A segmented temperature control tempering process for AT bainite high-speed turnout rail

Abstract

The application discloses a segmented temperature control tempering process for AT bainite high-speed turnout rail, and belongs to the technical field of metal material heat treatment. The process comprises the following steps: (1) a temperature rising homogenization stage: the AT rail is heated from room temperature to 200-230 DEG C at a temperature rising rate of less than or equal to 80 DEG C / h, and the holding time is calculated according to the maximum wall thickness of the rail, and the holding time is 30-40 min per 25 mm; (2) a segmented isothermal tempering stage: first heated to 240-280 DEG C and held for 2-4 hours, then heated to 300-340 DEG C and held for 4-6 hours, and the temperature rising rate between the two stages is 30-50 DEG C / h; (3) a ladder cooling stage: cooled to 200 DEG C at a cooling rate of less than or equal to 50 DEG C / h, and then air-cooled to room temperature. The process adopts the segmented temperature control strategy of "temperature rising homogenization+segmented isothermal+ladder cooling", and realizes efficient release of residual stress, controllable decomposition of residual austenite and stabilization treatment of bainite matrix.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and particularly relates to a segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails. Background Technology

[0002] AT-type turnout rails are key load-bearing components in high-speed railway turnout sections. They have an asymmetrical cross-section with a wide rail head, a narrow rail base, and a variable cross-section transition area. During service, the rails bear multiple complex loads, including wheel-rail contact stress, bending stress, and temperature stress from high-speed train passage.

[0003] Bainitic steel has become an important development direction for high-speed turnout rails due to its excellent strength-toughness ratio and resistance to rolling contact fatigue. However, during the rolling and online controlled cooling processes of bainitic steel rails, the significant differences in cooling rates at different parts of the AT section make it easy to generate large residual stresses in the rail head core, the rail base triangular region, and the variable cross-section transition zone. At the same time, the residual austenite in the microstructure is not stable enough, which may lead to phase transformation-induced plastic instability or microcrack initiation during long-term service.

[0004] Existing tempering processes mostly employ isothermal tempering, failing to differentiate control based on the structural characteristics of AT (austenitic) rail sections. This makes it difficult to simultaneously achieve effective residual stress relief and microstructural stability at both the rail head and base. If the tempering temperature is too low, the residual austenite decomposes incompletely, resulting in poor microstructural stability; if the tempering temperature is too high, bainite laths coarsen and carbide precipitation and growth occur, leading to decreased toughness. Therefore, developing a segmented temperature-controlled tempering process for AT-section bainitic rails to achieve precise residual stress release and microstructural stabilization is of great significance for improving the service reliability of high-speed turnout rails. Summary of the Invention

[0005] The purpose of this invention is to provide a segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails. Based on the heat conduction characteristics and microstructure evolution of different parts of the AT cross section, this process adopts a segmented temperature control strategy of "heating homogenization + segmented isothermal + step cooling" to achieve efficient release of residual stress, controllable decomposition of residual austenite, and stabilization of the bainitic matrix.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails, comprising the following steps:

[0008] (1) Heating and homogenization stage: The AT rail is heated from room temperature to 200-230℃ at a heating rate of ≤80℃ / h. The holding time is calculated based on the maximum wall thickness of the rail, and the holding time is 30-40min per 25mm.

[0009] (2) Segmented isothermal tempering stage: First heat to 240-280℃ and hold for 2-4 hours, then heat to 300-340℃ and hold for 4-6 hours. The heating rate between the two stages is 30-50℃ / h.

[0010] (3) Step cooling stage: Cool to 200℃ at a cooling rate of ≤50℃ / h, then air cool to room temperature;

[0011] Specifically, during the tempering process, the temperature of the AT rail head, web, and base is monitored and compensated to ensure that the cross-sectional temperature difference is ≤15℃.

[0012] Furthermore, it is suitable for high-purity bainitic steel rails, whose typical chemical composition by weight percentage is: C 0.18%–0.28%, Si 1.20%–1.80%, Mn 1.80%–2.50%, Cr 0.80%–1.50%, Mo 0.20%–0.60%, Ni 0.50%–1.20%, V 0.05%–0.12%, Nb 0.020%–0.050%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the initial microstructure of the AT bainitic high-speed turnout rail is carbide-free bainite + 5% to 15% residual austenite by volume, and the initial residual stress peak is located in the center of the rail head.

