AU2019268045B2

AU2019268045B2 - A bainitic rail welded joint and post-welding heat treatment method to control “white block” structure thereof

Info

Publication number: AU2019268045B2
Application number: AU2019268045A
Authority: AU
Inventors: Jian Deng; Dadong Li; Ruoyu WANG; Feixiang Xu
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2018-11-20
Filing date: 2019-11-18
Publication date: 2021-08-05
Anticipated expiration: 2039-11-18
Also published as: CN109207708B; AU2019268045A1; CN109207708A

Abstract

The invention provides a bainitic rail welded joint, comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic 5 metallography; wherein the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field. The invention adopts a specific post-welding heat treatment method for bainitic rail welded joint and a three-stage cooling method with specific process to produce bainitic rail welded joint with smaller and less white block defect. With the specific post-welding 10 heat treatment method, the invention can significantly reduce the number of "white block" defects in the microstructure of welded joint, avoiding crack extension at welded joint caused by defects of welding area and contributing to improvement of mechanical property of bainitic rail welded joint, and further alleviating "saddle" abrasion and wheel-rail impact caused by low hardness of rail welding area during 15 railway line operation to extend the service life of rail and ensure the safety of railway operation

Description

A Bainitic Rail Welded Joint and Post-Welding Heat Treatment Method to Control "White Block" Structure Thereof

Related Application

[0001] This application claims priority from Chinese Application No. 201811382920.9 filed on 20 November 2018, the contents of which are to be taken as incorporated herein by this reference.

Field of the Invention

[0002] Embodiments of the invention belong to the technical field of bainitic rail welding, and relates to a bainitic rail welded joint and post-welding heat treatment method therefor, in particular to a bainitic rail welded joint and post-welding heat treatment method to control "white block" structure thereof.

Background

[0003] When austenite is supercooled to a temperature lower than pearlite transition temperature and higher than martensite transition temperature, a transformation from shear phase change to short-range diffusion occurs, and the transformation product is called bainite. The bainite in steel is a mixed structure of ferrite and carbide.

[0004] Over the recent decades, the bainitic rail has been a hot research in countries around the globe. It is expected to replace the traditional pearlite rail due to its high tenacity, wear resistance and long service life, and widely used for railway switch parts and small-radius curve sections of heavy load lines. Currently, seamless rail has become an irresistible trend. As a key procedure of rail seamless process, rail welding quality is directly related to the service life of railway lines and even to train operation safety. During service of rail, long welded rail mainly fractures at the welded joints due to low welding quality and complexity of actual operation conditions of lines, thus welded joints become a vulnerable part of seamless line. Seamless lines are paved by such rails through welding. Due to high alloy content in the bainitic rail, Mn segregation zone is likely to occur in the bainitic rail web and other parts, causing martensite banded structure. In particular, abnormal bright white defects, commonly known as "white block" in the industry, are often found near the weld seam in flash welding process, which brings potential harm to the safe operation of railway. Such bright white defects have complicated microstructure, and are the result of element gathering on the grain boundary of the welding semi-molten area. The bright white color is caused by element gathering and structure difference. The "white block" is actually a high alloy area on the grain boundary with element gathering and a martensitic structure produced under welding air-cooling condition by melting or liquefaction on grain boundary of high alloy area under high temperature of rail welding. The white color results from corrosion caused by the corrosive resistant to metallographic phase. Banded segregation will influence the impact tenacity of joints and the "white block" is commonly seen in the banded structures. In addition, due to the melting process and high temperature, after the bainitic rail produced based on the principle of fine grain strengthening is subjected to the welding heat cycle, and the hardening layer in the weld area disappears and a wide low-hardness area appears. Austenitic grain coarsening in the overheating area of rail welding results in the hardness of weld seam and heat affected area being much lower than that of the rail base material. "Saddle" abrasion is likely to occur firstly on the tread during service of softened rail welded joint, which increases the wheel-rail impact, reduces the service life of rail and even endangers train operation safety. Therefore, as specified by the current railway industry standards in China, TB/T1632.2-2014, Welding of Rails Part 2: Flash Welding, and TB/T1632.3-2014, Welding of Rails - Part 3: Thermit Welding, average hardness of the welding area of heat-treated rail should not be lower than 90% of that of rail base material, and hazardous structures such as martensite or bainite should not appear in the microstructures of weld seam and heat affected area. The above two standards relating to welding of rails are targeted to pearlite rail. Considering that there are no national or international standards for welding of bainitic rail, it is not appropriate to assess the mechanical property of bainitic rail welded joint in full accordance with the current national standards for welding of rails.

[0005] More importantly, more "white block" structures with bigger size exist in the microstructure of the welded joint generated via the prior art of bainitic rail welding. Micro-crack is likely to occur on such "white block", which is exactly a main reason for damage of the bainitic rail welded joint. The crack extends along the direction of banded structure of welded joint to form railhead transverse crack. Being a major cause for rail fracture, the railhead transverse crack can further reduce the service life of the rail.

