Heat treatment process for optimizing residual stress of EA4T axle and EA4T axle

Abstract

The invention provides a heat treatment process for optimizing residual stress of an EA4T axle and the EA4T axle. The heat treatment comprises austenite homogenization normalizing, quenching, tempering and subcritical quenching. Wherein the austenite homogenization normalizing comprises the following steps: heating to 890-910 DEG C, preserving heat, and then air-cooling to 200 DEG C or below according to a cooling rate of 360-400 DEG C / h; the quenching comprises the following steps: heating to 870-890 DEG C, and water-cooling to room temperature; the tempering is as follows: heating to 630-650 DEG C, and air-cooling to room temperature; the subcritical quenching comprises the following steps: heating to 540-560 DEG C, and cooling to room temperature by water. Compared with the prior art, the axle subjected to heat treatment through the designed heat treatment method has the advantages that under the condition that the chemical components, mechanical properties and microstructures of the axle are not changed, uniform residual compressive stress is obtained on the surface, meanwhile, the residual compressive stress is greatly increased, and it is ensured that the residual stress on the surface layer of the axle is smaller than-50 MPa; the fatigue strength and the service life of the axle can be effectively improved.

Description

Technical Field

[0001] The invention belongs to the field of rail transit axle steel production, and particularly relates to a heat treatment process for optimizing residual stress of an EA4T axle and an EA4T axle. Background Art

[0002] In recent years, with the rapid development of the global economy, the railway transportation industry has also experienced rapid growth. With its unique advantages, including low cost, large capacity, high speed, low energy consumption, and environmental friendliness, railway transportation occupies an indispensable position in the diversified global transportation system. Axles are crucial components for the safe operation of railway locomotives. With the widespread speed increases and the increase in heavy-load trains on China's railways, the requirements for axle strength and reliability are becoming increasingly stringent. Various axle damages during high-speed locomotive operation are closely related to the distribution of residual stress.

[0003] Related research shows that the residual stress state of the axle surface has an important influence on the fatigue strength of the axle. If there is residual tensile stress σ r , when the axle is subjected to an external cyclic load, the surface stress level increases, and the maximum stress on the surface is reduced by σ max Increase to σ max +σ r , it is possible to exceed the fatigue strength of the axle and cause fatigue failure; on the contrary, if there is residual compressive stress σ on the axle surface r , when the axle is subjected to an external cyclic load, the surface stress level decreases, and the maximum stress on the surface is reduced by σ max Reduced to σ max -σ r , the stress amplitude remains unchanged, but the average stress is reduced, thus improving the fatigue strength and life of the axle.

[0004] European standard EN 13262 clearly defines residual stress within axle products, specifying the magnitude and distribution of residual stress beneath the axle surface. It considers residual stress a key quality criterion. The standard stipulates that for finished axles, the residual stress on the axle surface should be ≤ +100 MPa. The difference between the residual stress values ​​measured at two different points 2 mm below the axle surface should be ≤ +40 MPa.

[0005] Chinese patent application number CN 104513893 A, published on April 15, 2015, discloses a "Method for Heat Treatment of EA4T Steel Axles." The heat treatment process disclosed is as follows: The furnace temperature is raised to 870±10°C, and the axle blank is loaded into the furnace while its temperature is no higher than 200°C. The furnace temperature is maintained at 870±10°C for 2±0.5 hours. The axle blank is then removed from the furnace and placed in a quenching liquid to cool to the same temperature as the quenching liquid, with the quenching liquid temperature maintained between 15 and 40°C. The quenched axle blank is then placed in a furnace at 630±10°C while its temperature is no higher than 200°C. The furnace temperature is maintained at 630±10°C for 5.5±0.5 hours, and the axle blank is removed from the furnace and allowed to cool naturally in air. This process primarily defines the process temperatures and times for quenching and tempering EA4T steel axles, but does not address the issue of residual stress. Summary of the Invention

[0006] The purpose of the present invention is to provide a heat treatment process for optimizing the residual stress of the EA4T axle, which first performs austenite homogenization normalizing, and then performs quenching, tempering and subcritical quenching, so that the axle obtains uniform residual compressive stress on the surface without changing the chemical composition, mechanical properties and microstructure, and at the same time greatly increases the residual compressive stress, so that when the axle is subjected to an external cyclic load, the resultant stress level of the surface layer is significantly reduced, the stress amplitude remains unchanged, but the average stress is reduced, which can effectively improve the fatigue strength and life of the axle.

