The body may also form part of a ball bearing or a roller bearing. With all these examples, a curved metal tread rolls over a metal surface, wherein the contact area between said tread and said surface is substantially linear if said tread is cylindrical and said surface is flat. With other shapes of the tread and/or of the surface, the contact area may be substantially elliptical or circular.

When the tread rolls on the surface, wear may occur for all kinds of reasons. Treadwear may be caused by friction contact with the surface, whereby loss of material of the tread may occur. The loss of material may also be caused by friction resulting from the presence of material particles between the tread and the surface, whereby said friction in fact takes place between said material particles and said surface.

In practice it has become apparent that the loss of material caused by friction often results in polygonization of the tread, that is, that the curved tread takes the form of a polygon, which may or may not be a regular polygon.

Polygonization of a curved tread may be caused by wear, but it may also be caused, whether or not in combination with wear, by heating or plastic deformation of the tread material, which may or may not take place locally. Said

local heating or plastic deformation may for example occur when the tread is subjected to varying loads, or if the surface is subjected to a friction force. Such a friction force may be the result of friction contact with the aforesaid surface. A curved metal tread of a train wheel can be loaded and/or heated by friction contact with brake shoes which are pressed against the tread in order to slow down the train.

Deformation and/or heating of the tread may effect a change in the crystal structure of the metal at the surface of said tread. In practice it has become apparent that in the case of a steel train wheel, heating and/or deformation may lead to conversion of the crystal structure at the surface into a harder crystal structure, for example a martensitic structure, whilst areas comprising a comparatively softer metal, for example ferrite and/or pearlite, are present between the surface parts comprising the relatively hard metal. In practice, such a conversion of the crystal structure of the metal at the tread surface often leads to polygonization, on the one hand because the volume of the material changes locally, and on the other hand because the softer parts of the material surface wear down more quickly than the harder parts.

The resulting polygonization of the tread may inter alia lead to an increased noise production by the rolling tread. Thus, it has become apparent that a considerable part of the noise emission from a passing train may be caused by undesirable polygonization of the train wheels.

This noise emission will increase further with use of the train, since the polygonization effect, once it has been initiated, is further enhanced during driving and braking of the train.

Polygonization of the tread furthermore causes vibrations, which vibrations may be undesirable again for all kinds of reasons.

The objective of the invention is to reduce the noise emission and other negative consequences of the rolling of a metal curved tread over a surface by preventing the development of an undesirable form of polygonization or by lessening the consequences thereof.

According to the invention, predetermined parts of the tread are given a harder crystal structure than other parts of the tread. In this way it is possible to initiate a form of polygonization which is less disadvantageous because it fewer vibrations and/or less noise are produced. Even when the initiated polygonisation is further enhanced with use of the body comprising the curved tread, the noise emission and the vibrations being produced will be less than in the case of spontaneously developed polygonization, because the predetermined parts of the tread can be selected such that, even as the unroundness of the curved tread increases, the form of said tread will be more advantageous than that of a tread which does not comprise predetermined processed parts.

Preferably, the tread consists of steel, and the harder crystal structure is martensitic. Said martensitic structure can be provided by briefly heating the respective parts of the tread.

Preferably, the parts comprising a harder crystal structure extend in strips across the curved surface. The development of vibrations and noise can be prevented by varying the spacing between successive strips with a predetermined regularity.

Preferably, the strips extend parallel to each other, at an acute angle to the direction of running, which is the direction in which the tread rolls over the surface, which acute angle is preferably smaller than 80° and more preferably ranges between 40° and 75°. In a preferred embodiment, said angle is approximately 70°.

Preferably, the strips overlap, in the sense that a part having the harder crystal structure is present in the width direction of the tread at all times. The oblique strips can overlap in such a manner thereby that one, two, or even three strips lie side by side in the width direction at all times. The tread may substantially have the shape of a cylindrical surface thereby, which tread can roll over a substantially flat surface.