[0006] Therefore, it has become one of the urgent problems in the field of railway engineering to find a way to control the "white block" near the bainitic rail welded joint and further to improve the service performance of bainitic rail welded joint and ensure railway operation safety.

[0007] A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of the Invention

[0008] Embodiments of the invention may provide a bainitic rail welded joint and post-welding heat treatment method therefor, in particular a post-welding heat treatment method to control the "white block" structure at the bainitic rail welded joint. The post-welding heat treatment process provided by the invention may be used to effectively control the "white block" near the bainitic rail welded joint by significantly reduce the number and size of the "white block", further to improve the service performance of the bainitic rail welded joint and ensure railway operation safety. Besides, the process maybe simply implemented under mild condition and can be easily controlled, making it suitable for promotion in and application for large-scale production.

[0009] According to one aspect of the invention, there is provided a post-welding heat treatment method for bainitic rail welded joint, comprising the steps of:

1) implementing first-stage cooling for a to-be-cooled bainitic rail welded joint obtained from welding to obtain the welded joint after the first-stage cooling;

the first-stage cooling being slow cooling;

the first-stage cooling having a cooling rate of substantially 0.8 1.4°C/min;

2) heating the welded joint after the first-stage cooling obtained in the above step, and implementing second-stage cooling to obtain the welded joint after the second-stage cooling;

the second-stage cooling being quick cooling;

the second-stage cooling having a cooling rate of substantially 0.9 2.1°C/s

3) implementing third-stage cooling for the welded joint after the second stage cooling obtained in the above step to obtain bainitic rail welded joint;

the third-stage cooling being natural cooling;

the third-stage cooling having a cooling rate of substantially 0.3-0.9°C/s;

the bainitic rail welded joint comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography;

wherein, the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field.

[0010] Preferably, the temperature of the to-be-cooled bainitic rail welded joint obtained from the welding is 1400-1600°C; and wherein the step of heating is heating for the entire section.

[0011] Preferably, the initial cooling temperature of the second-stage cooling is higher than or equal to 800°C; and wherein the step heating provides a heating rate of substantially 6.2-10.80C/s.

[0012] Preferably, during the heating, the temperature difference between a rail head and a rail bottom associated with the welded joint is controlled substantially within 50°C;

the temperature of the welded joint after the first-stage cooling is lower than or equal to 290°C;

the temperature after the heating is lower than or equal to 970°C; the temperature of the welded joint after the second-stage cooling is lower than or equal to 2800C.

[0013] Preferably, the temperature of the welded joint after the first-stage cooling is substantially 150-280OC;

the temperature after the heating is substantially 880-960OC;

the temperature of the welded joint after the second-stage cooling is substantially 180-270OC;

the first-stage cooling is indirect cooling; and

the third-stage cooling is direct cooling.

[0014] Preferably, the first-stage cooling is in particular to achieve slow cooling at a cooling speed controlled by a heat insulation device;

the heating comprises heating by medium-frequency induction simulation electric heating coil and/or heating by oxy-acetylene flame simulation heater;

the second-stage cooling is quick cooling by ejecting one or more of compressed air, mist and water-mist mixture as the cooling medium.

[0015] According to another aspect of the invention, there is provided a bainitic rail welded joint, comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography;

wherein, the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field; and wherein

5a the welded joint is obtained from heat treatment of the to-be-cooled bainitic rail welded joint obtained from welding; and

the heat treatment is based on the method as described herein.

[0016] Preferably, the size of the white block structure is smaller than or equal to 2000pm;

the width of the weld seam is substantially 101-772pm;

the white block structures are independent of each other.

[0017] Preferably, the weld seam structure is bright white decarburized layer;

the weld seam structure comprises ferritic structure and bainitic structure;

the white block structure has irregular shape;

the heat treatment comprises three-stage cooling steps.

[0018] Preferably, the white block structure distributed in and around the weld seam is in particular the white micro-area irregularly distributed in areas near the weld seam;

the welding comprises flash welding and/or gas pressure welding;

the heat treatment comprises first-stage cooling, heating, second-stage cooling and third-stage cooling.

[0019] Preferably, the heat treatment is implemented according to the abovementioned post-welding heat treatment method for bainitic rail welded joint.

[0020] There is also disclosed herein a bainitic rail welded joint, comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography; wherein the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field. Compared with the prior art, embodiments of the invention may be targeted at the defects of the prior art of bainitic rail welding, by which the welded joint produced contains microstructure with a large number of "white block" structures and is prone to micro-crack. Such micro-crack extends along the direction of banded structure of welded joint to form railhead transverse crack, thus causing rail fracture and reducing its service life.

[0021] Embodiments of the invention may creatively adopt a specific post-welding heat treatment method for bainitic rail welded joint and a three-stage cooling method with specific process to produce bainitic rail welded joint with smaller and less white block defect. With the post-welding heat treatment method to control the "white block" structure near the bainitic rail welded joint, embodiments of the invention may significantly reduce the number of "white block" defects in the microstructure of bainitic rail welded joint, avoiding crack extension at welded joint caused by defects of welding area and contributing to improvement of mechanical property of bainitic rail welded joint, and further alleviating "saddle" abrasion and wheel-rail impact caused by low hardness of rail welding area during railway line operation to extend the service life of rail and ensure the safety of railway operation.

