Welded joint of heterogeneous strength hot-rolled proeutectoid pearlite rail and post-weld cooling method thereof

Abstract

The present application relates to the field of steel rail production, and discloses a dissimilar strength hot-rolled type eutectoid pearlite rail welded joint and a post-weld cooling method thereof, which comprises the following steps: first stage cooling is performed on the welded joint formed by welding a low-strength hot-rolled type eutectoid pearlite rail base material and a high-strength hot-rolled type eutectoid pearlite rail base material, so that the surface temperature of the welded joint is reduced from 1000-1100 DEG C to 400-500 DEG C; second stage cooling is performed on the welded joint, so that the surface temperature of the welded joint is reduced from 400-500 DEG C to less than or equal to 220 DEG C; and the welded joint is naturally cooled to ambient temperature. The present application can make the heat-affected zones on both sides of the rail joint weld keep high hardness, ensure that the heat-affected zones on both sides of the rail joint weld have no harmful martensite, and also can realize good hardness matching between the regions on both sides of the weld and the corresponding base materials, and good hardness matching between the heat-affected zones on both sides of the weld.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel rail production, in particular to a dissimilar strength hot-rolled type eutectoid pearlite steel rail welded joint and a post-weld cooling method thereof. BACKGROUND

[0002] Different railway line conditions have different requirements for the material and performance of steel rails. According to the differences in production processes, steel rails can be divided into hot-rolled type and heat-treated type. For straight sections and large curve radius sections of ordinary speed railways (railways with a running speed below 200 km / h) and high-speed railways (railways with a running speed of 200-350 km / h), hot-rolled type steel rails are more suitable, while small curve radius sections usually use heat-treated type steel rails with better wear resistance. For some railways with complex line conditions, the use of high-strength heat-treated type steel rails may make the contact fatigue crack propagation more in-depth, which may in turn reduce the service life of the steel rails. Although the use of hot-rolled type steel rails with slightly lower wear resistance increases natural wear, it also significantly reduces the damage caused by the in-depth propagation of contact fatigue cracks to the service of the steel rails. Therefore, at present, hot-rolled type steel rails and heat-treated type steel rails are commonly used for ordinary speed and high-speed railways in China, which involves the welding between steel rails of different materials / strength grades.

[0003] At present, mobile flash welding has become the mainstream on-line welding technology for rail construction sites at home and abroad. For hot-rolled steel rails, the heat-affected zone is equivalent to being heat treated due to the effect of welding thermal cycle. In addition, the effect of multi-element alloy in the steel rail leads to the fact that the hardness of the welding heat-affected zone is higher than that of the steel rail base material as a whole, that is, the hardness of the welding heat-affected zone of the hot-rolled steel rail is higher than that of the steel rail base material as a whole. When the hardness of the welding heat-affected zone of the hot-rolled steel rail is more than 1.2 times the hardness of the steel rail base material, the base material hardness is low, and damage is prone to occur in the base material area. Therefore, for the welding of hot-rolled eutectoid pearlite steel rails, the hardness of the welding heat-affected zone is usually controlled to be 1.0-1.1 times the hardness of the base material. For heat-treated steel rails, the quenched layer originally belonging to the steel rail base material is destroyed under the effect of welding thermal cycle, resulting in the fact that the austenite grain size of the welding heat-affected zone is coarse and the pearlite interlamellar spacing is higher than that of the base material, and thus the hardness of the welding heat-affected zone is lower than that of the steel rail base material as a whole, that is, the hardness of the welding heat-affected zone of the heat-treated steel rail is lower than that of the steel rail base material as a whole. When the hardness of the welding heat-affected zone of the heat-treated steel rail is less than 0.9 times the hardness of the steel rail base material, the hardness of the welding heat-affected zone is low, and damage is prone to occur in the welding heat-affected zone. In addition, when the hardness of the welding heat-affected zone of the heat-treated steel rail is more than 1.0 times the hardness of the steel rail base material (such as 1.1-1.2 times), it is likely that high-hardness martensite is formed in the welding heat-affected zone. The stress in the martensite is high, which is prone to form cracks inside the structure under the wheel-rail contact pressure, which is not conducive to the overall service safety of the rail joint. In addition, the martensite is also prone to cause the overall flaky peeling of the rail head tread. Therefore, for the welding of heat-treated eutectoid pearlite steel rails, the hardness of the welding heat-affected zone is usually controlled to be between 0.9 and 1.0 times the hardness of the base material.

[0004] When new varieties of steel rails are laid on existing lines for old rail replacement and new rail trial laying, the welding between new varieties of steel rails and existing varieties of steel rails on the line is involved. For two kinds of steel rails with different materials and strengths, the difference between the base materials brings challenges to their welding. During the service of the rail welding joint on the railway line, the "saddle-type" wear is prone to occur in the low-hardness area of the rail head tread, which not only increases the wheel-rail impact, but also affects the service life of the rail, and even endangers the safety of train operation. Therefore, after the welding of the steel rail is completed, how to restore the strength and hardness reduced due to welding becomes a prerequisite for the application of the steel rail.

[0005] In summary, when rails with different materials and / or strengths are welded together, the welding of the formed dissimilar rail joint and the subsequent performance of the microstructure through post-weld heat treatment are both problems that need to be considered. Considering that the welding quality of the rail determines whether the seamless line can withstand the test of railway operation, even the safety of train operation. Therefore, there is a need for improvement in the welding and post-weld heat treatment technology of dissimilar strength grade and / or dissimilar material rails in the prior art. SUMMARY

[0006] The main purpose of the present application is to provide a post-weld cooling method for dissimilar strength hot-rolled eutectoid pearlite rail welded joints, to solve the problem of how to cool the dissimilar strength hot-rolled eutectoid pearlite rail welded joints after welding to ensure the performance of the rail.

[0007] According to one aspect of the present application, a post-weld cooling method for dissimilar strength hot-rolled eutectoid pearlite rail welded joints is provided, comprising the following steps in sequence:

[0008] S1, the first stage cooling of the welded joint formed by welding the low strength hot-rolled eutectoid pearlite rail base material and the high strength hot-rolled eutectoid pearlite rail base material, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃, wherein the first heat affected zone corresponding to the low strength hot-rolled eutectoid pearlite rail base material is cooled at a first cooling speed, and the second heat affected zone corresponding to the high strength hot-rolled eutectoid pearlite rail base material is cooled at a second cooling speed less than the first cooling speed;

[0009] S2, the second stage cooling of the welded joint, so that the surface temperature of the welded joint is reduced from 400-500℃ to less than or equal to 220℃, wherein the cooling speed of the welded joint is less than or equal to 1.4℃ / s;

[0010] S3, the natural cooling of the welded joint to ambient temperature.

[0011] According to one embodiment of the present application, the first cooling speed is 3.0-4.0℃ / s, and the second cooling speed is 1.5-2.5℃ / s.

[0012] According to one embodiment of the present application, the cooling speed of the welded joint in step S2 is 0.8-1.4℃ / s.

[0013] According to one embodiment of the present application, the method is carried out in a low-temperature environment of 1-15℃ in the field, and a profiled temperature control device is used to compensate for the heat of the entire cross section of the welded joint in steps S1 and S2.

[0014] According to one embodiment of the present application, the profiled temperature control device comprises:

[0015] The two housings are pivotally connected, each of the two housings has a shape matching the first heat affected zone and the second heat affected zone, and each of the two housings is provided with a plurality of heating portions;

[0016] The control unit is configured to control the heating portions of the two housings respectively.

[0017] According to an embodiment of the present application, the heating portions of the two housings corresponding to the regions of the rail head are smaller in size and more densely distributed than the heating portions corresponding to the regions of the rail waist and the rail bottom.

[0018] According to an embodiment of the present application, the low-strength hot-rolled type of the pearlitic rail base material has a tensile strength of 980-1050 MPa and a hardness of 280-310 HV at room temperature; and the high-strength hot-rolled type of the pearlitic rail base material has a tensile strength of 1100-1180 MPa and a hardness of 320-350 HV at room temperature.

[0019] According to an embodiment of the present application, the chemical composition of the low-strength hot-rolled type of the pearlitic rail base material contains, by mass percentage, 0.68-0.74% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.04-0.08% of V, and the balance of Fe and inevitable impurities.

[0020] The chemical composition of the high-strength hot-rolled type of the pearlitic rail base material contains, by mass percentage, 0.76-0.82% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.30-0.50% of Cr, 0.04-0.08% of V, and the balance of Fe and inevitable impurities.

