WO2013060866A1

WO2013060866A1 - A bearing component

Info

Publication number: WO2013060866A1
Application number: PCT/EP2012/071293
Authority: WO
Inventors: Mohamed Sherif; John Beswick
Original assignee: Aktiebolaget Skf
Priority date: 2011-10-28
Filing date: 2012-10-26
Publication date: 2013-05-02

Abstract

A bearing component formed from a steel composition comprising of carbon, silicon, manganese, molybdenum, chromium, cobalt, aluminium, vanadium, optionally one or more of the following elements sulphur, phosphorous, arsenic, tin, and the balance iron, together with unavoidable impurities.

Description

A Bearing Component

Technical field The present invention relates generally to the field of metallurgy and to a bearing component such as a rolling element or ring formed from a bearing steel. The bearing steel has a microstructure comprising a very fine bainitic matrix and vanadium carbide precipitates.

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 fatigue, wear and creep .

Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished or semi-finished components so as to improve, for example, wear resistance and fatigue

resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue performance.

An alternative to case-hardening is through-hardening.

Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened

components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example. For through-hardened bearing steel components, two heat- treating methods are available: martensite hardening or austempering . Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed.

The martensite through-hardening process involves

austenitising the steel prior to quenching below the

martensite start temperature. The steel may then be low- temperature tempered to stabilize the microstructure.

The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels instead of martensite through- hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance.

Numerous conventional heat-treatments are known for

achieving martensite through-hardening and bainite through- hardening .

WO 01/79568 describes a method for the production of a part for a rolling bearing. Summary

The present invention provides a bearing component formed from a steel composition comprising:

(a) from 0. 9 - 1.8 wt . % carbon,

(b) from 1. 0 - 2.0 wt . % silicon,

(c) from 1. 0 - 2.0 wt . % manganese,

(d) from 0. 1 - 0.4 wt . % molybdenum,

(e) from 1. 0 - 3.0 wt . % chromium,

(f) from 0. 2 - 1.5 wt . % cobalt,

(g) from 1. 0 - 2.0 wt . % aluminium,

(h) from 0. 1 - 5 wt .. % vanadium,

(i) optionally one or more of the f

from 0 - 0 .1 wt ..% sulphur,

from 0 - 0 .1 wt ..% phosphorous,

from 0 - 0 .075 wt . % arsenic,

from 0 - 0 .075 wt . % tin,

from 0 - 0 .075 wt . % antimony,

(j) the balance iron, together with unavoidable impurities.

The bearing component is formed from the alloy as herein described and preferably comprises lower bainite as the main phase (typically at least 60% bainite, more typically at least 80% bainite) . The plates of bainite are very fine.

In particular, the material preferably has a microstructure comprising plates of bainite (preferably lower bainite) of less than 100 nm thickness, typically from 10 to 50 nm, more typically from 20 to 40 nm. The plates of bainite are advantageously interspersed with retained austenite thin films. The bainite typically forms at least 60% of the microstructure, more typically at least 80% (by volume) . The steel preferably also contains vanadium carbide

precipitates and/or vanadium carbonitride precipitates.

Typically, the microstructure will comprises at least 3% carbides, more typically at least 5% (by volume) .

The microstructure and resulting mechanical properties lead to improved rolling contact fatigue performance in the bearing component.

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 steel composition preferably comprises from 0.9 - 1.7 wt . % carbon, more preferably from 0.9 to 1.6 wt . % carbon, still more preferably from 1.0 to 1.5 wt . % carbon. In combination with the other alloying elements, this results in the desired fine (lower) bainite microstructure. Carbon acts to lower the bainite start transformation temperature. Carbon also forms desirable carbide precipitates with vanadium, which improves the mechanical properties.

The steel composition preferably comprises 1.2 - 1.8 wt . % silicon, more preferably from 1.3 - 1.7 wt . % silicon, still more preferably from 1.4 - 1.6 wt . % silicon. In combination with the other alloying elements, this results in the desired microstructure . Silicon helps to suppress the precipitation of cementite. However, too high a silicon content may result in undesirable surface oxides and a poor surface finish. For this reason, the maximum silicon content is 2 wt.%, more preferably 1.9 wt . % .

The steel composition preferably comprises 1.2 - 1.8 wt.% manganese, more preferably from 1.3 - 1.7 wt.% manganese, still more preferably from 1.4 - 1.6 wt.% manganese.

Manganese acts to increase the stability of austenite relative to ferrite. Manganese may also increase the hardenability .

