Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2001-06-22
2003-05-13
King, Roy (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S232000
Reexamination Certificate
active
06562151
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a rolling shaft, and in particular relates to the rolling shaft corresponding to an inner ring of a rolling bearing (in particular, a radial needle bearing).
Conventionally, as the rolling shaft corresponding to the inner ring of the radial needle bearing, such a material has been used that a through-hardening steel as SUJ2 which is further performed with a quenching and a tempering so as to provide a Vickers hardness being Hv650 or more. In this case, for providing the Vickers hardness of Hv650 or more, in response to processing requirements as a lathe turning, the rolling shaft having Vickers hardness Hv300 or less is performed to an outer periphery thereof with the lathe turning, and subsequently the outer periphery is performed with a high frequency quenching.
However, recently the radial needle bearing has been very often used under high load. In this case, a rolling fatigue resistance of the rolling shaft corresponding to the inner ring of the radial needle bearing is not sufficient with the conventional through-hardening steel such as SUJ2 or the ordinary high frequency-quenched steel. Major part of such insufficient rolling fatigue resistance is concerned with tolerance (surface fatigue resistance) against a surface fatigue which is caused in a surface of a raceway of the rolling shaft by contamination of a lubricant or supply shortage thereof.
In the radial needle bearing, such surface fatigue is caused in a portion from the surface thereof till a depth corresponding to 2% of the diameter Da of the rolling element (called said depth as “2% Da” hereafter), or in a portion from the surface till 0.1 mm in the depth of an absolute value, and in particularly a large surface fatigue occurs in a portion until 0.05 mm from the surface. In order to improve the surface fatigue resistance, it is necessary that a residual austenite is 15 to 40 vol % (preferably, 20 to 35 vol %) in the surface layer which is effected with fatigue. However, there is a problem that, for rendering the residual austenite to be 15 to 40 vol % (preferably 20 to 35 vol %) by the through-hardening or the ordinary high frequency-quenching, the quenching temperature must be heightened, whereby austenite crystal particles in a hardened part by quenching are coarsened, so that the surface fatigue resistance goes down.
It has been found out that, even if the residual austenite is present and a stress caused in the rolling shaft is within a limit of elasticity, the rolling shaft comprising the bearing steel (through-hardening) of SUJ2 which is worked under high load, is affected with a plastic deformation by a time-passing decomposition (transformation into martensite) of the residual austenite by the stress.
Further, the conventional rolling shaft is involved with another problem that it is made of a material (SC, SCr, SCM, SNCM) having Vickers hardness of Hv300 or less and carbon concentration of 0.4 wt % or less, which is carried out on the outer periphery with the high frequency-quenching (hardness of surface layer is Hv650 or more), and since part (core portion) other than the surface layer has hardness Hv300 or less and if a high impact load is given, the plastic deformation probably occurs.
SUMMARY OF THE INVENTION
It is, accordingly, an object of the invention to solve the problems of the conventional rolling shaft and provide a rolling shaft which is excellent in the rolling fatigue resistance and is least to generate the plastic deformation.
For solving the problems, the rolling shaft of the invention relatively rolling with respect to an opposite member, is characterized in that the rolling shaft comprises a steel containing 0.5 to 1.2 wt % carbon and is provided with a surface layer containing 0.05 to 0.4 wt % nitrogen, having Vickers hardness being Hv650 or more through a high frequency quenching performed thereto and a residual austenite being 15 to 40 vol %, and a core portion of the rolling shaft contains 0 vol % of the residual austenite.
If the rolling shaft is produced in the way that, after nitrogen is 0.05 to 0.4 wt % nitrified into the surface, the high frequency-quenching is performed to provide a surface layer having Vickers hardness of Hv650 or more and the residual austenite of 15 to 40 vol %, the residual austenite may be formed without coarsening the austenite crystal particles in the surface layer. Thus the rolling fatigue resistance may be increased in the raceway surface (the above mentioned surface layer).
Further, in case the rolling shaft is produced in the way that, after nitrogen is 0.05 to 0.4 wt % nitrified into the surface, the quenching and the tempering are performed to thermally refine the whole hardness of the rolling shaft to be Hv300 to 500 (preferably Hv400 to 500), and subsequently the high frequency-quenching is practiced to provide the surface layer having Vickers hardness of Hv650 or more and the residual austenite of 15 to 40 vol %, the rolling fatigue resistance in the raceway surface (the above mentioned surface layer) may be increased as in the above mentioned way, and at the same time, the hardness of the other part (core portion) than the surface layer may be made Hv300 to 500 (preferably Hv400 to 500) and the residual austenite in the other part (core portion) than the surface layer may be made 0 vol %, whereby the plastic deformation in the rolling shaft may be prevented, which is caused together with the time-passing decomposition of the residual austenite owing to the stress (stress within the elasticity limit) occurring in the rolling shaft, and further, the plastic deformation when adding the high impact load may be prevented.
The surface layer referred to in the invention is meant by the portion from the surface to 2% Da, or the portion from the surface until 0.1 mm (in particular 0.05 mm) in the depth of the absolute value.
Herein, critical meanings of these values will be described.
[Carbon Concentration in the Steel: 0.5 to 1.2 wt %]
If the carbon concentration is less than 0.5 wt %, it is difficult to obtain a stabilized hardness Hv650 (Hrc58) or more in the surface layer and in the high frequency-quenched portion. For obtaining the preferred hardness Hv650 (Hrc58) or more, regardless of the dimensions of the rolling shaft, the lower limit should be 0.5 wt %.
Further, when performing a carbonization by way of a carbonitriding process, 0.5 wt % or more of carbon is needed for forming fine (0.5 to 1.0 &mgr;m) carbonitrides in the surface layer.
Further, if the carbon concentration is more than 1.2 wt %, macro carbides are easily produced in the steel to decrease a rolling life.
[Nitrogen Concentration in the Surface Layer: 0.05 to 0.4 wt %]
If nitrogen is made solid together with carbon in a quenched structure, it has an effect strengthening a matrix. From this fact, as the surface hardness is increased and the tempering resistibility is also increased, an abrasion resistance is available over a wide temperature range, so that the rolling shaft may have a long service life.
If the nitrogen concentration is less than 0.05 wt %, the abrasion resistance is insufficient and it is difficult to obtain 15 vol % or more of the residual austenite in the surface layer. Exceeding 0.4 wt %, it takes a long time for processing (polishing or grinding) after the heat treatment. A post processing cost is heightened, accordingly.
In order to make the abrasion resistance and the post processing cost optimum, preferable is 0.1 to 0.3 wt %.
Particularly, it is preferable that the surface layer for supporting the rolling faces and making difficult to generate the surface fatigue is made of the portion of 0.05 mm or more, or 2% Da from the surface of the completed rolling shaft, and the nitrogen concentration at the positions (the portion of more than 0.05 mm, or 2% Da from the surface) is 0.1 wt % or more, preferably 0.2 wt % or more.
[Hardness in the Surface Layer: Vickers Hardness Hv650 (Hrc58) or more]
If the hardness in the surface layer is less than Hv650, it is insufficie
Fukushima Hiroshi
Hirakawa Kiyoshi
King Roy
NSK Ltd.
Sughrue & Mion, PLLC
Wilkins, III Harry D.
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