CN112226609B - Construction method for heat treatment of post-welded joints of dissimilar steel rails - Google Patents
Construction method for heat treatment of post-welded joints of dissimilar steel rails Download PDFInfo
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- C—CHEMISTRY; METALLURGY
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D1/84—Controlled slow cooling
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- 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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- 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
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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Abstract
The invention discloses a construction method for heat treatment of a post-weld joint of a dissimilar steel rail, and belongs to the technical field of post-weld heat treatment of steel rails. The invention provides a construction method for heat treatment of a welded joint of a dissimilar steel rail, which has low cost and good mechanical property of the welded joint after heat treatment, and comprises the following steps: carrying out first cooling on the welding joint to 100-200 ℃; then normalizing and heating to 920-980 ℃; then carrying out second cooling to 380-450 ℃; then carrying out third cooling to 100-180 ℃; then heating and tempering to 280-350 ℃, and preserving heat; 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, has no obvious large-size martensite sign in the steel rail welding heat affected zone, has good joint toughness, and is 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 construction method for heat treatment of post-weld joints of dissimilar steel rails.
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 construction method suitable for heat treatment of welded joints of dissimilar steel rails is urgently needed to improve the tread hardness of the steel rails reduced by welding and improve the impact toughness of the steel rail joints so as to ensure the service performance and the railway operation safety of the welded joints of the dissimilar steel rails.
Disclosure of Invention
The invention aims to solve the technical problem of providing a construction method for heat treatment of a welded joint of dissimilar steel rails, which has low cost and good mechanical property of the welded joint after heat treatment.
In order to solve the problems, the technical scheme adopted by the invention is to provide a construction method for heat treatment of a post-weld joint of a dissimilar 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 1000-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 100-200 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 0.1-15.0 ℃/s;
b. after cooling in the first stage, normalizing and heating, and stopping heating when the surface temperature of the joint is heated to 920-980 ℃;
c. after the normalizing heating is finished, carrying out second-stage cooling, wherein the starting cooling temperature of the second-stage cooling is not lower than 780 ℃, high-pressure cooling gas or water mist mixed gas is used as a cooling medium, the cooling speed is 2.5-3.0 ℃/s, and the final cooling temperature is 380-450 ℃;
d. after the second stage of cooling, performing third stage of cooling, and taking high-pressure cooling gas or water mist mixed gas as a cooling medium to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-0.8 ℃/s;
e. 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 construction method for the heat treatment of the welded joints of the dissimilar steel rails, the tensile strength of the bainite steel rail is more than 1300MPa, and the chemical components of the mother materials in percentage by weight are as follows: 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 construction method for the heat treatment of the welded joints of the dissimilar steel rails, in the step a, in the first-stage cooling, when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s.
In the step b, the normalizing heating is full-section normalizing heating of the welded joint of the steel rail by adopting a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater.
In the construction method for the heat treatment of the welded joints of the dissimilar steel rails, in the step c, in the cooling of the second stage, the distance between a nozzle type cooling device matched with the normalizing machine set and the surfaces of the welded joints is 30-50 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.30-0.60 MPa.
In the construction method for the heat treatment of the welded joints of the dissimilar steel rails, in the step d, in the third stage of cooling, a nozzle type cooling device matched with a normalizing machine set is 30-50 mm away from the surfaces of the welded joints; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.05-0.10 MPa.
In the construction method for the heat treatment of the welded joints of the dissimilar steel rails, in the step e, tempering treatment is carried out by adopting an open type electric heating tempering device, and the distance between the open type electric heating tempering device and the surfaces of the welded joints is 5-10 mm.
In the above construction method for heat treatment of the welded joint of the dissimilar steel rail, in step e, the structure of the open type electric heating tempering device includes: 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 construction method for the heat treatment of the welded joint of the dissimilar 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 by a steel rail moving flash welding machine.
