GB2521220A

GB2521220A - Process for treating steel components

Info

Publication number: GB2521220A
Application number: GB1322174.2A
Authority: GB
Inventors: Jurgen Gegner
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2013-12-16
Filing date: 2013-12-16
Publication date: 2015-06-17
Also published as: WO2015090958A1; GB201322174D0; EP3110979A1

Abstract

A method of treating a contact surface of a steel component of a rolling contact loaded machine element (e.g. a rolling bearing element) which has a surface layer predominantly having a martensitic and/or bainitic microstructure by cold-working the region of the element adjacent the surface layer to introduce a compressive residual stress of at least 400 MPa to a depth of at least 50 microns, machining the contact surface and heat treating the cold-worked and machined steel component below the tempering or transformation temperature of the steel. Cold working can be by shot peening, burnishing or deep rolling and machining can be by grinding, honing, polishing, lapping, super-finishing, cutting or turning. The heat treatment can be at a temperature in the range 140-230 0C for a duration in the range 5 minutes to 2 hours. The component can be coated, e.g. black oxide or PVD before or during heat treatment.

Description

Process for treating steel components

Technical Field

The present invention relates to the field of metallurgy and bearing steels. More specifically, the present invention relates to a process for treating a steel component of a rolling-contact loaded machine element to provide an increased effective lifetime of the component. The invention relates in particular to a treatment for steel bearing components.

Background

Bearings are devices that permit constrained relative motion between two parts.

Rolling element bearings, for example, comprise inner and outer raceways and a plurality of rolling elements (balls or rollers) disposed therebetween. Rolling-contact loaded machine elements include more broadly both steel bearing components, such as rolling bearings, and continuously variable transmissions.

For long-term reliability and performance it is important that the various elements have a high resistance to rolling contact fatigue, friction, wear and creep.

The components of bearings which include contact surfaces, suffer in tribologically demanding mixed friction applications. Examples of such components are discs and rollers of continuously variable transmissions, rolling element bearings in paper or printing machines, cam shafts in diesel generators and gearings in industrial gearboxes. It has been found that, under demanding conditions, surface cracking can be initiated and this leads to corrosion fatigue crack growth, which in turn leads to damage propagation and failure by, for example, spalling, micropitting or fracture. As a result, the nominal component life may not be reached.

Different approaches have been applied to solving the surface-cracking-initiated failure mechanisms and to avoid this reduced performance. In relation to tensile-stress-induced surface crack initiation, for instance, tougher (e.g. Ni and/or W alloyed, bainite hardened) steels, steels with increased retained austenite content (N alloying, case hardening) or partial ceramic (hybrid) contacts, for example, with Si3N4 or Y-stabilized Zr02 roller(s), can be used. Micropitting up to grey staining from, for example, shear-stress-induced fatigue micro-cracking may be counteracted by lubricant selection, improving the surface and profile quality, or adjusting the hardness or physical vapour deposition (PVD) coating of the contact steel bodies. However, no tailored combined solution is known for both competing surface crack initiation mechanisms.

It is an object of the present invention to provide an improved process for improving the longevity of steel components of rolling contact loaded machine elements, or a process to tackle the drawbacks associated with the prior art, or at least provide a commercially viable alternative thereto.

Summary of the Invention

According to a first aspect, there is provided a process for treating a contact surface of a steel component of a rolling contact loaded machine element, the process comprising: providing a steel component having a surface layer predominantly having a martensitic and/or bainitic microstructure and a contact surface adjacent the surface layer; cold-working said contact surface to introduce a compressive residual stress in said steel component of at least 400 MPa to a depth of at least 50 microns from said contact surface; machining said contact surface; and heating the cold-worked and machined steel component below the tempering or transformation temperature of the steel.

The present disclosure will now be described further. In the following passages different aspects/embodiments of the disclosure are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to a process for treating a contact surface of a steel component of a rolling contact loaded machine element, in particular a steel bearing component. Such machine elements are well known in the art. It should be noted that all aspects discussed herein with relation to steel bearing components apply more broadly to steel components of rolling contact loaded machine elements, of which a steel bearing component is a preferred embodiment.

