CN112251587A - Heat treatment method for welded joint of bainite steel rail and eutectoid pearlite steel rail - Google Patents
Heat treatment method for welded joint of bainite steel rail and eutectoid pearlite steel rail Download PDFInfo
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
- C21D9/505—Cooling thereof
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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Abstract
The invention discloses a heat treatment method for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail, and belongs to the technical field of post-weld heat treatment of steel rails. The invention provides a heat treatment method of a bainite steel rail and eutectoid pearlite steel rail welding joint, which is low in cost and good in mechanical property of the welding joint after heat treatment, and comprises the following steps: carrying out first cooling on the welding joint to 720-760 ℃; then carrying out second cooling to 350-400 ℃; thirdly, cooling to 100-180 ℃; then carrying out tempering heat treatment; and finally, naturally cooling to room temperature. The method can improve saddle-shaped abrasion of the steel rail welding joint caused by low hardness of the welding area in the line service process of the steel rail, the steel rail welding heat affected zone has no obvious martensite sign, and the joint has good toughness, thereby being beneficial to ensuring the railway operation safety.
Description
Technical Field
The invention belongs to the technical field of post-weld heat treatment of steel rails, and particularly relates to a heat treatment method for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail.
Background
At present, heavy haul railway lines at home and abroad mostly adopt eutectoid pearlite steel rails, the carbon content of the steel rails is usually within the range of 0.72-0.82% by weight, and the steel rails have the characteristics of good obdurability matching, moderate comprehensive mechanical property index and the like, and the metallographic structure is pearlite. With the rapid development of railways, heavy-duty lines with large axle loads have higher requirements on the service performance of steel rails. As the comprehensive mechanical property and the welding property of the traditional pearlite steel rail are almost developed to the limit, under the condition, the strength grade is higher, the bainite steel rail with good wear resistance and contact fatigue resistance is produced at the same time, the carbon content of the steel rail is usually within the range of 0.20-0.30% by weight, and the metallographic structure is a complex phase structure consisting of bainite, a small amount of martensite (or martensite island) and residual austenite. At present, steel rail mobile flash welding has become the mainstream steel rail on-line welding technology in railway construction sites at home and abroad, and for two kinds of steel rails with different strength grades and materials, the difference between the properties of parent metals brings great challenges to the welding. In addition, after the rail is subjected to welding heat cycle, the hardened layer of the welding area disappears and low hardness areas with larger width are formed on two sides of the welding seam, so that the hardness of the welding seam and the heat affected area is lower than that of the rail base metal. In the service process of a steel rail, saddle-shaped abrasion is easily formed on the head tread of a welded joint preferentially, so that the impact of a wheel rail is increased, the service life of the steel rail is seriously influenced, and even the driving safety is endangered. Therefore, the precondition for the application of the steel rail is that how to recover the mechanical property of the steel rail reduced by welding.
CN201610909362.1 discloses a postweld heat treatment method for a hypereutectoid steel rail and PG4 heat treatment eutectoid pearlite steel rail welded joint, which comprises the steps of firstly cooling a steel rail welded joint to be cooled obtained by welding to below 400 ℃, then heating the steel rail welded joint after the first cooling to 860-930 ℃, and then carrying out second cooling until the tread temperature of the steel rail welded joint is 410-450 ℃. The dissimilar steel rail welding joint obtained by the method can meet the current national railway industry standard TB/T1632.2-2014 steel rail welding part 2: flash welding is the test requirement for fatigue, tensile, impact and static bending tests. However, the invention relates to the process of normalizing heat treatment after welding the steel rail, and needs to adopt the heat treatment equipment after welding the steel rail to locally heat the welded joint of the steel rail, so that the operation and implementation processes are complex, and the cost is higher.
201210394058.X discloses a heat treatment method of a bainite steel rail, which comprises the steps of naturally cooling a steel rail after finish rolling so as to reduce the surface temperature of a rail head of the steel rail to 460-490 ℃; forcibly cooling the steel rail at the cooling speed of 2.0-4.0 ℃/s so as to reduce the surface layer temperature of the rail head of the steel rail to 250-290 ℃; naturally raising the temperature of the steel rail until the surface temperature of the rail head of the steel rail reaches more than 300 ℃; placing the steel rail in a heating furnace with the hearth temperature of 300-350 ℃ for tempering treatment for 2-6 h; and cooling the steel rail to room temperature in air. The invention aims to obtain a bainite steel rail with good comprehensive mechanical properties, belongs to a steel rail production heat treatment process, and is not suitable for postweld heat treatment of a bainite and pearlite steel rail welding joint.