In one preferred embodiment, the body consists of a wheel for a tram or a train, which wheel can run on a rail.

The strips may have a width of 5-20 mm, for example, whilst parts comprising ferrite and pearlite, which materials are comparatively soft, are present between said strips, wherein the width of said parts is in the same order of or larger than that of the strips.

The invention furthermore relates to a vehicle, for example train, provided with wheels in the form of a body as described before.

The invention furthermore relates to a method for manufacturing a body comprising a curved metal tread which can roll over a surface, according to which method predetermined parts of the tread are given a harder crystal structure than other parts of the tread. To that end, the tread material is locally subjected to a heat treatment.

Preferably, said local heating takes place by means of a laser beam, which is moved across the surface. Such a laser beam may have a diameter of 5 mm, and the speed at which the laser beam moves across the surface may be 0.5-20 mm/sec. The material at the tread surface can thereby be heated to a temperature of 800 °C, for example, after which it cools down quickly, so that martensite can be formed. The depth of case, that is, the depth to which martensite has been transformed, is 0.5 mm, for example.

The invention furthermore relates to a method for reducing the noise emission from a vehicle having metal wheels, such as a train or a. tram, which is moved over metal rails, wherein the wheels are provided with a tread of which predetermined parts have a harder structure than other parts of the tread.

For a better understanding of the invention, a few examples of a curved metal tread will be described hereafter with reference to the drawing.

Figures 1 and 2 show a part of a tread, and Figure 3 shows a train wheel.

The figures are merely schematic representations, wherein corresponding parts are indicated by the same numerals.

Those parts of the tread that have a martensitic crystal structure are hatched in the figures. The other parts of the tread have a softer crystal structure.

Figure 1 shows a part of a tread wherein those parts of the tread that have a martensitic crystal structure are made up of oblique strips 1. The other parts 2 of the tread have a softer crystal structure, they will consist of ferrite and/or pearlite, for example.

The conversion of the hatched parts 1 into a martensitic structure to a depth of approximately 0.5 mm has not only increased the hardness of the material in those parts, but it has furthermore resulted in the formation of a profile, because material slightly expands upon being converted to the martensitic structure. As a result, the hatched parts 1 are slightly higher than the parts 2 located therebetween. This difference in height can become larger because of wear, since the softer parts 2 wear down more quickly than the harder parts 1.

According to Figure 1, the strips 1 having a martensitic structure extend obliquely, so that at least one strip 1 will at all times be in contact with the surface over which the body rolls.

Figure 2 shows an example wherein the strips 1 comprising martensite extend in transverse direction with respect to the direction in which the tread rolls. The spacing between the strips 1 is varied in such a manner that the noise emission and the vibrations produced by the rolling movement are reduced to a minimum. By means of experiments a variation in the spacing between the strips can be determined at which the noise production is minimal. Then said predetermined variation can be provided on the tread to be monitored.

Figure 3 is a perspective view of a train wheel 3, which train wheel 3 is provided with a flange 4. The tread of the train wheel comprises strips 1 having a martensitic crystal structure, whilst the other parts 2 of the tread have a softer crystal structure. The strips 1 having the martensitic structure are provided by means of a laser 5, which aims a laser beam 6 at the surface of the train wheel.

Said laser beam 6 can locally apply so much energy to the tread material (steel) that said material is heated to a

temperature above 800 °C. As a result of the good heat conduction of the steel, the subsequent temperature decrease takes place sufficiently quickly to cause the formation of a martensitic structure.

Strips 1 can be provided in the desired direction by rotating the train wheel (arrow 7) about axis of rotation 8 and simultaneously moving laser 5 in the direction indicated by arrows 9.

The above-described embodiments are to be considered as examples, a great many variations are possible wherein the metal tread of a train wheel or of a roller bearing or of another rolling body is locally given a harder crystal structure, thus minimizing the vibrations and the noise that are produced.