CN113458568B

CN113458568B - Method for welding medium carbon steel rail in field

Info

Publication number: CN113458568B
Application number: CN202110909264.9A
Authority: CN
Inventors: 白威; 李大东; 陆鑫; 徐飞翔
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2021-08-09
Filing date: 2021-08-09
Publication date: 2022-10-21
Anticipated expiration: 2041-08-09
Also published as: CN113458568A

Abstract

The invention discloses a field welding method of a medium carbon steel rail. The method comprises the following steps: welding a plurality of steel rails made of medium carbon steel rail base materials, controlling the welding upset forging amount of the steel rails to be kept at 13.1-13.9mm, and adopting heat input amount of 12.1-13.8MJ for welding; cooling the welded rail joint in a predetermined manner; and carrying out heat treatment on the cooled welded joint, wherein the heat treatment comprises heating the whole section of the steel rail joint, heating the surface temperature of the steel rail joint to 900-940 ℃, stopping heating, cooling the steel rail joint in a field construction environment, heating the steel rail joint to 460-500 ℃ again when the surface temperature of the joint is reduced to 400-450 ℃, and slowly cooling the steel rail joint to the ambient temperature at a cooling speed of 0.5-1.0 ℃/s in a thermal compensation and heat preservation mode.

Description

Method for welding medium carbon steel rail in field

Technical Field

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

Background

At present, the steel rail for the railway is mainly a high-carbon steel rail with the carbon content of 0.7-1.1%, the strength of the steel rail is generally required to be more than or equal to 880MPa, the steel rail has good wear resistance, and the steel rail is mainly suitable for plain areas. For road sections such as Sichuan-Tibet railways with extremely harsh line service conditions, the special natural conditions such as long-uphill and long-downhill terrain conditions, annual temperature difference, large day-night temperature and the like, higher requirements are put forward on the shock resistance and fatigue resistance of the steel rail. In addition, the train can locally heat the surface of the rail under long-term traction and braking, which also requires the rail to have sufficient fatigue resistance. At normal temperature, the U-shaped impact value of the U71Mn hot rolled steel rail head is 25-30J, while the U-shaped impact value of the rail head is only 5-8J at the low temperature of-40 ℃, and the impact toughness is greatly reduced.

At present, with the increase of railway transportation volume, the steel rail serving in a low-temperature environment puts new requirements on wear resistance, low-temperature toughness and the like. When the existing welding operation method is used for welding in the low-temperature environment, due to the fact that the field environment temperature is low, the welding process is limited, low-temperature brittle fracture is easily generated on a welding joint, and the quality of the welding joint is unqualified.

Disclosure of Invention

Aiming at the problems, the invention provides a field welding method of a medium carbon steel rail. The method significantly improves the low temperature toughness of the weld joint and the weld heat affected zone by controlling the welding process and the post-weld heat treatment.

According to one aspect of the invention, a field welding method for a medium carbon steel rail is provided, which comprises the following steps:

step 1): welding a plurality of steel rails made of medium carbon steel rail base materials, controlling the welding upsetting amount of the steel rails to be kept between 13.1 and 13.9mm, and adopting the heat input amount of 12.1 to 13.8MJ for welding;

step 2): cooling the welded rail joint welded in the step 1) in a preset mode;

step 3): and (3) carrying out heat treatment on the welded joint cooled in the step 2), wherein the heat treatment comprises heating the whole section of the steel rail joint, heating the surface temperature of the steel rail joint to 900-940 ℃, stopping heating, cooling the steel rail joint in a field construction environment, heating the steel rail joint to 460-500 ℃ again when the surface temperature of the joint is reduced to 400-450 ℃, and slowly cooling the steel rail joint to the ambient temperature at a cooling speed of 0.5-1.0 ℃/s in a thermal compensation and heat preservation mode.

According to one embodiment of the present invention, the C content in the base material of the medium carbon steel rail is 0.50 to 0.62%.

According to one embodiment of the invention, the tensile strength of the steel rail base material at room temperature is 1050-1130MPa, the elongation is 17-25%, the U-shaped impact energy is 30-42J, and the U-shaped impact energy of the steel rail base material is 18-24J at-20 ℃.

According to one embodiment of the invention, the welding is performed by mobile flash welding.

According to an embodiment of the invention, the cooling mode in the step 2) comprises the step of coating the rail joint head, the rail web and the rail bottom by using the split device immediately after the welding push-button of the rail joint is finished, but power is not supplied to the split device so as to slowly cool the rail joint.

According to one embodiment of the invention, the full-face heating operation of step 3) is started when the post-welded rail head surface temperature is reduced to 200-260 ℃, and the rail joint is full-face heated by supplying power to the split device.

According to one embodiment of the invention, the cooling in step 3) is carried out with the rail joint being wrapped by a split device, wherein the cooling rate is 3.0-10.0 ℃/s.

According to one embodiment of the invention, the reheating, the thermal compensation and the incubation in step 3) are performed using a split device.

According to the invention, the medium carbon steel rail welded by the method is also provided.

