JPS59116321A - Heat treatment for improving resistance to fracture of upper filler of rail - Google Patents

Abstract

PURPOSE:To improve the resistance to fracture of the upper filler of a rail in producing a hard top rail by transforming the entire top of the rail to martensite then heating the same to tempered martensite thereby decreasing the residual tensile stress in the upper filler of the rail or converting said stress to residual compressive stress. CONSTITUTION:The entire top part of a rail consisting of a rail head 2, a head side face 3 and an upper filler 1 is heated to the temp. higher than the A1 transformation point by a heater 7 of a hardening device 6. When the rail R attains the temp. before it falls from the Av1 transformation point, water or the like is ejected from a cooling apparatus 8 to the faces 2, 3 and the upper filler 1 to cool approximately uniformly the same until the martensite transformation ends. The entire head of the hardened rail is heated to 400 deg.C-A1 transformation point by a heater 10 of the succeeding tempering device 9 to charge the martensite structure to tempered martensite structure. The upper filler 1 and the faces 2, 3 are uniformly and quickly cooled by the cooling similar to the above in a cool apparatus 11 right after said heating. The residual tensile stress in the upper filler is then decreased or converted to residual compressive stress.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for improving fracture resistance under a rail chin.

The most common type of damage to rails is breakage from the rail ends.For this reason, the rail ends must be carefully tightened to prevent damage such as dents and abrasion. A so-called temporal rail with a heat-treated head is increasingly being used.

On the other hand, rails are generally used to connect rails using joint plates, but in the side rails, fatigue cracks occur starting at the contact area with the joint plate or under the rail chin in the vicinity, resulting in failure. There are scattered examples of this. This phenomenon is a problem unique to temporal rails that is not seen in ordinary rolled rails.

The inventors of the present invention conducted various studies on the cause of this problem, and found that an excessive tensile residual stress of about 10% to 30"/mJ was generated under the rail chin of the temporal rail in the longitudinal direction. It has become clear that when this is combined with the fluctuating stress caused by trains passing over the rails, and also fretting corrosion with the joint plates, fatigue cracks develop and propagate from under the rail chin, leading to failure. Therefore, it has been found that by reducing this excessive tensile residual stress to 5 K24A or less or converting it to compressive residual stress, it is possible to significantly improve the fatigue fracture that occurs under the rail chin.

The generation of excessive tensile residual stress under the rail chin of the temporal rail has been caused by the fact that conventionally the main focus has been on making the rail head stronger and harder, without paying any attention to the area under the rail chin. be.

The present invention was made based on the above findings, and provides a rail heat treatment method that improves the fracture resistance under the rail chin. However, the gist of this is that in manufacturing temporal heat-treated rails, the top surface of the rail head, the side surface of the rail head, and the entire rail head under the rail chin are heated to a temperature above the A1 transformation point, and then uniformly rapidly cooled to produce martensite. After causing metamorphosis, the entire head of the rail is heated again at 400°C or above A.
The rail chin is reheated to a temperature that does not exceed the transformation point to create a martensitic structure with high strength and toughness, and then immediately cooled rapidly under the rail chin, evenly on the rail top surface and the rail head side surface, to form the rail chin. This improves the fracture resistance under the rail chin by imparting compressive residual stress to the rail chin.

(The invention will be described in detail with reference to an illustrative embodiment shown in FIG.

For convenience of explanation, the names of each part of the rail base will be described with reference to FIG. 1 is the underside of the rail, 2 is the top surface of the rail, 3 is the side surface of the rail head, 4 is the neutral axis of the rail, and 5 is the bottom of the rail.

The heat treatment of the entire head of the rail including the rail under the chin of the present invention may be applied only to the rail fastening part where damage occurs, but sometimes long rails are cut from the middle and used as short rails. Therefore, it is desirable to carry out the test over the entire length of the rail. Therefore, as an example, a case will be described with reference to FIG. 2 in which the method is implemented over the entire length of the rail.

In this figure, the quenching device 6 is composed of a heating device 7, a cooling device 8, and a force 1. Reference numeral 9 denotes a tempering device, which is composed of a caloric device 10 and a cooling device 11.

The quenching device 6 and the tempering device 9 are installed in series on the rail transfer line, and the heat treatment is performed continuously.

The heating devices 7 and 10 of the quenching device 6 and the tempering device 9 are, for example, high-frequency induction heating or flame heating devices, and the rail top surface 2, head 11t11 surface;
3 and the entire head of the rail of the jaw 1. , 1f, the respective cooling devices t8 and 11 of the quenching device 6 and the tempering device 9 are devices that spout a refrigerant, such as water, capable of rapid cooling, and the refrigerant jetting direction is between the rail top surface 2 and the rail head. The side surface 3, and the rail chin 1 are directed toward the refrigerant, and the rail chin 1 is directed toward the rail top surface 2 and the rail chin 1t1.1. l ni
3. Rapidly cool evenly.

The heat treatment method will be briefly described. The housing rail is heated by the heating device 7 of the quenching device 6, and the rail top surface 2 and the head 9111 surface 3 are heated.
The reason why the entire head of the rail consisting of the A and the lower part of the chin must be heated to a temperature above the A1 transformation point, for example 800°C, is to be heated above the A1 transformation point during the subsequent rapid cooling. In addition, the rail jaw T1 is transformed into martensite at the same time as the rail top surface 2 and the class 111.11 surface 3.

