CN112063825B

CN112063825B - Heat treatment method for 1100 MPa-level low-alloy heat-treated steel rail after welding

Info

Publication number: CN112063825B
Application number: CN202010886017.7A
Authority: CN
Inventors: 白威; 李大东; 王若愚; 邓健
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2020-08-28
Filing date: 2020-08-28
Publication date: 2022-07-19
Anticipated expiration: 2040-08-28
Also published as: CN112063825A

Abstract

The invention relates to the technical field of railway steel rail manufacturing, and discloses a postweld heat treatment method for a 1100 MPa-level low-alloy heat-treated steel rail. The method comprises the following steps: (1) cooling the welded steel rail welding joint with residual temperature of 1150-1300 ℃ in the first stage to reduce the surface temperature of the steel rail welding joint to 580-640 ℃; (2) cooling the steel rail welding joint in the second stage to reduce the surface temperature of the steel rail welding joint to 410-470 ℃; (3) and cooling the welded joint of the steel rail in the third stage to reduce the surface temperature of the welded joint of the steel rail to 10-30 ℃. The invention carries out heat treatment by using the welding waste heat without reheating, thereby simplifying the heat treatment process and reducing the cost; the percentage content of martensite structures possibly appearing in the metallographic structure of the steel rail welded joint can be controlled within the range of less than or equal to 3%, and meanwhile, the physical fatigue life of the steel rail joint can reach 250 ten thousand times.

Description

Postweld heat treatment method for 1100 MPa-level low-alloy heat-treated steel rail

Technical Field

The invention relates to the technical field of railway steel rail manufacturing, in particular to a postweld heat treatment method for a 1100 MPa-level low-alloy heat-treated steel rail.

Background

Along with the continuous improvement of the high-speed railway network in China, the heavy-duty transformation of the existing passenger-cargo mixed transportation trunk line is gradually implemented, and the large transportation volume, the large axle weight and the high density are the development directions of future heavy-duty railways. The rapid development of railways puts higher requirements on the service performance of steel rails, and the common carbon steel rails are difficult to meet the requirements of railway speed increase and heavy-load transportation.

The steel rail can be strengthened by two ways of heat treatment and alloying. Heat treatment is the most economical and effective way to improve rail performance. The alloying has the advantages of simple production process, delivery in a hot rolling state generally and heat energy saving. Research shows that steel rails with higher strength can be produced by only utilizing alloy reinforcement, but the ductility and toughness are lower. The high-strength steel rail with higher strength grade and better plastic toughness can be produced by adopting a mode of combining low alloy and heat treatment. Based on the steel rail online heat treatment technology, the low-alloy heat treatment steel rail with the tensile strength of 1100MPa can be produced by proportioning low-content alloy elements, has the advantages of good toughness, plasticity, wear resistance, contact fatigue resistance and the like, and is particularly suitable for freight and passenger-cargo mixed transportation railways with medium axle weight.

The addition of alloy elements in the rail steel increases the stability of the super-cooled austenite, so that the CCT curve is shifted to the right, and the hardenability is obviously improved. Therefore, when the production process is not controlled tightly or the cooling rate is not controlled well, the martensite is formed in the region of the composition segregation by exceeding the critical cooling rate even in the case of cooling at the normal cooling rate or even in the case of air cooling, which is a problem that cannot be ignored in the production of alloy rails. The local segregation of the rail steel is inevitable due to factors such as the steel-making process, the steel homogeneity, the cleanliness and the like. Due to segregation, chemical compositions of all micro regions are different, so that Ms points are different, the martensite transformation is different, and martensite is generated in partial regions. At the same time, the segregation can cause the steel rail to generate martensite due to the segregation locally under the action of the welding heat cycle. Even if the final cooling temperature in the process of rapid cooling of the steel rail after welding heat treatment is higher than the Ms (martensite start temperature) temperature of the steel rail, the CCT curve is shifted to the right due to the existence of local segregation, and martensite is formed. Therefore, for the steel rail with component segregation, the reasonable control of the cooling speed and the final cooling temperature in the post-weld heat treatment cooling process is beneficial to reducing and even avoiding the influence of the segregated martensite on the service performance of the steel rail joint.

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. Meanwhile, after the steel rail is subjected to the action of welding heat cycle, a hardening layer of a welding area disappears, and low-hardness areas with larger width are formed on two sides of a welding line, so that the hardness of the welding line and a heat affected area is lower than that of a steel 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.

Chinese patent CN106544933A 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: testing requirements for fatigue, tension, impact and static bending tests in flash welding; 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. It should be noted that the patent relates to a post-weld normalizing heat treatment process for rails, and since the welded region of the rail is reheated above the austenitizing temperature during the heating process, the effect of the welding process on the structural properties of the rail joint does not need to be considered. However, the steel rail welded joint needs to be locally heated by steel rail postweld heat treatment equipment, the operation and implementation processes are complex, and the cost is high.

Chinese patent CN103898310A discloses a method for heat treatment of welded joint of bainite steel rail, which comprises first cooling the welded joint of bainite steel rail to be cooled to a first temperature not higher than 450 ℃, then heating the first cooled welded joint to a second temperature, and then performing second cooling, wherein the second temperature is higher than the first temperature and not higher than 510 ℃. The method mainly aims at a postweld heat treatment process of the bainite steel rail welded joint, wherein the cooling starting temperature of the bainite steel rail is 1300-1380 ℃, and the cooling ending temperature after the second cooling is room temperature. However, it should be noted that the bainitic steel rails referred to in the above patent and the hypoeutectoid steel rails referred to in the present application have different composition systems and distinct metallurgical structure and mechanical property characteristics. In addition, the above patent also relates to the process of post-weld normalizing heat treatment of the steel rail, and the steel rail post-weld heat treatment equipment is required to be adopted to carry out local heating and cooling on the welded joint of the steel rail, so that the operation and implementation process is complex, and the cost is higher.

