AU2020210190B2 - Process for controlling microstructure of flash butt welded joints of r350ht rail with a medium-upper limit of manganese content - Google Patents

Abstract

The invention discloses a process for controlling a microstructure of a flash butt welded joint of R350HT steel rail with a medium-upper limit of Manganese content, belonging to the technical field of rail flash butt welding. In order to solve the technical problem in the prior art that martensite can easily develop in the flash butt welded joints of R350HT rail, the invention provides a process for controlling a microstructure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content, comprising a welding process and post-weld treatment, wherein, in the welding process, the pulsating or preheating flash butt welding with a welding heat-input of 7.0-8.5 MJ is controlled and a welding upset rail-used is 14.5-15.5 mm; and in the post-weld treatment, the joint is naturally cooled to 525-560 °C, then cooled to 250°C or less under heat insulation, and finally naturally cooled to room temperature. The chemical compositions, hot rolling process, welding process and post-welding cooling are comprehensively controlled to eliminate abnormal microstructures such as martensite and the like in the flash butt welded rail joints and ensure their mechanical properties. 1/1 Fig. 1

Description

1/1

Fig. 1

PROCESS FOR CONTROLLING MICROSTRUCTURE OF FLASH BUTT WELDED

JOINTS OF R350HT RAIL WITH A MEDIUM-UPPER LIMIT OF MANGANESE

CONTENT

Technical Field of the Invention

[0001] The disclosure belongs to the technical field of rail flash butt welding, and in particular relates to a process for controlling a microstructure of a flash butt welded joint

of R350HT rail with a medium-upper limit of Manganese content.

Background of the Invention

[0002] Relevant authorities are attaching more importance to the quality of rail joints with rapid development of seamless railway technology in passenger transport and freight transport, as well as high-speed and heavy-haul railway construction worldwide. The railway line is the direct carrier of train operation and its reliable quality is the key to the safe operation of trains. Flash butt welded rail joint, as a weak link of the whole line, has a direct influence on the safety of the railway. However, in the matrix of flash butt welded rail joint, the martensitic microstructure belongs to hard brittle phase with very high hardness and very low toughness, thus it is a heterogeneous microstructure that can severely affect the base metal and joint during rail welding and production. It can cause breakage, chunking and even peeling in the rails installed on the line and seriously affect the safe operation of trains.

[0003] The morphology and content of martensite in joints are specified in the flash butt welding standards of rails widely accepted at home and abroad, as well as in the technical conditions of enterprises. According to Railway Applications-Track-FlashButt Welding of Rails. Part2: New R220 R260 R260Mn and R350HT Grade Rails by Mobile Welding Machines at Sites other than a Fixed Plant, specimens shall be free from harmful microstructures such as martensite or bainite under a 100x optical microscope, and they shall be taken from the vertical center surface of rail head and rail foot, i.e. One specimen (size: 20x20mm) from the rail head, two specimens (size: 10x20mm) from two rail feet respectively. Based on European Standard, R350HT rail is the heat-treated rail specified in Railway Applications-Track-FlashButt Welding of Rails. Part2: New R220 R260 R260Mn and R350HT GradeRails by Mobile Welding Machines at Sites other than a Fixed Plant, and contains the following contents: Carbon: 0.70 - 0.82%., Manganese: 0.65 - 1.25%, Silicon: 0.13 - 0.60%, and Chromium < 0.15%; and R350HT rail has the following mechanical properties: Rm > 1175MPa, A > 9%, and rail head running surface hardness > 350HB. Thus, they are widely accepted in foreign national railways and subway lines. Moreover, many countries in the world pose very stringent requirement on the morphology and content of martensite in flash butt welded rail joints. How to inhibit and eliminate martensite in flash butt welded rail joints through welding process and post-weld treatment process is an important factor for judging whether the flash butt welded rail joint quality meets the standard.

[0004] Nevertheless, R350HT rail covers a relatively wide composition in EN standard. Alloy elements with high content of Manganese are often used to design the chemical composition of steel. But only considering that the mechanical properties of hot-rolled rails comply with EN standard but that the change of the user's welding acceptance standard will cause the martensite occurred in the metallographic microstructure of the central surface under the tread surface of flash butt welded joints cannot make the rail pass welding examination.

