AU2019204691A1 - Heat treatment method for rail flash butt welding joint - Google Patents
Heat treatment method for rail flash butt welding joint Download PDFInfo
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- AU2019204691A1 AU2019204691A1 AU2019204691A AU2019204691A AU2019204691A1 AU 2019204691 A1 AU2019204691 A1 AU 2019204691A1 AU 2019204691 A AU2019204691 A AU 2019204691A AU 2019204691 A AU2019204691 A AU 2019204691A AU 2019204691 A1 AU2019204691 A1 AU 2019204691A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
- C21D9/505—Cooling thereof
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
Abstract
Abstract The invention discloses a heat treatment method for a rail flash butt welding joint, and belongs to the technical field of rail welding. In order to solve technical problems of "saddle" profile wear and early fatigue fracture of the welded joint caused by low hardness of a welding area or abnormal microstructure of the joint in the service process of a rail, the invention provides a heat treatment method for a rail flash butt welding joint, wherein the welded joint enters the first cooling stage, the second cooling stage and the third cooling stage sequentially, and the cooling speed and temperature of each stage are controlled to remove any abnormal structure such as martensite and bainite from metallographic structure of the welded joint, so as to improve the "saddle" profile wear and early fatigue fracture mentioned above, and ensure safe operation of railway lines. * Eutectoid pearlite rail 600 -a---Hypereutectoid rail - Softening zone measuring line of eutectoid pearlite rail - Softening zone measuring line of hyperetectoid rail p50 I -40 -30 -20 -10 0 10 20 30 40 Distance from center of weld seam/mm Fig. 1 Euteetoid pearlite rail -a- Hypereutectoid rail - Softening zone measuring line of eutectoid pearlite rail - Softening zone measuring line of hypereutectoid rail 45\ -40 -30 -20 -10 0 10 20 30 40 Distance from center of weld seam/mm Fig. 2 ,-Eutectoid pearlite rail r_00 -A- Hypereutectoid rail -Softening zone measuring line of eutectoid pearlite rail -4Softening zone measuringline of hypereutectoid rail -40 -30 -20 -10 0 10 20 30 40 Distance from center of weld seam/mm Fig. 3
Description
Field of the Invention
The invention belongs to the technical field of rail welding, in particular to a heat treatment method for a rail flash butt welding joint.
Background of the Invention
Eutectoid pearlite rails are mostly used in domestic and overseas heavy haul railway lines. As the carbon content is usually in the range of 0.72-0.82% by weight, and the metallographic structure is pearlite, such rails have the features of strong toughness, good compatibility and moderate comprehensive mechanical performance index. With the rapid development of the railway, the heavy haul railway line with large axle load is imposed with stricter requirements on the rail service performance; the comprehensive mechanical property and welding property of the traditional pearlite rails have almost reached the limit. In this case, hypereutectoid rails with higher strength grade, good wear resistance, contact fatigue and other comprehensive properties are manufactured; their carbon content is usually in the range of 0.90-1.10% by weight, and the metallographic structure is composed of pearlite and a small amount of proeutectoid cementite. The rail flash butt welding joint has become the in-service welding technology of mainstream rails at the domestic and overseas railway construction sites at this stage. For two kinds of rails with different strength grades and materials, the difference in the performance of the base metal brings great challenges to their welding. Due to the welding thermal cycle, the hardened layer in the welding area of rails disappears and a low hardness area with a larger width is formed on both sides of the weld seam, resulting in the hardness of the weld seam and the heat affected area being lower than that of the rail base metal. In the service process of railway line, the rail head tread of welded joints is intended to preferentially form a saddle profile, which will increase the wheel-rail impact, seriously affect the service life of the rail, and even endanger the safe operation of railway line. Therefore, how to restore the mechanical properties of the rail reduced by welding is the premise for the rail to be applied.
Few reports and literatures on the welding and post-welding heat treatment process of hypereutectoid rail and eutectoid rail are available now. The post-weld heat treatment method for hypereutectoid rail and PG4 heat treatment eutectoid pearlite welded joint disclosed in
CN201610909362.1, comprises the following steps: the welded joint of rail is cooled to below
400°C in the first stage, then heated to 860-930°C, and cooled in the second stage until the tread of the welded joint reaches 410-450°C. The dissimilar welded joints obtained by the method can meet the requirements for fatigue test, tensile test, impact test and static bending tests in the existing i
2019204691 01 Jul 2019 domestic railway industry standard TB/T1632.2-2014 Welding of Rails Part 2: Flash Butt Welding.
However, the above invention relates to rail post-weld normalizing heat treatment, the rail post-weld heat treatment equipment is required to locally heat welded joints, which is complicated in operation and implementation process and has high cost.