Therefore, in the field of railway engineering, a postweld heat treatment method capable of effectively improving the hardness of the longitudinal section of a low-alloy heat-treated steel rail welded joint is urgently needed so as to improve the service performance of the rail welded joint and ensure the running safety of railways.

Disclosure of Invention

The invention aims to solve the problems that the heat treatment method for the welded joint of the steel rail in the prior art is complex in operation process, high in cost, poor in mechanical property of the welded joint after heat treatment and short in physical fatigue life of the steel rail joint, and provides the heat treatment method for the welded joint of the 1100 MPa-level low-alloy heat-treated steel rail, the heat treatment method is used for carrying out heat treatment on the welded joint of the 1100 MPa-level low-alloy heat-treated steel rail, the heat treatment cost is low, and the mechanical property of the welded joint after heat treatment is good.

In order to achieve the purpose, the invention provides a heat treatment method after welding for 1100 MPa-level low-alloy heat-treated steel rails, which comprises the following steps:

(1) cooling the welded steel rail welding joint with residual temperature of 1150-;

(2) cooling the steel rail welding joint at the second stage to reduce the surface temperature of the steel rail welding joint to 410-470 ℃, wherein the cooling at the second stage is carried out by adopting a steel rail head profiling cooling device, the cooling medium is compressed air or water mist mixed gas, and the cooling speed is 1.2-2.8 ℃/s;

(3) Cooling the welded joint of the steel rail at the third stage, so that the surface temperature of the welded joint of the steel rail is reduced to 10-30 ℃, wherein the cooling at the third stage is carried out by adopting a profiling cooling device of a rail head of the steel rail, a cooling medium is compressed air or water mist mixed gas, and the cooling speed is 0.2-0.6 ℃/s;

the tensile strength of a steel rail base metal of the steel rail welding joint is 1100MPa, and the chemical components of the steel rail base metal comprise 0.65-0.72 wt% of C, 0.9-1.1 wt% of Si, 1.05-1.2 wt% of Mn, 0.5-0.7 wt% of Cr, less than or equal to 0.02 wt% of P, less than or equal to 0.02 wt% of S, less than or equal to 0.01 wt% of V, and the balance of Fe and inevitable impurities.

Preferably, in step (1), the rail welded joint is formed by welding with a rail mobile flash welder.

Preferably, in the step (1), the welded steel rail joint with the residual temperature of 1200-1250 ℃ is subjected to first-stage cooling.

Preferably, in step (1), the cooling rate of the first stage cooling is 7-7.5 ℃/s.

Preferably, when the second stage cooling is carried out in the step (2), the distance between the rail head profiling cooling device and the rail head tread is 20-30 mm.

Preferably, when the second stage cooling is performed in the step (2), the pressure of the compressed air or the water mist mixture sprayed by the rail head profiling cooling device is 0.2-0.4 MPa.

Preferably, in step (2), the cooling rate of the second stage cooling is 2-2.5 ℃/s.

Preferably, in the step (3), the welded rail joint is subjected to a third stage of cooling, so that the surface temperature of the welded rail joint is reduced to 20-25 ℃.

Preferably, when the third stage of cooling in step (3) is carried out, the distance between the rail head profiling cooling device and the rail head tread is 20-30 mm.

Preferably, when the third stage cooling is performed in the step (3), the pressure of the compressed air or the water mist mixture sprayed by the rail head profiling cooling device is 0.04-0.15 MPa.

More preferably, when the third stage cooling is performed in the step (3), the pressure of the compressed air or the water mist mixture sprayed by the steel rail head profiling cooling device is 0.06-0.1 MPa.

Compared with the prior art, the invention has the following advantages:

(1) the invention carries out heat treatment by utilizing the welding waste heat of the welding joint, and does not need to be reheated in the heat treatment process, thereby simplifying the heat treatment process and reducing the cost.

(2) The method can ensure that the longitudinal average hardness of the steel rail joint in the area which is +/-20 mm away from the center of the welding seam meets the +/-30 HV range of the average hardness of the corresponding steel rail base metal (excluding the decarburized welding seam center line: the hardness is low due to decarburization of the welding seam center and element burning loss caused by high temperature of steel rail welding), the widths of the softening areas at two sides of the welding seam of the joint are not more than 15mm, and saddle-shaped abrasion of the steel rail joint caused by low hardness of the welding area in the service process of the steel rail line can be improved. Meanwhile, the percentage content of martensite structure possibly appearing in the metallographic structure of the welded joint of the steel rail can be controlled within the range of less than or equal to 3 percent, and the control of martensite formed due to segregation of alloy elements is facilitated. Meanwhile, the physical fatigue life of the steel rail joint can reach 250 ten thousand times, which is beneficial to ensuring the railway operation safety.

In addition, the invention and Chinese patent CN110016544A both relate to a cooling mode of three-step cooling after the flash welding of the steel rail. However, it should be noted that the present invention is significantly different from chinese patent CN110016544A, and the specific comparison is shown in table a.

Table a this application compares with chinese patent CN110016544A

The results in table a show that, the present invention and chinese patent CN110016544A are both processes for performing heat treatment on a rail joint with high welding residual temperature by using the welding residual heat as a heat source, but the two documents have different rail materials and different cooling processes, which result in different microstructure changes and properties of the rail joint, i.e. different implementation effects. Therefore, the invention is significantly different from the Chinese patent CN 110016544A.

