A flash welding and joint heat treatment process for pearlite rails containing Cr, Nb and RE

Abstract

The present invention discloses a flash welding and joint heat treatment process for pearlite rails containing Cr, Nb, and RE, including: a flash welding process and a joint heat treatment process. After using the flash welding joint process and the joint heat treatment process of the present invention, it can be ensured that the pearlite rail joints containing Cr, Nb, and RE are all normal pearlite structures, without abnormal structures such as bainite and martensite; the present invention solves the problems of low hardness of high-strength and high-hardness rail joints, poor joint drop hammer performance, and low joint collapse, and can effectively improve the performance matching between rail joints and parent materials, improve the comprehensive mechanical properties and contact fatigue performance of joints, thereby effectively improving the service performance of joints.

Description

Technical Field

[0001] The invention relates to the technical field of rail welding, in particular to a flash welding and joint heat treatment process of a pearlite rail containing Cr, Nb and RE. Background Art

[0002] Flash welding is one of the important ways to connect rails and is widely used in the production of 500-meter-long rails. The welding performance of steel is mainly affected by the chemical composition of steel. The research and development of pearlite rails in my country are all "high carbon, low silicon, and medium manganese", with chromium as the main element, and alloy strengthening elements such as chromium and vanadium are added according to different strength and hardness requirements. Due to the high carbon content of high-strength and high-hardness pearlite rails for heavy-duty railways, their welding performance is poor, mainly manifested as follows: ① There are many defects in the welding overheating zone. There is segregation of carbon and manganese components along the austenite grain boundary in the overheating zone of flash welding, resulting in abnormal microstructures such as bainite or martensite in the joint under the supercooled state, which increases the brittleness of the joint. ② Due to the cyclic heating of the welding heat-affected zone, the grain size is significantly increased, and the non-equilibrium process of rapid heating and cooling of the joint leads to a decrease in the toughness of the flash welded joint. The impact of the joint is only about 50% of the parent material. When the rail line is in service, there is a risk of rail breakage at the joint. ③ Due to the burning of carbon and alloy during the welding process, the mechanical properties of the joint are significantly reduced, especially the hardness of the joint is significantly reduced, the performance match between the joint and the parent material is poor, and the high strength and high hardness characteristics of the heavy-load line rails cannot be reflected. During the service of the line, saddle wear and joint peeling will occur, which seriously affects the smoothness of the seamless line.

[0003] 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.

[0004] The present invention is aimed at the flash welding and joint heat treatment process of high-strength and high-hardness pearlite rails containing Cr, Nb and RE for heavy-duty railways, which can ensure that the joints are all normal pearlite structures without abnormal structures such as bainite and martensite. The present invention solves the problems of low hardness, poor drop hammer performance and low collapse of the joints of high-strength and high-hardness pearlite rails containing Cr, Nb and RE, and obtains an excellent joint that matches the performance of the parent material, so that the joint performance meets the inspection requirements for joint mechanical properties and drop hammer in "TB / T 1632.2-2014 Rail Welding Part 2: Flash Welding".

[0005] The patent with publication number CN201810708275.9 discloses a heat treatment method for rail flash welded joints. The method is for dissimilar rail welded joints of hypereutectoid rails and eutectoid rails. After welding, the high-temperature welded joints are directly cooled in three stages. The temperature is reduced from 1000-1400°C to 650-720°C in the first cooling stage, the temperature is reduced to 350-410°C in the second cooling stage, and the temperature is reduced to 10-30°C in the third cooling stage. This patent is an invention for rail mobile flash welder welding, and the cooling medium can be a water mist mixture. According to the requirements of TB / T 1632.2 standard, the mobile flash welded joint drop hammer is required to be continuous for 15 times, and the joint cooling treatment can only use compressed air, which does not meet the standard requirements for joint heat treatment process, and the invention does not meet the requirement of 25 times continuous for fixed flash welded joints.

