CN112251592A

CN112251592A - Construction method for heat treatment of dissimilar steel rail flash welded joint

Info

Publication number: CN112251592A
Application number: CN202011146997.3A
Authority: CN
Inventors: 白威; 李大东; 陆鑫; 徐飞翔
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2020-10-23
Filing date: 2020-10-23
Publication date: 2021-01-22
Anticipated expiration: 2040-10-23
Also published as: CN112251592B

Abstract

The invention relates to the field of steel rail rolling production process methods, in particular to a construction method for heat treatment of a joint after dissimilar steel rail flash welding, which has low cost and good mechanical property of the treated welding joint, and comprises the following steps: a. carrying out first-stage cooling on a welded joint formed by welding the bainite steel rail a and the eutectoid pearlite steel rail b and having the temperature of 1310-1500 ℃; b. after the first cooling, carrying out full-section normalizing heating on the steel rail welding joint b; c. carrying out secondary cooling after the normalizing heating is finished; d. after the welding joint is cooled in the second stage, cooling in the third stage is carried out; e. and (4) carrying out tempering heat treatment, and reducing the surface temperature of the welded joint to the room temperature of 10-30 ℃ after the tempering process is finished. The invention is helpful to ensure the safety of railway operation, and is particularly suitable for the treatment process after the welding of dissimilar steel rails.

Description

Construction method for heat treatment of dissimilar steel rail flash welded joint

Technical Field

The invention relates to the field of steel rail rolling production process methods, in particular to a construction method for heat treatment of a dissimilar steel rail flash welded joint.

Background

At present, high-speed, quasi-high-speed and passenger-cargo mixed transportation railways at home and abroad are mostly eutectoid pearlite steel rails, the carbon content of the steel rails is usually within the range of 0.65-0.82% by weight, and the metallographic structure is pearlite, so that the steel rails have the characteristics of good obdurability matching, moderate comprehensive mechanical property index and the like. The rapid development of railways puts higher requirements on the service performance of steel rails. As the mechanical property and the welding property of the traditional pearlite steel rail are almost developed to the limit, under the condition, the strength grade is higher, the bainite steel rail with good wear resistance and contact fatigue resistance is produced at the same time, the carbon content of the steel rail is usually within the range of 0.10-0.30% by weight, and the metallographic structure is a complex phase structure consisting of bainite, a small amount of martensite (or martensite island) and residual austenite. At present, the steel rail mobile flash welding has become the mainstream steel rail on-line welding technology in railway construction sites at home and abroad. Along with the gradual application of the bainite steel rail, the bainite steel rail and the existing pearlite steel rail on the railway line are mixed for use, which undoubtedly brings great challenges to the welding of the steel rails made of different materials. In addition, after the rail is subjected to welding heat cycle, the hardened layer of the welding area disappears and low hardness areas with larger width are formed on two sides of the welding seam, so that the hardness of the welding seam and the heat affected area is lower than that of the rail base metal. In the service process of a steel rail, saddle-shaped abrasion is easily formed on the head tread of a welded joint preferentially, so that the impact of a wheel rail is increased, the service life of the steel rail is seriously influenced, and even the driving safety is endangered. Therefore, the precondition for the application of the steel rail is that how to recover the mechanical property of the steel rail reduced by welding.

Under the influence of melting process and high temperature, austenite grains in a rail welding overheating zone are coarse, so that the hardness of the zone is obviously lower than that of a base material. In the service process of the softened steel rail welding joint, saddle-shaped abrasion is easily formed on the tread part of the joint rail head preferentially, the impact of a wheel rail is increased, the service life of the steel rail is influenced, and even the driving safety is endangered. Therefore, the current national railroad industry standard TB/T1632.2-2014 section 2 of rail welding: flash welding and TB/T1632.4-2014 Rail welding part 4: the gas pressure welding stipulates that the average hardness of a welding area of a heat-treated steel rail is not less than 90% of the average hardness of a steel rail base metal, and harmful structures such as martensite and bainite should not exist in microstructures of a welding line and a heat affected zone. The objects related to the two steel rail welding standards are pearlite steel rails, but at present, no welding standard suitable for bainite steel rails exists at home and abroad. Therefore, it is not suitable to evaluate the mechanical properties of the welded bainitic rail joints in full compliance with the current rail welding technical standards in China, and too high longitudinal hardness of the welded joint rail head tread will result in reduced fatigue properties of the joints and early fracture.

At present, after the domestic steel rail welding is finished, the steel rail welding part 2 of TB/T1632.2-2014 is generally followed: flash welding and TB/T1632.4-2014 Rail welding part 4: after-welding normalizing heat treatment is carried out according to the standard of air pressure welding, after a steel rail welding joint is heated to a temperature higher than the austenitizing temperature by taking medium-frequency induction heating or oxyacetylene flame heating as a heat source, the tread hardness of a steel rail welding area is further improved by adopting an air cooling or air cooling process.

At present, there are also few published reports and literature documents on the heat treatment method of welded joints of bainitic and eutectoid pearlitic rails. CN201610909362.1 discloses a postweld heat treatment method for a hypereutectoid steel rail and PG4 heat treatment eutectoid pearlite steel rail welded joint, which comprises the steps of firstly cooling a steel rail welded joint to be cooled obtained by welding to below 400 ℃, then heating the steel rail welded joint after the first cooling to 860-930 ℃, and then carrying out second cooling until the tread temperature of the steel rail welded joint is 410-450 ℃. The dissimilar steel rail welding joint obtained by the method can meet the current national railway industry standard TB/T1632.2-2014 steel rail welding part 2: flash welding is the test requirement for fatigue, tensile, impact and static bending tests. Unlike the present invention, the above-described invention relates to a welded joint for a pearlite rail, and the present invention relates to a welded joint for a bainite rail and a eutectoid pearlite rail. In addition, the invention also relates to a process for tempering the steel rail welded joint so as to further improve the impact toughness of the welded joint.