[0021] According to an embodiment of the present application, the low-strength hot-rolled type of the pearlitic rail base material and the high-strength hot-rolled type of the pearlitic rail base material have the same rail type and a specification of 60-75 kg / m, and the welded joint is formed by welding with a moving flash butt welder.

[0022] According to another aspect of the present application, a welded joint of a dissimilar strength hot-rolled type of the pearlitic rail is provided, which is cooled by the post-weld cooling method as described above; the heat affected zones on both sides of the weld of the welded joint are both pearlite without martensite; the longitudinal hardness of the region corresponding to the low-strength hot-rolled type of the pearlitic rail base material and from the center of the weld to 15 mm away from the center of the weld reaches 106-110% of the average hardness of the low-strength hot-rolled type of the pearlitic rail base material; the longitudinal hardness of the region corresponding to the high-strength hot-rolled type of the pearlitic rail base material and from the center of the weld to 15 mm away from the center of the weld reaches 100-105% of the average hardness of the low-strength hot-rolled type of the pearlitic rail base material; and the hardness difference of the heat affected zones on both sides of the weld is within 30 HV.

[0023] In the technical scheme of the present application, by sequentially performing the first stage cooling, the second stage cooling and the natural cooling, and controlling the open cooling temperature, the final cooling temperature and the cooling speed of the first stage cooling and the second stage cooling, the following beneficial effects can be achieved: the heat affected zone on both sides of the rail joint weld maintains a higher hardness, thereby ensuring the wear resistance of the rail joint; the heat affected zone on both sides of the rail joint weld can be ensured to be pearlite, without harmful martensite; the longitudinal hardness of the rail joint within the area of ±15mm from the center of the weld can reach 106-110% and 100-105% of the average hardness of the low-strength hot-rolled type eutectoid pearlite rail and the high-strength hot-rolled type eutectoid pearlite rail base material, respectively, and the hardness difference of the heat affected zone on both sides of the rail joint weld is within 30HV, the hardness difference of the heat affected zone on both sides of the rail joint weld is improved, thereby realizing good hardness matching of the heat affected zone on both sides of the rail joint weld of the dissimilar strength rail, and ensuring the safety of railway operation. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical scheme in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0025] Figure 1 A flow chart of the post-weld cooling method of the dissimilar strength hot-rolled type eutectoid pearlite rail welded joint according to the embodiment of the present application is shown;

[0026] Figure 2 A longitudinal hardness test position schematic diagram of the rail joint rail head tread below 5mm is shown;

[0027] Figure 3 A metallographic specimen cutting position schematic diagram in each embodiment and the comparative example is shown;

[0028] Figure 4 A whole schematic diagram of the split type profiled temperature control device according to the embodiment of the present application is shown;

[0029] Figure 5 A schematic diagram of the distribution of the electric heating sheet at the rail head part of the split type profiled temperature control device according to the embodiment of the present application is shown. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the present application more clear, the embodiments of the present application are further described in detail below, with reference to the drawings.

[0031] It should be noted that all the expressions of "first" and "second" in the embodiments of the present application are used to distinguish two same name different entities or different parameters, and the "first" and "second" are only for the convenience of description, and should not be understood as the limitation of the embodiments of the present application, and the subsequent embodiments will not be described one by one.

[0032] In some countries' steel rail welding standards, such as Australian steel rail welding standard AS1085.20-2012, for some high-strength grades, high-carbon content and high-alloy content steel rails, under the observation magnification of 100x of metallographic microscope, the percentage content of martensite organization in the most serious area of martensite appearing in the steel rail welded joint shall not be higher than 5%, otherwise the joint will be caused by a large amount of hardened martensite organization to cause premature fatigue fracture, which seriously affects the safety of railway operation. And for Chinese railway steel rail welding standard TB / T1632.2-2014, it is provided that the martensite organization shall not appear in the whole cross section of the steel rail welded joint. Therefore, strictly controlling the martensite content in the steel rail welding organization is of great importance to the stable operation of the railway line. In addition, as mentioned in the above background, for dissimilar strength steel rail welded joints, it is necessary to realize the good hardness matching of the regions on both sides of the weld with the corresponding base metal, and also to realize the good hardness matching of the heat affected zones on both sides of the weld. The one or more embodiments to be described below are proposed to achieve these purposes.

[0033] Reference Figure 1 The present application proposes a post-weld cooling method for dissimilar strength hot-rolled eutectoid pearlite steel rail welded joints, which comprises the following steps performed in sequence:

[0034] S1, the welded joint formed by welding the low-strength hot-rolled eutectoid pearlite steel rail base material and the high-strength hot-rolled eutectoid pearlite steel rail base material is subjected to first stage cooling, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃, wherein the first heat affected zone corresponding to the low-strength hot-rolled eutectoid pearlite steel rail base material is cooled at a first cooling speed, and the second heat affected zone corresponding to the high-strength hot-rolled eutectoid pearlite steel rail base material is cooled at a second cooling speed smaller than the first cooling speed;

[0035] S2, the welded joint is subjected to second stage cooling, so that the surface temperature of the welded joint is reduced from 400-500℃ to less than or equal to 220℃, wherein the cooling speed of the welded joint is less than or equal to 1.4℃ / s;

[0036] S3, the welded joint is naturally cooled to ambient temperature.

[0037] In the embodiment of the present application, the welded joint is a region with a length in the range of 60-80 mm, including a weld and heat-affected zones, the center of which is the weld. The heat-affected zones include a first heat-affected zone corresponding to the low-strength hot-rolled type of the eutectoid pearlite rail base material and a second heat-affected zone corresponding to the high-strength hot-rolled type of the eutectoid pearlite rail base material on both sides of the weld, respectively.

[0038] The inventors of the present application have realized that the martensite transformation start temperature of the low-strength hot-rolled type of the eutectoid pearlite rail steel is 280-320℃, and the martensite transformation critical cooling rate is 3.0-4.0℃ / s. The martensite transformation start temperature of the high-strength hot-rolled type of the eutectoid pearlite rail steel is 220-250℃, and the martensite transformation critical cooling rate is 1.5-2.5℃ / s. In order to avoid the occurrence of abnormal structures such as martensite in the rail welded joint, when the welded joint of the low-strength hot-rolled type of the eutectoid pearlite rail and the high-strength hot-rolled type of the eutectoid pearlite rail is heat treated, the final cooling temperature during the rapid cooling process of the heat treatment needs to be controlled to be above the martensite transformation start temperature of the relatively high low-strength hot-rolled type of the eutectoid pearlite rail steel. At the same time, the cooling rate during the post-weld heat treatment process must be limited to the martensite transformation critical cooling rate of the high-strength hot-rolled type of the eutectoid pearlite rail steel with a relatively low critical cooling rate, otherwise the joint may be prone to premature fatigue fracture due to the quenched martensite structure.

[0039] In the embodiment of the present application, the open cooling temperature of the first stage cooling is 1000-1100℃, and the higher open cooling temperature ensures sufficient driving force for the subsequent cooling process of the pearlite phase transformation, so that the pearlite lamellar spacing is fully refined, and the hardness of the heat-affected zones on both sides of the weld can be effectively improved. The present application utilizes the residual heat of the rail welding to realize the post-weld heat treatment process of the rail. The post-weld accelerated cooling is implemented for the rail joint obtained by welding with a relatively high residual temperature, so as to reduce the phase transformation temperature of the rail head from austenite to pearlite and improve the hardness of the austenite recrystallization zone. The high or low of the surface residual temperature of the rail joint after welding can be realized by adjusting the process parameters such as the rail welding heat input, prolonging or shortening the high-temperature residence time. Based on the principles of metallurgy, under the condition of high-temperature rapid cooling after welding, the rail joint has a certain dynamic supercooling degree, which causes the phase transformation temperature of the austenite to pearlite transformation under the non-equilibrium state to be lowered, and the phase transformation temperature gradually decreases with the increase of the supercooling degree. Therefore, even in the second stage cooling with a lower open cooling temperature, the organization transformation from austenite to pearlite can still occur in the joint.

[0040] The final cooling temperature of the first stage cooling is set to 400-500℃. Firstly, the final cooling temperature is controlled to be higher than 80℃ of the higher temperature of the martensite transformation start temperature of the two kinds of rail steels (i.e. 280-320℃), so as to avoid the formation of harmful martensite structure during the first stage cooling. Secondly, if the final cooling temperature is too high (for example, 530℃), the pearlite structure obtained during the cooling process may not be refined enough, and the hardness may not be improved obviously. Therefore, the final cooling temperature of the first stage cooling is set to 400-500℃.