[0006] The patent with publication number CN202010886023.2 discloses a post-weld heat treatment method for 1300MPa low-alloy heat-treated rails. This method cools the rail welded joints with a residual temperature of 900-1100°C formed by the welded joints in three stages: natural cooling to 650-720°C in the first stage, controlled cooling to 480-550°C in the second stage, and controlled cooling to 10-30°C in the second stage. This patent is an invention for the welding of mobile flash welders for rails. The cooling medium can be a water mist mixture, and the joint hardness is tested by Vickers hardness, which does not meet the heat treatment process and performance inspection requirements of the TB / T1632.2 standard for fixed flash welded joints. In addition, a certain proportion of abnormal point-like martensitic structures appear in the joints of this invention, which not only fails to meet the standard requirements but also seriously affects the safety of the joint line operation.

[0007] The patent with the publication number CN201510319185.7 discloses a method for post-weld heat treatment of hypereutectoid rail welded joints, which is to use a medium frequency or oxyacetylene flame to quickly heat the weld area to 900-920°C for the hypereutectoid rail flash welded joint or gas pressure welded joint, cool the joint to 430-450°C at a cooling rate of 2.2-2.8°C / s, and then cool it to room temperature in air. The full-section weld impact energy Aku of the patent is ≥9J, which meets the standard requirement of ≥6.5J, but is lower than the parent material impact performance index; the joint hardness after heat treatment reaches 105% of the parent material. The hypereutectoid rail welded joint has too high hardness, which will inevitably lead to abnormal structure in the joint, high hardness and low toughness in the joint, which seriously affects the service performance of the joint and the safety of line operation. Summary of the invention

[0008] The purpose of the present invention is to provide a flash welding and joint heat treatment process for pearlite rails containing Cr, Nb and RE. After using the flash welding joint process and joint heat treatment process of the present invention, it can be ensured that the pearlite rail joints containing Cr, Nb and RE have normal pearlite structures without abnormal structures such as bainite and martensite. The present invention solves the problems of low hardness, poor drop hammer performance and low collapse of high-strength and high-hardness rail joints, and can effectively improve the performance matching between the rail joint and the parent material, improve the comprehensive mechanical properties and contact fatigue performance of the joint, thereby effectively improving the service performance of the joint.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] The present invention discloses a flash welding and joint heat treatment process for pearlite rails containing Cr, Nb and RE, comprising:

[0011] Flash welding process:

[0012] GAAS80 / 580 fixed flash welder is used for rail welding; the welding machine oil temperature is controlled at about 40℃, the water temperature is controlled at about 20℃, and the grid voltage is between 416 and 425V;

[0013] The seam of rail welds is controlled between 10 and 12 mm, the arching is controlled between 0.35 and 0.55 mm, the number of flash welding preheating times is between 11 and 13 times, the secondary preheating current is controlled between 58 and 63 KA, the flash welding upsetting amount is controlled between 14 and 16.5 mm, the upsetting force is controlled between 515 and 670 KN, the welding time is controlled between 110 and 120 s, and the final melting speed is controlled between 2.60 and 2.80 mm / s;

[0014] Air cool the high-temperature welded joint to below 200°C to ensure complete phase change at every position of the joint. When the joint is ≤280°C, accelerate the cooling appropriately.

[0015] Joint heat treatment process:

[0016] Use the joint dual-frequency normalizing machine for induction heating. First, perform low-frequency induction heating on the joint: use 57-65kW low-frequency power for heating for 70-90s to heat the joint from 0-200℃ to 810-840℃; use low-frequency heating to ensure that the center of the rail head can be heated to the required temperature;

[0017] After the joint reaches the target temperature, it is immediately subjected to high-frequency induction heating: the normalizing machine is adjusted to 68-77kW high-frequency power for heating for 65-80s, and the joint is heated from 810-840℃ to 890-910℃; high-frequency heating ensures that the temperature within the range of 0-15mm below the tread of the rail head can be heated to the target temperature, and the total joint heating time is controlled between 135-170s;

[0018] After the joint reaches the target temperature of 890-910℃, stop heating and immediately perform air jet accelerated cooling on the joint and rail waist. The air jet cooling nozzle is 20-30mm away from the rail head and 50-60mm away from the rail waist; the air jet pressure is 0.34-0.40MPa, and the air jet time is 110-130s; the final air jet cooling temperature of the joint should be controlled between 350-430℃, then stop air jet cooling, and let the joint cool naturally to room temperature.

