AU2018214003B2 - Post-weld heat treatment method for welded joints of bainitic rails - Google Patents

Post-weld heat treatment method for welded joints of bainitic rails Download PDF

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AU2018214003B2
AU2018214003B2 AU2018214003A AU2018214003A AU2018214003B2 AU 2018214003 B2 AU2018214003 B2 AU 2018214003B2 AU 2018214003 A AU2018214003 A AU 2018214003A AU 2018214003 A AU2018214003 A AU 2018214003A AU 2018214003 B2 AU2018214003 B2 AU 2018214003B2
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cooling
welded joints
rails
bainitic
heat treatment
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AU2018214003A1 (en
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Wei Bai
Jian Deng
Dadong Li
Xin Lu
Ruoyu WANG
Feixiang Xu
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Pangang Group Research Institute Co Ltd
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Pangang Group Research Institute Co Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/002—Resistance welding; Severing by resistance heating specially adapted for particular articles or work
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/02—Pressure butt welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/04—Flash butt welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K31/00—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups
    • B23K31/02—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups relating to soldering or welding
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C21D1/42—Induction heating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C21D1/52—Methods of heating with flames
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/613—Gases; Liquefied or solidified normally gaseous material
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • C21D9/505—Cooling thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/26—Railway- or like rails
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/02—Iron or ferrous alloys
    • B23K2103/04—Steel or steel alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
  • Machines For Laying And Maintaining Railways (AREA)
  • Pressure Welding/Diffusion-Bonding (AREA)

Abstract

The invention relates to the field of manufacturing technology of railway rails, and provides a post-weld heat treatment method for welded 5 joints of bainitic rails, which can control the longitudinal hardness of the tread of welded joints of bainitic rails within a reasonable range. The post-weld heat treatment method in the invention comprises the following steps sequentially performed: A. performing a first cooling for the welded joints with a temperature of 1500-1600°C to 150-250°C, wherein 10 the first cooling is naturally realized by air cooling; B. heating the welded joints to 880-960°C by an intermediate-frequency induction profiling electric heating coil and/or an oxygen-acetylene flame profiling heater; C. performing a second cooling for the welded joints, stopping the second cooling when the welded joints are cooled to 180-250 °C , and 15 immediately performing a third cooling to the room temperature of 5-40°C; wherein the second cooling is rapidly realized by applying a cooling medium, the initial cooling temperature of the second cooling is above 800°C, and the third cooling is naturally realized by air cooling. -- Longitudinal hardness curve of the rail 50-- head tread of welded joints -Measurement line of softened zone of welded joints -110 -100 -90 -80 -70 -50 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 Distance from the center of the welding seam/nmm Fig. I -4-Longitudinal hardness curve of the rail head tread ofweldedjoints - Measurement line of softened zone of weldedjoints -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 Distance from the center ofthe welding seam/mm Fig. 2

Description

Distance from the center of the welding seam/mm
Fig-2
Post-weld Heat Treatment Method for Welded Joints of Bainitic Rails
Technical Field
The invention relates to the field of manufacturing technology of railway rails, and in particular to a post-weld heat treatment method for welded joints of bainitic rails.
Background
Bainitic rail is a global research hotspot in recent decades. Due to its high toughness, wear resistance, long service life and other properties, it is expected to replace traditional pearlite rails, widely used in railway turnout components and small radius curves of heavy haul lines. Currently, the seamlessness of rails has become an inevitable trend. As an important process in the seamlessness of rails, the quality of rail welding is directly related to the service life of railway lines and even traffic safety. In the service process, the welded long rails always suffer from joint fracture with the effects of poor welding quality and complex operating conditions of the line, so the welded joint is the weak link of the seamless line.