The field welding method for the medium carbon steel rail disclosed by the invention can effectively reduce the generation probability of welding dust spots and obviously improve the low-temperature toughness of a welding joint by comprehensively controlling chemical components, a rolling process, a welding process and postweld heat treatment. The steel rail welding joint obtained by the welding method has the full-section tensile strength Rm of more than or equal to 900MPa at room temperature (20-30 ℃), the average hardness of the longitudinal section of the joint reaches more than 90% of the hardness of a steel rail base metal, the joint can continuously pass a drop hammer test for 2 times without cracking, and the average value of the U-shaped impact energy of the full-section of the welding seam of the joint is more than or equal to 20J under the condition of-20 ℃, so that compared with the existing welding process, the low-temperature toughness is greatly improved.

Drawings

FIG. 1 is a schematic view of the various zones of a rail weld joint.

Fig. 2 is a schematic diagram of a metallographic sample in each embodiment in a sectioned position.

FIG. 3 is a schematic diagram showing the distribution of electric heaters on the head of the split rail heating apparatus.

Fig. 4 is an overall schematic view of the split rail heating apparatus.

FIG. 5 is a metallographic structure diagram of a weld heat-affected zone in example 1.

FIG. 6 is a metallographic structure diagram of a weld heat-affected zone in example 2.

FIG. 7 is a metallographic structure diagram of a weld heat-affected zone in example 3.

FIG. 8 is a metallographic structure diagram of a weld heat-affected zone in example 4.

FIG. 9 is a metallographic structure diagram of a weld heat-affected zone in example 5.

FIG. 10 is a metallographic structure diagram of a weld heat-affected zone of comparative example 1.

FIG. 11 is a metallographic structure diagram of a weld heat affected zone of comparative example 2.

FIG. 12 is a metallographic structure diagram of a weld heat-affected zone of comparative example 3.

FIG. 13 is a tensile fracture plot of the weld heat affected zone of comparative example 4.

Fig. 14 is a tensile fracture diagram of the 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 do not limit the invention.

The brittle martensite structure generated in the steel rail welding process is directly related to the service performance of the steel rail and even harms the driving safety. Therefore, the martensite structures which can appear in the domestic and foreign railway steel rail welding standards are strictly regulated. The steel rail joint welding seam and the heat affected zone structure of the current railway industry steel rail welding standard TB/T1632.2-2014 in China are mainly pearlite and can generate a small amount of ferrite. No harmful structures such as martensite or bainite should appear. Australia rail welding Standard AS1085.20-2012 specifies: for some steel rails with high strength grade, high carbon content and high alloy content, under the observation magnification of a metallographic microscope of 100x, the percentage content of a martensite structure in the most serious area of a steel rail welding joint is not higher than 5%, otherwise, the joint can cause early fatigue fracture due to a large amount of quenched martensite structures, and the running safety of the railway is seriously influenced. The welding dust spot is the main cause of the drop weight and tensile fracture of the rail joint.

The martensite transformation critical cooling speed of the medium carbon steel rail is 1.2-2.8 ℃/s, and the Ms temperature (the starting temperature for martensite formation) of the rail steel is 260-320 ℃. When the steel rail welding construction is carried out under the condition of normal temperature of 10-30 ℃, because the cooling speed of the construction environment is relatively slow, a martensite structure is not formed in a steel rail welding heat affected zone usually. When the steel rail welding construction is carried out in a low-temperature environment in the field (such as +5 to-20 ℃), because the environment cooling rate is higher (generally higher than 3 ℃/s and higher than the martensite transformation critical cooling rate of a steel rail material), a martensite structure is easily formed in a welding heat affected zone due to the higher environment cooling rate in the steel rail welding and postweld heat treatment cooling stages in the field. Therefore, when steel rail welding and postweld heat treatment construction is carried out in a field low-temperature environment (such as +5 to-20 ℃), measures for slowly cooling steel rail welding and postweld heat treatment are required to avoid martensite in a joint, and available measures comprise carrying out a welding test by adopting larger welding heat input and reducing the cooling rate of a steel rail welding heat affected zone. After the post-welding heat treatment of the steel rail is finished, asbestos with soft material and certain thickness can be adopted to coat the welding area of the steel rail so as to prevent the steel rail joint from forming a martensite structure in the post-welding heat treatment process. In addition, under the combined action of a low-temperature environment and the asbestos coating layer, the cooling speed of the joint in the coating layer is moderate, and the purposes of refining the interlayer spacing of pearlite sheets in a welding heat affected zone and ensuring that the joint obtains enough strength, toughness and hardness can be achieved. Meanwhile, the occurrence of martensite structure is avoided, and the railway operation safety is ensured.

It should be noted that the post-weld cooling control is to use a split device to coat the rail head, the rail web and the rail foot of the rail joint immediately after the welding and the beading of the rail joint are finished, so as to slowly cool the rail joint and prevent the joint from forming martensite during the post-weld cooling process.