[0019] Furthermore, before welding, the end faces of two rails of the same material and with the same cross-section are ground and rust-free, and the electrode clamping ends are ground and rust-free.

[0020] Further, the chemical composition of the flash welded rail is C: 0.73-0.83%, Si: 0.50-0.80%, Mn: 0.80-1.10%, Cr: 0.30-0.60%, Nb: 0.01-0.03%, RE: 0.0005-0.0020%, P≤0.010%, S≤0.005%, Al: ≤0.004%, and the rest is Fe.

[0021] Furthermore, the rolling compression ratio of the rail should not be less than 9:1, and the final rolling temperature should not be higher than 950°C, so as to ensure the grain size of the original austenite of the rail.

[0022] Furthermore, the joint tensile strength Rm ≥ 1139MPa, the joint extension A ≥ 12.0%, and the joint impact KU 2 ≥15.7J, joint fatigue load cycle 2×10 6 The average hardness of the joint is H J Average hardness of base material H P The ratio satisfies: H J / H P ≥0.91, average hardness of joint soft spot HJ1 Average hardness of base material H P Ratio: H J1 / H P ≥0.82, softening area W≤20mm, the joint was not broken after 25 hammer drops, the weld and heat affected zone are typical pearlite structures, there is no martensite or bainite harmful structure, and all properties meet the standard requirements.

[0023] Compared with the prior art, the beneficial technical effects of the present invention are:

[0024] Based on the method of the present invention, the tensile strength of the joint Rm ≥ 1139MPa, the extension of the joint A ≥ 12.0%, and the impact strength of the joint KU 2 ≥15.7J, joint fatigue load cycle 2×10 6 The average hardness of the joint is H J Average hardness of base material H P The ratio satisfies: H J / H P ≥0.91, average hardness of joint soft spot H J1 Average hardness of base material H P Ratio: H J1 / H P ≥0.82, softening area W≤20mm, the joint was not broken after 25 hammer drops, the weld and heat affected zone are typical pearlite structures, there is no martensite or bainite harmful structure, and all properties meet the standard requirements.

[0025] The present invention can effectively improve the mechanical properties of the flash welded joint of pearlite rails containing Cr, Nb, and RE, improve the strength, hardness, and toughness of the joint, and make it match the performance of the parent material. The welded joint of the present invention can meet the requirement that the joint does not break after 25 hammer drops, improve the risk of "saddle-shaped" wear or fracture caused by abnormal microstructure of the joint due to excessive hardness difference in the welding area during the service of the line, and give full play to the high strength, high hardness, and high toughness of the pearlite rails containing Cr, Nb, and RE and the joints, ensuring that the rails and joints meet the service requirements of heavy-duty railways and the safety of line operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below in conjunction with the accompanying drawings.

[0027] Figure 1 It is the CCT curve of pearlite rail containing Cr, Nb and RE;

[0028] Figure 2 The test results of the longitudinal section of the joint (test line 1) of Example 1 are as follows;

[0029] Figure 3The test results of the longitudinal section of the joint (test line 1) of Example 2;

[0030] Figure 4 The test results of the longitudinal section of the joint (test line 1) of Example 3. DETAILED DESCRIPTION

[0031] A flash welding and joint heat treatment process for pearlite rails containing Cr, Nb and RE:

[0032] The welded pearlite rail containing Cr, Nb and RE of the present invention has C, Si, Mn, Cr, Nb and RE as main alloy elements, and the weight percentage of the chemical composition of the rail is C: 0.73-0.83%, Si: 0.50-0.80%, Mn: 0.80-1.10%, Cr: 0.30-0.60%, Nb: 0.01-0.03%, RE: 0.0005-0.0020%, P≤0.010%, S≤0.005%, Al:≤0.004%. The rail is produced by smelting, continuous casting, slow cooling of steel billets, rolling, heat treatment and the like, wherein the rolling compression ratio of the rail is ≥11, the final rolling temperature is ≤950°C, and the final cooling temperature is ≤510°C, so as to ensure the grain size, pearlite sheet spacing, tensile strength and hardness of the rail.

