CN114891967A

CN114891967A - Method for welding medium-low carbon pearlite steel rail

Info

Publication number: CN114891967A
Application number: CN202210695199.9A
Authority: CN
Inventors: 白威; 李大东; 陆鑫; 邓健
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: 2022-06-20
Filing date: 2022-06-20
Publication date: 2022-08-12
Anticipated expiration: 2042-06-20
Also published as: WO2023246359A1; CN114891967B; AU2023287051A1

Abstract

The invention discloses a method for welding a steel rail with medium and low carbon pearlite. The method comprises the following steps: welding a plurality of steel rails made of medium-low carbon pearlite steel rail base materials; cooling the welded joint of the welded rail to a first predetermined temperature; placing a rail head portion of the welded joint into a heating region of a first electromagnetic induction coil, placing a rail web portion and a rail foot portion of the welded joint into a heating region of a second electromagnetic induction coil, and simultaneously turning on the first electromagnetic induction coil and the second electromagnetic induction coil to heat the rail head portion, the rail web portion and the rail foot portion to a second predetermined temperature, wherein a first heating frequency of the first electromagnetic induction coil is set to be higher than a second heating frequency of the second electromagnetic induction coil; and cooling the heated welding joint. The method achieves the purpose of obviously improving the impact toughness of the welding seam and improves the service safety of the railway by respectively controlling the normalizing and heating processes of the rail head, the rail web and the rail bottom of the steel rail joint.

Description

Method for welding medium-low carbon pearlite steel rail

Technical Field

The invention relates to the technical field of railway steel rail manufacturing, in particular to a method for welding a medium-low carbon pearlite steel rail.

Background

In recent years, the development of railway systems at home and abroad towards high speed and heavy load direction puts high requirements on the comprehensive properties of steel rail base metals and welded joints. At present, the carbon content of a railway rail is mainly concentrated to 0.7 to 1.1%, and the rail has a structure of full pearlite or pearlite and a small amount of proeutectoid ferrite (or proeutectoid cementite), and is generally required to have a rail strength of not less than 880MPa and to have good wear resistance. And for severe road sections with special natural conditions such as cold regions in winter, large annual temperature difference and day-night temperature difference, new requirements on the impact toughness and the fatigue damage resistance of the steel rail are provided. At present, with the great increase of the axle weight, the total transportation amount and the transportation frequency of a railway train, new requirements on the impact toughness, the strength, the wear resistance and the like of a steel rail welding joint are provided.

Under the condition, the medium-low carbon pearlite steel rail with higher impact toughness and better fatigue damage resistance is produced. However, similar to other varieties of pearlitic rails, such rail welding applications also suffer from poor full-face impact toughness of the post-weld joint. The post-weld heat treatment is an effective means for improving the impact toughness of the steel rail joint and improving the service performance of the joint. The conventional steel rail postweld normalizing technology can improve the impact toughness of the joint to a certain extent. However, compared with the weld of the normalizing joint rail head, the weld of the rail web and the rail bottom still has relatively low impact toughness, which is not beneficial to the running safety of the railway.

Therefore, in the field of railway engineering, a heat treatment method for improving the post-welding full-section impact toughness matching performance of medium-low carbon pearlite steel rails is needed to improve the performances of the steel rails, such as impact toughness, hardness and the like, which are reduced due to welding, so as to ensure the service performance of a steel rail welding joint and the railway operation safety.

Disclosure of Invention

Aiming at the problems, the invention provides a method for welding a steel rail with medium-low carbon pearlite. The method achieves the purpose of obviously improving the impact toughness of the welding seam and improves the service safety of the railway by respectively controlling the normalizing and heating processes of the rail head, the rail web and the rail bottom of the steel rail joint.

According to an aspect of the present invention, there is provided a method for welding a medium-low carbon pearlite steel rail, the method comprising the steps of:

step 1): welding a plurality of steel rails made of medium-low carbon pearlite steel rail base materials;

step 2): cooling the welded joint of the steel rail welded in the step 1) to a first preset temperature;

step 3): after step 2) is completed, placing the head portion of the welded joint in the heating zone of the first electromagnetic induction coil, placing the web portion and the foot portion of the welded joint in the heating zone of the second electromagnetic induction coil, and simultaneously turning on the first electromagnetic induction coil and the second electromagnetic induction coil to heat the head portion, the web portion and the foot portion to a second predetermined temperature, wherein a first heating frequency of the first electromagnetic induction coil is set to be higher than a second heating frequency of the second electromagnetic induction coil;

step 4): and cooling the heated welding joint.

According to one embodiment of the invention, the first heating frequency and the second heating frequency are set to enable synchronization of temperature changes of the head portion and the web portion and the foot portion of the weld joint. Wherein the specific values of the first heating frequency and the second heating frequency can be set according to the specification of the steel rail.

According to one embodiment of the invention, the parent material comprises the following components in weight percent: 0.56-0.74% of C, 0.40-0.70% of Si, 0.60-1.00% of Mn, 0.15-0.45% of Cr, 0.10-0.40% of Cu, 0.05-0.35% of Ni, 0.02-0.08% of V, and the balance of Fe and inevitable impurities.

According to one embodiment of the invention, the cooling in step 4) comprises:

cooling the rail head of the welding joint to the surface temperature of 380-450 ℃ by taking compressed air or water mist mixed gas with the pressure of 0.1-0.5MPa as a cooling medium, and naturally cooling the welding joint to the ambient temperature; and

and naturally cooling the rail web and the rail bottom to the ambient temperature.

According to one embodiment of the invention, the compressed air or water mist mixture cools the railhead at a cooling rate of 4.0-10.0 ℃/s.

According to an embodiment of the present invention, the first predetermined temperature is 200-300 ℃.

According to one embodiment of the present invention, the second predetermined temperature is 900-.