[0022] To remedy the defects of the prior art of bainitic rail welding, embodiments of the invention may provide a post-welding heat treatment method to control the "white block" structure near the bainitic rail welded joint. Based on the high sensitivity of bainitic rail to temperature and the direct influence of each cooling process of the joint on the mechanical properties of bainitic rail welded joint, embodiments of the invention, on the premise of temperature control, may provide a post-welding heat treatment method for the whole process of rail welded joint from the beginning of welding to the final cooling to room temperature, which can effectively control the number of bright white defects near the bainitic rail welded joint, so as to ensure the safety of railway operation.

[0023] The post-welding heat treatment method for bainite rail welded joint may effectively overcome the defect of post-welding heat treatment method for bainite rail welded joint in the prior art that it can only reduce the martensitic structure content in the bainitic rail welded joint. The post-welding heat treatment method described herein may improve the mechanical properties of joints reduced by bainitic rail welding, and may achieve the purpose of reducing the "white block" structure in the micro-area under different conditions by controlling different post-welding heat treatment process parameters during the welding process.

[0024] As shown by experiment results, the post-welding heat treatment method may effectively control the content of abnormal "white block" structure in the bainitic rail welded joint. The physical fatigue life of bainitic rail welded joint is not less than 2 million times, as stipulated by TB1632, and the longitudinal hardness under the tread of forced-air-cooled area at the weld seam of bainitic rail welded joint is 85%-90% of the average hardness of rail base material.

Description of the Drawings

[0025] Fig. 1 is the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in example 1 of an embodiment of the invention;

[0026] Fig. 2 is the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in example 2 of an embodiment of the invention;

[0027] Fig. 3 is the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in comparative example 1 of an embodiment of the invention;

[0028] Fig. 4 is the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in comparative example 2 of an embodiment of the invention.

Detailed Description of the Preferred Examples

[0029] In order to better understand the invention, the preferred examples of the invention will be further described in combination with the examples. However, it should be understood that such description is only to further illustrate the features and advantages of embodiments the invention, but not intended to limit the claims.

[0030] There is no specific restriction for sources of all the raw materials. Any raw material purchased in the market or prepared according to the conventional method well known by those skilled in the art maybe acceptable.

[0031] There is no specific restriction for purity of all the raw materials, and those compliant with industrial purity requirement or conventional purity requirement for the technical field of bainitic rail welding are preferred.

[0032] The abbreviations of the processes and products used are the conventional abbreviations in the art. Detailed steps and conventional parameters of each abbreviation are clear and definite in their relevant art and can be achieved by those skilled in the art through conventional methods according to the abbreviations.

[0033] There is provided a bainitic rail welded joint, comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography; wherein, the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field.

[0034] There is no specific restriction for definition and composition of the bainitic rail. Any definition and composition of the bainitic rail well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the definition and composition based on particular application as well as product and quality requirements. The chemical composition of the bainitic rail may be: C: 0.18-0.30 wt%, Si: 0.8-1.8 wt%; Mn: 1.5-2.5 wt%; Cr: 0.50-1.60 wt%; Mo: 0.20-0.50 wt%. Wherein, C content can also be 0.20-0.28 wt% or 0.22-0.25 wt%. Si content can also be 1.0 1.6 wt% or 1.2-1.4 wt%. Mn content can also be 1.7-2.3 wt% or 1.9-2.1 wt%. Cr content can also be 0.70-1.40 wt% or 0.90-1.20 wt%. Mo content can also be 0.25 0.45 wt% or 0.30-0.40 wt%.

[0035] There is no specific restriction for definition of the microscopic metallography. Any microscopic metallographic structure of the metal material well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the microscopic metallography based on particular application as well as product and quality requirements. The microscopic metallography may be reflected by the microscopic metallographic pictures. There is no specific restriction for definition of the white block structure. Any white block defect of the bainitic rail well known by those skilled in the art is suitable. Those skilled in the art can select the white block defect based on particular application as well as product and quality requirements. The white block refers to the bright white defect that, after being corroded by 3% nitric acid alcoholic solution, presents white block structure brighter than martensitic structure.

[0036] In principle, there is no specific restriction for composition and size of the weld seam structure. Those skilled in the art can select and adjust the composition and size based on particular application as well as product and quality requirements. The weld seam structure is a bright white decarburized layer and preferably comprises ferritic structure and bainitic structure. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may control the number and size of "white block" structure in the micro-area under different conditions. The width of the weld is preferably 101-772pm, more preferably 201 672pm, more preferably 301-572pm and more preferably 401-472pm. The "welded joint" at the macro level refers to an area with a width of 80-120mm (85-115mm or 90-110mm) with weld seam and/or normalizing-affected area included. The center of the area is the rail weld seam.