[0005] The post-weld heat treatment is commonly used at home and abroad to inhibit and eliminate martensitic microstructure of flash butt welded rail joints, i.e. after post-weld cooling of joints to a lower temperature, the weld zone is reheated to a certain temperature by an electromagnetic induction coil or a flame heating device, and then the heated weld zone is cooled at a certain cooling rate. TB/T 1632 provides that post-weld heat treatment of pearlite rails includes normalizing and post-weld slack quenching for restoring the hardness of rail head. Specifically, the fixed-type flash butt welded joint shall be heated by medium frequency induction; while the mobile flash butt welded joint shall be heated by medium frequency electric induction or swinging a flame heater. In the case of medium frequency electric induction, the starting temperature of rail head surface should be less than 500°C, and the heating temperature should be preferably 900±20°C. If the joint is heated by swing a flame heater, the starting temperature of the rail head surface shall be less than 500°C, the heating width shall be 50±10mm, and the heating temperature shall be preferably 850 - 950°C. In case of rail head slack quenching, the joint shall be cooled by spraying compressed air. However, the method adopted by other domestic scholars is post-weld heat treatment without exception, i.e. different purposes are achieved by controlling heating rate, peak temperature, cooling rate and final temperature of forced cooling. Foreign countries only focus on strengthening heat treatment of flash butt welded rail joints. In the post-weld natural cooling process of joints, the weld zone is forcedly cooled with compressed air to achieve the purpose of improving joint hardness, which is not helpful to inhibit and eliminate martensite though. If not properly controlled, it will increase the martensite content of joints. No post-weld treatment measures are taken against some heat-treated rails and all heat-treated rail varieties, i.e. only post-weld natural cooling is performed.

[0006] It is against this background that the present disclosure has been developed.

Summary of the Invention

[0007] According to a first aspect, there is provided a process for controlling a structure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content, comprising a welding process and a post-weld treatment, wherein the Manganese content of R350HT rail base metal is 1.05-1.15 wt%; in the welding process, the pulsating or preheating flash butt welding with a welding heat-input of 7.0-8.5 MJ is controlled and a welding upset rail-used is 14.5-15.5 mm; in the post-weld treatment, the flash butt welded joint is naturally cooled to enable the rail tread weld center to reach 525-560°C, then cooled under heat insulation to enable the rail tread weld center to be not more than 250 C, and finally naturally cooled to room temperature; and Carbon content of the R350HT rail base metal is 0.72-0.78 wt%, and Silicon content is < 0.30 wt%.

[0008] In one form, in the process for controlling the microstructure of a flash butt welded joint of R35OHT rail with a medium-upper limit of Manganese content, the weld center is a zone with 150 - 200 mm wide respectively on both sides of a joint bond line.

[0009] In one form, in the process for controlling the structure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content, the cooling under heat insulation is achieved by a thermal insulation material an aluminosilicate fiber cotton with a thickness of> 30mm.

[00010] In one form, in the process for controlling the structure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content, Carbon content

of the R350HT rail base metal is at the lower and medium limit, and Silicon content is <

0.30 wt%.

[00011] In one form, in the process for controlling the structure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content, the R350HT rail

base metal is produced by a universal rolling line and an on-line heat treatment line, and

the mechanical properties thereof meet the conditions below: Rm > 1175MPa, A > 9%,

and rail head running surface hardness > 350HB.

[00012] The disclosure has the following beneficial effects:

[00013] The disclosure provides a comprehensive control of chemical composition, hot rolling process, welding process and post-welding cooling of R350HT rail with a

medium-upper limit of Manganese content; as a result, when the Manganese content of

R350HT rail is 1.05 - 1.15wt%, joints may not be reheated, which thus eliminates

abnormal structures such as martensite and the like in the flash butt welded rail joints,

and ensures the compliance of joint hardness, slow bend test and fatigue performance

with the technical requirements of EN 14587-2:2009. The disclosure realizes cooling

under heat insulation by a simple thermal insulation device, so that the post-weld cooling

rate is well controlled. The disclosure has simple control process applicable to both fixed

flash butt welding and mobile flash butt welding, and can solve major welding technical

problems on the promotion and application of R350HT rail. Thus, the disclosure has

good promotion and application prospects.

Brief Description of the Drawings

[00014] Fig. 1 is a metallographic structure diagram of a flash butt welded joint of R350HT rail.

Detailed Description

[00015] Specifically, the process for controlling a microstructure of a flash butt welded joint of R350HT rail with a medium-upper limit of Manganese content comprises welding process and post-weld treatment, wherein the Manganese content of R350HT rail base metal is 1.05-1.15 wt%; in the welding process, the pulsating or preheating flash butt welding with a welding heat-input of 7.0-8.5 MJ is controlled and a welding upset rail-used is 14.5-15.5 mm.