According to one embodiment of the invention, the steel rail base material microstructure is controlled to include 95-99% pearlite and 5-1% pro-eutectoid ferrite.

According to one embodiment of the invention, the welding upset amount of the steel rail in the step 1) is kept between 10.2 and 12.2mm, and the welding of the steel rail is carried out by using the heat input amount of 7.5 to 9.0 MJ.

According to one embodiment of the invention, the welded rail in step 2) is naturally cooled.

The method for welding the medium-low carbon pearlite steel rail disclosed by the invention achieves the purpose of obviously improving the impact toughness of the welding seam by respectively controlling the normalizing and heating processes of the rail head, the rail web and the rail bottom of the steel rail joint; further, by comprehensively controlling chemical components, welding process and post-welding heat treatment, the aims of remarkably improving the room-temperature impact toughness of the rail web and the rail bottom of the steel rail joint and improving the matching property of the full-section impact toughness are fulfilled. The numerical range of the impact power of the welding seam of the rail head of the joint processed by the method for welding the steel rail with the medium and low carbon pearlite is 35-50J, the numerical range of the impact power of the welding seam of the rail web and the rail bottom is 18-26J, and the impact power of the welding seam of the full section of the joint and the impact power of the corresponding position of the steel rail base metal reach the same level. Compared with the prior steel rail welding and postweld heat treatment process, the impact toughness of the steel rail joint adopting the medium-low carbon pearlite steel rail welding method is greatly improved. Meanwhile, the welding heat affected zone of the normalizing joint has no abnormal structures such as martensite and the like, and the safety of railway operation is favorably ensured.

Drawings

FIG. 1 is a flow chart of a method for welding a medium and low carbon pearlite steel rail according to the present invention;

FIG. 2 shows a schematic diagram of a split heating operation;

FIG. 3 is a side view of a rail head cooling apparatus for use in accordance with an embodiment of the present invention;

FIG. 4 is a bottom plan view of a rail joint cooling apparatus used in accordance with an embodiment of the present invention;

FIG. 5 is a schematic view of a steel rail welded joint impact specimen sampling position;

FIG. 6 is a schematic diagram of the cutting positions of gold phase samples in each of examples and comparative examples;

FIG. 7 is a metallographic structure diagram of a weld heat-affected zone in example 1;

FIG. 8 is a metallographic structure diagram of a weld heat affected zone of comparative example 2;

FIG. 9 is a metallographic structure diagram of a weld heat-affected zone of comparative example 3;

FIG. 10 is a metallographic structure diagram of a weld heat-affected zone of comparative example 4;

FIG. 11 is a metallographic structure diagram of a weld heat-affected zone of comparative example 5.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Under the action of welding thermal cycle, a hardening layer originally belonging to a steel rail base metal disappears, the impact toughness, the strength, the hardness and the like of a welding area are greatly reduced, and a welding joint becomes a weak link of a railway system. Meanwhile, the brittle martensite structure caused by improper cooling and the like in the welding process can also directly relate to the service performance of the steel rail. Therefore, the steel rail joint welding seam and the heat affected zone structure are mainly pearlite and a small amount of ferrite can appear according to the steel rail welding standard TB/T1632.2-2014 of the current railway industry in China. Harmful structures such as martensite or bainite should not appear, otherwise the joint will be subjected to early fatigue fracture due to hardened martensite, and the railway running safety is seriously influenced. In addition, the performance reduction of the welding area directly influences the service performance of the steel rail joint and even harms the railway running safety. Therefore, after the flash welding of the steel rail is finished according to the steel rail welding standard TB/T1632.2-2014 in the current railway industry in China, normalizing heat treatment is required to be carried out so as to improve the indexes of impact toughness, strength, hardness and the like of the joint, improve the reduced mechanical property of the steel rail after the flash welding and improve the service safety of the joint.

The martensite transformation critical cooling speed of the medium-low carbon pearlite rail steel related in the invention is 1.0-1.7 ℃/s theoretically, and the Ms temperature (the start temperature of martensite formation) of the rail steel is 220-260 ℃. When the steel rail flash welding construction is carried out at the ambient temperature of 20-30 ℃, because the ambient cooling speed is relatively slow, a martensite structure is not formed in a steel rail welding heat affected zone generally.

It should be noted that the natural cooling after the welding of the steel rail is to cool the steel rail joint to below 200 ℃ along with the environment without any device after the flash welding of the steel rail finishes the welding and pushing up the beading.

The martensite formation condition in the steel is a product in which the steel is cooled to below the Ms (martensite start) temperature at a temperature higher than the austenitizing temperature and higher than the martensite formation critical cooling rate. In the case of not considering the composition segregation, the rail steel does not form a martensite structure if the cooling rate in the welding and post-welding heat treatment cooling process is lower than the martensite transformation critical cooling rate. In actual steel production, micro-region component segregation in steel is difficult to avoid in the steel alloying process, a partial micro-region cct curve (a supercooled austenite continuous cooling transformation curve) in the steel is caused to move to the right, the martensite transformation starting temperature is reduced, and martensite is easier to form in the micro-region of the steel when the component segregation exists. Therefore, in order to avoid martensite in the welding heat affected zone during the post-weld heat treatment of the steel rail, the final cooling temperature of the post-weld heat treatment is usually set to be more than 100 ℃ higher than the theoretical Ms temperature of the steel rail steel so as to avoid the formation of welding segregation martensite.

