A method for improving the performance of U71Mn rail fixed flash welding joints at low temperature

Abstract

The present invention discloses a method for improving the performance of U71Mn rail fixed flash welding joints at low temperature, which includes joint fixed flash welding and joint heat treatment processes, and a forging stage is added between the welding process and the heat treatment process, in which the rail consumption is set to 1.0mm-3.0mm, the forging time is 3.0s-5.0s, the average speed is 0.6mm / s-1.5mm / s, and the forging pressure is 600KN-800KN. After using the present invention, it can be guaranteed to improve the problems of low hardness of U71Mn rail fixed flash welding joints at low temperature and poor joint drop hammer performance, thereby effectively improving the service performance of the joint.

Description

Technical Field

[0001] The invention relates to the technical field of rail welding, and in particular to a method for improving the performance of a fixed flash welding joint of a U71Mn rail at low temperature. Background Art

[0002] Rail flash welding process and joint heat treatment process directly affect the joint microstructure performance and service status. Improper flash welding process will cause defects such as overburning, unwelded, excessive gray spots, and liquefaction cracks in the joint. The metallographic grain size of the welded joint is relatively coarse, only about 1-2 levels, and the joint performance is very poor. The welded joint is subjected to normalizing heat treatment, and the weld microstructure is significantly refined, and the grain size is increased from level 1 to more than level 8, which greatly improves the toughness of the flash welded joint. However, improper weld heat treatment process, such as high or low heating temperature, fast or slow heating speed, long or short high temperature residence time, uneven temperature distribution, and continuous cooling under air cooling conditions, is prone to coarse organization in the joint, especially in the heat-affected zone, and even easy to form harmful organizations such as martensite, causing hardening and embrittlement at the joint. During the service of the line, under the action of wheel-rail stress, the joint is very likely to break, becoming a weak link in line operation.

[0003] At present, with the increase in railway transportation volume, rails serving in low-temperature environments have put forward new requirements for wear resistance, low-temperature toughness, etc. When welding in such a low-temperature environment, the existing welding method is prone to low-temperature brittle fracture of the welded joint due to the low ambient temperature and limited welding process, resulting in unqualified welded joint quality.

[0004] Patent document CN202210912903.1 discloses a post-weld heat treatment method for a U71MnG high-speed rail welded joint, which is to stop heating the joint after heating it to 880-910°C, use air jet cooling, and stop air jet cooling after the rail joint tread temperature drops to 410-455°C. This patent document is an invention for U71MnG high-speed rails, and it only provides hardness indicators, without describing key indicators such as the joint drop hammer performance under low-temperature welding environment. Summary of the invention

[0005] The purpose of the present invention is to provide a method for improving the performance of U71Mn rail fixed flash welding joints at low temperature. The flash welding joint process of the present invention is used, and a forging stage is added later to obtain rail welding joints with good plasticity and toughness. After the joint heat treatment process, it can be ensured that the rail fixed flash welding joints are all normal pearlite structures, and there are no abnormal structures such as bainite and martensite; the present invention solves the problems of low hardness of rail joints and poor drop hammer performance of joints, and can effectively improve the performance matching of rail joints and parent materials, improve the comprehensive mechanical properties and contact fatigue performance of joints, and thus effectively improve the service performance of joints.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for improving the performance of a U71Mn rail fixed flash welding joint at low temperature, comprising:

[0008] (1) Grinding and removing rust from the end face of the rolled U71Mn rail and the rust removal and grinding of the electrode clamping end;

[0009] (2) Use GAAS80 / 580 fixed flash welder for rail welding; the grid voltage is between 410-420V;

[0010] (3) Joint fixing flash welding process: The rail weld seam is controlled between 9-13 mm, the arch is controlled between 0.45-0.60 mm, the flash welding preheating times are controlled between 10-12 times, the secondary preheating current is controlled between 60-70 KA, the flash welding upsetting amount is controlled between 12-16 mm, the upsetting force is controlled between 580-700 KN, the welding time is controlled between 130-140 s, and the final melting speed is controlled between 2.40-2.60 mm / s;

[0011] (4) Add a forging stage, in which the rail consumption is set to 1.0 mm-3.0 mm, the forging time is set to 3.0 s-5.0 s, the average speed is set to 0.6 mm / s-1.5 mm / s, and the forging pressure is set to 600 KN-800 KN;

[0012] (5) Air cool the high-temperature welded joint to below 200°C to ensure complete phase change at every position of the joint;