Drawings

FIG. 1 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the tread of a rail head of a welded joint of a low alloy heat treated steel rail under post-weld heat treatment conditions obtained by the method of example 1;

FIG. 2 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the tread of the rail head of a welded joint of a low alloy heat treated steel rail obtained by the method of example 2 under the conditions of post-weld heat treatment;

FIG. 3 is a graph showing the effect of longitudinal hardness at a position 3-5mm below the tread of the rail head of a welded joint of a low alloy heat treated steel rail obtained by the method of comparative example 1 under post-weld air cooling conditions;

FIG. 4 is a graph showing the effect of longitudinal hardness at a position 3-5mm below the tread of the head of a welded joint of a low alloy heat treated steel rail obtained by the method of comparative example 2 under the conditions of post-weld heat treatment;

FIG. 5 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the tread of the rail head of a welded joint of a low alloy heat treated rail obtained by the method of comparative example 3 under the condition of post-weld heat treatment;

FIG. 6 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the tread of the rail head of a welded joint of a low alloy heat treated rail obtained by the method of comparative example 4 under the condition of post-weld heat treatment;

FIG. 7 is a graph showing the effect of longitudinal hardness at a position 3 to 5mm below the tread of the rail head of a welded joint of a low alloy heat treated rail obtained by the method of comparative example 5 under the condition of post-weld heat treatment;

FIG. 8 is a schematic diagram showing the position of a longitudinal hardness detection point at a position 3-5mm below a rail head tread of the welded joint for rails according to the present invention;

FIG. 9 is a schematic view of a metallographic specimen sampling site on a rail head tread of a welded joint for a steel rail according to the present invention;

FIG. 10 is a schematic view of a rail head profiling cooling apparatus according to the present invention;

fig. 11 is a bottom schematic view of a rail head profiling cooling apparatus for use with the present invention.

Description of the reference numerals

1 a medium channel; 2 a top nozzle; 3 a medium channel; 4 side nozzles; a recrystallization region; b, rail head tread; c, welding; d, metallographic test inspection surface.

Detailed Description

The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

In the previous research, the inventor finds that the critical cooling speed of the martensite transformation in the continuous cooling transformation process of the 1100 MPa-grade low-alloy heat-treated rail steel related to the invention is about 1.5-2.5 ℃/s, and the Ms temperature (the temperature at which the martensite structure begins to form) is about 230-280 ℃. In order to avoid the abnormal structures of martensite, bainite and the like of a steel rail welded joint, when the steel rail welded joint is subjected to postweld heat treatment, the final cooling temperature in the rapid cooling process of the steel rail postweld heat treatment needs to be controlled to be higher than the Ms temperature of the steel rail. When the rail joint above the austenitizing temperature is rapidly cooled at a cooling speed higher than the rail steel martensite transformation critical cooling speed, the final cooling temperature is controlled to be higher than the Ms temperature of the rail steel, and the subsequent cooling speed is lower than the rail steel martensite transformation critical cooling speed. Otherwise, the joint will fail prematurely due to the high amount of hardened martensite. On the premise of not considering the component segregation of the steel rail, when the steel rail joint above the austenitizing temperature is rapidly cooled to the temperature below Ms by adopting the cooling speed lower than the martensite transformation critical cooling speed of the steel rail, the martensite is not formed in the steel rail joint. Thus, the rail welding standard is AS specified in australian rail welding standard AS 1085.20-2012: for some steel rails with high strength grade, high carbon content and high alloy content, under the observation magnification of a metallographic microscope of 100X, the percentage content of a martensite structure in the most serious area of a steel rail welding joint is not higher than 5%, otherwise, the joint can be subjected to early fatigue fracture due to a large amount of hardened martensite structures, and the running safety of the railway is seriously influenced. 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 above findings, the inventors have completed the present invention.

It should be noted that the thickness of the rail web and the rail base of the steel rail is relatively thin, and the temperature is rapidly reduced in the cooling process. Meanwhile, the rail web region is also generally the region with the most serious rail segregation, and the martensite structure is also most easily formed in the rail web. In order to avoid the deterioration of the service performance of the steel rail welding joint caused by a large amount of brittle and hard martensite structures, the post-welding heat treatment of the steel rail is only carried out on the rail head tread of the steel rail joint and the side face of the rail head adjacent to the rail head tread, and the rail web and the rail bottom of the steel rail joint are naturally cooled.

The invention provides a heat treatment method after welding for 1100 MPa-level low-alloy heat-treated steel rail, which comprises the following steps:

(2) cooling the steel rail welding joint at the second stage to reduce the surface temperature of the steel rail welding joint to 410-470 ℃, wherein the cooling at the second stage is carried out by adopting a steel rail head profiling cooling device, a cooling medium is compressed air or water mist mixed gas, and the cooling speed is 1.2-2.8 ℃/s;

(3) And cooling the welded joint of the steel rail at the third stage to reduce the surface temperature of the welded joint of the steel rail to 10-30 ℃, wherein the cooling at the third stage is carried out by adopting a profiling cooling device of the head of the steel rail, the cooling medium is compressed air or water mist mixed gas, and the cooling speed is 0.2-0.6 ℃/s.

In the invention, the tensile strength of a steel rail base material of the steel rail welding joint is 1100MPa, and the chemical components of the steel rail base material comprise 0.65-0.72 weight percent of C, 0.9-1.1 weight percent of Si, 1.05-1.2 weight percent of Mn, 0.5-0.7 weight percent of Cr, less than or equal to 0.02 weight percent of P, less than or equal to 0.02 weight percent of S, less than or equal to 0.01 weight percent of V, and the balance of Fe and inevitable impurities.

