A heat treatment method for medium carbon and low manganese rail welded joints using single frequency induction heating

Abstract

A heat treatment method for a medium-carbon low-manganese rail welded joint using single-frequency induction heating is disclosed, comprising: using single-frequency medium-frequency induction to quickly heat the weld and its two sides; immediately after the heating is completed, high-pressure, medium-distance jet cooling is performed on the heated area and the rail head treads on both sides of the heated area, so that the area under the rail head tread is cooled to the final cooling temperature, and the t8 / 5 cooling rate of the weld tread during jet cooling is 5.5-4.5°C / s, and the supercooling temperature is controlled to be 80-120°C; the welded joint is air-cooled to room temperature. The carbon content in the rail base material is 0.55-0.70wt%, and the manganese content is 0.50-0.80wt%. This method can control the longitudinal hardness H of the rail joint. J / H P Reach 0.92~0.95, the width of the left and right softening zones of the joint is ≤15mm. The microstructures of the rail head, rail bottom triangle area and rail bottom foot of the joint are pearlite and trace proeutectoid ferrite, with good service performance.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment after rail welding, and particularly relates to a heat treatment method for a medium-carbon and low-manganese rail welding joint using single-frequency induction heating. Background Art

[0002] The carbon content of pearlite rails reaches 0.68% to 0.82%, the hot-rolled strength ranges from 880 to 1080 MPa, the hardness is 260 to 360 HBW, the heat-treated rail strength is 1080 to 1280 MPa, and the hardness is 320 to 400 HB. The main representative rail grades are U71Mn, U75V, and U78CrV. The carbon content of hypereutectoid rail U95Cr reaches 0.90% to 1.0%, its strength is 1350 MPa, and its hardness is about 420 HBW. This type of rail is mainly used for domestic high-speed railways, mixed passenger and freight transportation, and heavy-load lines. In order to have a certain wear resistance, this type of rail generally has a carbon content of 0.68% to 1.0% and a manganese content of 0.75% to 1.20%. However, when this type of steel is used for rails laid on ultra-long downhill sections in plateaus and high-cold areas, the rail tread is prone to abrasion of the martensitic structure, resulting in the risk of rail breakage. In response to this problem, medium carbon low manganese rails were developed. Due to the special composition design of the rails with carbon content (0.55-0.68%) at the middle limit and Mn content (0.50-0.80%) at the lower limit, this medium carbon low manganese rail is less likely to have a martensitic white layer hardened structure that causes wheel-rail contact scratches on ultra-long downhill sections.

[0003] However, when the existing welded joint heat treatment method and process of a rail whose base material is pearlite rail or other components is used to heat treat the welded joint of such material, the obtained joint generally has a longitudinal hardness of H J / H P =0.85~0.89, the width of the softening zone is 25mm~35mm, and point-like martensite structure often appears in the area 20mm~30mm below the rail head tread, which cannot meet the following requirements specified in TB / T1632.2-2014 standard: H J / H P ≥0.90, the width of the softening zone ≤20mm, and the rail head, rail bottom triangle area and rail bottom foot are pearlite or pearlite plus trace ferrite structure, without martensite and bainite abnormal structure.

[0004] Therefore, there is an urgent need in the art to develop a post-weld heat treatment method for medium carbon low manganese rail welded joints so that the performance of the welded joints can meet the specified requirements. Summary of the invention

[0005] In order to solve at least one of the existing technical problems, the present invention proposes a single-frequency induction heating heat treatment method for medium-carbon low-manganese rail welded joints. The method of the present invention can control the longitudinal hardness H of the rail joint. J / H P Reach 0.92~0.95, the width of the left and right softening zones of the joint is ≤15mm. The microstructure of the rail head, rail bottom triangle area and rail bottom foot of the joint is pearlite and trace proeutectoid ferrite, with good service performance.