The steel composition preferably comprises from 0.15 - 0.35 wt.% molybdenum, more preferably from 0.2 - 0.3 wt.% molybdenum. Molybdenum acts to avoid austenite grain boundary embrittlement owing to impurities such as, for example, phosphorus. Molybdenum also acts to increase hardenability and reduce the bainite start temperature

The steel composition preferably comprises 1.5 to 2.5 wt.% chromium, more preferably from 1.7 - 2.3 wt.% chromium, still more preferably from 1.8 - 2.0 wt.% chromium.

Chromium acts to increase hardenability and reduce the bainite start temperature.

The steel composition preferably comprises from 0.3 - 1.2 wt.% cobalt, more preferably from 0.5 - 1.0 wt.% cobalt, still more preferably 0.6 - 0.9 wt.% cobalt. Cobalt has been found to improve the corrosion resistance of the bearing component. This is very important for bearing components for wind turbines or marine pods, for example. Such bearings may become contaminated by sea water, which can drastically reduce the service life of the bearing.

Cobalt also acts to accelerate the transformation to

bainite. However, too much cobalt (in excess of 1.5 wt . ~6 ) in conjunction with the other alloy elements herein described may result in too high an austenitising temperature.

The steel composition preferably comprises from 1.1 - 1.7 wt . % aluminium, more preferably from 1.2 - 1.5 wt . %

aluminium, still more preferably 1.3 - 1.4 wt . % aluminium. Aluminium has been found to improve the intrinsic toughness of the bearing component, possibly due to it suppressing carbide formation. The steel composition preferably comprises from 0.2 - 4.0 wt . % vanadium, more preferably from 0.2 - 3.0 wt . % vanadium, still more preferably 0.3 - 1.0 wt . % vanadium. Vanadium combines with carbon to form vanadium carbide precipitates. The vanadium carbide precipitates increase the yield

strength, tensile strength and/or hardness of the material. Vanadium carbonitride precipitates may also be present.

The steel composition may optionally include one or more of the following elements

from 0 - 0.1 wt . % sulphur,

from 0 0.1 wt • %o phosphorous,

from 0 0.075 wt . % arsenic,

from 0 0.075 wt o

• o tin, and

from 0 0.075 wt . % antimony.

The steel composition preferably comprises little or no sulphur, for example from 0 - 0.015 wt . % sulphur. The steel composition preferably comprises little or no phosphorous, for example from 0 - 0.02 wt . % phosphorous. The sum of arsenic, tin and antimony is preferably no more than 0.075 wt . % .

The steel composition preferably comprises ≤ 15 ppm oxygen. Oxygen may be present as an impurity.

The steel composition preferably comprises ≤ 30 ppm

titanium. Titanium may be present as an impurity.

The steel composition preferably comprises ≤ 50 ppm calcium. Calcium may be present as an impurity but may also be added intentionally in very small amounts.

The steel composition may also contain very small amounts of nitrogen .

The steel composition may be made by the following route, provided by way of example. The steel can be cast into moulds followed by high temperature soaking and then hot rolling. This results in the vanadium being dissolved in the austenite phase. Slow cooling precipitates out vanadium carbide and possibly also vanadium carbonitride, while the austenite mostly transforms to pearlite. A step of

austenitising results in austenite being formed together with retained vanadium carbide precipitates (and possibly also vanadium carbonitride precipitates) . The step of austenitising is suitably performed at a temperature of less than 1050 ^°C - preferably at a temperature of less than 950 ^°C - to prevent excessive austenite grain growth and to prevent the dissolution of the retained vanadium carbide precipitates (and any vanadium carbonitride precipitates) . The material may subsequently undergo austempering/bainite through-hardening to transform the austenite to bainite

(lower bainite) .

The microstructure of the steel composition preferably comprises a very fine bainitic matrix and vanadium carbide precipitates. In particular, the material preferably has a microstructure comprising plates of bainite (preferably lower bainite) of less than 100 nm thickness, typically from 10 to 50 nm, more typically from 20 to 40 nm. The plates of bainite are advantageously interspersed with retained austenite thin films. In addition to the vanadium carbide precipitates, vanadium carbonitride precipitates may also be present .

It will be appreciated that the steel for use in the bearing component according to the present invention may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt . % of the composition. Preferably, the alloys contain 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. The phosphorous and sulphur contents are preferably kept to a minimum.

The alloys according to 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 bearing component according to the present invention is formed from a steel that transforms to bainite at a

temperature of typically 110 to 350°C, more typically 115 to 250°C. The transformation time for bainite formation to cease is typically from 3 hours to 80 days, more typically from 6 hours to 60 days. The transformation time depends on the transformation temperature: the time is longer for lower temperatures. The amount of bainite that is formed depends on the transformation temperature: more bainite is formed at lower temperatures. The amount of retained austenite increases for higher transformation temperatures.