[00016] The flash butt welding process of the rail is as follows: first of all, the cross section of the rail to be welded is leveled and cleaned through the beam blasting in the flash leveling stage, thus providing relatively even and smooth favorable conditions for subsequent flash and heating; secondly, a certain temperature gradient is formed in the longitudinal direction of the rail through heat accumulated during the flash process of high current and low voltage in the preheating stage, and the end face of rail is heated to a sufficient temperature, thus creating conditions for accelerating the flash homogenization process; and finally, a protective atmosphere is created through accelerated flash burning at the end stage so as to prevent end surface oxidation in the whole weld zone, and a proper temperature field is distributed to get ready for upsetting. The welding heat-input is mainly used to control the heat accumulated during the flash process of high current and low voltage in the preheating stage, which directly affects the temperature gradient of the rail end.

[00017] Based on a suitable temperature gradient obtained, a force is applied along the longitudinal direction of two rails to bring them together. In this point, the rail with plastic deformation capability is extruded and deformed. The upset rail-used refers to the amount of metal consumed in the front section of the rail before and after upsetting. As a rule, at a certain heat-input, the greater the upset force, the greater the upset rail-used. Tests show that when the Manganese content of R350HT rail is in the medium-upper limit (1.05 - 1.15wt%), the best match can be achieved by the welding heat-input of 7.0 8.5MJ and the welding upset rail-used of 14.5 - 15.5mm, and the rail joint has the maximum bonding strength and the most stable joint quality.

[00018] In the rail smelting process, Manganese and S are easily turned into manganese sulfide, which can reduce the harm of sulfur in steel to a certain extent as a good deoxidizer and desulfurizer. Moreover, Manganese dissolves into ferrite to strengthen solid solution and thus improve the hardness and strength of hot-rolled steel. However, obvious tempering brittleness occurs when steel contains a large amount of Manganese. Besides, Manganese can promote grain growth, making steel sensitive to overheating, and when the mass fraction of Manganese exceeds 1%, the weldability of steel will be reduced. In the cooling process of flash butt welded rail joint, martensite is easily distributed in the form of spot or strip in the micro-segregation area of Manganese element, which seriously affects the joint performance. In general, the higher the average mass fraction of Manganese in steel, the greater the probability of Manganese segregation, i.e. the higher the Manganese content, the easier it is to develop martensite.

[00019] Martensite is essentially a supersaturated interstitial solid solution of carbon in a-Fe. Martensite exists in two morphologies mainly, i.e. lath martensite and plate martensite (the martensite mentioned in the disclosure is mainly plate martensite). Plate martensite generally develops in high carbon steel with Carbon content greater than 0.6%. Due to the sample phase, it is sometimes needle-like or bamboo-leaf-like under an optical microscope, so the plate martensite is also called needle-like martensite or bamboo-leaf-like martensite. Another important characteristic of plate martensite lies in a large number of microcracks, which increase the brittleness of high carbon steel parts. Due to internal stress, microcracks will gradually expand into macro cracks, which can lead to joint cracking or obvious reduction of joint fatigue life. Like other solid-state phase transformation, martensitic transformation requires sufficient undercooling to make the driving force of phase transformation greater than its resistance so as to cause austenite-martensite transformation. However, the difference is that large undercooling must be below the Martensite-start (Ms) temperature. Martensite transformation occurs without diffusion, and extremely fast transformation speed is observed at a relatively low temperature. When austenite in steel is transformed into martensite, only face-centered cubic lattice is reorganized into body-centered cubic lattice without composition change. At the same time, martensitic transformation takes place within a certain temperature range. The martensitic transformation in R350HT rail in the disclosure occurs in a continuous (i.e., temperature change) cooling process. Specifically, a certain amount of martensite is developed immediately when austenite is cooled below Martensite-start (Ms) temperature at a speed greater than the critical quenching speed, and a certain amount of martensite is developed again as the temperature decreases, but the martensite developed first does not grow; and the amount of martensitic transformation gradually increases as the temperature drops until below Martensite-finish (Mf) temperature.

