CN114891967B

CN114891967B - Welding method for medium-low carbon pearlite steel rail

Info

Publication number: CN114891967B
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: 2023-10-13
Anticipated expiration: 2042-06-20
Also published as: WO2023246359A1; CN114891967A

Abstract

The invention discloses a welding method of a medium-low carbon pearlite steel rail. 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 steel rail to a first preset temperature; placing the rail head portion of the weld joint in the heating zone of the first electromagnetic induction coil, placing the web portion and the rail foot portion of the weld 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 rail head portion, the 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 aim of remarkably improving the impact toughness of the welding seam and improves the service safety of the railway by respectively controlling the normalizing heating process of the rail head, the rail web and the rail bottom of the rail joint.

Description

Welding method for medium-low carbon pearlite steel rail

Technical Field

The invention relates to the technical field of railway steel rail manufacturing, in particular to a welding method of 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 has put high demands on the comprehensive performance of rail base materials and welded joints. At present, the carbon content of the railway steel rail is mainly concentrated to 0.7-1.1%, and the railway steel rail has full pearlite or pearlite and a small amount of proeutectoid ferrite (or proeutectoid cementite) structure, and the strength of the railway steel rail is generally not lower than 880MPa and has good wear resistance. And the steel rail has new requirements on impact toughness and fatigue damage resistance of the steel rail for severe road sections with special natural conditions such as cold areas in winter, large annual temperature difference, large day-night temperature difference and the like. At present, with the great increase of the axle weight, the total transportation amount and the transportation frequency of railway trains, new requirements on the impact toughness, the strength, the wear resistance and the like of steel rail welding joints are provided.

In this case, a medium-low carbon pearlite rail having higher impact toughness and better fatigue damage resistance has been developed. However, similar to other varieties of pearlitic rails, there is also the problem of poor full face impact toughness of the post-weld joint in such rail welding applications. Post-welding heat treatment is an effective means for improving the impact toughness of the rail joint and improving the service performance of the joint. Conventional post-weld normalizing techniques for rails can improve joint impact toughness to some extent. However, the rail web and rail foot welds are still relatively low in impact toughness compared to the normalizing joint rail head welds, which is detrimental to railway operation safety.

Therefore, a heat treatment method for improving the full-section impact toughness matching property of the welded middle-low carbon pearlite steel rail is needed in the railway engineering field so as to improve the properties of the steel rail, such as reduced impact toughness, hardness and the like caused by welding, and ensure the service performance of the welded joint of the steel rail and the railway operation safety.

Disclosure of Invention

The invention provides a welding method for a medium-low carbon pearlite steel rail. The method achieves the aim of remarkably improving the impact toughness of the welding seam and improves the service safety of the railway by respectively controlling the normalizing heating process of the rail head, the rail web and the rail bottom of the rail joint.

According to an aspect of the present invention, there is provided a method for welding a medium and low carbon pearlite 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 welded steel rail in the step 1) to a first preset temperature;

step 3): placing the rail head portion of the weld joint in the heating zone of the first electromagnetic induction coil after step 2) is completed, placing the web portion and the rail foot portion of the weld 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 rail head portion, the 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;

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 such that temperature changes of the head portion and the web portion and the foot portion of the weld joint can be synchronized. 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 base 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 unavoidable impurities.

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

cooling the rail head of the welding joint to 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

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 rail head at a cooling rate of 4.0-10.0 ℃/s.

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

According to one embodiment of the invention, the second predetermined temperature is 900-960 ℃.

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

According to one embodiment of the invention, the upsetting amount of the steel rail welding in the step 1) is kept between 10.2 and 12.2mm, and the steel rail welding is performed by adopting 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 cooled naturally.

The middle-low carbon pearlite steel rail welding method disclosed by the invention achieves the aim of obviously improving the impact toughness of a welding line by respectively controlling the normalizing heating process of the rail head, the rail web and the rail bottom of the steel rail joint; further, the comprehensive control of chemical components, welding process and postweld heat treatment achieves the purposes of remarkably improving the room temperature impact toughness of rail web and rail bottom of the steel rail joint and improving the full section impact toughness matching property. The impact power value range of the welding seam of the head of the joint treated by the welding method of the medium-low carbon pearlite steel rail is 35-50J, the impact power value range of the welding seam of the waist and the bottom of the rail 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 base metal of the steel rail reach the same level. Compared with the existing steel rail welding and post-welding heat treatment process, the steel rail joint impact toughness of 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 is beneficial to guaranteeing the running safety of a railway.