[0013] (6) Joint heat treatment process: Use a dual-frequency normalizing machine to perform induction heating on the welded joint. First, use low-frequency induction heating to treat the joint: use 53-58kW low-frequency power, induction heating for 75-85s, and heat the joint from 0-200℃ to 830-850℃. Low-frequency induction heating can ensure that the core of the rail head can be heated to the required temperature; after the low-frequency induction heating of the joint reaches 825-840℃, immediately perform high-frequency induction heating on the joint: adjust the normalizing machine frequency to 70-78kW high-frequency power for 50-60s, and heat the joint from 825-840℃ to 91 0-930℃, the total double-frequency normalizing heating time of the joint is controlled between 136-150s; after the joint reaches the target temperature of 910-930℃, stop heating, and immediately perform air jet accelerated cooling treatment on the joint and rail waist. The air jet cooling nozzle is 20-30mm away from the top surface of the rail, 40-50mm away from the side of the rail head, and 50-60mm away from the rail waist; the air jet pressure is 0.35-0.40MPa, the air jet time is 170-185s, the final cooling temperature of the joint air jet should be controlled between 300-350℃, and then the air jet cooling is stopped, and the joint is naturally cooled to room temperature.

[0014] Furthermore, the low temperature environment temperature is below -15°C.

[0015] Furthermore, the ratio of the average hardness Hj of the joint to the average hardness Hp of the base material satisfies: Hj / Hp≥0.92, and the ratio of the average hardness Hj1 of the joint soft spot to the average hardness Hp of the base material satisfies: Hj1 / Hp≥0.85.

[0016] Furthermore, the softening area w≤16mm.

[0017] Furthermore, the performance test at a hammer drop height of 5.2 meters was carried out: after welding, the joint was not broken after 25 hammer drops, and the weld and heat-affected zone were typical pearlite structures, without any harmful martensite or bainite structures.

[0018] Furthermore, the normalizing temperature is: 830°C for low frequency heating and 920°C for high frequency heating.

[0019] Furthermore, the normalizing temperature is: 825°C for low frequency heating and 925°C for high frequency heating.

[0020] Compared with the prior art, the beneficial technical effects of the present invention are:

[0021] The method of the present invention adds a forging stage, so that metal atoms on the end surface of the steel rail to be welded penetrate and diffuse each other under the action of the forging force to form a connection between the metal atoms, and complete crystallization under the action of pressure, so that the joint forms a forged structure, which greatly improves the quality of the joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described below in conjunction with the accompanying drawings.

[0023] Figure 1 The Rockwell hardness performance of the longitudinal section of the rail head is measured for the flash welded joint after normalizing treatment in Example 4;

[0024] Figure 2 The Rockwell hardness performance of the longitudinal section of the rail head is measured for the flash welded joint after normalizing treatment in Example 5;

[0025] Figure 3 The Rockwell hardness performance of the longitudinal section of the rail head was measured for the flash welded joint after normalizing treatment in Example 6. DETAILED DESCRIPTION

[0026] A method for improving the performance of U71Mn rail fixed flash welding joints at low temperatures:

[0027] By using the flash welding joint process of the present invention, a forging stage is added to obtain a rail welding joint with good plasticity and toughness. After the joint heat treatment process, the problem of low hardness of the rail joint and poor drop hammer performance of the joint is further solved, and the performance matching between the rail joint and the parent material can be effectively improved. The specific steps are as follows:

[0028] (1) Grinding and removing rust from the end face of the rolled U71Mn rail and the rust removal and grinding of the electrode clamping end;

[0029] (2) Use GAAS80 / 580 fixed flash welder for rail welding; the grid voltage is between 410-430V;

[0030] (3) Joint fixing flash welding process: The rail weld seam is controlled between 9-13 mm, the arch is controlled between 0.45-0.60 mm, the flash welding preheating times are controlled between 10-12 times, the secondary preheating current is controlled between 60-70 KA, the flash welding upsetting amount is controlled between 12-16 mm, the upsetting force is controlled between 580-700 KN, the welding time is controlled between 130-140 s, and the final melting speed is controlled between 2.40-2.60 mm / s;

[0031] (4) Add a forging stage, in which the rail consumption is set to 1.0mm-3.0mm, the forging time is set to 3.0s-5.0s, the average speed is set to 0.6mm / s-1.5mm / s, and the forging pressure is set to 600KN-800KN.