[00020] The martensitic structure of the disclosure mainly develops within 5mm along the vertical center surface of the rail and both sides of the bond line. Since Carbon and

Manganese are segregation elements, when the Manganese content of the rail is

increased to 1.05 - 1.15 wt%, the base metal of the rail containing a high content of

Manganese will inevitably cause Manganese minor segregation and the high content of

chemical components in the segregation region will cause the CCT curve to shift heavily

to the right; as a result, the spot martensitic structure can be developed easily in the air

cooling process of the welded joint. Therefore, after the flash butt welding, the flash butt

welded joint is naturally cooled to enable the rail tread weld center to reach 525-560°C,

then cooled under heat insulation to enable the rail tread weld center to be not more than

250°C, and finally naturally cooled to room temperature. In the cooling process of the

rail joint, the cooling rate is controlled within an appropriate range (lower than the

critical cooling rate) to ensure excellent joint performance and eliminate martensitic

transformation.

[00021] The specific heat treatment steps are as follows: first of all, measuring the temperature of the rail tread weld center of the naturally cooled rail joint with an infrared

thermometer after the flash butt welding; when the temperature drops to 525-560°C,

holding the heat of the weld zone immediately with a thermal insulation device and

monitoring the temperature of joint continuously; when the temperature at the center of

the tread weld is not more than 250°C, removing the thermal insulation device; and

finally, cooling the joint naturally in air to room temperature, wherein the weld center is

a zone with 150-200 mm wide respectively on both sides of the joint bond line; and an

aluminosilicate fiber cotton with a thickness of > 30mm is used as a thermal insulation

material for achieving thermal insulation and cooling.

[00022] The natural cooling is fast (3 -5°C/s) in the high temperature section and the

slow natural cooling is slow (<0.2 °C/s) in the low temperature section. The heat insulation and cooling rate can reach 1 - 2°C/s by controlling the starting temperature of heat insulation and the device thereof.

[00023] The disclosure is applicable to R350HT rail with Carbon content of 0.70-0.82 wt% and Si content of 0.13-0.60 wt%. In addition, the inventor found in practice that when Carbon content of R350HT rail base metal is at the lower and middle limit (Carbon content is generally 0.72-0.78 wt%) and Si is < 0.30wt%, the metallographic structure of joint can be better controlled by the method of the disclosure.

[00024] The R350HT rail adopted by the disclosure is produced by a universal rolling line and an on-line heat treatment line, and its mechanical properties meet the conditions below: Rm > 1175MPa, A > 9%, and rail head running surface hardness > 350HB.

[00025] In the disclosure, the contents are all mass percent.

[00026] The disclosure will be described in further detail by the following embodiments, but the protection scope of the disclosure is not limited to the described embodiments.

[00027] In the embodiment, the rail slow bend test is the most widely used method for evaluating the overall performance of rail joints at home and abroad, and all inspection standards for rail joints mainly include two indexes, namely, load and deflection.

[00028] In the slow bend test, a load is applied to a rail joint at a certain loading rate with a three-point or four-point support method. The slow bend test performance of the joint can be judged acceptable if the joint cannot be broken while the load reaches the standard specified value continuously and the maximum deflection is greater than the standard specified value, and different rail types correspond to different load and deflection values. Among international standards, Russia STO RZD 1.08.002:2009 poses the most stringent quality standard, i.e. the minimum load and deflection of a joint should be not less than 2100kN and 30mm respectively (65 kg/m rail, with the rail head stressed), which translates into the minimum load of 1907kN and 30 mm when 60kg/m rail head is stressed. European standard BS EN14587-2:2009 provides that the minimum load and deflection of a joint should be not less than 1600KN and 20 mm respectively (60 kg/m rail, with the rail head stressed). Australian Standard AS1085.20-2012 provides that the minimum stress at a joint rail foot is 900MPa, and the converted minimum load and deflection are not less than 1670KN and 20 mm respectively (60 kg/m rail, with the rail head stressed). Chinese standards provide that the slow bend test breaking load is not less than 1450 kN (60 kg/m rail, with the rail head stressed), but specify no requirement for deflection.

[00029] The slow bend test load mainly reflects the joint bonding strength, joint appearance and internal defect index, while deflection mainly reflects the joint strength and toughness index. Poor joint welding process will cause the joint fracture before it reaches the load value specified in the standard due to defects such as gray spots, lack of fusion or over-burning, or cause the joint harder or softer and further making its deflection not meet the standard requirements due to improper matching of heat-input and upset forging quantity and improper post-weld treatment method.

