JP5531845B2 - Post-heat treatment method near the flash butt weld - Google Patents

Description

The present invention provides a method for improving the reliability in the vicinity of a rail weld.

  When rails are used on railroads, it is the rail joints that are most prone to damage and cost of maintenance. The seam is the main source of noise and vibration generated when passing through the train. The speed of passenger railways and heavy loading of freight railways are being promoted in Japan and overseas, and the long rail technology that makes rail joints having the above-mentioned problems continuous by welding has become common.

  A long rail welded part and a rail cross section name are demonstrated using FIG. Fig.1 (a) is a side view of the welding part in the longitudinal direction. Long rails are manufactured by welding at least two rails. For this reason, the welded portion 7 is included in the long rail. The weld 8 has a bead 8.

  FIG. 1B is a cross-sectional view perpendicular to the rail longitudinal direction. The rail has a head 1 at the top of the rail where contact with the wheel occurs, a foot 3 at the bottom of the rail that contacts the sleepers, and a pillar 2 that is a vertical portion between the head 1 and the foot 3. The highest point 4 on the head is also called the top of the head and the upper surface 5 of the foot, and the back 6 of the foot is also called the sole or bottom.

As a main welding method of the rail, there is flash butt welding (for example, Patent Document 1).
As shown in FIG. 2, in the flash butt welding method, a voltage is applied to the workpiece 10 placed opposite to each other through the electrode 9 to generate an arc between the end faces to melt the end face of the workpiece. This is a welding method in which when a material to be welded is sufficiently heated, the material to be welded is joined by pressing the material in the axial direction.

  The rails are first welded at a welding plant up to 200-500m, and then transported to the laying site with a freight car dedicated to long rails. The flash butt welding method targeted by the present invention is used in both factory welding and on-site welding.

JP 2001-219177 A

  When the rail is used, surface damage 14A as shown in FIG. 3 occurs on the head surface 100 mm to 200 mm away from the joint surface of the flash butt weld, or at a similar distance from the joint surface, as shown in FIG. The fatigue crack 14B may occur from the bottom surface of the rail or the column portion. Since these damage locations are far outside the weld heat affected zone, it may be determined whether the rail base material has been damaged. The present invention provides a technique for preventing damage in the vicinity of these welds.

The present invention has been made in consideration of rail damage in the prior art, and an object thereof is to provide a method for manufacturing a long rail having improved reliability in the vicinity of a welded portion as compared with the conventional art. .

As a result of investigating the damage occurring at a position around 100 mm from the joint surface, it was found that a heat-affected zone was present at the starting point of the damage and a martensite structure was generated.
In flash butt welding, an electrode is attached to a rail, a voltage is applied through the electrode, and an arc is generated between the end surfaces of the rail.

  When the contact between the electrode and the rail is poor, arcing may occur between the electrode and the rail as shown in FIG. This phenomenon is called die burn, electrode burn, or electrode burn. In the present specification, this phenomenon is called diburn. The arc-generated rail surface is rapidly heated to the austenite region, and the temperature is rapidly lowered by heat conduction to the surroundings. Rail steel has high hardenability, and it is thought that martensite is generated by this heat cycle. Rail steel has a high C content, and its martensite is extremely hard and brittle. For this reason, it is considered that the rail steel in which the martensite structure is generated is damaged by a lighter load than the normal rail steel.

As a countermeasure, martensite is rendered harmless by the following method.
(1) After the stationary flash welding the vicinity of the rail to a heat treatment method, 250 ° C. or higher 600 ° C. the rail head surface of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld A post-heat treatment method in the vicinity of the flash butt weld, which is reheated below.
(2) After the stationary flash welding the vicinity of the rail to a heat treatment method, the rail foot backside surface of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, solid A post-heat treatment method in the vicinity of a flash butt weld, characterized by reheating to a temperature below the phase line temperature.
(3) After the stationary flash welding the vicinity of the rail to a heat treatment method, the rail foot backside surface of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, solid The post-heat treatment method in the vicinity of the flash butt weld as described in (1) above, wherein reheating is performed below the phase line temperature.
(4) After the movable flash welding the vicinity of the rail to a heat treatment method, the rail pillar portion surfaces of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, solid A post-heat treatment method in the vicinity of a flash butt weld, characterized by reheating to a temperature below the phase line temperature.

  According to the present invention, martensite generated by die burn is rendered harmless, and damage to the welded portion is difficult to occur.

