GB2512838A

GB2512838A - Bearing component formed from steel alloy

Info

Publication number: GB2512838A
Application number: GB1306311.0A
Authority: GB
Inventors: Aidan Kerrigan; Staffan Larsson; Pedro Miguel Marques Vieira Raposo Marques; Stefano Richaud; Mark Verbakel; Rolando Velazquez
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2013-04-08
Filing date: 2013-04-08
Publication date: 2014-10-15
Also published as: GB201306311D0

Abstract

A bearing component formed of a steel comprising (by weight): 0.8-1.2 % carbon, 1.1-1.5 % silicon, 0.5-1.5 % manganese 1 to 2 % chromium, 0-0.8 % molybdenum, 0-0.6 % vanadium, 0-0.2 % niobium, 0-0.3 % tantalum, 0-0.4 % nickel, 0-0.5 % copper, 0-0.05 % nitrogen, 0-0.04 % phosphorus, 0-0.04 % sulphur, 0-0.050 % aluminium, up to 15 ppm oxygen, up to 50 ppm titanium and up to 10 ppm calcium, with the balance being iron and unavoidable impurities. The steel comprises a martensitic microstructure, optionally also with retained austenite and/or carbides, for example at least 60 % by volume martensite, up to 20 % by volume austenite and up to 20 % by volume carbides. A surface region of the steel has been carbonitrided, this region may also be burnished.

Description

Bearing component formed from steel alloy

Technical Field

The present invention relates generally to the field of metallurgy and to a bearing component formed from a steel alloy.

Background

Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (balls or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling contact fatigue, wear and creep.

The bearing steel known as lOOCr6 has the following nominal composition: 1.0 wt% carbon, 0.25 wt% silicon, 0.35 wt% manganese, 0.05 wt% molybdenum, 1.45 wt% chromium, 0.15 wt% nickel, 0.035 wt% aluminium, 0.15 wt% copper, 0.015 wt% phosphorus and 0.010 wt% sulphur, the balance being iron (and any unavoidable impurities). This steel exhibits high hardness and is suitable for use in bearings.

WO 03/01 21 56 discloses a bearing steel comprising from 0.12 to 0.30 wt.% carbon, from 0.8 to 1.5 wt.% silicon, from 1.0 to 1.6 wt.% manganese, optionally one or more of from 0 to 0.30 wt.% molybdenum, from 0 to 0.6 wt.% nickel, from 0 to 0.6 wt.% molybdenum, from 0 to 0.6 wt.% nickel, from 0 to 0.06 wt.% aluminium, from Oto 0.30 wt.% copper, from 0 to 0.10 wt% sulfur, from 0 to 0.03 wt.% phosphorous, from 0 to 0.050 wt% niobium, and the balance iron, together with unavoidable impurities. A bearing component formed of the steel and subjected to low pressure carburising or carbonitriding exhibits high resistance to rolling contact fatigue. However, the resistance to rolling contact fatigue is reduced at elevated temperatures.

It is an object of the present invention to address or at least mitigate some of the problems associated with prior art, or at least to provide a commercially useful alternative thereto.

S

Summary

The present invention provides a bearing component formed of a bearing steel alloy comprising: from 0.8 to 1.2 wt.% carbon, from 1.1 to 1.5 wt.% silicon, from 0.5 to 1.5 wt.% manganese, and from 1 to 2 wt.% chromium, optionally one or more of from 0 to 0.8 wt% molybdenum, from 0 to 0.6 wt% vanadium, from 0 to 0.2 wt% niobium, from 0 to 0.3 wt% tantalum, from 0 to 0.4 wt% nickel, from 0 to 0.5 wt% copper, from 0 to 0.05 wt% nitrogen, from 0 to 0.04 wt% phosphorus, from 0 to 0.04 wt% sulphur, from 0 to 0.050 wt.% aluminium, upto 15 ppm oxygen, up to 50 ppm titanium, upto 10 ppm calcium, and the balance iron, together with any unavoidable impurities, wherein the microstructure of the alloy comprises martensite, optionally retained austenite and optionally carbides, and wherein a surface region of the bearing component is carbonitrided.

The alloying elements in combination with the microstructure provides the bearing component with high resistance to rolling contact fatigue, even at elevated temperatures.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects 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 microstructure of the steel alloy comprises martensite as the main phase.

Retained austenite and/or carbides may also be present. In addition, a surface region of the bearing component is carbonitrided. Carbonitriding enriches the surface of the bearing component with carbon and nitrogen. The microstructure of a carbonitrided surface typically comprises a higher volume of enlarged carbides compared to a standard through-hardened bearing steel.

Such a surface microstructure may increase resistance to damage from solid particles in lubrication. Accordingly, the bearing component may exhibit high resistance to rolling contact fatigue in particle-contaminated lubrication conditions.

