DE102015201644B4 - Method for deep rolling a metallic object, in particular a raceway of a roller bearing - Google Patents

Abstract

A method of rolling a metallic object, comprising the steps of: - first traversing a surface of the metallic object with a first rolling body having a first mean Hertzian pressure and a first frequency; and second passing over the surface of the metallic object with a second rolling body having a second mean Hertzian pressure and a second frequency; wherein the first average Hertzian pressure is greater than the second average Hertzian pressure, and wherein the first frequency is less than the second frequency.

Description

The invention relates to a method for deep rolling a metallic object, in particular a raceway of a bearing, such as a rolling bearing.

Radial and axial bearings are used for the rotatable mounting of two objects to each other. Rolling bearings have two bearing rings, which can be arranged on each one of the objects to be stored and rotatable relative to each other about a common axis. In order to enable as frictionless sliding of the bearing rings together as possible, rolling elements are arranged between the bearing rings, which roll or roll on running surfaces of the bearing rings. Linear guides or ball screws operate essentially according to the same principle, whereby in linear guides a slide of a guide rail and in the case of ball screws a nut slides on a spindle.

To reduce the rolling resistance of bearings, linear guides or ball screws, to ensure a quiet and smooth running, to reduce operating temperatures and to achieve low wear, the rolling elements and the running surfaces as smooth as possible and uniform surface and are made of a hard, low-wear , preferably made of corrosion-resistant material. Furthermore, bearings, linear guides and ball screws have lubrication, z. As a grease or oil lubrication to further reduce the rolling resistance.

Bearings of bearings, in particular rolling bearings of wind energy gear, are often exposed during operation high mechanical and tribological stresses. Tensile stresses can, for. B. under the influence of mixed friction, lead to the raceway surfaces by brittle violent fracture to the formation of cracks that grow in depth and destroy the camp eventually. This results in high costs through maintenance, replacement and shutdown of the device on which the damaged bearing is located.

In the WO 2008/042161 A2 discloses a method in which the deep compressive stresses generated in the Hertzian contact in a component to be treated are increased by plastic surface deformation in the outer peripheral layer in order to achieve the most uniform distribution possible. In particular, a combination of long- or kurzreichweitigem deep rolling and shot peening is provided.

The DE 102 16 492 A1 describes a method for improving the material properties in the z. B. in chipless machining of a component influenced edge layer by subsequent surface treatment (vibratory grinding) at elevated temperature.

The object of the present invention is to provide a method for processing a metallic object with which metallic objects can be produced which at least partially do not have the above disadvantages. The object is therefore to provide a method for processing a metallic object, in particular a running surface of a part of a bearing, such as a rolling bearing, by which reduces the risk of formation of cracks by brittle violent fracture of the metallic object and the propagation speed of possible such cracks becomes.

• – Erstes Überfahren einer Oberfläche des metallischen Objekts mit einem ersten Rollkörper mit einer ersten mittleren Hertzschen Pressung und einer ersten Häufigkeit; und
• – Zweites Überfahren der Oberfläche des metallischen Objekts mit einem zweiten Rollkörper mit einer zweiten mittleren Hertzschen Pressung und einer zweiten Häufigkeit.

The object is achieved by a method having the features of claim 1. The dependent claims 2 to 10 represent preferred embodiments of the method. Accordingly, the object is achieved by a method for deep-rolling a metallic object, comprising the steps:

• - First driving over a surface of the metallic object with a first rolling body having a first mean Hertzian pressure and a first frequency; and
• Second passing over the surface of the metallic object with a second rolling body having a second mean Hertzian pressure and a second frequency.

Here, the first mean Hertzian pressure is greater than the second mean Hertzian pressure and the first frequency less than the second frequency.

To avoid tension-controlled cracking by brittle violent fracture z. B. under the influence of mixed friction on raceway surfaces of rolling bearings from wind turbine gear compressive stresses of the material can be produced by work hardening. Such strain hardening is particularly advantageous in an edge layer of the track, since such edge layers have to withstand particularly high mechanical loads.

According to the invention, a method for deep-rolling a metallic object is used for this purpose, in which a rolling element is guided with varying frequency under different Hertzian pressure over a surface of the metallic object to be machined. It is exploited that the built-up compressive residual stresses of the metallic object of the frequency of overrun by the rolling body are dependent.