[0037] In principle, there is no specific restriction for the number of white block structure. Those skilled in the art can select and adjust the number based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be controlled. The number of white blocks with a size greater than or equal to 100pm is less than 10/ single microscopic metallographic field. The single microscopic metallographic field refers to the single optical metallographic field in a metallographic microscope system, i.e., the area of the field directly observed one time by the eyepiece with a metallographic microscope. In a single optical metallographic field of the metallographic microscope system, the number of white block structure with a size greater than or equal to 100pm can be less than 10. Wherein, the size of white block structure is preferably greater than or equal to 150pm, and more preferably greater than or equal to 200pm; the number of white block structure is more preferably less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 5.

[0038] There is no specific restriction for particular multiple of the single microscopic metallographic field. Any conventional multiple of conventional microscopic metallographic picture of such welded joint well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the multiple based on particular application as well as product and quality requirements. The endpoint value of the specific multiple of the microscopic metallographic field is subject to the white block structure with clear indication of 100pm. Too high or low endpoint value can cause the 100pm white block structure to be indicated unclearly or incompletely.

[0039] In principle, there is no specific restriction for size of the white block structure. Those skilled in the art can select and adjust the size based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may control the number and size of "white block" structure in the micro-area under different conditions. The size of the white block structure is preferably smaller than or equal to 2000pm, more preferably smaller than or equal to 1000pm, more preferably smaller than or equal to 500pm, more preferably smaller than or equal to

200pm, more preferably smaller than or equal to 100pm, and specifically smaller than or equal to 0.1-1OOpm.

[0040] In principle, there is no specific restriction for the shape of white block structure. Those skilled in the art can select and adjust the shape based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the embodiments of the invention may control the number and size of "white block" structure in the micro area under different conditions. The white block structures are independent of each other. The white block structure has irregular shape. The white block structure distributed in and around the weld seam can be specifically the white micro-area irregularly distributed in areas near the weld seam.

[0041] In principle, there is no specific restriction for preparation forms of the welded joints. Any preparation form for bainitic rail welded joint well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the preparation form based on particular application as well as product and quality requirements. The welding preferably comprises flash welding and/or gas pressure welding, more preferably flash welding or gas pressure welding. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may control the number and size of "white block" structure in the micro-area under different conditions. The welded joint is particularly obtained from heat treatment for the to-be-cooled bainitic rail welded joint obtained from welding. The heat treatment preferably comprises three-stage cooling steps. Wherein, the heat treatment particularly and preferably comprises first-stage cooling, heating, second stage cooling and third-stage cooling.

[0042] Considering that the forced-air cooling process will directly influence the recovery of longitudinal hardness under the tread of welded joint during the post welding normalizing heat treatment for the rail joint, it was discovered through study that excessively high forced-air cooling intensity will cause fatigue damage and even fracture of the welded joint due to inferior tenacity and plasticity; excessively low forced-air cooling intensity will cause sunken tread at the welded joint due to the hardness of the welding area lower than 90% of the average hardness of base material, thus influencing smooth operation of the line, even train operation safety.

[0043] Therefore, starting with processes and parameters of heat treatment, creative selection and combination has been made, and a post-welding heat treatment method for bainitic rail welded joint, comprising the following steps is provided:

1) implementing first-stage cooling for the to-be-cooled bainitic rail welded joint obtained from welding, as so to obtain the welded joint after the first-stage cooling;

3) implementing third-stage cooling for the welded joint after the second stage cooling obtained in the above step to obtain bainitic rail welded joint.

[0044] The composition, shape, size, status selection and proportion of the materials used in the post-welding heat treatment method for the abovementioned bainitic rail welded joint and the relevant preference principles are corresponding to those used for the aforementioned bainitic rail welded joint and relevant preference principles. Thus they will not be described herein.

[0045] The to-be-cooled bainitic rail welded joint obtained from welding is firstly subject to the first-stage cooling, as so to obtain the welded joint after the first-stage cooling;

[0046] There is no specific restriction for initial temperature of the to-be-cooled bainitic rail welded joint. Any temperature before the conventional heat treatment for the to-be-cooled bainitic rail welded joint well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the temperature based on particular production as well as product and quality requirements. The temperature of the to-be-cooled bainitic rail welded joint obtained from welding is preferably 1400 1600°C, more preferably 1420-1580°C, more preferably 1450-1550°C, and more preferably 1470-1530°C.

[0047] In principle, there is no specific restriction for the form and method of the first-stage cooling. Those skilled in the art can select and adjust the form and method based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may reduce the number and size of "white block" structure in the micro-area under different conditions. The first-stage cooling is preferably slow cooling. The first-stage cooling is preferably indirect cooling. Slow cooling can be carried out at a cooling speed controlled by an insulation device. Specifically, the bainitic rail welded joint with higher temperature is indirectly cooled, and the cooling speed for welded joint at a higher temperature after heat treatment is controlled by a special insulation device. More specifically, the time taken for the bainitic rail welded joint with a higher residual temperature to be cooled to the temperature after the first-stage cooling is retarded by an asbestos-containing insulating cover.