It is explanatory drawing of the name of a welding part and a rail cross section, (a) is the side view which looked at the long rail from the horizontal direction, (b) is AA 'sectional drawing of (a). It is a schematic diagram of flash butt welding, (a) is a flushing process, (b) is an upset process, (c) is a trimming process. It is a schematic diagram of the surface damage of the crown part of a rail welding part. It is an example of the surface damage from the sole part or pillar part of a rail welding part. It is a schematic diagram which shows the generation | occurrence | production mechanism of the die burn in flash butt welding. The example of arrangement | positioning of the electrode in flash butt welding is shown. (A) is a type in which electrodes are attached to the rail head top surface and the back of the foot, which are frequently used in factory welding machines, and (b) is a type in which electrodes are frequently used in field welding and the electrodes are attached to the column part from the left and right. It is a schematic diagram which shows the contact state of a wheel and a rail. (A) shows that the contact between the wheel and the rail has a contact surface, and (b) shows that in-plane stress is generated on the contact surface. It is explanatory drawing in which a bending stress and a shear stress act on a rail when a train passes. The continuous cooling diagram of eutectoid steel is shown. Explanatory drawing of the metal structure based on the difference in heating temperature, and a hardness difference. A method for deliberately generating a diburn and investigating the effect of countermeasures is shown. (A) is a schematic diagram of the electrode which has the recessed part which made the clearance gap in the surface where an electrode contacts a rail. (B) is a schematic diagram which shows the generation | occurrence | production state of the die burn at the time of mounting | wearing with the electrode which has the said recessed part, and welding. (C) shows the relationship between the contact position of the electrode and its recess and the collection position of the macro sample. (D) shows the generation state of martensite in the microscopic section. It is a schematic diagram which shows the reheating range of the dieburn part in an Example. It is a schematic diagram which shows the microscopic position and hardness measurement position in an Example. It is a schematic diagram which shows the method of a surface damage resistance test. It is a schematic diagram which shows the method of a fatigue test. It is a schematic diagram which shows the arrangement position of the electrode which has a recessed part in Example A. It is a schematic diagram which shows the arrangement position of the electrode which has a recessed part in Example B. FIG. 6 is a schematic diagram illustrating an arrangement position of an electrode having a recess in Example C. It is a schematic diagram which shows the electrode of the format used for Example D and the electrode which has the recessed part used for the rail pillar part. (A) The figure is the state before mounting | wearing with a rail, (b) The figure is the state mounted | worn with a rail. FIG. 6 is a schematic diagram showing the arrangement position of an electrode having a recess in Example D. (A) A figure is sectional drawing seen from the rail longitudinal direction axis | shaft, (b) A figure shows the AA cross section in (a) figure.

<Flash butt welding method>
First, flash butt welding will be described in more detail with reference to FIG. The first step of the flash butt welding method is a step of generating an arc continuously between the end faces shown in FIG. 2A and is called a flushing step. In this step, an arc is generated between the end faces of the material to be welded by the voltage applied via the electrode 9. The portion where the arc is generated is melted locally, and a part of the melted metal is released to the outside as a sputter, and the rest remains on the end face. A dent called a crater occurs in the part melted by the arc. The material to be welded is gradually approached, and arcs are successively generated at new contact portions, and the material is gradually shortened by repeated local melting and scattering. In this process, the materials to be welded are brought closer to each other so that the material interval is kept substantially constant.

  In the middle of the flushing process, there is a case where a process is adopted in which the material end surface is intentionally brought into contact and the base material temperature is increased by a large current caused by direct energization. The purpose is to smoothen the temperature distribution in the vicinity of the end face and proceed to the upset process more efficiently. This process is called a “preheating process” and is repeated several times for contact energization for about 2 to 5 seconds and for stopping the current by separating the end faces.

  By continuing the flushing process for several tens of seconds to several minutes, the entire end face of the workpiece is melted. Further, the material in the vicinity of the end surface softens due to the temperature rise. When this state is reached, pressurization in the axial direction is performed as shown in FIG. By this pressurization called upset, the crater uneven surface formed on the end faces is crushed, and the molten metal existing between the end faces is pushed out of the system. In the vicinity of the softened end surface, the cross section increases due to plastic deformation, and a bead 11 is formed around the weld surface.

  This bead is sheared and removed by the trimmer 12 during the high temperature period immediately after welding as shown in FIG. This process is called trimming. After trimming, a thin bead 8 having a height of several mm and a width of about 10 to 30 mm remains around the weld.

  The thin bead remaining after trimming the rail head in contact with the wheel is smoothed and polished with a grinder. The rail pillar and foot bead are treated differently depending on the railway company, such as complete smoothing by grinder polishing, thinning by grinder polishing, no maintenance.

  The mounting position of the electrode 9 on the rail is such that the rail head 1 and the bottom surface 3 are sandwiched from above and below, as shown in FIG. 6A, and the rail pillar portion from the side as shown in FIG. 6B. There is a method of sandwiching 2. A form in which the rail head 1 and the bottom 3 are sandwiched from above and below is frequently used in factory welding. On the other hand, the type in which the rail is sandwiched from the left and right does not require equipment space below the rail and is often used as a field welding machine. In either method, the electrode 9 is brought into close contact with the rail with a load of 10 tons or more. The position where the die burn is generated is the mounting position of these electrodes. The distance LE between the joint surface and the electrode 9 and the length Le in the rail longitudinal direction of the electrode 9 are both about 100 mm. The width Wc of the electrode 9 in the direction perpendicular to the longitudinal direction of the rail varies depending on the mounting part on the rail, and is about 40 to 80 mm for the rail head 1 and the pillar 2 and about 60 to 160 mm for the rail sole 3. . The surface on which the electrode 9 is in contact with the rail is a flat surface if it is the sole portion 3, a convex curved surface if it is the column portion 2, or a flat surface or a concave curved surface if it is the top of the head.