The application number is 201210394058.X, and the invention is a prior patent technology of a heat treatment method of a bainite steel rail, and discloses a heat treatment method of the bainite steel rail, which comprises the steps of naturally cooling a steel rail after finish rolling to reduce the temperature of the surface layer of a rail head of the steel rail to 460-490 ℃; forcibly cooling the steel rail at a cooling speed of 2.0-4.0 ℃/s to reduce the temperature of the surface layer of the rail head of the steel rail to 250-290 ℃; naturally raising the temperature of the steel rail until the surface temperature of the rail head of the steel rail reaches more than 300 ℃; placing the steel rail in a heating furnace with the hearth temperature of 300-350 ℃ for tempering treatment for 2-6 h; and cooling the steel rail to room temperature in air. The invention aims to obtain a bainite steel rail with good comprehensive mechanical properties, belongs to a steel rail production heat treatment process, and is not suitable for postweld heat treatment of a bainite and pearlite steel rail welding joint.

The application number is 201810480790.6, the invention discloses a heat treatment method for a bainite welded joint after welding, and the method specifically comprises the steps of cooling the surface temperature of a tread of the welded joint to be cooled to a temperature range of 850-500 ℃, then carrying out air cooling on the rail head part of the joint, cooling the surface temperature of the tread to be cooled to 270-210 ℃, and then finishing the air cooling to naturally cool the joint to the room temperature. The method is characterized in that air cooling is directly carried out on the joint after the bainite steel rail is welded, no heating process is carried out after the steel rail is welded, and the heat treatment effect is uncertain.

The bainite steel rail related to the bainite steel rail joint microstructure and damage research in the Zhao national Master academic paper of China railway science research institute belongs to different component systems from the bainite steel rail in the invention. The following table shows that the contents of Si, Ni, Cr and Mo in the two bainite steel rail base metal components are obviously different. For steel rails with different component systems, the corresponding post-weld heat treatment processes are also quite different.

Chemical component comparison (mass fraction%) of bainite steel rail

Classification

C

Si

Mn

Ni

Cr

Mo

V

Al

P、S

Comparison document

0.1～ 0.3

0.5～ 0.9

1.0～ 2.3

0.30～ 0.75

0.6～ 1.3

0.2～ 0.7

/

0.004

≤0.015

Steel of the invention Rail

0.20～ 0.30

0.8～ 1.8

1.5～ 2.5

/

0.5～ 1.6

0.2～ 0.5

/

≤ 0.005

≤0.010

Note that: "/" is the absence of such elements.

Application No.: CN201410135909.8, inventive name: a heat treatment method for bainite steel rail welded joint after welding is disclosed, and the heat treatment principle of steel rail joint is introduced, in the heat treatment process after welding, the steel rail joint is heated to above austenitizing temperature, and the welding area is rapidly cooled by using compressed air as a cooling medium to recover the mechanical property of the steel rail reduced by welding. Also, no specific values of impact toughness for the bainite welded joint are described in this patent. The invention uses the steel rail welding residual heat as the heat source of the joint postweld heat treatment, controls the phase change process by controlling the cooling speed of different temperature sections, and realizes the postweld heat treatment of the dissimilar steel rail joint. The existing research shows that: the heat treatment after rolling, particularly the tempering treatment, has obvious influence on the structure and the performance of the low-carbon bainite steel, and is one of important means for improving the comprehensive mechanical performance of the low-carbon bainite steel. According to the invention, the application of the split electric heating tempering device can greatly improve the impact toughness of the steel rail welding joint. In addition, the present application is applicable to a joint formed by welding a bainite rail and a eutectoid pearlite rail, and the bainite rail of a single material is not disposed on both sides of the weld.

Application No.: CN201810581145.3, inventive name: the postweld heat treatment method of the hypereutectoid steel rail and the eutectoid steel rail welded joint, application number: CN201810720765.0, inventive name: heat treatment method of steel rail welded joint, application number: CN201810710040.3, inventive name: a heat treatment method for welded joints of rails made of dissimilar materials is disclosed in the above 3 patents, but the structures of two rails, namely hypereutectoid steel rails and eutectoid steel rails, are mainly pearlite, and the post-weld heat treatment principle is that compressed air or water mist mixed gas is used as a cooling medium, and rail joints at austenitizing temperature are rapidly cooled, so that the purposes of refining pearlite lamellar spacing and improving the thermoplasticity of rail welding areas are achieved. The invention relates to a joint formed by welding a bainite steel rail and a eutectoid pearlite steel rail, and the difference of the bainite steel rail and the eutectoid pearlite steel rail in material causes that the scheme cannot be directly applied to the patent application. In addition, the application of split electrical heating tempering equipment in this application can promote rail welded joint impact toughness by a wide margin when stabilizing welded joint tissue, eliminating welding residual stress, helps improving xenogenesis rail joint performance in service.

In summary, in the field of railway engineering, a heat treatment method suitable for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail is urgently needed to improve the tread hardness of the steel rail reduced due to welding and improve the impact toughness of the steel rail joint so as to ensure the service performance and the railway operation safety of a welded joint of dissimilar steel rails.