[00030] The fatigue performance of rail is similar to that of slow bend test. An alternating load is applied to a rail joint at a certain frequency and amplitude with the three-point or four-point support method. The slow bend test performance of the joint can be judged acceptable if the joint cannot be broken while the cycle times reach the standard specified value. Different rail types correspond to different maximum stresses at the rail foot.

Embodiment 1

[00031] Flash butt welding process with welding heat-input of 7.0 MJ and upset rail-used of 14.5 mm is adopted to treat European standard R350HT heat-treated rail with Manganese mass fraction of 1.05% (Carbon content was at the medium and lower limit, Si < 0.30wt%, Rm greater than or equal 1175MPa, A greater than or equal 9%, and rail head running surface hardness greater than or equal 350HB). After the flash butt welding, first of all, the temperature of the rail tread weld center of the naturally cooled rail joint is measured with an infrared thermometer after the flash butt welding. When the temperature dropped to 550°C, the two 200mm wide zones respectively on both sides of the joint bond line are insulated immediately by an aluminosilicate fiber cotton thermal insulation device with a thickness of > 30mm, during which the temperature of joint was continuously monitored. When the temperature at the tread weld center was lower than 250°C, the device could be removed. Finally, the joint was naturally cooled in air to room temperature.

[00032] The joint obtained by the above welding process and post-weld treatment method was inspected and analyzed according to EN 14587.2. It was found that the

microstructure of the rail head and rail foot on the center surface of the joint was pearlite,

and no harmful microstructure such as martensite is observed. Except for the softening

zone, the joint hardness is in the range of the average hardness of the base metal -30HV

to +60HV. Based on continuous three-point slow bend test at 2000kN, the maximum

deflection is 26.8mm. After the three-point support fatigue test for five million times, the

rail is not broken and all indexes met the requirements of EN standard.

Embodiment 2

[00033] Flash butt welding process with welding heat-input of 7.9 MJ and upset rail-used of 14.8 mm is adopted to treat European standard R350HT heat-treated rail with

Manganese mass fraction of 1.13% (Carbon content is at the medium and lower limit, Si

< 0.30wt%, Rm > 1175MPa, A > 9%, and rail head running surface hardness > 350HB).

After the flash butt welding, first of all, the temperature of the rail tread weld center of

the naturally cooled rail joint is measured with an infrared thermometer after the flash

butt welding. When the temperature dropped to 560°C, the two 200mm wide zones

respectively on both sides of the joint bond line are insulated immediately by an

aluminosilicate fiber cotton thermal insulation device with a thickness of > 30mm,

during which the temperature of joint is continuously monitored. When the temperature

at the center of the tread weld is lower than 250°C, the device could be removed. Finally,

the joint is naturally cooled in air to room temperature.

[00034] The joint obtained by the above welding process and post-weld treatment

method is inspected and analyzed according to EN 14587.2. It is found that the

microstructure of the rail head and rail foot on the center surface of the joint is pearlite,

and no harmful microstructure such as martensite is observed. Except for the softening

zone, the joint hardness is in the range of the average of the base metal -30HV to +60HV.

Based on continuous three-point slow bend test at 2000kN, the maximum deflection is

26.9 mm. After the three-point support fatigue test for five million times, the rail is not broken and all indexes met the requirements of EN standard.

Embodiment 3

[00035] Flash butt welding process with welding heat-input of 8.5 MJ and upset rail-used of 15.5 mm is adopted to treat European standard R350HT heat-treated rail with Manganese mass fraction of 1.15% (Carbon content is at the medium and lower limit, Si < 0.30wt%, Rm > 1175MPa, A > 9%, and rail head running surface hardness > 350HB). After the flash butt welding, first of all, the temperature of the rail tread weld center of the naturally cooled rail joint is measured with an infrared thermometer after the flash butt welding. When the temperature dropped to 525°C, the two 200mm with zones respectively on both sides of the joint bond line are insulated immediately by an aluminosilicate fiber cotton thermal insulation device with a thickness of > 30mm, during which the temperature of joint is continuously monitored. When the temperature at the tread weld center is lower than 250°C, the device could be removed. Finally, the joint is naturally cooled in air to room temperature.

[00036] The joint obtained by the above welding process and post-weld treatment method is inspected and analyzed according to EN 14587.2. It is found that the microstructure of the rail head and rail foot on the center surface of the joint is pearlite, and no harmful microstructure such as martensite is observed. Except for the softening zone, the joint hardness is in the range of the average of the base metal -30HV to +60HV. Based on continuous three-point slow bend test at 2000kN, the maximum deflection is 27.1 mm. After the three-point support fatigue test for five million times, the rail is not broken and all indexes met the requirements of EN standard.