The conventional post-welding normalizing process for the steel rail integrally heats the whole cross section of the joint by using one heating device, and the temperature acquisition (namely temperature measurement) position is the tread of the rail head of the welded joint of the steel rail. When the conventional normalizing equipment is adopted to heat the steel rail joint, the thicknesses of the rail web and the rail bottom area are thinner, and the set normalizing heating temperature is easily reached preferentially. The thickness of the rail head area is large, the heat conduction is relatively slow, and the heating process is obviously delayed compared with the rail web and the rail bottom. It is therefore often the case that the web and foot regions of the welded joint have preferentially reached the set normalizing temperature, while the head region has not yet reached the set temperature. When the rail head of the rail joint reaches the set temperature, the rail web and the rail bottom are heated for a long time, and the temperature exceeds the set temperature. In this case, the impact toughness of the weld heat affected zone at the rail web and the rail bottom of the rail joint is easily reduced due to the excessively high heating temperature and the excessively long high temperature retention time in the normalizing process. In addition, the difference of the temperature distribution of the rail head, the rail web and the rail bottom of the welded joint in the normalizing process can indirectly cause the difference of microstructures, austenite grain sizes, residual stress and the like at the rail head, the rail web and the rail bottom, and the uniformity of the mechanical performance of the whole section of the steel rail joint is influenced, so that the difference of the service performance of the whole section of the steel rail joint is further caused, and the operation safety of a railway is not facilitated.

The basic principle of the invention is that the flash welding joint of the steel rail is normalized (re-austenitized), so that the structure of the normalized heating area is transformed again and recrystallized to refine grains and improve the toughness of the joint. Meanwhile, the rail joint head, the rail web and the rail bottom are respectively controlled to be heated to avoid insufficient heating of the rail joint head and overheating of the rail web and the rail bottom. The impact toughness difference of the full-section of the steel rail joint is reduced while the impact toughness of the rail web and the rail bottom of the steel rail joint is improved, so that the aim of improving the matching performance of the impact toughness of the steel rail base metal and the full-section of the joint is fulfilled.

In the invention, the rail welding joint is a region which is obtained after welding and has the total length of 70-110mm including a welding seam. The full section refers to the whole section of the steel rail welding joint, including the welding seam, with the total length being in the range of 70-110mm, and comprises a rail head, a rail web and a rail bottom.

The medium-low carbon pearlite steel rail controls the microstructure of a steel rail base material to be 95-99% of pearlite and 5-1% of pro-eutectoid ferrite (volume percentage). The chemical components of the steel rail base metal for obtaining the microstructure need to meet the following conditions (by mass percent): 0.56-0.74% of C, 0.40-0.70% of Si, 0.60-1.00% of Mn, 0.15-0.45% of Cr, 0.10-0.40% of Cu, 0.05-0.35% of Ni, 0.02-0.08% of V, and the balance of Fe and inevitable impurities.

As shown in fig. 1, the method for welding a medium/low carbon pearlite steel rail according to the present invention generally includes:

step 3): after step 2) is completed, placing the rail head part of the welded joint in a heating area of a first electromagnetic induction coil, placing the rail web part and the rail foot part of the welded joint in a heating area of a second electromagnetic induction coil, and simultaneously turning on the first electromagnetic induction coil and the second electromagnetic induction coil to heat the rail head part, the rail web part and the rail foot part to a second predetermined temperature, wherein a first heating frequency of the first electromagnetic induction coil is set to be higher than a second heating frequency of the second electromagnetic induction coil;

step 4): and cooling the heated welding joint.

The operation of each step is described in more detail below.

In step 1), a plurality of rails made of a medium-low carbon pearlite rail base material are welded. The welding upset amount is controlled to be kept at 10.2-12.2mm, and the welding adopts the heat input amount of 7.5-9.0 MJ. If the upsetting amount is less than 10.2mm, large-size welding seam dust spots, welding non-metallic inclusions and the like are easily caused and cannot be discharged in time, and the impact toughness of the welding seam is reduced. If the welding upset is higher than 12.2mm, excessive discharge of weld metal is easily caused, a cold joint is formed, and further the impact toughness of the weld is reduced. The welding heat input is controlled to be 7.5-9.0MJ, so that the defects of large-size gray spots can be effectively avoided while martensite in a welding heat affected zone of the steel rail in a welding state is avoided. When the welding heat input is lower than 7.5MJ, martensite randomly distributed in the welding heat affected zone of the steel rail in a welding state appears. When the welding heat input is higher than 9.0MJ, large-size dust spots may appear at the welding seam of the steel rail joint in a welding state, and the impact toughness of the joint is influenced.

In step 2), the welded joint of the rail welded in step 1) is cooled to a first predetermined temperature. Wherein, the cooling adopts natural cooling, and the first preset temperature can be 200-300 ℃.

In step 3), the rail head portion of the welded joint is placed in the heating area of the first electromagnetic induction coil, the rail web portion and the rail foot portion of the welded joint are placed in the heating area of the second electromagnetic induction coil, and the first electromagnetic induction coil and the second electromagnetic induction coil are simultaneously turned on to heat the rail head portion, the rail web portion and the rail foot portion to a second predetermined temperature, wherein the first heating frequency of the first electromagnetic induction coil is set to be higher than the second heating frequency of the second electromagnetic induction coil. Wherein, the rail head, the rail web and the rail bottom of the welding joint can be simultaneously heated by adopting a separated normalizing heating coil. Specifically, the rail head of the welded joint separately adopts a set of medium frequency induction heating coils along the profile, and the rail web and the rail base share a set of medium frequency induction heating coils distributed along the profile. The frequency of the induction coil heating the head is higher than the frequency of the coil heating the web and foot, thereby enabling the head and web and foot temperature changes of the weld joint to be synchronized. The heating may be stopped after the rail head, rail web and rail foot surfaces of the weld joint are all heated to some same temperature between 900-.