The invention has the beneficial effects that:
in the invention, one-time normalizing heating, three-time cooling and one-time tempering are adopted to carry out postweld heat treatment on the welding joint of the dissimilar steel rail in the heat treatment process, so that no martensite exists in the welding heat affected zone on one side of the eutectoid pearlite steel rail of the welding joint of the dissimilar steel rail, and no obvious large-size blocky martensite exists in the welding heat affected zone on one side of the bainite steel rail of the steel rail joint. The width of the softening zone at one side of the bainite steel rail of the dissimilar steel rail joint is less than or equal to 50mm, and the width of the softening zone at one side of the eutectoid pearlite steel rail of the steel rail joint is less than or equal to 30 mm. The average hardness of one side of the bainite steel rail of the dissimilar material steel rail joint can reach more than 85 percent of the base material, and the average hardness of one side of the pearlite steel rail can reach more than 90 percent of the base material; meanwhile, the room temperature impact energy of the full-section welding seam of the welding joint reaches 15J, which is higher than 6.5J specified by TB/T1632.2-2014.
The invention is beneficial to improving saddle-shaped abrasion of a steel rail welding joint caused by low hardness of a welding area in a line service process, has good joint toughness and is beneficial to ensuring the running safety of a railway.
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 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. 8 is a schematic view of the open electric tempering apparatus of the present invention, wherein A1 and A2 are left terminals; b1 and B2 are right side posts; 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 construction method for heat treatment of the post-weld joint of the dissimilar 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 1000-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 100-200 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 0.1-15.0 ℃/s;
b. after cooling in the first stage, normalizing and heating, and stopping heating when the surface temperature of the joint is heated to 920-980 ℃;
c. after the normalizing heating is finished, carrying out second-stage cooling, wherein the starting cooling temperature of the second-stage cooling is not lower than 780 ℃, high-pressure cooling gas or water mist mixed gas is used as a cooling medium, the cooling speed is 2.5-3.0 ℃/s, and the final cooling temperature is 380-450 ℃;
d. after the second stage of cooling, performing third stage of cooling, and taking high-pressure cooling gas or water mist mixed gas as a cooling medium to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-0.8 ℃/s;
e. 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 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. When the first stage carries out natural cooling, because there is the cold rate of high temperature stage very fast, the cold rate of low temperature stage is the slow condition relatively, and the cooling rate of each temperature section is all inequality, and the cooling rate of each temperature section is this moment: when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s. Without considering the component segregation, a welded rail joint generally does not exhibit a martensite structure in a weld heat affected zone under a post-weld air cooling condition.
In the invention, the normalizing heat treatment refers to a heat treatment process of heating the metal workpiece to be 30-50 ℃ above Ac3 (the final temperature of ferrite transformed into austenite during heating), keeping the temperature for a period of time, taking the metal workpiece out of a furnace, and naturally cooling, or spraying compressed air for cooling in the air. The post-weld normalizing heat treatment of the welded joint of the steel rail is different from the heat treatment process used by common small-size workpieces, and the normalizing heat treatment of the welded joint of the steel rail cannot keep the temperature (the temperature above the austenitizing temperature) for a long time after the target temperature is reached because the length of a sample after the steel rail is welded can be as high as hundreds of meters. Therefore, the steel rail welding joint is heated to the target temperature at a temperature slightly higher than the conventional normalizing temperature, and then the air cooling or air cooling heat treatment process is carried out, wherein the optimal normalizing heating temperature is 920-980 ℃.
According to the invention, after the steel rail joint is heated to the normalizing temperature of 920-980 ℃, the steel rail joint is quickly conveyed to the position of a cooling device through a conveying channel after the normalizing heating is finished, cooling is immediately carried out under natural conditions, and the surface temperature of the steel rail joint is monitored in real time by adopting a handheld infrared temperature measuring gun along with the temperature reduction process so as to ensure that the surface temperature of the joint is above 780 ℃. The normalizing in the parameters is to ensure that the steel rail joint can be fully austenitized, provide dynamic conditions for the subsequent rapid cooling and thinning of a welded structure, and rapidly cool the surface of the steel rail joint at a proper cooling speed so as to fulfill the aims of thinning a weld zone structure and improving the strength and hardness of the joint.
