Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
1998-09-21
2001-03-06
Sheehan, John (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S906000, C384S492000, C384S625000, C384S912000
Reexamination Certificate
active
06197128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rolling bearing and particularly to a rolling bearing consisting of carburized steel.
2. Description of the Background Art
Carburized steel commonly used as a bearing material has a longer rolling contact fatigue life than bearing steel. Thus a rolling bearing made of carburized steel is generally used as a long life bearing.
A long life of a carburized steel bearing is assumed to be attributable to a large amount of retained austenite produced by carbulizing and the presence of compressive residual stress in its surface layer. It is assumed that a conventional steel product has a large amount of non-metallic inclusion, which acts as a source of stress concentration, whereas in the carburized steel rolling bearing, compressive residual stress and retained austenite produced by carburizing of the surface layer serve to relax the stress concentration.
Recent development in steelmaking technique, however, has resulted in the improvement in steel quality and has enhanced the life of bearing steel. Thus the advantage of carburized steel over bearing steel in terms of service life has almost eliminated.
Although it has been known that carburized steel(e.g. M50NiL steel) containing a large amount of Cr(chromium), Mo(molybdenum) and/or V(vanadium), exhibits a life even longer, the use of these expensive chemical components, such as Cr, Mo, and V is not practical.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an inexpensive long life rolling bearing made of carbulized steel by the combination of relatively inexpensive chemical components.
Through a careful investigation, the inventor of the present application has found that inexpensive carburized steel with a long rolling contact life allowing rapid carburizing can be obtained by adding a prescribed amount of chemical components including C(carbon), Si(silicon), Mn(manganese), Cr(chromium), Ni(nickel) and Mo(molybdenum).
Therefore the present invention provides a rolling bearing having a rolling bearing ring and a rolling element constituting a rolling contact surface, at least one of the rolling bearing ring and the rolling element consisting of carburized steel containing at least 0.3 wt. % and at most 0.6 wt. % of C, at least 0.1 wt. % and at most 0.35 wt. % of Si, at least 1.1 wt % and at most 1.5 wt. % of Mn, at least 0.5 wt % and at most 2.0 wt. % of Cr, at least 0.2 wt % and at most 0.6 wt. % of Ni and at least 0.15 wt % and at most 0.5 wt. % of Mo.
The amounts of added chemical components are determined according to the following reasons.
C is an important element determining a basic toughness and hardness of carburized steel of the present invention. Through quenching, C undergoes the Martensitic transformation, whereby hardness is improved. C is also an austenite stabilizing element which prevents the degradation of toughness when being heated up to the austenitizing temperature. On the other hand, when the content of C is too high, resulting excessive hardness causes toughness to decrease. In the present invention, the lower limit of the C content is 0.3 wt. % because the carbide production, the quenching hardness of a core and the carburizing speed are enhanced by a higher C content. The upper limit is 0.6 wt. % to ensure toughness necessary for a rolling bearing.
The influence of the amount of initially added C on the rolling contact life is not evident, as C content in the surface layer produced by carburizing has a greater influence.
Si is known as a deoxidating element or an element which enhances hardness and heat resistance. As a high Si content suppresses carburizing, the upper limit of Si content is 0.35 wt. %. Because a higher Si content is preferred with regard to the rolling contact life, the lower limit is 0.1 wt. %.
When the Mn content is low, both the rolling contact life characteristic and the carburizing speed are impaired. On the other hand, a high Mn content degrades workability. Thus the Mn content is set in the range of 1.1-1.5 wt. %.
Though a high Cr content is favorable with regard to both the carbide production and the heat resistance, a higher Cr content means a higher cost as it is an expensive chemical component. When the Cr content is low, effect on the carbide production and heat resistance is decreased and the rolling contact life is shortened in the clean oil lubricating condition under high contact pressure and the debris contamination lubrication condition. When the Cr content is too high, cost rises and the rolling contact life under the debris contamination lubrication condition is shortened by formation of large carbide. Thus the range of Cr content is 0.5-2.0 wt. %.
Ni is necessary to some extent in order to enhance the toughness, but when the content is too high, increased retained austenite degrades the hardness. The Ni content must be at least 0.2 wt. % in order to prevent decrease in rolling contact life under the condition of the point contact (high contact stress) or the debris contamination and to avoid the lowering of toughness enhancement effect. The Ni content must be at most 0.6 wt. % in order to avoid the hardness degradation.
Mo is a life extending element which produces sub-carbide at the coexistence with Cr and enhances the tempering resistance. When the Mo content is too low, the rolling contact life under the point contact (high contact stress) condition decreases. When the Mo content is too high, the rolling contact life under the debris contamination condition decreases and cost becomes prohibitive. Therefore the range of Mo content is 0.15-0.5 wt. %.
In carburized steel, carburizing speed can be enhanced by decreasing the Si content and increasing the C content. In addition, by increasing the Mn content and the Cr content and decreasing the Ni content and the Mo content, a certain amount of retained austenite necessary for the enhancement of the rolling contact life can be secured and the austenite stabilization and high hardness due to the carbide precipitation can be attained with relatively inexpensive materials.
In the application where the use under high temperature is expected, a bearing having a surface layer with a long rolling contact life and an core with superior dimensional stability can be provided by carbo-nitriding and following high temperature tempering of the material of the present invention.
In the above-mentioned aspect, carburized steel preferably has a distribution of carbide having a maximum size of not more than 3.5 &mgr;m directly below the rolling contact surface and not more than 8 &mgr;m below a mill scale surface. Thus the stress concentration can be relaxed without sacrificing the anti-crack strength. Such carbide distribution can be achieved by carbo-nitriding the above described carburized steel.
In the aspect described above, preferably the compressive stress of the surface layer of carburized steel is at most −150 MPa to obtain a long life bearing. The feature of this carburized steel can be obtained by high temperature tempering.
In the aspect described above, carburized steel has 20-30% retained austenite in the surface layer and at most 5% retained austenite in the core. Thus a rolling bearing exhibiting a favorable dimensional stability and a long rolling contact life even under a high temperature can be obtained.
REFERENCES:
patent: 4871268 (1989-10-01), Furamura et al.
patent: 5352303 (1994-10-01), Murakami et al.
patent: 5413643 (1995-05-01), Murakami et al.
patent: 5560787 (1996-10-01), Takagi et al.
patent: 5989694 (1999-11-01), Mitamura et al.
patent: 8-303470 (1996-11-01), None
McDermott & Will & Emery
NTN Corporation
Sheehan John
LandOfFree
Rolling bearing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rolling bearing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rolling bearing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2498963