[0048] In principle, there is no specific restriction for the detailed parameters of the first-stage cooling. Those skilled in the art can select and adjust the parameter based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The cooling rate of the first-stage cooling is preferably 0.8 1.4°C/min, more preferably 0.9-1.3°C/min, and more preferably 1.0-1.2°C/min. The temperature of the welded joint after the first-stage cooling is preferably lower than or equal to 2900C, more preferably 150-280°C, more preferably 180-250°C, and more preferably 200-2300 C.

[0049] The welded joint after the first-stage cooling obtained in the above step is heated and then subject to the second-stage cooling to obtain the welded joint after the second-stage cooling;

[0050] In principle, there is no specific restriction for the form and method of heating. Those skilled in the art can select and adjust the form and method based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may reduce the number and size of "white block" structure in the micro area under different conditions. The heating is preferably heating for the entire section. The heating form preferably comprises heating by medium-frequency induction simulation electric heating coil and/or heating by oxy-acetylene flame simulation heater, and more preferably heating by medium-frequency induction simulation electric heating coil or heating by oxy-acetylene flame simulation heater. During heating, the temperature difference between the railhead and the rail bottom is controlled within 50°C, more preferably within 400C, and more preferably within 300C. The heating for the entire section refers to heating for the entire section of the rail welded joint with a length of around 80-120mm, including weld seam.

[0051] In principle, there is no specific restriction for the detailed parameters of the heating. Those skilled in the art can select and adjust the parameters based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The heating rate of the heating is preferably 6.2-10.80C/s, more preferably 7.2-9.80C/s, and more preferably 8.2-8.80C/s. The temperature of the welded joint after the heating is preferably lower than or equal to 9700C, more preferably 880 9600C, more preferably 890-9500C, more preferably 900-9400C, and more preferably 910-9300C.

[0052] Normalizing heat treatment usually refers to the heating process to heat metal workpiece with conventional methods to 30-500C above Ac3 (the finishing temperature when ferrite turns into austenite during heating), after a period of time, take out the metal workpiece and implement natural cooling or cooling by spray or ejected compressed air. The post-welding normalizing heat treatment for rail welded join is different from the heat treatment for small workpiece. The length of specimen can be up to hundreds of meters after rail welding, so the target temperature cannot be maintained for a long time (the temperature above austenization temperature) in the normalizing heat treatment for rail welded joint. Therefore, a temperature higher than conventional normalizing heat treatment is employed to heat the rail welded joint to the target temperature, and then the heat treatment by air-cooling or forced-air cooling is carried out. The temperature of the heating, i.e., the normalizing heating is preferably 880-9600C.

[0053] In principle, there is no specific restriction for the initial temperature of the second-stage cooling for the bainitic rail welded joint. Those skilled in the art can select and adjust the initial temperature based on particular production as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The initial cooling temperature of the second-stage cooling is preferably higher than or equal to 8000C, more preferably higher than or equal to 8200C, and more preferably higher than or equal to 8400C.

[0054] In principle, there is no specific restriction for the form and method of the second-stage cooling. Those skilled in the art can select and adjust the form and method based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The second-stage cooling is preferably quick cooling. The second-stage cooling is preferably quick cooling with one or more of the ejected compressed air, mist and water-mist mixture as the cooling medium, and specifically the quick cooling with one of the ejected compressed air, mist and water-mist mixture as the cooling medium.

[0055] The welded joint after the heating is subject to the second-stage cooling to obtain the welded joint after the second-stage cooling. In addition to the normalizing heating area, the cooling area preferably includes the railhead tread and the sides of the rail within a length range of 80mm (more preferably 60mm, more preferably 50mm) on the both sides outside the heating area.

[0056] In principle, there is no specific restriction for the detailed parameters of the second-stage cooling. Those skilled in the art can select and adjust the parameters based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The cooling rate of the second-stage cooling is preferably 0.9 2.1°C/s, more preferably 1.1-1.90C/s, and more preferably 1.3-1.70C/s. The temperature of the welded joint after the second-stage cooling is preferably lower than or equal to 2800C, more preferably 180-2700C, more preferably 200-2500C, and more preferably 220-2300C.

[0057] The welded joint after the second-stage cooling obtained in the above step is finally subject to the third-stage cooling to obtain bainitic rail welded joint.

[0058] In principle, there is no specific restriction for the form and method of the third-stage cooling. Those skilled in the art can select and adjust the form and method based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, embodiments of the invention may reduce the number and size of "white block" structure in the micro-area under different conditions. The third-stage cooling is preferably natural cooling. The third-stage cooling is preferably direct cooling, and can be specifically direct natural cooling in the air to room temperature.

[0059] There is no specific restriction for definition of the room temperature. Any conventional definition for room temperature well known by those skilled in the art is acceptable. Those skilled in the art can select and adjust the definition based on particular production as well as product and quality requirements. The room temperature is preferably 5-40°C, more preferably 10-35°C, more preferably 15 30°C, and more preferably 20-25°C.

[0060] In principle, there is no specific restriction for the detailed parameters of the third-stage cooling. Those skilled in the art can select and adjust the parameters based on particular application as well as product and quality requirements. To better improve the mechanical properties of joints reduced by bainitic rail welding, the number and size of "white block" structure in the micro-area under different conditions may be reduced. The cooling rate of the third-stage cooling is preferably 0.3-0.9°C/s, more preferably 0.4-0.8°C/s, and more preferably 0.5-0.7°C/s.