<Rail damage occurrence mode>
The railroad track is composed of a crushed ballast, sleepers, a rail and sleeper fastening device, and rails. When the train passes on the rail, a load distributed from the wheels of many trains is applied to the rail.

  In order to consider the cause of the above-mentioned damage, it is necessary to consider the load state from the wheel to the rail (FIG. 7).

  When the wheel 15 passes, the wheel 15 and the rail 1 have a contact area 16 having an area. This is because both the wheel and the rail are elastic bodies, and both elastically deform. The wheel shape in FIG. 7 exaggerates a contact surface with the rail, a conical surface called a tread surface, or a curved rotation surface. Due to the difference in shape between the wheel 15 and the rail 1, tangential stresses 17 and 18 are generated in the contact surface 16. As a result, plastic flow, so-called metal flow, progresses on the rail surface layer. Excessive plastic flow can cause surface damage such as cracking and delamination. In addition, in the curved track, the circumferential speed of the wheel is the same regardless of the contact track length between the inner rail and the outer rail of the track, and the tangential stress is further increased. Furthermore, in the vicinity of the station and in the acceleration / deceleration section in the gradient section, stress is exerted by the wheels pushing the rail in the tangential direction. Such an increase in tangential stress increases the plastic flow of the rail surface and causes wear and damage.

  Further, as shown in FIG. 8, the bending stress distribution 37 is generated in the rail due to the passage of the wheels, the longitudinal compression is generated in the rail head 1, and the longitudinal tension is generated in the rail foot 3. The tensile stress of the rail foot 3 is generated every time the wheel 15 passes, and the rail foot 3 needs to be considered for the occurrence of fatigue cracks. In cases where damage occurs from the sole side starting from the dieburn, it is considered that this repeated bending load is probably influencing.

  In addition, although the bending stress is neutral when the wheel passes through the column part 2, it is considered that the shear stress acts evenly on the entire cross section of the rail, and fatigue cracks may be generated from the column part 2 due to repeated shear loads. It is done.

<About rail material>
Next, rail steel will be described. The rail steel is generally hypoeutectoid steel or eutectoid carbon steel containing 0.5 to 0.8% by mass of carbon, as defined in JIS E1101 and JIS E1120. Recently, overeutectoid composition rail steel containing more than 0.8% by mass of carbon, which has improved wear resistance for heavy-duty cargo lines in overseas mining railways, is also becoming popular. .

Below, the preferable component range of the rail steel which this invention makes object is described. The content of the component is mass%.
C: An essential element for securing strength and hardness in rail steel. If it is less than 0.5%, it is difficult to obtain a high-strength pearlite structure necessary for train weight maintenance and wear resistance, and if it exceeds 1.2%, harmful proeutectoid cementite that embrittles austenite grain boundaries is generated. Therefore, it is not preferable.

Si: Fixes oxygen which is harmful to the properties of steel and has an effect of increasing strength by strengthening solid solution in the ferrite phase in the pearlite structure. If it is less than 0.1%, those effects are small, and if it exceeds 1.2%, embrittlement occurs and weldability is also lowered.
Mn: Strengthens by lowering the pearlite transformation temperature and densifying the pearlite structure. However, if it is less than 0.1%, the effect is small, and if it exceeds 1.2%, a martensite structure is easily generated in the segregated portion, which is not preferable.

  In addition to the above components, one or more Cr, Mo, V may be added as necessary, and accelerated cooling in the cooling process can achieve higher strength and higher toughness. Desirable ranges of these elements will be described below.

  Cr: Increasing strength by lowering the pearlite transformation temperature and containing 0.05% or more is effective from the viewpoint of preventing weld joint softening. On the other hand, if the content exceeds 1.0%, bainite and martensite are generated near the corners of the element segregation portion and the rail head portion that is easily overcooled during forced cooling, resulting in a decrease in toughness.

  Mo: There is an effect of suppressing the pearlite transformation speed and miniaturizing the pearlite block size. However, if it is less than 0.005%, this effect is small. On the other hand, if Mo exceeds 0.2%, the pearlite transformation is excessively delayed in the segregation part, and a bainite structure or a martensite structure is generated, which is not preferable.

  V: V carbonitrides that become pearlite transformation nuclei are precipitated, and there is an effect of refining the pearlite structure by promoting the formation of transformation nuclei from the austenite grain boundaries and within the grains. However, if V is less than 0.005%, this effect is weak, and if it is 0.07% or more, V carbonitride becomes coarse and becomes a starting point of fracture, which is not preferable.

  Al, N, Ni, Cu, Nb, Mg, and Ca are elements inevitably mixed in the raw material melting step. Some of these elements have been found to be effective in factories with increased strength, improved ductility and toughness, and corrosion resistance, depending on specific manufacturing conditions. However, when it is contained in a large amount, it may cause an adverse effect. Generally, it is not used for improving the quality of the rail steel, and is generally controlled to 0.2% or less.