A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Summary of the Invention
Embodiments of the invention may provide a heat treatment method for a rail flash built welding joint to solve technical problems of saddle profile wear and early fatigue fracture of the welded joint caused by low hardness of a welding area or abnormal microstructure of the joint in the service process of a rail.
According to one aspect of the invention, there is provided a heat treatment method for a rail flash butt welding joint, comprising the following steps: a welded joint is cooled in the first stage at a first cooling speed to reduce the surface temperature of the rail head thereof to 650-720°C; secondly, the welded joint is cooled in the second stage at a second cooling speed to reduce the surface temperature of the rail head thereof to 350-410°C; and finally, the welded joint is cooled in the third stage at a third cooling speed to reduce the surface temperature of the rail head thereof to 10-30°C.
Optionally, in the heat treatment method for the rail flash butt welding joint, the welded joint is a dissimilar welded joint welded from a hypereutectoid rail and a eutectoid rail with the a same rail profile.
Preferably, in the post-weld heat treatment method of the welded joints welded from hypereutectoid rails and eutectoid rails, the profile of the hypereutectoid rails is 60 - 75kg/m and that of eutectoid rails is 60 - 75kg/m.
Optionally, in the heat treatment method for the rail flash butt welding joint, the initial temperature of the welded joint is 1000-1400°C.
Optionally, in the heat treatment method for the rail flash butt welding joint, the cooling method of the first stage is natural cooling in air.
Optionally, in the heat treatment method for the rail flash butt welding joint, the first cooling speed is 5.0-9.0°C/s.
Optionally, in the heat treatment method for the rail flash butt welding joint, a rail head
2019204691 01 Jul 2019 profiling cooling device is used in the second cooling stage, compressed air or water-mist mixture is taken as a cooling medium to cool the tread and side of the rail head of the welded joint, the distance between the cooling device and the rail head tread is 20-50 mm, and the pressure of compressed air or water-mist mixture sprayed by the cooling device is 0.40-0.80 MPa.
In the heat treatment method for the rail flash butt welding joint, the second cooling speed may be 1.5-2.5°C/s.
Optionally, in the heat treatment method for the rail flash butt welding joint, a rail head profiling cooling device is used in the third cooling stage, compressed air or water-mist mixture is taken as a cooling medium to cool the tread and side of the rail head of the welded joint, the distance between the cooling device and the rail head tread is 20-50 mm, and the pressure of compressed air or water-mist mixture sprayed by the cooling device is 0.05-0.25 MPa.
In the heat treatment method for the rail flash butt welding joint, the third cooling speed may be 0.05-0.50°C/s.
In the heat treatment method for the rail flash butt welding joint, the welded joint may be welded from mobile rail flash butt welder.
The beneficial effects of the invention are may include:
The method of the invention can improve the saddle wear of rail joints caused by the low hardness of the welding area in the service process of the rail, ensure that the longitudinal hardness of rail joints within ±10mm area from the center of weld seam is within ±30HV of the average hardness of the corresponding base metal of eutectoid pearlite rail and hypereutectoid rail (excluding the center line of decarburized weld seam. Due to the high welding temperature of the rail, the center of weld seam is decarburized and has melting loss of elements, resulting in low hardness), with the width of the softening area on both sides of the weld seam of joint lower than 15mm, and ensure that abnormal structures such as martensite and bainite are removed from the metallurgical structure of welded joints, which is beneficial to the safe operation of railway line.
Brief Description of the Drawings
Fig. 1 is a longitudinal hardness effect diagram of the welded joint of hypereutectoid rail and eutectoid pearlite rail at a position 3-5mm below the rail head tread under the post-weld heat treatment condition obtained by the method in example 1.
Fig. 2 is a longitudinal hardness effect diagram of the welded joint of hypereutectoid rail and eutectoid pearlite rail at a position 3-5mm below the rail head tread under the post-weld heat
2019204691 01 Jul 2019 treatment condition obtained by the method in example 2.
Fig. 3 is a longitudinal hardness effect diagram of the welded joint of hypereutectoid rail and eutectoid pearlite rail at a position 3-5mm below the rail head tread under the post-weld air cooling condition obtained by the method in comparative example 1.
Fig. 4 is a longitudinal hardness effect diagram of the welded joint of hypereutectoid rail and eutectoid pearlite rail at a position 3-5mm below the rail head tread under the post-weld heat treatment condition obtained by the method in comparative example 2.
Fig. 5 is a longitudinal hardness effect diagram of the welded joint of hypereutectoid rail and eutectoid pearlite rail at a position 3-5mm below the rail head tread under the post-weld heat treatment condition obtained by the method in comparative example 3.