201810480790.6 discloses a heat treatment method after welding for bainite welded joint, which is to cool the surface temperature of the welded joint tread to be cooled to a temperature range of 850-500 ℃ and then wind cool the rail head part of the joint, and when the surface temperature of the tread is cooled to 270-210 ℃, wind cool is finished, and the joint is naturally cooled to room temperature. The method is characterized in that air cooling is directly carried out on the joint after the bainite steel rail is welded, no heating process is carried out after the steel rail is welded, and the heat treatment effect is uncertain.
CN201410135909.8 discloses a heat treatment method of bainite steel rail welded joint and introduces the heat treatment principle of steel rail joint, in the post-welding heat treatment process, the steel rail joint needs to be heated to above austenitizing temperature, and compressed air is taken as cooling medium to carry out rapid cooling on the welding area so as to recover the mechanical property of the steel rail reduced by welding; however, the method does not describe the specific impact toughness value of the bainite welded joint, and the method is applicable to bainite steel rails with the same material, and is not applicable to dissimilar joints formed by welding the bainite steel rail and eutectoid pearlite steel rail.
CN201810581145.3 discloses a postweld heat treatment method for a hypereutectoid steel rail and eutectoid steel rail welded joint, CN201810720765.0 discloses a heat treatment method for a steel rail welded joint, and CN201810710040.3 discloses a heat treatment method for a dissimilar material steel rail welded joint, which introduce the postweld heat treatment method for the hypereutectoid steel rail and eutectoid steel rail welded joint, but the structures of the hypereutectoid steel rail and the eutectoid steel rail are mainly pearlite, the postweld heat treatment principle is that compressed air or water mist mixed gas is used as a cooling medium, and the rail joint at austenitizing temperature is rapidly cooled to achieve the purpose of thinning the pearlite interlayer spacing and improve the thermoplasticity of the rail welding area, and is not suitable for the joint formed by welding bainite and eutectoid steel rail.
In summary, in the field of railway engineering, a heat treatment method suitable for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail is urgently needed to improve the tread hardness of the steel rail reduced due to welding and improve the impact toughness of the steel rail joint so as to ensure the service performance and the railway operation safety of a welded joint of dissimilar steel rails.
Disclosure of Invention
The invention aims to provide a heat treatment method of a bainite steel rail and eutectoid pearlite steel rail welding joint, which has low cost and good mechanical property of the welding joint after heat treatment.
In order to solve the problems, the technical scheme adopted by the invention is to provide a heat treatment method for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail, which comprises the following steps:
a. welding a welded joint formed by welding a bainite steel rail and a eutectoid pearlite steel rail and having the temperature of 1100-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 720-760 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 10.0-13.0 ℃/s;
b. cooling the welded joint in the second stage to reduce the surface temperature of the welded joint to 350-400 ℃, wherein the cooling in the second stage is performed by adopting a profiling cooling device, the profiling cooling device sprays compressed air or water mist mixed gas to the welded joint, and the cooling speed is 2.5-3.0 ℃/s;
b. cooling the welded joint in the third stage to reduce the surface temperature of the welded joint to 100-180 ℃, wherein the cooling in the third stage is performed by adopting a profiling cooling device, and the profiling cooling device sprays compressed air or water mist mixed gas to the welded joint at the cooling speed of 0.6-0.8 ℃/s;
d. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature reduced to 100-180 ℃, wherein the tempering temperature is 280-350 ℃, the heat preservation time is 1-2 h, then placing the welded joint in air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed at the stage is 0.05-0.5 ℃/s.
In the heat treatment method of the bainite steel rail and eutectoid pearlite steel rail welded joint, the tensile strength of the bainite steel rail is more than 1300MPa, and the chemical components of the parent materials are as follows by weight percent: c: 0.20 to 0.30%, Si: 0.8-1.8%, Mn: 1.5-2.5%, Cr: 0.50 to 1.60%, Mo: 0.20-0.50%; the tensile strength of the eutectoid pearlite steel rail is more than 1250MPa, and the chemical components of the base material comprise the following components in percentage by weight: c: 0.75-0.82%, Si: 0.70-1.10%, Mn: 0.5-0.9%, Cr: 0.30-0.50%, V: 0.05 to 0.10 percent.