The inventors have found that under certain mixed friction operating conditions reduced working life time of contact surfaces of bearings and other Hertzian rolling contact loaded machine elements (e.g. continuously variable transmissions, camshafts) can be attributed to both (i) deep (about 50 to 250 pm) and (ii) shallow (up to about 10 pm) tensile-stress-induced and spontaneous cyclic shear stress induced fatigue surface crack initiation. In particular, in tribologically demanding application of several steel-made Hertzian rolling contact loaded machine elements of all sizes, competing mechanisms of (near-) surface failure-initiation can occur: frictional tensile-stress-induced brittle spontaneous initiation of surface cracks having a depth of from about 50 to 250 pm; and shallow (up to about 10pm) cyclic-shear-stress-induced surface micro cracks of up to 10pm in depth. The risk of tensile-stress-induced damage even increases with rising contact width, i.e. essentially the size (diameter) of the rolling element.

The inventors have found that it is possible to combine techniques of cold-working compressive residual stress formation and surface machining with final reheating to successfully improve the working lifetime of a steel component under the above-described tribologically demanding running conditions.

The present invention provides a process for treating a contact surface of a steel component of a rolling contact loaded machine element, especially a steel bearing component. The machine element may be, for example, at least a portion of a continuously variable transmission. Bearing components are well known in the art. Preferably the steel bearing component is one or more of an inner raceway, an outer raceway, a roller, and/or a ball bearing.

Steel compositions suitable for use in bearing components are well known in the art. Preferably, the steel component is formed of a standard bearing steel such as, for example, through hardenable lOOCrS or higher alloyed derivatives like l000rMn6 or lOOCrMo7-3, l8NiCrMol4-6 or other suitable case hardenable steels, or SOCrMo4 or other suitable induction hardenable (tempering) steels.

The steel component has a surface layer predominantly having a martensitic and/or bainitic microstructure. The surface layer is the region of the steel component adjacent the contact surface. The surface layer is at least 50 microns deep from the contact surface, preferably at least 100 microns, more preferably at least 200 microns and most preferably at least 300 microns. The surface layer can be provided with the predominantly martensitic and/or bainitic microstructure by a heat treatment step. Such heat treatment steps are well known and include martensite heat treatments, bainite heat treatments, case or induction hardening heat treatment. By predominantly it is meant that the layer comprises at least SOvol% of martensite and/or bainite, preferably at least 75vo1%, preferably at least 9Ovol% and is preferably substantially entirely of martensite and/or bainite.

The process according to the present invention comprises treatments applied to the contact surface of the steel component. These treatments will affect at least a portion of the surface layer adjacent the contact surface. The term contact surface as used herein is, therefore, intended to encompass the surface portion of the component which, in use, contacts another surface, such as a surface of another part of the bearing, where there may be movement between the surfaces. An example would be the inner surface of a raceway which, in use, contacts a surface of a rolling element (ball or roller bearing).

The term surface" encompasses the contact surface and also the portion of the surface layer of the steel directly adjacent the free surface. This is because the process according to the present invention affects the microstructure at the free surface and also in the surface layer (i.e., edge zone) of the component.

The present invention combines two tailored preventive measures against the failure-causing surface-crack initiation mechanisms described above. These steps are cold-working and machining. The cold-working introduces a compressive residual stress to a depth of at least 50 microns. The machining is carried out to enable an effective thermal static strain aging. The machining, for instance grinding, affects the microstructure of the contact surface to a depth of at least 5 microns and up to at least 10 microns.

The steps of cold-working and machining can be carried out in any order. They are preferably carried out such that the step before the heating step is machining.

However, machining can be carried out before and/or after cold-working.

Performing a surface machining before or after cold rolling allows for rough machining to remove scale, alleviate distortion effects and adjust dimensions or for last profile correction and roughness reduction, respectively introducing or increasing the amount of plastic yielding in the outermost near-surface material layer.