The microstructure of the as-transformed (bainitically heat treated) steel is different from ordinary bainitic bearing steel structures in two respects. First, the presence of vanadium carbides (and/or vanadium carbonitrides ) increases the yield strength and hardness of the bearing material. An appreciably amount of vanadium carbide precipitation takes place during the hot-rolling schedule of the steel during manufacture. A typical heating temperature is 1150°C, at which a portion of the vanadium becomes soluble in the austenitic phase. The "in solid solution" vanadium then precipitates during hot-rolling, thereby strengthening the steel structure. The steel is then allowed to cool slowly from a temperature of around 900°C to room temperature in order to: prevent the vanadium precipitates coarsening or undergoing dissolution if the hot-rolled material is exposed to temperatures higher than 900°C; allow further precipitation of vanadium precipitates; and prevent the formation of bainite and/or martensite.

During subsequent heat treatment (partial austenitisation) , the vanadium carbide precipitates are not dissolved and are retained to improve both strength and hardness.

During partial austenitisation, while taking into account the proportion of alloying elements lost to the formation of precipitates, the matrix austenitic phase has a chemical composition which still allows it to subsequently transform into very fine nano-structured bainite upon isothermal holding at the appropriate temperature. The process for the manufacture of the steel for the bearing component avoids rapid cooling so that residual stresses can be avoided in large component pieces.

If desired, various mechanical properties can be improved by carrying out any of the conventional post-bainite

transformation steps. For example, in some cases, the yield strength can be improved by carrying out a post-bainite transformation deformation step followed by tempering. The bearing component may be part of a rolling element bearing, for example the bearing inner or outer ring, or the ball or roller element. The bearing component could also be part of a linear bearing such as ball and roller screws. The present invention also provides a bearing comprising a bearing component as herein described. Example

An example of a suitable bainitic steel composition for use in the present invention includes (the balance being Fe and any unavoidable impurities) :

1.0 wt . % carbon,

1.5 wt . % silicon,

1.5 wt . % manganese,

1.9 wt . % chromium,

0.25 wt . % molybdenum,

0.35 wt . % vanadium, and

0.75 wt . % cobalt. A thermodynamic calculation, at equilibrium, is provided below which demonstrates the level of vanadium solubility in austenite at the initial hot-rolling temperature (0.23 wt% out of the bulk's 0.35 wt%) :

Conditions :

T=1423.15, N=l, P=1E5, W(C)=lE-2, W (MN) =1.5E-2 ,

W (SI) =1.5E-2, W (CR) =1.9E-2,

W (CO) =7.5E-3, W (AL) =1.35E-2, W (MO) =2.5E-3 ,

W (V) =3.5E-3

DEGREES OF FREEDOM 0

Temperature 1423.15 K (1150.00 C) , Pressure

1.000000E+05

Number of moles of components l.OOOOOE+00, Mass in grams 5.23735E+01

Total Gibbs energy -8.24142E+04, Enthalpy

3.73294E+04, Volume 6.70803E-06 Component Moles W-Fraction

Activity Potential Ref.stat

AL 2.6205E-02 1.3500E-02

7.7211E-07 -1.6654E+05 SER

C 4.3605E-02 1.0000E-02

5.7676E-02 -3.3758E+04 SER

CO 6.6652E-03 7.5000E-03

1.0706E-05 -1.3542E+05 SER

CR 1.9138E-02 1.9000E-02

1.1552E-04 -1.0728E+05 SER

FE 8.5715E-01 9.1400E-01

1.5103E-03 -7.6859E+04 SER

MN 1.4300E-02 1.5000E-02

9.4585E-06 -1.3689E+05 SER

MO 1.3647E-03 2.5000E-03

2.0545E-05 -1.2771E+05 SER

SI 2.7972E-02 1.5000E-02

2.4349E-07 -1.8019E+05 SER

V 3.5984E-03 3.5000E-03

1.5137E-06 -1.5857E+05 SER

FCC_A1#1 Status ENTERED Driving force O.OOOOE+00

Moles 9.9701E-01, Mass 5.2271E+01, Volume fraction

9.9985E-01 Mass fractions:

FE 9.15728E-01 MN 1.50281E-02 CO 7.51436E-03 CR 1.88275E-02 AL 1.35264E-02 MO 2.34695E-03 SI 1.50294E-02 C 9.69794E-03 V 2.30145E-03

FCC_A1#2 Status ENTERED Driving force O.OOOOE+00 Moles 2.9917E-03, Mass 1.0229E-01, Volume fraction 1.4519E-04 Mass fractions:

V 6.15950E-01 MO 8.07096E-02 CO 1.64341E-04 C 1.64349E-01 FE 3.10303E-02 SI 2.09655E-07 CR 1.07137E-01 MN 6.60349E-04 AL 8.32397E-08

Figure 1 contains a plot showing the carbon and vanadium composition ranges within which partial austenitisation to yield austenite and vanadium carbide is possible at 920°C and 1 atm, highlighted.