[0041] In order to realize the good hardness matching of the regions on both sides of the weld with the corresponding base metal (for example, the longitudinal hardness of the rail joint in the region of ±15mm from the center of the weld is respectively 106-110% and 100-105% of the average hardness of the base metal of the low-strength hot-rolled type eutectoid pearlite rail and the high-strength hot-rolled type eutectoid pearlite rail), and realize the good hardness matching of the heat-affected zones on both sides of the weld (for example, the hardness difference of the heat-affected zones on both sides of the weld of the rail joint is within 30HV), the cooling speed of the first heat-affected zone (corresponding to the low-strength hot-rolled type eutectoid pearlite rail base metal) is greater than the cooling speed of the second heat-affected zone (corresponding to the low-strength hot-rolled type eutectoid pearlite rail base metal) during the first stage cooling, so that the hardness of the first heat-affected zone can be improved more than the second heat-affected zone, so as to meet the hardness matching requirements.

[0042] In some embodiments, the first cooling speed is 3.0-4.0℃ / s, and the second cooling speed is 1.5-2.5℃ / s. By adopting appropriate cooling speeds, the heat-affected zones on both sides of the weld can have higher hardness, while the heat-affected zones on both sides of the weld are prevented from having too high hardness (for example, the hardness of the heat-affected zones on both sides of the weld is more than 1.1 times of the hardness of the hot-rolled type rail base metal) due to too fast cooling speed. Since the hardness of the heat-affected zone can be improved after the hot-rolled type rail is welded under the condition of air natural cooling, and when the ambient temperature is too low, the cooling speed under the condition of natural cooling is too low, the hardness of the heat-affected zone will be greatly improved to more than 1.1 times of the hardness of the hot-rolled type rail base metal, which is not conducive to the service safety of the rail joint. In order to improve the hardness of the heat-affected zone, the first stage cooling adopts the cooling speed of the critical cooling speed of the martensite transformation of the two kinds of rail steels to cool the heat-affected zones on both sides of the weld of the rail joint.

[0043] In the embodiments of the present application, the open cooling temperature of the second stage cooling is 400-500℃, the final cooling temperature is less than or equal to 220℃, and the cooling speed is less than or equal to 1.4℃ / s. The second cooling stage adopts a lower cooling speed than the first stage to cool the rail welded joint. The cooling speed of this stage is below the critical cooling speed of the martensite transformation of the two kinds of rail steels, so as to avoid the formation of harmful martensite structure during the cooling process of the second cooling stage.

[0044] In some embodiments, the cooling rate of the welded joint in step S2 is 0.8-1.4°C / s. When the cooling rate is lower than 0.8°C / s, the subsequent steel rail welding production will be affected due to the excessively slow cooling rate and long time consumption. When the cooling rate is higher than 1.4°C / s, such as 1.5°C / s, the 1.5°C / s cooling rate will possibly result in formation of martensite structure in the post-weld continuous cooling process of the rail joint, affecting the service safety of the rail joint, since the martensite transformation critical cooling rate of the high-strength hot-rolled eutectoid pearlite rail steel in the present application is 1.5°C / s. Therefore, in some embodiments of the present application, the second-stage cooling is performed at a cooling rate of 0.8-1.4°C / s.

[0045] When the surface temperature of the welded joint is reduced to lower than or equal to 220°C, the heat treatment of the welded joint is completed, and the rail joint can be naturally cooled to ambient temperature. The stage of naturally cooling the joint to ambient temperature does not have the conditions for formation of martensite structure.

[0046] It should be noted that, to avoid formation of harmful martensite structure in the heat-affected zone during the rail welding and post-weld heat treatment, the cooling rate during the welding and post-weld heat treatment can be controlled to be lower than the martensite transformation critical cooling rate of the rail steel, or the final cooling temperature of the rapid cooling stage of the welding and post-weld heat treatment can be controlled to be above the martensite transformation start temperature of the rail steel by artificial intervention, and then the rail steel is cooled to ambient temperature at a cooling rate lower than the martensite transformation critical cooling rate. During the rail welding and post-weld heat treatment, to improve the hardness and toughness of the welded region, the accelerated cooling method (such as spraying of compressed air as a cooling medium) is usually adopted to make the welded heat-affected zone obtain a pearlite structure with a fine lamellar spacing.

[0047] In some embodiments, the post-weld cooling method of the present application is carried out in a low-temperature environment of 1-15℃ in the wild, and the profiled temperature control device is used in step S1 and step S2 to compensate the heat of the whole section of the welded joint. It should be noted that when the rail welding operation is carried out in a normal temperature (20-30℃) environment, the rail is naturally cooled (air-cooled) after welding, and the harmful martensite structure will not be formed in the heat affected zone of the rail due to the too fast cooling speed. However, when the rail welding and post-weld heat treatment are carried out in a low-temperature (1-15℃) environment in the wild, the cooling speed in the process is extremely fast (the cooling speed can reach 3.5-8.0℃ / s) due to the environment temperature, which creates conditions for the formation of martensite. Moreover, when the environment temperature is too low to cause the cooling speed to be too low under the natural cooling condition, the hardness of the heat affected zone will be greatly increased to more than 1.1 times of the hardness of the hot-rolled rail base material, which is not conducive to the service safety of the rail joint. Therefore, when the rail welding and post-weld heat treatment are carried out in a low-temperature environment in the wild, special attention should be paid to the problems of the formation of abnormal martensite structure and the excessive increase of hardness, so that the rail welding and post-weld heat treatment process needs to be strictly controlled. In order to achieve the cooling control in different stages as described above, the first-stage cooling and the second-stage cooling of the present application both use the profiled temperature control device to compensate the heat of the whole section of the welded joint, so as to control the cooling speed of the rail joint. After the second-stage cooling is completed, the rail profiled temperature control device can be removed, and the rail joint is placed in the low-temperature environment in the wild for natural cooling treatment, so that the joint is naturally cooled to the environment temperature.

[0048] In some embodiments, the profiled temperature control device comprises: two pivotally connected housings, the two housings respectively have shapes matched with the first heat affected zone and the second heat affected zone, and are each provided with a plurality of heating portions; and a control portion configured to control the heating portions of the two housings respectively, so as to control the heating capacities (for example: heating rate, heating temperature) of the two housings respectively. The two housings are pivotally connected so as to be relatively rotatable to be unfolded for being removed from the welded joint, and also relatively rotatable to be closed for covering the welded joint to achieve heat compensation. Under the control of the control portion, the two housings can independently heat the first heat affected zone and the second heat affected zone respectively, so as to control the different cooling speeds of the two zones in the first-stage cooling process.

[0049] In view of the problem that the heat transfer is slow due to the relatively thick thickness of the rail head relative to the rail waist and the rail bottom, in some embodiments of the present application, the heating portions of the two housings corresponding to the area of the rail head are smaller in size and more densely distributed than the heating portions corresponding to the areas of the rail waist and the rail bottom. Thus, the heating temperature difference between the rail head and the rail waist and the rail bottom can be eliminated, and the rail head and the rail waist and the rail bottom of the rail joint can reach the same heating temperature at the same time.

[0050] Figure 4 Fig. 1 shows a schematic diagram of a split profiled temperature control device according to an embodiment of the present application, Figure 5The distribution of the electric heating sheet of the split type profiled temperature control device at the rail head is shown. A1 / A2 / B1 / B2 are terminal posts, C is a circular track type ceramic electric heating sheet at the rail waist, D is a rotating shaft, E is a fixed clasp, F is a device shell, G is an asbestos insulation layer, A3 is a rail head tread heating area, B3 is a rail head side heating area, C3 is a rail head lower jaw heating area, D3 is a circular track type ceramic electric heating sheet, and E3 is a rail joint rail head weld positioning line. Figure 4 and Figure 5 As shown, the device shell includes a first shell and a second shell connected by a rotating shaft. The first shell and the second shell are respectively provided with a fixed clasp and a corresponding matching structure, and the first shell and the second shell are fixedly connected through the cooperation of the fixed clasp and the matching structure. The inside of the device shell is distributed with densely arranged small size circular plate type track type ceramic electric heating sheets, which can be closely combined with the surface of the steel rail to achieve good heat conduction, and the heat compensation process of the rail joint is realized under the joint action of the multiple rows of parallel distributed circular plate type track type ceramic electric heating sheets. In addition, the device metal shell and the ceramic electric heating sheet are also attached with an asbestos insulation layer with a thickness of 2 cm, which has a certain heat preservation function. The split type profiled temperature control device is fixed with the rail head weld center of the rail welded joint (weld positioning line) as the boundary. A1 terminal post and A2 terminal post form a loop and control heating of the left half of the device (rail welded joint) in full section; B1 terminal post and B2 terminal post form a loop and control heating of the right half of the device (rail welded joint) in full section.