Fig. 6 is a schematic diagram of longitudinal hardness tested at a position 3-5mm below the rail head tread of the welded joint of the invention.
Fig. 7 is a schematic diagram of the sampling position of the metallographic specimen of the rail head tread of the rail joint of the invention.
Detailed Description of the Preferred Embodiments
The heat treatment method for a rail flash butt welding joint comprises the following steps: a welded joint of rail is cooled in the first stage at a first cooling speed to reduce the surface temperature of the rail head thereof to 650-720°C; secondly, the welded joint is cooled in the second stage at a second cooling speed to reduce the surface temperature of the rail head thereof to 350-410°C; and finally, the welded joint is cooled in the third stage at a third cooling speed to reduce the surface temperature of the rail head thereof to 10-30°C.
The critical cooling speed of martensitic transformation during continuous cooling transformation of eutectoid pearlite rail steel is approximately 0.7-1.5°C/s; whereas, the critical cooling speed of martensitic transformation during continuous cooling transformation of hypereutectoid rail steel is approximately 1.8-3.0°C/s. The Ms temperature of eutectoid pearlite rail (starting temperature of forming martensite structure ) is approximately 250°C; whereas, that of hypereutectoid rail is approximately 190°C. In order to avoid abnormal structures such as martensite in the welded joints, when the welded joints of hypereutectoid rail and eutectoid rail are subjected to post-weld heat treatment, the final cooling temperature in the rapid cooling process of post-weld heat treatment shall be controlled above the Ms temperature of eutectoid pearlite rail. Meanwhile, the cooling speed in the post-weld heat treatment process must be limited to eutectoid
2019204691 01 Jul 2019 pearlite rail steel with relatively low critical cooling speed; otherwise, the joints will suffer premature fatigue fracture due to the hardened martensite structure.
In view of this, the welding standard of rail, for example, Railway Track Material - Welding of Steel Rail (AS1085.20-2012), specifies as follows: when such rails with high strength grade, high carbon content and high alloy content are observed under a lOOx metallographic microscope, the percentage of martensite structure in the area of the welded joint where the maximum martensite is found shall be not more than 5%; otherwise, the joint may be subjected to premature fatigue fracture due to a large amount of hardened martensite structure, which will seriously affect the safe operation of railway line. It is thus clear that, the strict control of the martensite content in the welded structure of rail is greatly significant for stable operation of railway line. In addition, there is a small amount of secondary cementite in the base metal of hypereutectoid rail. In order to prevent secondary cementite from reticular intergranular precipitation during rail welding, the joint must be rapidly cooled before austenite-to-pearlite transformation begins. Based on the above findings, the inventors have completed the invention.
In the invention, the post-welding heat treatment process of the rail is realized by its welding waste heat. The rail joints with high residual temperature (1000-1400°C) are subjected to the post-weld accelerated cooling to reduce the phase-transition temperature of austenite-to-pearlite transformation, and improve the hardness of austenite recrystallization zone. Based on the principles of metallography, the rail joint has a certain degree of dynamic supercooling under the post-weld high temperature and rapid cooling condition, so that the phase-transition temperature of austenite-to-pearlite transformation moves down in the non-equilibrium state, and the phase-transition temperature gradually decreases with the increase in degree of supercooling. As a result, the joint rail head may still be structurally transformed from the austenite to pearlite, even if it is cooled at the second stage where the initial cooling temperature is relatively low.
In the invention, the first cooling stage is natural cooling in air (or at room temperature) at the cooling speed of 5.0-9.0°C/s, which can be controlled by adjusting the test environment temperature (e.g. central air conditioning); and the final cooling temperature of the first cooling stage of the welded joint can be controlled at 650-720°C by setting the welder or manual operation. The initial cooling temperature of the second cooling stage is 650-720°C. It should be noted that, in the invention, the final cooling temperature of the second cooling stage is higher than the initial temperature of martensitic transformation (Ms temperature) of hypereutectoid rail steel and
2019204691 01 Jul 2019 eutectoid pearlite rail steel; the final cooling temperature of the second cooling stage is 350-4 KFC, and the cooling speed is 1.5-2.5°C. In the third stage, the rail joint is slowly cooled to room temperature (10-30°C) at the cooling speed of 0.05-0.50°C/s, which is lower than the critical cooling speed of martensitic transformation of eutectoid pearlite rail steel, so as to avoid any hardened martensitic structure. Based on the three-step cooling method, the invention can limit the cooling speed and cooling temperature at different cooling stages, improve the hardness of the welded joint, and eliminate abnormal structures such as martensite and bainite in the welded joint.