  P, S, and O are inevitably contained in the steel. In either case, the steel is embrittled and the impact characteristics are deteriorated, so that the content is preferably 0.025% or less.

<Structure change by continuous cooling diagram, description of martensite formation process in dieburn part>
In general, the phase change in the cooling process varies depending on the steel composition and the cooling rate. FIG. 9 schematically shows the structure change of the eutectoid steel in the continuous cooling state with a CCT diagram.

  When the cooling rate is moderate as shown by the curve (1), the pearlite transformation starts on the Ps line and the pearlite transformation is completed on the Pf line. When the cooling rate increases, as shown in the cooling curve (3), the pearlite transformation stops at the temperature (B), and a part of bainite structure may be formed, but the untransformed part is supercooled as austenite, and (C The martensitic transformation occurs at points) to (D). Further, when the cooling rate is high, as shown in the cooling curve (5), the steel is supercooled to the Ms point with the austenite structure, and then martensitic transformation occurs.

  When die burn occurs, the rail surface layer is heated to the austenite region by arcing to the rail surface, and then rapidly cooled as shown by the line (5), which is considered to produce a martensite structure.

  According to the investigation of the cross-sectional structure of the site where the die burn occurred, the martensite region formed by the die burn has a case where the depth from the rail surface layer is about 15 mm at the maximum, and the width and length are at most 50 mm. The size of the region is considered to be determined by the arc discharge intensity and the number of durations.

<Martensite tempering process, deburning harmless>
Martensite is a structure in which iron atoms are displaced directly from austenite, which is a face-centered lattice, and is transformed into a tetragonal lattice, and a large amount of lattice defects such as transition are introduced during the transformation. Martensite does not contain carbides, and carbon is dissolved in the lattice. The strain of the lattice increases according to the amount of carbon, and the higher the amount of carbon, the harder. For this reason, the martensitic structure of high carbon steel such as rails is extremely hard and brittle, and rails often cause damage during use. When the martensite structure is heated, ε carbide precipitates at about 200 ° C., and the lattice strain is alleviated, so that the hardness is lowered, the brittleness is alleviated and the toxicity is lowered. Therefore, in order to make the martensite produced by the die burn harmless, it is necessary to heat at least 200 ° C. or higher, and the martensite structure fraction needs to be 20% or less as an area ratio by cross-sectional observation.

In addition, it is desirable that the range where the die burn is tempered extends to a depth of at least 5 mm from the surface. When heating is performed from the surface, the internal temperature is lower than the surface, so that the surface temperature needs to be 250 ° C. or higher in order to make the inner 5 mm or more 200 ° C. or higher.
In the region where the steel surface is heated, there is a risk of die burn in the range where the electrode is mounted on the rail, and it is desirable to heat the region and its outside by about 10 mm.

  When the heating temperature of the martensite portion is increased, the change in the composition of carbides, the increase in particle diameter, and the disappearance of lattice defects proceed, and the hardness decreases monotonously. The ductility tends to increase globally.

<Structure and hardness change due to heating of rail base material>
FIG. 10 schematically shows the relationship between the heating temperature, the structure and the hardness of the pearlite steel base material. The left end of the figure is a rail base material that is not affected by heat, and the right end shows a state heated to the solidus temperature.

  The region heated above the austenitization completion temperature is all pearlite transformed during subsequent cooling. The hardness of the portion varies depending on the cooling rate during cooling.

  When the heating temperature is between the austenitization start temperature and the austenitization completion temperature, the austenite phase and the untransformed ferrite phase or cementite phase are mixed at the time of heating. The portion transformed into austenite is transformed into pearlite by subsequent cooling, but the untransformed ferrite phase and the undissolved spheroidized cementite remain at room temperature. These structures have a lower hardness than a normal pearlite structure transformed from the austenite phase.

  Further, even in a region where the heating temperature is low and does not reach the austenitizing start temperature, the cementite in the pearlite is spheroidized and the hardness is lowered in the region heated to 500 ° C. or higher. As the heating temperature is further lowered, the degree of spheroidization becomes smaller and gradually approaches the hardness of the base material.

  The spheroidizing region from the heating temperature of 500 ° C to the austenitizing start temperature is not much different from the base material in the macro structure, but when it reaches the austenitizing start temperature, it becomes a fine grain due to the mixed phase region and is clearly different by etching with nitrate nitrate etc. Can be determined. The region heated above the austenitization completion temperature tends to increase the crystal grain size by high-temperature heating, but exhibits a structure close to the base material in terms of macro structure. In the region from 500 ° C. to the austenitization start temperature, spheroidized cementite can be confirmed by a scanning electron microscope (SEM).

  For the rail head 1 that is exposed to a wear environment due to contact with the wheels, it is necessary to limit the rail head 1 to about 600 ° C. at which the hardness is not significantly reduced. on the other hand. There is no substantial adverse effect of a decrease in hardness associated with an increase in heating temperature, and there is no problem with heating to the solidus temperature at which the rail begins to melt. However, from the viewpoint of detoxifying martensite, it is sufficient to heat to 600 ° C.