[0032] (5) Air cool the high-temperature welded joint to below 200°C to ensure that each position of the joint undergoes a complete phase change. When the joint temperature is less than 300°C, the cooling can be accelerated appropriately;

[0033] (6) Joint heat treatment process: Use a dual-frequency normalizing machine to perform induction heating on the welded joint. First, use low-frequency induction heating to treat the joint: use 53-58kW low-frequency power, induction heating for 75-85s, and heat the joint from 0-200℃ to 830-850℃. Low-frequency induction heating can ensure that the core of the rail head can be heated to the required temperature; after the low-frequency induction heating of the joint reaches 825-840℃, the joint is immediately subjected to high-frequency induction heating:

[0034] (7) The normalizing machine frequency is adjusted to 70-78kW high-frequency power for heating for 50-60s, and the joint is heated from 825-840℃ to 910-930℃. The total double-frequency normalizing heating time of the joint is controlled between 136-150s. After the joint reaches the target temperature of 910-930℃, the heating is stopped, and the joint and rail waist are immediately cooled by air spray;

[0035] (8) The air jet cooling outlet is 20-30mm away from the rail top, 40-50mm away from the rail head side, and 50-60mm away from the rail waist. The air jet pressure is 0.35-0.40MPa, the air jet time is 170-185s, and the final air jet cooling temperature of the joint should be controlled between 300-350℃, then stop the air jet cooling, and let the joint cool naturally to room temperature.

[0036] The content of the present invention is described in detail below through specific examples. The examples are intended to help understand the present invention but are not intended to limit the content of the present invention.

[0037] Example

[0038] The main adjustment parameters of the welding process in the embodiment and the corresponding drop hammer test results are shown in Table 1.

[0039] Table 1: Drop hammer test results of different welding processes in the embodiment

[0040]

[0041] As can be seen from Table 1, the performance test of the joints under different welding processes was carried out at a hammer drop height of 5.2 meters: Example 1 rail flash welding process; Example 1 and Example 2 rail flash welding processes were followed by a forging stage, and no fracture occurred during the drop hammer; Example 3 rail flash welding process was followed by a forging stage, but the setting parameters were large, the plastic deformation of the welded joint was excessive, and the metal grains may be deformed and elongated due to excessive extrusion, resulting in fiberization of the metal structure in the joint area, grain coarsening, or concentrated inclusions at the grain boundaries, which reduced the plasticity. Performance degradation. The number of drop hammer fractures was 3. In contrast to the rail flash welding process 1, the number of drop hammer fractures was 6, and obvious gray spots or liquefaction cracks were found on 4 fractures.

[0042] The grain size of the rail fixed flash welded joint in the welded state is relatively coarse, only about 1-2 levels, and the mechanical properties of the joint are very poor. Normalizing heat treatment of the welded joint can refine the weld microstructure and effectively improve the overall mechanical properties of the flash welded joint. The comparison schemes of different joint heat treatment processes in Examples 4-6 (all three use the welding process of Example 2) are shown in Table 2.

[0043] Table 2: Comparison of different heat treatment processes in the examples

[0044]

[0045] The flash welded joints after normalizing in Example 4 to Example 6 were subjected to comparative analysis of Rockwell hardness performance and softening zone width of the rail head longitudinal section according to "TB / T 1632.2-2014 Rail Welding Part 2: Flash Welding". The comparative analysis results are shown in Table 3 to Table 5. Figure 1-Figure 3 .

[0046] Table 3: Test results of joint longitudinal section (test line 1) of Example 4

[0047]

[0048] From Table 3 and Figure 1 It can be seen that in Example 4, due to the short air jet pressure and air jet time of the joint, the final cooling temperature of the joint is high, resulting in the joint hardness ratio: Hj / Hp=0.88, Hj1 / Hp=0.79, and the softening zone width is 21mm and 25mm, which does not meet the standard requirements of TB / T1632.

[0049] Table 4: Test results of joint longitudinal section (test line 1) of Example 5

[0050]

[0051]

[0052] From Table 4 and Figure 2 It can be seen that, compared with Example 4, the joint in Example 5 increases the air jet pressure and air jet time, and the final cooling temperature of the joint is reduced, but the joint hardness ratio: Hj / Hp=0.89, the softening zone width is 21mm and 22mm, which still does not meet the standard requirements of TB / T 1632.

