JPH05255735A - Improvement of fatigue strength of weld zone of rail - Google Patents

Abstract

PURPOSE:To effectively improve fatigue strength in a weld zone on an actual line which is the weakest point at the time of forming a track into long rail. CONSTITUTION:Repeated load is applied to welded rail in a vertical direction of the rail by the times between once and ten times so that the surface of a railhead and the surface of a rail foot are subjected to plastic deformation and compressive residual stress is applied to the rail foot side of the weld zone of rail, by which fatigue strength in the weld zone is improved. A tensile residual stress of about 250MPa is established at the surface of the rail foot in the as-welded weld zone. The residual stress at the surface of the rail foot of the weld zone of a rail produced by this invention is formed, on the contrary, into a compressive residual stress of about 200MPa, and as a result, fatigue strength can be improved by >=1.3 times the fatigue strength of as-welded state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the fatigue strength of rail welds.

[0002]

2. Description of the Related Art With the aim of improving the efficiency of railway transportation, further speeding up of trains, maintenance-free operation, and comfortable operation are being aimed at. From these trends, there is a demand for even greater damage resistance for rails that support trains, and at the same time, longer rails are being made by welding the rails.

Commonly used rail welding methods include flash butt welding, gas pressure welding, enclosed arc welding and thermite welding. The former two methods are generally carried out at the plants (bases) of JR and private railway companies, and the latter two methods are generally carried out locally on actual routes. Flash butt welding is a method in which the vicinity of both rail ends to be welded is sandwiched by electrodes, and the rail ends are melted by discharging the joint ends, and then both rails are pressed to join. Gas pressure welding is basically the same as flash butt welding as a joining method, and differs from flash butt welding only in that gas is used as a melting heat source at the rail end. These welding methods carried out in factories (bases) are basically less likely to cause defects due to melt pressure welding, and have high reliability because the welding environment and welding execution conditions are stable.
Therefore, the fatigue strength of the rail weld is also very good.

However, in the case of the latter two welding methods, the enclosed arc welding is a kind of manual arc welding, and the quality of the welded portion depends on the technique of the welder. Since this is a kind of casting that is made by casting a mold between rails, it has drawbacks such as easy occurrence of casting defects. Furthermore, due to the time constraints such as sewing between train passages and the fact that the welding environment is bad because it is carried out on an actual route, these welds have low reliability. In reality, the fatigue strength was low. Therefore, the points where the damage occurs on the actual route are these field welding points, and improvement of the fatigue strength of the field welding points such as enclosed arc welding and thermite welding has been the biggest issue.

In order to improve the fatigue strength of these welds, conventionally, the welding technique has been improved in the case of enclosed arc welding, and the casting defect mitigation technique has been achieved in the case of thermite welding by improving the mold. Came. As a result, the fatigue damage from these welds on the actual line has been considerably reduced, but the fatigue damage from these welds has still been observed. Since then, measures have been taken to reduce stress concentration by removing excess welds, and to make the hardness distribution uniform by post-weld heat treatment, until the fatigue strength of these welds is drastically improved. Had not reached.

[0006]

Therefore, the present inventors have found that
We investigated in detail about the main cause of fatigue damage in the field welding spots such as enclosed arc welding and thermite welding for the actual damage rail. As a result, (1) the starting point of fatigue damage is not a welding defect or the like, but in the case of enclosed arc welding, it is a grinding defect of welding excess,
In the case of thermite welding, it should be the stress concentration point at the weld toe end. (2) A large tensile residual stress exists in the rail longitudinal direction in both the enclosed arc welding and thermite welding on the rail bottom side of the weld that is the starting point of fatigue failure. Clarified the above. Therefore, in order to improve the fatigue strength of these on-site welds, it is important to minimize the stress concentration in the weld and to reduce or change the tensile residual stress in the rail longitudinal direction at the bottom of the weld to the compression side. I found out.

[0007] Based on the above knowledge, firstly, efforts were made to improve the fatigue strength of the weld by minimizing the stress concentration in the weld. In the case of enclosed arc welding, the fatigue strength was investigated for rails in which the grinder polishing machine was changed from # 25 to # 120 in order to minimize the excess welding grinding flaw. As a result, it was found that the smaller the roughness of the polishing plate, the higher the fatigue strength. However, when welding in a factory (base) such as flash butt welding or gas pressure welding, it can be carefully finished with a polishing machine with a small roughness, but on the actual line site it is done by sewing between train passages. , We can't expect a polite finish over time like a welded part in a factory (base). Therefore, it was found that it is necessary to improve the fatigue strength by reducing the tensile residual stress in the rail longitudinal direction at the bottom of the welded portion or changing it to the compression side.

