Heat treatment method for flash-welded joint of low-alloy U75VH heat-treated steel rail

Abstract

The invention discloses a low-alloy U75VH heat treatment steel rail flash welding joint heat treatment method which comprises the following steps: a heating process: normalizing a joint by using double-frequency normalizing induction heating equipment, firstly performing low-frequency induction heating: heating the welded joint from room temperature to 830-850 DEG C under the conditions that the low-frequency frequency is 1000-1400Hz, the low-frequency power is 70-90kW and the heating time is 60-90s; after heating to the temperature, low-frequency heating is adjusted to high-frequency induction heating, the heating frequency ranges from 2000 Hz to 2300 Hz, the high-frequency power ranges from 50 kW to 70 kW, the heating time ranges from 50 s to 90 s, the connector is heated to 890 DEG C to 910 DEG C from 830 DEG C to 850 DEG C, and the total heating time is controlled to range from 110 s to 180 s; the method further comprises a cooling process. According to the heat treatment method for the joint, the mechanical property and the microscopic structure of the low-cost and low-alloy U75VH heat treatment steel rail joint produced by welding can meet the standard requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of rail welding, and particularly to a heat treatment method for flash welding joints of low-alloy U75VH heat-treated rails. Background Art

[0002] At present, domestic U75VH heat-treated rails of H340 grade are suitable for most heavy-haul lines (100 - 300 million tons), passenger-cargo mixed lines, speed-up lines, etc. in China. The wear resistance and service performance of U75VH heat-treated rails can meet the transportation requirements of existing lines. In the requirements of TB / T 2344.1-2020 standard: for U75VH heat-treated rails of H340 grade, the strength ≥ 1180 MPa, and the hardness is between 340 - 400 HBW. Currently in China, the strength of produced H340 grade rails ≥ 1250 MPa, the hardness ≥ 360 HBW, and the alloy Si + Mn + V ≥ 1.67%. There is a large margin in the alloying elements and mechanical properties of the rails. Combining with the online heat treatment control technology, there is a large room for optimization in the alloy composition and mechanical properties of U75VH heat-treated rails. By reducing the content of alloying elements in the rails and optimizing the online controlled cooling process, the production cost of U75VH heat-treated rails can be further reduced, which also conforms to the low-carbon and green development trend of iron and steel enterprises. However, the change in composition and the optimization of the production process will inevitably affect the welding performance of the rails. The rail welding process should be appropriately adjusted to meet the standard and line service requirements, and at the same time ensure that the mechanical properties of the joints can be excellently matched with the base metal.

[0003] The important performance indicators of flash welding joints of low-alloy U75VH heat-treated rails include: drop hammer, static bending, joint tension, joint hardness, joint impact, etc. Compared with the existing U75V rails, the reduction of the alloy content in the rail base metal and the optimization of the production process ensure the strength and hardness of the rail base metal. However, during the flash welding process, due to the further burning loss of carbon and alloy, the hardness of the joint drops linearly. In order to make the strength and hardness of the joint meet the standard requirements, it is necessary to adjust the rail welding process and the joint heat treatment process to ensure that the mechanical properties and microstructure of the joint both meet the standard requirements. Summary of the Invention

[0004] The purpose of the present invention is to provide a heat treatment method for flash welding joints of low-alloy U75VH heat-treated rails. Through the joint heat treatment method of the present invention, the mechanical properties and microstructure of the joints of low-cost, low-alloy U75VH heat-treated rails produced by welding can meet the standard requirements, solve the problem that the mechanical properties of the joints are reduced due to the reduction of the alloy content in the rails and the burning loss of alloying elements during the welding process, and the obtained mechanical properties of the joints can be excellently matched with the base metal.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A heat treatment method for the flash welded joint of low alloy U75VH heat treated rail of the present invention includes:

[0007] The heating process is as follows: Use a dual - frequency normalizing induction heating device to perform normalizing treatment on the joint. First, perform low - frequency induction heating on the joint: The low - frequency frequency is between 1000 - 1400Hz, the low - frequency power is between 70 - 90kW, and the heating time is between 60 - 90s. Heat the as - welded joint from room temperature to 830 - 850°C; After the low - frequency heating reaches the temperature, immediately adjust the induction heating parameters of the joint normalizer, change from low - frequency heating to high - frequency induction heating. The heating frequency is between 2000 - 2300Hz, the high - frequency power is between 50 - 70kW, and the heating time is between 50 - 90s. Heat the joint from 830 - 850°C to between 890 - 910°C. The total heating time of the joint normalizing heat treatment is controlled between 110 - 180s;

[0008] The cooling process is as follows: After the joint induction heating reaches the temperature of 890 - 910°C, stop heating, and immediately perform air - jet accelerated cooling treatment on the rail head and rail web of the joint. The air - jet time is controlled between 100 - 130s, and the air - jet pressure is between 0.35 - 0.45MPa. Cool the joint from 890 - 910°C to between 400 - 450°C. After the cooling reaches the target temperature, naturally cool to room temperature.

[0009] Further, the welded joint of the low alloy U75VH heat treated steel is heat - treated after cooling to below 300°C after welding.

[0010] Further, the low alloy U75VH heat treated steel takes C, Si, Mn, V as the main elements. Among them, the weight percentages of the chemical components of the rail are C: 0.75 - 0.79%, Si: 0.60 - 0.70%, Mn: 0.80 - 0.96%, V ≤ 0.04 - 0.05%, P ≤ 0.015%, S ≤ 0.004%, Al: ≤ 0.004%, and the rest is Fe and unavoidable impurities.

[0011] Specifically, it includes: Adopt a dual - frequency heating method to heat the flash welded joint area of the low alloy U75VH heat treated rail. First, use a low - frequency power of 80kW to heat to 840°C, with a heating time of 85s, then use a high - frequency power of 60kW to heat to 900°C, with a heating time of 70s. After stopping heating, immediately perform air - jet accelerated cooling treatment on the rail head and rail web of the joint. The air - jet wind pressure is 0.40MPa, and the air - jet time is 120s. When the temperature of the joint tread is air - cooled to the final cooling temperature of 420°C, stop the air - jet cooling, and then the rail joint is naturally cooled to room temperature. Refer to the requirements of TB / T 1632.2 - 2014 standard, use an HR - 150A Rockwell hardness tester to test the Rockwell hardness 5mm below the tread of the rail head on the longitudinal section.

[0012] Furthermore, the ratio of the average hardness HJ of the hybrid welded joint to the average hardness HP of the base metal is 0.97, the ratio of the average hardness HJ1 of the soft spots in the joint to the average hardness HP of the base metal is 0.83, the width of the softened zone on the left side of the joint is 15 mm, and the width of the softened zone on the right side is 10 mm. Both the hardness of the joint and the width of the softened zone meet the standard requirements.

[0013] Furthermore, to improve the hardness of the welded joint of the low-alloy U75VH heat-treated rail, the ratio of the average hardness HJ of the joint to the average hardness HP of the base metal satisfies HJ / HP≥0.95; the ratio of the average hardness HJ1 of the soft spots in the joint to the average hardness HP of the base metal satisfies HJ1 / HP≥0.83.

[0014] Furthermore, after heat treatment: the width of the softened area ≤ 20 mm, meeting the standard requirements.