In step 4), the heated welded joint is cooled. Cooling the rail head of the welding joint to the surface temperature of 380-450 ℃ at the cooling speed of 4.0-10.0 ℃/s by taking compressed air or water mist mixed gas with the pressure of 0.1-0.5MPa as a cooling medium, and then naturally cooling the joint to the ambient temperature. And after the rail web and the rail bottom of the steel rail joint are normalized and heated, naturally cooling to the ambient temperature. The final cooling temperature of the postweld heat treatment is set to be more than 100 ℃ of the theoretical Ms temperature of the rail steel so as to avoid martensite formation in a heat affected zone in the joint cooling process. After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature. In the process of accelerated cooling of rail joints, pearlite transformation is substantially completed when the surface temperature of the rail head, web and rail base is reduced to below 500 ℃.

Fig. 2 shows a schematic diagram of a split heating operation. The first electromagnetic induction coil R1 is symmetrically arranged at the left side and the right side of the weld joint center C, and the first electromagnetic induction coil R1 extends from the front side surface of the rail head to the top of the rail head and then continuously extends to the rear side surface of the rail head, and the first electromagnetic induction coil R1 generally extends around the outer contour of the rail head. The second electromagnetic induction coils R2 are symmetrically arranged at the left side and the right side of the weld joint center C, the second electromagnetic induction coils R2 extend from the front side surface of the rail web to the rail bottom, extend to the rear side surface of the rail bottom along the bottom surface, continue to extend upwards to the rear side surface of the rail web, and extend around the outer contour of the rail web and the rail bottom in general. The ends of the first electromagnetic induction coil R1 and the second electromagnetic induction coil R2 are fixed to the steel rail with fixing bolts F. Current is applied to the first and second electromagnetic coils R1 and R2, respectively, to heat the head, web and foot. In the figure, the coil a1-a2 is positioned in a medium frequency induction heating area on the tread surface and the side surface of the rail head of the steel rail, and the coil b1-b2 is positioned in a medium frequency induction heating area on the rail web and the rail bottom of the steel rail. The parallel distance between the red copper coils distributed along the profile of the rail head, the rail web and the rail bottom of the steel rail and the surface of the steel rail is 20mm, and the surface of each coil is wound with a high-temperature-resistant insulating adhesive tape. The inside of the red copper coil is attached with a cooling circulating water pipeline, so that the coil can not be burnt and damaged due to high temperature caused by induction heating. It should be noted that the rail head, the rail web and the rail foot intermediate frequency induction heating coil are respectively externally connected with a set of multi-turn-ratio intermediate frequency quenching transformer and a circulating water cooling system so as to realize the control heating of the rail head, the rail web and the rail foot of the rail joint respectively. The actual size and the arrangement condition of the heating coil can be adjusted according to the actual size of the steel rail with different profiles. In the test process, a temperature controller is adopted to control and adjust the heating temperature. The working temperature range of the device is 200-1100 ℃. In the test process, two infrared thermometers are adopted to respectively monitor the temperature of the tread and the web of the rail head of the steel rail joint, and the temperature controller can adjust the heating frequency in time according to the actual temperature difference of the rail head, the web and the rail bottom, so that the temperature of the full section of the steel rail joint can be adjusted in time, and finally the full section of the steel rail has the same normalizing heating temperature. When the surface temperature of the rail head is reduced to about 200-plus-300 ℃ after the flash welding of the steel rail joint, the pair of separated medium-frequency induction devices are adopted to respectively and simultaneously heat the rail head, the rail web and the rail bottom of the steel rail joint, and the surfaces of the rail head, the rail web and the rail bottom of the steel rail joint are respectively heated to a certain same set temperature between 900-plus-960 ℃ and then are stopped heating.

Figure 3 is a schematic view of a rail head cooling arrangement for use in accordance with an embodiment of the present invention. Fig. 4 is a bottom view of a rail head cooling apparatus used in accordance with an embodiment of the present invention. The device only cools the tread and the side face of the rail head of the steel rail, and the size and the shape of the aperture of the air outlet can be designed, processed and changed according to actual requirements, so that different cooling strengths (cooling speeds) are realized. The pressure of the cooling medium flowing through the channel 1 and the channel 3 can be monitored through a pressure gauge and other devices, the pressure of the medium can be adjusted according to actual needs, and the cooling medium is sprayed to the tread of the rail head and the side face of the rail head through the top nozzle 2 and the side face nozzle 4 respectively.

FIG. 5 is a schematic view of a steel rail welded joint impact specimen sampling position. In the examples and comparative examples, the impact toughness at the rail head, web and foot of the rail weld joint was the average of the room temperature impact energy of the impact specimens at the rail head, web and foot of the rail weld joint. The impact toughness of the rail head of the welding joint is the average value of the impact work of the corresponding 1# -4# sample, the impact toughness of the rail web of the welding joint is the average value of the impact work of the corresponding 5# -8# sample, and the impact toughness of the rail bottom of the welding joint is the average value of the impact work of the corresponding 9# -14# sample. The rail joint impact specimen was sampled in the manner shown in fig. 5 when the impact test was performed. The obtained steel rail joint subjected to postweld heat treatment is machined into a Charpy U-shaped impact sample, and a welding seam is positioned in the center of the sample. Impact tests were carried out on rail joint impact specimens using a SANS ZBC2000 impact tester at room temperature (20-30 ℃).

Fig. 6 is a schematic diagram of the position of the metallographic specimen cut out in each of the examples and the comparative examples, wherein the position c is the center of the weld joint, and the position d is the sampling position of the metallographic specimen on the rail head tread of the steel rail welded joint. The sampling method is that 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, 3% nitric acid alcohol solution is adopted to carry out etching on the metallographic structure sample of the steel rail joint, and a German Leica MeF3 optical microscope is adopted to observe the metallographic structure of the steel rail joint.

The following are specific examples and comparative examples of the method for welding a medium-low carbon pearlite rail according to the present invention.