Affected by the melting process and high temperature, the austenite grains are coarse in the overheated welded zone of the rail, resulting in the hardness of this zone significantly lower than that of the base metal. In the service process, the softened welded joint of the rail is prone to preferentially form a “saddle-type” wear on the rail head tread of the joint, which increases the wheel-rail impact, affects the service life of the rail, and even jeopardizes the traffic safety. Therefore, the current domestic railway industry standards TB/T1632.2-2014, Welding of rails - Part 2: Flash welding, and TB/T1632.4-2014, Welding of rails - Part 4: Gas pressure welding, stipulate that, for heat treatment of rails, average hardness of the welded zones should not be less than 90% of the average hardness of the base metal, and there should be no harmful structure such as martensite or bainite in the micro structure of the welding seams and heat affected zones. The objects that the above two standards for welding of rails are related to pearlite rails, while there is no i
welding standard applicable for bainitic rails at home and abroad. Therefore, it is not appropriate to evaluate the mechanical properties of the welded joints of bainitic rails in full compliance with the current domestic technical standards for welding of rails, and excessive longitudinal hardness of rail head tread of the welded joints (higher than 90% of the average hardness of the base metal) will lead to decrease in the fatigue properties of the welded joints and occurrence of early fractures.
Generally, chemical components of bainitic rails are: C content of 0.20-0.30% by weight, Si content of 0.8-1.8% by weight, Mn content of 1.5-2.5% by weight, Cr content of 0.50-1.60% by weight, and Mo content of 0.20-0.50% by weight. Employing the heat treatment process technology, the bainitic rails produced on the basis of the fine grain strengthening principle are subject to the welding thermal cycle, then the quench-hardened layer in the welding seam zone disappears and a wide low-hardness zone appears, resulting in that the hardness of the welding seam and the heat affected zone is far below that of the rail base metal, so post-weld heat treatment of welded joints of bainitic rails is the most effective means of restoring the hardness of the welded zone of the rail.
At present, after the completion of welding of rails at home, post-weld normalizing heat treatment is performed generally following standards TB/T1632.2-2014, Welding of rails - Part 2: Flash welding, and TB/T1632.4-2014, Welding of rails - Part 4: Gas pressure welding, and after the welded joints of rails are heated to a temperature above the austenitizing temperature by using intermediate-frequency induction electric heating or oxygen-acetylene flame heating as the heat source, the tread hardness of the welded zone of the rails is further improved by air cooling or wind cooling process.
Application No. 201410135909.8, entitled Post-weld heat treatment method for welded joints of bainitic rails, discloses a post-weld heat treatment method for joints of bainitic rails, which comprises the following steps: performing a first cooling for the welded joints of bainitic rails to a first
2018214003 15 Jan 2020 temperature not higher than 450°C, heating the welded joints to a second temperature, and performing a second cooling, wherein the second temperature is higher than the first temperature and not higher than 510°C, wherein the initial cooling temperature of the bainitic rails is 1300-1380°C, and the final 5 cooling temperature after the second cooling is room temperature. The method involves the post-weld heat treatment process of the welded joints of bainitic rails, which can significantly reduce the content of martensite structure in the welded joints of bainitic rails and increase impact toughness of the welded joints. Whereas, the present invention relates to a post-weld heat treatment io method for welded joints of bainitic rails in order to restore joint hardness which is reduced in welding of the bainitic rails. Although both methods belong to the post-weld heat treatment process of bainitic rails, the purpose of realization and the process control procedure are different.
Application No. 201210394058.X, entitled Heat treatment method for 15 bainitic rails, discloses a heat treatment method for bainitic rails, which comprises the following steps: naturally cooling the rail after final rolling to reduce the surface temperature of the rail head to 460-490 °C; forced cooling the rail at a cooling rate of 2.0-4.0°C/s, so that the surface temperature of the rail head is reduced to 250-290°C; allowing the rail temperature to naturally rise 20 until the surface temperature of the rail head reaches 300°C above; placing the rail into a heating furnace with a temperature of 300-350°C for tempering treatment for 2-6h; air cooling the rail to room temperature. The invention aims to obtain bainitic rails with good comprehensive mechanical properties, and belongs to the heat treatment process for rail production rather than the 25 post-weld heat treatment method for welded joints of bainitic rails.
In summary, there is an urgent need in the field of railway engineering for a post-weld heat treatment method to ensure the service life of welded joints of rails by the way of controlling the tread hardness of welded joints of bainitic rails within a reasonable range, so as to ensure the service performance of 30 welded joints of bainitic rails and the safety of railway operation.
It is against this background and the problems and difficulties associated therewith that the present invention has been developed.
Certain objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.