In the method of the present invention, the structure of the rail head profiling cooling device described in the step (2) and the step (3) is shown in fig. 10 to 11, and comprises a medium channel 1, a top nozzle 2, a medium channel 3 and a side nozzle 4, wherein the medium channel 1 is connected with the top nozzle 2, and the medium channel 3 is connected with the side nozzle 4. The device only cools the tread of the rail head of the steel rail and the side surface of the rail head, and the shape and the size of the aperture of the device can be designed, processed and modified according to actual requirements, so that different cooling strengths are realized. The pressure of the medium flowing through the

medium channels

1 and 3 can be monitored by means of a pressure detection device in question and the pressure of the medium can be adjusted as required.

According to the invention, an infrared thermometer is adopted to collect temperature signals of a rail head tread of a steel rail, wherein the rail head tread of the steel rail is a contact part of a wheel and the steel rail; the hardness value corresponding to the softening area width measurement line in the longitudinal hardness curve of the steel rail joint is the hardness obtained by subtracting 25HV from the average hardness Hp of the steel rail base metal; the width of the softened region in the hardness curve is the intercept of the hardness curve and the measurement line of the width of the softened region.

In the present invention, unless otherwise specified, the "welded rail joint" is a welded region having a length of 60 to 80mm including a weld and/or a heat-affected zone, and the center of the region is the weld of the rail.

The method of the invention carries out heat treatment on the welded joint of the 1100 MPa-level low-alloy heat-treated steel rail, adopts three stages of cooling processes to treat the welded joint, reduces the surface temperature of the welded joint of the steel rail in each stage of cooling process to proper temperature, reasonably controls the cooling speed in each stage of cooling process, and adopts proper cooling devices and proper cooling modes, thereby effectively improving the hardness of the longitudinal section of the welded joint of the low-alloy heat-treated steel rail, improving the service performance of the welded joint of the steel rail and ensuring the running safety of the railway.

The invention aims at the rail joint with higher residual temperature obtained by welding to implement post-welding accelerated cooling so as to reduce the transformation temperature of the joint rail head from austenite to pearlite and improve the hardness of an austenite recrystallization zone. 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. It should be noted that the infrared thermometer is only used on the surface of the rail head tread, and the temperature of the rail core is usually 50-80 ℃ higher than that of the surface under the rapid cooling action. Even when the rail surface temperature is below the transformation temperature, the transformation process can still occur due to the core temperature being still high. Therefore, even if the joint railhead is cooled in the second stage where the opening cooling temperature is relatively low, the structural transformation from austenite to pearlite can occur. In the invention, the first cooling is natural cooling in air, the control of the cooling speed in the first stage can be realized by adjusting the test environment temperature (such as adopting a central air-conditioning temperature control and the like), and the final cooling temperature of the first cooling of the steel rail welding joint can be controlled at 580-640 ℃ by adjusting the setting of a welding machine or manual operation. The opening temperature of the second cooling is 580-640 ℃. In the invention, the final cooling temperature of the second cooling is above the Ms temperature of the rail steel by 410-470 ℃. When the rail joint is cooled in the third stage, in order to avoid the joint from generating a large amount of hardened martensite, the invention selects the cooling speed of 0.2-0.6 ℃/s lower than the rail steel martensite transformation critical cooling speed to cool the joint.

In the metallurgical principle, the martensitic structure of steel is a product of cooling steel at a cooling rate higher than the martensite transformation critical cooling rate to a temperature not higher than the Ms temperature (the starting temperature for formation of the martensitic structure). In order to avoid a large amount of brittle and hard martensite in the steel rail joint, when the steel rail joint is subjected to postweld heat treatment, the final cooling temperature in the postweld heat treatment rapid cooling process is controlled to be higher than the Ms temperature of the steel rail in the second cooling stage. When the joint is subjected to heat treatment in the second cooling stage at a cooling rate higher than the rail steel martensite formation critical cooling rate, the final cooling temperature in the stage is higher than the rail steel Ms temperature, and the cooling rate in the third cooling stage is lower than the rail steel martensite formation critical cooling rate. Although inevitable element segregation exists in the steel rail welding process, only a small amount of martensite is generated due to the high final cooling temperature in the postweld heat treatment cooling process, and when the martensite is less than 5% in percentage and is dispersed (under the condition of 100X observation of a metallographic microscope), the fatigue life of a steel rail joint is not obviously influenced. Meanwhile, the cooling speed of the second cooling stage of the post-weld heat treatment is relatively high, and the high supercooling degree is beneficial to improving the toughness of the joint, so that the fatigue life of the steel rail joint is long.

In the method of the invention, in the step (1), the rail welding joint is formed by welding the rails through a rail mobile flash welding machine.

The invention carries out heat treatment by utilizing the welding waste heat of the welding joint. In particular embodiments, the welded rail joint may be subjected to a first stage of cooling at a residual temperature of 1150 ℃, 1170 ℃, 1190 ℃, 1200 ℃, 1230 ℃, 1250 ℃, 1170 ℃ or 1300 ℃.

In a preferred embodiment, in the step (1), the welded joint of the steel rail with the residual temperature of 1200-1250 ℃ obtained by welding is subjected to first-stage cooling.

In the method, the cooling temperature and the cooling speed during cooling at each stage need to be reasonably controlled, so that the hardness of the longitudinal section of the welded joint of the 1100 MPa-level low-alloy heat-treated steel rail is improved, the percentage content of martensite structures possibly appearing in the metallographic structure of the welded joint of the steel rail can be controlled within the range of less than or equal to 3%, and meanwhile, the fatigue life of the steel rail joint reaches 250 ten thousand times.

In particular embodiments, after the first stage cooling, the surface temperature of the rail weld joint may be reduced to any value in the range of 580 ℃, 590 ℃, 600 ℃, 610 ℃, 620 ℃, 630 ℃, 640 ℃ and any two of these values.

In a preferred embodiment, after the first stage cooling, the surface temperature of the rail weld joint may be reduced to 600 ℃ to 640 ℃.

