A method for gradient heat treatment strengthening of heavy load rail

Abstract

The application discloses a heavy load rail gradient heat treatment strengthening method and belongs to the technical field of rail heat treatment. The method utilizes rolling residual heat, adopts three-section gradient air cooling+low-temperature tempering, precisely controls the rail head structure and hardness gradient, forms a fine pearlite gradient structure, and realizes the collaborative promotion of high hardness, high toughness, high wear resistance and high fatigue resistance. The application solves the problems of uneven hardness, insufficient toughness and easy fatigue peeling of traditional rails in heat treatment, is suitable for heavy load, high cold and high altitude railway rail production, and significantly prolongs the service life.

Description

Technical Field

[0001] This invention belongs to the field of rail manufacturing and heat treatment technology, and particularly relates to a gradient heat treatment method for strengthening and toughening heavy-duty rails. It is applicable to improving the wear resistance, fatigue resistance, and toughness of rail heads of heavy-duty rails such as U75V, U78CrV, and R350HT. Background Technology

[0002] Heavy-haul railway rails bear high axle loads and high-frequency cyclic loads, making the rail heads prone to wear, peeling, cracks, and contact fatigue.

[0003] Traditional residual heat quenching or overall quenching has the following characteristics:

[0004] 1) The rail head has high hardness but insufficient toughness, making it prone to chipping and peeling.

[0005] 2) The hardness gradient is unreasonable, and the surface and core properties do not match;

[0006] 3) The microstructure is coarse, the pearlite lamellae are uneven, and the fatigue life is short;

[0007] 4) Unreasonable residual stress distribution can easily lead to transverse cracks in the rail head.

[0008] There is an urgent need for a heat treatment method that can precisely control the gradient of rail head microstructure, match strength and toughness, and synergistically improve wear resistance and fatigue resistance.

[0009] Application No. 202511800466.4 discloses a production method for improving the special properties of steel rails, including the steel smelting process; rolling process; converter smelting using aluminum-free deoxidation alloying, with argon blowing throughout the refining process; vacuum degree ≤0.10KPa, deep vacuum time ≥18min, superheat AT ≤30°C; rare earth alloy precisely added at the VD vacuum refining station; online residual heat quenching cooling medium is a mixture of gas mist and air; the residual heat treatment temperature after final rolling of the steel rail is 500-600°C; after exiting the heat treatment production line, the rail web is induction heated until it reaches 660°C, at which point heating is stopped; the composition of the steel rail is also specified. The purpose of this invention is to provide a production method for improving the special properties of steel rails, effectively ensuring the structural safety and crack propagation stability of online heat-treated steel rails.

[0010] Application No. 202512052370.0 discloses a low-temperature toughness optimization process for ultrafine pearlitic steel rails based on synergistic control of vanadium microalloying and gradient air cooling. 1) The chemical composition of the steel rail, by mass percentage, includes: C: 0.60~0.69%; Si: 0.30~0.65%; Mn: 0.80-1.10%; Cr: 0.10~0.35%; V: 0.01~0.08%; P≤0.025%, S≤0.025%, with the remainder being Fe and unavoidable impurities; 2) Online gradient air cooling process. The purpose of this invention is to provide a low-temperature toughness optimization process centered on "vanadium microalloying + online gradient air cooling," achieving synergistic control of ultrafine pearlitic microstructure and high / low temperature fracture toughness through precise composition design and process parameter optimization.

[0011] Application No. 202511968783.7 discloses a method for preparing high-toughness steel rails for extreme cold conditions based on dual-phase microstructure control, including: (1) smelting process; (2) rolling process; (3) heat treatment process: adopting a three-step method of "reheating and controlled cooling and tempering"; tempering: adopting a traditional continuous tempering process, holding at 200°C for 4~6 hours; the chemical composition of the steel rail by mass percentage is: C: 0.70~0.76%, Si: 0.45~0.55%, Mn: 1.00~1.20%.