Disclosure of Invention

The invention aims to solve the technical problem of providing a construction method for heat treatment of a joint after dissimilar steel rail flash welding, which has low cost and good mechanical property of the treated welding joint.

The technical scheme adopted by the invention for solving the technical problems is as follows: the construction method for the heat treatment of the joint after the flash welding of the dissimilar steel rail comprises the following steps: a. welding a welded joint formed by welding the bainite steel rail a and the eutectoid pearlite steel rail b and having a temperature of 1310-1500 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 100-200 ℃, wherein the cooling speed is 0.1-15.0 ℃/s; b. after the first cooling, carrying out full-section normalizing heating on the steel rail welding joint b, and stopping heating when the surface temperature of the joint is heated to 920-; c. carrying out second cooling after the normalizing heating is finished, wherein the opening cooling temperature of the second cooling is not lower than 780 ℃, and the cooling length comprises the range of 60-70 mm on each of two sides with the welding seam as the center, wherein the second cooling is rapid cooling, the cooling speed is 3.5-6.0 ℃/s, and the final cooling temperature is 460-520 ℃; d. after the second-stage cooling of the welding joint is finished, performing third-stage cooling to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-1.0 ℃/s; e. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 100-180 ℃ at 280-350 ℃ for 0.5-1.0 h, taking down the tempering device after the tempering process is finished, naturally cooling the welded joint in the air to reduce the surface temperature of the welded joint to the room temperature of 10-30 ℃, wherein the cooling speed at the stage is 0.1-0.9 ℃/s.

Further, in the step b, a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater are/is adopted to carry out full-section normalizing heating on the steel rail welding joint.

Further, in the step c, the second cooling is artificial cooling using compressed air or a water mist mixture, and in the step d, the third cooling is artificial cooling using compressed air or a water mist mixture.

Furthermore, in the step e, a split electric heating tempering device is adopted for tempering heat treatment.

Further, in the step a, the first-stage cooling is natural cooling in air, wherein when the natural cooling is performed, the cooling speed in the high-temperature stage is fast, and the cooling speed in the low-temperature stage is slow, that is: when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃/s; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃/s; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃/s; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s.

Furthermore, in the second cooling process in the step c, the distance between a nozzle type cooling device matched with the normalizing unit and the surface of the welding joint b is 30-40 mm, and the pressure of compressed air or water mist mixed gas sprayed by the cooling device is 0.30-0.60 MPa; in the step d, the distance between the nozzle type cooling device matched with the normalizing unit and the surface of the welded joint b is 30-40 mm in the third cooling process, and the pressure of compressed air or water mist mixed gas sprayed by the cooling device is 0.05-0.10 MPa.

Further, the welded joint b of the dissimilar steel rail is formed by welding the bainite steel rail a and the eutectoid pearlite steel rail b.

Furthermore, the dissimilar steel rail welding joint b is formed by welding a bainite steel rail a and a eutectoid pearlite steel rail b which have the same steel rail shape and the specification of 60-68 kg/m through a steel rail moving flash welding machine.

Furthermore, the tensile strength of the bainite steel rail a is 1200MPa, and the chemical components of the base metal are as follows: c content of 0.10-0.19 wt%, Si content of 0.5-1.2 wt%, Mn content of 0.5-0.8 wt%, Cr content of 0.50-1.40 wt% and Mo content of 0.20-0.50 wt%; the tensile strength of the eutectoid pearlite steel rail is 1100MPa, and the chemical components of the base metal are as follows: c content of 0.60-0.74 wt%, Si content of 0.15-0.90 wt%, and Mn content of 0.7-1.2 wt%.

Further, in the step e, the tempering device is a profiling electric heating tempering device, and the distance between the profiling electric heating tempering device and the surface of the welding joint b is 5-10 mm; the tempering heat treatment temperature is 280-350 ℃, and the heat preservation time is 0.5-1.0 h.

The invention has the beneficial effects that: in the heat treatment process, the flash welding joints of the dissimilar steel rails are subjected to postweld heat treatment by adopting 'primary normalizing heating, tertiary cooling and primary tempering', so that no martensite exists in the welding heat affected zone on one side of the eutectoid pearlite steel rails of the welding joints of the dissimilar steel rails, and no obvious large-size blocky martensite exists in the welding heat affected zone on one side of the bainite steel rails of the steel rail joints. The width of the softening zone at one side of the bainite steel rail of the dissimilar steel rail joint is less than or equal to 55mm, and the width of the softening zone at one side of the eutectoid pearlite steel rail of the steel rail joint is less than or equal to 35 mm. The average hardness of the bainite steel rail side and the pearlite steel rail side of the dissimilar material steel rail joint can reach more than 85 percent of that of the parent metal. Meanwhile, the room temperature impact energy of the full-section weld joint of the welded joint reaches 18.0J, which is higher than 6.5J specified by TB/T1632.2-2014. The invention is beneficial to improving saddle-shaped abrasion of a steel rail welding joint caused by low hardness of a welding area in the line service process, has good impact toughness of the joint, is beneficial to ensuring the running safety of a railway, and is particularly suitable for a treatment process after welding of dissimilar steel rails.

Drawings

FIG. 1 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of a welded rail joint obtained by the method of example 1 of the present invention.

FIG. 2 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of a welded rail joint obtained by the method of example 2 of the present invention.

FIG. 3 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of the welded rail joint obtained by the method of comparative example 2.