On the other hand, in the case of thermite welding, it was examined how much the fatigue strength could be improved by punching out the weld surplus by shearing and reducing the stress concentration at the surplus toe. As a result, it was found that the fatigue limit could be improved by about 20 to 30 MPa. In order to further improve the fatigue strength, it was found that residual stress control of rail welds is necessary as in the case of enclosed arc welding.

As a method for reducing the tensile residual stress in the longitudinal direction of the rail at the bottom of the welded portion or changing it to the compression side, JP-A-59-93837 and JP-A-62-22704 are known.
There is a method of controlling residual stress by welding heat or reheating after welding as in Japanese Patent Laid-Open No. However, in these methods, it is difficult to control whether or not the residual stress is reliably controlled, and in the case of reheating or the like, a heating device and a cooling device are required, and the residual stress control is costly. There are some drawbacks.

Therefore, the present inventors sought a low cost and simple rail residual stress control method, which is an alternative to the above method. As a result, focusing on the fact that when the rail is bent, a residual compressive stress is generated on the side that is plastically deformed in tension, and the joint part of the welded rail is plastically deformed on the rail head surface and bottom surface in the rail vertical direction. It was found that a method of applying a tensile stress to the rail bottom surface to straighten the rail and simultaneously applying a compressive residual stress to the rail bottom surface is obtained by applying a tensile plastic deformation to the rail bottom surface.

[0011]

The present invention has been made on the basis of such knowledge, and the gist thereof is as follows.
Repeated load is applied to the welded rail joint in the vertical direction of the rail within a range of 1 to 10 times so that the rail head surface and bottom surface are plastically deformed, and the rail bottom surface is pulled by the final load. This is a method for improving the fatigue strength of rail welds that causes plastic deformation.

The present invention will be described in detail below with reference to the drawings. FIG. 1 shows a cross section of the rail. First, for convenience of description, the names of each part of the rail will be described. In FIG. 1, 1
The range 2 is the head, the range 2 is the abdomen, and the range 3 is the bottom. FIG. 2 shows a side view of the welded rail. Y indicates a welded portion. Even if the welding rail has welding excess, it does not matter if the excess is ground and removed.

The reason why repeated load is applied so that the rail head surface and the bottom surface are plastically deformed in the vertical direction of the rail is that the rail welded almost straightly and finally the surface of the rail bottom side is plastically deformed by tension. To give. Therefore, the shortest number of repetitions in the vertical direction of the rail is one. That is, first, a span is provided on the rail head side so that the rail head surface is subjected to tensile plastic deformation, and the rail bottom portion side is loaded, and then a rail bottom surface side span is provided and the rail head portion side is loaded to load the rail bottom portion. The side surface is subjected to tensile plastic deformation. As a result, compressive residual stress can be applied to the rail bottom surface. The reason why the number of repetitions in the vertical direction of the rail is limited to 10 times is that when a repeated load of 10 times or more is applied until the rail head surface and the bottom surface are plastically deformed, fatigue damage occurs on the rail head surface and the bottom surface. However, if the number of repetitions increases, the residual stress on the rail surface may gradually decrease. Therefore, the number of repetitions is preferably as small as possible.

The reason why the last load must be applied from the rail head side to give tensile plastic deformation to the rail bottom surface is to compress the rail bottom surface by tensile plastic deformation. This is because the residual stress of Although the magnitude of the load in the vertical direction of the rail is not particularly limited, the point is that the load that plastically deforms the rail head surface and the bottom surface must be applied. The load can be selected from those in which the relationship between the load and the displacement has been obtained in advance in relation to the rail shape, steel type, load supporting span, rail welding method, and the like. It is preferable to select a load in which the surface strain of the rail head and the surface strain of the bottom are within the range of 0.5% to 3% from the viewpoint of preventing sudden breakage during bending and residual stress.

FIG. 3 shows the load condition of the welded rail.
In FIG. 3, A and B are welded rails, G is a load jig, and S is a load support jig. FIG. 4 shows the relationship between the load of the welding rail due to the load shown in FIG. 3 and the displacement of the rail bottom surface. The upper part of the vertical axis represents tensile load, the lower part represents compressive load, the right part of the horizontal axis represents tensile displacement, and the left part represents compressive displacement. A solid line A in FIG. 3 indicates a load supporting jig S on the rail head side by the method of the present invention.
Is provided to show the deformed state of the rail loaded from the bottom side of the rail. A solid line a in FIG. 4 conceptually shows a load-deformation curve of the bottom of the rail welded portion corresponding to the solid line A in FIG. After loading from the rail bottom side, a load supporting jig S is provided on the rail bottom side this time, and a deformed state of the rail loaded from the rail head side is shown by a broken line B in FIG. A broken line b in FIG. 4 conceptually shows a load-deformation curve of the bottom portion of the rail welded portion corresponding to the broken line B in FIG. It can be seen that a residual tensile strain remains on the bottom surface of the rail welded portion, so that a compressive residual stress occurs in that portion.