The process according to the present invention involves a cold-working step. In this step, the tensile-stress-induced initiation of brittle spontaneous cracks of typically 50 to 250 pm depth is counteracted. Cold-working is well known in the art. Preferably the cold-working is performed by shot peening, low or high plasticity burnishing, deep rolling, ball or roller burnishing and/or laser-shock peening. The cold-working is applied to the contact surface to introduce a compressive residual stress in said steel component of at least 400 MPa to a depth of at least 50 microns, preferably at least 100 microns, from said contact surface. This residual stress is retained after the final heat treatment step and is present in the final condition of the component on delivery.

Preferably, the cold-working is performed to introduce a compressive residual stress in said steel component of from 500 MPa to 1000 MPa, more preferably from 600 MPa to 800 MPa. In order to avoid any pre-damage of the microstructure, a maximum value the compressive residual stresses of 1000 MPa or less, preferably 800 MPa or less should be achieved.

Preferably, the cold-working is performed to introduce the compressive residual stress in said steel component to a depth of from 50 to 250 microns from the contact surface, more preferably from 100 to 200 microns. The depth can be selected based upon the depth of the occurring brittle spontaneous incipient cracks under the specific loading conditions of a certain application.

Such residual stresses in metals can be determined by X-ray diffraction applying, for instance, the sin2kv method. For hardened steels, the shift of the Bragg angle 0 (diffraction angle 20z156.1°) of the a-Fe (211) lattice planes is commonly measured for this purpose, e.g. on a diffractometer with scintillation type counter tube and focusing Bragg-Brentano coupled 0-20 diffraction geometry, for variable specimen orientations, which are characterized by the tilt angle ii. The use of exciting long-wave Cr Kcz radiation of wavelength of 0.229 nm in the back scattering region is well suited. As the relevant effective penetration depth with this X-ray source equals around 5 pm, distance curves of the residual stresses can be measured with appropriate spatial resolution by means of gradual electra-polishing material removal. Additionally, the peak width of the a-Fe (211) X-ray diffraction line, expressed e.g. as full width at half maximum (FWHM) intensity, can be measured in the direction of kji=O°.

The process according to the present invention involves a machining step.

Machining is carried out to specifically pie-condition the microstructure in the outermost material layer of 5 to 10 pm depth for final static strain aging reheating by plastic deformation. That is the outermost 5 to 10 pm deep layer is conditioned, i.e. microstructurally prepared, for subsequent static strain aging by the machining step. Machining is well known in the art. It has been found that this step creates the prerequisites for reducing shear-stress-induced initiation of fatigue micro cracks of typically 5 to 10 pm depth.

Thermal static strain aging is well known in the art and is discussed in, for example, US7037383, which is incorporated herein by reference. The machining is preferably carried out in such a way that subsequent reheating at a testing temperature of 150°C for 1 hour results in a reduction of the full width at half maximum (FWHM) peak width of the a-Fe (211) X-ray diffraction line, excited by the Chromium K-alpha X-rays, at the surface by minimum 0.1°. This material response proves the effectiveness of microstructural preconditioning by the machining step, regardless of the eventually applied temperature and time of the final reheating step in the actual production process of the steel component.

The machining is preferably performed by grinding and/or honing the contact surface. Other processes include polishing, lapping, superfinishing, high speed cutting and/or hard turning, possibly followed by grinding and/or honing.

The process according to the present invention further comprises heating the cold-worked and machined steel component below the tempering or transformation temperature of the hardening heat treatment of the steel. This preferably leads to little or no loss in hardness. The heat treatment thermally stabilises the microstructure in the surface layer which is mechanically pre-conditioned in the previous treatment steps.

The step of heating the cold-worked and machined steel component is preferably conducted atatemperature of from 140 to 23000, more preferably from 150 to 200°C, and typically for a duration of from 5 minutes to 2 hours, preferably from minutes to 1 hour. The minimum duration of final reheating is defined by the time required to reduce the full width at half maximum (FWHM) peak width of the a-Fe (211) X-ray diffraction line at the surface by at least 0.10, which ensures effective thermal static strain aging. Preferably, the heating step is carried out at a temperature and time sufficient to reduce the full width at half maximum (FWHM) peak width of the a-Fe (211) X-ray diffraction line at the surface by at least 0.10, more preferably by at least 0.2°. Provided that the hardness remains virtually unchanged, the achieved FWHM decrease provides a measure of the intensity of the desired microstructural stabilization by thermal static strain aging.