FIG. 4 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of the welded rail joint obtained by the method of comparative example 3.

Fig. 5 is a schematic view of the positions of the respective portions according to the present invention.

Fig. 6 is a schematic view of the sampling position of the metallographic specimen of the rail head tread of the rail joint according to the present invention.

FIG. 7 is a schematic view showing the distribution of electric heaters on the head of the split rail heating apparatus.

Fig. 8 is an overall schematic view of the split rail heating apparatus.

The labels in the figure are: the steel rail comprises a bainite steel rail at a position a, a welding joint at a position B, a eutectoid pearlite steel rail at a position C, a rail head tread at a position D, a weld joint center at a position E, a metallographic specimen sampling position at a position F, A3, a side face heating area of a rail head, B3, C3, a chin heating area of the rail head, D3, a crawler-type ceramic electric heater, A1/A2, a left terminal post, B1/B2, a crawler-type ceramic heater, a rotating shaft, a fixed snap ring, an apparatus shell, F and an asbestos insulation layer.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

The construction method for the heat treatment of the joint after the flash welding of the dissimilar steel rail comprises the following steps: a. welding a welded joint formed by welding the bainite steel rail a and the eutectoid pearlite steel rail b and having a temperature of 1310-1500 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 100-200 ℃, wherein the cooling speed is 0.1-15.0 ℃/s; b. after the first cooling, carrying out full-section normalizing heating on the steel rail welding joint b, and stopping heating when the surface temperature of the joint is heated to 920-; c. carrying out second cooling after the normalizing heating is finished, wherein the opening cooling temperature of the second cooling is not lower than 780 ℃, and the cooling length comprises the range of 60-70 mm on each of two sides with the welding seam as the center, wherein the second cooling is rapid cooling, the cooling speed is 3.5-6.0 ℃/s, and the final cooling temperature is 460-520 ℃; d. after the second-stage cooling of the welding joint is finished, performing third-stage cooling to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-1.0 ℃/s; e. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 100-180 ℃ at 280-350 ℃ for 0.5-1.0 h, taking down the tempering device after the tempering process is finished, naturally cooling the welded joint in the air to reduce the surface temperature of the welded joint to the room temperature of 10-30 ℃, wherein the cooling speed at the stage is 0.1-0.9 ℃/s.

Specifically, the heat treatment work of the joint after flash welding of the dissimilar steel rail is completed through one-time normalizing heating, three-time cooling and one-time tempering on the basis of welding the joint of the bainite steel rail and the eutectoid pearlite steel rail which are obtained by welding and kept at the temperature of 1310-1500 ℃. In order to obtain better treatment effect, the following scheme is preferred: and the primary normalizing heating is carried out after the dissimilar steel rail flash welding joint is cooled for the first time and the temperature is reduced to 100-200 ℃. The primary normalizing heating is preferably carried out by adopting a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater, and is completed by heating the dissimilar steel rail welding joint with the primary cooling temperature reduced to 100-200 ℃ to 920-980 ℃. And preferably, carrying out secondary cooling immediately after the primary normalizing heating is finished, wherein the opening cooling temperature of the secondary cooling is not lower than 780 ℃, and the cooling length comprises the range of 60-70 mm on two sides of the welding seam as the center. Preferably, the second cooling is rapid cooling by taking high-pressure cooling gas or water mist mixed gas as a cooling medium, and the final cooling temperature is 460-520 ℃. In a preferred embodiment of the above aspect, the first cooling and the third cooling are both performed by natural cooling in air. In the primary normalizing heating process, the length of the full-section heating preferably ranges from 40mm to 60 mm on two sides of the full-section heating by taking the welding seam as a center. The preferable one-time tempering process is to adopt a split electric heating tempering device to carry out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 100-180 ℃ at the temperature of 280-350 ℃ for 0.5-1.0 h. And after the tempering process is finished, taking down the tempering device. And naturally cooling the welding joint in the air to reduce the surface temperature of the welding joint to the room temperature of 10-30 ℃, wherein the cooling speed at the stage is 0.1-0.9 ℃/s. The initial temperature of the steel rail welding joint is preferably 1310-1500 ℃, and the steel rail welding joint can be controlled by adjusting steel rail welding process parameters. Preferably, the bainite steel rail (a) has a tensile strength of 1200MPa, and the base metal comprises the following chemical components: c content of 0.10-0.19 wt%, Si content of 0.5-1.2 wt%, Mn content of 0.5-0.8 wt%, Cr content of 0.50-1.40 wt% and Mo content of 0.20-0.50 wt%; the tensile strength of the eutectoid pearlite steel rail is 1100MPa, and the chemical components of the base metal are as follows: c content of 0.60-0.74 wt%, Si content of 0.15-0.90 wt%, and Mn content of 0.7-1.2 wt%. In the preferable step e, the tempering device is a profiling electric heating tempering device, and the distance between the profiling electric heating tempering device and the surface of the welding joint b is 5-10 mm; the tempering heat treatment temperature is 280-350 ℃, and the heat preservation time is 0.5-1.0 h.

The joint can be formed by welding a bainite steel rail a and a eutectoid pearlite steel rail c which have the same rail type and the specification of 60-68 kg/m through a steel rail mobile flash welding machine. The welded joint b comprises a welding seam and/or a region with the length of the heat affected zone within the range of 80-120 mm, and the center of the region is the welding seam. In the invention, the room temperature is 10-30 ℃.