[0015] Furthermore, the weld and the heat-affected zone are typical pearlite structures, and there are no harmful structures such as martensite, bainite, or secondary cementite. The performance of the joint meets the standard requirements.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are:

[0017] The method of the present invention improves the hardness of the welded joint of the low-alloy U75VH heat-treated rail by controlling the heating temperature, heating power, heating time, air-blowing wind pressure, and final cooling temperature. The ratio of the average hardness HJ of the joint to the average hardness HP of the base metal satisfies HJ / HP≥0.95; the ratio of the average hardness HJ1 of the soft spots in the joint to the average hardness HP of the base metal satisfies HJ1 / HP≥0.83; the width of the softened area ≤ 20 mm, meeting the standard requirements. The weld and the heat-affected zone are typical pearlite structures, and there are no harmful structures such as martensite, bainite, or secondary cementite. The performance of the joint meets the standard requirements. Detailed implementation manners

[0018] A heat treatment method for the flash welded joint of low-alloy U75VH heat-treated rail. The low-alloy U75VH heat-treated rail flash welded in the present invention mainly contains elements C, Si, Mn, and V. Among them, the weight percentages of the chemical components of the rail are C: 0.75 - 0.79%, Si: 0.60 - 0.70%, Mn: 0.80 - 0.96%, V≤0.04 - 0.05%, P≤0.015%, S≤0.004%, Al: ≤0.004%, and the rest is Fe.

[0019] For the welded joint of the low-alloy U75VH rail heat-treated in the present invention, heat treatment shall be carried out after cooling to below 300°C after welding.

[0020] The heating process is as follows: Use a dual-frequency normalizing induction heating device to perform normalizing treatment on the joint. First, perform low-frequency induction heating on the joint: the low-frequency frequency is between 1000 - 1400 Hz, the low-frequency power is between 70 - 90 kW, the heating time is between 60 - 90 s, and heat the as-welded joint from room temperature to 830 - 850 °C. After the low-frequency heating reaches the temperature, immediately adjust the induction heating parameters of the joint normalizing machine, switch from low-frequency heating to high-frequency induction heating. The heating frequency is between 2000 - 2300 Hz, the high-frequency power is between 50 - 70 kW, and the heating time is between 50 - 90 s. Heat the joint from 830 - 850 °C to between 890 - 910 °C. The total heating time for the joint normalizing heat treatment is controlled between 110 - 180 s.

[0021] The cooling process is as follows: After the joint induction heating reaches the temperature of 890 - 910 °C, stop heating, and immediately perform air-blowing accelerated cooling treatment on the rail head and rail waist of the joint. The air-blowing time is controlled between 100 - 130 s, and the air-blowing pressure is between 0.35 - 0.45 MPa. Cool the joint from 890 - 910 °C to between 400 - 450 °C. After cooling reaches the target temperature, naturally cool to room temperature.

[0022] Example 1

[0023] Adopt the dual-frequency heating method to heat the flash welding joint area of the low-alloy U75VH heat-treated rail. First, use a low-frequency power of 65 kW, heat to 870 °C, and the heating time is 100 s. Then, use a high-frequency power of 50 kW, heat to 890 °C, and the heating time is 50 s. After stopping heating, immediately perform air-blowing accelerated cooling treatment on the rail head and rail waist of the joint. The air-blowing wind pressure is 0.25 MPa, and the air-blowing time is 120 s. When the tread surface temperature of the joint is air-cooled to the final cooling temperature of 500 °C, stop air-blowing cooling, and then the rail joint is naturally cooled to room temperature. Referring to the requirements of the TB / T 1632.2 - 2014 standard, use an HR-150A Rockwell hardness tester to measure the Rockwell hardness 5 mm below the tread surface of the rail head on the longitudinal section. In Example 1, the ratio of the average hardness HJ of the joint to the average hardness HP of the base metal is 0.87, the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.78, the width of the softened area on the left side of the joint is 23 mm, and the width of the softened area on the right side of the joint is 24 mm. Both the hardness of the joint and the width of the softened area do not meet the standard requirements.