In the invention, the start cooling temperature of the second cooling stage is not lower than 780 ℃, the finish cooling temperature of the second cooling stage is higher than the martensite transformation starting temperature (Ms temperature) of the bainite rail steel and the eutectoid pearlite rail steel, the finish cooling temperature of the second cooling stage is 380-450 ℃, and the cooling speed of the second cooling stage is controlled at 2.5-3.0 ℃/s, so that the mechanical property of a welding region is improved by performing accelerated cooling on super-cooled austenite to refine the pearlite interlayer spacing and refine the bainite structure. 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 nozzle type cooling device matched with the normalizing unit and the surface of the welding joint is 30-50 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.30-0.60 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 nozzle type cooling device matched with the normalizing unit and the surface of the welding joint is 30-50 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.05-0.10 MPa.
In step e of the present invention, the tempering temperature is low, generally below 500 ℃, and is usually used to improve the toughness of the rail joint and remove the welding residual stress. According to the principle of metallurgy: the low temperature tempering temperature is 150-250 ℃, the medium temperature tempering temperature is 350-500 ℃, and the high temperature tempering temperature is 500-650 ℃. Through tests: when the tempering temperature is more 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 e, 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 set to be 280-350 ℃, and the heat preservation time is set to be 1-2 h. The profiling device can be tightly attached to the surface of a steel rail welding joint, good heat conduction is achieved, and a tempering heat treatment process can be achieved under the combined action of the crawler-type heater. And when the tempering and heat preservation process is finished, taking down the tempering device. And (3) placing the welded joint in air for natural cooling, so that the surface temperature of the welded joint is reduced to 10-30 ℃, and the cooling speed in the stage is 0.05-0.5 ℃/s.
It should be noted that: the heating device of fig. 7 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 8 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 mobile flash welding machine or a gas pressure welding machine. The welded joint comprises a welding seam and/or a region with the length of the heat affected zone within the range of 80-120 mm, and the center of the region is the welding seam. In the invention, the room temperature is 10-30 ℃.
The present invention is further illustrated by the following examples, which are not intended to limit the scope of the invention.
In the following examples, both the bainitic rail and the eutectoid pearlite rail were heat-treated rails, and were produced by Pan Steel group.
The invention relates to a postweld heat treatment process method of a bainite steel rail welded joint, which refers to TB/T1632.2-2014 part 2 of steel rail welding: flash welding and TB/T1632.4-2014 Rail welding part 4: according to the standard of air pressure welding, a steel rail welding joint is machined into a longitudinal section hardness sample, the Rockwell hardness of the longitudinal section of the welding joint is detected at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, the distance between the measuring points is 5mm, the Rockwell hardness method is carried out according to the regulation of GB/T230.1-2009, an HRC scale is adopted, Hp represents the average hardness value of a steel rail base metal, Hj represents the average hardness value of the joint, and the position where the hardness of the joint is lower than 0.9Hp represents a softening region.
Example 1
After the upsetting and the beading in the moving flash welding process of the dissimilar steel rail with the specification of 75kg/m are finished, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, placing a dissimilar steel rail joint with the residual temperature of 1200 ℃ obtained by welding in air for natural cooling so as to reduce the temperature of the surface layer of a rail head of the steel rail joint to 200 ℃; then, heating the whole section of the steel rail welding joint area by adopting a medium-frequency induction profiling electric heating coil, and stopping heating when the tread temperature of the steel rail reaches 950 ℃; then, compressed air is taken as a cooling medium, and rapid cooling is carried out at a cooling speed of 3.0 ℃/s, wherein the final cooling temperature of the stage is 430 ℃; then, carrying out third-stage cooling at a third cooling speed of 0.8 ℃/s to reduce the temperature of the surface layer of the rail head of the steel rail joint to 170 ℃; and then, tempering heat treatment is carried out on the welded joint of the steel rail with the surface temperature of 170 ℃ at 350 ℃ for 1h by adopting a split electric heating tempering device, after the tempering heat preservation process is finished, the tempering device is taken down, and the joint is placed in the air and naturally cooled to the room temperature at the cooling speed of 0.3 ℃/s, so that the welded joint of the steel rail subjected to heat treatment in the embodiment is obtained.