In the method of the invention, a rail head profiling cooling device is used in the second cooling stage and the third cooling stage, compressed air or water-mist mixture is taken as a cooling medium to cool the tread and side of the rail head of the welded joint, and the distance between the cooling device and the rail head tread is 20-50 mm. The pressure of compressed air or water-mist mixture sprayed by the cooling device is 0.40-0.80 MPa at the second cooling stage, and this value is 0.05-0.25MPa at the third cooling stage.
In the invention, the hypereutectoid rail and the eutectoid rail used for welding are of the same specification, in particular, 60-75 kg/m. The welded joint is a dissimilar welded joint welded from a mobile rail flash butt welder by adopting the same welding procedure.
In the invention, unless otherwise specified, the welded joint is an area with a length ranging from 70 mm to 100 mm, including a weld seam and/or a heat-affected zone obtained by welding, and the center of the area is a weld seam of a rail.
In the invention, an infrared thermometer is adopted to collect a temperature signal of a rail head tread, wherein the rail head tread is a contact part between a wheel and a rail; the hardness value corresponding to the softening zone width measuring line in the longitudinal hardness curve of the rail joint indicates the hardness after the average hardness of the rail base metal is deducted by 25HV; and the width of softening zone in the hardness curve is an intercept between the hardness curve and the softening zone width measuring line.
The invention will be further described in detail in combination with examples, but the protection scope of the invention is not limited to the scope of the described examples.
In the examples and the comparative examples of the invention, during the hardness test of the welded joint, the sampling position of a longitudinal section hardened sample of a welded joint is shown in Fig. 6. When the hardness value of the longitudinal section of the welded joint is tested, the measuring points are symmetrically arranged at the left and right sides with the weld seam as
2019204691 01 Jul 2019 the center, and the distance between the measuring points is 2mm. Longitudinal section of the welded joint shall be tested for Vickers hardness according to GB/T 4340.1-2009, with a test force value of 294.2 N. The recrystallization zone is ±10mm away from the center of weld seam. The hardness test shall cover the whole area of the welded joint, including the base metal, heat affected zone and weld seam, and an area extending to 20mm of the rail base metal on each side of the welded joint.
Example 1
After the 60kg/m rail was completely upset and knob-removed in the mobile flash butt welding process, the welded joint was subjected to post-weld heat treatment. Firstly, the rail joint with a residual temperature of 1100°C obtained by welding was cooled in the first stage at the first cooling speed of 7.0°C/s to reduce the surface temperature of the rail head to 660°C; secondly, the rail joint was cooled in the second stage at the second cooling speed of 2.0°C/s to reduce the surface temperature of the rail head to 400°C; and finally, the rail joint was cooled in the third stage at the third cooling speed of 0.4°C/s to reduce the surface temperature of the rail head to room temperature of 25°C, so as to obtain the post-weld heat-treated rail joint of the invention.
In the post-weld heat treatment process, the first cooling was natural cooling in air; whereas, in the second cooling process and the third cooling process, a rail head profiling cooling device was used to cool the tread and side of the rail head of the welded joint with compressed air as a cooling medium, and the distance between the cooling device and the rail head tread was 42mm. The pressure of compressed air sprayed by the cooling device was 0.62MPa in the second cooling process and 0.20MPa in the third cooling process. An infrared thermometer was used to monitor the tread temperature of rail head.
The post-weld heat-treated rail joint obtained in this example was machined into a longitudinal hardened sample. The Vickers hardness tester (Shandong Laizhou Testing Instrument Factory, Model: HBV-30A) was used to test the longitudinal Vickers hardness of the hardened sample at a position 4mm below the rail head tread at an interval of 2 mm between measuring points. The measuring points were symmetrically arranged on the left and right sides with the weld seam as the center. The Vickers hardness test method is as described in GB/T 4340.1-2009 Metallic Materials-Vickers Hardness Test-. Part 1: Test Method, and an HV scale shall be used. The hardness test data is shown in Table 1, and the longitudinal hardness distribution effect of joints is shown in Fig. 1.
2019204691 01 Jul 2019
Table 1 Hardness test data of welded joint in example 1 Distance from center of weld seam/mm
| Le ft | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 372 | 462 | 461 | 460 | 451 | 433 | 350 | 320 | 392 | 404 | 420 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 432 | 439 | 440 | 438 | 441 | 441 | 439 | 440 | 442 | 441 | / | |
| Ri ght | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 372 | 456 | 458 | 461 | 455 | 428 | 325 | 342 | 422 | 435 | 451 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 452 | 450 | 452 | 454 | 454 | 452 | 453 | 455 | 456 | 455 | / |
As can be seen from Table 1 and Fig. 1, when the welded joints are treated by the invention, the longitudinal hardness of rail joints within ±10mm area from the center of weld seam is respectively within ±30HV of the average hardness of the corresponding base metal of eutectoid pearlite rail and hypereutectoid rail (excluding the center line of decarburized weld seam. Due to the high welding temperature of the rail, the center of weld seam is decarburized and has melting loss of elements, resulting in low hardness). The width of the softening zone on the eutectoid pearlite rail side of the joint weld seam is 10mm, the width of the softening zone on the
2019204691 01 Jul 2019 hypereutectoid rail side is 6mm, and the width of the softening zone on both sides of the joint weld is less than 15 mm.