[0006] According to the present invention, a single-frequency induction heating heat treatment method for a medium-carbon low-manganese rail welded joint is provided, comprising the following steps:

[0007] Use single-frequency medium-frequency induction to quickly heat the weld and the predetermined length areas on both sides of the weld to a peak temperature of 900℃~960℃;

[0008] After heating is completed, high-pressure and medium-distance jet cooling is immediately performed on the heating zone and the rail head tread within a length of 60mm on both sides of the heating zone, so that the area within a predetermined range under the rail head tread is cooled to the final cooling temperature. During the jet cooling, the cooling rate of the rail joint weld tread t8 / 5 is 5.5℃ / s~4.5℃ / s, the cooling time is 80s~95s, and the supercooling temperature is controlled at 80℃~120℃;

[0009] Air cool the welded joint to room temperature;

[0010] The carbon content of the base material of the medium-carbon low-manganese rail weld joint is 0.55wt% to 0.70w%, and the manganese content is 0.50wt% to 0.80wt%.

[0011] According to one embodiment of the present invention, the single-frequency medium-frequency induction rapid heating includes using a frequency of 1200Hz to 1600Hz and auto-coupling rising changes with rising temperature, a constant power of 120kVA to 150kVA, and a heating time of about 120 to 160s.

[0012] According to one embodiment of the present invention, the base material of the medium carbon low manganese rail weld joint comprises the following components in weight percentage: C content is 0.55% to 0.70%, Si content is 0.30% to 0.60%, Mn content is 0.50% to 0.80%, the sum of Cr, Ni, Cu and V content is 0.30% to 1.30%, P and S content are both lower than 0.025%, and the balance is Fe and unavoidable impurities.

[0013] According to one embodiment of the present invention, the total length of the air jet cooling is controlled at 200 mm.

[0014] According to one embodiment of the present invention, the air jet pressure is controlled to be 0.25MPa-0.35MPa, and the distance between the air jet and the tread is controlled to be 30mm-40mm.

[0015] According to one embodiment of the present invention, the start-up temperature of the air jet cooling is controlled at 830°C to 870°C.

[0016] According to one embodiment of the present invention, the final cooling temperature of the air jet cooling is controlled at 420°C to 470°C.

[0017] According to one embodiment of the present invention, the method further comprises controlling the surface recovery temperature of the joint weld to be 520°C to 550°C.

[0018] According to one embodiment of the present invention, the area within the predetermined range below the rail head tread includes an area within 10 mm below the rail head tread.

[0019] According to one embodiment of the present invention, the predetermined length areas on the left and right sides of the weld are areas of 40 mm on the left and right sides of the weld.

[0020] Due to the adoption of the above technical solution, the present invention has at least one of the following beneficial effects compared with the prior art:

[0021] (1) Medium carbon low manganese rails have low C and Mn contents. The rail flash welded joints are subjected to conventional normalizing air cooling and longitudinal hardness H J / H P ≥0.90 technical index is difficult to achieve. Through the method of the present invention, single-frequency medium-frequency heating normalizing and 80℃~120℃ large undercooling air cooling are adopted to achieve the longitudinal hardness H of the rail joint. J / H P Reach 0.92~0.95, the width of the left and right softening zones of the joint is ≤15mm, meeting the technical indicators of the standard technical requirements of TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding";

[0022] (2) The microstructure of the rail head, rail bottom triangle area and rail bottom foot of the rail welded joint treated by the method of the present invention at the specified inspection positions is pearlite plus a trace amount of proeutectoid ferrite, without abnormal structures such as martensite and bainite, and meets the structural requirements of the standard technical provisions of TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding". BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram showing the longitudinal hardness test results of the welded joint after heat treatment according to Example 1;

[0024] Figure 2 is a graph showing the longitudinal hardness test results of the welded joint after heat treatment according to Example 2;

[0025] Figure 3 This is a diagram showing the longitudinal hardness test results of the welded joint after heat treatment according to Example 3;

[0026] Figure 4 is a graph showing the longitudinal hardness test results of the welded joint after heat treatment according to Example 4;

[0027] Figure 5 This is a metallographic structure test result diagram of the welded joint after heat treatment according to Example 1;

[0028] Figure 6 This is a metallographic structure test result diagram of the welded joint after heat treatment according to Example 2;

[0029] Figure 7 This is a diagram showing the metallographic structure test results of the welded joint after heat treatment according to Example 3;

[0030] Figure 8 This is a diagram showing the metallographic structure test results of the weld joint after heat treatment according to Example 4. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] The present invention provides a single-frequency induction heating heat treatment method for a medium-carbon low-manganese rail welded joint. The method generally comprises a plurality of steps described below.