[0051] The heating capacity (heating rate, heating temperature) of the left half and the right half of the profiled temperature control device can be the same or different, and the specific heating temperature can be set according to the specific program. The size and distribution density of the circular plate type track type electric heating sheets distributed on the left and right sides of the split type device can be the same. The layout of the rail head, rail waist and rail bottom heating areas of the split type profiled temperature control device is similar. Because the rail head is thicker, the heat transfer is slower, so the number of circular plate type track type ceramic electric heating sheets covering the rail head is more than that of the track type ceramic electric heating sheets in the rail waist and rail bottom areas, the distribution is more dense, and the diameter size of the electric heating sheet is smaller, and the heating temperature is higher, so as to ensure sufficient heating of the rail joint rail head. The size of the ceramic electric heating sheets in the rail waist and rail bottom areas of the profiled device is larger and the distribution is sparser than that in the rail head, so the heating temperature of the rail waist and rail bottom areas of the profiled device is slightly lower than that of the rail head area. By reasonably arranging the size, number and distribution distance of the ceramic electric heating sheets embedded in the split type device, the heating temperature difference between the rail head and the rail waist and the rail bottom is eliminated, so that the rail head and the rail waist and the rail bottom of the rail joint reach the same heating temperature at the same time.

[0052] As shown in Figure 4The shown split type profiled temperature control device has the advantages of small size, flexibility, low cost and the like, is convenient for field construction, can adopt diesel generator 380V voltage or 220V AC mains power supply, and has a rated power of 10kW. The device takes a commercial LCD disc-shaped track type ceramic heating sheet as a heat source, and a disc-shaped track type ceramic heating sheet with a diameter of φ5mm and a thickness of 5mm is used for the rail head area of the split type device, and a disc-shaped track type ceramic heating sheet with a diameter of φ8mm and a thickness of 5mm is used for the rail waist and rail bottom areas of the split type device. With asbestos heat preservation material and a steel structure shell, a steel rail profiled split type heater is made, which is convenient for assembly and disassembly and is suitable for local heating of a steel rail welded joint. The actual size of the heating device and the size and distribution of the heater can be adjusted and changed according to the actual profile of the steel rail. In the design process of the device, the disc-shaped track type ceramic heating sheets in multiple rows are evenly fixed inside the device with a similar steel rail profile, so that the heating sheets evenly cover and fully adhere to the surface of the steel rail to achieve good heat conduction during the heating process, and the device is used to realize heat compensation of the steel rail joint. In the test process, a temperature controller is used to control the heating temperature. The working temperature range of the device is 200-1000℃, and the device can rotate by 180° around the rotation axis.

[0053] In the embodiment of the present application, in order to avoid the formation of harmful martensite structure in the cooling process of the steel rail after welding in the low-temperature field construction environment, the first stage cooling process is adopted, in which the temperature of the joint surface is reduced from 1000-1100℃ to 400-500℃ after the completion of the rail welding, the full-section cladding of the rail head, rail waist and rail bottom is carried out by using the split-type profiled temperature control device to slow down the cooling speed of the rail joint after welding, and the under-speed cooling is carried out, and at the same time, the left half part (corresponding to the low-strength hot-rolled type eutectoid pearlite steel rail on one side of the welding heat affected zone) and the right half part (corresponding to the high-strength hot-rolled type eutectoid pearlite steel rail on one side of the welding heat affected zone) of the welded joint are controlled and cooled respectively by using the hot compensation method in the split-type device, and the cooling speed of the steel rail welding heat affected zone on both sides of the weld is slowed down by the form of ceramic electric heating sheet heat supplement. For the left half part of the device, the cooling speed is 3.0-4.0℃ / s. For the right half part of the device, the cooling speed is 1.5-2.5℃ / s. It should be noted that the heat treatment object in the present application is two kinds of hot-rolled type eutectoid pearlite steel rails, and when the cooling speed of this stage is too high, the hardness of the corresponding steel rail welding heat affected zone will exceed 1.1-1.2 times of the hardness of the corresponding steel rail base material, which is not conducive to the service safety of the steel rail welded joint. And for the second stage cooling process in which the steel rail welded joint is reduced from 400-500℃ to less than or equal to 220℃, the full-section cladding of the rail head, rail waist and rail bottom is carried out by using the split-type profiled temperature control device to slow down the cooling speed of the steel rail welded joint after welding, and at the same time, the left half part (corresponding to the low-strength hot-rolled type eutectoid pearlite steel rail on one side of the welding heat affected zone) and the right half part (corresponding to the high-strength hot-rolled type eutectoid pearlite steel rail on one side of the welding heat affected zone) of the welded joint are controlled and cooled respectively by using the hot compensation method in the split-type device, and the cooling speed of the steel rail welding heat affected zone on both sides of the weld is slowed down by the form of ceramic electric heating sheet heat supplement. The second stage cooling is still the cooling by using the ceramic electric heating sheet heat supplement. The cooling speed of this stage is 0.8-1.4℃ / s, which is lower than the critical cooling speed of the martensite transformation of the two kinds of steel rails. At the same time, the final cooling temperature of the second stage cooling is above the martensite transformation start temperature of the two kinds of steel rails, and then the heat treatment of the welded joint is completed. When the surface temperature of the steel rail welded joint is reduced to less than or equal to 220℃, the profiled temperature control device is removed, and the steel rail joint is placed in the low-temperature environment for natural cooling treatment, so that the joint is naturally cooled to the ambient temperature of 1-15℃. In the cooling process, the temperature signal of the rail head tread can be collected by using the infrared temperature measuring instrument, and the rail head tread is the contact part of the wheel and the rail.

[0054] In some embodiments, the low-strength hot-rolled type of eutectoid pearlite rail base material has a tensile strength of 980-1050 MPa and a hardness of 280-310 HV at room temperature (20-30℃); the high-strength hot-rolled type of eutectoid pearlite rail base material has a tensile strength of 1100-1180 MPa and a hardness of 320-350 HV at room temperature (20-30℃).

[0055] The low-strength hot-rolled type of eutectoid pearlite rail base material and the high-strength hot-rolled type of eutectoid pearlite rail base material have different chemical composition intervals. In some embodiments, the chemical composition of the low-strength hot-rolled type of eutectoid pearlite rail base material contains, in mass percentage, 0.68-0.74% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.04-0.08% of V, and the balance of Fe and inevitable impurities; the chemical composition of the high-strength hot-rolled type of eutectoid pearlite rail base material contains, in mass percentage, 0.76-0.82% of C, 0.50-0.80% of Si, 0.70-1.0% of Mn, 0.30-0.50% of Cr, 0.04-0.08% of V, and the balance of Fe and inevitable impurities.

[0056] In some embodiments, the low-strength hot-rolled type of eutectoid pearlite rail base material and the high-strength hot-rolled type of eutectoid pearlite rail base material have the same rail type and a specification of 60-75 kg / m, and the welded joint is formed by moving flash welding

[0057] It should be noted that the heat treatment technology itself is a process of controlling various factors in the heating and cooling process, and the steps in the heat treatment technology are related and influence each other. There may be inevitable process parameter overlaps and coincidences between the present application and other patent documents, but the applicable objects, heat treatment implementation equipment, etc. are different between each patent, so data cannot be simply applied and compared. The chemical composition of steel rails and heat treatment processes developed by countries around the world inevitably overlap, and are affected by factors such as smelting capacity, heat treatment equipment, and personnel operation level, so the applicable objects of each invention patent are different (including the mechanical properties of the steel rail, the temperature distribution, etc.), the cooling devices used and the implementation process are also different, which produces essential differences, leading to the fact that these processes cannot be simply applied. In addition, according to the characteristics of the hardness change difference of the heat-affected zone in the welding and continuous cooling process of the hot-rolled type of eutectoid pearlite rail steel and the heat-treated type of eutectoid pearlite rail steel, the present application adopts different strength cooling methods for the heat-affected zone on both sides of the rail joint weld, limits the cooling speed and cooling temperature of each cooling stage, improves the "saddle-type" wear of the rail joint caused by the low hardness of the welded area during the service of the rail on the line, and at the same time avoids the harm to the service performance of the rail joint caused by the generation of martensite structure, so the present application has made significant progress compared with other patent applications.