When the heated rail base reaches a temperature below the Ar1 transformation point, for example, about 750°C, a coolant capable of rapid cooling, such as water, is supplied from the cooling device 8 to the rail top surface 2.
The water is ejected onto the sides 3 and under the chin 1 of the head so that it is uniformly cooled until the martensitic transformation is completed. The purpose of rapid cooling is to obtain a martensitic structure.

The hardened rail is heated by the heating device 1 of the tempering device 9.
400℃ or more A at 0, temperature not exceeding the transformation point, e.g. 6
The martensitic structure is heated to 00°C to change the martensitic structure to a tempered martensitic structure, resulting in a structure having high strength and high toughness. The reason for using a tempered martensitic structure is that it has higher wear resistance than a regular rolled rail, and has a lower force, higher strength, and higher toughness, which improves the fatigue limit. The reason why the heating temperature is set to 400℃ or higher is that 400℃
At lower temperatures, the martensitic structure generated in the quenching device 6 is not completely tempered and becomes brittle, and for another reason.
One reason is that a larger compressive residual stress cannot be obtained in the next rapid cooling of the cooling device 11. -1 Near L, the reason why the temperature is set so that it does not exceed the AI transformation point is that if the temperature exceeds the A1 transformation point, the heated part will become austenite,
The quenching in the quenching device 6 becomes meaningless.

After heating the entire head of the rail consisting of the top surface of the rail head, the side surface 3 of the head and the bottom of the chin to approximately 600'C, using the heating device 10,
Immediately, the cooling device 11 blows out a refrigerant with a high cooling rate, such as water, to rapidly cool the rail chin 1 uniformly with the rail top surface 2 and the rail head side surface 3. Rapid cooling generates larger compressive residual stress, and the faster the cooling rate, the greater the compressive residual stress, so a refrigerant with a faster cooling rate is best. In addition, when cooling, it is possible to rapidly cool it to room temperature, but it is also possible to rapidly cool it to about 200℃,
After that, you can leave it to cool.

Compressive residual stress acts as an average stress, so even if repeated tensile stress acts, a portion of the compressive residual stress is tensile! 1
The cyclic stress per piece is reduced, resulting in an improvement in the fatigue limit.

As described above, the rail jaw F manufactured according to the present invention has a tempered martensitic structure similar to the top surface of the rail head and the side surface of the rail head, so it has high strength, high toughness, and The fatigue limit is greatly improved when compressive residual stress is applied, and the fracture resistance is improved.

An embodiment of the present invention will be briefly described.

Using a JIS5ON ordinary carbon steel rail, the high-frequency induction heating device 7 is used to heat the underside of the rail, the top surface and side surface of the rail head.
After heating to 800°C, the heated parts are rapidly cooled uniformly with water in the cooling device 8 to transform into martensite.
Thereafter, the area under the rail chin, the top surface of the rail head, and the side surface of the rail head are again heated to 600°C with the heating device 10, and then water is immediately uniformly jetted onto these heated areas to rapidly cool them to room temperature. As a result, as shown in Figure 3, there is a U2 under the rail chin of the conventional temporal rail (quenched/tempered rail 2 code B).
5 KZ", while there is a tensile residual stress of u 10 KR," under the rail chin of the rail of the present invention (symbol A).
Next, the rail A of the present invention and the conventional side rail B (quenched/tempered rail) were subjected to an actual bending fatigue test (load P 20 tons of potato, 65 tons of P, and up to 107 repetitions). As a result, as shown in Table 1, as in the case of the conventional temporal rail B i actual track, cracks were observed to occur from the bottom of the rail chin which is in contact with the joint plate, whereas the present invention rail Aid 107 times Even after a number of repetitions, no cracks were observed, indicating that the fracture resistance under the rail chin was improved.

[Brief explanation of the drawing]

FIG. 1 is a front view of the rail, and FIG. 2 is a first diagram illustrating the heat treatment method for the rail according to the present invention, in which (a) is a side view and (b) is a sectional view taken along line X-X. FIG. 3 is a chart showing the effects of the embodiment of the present invention in comparison with the conventional example, and FIG. 4 is an explanatory diagram of test conditions in a crack generation test between the present invention and the conventional example. φ・Rail, 1...Rail under the chin, 2...Rail on the top of the head,
3...Rail head side, 4...Rail neutral axis, 5...
-Rail bottom, 6... Quenching device, 7, 10... Heating device, 8, 11... Cooling device, 9... Tempering device. Patent Applicant Representative Patent Attorney Tomoyuki Yafuki (1 person)) No. 1 Illustration No. 2rA Fig. 3 Fig. 4

Claims (1)

[Claims] When heat treating a rail, the top surface of the rail. The rail on the side of the rail head and under the rail chin, and the whole head is A
After heating to a temperature higher than the A1 transformation point, uniformly and rapidly cool it to cause martensitic transformation, and after that, reheating the entire rail head to a temperature of 400° C. or higher but not exceeding the A1 transformation point,
A heat treatment method for improving fracture resistance under a rail chin, which is characterized in that immediately after that, the area under the rail chin is rapidly cooled uniformly with the top surface of the rail head and the side surface of the rail head.