[0007] Another object of the present invention is to provide an EA4T axle, which is produced using the above-mentioned heat treatment process for optimizing the residual stress of the EA4T axle. The product stably meets the requirement that the maximum difference in residual stress at 6 measuring points in one circumference is less than 35 MPa, and the surface residual stress is less than -50 MPa, thereby improving the fatigue strength and life of the axle.

[0008] The specific technical solutions of the present invention are as follows:

[0009] An EA4T axle, wherein the maximum difference in residual stress at 6 measuring points in a circumferential circle 2 mm below the surface of the EA4T axle is less than 35 MPa, and the residual stress of the axle is uniform. Preferably, the maximum difference in residual stress at 6 measuring points in a circumferential circle 2 mm below the surface of the EA4T axle is less than 30 MPa.

[0010] The surface residual stress of the EA4T axle is less than -50 MPa, preferably less than -80 MPa.

[0011] The EA4T axle includes the following components in mass percentage: C 0.24-0.29%, Si 0.28-0.37%, Mn 0.60-0.77%, P ≤ 0.015%, S ≤ 0.010%, Cr 1.00-1.20%, Mo 0.18-0.28%, Ni 0.15-0.28%, Al 0.015-0.040%, Cu ≤ 0.30%, V 0.030-0.060%, T [O] ≤ 15ppm, [H] ≤ 2.0%, [N] ≤ 80ppm, and the balance is Fe and unavoidable impurities.

[0012] The present invention provides a heat treatment process for optimizing the residual stress of an EA4T axle, which comprises austenite homogenization normalizing, quenching, tempering and subcritical quenching.

[0013] Specifically, the austenite homogenization normalizing process involves heating the rough axle to a temperature of 890-910°C at a heating rate of 70-100°C / h. The heating and holding time (T1) within this temperature range is calculated as the axle diameter (D) x 1.2-1.6 min / mm, where the axle diameter (D) is expressed in mm and the heating and holding time (T1) is expressed in minutes. The axle is then air-cooled to below 200°C at a cooling rate of 360-400°C / h. This normalizing process not only refines the grain size but also improves microstructure heterogeneity, preparing the axle for the subsequent final heat treatment.

[0014] The quenching is specifically as follows: heating the blank axle to a temperature of 870-890°C at a heating rate of 70-100°C / h, and the heating and holding time T2 in this temperature range is calculated according to the axle diameter D×1.4-1.8min / mm, where the unit of the axle diameter D is mm and the unit of the heating and holding time T2 is min; then water-cooling to room temperature; the temperature of the cooling water used for the water cooling is maintained at 10-40°C.

[0015] The tempering is specifically as follows: heating the blank axle to a temperature of 630-650°C at a heating rate of 70-100°C / h, and calculating the heating and holding time T3 in this temperature range according to the axle diameter D×2.1-2.5min / mm, where the unit of the axle diameter D is mm and the unit of the heating and holding time T3 is min; and then air cooling to room temperature.

[0016] The subcritical quenching process specifically involves heating the axle blank to a temperature of 540-560°C at a heating rate of 70-100°C / h. The heating and holding time T4 within this temperature range is calculated as: diameter D x 1.5-1.9 min / mm, where the axle diameter D is measured in mm and the heating and holding time T4 is measured in minutes. The axle blank is then water-cooled to room temperature. The cooling water used for water cooling is maintained at a temperature of 20-40°C.

[0017] In the present invention, after the above-mentioned normalizing, not only the grains are refined, but also the heterogeneity of the organization is improved, so as to prepare the organization for the subsequent final heat treatment. And by air cooling at a cooling rate of 360 to 400 ° C / h, a uniform fine-grained organization is formed, the initial organizational heterogeneity is reduced, the thermal stress gradient during subsequent quenching is reduced, and the circumferential residual stress difference is reduced. In the subsequent quenching, by controlling the temperature and time, the austenite is ensured to be homogenized, the water cooling temperature is controlled, the circumferential cooling difference is reduced, and the surface compressive stress distribution is made consistent. During tempering, the internal stress of quenching is released, and air cooling is used to avoid the introduction of new stress, balance the stress difference between the core and the surface, and further homogenize the overall stress distribution. Finally, through subcritical quenching, partial phase transformation supplements the surface compressive stress, while avoiding excessive thermal stress, further increasing the surface compressive stress, and reducing the circumferential stress fluctuation by gentle cooling (water temperature of 20 to 40 ° C). In addition, the present invention controls the heating rate at each stage (70-100°C / h), and the designed holding time is calculated based on the diameter to ensure that the temperature gradient is minimized and the accumulation of thermal stress is reduced. Through multiple stress adjustments, the structure is refined by normalizing, compressive stress is introduced by quenching, stress is relaxed by tempering, and compressive stress is supplemented by subcritical quenching to form a stable and uniform stress field. The elements such as Cr, Mo, and V in the axle composition are combined to improve the hardenability, ensure the uniformity of the surface and core structure of large-section axles, and enhance fatigue resistance. Ultimately, the residual stress of the axle is homogenized, so that it stably meets the maximum difference of the residual stress of 6 measuring points in a circumferential circle of less than 35MPa; the residual compressive stress on the surface of the axle is greatly increased, ensuring that the surface residual stress is less than -50MPa, thereby improving the fatigue strength and life of the axle.