[0012] Cr: 0.15~0.20%, V: 0.08~0.12%, N: 0.010~0.015%, P≤0.020%, S≤0.015%, with the remainder being Fe and unavoidable trace elements. This invention, through optimized composition and synergistic processing, achieves excellent comprehensive performance with an impact energy ≥27J and a tensile strength ≥1190MPa at -60°C. Summary of the Invention

[0013] The purpose of this invention is to provide a three-stage temperature control + gradient air cooling heat treatment method. By differentially cooling the rail head surface and the core in stages, a gradient structure of fine pearlite on the surface, uniform pearlite in the transition layer, and fine grains in the core is obtained, thereby achieving an integrated improvement in high hardness, high toughness, high wear resistance, and high resistance to contact fatigue.

[0014] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0015] This invention provides a method for strengthening and toughening heavy-duty rails through gradient heat treatment, comprising: 1) utilizing the residual heat after final rolling of the rails, with temperature controlled at 820-860℃;

[0016] 2) First stage rapid cooling: The rail head is cooled to 600-630℃ at a rate of 3.0-4.5℃ / s to refine the pearlite lamellars;

[0017] 3) Second stage intermediate cooling: cooling to 480-520℃ at 1.5-2.5℃ / s to stabilize the microstructure and eliminate stress;

[0018] 4) Third stage slow cooling: Cooling to below 150℃ at 0.2-0.6℃ / s, then air cooling to room temperature;

[0019] 5) Stress-relief tempering: Hold at 280-320℃ for 2-3 hours to reduce residual tensile stress and improve toughness.

[0020] Furthermore, taking 60kg / m U78CrV heavy-duty steel rails as the target, universal rolling is used with the final rolling temperature controlled at 840℃ before entering the online gradient heat treatment unit.

[0021] Furthermore, the rail head undergoes a first-stage enhanced air cooling process, employing adjustable airflow nozzles to centrally cool the working layer of the rail head, controlling the cooling rate at 3.8℃ / s, rapidly cooling it to 610℃, inhibiting austenite grain growth, promoting pearlite transformation, and significantly refining the interlamellar spacing, thus laying the microstructure foundation for high hardness and high toughness.

[0022] Furthermore, the second stage of medium-speed cooling is then initiated, reducing the nozzle airflow and the cooling rate to 2.0℃ / s, cooling to 500℃ to ensure uniform microstructure transformation, eliminate the internal stress generated by the rapid cooling in the first stage, and prevent microstructure abnormalities and quenching cracks.

[0023] Furthermore, the system enters the third stage of slow cooling, where forced air cooling is turned off, and natural heat dissipation is used to slowly cool the system to 120°C at a rate of 0.4°C / s. Subsequently, it is air-cooled to room temperature, completing the entire gradient cooling process.

[0024] Furthermore, the cooled rails are fed into a roller hearth tempering furnace and held at 300°C for 2.5 hours to fully eliminate residual stress from heat treatment, stabilize dimensions and microstructure, and further improve impact toughness and fatigue resistance.

[0025] Furthermore, after treatment, multiple tests were conducted on the rail head tread, inner side, and corners: average hardness 345 HBW, hardness fluctuation ±12 HBW, impact energy at -40℃ 55 J, microstructure is uniform and fine pearlite, lamellar spacing is small and continuous, no network cementite, no martensite / bainite.

[0026] Furthermore, it is applicable to U75V, U78CrV, and R350HT heavy-duty rails.

[0027] Furthermore, the rail head hardness is 320–360 HBW, with fluctuation ≤ ±15 HBW.

[0028] Furthermore, the structure consists of uniform and dense pearlite with small and continuous interlamellar spacing.

[0029] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0030] Rail head hardness 320–360 HBW, uniformity ±15 HBW;

[0031] Impact toughness is increased by 30-50%, and contact fatigue life is increased by more than 50%.

[0032] Wear resistance is improved by 40%, and wear loss is significantly reduced;

[0033] The residual stress is reasonably distributed, making it less prone to peeling and cracking. Detailed Implementation