[0033] Step S1: using single-frequency medium-frequency induction to rapidly heat the weld and the predetermined length regions on both sides of the weld to a peak temperature of 900° C. to 960° C.

[0034] Specifically, during the post-weld heat treatment of the rail flash welded joint, a single-frequency medium-frequency induction coil can be used to rapidly heat the 40 mm length region on the left and right of the weld to a peak temperature of 900° C. to 960° C. The flash welding can be mobile flash welding or fixed flash welding.

[0035] The related operation of medium frequency induction heating is to use a frequency of 1200Hz~1600Hz and auto-couple the rising change with the rising temperature. The power is constant at 120kVA~150kVA, and the heating time is about 120~160s.

[0036] Optionally, in some embodiments of the present invention, the base material of the medium carbon low manganese rail welded joint includes the following components by weight percentage: C content is 0.55% to 0.70%, Si content is 0.30% to 0.60%, Mn content is 0.50% to 0.80%, the sum of Cr, Ni, Cu and V content is 0.30% to 1.30%, P and S content are both lower than 0.025%, and the remainder is Fe and unavoidable impurities. Due to the special composition design that the carbon content (0.55 to 0.70%) of the rail is in the middle limit and the Mn content (0.50 to 0.80%) is in the lower limit, this medium carbon low manganese rail is not prone to the martensitic white layer hardened structure of wheel-rail contact scratches on super-long downhill sections.

[0037] Step S2: Rapid cooling by air spraying.

[0038] Specifically, after the heating is completed, the rail head tread in the heating zone and the rail head tread within a length of 60 mm on both sides outside the heating zone is immediately subjected to high wind pressure and medium distance jet cooling, so that the area within a predetermined range under the rail head tread is cooled to the final cooling temperature. During the jet cooling, the cooling rate of the rail joint weld tread t8 / 5 is 5.5℃ / s~4.5℃ / s, the cooling time is 80s~95s, and the supercooling temperature is controlled to be 80℃~120℃. The supercooling temperature has a great influence on the longitudinal hardness and structure of the rail joint after heat treatment. The C and Mn contents in the medium carbon low manganese rail of the present invention are low, and a large supercooling degree of 80℃~120℃ air cooling is required.

[0039] In some embodiments, the total length of the air jet cooling is controlled at 200 mm. The air jet pressure is controlled to be a high air pressure of 0.25 MPa to 0.35 MPa, and the distance between the air jet and the tread is controlled to be a medium distance of 30 mm to 40 mm. The start cooling temperature of the air jet cooling is controlled at 830°C to 870°C. The final cooling temperature of the air jet cooling is controlled at 420°C to 470°C. The surface recovery temperature of the joint weld is controlled to be 520°C to 550°C.

[0040] Step S3: air-cooling the welded joint to room temperature.

[0041] Specifically, the room temperature may be about 20°C to about 25°C.

[0042] The following is a specific embodiment of the heat treatment method of single-frequency induction heating of medium-carbon low-manganese rail welded joints according to the present invention. Unless otherwise specified, the raw materials, equipment, consumables, etc. used in the following embodiments can be obtained by conventional commercial means.

[0043] For the parts involving numerical ranges, those skilled in the art can select any value within the numerical range defined by the present invention according to actual needs, and are not limited to the numerical values ​​listed in the specific embodiments.

[0044] In the following examples, sampling is carried out according to the TB / T1632.2-2014 "Rail Welding Part 2: Flash Welding" standard. After the longitudinal section hardness specimen is processed, it is corroded with nitric acid to find the weld position. The hardness measurement points are symmetrically arranged on the left and right sides with the weld as the center, and the measurement point spacing is 5mm. Rockwell hardness is tested according to GB / T230.1-2018 "Rockwell hardness test for metallic materials Part 1: Test method".

[0045] Example 1

[0046] The medium carbon low manganese rail base material used in this embodiment includes: C content of 0.70%, Si content of 0.30%, Mn content of 0.80%, the sum of Cr, Ni, Cu and V content of 0.90%, P and S content of less than 0.025%, and the balance of Fe and unavoidable impurities.