[0018] Compared with the prior art, the axle heat-treated by the designed heat treatment method of the present invention obtains uniform residual compressive stress on the surface without changing the chemical composition, mechanical properties and microstructure of the axle, and at the same time greatly increases the residual compressive stress, so that when the axle is subjected to an external cyclic load, the resultant stress level of the surface layer is significantly reduced, the stress amplitude remains unchanged, but the average stress is reduced, ensuring that the residual stress on the surface layer of the axle is less than -50MPa, which can effectively improve the fatigue strength and life of the axle and can stably meet the requirements of the EN 13262 standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the location of residual stress measurement points on axles in the EN 13262 standard;

[0020] Figure 2 is the average value of the residual compressive stress on the axle surface of Examples 1-3 and Comparative Examples 1-3;

[0021] Figure 3 The maximum difference in residual stress at 2 mm below the axle surface between Examples 1-3 and Comparative Examples 1-3; DETAILED DESCRIPTION

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0023] Example 1-Example 4

[0024] An EA4T axle comprises the following components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities, in Table 1 (T[O], [N], [H] units are ppm, others are wt%), and the axle body size is 160 to 280 mm.

[0025] Comparative Example 1-Comparative Example 4

[0026] An EA4T axle comprises the following components by mass percentage: as shown in Table 1, with the remainder not shown in Table 1 being Fe and unavoidable impurities (T[O], [N], and [H] are in ppm, and the others are in wt%). The axle body size is 160 to 280 mm.

[0027] Table 1 Chemical composition content of each embodiment and comparative example

[0028]

[0029]

[0030] The components of Example 1 and Comparative Example 1 are the same, the components of Example 2 and Comparative Example 2 are the same, the components of Example 3 and Comparative Example 3 are the same, and the components of Example 4 and Comparative Example 4 are the same.

[0031] The production method of the axles of each embodiment and comparative example includes the following process flow:

[0032] Electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating furnace heating → axle billet rolling → axle billet forging → rough turning of blank axle → axle end facing → austenite homogenization normalizing (890℃~910℃) + quenching (870℃~890℃) + tempering (630℃~650℃) + subcritical quenching (540℃~560℃) heat treatment → axle outer circle finishing turning → outer circle grinding → flaw detection.

[0033] The conventional process includes smelting in electric arc furnace or converter → refining in LF furnace → vacuum degassing in RH or VD → continuous casting → heating in billet heating furnace → rolling of axle billet → forging of axle billet → rough turning of blank axle → end face processing of axle.

[0034] The austenite homogenization normalizing is specifically as follows: the blank axle is heated to a temperature of 890-910°C at a heating rate of 70-100°C / h, and the heating and holding time T1 in this temperature range is calculated according to the axle diameter D×1.2-1.6min / mm, where the unit of the axle diameter D is mm and the unit of the heating and holding time T1 is min; then, the blank axle is air-cooled to below 200°C at a cooling rate of 360-400°C / h.

[0035] The quenching is specifically as follows: heating the blank axle to a temperature of 870-890°C at a heating rate of 70-100°C / h, and the heating and holding time T2 in this temperature range is calculated according to the axle diameter D×1.4-1.8min / mm, where the unit of the axle diameter D is mm and the unit of the heating and holding time T2 is min; then water-cooling to room temperature; the temperature of the cooling water used for the water cooling is maintained at 10-40°C.

[0036] The tempering is specifically as follows: heating the blank axle to a temperature of 630-650°C at a heating rate of 70-100°C / h, and calculating the heating and holding time T3 in this temperature range according to the axle diameter D×2.1-2.5min / mm, where the unit of the axle diameter D is mm and the unit of the heating and holding time T3 is min; and then air cooling to room temperature.

