METHOD FOR PRODUCING A ROLLING BEARING ELEMENT

Description

[0001] The present invention relates to a method for producing a rolling bearing element, in particular a raceway element for a large rolling bearing.

[0002] In today's slewing bearings, the raceway elements are generally made of case-hardened or through-hardened steel. When case-hardening steel is used, the inherently low-alloy steel is carburized in a surface layer in a carbon atmosphere so that it can be subsequently hardened. Due to the constant stress on the raceway elements from the rotating rolling elements, slewing bearings require a high surface hardness in the raceway area to ensure a long service life. However, case-hardening or through-hardening of the raceway elements involves relatively high energy consumption.

[0003] For pivot bearings, however, it is known to produce hardened surface layers using induction processes. However, the surface hardness requirements for these bearings are not as stringent as for slewing bearings due to the different loads from the rolling elements. Traditionally, different steel grades are used for pivot bearings than for slewing bearings. Therefore, the induction hardening processes used for these types of bearings cannot be transferred to the production of slewing bearings.

[0004] It is also known to apply inductive surface hardening to the steel grades used in smaller rolling bearing diameters, such as 100Cr6. Here, too, the known processes for inductive hardening of the surface layers cannot be transferred to slewing bearings from an economic perspective. Furthermore, vehicle wheel bearings with hardened raceways are known from US 2009 / 052823 A1 and JP 2003193139A. WO 03 / 060170 A1 discloses further prior art.

[0005] It is an object of the present invention to provide a method for producing a rolling bearing element for a slewing bearing, in which a long service life of the rolling bearing element is ensured while at the same time being simple to manufacture.

[0006] This problem is solved by a method having the features of the main claim. Advantageous embodiments are the subject of the dependent subclaims.

[0007] According to one embodiment of the invention, a method for producing a rolling bearing element, in particular a raceway element, for a large rolling bearing is provided, comprising the following steps: Selection of a steel having a carbon admixture of at least 0.46 mass%; Determination of a contact zone for a rolling bearing counter-element; Determination of a stress profile in the contact zone generated by a Hertzian contact between the rolling bearing element and the rolling bearing counter-element based on an expected load on the bearing; Induction hardening of the contact zone, whereby the contact zone receives a basic structure and an induction-hardened surface layer, wherein the induction hardening is carried out in such a way that ∘ the surface layer has a maximum hardness that is lower than the maximum achievable hardness of the steel used, wherein the hardness is the same within manufacturing tolerances; and a depth of the hardened surface layer covers the stress curve generated by the Hertzian contact, in particular extends deeper into the material than the stress curve.

[0008] In contrast to the costly use of case-hardening steel, which generally has a carbon content of less than 0.2 mass% outside the surface layer and can only be sufficiently hardened for use in highly loaded rolling bearings through complex carburization in the surface layer to a carbon content of, for example, 0.8 mass% in this layer, a more cost-effective, so-called heat-treatable steel with a carbon admixture of at least 0.46 mass% can be used here, with the contact zone of the rolling bearing element being subsequently hardened using an inductive process. This hardening process is associated with significantly less effort than carburizing the surface layer. The raceway elements and / or the rolling elements of the large-diameter bearing can be designed as rolling bearing elements according to the invention, in particular the raceway elements and / or the rolling elements of the large-diameter bearing.In particular, the elements of the large-diameter bearing subject to rolling contact benefit from the design according to the invention. The rolling bearing counter element is the rolling bearing element in rolling contact with the rolling bearing element and is preferably also designed according to the invention. The large-diameter bearing preferably has a diameter of at least 250 mm.

[0009] The invention is essentially based on the finding that with carbon contents lower than 0.46 mass%, induction hardening of the surface layers does not achieve sufficient surface hardness with a sufficiently stable microstructure to ensure a sufficiently long service life of the raceway element. In particular, with lower carbon contents in the surface layer, a disadvantageously unstable microstructure can arise during induction hardening, which leads to premature failure of the rolling bearing element and thus of the slewing bearing. In addition, particularly with slewing bearings, a material distortion that can no longer be ignored occurs during heat treatment, so that high running accuracy for the rolling elements on the raceways is no longer guaranteed. After heat treatment, the distortion must be removed again by further machining, for example, machining, which further increases the production costs for corresponding raceway elements.However, even after machining, i.e., the removal of material from the surface, it must be ensured that the maximum hardness of the surface corresponds to the design of the large bearing selected based on the expected load, so that the required load-bearing capacity is met. Therefore, the thickness of the surface layer should be greater, especially for heavily loaded large bearings, than for smaller bearings.