In the step b, the distance between the profiling cooling device and the welded joint is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.40-0.80 MPa.
In the step c, the distance between the profiling cooling device and the welded joint is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.10-0.25 MPa.
In the step d, tempering is carried out by adopting a split electric heating tempering device, wherein the distance between the split electric heating tempering device and the surface of the welded joint is 5-10 mm.
In the heat treatment method of the welded joint of the bainite steel rail and the eutectoid pearlite steel rail, in the step d, the structure of the open type electric heating tempering device comprises the following steps: the device comprises left terminals A1 and A2, right terminals B1 and B2, a crawler-type ceramic heater C, a rotating shaft D, a fixed snap ring E, a metal sheet shell F and an asbestos insulation layer G; wherein, the terminal A1 and the terminal A2 form a loop; terminal B1 and terminal B2 form a circuit; the electric heaters arranged in parallel are connected in parallel.
In the heat treatment method of the welded joint of the bainite steel rail and the eutectoid pearlite steel rail, the welded joint is formed by welding the bainite steel rail and the eutectoid pearlite steel rail which have the same steel rail shape and the specification of 60-75 kg/m through a steel rail moving flash welding machine.
The invention has the beneficial effects that:
according to the invention, the joint is subjected to tempering heat treatment after welding, so that the joint is beneficial to further improving the impact toughness of the joint while keeping higher strength and hardness; the martensite is not generated in the welding heat affected zone on one side of the eutectoid pearlite steel rail of the steel rail joint, and obvious block martensite is not generated in the welding heat affected zone on one side of the bainite steel rail of the steel rail joint; the width of a softening region at one side of the bainite steel rail of the steel rail joint is less than or equal to 25mm, and the width of a softening region at one side of the eutectoid pearlite steel rail of the steel rail joint is less than or equal to 10 mm; the room temperature impact energy of the full-section welding line of the welding joint reaches 10J, which is beneficial to ensuring the railway operation safety.
Drawings
FIG. 1 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of a welded rail joint obtained under the conditions of the post-weld heat treatment in example 1.
FIG. 2 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of the welded joint for a steel rail obtained under the conditions of the post-weld heat treatment in example 2.
FIG. 3 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of the welded joint for a steel rail obtained under the condition of the postweld heat treatment in comparative example 2.
FIG. 4 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the rail head tread of the welded joint for a steel rail obtained under the condition of the post-weld heat treatment in comparative example 3.
Fig. 5 is a schematic view of each part or position of the steel rail welded joint of the present invention, wherein a is a bainite steel rail, b is the welded joint, c is a eutectoid pearlite steel rail, d is a rail head tread, and e is a weld center.
FIG. 6 is a schematic diagram of a metallographic specimen sampling position of a rail head tread of a welded joint of a steel rail according to the present invention, wherein e is a weld center and f is the metallographic specimen sampling position.
Fig. 7 is a schematic three-dimensional structure diagram of the profiling cooling device for the rail head of the steel rail of the invention.
Fig. 8 is a schematic view of the use of the profiling cooling device for the head of a steel rail of the present invention.
FIG. 9 is a schematic diagram showing the distribution of electric heaters on the railhead part of the open type electric heating tempering device, wherein 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.
FIG. 10 is a schematic view of the open electric tempering apparatus of the present invention, wherein A1/A2 is a left side binding post; B1/B2 is a right side 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 insulating layer; terminal a1 and terminal a2 form a circuit; terminal B1 and terminal B2 form a circuit; a plurality of electric heaters arranged in parallel are connected together in a parallel connection mode.
Detailed Description
In the invention, the martensite transformation critical cooling speed in the continuous cooling process of the bainite rail steel is 0.8-1.2 ℃/s, and the Ms temperature (the start temperature of martensite formation) of the rail steel is 330-370 ℃; the critical cooling speed of martensite transformation in the continuous cooling process of the eutectoid pearlite rail steel is 1.0-1.5 ℃/s, and the Ms temperature (the start temperature of martensite formation) of the rail steel is 220-280 ℃. In order to avoid the occurrence of abnormal structures such as martensite in the welded joint of the steel rail, when the welded joint of the bainite steel rail and the eutectoid pearlite steel rail is subjected to postweld heat treatment, the final cooling temperature in the postweld heat treatment rapid cooling process needs to be controlled to be higher than the Ms temperature of the bainite steel rail. At the same time, the cooling rate during the post-weld heat treatment must be limited to bainitic rail steels with relatively low critical cooling rates, otherwise the joint will fail prematurely due to the large amount of hardened martensite.