In the present invention, the loading method is not specified. For example, it is possible to provide a span corresponding to the interval between the sleepers of the actual line, set the load support jig S on the span portion, and gradually load the load support jig S from the rail head side and the bottom side. For the load, a hydraulic jack or a mechanical screw jack can be used. Regarding the final load level, if the relationship between the rail springback amount and the load is obtained in advance in the laboratory by changing the rail shape, steel type, welding method, etc., the rail is loaded from the head side and the rail bottom side is loaded. The load can be selected so that the rail becomes straight when unloading after the surface is plastically deformed by tension.

How the residual stress of the rail according to the present invention differs from that of the as-welded case was determined using X-rays, and the results are shown in Table 1. When the rail is welded without correction, the residual stress in the longitudinal direction of the rail shows a tensile residual stress of about 196 MPa to 250 MPa at the center of the rail head and the center of the bottom. On the other hand, the rail according to the method of the present invention is 167 at the center of the rail head.
MPa and 176MPa, which are less than as-welded, and the rail bottom surface that affects fatigue strength is 196MPa and 206MPa.
It can be seen that the compressive residual stress has changed to MPa. It is known that the compressive residual stress has a very effective effect on fatigue strength.
Fatigue failure will not occur unless a load such as a tensile stress above a is applied from the rail head side. Regarding the tensile residual stress on the surface of the rail head side, the residual stress on the rail head side welded part is applied with the excess load applied, and after the rail is straightened, the weld excess is ground to remove the tensile residual stress. Can be removed.

[0018]

[Table 1]

[0019]

EXAMPLES Specific examples will be described below. In order to confirm how much the fatigue strength improving effect according to the present invention is, 60 kg / m ordinary carbon steel rail was welded by the enclosed arc welding method and thermite welding method which are generally used as the field welding method. . Table 2 shows the residual stress measurement results in the rail longitudinal direction by X-ray after welding. Further, the results of measurement of residual stress in the longitudinal direction of the rail by X-ray of the rail to which the fatigue strength improving method according to the present invention is applied are also shown in Table 2. A three-point bending fatigue test was performed on these rails using a 980 KN hydraulic fatigue tester. The cyclic stress range was 294 MPa, which is the lower limit of the fatigue limit of a sound enclosed arc welded joint. The fatigue test results are also shown in Table 2.

As is clear from Table 2, the fatigue strength improving method according to the present invention is very effective. That is, in the as-welded state, even if the rail bottom side is ground and finished using a # 80 grinder, both the enclosed arc welded joint and the thermite welded joint undergo fatigue fracture, whereas
In the method of the present invention, no fatigue fracture was observed in any of the welding methods, and it can be seen that a very excellent fatigue strength is exhibited.

As described above, the reason why the fatigue strength of each welded joint is excellent is that it is based on the compressive residual stress of about 200 MPa generated by the plastic deformation by tensile deformation on the rail bottom surface. Therefore, it is expected that the fatigue strength of the rail bottom side can be significantly improved by the method of the present invention even with excess welding, and the construction process can be largely omitted and the cost can be reduced.

[0022]

[Table 2]

[0023]

As described above, according to the present invention, a load is applied to the welded rail repeatedly in the vertical direction so as to cause reverse deformation, thereby imparting a compressive residual stress to the surface of the rail bottom side, and Fatigue strength can be significantly improved. In addition, the construction process can be largely omitted, and the construction on site can be facilitated.

[Brief description of drawings]

FIG. 1 is a view showing a cross section of a rail.

FIG. 2 is a view showing a side surface of a welded rail.

FIG. 3 is an explanatory view showing a welding rail load state of the present invention.

FIG. 4 is a diagram showing the relationship between the load due to the load on the welding rail and the displacement of the rail bottom surface.

[Explanation of Codes] 1: Rail head 2: Rail abdomen 3: Rail bottom A, B: Weld rail after loading G: Load jig S: Load support jig Y: Weld section

Claims (1)

[Claims] 1. A welded rail joint portion is repeatedly loaded in the vertical direction of the rail within a range of 1 to 10 times so that the rail head surface and the bottom surface are plastically deformed, and the rail bottom portion is subjected to the final load. A method for improving the fatigue strength of rail welds, characterized by applying tensile plastic deformation to the side surface.