Whereas the small thermal reduction is of no practical relevance, reheating makes the cold-working compressive residual stresses more stable against decrease by cyclic relaxation during rolling contact loading. Thermal static strain aging of the (surface machining) pre-conditioned near-surface microstructure by final reheating increases the fatigue resistance of the outermost layer of minimum to 10 pm depth and therefore strengthens the treated rolling contact component against fatigue surface micro-crack initiation.

Optionally the process according to the present invention further comprises a step of surface coating the contact surface of the steel component. This can be applied before, during or after the heating step, provided that the step does not affect the microstructure of the steel. Suitable coatings are well known in the art and include, for example, black oxide or physical vapour deposition coating to improve the running-in or poor-lubrication behaviour or to reduce local sliding friction under vibration loading. This coating step can optionally be interposed before or even carried out as final reheating -provided that an appropriate process temperature and time is applied.

According to a further aspect, there is provided a steel component of a rolling contact loaded machine element obtainable by the process described herein.

Preferably the steel component of a rolling contact loaded machine element is a steel bearing component, preferably an inner raceway, an outer raceway, or a rolling element of a roller or a ball bearing.

Figures The present disclosure will be described in relation to the following non-limiting figures, in which: Figure 1 is an exemplary flow chart of a process according to the present invention.

Figure 2 illustrates schematic depth distributions of the residual stress before and after final reheating, plotted from the surface to the core of the material in the condition on delivery of a steel bearing component.

Figure 3 illustrates the X-ray diffraction line width before and after final reheating, plotted from the surface to the core of the material in the condition on delivery of a steel bearing component.

Figure 1 includes the following steps: A -heat treatment of a steel bearing component, such as martensite heat treatment, banite heat treatment, case or induction hardening treatment; B -Surface machining the component, such as high speed cutting, hard turning, grinding, honing or superfinishing; C -Cold-working the component, such as shot peening, deep cold rolling, or low plasticity burnishing; D -Surface machining the component, such as high speed cutting, hard turning, grinding, honing or superfinishing; E -Low temperature coating the component (optional); and F -Reheating the component below the tempering or transformation temperature for, e.g. one hour.

With respect to Figures 2 and 3, the intended reheating effect of thermal static strain aging is reflected in the decrease of the FWHM. The change on the surface, indicated by the arrow in Figure 3, is a measure of the effectiveness of microstructural stabilization against fatigue surface micro-crack formation. The minimum limit of 400 MPa up to 50 pm depth of the compressive residual stresses is included in Figure 2. The core value, reached in the example at about 250 microns, is characteristic of the applied heat treatment. In Figure 2, low tensile residual stresses around 100 MPa indicate martensitic hardening.

Examples

The present disclosure will now be described in relation to the following non-

limiting examples.

Bearing components were prepared from a standard bearing steel lOOCr6 (SAE 52100). The bearing components provided inner and outer raceways for a roller bearing.

In a first example, the bearing components were pre-treated with a martensitic hardening process, involving tempering at 220 °C for 4 hours. The resulting hardness was around 60 HRC.

The bearing components were then subjected to a number of process steps.

These were: 1. Grinding surface machining; 2. Cold-working by shot peening, although other techniques such as deep rolling or low plasticity burnishing could be used; 3. Honing to finally produce the residual stress and X-ray diffraction peak width distribution plotted as the dashed line in Figures 2 and 3, respectively.

No metallographically detectable changes occurred in the microstructure by this procedure but it was found that the dislocations were rearranged into, for example, multipole configurations due to yielding in the plastically deformed zone. This can be observed on a transmission electron microscope. Such measurement techniques are well known in the art. The suitability of mechanical pre-conditioning by machining was verified by heating a sample of the machined surface zone at 150°C for 1 hour in a laboratory furnace in air. This test treatment resulted in a reduction of the X-ray diffraction peak width on the surface by 0.15°, as measured on a diffractometer. -10-

4. Optionally subjecting the components to a low temperature, e.g. PVD or "hot" black oxide, coating respectively at 200°C and 140°C.