[0053] Table 5: Test results of joint longitudinal section (test line 1) of Example 6

[0054]

[0055] From Table 5 and Figure 3It can be seen that, compared with Examples 4 and 5, Example 6 further increases the joint air jet pressure and air jet time, and the joint final cooling temperature is further reduced. The joint hardness ratio after normalizing is: Hj / Hp=0.93, Hi1 / Hp0.87, and the softening zone width is 15mm and 15mm, which fully meets the standard requirements of TB / T1632. The joint of Example 6 can achieve excellent hardness matching with the parent material. The actual fatigue load cycle of this joint is 2x10 6 The joint did not break after 25 consecutive hammer drops. The mechanical properties and metallographic structure of the joint met the requirements of TB / T1632.2-2014 standard.

[0056] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving the performance of U71Mn rail fixed flash welding joints at low temperature, characterized in that: include: (1) Grinding and removing rust from the end face of the rolled U71Mn rail and the rust removal and grinding of the electrode clamping end; (2) Use GAAS80 / 580 fixed flash welder for rail welding; the grid voltage is between 410-420V; (3) Joint fixing flash welding process: The rail weld seam is controlled between 9-13 mm, the arch is controlled between 0.45-0.60 mm, the flash welding preheating times are controlled between 10-12 times, the secondary preheating current is controlled between 60-70 KA, the flash welding upsetting amount is controlled between 12-16 mm, the upsetting force is controlled between 580-700 KN, the welding time is controlled between 130-140 s, and the final melting speed is controlled between 2.40-2.60 mm / s; (4) Add a forging stage, in which the rail consumption is set to 1.0 mm-3.0 mm, the forging time is set to 3.0 s-5.0 s, the average speed is set to 0.6 mm / s-1.5 mm / s, and the forging pressure is set to 600 KN-800 KN; (5) Air cool the high-temperature welded joint to below 200°C to ensure complete phase change at every position of the joint; (6) Joint heat treatment process: Use a dual-frequency normalizing machine to perform induction heating on the welded joint. First, use low-frequency induction heating to treat the joint: use 53-58kW low-frequency power, induction heating for 75-85s, and heat the joint from 0-200℃ to 830-850℃. Low-frequency induction heating can ensure that the core of the rail head can be heated to the required temperature; after the low-frequency induction heating of the joint reaches 825-840℃, immediately perform high-frequency induction heating on the joint: adjust the normalizing machine frequency to 70-78kW high-frequency power for 50-60s, and heat the joint from 825-840℃ to 91 0-930℃, the total double-frequency normalizing heating time of the joint is controlled between 136-150s; after the joint reaches the target temperature of 910-930℃, stop heating, and immediately perform air jet accelerated cooling treatment on the joint and rail waist. The air jet cooling nozzle is 20-30mm away from the top surface of the rail, 40-50mm away from the side of the rail head, and 50-60mm away from the rail waist; the air jet pressure is 0.35-0.40MPa, the air jet time is 170-185s, the final cooling temperature of the joint air jet should be controlled between 300-350℃, and then the air jet cooling is stopped, and the joint is naturally cooled to room temperature.

2. The method for improving the performance of U71Mn rail fixed flash welding joints at low temperature according to claim 1, characterized in that: The low temperature environment temperature is below -15°C.

3. The method for improving the performance of U71Mn rail fixed flash welding joints at low temperature according to claim 1, characterized in that: The ratio of the average hardness Hj of the joint to the average hardness Hp of the base material satisfies: Hj / Hp≥0.92, and the ratio of the average hardness Hj1 of the joint soft spot to the average hardness Hp of the base material satisfies: Hj1 / Hp≥0.

85.

4. The method for improving the performance of U71Mn rail fixed flash welding joints at low temperature according to claim 1, characterized in that: Softening area w≤16mm.

5. The method for improving the performance of U71Mn rail fixed flash welding joints at low temperature according to claim 1, characterized in that: 5.2m hammer drop height performance test: After welding, the joint was not broken after 25 hammer drops. The weld and heat affected zone are typical pearlite structures, and there is no harmful martensite and bainite structures.

6. The method for improving the performance of U71Mn rail fixed flash welded joints at low temperature according to claim 1, characterized in that: Normalizing temperature: low frequency heating temperature 830℃, high frequency heating temperature 920℃.

7. The method for improving the performance of U71Mn rail fixed flash welded joints at low temperature according to claim 1, characterized in that: Normalizing temperature: low frequency heating temperature 825℃, high frequency heating temperature 925℃.

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