The martensitic structure is formed by cooling the steel to a temperature not lower than the Ms (martensite start) temperature under the condition of a temperature not lower than the austenitizing temperature and a temperature higher than the martensite transformation critical cooling rate. Without considering the component segregation, the rail steel does not cause the formation of 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 addition, the invention utilizes the steel rail mobile flash welding machine and adopts the larger heat input quantity of 12.1-13.8MJ to carry out the welding test. If the heat input is too large, the retention time of the welding high temperature is too long, which easily causes the serious low hardness and collapse of the joint and the too wide width of the softening area. If the heat input is too small, the joint may have a detrimental martensite structure in the heat-affected zone due to too high a cooling rate after welding.

Wherein the welding process controls the welding upsetting amount to be kept between 13.1 and 13.9mm. The invention enters the welding process control stage after the steel rail is rolled, and the upset forging amount is specially controlled. If the welding upset forging amount is too small, the welding seam has unremoved dust spots and welding slag, and the mechanical property of the joint is reduced; if the welding upset amount is controlled too much, a cold joint is easy to form, and the mechanical property of the joint is also reduced.

Fig. 1 shows a schematic view of the various zones of a rail weld joint. In the examples and comparative examples, the longitudinal section hardness detection point position at a position 3 to 5mm below the rail head tread of the welded rail joint is shown in FIG. 1. In fig. 1, a and c are rail base metal, b is a welded joint, and d is a rail head tread. The position e in figure 1 is the center of the weld. Fig. 2 shows the sampling position of the metallographic specimen of the rail head tread of the steel rail welding joint, and the position e is the center of the welding seam.

FIG. 3 is a schematic diagram showing the distribution of electric heaters on the head of the split rail heating apparatus. In the drawings,: a is a railhead tread heating area; b is a rail head side heating area; c is a railhead lower jaw heating area; d is a crawler-type ceramic electric heater. It should be noted that the layout of the heating areas of the head, web and foot of the heating device is similar. Because the rail head is thicker and slower in heat transfer, the number of the ceramic heaters for coating the rail head is larger than that of the ceramic heaters in the rail waist and rail bottom areas, so that the full section of the steel rail is fully heated.

Fig. 4 is an overall schematic view of the split rail heating apparatus. The device has the advantages of small size, flexibility, low cost and the like, and is convenient for field construction. The power can be supplied by a diesel generator with 380V voltage or 220V alternating current commercial power, and the rated power is 10kW. The device uses a commercial LCD crawler-type ceramic heater as a heat source, and the heater is a ceramic wafer with the size of 10mm (length) multiplied by 10mm (width) multiplied by 7mm (thickness). The split-type heater is made of a ring-shaped split-type heater by matching with a heat insulation material and a steel structure shell, is convenient to assemble and disassemble, and is suitable for local heating of a steel rail welding joint. The actual size of the heating device and the specification and distribution condition of the heater can be adjusted according to the actual size of the steel rail profile. In the device design process, evenly fix the device inboard that has similar rail profile shape with multiunit crawler-type heater, make the heater cladding on the rail surface and with rail surface fully laminate in order to realize the good heat-conduction in the heating process, can realize the heat treatment process to the rail joint based on the device. In the test process, a temperature controller is adopted to control the heating temperature. The working temperature range of the device is 200-990 ℃. The split device can be rotated by 180 DEG at maximum about the axis of rotation. In fig. 4: A1/A2 is a left binding post; B1/B2 is a right binding post; c is a crawler-type ceramic heater; d is a turning shaft; e is a fixed snap ring; f is a device shell which is formed by welding metal sheets; g is an asbestos insulation layer. Note that the terminal A1 and the terminal A2 form a circuit. The terminal B1 and the terminal B2 form a circuit. A plurality of electric heaters arranged in parallel are connected together in a parallel connection mode.

The post-welding heat treatment is controlled by adopting a split device as shown in figure 4 to coat the rail head, the rail web and the rail bottom of the rail joint immediately after the welding beading of the rail joint is finished, so that the joint is slowly cooled. It should be noted that, in the process of using the split device to slowly cool the rail joint after the rail welding is completed, the device is not used for heating the joint, and the device can play a role in monitoring the surface temperature of the sample at this stage. Due to the existence of the asbestos coating layer with a certain thickness in the split device, the device can also play a role in slowly cooling the steel rail joint under the condition of not heating so as to avoid the formation of a martensite structure in the cooling process of the steel rail joint after welding due to the overhigh cooling speed of the construction environment.

When the surface temperature of the rail head after the welding of the rail joint is reduced to 200-260 ℃, the full section of the rail joint is heated by adopting the split device, and the heating is stopped after the surface temperature of the rail joint is heated to 900-940 ℃. And then naturally cooling the steel rail joint wrapped by the split device in a field construction environment (+ 5-20 ℃), wherein the cooling speed of the steel rail joint in the cooling stage is 3.0-10.0 ℃/s. When the temperature of the joint surface is reduced to 400-450 ℃, the steel rail joint is heated to 460-500 ℃ again by adopting the split device so as to austenitize the steel rail again, thereby reducing the possibility of martensite in the subsequent process. In addition, the matrix structure is transformed and recrystallized again by reheating so as to refine grains and enhance the toughness of the material. The steel rail joint is slowly cooled to the ambient temperature (+ 5 to-20 ℃) at the cooling speed of 0.5 to 1.0 ℃/s by means of thermal compensation and heat preservation of asbestos in the split type device, so that the welding and post-welding heat treatment processes of the medium carbon steel rail are completed. Wherein, the rail joint finishes pearlite transformation at 460-500 ℃, and no martensite structure is generated when the rail steel is continuously cooled by adopting a cooling speed lower than the martensite transformation critical cooling speed.

