BEARING COMPONENT
Bearing component (10) containing steel that comprises, by weight, 10-30 ppm Ca, max 20 ppm S and max 15 ppm O and in that said steel includes sulphide inclusions and less than 5% of the sulphide inclusions contain encapsulated or embedded oxide inclusions.
The present invention concerns a bearing component, i.e. a component that constitutes at least part of one of the following: a ball bearing, roller bearing, ball thrust bearing, roller thrust bearing, tapered roller thrust bearing, ball screw, or a component for an application in which is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding.
BACKGROUND OF THE INVENTIONStringent requirements have been established for the quality of bearing steel, especially as regards its content of non-metallic inclusions. Bearing components can namely suffer from contact fatigue, i.e. the cracking and subsequent pitting of surfaces subjected to alternating Hertzian stresses such as rolling contact or combined rolling and sliding, and non-metallic inclusions, such as certain sulphide inclusions, have been found to initiate sub-surface damage that may lead to crack growth and spalling. Furthermore, since sulphides in bearing steels have an elongated shape, they tend to become oriented and thus make the steel anisotropic.
In order to avoid these problems with sulphides it is however not advantageous to reduce the sulphur content of the bearing steel to as close to zero as possible since this results in magnesium and calcium in the melt entering oxide inclusions in the form of aluminates and forming undesired complex aluminate inclusions. Pure aluminates are hard and brittle; they will break during rolling and do not therefore pose a substantial problem to manufacturers of bearing components with high degrees of forming deformation. However, complex aluminates can be hard but they are not brittle so they will remain intact during rolling and will therefore be incorporated into the finished bearing component. If a complex aluminate inclusion becomes located in an area of the bearing component subject to heavy loading, this is where a fatigue failure will start.
European patent application no. EP 1 094 124 concerns a super clean steel for use in objects subjected to heavy loads in all directions. The steel disclosed therein comprises, in weight, between 8 ppm to 12 ppm S. A sulphur content of 8 ppm or above counteracts the release of calcium from slag and thus the formation of globular oxide inclusions, and a sulphur content below 12 ppm reduces the enrichment of dangerous sulphides in castings.
SUMMARY OF THE INVENTIONAn object of the invention is to provide a bearing component with improved fatigue properties.
The object of the invention is achieved by a bearing component comprising steel that comprises by weight, 10-30 ppm Ca, max 20 ppm S and max 15 ppm O. Preferably the steel comprises 10-30 ppm Ca, max 20 ppm S and max 10 ppm O, or most preferably 10-30 ppm Ca, max 20 ppm S and max 8 ppm O.
In conventional bearing steels, the dominating inclusion type is sulphides due to the fact that the sulphur content normally is higher than the oxygen content. A very large number of the sulphide inclusions contain embedded or encapsulated oxide inclusions, often at the sulphide inclusion ends. When these embedded or encapsulated oxide inclusions have matrix contact, these oxy-sulphides will act as crack initiators. The life time of a bearing component is most adversely affected by oxygen-containing inclusions, also such as sulphide inclusions containing encapsulated or embedded oxide inclusions.
The addition of calcium to bearing steel (i.e. steel suitable for use in a bearing component) after the sulphur content has been reduced to a level substantially of the same order as the oxygen content, will reduce the total number of sulphide inclusions remaining in the steel and it will modify the shape of the remaining inclusions into one that is less detrimental to mechanical properties in the final bearing component. It has namely been found that the addition of calcium to bearing component steel will result in less than 5% of the sulphide inclusions containing encapsulated or embedded oxide inclusions. The addition of calcium will also result in all of the sulphide inclusions having an aspect ratio of less than 3:1 (i.e. the ratio of the inclusion's largest diameter to the inclusion's shortest diameter) and in the maximum sulphide inclusion length being 125 μm at a Reduced Variate equal to 3 evaluated using the ASTM E2283-03 Extreme Value Analysis Standard. Such sulphide inclusions are less damaging to a bearing component as regards its fatigue properties. Furthermore, the absence of sulphide inclusions having an aspect ratio greater than 3:1 and having a maximum length greater than 125 μm gives the steel more uniform properties in all directions and thereby reduces directional anisotropy in the steel.
According to an embodiment of the invention the bearing component comprises steel that contains between 0.70 to 1.20% C. The steel may for example be of ASTM A295/A295M-09 type, such ISO 683-17:1999 ASTM A-295-98, or any other high-carbon through-hardened steel that is suitable for an application in which it is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding.
According to an embodiment of the invention the bearing component constitutes at least part of one of the following: a ball bearing, roller bearing, ball thrust bearing, roller thrust bearing, tapered roller thrust bearing, ball screw, or a component for an application in which is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding, for example a roller bearing application, a linear motion application, a gear box application or a torque transmitting application.
The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures in which;
It should be noted that the drawing has not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.
DETAILED DESCRIPTION OF EMBODIMENTSThe steel from which at least part of the ball bearing 10 is manufactured may for example contain between 0.70 to 1.20% carbon. For example the steel may have the following 20 composition:
the remainder being Fe, and normally occurring impurities comprising 10-30 ppm Ca, max 20 ppm S and max 15 ppm O, preferably max 10 ppm O or most preferably max 8 ppm O.
Claims
1. A bearing component that comprises steel, and wherein the steel comprises, by weight,
- 10-30 ppm Ca,
- a maximum of 20 ppm S, and
- a maximum of 15 ppm O, and wherein
- the steel includes sulphide inclusions, and wherein
- less than 5% of the sulphide inclusions contain one of an encapsulated and embedded oxide inclusions.
2. The bearing component according to claim 1, wherein the steel includes sulphide inclusions and the maximum length of said sulphide inclusions is 125 μm at a Reduced Variate equal to 3.
3. The bearing component according to claim 1, wherein the steel comprises:
- 10-30 ppm Ca,
- maximum 20 ppm S, and
- a maximum 10 ppm O.
4. The bearing component according to claim 1, wherein the steel includes sulphide inclusions and all of said sulphide inclusions have an aspect ratio less than 3:1.
5. The bearing component according to claims 1, wherein the steel contains between 0.07 to 1.20% C.
6. The Bearing component according to any of the preceding claims, characterized in that it constitutes at least part of one of the following: A ball bearing, roller bearing, ball thrust bearing, roller thrust bearing, tapered roller thrust bearing, ball screw, or a component for an application in which is subjected to alternating Hertzian stresses, such as rolling contact or combined rolling and sliding.
7. The bearing component according to claim 1, wherein the steel comprises:
- 10-30 ppm Ca,
- a maximum 20 ppm S, and
- a maximum of 8 ppm O.
Type: Application
Filed: May 20, 2010
Publication Date: Mar 15, 2012
Inventors: Thore Lund (Goteborg), Ingemar Strandell (Savedalen)
Application Number: 13/321,410
International Classification: C22C 38/00 (20060101);