Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-08-16
2004-10-26
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
Reexamination Certificate
active
06808571
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat resistant carburized rolling bearing component and to a manufacturing method thereof.
2. Description of the Background Art
A rolling bearing used in a power transmitting portion or an engine portion of vehicles, airplanes, ships and boats, industrial machines and the like is used under a severe environment. Even under the severe environment, superior rolling fatigue life and reliability are required. In the rolling bearing used for the applications mentioned above, foreign matters such as particles, dusts and iron particles may possibly be involved, and in such an environment, the rolling fatigue life becomes significantly shorter than in a clean environment. As a countermeasure, recently, carbo-nitriding process is performed on high-carbon chromium bearing steel such as SUJ2 or a case-hardened steel such as SCM420 and SNCM815 to generate an appropriate amount of retained austenite immediately below the rolling surface, so as to improve life in an environment where foreign matters are involved.
General carbo-nitriding process, however, requires process time longer than the quenching and tempering process applied to SUJ2, for example, and therefore manufacturing cost of the rolling bearing subjected to the carbo-nitriding process is significantly increased as compared with the rolling bearing manufactured by the common quenching and tempering.
The rolling bearing used for vehicles and airplanes is used in an environment involving not only foreign matters but also high temperature. Therefore, excellent rolling fatigue life is required under very severe conditions of use. Generally, a rolling bearing used in a high temperature environment is prepared by quenching high-carbon chromium bearing steel such as SUJ2 followed by high temperature tempering to attain dimensional stability. High temperature tempering is also performed after carburization and quenching on the case-hardened steel such as SCM420 and SNCM815. When such materials are subjected to high temperature tempering, however, hardness degrades significantly, and therefore, a prescribed hardness required of a rolling bearing cannot be attained, resulting in shorter rolling fatigue life and lower wear resistance. For this reason, a precipitation hardening type steel material such as M50 is used for a rolling bearing used in a high temperature range. However, the cost of the material and manufacturing of such a rolling bearing is high, and in addition, the scope of application is limited, so that it has been impossible to satisfy various needs.
In a rolling bearing subjected to carbo-nitriding process, retained austenite is generated directly below the rolling portion after heat treatment, stress concentration caused by biting of foreign matters is relaxed by the function of the retained austenite, and in addition, resistance to temper softening is improved by nitrogen introduced to the steel, so as to improve rolling fatigue life. As described above, however, a rolling bearing used at a high temperature must be subjected to high temperature tempering, by which the retained austenite is decomposed and the amount thereof is reduced. Therefore the effect mentioned above is not expected. Further, there is a limit to prevent temper softening by the introduced nitrogen, and therefore sufficient performance cannot be attained in an environment involving foreign matters and high temperature.
Recently, development of an engine having high output and small size is in rapid progress in the field of vehicles, for example, and at the same time, the environment of use of the rolling bearing has become increasingly severe. The temperature range at which the rolling bearing is used in the engine portion is generally about 130° C. However, instantaneous increase of temperature to 160° C. is expected. As the engine comes to have higher output, the temperature range of the rolling bearing will be increased to about 160° C. for normal use and up to 200° C. or more instantaneously. Therefore, when higher output and reduction in weight of the engine are promoted, it would be impossible to maintain sufficient rolling fatigue life under the expected environment involving foreign matters and the high temperature, by the present high-carbon chromium bearing steel or by the carburizing or carbo-nitriding process.
Further, the precipitation hardening type bearing steel such as M50 is disadvantageous because of the high cost. Therefore, a rolling bearing that is inexpensive and has sufficient rolling fatigue life even under a severe environment of use is desired.
SUMMARY OF THE INVENTION
The present invention was made to solve the above-described problems, and an object is to provide a heat resistant carburized rolling bearing component having excellent rolling fatigue life, wear resistance and dimensional stability even under an environment involving foreign matters and an environment of high temperature, and is inexpensive as compared with the prior art examples, as well as to provide a manufacturing method thereof.
Through intensive study, the inventors of the present invention have found combinations and respective contents of composition elements that can provide inexpensive heat-resistant carburized rolling bearing component having excellent rolling fatigue life under the environment involving foreign matters and environment involving high temperature.
The present invention provides a component of a heat resistant carburized rolling bearing having an inner ring, an outer ring and a rolling element, formed of a steel material, at least containing, as alloy elements in a matrix, by mass %, at least 0.1% and at most 0.4% of C (carbon), at least 0.3% and at most 3.0% of Si (silicon), at least 0.2% and at most 2.0% of Mn (manganese), at most 0.03% of P (phosphorus), at most 0.03% of S (sulfur), at least 0.3% and less than 2.5% of Cr (chromium), at least 0.1% and less than 2.0% of Ni (nickel), at most 0.050% of Al (aluminum), at most 0.003% of Ti (titanium), at most 0.0015% of O (oxygen) and at most 0.025% of N (nitrogen) and a remaining part of Fe and an unavoidable impurity, formed by carburizing or carbo-nitriding process followed by quenching, followed in turn by tempering at a tempering temperature of at least 200° C. and at most 350° C., and having a surface hardness of at least HRC57 after the tempering process.
The heat resistant carburized rolling bearing component of the present invention having the above described composition can attain surface hardness as high as HRC57 or higher even after high temperature tempering, and therefore, satisfactory rolling fatigue life and wear resistance can be attained even under the environment involving high temperature and foreign matters. As high temperature tempering process is performed, retained austenite that is instable to heat can be decomposed in advance, and therefore, dimensional stability at a high temperature environment is ensured.
Further, the steel having the above described composition is inexpensive as compared with the precipitation hardening type bearing steel such as M50.
From the foregoing, it is understood that an inexpensive heat resistant carburized rolling bearing component having excellent rolling fatigue life, wear resistance and dimensional stability under the environment involving foreign matters and the environment involving high temperature can be obtained.
The temperature for tempering process is at least 200° C. and at most 350° C. A rolling bearing used in a high temperature environment may possibly heated to 200° C. or higher. Therefore, tempering process is performed at a temperature not lower than 200° C. to ensure dimensional stability. When the temperature for tempering process exceeds 350° C., surface hardness will be lower than HRC57, and the life of the rolling bearing abruptly decreases.
By performing carbo-nitriding process in place of carburizing process, it is possible to attain further improved rolling fatigue life, wear resistance and dimensional stability in the e
Kinami Tosiya
Maeda Kikuo
Nakamura Sadayuki
Tanaka Hiromasa
King Roy
McDermott Will & Emery LLP
NTN Corporation
Wilkins, III Harry D
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