In the invention, the martensite transformation critical cooling speed in the continuous cooling process of the bainite rail steel is 1.2-1.9 ℃/s, and the Ms temperature (the start temperature of martensite formation) of the rail steel is 380-440 ℃; the critical cooling speed of martensite transformation in the continuous cooling process of the eutectoid pearlite rail steel is 1.3-1.7 ℃/s, and the Ms temperature (the starting temperature of martensite formation) of the rail steel is 210-260 ℃. In order to avoid the occurrence of abnormal structures such as martensite in the welded joint of the steel rail, when the welded joint of the bainite steel rail and the eutectoid pearlite steel rail is subjected to postweld heat treatment, the final cooling temperature in the postweld heat treatment rapid cooling process needs to be controlled to be higher than the Ms temperature of the bainite steel rail. At the same time, the cooling rate during the post-weld heat treatment must be limited to bainitic rail steels with relatively low critical cooling rates, otherwise the joint will fail prematurely due to the large amount of hardened martensite. In some national rail welding standards, such AS specified in australian rail welding standard AS1085.20-2012, for some high-strength-grade, high-carbon-content and high-alloy-content rails, under an observation magnification of a metallographic microscope of 100x, for the most severe region where martensite appears in a rail welded joint, the percentage content of martensite structure is not higher than 5%, otherwise the joint will cause premature fatigue fracture due to a large amount of hardened martensite structure, and the operation safety of the rail is seriously affected. Therefore, strict control of the martensite content in the welded structure of the steel rail is important for stable operation of the railway line.

In order to solve the technical problem, as a preferable scheme, the method can select a welding joint formed by welding the bainite steel rail and the eutectoid pearlite steel rail and having the temperature of 1310-1500 ℃ to carry out first-stage cooling, so that the surface temperature of the steel rail welding joint is reduced to 100-200 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 0.1-15.0 ℃/s. It should be noted that: when natural cooling is performed, the cooling speed in the high-temperature stage is higher, the cooling speed in the low-temperature stage is relatively lower, and the cooling speeds of the temperature stages are different. The method specifically comprises the following steps: when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃/s; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃/s; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃/s; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s. Without taking into account compositional segregation, welded rail joints typically do not exhibit a brittle and hard martensite structure in the weld heat affected zone under post-weld air cooling conditions. After the first cooling, carrying out full-section normalizing heating on the steel rail welding joint by adopting a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater, and stopping heating when the surface temperature of the joint is heated to 920-980 ℃ so as to finish the normalizing heating; and carrying out secondary cooling after the normalizing heating is finished, wherein the opening cooling temperature of the secondary cooling is not lower than 780 ℃, and the cooling length comprises the range of 60-70 mm on each of two sides with the welding seam as the center. The second cooling is rapid cooling by taking high-pressure cooling gas or water mist mixed gas as a cooling medium, the cooling speed is 3.5-6.0 ℃/s, and the final cooling temperature is 460-520 ℃; after the second stage cooling of the welding joint is finished, placing the welding joint in air for third stage cooling, so that the surface temperature of the welding joint is reduced to 100-180 ℃, and the cooling speed of the welding joint in the stage is 0.1-0.9 ℃/s; and tempering heat treatment at the temperature of 280-350 ℃ and the heat preservation time of 1-2 h is carried out on the steel rail welding joint with the surface temperature of 100-180 ℃ by adopting a split electric heating tempering device. And after the tempering process is finished, taking down the tempering device. And naturally cooling the welding joint in the air to reduce the surface temperature of the welding joint to the room temperature of 10-30 ℃, wherein the cooling speed at the stage is 0.1-0.9 ℃/s.

In the invention, the normalizing heat treatment generally refers to a heat treatment process of heating a metal workpiece to be 30-50 ℃ above Ac3 (the final temperature of ferrite transformed into austenite during heating) by a conventional mode, taking the metal workpiece out of a furnace after keeping the temperature for a period of time, and naturally cooling the metal workpiece in air, or spraying compressed air for cooling. The post-weld normalizing heat treatment of the welded joint of the steel rail is different from the heat treatment process used by common small-size workpieces, and the normalizing heat treatment of the welded joint of the steel rail cannot keep the temperature (the temperature above the austenitizing temperature) for a long time after the target temperature is reached because the length of a sample after the steel rail is welded can be as high as hundreds of meters. Therefore, the steel rail welding joint is heated to a target temperature by generally adopting a heat treatment process of air cooling or air cooling after the temperature is slightly higher than the conventional normalizing temperature, and the optimal normalizing heating temperature of the invention is 920-980 ℃.

In the invention, the first-stage cooling is natural cooling in air, and the control of the first-stage cooling speed can be realized by adjusting the test environment temperature (for example, adopting a central air conditioner for temperature control). The reason why the temperature drop rate is still high in this stage though air cooling is because the temperature difference between the welded joint and the air is large. The second cooling stage has a cold start temperature of not less than 780 ℃. In the present invention, the finish cooling temperature of the second cooling is higher than the martensite transformation start temperature (Ms temperature) of the bainite rail steel and the eutectoid pearlite rail steel, and the finish cooling temperature of the second cooling is 460 to 520 ℃. The cooling speed of the second cooling stage is controlled to be 3.5-6.0 ℃/s, so that the mechanical property of a welding area is improved by performing accelerated cooling on the super-cooled austenite to refine the interlayer spacing of pearlite plates and refine the bainite structure. When the dissimilar steel rail joint is cooled in the third stage, in order to avoid the hardened martensite structure of the joint, the invention preferably adopts a natural cooling mode, and the joint is cooled at a cooling speed of 0.1-0.9 ℃/s which is lower than the martensite transformation critical cooling speed of the steel rail. When a profiling electric heating tempering device (split steel rail heating device) is adopted to carry out tempering heat treatment on the steel rail welded joint with the surface temperature of 100-180 ℃, the tempering temperature is set to be 280-350 ℃, and the heat preservation time is set to be 0.5-1.0 h. The profiling device can be tightly attached to the surface of a steel rail welding joint, good heat conduction is achieved, and a tempering heat treatment process can be achieved under the combined action of the crawler-type heater. And when the tempering and heat preservation process is finished, taking down the tempering device. And (3) placing the welded joint in air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed in the stage is 0.1-0.9 ℃/s.