Summary of the Invention
In one aspect, there is provided a post-weld heat treatment method for welded joints of bainitic rails, comprising the following steps sequentially performed:
A. performing a first cooling for the welded joints with a temperature of 1500-1600°C to 150-250°C, wherein the first cooling is realized by air cooling;
B. heating the welded joints to 880-960°C;
C. performing a second cooling for the welded joints, stopping the second cooling when the welded joints are cooled to 180-250°C, and immediately performing a third cooling to the room temperature of 5-40°C; wherein the second cooling is rapidly realized by applying a cooling medium, the initial cooling temperature of the second cooling is above 800 °C, and the third cooling is naturally realized by air cooling.
Further, in step C, the cooling medium of the second cooling is compressed air, which is sprayed to realize rapidly cooling.
Further, in step B, the welded joints are heated in a full-section way by an intermediate-frequency induction profiling electric heating coil and/or an oxygen-acetylene flame profiling heater.
Further, the welded joints are obtained in welding two bainitic rails by flash welding or gas pressure welding.
The beneficial effects of the invention are that the longitudinal hardness of the rail head tread of the welded joints of bainitic rails which are subject to heat treatment according to the method of the present invention is controlled within a reasonable range of 85-90% of the average hardness of the rail base metal. Appropriate hardness of the rail head tread makes welded joints not prone to
2018214003 15 Jan 2020 cause early fatigue damage due to excessive hardness and poor toughness and plasticity, and not easy to form “saddle-type” wear due to low hardness of the rail head tread. Thus, the invention can ensure the service performance of the welded joints of bainitic rails and the safety of railway operation. According to 5 the experimental results, the practical fatigue life is not less than 3 million times, much higher than 2 million times specified in standards TB/T1632.2-2014 and TB/T1632.4-2014.
Brief Description of the Drawings
Fig. 1 is a longitudinal hardness effect diagram at a position 5mm below a io rail head tread of welded joints of bainitic rails obtained by the method in embodiment 1.
Fig. 2 is a longitudinal hardness effect diagram at a position 5mm below a rail head tread of welded joints of bainitic rails obtained by the method in reference embodiment 1.
Fig. 3 is a longitudinal hardness effect diagram at a position 5mm below a rail head tread of welded joints of bainitic rails obtained by the method in reference embodiment 3.
Fig. 4 is a longitudinal hardness effect diagram at a position 5mm below a rail head tread of welded joints of bainitic rails obtained by the method in 20 reference embodiment 4.
Fig. 5 is a hardness sampling diagram of the longitudinal section specified by the relevant standard.
Description of the Preferred Embodiments
The invention will be further illustrated below with detailed description of 25 the preferred embodiments in combination with drawings.
In the invention, the welded joint is the zone obtained after welding, comprising the welding seam and the heat affected zone and having a length ranging from 80 to 120mm, and the center of the zone is the welding seam of the rail. In the invention, the room temperature is in the range of 5-40 °C.
The post-weld heat treatment method for welded joints of bainitic rails comprises the following steps sequentially performed:
2018214003 15 Jan 2020
A. performing a first cooling for the welded joints with a temperature of 1500-1600°C to 150-250°C, wherein the first cooling is naturally realized by air cooling;
B. heating the welded joints to 880-960°C by an intermediate-frequency 5 induction profiling electric heating coil and/or an oxygen-acetylene flame profiling heater;
C. performing a second cooling for the welded joints, stopping the second cooling when the welded joints are cooled to 180-250°C, and immediately performing a third cooling to the room temperature of 5-40°C; wherein the second cooling is rapidly realized by applying a cooling medium, the initial cooling temperature of the second cooling is above 800 °C, and the third cooling is naturally realized by air cooling.
In the invention, the welded joint is the zone obtained after welding, comprising the welding seam and the heat affected zone and having a length 15 ranging from 80 to 120mm, and the center of the zone is the welding seam of the rail. In the invention, the room temperature is in the range of 5-40°C.