Drawings

FIG. 1 is a flow chart of a method of welding a medium and low carbon pearlite 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 device used in accordance with one embodiment of the present invention;

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

FIG. 5 is a schematic view of the sampling location of the impact specimen of the rail weld joint;

FIG. 6 is a schematic view showing the cut-out positions of metallographic specimens in each of examples and comparative examples;

FIG. 7 is a metallographic view of a weld heat affected zone of example 1;

FIG. 8 is a metallographic view of the weld heat affected zone of comparative example 2;

FIG. 9 is a metallographic view of the weld heat affected zone of comparative example 3;

FIG. 10 is a metallographic view of the weld heat affected zone of comparative example 4;

FIG. 11 is a metallographic view of the weld heat affected zone of comparative example 5.

Detailed Description

The present invention will be described in further detail with reference to specific embodiments in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

Under the action of welding thermal cycle, the hardening layer originally belonging to the base material of the steel rail disappears, the impact toughness, strength, hardness and the like of the welding area are greatly reduced, and the welding joint becomes a weak link of a railway system. Meanwhile, brittle martensitic structure caused by improper cooling and the like in the welding process can also be directly related to service performance of the steel rail. Therefore, the steel rail welding standard TB/T1632.2-2014 of the current railway industry in China stipulates that the steel rail joint weld joint and the heat affected zone structure should be mainly pearlitic, and a small amount of ferrite can appear. The martensite or bainite and other harmful structures should not appear, otherwise, the joint can be broken by premature fatigue caused by quenched martensite, and the railway operation safety is seriously affected. In addition, the performance degradation of the welding area directly affects the service performance of the rail joint and even jeopardizes the running safety of the railway. Therefore, after the steel rail welding standard TB/T1632.2-2014 in the current railway industry in China prescribes that the steel rail is subjected to flash welding, the normalizing heat treatment is required to improve the indexes such as impact toughness, strength, hardness and the like of the joint, improve the mechanical property reduced after the steel rail flash welding and improve the service safety of the joint.

The theoretical martensitic transformation critical cooling speed of the medium-low carbon pearlite rail steel is 1.0-1.7 ℃/s, and the rail steel Ms temperature (the starting temperature of martensite formation) is 220-260 ℃. When the steel rail flash welding construction is carried out at the environment temperature of 20-30 ℃, a martensitic structure is not usually formed in a steel rail welding heat affected zone due to the relatively slow ambient cooling speed.

It should be noted that natural cooling after rail welding is to cool rail joints to below 200 ℃ along with the environment without adopting any device after rail flash welding completes welding and pushing.

The martensite formation condition in the steel is a product of cooling the steel above the austenitizing temperature and below the Ms (martensite start) temperature at a temperature above the critical cooling rate for martensite formation. The cooling rate of the rail steel during welding and post-welding heat treatment cooling is lower than the martensitic transformation critical cooling rate without considering the component segregation, so that a martensitic structure is not formed. In actual steel production, the micro-zone component segregation in the steel is difficult to avoid due to the alloying process of the steel, so that the cct curve (supercooled austenite continuous cooling transition curve) of the part of the steel moves to the right, and the martensitic transformation starting temperature is reduced, namely the micro-zone of the part of the steel is more prone to form martensite when the component segregation exists. Therefore, to avoid martensite in the weld heat affected zone during the post weld heat treatment of the rail, the post weld heat treatment finish cooling temperature is typically set above 100 ℃ above the theoretical Ms temperature of the rail steel to avoid the formation of weld segregated martensite.

