materials, there are non-uniformities and geometric non-uniformities. As a result, the rail weld joint will produce height differences in the local area of ​​the rail top under the action of multiple-frequency contact between the vehicle body and the wheelset, and thus form an uneven area along the longitudinal direction of the rail in the rail weld area.

[0004] In recent years, bainitic rails, with their excellent strength-toughness balance, have demonstrated outstanding performance in improving rail wear resistance and toughness. However, controlling the soft zone in their welded joints has become a bottleneck restricting the technological advantage. During welding, the heat-affected zone (HAZ) of bainitic rail welded joints is susceptible to microstructural degradation due to thermal cycling, forming a softening zone with a hardness significantly lower than that of the parent material. Under existing processes, the soft zone in bainitic rail welded joints is generally wide (≥2.5mm), resulting in reduced load-bearing capacity and unbalanced stress distribution. This, under repeated high-frequency wheel-rail impact loads, can easily induce rail head crushing, corrugation, and even transverse crack propagation. Furthermore, an excessively wide soft zone accelerates the accumulation of microscopic damage, making the joint area the weak link in the overall rail lifespan, forcing maintenance personnel to frequently intervene for repairs or replacements, significantly increasing costs. This contradiction is particularly pronounced on ultra-high-speed lines with speeds exceeding 350 km / h.

[0005] How to control the width of the softening zone of bainite rail weld joints has become an urgent problem to be solved in this field. Summary of the Invention

[0006] In view of the above problems, the present invention provides a method for controlling the softening zone of a bainite rail weld joint.

[0007] According to one aspect of the present invention, a method for controlling the softening zone of a bainitic rail weld joint is provided, the method comprising the following steps: The bainite rails are welded using flash welding. The flash welding process includes a preheating stage, a continuous sintering stage, and an accelerated sintering stage. The preheating stage allows the molten layer thickness of the weld joint to be 1.5-2.5 mm. The continuous sintering stage controls the molten pool temperature at 1480-1530°C. The high-temperature rail weld joint after welding is subjected to three-stage gradient cooling. In the first stage, the weld joint is cooled to the bainite phase transformation point temperature range at a cooling rate of 18~22℃ / s. In the second stage, the weld joint is cooled for a predetermined time at a cooling rate of 0.08~0.12℃ / s. In the third stage, the weld joint is cooled to room temperature at a cooling rate of less than 0.5℃ / s.

[0008] According to one embodiment of the present invention, the bainitic rail base material includes the following components by weight: 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%, and the balance is Fe and unavoidable impurities. The specification of the bainitic rail is 60 kg / m.

[0009] According to one embodiment of the present invention, the voltage of the preheating stage is controlled to be 380-400V, the voltage of the continuous burning stage is controlled to be 360-380V, and the voltage of the accelerated burning stage is controlled to be 380-400V.

[0010] According to one embodiment of the present invention, the flash speed in the preheating stage is 4.0-4.8 mm / s, and the flash speed in the continuous burning stage is 1.5-2.0 mm / s.

[0011] According to one embodiment of the present invention, the upsetting is performed in the high-temperature plastic rheological section at 900-800° C. using a pressure of 340-360 MPa for 3-5 seconds.

[0012] According to one embodiment of the present invention, a pressure of 400-450 MPa is used for upsetting below 800° C., and the dynamic displacement compensation amount is controlled to be 18-22 mm.

[0013] According to one embodiment of the present invention, the cooling in the first stage is controlled by injecting nitrogen.

[0014] According to one embodiment of the present invention, the cooling in the second stage is controlled by atomized water cooling, and the cooling time in the second stage is 1300-1500 seconds.

[0015] According to one embodiment of the present invention, the cooling in the third stage is controlled by infrared radiation.

[0016] According to one embodiment of the present invention, the width of the softening zone of the bainite rail weld 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 7 Second-rate.