The addition of nitrogen to the surface microstructure may reduce the reduction in hardness caused by elevated temperatures. For example, after tempering at temperatures of about 220 °C, or even about 250 °C, the hardness of the nitrogen-enriched surface may be maintained. Accordingly, the bearing component may exhibit high resistance to rolling contact fatigue at elevated temperatures.

Carbonitriding may also provide the surface region with a compressive residual stress, which is beneficial in offsetting the shear stresses from rolling contact fatigue. The magnitude of the surface compressive residual stress may be enhanced by applying a martempering treatment, a partial bainite transformation or a complete bainite transformation prior to cooling to room temperature.

Typically substantially the entire surface of the bearing component is carbonitrided, more typically the entire surface of the bearing component is carbonitrided. Carbonitriding techniques are well known in the art.

The microstructure and carbonitrided surface region may be obtained, for example, as follows. The steel alloy is provided in spheroidised and annealed condition. A heat-treatment at a temperature of from 840 to 900°C is then carried out in an atmosphere containing carbon and nitrogen. For example, an atmosphere comprising a hydrocarbon and ammonia may be used. The furnace carbon potential is generally in the range of from 0.8 to 1.4 wt.%, for example about 1 wt.%. As a result of the carbonitriding, the surface of the steel alloy typically comprises from 0.9 to 1.5 wt.% carbon and from 0.05 to 0.5 wt.% nitrogen. The alloy is then quenched in a suitable media such as, for example, oil, typically to a temperature of less than 1 00°C to obtain a martensitic structure. A tempering treatment is then optionally carried out, for example at a temperature of from 200 to 300°C.

The steel alloy comprises from 0.8 to 1.2 wt.% carbon. Preferably, the steel alloy comprises from 0.9 to 1.1 wt.% carbon, more preferably from 0.95 to 1.05 wt.% carbon, even more preferably about 1 wt.% carbon. The presence of carbon in the specified amounts, in conjunction with the other alloying elements, results in the desired microstructure. The carbon also serves to increase the hardness of the steel alloy.

The steel alloy comprises from 1.1 to 1.5 wt.% silicon. Preferably the steel alloy comprises from 1.2 to 1.4 wt.% silicon, more preferably from 1.25 to 1.35 wt.% silicon, even more preferably about 1.3 wt.% silicon. The presence of silicon in the specified amount serves to resist the tempering effects of elevated temperatures. Accordingly, the bearing component may exhibit high resistance to rolling contact fatigue at elevated temperatures. In addition, the presence of silicon may act to increase the strength and toughness of the alloy. However, too high a silicon content may result in undesirable surface oxides and a poor surface finish.

The steel alloy comprises from 0.5 to 1.5 wt.% manganese. Preferably the steel alloy comprises from 0.7 to 1.3 wt.% manganese, more preferably from 0.9 to 1.1 wt.% manganese, even more preferably about 1 wt.% manganese.

The manganese increases hardness and also contributes to strength.

Manganese may also have a beneficial effect on hardenability.

The steel alloy comprises from 1 to 2 wt.% chromium. Preferably the alloy comprises from 1.1 to 1.8 wt.% chromium, more preferably from 1.3 wt.% to 1.5 wt.% chromium, even more preferably about 1.4 wt.% chromium. The presence of chromium in the specified amounts improves rolling contact fatigue resistance. Chromium may also have a beneficial effect on hardenability.

The alloy may optionally comprise from 0 to 0.8 wt% molybdenum, for example from 0.01 to 0.8 wt.% molybdenum. Molybdenum may act to increase the hardenability of the alloy. In addition, Molybdenum may improve grain boundary cohesion.

The alloy may optionally comprise from 0 to 0.6 wt% vanadium, for example from 0.01 to 0.6 wt.% vanadium. Vanadium may act to increase the hardness of the alloy and preferably also the yield strength and/or tensile strength.

The alloy may optionally comprise from 0 to 0.4 wt% nickel, for example from 0.01 to 0.4 wt.% nickel. Nickel may act to increase hardenability and impact strength.

The alloy may optionally comprise from 0 to 0.5 wt% copper, for example from 0.01 to 0.5 wt.% copper. The copper may act to provide improved corrosion resistance.

The alloy may optionally comprise from 0 to 0.05 wt.% aluminium, for example from 0.01 to 0.05 wt.% aluminium. Aluminium may be used as a deoxidizer.

Aluminium may also act to control grain size in the alloy. In addition, aluminium may improve the intrinsic toughness of the bearing component.

The alloy may optionally comprise one or more of from 0 to 0.2 wt% niobium, for example from 0.01 to 0.2 wt.% niobium, from 0 to 0.3 wt% tantalum, for example from 0.01 to 0.13 wt.% tantalum, from 0 to 0.05 wt% nitrogen, for example from 0.01 to 0.05 wt.% nitrogen, up to 50 ppm titanium, for example from 1 to 50 ppm nitrogen, and up to 10 ppm calcium, for example from 1 to ppm calcium.