The conventional steel rail post-welding normalizing process uses a heating device to integrally heat the whole section of the joint, and the temperature acquisition (i.e. temperature measurement) position is the tread of the rail head of the steel rail welding joint. When the conventional normalizing equipment is used for heating the rail joints, the rail web and the rail bottom area are thinner, and the set normalizing heating temperature is easy to be preferentially reached. While the head region has a greater thickness and relatively slow heat transfer, the heating process is significantly retarded compared to the web and foot. It is therefore common for the web and foot regions of the weld joint to 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 is already higher than the set temperature. In this case, the weld heat affected zone at the web and foot of the rail joint is susceptible to reduced impact toughness due to excessive heating temperatures and excessive high temperature residence times during normalizing. In addition, the difference of temperature distribution of the rail head, the rail web and the rail bottom of the welding joint in the normalizing process can indirectly cause the difference of microstructure, austenite grain size, residual stress and the like at the rail head, the rail web and the rail bottom, and the uniformity of the mechanical property of the full section of the rail joint is affected, so that the difference of the service property of the full section of the rail joint is further caused, and the safety of railway operation is not facilitated.

The basic principle of the invention is that the normalizing heating area structure is re-transformed and recrystallized to refine grains and improve the joint toughness by normalizing (re-austenitizing) the steel rail flash welding joint. Meanwhile, the rail joint head, the rail web and the rail bottom are heated by controlling the heating respectively so as to avoid insufficient heating of the rail joint head and overheating of the rail web and the rail bottom. The gap between the full-section impact toughness is reduced while the impact toughness of the rail web and the rail bottom of the rail joint is improved, so that the aim of improving the matching property of the full-section impact toughness of the base metal of the rail and the joint is fulfilled.

In the present invention, the steel rail welded "joint" is a region obtained after welding and including a welded joint, and having a total length ranging from 70 mm to 110 mm. Full section refers to the entire cross section of the rail welded joint, including the head, web, and foot, including the weld joint, over a total length of about 70-110 mm.

The medium-low carbon pearlite steel rail controls the microscopic structure of the steel rail parent metal to be 95-99% pearlite and 5-1% proeutectoid ferrite (volume percentage). The chemical composition of the steel rail parent metal for obtaining the microstructure needs to meet the following conditions (in 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 unavoidable impurities.

As shown in fig. 1, the welding method of the medium-low carbon pearlite steel rail according to the present invention generally comprises:

step 4): and cooling the heated welding joint.

The operation of the individual steps 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 upsetting amount of welding is controlled to be 10.2-12.2mm, and the heat input amount of 7.5-9.0MJ is adopted for welding. If the upsetting amount is less than 10.2mm, large-size weld ash spots, welding nonmetallic inclusions and the like are easy to cause and cannot be discharged in time, and the impact toughness of the weld is reduced. If the welding upsetting amount is higher than 12.2mm, the excessive discharge of weld metal is easily caused, a cold joint is formed, and the impact toughness of the weld is further reduced. The welding heat input is controlled at 7.5-9.0MJ, so that the formation of large-size gray specks can be effectively avoided while the martensite is avoided in the welding heat affected zone of the steel rail in a welding state. When the welding heat input is lower than 7.5MJ, martensite distributed randomly in the welding heat affected zone of the steel rail occurs in the welding state. When the welding heat input is higher than 9.0MJ, large-size gray spots can appear at the joint seams of the steel rail joints in a welding state, and the impact toughness of the joints is affected.

In step 2), the welded joint of the rails 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 weld joint is placed in the heating zone of the first electromagnetic induction coil, the web portion and the rail foot portion of the weld joint are placed in the heating zone 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 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. Wherein, separate normalizing heating coils can be adopted to heat the rail head, the rail web and the rail bottom of the welding joint respectively and simultaneously. Specifically, the rail head of the welding joint adopts a set of medium frequency induction heating coils along the profile of the rail head, and the rail web and the rail bottom share a set of medium frequency induction heating coils distributed along the profile of the rail web and the rail bottom. The frequency of the induction coil heating the head is higher than the frequency of the coils heating the web and the foot, thereby enabling the temperature changes of the head and web and foot of the weld joint to be synchronized. The heating may be stopped after the head, web and foot surfaces of the weld joint are all heated to some same temperature between 900-960 c.

In step 4), the heated welded joint is cooled. And cooling the rail head of the welding joint to 380-450 ℃ at a 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 naturally cooling the joint to the ambient temperature. And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint. The final cooling temperature of the postweld heat treatment is set above the theoretical Ms temperature of the rail steel of 100 ℃ so as to avoid martensite formation in a heat affected zone in the joint cooling process. And after normalizing heating of the rail web and the rail bottom of the rail joint is finished, naturally cooling to the ambient temperature. During the accelerated cooling of the rail joint, the pearlitic transformation is substantially completed when the rail head surface, web and foot surface temperatures drop below 500 ℃.