[0061] The cooling method for the first-stage cooling, the second-stage cooling and the third-stage cooling may be at least one of the air-cooling, forced-air cooling and other cooling methods. To improve the relevant mechanical properties of the welded joint, embodiments of the invention preferably employs slow cooling for the first-stage cooling, forced-air cooling for the second-stage cooling, and air cooling for the third-stage cooling. Wherein, the first-stage cooling is indirect slow cooling by using relevant media, and the third-stage cooling is direct natural cooling for the bainitic rail welded joint obtained with relatively high residual temperature.

[0062] To remedy the defects of the prior art of bainitic rail welding, in the above steps, there may be provided a bainitic rail welded joint and post-welding heat treatment method to control "white block" structure thereof. Based on the high sensitivity of bainitic rail to temperature and the direct influence of each cooling process of the joint on the mechanical properties of bainitic rail welded joint, embodiments of the invention, on the premise of temperature control, may provide a post-welding heat treatment method for the whole process of rail welded joint from the beginning of welding to the final cooling to room temperature. The method can effectively control the number of bright white defects near the bainitic rail welded joint, so as to ensure the safety of railway operation. The detailed steps can be: implementing the first-stage cooling for the bainitic rail welded joint with relatively high residual temperature obtained after welding to a first temperature, heating the welded joint obtained from the first-stage cooling to a certain temperature, and implementing the second-stage cooling and stopping at a second temperature, and then immediately implementing the third-stage cooling to room temperature, wherein the first temperature is not higher than 2900C, the certain temperature is higher than the first temperature but not higher than 9700C, the second temperature is not higher than 2800C, the first-stage cooling is slow cooling, the second-stage cooling is quick cooling by using media, the third-stage cooling is natural cooling in the air, and the initial cooling temperature of the second-stage cooling is controlled above 8000C of the welded joint heated from the first temperature to the second temperature.

[0063] Embodiments of invention has made creative improvement for the post welding heat treatment for the bainitic rail welded joint and adopts a three-stage cooling method with specific process and the combination of slow cooling, heating for the entire section, quick cooling and natural cooling under the combination of specific temperature and cooling rate to produce bainitic tail welded joint with smaller and less white block defect. With the post-welding heat treatment method to control the "white block" structure near the bainitic rail welded joint, embodiments of the invention may significantly reduce the number of "white block" defects in the microstructure of bainitic rail welded joint, avoiding crack extension at welded joint caused by defects of welding area and contributing to improvement of mechanical property of bainitic rail welded joint, and further alleviating "saddle" abrasion and wheel-rail impact caused by low hardness of rail welding area during railway line operation to extend the service life of rail and ensure the safety of railway operation.

[0064] Based on the high sensitivity of bainitic rail to temperature and the direct influence of each cooling process of the joint on the mechanical properties of bainitic rail welded joint, embodiments of the invention, on the premise of temperature control, may provide a post-welding heat treatment method for the whole process of rail welded joint from the beginning of welding to the final cooling to room temperature, which can effectively control the number of bright white defects near the bainitic rail welded joint, improve the composition of the structures in the bainitic rail welded joint and extend the physical fatigue life of the rail welded joint, so as to avoid sunken tread at the welded joint due to the low hardness of the welding area, extend the service life of rail and ensure the safety of railway operation.

[0065] The post-welding heat treatment method for bainite rail welded joint effectively overcomes the defect of post-welding heat treatment method for bainite rail welded joint in the prior art that it can only reduce the martensitic structure content in the bainitic rail welded joint. The post-welding heat treatment method can improve the mechanical properties of joints reduced by bainitic rail welding, and can achieve the purpose of reducing the "white block" structure in the micro-area under different conditions by controlling different post-welding heat treatment process parameters during the welding process.

[0066] As shown by experiment results, the post-welding heat treatment method can effectively control the content of abnormal "white block" structure in the bainitic rail welded joint. The fatigue life of bainitic rail welded joint is not less than 2 million times, as stipulated by TB1632, and the longitudinal hardness under the tread of forced-air-cooled area at the weld seam of bainitic rail welded joint is 85%-90% of the average hardness of rail base material.

[0067] The following will further describe the bainitic rail welded joint and post welding heat treatment therefor in combination with the examples. But it should be understood that the examples are to be carried out on the premise of the technical scheme of embodiments the invention, and that the detailed examples and the specific operation process provided herein are only to further describe the features and advantages of embodiments of invention, but not to limit the claims and the scope of protection of the invention.

[0068] The raw and auxiliary materials used in the following examples are all commercially available. Wherein, the bainitic rail is PB2 heat-treated bainitic rail produced by Pangang Group.