[0017] Due to the adoption of the above technical solution, the method for controlling the softening zone of a bainite rail weld joint provided by the present invention has at least one of the following beneficial effects: (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 1.5-2.5 mm. Compared with the 4.2 mm thick molten layer in the prior art, the amount of burn-in is significantly reduced, avoiding excessive melting of the rail base material and reducing the expansion of the heat-affected zone. During the continuous burn-in stage, the temperature fluctuation of the molten pool is strictly controlled to ensure the uniformity of the material microstructure, achieving a softening zone width of ≤0.5 mm (80% reduction compared with the Bombardier BWR system), and a contact fatigue life exceeding 1.1×10 7 The cycle (load-stress ratio R=0.1) is 300% higher than that of the traditional process, meeting the ultimate load requirements of track welds for 400km / h high-speed railways; (2) The method of the present invention adopts three-stage gradient controlled cooling after welding to construct a bainite phase transformation and precipitation phase self-catalytic circulation system, and uses the dual strengthening of fine grain strengthening and precipitation strengthening in the heat-affected zone to achieve an overall improvement in the strength and toughness of the welding softening zone, thereby avoiding the softening zone from being easily worn during service due to its hardness being significantly lower than that of the base material. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The figure is a flow chart of a method for controlling the softening zone of a bainitic rail weld joint according to one embodiment of the present invention. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0020] Flash welding technology offers high thermal efficiency, allowing for the welding of larger parts. Heating time is also uniform across the weld face, making it suitable for both compact and expanded cross-sections. Rails, with their compact top section and expanded waist and foot sections, are well-suited for flash welding.

[0021] During flash welding, the electrodes clamp the rails, a transformer is connected, and the rail ends are gradually brought closer together until they are in partial contact. The contact point heats up, creating a liquid metal lintel that explodes and produces sparks, forming a continuous flash. Once the flash heat reaches the appropriate temperature, an upsetting force is rapidly applied to press the rail ends together. The current is then cut off, causing intense plastic deformation in the weld zone and cross-crystallization of the bonded surfaces to form the weld joint.

[0022] The bainite rail flash welding employed in this invention is primarily divided into five stages: flash flattening, burnout, upsetting, and pressure holding. Each stage can include multiple sub-stages. The preheating, continuous burnout, and accelerated burnout stages described in this invention primarily refer to the burnout stage. Bainite flash welding uses the current in the main welding circuit as a control reference to control the speed and displacement of the movable frame, achieving a heating method that combines short-circuiting and flash welding. This method, while maintaining sufficient heat input, increases the heat generation ratio of contact resistance, thereby improving the temperature uniformity of the weld joint end face.

[0023] like Figure 1 As shown, the method for controlling the softening zone of a bainite rail weld joint provided by the present invention generally includes the following steps: Step S1: Flash welding is used to weld the bainite rail. The flash welding process includes a preheating stage, a continuous sintering stage, and an accelerated sintering stage. The preheating stage ensures that the thickness of the molten layer of the weld joint is 1.5-2.5 mm. The continuous sintering stage controls the molten pool temperature to 1480-1530°C. Step S2: The high-temperature rail weld joint after welding is subjected to three-stage gradient cooling. In the first stage, the weld joint is cooled to near the bainite transformation point at a cooling rate of 18-22°C / s. In the second stage, the weld joint is cooled for a predetermined time at a cooling rate of 0.08-0.12°C / s. In the third stage, the weld joint is cooled to room temperature at a cooling rate of less than 0.5°C / s.

[0024] The method of the present invention strictly controls heat input during the preheating phase, limiting the thickness of the molten layer to 1.5-2.5 mm. Compared with the 4.2 mm thick molten layer used in the prior art, this significantly reduces the amount of burn-in, preventing excessive melting of the rail parent material and reducing the expansion of the heat-affected zone. If the molten layer thickness is less than 1.5 mm, insufficient penetration of the molten pool during fine firing will result, affecting material density. During the continuous firing phase, strict control of molten pool temperature fluctuations ensures uniformity of the material's microstructure, achieving a softening zone width of ≤ 0.5 mm (an 80% reduction compared to the Bombardier BWR system), and a contact fatigue life exceeding 1.1 × 10 7 The cycle (load-stress ratio R=0.1) is 300% higher than that of the traditional process, meeting the ultimate load requirements of track welds for 400km / h high-speed railways.