<About the appropriate reheating temperature range according to the electrode mounting position during welding>
When the martensite structure generated by dieburn is detoxified in the electrode part, the martensite structure changes according to the reheating temperature as described above. In addition, the base material pearlite structure around the martensite also undergoes structural change depending on the temperature due to reheating.

On the other hand, the rails share different functions for each part, and their desirable metal structures are different. Also in the welded portion, the function to be performed for each rail part is the same as that of the base material.
For example, since the rail head is exposed to wear, metal flow, and cracking due to contact with the wheels, it is preferable that the strength and hardness are high. For this reason, it is not preferable that the pearlite structure is spheroidized and softened by reheating. For this reason, in the case of the welding method in which the electrode at the time of welding is in contact with the rail head, the temperature at which the electrode mounting site is reheated needs to be at least 250 ° C. or more as described above. Even when the structure is spheroidized, it is desirable that the degree of strength decrease is 600 ° C. or less.

The effect of reheating temperature will be described in more detail. In the range of 250 ° C to 350 ° C, martensite is precipitated as solute carbon as carbides, but still contains a large amount of transition, and the altitude is higher than that of the rail base material. is there. In this state, martensite is tempered, and the risk of serious damage is avoided, but there is a difference in hardness from the base metal part of the rail, and when the wheel passes, the amount of strain that occurs in both is reduced. A different phenomenon occurs. This leaves some risk that cracking can occur with long-term use. On the other hand, when the reheating temperature exceeds 550 ° C., the pearlite of the rail base material starts to be spheroidized and the hardness decreases, and the hardness is lower than that of the base material portion although the degree is small.
To further reduce the risk of cracking when the rail base material and the tempered portion of the martensite deform evenly when passing through the train, the hardness difference between the tempered portion and the rail base material is smaller. It is desirable to use heating conditions. From the above viewpoint, the reheating temperature is most preferably 350 ° C. to 550.

  On the other hand, since the rail pillar portion and the foot portion do not come into contact with the wheel, high strength as in the rail head portion is not necessary. For this reason, when the electrode at the time of welding is in contact with the rail column part or the bottom part, the temperature when reheating the electrode mounting portion needs to be at least 250 ° C. or more as described above. It can be raised to the solidus temperature at which the material begins to melt. When the temperature is raised to the solidus temperature or higher, the rail surface melts and irregularities are generated on the surface, which is not preferable because it becomes a starting point of fatigue cracks during use.

  However, also in the rail column part and the foot part, it is desirable that the difference in hardness between the rail base material part and the reheating part is smaller as described in the reheating of the rail head part in the previous section. Although contact with the wheels does not occur when the train passes through the rail pillars and feet, fatigue cracks may occur in these parts depending on the track conditions, and the strain and stress are uniform in the rail base material and tempered part. This is because it is desirable. Therefore, it is most desirable that the reheating temperature is 350 ° C. to 550 also in the rail pillar portion and the foot portion.

<Dieburn heat treatment method>
Means for detoxifying martensite generated by die burn should not be particularly limited, and any of heating by a combustible gas, high-frequency induction heating, electric heating and the like can be used.

Examples of the present invention and comparative examples are shown below.
(Rail steel)
Table 1 shows three types of rails used in the examples of the present specification. Rail steel A is a steel type commonly known as a normal rail, and is a hypoeutectoid steel containing 0.65 to 0.75% by weight of carbon, and is a rolled product with a rail head hardness of 260 Vickers hardness. ~ 290. Rail steel B is a wear-resistant rail that has been heat-treated after rolling, and is a eutectoid steel containing 0.75 to 0.85% by weight of carbon. It is. Rail steel C is a hypereutectoid steel containing carbon content of 0.85 to 0.95%, and is a wear-resistant rail that has been heat-treated after rolling. It is. The rail size used was a heavy load railway size with a metric unit weight of 70 kg / m.

(Welding method)
The welding pattern is as follows: (1) First, the rail end faces are brought close to each other for 60 seconds at a flushing speed of 0.1 mm / s, and the contacted portion is melted and scattered by flushing. In this process, even if the end surface of the original rail to be welded is not parallel, the convex portion is preferentially flushed and melted and scattered, and the end surfaces of both rails become parallel by flushing for 60 seconds. This process is necessary to cause contact between the end faces uniformly over the entire cross section at the time of the next short circuit between the end faces. Next, (2) a combination of short-circuiting for 2 seconds between rail end faces and opening for 1 second is caused 8 times, a large current is intermittently passed between the rails to heat the rails, and (3) 0. After the final flushing for approaching between the rail end faces at a moving speed of 3 mm / s for 120 seconds, (4) the rails were pressurized and welded with a load of 60 tons in the upset process. This welding pattern and welding conditions were common to all the following examples.