In specific embodiments, in step (1), the cooling rate of the first stage cooling may be 6.5 ℃/s, 6.7 ℃/s, 6.9 ℃/s, 7 ℃/s, 7.2 ℃/s, 7.4 ℃/s, 7.6 ℃/s, 7.8 ℃/s, or 8.0 ℃/s.

In a preferred embodiment, in step (1), the cooling rate of the first stage cooling is 7 to 7.5 ℃/s.

In the method of the present invention, in the step (2), the surface temperature of the welded steel rail joint may be reduced to any value within a range of 410 ℃, 420 ℃, 430 ℃, 440 ℃, 450 ℃, 460 ℃, 470 ℃ and any two of these values when the welded steel rail joint is subjected to the second stage cooling.

In particular embodiments, the distance between the rail head profiling cooling apparatus and the rail head tread when performing the second stage cooling in step (2) may be 20mm, 22mm, 24mm, 26mm, 28mm or 30 mm.

In the method, when the second stage cooling is carried out in the step (2), the pressure of the compressed air or the water mist mixed gas sprayed by the profiling cooling device is 0.2-0.4 MPa; specifically, for example, the pressure may be 0.2MPa, 0.3MPa, 0.4 MPa; preferably, when the second-stage cooling is performed in step (2), the pressure of the compressed air or the water mist mixture ejected from the pattern cooling device is 0.3 MPa.

In specific embodiments, in step (2), the cooling rate of the second stage cooling may be 1.2 ℃/s, 1.4 ℃/s, 1.6 ℃/s, 1.8 ℃/s, 2 ℃/s, 2.2 ℃/s, 2.4 ℃/s, 2.6 ℃/s, or 2.8 ℃/s.

In a preferred embodiment, in step (2), the cooling rate of the second stage cooling is 2 to 2.5 ℃/s.

In particular embodiments, in step (3), the surface temperature of the welded rail joint may be reduced to 10 ℃, 14 ℃, 18 ℃, 22 ℃, 24 ℃, 26 ℃ or 30 ℃ when the welded rail joint is subjected to the third stage of cooling.

In a preferred embodiment, in the step (3), the surface temperature of the welded rail joint is reduced to 20 to 25 ℃ when the welded rail joint is subjected to the third-stage cooling.

In the method of the invention, the distance between the rail head profiling cooling device and the rail head tread when the third stage cooling is carried out in step (3) may be 20mm, 22mm, 24mm, 26mm, 28mm or 30 mm.

In the method, when the third stage of cooling is carried out in the step (3), the pressure of compressed air or water mist mixed gas sprayed by the steel rail head profiling cooling device is 0.04-0.15 MPa; specifically, for example, it may be 0.04MPa, 0.06MPa, 0.08MPa, 0.1MPa, 0.12MPa, 0.14MPa or 0.15 MPa; preferably, when the third stage of cooling is performed in step (3), the pressure of the compressed air or the water mist mixture sprayed by the rail head profiling cooling device is 0.06-0.1 MPa.

In connection with the present invention, it should be noted that the heat treatment technique itself is a process for controlling each factor in the heating and cooling processes, and each step in the heat treatment technique is related to and affects each other. The application may have unavoidable process parameter coincidence with other patent documents, but applicable objects, heat treatment implementation equipment and the like of all patents are different, so that data can not be simply applied and compared. The chemical components, heat treatment process and the like of the steel rails developed in various countries in the world are inevitably overlapped, and are influenced by factors such as smelting capacity, heat treatment equipment, personnel operation level and the like, the applicable objects (including the mechanical property, the temperature distribution and the like of the steel rails) of the invention patents are different, the adopted cooling devices and the implementation process are different, and essential difference is generated, so that the processes cannot be simply applied. In addition, according to the continuous cooling characteristic of the low-alloy heat-treated rail steel, a three-step cooling mode (post-welding normalizing heat treatment is not needed) is adopted, the cooling speed and the cooling temperature of each cooling stage are limited, and saddle-shaped abrasion of a steel rail joint caused by low hardness of a welding area in the service process of a steel rail on a line is improved, so that the method has remarkable progress compared with other patent applications.

The present invention will be described in detail by way of examples, but the scope of the present invention is not limited thereto.

In the examples and comparative examples of the present invention, the sampling positions of the metallographic specimens on the rail head tread of the welded joint of steel rails are shown in fig. 9. The longitudinal hardness detection point 3-5mm below the rail head tread of the steel rail welded joint is shown in figure 8, wherein a is a recrystallization region, b is the rail head tread, c is a welding line, and d is a metallographic test inspection surface.

In the embodiment and the comparative example of the invention, the 1100 MPa-level low-alloy heat-treated steel rail for welding has the specification of 60-75kg/m, and the steel rail welding joint is a welding joint formed by welding a steel rail mobile flash welding machine by adopting the same welding process.

The invention adopts a pulsating bending fatigue test, the load frequency is 5Hz, and the load ratio is 0.2. The maximum load and the minimum load were determined from TB/T1632.1-2014. A three-point bending fatigue test is carried out on a steel rail welding joint by adopting an MTS-FT310 type fatigue testing machine, and the test target that the welding joint does not generate fatigue fracture when the cyclic load is loaded for 250 ten thousand times is taken as a test target.