[0025] Conventional flash welding processes use a single-stage cooling process, resulting in coarsening of the weld joint structure (original austenite grains ≥ 35μm) and a tensile strength loss of 8%-12% (only 1230-1290MPa for a 1400MPa base metal). The method of the present invention employs a three-stage gradient controlled cooling process after welding, aligning the cooling rate with the kinetics of the bainite transformation to reduce fluctuations in the nucleation rate of the precipitate phase. During the bainite transformation, a self-catalytic cycle system of bainite phase transformation and precipitation phase is constructed: bainite nucleation expel carbon to austenite, inducing the preferential precipitation of (V, Mo) (C, N) composite phase at the bainite / austenite interface, thereby enhancing the pinning force of the precipitate on the grain boundary, while restricting the growth of bainite laths and refining the lath structure; during the bainite phase transformation, the carbon expulsion effect accelerates the formation of precipitation phase, and the formation of precipitation phase reduces the local carbon concentration, thereby promoting bainite nucleation. This cycle forms a positive feedback self-catalytic strengthening mechanism, achieving dual strengthening of grain refinement and precipitation strengthening in the heat-affected zone, and realizing an overall improvement in the strength and toughness of the welding softening zone, thereby avoiding the softening zone from being easily worn during service due to its significantly lower hardness than the base material.

[0026] In some embodiments of the present invention, the bainitic rail base material comprises the following components, by weight: 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 form carbonitride precipitations with the C and N elements during the post-weld cooling process. These precipitations synergize with dislocations, grain boundaries, and other factors to enhance the strength of the joint region. The bainitic rail has a specification of 60 kg / m.

[0027] In some embodiments of the present invention, the preheating stage voltage is controlled at 380-400V, lasting approximately 10-20 seconds, and the flash speed during the preheating stage is 4.0-4.8 mm / s. This stage initially softens the end face, establishes conductive contact points, and uniformly raises the end face temperature to 1200-1300°C. The combination of a higher voltage and a higher movement speed during the preheating stage shortens the thermal exposure time, prevents 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 sintering while minimizing the expansion of the heat-affected zone.

[0028] In some embodiments of the present invention, the voltage during the continuous sintering phase is controlled at 360-380V, and the flash speed is 1.5-2.0mm / s. This phase utilizes lower voltage and lower movement speed to ensure sufficient time for the molten pool to reach the target temperature and achieve uniform molten pool temperature. This allows for precise control of the molten pool temperature, minimizes the amount of base metal sintered, and inhibits the expansion of the heat-affected zone.

[0029] In some embodiments of the present invention, the voltage during the accelerated burning phase is controlled to be 380-400V, the duration is approximately 2-4 seconds, and the burning speed is above 3.0 mm / s. The accelerated burning phase forces the discharge of oxides to prevent gray spots.

[0030] In some embodiments of the present invention, during the upset forging process, a pressure of 340-360 MPa is applied for 3-5 seconds during the high-temperature plastic rheology stage at 900-800°C. Thereafter, the upset forging is performed at a pressure of 400-450 MPa, and the dynamic displacement compensation amount is controlled to be 18-22 mm. By adopting dynamic upset forging, a lower pressure is applied at a high temperature to fully soften the material, promote grain boundary sliding and dislocation movement, and form a uniform initial plastic rheology. At this stage, local stress concentration caused by high pressure is avoided, and the risk of folding and crack initiation is reduced; thereafter, the pressure is increased to 400-450 MPa, and the low rheological stress characteristics of the material at high temperature are utilized to accelerate the densification process. Dynamic pressure adjustment can match the real-time deformation resistance of the material, avoid flow stagnation caused by strain hardening, and avoid the formation of defects.