In a first method step, a first overrunning or rolling over of the surface of the metallic object takes place by means of a rolling body. In this case, the surface is overrun or overrun relatively a few times with a relatively high first pressure. Here, high residual compressive stresses are generated, which follow a depth distribution of a von Mises voltage. It is of great importance that the surface is run over or overrun only once or a few times at the relatively high first pressure, in order to avoid excessive internal compressive stresses which are above 1000 MPa and would thus damage the metallic object.

In a second method step, a second overrunning or rolling over of the surface of the metallic object takes place by means of the same or another rolling body. In this case, the surface is overrun or overrun relatively many times, more frequently than in the first method step, with a relatively low second pressure. As a result, residual compressive stresses generated in the near-surface zones of the metallic object by the first method step can be distributed uniformly over the depth.

This method has the advantage that particularly uniform compressive residual stresses, for example between about 400 MPa and 1000 MPa, preferably between 500 MPa and 800 MPa, can be introduced into a metallic component, in particular of hardened steel, in particular into an edge layer of the component. These residual compressive stresses can be introduced, for example, into the component at a depth of approximately 200 μm to 300 μm. In this case, a particularly uniform depth profile of these compressive residual stresses in the metallic object can be formed. As a result, the risk of formation of cracks due to brittle fracture of the metallic object, z. B. under the influence of mixed friction on raceway surfaces of rolling bearings from wind turbine gearboxes, greatly reduced and slowed the growth of possible cracks. Excessive compressive stresses, especially near the yield point, which is about 1500 MPa for hardened steel, can damage the material and can be avoided by the process.

It is preferable that the metallic object is a part of a bearing and the surface is a running surface of the bearing. Such bearings have raceway surfaces, which are often particularly high mechanical and tribological stresses, z. B. under operating conditions with mixed friction exposed.

Preferably, the first run over with a Hertzian pressure between 6000 MPa and 4000 MPa, more preferably about 5000 MPa, and / or the second run over with a Hertzian pressure between 4500 MPa and 2500 MPa, more preferably about 3500 MPa. Machining the surface of the metallic object with such Hertzian pressure enables the introduction of residual compressive stresses in the metallic object, in particular hardened bearing steel, with a particularly uniform depth profile. In each case, the influence depths measured from the surface with compressive residual stresses of from 400 MPa to about 1000 MPa, preferably between 500 MPa and 800 MPa, are preferably between about 40-50 μm and 250 μm, more preferably between 0 μm and about 300 μm.

In a preferred variant of the method, the Hertzian pressure during the first overrun and / or the second overrun is substantially constant.

In an alternative variant of the method, the Hertzian pressure is changed during the first pass and / or the second pass. As a result, the introduction of residual compressive stresses into the metallic object with a particularly uniform depth profile can be favorably promoted.

Preferably, the change in Hertzian pressure is from a relatively high Hertzian pressure to a relatively low Hertzian pressure. Preference is given to deep-rolling with step-by-step or continuously decreasing Hertzian pressure so as not to relieve compressive stresses due to stress relaxation near the surface. Alternatively, with increasing Hertzian pressure or in any stages, for. B. with the lowest Hertzian pressure as the last step, or be rolled with stepless change in contact pressure. As a result, the introduction of residual compressive stresses in the metallic object is favored with a particularly uniform depth profile in an advantageous manner.

More preferably, the first frequency is between 1 and 10 repetitions and / or the second frequency is between 2 and 200 repetitions. As a result, the introduction of residual compressive stresses in the metallic object is favored with a particularly uniform depth profile in an advantageous manner.

It is particularly preferred that the first rolling body and the second rolling body are the same or the same rolling body. The use of the same rolling body has the advantage that an exchange of the rolling body between the first driving over and the second driving over is not required and thus eliminates change times and manufacturing costs are reduced. The rolling body preferably has a diameter of between 4 mm and 8 mm, particularly preferably about 6 mm.

It is advantageous if after the second run over at least one edge region of the surface is removed by grinding and / or honing. As a result, for example, the outermost edge layer of z. B. 40 .mu.m to 50 .mu.m, in which the compressive stresses corresponding to the course of von Mises tension during deep rolling are less high, are removed. As a result, a particularly uniform depth profile of the residual compressive stresses can be generated in an advantageous manner. In addition, the surface quality (eg roughness) can be optimized by a final grinding and / or honing.