In the present invention, a rail welding "joint" is a region of 70 to 110mm in length including a weld seam obtained by welding. The full section refers to the whole section of the steel rail welding joint with the length of the welding seam within the range of 70-110 mm, and comprises a rail head, a rail web and a rail bottom.

The inventor finds out in research that the welding heat input amount needs to be strictly controlled in the steel rail welding process, and the phenomenon that the joint is cooled quickly after welding due to too low heat input is avoided, so that a martensite or bainite structure is formed. Meanwhile, the performance control of the steel rail welding joint needs to be matched with proper upsetting amount so as to fully eliminate the defects of welding dust spots, welding slag inclusion and the like possibly formed in the welding line and reduce the influence of the welding defects on the mechanical property of the steel rail joint. In addition, the heat treatment process for the welded joint after welding has a significant effect on the low-temperature toughness of the joint.

Example 1

Controlling the content of C in the steel rail base metal to be 0.5 percent. The rail is flash welded by a rail mobile flash welding machine with heat input of 12.1MJ, the actual welding upset amount is kept at 13.1mm, and after the rail welding push beading is finished, a split device is used for coating the full section of the rail joint with the residual temperature of more than 600 ℃ after welding. When the rail head surface temperature drops to 200 ℃, the rail joint is heated in its full section using a split device as shown in fig. 4. And (3) heating the surface of the steel rail joint to 900 ℃, and then stopping heating. The rail joint wrapped by the split device was then allowed to cool naturally in the field environment (-10 ℃) at a cooling rate of 8.0 ℃/s during this cooling phase. When the surface temperature of the joint is reduced to 400 ℃, the steel rail joint is heated to 460 ℃ again by adopting the split device, and then the steel rail joint is slowly cooled to the ambient temperature (-10 ℃) at the cooling speed of 0.5 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, thereby completing the welding of the medium carbon steel rail and the heat treatment process after welding.

The post-weld heat treated rail joint obtained in this example was machined into a longitudinal hardness test specimen. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general plant of testing machines in Laizhou, shandong, model HBV-30A) at a position 5mm below a rail head tread of a steel rail at a measuring point interval of 2mm, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 part 1 of metal Vickers hardness test: test methods "were performed using HV scale. Performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a steel rail welding joint drop hammer test is carried out according to the current steel rail welding standard TB/T1632.2-2014 of the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammer is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the medium carbon steel rail welded joint obtained by the welding method of the present invention, as shown in fig. 5, no martensite structure appears in the heat affected zone of the rail joint at an 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 tensile strength of the full section of the steel rail flash welding joint in a welding state at room temperature (20 ℃) is 905MPa, the average hardness of the longitudinal section of the joint reaches 90% of the hardness of the base metal of the steel rail, the joint continuously passes a drop hammer test for 2 times without breaking, and the average value of the U-shaped impact energy of the full section of the welding joint of the joint at the temperature of-20 ℃ is 21J, so that various indexes required by the steel rail welding joint in a low-temperature environment are met.

Example 2

Controlling the content of C in the steel rail base metal to be 0.55%. The rail is welded by a movable flash welding machine and the flash welding of the rail is carried out by adopting the heat input of 12.5MJ, the actual welding upset forging quantity is kept at 13.5mm, and after the welding push beading of the rail is finished, the full section of the rail joint with the residual temperature of more than 600 ℃ after welding is coated by a split device. When the rail head surface temperature drops to 220 ℃, the rail joint is subjected to full-face heating by using a split device as shown in fig. 4. And (3) heating the surface of the steel rail joint to 920 ℃, and then stopping heating. The rail joint wrapped by the split device is then allowed to cool naturally in the field environment (-10 ℃) at a cooling rate of 5.0 ℃/s during this cooling phase. When the surface temperature of the joint is reduced to 420 ℃, the steel rail joint is heated to 490 ℃ again by adopting the split device, and then the steel rail joint is slowly cooled to the ambient temperature (-10 ℃) at the cooling speed of 0.8 ℃/s in a thermal compensation and asbestos heat preservation mode in the split device, so that the welding and post-welding 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 longitudinal hardness test specimen. A hardness sample is longitudinally subjected to Vickers hardness detection by using a Bravicer hardness tester (model HBV-30A, general plant of testing machines in Lyzhou city, shandong) at a position 5mm below a tread of a rail head of a steel rail by taking 2mm as a measuring point interval, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 part 1 of metal Vickers hardness test: test methods "were performed using HV scale. Performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a rail welding joint drop hammer test is carried out according to the current rail welding standard TB/T1632.2-2014 in the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammer is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the medium carbon steel rail steel welded joint obtained by the welding method of the present invention, as shown in fig. 6, no martensite structure appears in the heat affected zone of the rail joint at an 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 tensile strength of the full section of the steel rail flash welding joint in a welding state at room temperature (20 ℃) is 915MPa, the average hardness of the longitudinal section of the joint reaches 90% of the hardness of the base metal of the steel rail, the joint continuously passes a drop hammer test for 2 times without breaking, and the average value of the U-shaped impact power of the full section of the welding joint of the joint at the temperature of-20 ℃ is 22J, so that various indexes required by the steel rail welding joint in a low-temperature environment are met.