Based on the sampling method shown in Fig. 7, the metallographic specimen of rail joint was tested for metallographic structure according to GB/T13298-2015 Inspection Methods of Microstructure for Metals. Specifically, the metallographic specimen of rail joint was etched with 3% nital, and the metallographic structure of rail joint was observed under a German Leica MeF3 optical microscope. The results show that there are not abnormal structures such as martensite and bainite in the metallographic structure of rail joint.
Example 2
After the 68kg/m rail was completely upset and knob-removed in the mobile flash butt welding process, the welded joint was subjected to post-weld heat treatment. Firstly, the rail joint with a residual temperature of 1000°C obtained by welding was cooled in the first stage at the first cooling speed of 6.5°C/s to reduce the surface temperature of the rail head to 680°C; secondly, the rail joint was cooled in the second stage at the second cooling speed of 2.2°C/s to reduce the surface temperature of the rail head to 380°C; and finally, the rail joint was cooled in the third stage at the third cooling speed of 0.10°C/s to reduce the surface temperature of the rail head to room temperature of 25°C, so as to obtain the post-weld heat-treated dissimilar rail joint of the invention.
In the post-weld heat treatment process, the first cooling was natural cooling in air; whereas, in the second cooling process and the third cooling process, a rail head profiling cooling device was used to cool the tread and side of the rail head of the welded joint with water-mist mixture as a cooling medium, and the distance between the cooling device and the rail head tread was 40mm. The pressure of water-mist mixture sprayed by the cooling device was 0.65MPa in the second cooling process and 0.09MPa in the third cooling process. An infrared thermometer was used to monitor the tread temperature of rail head.
The post-weld heat-treated rail joint obtained in this example was machined into a longitudinal hardened sample. The Vickers hardness tester (Shandong Laizhou Testing Instrument Factory,
Model: HBV-30A) was used to test the longitudinal Vickers hardness of the hardened sample at a position 4mm below the rail head tread at an interval of 2 mm between measuring points. The measuring points were symmetrically arranged on the left and right sides with the weld seam as the center. The Vickers hardness test method is as described in GB/T 4340.1-2009 Metallic
Materials-Vickers Hardness Test-. Part 1: Test Method, and an HV scale shall be used. The hardness
2019204691 01 Jul 2019 test data is shown in Table 2, and the longitudinal hardness distribution effect of joints is shown in
Fig. 2.
Table 2 Hardness test data of welded joint in example 2 Distance from center of weld seam/mm
| Le ft | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 380 | 466 | 465 | 463 | 458 | 438 | 359 | 332 | 397 | 411 | 422 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 436 | 437 | 442 | 442 | 438 | 441 | 440 | 441 | 439 | 440 | / | |
| Ri ght | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 380 | 464 | 460 | 461 | 466 | 432 | 330 | 355 | 422 | 438 | 451 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 455 | 454 | 454 | 452 | 453 | 455 | 451 | 452 | 455 | 452 | / |
As can be seen from Table 2 and Fig. 2, when the welded joints are treated by the invention, the longitudinal hardness of rail joints within ±10mm area from the center of weld seam is respectively within ±30HV of the average hardness of the corresponding base metal of eutectoid pearlite rail and hypereutectoid rail (excluding the center line of decarburized weld seam. Due to the high welding temperature of the rail, the center of weld seam is decarburized and has melting io
2019204691 01 Jul 2019 loss of elements, resulting in low hardness). The width of the softening zone on the eutectoid pearlite rail side of the joint weld seam is 8mm, the width of the softening zone on the hypereutectoid rail side is 6mm, and the width of the softening zone on both sides of the joint weld is less than 15 mm.
Based on the sampling method shown in Fig. 7, the metallographic specimen of rail joint was tested for metallographic structure according to GB/T13298-2015 Inspection Methods of Microstructure for Metals. Specifically, the metallographic specimen of rail joint was etched with 3% nital, and the metallographic structure of rail joint was observed under a German Leica MeF3 optical microscope. The results show that there are not abnormal structures such as martensite and bainite in the metallographic structure of rail joint.