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 welded steel rail joint with residual temperature of 1250 ℃ at a first cooling speed of 7.5 ℃/s to reduce the rail head surface layer temperature of the steel rail joint to 640 ℃, then carrying out second-stage cooling on the steel rail joint at a second cooling speed of 2.5 ℃/s to reduce the rail head surface layer temperature of the steel rail joint to 450 ℃, and finally carrying out third-stage cooling on the steel rail joint at a third cooling speed of 0.40 ℃/s to reduce the rail head surface layer temperature of the steel rail joint to room temperature of 25 ℃, thereby obtaining the welded steel rail joint subjected to post-welding heat treatment. In the postweld heat treatment process, the first stage of cooling is natural cooling in the air; in the second stage cooling and the third stage cooling, a steel rail head profiling cooling device is adopted to cool the rail head tread and the rail head side surface of the 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 30 mm; in the second stage cooling process, the gas pressure of the compressed air sprayed by the cooling device is 0.32 MPa; in the third stage cooling process, the gas pressure of the compressed air injected by the cooling device is 0.1 MPa. And monitoring the temperature of the tread of the rail head of the steel rail by adopting an infrared thermometer.

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

As is clear from table 1 and fig. 1, the rail base metal had an average hardness of 411 HV. For the steel rail welded joint treated by the method, the average longitudinal hardness of the steel rail joint in a region which is +/-20 mm away from the center of a welding seam is 387HV, and the range of +/-30 HV of the average hardness of a steel rail base metal is met (excluding a decarburization welding seam center line: the center of the welding seam is decarburized and generates element burning loss under the influence of high welding temperature of the steel rail, so that the hardness is lower). The width of the softening area on the left side of the joint welding line is 8.0mm, the width of the softening area on the right side of the joint welding line is 8.0mm, and the widths of the softening areas on the two sides of the joint welding line are not more than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination is carried out on the metallographic structure sample of the rail joint according to GB/T13298-2015 & lt & gt method for metal microstructure examination, etching is carried out on the metallographic sample of the rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint is observed by adopting a German Leica MeF3 optical microscope. The result shows that under the observation magnification of a metallographic microscope of 100X, only a small amount of punctiform martensite is produced in the most severe martensite area in the heat affected zone of the joint, and the percentage content of the martensite is 2.0%. Meanwhile, the fatigue life of the steel rail joint can reach 250 ten thousand times, which is beneficial to ensuring the railway operation safety.

Example 2

After the upsetting and the pushing of the beading in the moving flash welding process of the steel rail with the specification of 60kg/m is finished, 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 7 ℃/s so as to reduce the rail head surface layer temperature of the steel rail joint to 600 ℃, then carrying out second-stage cooling on the steel rail joint at a second cooling speed of 2 ℃/s so as to reduce the rail head surface layer temperature of the steel rail joint to 430 ℃, and finally carrying out third-stage cooling on the steel rail joint at a third cooling speed of 0.2 ℃/s so as to reduce the rail head surface layer temperature of the steel rail joint to the room temperature of 20 ℃, thereby obtaining the post-welding heat-treated steel rail welded joint. In the process of postweld heat treatment, the first stage of cooling is natural cooling in the air; in the second stage cooling and the third stage cooling, 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 20 mm; in the second stage cooling process, the gas pressure of the water mist mixed gas sprayed by the cooling device is 0.26 MPa; in the third stage of cooling process, the gas pressure of the water mist mixed gas sprayed by the cooling device is 0.04 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer.

The post-weld heat treated rail joint obtained in this example was machined into a longitudinal hardness test specimen. A hardness sample is longitudinally subjected to Vickers hardness detection by using a Bravicer hardness tester (model HBV-30A, general plant of testing machines in Lyzhou city, Shandong) at a position 4mm below a tread of a rail head of a steel rail by taking 2mm as a measuring point interval, and measuring points are symmetrically arranged towards the left side and the right side by taking a welding line as a center. The Vickers hardness test method refers to GB/T4340.1-2009 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. The hardness test data are shown in Table 2, and the effect of the distribution of the longitudinal hardness of the joints is shown in FIG. 2.

TABLE 2

As is clear from Table 2 and FIG. 2, the average hardness of the rail base metal is 411 HV. For the steel rail welding joint treated by the method, the longitudinal average hardness of the steel rail joint in a region which is +/-20 mm away from the center of a welding seam is 389HV, and the range of +/-30 HV of the average hardness of a steel rail base metal is met (the center line of the welding seam which does not contain decarburization is influenced by high temperature of steel rail welding, the center of the welding seam is decarburized and generates element burning loss, so that the hardness is lower). The width of the softening area on the left side of the joint is 8.0mm, the width of the softening area on the right side of the joint is 8.0mm, and the widths of the softening areas on the two sides of the welding line of the joint are not more than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination was performed on the metallographic specimen of the rail joint according to GB/T13298-2015 "Metal microstructure examination method", etching was performed on the metallographic specimen of the rail joint by using a 3% nitric acid alcoholic solution, and the metallographic structure of the rail joint was observed by using a German Leica MeF3 optical microscope. The result shows that under the observation magnification of a metallographic microscope of 100X, only a small amount of punctiform martensite is produced and the percentage content of the martensite is only 2.5% for the area with the most severe martensite in the heat affected zone of the joint. Meanwhile, the fatigue life of the steel rail joint can reach 250 ten thousand times, which is beneficial to ensuring the railway operation safety.

Comparative 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, 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 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 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

As is clear from Table 3 and FIG. 3, the average hardness of the base material was 411 HV. For a steel rail welding joint which is not treated by the post-welding heat treatment method provided by the invention, compared with the hardness of steel rail base materials on two sides of a welding seam, the whole welding area is in a softening state. The average longitudinal hardness of the steel rail joint in a region which is +/-20 mm away from the center of the welding seam is 356HV, and the range of +/-30 HV of the corresponding average hardness of the steel rail base metal cannot be met (the center line of the welding seam which does not contain decarburization is influenced by high welding temperature of the steel rail, the center of the welding seam is decarburized, element burning loss is generated, and the hardness is lower). The width of the softened region on the left side of the joint weld was 18.0mm and the width of the softened region on the right side of the joint weld was 18.0 mm. In the service process of the line, the welded joint obtained by the comparative example is easy to preferentially form low collapse of the tread of the rail head of the steel rail in the joint softening area, so that saddle-shaped abrasion is caused, and the smoothness and the driving safety of the line are influenced.