[0058] By the technical scheme, the application can realize the following beneficial effects:

[0059] 1. The application can keep the heat affected zone on both sides of the weld of the rail joint at a high hardness, thereby ensuring the wear resistance of the rail joint.

[0060] 2. The application can ensure that the heat affected zone on both sides of the weld of the rail joint is pearlite, without harmful martensite.

[0061] 3. The application can make the longitudinal hardness of the rail joint within the area of ±15mm from the center of the weld reach 106-110% and 100-105% of the average hardness of the low-strength hot-rolled type eutectoid pearlite rail and the high-strength hot-rolled type eutectoid pearlite rail, respectively, and the hardness difference of the heat affected zone on both sides of the weld is within 30HV, thereby improving the hardness difference of the heat affected zone on both sides of the weld of the rail joint, and realizing good hardness matching of the heat affected zone on both sides of the weld of the rail joint, and ensuring the safety of railway operation.

[0062] According to another aspect of the application, a rail joint of different strength hot-rolled type eutectoid pearlite rails is provided, which is cooled by the post-weld cooling method as described above; the heat affected zone on both sides of the weld of the joint is pearlite, without martensite; the longitudinal hardness of the area corresponding to the low-strength hot-rolled type eutectoid pearlite rail and from the center of the weld to 15mm from the center of the weld reaches 106-110% of the average hardness of the low-strength hot-rolled type eutectoid pearlite rail; the longitudinal hardness of the area corresponding to the high-strength hot-rolled type eutectoid pearlite rail and from the center of the weld to 15mm from the center of the weld reaches 100-105% of the average hardness of the low-strength hot-rolled type eutectoid pearlite rail; and the hardness difference of the heat affected zone on both sides of the weld is within 30HV.

[0063] The following will be described according to specific examples and comparative examples. In the following examples and comparative examples, the low-strength hot-rolled type eutectoid pearlite rail and the high-strength hot-rolled type eutectoid pearlite rail are both produced by Pansteel Group. Figure 2 The following is a schematic diagram of the longitudinal hardness test position of the rail joint below the rail head tread, wherein a is the low-strength hot-rolled type eutectoid pearlite rail, b is the joint, c is the high-strength hot-rolled type eutectoid pearlite rail, d is the rail joint rail head tread, and e is the center of the weld. Figure 3 The following is a schematic diagram of the position of the metallographic sample in each example and comparative example, wherein e is the center of the weld, and f is the position of the metallographic sample.

[0064] Example 1

[0065] In this embodiment, the low-strength hot-rolled type of eutectoid pearlite rail base material contains 0.68% of C, 0.50% of Si, 0.70% of Mn, 0.04% of V, and the balance of Fe and inevitable impurities in terms of mass percentage. The tensile strength of the low-strength hot-rolled type of eutectoid pearlite rail base material at room temperature (20-30℃) is 980 MPa, and the hardness is 280 HV. The high-strength hot-rolled type of eutectoid pearlite rail base material contains 0.76% of C, 0.50% of Si, 0.70% of Mn, 0.30% of Cr, 0.04% of V, and the balance of Fe and inevitable impurities in terms of mass percentage. The tensile strength of the high-strength hot-rolled type of eutectoid pearlite rail base material at room temperature (20-30℃) is 1100 MPa, and the hardness is 320 HV.

[0066] After the top forging and the nub pushing during the moving flash welding process of the rail with a specification of 60 kg / m, the rail welding joint formed by the welding at a surface temperature of 1100℃ is subjected to the first stage cooling by using the profiling temperature control device. During the cooling process, the low-strength hot-rolled type of eutectoid pearlite rail side and the high-strength hot-rolled type of eutectoid pearlite rail side are cooled at a first cooling speed of 4.0℃ / s and 2.5℃ / s respectively, so that the surface temperature of the corresponding rail welding heat affected zone is cooled to 500℃. Then, the second stage cooling is entered. During the cooling process, the low-strength hot-rolled type of eutectoid pearlite rail side and the high-strength hot-rolled type of eutectoid pearlite rail side are cooled at a first cooling speed of 1.4℃ / s and 1.4℃ / s respectively, so that the surface temperature of the corresponding rail welding heat affected zone is cooled to 220℃. Then, the heat treatment of the welding joint is completed. The rail profiling temperature control device is removed, and the rail joint is placed in the outdoor low-temperature environment for the natural cooling treatment, so that the joint is naturally cooled to the ambient temperature of 1℃, thereby obtaining the heterogeneous rail welding joint subjected to the post-weld cooling treatment in this embodiment.

[0067] During the post-weld cooling treatment process, the first stage cooling is the under-speed cooling of the welding joint by using the rail profiling temperature control device in a heat compensation manner. The profiling temperature control device controls the cooling of the left half part (corresponding to the low-strength hot-rolled type of eutectoid pearlite rail side welding heat affected zone) and the right half part (corresponding to the high-strength hot-rolled type of eutectoid pearlite rail side welding heat affected zone) of the welding joint respectively with the weld center as the dividing line. For the left half part of the device, the cooling speed is 4.0℃ / s. For the right half part of the device, the cooling speed is 2.5℃ / s. The second stage cooling is the cooling by using the rail profiling temperature control device. The cooling speed of the heat affected zone on both sides of the weld is 1.4℃ / s respectively. When the surface temperature of the welding joint is reduced to 220℃, the heat treatment of the welding joint is completed. The rail profiling temperature control device is removed, and the rail joint is placed in the outdoor low-temperature environment for the natural cooling treatment, so that the joint is naturally cooled to the outdoor low-temperature environment temperature of 1℃. The infrared temperature measuring instrument is used to monitor the rail head tread temperature.

[0068] The post-weld heat-treated rail joint obtained in this example was machined into a longitudinal hardness sample. A Brinell hardness tester (Shandong Laizhou Test Machine Factory, model HBV-30A) was used to detect the longitudinal Brinell hardness of the hardness sample at a position 5 mm below the rail head tread, with a 2 mm spacing between the test points. The test points were symmetrically arranged to the left and right of the weld. The Brinell hardness test method was in accordance with GB / T 4340.1-2009 “Metallic Materials-Brinell Hardness Test-Part 1: Test Method”, and the HV scale was used. The sampling method shown in FIG. 1 was used to test the metallographic structure of the rail joint according to GB / T 13298-2015 “Metallic Materials-Metallographic Examination Methods”. A 3% nitric acid alcohol solution was used to etch the rail joint metallographic sample, and a German Leica MeF3 optical microscope was used to observe the metallographic structure of the rail joint. Figure 3

[0069] For the dissimilar steel rail welded joint treated in this example, the longitudinal hardness of the rail joint within ±15 mm of the weld center meets 106% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlite rail and high-strength hot-rolled eutectoid pearlite rail base metal, respectively, and the average hardness of the high-strength hot-rolled eutectoid pearlite rail side weld heat-affected zone is 23 HV higher than that of the low-strength hot-rolled eutectoid pearlite rail side weld heat-affected zone. The metallographic structure of the rail weld heat-affected zone on both sides is visible pearlite, without abnormal structures such as martensite.

[0070] Example 2

[0071] In this example, the low-strength hot-rolled eutectoid pearlite rail base metal contains 0.74% C, 0.80% Si, 1.0% Mn, 0.08% V, and the balance Fe and unavoidable impurities, by mass percent. The tensile strength of the low-strength hot-rolled eutectoid pearlite rail base metal at room temperature (20-30°C) is 1050 MPa, and the hardness is 310 HV. The high-strength hot-rolled eutectoid pearlite rail base metal contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, 0.08% V, and the balance Fe and unavoidable impurities, by mass percent. The tensile strength of the high-strength hot-rolled eutectoid pearlite rail base metal at room temperature (20-30°C) is 1180 MPa, and the hardness is 350 HV.

[0072] ​After the top upsetting and the nub pushing in the moving flash welding process of the rail with the gauge of 60 kg / m, the first stage cooling is performed on the rail welded joint with the surface temperature of 1100℃ by using the profiling temperature control device. During the cooling process, the surface temperature of the corresponding rail welded heat affected zone is cooled to 500℃ at the first cooling speed of 4.0℃ / s and 2.5℃ / s for the low-strength hot-rolled ferrite-pearlite rail side and the high-strength hot-rolled ferrite-pearlite rail side respectively. Then the second stage cooling is performed, during which the surface temperature of the corresponding rail welded heat affected zone is cooled to 220℃ at the first cooling speed of 1.4℃ / s and 1.4℃ / s for the low-strength hot-rolled ferrite-pearlite rail side and the high-strength hot-rolled ferrite-pearlite rail side respectively, and then the heat treatment of the welded joint is completed. The rail profiling temperature control device is removed, and the rail welded joint is placed in the outdoor low-temperature environment for natural cooling treatment, so that the joint is naturally cooled to the ambient temperature of 1℃, thereby obtaining the dissimilar rail welded joint after the post-weld cooling treatment of the present embodiment.