[0014] Furthermore, after tempering treatment, the volume fraction of residual austenite in the rail is stably controlled at 3% to 8%, the longitudinal residual stress at the rail head is ≤120MPa, and the longitudinal residual stress at the rail bottom is ≤100MPa.

[0015] Furthermore, after tempering, the rail head tensile strength is ≥1350MPa, the elongation after fracture is ≥13%, and the low-temperature impact absorption energy at -20℃ is ≥70 J.

[0016] Furthermore, in the segmented isothermal tempering stage, auxiliary heating or heat insulation measures are set for the transition area of ​​the AT rail with variable cross-section to control the uniformity of the temperature field.

[0017] Furthermore, after rolling and online controlled cooling, the rail has a microstructure of carbide-free bainite and retained austenite, with a rail head surface hardness of 42-48 HRC. The initial residual stress distribution exhibits the characteristics of tensile stress in the rail head core and compressive stress on the rail bottom surface.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] 1. Design for AT cross-section structural characteristics: Taking into full account the non-uniform temperature field of the asymmetric cross-section of AT rail during heat treatment, zoned monitoring and compensation measures are adopted to achieve uniform temperature control of complex cross-section rails and avoid adding residual stress during tempering.

[0020] 2. Segmented isothermal tempering strategy: Through two-stage isothermal treatment of low-temperature stabilization and medium-temperature stress relief, the controllable decomposition of residual austenite is achieved, while avoiding carbide coarsening, resulting in a stable bainitic structure and excellent strength and toughness matching.

[0021] 3. Efficient release of residual stress: Combining stepped heating and cooling with long-term medium-temperature insulation, the residual stress of the rail is released fully and uniformly, significantly reducing the risk of stress corrosion cracking and fatigue failure during service.

[0022] 4. High operability of the process: The tempering process described in this invention is applicable to existing roller hearth continuous heat treatment furnaces or bogie hearth furnaces, without the need for large-scale equipment modification, and has good prospects for industrial promotion and application. Detailed Implementation

[0023] A segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails is proposed. Based on the heat conduction characteristics and microstructure evolution of different parts of the AT cross section, the process adopts a segmented temperature control strategy of "heating homogenization + segmented isothermal + step cooling" to achieve efficient release of residual stress, controllable decomposition of residual austenite, and stabilization of the bainitic matrix.

[0024] I. Applicable Rail Materials and Initial Conditions

[0025] This invention is applicable to high-purity bainitic steel rails, whose typical chemical composition by weight percentage is: C 0.18%–0.28%, Si 1.20%–1.80%, Mn 1.80%–2.50%, Cr 0.80%–1.50%, Mo 0.20%–0.60%, Ni 0.50%–1.20%, V 0.05%–0.12%, Nb 0.020%–0.050%, with the balance being Fe and unavoidable impurities.

[0026] After rolling and online controlled cooling, the microstructure of the rail is carbide-free bainite + retained austenite (volume fraction 5% to 15%), the surface hardness of the rail head is 42 to 48 HRC, and the initial residual stress distribution shows the characteristics of tensile stress in the rail head core and compressive stress on the rail bottom surface.

[0027] II. Segmented Temperature Control Tempering Process

[0028] The segmented temperature-controlled tempering process described in this invention is carried out in a bogie-type heat treatment furnace or a roller hearth continuous heat treatment furnace with zoned temperature control function. Based on the asymmetric characteristics of the AT rail cross-section, a differentiated temperature control strategy is adopted. Specifically, it includes the following three stages:

[0029] (1) Heating and homogenization stage

[0030] The AT rails are slowly heated from room temperature to 200–230°C at a heating rate of ≤80°C / h. The holding time is calculated based on the maximum wall thickness of the rail, with 30–40 minutes per 25mm section. The purpose of this stage is to ensure uniform temperature across the entire rail, preventing excessive thermal stress caused by large temperature differences across the cross-section due to rapid heating. During the heating process, independent thermocouples are used to monitor the rail head, rail web, and rail base to ensure that the temperature difference across the cross-section is ≤15°C.