[0033] Specific flash welding and joint heat treatment processes include:

[0034] GAAS80 / 580 fixed flash welder is used for rail welding. The oil temperature of the welder is controlled at about 40℃, the water temperature is controlled at about 20℃, and the grid voltage is between 416 and 425V.

[0035] Select two rails of the same material and section (60kg / m or 75kg / m), grind and remove rust from the end faces and the electrode clamping end. The weld seam of the rail should be controlled between 10 and 12 mm, and the arching should be controlled between 0.35 and 0.55 mm.

[0036] The flash welding preheating times are between 11 and 13 times, the secondary preheating current is controlled between 58 and 63KA, the flash welding upsetting amount is controlled between 14 and 16.5mm, the upsetting force is controlled between 515 and 670KN, the welding time is controlled between 110 and 120s, and the final melting speed is controlled between 2.60 and 2.80mm / s.

[0037] The time from flash welding upsetting to embossing should be controlled within 15s. The maximum embossing allowance allowed for the rail head is 2.0mm, the maximum embossing allowance allowed for the rail head jaw and rail waist is 2.0mm, and the maximum embossing allowance allowed for the rail bottom is 1.5mm.

[0038] Air cool the high-temperature welded joint to below 200℃ to ensure that each position of the joint is completely phase-changed. When the joint is ≤280℃, the cooling can be accelerated appropriately. After the joint is cooled, grind the joint weld bar flat, that is, grind the convex allowances at the rail head and rail bottom flat.

[0039] Use a dual-frequency normalizing machine to normalize the welded joint. First, use low-frequency induction heating: use 57-65kW low-frequency power to heat for 70-90s, and heat the joint from 0-200℃ to 810-840℃.

[0040] After the low-frequency induction heating of the joint reaches the target temperature, the low-frequency heating is turned off, and the joint is immediately subjected to high-frequency induction heating. The normalizing machine is adjusted to 68-77kW high-frequency power for heating for 65-80s, and the joint is heated from 810-840℃ to 890-910℃. The total normalizing heat treatment time of the joint is controlled between 135 and 170s.

[0041] When the joint reaches the target temperature of 890-910℃ after normalizing heat treatment, 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 rail head and 50-60mm away from the rail waist. The air jet pressure is 0.34-0.40MPa and the air jet time is 110-130s. The final air jet cooling temperature of the joint should be controlled between 350-430℃, then stop air jet cooling and let the joint cool naturally to room temperature.

[0042] Process comparison during implementation:

[0043] Pearlite rails containing Cr, Nb, and RE have high carbon and alloy contents. Segregation of elements such as C and Mn and microstructural defects are easily generated in the overheating zone during flash welding. Therefore, in the welding process parameter adjustment link, key parameters such as preheating times, top amount, and tail burning rate are adjusted, combined with comparative analysis of joint mechanical properties and drop hammer results, so as to determine the rail flash welding process. The main adjustment parameters of the welding process during implementation and the corresponding joint hardness and drop hammer test results are shown in Table 1.