[0073] During the post-weld cooling treatment, the first stage cooling is the under-speed cooling of the welded joint in the form of heat compensation by using the rail profiling temperature control device. The left half part (corresponding to the welded heat affected zone of the low-strength hot-rolled ferrite-pearlite rail side) and the right half part (corresponding to the welded heat affected zone of the high-strength hot-rolled ferrite-pearlite rail side) of the welded joint are controlled and cooled respectively by the profiling temperature control device with the weld center as the dividing line. For the left half part of the device, the cooling speed is 4.0℃ / s. For the right half part of the device, the cooling speed is 2.5℃ / s. The second stage cooling is the cooling performed by using the rail profiling temperature control device. The cooling speed of the heat affected zone on both sides of the weld is 1.4℃ / s respectively. When the surface temperature of the welded joint is reduced to 220℃, the heat treatment of the welded joint is completed immediately. The rail profiling temperature control device is removed, and the rail welded joint is placed in the outdoor low-temperature environment for natural cooling treatment, so that the joint is naturally cooled to the outdoor low-temperature environment temperature of 1℃. The infrared temperature measuring instrument is used to monitor the temperature of the rail head tread.

[0074] The rail welded joint after the post-weld heat treatment obtained in the present embodiment is machined into a longitudinal hardness sample. The longitudinal Vickers hardness of the hardness sample is detected by using the Brinell hardness tester (Shandong Laizhou Test Machine Factory, model HBV-30A) at the position 5mm below the rail head tread with a measuring point spacing of 2mm. The measuring points are symmetrically arranged to the left and right sides with the weld as the center. The Vickers hardness test method is performed in accordance with GB / T 4340.1-2009 "Metallic Vickers Hardness Test Part 1: Test Method", and the HV scale is used. The longitudinal hardness of the rail welded joint is determined in accordance with GB / T 13249-91 "Rail Hardness Test". Figure 3The sampling method shown in GB / T13298-2015 "Metal Microstructure Test Method" for steel rail joint metallographic sample for metallographic structure test, using 3% nitric acid alcohol solution for steel rail joint metallographic sample to carry out etching, using German Leica MeF3 optical microscope to observe the steel rail joint microstructure.

[0075] For the dissimilar steel rail welded joint treated by the embodiment, the rail joint longitudinal hardness in the area of ±15mm from the weld center meets 110% and 105% of the average hardness of the corresponding low-strength hot-rolled type eutectoid pearlite steel rail and high-strength hot-rolled type eutectoid pearlite steel rail base material, respectively, and the average hardness of the high-strength hot-rolled type eutectoid pearlite steel rail side weld heat affected zone is 27HV higher than that of the low-strength hot-rolled type eutectoid pearlite steel rail side weld heat affected zone, and the steel rail weld heat affected zone on both sides of the weld is visible pearlite, without martensite and other abnormal structures.

[0076] Example 3

[0077] In the embodiment, the low-strength hot-rolled type eutectoid pearlite steel rail base material contains 0.74% of C, 0.80% of Si, 1.0% of Mn, 0.08% of V, and the balance of Fe and unavoidable impurities in mass percentage. The tensile strength of the low-strength hot-rolled type eutectoid pearlite steel rail base material at room temperature (20-30℃) is 1050MPa, and the hardness is 310HV. The high-strength hot-rolled type eutectoid pearlite steel rail base material contains 0.76% of C, 0.50% of Si, 0.70% of Mn, 0.30% of Cr, 0.04% of V, and the balance of Fe and unavoidable impurities in mass percentage. The tensile strength of the high-strength hot-rolled type eutectoid pearlite steel rail base material at room temperature (20-30℃) is 1100MPa, and the hardness is 320HV.

[0078] After the top upsetting and the nub pushing in the moving flash welding process of the rail with the gauge of 60 kg / m, the first stage cooling is performed on the rail welded joint with the surface temperature of 1100℃ by using the profiling temperature control device. During the cooling process, the surface temperature of the corresponding rail welded heat affected zone is cooled to 500℃ at the first cooling speed of 4.0℃ / s and 2.5℃ / s for the low-strength hot-rolled ferrite-pearlite rail side and the high-strength hot-rolled ferrite-pearlite rail side respectively. Then the second stage cooling is performed. During the cooling process, the surface temperature of the corresponding rail welded heat affected zone is cooled to 220℃ at the first cooling speed of 1.4℃ / s and 1.4℃ / s for the low-strength hot-rolled ferrite-pearlite rail side and the high-strength hot-rolled ferrite-pearlite rail side respectively, and then the heat treatment of the welded joint is completed. The rail profiling temperature control device is removed, and the rail welded joint is placed in the outdoor low-temperature environment for natural cooling treatment, so that the joint is naturally cooled to the ambient temperature of 1℃, thereby obtaining the heterogeneous rail welded joint after the post-weld cooling treatment in the example.

[0079] During the post-weld cooling treatment process, the first stage cooling is the under-speed cooling of the welded joint in the heat compensation mode by using the rail profiling temperature control device. The left half part (corresponding to the welded heat affected zone of the low-strength hot-rolled ferrite-pearlite rail side) and the right half part (corresponding to the welded heat affected zone of the high-strength hot-rolled ferrite-pearlite rail side) of the welded joint are controlled and cooled respectively by the profiling temperature control device with the weld center as the dividing line. For the left half part of the device, the cooling speed is 4.0℃ / s. For the right half part of the device, the cooling speed is 2.5℃ / s. The second stage cooling is the cooling performed by using the rail profiling temperature control device. The cooling speed of the heat affected zone on both sides of the weld is 1.4℃ / s respectively. When the surface temperature of the welded joint is reduced to 220℃, the heat treatment of the welded joint is completed immediately. The rail profiling temperature control device is removed, and the rail welded joint is placed in the outdoor low-temperature environment for natural cooling treatment, so that the joint is naturally cooled to the outdoor low-temperature environment temperature of 1℃. The infrared temperature measuring instrument is used to monitor the temperature of the rail head tread.

[0080] The rail welded joint after the post-weld heat treatment obtained in the example is machined into a longitudinal hardness sample. The longitudinal Vickers hardness of the hardness sample is detected by using the Brinell hardness tester (Shandong Laizhou Test Machine Factory, model HBV-30A) at the position 5mm below the rail head tread with a measuring point spacing of 2mm. The measuring points are symmetrically arranged to the left and right sides with the weld as the center. The Vickers hardness test method is performed according to GB / T 4340.1-2009 “Metallic Vickers Hardness Test Part 1: Test Method”, and the HV scale is used. The longitudinal hardness of the rail welded joint is determined according to GB / T 13249-91 “Rail Hardness Test”. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0081] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 110% and 105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is 5HV higher than the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0082] Example 4

[0083] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, contains 0.74% C, 0.80% Si, 1.0% Mn, 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0084] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0085] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0086] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0087] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 106% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is 9HV higher than the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0088] Example 5

[0089] In this embodiment, the low-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). The high-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0090] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. Remove the rail conformal temperature control device and place the rail joint in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thereby obtaining the dissimilar rail welded joint of this embodiment after post-weld cooling treatment.

[0091] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0092] The rail joint obtained in this embodiment after post-weld heat treatment was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0093] For dissimilar rail welded joints processed in this embodiment, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 107% and 103% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 28HV higher than the average hardness of the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the weld heat-affected zones on both sides of the rail is visible pearlite, without abnormal structures such as martensite.

[0094] Comparative Example 1

[0095] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0096] After the upsetting and push-off processes of the moving flash welding process, the dissimilar rail welded joint with a specification of 60kg / m is directly air-cooled (naturally cooled) to an ambient temperature of 1℃ from the residual temperature of 1100℃, thus obtaining the dissimilar rail welded joint obtained in this comparative example.

[0097] The rail joint obtained in this comparative example was machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0098] For the rail welded joint in this comparative example, compared with the base material of the rail on both sides of the weld, the entire weld heat-affected zone shows an increasing hardness trend. Within a 15mm radius from the weld center, the longitudinal hardness of the rail joint meets 118% and 123% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 63 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, due to the lack of cooling mitigation measures during the joint cooling process, coupled with an excessively low ambient temperature of 1℃ and a rapid cooling rate, a significant amount of martensite, in addition to pearlite, appears in the metallographic structure of the weld heat-affected zone on both sides of the rail joint weld. Under this process, the excessive hardness difference between the weld heat-affected zones on both sides of the rail joint weld, and the presence of a significant amount of martensite in both weld heat-affected zones, are detrimental to railway operation safety.