[0031] In some embodiments of the present invention, the first stage of cooling is controlled by nitrogen injection, with a flow rate of approximately 20-30 m / s, achieving a cooling rate of 18-22°C / s, thereby suppressing the formation of proeutectoid ferrite and pearlite. The second stage of cooling is controlled by atomized water cooling, with a cooling rate of 0.08-0.12°C / s, precisely matching the phase nucleation rate. The second stage cooling time is 1300-1500 s, ensuring a bainite transformation of ≥98%. The combination of nitrogen injection and atomized water cooling allows for real-time adjustment of cooling intensity deviations to ≤5%, avoiding performance fluctuations caused by traditional processes relying on empirical judgment. The third stage of cooling is controlled by infrared radiation, suppressing the decomposition of retained austenite (volume fraction ≤2.5%).

[0032] In some embodiments of the present invention, the softening zone width of the bainite 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 7 Tensile strength ≥1400MPa, elongation after fracture ≥12%, impact energy at -40℃ ≥110J, HAZ hardness ≥96% of parent material, hardness matching ≥96%.

[0033] The present invention is further illustrated by the following examples, but the scope of protection of the present invention is not limited thereto. The width of the softening zone of the welded joint in the following examples was measured using the microhardness profile method, and the test conditions for the contact fatigue life were a load-stress ratio R = 0.1 and a Hertz contact stress of 1500 MPa.

[0034] Example 1 The rail base material of this embodiment comprises the following components, by weight: 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 base material has a tensile strength of 1435 MPa and a hardness of 390 HB.

[0035] Bainitic rails were flash welded. During the preheating phase, the voltage was controlled at 380V ± 0.5V, the flash rate at 4.0mm / s, the weld joint molten layer thickness at 1.6mm, and the duration was 20s. During the continuous sintering phase, the voltage was 360V ± 0.5V, the flash rate at 1.5mm / s, and the melt pool temperature was 1480°C (measured with an infrared thermometer, with a fluctuation of ±18°C). During the accelerated sintering phase, the voltage was 380V ± 0.5V, the flash rate at 3.0mm / s, and the duration was 4s. During the upset forging operation, the pressure during the high-temperature plastic flow stage from 900 to 800°C was 340MPa for 5 seconds (pressure sensor error ≤ ±5MPa). Below 800°C, a 400MPa pressure was used, with a dynamic displacement compensation of 18mm (controlled by a laser displacement sensor in closed-loop control).

[0036] The high-temperature rail weld joint after welding was subjected to three-stage gradient cooling. In the first stage, nitrogen was sprayed at a cooling rate of 18°C / s to cool it to 470°C. In the second stage, atomized water cooling was used with a controlled cooling rate of 0.08°C / s for 1500s. In the third stage, infrared slow cooling was used to cool the weld joint to room temperature at a cooling rate of about 0.4°C / s.

[0037] Performance test results: The width of the softening zone of the weld joint is about 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 7 Second cycle.

[0038] Example 2 The rail base material of this embodiment comprises the following components, by weight: 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 base material has a tensile strength of 1450 MPa and a hardness of 395 HB.

[0039] Bainitic rails were flash welded. The preheating voltage was controlled at 390V ± 0.5V, the flash rate at 4.4mm / s, and the weld joint molten layer thickness was 2.0mm, lasting 15s. The continuous sintering phase was controlled at 370V ± 0.5V, a sintering rate of 1.8mm / s, and a melt pool temperature of 1500°C (measured with an infrared thermometer, with a fluctuation of ±15°C). The accelerated sintering phase was controlled at 390V ± 0.5V, a sintering rate of 3.0mm / s, and lasting 3s. During the upset forging operation, the pressure during the high-temperature plastic flow stage from 900 to 800°C was 350MPa for 4 seconds (pressure sensor error ≤ ±5MPa). Below 800°C, a 430MPa pressure was used, with a dynamic displacement compensation of 20mm (controlled by a closed-loop laser displacement sensor).

[0040] The high-temperature rail weld joint after welding was subjected to three-stage gradient cooling. In the first stage, nitrogen was sprayed at a cooling rate of 20℃ / s to cool to 480℃. In the second stage, atomized water cooling was used with a controlled cooling rate of 0.10℃ / s for 1400s. In the third stage, infrared slow cooling was used to cool the weld joint to room temperature at a cooling rate of about 0.5℃ / s.

[0041] Performance test results: The width of the softening zone of the weld joint is about 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 7 Second cycle.