In the invention, the heating temperature of step B is close to the conventional normalizing temperature, however the difference exists as follows: normalizing heat treatment generally refers to heating the metal workpiece to 20 30-50°C above Ac3 (the final temperature of transformation from ferrite to austenite during heating) in a conventional manner; after a period of heat preservation, the metal workpiece is taken out of the furnace for heat treatment process such as air cooling, or spraying cooling, or compressed air cooling. However, the post-weld normalizing heat treatment of welded joints of rails is 25 different from the heat treatment process usually used for small-sized workpieces. Because the specimen of welded rails can be as long as hundreds of meters, the normalizing heat treatment of the welded joints of the rails is impossible to involve heat preservation for a long time after reaching a target temperature (temperature above the austenitizing temperature). Therefore, 30 generally the welded joints of the rails are heated at a temperature which slightly higher than the conventional normalizing temperature to the target
2018214003 15 Jan 2020 temperature, and then treated by air cooling or compressed air cooling, that is, the heating temperature of step B is 880-960°C.
In step C, the second cooling is rapidly realized by applying a cooling medium. Since there are a variety of cooling media, the present invention 5 recommends that the second cooling is realized by spraying compressed air as the cooling medium. In addition to the heating zone heated in step B, the cooling zone of the second cooling includes the tread and side surfaces of the rail head in the range of 80mm in length on both sides of the heating zone.
In the step B, the welded joints are heated in a full-section way. The 10 full-section heating refers to heating the entire section of the welded joints of the rails including the welding seam and having a length in the range of about 80-120mm.
The method of the invention can be used for welded joints of bainitic rails obtained by welding at various temperatures, preferably welded joints of 15 bainitic rails having an initial temperature of 1500-1600°C. The method of the invention can be used for welded joints of bainitic rails obtained by various welding methods at a temperature of 1500-1600 °C after welding, preferably welded joints of bainitic rails obtained by at least one welding method of flash welding and gas pressure welding at a higher residual temperature after 20 welding.
The invention is further described below in combination with the embodiments and reference embodiments, and reference is made to standards TB/T1632.2-2014, Welding of rails - Part 2: Flash welding, and TB/T1632.4-2014, Welding of rails - Part 4: Gas pressure welding in the 25 embodiments and reference embodiments, and the longitudinal hardness data detection and three-point bending fatigue test are performed on the welded joints of bainitic rails obtained by the invention, with a test target that no fatigue fracture occurs in welded joints when the cyclic load is 3 million times. Figure 5 is a hardness sampling diagram of the longitudinal section specified in 30 the above standards.
Embodiment 1:
2018214003 15 Jan 2020
The welded joints of bainitic rails obtained by flash welding at a temperature of 1500-1600°C are air-cooled. When the welded joints are cooled from 1550°C to 200°C, the welded joints of rails are heated in a full section way by an intermediate-frequency induction profiling electric heating coil.
When the tread temperature of the rails reaches 950°C, the heating is stopped, then the obtained welded joints of the rails are immediately cooled by compressed air to 220°C, and finally the welded joints are air-cooled to room temperature (about 23°C), thus obtaining the welded joints of bainitic rails of the invention subject to post-weld heat treatment.
io The welded joints of bainitic rails in this embodiment are machined into longitudinal hardness specimens, the longitudinal Rockwell hardness is detected on longitudinal hardness specimen at a position 5mm below the tread, the detection points are symmetrically arranged to the left and right sides with the welding seam as the center, and the spacing between the detection points is
5mm. The Rockwell hardness test method is carried out following stipulations of GB/T 230.1-2009, using the HRC scale. For the longitudinal hardness data at the position 5mm below the rail head tread of the welded joint, see Table 1. The distribution effect of the longitudinal hardness is shown in Fig. 1.
Table 1:
Distance from the center of the we ding seam/mm
Left From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness /HRC 31.0 35.0 35.6 35.7 34.0 35.4 27.6 30.3 33. 6 35. 2 35.6 37.2
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 38.4 39.2 39.8 41.5 42.6 42.8 42.6 42.8 43. 3 43. 1 43.2 /
Right From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness /HRC 31.0 35.2 35.0 34.8 33.4 28.4 31.0 33.1 34. 7 35. 7 37.0 38.0
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 38.7 39.5 41.2 42.1 42.8 42.6 42.9 43.3 43. 2 42. 8 43.3 /
2018214003 15 Jan 2020
It can be seen from Table 1 and Fig. 1 that the hardness of the welded zone of the joint of bainitic rail can reach 87% of the average hardness of the base metal and fall within 85-90% of the average hardness of the base metal, when the welded joint of bainitic rail obtained by flash welding at a temperature of 5 1500-1600°C is treated by the post-weld heat treatment method of the present invention. The welded joint specimen of bainitic rail in this embodiment is able to pass a fatigue test with the cycle times of 3 million.