Fig. 2 shows a schematic diagram of a split heating operation. The first electromagnetic induction coils R1 are symmetrically arranged at the left side and the right side of the welding line center C, the first electromagnetic induction coils R1 extend from the front side surface of the rail head to the top of the rail head and then continuously extend to the rear side surface of the rail head, and the first electromagnetic induction coils R1 generally extend 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 welding line 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, and extend upwards to the rear side surface of the rail web, and generally extend around the outer contours of the rail web and the rail bottom. The ends of the first electromagnetic induction coil R1 and the second electromagnetic induction coil R2 are fixed to the rail with fixing bolts F. Current is applied to the first and second electromagnetic induction coils R1 and R2, respectively, to heat the head, web and foot. In the figure, the positions of the coils a1-a2 are the rail head tread and rail head side surface medium frequency induction heating areas, and the positions of the coils b1-b2 are the rail web and rail bottom medium frequency induction heating areas. The parallel distance between the red copper coils distributed along the rail head, the rail web and the rail bottom profile of the steel rail and the surface of the steel rail is 20mm, and the surface of the coils is wound with high-temperature-resistant insulating adhesive tapes. The copper coil is internally provided with a cooling circulating water pipeline, so that the coil can be ensured not to be burnt due to high temperature caused by induction heating. The rail head, the rail web and the rail bottom medium frequency induction heating coils are respectively externally connected with a set of multi-turn ratio medium 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 bottom of the rail joint. The actual size and arrangement of the heating coils can be adjusted according to the actual sizes of the steel rails with different profiles. In the test process, a temperature controller is adopted to control and regulate the heating temperature. The working temperature range of the device is 200-1100 ℃. In the test process, two infrared thermometers are adopted to monitor the temperature of the rail head tread and the rail web of the rail joint respectively, and a temperature controller can timely adjust the heating frequency according to the actual temperature difference of the rail head, the rail web and the rail bottom, so that the temperature of the whole section of the rail joint can be timely adjusted, and finally the whole section of the rail has the same normalizing heating temperature. When the surface temperature of the rail head after the flash welding of the rail joint is reduced to about 200-300 ℃, the split type medium frequency induction device is adopted to heat the rail head, the rail web and the rail bottom of the rail joint respectively and simultaneously, and the surface of the rail head, the rail web and the rail bottom of the rail joint are respectively heated to a certain same set temperature between 900-960 ℃ and then are stopped heating.

Fig. 3 is a schematic view of a rail head cooling device used in accordance with an embodiment of the present invention. Fig. 4 is a bottom view of a rail head cooling device used in accordance with one embodiment of the present invention. The device only cools the tread of the rail head 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, thereby realizing different cooling strengths (cooling speeds). The pressure of the cooling medium flowing through the channels 1 and 3 can be monitored by devices such as pressure gauges, the pressure of the medium can be regulated according to actual needs, and the cooling medium is sprayed to the rail head tread and the rail head side through the top nozzle 2 and the side nozzle 4 respectively.

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

Fig. 6 is a schematic diagram of the cutting position of the metallographic specimen in each example and comparative example, wherein the c position is the center of the welding seam, and the d position is the sampling position of the metallographic specimen of the tread of the rail head of the steel rail welding joint. The sampling method is used for carrying out metallographic structure inspection on the steel rail joint metallographic specimen according to GB/T13298-2015 metal microscopic structure inspection method, carrying out etching on the steel rail joint metallographic specimen by adopting 3% nitric alcohol solution, and observing the steel rail joint metallographic structure by adopting a German Laika MeF3 optical microscope.

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

Example 1

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

And carrying out flash welding on the steel rail by using a steel rail moving flash welding machine and adopting a heat input amount of 7.5MJ, wherein the actual welding upsetting amount is kept at 10.5mm. 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 welding joint are heated simultaneously by adopting a separated normalizing heating mode as shown in fig. 2. The heating was stopped when the head, web and foot surface temperatures of the weld joint were heated to 930 ℃. Then, the head tread and the head side of the rail joint were cooled to a surface temperature of 445 ℃ at a cooling rate of 9.0 ℃/s by using a cooling device with compressed air having a pressure of 0.45MPa as a cooling medium, and then the head was naturally cooled to an ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

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 steel rail joint metallographic specimen is inspected according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 6.