In the rail welding standards, AS specified in australian rail welding standard AS1085.20-2012, for some high-strength-grade, high-carbon-content and high-alloy-content rails, under the observation magnification of a metallographic microscope of 100x, for the most severe region where martensite appears in a rail welded joint, the percentage content of a martensite structure is not higher than 5%, otherwise the joint will cause premature fatigue fracture due to a large amount of hardened martensite structures, and the operation safety of the railway is seriously affected. Therefore, strict control of the martensite content in the welded structure of the steel rail is important for stable operation of the railway line.
Based on the discovery, the invention designs a heat treatment method for a welded joint of a bainite steel rail and a eutectoid pearlite steel rail, which comprises the following steps: a. welding a welded joint formed by welding a bainite steel rail and a eutectoid pearlite steel rail and having the temperature of 1100-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 720-760 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 10.0-13.0 ℃/s;
b. cooling the welded joint in the second stage to reduce the surface temperature of the welded joint to 350-400 ℃, wherein the cooling in the second stage is performed by adopting a profiling cooling device, the profiling cooling device sprays compressed air or water mist mixed gas to the welded joint, and the cooling speed is 2.5-3.0 ℃/s;
c. cooling the welded joint in the third stage to reduce the surface temperature of the welded joint to 100-180 ℃, wherein the cooling in the third stage is natural cooling in the air, and the cooling speed is 0.6-0.8 ℃/s;
d. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature reduced to 100-180 ℃, wherein the tempering temperature is 280-350 ℃, the heat preservation time is 1-2 h (after the tempering heat preservation process is finished, the tempering device is taken down), then placing the welded joint in the air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed at the stage is 0.05-0.5 ℃/s.
In the invention, the first-stage cooling is natural cooling in air, and the control of the first-stage cooling speed can be realized by adjusting the test environment temperature (for example, adopting a central air conditioner for temperature control). The reason why the temperature drop rate is still high in this stage though air cooling is because the temperature difference between the welded joint and the air is large.
The invention realizes the post-welding heat treatment process of the steel rail by using the welding waste heat of the steel rail. And performing post-welding accelerated cooling on the welded rail joint with higher residual temperature so as to reduce the transformation temperature of the rail joint from austenite to pearlite/bainite, thereby improving the hardness of an austenite recrystallization region. Based on the principle of metallurgy, the steel rail joint has certain dynamic supercooling degree under the high-temperature rapid cooling condition after welding, so that the phase transition temperature of transformation from austenite to pearlite in a non-equilibrium state moves downwards, and the phase transition temperature is gradually reduced along with the increase of the supercooling degree. Therefore, even if the joint railhead is cooled in the second stage in which the opening temperature is relatively low, the structural transformation from austenite to pearlite/bainite can occur.
In the invention, the start cooling temperature of the second cooling stage is 720-760 ℃, and the finish cooling temperature of the second cooling is controlled to be higher than the martensite transformation starting temperature (Ms temperature) of the bainite rail steel and the eutectoid pearlite rail steel, namely the finish cooling temperature of the second cooling is 350-400 ℃. In the second cooling stage, the following method can be adopted to ensure that the cooling speed is 2.5-3.0 ℃/s: the distance between the profiling cooling device and the welding joint is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.40-0.80 MPa.
In the invention, when the steel rail joint is cooled in the third stage, in order to avoid the hardened martensite structure of the joint, the joint is cooled at the cooling speed of 0.6-0.8 ℃/s which is lower than the martensite transformation critical cooling speed of the steel rail. In the third stage of cooling, the following method can be adopted to ensure that the cooling speed is 0.6-0.8 ℃/s: the distance between the profiling cooling device and the welded joint railhead tread is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.10-0.25 MPa.
It should be noted that: the device of fig. 8 only cools the rail head tread and the rail head side, and the hole diameter can be designed and processed according to actual requirements, so that different cooling strengths are realized. The pressure of the gas flowing through the first compressed air or water mist mixed gas channel and the second compressed air or water mist mixed gas channel can be monitored through related pressure detection devices, and the pressure of the gas flowing through the first compressed air or water mist mixed gas channel and the second compressed air or water mist mixed gas channel is adjusted according to actual needs.