Example 3

Controlling the content of C in the steel rail base metal to be 0.61 percent. The rail is flash welded by a rail mobile flash welding machine with heat input of 12.8MJ, the actual welding upset amount is kept at 13.2mm, and after the rail welding push beading is finished, a split device is adopted to coat the full section of the rail joint with the residual temperature of more than 600 ℃ after welding. When the rail head surface temperature drops to 240 ℃, the rail joint is subjected to full section heating using a split device as shown in fig. 4. And (3) stopping heating after the surface temperature of the steel rail joint is heated to 910 ℃. The rail joint wrapped by the split unit is then allowed to cool naturally in the field environment (-10 ℃) at a cooling rate of 3.0 ℃/s during this cooling phase. When the surface temperature of the joint is reduced to 430 ℃, the steel rail joint is heated to 470 ℃ again by adopting the split device, and then the steel rail joint is slowly cooled to the ambient temperature (-10 ℃) at the cooling speed of 0.9 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, thereby completing the welding and post-welding heat treatment process of the medium carbon steel rail.

The post-weld heat treated rail joint obtained in this example was machined into a longitudinal hardness test specimen. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general plant of testing machines in Laizhou, shandong, model HBV-30A) at a position 5mm below a rail head tread of a steel rail at a measuring point interval of 2mm, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 part 1 of metal Vickers hardness test: test methods "were performed using HV scale. Performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a steel rail welding joint drop hammer test is carried out according to the current steel rail welding standard TB/T1632.2-2014 of the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the medium carbon steel rail steel welded joint obtained by the welding method of the present invention, as shown in fig. 7, no martensite structure appears in the heat affected zone of the rail joint at an 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 tensile strength of the full section of the steel rail flash welding joint in a welded state at room temperature (25 ℃) is 920MPa, the average hardness of the longitudinal section of the joint reaches 91% of the hardness of the base metal of the steel rail, the joint continuously passes a drop hammer test for 2 times without breaking, and the average value of the U-shaped impact energy of the full section of the welding joint of the joint at the temperature of-20 ℃ is 23J, so that various indexes required by the steel rail welding joint in a low-temperature environment are met.

Example 4

Controlling the content of C in the steel rail base metal to be 0.60 percent. The steel rail is welded by a mobile flash welding machine and the heat input of 13.2MJ, the actual welding upset forging quantity is kept at 13.4mm, and after the steel rail welding push beading is finished, a split device is used for coating the whole section of the steel rail joint with the residual temperature of more than 600 ℃ after welding. When the rail head surface temperature drops to 250 ℃, the rail joint is subjected to full section heating using a split device as shown in fig. 4. And (3) heating the surface of the steel rail joint to 930 ℃, and then stopping heating. The split unit wrapped rail joint was then allowed to cool naturally in the field environment (-18℃.) at a cooling rate of 4.5C/s during this cooling phase. When the surface temperature of the joint is reduced to 440 ℃, the steel rail joint is heated to 485 ℃ again by adopting the split device, and the steel rail joint is slowly cooled to the ambient temperature (-18 ℃) at the cooling speed of 0.8 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, 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 longitudinal hardness test specimen. A hardness sample is longitudinally subjected to Vickers hardness detection by using a Bravicer hardness tester (model HBV-30A, general plant of testing machines in Lyzhou city, shandong) at a position 5mm below a tread of a rail head of a steel rail by taking 2mm as a measuring point interval, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. According to the sampling method shown in the reference figure 2, metallographic structure inspection is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method, etching is carried out on the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope; a rail welding joint drop hammer test is carried out according to the current rail welding standard TB/T1632.2-2014 in the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the medium carbon steel rail steel welded joint obtained by the welding method of the present invention, as shown in fig. 8, no martensite structure appears in the heat affected zone of the rail joint at an 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 tensile strength of the full-section of the steel rail flash welding joint in a welded state at room temperature (25 ℃) is 905MPa, the average hardness of the longitudinal section of the joint reaches 92% of the hardness of a steel rail base metal, the joint continuously passes a drop hammer test for 2 times without breaking, and the average value of the U-shaped impact energy of the full-section of the joint welding joint at the temperature of-20 ℃ is 21J, so that various indexes required by the steel rail welding joint in a low-temperature environment are met.