[0047] After the above-mentioned medium carbon low manganese rails are flash welded at the rail base, the joints are heated by single-frequency induction heating, and the weld and the area 40mm to the left and right of the weld are heated until the tread temperature reaches a peak temperature of 900℃. After the heating is completed, the rail head tread within the heating zone and the length of 60mm on both sides outside the heating zone (a total of 200mm) is immediately subjected to 0.35MPa high wind pressure and 30mm away from the tread for rapid forced cooling by spraying. The opening temperature is controlled at about 830℃, and the rail head tread is sprayed for cooling, and the spray time is 80s, so that the area within 10mm below the rail head tread is quickly cooled to the final cooling temperature of 440℃, and then the spray cooling is immediately stopped and air cooled to room temperature. The cooling rate of the t8 / 5 of the rail joint weld tread in the spray cooling section reaches 5.5℃ / s, and the supercooling temperature reaches 120℃, ensuring that the surface temperature of the joint weld rises to 550℃.

[0048] like Figure 1 As shown, the above method can control the longitudinal hardness H of the rail joint J / H P Reach 0.95, H J1 / H P =0.84, the width of the softening zone on the left and right sides of the joint is ≤10mm. Figure 5 As shown, the microstructure of the specified inspection positions of the rail head, rail bottom triangle area and rail bottom foot of the joint is pearlite + trace proeutectoid ferrite, without abnormal structures such as martensite and bainite.

[0049] Example 2

[0050] The medium carbon low manganese rail base material used in this embodiment includes: C content of 0.55%, Si content of 0.60%, Mn content of 0.50%, the sum of Cr, Ni, Cu and V content of 0.30%, P and S content of less than 0.020%, and the balance of Fe and unavoidable impurities.

[0051] After the above-mentioned medium carbon low manganese rails are flash welded at the rail base, the joints are heated by single-frequency induction heating, and the weld and the area 40mm to the left and right of the weld are heated until the tread temperature reaches a peak temperature of 920℃. After the heating is completed, the rail head tread within the heating zone and the length of 60mm on both sides outside the heating zone (a total of 200mm) is immediately subjected to 0.30MPa high wind pressure and 40mm away from the tread for rapid forced cooling. The opening temperature is controlled at about 830℃, and the rail head tread is sprayed for cooling, and the spray time is 87s, so that the area within 10mm below the rail head tread is quickly cooled to the final cooling temperature of 450℃, and then the spray cooling is immediately stopped and air cooled to room temperature. The cooling rate of the tread t8 / 5 of the rail joint weld in the spray cooling section reaches 5.0℃ / s, and the supercooling temperature reaches 100℃, ensuring that the surface temperature of the joint weld rises to 550℃.

[0052] like Figure 2 As shown, the above method can control the longitudinal hardness H of the rail joint J / H P Reach 0.94, H J1 / H P =0.85, the width of the softening zone on the left and right sides of the joint is ≤10mm. Figure 6 As shown, the microstructure of the specified inspection positions of the rail head, rail bottom triangle area and rail bottom foot of the joint is pearlite + trace proeutectoid ferrite, without abnormal structures such as martensite and bainite.

[0053] Example 3

[0054] The medium carbon low manganese rail base material used in this embodiment includes: C content of 0.65%, Si content of 0.50%, Mn content of 0.70%, the sum of Cr, Ni, Cu and V content of 0.60%, P and S content of less than 0.020%, and the balance of Fe and unavoidable impurities.

[0055] After the above-mentioned medium carbon low manganese rails are flash welded at the rail base, the joints are heated by single-frequency induction heating, and the weld and the area 40mm to the left and right of the weld are heated until the tread temperature reaches a peak temperature of 940℃. After the heating is completed, the rail head tread within the heating zone and the length of 60mm on both sides outside the heating zone (a total of 200mm) is immediately subjected to 0.28MPa high wind pressure and 35mm away from the tread for rapid forced cooling by spraying. The opening temperature is controlled at about 850℃, and the rail head tread is sprayed for cooling for 90s, so that the area within 10mm below the rail head tread is quickly cooled to the final cooling temperature of 440℃, and then the spray cooling is immediately stopped and air cooled to room temperature. The cooling rate of the t8 / 5 of the rail joint weld tread in the spray cooling section reaches 5.0℃ / s, and the supercooling temperature reaches 100℃, ensuring that the surface temperature of the joint weld rises to 540℃.