As shown in fig. 7, the heating device head, web and foot heating areas of the open rail heating device shown in fig. 7 are similarly arranged. Because the rail head is thicker and slower in heat transfer, the number of the ceramic heaters for coating the rail head is larger than that of the ceramic heaters in the rail waist and rail bottom areas, so that the full section of the steel rail is fully heated. The open type steel rail heating device shown in fig. 8 has the advantages of small size, flexibility, low cost and the like, is convenient for field construction, can be powered by 380V voltage or 220V alternating current mains supply of a diesel generator, and has the rated power of 10 kW. The device uses a commercial LCD crawler-type ceramic heater as a heat source, and the heater is a ceramic wafer with the size of 10mm (length) multiplied by 10mm (width) multiplied by 7mm (thickness). The split-type heater is made of a ring-shaped split-type heater by matching with a heat insulation material and a steel structure shell, is convenient to assemble and disassemble, and is suitable for local heating of a steel rail welding joint. The actual size of the heating device and the specification and distribution condition of the heater can be adjusted according to the actual size of the steel rail profile. In the device design process, evenly fix the device inboard that has similar rail profile shape with multiunit crawler-type heater, make the heater cladding on the rail surface and with rail surface fully laminate in order to realize the good heat-conduction in the heating process, can realize the heat treatment process to the rail joint based on the device. In the test process, a temperature controller is adopted to control the heating temperature. The working temperature range of the device is 200-750 ℃. The split device can be rotated by 180 DEG at maximum about the axis of rotation. Wherein, the terminal A1 and the terminal A2 form a loop. The terminal B1 and the terminal B2 form a loop, and a plurality of electric heaters which are arranged in parallel are connected together in a parallel connection mode.

The invention relates to a postweld heat treatment process method of a bainite steel rail welded joint, which refers to TB/T1632.2-2014 part 2 of steel rail welding: flash welding and TB/T1632.4-2014 Rail welding part 4: according to the standard of air pressure welding, a steel rail welding joint is machined into a longitudinal section hardness sample, the Rockwell hardness of the longitudinal section of the welding joint is detected at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, the distance between the measuring points is 5mm, the Rockwell hardness method is carried out according to the regulation of GB/T230.1-2009, an HRC scale is adopted, Hp represents the average hardness value of a steel rail base metal, Hj represents the average hardness value of the joint, and the position where the hardness of the joint is lower than 0.9Hp represents a softening region.

Examples

Example 1

After the upsetting and the beading in the moving flash welding process of the dissimilar steel rail with the specification of 60kg/m are finished, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, placing the welded dissimilar steel rail joint with the residual temperature of 1350 ℃ in air for natural cooling so as to reduce the surface temperature of the rail head of the steel rail joint to 200 ℃, and then adopting a medium-frequency induction profiling electric heating coil to carry out full-section heating on the welded joint area of the steel rail. And stopping heating when the tread temperature of the steel rail reaches 950 ℃. Then, the cooling is carried out rapidly by using compressed air as a cooling medium at a cooling rate of 5.0 ℃/s, and the final cooling temperature of the stage is 500 ℃. Then, a third stage cooling is performed at a third cooling rate of 1.0 ℃/s to reduce the rail head surface temperature of the rail joint to 170 ℃. Then, a split electric heating tempering device is adopted to carry out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 170 ℃ at the temperature of 350 ℃ for 1.0 h. And after the tempering and heat preservation process is finished, taking down the tempering device, and naturally cooling the joint to room temperature in the air at a cooling speed of 0.8 ℃/s to obtain the welded steel rail joint subjected to postweld heat treatment.

The first cooling is natural cooling carried out in air, the nozzle type cooling device matched with the steel rail post-welding heat treatment unit is adopted in the second cooling and the third cooling processes to cool the rail head tread and the side face of the rail head of the steel rail joint by taking compressed air or water mist mixed gas as a cooling medium, and the distance between the nozzle of the cooling device and the rail head tread is 40 mm. In the second cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.50 MPa; in the third cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.10 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.

The bainite steel rail welding joint obtained in the embodiment 1 is machined into a longitudinal hardness test sample, longitudinal Rockwell hardness detection is carried out on the welding joint at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 1, and the distribution effect of the longitudinal hardness is shown in FIG. 1.

TABLE 1

As can be seen from table 1 and fig. 1, the width of the bainite rail side softening zone and the 35mm eutectoid pearlite rail side softening zone of the rail joint were 55m and 35mm, respectively, for the rail joint treated according to the present invention. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 85% of that of the base material, and the average hardness of the pearlite steel rail side can reach 88% of that of the base material.

Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 18.5J, which is beneficial to ensuring the railway operation safety.