[0031] (2) Segmented isothermal tempering stage

[0032] Based on the cross-sectional structural characteristics and performance requirements of AT rails, isothermal tempering is divided into two temperature ranges:

[0033] First isothermal stage (low-temperature stabilization stage): The rail is heated to 240–280℃ and held for 2–4 hours. This stage mainly promotes the decomposition of unstable blocky retained austenite in the microstructure, transforming it into tempered martensite or bainite, while initially releasing residual stress from rolling and phase transformation. This temperature range effectively prevents premature precipitation and coarsening of carbides, preserving the toughness characteristics of bainitic laths.

[0034] The second isothermal stage (stress relief stage): The rail is heated to 300–340℃ and held for 4–6 hours. This stage further promotes the dispersed precipitation of carbides in the bainitic matrix, stabilizing the bainitic structure, while simultaneously utilizing the high-temperature creep effect to fully release residual stress within the rail. For areas of concentrated residual tensile stress in the core of the AT rail head, localized slow cooling measures can be supplemented in the second isothermal stage to ensure uniform stress release.

[0035] A stepped heating method is used between the two isothermal sections, with the heating rate controlled at 30-50℃ / h to prevent uneven tissue transformation caused by thermal shock.

[0036] (3) Stepped cooling stage

[0037] After tempering and heat preservation, a staged cooling method is adopted:

[0038] First stage cooling: Slowly cool from the tempering temperature to 200℃ at a cooling rate of ≤50℃ / h. This stage is a stress-sensitive area and the cooling rate must be strictly controlled.

[0039] Second-stage cooling: Cooling can be accelerated appropriately below 200℃, using natural air cooling to room temperature.

[0040] Throughout the cooling process, the temperature difference between the rail head and the rail base of the AT rail is controlled within 20°C to prevent the generation of new residual stress during the cooling process.

[0041] III. Differentiated Temperature Control Strategy for Cross-Sections

[0042] Given the asymmetry of the AT rail cross-section, temperature monitoring and compensation are performed separately for the rail head and rail base during the tempering process:

[0043] The rail head section has a large cross-section and high heat capacity, resulting in slow heating and cooling response. During the heating phase, the heat preservation time can be appropriately extended.

[0044] The bottom section of the rail has a small cross-section and dissipates heat quickly. During the cooling phase, heat insulation or slow cooling measures need to be taken to avoid excessively fast cooling rate.

[0045] In the variable cross-section transition area, auxiliary heating devices or insulation layers are installed to ensure uniform temperature field.

[0046] IV. Microstructure and Properties after Tempering

[0047] After the above-mentioned segmented temperature-controlled tempering process, the microstructure characteristics of the AT bainitic steel rail are as follows:

[0048] ① The volume fraction of retained austenite is stably controlled at 3% to 8%, and it is uniformly distributed in a thin film between the bainite laths, which improves the stability of the microstructure;

[0049] ② The bainitic laths maintain a fine lath morphology, with diffusely distributed ε-carbides or transitional carbides precipitated between the laths, with a size ≤50nm;

[0050] ③ The original austenite grain size remained finer than grade 8, with no obvious coarsening.

[0051] Mechanical performance indicators:

[0052] ① The tensile strength of the rail head is ≥1350 MPa, and the elongation after fracture is ≥13%;

[0053] ② Low-temperature shock absorption energy (KU2) at -20℃ ≥ 70 J;

[0054] ③ The longitudinal residual stress at the rail head is ≤120 MPa, and the longitudinal residual stress at the rail bottom is ≤100 MPa, which is more than 50% lower than before tempering.

[0055] ④ The rolling contact fatigue life is increased by more than 25% compared with the untempered state, and the structural stability is significantly enhanced.

[0056] Example 1

[0057] This embodiment uses 60kg / m specification AT section bainitic turnout rails, whose initial microstructure is carbide-free bainite + retained austenite (volume fraction of about 11%), the surface hardness of the rail head is 46 HRC, and the initial residual stress peak is located in the center of the rail head, which is about 240MPa (tensile stress).

[0058] Segmented temperature-controlled tempering process is adopted:

[0059] Heating and homogenization stage: Heating to 220℃ at 60℃ / h and holding for 90min (maximum rail wall thickness is about 70mm).

[0060] First isothermal stage: Heat to 260℃ and hold for 3 hours;

[0061] Second isothermal section: Heat to 320℃, hold for 5 hours, heating rate 40℃ / h;

[0062] Step cooling: Cool to 200℃ at 40℃ / h, then air cool to room temperature.

[0063] Post-tempering inspection:

[0064] The volume fraction of retained austenite decreased to 6.5%, and it was uniformly distributed in a thin film.