[0056] like Figure 3 As shown, the above method can control the longitudinal hardness H of the rail joint J / H P Reach 0.93, H J1 / H P =0.83, the width of the softening zone on the left and right sides of the joint is ≤10mm. Figure 7 As shown in the figure, the microstructure of the rail head, rail bottom triangle area and rail bottom foot of the joint is pearlite + trace proeutectoid ferrite, which has good service performance.

[0057] Example 4

[0058] The medium carbon low manganese rail base material used in this embodiment includes: C content of 0.60%, Si content of 0.40%, Mn content of 0.60%, the sum of Cr, Ni, Cu and V content of 1.30%, P and S content of less than 0.025%, and the balance of Fe and unavoidable impurities.

[0059] After the above-mentioned medium carbon low manganese rails are flash welded at the rail base, the joints are heated by single-frequency induction heating, and the weld and the area 40mm to the left and right of the weld are heated until the tread temperature reaches a peak temperature of 960℃. After the heating is completed, the rail head tread within the heating zone and the length of 60mm on both sides outside the heating zone (a total of 200mm) is immediately subjected to 0.25MPa high wind pressure and 30mm away from the tread for rapid forced cooling. The opening temperature is controlled at about 850℃, and the rail head tread is sprayed for cooling, and the spray time is 90s, so that the area within 10mm below the rail head tread is quickly cooled to the final cooling temperature of 470℃, and then the spray cooling is immediately stopped and air cooled to room temperature. The cooling rate of the t8 / 5 of the rail joint weld tread in the spray cooling section reaches 5.0℃ / s, and the supercooling temperature reaches 80℃, ensuring that the surface temperature of the joint weld rises at 550℃.

[0060] like Figure 4 As shown, the above method can control the longitudinal hardness H of the rail joint J / H P Reach 0.93, H J1 / H P =0.85, the width of the softening zone on the left and right sides of the joint is ≤15mm. Figure 8 As shown in the figure, the microstructure of the rail head, rail bottom triangle area and rail bottom foot of the joint is pearlite + trace proeutectoid ferrite, which has good service performance.

[0061] The above embodiments only express the implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the attached claims.

Claims

1. A heat treatment method for medium carbon low manganese rail welded joints by single frequency induction heating, characterized in that: The following steps are involved: Use single-frequency medium-frequency induction to quickly heat the weld and the 40mm area on both sides of the weld to a peak temperature of 900℃~960℃; After heating is completed, high-pressure and medium-distance jet cooling is immediately performed on the heating zone and the rail head tread within a length of 60mm on both sides of the heating zone. The distance between the air jet and the tread is controlled to be 30mm to 40mm, so that the area within 10mm below the rail head tread is cooled to a final cooling temperature of 420℃ to 470℃. During the jet cooling, the cooling rate of the rail joint weld tread t8 / 5 is 5.5℃ / s to 4.5℃ / s, the cooling time is 80s to 95s, and the supercooling temperature is controlled to be 80℃ to 120℃; Air cool the welded joint to room temperature; The carbon content of the base material of the medium-carbon low-manganese rail weld joint is 0.55wt% to 0.70w%, and the manganese content is 0.50wt% to 0.80wt%.

2. The method according to claim 1, characterized in that The single-frequency medium-frequency induction rapid heating includes adopting a frequency of 1200 Hz to 1600 Hz and auto-coupling rising changes with temperature rising, a power constant at 120 kVA to 150 kVA, and a heating time of 120 to 160 seconds.

3. The method according to claim 1, characterized in that: The base material of the medium carbon low manganese rail weld joint comprises the following components by weight percentage: C content is 0.55% to 0.70%, Si content is 0.30% to 0.60%, Mn content is 0.50% to 0.80%, the sum of Cr, Ni, Cu and V content is 0.30% to 1.30%, P and S content are both lower than 0.025%, and the balance is Fe and unavoidable impurities.

4. The method according to claim 1, characterized in that: The total length of the air jet cooling is controlled at 200mm.

5. The method according to claim 1, characterized in that The air spray pressure is controlled at 0.25MPa~0.35MPa.

6. The method according to claim 1, characterized in that The cooling temperature of the air jet cooling is controlled at 830°C to 870°C.

7. The method according to claim 1, characterized in that The method further includes controlling the surface recovery temperature of the joint weld to be 520°C to 550°C.

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