Other elements that may be present include oxygen, phosphorus and sulphur.

Preferably, the presence of these elements is kept to a minimum. If phosphorus is present, the content thereof should generally not exceed 0.04 wt%. If sulphur is present, the content should generally not exceed 0.04 wt%.

If oxygen is present, the content should generally not exceed 15 ppm.

It will be appreciated that the steel alloy may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt.% of the composition.

Preferably, the alloy contains unavoidable impurities in an amount of not more than 0.3 wt.% of the composition, more preferably not more than 0.1 wt.% of the composition. As noted above, the phosphorus, sulphur and oxygen contents are preferably kept to a minimum.

In a preferred embodiment, the steel alloy comprises: about 1 wt.% carbon, about 1.3 wt.% silicon, about 1 wt.% manganese, about 1.4 wt.% chromium, and the balance iron, together with any unavoidable impurities.

The steel alloys for use in the present invention may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements which are mandatory other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.

The microstructure of the steel alloy typically comprises at least 60 vol.% martensite, more typically at least 70 vol.% martensite, still more typically at least 80 vol.% martensite. Retained austenite and/or carbides may also be present. For example, the microstructure of the alloy may comprise up to 20 vol.% retained austenite, preferably from 1 to 15 vol.% retained austenite, more preferably from 5 to 10 vol.0!0 retained austenite.

Similarly, the microstructure of the alloy may comprise up to 20 vol.% carbides, preferably from 2 to 15 vol.% carbides, more preferably from 5 to 10 vol.% carbides. If the microstructure of the alloy comprises carbides, then typically most of the carbides have an equivalent circle diameter (ECD) of up to 10 microns, for example 1 to 10 microns.

Carbonitriding enriches the surface of the bearing component with carbon and nitrogen. The microstructure of a carbonitrided surface typically comprises a higher volume of enlarged carbides compared to the standard through-hardened bearing steel. This surface microstructure is beneficial in terms of increased resistance to damage from solid particles in lubrication.

The structure of the steel alloys described herein can be determined by conventional microstructural characterisation techniques such as, for example, light optical microscopy, TEM, SEM, AP-FIM, TDA and X-ray diffraction, including combinations of two or more of these techniques.

Preferably a surface region (for example a carbonitrided surface region) of the bearing component is burnished. Burnishing may enhance the surface compressive stress. This may provide additional resistance to surface initiated failures, particularly when operating in thin lubrication films.

Accordingly, the bearing component may exhibit high resistance to rolling contact fatigue in thin lubrication conditions. Burnishing techniques are known in the art. Burnishing may be carried out, for example, using balls of diameter 4-10 mm and at a pressure of 50 -400 bar.

The bearing component may be at least one of a rolling element, an inner ring, and an outer ring.

The present invention provides a bearing comprising a bearing component as described herein.

Figures The present invention will now be described further, by way of example, with reference to the following figures: Figure 1 shows a plot of X-ray diffraction results indicating residual stress and fatigue damage index (FDI) at various depths of a steel alloy.

Figure 2 shows a light optical micrograph image of a carbonitrided surface of an example of an alloy as described herein.

Figure 3 shows a light optical micrograph image of a standard through hardened bearing steel.

Figure 4 shows hardness profiles of an alloy as described herein in the carbonitrided and tempered condition.

Examile The invention will now be explained with reference to the following non-limiting

example.

A steel with the following composition (approximate weight %) was prepared: 1% carbon, 1.3% silicon, 1% manganese, 1.4% chromium and the balance iron and unavoidable impurities. The steel was then subjected to spheroidising and annealing treatments, followed by a heat-treatment (and carbonitriding) at a temperature of from 840 to 900°C in an atmosphere comprising carbon and nitrogen. (Ammonia was used as the nitrogen source.) The furnace carbon potential was approximately 1 wt.%. The steel was then quenched in oil to a temperature of about 100 °C, followed by tempering at a temperature of from to 300°C. The obtained microstructure comprised martensite as a main phase, together with some retained austenite and also carbides. A carbonitrided surface region was also present.