[0037] The subcritical quenching process specifically involves heating the axle blank to a temperature of 540-560°C at a heating rate of 70-100°C / h. The heating and holding time T4 within this temperature range is calculated as: diameter D x 1.5-1.9 min / mm, where the axle diameter D is measured in mm and the heating and holding time T4 is measured in minutes. The axle blank is then water-cooled to room temperature. The cooling water used for water cooling is maintained at a temperature of 20-40°C.

[0038] The specific axle heat treatment processes of each embodiment and comparative example are as follows:

[0039] Example 1

[0040] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0041] The diameter of the heat-treated blank axle is D=250mm.

[0042] Austenite homogenization normalizing: heat at 80℃ / h to 890℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0043] Quenching: heat at 80℃ / h to 870℃, heating and holding time 400min, water temperature 20℃, water cooling to room temperature.

[0044] Tempering: Heat at 80℃ / h to 630℃, heat and hold for 560min, and air cool to room temperature.

[0045] Subcritical quenching: heat to 540℃ at 80℃h, heating and holding time for 420min, water temperature 30℃, and water cooling to room temperature.

[0046] Example 2

[0047] The diameter of the heat-treated blank axle is D=250mm.

[0048] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0049] Austenite homogenization normalizing: heat at 80℃ / h to 900℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0050] Quenching: heat at 80℃ / h to 880℃, heating and holding time 400min, water temperature 20℃, water cooling to room temperature.

[0051] Tempering: Heat at 80℃ / h to 640℃, heat and hold for 560min, and air cool to room temperature.

[0052] Subcritical quenching: heat to 550℃ at 80℃h, heating and holding time for 420min, water temperature 25℃, and water cooling to room temperature.

[0053] Example 3

[0054] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0055] The diameter of the heat-treated blank axle is D=250mm.

[0056] Austenite homogenization normalizing: heat at 80℃ / h to 910℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0057] Quenching: heat at 80℃ / h to 890℃, heating and holding time 400min, water temperature 25℃, water cooling to room temperature.

[0058] Tempering: Heat at 80℃ / h to 650℃, heat and hold for 560min, and air cool to room temperature.

[0059] Subcritical quenching: heat to 560℃ at 80℃h, heating and holding time for 420min, water temperature 25℃, and water cooling to room temperature.

[0060] Example 4

[0061] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0062] The diameter of the heat-treated blank axle is D=250mm.

[0063] Austenite homogenization normalizing: heat at 80℃ / h to 900℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0064] Quenching: heat at 80℃ / h to 890℃, heating and holding time 400min, water temperature 25℃, water cooling to room temperature.

[0065] Tempering: Heat at 80℃ / h to 650℃, heat and hold for 560min, and air cool to room temperature.

[0066] Subcritical quenching: heat to 560℃ at 80℃h, heating and holding time for 420min, water temperature 30℃, and water cooling to room temperature.

[0067] The finished axle diameter is 221.8mm.

[0068] The heat treatment processes of Comparative Examples 1 to 4 include normalizing, quenching and tempering, and the heat treatment process parameters are as follows:

[0069] Comparative Example 1

[0070] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0071] The diameter of the heat-treated blank axle is D=250mm.

[0072] Austenite homogenization normalizing: heat at 80℃ / h to 890℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0073] Quenching: heat at 80℃ / h to 870℃, heating and holding time 400min, water temperature 20℃, water cooling to room temperature.

[0074] Tempering: Heat at 80℃ / h to 630℃, heat and hold for 560min, and air cool to room temperature.

[0075] Comparative Example 2

[0076] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0077] The diameter of the heat-treated blank axle is D=250mm.

[0078] Austenite homogenization normalizing: heat at 80℃ / h to 900℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0079] Quenching: heat at 80℃ / h to 880℃, heating and holding time 400min, water temperature 20℃, water cooling to room temperature.

[0080] Tempering: Heat at 80℃ / h to 640℃, heat and hold for 560min, and air cool to room temperature.

[0081] Comparative Example 3

[0082] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0083] The diameter of the heat-treated blank axle is D=250mm.

[0084] Austenite homogenization normalizing: heat at 80℃ / h to 910℃, heat and hold for 380min, then air cool to below 200℃ at 380℃ / h.

[0085] Quenching: heat at 80℃ / h to 890℃, heating and holding time 400min, water temperature 25℃, water cooling to room temperature.

[0086] Tempering: Heat at 80℃ / h to 650℃, heat and hold for 560min, and air cool to room temperature.

[0087] Comparative Example 4

[0088] A heat treatment process for optimizing residual stress in an EA4T axle includes the following steps:

[0089] The diameter of the heat-treated blank axle is D=250mm.