(Die burn generation method)
As shown in FIG. 11 (a), a recess 9B having a depth of 0.5 mm, a width of 20 mm in the longitudinal direction of the rail, and a width of 20 mm is formed in the electrode of the welding machine, and 0.5 mm is formed on the contact surface with the rail surface layer 20. The gap was made. There are four electrodes in the welding machine, and the electrode 9A having the recess is attached to one or two places of the welding machine for each embodiment, and the die burn 21 is surely formed in this poor contact portion during welding. Was generated. The situation at the time of the welding is schematically shown in FIG. After welding, as shown in FIG. 11 (c), when the vertical section 24 was cut out on the rail surface in the longitudinal direction from the mounting portion 22 of the electrode 9A having the concave portion, the portion 23 where the concave portion of the electrode was located was confirmed. It was confirmed that the martensitic structure 25 was generated in a range corresponding to 5 to 20 mm in the longitudinal direction of the rail, 5 to 20 mm in the perpendicular direction, and a maximum depth of 15 mm from the surface (FIG. 11D )). Around the martensitic structure 25, a fine-textured zone 26 heated to the middle between the austenitization start temperature and the austenitization completion temperature was observed.

(Post-heat treatment method for die burner)
The heating of the die burner generating part after welding was performed using an induction heating method in which an induction current was generated on the rail surface using an electromagnetic coil, and the Joule heat generation was generated. The induced current and ultimate temperature generated on the rail surface vary depending on the shape of the electromagnetic coil used, the exciting current flowing through the coil, and the exciting time. Prior to the experiment, the relationship between the current, the excitation time and the temperature reached on the rail surface was obtained in advance, and the excitation current and the excitation time were changed so that the target rail surface temperature was obtained. As shown in FIG. 12, the heating range 27 was set to a range of 100 mm in the rail longitudinal direction and 50 mm in the direction perpendicular to the rail longitudinal direction, including the electrode mounting range 22. It was kept at the heating set temperature for 30 seconds and allowed to cool naturally after completion of heating. The cooling rate at that time is about 1.0 ° C./s.

(Evaluation test method)
Depending on the example, 2 to 3 welds were reheated under the same conditions. One sample was cut out from a die burn generation part to examine the hardness and metal structure, and the other test welds were subjected to a surface damage resistance evaluation test, a bending fatigue test, or both.

(Hardness, metal structure test)
As shown in FIG. 11C, the hardness and metal structure were inspected by cutting out a longitudinal section perpendicular to the rail surface where the recess 9B of the electrode 9A was located. This section was mirror-polished and measured with a measurement load of 100 N with a Vickers hardness meter at a position 30 mm from the rail surface at a die burn generation position as shown in FIG.

  In curved sections and heavy-duty railways, rail steels B and C having a rail head hardness of around Hv400 are often used. In the welding for mounting the electrode on the rail head, when heat treatment is performed on the die burner of the electrode mounting portion, if the reheating temperature is excessive, softening occurs due to spheroidization of pearlite. As a result, wear progresses significantly over a wide range during use, and the wear depth also increases. When heat-treating the head using a high-hardness rail, if the hardness after the heat treatment is less than Hv300, the difference in the amount of wear with the base material is not desirable.

  The metal structure was observed by etching the hardness measurement sample with 3% nitrate alcohol (Nital solution). Since there is no carbide in the martensite structure, it is not etched with the nital solution and appears white under a microscope. On the other hand, the pearlite structure, the bainite structure, and the tempered martensite structure are etched by the nital liquid according to the form of the carbide and are clearly distinguished from the martensite.

The structure fraction of the metal structure was calculated by the point counting method (point counting method), with the observation region 20 × 20 mm near the rail surface, directly under the electrode recess, observed at a magnification of 50 times. The point calculation was carried out with reference to the microscopic examination method for inclusions shown in Annex 1 (normative) of JIS G0555. The number of inclusions indicated by JIS was replaced with the number of each structure, and the area ratio was calculated.
As described above, when the martensite structure exceeds 20% in the structure fraction, the material is remarkably deteriorated.

  When the heating temperature exceeded the austenitization completion temperature, the martensite structure generated by the diburning disappeared completely. Also in this case, the structure observation and the hardness measurement were performed in the vicinity where the concave portion 9B of the electrode was located, where the die burn was supposed to exist.

(Surface damage resistance test method)
The surface damage resistance evaluation test was performed with a tester schematically shown in FIG. In this test, the test rail is fixed to a horizontally moving carriage 33 that slides. The horizontally movable carriage 33 has a roller 34 and reciprocates horizontally on the surface plate 35. The horizontal movement of the horizontal movement carriage 33 is performed by a hydraulic cylinder.

  A test rail is installed on the moving carriage 33 so that the electrode having a concave portion is attached at the time of welding, and the area where the die carriage is generated becomes an intermediate point when the moving carriage 33 reciprocates. Each side was 500 mm from the part. The moving speed of the carriage 33 was 800 mm / s.

  A vertical load 32 is applied to the wheel 15 downward by hydraulic pressure. In this test, it was set at 15 tons, which is a common wheel load on heavy freight railways. This vertical load 32 is equal to the vertical load Py that the rail receives from the wheel 15. The vertical load was applied when the carriage moved in the forward direction, and the wheel 15 was lifted on the return path to make no load. That is, the plastic flow on the rail surface was caused to occur in a certain direction.