[0010] In a preferred embodiment of the invention, the surface layer has a maximum hardness that is lower than the maximum achievable hardness of the steel used. With the steel used, a significantly higher hardness can generally be achieved than with the steels known for use in large bearings, such as 42CrMo4. However, this advantage is preferably not utilized, and only a hardness equivalent to that of known rolling bearing elements made of 42CrMo4 for large bearings is produced using the inductive hardening process. As a result, the steel used in the invention is not pushed to its material limits during hardening, as is often the case when using 42CrMo4. This results in a maximum hardness in the surface layer that is comparable to that of known large bearings, but a different hardness progression with increasing depth.While with known steel grades for large bearings the hardness drops sharply after a comparatively thin surface layer and quickly transitions into the hardness of the unhardened area, the steel used in the invention achieves a much flatter transition. Consequently, as the hardness decreases, the hardened surface layer extends significantly deeper into the rolling bearing element than with known large bearings. At the same time, this also creates a relatively fine and stable microstructure. On the one hand, this results in a significantly increased service life of the large bearings. On the other hand, when using the steel, which is inherently more highly hardenable and contains more than 0.46 mass% carbon, a much simpler and more material-friendly process is necessary to achieve the required hardness, so that the steel is subjected to less stress.This may be due, for example, to the fact that the steel does not need to be heated as much and is quenched as quickly in order to show the desired flat hardness profile.

[0011] According to the invention, it is important to ensure that the load-bearing capacity of the hardened layer, and thus its depth, covers the stress distribution generated by Hertzian contact with the rolling bearing counter-elements. The stress decreases with increasing depth. Consequently, the thickness of the hardened layer must be matched to the expected load on the bearing. This is achieved by creating thicker surface layers for higher expected loads, i.e., a deeper stress distribution, than for lower loads.

[0012] In an advantageous embodiment of the invention, the steel contains admixtures of manganese, chromium, and / or molybdenum. In particular, admixtures of these alloying elements provide a steel that is readily inductively hardenable and provides a sufficient hardening depth, as well as a suitable microstructure for high load-bearing capacity and a long service life of the raceway element. It is particularly advantageous if the steel contains a molybdenum admixture of at least 0.12 mass%. The advantageous hardenability cannot be further increased above a molybdenum content of 0.35 mass%, which is why the admixture is preferably in the range of 0.12 to 0.35 mass%.

[0013] In an advantageous embodiment of the invention, the hardened surface layer has a maximum hardness of at least 58 HRC. Especially at this surface hardness, the raceway element has a sufficiently stable raceway to ensure a long service life.

[0014] According to one embodiment of the invention, a large-diameter rolling bearing arrangement is specified which has at least one rolling bearing element according to one of the preceding claims 1 to 5 and thus benefits from the advantageous properties.

[0015] Further advantages and embodiments of the invention will become apparent from the following embodiment described in conjunction with the accompanying figures. Figures 1 to 3 various views and components of the embodiment of the invention.

[0016] According to one embodiment of the invention, in the Figure 1A slewing bearing 1 with a diameter of at least 250 mm is described. The slewing bearing comprises two raceway elements, designed as an outer ring 3 and an inner ring 5. Rolling elements designed as balls 7 are arranged between the raceway elements. When the two raceway elements move, the balls 7 roll on the raceway elements. The slewing bearing is typically used in installation situations where there is a continuous rotation of the outer ring 3 relative to the inner ring 5, or vice versa. This can be the case, for example, in a wind turbine.

[0017] In the Figure 2The inner ring 5 is shown in detail. It has a raceway 21 on which the balls 7 roll during operation of the slewing bearing 1. The raceway 21 has a hardened surface layer that can withstand the loads caused by contact with the balls 7 and ensures a long service life of the slewing bearing. The same applies to the Figure 3 shown outer ring 3, which also has a surface-hardened raceway 23.

[0018] The surfaces of raceways 21 and 23 are constantly loaded due to rolling contact with the rolling elements. The bearing rings are made of a steel with a carbon admixture of 0.46 to 1.0 mass% and a molybdenum admixture of 0.12 to 0.35 mass%. Further admixtures are preferably manganese between 0.5 and 1.0 mass% and / or chromium between 0.9 and 1.5 mass%. Such materials are known per se; their use for the manufacture of raceway elements with appropriate induction hardening of the surface layer provides a more cost-effective and thus simpler manufacturing method compared to known raceway elements for slewing bearings. The steel can, for example, correspond to the specification 50CrMo4, whereas 42CrMo4 is primarily used for known slewing bearings.

[0019] The raceways of the bearing rings are hardened using an inductive process after they have been formed. For example, an inductor is passed close to the raceway, heating it. A phase transformation occurs, forming a particularly harder material. The heated material is quenched using a subsequent spray, thus preserving the harder structure. This inductive hardening process is preferably carried out slip-free according to known methods, so that no unhardened zone remains on the raceway. The raceway thus has the same hardness along the entire raceway element, within the limits of manufacturing tolerances.