[0065] The rail head has a tensile strength of 1410 MPa, an elongation of 13.8%, and an impact energy of 76 J at -20℃.

[0066] The residual stress at the rail head core was reduced to 95 MPa, and the residual stress at the rail base was reduced to 72 MPa, with a stress release rate exceeding 60%.

[0067] No carbide coarsening or bainite lath growth was observed in the microstructure.

[0068] Example 2

[0069] Adjusting tempering process parameters to investigate the effect of different isothermal temperatures on performance:

[0070] The temperature of the first isothermal section was adjusted to 250℃ (held for 3 hours), and the temperature of the second isothermal section was adjusted to 310℃ (held for 6 hours). The rest was the same as in Example 1.

[0071] Test results:

[0072] The volume fraction of retained austenite decreased to 5.8%;

[0073] Tensile strength 1430 MPa, elongation 13.2%, impact energy at -20℃ 72J;

[0074] The residual stress has been fully released, with the residual stress in the rail head core at 88 MPa.

[0075] The results show that appropriately lowering the temperature of the second isothermal zone slightly improves the strength while maintaining excellent toughness. The process parameters can be adaptively adjusted according to the specific service environment.

[0076] Comparative Example

[0077] The traditional integral isothermal tempering process was adopted, with a tempering temperature of 300℃ and a holding time of 6 hours, performed in a single isothermal treatment. Test results show:

[0078] The volume fraction of retained austenite decreased to 4.2%;

[0079] Tensile strength 1380 MPa, elongation 11.5%, impact energy at -20℃ 58J;

[0080] The residual stress at the rail head core is 135 MPa, and the residual stress at the rail base is 110 MPa, indicating uneven stress release.

[0081] As can be seen from the comparison, although the traditional integral tempering process can reduce residual stress, the toughness decreases significantly, and the stress release effect is not as uniform as that of the segmented temperature control process.

[0082] 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 segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails, characterized in that: Includes the following steps: (1) Heating and homogenization stage: The AT rail is heated from room temperature to 200-230℃ at a heating rate of ≤80℃ / h. The holding time is calculated based on the maximum wall thickness of the rail, and the holding time is 30-40min per 25mm. (2) Segmented isothermal tempering stage: First heat to 240-280℃ and hold for 2-4 hours, then heat to 300-340℃ and hold for 4-6 hours. The heating rate between the two stages is 30-50℃ / h. (3) Step cooling stage: Cool to 200℃ at a cooling rate of ≤50℃ / h, then air cool to room temperature; Specifically, during the tempering process, the temperature of the AT rail head, web, and base is monitored and compensated to ensure that the cross-sectional temperature difference is ≤15℃.

2. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 1, characterized in that: Suitable for high-purity bainitic steel rails, its typical chemical composition by weight percentage is: C 0.18%~0.28%, Si 1.20%~1.80%, Mn 1.80%~2.50%, Cr 0.80%~1.50%, Mo 0.20%~0.60%, Ni 0.50%~1.20%, V 0.05%~0.12%, Nb 0.020%~0.050%, with the balance being Fe and unavoidable impurities.

3. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 1, characterized in that: The initial microstructure of the AT bainitic high-speed turnout rail is carbide-free bainite plus 5% to 15% residual austenite by volume, and the initial residual stress peak is located in the center of the rail head.

4. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 1, characterized in that: After tempering, the volume fraction of residual austenite in the rail is stably controlled at 3% to 8%, the longitudinal residual stress at the rail head is ≤120MPa, and the longitudinal residual stress at the rail bottom is ≤100MPa.

5. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 1, characterized in that: After tempering, the rail head tensile strength is ≥1350MPa, the elongation after fracture is ≥13%, and the low-temperature impact absorption energy at -20℃ is ≥70 J.

6. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 1, characterized in that: During the segmented isothermal tempering stage, auxiliary heating or insulation measures are set for the transition area of ​​the AT rail with variable cross-section to control the uniformity of the temperature field.

7. The segmented temperature-controlled tempering process for AT bainitic high-speed turnout rails according to claim 2, characterized in that: After rolling and online controlled cooling, the rail has a microstructure of carbide-free bainite and retained austenite. The surface hardness of the rail head is 42-48 HRC, and the initial residual stress distribution shows the characteristics of tensile stress in the rail head core and compressive stress on the rail bottom surface.