[0024] Longitudinal section Rockwell hardness test data and results of the welded joint in Example 1

[0025]

[0026]

[0027] Example 2

[0028] The flash welding joint area of low-alloy U75VH heat-treated steel rail is heated by a dual-frequency heating method. First, a low-frequency power of 70 kW is used to heat it to 860 °C for 95 s, and then a high-frequency power of 65 kW is used to heat it to 910 °C for 68 s. Immediately after stopping heating, the rail head and rail web of the joint are subjected to accelerated cooling by air spraying. The air spraying wind pressure is 0.30 MPa, and the air spraying time is 120 s. When the tread temperature of the joint is air-cooled to the final cooling temperature of 460 °C, the air spraying cooling is stopped, and then the rail joint is naturally cooled to room temperature. Referring to the requirements of TB / T 1632.2-2014 standard, an HR-150A Rockwell hardness tester is used to measure the Rockwell hardness 5 mm below the tread of the rail head on the longitudinal section. In Example 2, the ratio of the average hardness HJ of the hybrid welded joint to the average hardness HP of the base metal is 0.90, the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.79, the width of the softened area on the left side of the joint is 20 mm, and the width of the softened area on the right side of the joint is 19 mm. The hardness of the soft spot of the joint does not meet the standard requirements, and the width of the softened area of the joint is also close to the upper limit of the standard requirements.

[0029] Longitudinal Section Rockwell Hardness Test Data and Results of the Welded Joint in Example 2

[0030]

[0031]

[0032] Example 3

[0033] The flash welding joint area of low-alloy U75VH heat-treated steel rail is heated by a dual-frequency heating method. First, a low-frequency power of 80 kW is used to heat it to 840 °C for 85 s, and then a high-frequency power of 60 kW is used to heat it to 900 °C for 70 s. Immediately after stopping heating, the rail head and rail web of the joint are subjected to accelerated cooling by air spraying. The air spraying wind pressure is 0.40 MPa, and the air spraying time is 120 s. When the tread temperature of the joint is air-cooled to the final cooling temperature of 420 °C, the air spraying cooling is stopped, and then the rail joint is naturally cooled to room temperature. Referring to the requirements of TB / T 1632.2-2014 standard, an HR-150A Rockwell hardness tester is used to measure the Rockwell hardness 5 mm below the tread of the rail head on the longitudinal section. In Example 3, the ratio of the average hardness HJ of the hybrid welded joint to the average hardness HP of the base metal is 0.97, the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.83, the width of the softened area on the left side of the joint is 15 mm, and the width of the softened area on the right side of the joint is 10 mm. Both the hardness of the joint and the width of the softened area meet the standard requirements.

[0034] Longitudinal Section Rockwell Hardness Test Data and Results of the Welded Joint in Example 3

[0035]

[0036]

[0037] As can be seen from different embodiments, the hardness of the joints and the width of the soft zone obtained in Embodiment 1 and Embodiment 2 do not meet the standard requirements. The joint obtained in Embodiment 3 has good hardness. The ratio of the average hardness HJ of the joint to the average hardness HP of the base material satisfies HJ / HP≥0.95; the ratio of the average hardness HJ1 of the soft spots of the joint to the average hardness HP of the base material satisfies HJ1 / HP≥0.83; the width of the softened area ≤20 mm, meeting the standard requirements. The method of the present invention improves the hardness of the flash welded joints of low alloy U75VH heat-treated steel rails by controlling the heating temperature, heating power, heating time, air-blowing wind pressure and final cooling temperature. It can ensure that the mechanical properties of the flash welded joints of low-cost, low-alloy U75VH heat-treated steel rails meet the standards and the service requirements of the line. The mechanical properties of the obtained joints can achieve excellent matching with the base material. The joint performance of the present invention is suitable for most heavy-haul lines (100 - 300 million tons), passenger-cargo mixed lines, speed-up lines, etc. in China.