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

Example 2

The steel rail parent metal microstructure is controlled to be 99% pearlite and 1% proeutectoid ferrite. The room temperature (20-25 ℃) tensile strength of the base metal of the steel rail is 1240MPa, the extensibility is 12.5%, the U-shaped impact power value range of the base metal rail head room temperature is 36J, and the U-shaped impact power value range of the rail web and the rail bottom room temperature is 16J. The chemical components of the steel 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 unavoidable impurities.

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.8MJ, the actual welding upsetting quantity is kept at 12.0mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the welding joint at the same time. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 950 c. Then, the water mist mixed gas with the pressure of 0.30MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 390 ℃ at the cooling speed of 7.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the method of the present invention, no martensitic structure appears in the rail joint heat affected zone under 100X observation magnification. Wherein, the weld joint structure is pearlite and peritectic proeutectoid ferrite, and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite. The average value of the room temperature impact power of the welding seam at the rail head and the welding seam at the rail web and the rail bottom of the air-cooled joint after flash welding is 9.5J, and the average value of the room temperature impact power of the welding seam 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 power of the welding seam of the rail head of the normalizing joint is 38J, the average value of the impact power of the welding seam of the rail web and the rail bottom is 17J, and the impact power of the welding seam of the full section of the normalizing joint and the impact power of the corresponding position of the base metal of the steel rail reach the same level.

Example 3

The steel rail parent metal microstructure is controlled to be 96% pearlite and 4% proeutectoid ferrite. The room temperature (20-25 ℃) tensile strength of the base metal of the steel rail is 1120MPa, the extensibility is 16.5%, the U-shaped impact power value range of the base metal rail head room temperature is 43J, and the U-shaped impact power value range of the rail web and the rail bottom room temperature is 21J. The chemical components of the steel 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.

The flash welding of the steel rail is carried out by using a steel rail moving flash welding machine and adopting a heat input quantity of 7.5MJ, the actual welding upsetting quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 300 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 910 c. Then, the rail head tread and the rail head side face of the rail joint are cooled to 440 ℃ at a cooling speed of 5.0 ℃/s by adopting a cooling device and 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 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

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

Example 4

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

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.5MJ, the actual welding upsetting quantity is kept at 11.5mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 930 ℃. Then, the water mist mixed gas with the pressure of 0.30MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 400 ℃ at the cooling speed of 7.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

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

Comparative example 1

The material of the flash welding head of the steel rail in the welded state used in this comparative example was exactly the same as in example 1. In contrast, this comparative example uses a conventional integral heating coil to heat the full face of the rail joint, in sharp contrast to the split heating coil described in this invention.

In example 1, the flash welding of the rail was performed using a rail-moving flash welder with a heat input of 7.5MJ, and the actual welding upsetting amount was kept at 10.5mm. 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 welding joint are heated simultaneously by adopting a separated normalizing heating mode as shown in fig. 2. The heating was stopped when the head, web and foot surface temperatures of the weld joint were heated to 930 ℃. Then, the head tread and the head side of the rail joint were cooled to a surface temperature of 445 ℃ at a cooling rate of 9.0 ℃/s by using a cooling device with compressed air having a pressure of 0.45MPa as a cooling medium, and then the head was naturally cooled to an ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