Example 2

After the upsetting and the beading in the moving flash welding process of the dissimilar steel rail with the specification of 68kg/m are finished, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, placing the dissimilar steel rail joint with the residual temperature of 1400 ℃ obtained by welding in air for natural cooling so as to reduce the surface layer temperature of the rail head of the steel rail joint to 150 ℃. And then, heating the whole section of the steel rail welding joint area by adopting a medium-frequency induction profiling electric heating coil. And stopping heating when the tread temperature of the steel rail reaches 930 ℃. Then, the cooling is carried out rapidly by using compressed air as a cooling medium at a cooling rate of 4.0 ℃/s, and the final cooling temperature of the stage is 480 ℃. Then, a third stage cooling is carried out at a third cooling rate of 0.8 ℃/s to reduce the rail head surface temperature of the rail joint to 160 ℃. Then, a split electric heating tempering device is adopted to carry out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 160 ℃ at the temperature of 320 ℃ for 1.0 h. And after the tempering and heat preservation process is finished, taking down the tempering device, and naturally cooling the joint to room temperature in the air at a cooling speed of 0.6 ℃/s to obtain the welded steel rail joint subjected to postweld heat treatment.

The first cooling is natural cooling carried out in air, the nozzle type cooling device matched with the steel rail post-welding heat treatment unit is adopted in the second cooling and the third cooling processes to cool the rail head tread and the side face of the rail head of the steel rail joint by taking compressed air or water mist mixed gas as a cooling medium, and the distance between the nozzle of the cooling device and the rail head tread is 40 mm. In the second cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.40 MPa; in the third cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.08 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.

The bainite steel rail welding joint obtained in the embodiment is machined into a longitudinal hardness test sample, longitudinal Rockwell hardness detection is carried out on the welding joint at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 2, and the distribution effect of the longitudinal hardness is shown in FIG. 2.

TABLE 2

As can be seen from table 2 and fig. 2, the width of the bainite rail side softening zone and the 35mm eutectoid pearlite rail side softening zone of the rail joint were 55m and 35mm, respectively, for the rail joint treated according to the present invention. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 88% of that of the base material, and the average hardness of the pearlite steel rail side can reach 89% of that of the base material.

Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 19.0J, which is beneficial to ensuring the railway operation safety.

Comparative example 1

The process conditions involved in the steel rail welding and post-weld cooling processes in comparative example 1 and example 2 are the same, except that the post-weld tempering heat treatment is not performed on the steel rail welded joint after the steel rail welded joint is cooled to the rail head surface layer temperature of 160 ℃ in the third stage.

The rail joint obtained in comparative example 1 was processed into a longitudinal hardness test specimen, and the hardness distribution curve was in accordance with fig. 2. The metallographic examination result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. The difference is that the impact energy at room temperature of the full-section welding seam of the dissimilar steel rail joint in the embodiment 2 is 14J, while the impact energy at room temperature of the full-section welding seam of the dissimilar steel rail joint in the comparative example is only 7J, and the toughness of the joint is relatively poor, so that the safety of railway operation is not facilitated.

Comparative example 2

The bainite rail weld joint was post-weld heat treated as in example 1, heating the rail weld joint to 950 ℃ using medium frequency induction profiling electric heating coils, except that the air cooling was stopped at a second cooling to 150 ℃, and then the weld joint was air cooled to room temperature (about 23 ℃).

Taking a longitudinal hardness test sample of the bainite steel rail welded joint obtained in the comparative example, and carrying out longitudinal Rockwell hardness detection at a position 5mm below a steel rail head tread, wherein the measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 3, and the effect of the longitudinal hardness distribution is shown in FIG. 3.

TABLE 3

As is clear from table 3 and fig. 3, the rail joint treated by the present comparative example had a softening zone width on the bainitic rail side of the rail joint of 40m and a softening zone width on the eutectoid pearlitic rail side of 30 mm. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 84% of that of the base material, and the average hardness of the pearlite steel rail side can reach 88% of that of the base material.

Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, a large amount of martensite appears in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail, and the percentage content of the martensite reaches 10%. Significant blocky martensite also occurs in the weld heat affected zone on one side of the bainitic rail of the rail joint. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is only 4J, which is not beneficial to the running safety of railways.

Comparative example 3

And directly air-cooling the bainite steel rail welded joint which is obtained by adopting gas pressure welding and has the temperature of 1500 ℃ to room temperature (about 23 ℃) so as to obtain the bainite steel rail welded joint under the air-cooling (natural cooling) condition.

Taking a longitudinal hardness test sample of the bainite steel rail welded joint obtained in the comparative example, and carrying out longitudinal Rockwell hardness detection at a position 5mm below a steel rail head tread, wherein the measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 4, and the effect of the longitudinal hardness distribution is shown in FIG. 4.

TABLE 4

As is clear from table 4 and fig. 4, the rail joint treated by the present comparative example had a softening zone width on the bainitic rail side of the rail joint of 63m and a softening zone width on the eutectoid pearlitic rail side of the rail joint of 65 mm. Compared with the hardness of the steel rail base metal, the hardness of the whole welding area is lower, and the hardness is distributed in a W shape. The welding joint obtained by the comparative example is easy to cause saddle-shaped abrasion of a joint railhead in the service process of a line, and the smoothness of the line and the driving safety are influenced. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 84% of that of the base material, and the average hardness of the pearlite steel rail side can reach 88% of that of the base material.

Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the microstructure of the welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and no abnormal structures such as martensite, bainite and the like appear. No significant blocky martensite is present in the weld heat affected zone on one side of the bainitic rail of the rail joint. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is only 4J, which is not beneficial to the running safety of railways.