Example 1

The microstructure of the steel rail base material is controlled to be 98 percent of pearlite and 2 percent of proeutectoid ferrite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1130MPa, the elongation is 15%, the numerical range of the U-shaped impact energy of the rail head of the base metal at room temperature is 43J, and the numerical range of the U-shaped impact energy of the rail web and the rail bottom at room temperature is 21J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.62% by weight of C, 0.45% by weight of Si, 0.92% by weight of Mn, 0.24% by weight of Cr, 0.25% by weight of Cu, 0.20% by weight of Ni, 0.05% by weight of V, and the balance Fe and unavoidable impurities.

The rail is flash welded by a movable flash welding machine of the rail and adopting the heat input quantity of 7.5MJ, and the actual welding upset forging quantity is kept at 10.5 mm. After the welding of the steel rail is finished, when the surface temperature of the rail head is reduced to about 250 ℃, the rail head, the rail web and the rail bottom of the welded joint are simultaneously heated by adopting a separated normalizing heating mode as shown in figure 2. And stopping heating when the surface temperature of the rail head, the rail web and the rail bottom of the welded joint is heated to 930 ℃. Then, the rail head tread and the rail head side surface of the rail joint are cooled to the surface temperature of 445 ℃ at the cooling speed of 9.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.45MPa as a cooling medium, and then the rail head is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The post-weld heat treated rail joint obtained in this example was machined into a charpy U-shaped impact specimen according to the sampling position shown in fig. 5. The metallographic structure of the metallographic specimen of the rail joint was examined by the method for examining the metallographic structure of the metallic structure described in GB/T13298-2015 "method for examining the metallographic structure of the metallic structure" with reference to the sampling method shown in FIG. 6.

The results show that: for the medium-low carbon pearlite steel rail normalizing joint obtained by the medium-low carbon pearlite steel rail welding method, as shown in fig. 7, no martensite structure appears in the heat affected zone of the steel rail joint under the observation magnification of 100X. The weld structure (the area marked by the left-hand oval dotted line) is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone (the area around the weld, shown enlarged in the right-hand figure) structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the rail head room-temperature impact energy of the air-cooled joint after flash welding is 10J, and the average value of the rail web and the rail bottom welding seam room-temperature impact energy is 7J. After the steel rail post-welding heat treatment method is adopted for treatment, the average value of the impact energy of the welding seam of the head of the normalizing joint is 43J, the average value of the impact energy of the welding seam of the web and the bottom of the rail is 22J, and the impact energy of the welding seam of the full section of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal reach the same level.

Example 2

The microstructure of the steel rail base material is controlled to be 99% of pearlite and 1% of proeutectoid ferrite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1240MPa, the elongation is 12.5%, the numerical range of the U-shaped impact energy of the base metal rail head at room temperature is 36J, and the numerical range of the U-shaped impact energy of the rail web and the rail base at room temperature is 16J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.74% of C, 0.58% of Si, 0.98% of Mn, 0.36% of Cr, 0.18% of Cu, 0.30% of Ni, 0.08% of V, and the balance of Fe and inevitable impurities.

A steel rail mobile flash welding machine is utilized, the heat input quantity of 8.8MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 12.0mm, after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a welding joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 950 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to 390 ℃ at a cooling speed of 7.0 ℃/s by using a cooling device and taking water mist mixed gas with the pressure of 0.30MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the steel rail normalizing joint with the medium-low carbon pearlite obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 9.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 6.3J. After the steel rail post-welding heat treatment method is adopted for treatment, the average value of the impact energy of the welding seam of the head of the normalizing joint is 38J, the average value of the impact energy of the welding seam of the web and the bottom of the rail is 17J, and the impact energy of the welding seam of the full section of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal reach the same level.

Example 3

The microstructure of the steel rail base metal is controlled to be 96 percent of pearlite and 4 percent of pro-eutectoid ferrite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1120MPa, the elongation is 16.5%, the numerical range of the U-shaped impact energy of the base metal rail head at room temperature is 43J, and the numerical range of the U-shaped impact energy of the rail web and the rail base at room temperature is 21J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.60% C, 0.45% Si, 0.75% Mn, 0.35% Cr, 0.40% Cu, 0.35% Ni, 0.05% V, and the balance Fe and unavoidable impurities.

A steel rail mobile flash welding machine is utilized, the heat input quantity of 7.5MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 300 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 910 ℃. Then, a cooling device is adopted to cool the tread and the side surface of the rail head of the steel rail joint to the surface temperature of 440 ℃ at the cooling speed of 5.0 ℃/s by taking compressed air with the pressure of 0.20MPa as a cooling medium, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the steel rail normalizing joint with the medium-low carbon pearlite obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 11.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 8.5J. After the steel rail postweld heat treatment method is adopted, the average value of the impact energy of the weld joint of the head of the normalizing joint is 44J, the average value of the impact energy of the weld joint of the web and the bottom of the rail is 24J, and the impact energy of the weld joint of the full section of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal reach the same level.

Example 4

The microstructure of the steel rail base material is controlled to be 97% of pearlite and 3% of proeutectoid ferrite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1150MPa, the elongation is 15.5%, the numerical range of the U-shaped impact energy of the base metal at the rail head at the room temperature is 40J, and the numerical range of the U-shaped impact energy of the rail web and the rail base at the room temperature is 18J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.68% by weight of C, 0.55% by weight of Si, 0.85% by weight of Mn, 0.35% by weight of Cr, 0.20% by weight of Cu, 0.15% by weight of Ni, 0.05% by weight of V, and the balance Fe and unavoidable impurities.