In the comparative example, the flash welding of the steel rail is carried out by using a steel rail moving flash welding machine and adopting a heat input amount of 7.5MJ, and the actual welding upsetting amount is kept at 10.5mm. 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 welding joint are heated by adopting a traditional integral normalizing heating mode. The heating was stopped when the head, web and foot surface temperatures of the weld joint were heated to 930 ℃. Then, the head tread and the head side of the rail joint were cooled to a surface temperature of 445 ℃ at a cooling rate of 9.0 ℃/s by using a cooling device with compressed air having a pressure of 0.45MPa as a cooling medium, and then the head was naturally cooled to an ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The post-weld heat treated rail joint obtained in this comparative example was machined into a charpy U-shaped impact specimen according to the sampling position shown in fig. 5. The metallographic structure of the steel rail joint metallographic specimen is inspected according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 6.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the method, a martensitic structure does not appear in a rail joint heat affected zone under 100X observation magnification, and the detection result is similar to that of the metallographic structure of FIG. 7. The average value of the room temperature impact power of the rail head of the air-cooled (in the welding state) joint after flash welding is 10J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 7J. For the normalized joint employing this comparative example, the average value of the rail head weld impact work was 43J, which is exactly the same as the normalized joint rail head weld impact work in example 1. In this comparative example, the average impact energy of the normalized joint web and foot weld was 14J, which is lower than the average impact energy of the normalized joint web and foot weld of example 1, which was 22J. The impact energy of the welding seam of the rail waist and the rail bottom of the normalizing joint obtained in the comparative example is lower than that of the welding seam of the rail waist and the rail bottom of the normalizing joint in the embodiment 1, so that the matching property of 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 base metal of the steel rail is further reduced, and the railway operation safety is not facilitated.

Comparative example 2

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

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.5MJ, the actual welding upsetting quantity is kept at 11.5mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, the whole section of the rail head, the rail web and the rail bottom of the steel rail joint is heated by adopting the existing integral normalizing heating coil. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 930 ℃. Then, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 400 ℃ at a cooling speed of 7.0 ℃/s by adopting a cooling device and 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 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the rail normalized joint obtained by the above method, as shown in fig. 8, no martensitic structure appears in the rail joint heat affected zone at 100X observation magnification. Wherein the weld structure (the area marked by the oval broken line in the left figure) is pearlite and eutectoid ferrite along the crystal, and the heat affected zone (the area around the weld is shown enlarged in the right figure) is pearlite and a small amount of eutectoid cementite. The average value of the room temperature impact power of the welding seam at the rail head and the rail bottom of the air-cooled joint after flash welding is 10.0J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 5J. After common normalizing heat treatment, the average impact power of the welding seam of the normalizing joint rail head is 12J, and the average impact power of the welding seam of the rail web and the rail bottom is 7J. The overall impact performance of the joint is relatively low, which is unfavorable for the running safety of the railway.

Comparative example 3

The flash welding of the steel rail is carried out by using a steel rail moving flash welding machine and adopting a heat input quantity of 7.5MJ, the actual welding upsetting quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 930 ℃. Then, the water mist mixed gas with the pressure of 0.45MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 195 ℃ at the cooling speed of 9.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the above method, as shown in fig. 9, no martensitic structure appears in the rail joint heat affected zone at 100X observation magnification. The weld joint structure is pearlite and along crystal proeutectoid ferrite (the area marked by the elliptic dotted line of the left graph), the heat affected zone structure is pearlite+a small amount of proeutectoid ferrite+a small amount of martensite (the area around the weld joint is shown in the right graph in an enlarged manner, and the inside of the rectangular frame is martensite). The average value of the room temperature impact power of the welding seam at the rail head and the rail bottom of the air-cooled joint after flash welding is 10J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 7J. After heat treatment, the average impact power of the welding seam of the rail head of the normalizing joint is 23J, and the average impact power of the welding seam of the rail web and the rail bottom is 15J. Because of the existence of martensite in the welding heat affected zone, the impact energy of the full-section weld joint of the normalizing joint has larger difference with the impact energy of the corresponding position of the base metal of the steel rail, and the joint has poorer matching property with the impact toughness of the full-section of the base metal, which is not beneficial to the safety of railway operation.

Comparative example 4

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.8MJ, the actual welding upsetting quantity is kept at 15.0mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 950 c. Then, the rail head tread and the rail head side face of the rail joint are cooled to 390 ℃ at a cooling speed of 7.0 ℃/s by adopting a cooling device and 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 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the post-weld heat treatment construction method of the present invention, as shown in fig. 10, no martensitic structure appears in the rail joint heat affected zone at 100X observation magnification. Wherein the weld structure is pearlite and along-grain proeutectoid ferrite (the area marked by the oval broken line in the left figure), and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite (the area around the weld is shown in the right figure in an enlarged manner). Because the amount of upsetting is excessive, excess weld metal is drained, forming a cold joint. The average value of the room temperature impact power of the welding seam at the rail head and the rail bottom of the air-cooled joint after flash welding is 9J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 5J. After heat treatment, the average value of impact energy of the welding seam of the rail head of the normalizing joint is 16J, the average value of impact energy of the welding seam of the rail web and the rail bottom is 10J, the impact energy of the welding seam of the full section of the normalizing joint has larger difference with the impact energy of the corresponding position of the base metal of the steel rail, and the matching property of the impact toughness of the joint and the full section of the base metal is poor, which is not beneficial to the running safety of the railway.