[0020] The maximum hardness of the surface layer is at least 58 HRC. This hardness can also be achieved with steel grades known for use in bearings. However, to achieve this minimum hardness, these steel grades must be hardened to the limits of their material-specific properties. With the steel used here, a maximum hardness of the surface layer could in principle be achieved that is significantly higher than 58 HRC. However, this hardness is not required for most applications for large bearings. In this respect, the steel required to achieve a hardness of 58 HRC is significantly less exploited in terms of its material possibilities than steel grades known for this purpose. In this respect, to achieve the desired maximum hardness, a hardening process that is significantly gentler on the material is possible compared to the known manufacturing processes for components for large bearings.This lowers the maximum temperature during hardening, and quenching can also be carried out with smaller temperature gradients, resulting in a flatter transition in hardness between the surface layer and the base structure. This material-friendly process leads, in particular, to better reproducibility of the results.

[0021] The induction hardening process is designed so that the depth of the hardened layer meets the requirements for the bearing's service life. In particular, the depth of the hardened layer is selected to cover the stress curve generated by Hertzian contact with the rolling elements. The hardened layer thus extends deeper into the material than the stress generated by rolling contact exceeds the load-bearing capacity of the unhardened base material. This prevents overloading of the base material; the increased stress and load are absorbed by the surface layer. This results in a long service life for the large-diameter bearing. Furthermore, the surface layer has a suitable structure for the bearing's high load-bearing capacity.

[0022] In a further embodiment of the invention, in addition to the raceway elements, the rolling elements are also designed according to the invention, i.e., they are hardened using an inductive process and consist of a corresponding steel. Alternatively, an embodiment of the invention is also possible in which only the rolling elements are designed according to the invention, but not the raceway elements. The invention can therefore be advantageously used in all components of a rolling bearing that are subjected to rolling contact. Furthermore, the invention is not limited to rolling bearings with balls as rolling elements, but is fundamentally applicable to all types of slewing bearings. List of reference symbols

[0023] 1Slewing bearing 3Outer ring 5Inner ring 7Ball 21, 23Raceway

Claims

1. Method of producing a rolling bearing element, in particular a raceway element, for a large rolling bearing, having the following steps: - selecting a steel including an addition of carbon of at least 0.46% by mass; - determining a contact zone for a rolling bearing counterpart element; - determining a stress profile in the contact zone, generated by Hertzian contact between rolling bearing element and rolling bearing counterpart element, on the basis of an expected load on the bearing; - inductively hardening the contact zone, which endows the contact zone with a basic microstructure and an inductively hardened surface layer, wherein the inductive hardening operation is conducted in such a way that ∘ the surface layer has a maximum hardness lower than the maximum achievable hardness of the steel used, where the hardness is the same within manufacturing tolerances; and ∘ a depth of the hardened surface layer covers the stress profile generated by the Hertzian contact, and in particular extends deeper into the material than the stress profile.

2. Method according to Claim 1, wherein the inductive hardening operation involves heating the surface layer of the contact zone with an inductor and quenching it with a downstream sprinkler, and hardness is determined directly after the quenching.

3. Method according to Claim 1 or 2, wherein a maximum temperature in the hardening operation is relatively low and the quenching is effected with a lower temperature gradient than is possible in the case of the steel used, such that the steel is not brought to its physical limits in the course of hardening.

4. Method according to Claim 3, wherein the maximum temperature in the course of hardening and the temperature gradient in the course of quenching are selected so as to give rise to a flat hardness profile between the basic structure and the surface layer.

5. Method according to Claim 3 or 4, wherein the maximum temperature in the course of hardening and the temperature gradient in the course of quenching are selected such that the surface layer has a fine and stable microstructure.

6. Method according to any of the preceding claims, wherein the steel used includes an addition of molybdenum, in particular at least 0.12% by mass of molybdenum.

7. Method according to Claim 6, wherein the inductive hardening operation is conducted in such a way that the surface layer of the contact zone after quenching has a maximum hardness which corresponds to the hardness of rolling bearing elements for large rolling bearings which are produced from 42CrMo4 steel but which is lower than the maximum achievable hardness of the steel used.

8. Method according to any of the preceding claims, wherein the steel used includes an addition of molybdenum and conforms to the 50CrMo4 specification, and wherein the inductive hardening operation is conducted in such a way that the surface layer of the contact zone after quenching has a maximum hardness which corresponds to the hardness of rolling bearing elements for large rolling bearings which are produced from 42CrMo4 steel, but which is lower than the maximum achievable hardness of the steel of the 50CrMo4 specification used.

9. Method according to any of the preceding claims, wherein the inductive hardening method is executed without slippage.

10. Method according to any of the preceding claims, wherein the surface layer of the contact zone is hardened to a hardness of at least 58 HRC.

11. Method according to any of the preceding claims, wherein the steel used includes an addition of manganese and / or chromium.

12. Method according to any of the preceding claims, wherein the method is used for production of a raceway element of a large rolling bearing having a diameter of at least 250 mm.