Claims

CLAIMS :

1. A bearing component formed from a steel composition comprising :

(a) from 0. 9 - 1.8 wt . % carbon,

(b) from 1. 0 - 2.0 wt . % silicon,

(c) from 1. 0 - 2.0 wt . % manganese,

(d) from 0. 1 - 0.4 wt . % molybdenum,

(e) from 1. 0 - 3.0 wt . % chromium,

(f) from 0. 2 - 1.5 wt . % cobalt,

(g) from 1. 0 - 2.0 wt . % aluminium,

(h) from 0. 1 - 5 wt .. % vanadium,

(i) optionally one or more of the f

from 0 - 0 .1 wt ..% sulphur,

from 0 - 0 .1 wt ..% phosphorous,

from 0 - 0 .075 wt . % arsenic,

from 0 - 0 .075 wt . % tin,

from 0 - 0 .075 wt . % antimony,

(j) the balance iron, together with unavoidable impurities

2. A bearing component as claimed in claim 2, wherein the sum of arsenic, tin and antimony is no more than 0.075 wt . %

3. A bearing component as claimed in claim 1 or claim 2, wherein the alloy comprises ≤ 15 ppm oxygen.

4. A bearing component as claimed in any one of the preceding claims, wherein the alloy comprises ≤ 30 ppm titanium .

5. A bearing component as claimed in any one of the preceding claims, wherein the alloy comprises ≤ 50 ppm calcium, preferably ≤ 10 ppm calcium. 6. A bearing component as claimed in any one of the preceding claims, comprises from 0.9 - 1.7 wt . % carbon, more preferably from 0.9 to 1.

6 wt . % carbon, still more

preferably from 1.0 to 1.5 wt . % carbon. 7. A bearing component as claimed in any one of the preceding claims, comprising from 1.2 - 1.8 wt . % silicon, more preferably from 1.3 - 1.

7 wt . % silicon, still more preferably from 1.4 - 1.6 wt . % silicon. 8. A bearing component as claimed in any one of the preceding claims, comprising from 1.2 - 1.

8 wt . % manganese, more preferably from 1.3 - 1.7 wt . % manganese, still more preferably from 1.4 - 1.6 wt . % manganese.

9. A bearing component as claimed in any one of the preceding claims, comprising from 0.15 - 0.35 wt . %

molybdenum, more preferably from 0.2 - 0.3 wt . % molybdenum.

10. A bearing component as claimed in any one of the preceding claims, comprising from 1.5 - 2.5 wt . % chromium, more preferably from 1.7 - 2.3 wt . % chromium, still more preferably 1.8 - 2.0 wt . % chromium.

11. A bearing component as claimed in any one of the preceding claims, comprising from 0.3 - 1.2 wt . % cobalt, more preferably from 0.5 - 1.0 wt . % cobalt, still more preferably 0.6 - 0.9 wt . % cobalt.

12. A bearing component as claimed in any one of the preceding claims, comprising from 1.1 - 1.7 wt . % aluminium, more preferably from 1.2 - 1.5 wt . % aluminium, still more preferably 1.3 - 1.4 wt . % aluminium.

13. A bearing component as claimed in any one of the preceding claims, comprising from 0.2 - 4.0 wt . % vanadium, more preferably from 0.2 - 3.0 wt . % vanadium, still more preferably 0.3 - 1.0 wt . % vanadium.

14. A bearing component as claimed in any one of the preceding claims, comprising from 0 - 0.015 wt . % sulphur.

15. A bearing component as claimed in any one of the preceding claims, comprising from 0 - 0.02 wt . % phosphorous.

16. A bearing component as claimed in any one of the preceding claims, wherein the microstructure of the steel composition comprises bainite.

17. A bearing component as claimed in any one of the preceding claims, wherein the microstructure of the steel comprises vanadium carbide precipitates.

18. A bearing component as claimed in any one of the preceding claims, wherein the microstructure of the steel comprises plates of bainite of less than 100 nm thickness.

19. A bearing component as claimed in any one of the preceding claims, wherein the microstructure of the steel comprises plates of bainite interspersed with austenite films .

20. A 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.

21. A bearing comprising a bearing component as claimed in any one of the preceding claims.