[0044] Table 1 Comparison results of different welding processes and performance during implementation

[0045]

[0046] As shown in Table 1, the performance test of the joints under different welding processes was carried out at a hammer drop height of 5.2 meters or 6.4 meters: Example 1 Rail Flash Welding Process, the number of broken rails by the hammer drop was 4, and obvious gray spots were found on most fractures, and the number was large, and the gray spot area of ​​some fractures exceeded the standard requirements; Example 2 Rail Flash Welding Process, the number of broken rails by the hammer drop was 2, and obvious gray spots were also found on the hammer drop fractures, and no single gray spot area and total gray spot area exceeded the standard; Example 3 Rail Flash Welding Process, no fracture occurred by the hammer drop, and the joint hardness also met the requirements. Increasing the number of preheating times and increasing the amount of upsetting can squeeze out the oxidation products produced during weld welding and minimize or eliminate gray spots.

[0047] The metallographic grain size of the pearlite rail joint containing Cr, Nb, and RE is relatively coarse in the welded state, 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 comprehensive mechanical properties of the flash welded joint. The CCT curve of the rail is an important basis for formulating the heat treatment process of steel. Combined with the CCT curve of the rail, it shows that the heating temperature of the rail joint reaches 679℃, which can achieve complete austenitization. Figure 1 The comparison scheme of different joint heat treatment processes during the implementation process is shown in Table 2.

[0048] Table 2 Comparison of different heat treatment processes during implementation

[0049]

[0050] The flash welded joints after normalizing treatment in Examples 1 to 3 were subjected to comparative analysis of the Rockwell hardness performance of the longitudinal section of the rail head according to "TB / T1632.2-2014 Rail Welding Part 2: Flash Welding". The comparative analysis results are shown in Tables 3 to 5. Figure 2-Figure 4 .

[0051] Table 3 Test results of joint longitudinal section (test line 1) of Example 1

[0052]

[0053] Table 4 Test results of joint longitudinal section (test line 1) of Example 2

[0054]

[0055]

[0056] Table 5 Test results of joint longitudinal section (test line 1) of Example 3

[0057]

[0058]

[0059] From Table 3-Table 5 and Figure 2-Figure 4 It can be seen that the average hardness ratio of the welded joints in Example 1 and Example 2 to the average hardness of the base material H J / H P The values ​​of the hardness of the joint and the width of the softening zone W are both less than 0.90, and the width of the softening zone W is both greater than 20 mm, which does not meet the standard requirements for heat-treated joints in "TB / T 1632.2-2014 Rail Welding Part 2: Flash Welding". The joint hardness is too low and the width of the softening zone is too wide. During the service of the line, the weld will experience low collapse, poor wear resistance, and horseshoe saddle-shaped joints, which will affect the smoothness of the joint and the safety of train operation. The joint hardness and softening zone width of Example 3 meet the standard requirements, and the joint and the parent material achieve excellent hardness matching.

[0060] According to the sampling method of "TB / T 1632.2-2014 Rail Welding Part 1: General Technical Conditions", the flash welded joints of Example 3 were tested for joint tensile and joint impact properties. The specific results are shown in Tables 5 and 6.

[0061] Table 5 Tensile properties of flash welded joint in Example 3

[0062]

[0063] Table 6 Impact properties of flash welded joints in Example 3

[0064]

[0065]

[0066] It can be seen from Tables 5 and 6 that the tensile properties and impact properties of the flash welded joint of Example 3 meet the standard requirements, and the joint has a large performance margin.

[0067] From the process comparison during the implementation process, it can be seen that the tensile strength of the joint in Example 3 is Rm≥1139MPa, the joint extension A≥12.0%, and the joint impact KU 2 ≥15.7J, the average hardness of the joint is H J Average hardness of base material H P The ratio satisfies: H J / H P ≥0.91, average hardness of joint soft spot H J1 Average hardness of base material H P Ratio: H J1 / H P≥0.82, softening area W≤20mm, joints were not broken after 25 hammer drops, and all performances met the standard requirements. The strength, hardness and toughness of the joints obtained in Example 3 match the performance of the parent material, can meet the requirements of heavy-duty railways for the smoothness of rail joints, improve the risk of "saddle-shaped" wear or joint microscopic abnormal structure caused by excessive hardness difference in the welding area during the service of the rails, give full play to the high strength, high hardness and high toughness of Cr, Nb and RE pearlite rails and joints, and ensure that the rails and joints meet the service requirements of heavy-duty railways and the safety of line operation.