Firstly, welding the steel rail, then carrying out accelerated cooling and then tempering in the step d, wherein the accelerated cooling is used for refining the interlayer spacing of pearlite plates in a welding area/refining bainite tissues in the welding area and improving the strength and hardness of a welding area of a joint, and the tempering is used for further improving the impact toughness of the joint and reducing the welding stress of the joint; tests show that when the tempering temperature is higher than 350 ℃, the impact toughness of the joint is not obviously improved, and when the tempering temperature is lower than 280 ℃, the impact of the joint is improved but the improvement amplitude is lower.
Therefore, in the step d, when an open type (profiling) electric heating tempering device is adopted to carry out tempering heat treatment on the steel rail welding joint with the surface temperature of 100-180 ℃, the tempering temperature is 280-350 ℃, and the heat preservation time is 1-2 hours. The profiling device can be tightly attached to the surface of a steel rail welding joint, good heat conduction is achieved, and a tempering heat treatment process can be achieved under the combined action of the crawler-type heater. And when the tempering and heat preservation process is finished, taking down the tempering device. And (3) placing the welded joint in air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed in the stage is 0.05-0.5 ℃/s.
It should be noted that: the heating device of fig. 9 has similar layouts of the head, web and foot heating zones. 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.
It should be noted that: the device in the figure 10 has the advantages of small size, flexibility, low cost and the like, is convenient for field construction, can be powered by a diesel generator or a 220V power supply, and has the rated power of 10 kW. The device uses a commercial LCD crawler-type ceramic heater as a heat source, the size of the heater can be 10mm (length) multiplied by 10mm (width) multiplied by 7mm (thickness), and a ring-shaped split heater is manufactured by matching with a heat insulation material and a steel structure shell, is convenient to assemble and disassemble, and is suitable for local heating of a steel rail welding joint. The actual size of the heating device and the specification and distribution condition of the heater can be adjusted according to the actual size of the steel rail profile. In the device design process, evenly fix the device inboard that has similar rail profile shape with multiunit crawler-type heater, make the heater cladding on the rail surface and with rail surface fully laminate in order to realize the good heat-conduction in the heating process, can realize the heat treatment process to the rail joint based on the device. In the test process, a temperature controller is adopted to control the heating temperature. The working temperature range of the device is 200-750 ℃. The split device can be rotated by 180 DEG at maximum about the axis of rotation. The split heaters A1 and A2 form a loop, and the split heaters B1 and B2 form a loop, and can be split into two symmetrical parts from the middle connecting position, so that the two sides can be heated independently.
The welding joint is formed by welding a bainite steel rail and a eutectoid pearlite steel rail which have the same rail type and the specification of 60-75 kg/m through a steel rail moving flash welding machine. The welded joint comprises a region with the length of 70-100 mm in the welding seam and/or the heat affected zone, and the center of the region is the welding seam. In the invention, the room temperature is 10-30 ℃.
The present invention is further illustrated by the following examples, which are not intended to limit the scope of the invention.
Example 1
After the upsetting and the push-button in the process of moving flash welding are finished on the steel rail with the specification of 68kg/m, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, carrying out first-stage cooling on a steel rail joint with the residual temperature of 1200 ℃ obtained by welding at a first cooling speed of 10.0 ℃/s so as to reduce the surface temperature of a rail head of the steel rail joint to 750 ℃, then carrying out second-stage cooling on the steel rail joint at a second cooling speed of 2.6 ℃/s so as to reduce the surface temperature of the rail head of the steel rail joint to 400 ℃, and finally carrying out third-stage cooling on the steel rail joint at a third cooling speed of 0.8 ℃/s so as to reduce the surface temperature of the rail head of the steel rail joint to 170 ℃; and then, tempering heat treatment at 330 ℃ for 1h is carried out on the welded joint of the steel rail with the surface temperature of 170 ℃ by adopting a profiling electric heating tempering device. And after the tempering and heat preservation process is finished, taking down the tempering device, placing the joint in the air for natural cooling, and reducing the surface temperature of the joint to the room temperature, thereby obtaining the welded steel rail joint subjected to postweld heat treatment.
