BEARING COMPONENT & METHOD

Abstract

Bearing component providing unaffected material that has a surface, which has been subjected to a hard machining process during where the temperature of the surface did not exceed the austenitizing temperature of the unaffected material. The surface of the bearing component includes a white layer formed during the hard machining process. The white layer has a nano-crystalline microstructure that includes grains having a maximum grain size up to 500 nm. The white layer is located directly adjacent to the unaffected material of the bearing component, where no dark layer is formed during the hard machining process.

Claims

1. A bearing component comprising: unaffected material having a surface that has been subjected to a hard machining process during which the temperature of the surface did not exceed the austenitizing temperature of the unaffected material, the surface of the bearing component includes a white layer formed during the hard machining process, wherein the white layer includes a nano-crystalline microstructure having grains having a maximum grain size up to 500 nm and the white layer is located on the unaffected material of the bearing component, wherein no dark layer having a hardness less than the hardness of the unaffected material is formed during the hard machining process.

2. The bearing component according to claim 1, wherein the white layer comprises the same amount of retained austenite as the unaffected material of the bearing component.

3. The bearing component according to claim 1, wherein the layer comprises less retained austenite than the unaffected material of the bearing component.

4. The bearing component according to claim 1, wherein the bearing component exhibits a hardness profile, wherein the hardness of bearing component is greatest at an as-machined surface of white layer (10), and decreases with depth below the as-machined surface, and wherein the hardness of the white layer is greater than the hardness of the unaffected material of bearing component.

5. The bearing component according to claim 1, wherein the layer extends up to 15 μm below the as-machined surface of the bearing component.

6. The bearing component according to claim 1, wherein the white layer has a Vickers hardness of 450-1500 HV1 and the unaffected material (14) of the bearing component has a Vickers hardness of 450 HV1 or more.

7. The bearing component according to claim 1, wherein the unaffected material has a hardness greater than or equal to 450 HV1.

8. The bearing component according to claim 1, further comprising that it constitutes at least a part of one of the following: a ball bearing, a roller bearing, a needle bearing, a tapered roller bearing, a spherical roller bearing, a toroidal roller bearing, a ball thrust bearing, a roller thrust bearing, a tapered roller thrust bearing, a wheel bearing, a hub bearing unit, a slewing bearing, a ball screw, cylindrical roller bearing, cylindrical axial roller bearing, spherical roller thrust bearing, spherical plane bearing, or a component for an application in which it is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding and/or an application that requires high wear resistance and/or increased fatigue and tensile strength.

9. A method for manufacturing a bearing component including an unaffected material, the method comprising the step of: subjecting a surface of a workpiece of the unaffected material to a hard machining process, wherein a white layer is formed during the hard machining process, and controlling at least one process parameter of the hard machining process to ensure that the temperature of the surface of the bearing component does not exceed the austenitizing temperature of the unaffected material during the hard machining process.

10. The method according to claim 9, wherein at least one process parameter of the hard machining process is one or more of the following: cutting speed, cutting force, cooling of cutting tool, cooling of the at least one part of the surface of the bearing component, cutting tool material, cutting tool condition, cutting direction, feed rate, depth.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0025] The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended schematic figures where;

[0026] FIG. 1 shows a cross sectional view of a typical subsurface microstructure of an as-machined workpiece according to the prior art,

[0027] FIG. 2 shows the hardness profile and the grain size of an as-machined workpiece according to the prior art with depth below the as-machined surface,

[0028] FIG. 3 shows a bearing component according to an embodiment of the present invention,

[0029] FIG. 4 shows the temperature of a surface of a workpiece against cutting speed,

[0030] FIG. 5 shows a cross sectional view of a typical sub-surface microstructure of an as-machined bearing component according to the present invention, and

[0031] FIG. 6 shows the hardness profile and the grain size of a bearing component according to the present invention with depth below its as-machined surface.

[0032] It should be noted that the drawings have not necessarily been drawn to scale and that the dimensions of certain features may have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF THE INVENTION

[0033] FIG. 1 shows a cross sectional view of a typical subsurface microstructure of an as-machined workpiece subjected to a hard machining process according to the prior art. The workpiece comprises a white layer 10, an underlying dark layer 12 directly adjacent to the white layer 10 and underlying unaffected material 14 directly adjacent to the dark layer 12.

[0034] The white layer 10 comprises evenly distributed carbides. The underlying dark layer 12, which is thicker than the white layer 12, also contains evenly distributed carbides. The unaffected material 14, which is unaffected by the hard machining process, comprises martensitic/bainitic needles having a length of about 2-3 μm and a width of about 0.5 μm. The martensitic/bainitic unaffected material also comprises evenly distributed carbides.

[0035] FIG. 2 shows the hardness profile 11 and the grain size 13 of an as-machined workpiece according to the prior art with depth below the as-machined surface, i.e. the uppermost surface of a thermally-induced white layer 10. It can be seen that the hardness of the dark layer 12 is lower than the hardness of the unaffected material 14, which may be detrimental to the performance of the workpiece when in use. The hardness of the dark layer 12 may be as much as 30% lower than the hardness of the unaffected material 14.