[0099] Comparative Example 2

[0100] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0101] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 300℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0102] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0103] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0104] For the welded joints of the rails in this comparative example, the final cooling temperature of 300℃ in the first cooling stage of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic steel rail is within the martensitic transformation initiation temperature range of 280~320℃ for low-strength hot-rolled eutectoid pearlitic steel rails, while this temperature is above 220~250℃ for high-strength hot-rolled eutectoid pearlitic steel rails. Simultaneously, the cooling rate of the HAZ on the low-strength hot-rolled eutectoid pearlitic steel rail side in the first cooling stage is 4.0℃ / s, while the cooling rate of the HAZ on the high-strength hot-rolled eutectoid pearlitic steel rail side in the first cooling stage is 2.5℃ / s. Therefore, a small amount of martensite forms in the HAZ on the low-strength hot-rolled eutectoid pearlitic steel rail side, while no martensite forms in the HAZ on the high-strength hot-rolled eutectoid pearlitic steel rail side. Hardness tests show that the longitudinal hardness of the rail joint within a 15mm radius from the weld center meets 125% and 112% of the average hardness of the base metal for the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 8 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. The formation of a small amount of brittle martensite in the weld heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is detrimental to the service safety of the rail joint in railway operations.

[0105] Comparative Example 3

[0106] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.68% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980 MPa and a hardness of 280 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0107] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 245℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0108] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0109] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0110] For the welded joints of the rails in this comparative example, the final cooling temperature of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic rail side (245℃) in the first cooling stage is below the martensitic transformation initiation temperature range of 280~320℃ for low-strength hot-rolled eutectoid pearlitic rails, while this temperature is between 220~250℃ for high-strength hot-rolled eutectoid pearlitic rails. Simultaneously, the cooling rate of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side in the first cooling stage is 4.0℃ / s, while the cooling rate of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side in the first cooling stage is 2.5℃ / s. Therefore, a small amount of martensite is formed in both the HAZ on the low-strength and high-strength hot-rolled eutectoid pearlitic rail sides. Hardness tests show that the longitudinal hardness of the rail joint within a 15mm radius from the weld center meets 130% and 117% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail base material and high-strength hot-rolled eutectoid pearlitic rail base material, respectively. Furthermore, the average hardness of the heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 10 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Because a small amount of brittle martensite structure forms in the heat-affected zones on both sides of the weld joint, it is detrimental to the service safety of the rail joint in railway operations.

[0111] Comparative Example 4

[0112] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, and 0.08% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1180 MPa and a hardness of 350 HV at room temperature (20–30°C).

[0113] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at a first cooling rate of 2.5℃ / s. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s respectively, and the heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0114] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 2.5℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0115] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0116] For the dissimilar rail welded joints in this comparative example, the cooling rate of the heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side is relatively low, resulting in insufficient hardness increase during cooling. Within a ±15mm radius of the weld center, the longitudinal hardness of the rail joint meets 105% of the average hardness of the base materials for both the low-strength and high-strength hot-rolled eutectoid pearlitic rails. Furthermore, the average hardness of the heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side is 42 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the metallographic structure of the heat-affected zones on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. The difference in average hardness between the weld heat-affected zone on one side of high-strength hot-rolled eutectoid pearlitic steel rail and the weld heat-affected zone on the other side of low-strength hot-rolled eutectoid pearlitic steel rail is significant. The average hardness of the weld heat-affected zone on the other side of low-strength hot-rolled eutectoid pearlitic steel rail is relatively low, which is detrimental to the service safety of the rail joint in the railway.

[0117] Comparative Example 5

[0118] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0119] After the upsetting and stub removal processes of the moving flash welding of 60kg / m rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded rail joint, whose surface temperature is 990℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0120] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0121] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0122] For the rail welded joint in this comparative example, the surface temperature of the rail welded joint was slightly lower before the start of the first stage of cooling, resulting in insufficient driving force for pearlite phase transformation during subsequent cooling, insufficient refinement of the pearlite lamellar spacing, and insufficient hardness improvement in the heat-affected zones on both sides of the weld. Hardness tests showed that the longitudinal hardness of the rail joint within a range of ±15mm from the weld center met 104% and 99% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone on the low-strength hot-rolled eutectoid pearlitic rail side was 6 HV higher than that on the high-strength hot-rolled eutectoid pearlitic rail side. At the same time, the metallographic structure of the heat-affected zones on both sides of the weld was visible pearlite, without any abnormal structures such as martensite. Because the hardness of the heat-affected zone on both sides of the weld joint obtained in this comparative example does not meet the requirement that the longitudinal hardness of the rail joint reaches 106-110% and 100-105% of the average hardness of the base material of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively, this comparative example is not conducive to the service safety of the rail joint in railways.

[0123] Comparative Example 6

[0124] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.74% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1050 MPa and a hardness of 310 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0125] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.2℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.2℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0126] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow, heat-compensated cooling of the weld joint. This device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.2℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at rates of 1.4℃ / s and 1.2℃ / s for the heat-affected zones on both sides of the weld, respectively. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. Infrared thermometers are used to monitor the temperature of the rail head tread.

[0127] The rail joints used in this comparative example were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0128] For the dissimilar rail welded joints in this comparative example, the longitudinal hardness of the rail joint within a ±15mm radius of the weld center meets 106% and 94% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the low-strength hot-rolled eutectoid pearlitic rail side is 28 HV higher than that on the high-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. However, since the hardness of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 94% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the high-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 100-105% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joints in railways.

[0129] Comparative Example 7

[0130] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0131] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 530℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 3.0℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0132] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 3.0℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0133] The rail joints used in this comparative example were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0134] For the dissimilar rail welded joints in this comparative example, the high final cooling temperature in the first stage resulted in insufficient refinement of the pearlitic structure during cooling, leading to a less significant increase in hardness. Hardness tests showed that the longitudinal hardness of the rail joint within a ±15mm radius of the weld center met 102% and 94% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the weld heat-affected zone on the high-strength hot-rolled eutectoid pearlitic rail side was 13 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the weld heat-affected zones on both sides of the rails was visible pearlite, without any abnormal structures such as martensite. Because the hardness of the heat-affected zone on both sides of the weld of the rail welded joint obtained in this comparative example does not meet the requirement that the longitudinal hardness of the obtained rail joint reaches 106-110% and 100-105% of the average hardness of the base material of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail, respectively, this comparative example is not conducive to the service safety of the rail joint in railways.

[0135] Comparative Example 8

[0136] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.71% C, 0.60% Si, 0.85% Mn, and 0.06% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1010 MPa and a hardness of 296 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.79% C, 0.60% Si, 0.85% Mn, 0.40% Cr, and 0.06% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1140 MPa and a hardness of 335 HV at room temperature (20–30°C).

[0137] After the upsetting and stub removal processes of the moving flash welding of 75kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1000℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 400℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 2.5℃ / s and 1.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0138] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 2.5℃ / s for the left half and 1.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 10℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0139] The rail joints obtained in this comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0140] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 104% and 103% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the high-strength hot-rolled eutectoid pearlitic rail side is 37 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 104% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0141] Comparative Example 9

[0142] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.75% C, 0.80% Si, 1.0% Mn, and 0.08% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1048 MPa and a hardness of 307 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.82% C, 0.80% Si, 1.0% Mn, 0.50% Cr, and 0.08% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1180 MPa and a hardness of 350 HV at room temperature (20–30°C).

[0143] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 1°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0144] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to 1℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0145] The rail joints obtained from the comparative example after post-weld heat treatment were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T 4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. Figure 3The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0146] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 113% and 105% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the high-strength hot-rolled eutectoid pearlitic rail side is 21 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 113% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0147] Comparative Example 10

[0148] In this comparative example, by mass percentage, the low-strength hot-rolled eutectoid pearlitic rail base material contains 0.67% C, 0.50% Si, 0.70% Mn, and 0.04% V, with the balance being Fe and unavoidable impurities. The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 977 MPa and a hardness of 278 HV at room temperature (20–30°C). By mass percentage, the high-strength hot-rolled eutectoid pearlitic rail base material contains 0.76% C, 0.50% Si, 0.70% Mn, 0.30% Cr, and 0.04% V, with the balance being Fe and unavoidable impurities. The high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100 MPa and a hardness of 320 HV at room temperature (20–30°C).