Comparative example 1
After the 68kg/m rail was completely upset and knob-removed in the mobile flash butt welding process, the rail joint with residual temperature of 1100°C was directly air-cooled to room temperature (about 25°C), thus obtaining the welded joint under the air cooling (natural cooling) condition.
The rail joints obtained under the post-weld air cooling condition in this comparative example were machined into longitudinal hardened samples. The Vickers hardness tester (Shandong Laizhou Testing Instrument Factory, Model: HBV-30A) was used to test the longitudinal Vickers hardness of the hardened sample at a position 5mm below the rail head tread at an interval of 2 mm between measuring points. The measuring points were symmetrically arranged on the left and right sides with the weld seam as the center. The Vickers hardness test method is as described in GB/T 4340.1-2009 Metallic Materials-Vickers Hardness Test-. Part 1: Test Method, and an HV scale shall be used. The hardness test data is shown in Table 3, and the longitudinal hardness distribution effect of joints is shown in Fig. 3.
Table 3 Hardness test data of welded joint in comparative example 1
| Distance from center of weld seam/mm | ||||||||||||
| Le ft | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne | 370 | 400 | 403 | 405 | 395 | 343 | 322 | 330 | 408 | 421 | 426 |
2019204691 01 Jul 2019
| ss/HV | ||||||||||||
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 430 | 436 | 437 | 439 | 439 | 440 | 440 | 442 | 441 | 441 | / | |
| Ri ght | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 370 | 403 | 400 | 390 | 380 | 320 | 300 | 390 | 420 | 432 | 438 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 445 | 453 | 455 | 451 | 454 | 453 | 452 | 453 | 454 | 456 | / |
As can be seen from Table 3 and Fig. 3, when the welded joints are not treated by the post-weld heat treatment method provided by the invention, the entire welded area is in a softened state, compared with the hardness of the rail base metal on both sides of the weld seam. The longitudinal hardness of rail joints within ±10mm area from the center of weld seam is not within ±30HV of the average hardness of the corresponding base metal of eutectoid pearlite rail and hypereutectoid rail (excluding the center line of decarburized weld seam. Due to the high welding temperature of the rail, the center of weld seam is decarburized and has melting loss of elements, resulting in low hardness). The width of the softening zone on the eutectoid pearlite rail side of the joint weld seam is 17mm, and this value on the hypereutectoid rail side is 18mm. The welded joints obtained by this comparative example are easy to form collapse of the rail head tread in the softening area on one side of the joint weld seam over the eutectoid steel rail in the service process of the railway line, thus affecting the smoothness of the railway line and driving safety.
Based on the sampling method shown in Fig. 7, the metallographic specimen of rail joint was tested for metallographic structure according to GB/T13298-2015 Inspection Methods of
2019204691 01 Jul 2019
Microstructure for Metals. Specifically, the metallographic specimen of rail joint was etched with 3% nital, and the metallographic structure of rail joint was observed under a German Leica MeF3 optical microscope. The results show that the metallographic structure of rail joints is normal and free from structures such as martensite and bainite.
Comparative example 2
After the 75kg/m rail was completely upset and knob-removed in the mobile flash butt welding process, the welded joint was subjected to post-weld heat treatment. Firstly, the rail joint with a residual temperature of 1050°C obtained by welding was cooled in the first stage at the first cooling speed of 6.0°C/s to reduce the surface temperature of the rail head to 680°C; secondly, the rail joint was cooled in the second stage at the second cooling speed of 2.8°C/s to reduce the surface temperature of the rail head to 220°C; and finally, the rail joint was cooled in the third stage at the third cooling speed of 0.10°C/s to reduce the surface temperature of the rail head to room temperature of 25°C, so as to obtain the post-weld heat-treated dissimilar rail joint of the comparative example.
In the post-weld heat treatment process, the first cooling was natural cooling in air; whereas, in the second cooling process and the third cooling process, a rail head profiling cooling device was used to cool the tread and side of the rail head of the welded joint with compressed air as a cooling medium, and the distance between the cooling device and the rail head tread was 45mm. The pressure of compressed air sprayed by the cooling device was 0.90MPa in the second cooling process and 0.13MPa in the third cooling process. An infrared thermometer was used to monitor the tread temperature of rail head.
The rail joints obtained under the post-weld air cooling condition in this comparative example were machined into longitudinal hardened samples. The Vickers hardness tester (Shandong Laizhou Testing Instrument Factory, Model: HBV-30A) was used to test the longitudinal Vickers hardness of the hardened sample at a position 4mm below the rail head tread at an interval of 2 mm between measuring points. The measuring points were symmetrically arranged on the left and right sides with the weld seam as the center. The Vickers hardness test method is as described in GB/T 4340.1-2009 Metallic Materials-Vickers Hardness Test-. Part 1: Test Method, and an HV scale shall be used. The hardness test data is shown in Table 4, and the longitudinal hardness distribution effect of joints is shown in Fig. 4.