Example 5

The content of C in the steel rail base metal is controlled to be 0.62 percent. The rail is flash welded by a movable flash welding machine with 13.8MJ heat input, the actual welding upsetting amount is kept at 13.9mm, and the full section of the rail joint with the residual temperature of more than 600 ℃ after welding is coated by a split device after the rail welding upsetting is finished. When the rail head surface temperature drops to 260 ℃, the rail joint is subjected to full section heating using a split device as shown in fig. 4. And (4) heating the surface of the steel rail joint to 940 ℃ and then stopping heating. The split unit wrapped rail joint is then allowed to cool naturally in the field environment (-18℃.) at a cooling rate of 10℃/s during this cooling phase. When the surface temperature of the joint is reduced to 450 ℃, the steel rail joint is heated to 500 ℃ again by adopting the split device, and the steel rail joint is slowly cooled to the ambient temperature (-18 ℃) at the cooling speed of 1.0 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, so that the welding and postweld heat treatment processes of the medium carbon steel rail are completed.

The results show that: for the medium carbon steel rail welded joint obtained by the welding method of the present invention, as shown in fig. 9, no martensite structure appears in the heat affected zone of the rail joint at an 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 tensile strength of the full section of the steel rail flash welding joint in a welding state at room temperature (25 ℃) is 920MPa, the average hardness of the longitudinal section of the joint reaches 94% of the hardness of the base metal of the steel rail, the joint continuously passes a drop hammer test for 2 times without breaking, and the average value of the U-shaped impact energy of the full section of the welding joint of the joint at the temperature of-20 ℃ is 23J, so that various indexes required by the steel rail welding joint in a low-temperature environment are met.

Comparative example 1

The content of C in the steel rail base metal is controlled to be 1.10 percent. The steel rail is welded by a mobile flash welding machine and the heat input of 13.2MJ, the actual welding upset forging quantity is kept at 13.4mm, and after the steel rail welding push beading is finished, a split device is used for coating the whole section of the steel rail joint with the residual temperature of more than 600 ℃ after welding. When the surface temperature of the rail head is reduced to 250 ℃, the full section of the rail joint is heated by adopting a split device as shown in fig. 4, and the heating is stopped after the surface temperature of the rail joint is heated to 930 ℃. The rail joint wrapped by the split device is then allowed to cool naturally in the field environment (-10 ℃) at a cooling rate of 4.5 ℃/s during this cooling phase. When the surface temperature of the joint is reduced to 440 ℃, the steel rail joint is heated to 485 ℃ again by adopting the split device, and the steel rail joint is slowly cooled to the ambient temperature (-10 ℃) at the cooling speed of 0.8 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, 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 longitudinal hardness test specimen. A hardness sample is longitudinally subjected to Vickers hardness detection by using a Bravicer hardness tester (model HBV-30A, general plant of testing machines in Lyzhou city, shandong) at a position 5mm below a tread of a rail head of a steel rail by taking 2mm as a measuring point interval, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 part 1 of metal Vickers hardness test: test methods "were performed using HV scale. Performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a steel rail welding joint drop hammer test is carried out according to the current steel rail welding standard TB/T1632.2-2014 of the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the rail welded joint treated by the comparative example, as shown in fig. 10, at an observation magnification of 100X, the critical cooling rate of rail steel martensite transformation is reduced due to the higher carbon content of the rail parent metal, so that a martensite structure appears in a rail welding heat affected zone within a range of ± 10mm from the center of a weld joint under a lower welding heat input condition, and the low-temperature toughness of the rail joint is affected. The weld structure is pearlite and pro-eutectoid ferrite along the crystal, and the heat affected zone structure in the region where no martensite is present is pearlite. Due to the existence of a martensite structure in a heat affected zone, the average value of the tensile strength of the full section of the obtained steel rail flash welding joint in a welding state is 750MPa, the average hardness of the longitudinal section of the joint reaches 93% of the hardness of a steel rail base metal, the 1 st drop hammer fracture occurs in the process of a joint drop hammer test, the average value of U-shaped impact energy of the full section of a welding line of the joint under the condition of 20 ℃ below zero is 11J, and the low-temperature toughness of the steel rail joint is poor.

Comparative example 2

Controlling the content of C in the steel rail base metal to be 0.60 percent. The steel rail is flash welded by a mobile flash welding machine of the steel rail and adopting the heat input of 13.2MJ, the actual welding upset forging quantity is kept at 13.4mm, and the steel rail is naturally cooled to the ambient temperature (-10 ℃) in the construction environment of minus 10 ℃ after the joint is pushed to form a flange.

The welded rail joint obtained in this comparative example was machined into a longitudinal hardness test specimen. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general plant of testing machines in Laizhou, shandong, model HBV-30A) at a position 5mm below a rail head tread of a steel rail at a measuring point interval of 2mm, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. According to the sampling method shown in the reference figure 2, metallographic structure inspection is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method, etching is carried out on the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope; a steel rail welding joint drop hammer test is carried out according to the current steel rail welding standard TB/T1632.2-2014 of the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the rail welded joint treated by this comparative example, as shown in fig. 11, at an observation magnification of 100X, the weld structure was pearlite and pro-eutectoid ferrite along the crystal. Because the construction environment temperature is low, the steel rail joint is not protected by a slow cooling device after welding, and the welded joint is not subjected to heat treatment after welding, the cooling speed of the joint is high, and a large amount of massive martensite structures appear in a heat affected zone within +/-20 mm from the center of a welding seam. Due to the formation of martensite structure, the average hardness of the longitudinal section of the obtained flash welding joint of the steel rail in a welding state reaches 95% of the hardness of a steel rail base metal, the average tensile strength of the full section of the flash welding joint of the steel rail is 580MPa, the 1 st drop weight is broken in the process of a joint drop weight test, the average U-shaped impact energy of the full section of the welding joint of the joint under the condition of-20 ℃ is 4J, and the low-temperature toughness is obviously insufficient.