In the postweld heat treatment process, the first cooling is natural cooling carried out in the air, the second cooling and the third cooling process adopt the steel rail railhead profiling cooling device to cool the railhead tread and the railhead side surface of the steel rail joint by taking compressed air or water mist mixed gas as a cooling medium, and the distance between the cooling device and the steel rail railhead tread is 40 mm. In the second cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.42 MPa; in the third cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.25 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.
The 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 factory of testing machines in Laizhou, Shandong, model HBV-30A) at a position 4mm 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. The hardness test data are shown in Table 1, and the effect of the distribution of the longitudinal hardness of the joint is shown in FIG. 1.
Table 1 example 1 hardness test data
As can be seen from table 1 and fig. 1, the width of the bainite rail side softening zone and the width of the eutectoid pearlite rail side softening zone of the rail joint were 24mm and 6mm, respectively, for the rail joint treated according to the present invention.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is 10J, which is beneficial to ensuring the running safety of the railway.
Example 2
After the upsetting and the push-button in the process of moving flash welding are finished on the steel rail with the specification of 60kg/m, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, the rail joint with the residual temperature of 1250 ℃ obtained by welding is subjected to first-stage cooling at a first cooling speed of 12.0 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 730 ℃, then the rail joint is subjected to second-stage cooling at a second cooling speed of 2.8 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 360 ℃, and finally the rail joint is subjected to third-stage cooling at a third cooling speed of 0.7 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 150 ℃. And then, tempering heat treatment at the temperature of 300 ℃ for 1.5h is carried out on the welded joint of the steel rail with the surface temperature of 150 ℃ by adopting a profiling electric heating tempering device. And after the tempering and heat preservation process is finished, taking down the tempering device, placing the joint in the air for natural cooling, and reducing the surface temperature of the joint to the room temperature, thereby obtaining the welded steel rail joint subjected to postweld heat treatment.
In the postweld heat treatment process, the first cooling is natural cooling in the air; in the second cooling process and the third cooling process, a rail head profiling cooling device is adopted to cool a rail head tread and a rail head side face of a rail joint by taking water-mist mixed gas as a cooling medium, and the distance between the cooling device and the rail head tread is 40 mm; in the second cooling process, the gas pressure of the water mist mixed gas sprayed by the cooling device is 0.60 MPa; in the third cooling process, the gas pressure of the water mist mixed gas sprayed by the cooling device is 0.20 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.
The 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 factory of testing machines in Laizhou, Shandong, model HBV-30A) at a position 4mm 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. The hardness test data are shown in Table 2, and the effect of the longitudinal hardness distribution of the joint is shown in FIG. 2.
Table 2 example 2 hardness test data
As is clear from table 2 and fig. 2, the width of the softened region on the bainitic rail side of the rail joint was 23mm and the width of the softened region on the eutectoid pearlitic rail side of the rail joint was 6mm for the rail weld joint treated according to the present invention.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is 11J, which is beneficial to ensuring the running safety of the railway.
Comparative example 1
The only difference is that the process conditions involved in the steel rail welding and postweld cooling processes in the comparative example are consistent with those involved in the steel rail welding and postweld cooling processes in the example 2, and the only difference is that the postweld tempering heat treatment is not carried out on the steel rail welding joint after the steel rail welding joint is cooled to the rail head surface layer temperature of 150 ℃ in the third stage, and the joint is directly placed in the air for natural cooling, so that the joint surface temperature is reduced to the room temperature.
The rail joint obtained in this comparative example was processed into a longitudinal hardness test specimen, and the hardness distribution curve was in accordance with fig. 2. Metallographic examination shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the eutectoid pearlite steel rail of the steel rail joint is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. The difference is that the room temperature impact energy of the welding seam of the rail head of the dissimilar steel rail joint in the embodiment 2 is 11J, while the room temperature impact energy of the welding seam of the full section of the dissimilar steel rail joint in the comparative example is only 7J, so that the toughness of the joint is poor, and the safety of railway operation is not facilitated.
Comparative example 2
After the upsetting and the push-button in the process of moving flash welding are finished on the steel rail with the specification of 68kg/m, directly air-cooling the steel rail joint with the residual temperature of 1100 ℃ to the room temperature (about 25 ℃) so as to obtain the steel rail welded joint under the air-cooling (natural cooling) condition.
