JP2579096B2 - Welding method to improve fatigue strength of rail joint - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art In order to increase the efficiency of rail transport, further speeding up, maintenance free, and comfortable operation of trains are being pursued. Due to these trends, rails supporting trains are required to have even higher damage resistance, and at the same time, long rails are being welded by welding rails.

[0003] As a method for welding rails, flash butt welding, gas pressure welding, enclosed arc welding, and thermite welding are commonly used. The former two laws are generally implemented at the plants (bases) of JR and private railway companies, and the latter two laws are generally implemented on actual routes.

[0004] Flash butt welding is a method in which the vicinity of both rail ends to be welded is sandwiched between electrodes, a discharge is applied to the joint end to bring the rail end into a molten state, and then the two rails are pressed and joined. The gas pressure welding is basically the same as the flash butt welding as a joining method, and is different from the flash butt welding only in that a gas is used as a heat source for melting at the end of the rail. These welding methods carried out at a factory (base) are basically less likely to generate defects due to fusion welding and are highly reliable because the welding environment and welding conditions are stable. Therefore, the fatigue strength of the rail weld is also very excellent.

However, in the latter two welding methods, enclosed arc welding is a kind of manual arc welding, and the quality of a welded portion is affected by the skill of a welder. Since it is a type of casting in which a mold is provided between the rails and cast, there are drawbacks such as easy occurrence of casting defects and the like. In addition, these welds have low reliability due to time constraints such as welding during the train passing because they are carried out on actual routes and the poor welding environment. In fact, the fatigue strength was also low. Therefore, the locations on the actual route where damage has occurred are these on-site welding locations, and improvement of the fatigue strength of on-site welding locations such as enclosed arc welding and thermit welding has been the biggest issue. Conventionally, in order to improve the fatigue strength of these welds, welding techniques have been improved in the case of enclosed arc welding, and techniques for reducing casting defects have been attempted in the case of thermite welding by improving the mold. As a result, although the fatigue failure from these welds on the actual route has been considerably reduced, fatigue failure from these welds has been still found. After that, further reduction of stress concentration by removing extra welded parts, uniformity of hardness distribution by heat treatment after welding,
Although such measures have been implemented, the fatigue strength of these welds has not been drastically improved.

[0006]

SUMMARY OF THE INVENTION Accordingly, the present inventors
The actual cause of fatigue failure at on-site welding spots such as enclosed arc welding and thermite welding was investigated in detail for actual damaged rails. As a result, (1) the starting point of fatigue failure is not a welding defect or the like; in the case of enclosed arc welding, it is a grinding flaw in the extra weld; in the case of thermite welding, it is a stress concentration point at the toe of the extra weld. thing. (2) A large tensile residual stress exists in the longitudinal direction of the rail in both the enclosed arc welding and the thermite welding on the rail bottom side of the welded portion that is the fatigue damage starting point. I clarified the above. Therefore, in order to improve the fatigue strength of these on-site welds, it is important to minimize the stress concentration at the weld and reduce the residual tensile stress in the longitudinal direction of the rail at the bottom of the weld or change it to the compression side. I understood that.

[0007] Based on the above knowledge, firstly, efforts were made to improve the fatigue strength of the welded portion by minimizing the stress concentration in the welded portion. In the case of the enclosed arc welding, in order to minimize the welding excess grinding flaw, the fatigue strength was investigated for rails in which the grinder polishing machine was changed from # 25 to # 120. As a result, it was found that the smaller the roughness of the polishing machine, the higher the fatigue strength. However, when welding at a factory (base) such as flash butt welding or gas pressure welding, it can be carefully finished with a small roughness polishing machine. It is not possible to expect timely and careful finishing like a weld at a factory (base). Therefore, it was found that it was necessary to improve the fatigue strength by reducing or changing the tensile residual stress in the longitudinal direction of the rail at the weld bottom to the compression side.

[0008] On the other hand, in the case of thermite welding, it was examined how much fatigue strength can be improved by punching out the excess weld by shearing and reducing the stress concentration at the excess weld 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 it was necessary to control the residual stress in the rail weld as in the case of enclosed arc welding.

As a method for reducing or changing the tensile residual stress in the longitudinal direction of the rail at the bottom of the welded portion to a compression side, Japanese Patent Application Laid-Open Nos. 59-93837 and 62-22704 disclose a method.
There is a method of controlling the residual stress by welding heat or reheating after welding as disclosed in Japanese Patent Application Publication No. 0-205. However, these methods increase the cost of residual stress control because it is difficult to control whether the residual stress is reliably controlled, and in the case of reheating, a heating device and a cooling device are required. There are drawbacks such as things.

Accordingly, the present inventors have sought a low-cost and simple rail residual stress control method which is an alternative to the above method. As a result, when bending the rail, the focus was on the generation of residual compressive stress on the plastically deformed side of the tension. Later, a method was proposed in which a load was applied from the rail head side to apply tensile plastic deformation to the rail bottom side surface to straighten the rail and simultaneously apply compressive residual stress to the rail bottom side. The present invention provides such a method for improving the fatigue strength of a rail weld.