[0068] 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 flash welding and joint heat treatment process for pearlite rails containing Cr, Nb and RE, characterized in that: include: Flash welding process: GAAS80 / 580 fixed flash welder is used for rail welding; the welding machine oil temperature is controlled at about 40℃, the water temperature is controlled at about 20℃, and the grid voltage is between 416 and 425V; The seam of rail welds is controlled between 10 and 12 mm, the arching is controlled between 0.35 and 0.55 mm, the number of flash welding preheating times is between 11 and 13 times, the secondary preheating current is controlled between 58 and 63 KA, the flash welding upsetting amount is controlled between 14 and 16.5 mm, the upsetting force is controlled between 515 and 670 KN, the welding time is controlled between 110 and 120 s, and the final melting speed is controlled between 2.60 and 2.80 mm / s; Air cool the high-temperature welded joint to below 200°C to ensure complete phase change at every position of the joint. When the joint is ≤280°C, accelerate the cooling appropriately. Joint heat treatment process: Use the joint dual-frequency normalizing machine for induction heating. First, perform low-frequency induction heating on the joint: use 57-65kW low-frequency power for heating for 70-90s to heat the joint from 0-200℃ to 810-840℃; use low-frequency heating to ensure that the center of the rail head can be heated to the required temperature; After the joint reaches the target temperature, it is immediately subjected to high-frequency induction heating: the normalizing machine is adjusted to 68-77kW high-frequency power for heating for 65-80s, and the joint is heated from 810-840℃ to 890-910℃; high-frequency heating ensures that the temperature within the range of 0-15mm below the tread of the rail head can be heated to the target temperature, and the total joint heating time is controlled between 135-170s; After the joint reaches the target temperature of 890-910℃, stop heating and immediately perform air jet accelerated cooling on the joint and rail waist. The air jet cooling nozzle is 20-30mm away from the rail head and 50-60mm away from the rail waist. The air jet pressure is 0.34-0.40MPa and the air jet time is 110-130s. The joint air jet final cooling temperature should be controlled between 350-430℃, then stop air jet cooling and let the joint cool naturally to room temperature. The chemical composition of the flash welded rail is as follows: C: 0.73-0.83%, Si: 0.50-0.80%, Mn: 0.80-1.10%, Cr: 0.30-0.60%, Nb: 0.01-0.03%, RE: 0.0005-0.0020%, P≤0.010%, S≤0.005%, Al: ≤0.004%, and the rest is Fe.

2. The flash welding and joint heat treatment process of pearlite rail containing Cr, Nb and RE according to claim 1 is characterized in that: Before welding, the end faces of two rails of the same material and with the same cross-section shall be polished and rust-free, and the electrode clamping ends shall be polished and rust-free.

3. The flash welding and joint heat treatment process of pearlite rail containing Cr, Nb and RE according to claim 1, characterized in that: The rolling compression ratio of the rail should not be less than 9:1, and the final rolling temperature should not be higher than 950℃, so as to ensure the grain size of the original austenite of the rail.

4. The flash welding and joint heat treatment process of pearlite rail containing Cr, Nb and RE according to claim 1 is characterized in that: The tensile strength of the joint is Rm ≥ 1139MPa, the elongation of the joint is A ≥ 12.0%, the impact of the joint is KU2 ≥ 15.7J, and the fatigue load cycle of the joint is 2×10 6 The average hardness of the joint is H J The average hardness of the base material H P The ratio satisfies: H J / H P ≥0.91, average hardness of joint soft spot H J1 Average hardness of base material H P Ratio: H J1 / H P ≥0.82, softening area W≤20mm, the joint was not broken after 25 hammer drops, the weld and heat affected zone are typical pearlite structures, there is no martensite or bainite harmful structure, and all properties meet the standard requirements.

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