The rail joint obtained in this comparative example under air cooling after welding was processed 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. The hardness test data are shown in Table 3, and the effect of the longitudinal hardness distribution of the joints is shown in FIG. 3.
Table 3 comparative example 2 hardness test data
As is apparent from Table 3 and FIG. 3, the width of the bainite rail-side softened region of the rail joint was 26mm, and the width of the eutectoid pearlite rail-side softened region of the rail joint was 18mm, for the welded rail joint of the rail which was not treated by the post-weld heat treatment method according to the present invention. In the service process of the line, the welded joint obtained by the comparative example is easy to preferentially form low-collapse steel rail head tread in a softening area on one side of the bainite steel rail of the joint weld seam, and the smoothness and driving safety of the line are influenced.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the average value of the room-temperature impact energy of the full-section welding seam of the welding joint is 5J, the impact toughness is poor, and the safety of railway operation is not facilitated.
Comparative example 3
After the upsetting and the push-button in the process of moving flash welding are finished on the steel rail with the specification of 68kg/m, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, the rail joint with the residual temperature of 1200 ℃ obtained by welding is subjected to first-stage cooling at a first cooling speed of 10.0 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 750 ℃, then the rail joint is subjected to second-stage cooling at a second cooling speed of 3.0 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 380 ℃, and finally the rail joint is subjected to third-stage cooling at a third cooling speed of 2.5 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to the room temperature of 25 ℃, so that the welded joint of the dissimilar materials of the rail in the comparative example is obtained.
In the postweld heat treatment process, the first cooling is natural cooling in the air; in the second cooling process and the third cooling process, a steel rail head profiling cooling device is adopted to cool a rail head tread and a rail head side face of a steel rail joint by taking compressed air as a cooling medium, and the distance between the cooling device and the steel rail head tread is 40 mm; in the second cooling process, the gas pressure of the compressed air sprayed by the cooling device is 0.80 MPa; in the third cooling process, the gas pressure of the compressed air injected by the cooling device is 0.40 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer.
The rail joint obtained in the comparative example under the air cooling condition after welding is processed into a longitudinal hardness sample. A hardness sample is subjected to longitudinal Vickers hardness detection by using a Bravicer hardness tester (general factory of testing machines in Laizhou, Shandong, model HBV-30A) at a position 4mm 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. The hardness test data are shown in Table 4, and the effect of the longitudinal hardness distribution of the joints is shown in FIG. 4.
Table 4 comparative example 2 hardness test data
As is apparent from Table 4 and FIG. 4, the width of the bainite rail-side softened region of the rail joint was 20mm, and the width of the eutectoid pearlite rail-side softened region of the rail joint was 6mm, for the welded rail joint not treated by the post-weld heat treatment method according to the present invention. In the service process of the line, the welded joint obtained by the comparative example is easy to preferentially form low-collapse steel rail head tread in a softening area on one side of the bainite steel rail of the joint weld seam, and the smoothness and driving safety of the line are influenced.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 Metal microstructure examination method, etching is carried out on the metallographic sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test results show that martensite appears in different degrees in the welding heat affected zone of the hypereutectoid steel rail and the eutectoid pearlite steel rail. Under the observation magnification of a metallographic microscope of 100x, for the most serious area with martensite structure in the joint, through statistics, the percentage content of martensite in the heat affected zone on one side of the joint bainite steel rail reaches 15%, and the percentage content of martensite in the heat affected zone on one side of the joint eutectoid pearlite steel rail reaches 10%, which is not beneficial to the operation safety of the railway. In addition, the average value of the room-temperature impact energy of the full-section welding seam of the welding joint is 4J, the impact toughness is poor, and the safety of railway operation is not facilitated.
As can be seen by comparing the weld joint railhead tread longitudinal stiffness and joint softening zone width in fig. 1-4: by adopting the postweld heat treatment method provided by the invention to carry out postweld heat treatment on the welded joint of the bainite steel rail and the eutectoid pearlite steel rail, no martensite exists in the welding heat affected zone on one side of the eutectoid pearlite steel rail of the steel rail joint, and no obvious block martensite exists in the welding heat affected zone on one side of the bainite steel rail of the steel rail joint. Meanwhile, the width of the softening region at one side of the bainite steel rail of the steel rail joint is less than or equal to 25mm, and the width of the softening region at one side of the eutectoid pearlite steel rail of the steel rail joint is less than or equal to 10 mm. In addition, the full-section room temperature impact energy of the welded joint railhead reaches 10J, which is beneficial to ensuring the railway driving safety.