[0036] FIG. 3 shows an example of a bearing component according to an embodiment of the invention, namely a rolling element bearing 16 that may range in size from 10 mm diameter to a few metres diameter and have a load-carrying capacity from a few tens of grams to many thousands of tonnes. The bearing component 16 according to the present invention may namely be of any size and have any load-carrying capacity. The illustrated bearing 16 has an inner ring 18 and an outer ring 20 and a set of rolling elements 22.

[0037] At least part of a surface of the inner ring 18, the outer ring 20 and/or the rolling elements 22 of the rolling element bearing 16, and preferably at least part of the surface of all of the rolling contact parts of the rolling element bearing 16 may have been subjected to one or more hard machining processes during which the temperature of said at least part(s) of the surface(s) did not exceed the austenitizing temperature of the unaffected material, which may be steel having a hardness greater than or equal to 450 HV1, measured using a conventional Vickers hardness indenter, such as AISI 52100 steel for example. One or more raceways of the bearing component 16 may for example be subjected to a method according to the present invention.

[0038] The surface(s) of a bearing component 16 subjected to a hard machining process will comprise a white layer 15 that comprises a nano-crystalline microstructure comprising randomly oriented grains having a maximum grain size up to 500 nm. For example, all of the grains in the white layer 15 will have a maximum transverse dimension of 5-500 nm measured using any conventional grain size-measuring technique. The white layer 15 will be located directly on the underlying unaffected material 14 of the bearing component 16, whereby no dark layer 12 having a hardness less than the hardness of the unaffected material 14 is formed during said hard machining process.

[0039] The white layer 15 of a bearing component 16 comprising AISI 52100 steel which is subjected to such a hard machining process will comprise bcc-(a) ferrite and orthorhombic-(8) cementite carbides whereby the martensite/bainite needles of the unaffected material 14 have been reoriented along the shear direction and broken-down into elongated sub-grains through dynamic recovery. A thermally-induced white layer 10 consists instead of fcc-(y) austenite, bcc-(a) martensite, and orthorhombic-(8) cementite carbides.

[0040] If the unaffected material 14 of the bearing component 16 comprises 0 volume-% retained austenite, then the white layer 15 formed during the hard machining process will also comprise 0 volume-% retained austenite. If the unaffected material 14 of the bearing component 16 comprises 10 volume-% retained austenite, then the white layer 15 formed during the hard machining process will comprise less than 10 volume-% retained austenite, for example 5 volume-% retained austenite.

[0041] FIG. 4 is a graph of the temperature of a surface of a workpiece against cutting speed. The graph indicated the phase transformation temperature 24, i.e. the austenitizing temperature of the unaffected material 14 of the workpiece. It can be seen that the higher cutting speeds result in the temperature of the surface of the workpiece exceeding the phase transformation temperature 24, whereupon an undesired thermally-induced white layer 10 will be formed. At lower cutting speeds the temperature will be suppressed as shown in FIG. 3, whereupon no phase transformation temperature 24 of the surface material occurs at the surface constituting the workpiece; a desired mechanically-induced white layer 15 will thus be formed.

[0042] Such information representing the effect of each, or a combination of process parameters on the temperature of a surface of a workpiece subjected to a hard machining process may be obtained from experimental data or by calculation. Process parameters may then be controlled in such a way as to produce a bearing component 16 having a white layer 15 having the desired microstructure and properties.

[0043] FIG. 5 shows a cross sectional view of a typical subsurface microstructure of an as-machined bearing component 16 subjected to a hard machining process according to the invention. The bearing component 16 comprises a white layer 15 located on the underlying unaffected material 14 without any discernible dark layer 12 having a hardness less than the hardness of the unaffected material therebetween. The white layer 15 comprises evenly distributed carbides. The unaffected material 14, which is unaffected by the hard machining process, comprises martensitic/bainitic needles having a length of about 2-3 μm and a width of about 0.5 μm. The martensitic/bainitic unaffected material 14 also comprises evenly distributed carbides.

[0044] FIG. 6 shows the hardness profile 26 and the grain size 28 of a bearing component 16 according to the present invention with depth below its as-machined surface. It can be seen that bearing component 16 exhibits a hardness profile in which the hardness is greatest at an as-machined surface of its mechanically-induced white layer 15. The hardness of the mechanically-induced white layer 15 is greater than the hardness of the unaffected material 14 (for example twice or three times the hardness of the unaffected material or more) and the hardness decreases smoothly with depth below the as-machined surface of the bearing component 16. The hardness of the mechanically-induced white layer 15 is namely never lower than the hardness of the unaffected material 14. The hardness decreases smoothly with depth below the as-machined surface of the bearing component 16. The thickness of the transition zone in which the hardness drops from its maximum at the as-machined surface side of the white layer 15 to its minimum at the unaffected material side of the white layer 15 can be up to 500 μm. There is namely no relatively soft dark layer 12 in between the mechanically-induced white layer 15 and the unaffected material 14. The size of the grains within the mechanically-induced white layer 15 is much lower than the size of the grains within the unaffected material 14. There is an abrupt and substantial change in grain size between the mechanically-induced white layer 15 and the unaffected material 14.

[0045] Such a mechanically-induced white layer 15 can extend from 1-15 μm below the as-machined surface of the bearing component 16 and can have a Vickers hardness of 450-1500 (HV1), whereby the unaffected material 14 of the bearing component 16 can have a Vickers hardness of 450 (HV1) or more measured using a conventional Vickers hardness test.

[0046] Further modifications of the invention within the scope of the claims would be apparent to a skilled person.