Embodiment 2:
The welded joints of bainitic rails obtained by gas pressure welding at a io temperature of 1500-1600°C are air-cooled. When the welded joints are cooled from 1530°C to 180°C, the welded joints of rails are heated in a full section way by an oxygen-acetylene flame profiling heater. When the tread temperature of the rails reaches 940°C, the heating is stopped, then the welded joints of the rails are cooled by compressed air to 210°C, and finally the welded joints are 15 air-cooled to room temperature (about 23°C), thus obtaining the welded joints of bainitic rails of the invention subject to post-weld heat treatment.
The welded joints of bainitic rails in this embodiment are machined into longitudinal hardness specimens, the longitudinal Rockwell hardness is detected on the longitudinal hardness specimen at a position 5mm below the rail 20 head tread, the detection points are symmetrically arranged to the left and right sides with the welding seam as the center, and the spacing between the detection points is 5mm. The Rockwell hardness test method is carried out following stipulations of GB/T 230.1-2009, using the HRC scale. Its hardness distribution effect is similar to the effect as shown in Fig. 1. The welded joint 25 specimen of bainitic rail in this embodiment is is able to pass a fatigue test with the cycle times of 3 million.
Reference Embodiment 1:
The welded joints of bainitic rails are subject to the post-weld heat treatment in accordance with the method of embodiment 1, and the welded 30 joints of the rails are heated to 950°C by using an intermediate-frequency induction profiling electric heating coil, except that the compressed air is
2018214003 15 Jan 2020 stopped when the temperature of the second cooling is 160°C, and then the welded joints are air-cooled to room temperature (about 23 °C).
The longitudinal hardness specimen of the welded joints of bainitic rails in this reference embodiment is taken to perform longitudinal Rockwell hardness 5 detection at a position 5mm below the rail head tread, the detection points are symmetrically arranged to the left and right sides with the welding seam as the center, and the spacing between the detection points is 5mm. The Rockwell hardness test method is carried out following stipulations of GB/T 230.1-2009, using the HRC scale. For the longitudinal hardness data at the position 5mm io below the rail head tread of the welded joint, see Table 2. The distribution effect of the longitudinal hardness is shown in Fig. 2.
Table 2:
Distance from the center of the welding seam/mm
Left From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness/ HRC 31.5 43.8 42.2 41.2 42.7 42.8 42.2 36.5 38. 0 33. 8 37.2 40.2
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 41.8 42.5 42.5 42.8 42.7 43.0 42.7 42.9 42. 7 43. 1 43.2 /
Righ t From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness /HRC 31.5 43.9 43.5 42.8 43.9 43.8 43.2 36.4 33. 4 36. 3 38.2 40.0
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 41.8 42.7 43.0 43.2 43.1 43.5 43.0 43.0 43. 2 43. 0 43.2 /
The longitudinal hardness specimen of the welded joints of bainitic rails obtained in this embodiment is taken to perform longitudinal Rockwell 15 hardness detection at a position 5mm below the rail head tread, and its hardness distribution effect is shown in Fig. 2. The average hardness of the welded joints of the rails in this embodiment reaches 93% of that of the base metal. However, a fatigue fracture occurs when the specimen of the welded joints of bainitic rails in this embodiment undergoes the fatigue test at 2.7 million times.
io
2018214003 15 Jan 2020
Reference Embodiment 2
The welded joints of bainitic rails are subject to the post-weld heat treatment in accordance with the method of embodiment 1, except that the air cooling is stopped when the welded joints of bainitic rails obtained by flash 5 welding at a temperature of 1550°C are subject to the first cooling to 100°C, then the tread of the rail is heated to 950 °C by using an intermediate-frequency induction profiling electric heating coil, then the heating is stopped, the welded joints of the rails are subject to the second cooling to 220°C, and finally the welded joints are air-cooled to room temperature (about 23 °C).
io The longitudinal hardness specimen of the welded joints of bainitic rails in this embodiment is taken to perform longitudinal Rockwell hardness detection at a position 5mm below the rail head tread, its hardness distribution effect is similar to the effect as shown in Fig. 1, but the average hardness of the welded zone reaches 92% of that of the base metal. However, a fracture occurs when 15 the specimen of the welded joints of bainitic rails in this embodiment undergoes the fatigue test at 2.8 million times.