A steel rail mobile flash welding machine is utilized, the heat input quantity of 8.5MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 11.5mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of the steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 930 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to 400 ℃ at a cooling speed of 7.0 ℃/s by using a cooling device and taking water mist mixed gas with the pressure of 0.30MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the steel rail normalizing joint with the medium-low carbon pearlite obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 10.0J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 7.5J. After the steel rail postweld heat treatment method is adopted, the average value of the impact energy of the weld joint of the head of the normalizing joint is 38J, the average value of the impact energy of the weld joint of the web and the bottom of the rail is 20J, and the impact energy of the weld joint of the full section of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal reach the same level.

Comparative example 1

The steel rail base material and the material of the steel rail flash welding joint in the welding state used in the comparative example are completely the same as those of example 1. In contrast, this comparative example uses a conventional integral heating coil to heat a rail joint at full section, in contrast to the split heating coil described in the present invention.

In example 1, the rail was flash welded using a rail moving flash welding machine with a heat input of 7.5MJ, and the actual amount of upset was kept at 10.5 mm. After the welding of the steel rail is finished, when the surface temperature of the rail head is reduced to about 250 ℃, the rail head, the rail web and the rail bottom of the welded joint are simultaneously heated by adopting a separated normalizing heating mode shown in figure 2. And stopping heating when the surface temperature of the rail head, the rail web and the rail bottom of the welded joint is heated to 930 ℃. Then, the rail head tread and the rail head side surface of the rail joint are cooled to the surface temperature of 445 ℃ at the cooling speed of 9.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.45MPa as a cooling medium, and then the rail head is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

In the comparative example, the rail is flash welded by using a rail mobile flash welding machine and adopting the heat input quantity of 7.5MJ, and the actual welding upset forging quantity is kept at 10.5 mm. After the steel rail is welded, when the surface temperature of the rail head is reduced to about 250 ℃, the rail head, the rail web and the rail bottom of the welded joint are heated by adopting a traditional integral normalizing heating mode. And stopping heating when the surface temperature of the rail head, the rail web and the rail bottom of the welded joint is heated to 930 ℃. Then, the rail head tread and the rail head side surface of the rail joint are cooled to the surface temperature of 445 ℃ at the cooling speed of 9.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.45MPa as a cooling medium, and then the rail head is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The post-weld heat treated rail joint obtained in this comparative example was machined into a charpy U-shaped impact specimen in accordance with the sampling positions shown in fig. 5. The metallographic structure of the metallographic specimen of the rail joint was examined by the method for examining the metallographic structure of the metallic structure described in GB/T13298-2015 "method for examining the metallographic structure of the metallic structure" with reference to the sampling method shown in FIG. 6.

The results show that: for the steel rail normalizing joint with the medium-low carbon pearlite obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X, and the detection result is similar to the metallographic structure shown in the figure 7. The average value of the rail head room-temperature impact energy of the obtained air-cooled (in a welding state) joint after flash welding is 10J, and the average value of the rail web and the rail bottom welding seam room-temperature impact energy is 7J. For the normalized joint using this comparative example, the average value of the work of percussion of the railhead weld was 43J, which was exactly the same as the work of percussion of the railhead weld of the normalized joint in example 1. In the comparative example, the average value of the impact energy of the weld joints at the rail web and the rail bottom of the normalized joint is 14J, and is lower than the average value of the impact energy of the weld joints at the rail web and the rail bottom of the normalized joint in the example 1, namely 22J. Namely, the impact energy of the normalized joint web and the rail bottom weld joint obtained in the comparative example is lower than that of the normalized joint web and the rail bottom weld joint in the embodiment 1, so that the matching performance of the impact energy of the normalized joint full-section weld joint and the impact energy of the corresponding position of the steel rail base metal is further reduced, and the operation safety of the railway is not facilitated.

Comparative example 2

The microstructure of the steel rail base metal is controlled to be 99.5 percent of pearlite and 0.5 percent of proeutectoid cementite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1400MPa, the elongation is 9%, the numerical range of the U-shaped impact power of the base metal at the rail head at the room temperature is 15J, and the numerical range of the U-shaped impact power of the rail web and the rail base at the room temperature is 9J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.98% by weight of C, 0.65% by weight of Si, 0.85% by weight of Mn, 0.55% by weight of Cr, 0.30% by weight of Cu, 0.20% by weight of Ni, 0.05% by weight of V, and the balance Fe and unavoidable impurities.

The steel rail is moved by a flash welding machine, the flash welding of the steel rail is carried out by adopting the heat input quantity of 8.5MJ, the actual welding upset forging quantity is kept at 11.5mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, the existing integral normalizing heating coil is adopted to carry out full-section heating on the rail head, the rail waist and the rail bottom of the steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 930 ℃. Then, the tread and the side surface of the rail head of the steel rail joint are cooled to the surface temperature of 400 ℃ at the cooling speed of 7.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.30MPa as a cooling medium, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: with respect to the rail normalized joint obtained by the above method, as shown in fig. 8, no martensite structure appears in the heat-affected zone of the rail joint at an observation magnification of 100X. The weld structure (the area marked by the left oval dotted line) is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone (the area around the weld, shown enlarged in the right drawing) structure is pearlite and a small amount of pro-eutectoid cementite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 10.0J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 5J. After the common normalizing heat treatment is carried out, the average value of the impact energy of the weld joint of the head of the normalizing joint is 12J, and the average value of the impact energy of the weld joint of the rail web and the rail bottom is 7J. The overall impact performance of the joint is relatively low, which is not beneficial to the railway operation safety.