As can be seen by comparing the weld joint railhead tread longitudinal hardness profiles in fig. 1-4: the process method provided by the invention can control the width of the softening region at one side of the bainite steel rail of the dissimilar steel rail joint to be less than or equal to 55mm, and the width of the softening region at one side of the eutectoid pearlite steel rail of the steel rail joint to be less than or equal to 35 mm. The average hardness of the bainite steel rail side and the pearlite steel rail side of the dissimilar material steel rail joint can reach more than 85 percent of that of the parent metal. Meanwhile, the metallographic structure in a welding heat affected zone on one side of the eutectoid pearlite steel rail of the steel rail joint is normal, and no martensite, bainite and other abnormal structures appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 18.0J, which is beneficial to ensuring the railway operation safety.

The embodiment and the comparative example can obtain that the steel rail welding joint can effectively improve the tread hardness of the steel rail reduced due to welding, simultaneously improve the impact toughness of the steel rail joint to ensure the service performance and the railway operation safety of a dissimilar steel rail welding joint, is also beneficial to improving saddle-shaped abrasion of the steel rail welding joint caused by low hardness of a welding area in the line service process, has good joint impact toughness, is beneficial to ensuring the railway operation safety, and is particularly suitable for the treatment process after welding the dissimilar steel rail. The invention has obvious technical advantages and wide market promotion prospect.

Claims

1. The construction method for the heat treatment of the joint after the flash welding of the dissimilar steel rail is characterized by comprising the following steps of:

a. performing first-stage cooling on a welded joint formed by welding the bainite steel rail (a) and the eutectoid pearlite steel rail (b) and having a temperature of 1310-1500 ℃ to reduce the surface temperature of the welded joint of the steel rail to 100-200 ℃, wherein the cooling speed is 0.1-15.0 ℃/s;

b. after the first cooling, carrying out full-section normalizing heating on the steel rail welding joint (b), and stopping heating when the surface temperature of the joint is heated to 920-;

c. carrying out second cooling after the normalizing heating is finished, wherein the opening cooling temperature of the second cooling is not lower than 780 ℃, and the cooling length comprises the range of 60-70 mm on each of two sides with the welding seam as the center, wherein the second cooling is rapid cooling, the cooling speed is 3.5-6.0 ℃/s, and the final cooling temperature is 460-520 ℃;

d. after the second-stage cooling of the welding joint is finished, performing third-stage cooling to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-1.0 ℃/s;

e. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature of 100-180 ℃ at 280-350 ℃ for 0.5-1.0 h, taking down the tempering device after the tempering process is finished, naturally cooling the welded joint in the air to reduce the surface temperature of the welded joint to the room temperature of 10-30 ℃, wherein the cooling speed at the stage is 0.1-0.9 ℃/s.

2. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, wherein: and in the step b, a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater are/is adopted to carry out full-section normalizing heating on the steel rail welding joint.

3. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, wherein: in step c, the second cooling is artificial cooling by using compressed air or a water mist mixture, and in step d, the third cooling is artificial cooling by using compressed air or a water mist mixture.

4. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, wherein: and e, tempering heat treatment is carried out by adopting a split electric heating tempering device.

5. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, 2, 3 or 4, wherein: in the step a, the first stage cooling is natural cooling in air, wherein when the natural cooling is performed, the cooling speed in a high temperature stage is high, and the cooling speed in a low temperature stage is low, namely: when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃/s; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃/s; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃/s; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s.

6. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, 2, 3 or 4, wherein: in the second cooling process in the step c, the distance between a nozzle type cooling device matched with the normalizing unit and the surface of the welding joint (b) is 30-40 mm, and the pressure of compressed air or water mist mixed gas sprayed by the cooling device is 0.30-0.60 MPa; in the step d, the distance between the nozzle type cooling device matched with the normalizing unit and the surface of the welding joint (b) is 30-40 mm in the third cooling process, and the pressure of compressed air or water mist mixed gas jetted by the cooling device is 0.05-0.10 MPa.

7. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, 2, 3 or 4, wherein: the welded joint (b) of the dissimilar steel rail is formed by welding a bainite steel rail (a) and a eutectoid pearlite steel rail (b).

8. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 7, wherein: the dissimilar steel rail welding joint (b) is formed by welding a bainite steel rail (a) and a eutectoid pearlite steel rail (b) which have the same steel rail shape and the specification of 60-68 kg/m by a steel rail moving flash welding machine.

9. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, 2, 3 or 4, wherein: the tensile strength of the bainite steel rail (a) is 1200MPa, and the chemical components of a base material are as follows: c content of 0.10-0.19 wt%, Si content of 0.5-1.2 wt%, Mn content of 0.5-0.8 wt%, Cr content of 0.50-1.40 wt% and Mo content of 0.20-0.50 wt%;

the tensile strength of the eutectoid pearlite steel rail is 1100MPa, and the chemical components of the base metal are as follows: c content of 0.60-0.74 wt%, Si content of 0.15-0.90 wt%, and Mn content of 0.7-1.2 wt%.

10. The construction method for the heat treatment of the flash welded dissimilar steel rail joint according to claim 1, 2, 3 or 4, wherein: in the step e, the tempering device is a profiling electric heating tempering device, and the distance between the profiling electric heating tempering device and the surface of the welding joint (b) is 5-10 mm; the tempering heat treatment temperature is 280-350 ℃, and the heat preservation time is 0.5-1.0 h.