Referring to the sampling method shown in FIG. 9, metallographic structure examination is carried out on the metallographic structure sample of the rail joint according to GB/T13298-2015 & lt & gt method for metal microstructure examination, etching is carried out on the metallographic sample of the rail joint by adopting a 3% nitric acid alcohol solution, and the metallographic structure of the rail joint is observed by adopting a German Leica MeF3 optical microscope. The result shows that the metallographic structure of the joint is normal and no abnormal structures such as martensite, bainite and the like exist. In this comparative example, the fatigue life of the rail joint was only 150 ten thousand times.

Comparative example 2

After the upsetting and the pushing of the beading in the moving flash welding process of the steel rail with the specification of 75kg/m is finished, 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 6 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 720 ℃, 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 180 ℃, and finally the rail joint is subjected to third-stage cooling at a third cooling speed of 0.2 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to the room temperature of 30 ℃, so that the welded rail joint subjected to post-welding heat treatment is obtained. In the process of postweld heat treatment, the first stage of cooling is natural cooling in the air; in the second stage cooling and the third stage cooling, 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 30 mm; in the second stage cooling process, the gas pressure of the compressed air sprayed by the cooling device is 0.4 MPa; in the third stage of cooling process, the gas pressure of the compressed air injected by the cooling device is 0.04 MPa. And monitoring the temperature of the tread 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

As can be seen from table 4 and fig. 4, for the welded joint of the steel rail which is not treated by the post-weld heat treatment method provided by the present invention, the width of the softening region on the left side of the obtained joint is 9.0mm, the width of the softening region on the right side is 9.0mm, and the widths of the softening regions on both sides of the weld joint of the joint are not greater than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination was performed on the metallographic specimen of the rail joint according to GB/T13298-2015 "Metal microstructure examination method", etching was performed on the metallographic specimen of the rail joint by using a 3% nitric acid alcoholic solution, and the metallographic structure of the rail joint was observed by using a German Leica MeF3 optical microscope. Metallographic examination shows that a large amount of hardened martensite structures appear in the heat affected zones on the left side and the right side of the joint welding line. The result shows that under the observation magnification of a metallographic microscope of 100X, the percentage content of the martensite structure reaches 8% for the most serious area in which the martensite structure appears. Under the comparative example, the fatigue life of the steel rail joint is only 180 ten thousand times, which is not beneficial to the safety of railway operation.

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 75kg/m, the post-welding heat treatment is carried out on the joint obtained by welding. Firstly, carrying out first-stage cooling on a welded rail joint with the residual temperature of 1200 ℃ at a first cooling speed of 7.0 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 680 ℃, then carrying out second-stage cooling on the rail joint at a second cooling speed of 4 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to 350 ℃, and finally carrying out third-stage cooling on the rail joint at a third cooling speed of 3 ℃/s so as to reduce the rail head surface layer temperature of the rail joint to the room temperature of 25 ℃, thereby obtaining the welded rail joint subjected to post-welding heat treatment. In the postweld heat treatment process, the first cooling is natural cooling in the air; in the second stage cooling and the third stage cooling, 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 30 mm; in the second stage cooling process, the gas pressure of the compressed air sprayed by the cooling device is 0.6 MPa; in the third stage cooling process, the gas pressure of the compressed air injected by the cooling device is 0.44 MPa. And monitoring the tread temperature of the rail head of the steel rail by adopting an infrared thermometer.

And (3) machining the steel rail joint obtained in the comparative example under the air cooling condition after welding into a longitudinal hardness test 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 section 1 of Metal Vickers hardness test: test methods "were performed using HV scale. The hardness test data are shown in Table 5, and the effect of the distribution of the longitudinal hardness of the joints is shown in FIG. 5.

TABLE 5

As is clear from Table 5 and FIG. 5, the average hardness of the rail base metal was 411 HV. For the steel rail welding joint which is not treated by the post-welding heat treatment method provided by the invention, the width of the softening area at the left side of the welding seam of the obtained joint is 14.0mm, the width of the softening area at the left side of the welding seam of the joint is 14.0mm, and the widths of the softening areas at both sides of the welding seam of the joint are not more than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination was performed on the metallographic specimen of the rail joint according to GB/T13298-2015 "Metal microstructure examination method", etching was performed on the metallographic specimen of the rail joint by using a 3% nitric acid alcoholic solution, and the metallographic structure of the rail joint was observed by using a German Leica MeF3 optical microscope. Metallographic examination shows that a large amount of hardened martensite structures appear in the heat affected zones on the left side and the right side of the joint welding line. The result shows that under the observation magnification of a metallographic microscope of 100X, the percentage content of the martensite structure reaches 10% for the most serious area in which the martensite structure appears. Under the comparative example, the fatigue life of the steel rail joint is only 80 ten thousand times, which is not beneficial to the running safety of the railway.

Comparative example 4

The process of example 1 was followed except that the second stage cooling reduced the rail head skin temperature of the rail joint to 350 ℃.

And (3) machining the steel rail joint obtained in the comparative example under the air cooling condition after welding into a longitudinal hardness test 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 6, and the effect of the longitudinal hardness distribution of the joints is shown in FIG. 6.