  Further, the rotation of the wheel 15 is driven by a motor 31. By adjusting the torque and the driving force for horizontal movement of the carriage 33, the contact surface between the rail and the wheel 15 can be loaded with a tangential force Px. In this test, Px was 1 ton.

  The test piece was reciprocated until the cumulative vertical load tonnage reached 15 million tons, and then the surface condition of the rail was observed, and if the damage occurred, the width and length were recorded. Further, when uneven wear occurred due to contact with the wheel 15, the amount of local wear was measured.

  Although minute surface damage may occur in the curved portion of the actual trajectory, it rarely results in surface peeling. However, damage caused by the generation of martensite such as diburn may lead to surface peeling or breakage, and the occurrence of damage itself is not preferable.

  In addition, if the amount of uneven wear is 0.5 mm or more, noise and vibration when passing through the wheel become significant, and it is assumed that the progress of uneven wear will be accelerated thereafter, and it may lead to surface damage. Is not preferable.

(Fatigue test method)
The evaluation test of bending fatigue strength was performed by a three-point bending method. FIG. 15 schematically shows the test method. A die-burn generating portion was placed at the center of the pedestal 38 set at a distance of 1 m, and a load was applied from the rail head at the midpoint of the pedestal 38 with the pushing jig 39. The curvature radius of the part which touches the rail of the base 38 and the pushing jig | tool 39 was 100 mmR. The test stress was set at the center of the sole of the rail. The minimum stress was 30 MPa, the maximum stress was 330 MPa, and the stress fluctuation range was 300 MPa. A normal flash butt weld joint has a fatigue life of up to 2 million times in a stress range of 300 MPa. The load repetition rate was 5 Hz, and the test was terminated when a crack occurred in the weld. Moreover, when the load repetition number was non-ruptured up to 2 million times, the test was terminated, and it was judged that the tire had sufficient fatigue performance.

<Example 1>
An example in which die burn is generated in the rail head during flash butt welding and the portion is reheated to various temperatures will be described below.

Steel B in Table 1 was used for the rail to be welded. The arrangement position of the electrode 9A provided with the recess is shown in FIG.
As an evaluation test, metal structure observation, hardness measurement, and surface damage resistance test were performed at the site where the die burn occurred. Table 2 shows the results of the reheating temperature condition and the surface damage resistance test.

  In Examples A1 to A3 in which the temperature of the die burn portion was heated to a range of 250 to 600 ° C., the martensite structure in the die burn portion was 20% or less, and as a result of the damage test, no surface damage was observed in the rail head.

  On the other hand, Comparative Example A1 did not reheat the die burn part, and martensite remained on the rail as it was, and surface damage occurred in the damage test.

  In Comparative Examples A2 and A3, the reheating temperature of the die burn part was low, and a large amount of martensite remained on the rail, and surface damage occurred in the damage test.

  In Comparative Example A4, the reheating temperature of the die burn portion was too high, and softening of hardness Hv300 or less occurred due to the spheroidization of the carbide, and local wear of 0.5 mm or more occurred in the damage test.

  In A5, the reheating temperature of the dieburn part is higher and the dieburn part has a pearlite structure, but in the region heated to 600 to 700 ° C. at the edge of the heating part, the hardness becomes Hv 300 or less due to the spheroidization of carbide. Softening caused local wear of 0.5 mm or more in the damage test.

<Example 2>
Next, in the flash butt welding, an example is shown in which a die burn is generated on the rail sole and the portion is reheated to various temperatures.

  Steel A in Table 1 was used for the rail to be welded. The arrangement position of the electrode 9A provided with the recess is shown in FIG. In the evaluation test, metal structure observation, hardness measurement, and bending fatigue test were performed at the site where the die burn occurred. Table 3 shows the reheating temperature conditions and the results of the bending fatigue test.

  In Examples B1 to B6 in which the temperature of the dieburn part was heated to a range of 250 ° C. to the solidus temperature or less, the martensite structure in the dieburn part was 20% or less, and as a result of the fatigue test, fatigue was repeated up to 2 million cycles. No damage occurred.

  On the other hand, Comparative Example B1 did not reheat the dieburn part, martensite remained on the rail as it was, and it broke early in the fatigue test.

  In Comparative Examples B2 and B3, the reheating temperature of the die burn part was low, and a large amount of martensite remained on the rail, and early fracture occurred in the fatigue test.

  In Comparative Example B4, the reheating temperature of the die burn portion was too high, the surface of the rail foot was melted, and early fracture occurred in the wear test.

<Example 3>
In this embodiment, when the rail is flash-butt welded, a die burn is generated in the rail head portion and the rail foot sole portion, and the portion is reheated to various temperatures.

  Steel C in Table 1 was used for the rail to be welded. The arrangement position of the electrode 9A provided with the recess is shown in FIG. In the evaluation test, metal structure observation, hardness measurement, and surface damage test were performed at the site where the die burn occurred. Table 4 shows the reheating temperature conditions, the cross-sectional structure, and the hardness test results, and Table 5 shows the results of the surface damage resistance test and the bending fatigue test.