In this embodiment, the first cooling is natural cooling performed in air, the nozzle-type cooling device used in conjunction with the rail post-weld heat treatment unit cools the rail head tread and the rail head side of the rail joint with compressed air or a water mist mixture as a cooling medium in the second cooling and the third cooling, and the distance between the nozzle of the cooling device and the rail head tread is 40 mm. In the second cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.60 MPa; in the third cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.10 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.
The bainite steel rail welding joint obtained in the embodiment is machined into a longitudinal hardness test sample, longitudinal Rockwell hardness detection is carried out on the welding joint at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 1, and the distribution effect of the longitudinal hardness is shown in FIG. 1.
Table 1 example 1 longitudinal stiffness data
As can be seen from table 1 and fig. 1, the width of the bainite rail side softening zone and the 30mm softening zone of the eutectoid pearlite rail side softening zone of the rail joint were 50mm and 30mm, respectively, for the rail joint treated according to the present invention. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 86% of that of the base material, and the average hardness of the pearlite steel rail side can reach 91% of that of the base material.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 15J, which is beneficial to ensuring the railway operation safety.
Example 2
After the upsetting and the beading in the moving flash welding process of the dissimilar steel rail with the specification of 68kg/m are finished, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, placing a dissimilar steel rail joint with residual temperature of 1250 ℃ obtained by welding in air for natural cooling so as to reduce the temperature of the surface layer of a rail head of the steel rail joint to 150 ℃; then, heating the whole section of the steel rail welding joint area by adopting a medium-frequency induction profiling electric heating coil, and stopping heating when the tread temperature of the steel rail reaches 930 ℃; then, taking compressed air as a cooling medium, and rapidly cooling at a cooling speed of 2.5 ℃/s, wherein the final cooling temperature of the stage is 400 ℃; then, carrying out third-stage cooling at a third cooling speed of 0.6 ℃/s to reduce the temperature of the surface layer of the rail head of the steel rail joint to 160 ℃; and then, tempering heat treatment is carried out on the welded joint of the steel rail with the surface temperature of 170 ℃ for 1h by adopting a split electric heating tempering device, after the tempering heat preservation process is finished, the tempering device is taken down, the joint is placed in the air and naturally cooled to the room temperature at the cooling speed of 0.5 ℃/s, and the welded and heat treated welded joint of the steel rail is obtained.
In this embodiment, the first cooling is natural cooling performed in air, the nozzle-type cooling device used in conjunction with the rail post-weld heat treatment unit cools the rail head tread and the rail head side of the rail joint with compressed air or a water mist mixture as a cooling medium in the second cooling and the third cooling, and the distance between the nozzle of the cooling device and the rail head tread is 40 mm. In the second cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.30 MPa; in the third cooling process, the gas pressure of the compressed air or the water mist mixed gas sprayed by the cooling device is 0.08 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer. During tempering heat treatment, the distance between a crawler-type ceramic heater in the profiling electric heating tempering device and the surface of a steel rail welding joint is 8mm, and the tempering device is provided with a temperature control system and can monitor the tempering heating temperature in real time.
The bainite steel rail welding joint obtained in the embodiment is machined into a longitudinal hardness test sample, longitudinal Rockwell hardness detection is carried out on the welding joint at a position 5mm below a tread, measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 2, and the distribution effect of the longitudinal hardness is shown in FIG. 2.
Table 2 example 2 longitudinal stiffness data
As can be seen from table 2 and fig. 2, the width of the bainite rail side softening zone and the 30mm rail side softening zone of the rail joint were measured at 48m and 30mm, respectively, for the rail joint treated according to the present invention. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 87% of that of the parent metal, and the average hardness of the pearlite steel rail side can reach 90% of that of the parent metal.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 14J, which is beneficial to ensuring the railway operation safety.