Example 1

[0069] The bainitic rail welded joint with relatively high residual temperature obtained from flash welding is subject to slow cooling and, when the welded joint is cooled from 15500C to 2000C, (the cooling rate is 0.8-1.4C/min), the rail welded joint area is subject to heating (the heating rate is 6.2-10.8°C/s) for the entire section by medium-frequency induction simulation electric heating coil. Heating is stopped when the temperature of rail tread reaches 9500C, and then the rail welded joint obtained therefrom is immediately subject to forced-air cooling to 2300C (the cooling rate is 0.9-2.1°C/s) and then air-cooling to room temperature (around 230C, the cooling rate is 0.3-0.9°C/s) to obtain the bainitic rail welded joint after post-welding heat treatment.

[0070] Representation is made to the bainitic rail welded joint obtained in example 1.

[0071] The bainitic rail welded joint obtained in the example is machined into metallographic specimen and then observed under microscope for its morphology of microstructure. The microstructure distribution is shown as Fig. 1.

[0072] Refer to Fig. 1, the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in example 1. Wherein, 1 is the weld seam.

[0073] Detection is made to the bainitic rail welded joint obtained in example 1.

[0074] By testing as per TB1632 standards, the physical fatigue life of the bainitic rail welded joint in example 1 is not less than 2 million times. The longitudinal hardness under the tread of forced-air-cooled area of weld seam of bainitic rail joint reaches 85%-90% of the average hardness of the rail base material, which is extremely close to the average hardness of the rail base material. Therefore, fracture does not occur on the rail welding specimen during the process of physical fatigue.

Example 2

[0075] The bainitic rail welded joint with relatively high residual temperature obtained from flash welding is subject to slow cooling and, when the welded joint is cooled from 1530°C to 1800C, (the cooling rate is 0.8-1.4C/min), the rail welded joint area is subject to heating (the heating rate is 6.2-10.8C/s) for the entire section by oxy-acetylene flame simulation heater. Heating is stopped when the temperature of rail tread reaches 9400C, and then the rail welded joint is subject to forced-air cooling to 2400C (the cooling rate is 0.9-2.1OC/s) and then air-cooling to room temperature (around 230C, the cooling rate is 0.3-0.90C/s)) to obtain the bainitic rail welded joint after post-welding heat treatment.

[0076] Representation is made to the bainitic rail welded joint obtained from example 2.

[0077] The bainitic rail welded joint obtained from the example is machined into metallographic specimen and then observed under microscope for its morphology of microstructure. The structure distribution results are similar to those indicated in example 1. The microstructure distribution is shown as Fig. 2.

[0078] Refer to Fig. 2, the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in example 2.

[0079] Detection is made to the bainitic rail welded joint obtained in Example 2.

[0080] By testing as per TB1632 standards, the physical fatigue life of the bainitic rail welded joint in example 2 is not less than 2 million times. The longitudinal hardness under the tread of forced-air-cooled area of weld seam of bainitic rail joint reaches 85%-90% of the average hardness of the rail base material, which is extremely close to the average hardness of the rail base material. Therefore, similarly, fracture does not occur on the rail welding specimen during the process of physical fatigue.

Comparative example 1

[0081] The bainitic rail welded joint is subject to post-welding heat treatment according the method of example 1, but unlike example 1, forced-air cooling is stopped when the welded joint is cooled from 9500 C to 1600C, and then the welded joint is air-cooled to room temperature (around 230C).

[0082] Representation is made to the bainitic rail welded joint obtained from comparative example 1.

[0083] The bainitic rail welded joint obtained from the comparative example is machined into metallographic specimen and then observed under microscope for its morphology of microstructure. The microstructure distribution is shown as Fig. 3.

[0084] Refer to Fig. 3, the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in comparative example 1.

[0085] Detection is made to the bainitic rail welded joint obtained from comparative example 1.

[0086] As proved by the testing, the average hardness range of the welded joint is 35.6-40.3HRC, which is 10% higher than that of the welding area. For bainitic rail welded joint, the one with an average hardness closer to that of rail base material is better, and an average hardness higher than that of the rail base material can cause more harm. Therefore, the rail welding specimen fractures during the process of physical fatigue.

Comparative example 2

[0087] The bainitic rail welded joint with relatively high residual temperature obtained from flash welding is subject to air-cooling and, when the welded joint is cooled from 15600C to 2000C, the rail welded joint area is subject to heating for the entire section by medium-frequency induction simulation electric heating coil. Heating is stopped when the temperature of rail tread reaches 9400C, and then the rail welded joint is subject to forced-air cooling to 1600C to obtain the bainitic rail welded joint after post-welding normalizing.

[0088] Representation is made to the bainitic rail welded joint obtained from comparative example 2.

[0089] The bainitic rail welded joint obtained from the comparative example is machined into metallographic specimen and then observed under microscope for its morphology of microstructure. The microstructure distribution is shown as Fig. 4.

[0090] Refer to Fig. 4, the microstructure picture under metallographic microscope of the bainitic rail welded joint obtained in comparative example 2. Wherein, 2 is the "white block".

[0091] As shown in Fig. 3 and Fig. 4, for the bainitic rail welded joint with relatively high residual temperature not treated by the post-welding heat treatment method, the metallographic specimen obtained therefrom shows significantly more "white block" structures and the martensite segregation band is obvious.