Rolling contact fatigue measurement To asses rolling contact fatigue, a reference example steel was prepared. The reference example steel was prepared in the same manner as above but without carbonitriding. Accordingly, rolling contact fatigue resistance was assessed with the alloy in the martensitically through-hardened condition but not carbonitrided. The test ran for 700 hours or 153 MRevs at a contact pressure of 2.35 OPa and a temperature of approximately 170°C. In comparison, for a 22220E spherical roller bearing (SRB) at 2.35 GPa, the calculated L10 life (the life at which 10% of bearings are expected to fail due to fatigue failure) would be 23 MRevs at normal operating temperatures. The component was investigated using X-ray diffraction and the results are shown in Figure 1. One line of the plot shows the residual stress and the other line the "Fatigue Damage Index" (FDI). Both indicate that there has been sub-surface fatigue damage accumulation. For a 1 OOCr6/521 00 steel, an FDI value of approximately 0.64 indicates that the has been reached. The critical FDI value for this silicon alloyed steel is not known, but an FDI value of 0.8 suggests that the fatigue life could be longer than 153 MRevs. The results indicate that this steel has a significantly longer life at elevated temperatures compared to standard bearings steels which form the basis for the calculated L10 life of 23 MRevs at normal operating temperatures.

Light optical microscopy Figure 2 shows a light optical microscope image of the surface of the steel alloy which had been subjected to carbonitriding. A higher volume fraction of enlarged carbides (white phase) is observed at the surface in comparison to a standard through hardened steel (Figure 3).

Hardness profile Figure 4 shows the shows the hardness profile of the steel in the as-carbonitrided and tempered condition. The vertical dashed line indicates the depth of material that will be removed during hard machining. A comparison of the hardness profiles after tempering at 220°C and 250°C indicates the increased temperature resistance provided by the surface nitrogen. The higher tempering temperature has a softening effect on the base steel but the nitrogen enriched surface maintains its hardness.

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

CLAIMS: 1. A bearing component formed of a bearing steel alloy comprising: from 0.8 to 1.2 wt.% carbon, from 1.1 to 1.5 wt.% silicon, from 0.5 to 1.5 wt.% manganese, and from 1 to 2 wt.% chromium, optionally one or more of from 0 to 0.8 wt% molybdenum, from 0 to 0.6 wt% vanadium, from 0 to 0.2 wt% niobium, from 0 to 0.3 wt% tantalum, from 0 to 0.4 wt% nickel, from 0 to 0.5 wt% copper, from 0 to 0.05 wt% nitrogen, from 0 to 0.04 wt% phosphorus, from 0 to 0.04 wt% sulphur, from 0 to 0.050 wt.% aluminium, upto 15 ppm oxygen, up to 50 ppm titanium, upto 10 ppm calcium, and the balance iron, together with any unavoidable impurities, wherein the microstructure of the alloy comprises martensite, optionally retained austenite and optionally carbides, and wherein a surface region of the bearing component is carbonitrided.I
2. The bearing component as claimed in claim 1, wherein the steel alloy comprises from 0.9 to 1.1 wt.% carbon, preferably from 0.95 to 1.05 wt.% carbon.
3. The bearing component as claimed in claim 1 or claim 2, wherein the steel alloy comprises from 1.2 to 1.4 wt.% silicon, preferably from 1.25 to 1.35 wt.% silicon.
4. The bearing component as claimed in any one of the preceding claims, wherein the steel alloy comprises from 0.7 to 1.3 wt.% manganese, preferably from 0.9 to 1.1 wt.% manganese.
5. The bearing component as claimed in any one of the preceding claims, wherein the steel alloy comprises from 1.1 to 1.8 wt.% chromium, preferably from 1.3 wt.% to 1.5 wt.% chromium.
6. The bearing component as claimed in any one of the preceding claims, wherein the alloy comprises about 1 wt.% carbon, about 1.3 wt.% silicon, about 1 wt.% manganese, about 1.4 wt.% chromium, and the balance iron, together with unavoidable impurities.
7. The bearing component as claimed in any one of the preceding claims, wherein the microstructure of the alloy comprises at least 60 vol.% martensite, preferably at least 70 vol.% martensite, more preferably at least 80 vol.% martensite.
8. The bearing component as claimed in any one of the preceding claims, wherein the microstructure of the alloy comprises up to 20 vol.% retained austenite, preferably from 1 to 15 vol.% retained austenite, more preferably from 5 to 10 vol.% retained austenite.
9. The bearing component as claimed in any one of the preceding claims, wherein the microstructure of the alloy comprises up to 20 vol.0!0 carbides, preferably from 2 to 15 vol.% carbides, more preferably from 5 to 10 vol.% carbides.
10. The bearing component as claimed in any one of the preceding claims, wherein the microstructure of the alloy comprises carbides, most of which have an equivalent circle diameter of up to 10 microns.
11. The bearing component as claimed in any one of the preceding claims, wherein a surface region of the bearing component is burnished.
12. The bearing component as claimed in claim 12, wherein said surface region of the bearing component which is burnished is also carbonitrided.
13. The bearing component as claimed in any one of the preceding claims which is at least one of a rolling element, an inner ring, and an outer ring.
14. A bearing comprising a bearing component as claimed in any one of the preceding claims.