Comparative example 3

Controlling the content of C in the steel rail base metal to be 0.60 percent. The rail is flash welded by a mobile flash welding machine with 13.5MJ heat input, and the actual welding upset amount is kept at 13.2mm. And after the steel rail welding push button is finished, a full section of the steel rail joint with the residual temperature of more than 600 ℃ after welding is coated by adopting a split device. When the rail head surface temperature is reduced to 230 ℃, the full section of the rail joint is heated by adopting a split device as shown in fig. 4, and the heating is stopped after the rail joint surface temperature is heated to 930 ℃. The split unit wrapped rail joint was then allowed to cool naturally in the field environment (-18℃.) at a cooling rate of 4.5C/s during this cooling phase. And then, when the surface temperature of the steel rail joint is reduced to 480 ℃, the heating and any operation of the split device are not adopted, so that the steel rail joint wrapped by the split device is naturally cooled to the ambient temperature (-18 ℃) in the field construction environment (-18 ℃).

The post-weld heat treated rail joint obtained in this example was machined into a longitudinal hardness test specimen. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general plant of testing machines in Laizhou, shandong, model HBV-30A) at a position 5mm below a rail head tread of a steel rail at a measuring point interval of 2mm, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 part 1 of metal Vickers hardness test: test method "was performed using HV scale; performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a rail welding joint drop hammer test is carried out according to the current rail welding standard TB/T1632.2-2014 in the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the rail welded joint treated by this comparative example, as shown in fig. 12, the weld structure was pearlite and pro-eutectoid ferrite along the crystal at an observation magnification of 100X. Although asbestos in the split device is slowly cooled after the steel rail joint is welded due to low construction environment temperature, a large amount of massive martensite structures appear in a heat affected zone within +/-20 mm from the center of a welding line due to high cooling speed and no manual intervention in the subsequent cooling process of the steel rail joint. Due to the formation of martensite structure, the average hardness of the longitudinal section of the obtained flash welding joint of the steel rail in a welding state reaches 97 percent of the hardness of the base metal of the steel rail, the average tensile strength of the full section of the flash welding joint of the steel rail is 450MPa, the 1 st drop hammer in the drop hammer test process of the joint is broken, the average value of U-shaped impact energy of the full section of the welding joint of the joint at the temperature of minus 20 ℃ is 3J, and the low-temperature toughness is poor.

Comparative example 4

Controlling the content of C in the steel rail base metal to be 0.60 percent. The rail is flash welded by a mobile flash welding machine with 13.2MJ heat input, and the actual welding upset amount is kept at 7.4mm. When the rail head surface temperature is reduced to 250 ℃ after the rail welding and the beading pushing are finished, the full section of the rail joint is heated by adopting a split device as shown in figure 4. And (3) heating the surface of the steel rail joint to 930 ℃, and then stopping heating. The split unit wrapped rail joint was then allowed to cool naturally in the field environment (-18℃.) at a cooling rate of 4.5C/s during this cooling phase. When the surface temperature of the joint is reduced to 440 ℃, the steel rail joint is heated to 485 ℃ again by adopting the split device, and the steel rail joint is slowly cooled to the ambient temperature (-18 ℃) at the cooling speed of 0.8 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, 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 by the comparison was machined into a longitudinal hardness test specimen. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general plant of testing machines in Laizhou, shandong, model HBV-30A) at a position 5mm below a rail head tread of a steel rail at a measuring point interval of 2mm, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. According to the sampling method shown in the reference figure 2, metallographic structure inspection is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method, etching is carried out on the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope; a rail welding joint drop hammer test is carried out according to the current rail welding standard TB/T1632.2-2014 in the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the steel rail welding joint treated by the comparative example, the welding seam and the heat affected zone are normally organized 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. In the welding process, the welding dust spot defect at the welding seam is not eliminated (refer to fig. 13) due to the fact that the welding upsetting amount of the steel rail is too small, the tensile property of the joint is reduced, the average value of the tensile strength of the whole section of the flash welding head of the steel rail is only 800MPa, the average hardness of the longitudinal section of the joint reaches 91% of the hardness of the base metal of the steel rail, the 1 st drop weight is broken in the drop weight test process of the joint due to the existence of the welding dust spot at the welding seam, the average value of U-shaped impact energy of the whole section of the welding seam of the joint at the temperature of-20 ℃ is 6J, and the low-temperature toughness is obviously insufficient.