[0090] Austenite homogenization normalizing: At 120℃ / h Heat to temperature 870℃ , heating and holding time is 380min, and air cooling is carried out at 380℃ / h to below 200℃.

[0091] Quenching: 120℃ / h Heat to temperature 830℃ , heating and holding time 400min, water temperature 25℃, water cooling to room temperature.

[0092] Tempering: Heat at 100℃ / h to 560℃, heat and hold for 560min, and air cool to room temperature.

[0093] Subcritical quenching: heat to 560℃ at 80℃h, heating and holding time for 420min, water temperature 30℃, and water cooling to room temperature.

[0094] Comparative Examples 1-3 and Examples 1-3 do not undergo subcritical quenching. Comparative Example 4 and Example 4 have the same process (also undergoing subcritical quenching), except that the parameters are improperly controlled.

[0095] The surface residual stress and the residual stress 2 mm below the axle surface were tested for the axles produced in each embodiment and comparative example. The results are shown in Tables 2 and 3.

[0096] Table 2 Surface residual stress of axles produced in various embodiments and comparative examples

[0097]

[0098] Table 3 Residual stress 2 mm below the surface of axles produced in various embodiments and comparative examples

[0099]

[0100] In summary, according to the optimized axle residual stress heat treatment production process of the present invention, the axles of Examples 1-4 fully meet the requirements of EN13262 standard. Not only is the residual stress of the axle uniform, the maximum difference in residual stress at 6 measuring points around the axle body is less than 35 MPa, and at the same time, the residual compressive stress on the surface of the axle is greatly improved, ensuring that the surface residual compressive stress is less than -50 MPa.

[0101] The underlined data do not meet the requirements of the present invention.

[0102] The above embodiments are described to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. An EA4T axle, characterized in that: The maximum difference in residual stress at six measuring points 2 mm below the surface of the EA4T axle is less than 35 MPa.

2. The EA4T axle according to claim 1, characterized in that: The surface residual stress of the EA4T axle is less than -50 MPa.

3. The EA4T axle according to claim 1 or 2, characterized in that: The EA4T axle includes the following components in mass percentage: C 0.24-0.29%, Si 0.28-0.37%, Mn 0.60-0.77%, P ≤ 0.015%, S ≤ 0.010%, Cr 1.00-1.20%, Mo 0.18-0.28%, Ni 0.15-0.28%, Al 0.015-0.040%, Cu ≤ 0.30%, V 0.030-0.060%, T [O] ≤ 15ppm, [H] ≤ 2.0%, [N] ≤ 80ppm, and the balance is Fe and unavoidable impurities.

4. A heat treatment process for optimizing the residual stress of an EA4T axle according to any one of claims 1 to 3, characterized in that: The heat treatment process includes austenite homogenization normalizing, quenching, tempering and subcritical quenching.

5. The heat treatment process according to claim 4, characterized in that: The austenite homogenization normalizing is specifically as follows: the blank axle is heated to a temperature of 890-910°C at a heating rate of 70-100°C / h, the heating and holding time T1 in this temperature range is calculated based on the axle diameter D×1.2-1.6min / mm, and then air-cooled to below 200°C at a cooling rate of 360-400°C / h.

6. The heat treatment process according to claim 4, characterized in that: The quenching is specifically as follows: heating the blank axle to a temperature of 870-890°C at a heating rate of 70-100°C / h, the heating and holding time T2 in this temperature range is calculated based on the axle diameter D×1.4-1.8min / mm, and then water-cooling to room temperature.

7. The heat treatment process according to claim 6, characterized in that: The temperature of the cooling water used in the water cooling during quenching is maintained at 10 to 40°C.

8. The heat treatment process according to claim 4, characterized in that: The tempering is specifically as follows: heating the blank axle to a temperature of 630-650°C at a heating rate of 70-100°C / h, heating and holding time in this temperature range, the heating and holding time T3 is calculated according to the axle diameter D×2.1-2.5min / mm, and then air cooling to room temperature.

9. The heat treatment process according to claim 4, characterized in that: The subcritical quenching is specifically as follows: heating the blank axle to a temperature of 540-560°C at a heating rate of 70-100°C / h, the heating and holding time T4 in this temperature range is calculated based on the diameter D×1.5-1.9min / mm, and then water-cooling to room temperature.

10. The heat treatment process according to claim 9, characterized in that: During subcritical quenching, the temperature of the cooling water used in the water cooling is maintained at 20 to 40°C.

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