[0092] It can be seen by comparing the metallography of each position of the welded joints in Fig. 1, Fig. 2, Fig. 3 and Fig. 4 that the content of "white block" defect can be significantly controlled by the post-welding heat treatment for the bainitic rail welded joint according to the process, and the physical fatigue life of the bainitic rail welded joint can be extended, thus ensuring safety of railway operation.

[0093] Detection is made to the bainitic rail welded joint obtained from comparative example 2.

[0094] As indicated by the testing, the rail welding specimen obtained in comparative example 2 fractures during the process of physical fatigue.

[0095] Detailed description has been made above for a bainitic rail welded joint and post-welding heat treatment method to control "white block" structure thereof. Specific examples are used herein to explain the concept and embodiments of the invention. The description of the above examples is only to help with understanding of the methods and core ideas of embodiments of the invention, including the best mode, and to enable those skilled in the art to implement the invention, including manufacture and usage of any device or system, as well as the method to implement any combination. It should be pointed out that some improvements and modifications which can be made by a person skilled in the art under the premise of not departing from the concept of the invention are within the protection scope of the claims. The protection scope of the invention is defined by the claims and can include other examples that those skilled in the art can come up with. If such other examples have structural elements that are not different from the textual representation of the claims, or if they have equivalent structural elements that are not materially different from the textual representation of the claims, such other examples should also be included in the scope of the claims.

[0096] Where any or all of the terms "comprise", "comprises", "comprised" or ''comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.

Claims

The claims defining the invention are as follows:

1. A post-welding heat treatment method for bainitic rail welded joint, comprising the steps of: 1) implementing first-stage cooling for a to-be-cooled bainitic rail welded joint obtained from welding to obtain the welded joint after the first-stage cooling; the first-stage cooling being slow cooling; the first-stage cooling having a cooling rate of substantially 0.8-1.4°C/min; 2) heating the welded joint after the first-stage cooling obtained in the above step, and implementing second-stage cooling to obtain the welded joint after the second stage cooling; the second-stage cooling being quick cooling; the second-stage cooling having a cooling rate of substantially 0.9-2.1°C/s; 3) implementing third-stage cooling for the welded joint after the second-stage cooling obtained in the above step to obtain bainitic rail welded joint the third-stage cooling being natural cooling; the third-stage cooling having a cooling rate of substantially 0.3-0.9C/s the bainitic rail welded joint comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography; wherein, the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field.

2. The post-welding heat treatment method according to Claim 1, wherein the temperature of the welded joint of the to-be-cooled bainitic rail obtained from welding is substantially 1400-1600°C; and wherein the step of heating is heating for the entire section.

3. The post-welding heat treatment method according to Claim 1, wherein an initial cooling temperature of the second-stage cooling is higher than or equal to 800°C; and wherein the step heating provides a heating rate of substantially 6.2 10.8 0C/s.

4. The post-welding heat treatment method according to Claim 1, wherein during the heating, the temperature difference between a rail head and a rail bottom associated with the welded joint is controlled substantially within 50°C; the temperature of the welded joint after the first-stage cooling is less than or equal to 290°C; the temperature after the heating is less than or equal to 970°C; the temperature of welded joint after the second-stage cooling is less than or equal to 2800 C.

5. The post-welding heat treatment method according to Claim 1, wherein the temperature of the welded joint after the first-stage cooling is substantiallyl50-280°C; the temperature after the heating is substantially 880-960°C; the temperature of the welded joint after the second-stage cooling is substantially 180-270°C; the first-stage cooling is indirect cooling; and the third-stage cooling is direct cooling.

6. The post-welding heat treatment method according Claim 1, wherein the first stage cooling is in particular to achieve slow cooling at a cooling speed controlled by a heat insulation device; the heating comprises heating by medium-frequency induction simulation electric heating coil and/or heating by oxy-acetylene flame simulation heater; the second-stage cooling is quick cooling by ejecting one or more of compressed air, mist and water-mist mixture as the cooling medium.

7. A bainitic rail welded joint, wherein comprising weld seam structure and white block structure distributed in and around the weld seam in terms of microscopic metallography; wherein, the number of white block structures with a size greater than or equal to 100pm is less than 10/single microscopic metallographic field; and wherein the welded joint is obtained from heat treatment of the to-be-cooled bainitic rail welded joint obtained from welding; and the heat treatment is based on the method according to any one of Claims 1 to 6.

8. The bainitic rail welded joint according to Claim 7, wherein the size of the white block structure is smaller than or equal to 2000pm; the width of the weld seam is substantially 101-772pm; the white block structures are independent of each other.

9. The bainitic rail welded joint according to Claim 8, wherein the weld seam structure is bright white decarburized layer; the weld seam structure comprises ferritic structure and bainitic structure; the white block structure has irregular shape; the heat treatment comprises three-stage cooling steps.

10. The bainitic rail welded joint according to Claim 7 or 8, wherein the white block structure distributed in and around the weld seam is in particular the white micro-area irregularly distributed in areas near the weld seam; the welding comprises flash welding and/or gas pressure welding; the heat treatment comprises first-stage cooling, heating, second-stage cooling and third-stage cooling.