Comparative example 5

Controlling the content of C in the steel rail base metal to be 0.60 percent. The rail flash welding is carried out by using a rail mobile flash welding machine and adopting the heat input quantity of 15.2MJ, and the actual welding upset forging quantity is kept at 13.5mm. And when the surface temperature of the rail head is reduced to 250 ℃ after the rail welding and the beading are finished, heating the full section of the rail joint by adopting a split device shown in fig. 4, and stopping heating after the surface temperature of the rail joint is heated to 930 ℃. The rail joint wrapped by the split unit is then allowed to cool naturally in the field environment (-18 ℃) at a cooling rate of 4.5 ℃/s during this cooling phase. When the surface temperature of the joint is reduced to 440 ℃, the steel rail joint is heated to 485 ℃ again by adopting the split device, and the steel rail joint is slowly cooled to the ambient temperature (-18 ℃) at the cooling speed of 0.8 ℃/s by adopting a thermal compensation and asbestos heat preservation mode in the split device, 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 by the comparison was machined into a longitudinal hardness test specimen. A hardness sample is longitudinally subjected to Vickers hardness detection by using a Bravicer hardness tester (model HBV-30A, general plant of testing machines in Lyzhou city, shandong) at a position 5mm below a tread of a rail head of a steel rail by taking 2mm as a measuring point interval, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. Performing metallographic structure inspection on a metallographic sample of the steel rail joint according to GB/T13298-2015 metal microstructure inspection method by referring to the sampling method shown in FIG. 2, etching the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and observing the metallographic structure of the steel rail joint by adopting a German Leica MeF3 optical microscope; a rail welding joint drop hammer test is carried out according to the current rail welding standard TB/T1632.2-2014 in the railway industry in China. In the drop hammer test process of the steel rail welding joint, the hammer head impacts the welding seam area of the steel rail joint. The weight of the hammerhead used is 1000kg, and the height of the free falling body is 3.1m.

The results show that: for the rail welded joint treated by the comparative example, the weld joint and the heat affected zone were normally constructed at an 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. In the welding process, because the welding heat input of the steel rail is overlarge, the width of a joint softening area of the steel rail is wide, a joint heat affected area is seriously softened, and the hardness is greatly reduced. The average value of the tensile strength of the whole section of the steel rail flash welding joint is 850MPa, and the average hardness of the longitudinal section of the joint reaches 86 percent of the hardness of the steel rail base metal. As shown in FIG. 14, the 2 nd drop hammer fracture in the drop hammer test process of the joint is caused by the existence of welding dust spots at the welding seam, and the average value of U-shaped impact energy of the full section of the welding seam of the joint at the temperature of-20 ℃ is 13J, which does not meet the safety requirement of railway operation.

As can be seen by comparing examples 1 to 5 with comparative examples 1 to 5: by adopting the welding process method provided by the invention, the generation probability of welding dust spots can be effectively reduced, and the martensite structure in the heat affected zone of the steel rail joint is avoided. The full-section tensile strength Rm of the welding joint obtained by the welding method is more than or equal to 900MPa at room temperature (20-30 ℃), the average hardness of the longitudinal section of the joint reaches more than 90% of the hardness of a steel rail base metal, the joint can continuously pass a drop hammer test for 2 times without breaking, and the average value of U-shaped impact power of the full-section of a welding line of the joint at the temperature of-20 ℃ is more than or equal to 20J, so that the operation safety of a railway is ensured.

The above examples only show the embodiments of the present invention, and the description thereof is 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 field welding method for a medium carbon steel rail is characterized by comprising the following steps:

step 2): cooling the welded joint of the steel rail welded in the step 1) in a preset mode, wherein the preset cooling mode comprises the steps of immediately coating the rail head, the rail web and the rail bottom of the steel rail joint by using a split device after the welding and the beading of the steel rail joint are finished, and supplying no power to the split device so as to slowly cool the steel rail joint in the split device;

step 3): and (3) performing heat treatment on the welded joint cooled in the step 2), wherein the heat treatment comprises heating the steel rail joint in a full section manner, heating the surface temperature of the steel rail joint to 900-940 ℃, stopping heating, cooling the steel rail joint in a field construction environment, heating the steel rail joint to 460-500 ℃ again when the surface temperature of the joint is reduced to 400-450 ℃, and slowly cooling the steel rail joint to the ambient temperature at a cooling speed of 0.5-1.0 ℃/s in a thermal compensation and heat preservation manner.

2. The method according to claim 1, wherein the C content in the base material of the medium carbon steel rail is 0.50 to 0.62%.

3. The method of claim 1, wherein the rail base material has a tensile strength of 1050-1130MPa at room temperature, an elongation of 17-25% and a U-shaped impact energy of 30-42J, and the rail base material has a U-shaped impact energy of 18-24J at-20 ℃.

4. Method according to claim 1, characterized in that the welding is performed by means of mobile flash welding.

5. The method of claim 1, wherein the full-face heating operation of step 3) is initiated when the rail head surface temperature drops to 200-260 ℃ after the rail joint is welded, and the rail joint is full-face heated by supplying power to the split device.

6. Method according to claim 1, characterized in that the cooling in step 3) is carried out with the rail joint being wrapped by a split device, wherein the cooling rate is 3.0 to 10.0 ℃/s.

7. The method according to claim 1, wherein the reheating, the thermal compensation and the incubation in step 3) are performed using a split device.

8. A medium carbon steel rail, characterized in that it is welded by the method of any one of claims 1 to 7.