5. Reheating at 210°C for 1 hour in air or nitrogen to produce the residual stress and X-ray diffraction peak width distribution plotted as the solid line in Figures 2 and 3, respectively.

In this example, the decrease in the X-ray diffraction peak width reflects the increased resistance of the outermost material layer to fatigue micro-crack initiation as a consequence of reheating-induced thermal static strain aging. This amounts to 0.3°, which exceeds the sufficient minimum value for an appropriate strengthening effect of 0.1° (cf. Figure 3). In the deeper zone influenced by cold-working, the compressive residual stresses are only slightly decreased by reheating (see Figure 2), but thermally stabilized against fatigue induced reduction during operation.

An alternative starting material could be a standard bearing steel lOOCr6 subjected to a bainitic hardening pre-treatment, with the transformation at 220 00 (lower bainite) for 5 hours. Also, case hardened steel l8NiCrMol4-6 or induction hardened steel 5OCrMo4 could be used. Other through, case or induction hardenable steels are possible as well.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

-11 -Claims: 1. A process for treating a contact surface of a steel component of a rolling contact loaded machine element, the process comprising: providing a steel component having a surface layer predominantly having a martensitic and/or bainitic microstructure and a contact surface adjacent the surface layer; cold-working said contact surface to introduce a compressive residual stress of at least 400 MPa to a depth of at least 50 microns from said contact surface; machining said contact surface; and heating the cold-worked and machined steel component below the tempering or transformation temperature of the steel.
2. The process according to claim 1 for treating a contact surface of a steel bearing component.
3. The process according to claim 1 or claim 2, wherein the step of machining said contact surface introduces plastic yielding to a depth of at least 5 microns from the contact surface, preferably to a depth of at least 10 microns.
4. The process according to any of the preceding claims, wherein the step of machining said contact surface introduces plastic yielding such that subsequent heating at 150°C for 1 hour results in a decrease of the full width at half maximum (FWHM) peak width of the alpha-iron (211) X-ray diffraction line, excited by the Chromium K-alpha X-rays, at the surface by at least 0.1°.
5. The process according to any of the preceding claims, wherein the step of heating the cold-worked and machined steel component results in a decrease of the full width at half maximum (FWHM) peak width of the alpha-iron (211) X-ray diffraction line, excited by the Chromium K-alpha X-rays, at the surface by at least 0.1°, and preferably by at least 0.2°.
6. The process according to any of the preceding claims, wherein the steel component is an inner raceway, an outer raceway, or a rolling element of a roller or a ball bearing.

-12 -
7. The process according to any of the preceding claims, wherein the steel component is formed of through or case hardenable bearing steel.
8. The process according to any of the preceding claims, wherein the steel component is provided following a martensite, bainite, case or induction hardening heat treatment.
9. The process according to any of the preceding claims, wherein the cold-
10. The process according to any of the preceding claims, wherein the cold-working is performed to introduce a compressive residual stress in said steel component of from 500 to 1000 MPa.
11. The process according to any of the preceding claims, wherein the cold-working is performed to introduce the compressive residual stress in said steel component to a depth of at least 100 microns from said contact surface.
12. The process according to any of the preceding claims, wherein the machining comprises grinding the contact surface.
13. The process according to any of the preceding claims, wherein the step of machining is carried out before and/or after the step of cold-working.
14. The process according to any of the preceding claims, wherein the step of heating the cold-worked and machined steel component below the tempering or transformation temperature of the steel is conducted at a temperature of from 140 to 230°C and/or for a duration of from 5 minutes to 2 hours.
15. The process according to any of the preceding claims, further comprising a step of surface-coating the contact surface of said steel component, preferably by black oxide or physical vapour deposition coating.
16. The process according to claim 15, wherein the step of surface coating is performed after cold-working and machining and before the heating step, and/or is performed together with heating. -13-17. A steel component of a rolling contact loaded machine element obtainable by a process as defined in any one of the preceding claims, preferably wherein the steel component is a steel bearing component, preferably an inner raceway, an outer raceway, or a rolling element of a roller or a ball bearing