[0042] Example 3 The rail base material of this embodiment comprises the following components, by weight: 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 base material has a tensile strength of 1465 MPa and a hardness of 405 HB.

[0043] Bainitic rails were flash welded. During the preheating phase, the voltage was controlled at 398V ± 0.5V, the flash rate at 4.8mm / s, the weld joint molten layer thickness at 2.5mm, and the duration was 10s. During the continuous sintering phase, the voltage was 360V ± 0.5V, the sintering rate at 2.0mm / s, and the melt pool temperature at 1530°C (measured with an infrared thermometer, with a fluctuation of ±13°C). During the accelerated sintering phase, the voltage was 400V ± 0.5V, the sintering rate at 3.0mm / s, and the duration was 2s. During the upset forging operation, the pressure during the high-temperature plastic flow stage from 900 to 800°C was 360MPa for 3 seconds (pressure sensor error ≤ ±5MPa). Below 800°C, a 450MPa pressure was used, with a dynamic displacement compensation of 22mm (controlled by a closed-loop laser displacement sensor).

[0044] The high-temperature rail weld joint after welding was subjected to three-stage gradient cooling. In the first stage, nitrogen was sprayed at a cooling rate of 22°C / s to cool it to 490°C. In the second stage, atomized water cooling was used with a controlled cooling rate of 0.12°C / s for 1300s. In the third stage, infrared slow cooling was used to cool the weld joint to room temperature at a cooling rate of about 0.5°C / s.

[0045] Performance test results: The width of the softening zone of the weld joint is about 0.42mm, the tensile strength of the heat-affected zone is 1440MPa, the hardness is 392HB, the elongation after fracture is 13.8%, and the contact fatigue life is 1.16×10 7 Second cycle.

[0046] The above-described embodiments merely illustrate the implementation methods 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 various modifications and improvements without departing from the spirit of the present invention, and all such modifications and improvements fall within the scope of protection of the present invention.

Claims

1. A method for controlling the softening zone of a bainitic rail weld joint, characterized in that: The following steps are involved: The bainite rails are welded using flash welding, which includes a preheating stage, a continuous sintering stage, and an accelerated sintering stage. The preheating stage allows the molten layer thickness of the weld joint to be 1.5-2.5 mm, and the continuous sintering stage controls the molten pool temperature at 1480-1530°C. The high-temperature rail weld joint after welding is subjected to three-stage gradient cooling. In the first stage, the weld joint is cooled to the bainite phase transformation point temperature range at a cooling rate of 18~22℃ / s. In the second stage, the weld joint is cooled for a predetermined time at a cooling rate of 0.08~0.12℃ / s. In the third stage, the weld joint is cooled to room temperature at a cooling rate of less than 0.5℃ / s.

2. The method according to claim 1, characterized in that The bainitic rail base material includes 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%, and the balance is Fe and unavoidable impurities. The specification of the bainitic rail is 60 kg / m.

3. The method according to claim 1, characterized in that The voltage of the preheating stage is controlled to be 380~400V, the voltage of the continuous burning stage is controlled to be 360~380V, and the voltage of the accelerated burning stage is controlled to be 380~400V.

4. The method according to claim 1, wherein The flash speed in the preheating stage is 4.0-4.8 mm / s, and the flash speed in the continuous burning stage is 1.5-2.0 mm / s.

5. The method according to claim 1, wherein The top forging is performed at 900~800℃ with a pressure of 340~360MPa for 3~5s.

6. The method according to claim 1, characterized in that The pressure of 400~450Mpa is used for upsetting below 800℃, and the dynamic displacement compensation is controlled at 18~22mm.

7. The method according to claim 1, characterized in that The first stage of cooling was controlled by sparging with nitrogen.

8. The method according to claim 1, characterized in that The second stage of cooling is controlled by atomized water cooling, and the second stage cooling time is 1300~1500s.

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

10. The method according to claim 1, characterized in that The width of the softening zone of the bainite rail weld joint is less than 0.5 mm, the hardness of the heat-affected zone is higher than 96% of the hardness of the parent material, and the contact fatigue life is ≥1.1×10 7 Second-rate.

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