[0011]

DISCLOSURE OF THE INVENTION The present invention has been made based on the above findings, and the gist of the present invention is that when the rails are opposed to each other and welded, the rail head side of the welded portion is welded. Welding by applying reverse strain so that the amount of angular deformation per 1 m length of the rail to be convex is 5 to 25 mm, and then loading from the rail head side until the amount of angular deformation disappears,
This is a welding method for improving the fatigue strength of a rail butt portion that applies tensile plastic deformation to the bottom surface of the rail welding portion.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 shows a cross section of the rail. First, the names of the rails will be described for convenience of explanation. In FIG. 1, 1
Is the head, 2 is the abdomen, 3 is the bottom. FIG. 2 shows a state of angular deformation after welding of the rail according to the present invention. Y indicates the welded portion, and S indicates the amount of angular deformation after welding.
Before the rail welding, reverse angular distortion S must be applied so that the amount of angular deformation S after welding is 5 to 25 mm per 1 m of rail length with the rail head side convex. The reason why the amount of angular deformation after welding must be convex on the rail head side is to straighten the rail while applying tensile plastic deformation to the rail bottom side surface by applying a load from the rail head side. This is because a residual stress of compression is applied to the surface of the rail bottom side by subjecting the rail bottom side surface to tensile plastic deformation. The amount of angular deformation S after welding is 5 per m of rail length.
The reason for limiting to 25 mm is that if the amount of angular deformation is 5 mm or less, the amount of tensile plastic deformation of the rail bottom side surface is small, and it is impossible to apply a large compressive residual stress effective for fatigue resistance to the rail bottom side surface. is there. Maximum value of angular deformation is 25
The reason for limiting to mm is that if the angular deformation exceeds 25 mm,
When trying to straighten the rail by applying load from the rail head side, there is a risk that static damage will occur in thermit welding, etc., and with an angular deformation of 25 mm or more, residual compression of the rail bottom side surface will no longer occur. This is because the stress value indicates saturation and is not so significant in practical use.

A solid line A in FIG. 3 shows a state in which a rail having angular deformation according to the present invention is loaded from the rail head side.
G indicates a load jig, and C indicates a load support jig. The present invention does not particularly define the loading method. A span corresponding to, for example, the interval between sleepers is provided on the rail bottom side, a load supporting jig C is set on the span section, and a load jig G is gradually applied from the rail head side. The load can be a hydraulic jack or a mechanical screw jack. To determine how much load is applied, if the relationship between the springback amount of the rail and the load is determined in advance in a laboratory by changing the rail shape, steel type, welding method, etc., the rail can be loaded from the head side. After the rail bottom side surface is plastically deformed by tension, a load can be selected such that the rail becomes straight when unloaded. The broken line B in FIG. 3 shows the rail of the present invention in a state where the load has been completed.

How the residual stress of the rail according to the present invention is different from that of the as-welded rail is determined by a cutting method using a strain gauge, and the results are shown in Table 1.

[0015]

[Table 1]

[0016] As is, the rail longitudinal residual stress shows a residual tensile stress of about 200 to 250 MPa at both the center of the rail head and the center of the bottom of the rail almost independently of the welding method. On the other hand, the rail according to the method of the present invention is almost the same as in the as-welded state at the center of the rail head, but it has been found that the residual stress on the rail bottom side surface, which affects the fatigue strength, changes to about 250 MPa. It is known that compressive residual stress has a very effective effect on fatigue strength. In short, a load that applies a tensile stress of about 250 MPa or more to the rail bottom surface is not applied from the rail head side. And fatigue failure does not occur. Regarding the tensile residual stress on the rail head side surface, the load is applied while the rail head side welded portion is left as it is, and after the rail is straightened, the residual weld stress is ground to reduce the tensile residual stress. Can be removed.

[0017]

EXAMPLES Specific examples will be described below. In order to confirm the effect of improving the fatigue strength according to the present invention, a 60 kg / m ordinary carbon steel rail was separately applied to each of the rails by an enclosed arc welding method and a thermite welding method which are generally used as field welding methods. Welding was performed by applying a reverse strain such that the head side became convex. Table 2 shows the results of measurement of the rail angular deformation and the residual stress in the rail longitudinal direction by X-rays after welding. Table 2 also shows the residual angular deformation of the rail to which the method for improving fatigue strength according to the present invention was applied and the measurement result of the residual stress in the rail longitudinal direction by X-ray.

These rails were subjected to a three-point bending fatigue test using a 980 KN hydraulic fatigue tester. The cyclic stress range applied was 294 MPa, which is the lower limit of the fatigue limit of a sound enclosure arc welded joint. Table 2 also shows the results of the fatigue test.

[0019]

[Table 2]

As is apparent from Table 2, the method for improving fatigue strength according to the present invention is very effective. In other words, as-welded, even if the rail bottom side is ground and finished using a # 80 polishing machine, both the enclosed arc welded joint and the thermite welded joint undergo fatigue fracture,
According to the method of the present invention, no fatigue fracture was observed in any of the welding methods, and it was found that the method exhibited extremely excellent fatigue strength. Thus, the reason why the fatigue strength of each welded joint in the present invention is excellent is that it is based on a compressive residual stress of about 245 MPa generated by tensile plastic deformation of the rail bottom surface. Therefore, it is expected that the method of the present invention can significantly improve the fatigue strength of the rail bottom side even if the welding is left as it is, so that the construction process can be largely omitted and the cost can be reduced.

[0021]

As described above, according to the present invention, it is possible to apply a compressive residual stress to the bottom side of the rail by extremely simple and low-cost means after welding, thereby providing a rail welding having extremely excellent fatigue strength. Department can be provided.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a state of angular deformation after welding of the rail of the present invention.

FIG. 3 is a diagram showing a situation in which the angularly deformed rail of the present invention is loaded from the rail head side.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rail head 2 Rail abdomen 3 Rail bottom A Angular deformation rail of the present invention B Rail after load C Load support jig G Load jig S Angular deformation

Claims (1)

(57) [Claims] When welding rails facing each other, welding is performed by applying a reverse strain so that the amount of angular deformation per 1 m length of a rail having a convex portion on a rail head side of a welding portion is 5 to 25 mm. A method for improving the fatigue strength of rail joints, wherein a load is applied from the rail head side until the amount of angular deformation disappears, and tensile plastic deformation is applied to the bottom side surface of the rail welded part.