[0149] After the upsetting and stub removal processes of the moving flash welding of 60kg / m steel rails, a contour-following temperature control device is used to perform the first stage of cooling on the welded joint, whose surface temperature is 1100℃. During the cooling process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 500℃ on the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 4.0℃ / s and 2.5℃ / s, respectively. Then, the second stage of cooling begins. During this process, the surface temperature of the corresponding heat-affected zone of the rail joint is cooled to 220℃ on both the low-strength hot-rolled eutectoid pearlitic rail side and the high-strength hot-rolled eutectoid pearlitic rail side at first cooling rates of 1.4℃ / s and 1.4℃ / s, respectively. The heat treatment of the welded joint is then completed. The rail conformal temperature control device was removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling treatment, allowing the joint to cool naturally to an ambient temperature of 15°C, thus obtaining the dissimilar rail welded joint of this comparative example after post-weld cooling treatment.

[0150] During post-weld cooling, the first stage of cooling involved a rail-contact temperature control device for slow cooling of the weld joint using heat compensation. The device controlled the cooling of the left half (corresponding to the heat-affected zone on one side of the low-strength hot-rolled eutectoid pearlitic rail) and the right half (corresponding to the heat-affected zone on one side of the high-strength hot-rolled eutectoid pearlitic rail) of the weld joint, using the weld center as the dividing line. The cooling rate was 4.0℃ / s for the left half and 2.5℃ / s for the right half. The second stage of cooling, also using the rail-contact temperature control device, involved cooling at a rate of 1.4℃ / s for both heat-affected zones on either side of the weld. Heat treatment was completed when the weld joint surface temperature reached 220℃. The rail-contact temperature control device was then removed, and the rail joint was placed in a low-temperature outdoor environment for natural cooling to a minimum ambient temperature of 15℃. An infrared thermometer was used to monitor the rail head tread temperature.

[0151] The rail joints used in the comparative analysis were machined into longitudinal hardness test specimens. A HBV-30A hardness tester (Shandong Laizhou Testing Machine Factory) was used to test the longitudinal Vickers hardness of the specimens at a distance of 5 mm below the rail head tread, with measuring points spaced 2 mm apart. The measuring points were symmetrically arranged to the left and right sides centered on the weld. The Vickers hardness test method was performed according to GB / T4340.1-2009 "Metallic materials - Vickers hardness testing - Part 1: Test methods", using the HV scale. ​The sampling method shown is to conduct metallographic examination of rail joint metallographic samples according to GB / T13298-2015 "Metallic Microstructure Test Method". The metallographic samples of rail joint are etched with 3% nitric acid alcohol solution, and the metallographic structure of rail joint is observed with a German Leica MeF3 optical microscope.

[0152] For the rail welded joint in this comparative example, the longitudinal hardness of the rail joint within a range of ±15mm from the weld center meets 104% and 100% of the average hardness of the corresponding low-strength hot-rolled eutectoid pearlitic rail and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. Furthermore, the average hardness of the heat-affected zone (HAZ) on the high-strength hot-rolled eutectoid pearlitic rail side is 31 HV higher than that on the low-strength hot-rolled eutectoid pearlitic rail side. Simultaneously, the microstructure of the HAZ on both sides of the weld is visible pearlite, without any abnormal structures such as martensite. Since the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint obtained in this comparative example is only 104% of the hardness of the corresponding rail base material, it does not meet the requirement that the hardness of the HAZ on the low-strength hot-rolled eutectoid pearlitic rail side of the rail welded joint reaches 106-110% of the hardness of the corresponding rail base material. Therefore, this comparative example is not conducive to the service safety of the rail joint in railways.

[0153] By comparing the longitudinal hardness of the rail head tread and the metallographic structure of the welded joint obtained from various embodiments and comparative examples, it can be seen that: The post-weld cooling treatment method provided by this invention, applied to welded joints of a low-strength hot-rolled eutectoid pearlitic rail and a high-strength hot-rolled eutectoid pearlitic rail, ensures that the heat-affected zone (HAZ) on both sides of the rail joint weld is composed solely of pearlite, without martensite or other abnormal structures. Simultaneously, the longitudinal hardness of the rail joint within ±15mm of the weld center can reach 106–110% and 100–105% of the average hardness of the corresponding low-strength and high-strength hot-rolled eutectoid pearlitic rail base materials, respectively. The hardness difference between the HAZ on both sides of the rail joint weld is within 30 HV, improving the hardness difference between the HAZ on both sides of the weld of dissimilar rail joints, thereby achieving good hardness matching between the HAZ on both sides of the weld of dissimilar strength rails and ensuring railway operation safety.

[0154] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A post-weld cooling method for a welded joint of hot-rolled eutectoid pearlitic steel rail of dissimilar strength, characterized in that, This includes the following steps performed sequentially: S1, the welded joint formed by welding low-strength hot-rolled eutectoid pearlitic steel rail base material and high-strength hot-rolled eutectoid pearlitic steel rail base material is subjected to a first stage of cooling, so that the surface temperature of the welded joint is reduced from 1000-1100℃ to 400-500℃. The first heat-affected zone corresponding to the low-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a first cooling rate, and the second heat-affected zone corresponding to the high-strength hot-rolled eutectoid pearlitic steel rail base material is cooled at a second cooling rate less than the first cooling rate. The first cooling rate is 3.0-4.0℃ / s, and the second cooling rate is 1.5-2.5℃ / s. S2, the welded joint is subjected to a second stage of cooling, so that the surface temperature of the welded joint is reduced from 400-500°C to below or equal to 220°C, wherein the cooling rate of the welded joint is less than or equal to 1.4°C / s; S3, allow the welded joint to cool naturally to ambient temperature.

2. The method according to claim 1, characterized in that, The cooling rate of the welded joint in step S2 is 0.8 to 1.4 °C / s.

3. The method according to claim 1, characterized in that, The method is carried out in a low-temperature outdoor environment with a temperature of 1 to 15°C. In steps S1 and S2, a contour-following temperature control device is used to perform full-section heat compensation on the welded joint.

4. The method according to claim 3, characterized in that, The contour-following temperature control device includes: Two housings are pivotally connected, each housing having a shape that matches the first heat-affected zone and the second heat-affected zone, and each housing is provided with multiple heating elements; The control unit is configured to control the heating units of the two housings respectively.

5. The method according to claim 4, characterized in that, The heating elements in the regions corresponding to the rail head of the two housings are smaller and more densely distributed than the heating elements in the regions corresponding to the rail web and rail bottom.

6. The method according to claim 1, characterized in that, The low-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 980~1050MPa and a hardness of 280~310HV at room temperature; the high-strength hot-rolled eutectoid pearlitic rail base material has a tensile strength of 1100~1180MPa and a hardness of 320~350HV at room temperature.

7. The method according to claim 1, characterized in that, The chemical composition of the low-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, comprises: 0.68~0.74% C, 0.50~0.80% Si, 0.70~1.0% Mn, 0.04~0.08% V, with the balance being Fe and unavoidable impurities; The chemical composition of the high-strength hot-rolled eutectoid pearlitic rail base material, by mass percentage, comprises: 0.76~0.82% C, 0.50~0.80% Si, 0.70~1.0% Mn, 0.30~0.50% Cr, 0.04~0.08% V, with the balance being Fe and unavoidable impurities.

8. The method according to claim 1, characterized in that, The low-strength hot-rolled eutectoid pearlitic rail base material and the high-strength hot-rolled eutectoid pearlitic rail base material have the same rail type and a specification of 60-75 kg / m. The welded joint is formed by welding with a mobile flash welding machine.

9. A welded joint for hot-rolled eutectoid pearlitic steel rails with dissimilar strength, characterized in that, The welded joint is cooled by the post-weld cooling method as described in any one of claims 1-8; the microstructure of the heat-affected zone on both sides of the weld joint is pearlite, without martensite; the longitudinal hardness of the region corresponding to the low-strength hot-rolled eutectoid pearlite rail base material and extending from the weld center to 15 mm from the weld center reaches 106-110% of the average hardness of the low-strength hot-rolled eutectoid pearlite rail base material; the longitudinal hardness of the region corresponding to the high-strength hot-rolled eutectoid pearlite rail base material and extending from the weld center to 15 mm from the weld center reaches 100-105% of the average hardness of the low-strength hot-rolled eutectoid pearlite rail base material; the hardness difference between the heat-affected zones on both sides of the weld is within 30 HV.

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