The invention is beneficial to improving saddle-shaped abrasion of a steel rail welding joint caused by low hardness of a welding area in a line service process, has good joint toughness and is beneficial to ensuring the running safety of a railway.
Claims (7)
1. The heat treatment method of the bainite steel rail and eutectoid pearlite steel rail welding joint is characterized in that: the method comprises the following steps:
a. welding a welded joint formed by welding a bainite steel rail and a eutectoid pearlite steel rail and having the temperature of 1100-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 720-760 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 10.0-13.0 ℃/s;
b. cooling the welded joint in the second stage to reduce the surface temperature of the welded joint to 350-400 ℃, wherein the cooling in the second stage is performed by adopting a profiling cooling device, the profiling cooling device sprays compressed air or water mist mixed gas to the welded joint, and the cooling speed is 2.5-3.0 ℃/s;
c. cooling the welded joint in the third stage to reduce the surface temperature of the welded joint to 100-180 ℃, wherein the cooling in the third stage is performed by adopting a profiling cooling device, and the profiling cooling device sprays compressed air or water mist mixed gas to the welded joint at the cooling speed of 0.6-0.8 ℃/s;
d. and (3) carrying out tempering heat treatment on the welded joint of the steel rail with the surface temperature reduced to 100-180 ℃, wherein the tempering temperature is 280-350 ℃, the heat preservation time is 1-2 h, then placing the welded joint in air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed at the stage is 0.05-0.5 ℃/s.
2. The heat treatment method for a welded joint of a bainitic steel rail and a eutectoid pearlite steel rail according to claim 1, characterized in that: the tensile strength of the bainite steel rail is more than 1300MPa, and the chemical components of the parent material are as follows by weight percent: c: 0.20 to 0.30%, Si: 0.8-1.8%, Mn: 1.5-2.5%, Cr: 0.50 to 1.60%, Mo: 0.20-0.50%; the tensile strength of the eutectoid pearlite steel rail is more than 1250MPa, and the chemical components of the base material comprise the following components in percentage by weight: c: 0.75-0.82%, Si: 0.70-1.10%, Mn: 0.5-0.9%, Cr: 0.30-0.50%, V: 0.05 to 0.10 percent.
3. The heat treatment method for a welded joint of a bainitic steel rail and a eutectoid pearlite steel rail according to claim 1, characterized in that: in the step b, the distance between the profiling cooling device and the welding joint is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.40-0.80 MPa.
4. The heat treatment method for a welded joint of a bainitic steel rail and a eutectoid pearlite steel rail according to claim 1, characterized in that: in the step c, the distance between the profiling cooling device and the welding joint is 20-40 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.10-0.25 MPa.
5. The heat treatment method for a welded joint of a bainitic steel rail and a eutectoid pearlite steel rail according to claim 1, characterized in that: and d, tempering by adopting a split electric heating tempering device, wherein the distance between the device and the surface of the steel rail welding joint is 5-10 mm.
6. The heat treatment method for a welded joint of a bainitic steel rail and an eutectoid pearlitic steel rail according to claim 5, characterized in that: in step d, the structure of the open type electric heating tempering device comprises: the device comprises left terminals A1 and A2, right terminals B1 and B2, a crawler-type ceramic heater C, a rotating shaft D, a fixed snap ring E, a metal sheet shell F and an asbestos insulation layer G; wherein, the terminal A1 and the terminal A2 form a loop; terminal B1 and terminal B2 form a circuit; the electric heaters arranged in parallel are connected in parallel.
7. The heat treatment method for a welded joint of a bainitic steel rail and a eutectoid pearlite steel rail according to any one of claims 1 to 6, characterized by comprising: the welding joint is formed by welding a bainite steel rail and a eutectoid pearlite steel rail which have the same steel rail type and the specification of 60-75 kg/m through a steel rail mobile flash welding machine.
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| CN113755670A (en) * | 2021-08-24 | 2021-12-07 | 中铁宝桥集团有限公司 | Quenching and cooling method for bainitic steel frog point rail |
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| CN115478149B (en) * | 2022-10-18 | 2023-08-25 | 包头钢铁（集团）有限责任公司 | Bainite steel rail and pearlite steel rail welding joint heat treatment process for heavy haul railway |
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