2019204691 01 Jul 2019
Table 4 Hardness test data of welded joint in comparative example 2 Distance from center of weld seam/mm
| Le ft | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 372 | 476 | 463 | 456 | 421 | 328 | 320 | 376 | 391 | 405 | 422 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 432 | 439 | 440 | 439 | 440 | 442 | 439 | 442 | 440 | 441 | / | |
| Ri ght | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 372 | 441 | 444 | 440 | 450 | 421 | 370 | 320 | 390 | 415 | 437 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 446 | 452 | 453 | 451 | 455 | 454 | 452 | 455 | 456 | 453 | / |
As can be seen from Table 4 and Fig. 4, when the welded joints are not treated by the post-weld heat treatment method provided by the invention, the width of the softening zone on the eutectoid rail side is 12mm, and this value on the hypereutectoid rail side is 10mm. The hardness of the joint weld seam on one side of the eutectoid pearlite rail is relatively high, and can reach 476HV
2019204691 01 Jul 2019 at a position 2mm from the center of weld seam, exceeding the average hardness of the base metal of the eutectoid pearlite rail by 36HV. In the service process of railway line, such joints are intended to preferentially form a collapse of the rail head tread in the softening area on one side of the hypereutectoid rail of the joint weld seam due to the relatively low hardness, which may result in “saddle” wear and affect the smoothness of railway line.
Based on the sampling method shown in Fig. 7, the metallographic specimen of rail joint was tested for metallographic structure according to GB/T13298-2015 Inspection Methods of Microstructure for Metals. Specifically, the metallographic specimen of rail joint was etched with 3% nital, and the metallographic structure of rail joint was observed under a German Leica MeF3 optical microscope. The results indicate the hardened martensite structure in the welding heat affected zone of the hypereutectoid rail and eutectoid pearlite rail, which is not conducive to the safe operation of railway lines.
Comparative example 3
After the 75kg/m rail was completely upset and knob-removed in the mobile flash butt welding process, the welded joint is subjected to post-weld heat treatment. Firstly, the rail joint with a residual temperature of 1050°C obtained by welding was cooled in the first stage at the first cooling speed of 7.0°C/s to reduce the surface temperature of the rail head to 670°C; secondly, the rail joint was cooled in the second stage at the second cooling speed of 2.4°C/s to reduce the surface temperature of the rail head to 390°C; and finally, the rail joint was cooled in the third stage at the third cooling speed of 2.5°C/s to reduce the surface temperature of the rail head to room temperature of 25°C, so as to obtain the post-weld heat-treated dissimilar rail joint of the comparative example.
In the post-weld heat treatment process, the first cooling was natural cooling in air; whereas, in the second cooling process and the third cooling process, a rail head profiling cooling device was used to cool the tread and side of the rail head of the welded joint with compressed air as a cooling medium, and the distance between the cooling device and the rail head tread was 40mm. The pressure of compressed air sprayed by the cooling device was 0.65MPa in the second cooling process and 0.80MPa in the third cooling process. An infrared thermometer was used to monitor the tread temperature of rail head.
The rail joints obtained under the post-weld air cooling condition in this comparative example were machined into longitudinal hardened samples. The Vickers hardness tester (Shandong Laizhou
2019204691 01 Jul 2019
Testing Instrument Factory, Model: HBV-30A) was used to test the longitudinal Vickers hardness of the hardened sample at a position 4mm below the rail head tread, at an interval of 2mm between measuring points. The measuring points were symmetrically arranged on the left and right sides with the weld seam as the center. The Vickers hardness test method is as described in GB/T 4340.1-2009 Metallic Materials-Vickers Hardness Test-. Part 1: Test Method, and an HV scale shall be used. The hardness test data is shown in Table 5, and the longitudinal hardness distribution effect of joints is shown in Fig. 5.
