Metal working – Method of mechanical manufacture – Process for making bearing or component thereof
Patent
1999-02-25
2000-08-15
Bryant, David P.
Metal working
Method of mechanical manufacture
Process for making bearing or component thereof
2989814, B21K 102
Patent
active
061017191
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of manufacturing rolling bearings which are used under severe environmental conditions such as equipment for steel works, engines of jet planes, gas turbines, and turbo chargers, and are required to have high reliability such as heat resistance, wear resistance, and core toughness.
BACKGROUND ART
In recent years, severe requirements have been imposed upon bearing materials for use at high temperatures, such as a heat-resisting carburized steel, in terms of higher temperature and higher speed operating conditions required of rolling bearings, or higher performance required of rolling bearings, which is obtained by adding core toughness and surface compressive residual stress to the rolling bearing materials.
Such a heat-resisting steel is a high alloy steel containing chromium (Cr), nickel (Ni), silicon (Si), molybdenum (Mo), vanadium (V), tungsten (W), etc. For example, there are known AISI M50 NiL as a material for use in engines of jet planes and gas turbines, and CBS 600 and Pyrowear 53 which contain Si in large amounts as materials for enhancing the heat resistance in particular.
In hardening the surface of a high alloy steel containing chromium (Cr), molybdenum (Mo), and silicon (Si) in large amounts such as stainless steel and tool steel, if ordinary gas carburization is employed, a dense oxide film is formed on the surface during carburization, or heavy intergranular oxidation occurs, making it difficult to carburize the steel using the ordinary gas carburization (cf. Journal of Japan Institute of Metals, Vol. 50, No. 4, p395-p402).
To carburize such a high alloy steel, a method has been employed, which includes carrying out pre-oxidation for forming an oxide layer on the surface of the steel in air at 955.degree. C., before carrying out a carburizing treatment, as described in "Specialty Carburizing Steels for Elevated Temperature Service" by C. F. Jatczak; "METAL PROGRESS", p70-p78, April, 1978. The pre-oxidation makes the dense oxide film porous, and accordingly the carburizing gas can contact the surfaces of the resulting bearing (inner ring, outer ring and rolling element).
On the other hand, as a method which does not use pre-oxidation, a plasma carburizing method as disclosed, e.g., in Laid-Open Patent Publication No. 1-225764 and Laid-Open Patent Publication No. 1-255656 can be used to carburize a high alloy steel. This method is widely employed.
Further, an example in which a vacuum carburizing method is applied to stainless steel has been reported (cf. Heat Treatment, Vol, 22, No. 3, p172-p174).
According to the gas carburizing method using the pre-oxidation, however, the chromium (Cr) content is a little bit larger (particularly, if it is 3 weight % or more), the effect of the pre-oxidation becomes reduced, making it difficult to perform carburizing.
Moreover, according to the gas carburization, very small amounts of C.sub.2 O and H.sub.2 O contained in the gas as gas ingredients cause formation of an intergranular oxide layer on the surface of the steel material, particularly if the Si content is large, spoiling the carburizability. Si is an element that enhances the tempering softening resistance, and therefore if the resulting bearing is used in a hot environment, the presence of Si largely affects the function of the bearing.
Besides, if the contents of Cr, Mo, V, etc. of the alloy increase, a dense carbide layer is formed on the steel surface, whereby carbon which is to penetrate and diffuse into the steel base is consumed in the formation of the carbide layer and accordingly the concentration of carbon solid-solved in the steel decreases, so that carbon cannot deeply penetrate and diffuse into the steel base, which requires prolonging the carburizing time. This, however, causes further enlargement of the intergranular oxide layer on the steel surface, leading to degraded strength.
On the other hand, the plasma carburizing method which does not include pre-oxidation does not only require the provision of equipment for carr
REFERENCES:
patent: 5139584 (1992-08-01), Gantois
patent: 5780165 (1998-07-01), Fukumoto et al.
Journal of Japan Institute of Metals, vol. 50, No. 4, pp. 395-402 (Apr. 1986).
C. Jatczak, Specialty Carburizing Steels for Elevated Temperature Service, Metal Progress, pp. 70-78 (Apr. 1978).
Heat Treatment, vol. 22, No. 3, pp. 172-174 (Jun. 1982).
"Surface Hardening", The Metal Surface Finishing Society of Japan, p. 10 (Jun. 1971).
"Iron and Steel and Alloy Elements", Last Volume edited by JSPS, Steel Making No. 19 Committee, p. 696-698 (Mar. 1966).
"Special Steel Handbook", 1st Edition, edited by Denki Seikou Kenkyujo, Rikogakusya Publishing Co., Ltd., 10-21 (1 page), (1969).
Kiuchi Akihiro
Okita Shigeru
Yamamura Kenji
Bryant David P.
Butler Marc W.
NSK Ltd.
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