Comparative example 5

The flash welding of the steel rail is carried out by using a steel rail moving flash welding machine and adopting a heat input quantity of 7.5MJ, the actual welding upsetting quantity is kept at 10.5mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint. The heating is stopped when the rail joint head, web and foot surface temperatures are heated to 850 ℃. Then, the water mist mixed gas with the pressure of 0.20MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 440 ℃ at the cooling speed of 5.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the above method, as shown in fig. 11, no martensitic structure appears in the rail joint heat affected zone at 100X observation magnification. Wherein the weld structure is pearlite and along-grain proeutectoid ferrite (the area marked by the oval broken line in the left figure), and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite (the area around the weld is shown in the right figure in an enlarged manner). The average value of the room temperature impact power of the welding seam at the rail head and the welding seam at the rail web and the rail bottom of the air-cooled joint after flash welding is 11.5J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 8.5J. By adopting the post-welding heat treatment method of the steel rail of the comparative example, the austenitizing process of the steel rail joint is incomplete due to the lower normalizing heating temperature. The average value of impact energy of the welding seam of the rail head of the normalizing joint is 24J, the average value of impact energy of the welding seam of the rail waist and the rail bottom is 17J, the impact energy of the welding seam of the full section of the normalizing joint has larger difference with the impact energy of the corresponding position of the base metal of the steel rail, and the matching property of the impact toughness of the joint and the full section of the base metal is poor, so that the safety of railway operation is not facilitated.

Comparative example 6

And carrying out flash welding on the steel rail by using a steel rail moving flash welding machine and adopting a heat input amount of 7.5MJ, wherein the actual welding upsetting amount is kept at 10.5mm. After the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the steel rail joint at the same time. The heating is stopped when the surface temperatures of the rail joint head, web and bottom are heated to 1150 ℃. Then, the water mist mixed gas with the pressure of 0.20MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 440 ℃ at the cooling speed of 5.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the method, no martensitic structure appears in the rail joint heat affected zone under 100X observation magnification. Wherein, the weld joint structure is pearlite and peritectic proeutectoid ferrite, and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite. The average value of the room temperature impact power of the welding seam at the rail head and the welding seam at the rail web and the rail bottom of the air-cooled joint after flash welding is 11.5J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 8.5J. By adopting the method, the high temperature residence time of the rail joint is too long due to the too high normalizing heating temperature, so that austenite grains are coarsened, and the impact toughness is reduced. The average value of impact power of the welding seam of the rail head of the normalizing joint is 25J, the average value of impact power of the welding seam of the rail waist and the rail bottom is 18J, the impact power of the welding seam of the full section of the normalizing joint has larger difference with the impact power of the corresponding position of the base metal of the steel rail, and the matching property of the impact toughness of the joint and the full section of the base metal is poor, so that the safety of railway operation is not facilitated.

Comparative example 7

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.8MJ, the actual welding upsetting quantity is kept at 12.0mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the 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 temperatures of the rail web and the rail bottom are heated to 800 ℃. Then, the rail head tread and the rail head side face of the rail joint are cooled to 390 ℃ at a cooling speed of 7.0 ℃/s by adopting a cooling device and 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 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the method, no martensitic structure appears in the rail joint heat affected zone under 100X observation magnification. Wherein, the weld joint structure is pearlite and peritectic proeutectoid ferrite, and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite. The average value of the room temperature impact power of the welding seam at the rail head and the welding seam at the rail web and the rail bottom of the air-cooled joint after flash welding is 9.5J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 6.3J. After heat treatment, normalizing the welding seam of the rail head of the normalizing joint is more sufficient, and the average value of impact energy is 38J; the average impact power value of the welding seam with the rail web and the rail bottom is only 10J due to low normalizing heating temperature, and the impact power of the welding seam with the full section of the normalizing joint has larger difference with the impact power of the corresponding position of the base metal of the steel rail, which is not beneficial to the safety of railway operation.