[0034] Example 1

[0035] Using 60kg / m U78CrV heavy-duty rails as the target material, universal rolling was employed with the final rolling temperature controlled at 840℃ before the rails entered the online gradient heat treatment unit. First, the rail head underwent a first-stage intensive air cooling, using adjustable airflow nozzles to centrally cool the working layer of the rail head at a controlled cooling rate of 3.8℃ / s, rapidly cooling to 610℃. This suppressed austenite grain growth, promoted pearlite transformation, and significantly refined the lamellar spacing, laying the microstructure foundation for high hardness and high toughness. The second stage involved medium-speed cooling, reducing the nozzle airflow to a cooling rate of 2.0℃ / s, cooling to 500℃ to ensure uniform microstructure transformation, eliminate internal stresses generated in the first-stage rapid cooling, and prevent microstructural abnormalities and quenching cracks. The third stage involved slow cooling, where forced air cooling was shut off, utilizing natural heat dissipation to slowly cool to 120℃ at 0.4℃ / s, followed by air cooling to room temperature, completing the entire gradient cooling process. After cooling, the rails are fed into a roller hearth tempering furnace and held at 300℃ for 2.5 hours to fully eliminate residual stress from heat treatment, stabilize dimensions and microstructure, and further improve impact toughness and fatigue resistance. After treatment, multiple points on the rail head tread, inner side, and corners are tested: average hardness 345 HBW, hardness fluctuation ±12 HBW, impact energy at -40℃ 55 J, microstructure is uniform and fine pearlite with small and continuous lamellar spacing, and no network cementite or martensite / bainite. Bench contact fatigue tests verify that the rails of this invention have a 58% longer fatigue life and a 42% lower wear rate compared to conventional residual heat quenched rails. Their overall performance meets the long-service requirements of heavy-haul railways with axle loads of 30 tons or more, significantly reducing rail replacement costs and track maintenance expenses.

[0036] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for strengthening and toughening heavy-duty rails through gradient heat treatment, characterized in that: include: 1) Utilize the residual heat after final rolling of the rails, with temperature controlled at 820-860℃; 2) First stage rapid cooling: The rail head is cooled to 600-630℃ at a rate of 3.0-4.5℃ / s to refine the pearlite lamellars; 3) Second stage intermediate cooling: cooling to 480-520℃ at 1.5-2.5℃ / s to stabilize the microstructure and eliminate stress; 4) Third stage slow cooling: Cooling to below 150℃ at 0.2-0.6℃ / s, then air cooling to room temperature; 5) Stress-relief tempering: Hold at 280-320℃ for 2-3 hours to reduce residual tensile stress and improve toughness.

2. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 1, characterized in that: Taking 60kg / mU78CrV heavy-duty steel rails as the target, universal rolling is used with the final rolling temperature controlled at 840℃, and the rails are then fed into an online gradient heat treatment unit.

3. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 2, characterized in that: The rail head undergoes a first-stage enhanced air cooling process, employing adjustable airflow nozzles to centrally cool the working layer of the rail head. The cooling rate is controlled at 3.8℃ / s, rapidly cooling to 610℃. This process inhibits austenite grain growth, promotes pearlite transformation, and significantly refines the interlamellar spacing, laying the microstructure foundation for high hardness and high toughness.

4. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 3, characterized in that: The second stage of medium-speed cooling is then initiated, reducing the nozzle airflow and the cooling rate to 2.0℃ / s, cooling to 500℃ to ensure uniform microstructure transformation, eliminate the internal stress generated by the rapid cooling in the first stage, and prevent microstructure abnormalities and quenching cracks.

5. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 4, characterized in that: Entering the third stage of slow cooling, the forced air cooling is turned off, and natural heat dissipation is used to slowly cool to 120°C at 0.4°C / s, followed by air cooling to room temperature, completing the entire gradient cooling process.

6. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 5, characterized in that: After cooling, the rails are fed into a roller hearth tempering furnace and held at 300°C for 2.5 hours to fully eliminate residual stress from heat treatment, stabilize dimensions and microstructure, and further improve impact toughness and fatigue resistance.

7. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 6, characterized in that: After treatment, multiple tests were conducted on the rail head tread, inner side, and corners: average hardness 345 HBW, hardness fluctuation ±12 HBW, impact energy at -40℃ 55 J, microstructure is uniform and fine pearlite, lamellar spacing is small and continuous, no network cementite, no martensite / bainite.

8. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 1, characterized in that: Suitable for U75V, U78CrV, and R350HT heavy-duty rails.

9. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 1, characterized in that: Rail head hardness 320–360 HBW, fluctuation ≤ ±15 HBW.

10. The method for strengthening and toughening heavy-duty rails through gradient heat treatment according to claim 1, characterized in that: The structure consists of uniform and dense pearlite with small and continuous interlamellar spacing.