TABLE 6

As is clear from table 6 and fig. 6, the rail base metal had an average hardness of 411 HV. For the steel rail welding joint treated by the post-weld heat treatment method provided by the comparative example, the width of the softening zone on the left side of the welding seam of the obtained joint is 8.0mm, the width of the softening zone on the left side of the welding seam of the joint is 8.0mm, and the widths of the softening zones on two sides of the welding seam of the joint are not more than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination was performed on the metallographic specimen of the rail joint according to GB/T13298-2015 "Metal microstructure examination method", etching was performed on the metallographic specimen of the rail joint by using a 3% nitric acid alcoholic solution, and the metallographic structure of the rail joint was observed by using a German Leica MeF3 optical microscope. Metallographic examination shows that a large amount of hardened martensite structures appear in the heat affected zones on the left side and the right side of the joint welding line. The result shows that under the observation magnification of a metallographic microscope of 100X, the percentage content of the martensite structure reaches 8% for the most serious area in which the martensite structure appears. Under the comparative example, the fatigue life of the steel rail joint is only 160 ten thousand times, which is not beneficial to the running safety of the railway.

Comparative example 5

The procedure of example 1 was followed except that the cooling rate in the first stage cooling was 5.5 ℃/s.

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 7, and the effect of the distribution of the longitudinal hardness of the joints is shown in FIG. 7.

TABLE 7

As is clear from table 7 and fig. 7, the rail base metal had an average hardness of 411 HV. With the welded steel rail joint treated by the present comparative example, the average longitudinal hardness of the welded steel rail joint in the region of + -20 mm from the center of the weld was 378HV, and could not satisfy the range of + -30 HV of the average hardness of the base steel rail material (excluding the center line of the decarburized weld: the center of the weld is decarburized and element burnout is generated under the influence of high temperature of welding the steel rail, resulting in a lower hardness). The width of the softening area on the left side of the joint welding line is 12.0mm, the width of the softening area on the right side of the joint welding line is 12.0mm, and the widths of the softening areas on the two sides of the joint welding line are not more than 15.0 mm.

Referring to the sampling method shown in FIG. 9, metallographic structure examination was performed on the metallographic samples of the rail joints according to GB/T13298-2015 "Metal microstructure examination method", etching was performed on the metallographic samples of the rail joints by using a 3% nitric acid alcoholic solution, and the metallographic structures of the rail joints were observed by using a German Leica MeF3 optical microscope. The result shows that under the observation magnification of a metallographic microscope of 100X, only a small amount of punctiform martensite is produced and the percentage content of the martensite is 2.0% for the area with the most severe martensite in the heat affected zone of the joint. Meanwhile, the fatigue life of a rail joint is 180 ten thousand times. In the comparative example, the hardness of the steel rail joint in the area of +/-20 mm from the center of the weld joint is low, and the fatigue life is low.

As can be seen by comparing the weld joint railhead tread longitudinal stiffness and joint softening zone width in fig. 1-7 and tables 1-7: by adopting the postweld heat treatment method provided by the invention to carry out postweld heat treatment on the joint of the hypoeutectoid steel rail, the longitudinal average hardness of the steel rail joint in a region which is +/-20 mm away from the center of a welding seam can meet the range of +/-30 HV of the average hardness of a corresponding steel rail base metal (excluding a decarburized welding seam center line, namely the center of the welding seam is decarburized and generates element burning loss to cause low hardness under the influence of high temperature of steel rail welding), and the widths of softening zones at two sides of the welding seam of the joint are not more than 15.0 mm. Meanwhile, the percentage content of martensite structures possibly appearing in the metallographic structure of the welded joint of the steel rail can be controlled within the range of less than or equal to 3 percent. Meanwhile, the fatigue life of the steel rail joint can reach 250 ten thousand times, which is beneficial to ensuring the running safety of the railway.

The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.

Claims

1. A heat treatment method for a post-weld of 1100 MPa-level low-alloy heat-treated steel rail is characterized by comprising the following steps:

(1) carrying out first-stage cooling on the welded steel rail welding joint with the residual temperature of 1200-1250 ℃ to reduce the surface temperature of the steel rail welding joint to 580-640 ℃, wherein the first-stage cooling mode is natural cooling in air, and the cooling speed is 6.5-8.0 ℃/s;

(2) cooling the steel rail welding joint in the second stage to reduce the surface temperature of the steel rail welding joint to 430-470 ℃, wherein the cooling in the second stage is carried out by adopting a steel rail head profiling cooling device, a cooling medium is compressed air or water mist mixed gas, the cooling speed is 1.2-2.8 ℃/s, and the pressure of the compressed air or the water mist mixed gas sprayed by the steel rail head profiling cooling device is 0.2-0.32 MPa;

2. The method of claim 1, wherein in step (1), the rail weld joint is formed by welding with a rail mobile flash welder.

3. The method as claimed in claim 1, wherein, in the step (1), the cooling rate of the first-stage cooling is 7-7.5 ℃/s.

4. A method according to claim 1, wherein the second stage of cooling in step (2) is carried out with the rail head profiling cooling means at a distance of from 20 to 30mm from the rail head tread.

5. The method according to claim 4, wherein in step (2), the cooling rate of the second stage cooling is 2-2.5 ℃/s.

6. The method of claim 1, wherein in step (3), the rail weld joint is subjected to a third stage of cooling to reduce the surface temperature of the rail weld joint to 20-25 ℃.

7. A method according to claim 5, wherein the third stage of cooling in step (3) is carried out with the rail head profiling cooling apparatus at a distance of from 20 to 30mm from the rail head tread.

8. The method of claim 7, wherein the third stage of cooling in step (3) is carried out with a compressed air or water mist mixture from the rail head profiling cooling unit at a pressure of 0.04 to 0.15 MPa.

9. The method of claim 7, wherein the third stage of cooling in step (3) is carried out with a compressed air or water mist mixture from the rail head profiling cooling unit at a pressure of 0.06 to 0.1 MPa.