Reference Embodiment 3
The welded joints of bainitic rails obtained by flash welding at a temperature of 1550 °C are directly air-cooled to room temperature (about 20 23 °C), thus obtaining the welded joints of bainitic rails under air cooling (natural cooling) condition.
The longitudinal hardness specimen of the welded joints of bainitic rails in this reference embodiment is taken to perform longitudinal Rockwell hardness detection at a position 5mm below the rail head tread, the detection points are 25 symmetrically arranged to the left and right sides with the welding seam as the center, and the spacing between the detection points is 5mm. The Rockwell hardness test method is carried out following stipulations of GB/T 230.1-2009, using the HRC scale. For the longitudinal hardness data at the position 5mm below the rail head tread of the welded joint, see Table 3. The distribution effect 30 of the longitudinal hardness is shown in Fig. 3.
2018214003 15 Jan 2020
Table 3:
Distance from the center of the we ding seam/mm
Left From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardnes s/HRC 31.0 35.0 35.6 35.7 34.0 35.4 27.6 30.3 33. 6 35. 2 35.6 37.2
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardnes s/HRC 38.4 39.2 39.8 41.5 42.6 42.8 42.6 42.8 43. 3 43. 1 43.2 /
Right From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardnes s/HRC 31.0 35.2 35.0 34.8 33.4 28.4 31.0 33.1 34. 7 35. 7 37.0 38.0
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardnes s/HRC 38.7 39.5 41.2 42.1 42.8 42.6 42.9 43.3 43. 2 42. 8 43.3 /
It can be seen from Table 3 and Fig. 3 that with respect to the welded joints of the bainitic rails obtained without the post-weld heat treatment method of the invention at a higher residual temperature, the hardness of welded zone of the joint of bainitic rails is 82% of the average hardness of the base metal and lower than 85-90% of the average hardness of the base metal. The entire welded zone io appears soft compared to the hardness of the base metal. The welded joints in this reference embodiment are prone to cause the rail head tread to collapse in the course of line service, which affects the smoothness of the line and the traffic safety.
Reference Embodiment 4
The welded joints of bainitic rails obtained by flash welding at a temperature of 1500-1600°C are air-cooled. When the welded joints are cooled from 1560°C to 200°C, the welded joints of rails are heated in a full section way by an intermediate-frequency induction profiling electric heating coil. When the tread temperature of the rails reaches 940°C, the heating is stopped,
2018214003 15 Jan 2020 then the welded joints of the rails are air-cooled to room temperature (about 23°C), thus obtaining the welded joints of bainitic rails under post-weld normalizing air cooling condition.
The longitudinal hardness specimen of the welded joints of bainitic rails in 5 this reference embodiment is taken to perform longitudinal Rockwell hardness detection at a position 5mm below the rail head tread, the detection points are symmetrically arranged to the left and right sides with the welding seam as the center, and the spacing between the detection points is 5mm. The Rockwell hardness test method is carried out following stipulations of GB/T 230.1-2009, io using the HRC scale. For the longitudinal hardness data at the position 5mm below the rail head tread of the welded joint, see Table 4. The distribution effect of the longitudinal hardness is as shown in Fig. 4.
Table 4:
Distance from the center of the welding seam/mm
Left From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness /HRC 32.0 37.1 36.4 36.2 35.5 35.2 33.0 35.3 32.2 35. 3 37.2 38.0
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 39.0 40.2 41.5 42.2 42.6 42.8 42.8 43.1 43.0 42. 9 43.2 /
Right From welding seam 0 5 10 15 20 25 30 35 40 45 50 55
Hardness /HRC 32.0 37.2 37.1 36.2 36.4 36.0 35.1 33.2 34.4 34. 5 31.4 35.8
From welding seam 60 65 70 75 80 85 90 95 100 105 110 /
Hardness /HRC 37.6 39.3 40.3 41.1 42.4 42.6 42.8 42.4 42.8 43. 2 43.3 /
It can be seen from Table 4 and Fig. 4 that with respect to the welded joints of the bainitic rails obtained without the use of the post-weld heat treatment method of the invention at a higher residual temperature, the hardness of welded zone of the joint reaches 84% of the average hardness of the base metal. The entire welded zone appears soft compared to the base metal. The welded joints in this reference embodiment are prone to cause the rail head tread to collapse in the course of line service, which affects the smoothness of the line and the traffic safety.