Embodiment 4

[00037] Flash butt welding process with welding heat-input of 8.4 MJ and upset rail-used of 15.0 mm is adopted to treat European standard R350HT heat-treated rail with Manganese mass fraction of 1.08% (Carbon content is at the middle and lower limit, Si < 0.30wt%, Rm > 1175MPa, A > 9%, and rail head running surface hardness > 350HB).

After the flash butt welding, first of all, the temperature of the rail tread weld center of

the naturally cooled rail joint is measured with an infrared thermometer after the flash

butt welding. When the temperature dropped to 540°C, the two 200mm wide zones

respectively on both sides of the joint bond line are insulated immediately by an

aluminosilicate fiber cotton thermal insulation device with a thickness of > 30mm, during which the temperature of joint is continuously monitored. When the temperature

at the tread weld center is lower than 250°C, the device could be removed. Finally, the

joint is naturally cooled in air to room temperature.

[00038] The joint obtained by the above welding process and post-welding treatment method is inspected and analyzed according to EN 14587.2. It is found that the

microstructure of the rail head and rail foot on the center surface of the joint is pearlite,

and no harmful microstructure such as martensite is observed. Except for the softening

zone, the joint hardness is in the range of the average of the base metal -30HV to +60HV.

Based on continuous three-point slow bend test at 2000kN, the maximum deflection is

27.2 mm. After the three-point support fatigue test for five million times, the rail is not

broken and all indexes met the requirements of EN standard.

[00039] The reference to any prior art in this specification is not, and should not be taken

as, an acknowledgement or any form of suggestion that such prior art forms part of the

common general knowledge.

[00040] It will be understood that the terms "comprise" and "include" and any of their derivatives (e.g. comprises, comprising, includes, including) as used in this specification,

and the claims that follow, is to be taken to be inclusive of features to which the term

refers, and is not meant to exclude the presence of any additional features unless

otherwise stated or implied.

[00041] In some cases, a single embodiment may, for succinctness and/or to assist in

understanding the scope of the disclosure, combine multiple features. It is to be

understood that in such a case, these multiple features may be provided separately (in separate embodiments), or in any other suitable combination. Alternatively, where separate features are described in separate embodiments, these separate features may be combined into a single embodiment unless otherwise stated or implied. This also applies to the claims which can be recombined in any combination. That is a claim may be amended to include a feature defined in any other claim. Further a phrase referring to "at least one of' a list of items refers to any combination of those items, including single members. As an example, "at least one of: a, b, or c" is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[00042] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application or applications described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.

Claims (4)

Claims

1. A process for controlling a microstructure of a flash butt welded joint of R350HT

rail with a medium-upper limit of Manganese content, comprising a welding process and a

post-weld treatment, wherein, the Manganese content of R350HT rail base metal is 1.05-1.15

wt%; in the welding process, the pulsating or preheating flash butt welding with a welding

heat-input of 7.0-8.5 MJ is controlled and a welding upset rail-used is 14.5-15.5 mm;

in the post-weld treatment, the flash butt welded joint is naturally cooled to enable the

rail tread weld center to reach 525-560°C, then cooled under heat insulation to enable the rail

tread weld center to be not more than 250 °C, and naturally cooled to room temperature; and

Carbon content of the R350HT rail base metal is 0.72-0.78 wt%, and Silicon content is <

0.30 wt%.

2. The process for controlling a microstructure of a flash butt welded joint of R350HT

rail with a medium-upper limit of Manganese content according to Claim 1, wherein the weld

center is a zone with 150 - 200 mm wide respectively on both sides of a joint bond line.

3. The process for controlling a microstructure of a flash butt welded joint of R350HT

rail with a medium-upper limit of Manganese content according to Claim 1, wherein the

cooling under heat insulation is achieved by the thermal insulation material aluminosilicate

fiber cotton with a thickness of>30mm.

4. The process for controlling a microstructure of a flash butt welded joint of R350HT

rail with a medium-upper limit of Manganese content according to any one of Claims 1 to 3,

wherein the R350HT rail base metal is produced by a universal rolling line and an on-line

heat treatment line, and the mechanical properties thereof meet the conditions below: The

average tensile strength (Rm) > 1175MPa, average reduction of area (A) > 9%, and average

rail head running surface hardness > 350HB.