According to a preferred development of the method, after the second run over or optionally after a final grinding and / or honing, a thermal treatment of the metallic object takes place for times between 5 minutes and 4 hours, preferably between 15 minutes and 3 hours, more preferably between 30 minutes and 2 h, at a temperature below the temperature of the last step of the previous heat treatment, that is, the tempering temperature at z. B. use, induction or martensitic hardening) or the transformation temperature at z. B. Bainithärten of the metallic object (eg., From bearing steel), preferably between 5 and 50 ° C below a tempering temperature or transition temperature. In this way, the residual compressive stresses and the microstructure of the metallic object are advantageously stabilized. At the selected temperature, the metallic object has no significant loss of hardness. A thermal degradation of residual compressive stresses can be minimized in this way.

In the following the invention will be explained in more detail with reference to a drawing. The drawing shows schematically:

1 A flowchart of an embodiment of the method according to the invention.

In 1 an embodiment of the method according to the invention is shown schematically in a flow chart. In a first process step 10 is a surface of a metallic object, such. B. a raceway of a rolling bearing, overrun with a rolling body about five times. Here is a Hertzian pressure of the rolling body on the surface at the beginning of the first process step 10 about 5500 MPa and at the end of the first process step 10 about 4500 MPa. In a preferred variant, the Hertzian pressure decreases during the first process step 10 constant from, in a likewise preferred variant, the Hertzian pressure decreases during the first process step 10 gradually, z. B. in steps of about 250 MPa, z. B. after each passing of the surface, from.

In a second process step 20 the surface of the metallic object is run over with the same rolling body about ten times. In this case, the Hertzian pressure of the rolling body on the surface at the beginning of the second process step 20 about 4500 MPa and at the end of the second process step 20 about 2500 MPa. In a preferred variant, the Hertzian pressure decreases during the second process step 20 constant from, in a likewise preferred variant, the Hertzian pressure decreases during the second process step 20 gradually, z. B. in steps of about 200 MPa, z. B. after each passing of the surface, from.

In a third process step 30 For example, a 40 μm thick surface zone of the metallic object, in which residual compressive stresses are less pronounced than in a deeper region below the surface, is mechanically removed by grinding and / or honing.

In a fourth process step 40 a thermal aftertreatment is carried out to stabilize the residual compressive stresses and the microstructure of the metallic object. In this case, the metallic object is heated for example for 2 h at a temperature which z. B. 10 ° C below the tempering (martensite) or transformation temperature (bainite) of the heat treatment during the previous hardening. A thermal degradation of the compressive residual stresses can be minimized in this way. The hardness of the metallic object remains substantially constant.

LIST OF REFERENCE NUMBERS

10

first process step

20

second process step

30

third process step

40

fourth process step

Claims (10)

A method of rolling a metallic object, comprising the steps of: - first traversing a surface of the metallic object with a first rolling body having a first mean Hertzian pressure and a first frequency; and second passing over the surface of the metallic object with a second rolling body having a second mean Hertzian pressure and a second frequency; wherein the first average Hertzian pressure is greater than the second average Hertzian pressure, and wherein the first frequency is less than the second frequency. A method according to claim 1, characterized in that the metallic object is a part of a bearing and the surface of a race of the bearing. A method according to claim 1 or 2, characterized in that the first run over with a Hertzian pressure between 6000 MPa and 4000 MPa and / or that the second run over with a Hertzian pressure between 4500 MPa and 2500 MPa. Method according to one of the preceding claims, characterized in that the Hertzian pressure during the first overrun and / or the second override is substantially constant. Method according to one of claims 1 to 3, characterized in that the Hertzian pressure during the first pass over and / or the second pass over is changed. A method according to claim 5, characterized in that the change in the Hertzian pressure from a relatively high Hertzian pressure to a relatively low Hertzian pressure occurs. Method according to one of the preceding claims, characterized in that the first frequency of 1-10 repetitions and / or the second frequency is between 2 and 200 repetitions. Method according to one of the preceding claims, characterized in that the first rolling body and the second rolling body are the same or the same rolling body. Method according to one of the preceding claims, characterized in that after the second run over at least one edge region of the surface is removed by grinding and / or honing. Method according to one of the preceding claims, characterized in that after the second run over a thermal treatment of the metallic object takes place between 5 min and 4 h at a temperature below the temperature of the last process step of the preceding heat treatment for hardening the metallic object.

Images