Comparative example 1
The comparative example is consistent with the process conditions involved in the steel rail welding and post-weld cooling processes in the example 2, and the only difference is that the post-weld 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 160 ℃ in the third stage.
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. The metallographic examination result shows that: under the observation magnification of a metallographic microscope of 100x, the metallographic structure in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and abnormal structures such as martensite and bainite do not appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. The difference is that the impact energy at room temperature of the full-section welding seam of the dissimilar steel rail joint in the embodiment 2 is 14J, while the impact energy at room temperature of the full-section welding seam of the dissimilar steel rail joint in the comparative example is only 7J, and the toughness of the joint is relatively poor, so that the safety of railway operation is not facilitated.
Comparative example 2
The bainite rail weld joint was post-weld heat treated as in example 1, heating the rail weld joint to 950 ℃ using medium frequency induction profiling electric heating coils, except that the air cooling was stopped at a second cooling to 150 ℃, and then the weld joint was air cooled to room temperature (about 23 ℃).
Taking a longitudinal hardness test sample of the bainite steel rail welded joint obtained in the comparative example, and carrying out longitudinal Rockwell hardness detection at a position 5mm below a steel rail head tread, wherein the measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 3, and the effect of the longitudinal hardness distribution is shown in FIG. 3.
Table 3 longitudinal hardness data for comparative example 2
As is clear from table 3 and fig. 3, the rail joint treated by the present comparative example had a softening zone width on the bainitic rail side of the rail joint of 40m and a softening zone width on the eutectoid pearlitic rail side of 30 mm. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 89% of that of the base material, and the average hardness of the pearlite steel rail side can reach 92% of that of the base material.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, a large amount of martensite appears in a welding heat affected zone on one side of the rail joint eutectoid pearlite rail, and the percentage content of the martensite reaches 10%. Significant blocky martensite also occurs in the weld heat affected zone on one side of the bainitic rail of the rail joint. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is only 4J, which is not beneficial to the running safety of railways.
Comparative example 3
And directly air-cooling the bainite steel rail welded joint which is obtained by adopting gas pressure welding and has the temperature of 1500 ℃ to room temperature (about 23 ℃) so as to obtain the bainite steel rail welded joint under the air-cooling (natural cooling) condition.
Taking a longitudinal hardness test sample of the bainite steel rail welded joint obtained in the comparative example, and carrying out longitudinal Rockwell hardness detection at a position 5mm below a steel rail head tread, wherein the measuring points are symmetrically arranged towards the left side and the right side by taking a welding seam as a center, and the distance between the measuring points is 5 mm. The Rockwell hardness test method is carried out according to the regulation of GB/T230.1-2009, and an HRC scale is adopted. The longitudinal hardness data of the welded joint at the position 5mm below the tread of the rail head are shown in Table 4, and the effect of the longitudinal hardness distribution is shown in FIG. 4.
Table 4 comparative example 3 longitudinal hardness data
As is clear from table 4 and fig. 4, the rail joint treated by the present comparative example had a softening zone width on the bainitic rail side of the rail joint of 63m and a softening zone width on the eutectoid pearlitic rail side of the rail joint of 65 mm. Compared with the hardness of the steel rail base metal, the hardness of the whole welding area is lower, and the hardness is distributed in a W shape. The welding joint obtained by the comparative example is easy to cause saddle-shaped abrasion of a joint railhead in the service process of a line, and the smoothness of the line and the driving safety are influenced. According to calculation, the average hardness of the bainite steel rail side of the dissimilar material steel rail joint can reach 84% of that of the base material, and the average hardness of the pearlite steel rail side can reach 87% of that of the base material.
Referring to the sampling method shown in FIG. 6, metallographic structure examination is carried out on the metallographic structure sample of the steel rail joint according to GB/T13298-2015 metal microstructure examination method, etching is carried out on the metallographic structure sample of the steel rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the steel rail joint is observed by adopting a German Leica MeF3 optical microscope. The test result shows that: under the observation magnification of a metallographic microscope of 100x, the microstructure of the welding heat affected zone on one side of the rail joint eutectoid pearlite rail is normal, and no abnormal structures such as martensite, bainite and the like appear. No significant blocky martensite is present in the weld heat affected zone on one side of the bainitic rail of the rail joint. In addition, the impact energy of the full-section welding seam of the welding joint at room temperature is only 4J, which is not beneficial to the running safety of railways.