[0038] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A heat treatment method for flash welded joints of low alloy U75VH heat treated rail, characterized in that: Including: The heating process is as follows: Use a dual-frequency normalizing induction heating device to perform normalizing treatment on the joint. First, perform low-frequency induction heating on the joint: The low-frequency frequency is between 1000 - 1400 Hz, the low-frequency power is between 70 - 90 kW, and the heating time is between 60 - 90 s. Heat the as-welded joint from room temperature to 830 - 850 °C. After the low-frequency heating reaches the temperature, immediately adjust the induction heating parameters of the joint normalizing machine, change from low-frequency heating to high-frequency induction heating. The heating frequency is between 2000 - 2300 Hz, the high-frequency power is between 50 - 70 kW, and the heating time is between 50 - 90 s. Heat the joint from 830 - 850 °C to between 890 - 910 °C. The total heating time of the joint normalizing heat treatment is controlled between 110 - 180 s. The cooling process is as follows: After the joint induction heating reaches the temperature of 890 - 910 °C, stop heating, and immediately perform air-blowing accelerated cooling treatment on the rail head and rail waist of the joint. The air-blowing time is controlled between 100 - 130 s, and the air-blowing pressure is between 0.35 - 0.45 MPa. Cool the joint from 890 - 910 °C to between 400 - 450 °C. After cooling reaches the target temperature, naturally cool to room temperature.

2. The heat treatment method for flash welded joints of low alloy U75VH heat treated steel rails according to claim 1, characterized in that: The welded joint of the low-alloy U75VH heat-treated steel is heat-treated after cooling to below 300 °C after welding.

3. The heat treatment method for the flash welded joint of low alloy U75VH heat treated steel rail according to claim 1, characterized in that: The low-alloy U75VH heat-treated steel has C, Si, Mn, and V as the main elements. Among them, the weight percentages of the chemical components of the rail are: C: 0.75 - 0.79%, Si: 0.60 - 0.70%, Mn: 0.80 - 0.96%, V ≤ 0.04 - 0.05%, P ≤ 0.015%, S ≤ 0.004%, Al: ≤ 0.004%, and the rest is Fe and inevitable impurities.

4. The heat treatment method for flash welded joints of low alloy U75VH heat treated steel rails according to claim 1, characterized in that: Specifically including: Adopt a dual-frequency heating method to heat the flash welding joint area of the low-alloy U75VH heat-treated rail. First, use a low-frequency power of 80 kW, heat to 840 °C, and the heating time is 85 s. Then use a high-frequency power of 60 kW, heat to 900 °C, and the heating time is 70 s. Immediately after stopping heating, perform air-blowing accelerated cooling treatment on the rail head and rail waist of the joint. The air-blowing wind pressure is 0.40 MPa, and the air-blowing time is 120 s. When the tread temperature of the joint is air-cooled to the final cooling temperature of 420 °C, stop the air-blowing cooling, and then the rail joint is naturally cooled to room temperature. Referring to the requirements of the TB / T 1632.2 - 2014 standard, use an HR-150A Rockwell hardness tester to measure the Rockwell hardness 5 mm below the tread on the longitudinal section of the rail head.

5. The heat treatment method for flash welded joints of low alloy U75VH heat treated steel rails according to claim 4, characterized in that: The ratio of the average hardness HJ of the welded joint to the average hardness HP of the base metal is 0.

97. The ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.

83. The width of the softened area on the left side of the joint is 15 mm, and the width of the softened area on the right side of the joint is 10 mm. The hardness and softened area width of the joint both meet the standard requirements.

6. The heat treatment method for flash welding joints of low-alloy U75VH heat-treated steel rails according to claim 3, characterized in that: Improve the hardness of the welded joint of low-alloy U75VH heat-treated rail, and the ratio of the average hardness HJ of the joint to the average hardness HP of the base metal satisfies HJ / HP≥0.95; the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal satisfies HJ1 / HP≥0.

83.

7. The heat treatment method for the flash welded joint of low alloy U75VH heat treated steel rail according to claim 1, characterized in that: After heat treatment: the width of the softened area ≤ 20 mm, meeting the standard requirements.

8. The heat treatment method for flash welding joints of low-alloy U75VH heat-treated steel rails according to claim 1, characterized in that: The weld and the heat-affected zone are typical pearlite structures, without harmful structures such as martensite, bainite or secondary cementite, and the joint performance meets the standard requirements.

Images