Table 5 Hardness test data of welded joint in comparative example 3 Distance from center of weld seam/mm
| Le ft | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 370 | 473 | 460 | 452 | 420 | 350 | 308 | 372 | 386 | 400 | 420 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 430 | 439 | 440 | 438 | 440 | 438 | 440 | 441 | 442 | 441 | / | |
| Ri ght | Distanc e from weld seam | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| Hardne ss/HV | 370 | 437 | 444 | 440 | 444 | 415 | 350 | 298 | 380 | 410 | 430 | |
| Distanc e from weld seam | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 | 38 | 40 | / | |
| Hardne ss/HV | 450 | 452 | 453 | 456 | 452 | 450 | 453 | 451 | 455 | 456 | / |
2019204691 01 Jul 2019
As can be seen from Table 5 and Fig. 5, when the welded joints are not treated by the post-weld heat treatment method provided by the invention, the width of the softening zone on the eutectoid rail side is 12mm, and this value on the hypereutectoid rail side is 11mm. The hardness of the joint weld seam on one side of the eutectoid pearlite rail is relatively high, and even exceeds the average hardness of the base metal of the eutectoid pearlite rail and hypereutectoid rail by 30HV at a position ±10mm from the center of weld seam. In the service process of railway line, such joints are intended to preferentially form a lower-part collapse of the rail head tread in the softening area on one side of the hypereutectoid rail of the joint weld seam due to the relatively low hardness, which may result in “saddle” wear and affect the smoothness of railway line.
Based on the sampling method shown in Fig. 7, the metallographic specimen of rail joint was tested for metallographic structure according to GB/T13298-2015 Inspection Methods of Microstructure for Metals. Specifically, the metallographic specimen of rail joint was etched with 3% nital, and the metallographic structure of rail joint was observed under a German Leica MeF3 optical microscope. The metallographic examination results indicate a large amount of lath martensite in the welding heat affected zone of the hypereutectoid rail and eutectoid pearlite rail, which is not conducive to the safe operation of railway lines.
By comparing the longitudinal hardness of the rail head tread and the softening zone width of welded joint in Fig. 1 to Fig. 5, it can be seen that: if the welded joints of hypereutectoid rail and eutectoid pearlite rail are subjected to the post-weld heat treatment according to the post-weld heat treatment method described in the invention, the longitudinal hardness of rail joints within ±10mm area from the center of weld seam is within ±30HV of the average hardness of the corresponding base metal of eutectoid pearlite rail and hypereutectoid rail respectively (excluding the center line of decarburized weld. Due to the high welding temperature of the rail, the center of weld seam is decarburized and has melting loss of elements, resulting in low hardness), and the width of the softening zone on both sides of the joint weld is less than 15 mm. Moreover, abnormal structures such as martensite and bainite are removed from the metallurgical structure of welded joints, which is beneficial to the safe operation of railway lines.
The above preferred examples of the invention have been described in detail. Although the post-weld heat treatment methods of rails according to the invention have been described with reference to the examples, it easily occurs to those skilled in the art that various modifications and combinations can be made to the invention without departing from the spirit and scope of the
2019204691 01 Jul 2019 invention.
Where any or all of the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.
Claims (10)
- The claims defining the invention are as follows:1. A heat treatment method for a rail flash butt welding joint, the method comprising the following steps: a welded joint of rail is cooled in a first stage at a first cooling speed to reduce the surface temperature of the rail head thereof to 650-720°C; secondly, the welded joint is cooled in a second stage at a second cooling speed to reduce the surface temperature of the rail head thereof to 350-410°C; and finally, the welded joint is cooled in a third stage at a third cooling speed to reduce the surface temperature of the rail head thereof to 10-30°C.
- 2. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein the welded joint is a dissimilar welded joint welded from a hypereutectoid rail and a eutectoid rail with the same rail profile.
- 3. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein the initial temperature of the welded joint is 1000-1400°C.
- 4. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein the cooling method of the first stage is natural cooling in air.
- 5. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein the first cooling speed is 5.0-9.0°C/s.
- 6. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein a rail head profiling cooling device is used in the second cooling stage, compressed air or water-mist mixture is taken as a cooling medium to cool the tread and side of the rail head of the welded joint, the distance between the cooling device and the rail head tread is 20-50 mm, and the pressure of compressed air or water-mist mixture sprayed by the cooling device is 0.40-0.80 MPa.
- 7. The heat treatment method for the rail flash butt welding joint according to claim 6, wherein the second cooling speed is 1.5-2.5°C/s.
- 8. The heat treatment method for the rail flash butt welding joint according to claim 1, wherein a rail head profiling cooling device is used in the third cooling stage, compressed air or water-mist mixture is taken as a cooling medium to cool the tread and side of the rail head of the welded joint, the distance between the cooling device and the rail head tread is 20-50 mm, and the2019204691 01 Jul 2019 pressure of compressed air or water-mist mixture sprayed by the cooling device is 0.05-0.25 MPa.
- 9. The heat treatment method for the rail flash butt welding joint according to claim 8, wherein the third cooling speed is 0.05-0.50°C/s.
- 10. The heat treatment method for the rail flash butt welding joint according to any one of claims 1 to 9, wherein the welded joint is welded from a mobile rail flash butt welder.
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