  Examples C1 to C9 in which the temperature of the head portion of the diburn portion was heated to a range of 250 to 600 ° C. and the temperature of the foot portion of the diburn portion were heated to a range of 250 ° C. to the solidus temperature were martensite in the diburn portion The organization became less than 20%. As a result of the damage test, no surface damage or abnormal wear was found on the rail head. Further, in the fatigue test, it did not break until the load was repeated 2 million times.

  On the other hand, in Comparative Example C1, the heating temperature of the die burner portion of the head was high, and local wear occurred in the damage test due to softening due to the spheroidization of carbide.

  In Comparative Example C2, the reheating temperature of the die burner portion on the sole was low, and a large amount of martensite remained on the rail, resulting in early fracture in the fatigue test.

  In Comparative Example C3, the reheating temperature of the dieburn part of the sole was too high, the surface of the rail sole melted and was prematurely broken in the wear test.

  In Comparative Example C4, the reheating temperature of the die burn portion was low, and a large amount of martensite remained on the rail, and surface damage occurred in the damage test. In addition, the reheating temperature of the die-burn part on the sole was low, and a large amount of martensite remained on the rail, resulting in early fracture in the fatigue test.

<Example 4>
Next, in welding using a movable flash butt welder for field welding in which electrodes are mounted on the rail column, die burn was generated in the rail column during welding, and the part was reheated to various temperatures Indicates. Steel C in Table 1 was used for the rail to be welded.

  FIG. 19 schematically shows a column electrode 40 and a column electrode 40A having a recess. The arrangement position of the electrode 40A provided with the recess is shown in FIG. In the evaluation test, metal structure observation, hardness measurement, and bending fatigue test were performed at the site where the die burn occurred. Table 6 summarizes the results of the reheating temperature condition and the bending fatigue test.

  In Examples D1 to D6 in which the temperature of the dieburn part was heated to a range of 250 ° C. to the solidus temperature or less, the martensite structure in the dieburn part was 20% or less. No damage occurred.

  On the other hand, Comparative Example D1 did not reheat the dieburn part, and martensite remained on the rail as it was, and it broke early in the fatigue test.

  In Comparative Examples D2 and D3, the reheating temperature of the die burn portion was low, and a large amount of martensite remained on the rail, and early fracture occurred in the fatigue test.

  In Comparative Example D4, the reheating temperature of the die burn part was too high, and a part of the surface of the rail column melted, resulting in early fracture in the fatigue test.

DESCRIPTION OF SYMBOLS 1 ... Rail head part, 2 ... Rail pillar part, 3 ... Rail foot part, 4 ... Rail top part, 5 ... Rail foot table, 6 ... Rail Sole, 7 ... weld, 8 ... weld bead, 9 ... electrode, 9A ... electrode with recess, 9B ... recess in electrode with recess, 10 ... rail to be welded , 11 ... weld bead by upset, 12 ... trimmer, 13 ... power supply, 14A ... surface damage, 14B ... fatigue crack, 15 ... wheels, 16 ... wheels and rails , Tangential stress in the contact surface, 19 ... sleepers, 20 ... surface layer portion of the rail on which the electrodes are mounted, 21 ... arc by diburn, 22. -Electrode contact range, 23 ... Corresponding position where the recess of the electrode was present, 24 ... Cross section of the structure inspection, 25 ... Martensite structure caused by dieburn, 26・ Fine-grained texture zone around martensite structure caused by dieburn, 27 ・ ・ ・ Reheating range in Examples, 28 ・ ・ ・ Heat-affected structure due to reheating, 29 ・ ・ ・ Speculum range, 30 ・ ・ ・ Hardness Measurement position, 31 ... Wheel drive motor of damage evaluation tester, 32 ... Vertical load, 33 ... Horizontal moving carriage, 34 ... Roller, 35 ... Surface plate, 36 ... Moving carriage Arrow indicating the reciprocating direction, 37 ... distribution of bending stress, 38 ... test piece support, 39 ... pressure jig, P ... test load, 40 ... pillar in the horizontal direction Electrode of the type to be attached to the electrode, 40A...

Claims (4)

A heat treatment method after flash butt welding the vicinity of the rail, reheating the weld heat affected zone outside the rail head surface of the electrodeposition GokuSo fixing position of the welding is of the weld 600 ° C. or less 250 ° C. or higher A post-heat treatment method in the vicinity of a flash butt weld. A heat treatment method after flash butt welding the vicinity of the rail, the rail foot backside surface of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, below the solidus temperature A post-heat treatment method in the vicinity of a flash butt weld, characterized by reheating. A heat treatment method after flash butt welding the vicinity of the rail, the rail foot backside surface of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, below the solidus temperature 2. The post heat treatment method in the vicinity of a flash butt weld according to claim 1, wherein reheating is performed. A heat treatment method after flash butt welding the vicinity of the rail, the rail pillar portion surfaces of the electrodeposition GokuSo fixing position of the welding is outside the weld heat affected zone of the weld 250 ° C. or higher, below the solidus temperature A post-heat treatment method in the vicinity of a flash butt weld, characterized by reheating.

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