As can be seen by comparing the weld joint railhead tread longitudinal hardness profiles in fig. 1-4: the process method provided by the invention can control the width of the softening region at one side of the bainite steel rail of the dissimilar steel rail joint to be less than or equal to 50m, and the width of the softening region at one side of the eutectoid pearlite steel rail of the steel rail joint to be less than or equal to 30 mm. Meanwhile, the metallographic structure in a welding heat affected zone on one side of the eutectoid pearlite steel rail of the steel rail joint is normal, and no martensite, bainite and other abnormal structures appear. The welding heat affected zone on one side of the bainite steel rail of the steel rail joint has no obvious block martensite. In addition, the room temperature impact energy of the full-section welding seam of the welding joint reaches 15J, which is beneficial to ensuring the railway operation safety.
Claims (9)
1. The construction method for heat treatment of the welded joints of the dissimilar steel rails is characterized by comprising the following steps of: 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 1000-1300 ℃ for first-stage cooling, so that the surface temperature of the steel rail welded joint is reduced to 100-200 ℃, the first-stage cooling is natural cooling in air, and the cooling speed is 0.1-15.0 ℃/s;
b. after cooling in the first stage, normalizing and heating, and stopping heating when the surface temperature of the joint is heated to 920-980 ℃;
c. after the normalizing heating is finished, carrying out second-stage cooling, wherein the starting cooling temperature of the second-stage cooling is not lower than 780 ℃, high-pressure cooling gas or water mist mixed gas is used as a cooling medium, the cooling speed is 2.5-3.0 ℃/s, and the final cooling temperature is 380-450 ℃;
d. after the second stage of cooling, performing third stage of cooling, and taking high-pressure cooling gas or water mist mixed gas as a cooling medium to reduce the surface temperature of the welding joint to 100-180 ℃, wherein the cooling speed of the stage is 0.6-0.8 ℃/s;
e. 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 construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: 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 construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: in the step a, in the first stage cooling, when the temperature is higher than 800 ℃, the cooling speed is 6.0-15.0 ℃; when the temperature is 800-500 ℃, the cooling speed is 3.0-5.8 ℃; when the temperature is 500-300 ℃, the cooling speed is 1.0-2.8 ℃; when the temperature is lower than 300 ℃, the cooling speed is 0.1-0.9 ℃/s.
4. The construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: in the step b, the normalizing heating is full-section normalizing heating of the steel rail welding joint by adopting a medium-frequency induction profiling electric heating coil and/or an oxy-acetylene flame profiling heater.
5. The construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: in the step c, in the second stage of cooling, the distance between a nozzle type cooling device matched with the normalizing machine set and the surface of the welded joint is 30-50 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.30-0.60 MPa.
6. The construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: in the step d, in the third stage cooling, the distance between a nozzle type cooling device matched with the normalizing machine set and the surface of the welded joint is 30-50 mm; the pressure of the compressed air or the water mist mixed gas ejected by the cooling device is 0.05-0.10 MPa.
7. The construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 1, wherein: and e, tempering by adopting an open type electric heating tempering device, wherein the distance between the open type electric heating tempering device and the surface of the welded joint is 5-10 mm.
8. The construction method for the heat treatment of the post-weld joint of dissimilar steel rails according to claim 7, wherein: in step e, 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.
9. The construction method for the heat treatment of the post-weld joint of the dissimilar steel rail according to any one of claims 1 to 8, characterized in that: 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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| CN115404333B (en) * | 2022-08-01 | 2024-04-30 | 包头钢铁（集团）有限责任公司 | High-strength and high-toughness bainitic steel rail aluminum thermal welding head heat treatment process for heavy haul railway |
| 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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