Comparative example 8

The flash welding of the steel rail is carried out by utilizing a steel rail moving flash welding machine and adopting a heat input quantity of 8.8MJ, the actual welding upsetting quantity is kept at 12.0mm, and after the steel rail is welded, when the surface temperature of the rail head is reduced to about 200 ℃, a separated normalizing heating coil shown in figure 2 is adopted to heat the rail head, the rail web and the rail bottom of the 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 temperatures of the rail web and the rail bottom are heated to 950 ℃. Then, the water mist mixed gas with the pressure of 0.30MPa is used as a cooling medium by adopting a cooling device, the rail head tread and the rail head side face of the rail joint are cooled to the surface temperature of 390 ℃ at the cooling speed of 7.0 ℃/s, and then the joint is naturally cooled to the ambient temperature (20-30 ℃). And naturally cooling to the ambient temperature after normalizing and heating the rail web and the rail bottom of the rail joint, thereby completing the welding and post-welding heat treatment processes of the medium carbon steel rail.

The results show that: for the medium-low carbon pearlite rail normalizing joint obtained by the method, no martensitic structure appears in the rail joint heat affected zone under 100X observation magnification. Wherein, the weld joint structure is pearlite and peritectic proeutectoid ferrite, and the heat affected zone structure is pearlite and a small amount of proeutectoid ferrite. The average value of the room temperature impact power of the welding seam at the rail head and the welding seam at the rail web and the rail bottom of the air-cooled joint after flash welding is 9.5J, and the average value of the room temperature impact power of the welding seam at the rail web and the rail bottom is 6.3J. After heat treatment, the normalizing joint rail head weld joint has low normalizing heating temperature, so that the average impact power value is only 11J, the rail web and rail bottom weld joint is normalized sufficiently, the average impact power value is 21J, and the impact power of the normalizing joint full-section weld joint and the impact power of the corresponding position of the rail base metal have large difference, thereby being unfavorable for railway operation safety.

As can be seen from comparative examples 1 to 4 and comparative examples 1 to 8: by adopting the welding method for the medium-low carbon pearlite steel rail, provided by the invention, the full-section impact toughness matching property of the steel rail joint can be improved. And meanwhile, the martensitic structure in the heat affected zone of the rail joint is avoided. The impact power of the welding line of the head of the joint thermally treated by the method is in the range of 35-50J, the impact power of the welding line of the waist and the bottom of the rail is in the range of 18-26J, the impact power of the welding line of the full section of the normalizing joint reaches the same level with the impact power of the corresponding position of the base metal of the medium-low carbon pearlitic steel rail related to the method, and the method is beneficial to ensuring the running safety of the railway.

The foregoing examples merely illustrate embodiments of the invention and are described in more detail and are not to be construed as limiting the scope of the invention. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the invention, which are all within the scope of the invention.

Claims

1. The welding method of the medium-low carbon pearlite steel rail is characterized by comprising the following steps of:

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

step 3): placing the head portion of the weld joint in a heating zone of a first electromagnetic induction coil after step 2) is completed, placing the web portion and foot portion of the weld joint in a heating zone of a second electromagnetic induction coil, while turning on the first electromagnetic induction coil and the second electromagnetic induction coil to heat the head portion, web portion, and 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, the first heating frequency and the second heating frequency being set such that temperature changes of the web portion and foot portion of the weld joint can be synchronized;

Step 4): and cooling the heated welding joint.

2. The method of claim 1, wherein the base material is composed of 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 unavoidable impurities.

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

naturally cooling the web and the foot to ambient temperature.

4. A method according to claim 3, wherein the compressed air or water mist mixture cools the rail head at a cooling rate of 4.0-10.0 ℃/s.

5. The method of claim 1, wherein the first predetermined temperature is 200-300 ℃.

6. The method of claim 1, wherein the second predetermined temperature is 900-960 ℃.

7. A method according to claim 1, wherein the rail parent metal microstructure is controlled to be 95-99% pearlite and 5-1% pro-eutectoid ferrite.

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

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