By comparing the longitudinal hardness distribution of the rail head treads of the welded joints in Figs. 1 to 4, it is found that the longitudinal hardness of the rail head tread of the welded joints of bainitic rails can be controlled within 85-90% of the average hardness of the base metal by using the method of the present invention. At the same time, the post-weld heat treatment method for rails of the invention can maintain the fatigue life of the welded joints of bainitic rails more than 3 million times, which is much higher than the technical requirement of 2 million times specified in TB1632, thereby achieving the goal of ensuring the service life of the rails.
Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.
It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims (4)

  1. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:
    1. A post-weld heat treatment method for welded joints of bainitic rails, the method comprising the following steps sequentially performed:
    A. performing a first cooling for the welded joints with a temperature of 1500-1600°C to 150-250°C, wherein the first cooling is naturally realized by air cooling;
    B. heating the welded joints to 880-960°C;
    C. performing a second cooling for the welded joints, stopping the second cooling when the welded joints are cooled to 180-250°C, and immediately performing a third cooling to the room temperature of 5-40°C; wherein the second cooling is rapidly realized by applying a cooling medium, the initial cooling temperature of the second cooling is above 800 °C, and the third cooling is naturally realized by air cooling.
  2. 2. The post-weld heat treatment method for welded joints of bainitic rails according to claim 1, wherein in step C, the second cooling is rapidly realized by spraying compressed air as the cooling medium.
  3. 3. The post-weld heat treatment method for welded joints of bainitic rails according to claim 1, wherein in step B, the welded joints are heated in a full-section way by an intermediate-frequency induction profiling electric heating coil and/or an oxygen-acetylene flame profiling heater.
  4. 4. The post-weld heat treatment method for welded joints of bainitic rails according to claim 1, wherein the welded joints are obtained in welding two bainitic rails by flash welding or gas pressure welding.
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CN108570550B (en) * 2018-05-18 2020-06-26 攀钢集团攀枝花钢铁研究院有限公司 Postweld heat treatment method for heat-treatment type bainite steel rail welded joint
CN109022746B (en) * 2018-10-24 2020-05-19 攀钢集团攀枝花钢铁研究院有限公司 Heat treatment method of U71MnH steel rail flash welding joint
CN109355482A (en) * 2018-10-26 2019-02-19 攀钢集团攀枝花钢铁研究院有限公司 The post weld heat treatment method of hypo eutectoid rail
CN109457101B (en) * 2018-11-20 2020-12-04 攀钢集团攀枝花钢铁研究院有限公司 Bainite steel rail welding joint and postweld heat treatment method for controlling white block structure of bainite steel rail welding joint
CN109207708B (en) * 2018-11-20 2020-12-04 攀钢集团攀枝花钢铁研究院有限公司 Bainite steel rail welding joint and postweld heat treatment method for controlling white block structure of bainite steel rail welding joint
CN110016546A (en) * 2019-05-28 2019-07-16 攀钢集团攀枝花钢铁研究院有限公司 Construction method for the heat treatment of bainite rail postwelding connector
CN110016544A (en) * 2019-05-28 2019-07-16 攀钢集团攀枝花钢铁研究院有限公司 Construction method for the heat treatment of bainite rail postwelding connector
CN110157890A (en) * 2019-05-28 2019-08-23 攀钢集团攀枝花钢铁研究院有限公司 Construction method for the heat treatment of bainite rail postwelding connector
CN110042218A (en) * 2019-05-28 2019-07-23 攀钢集团攀枝花钢铁研究院有限公司 Construction method for the heat treatment of bainite rail postwelding connector
CN111363897B (en) * 2020-04-23 2021-11-02 中国铁建重工集团股份有限公司 Heat treatment method for welded joint of dissimilar material steel rail
CN112226609B (en) * 2020-10-23 2022-03-22 攀钢集团攀枝花钢铁研究院有限公司 Construction method for heat treatment of post-welded joints of dissimilar steel rails
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