Comparative example 3

A steel rail mobile flash welding machine is utilized, the heat input quantity of 7.5MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 930 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to the surface temperature of 195 ℃ at the cooling speed of 9.0 ℃/s by adopting a cooling device and taking water mist mixed gas with the pressure of 0.45MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium and low carbon pearlite steel rail normalizing joint obtained by the method, as shown in fig. 9, no martensite structure appears in the heat affected zone of the steel rail joint under the observation magnification of 100X. The weld structure is pearlite and pro-eutectoid ferrite along the crystal (the area marked by the oval dotted line in the left drawing), and the heat affected zone structure is pearlite + a small amount of pro-eutectoid ferrite + a small amount of martensite (the area around the weld is shown in an enlarged manner in the right drawing, and the inside of the rectangular frame is martensite). The average value of the room temperature impact energy of the weld joint of the air-cooled joint rail head after flash welding is 10J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 7J. After heat treatment, the average value of the impact energy of the weld joint of the rail head of the normalized joint is 23J, and the average value of the impact energy of the weld joint of the rail web and the rail bottom is 15J. Due to the existence of martensite in the welding heat affected zone, the difference between the impact energy of the full-section welding line of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal is large, the matching performance of the full-section impact toughness of the joint and the base metal is poor, and the operation safety of the railway is not facilitated.

Comparative example 4

A steel rail mobile flash welding machine is utilized, the heat input quantity of 8.8MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 15.0mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 950 ℃. Then, the tread and the side surface of the rail head of the steel rail joint are cooled to the surface temperature of 390 ℃ at the cooling speed of 7.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.30MPa as a cooling medium, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium and low carbon pearlite steel rail normalizing joint obtained by the construction method of the postweld heat treatment of the invention, as shown in fig. 10, no martensite structure appears in the heat affected zone of the steel rail joint under the observation magnification of 100X. The weld structure is pearlite and pro-eutectoid ferrite along the crystal (the area marked by the oval dotted line in the left figure), and the heat-affected zone structure is pearlite and a small amount of pro-eutectoid ferrite (the area around the weld, shown enlarged in the right figure). Due to the excessive upset amount, the excess weld metal is expelled, forming a cold joint. The average value of the room temperature impact energy of the weld joint of the air-cooled joint rail head after flash welding is 9J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 5J. After heat treatment, the average value of the impact energy of the weld joint of the rail head of the normalizing joint is 16J, the average value of the impact energy of the weld joint of the rail web and the rail bottom is 10J, the difference between the impact energy of the weld joint of the full section of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal is large, the matching performance of the impact toughness of the full section of the joint and the base metal is poor, and the safety of railway operation is not facilitated.

Comparative example 5

The microstructure of the steel rail base metal is controlled to be 96% of pearlite and 4% of proeutectoid ferrite. The tensile strength of the steel rail base metal at room temperature (20-25 ℃) is 1120MPa, the elongation is 16.5%, the numerical range of the U-shaped impact energy of the base metal rail head at room temperature is 43J, and the numerical range of the U-shaped impact energy of the rail web and the rail base at room temperature is 21J. The chemical components of the rail steel for obtaining the microstructure and the mechanical property need to meet the following conditions: 0.60% C, 0.45% Si, 0.75% Mn, 0.35% Cr, 0.40% Cu, 0.35% Ni, 0.05% V, and the balance Fe and unavoidable impurities.

A steel rail mobile flash welding machine is utilized, the heat input quantity of 7.5MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. And stopping heating when the surface temperatures of the rail head, the rail web and the rail bottom of the steel rail joint are heated to 850 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to the surface temperature of 440 ℃ at the cooling speed of 5.0 ℃/s by adopting a cooling device and taking water mist mixed gas with the pressure of 0.20MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The post weld heat treated rail joint obtained in this example was machined into charpy U-shaped impact specimens in accordance with the sampling positions shown in fig. 5. The metallographic structure of the metallographic specimen of the rail joint was examined by the method of metallographic examination of metal microstructure described in GB/T13298-2015, with reference to the sampling method shown in FIG. 6.

The results show that: for the medium and low carbon pearlite steel rail normalizing joint obtained by the method, as shown in fig. 11, no martensite structure appears in the heat affected zone of the steel rail joint under the observation magnification of 100X. The weld structure is pearlite and pro-eutectoid ferrite along the crystal (the area marked by the oval dotted line in the left figure), and the heat-affected zone structure is pearlite and a small amount of pro-eutectoid ferrite (the area around the weld, shown enlarged in the right figure). The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 11.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 8.5J. By adopting the steel rail postweld heat treatment method of the comparative example, the normalizing heating temperature is lower, and the austenitizing process of the steel rail joint is incomplete. The average value of the impact energy of the welding line of the head of the normalizing joint is 24J, the average value of the impact energy of the welding line of the web of the rail and the bottom of the rail is 17J, the difference between the impact energy of the welding line of the full section of the normalizing joint and the impact energy of the corresponding position of the base metal of the rail is large, the matching performance of the impact toughness of the full section of the joint and the base metal is poor, and the operation safety of the railway is not facilitated.

Comparative example 6

The rail is flash welded by a movable flash welding machine of the rail and adopting the heat input quantity of 7.5MJ, and the actual welding upset forging quantity is kept at 10.5 mm. After the welding of the steel rail is finished, when the surface temperature of the rail head is reduced to about 200 ℃, the separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail web and the rail bottom of the steel rail joint. And stopping heating when the surface temperature of the rail head, the rail web and the rail bottom of the steel rail joint is heated to 1150 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to the surface temperature of 440 ℃ at the cooling speed of 5.0 ℃/s by adopting a cooling device and taking water mist mixed gas with the pressure of 0.20MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium-low carbon pearlite steel rail normalizing joint obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 11.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 8.5J. By adopting the method, because the normalizing heating temperature is too high, the high-temperature retention time of the steel rail joint is too long, austenite grains are coarsened, and the impact toughness is reduced. The average value of the impact energy of the welding line of the head of the normalizing joint is 25J, the average value of the impact energy of the welding line of the web of the rail and the bottom of the rail is 18J, the difference between the impact energy of the welding line of the full section of the normalizing joint and the impact energy of the corresponding position of the base metal of the rail is large, the matching performance of the impact toughness of the full section of the joint and the base metal is poor, and the operation safety of the railway is not facilitated.

Comparative example 7

A steel rail mobile flash welding machine is utilized, the heat input quantity of 8.8MJ is adopted to carry out steel rail flash welding, the actual welding upset forging quantity is kept at 12.0mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. Wherein, the heating is stopped when the surface temperature of the rail head of the rail joint is heated to 950 ℃, and the heating is stopped when the surface temperature of the rail web and the rail bottom is heated to 800 ℃. Then, the tread and the side surface of the rail head of the steel rail joint are cooled to the surface temperature of 390 ℃ at the cooling speed of 7.0 ℃/s by adopting a cooling device and taking compressed air with the pressure of 0.30MPa as a cooling medium, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, and therefore the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium-low carbon pearlite steel rail normalizing joint obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 9.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 6.3J. After heat treatment, normalizing the weld joint of the normalized joint railhead sufficiently, wherein the average value of the impact energy is 38J; the average value of the impact energy is only 10J due to the low normalizing heating temperature of the welding line of the rail web and the rail bottom, and the difference between the impact energy of the full-section welding line of the normalizing joint and the impact energy of the corresponding position of the steel rail base metal is large, so that the operation safety of the railway is not facilitated.

Comparative example 8

The method comprises the steps of carrying out steel rail flash welding by using a steel rail mobile flash welding machine and adopting 8.8MJ heat input quantity, wherein the actual welding upsetting quantity is kept at 12.0mm, and after the steel rail is welded, when the surface temperature of a rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to simultaneously heat the rail head, the rail waist and the rail bottom of a steel rail joint. Wherein, the heating is stopped when the surface temperature of the rail head of the rail joint is heated to 800 ℃, and the heating is stopped when the surface temperature of the rail web and the rail bottom is heated to 950 ℃. Then, cooling the tread and the side surface of the rail head of the steel rail joint to 390 ℃ at a cooling speed of 7.0 ℃/s by using a cooling device and taking water mist mixed gas with the pressure of 0.30MPa as a cooling medium, and naturally cooling the joint to the ambient temperature (20-30 ℃). After the rail web and the rail bottom of the steel rail joint are normalized and heated, the steel rail joint is naturally cooled to the ambient temperature, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium-low carbon pearlite steel rail normalizing joint obtained by the method, no martensite structure appears in a heat affected zone of the steel rail joint under the observation magnification of 100X. Wherein the weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure is pearlite and a small amount of pro-eutectoid ferrite. The average value of the room temperature impact energy of the weld joint of the air-cooled joint head after flash welding is 9.5J, and the average value of the room temperature impact energy of the weld joint at the rail web and the rail bottom is 6.3J. After heat treatment, the normalizing joint rail head welding line has a low normalizing heating temperature, so that the average value of the impact energy is only 11J, the normalizing of the rail web and rail bottom welding line is sufficient, the average value of the impact energy is 21J, the difference between the impact energy of the normalizing joint full-section welding line and the impact energy of the corresponding position of the steel rail base metal is large, and the operation safety of a railway is not facilitated.

As can be seen by comparing examples 1 to 4 with comparative examples 1 to 8: by adopting the method for welding the steel rail with the medium-low carbon pearlite provided by the invention, the full-section impact toughness matching property of the steel rail joint can be improved. Meanwhile, martensite structures in a heat affected zone of the steel rail joint are avoided. The numerical range of the impact energy of the welding line of the joint railhead heat-treated by the method is 35-50J, the numerical range of the impact energy of the welding line of the rail web and the rail bottom is 18-26J, and the impact energy of the full-section welding line of the normalized joint and the impact energy of the corresponding position of the medium-low carbon pearlite steel rail base material related by the invention reach the same level, thereby being beneficial to ensuring the running safety of the railway.

The above-mentioned embodiments only express the embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.

Claims

1. A method for welding a medium-low carbon pearlite steel rail is characterized by comprising the following steps:

and step 3): placing a head portion of the weld joint into a heating zone of a first electromagnetic induction coil, placing a web portion and a foot portion of the weld joint into a heating zone of a second electromagnetic induction coil, and simultaneously turning on the first electromagnetic induction coil and the second electromagnetic induction coil to heat the head portion, the web portion, and the foot portion to a second predetermined temperature, wherein a first heating frequency of the first electromagnetic induction coil is set higher than a second heating frequency of the second electromagnetic induction coil;

and step 4): cooling the heated weld joint.

2. The method of claim 1, wherein the first and second heating frequencies are set to enable synchronization of temperature changes of the head portion and the web and foot portions of the weld joint.

3. The method according to claim 1, wherein the parent material comprises the following components in weight percent: 0.56-0.74% of C, 0.40-0.70% of Si, 0.60-1.00% of Mn, 0.15-0.45% of Cr, 0.10-0.40% of Cu, 0.05-0.35% of Ni, 0.02-0.08% of V, and the balance of Fe and inevitable impurities.

4. The method of claim 1, wherein the cooling in step 4) comprises:

5. The method of claim 4, wherein the compressed air or water mist cools the railhead at a cooling rate of 4.0-10.0 ℃/s.

6. The method as claimed in claim 1, wherein the first predetermined temperature is 200-300 ℃.

7. The method as claimed in claim 1, wherein the second predetermined temperature is 900-960 ℃.

8. The method of claim 1, wherein the rail parent material microstructure is controlled to include 95-99% pearlite and 5-1% pro-eutectoid ferrite.

9. The method as claimed in claim 1, wherein the rail welding upset in step 1) is maintained at 10.2-12.2mm and rail welding is performed using a heat input of 7.5-9.0 MJ.

10. The method of claim 1, wherein the welded rail in step 2) is naturally cooled.