Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
1999-10-28
2001-06-26
King, Roy (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S906000, C148S233000, C148S662000
Reexamination Certificate
active
06251197
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to rolling or sliding parts, and more particularly to rolling parts suitable as antifriction bearing components, such bearing rings and rolling bodies, or as sliding parts suited as sliding bearing components, for use in soiled oil containing extraneous matter.
For example for use in such antifriction bearings, the present applicant has already proposed an antifriction bearing part which comprises a steel containing 0.5 to 1.2 wt. % of C and 0.7 to 3.0 wt. % of Cr and is given a surface with a C content of 1.5 to 3.0 wt. % and a surface hardness of at least 63 in Rockwell C hardness by carburizing treatment, the bearing part having a carburized layer containing a precipitated fine spheroidal carbide in an amount of 15 to 80% in area ratio. The spheroidal carbide is up to 10 &mgr;m in diameter. The matrix of the carburized layer contains 0.6 to 0.7 wt. % of C (JP-A No. 41934/1995).
However, we have found that the conventional part described still remains to be improved in life for use in antifriction bearings which are subjected to conditions of ever-increasing severity recently.
DISCLOSURE OF THE INVENTION
Accordingly, we have conducted extensive research with various experiments and found that the rolling life is greatly influenced by the particle-to-particle distance of the spheroidal carbide of the carburized layer in rolling or sliding parts. This finding has matured to the present invention.
An object of the present invention is to overcome the foregoing problem and provide a rolling or sliding part having a longer life than the conventional parts.
The present invention provides a rolling or sliding part comprising a steel material subjected to a heat treatment including carburization and containing a spheroidal carbide precipitated and dispersed in a carburized layer matrix, the particle-to-particle distance of the spheroidal carbide being up to 15 &mgr;m in terms of the distance between the most proximate particles.
With the rolling or sliding part, the particle-to-particle distance of the spheroidal carbide is limited to not greater than 15 &mgr;m in terms of the distance between the most proximate particles because it has been substantiated by experiments that this results in a lengthened rolling life in soiled oil and clean oil.
In the case of the rolling or sliding part of the present invention, the particle-to-particle distance of the spheroidal carbide is up to 15 &mgr;m in terms of the distance between the most proximate particles, so that the antifriction bearing comprising this part serves a prolonged rolling life in soiled oil and clean oil. Further when incorporating this part, the sliding bearing has an extended life.
The rolling or sliding part of the invention is suitable as antifriction bearing components, such bearing rings and rolling bodies, or as sliding bearing components, for use in soiled oil containing extraneous matter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The factor for giving the spheroidal carbide precipitated and dispersed in the carburized layer matrix of the rolling or sliding part a particle-to-particle distance of up to 15 &mgr;m in terms of the distance between the most proximate particles is thought to be involved in the alloy composition of the steel material to be used for making the part or in the conditions for the heat treatment including carburization, whereas research has yet to be made to clarify what influence the alloy composition or heat-treatment conditions produce on the particle-to-particle distance of the spheroidal carbide.
Examples of useful steel materials are known steels such as JIS SUJ2, JIS SCr420 and SAE 5120, and a steel comprising 0.15 to 0.45 wt. % of C, 1.2 to 1.6 wt. % of Cr, 0.35 to 0.55 wt. % of Si, 0.35 to 0.65 wt. % of Mn, and the balance Fe and inevitable impurities.
Useful for the heat treatment including carburization and to be conducted for a blank of rolling or sliding part prepared from such a steel material is, for example, a process comprising the first step of carburizing or carburizing and quenching, the second step of hardening to precipitate a fine spheroidal carbide in the resulting carburized layer, and the third step of high-concentration carburizing and quenching for forming a surface portion having a higher carbon concentration than the surface portion resulting from the first step. Preferably, the heating temperature of the third step is not higher than the heating temperature of the second step because if the heating temperature of the third step is higher than the heating temperature of the second step, the carbide precipitated by the second step is likely to partly dissolve in the matrix. Described more specifically, the process comprises the first step of heating the blank in a carburizing atmosphere containing 10 to 17 vol. % of C
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at a temperature of 930 to 950° C. for 3 to 5 hours, followed by oil quenching, the second step of heating the quenched blank at a temperature of 800 to 840° C. for 0.5 to 0.8 hour, followed by oil quenching, and the third step of heating the resulting blank in a carburizing atmosphere containing 10 to 17 vol. % of C
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at a temperature in the range of 790 to 840° C. and not higher than the heating temperature of the second step for 3 to 5 hours, followed by oil quenching. It is preferable to perform the third step by heating the blank in a carburizing atmosphere containing 10 to 17 vol. % of C
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at a temperature in the range of 790 to 820° C. and not higher than the heating temperature of the second step for 3 to 5 hours and thereafter heating the blank at an elevated temperature of 830 to 840° C. for 0.5 to 0.8 hour, followed by oil quenching. It is then possible to give the matrix an increased amount of carbide without producing coarser particles of carbide.
It is desired that the rolling or sliding part of the invention have a particle-to-particle distance of up to 10 &mgr;m in terms of the distance between the most proximate particles. In this case, the antifriction bearing comprising this part serves a further lengthened rolling life in soiled oil and clean oil, and the sliding bearing incorporating the part is given a further extended life.
Preferably the rolling or sliding part of the invention has a surface hardness of 58 to 67 in Rockwell C hardness (hereinafter referred to briefly as “HRC”). If less than 58 in HRC, the surface hardness is insufficient, rendering, for example, the antifriction bearing wherein the rolling part is used susceptible to surface indentations or flaws due to extraneous matter which will initiate spalling or separation and resulting in impaired abrasion resistance to shorten the life of the bearing, in the case where the bearing is used in soiled oil containing extraneous matter. If the hardness is over 67 in HRC, lower toughness will result. If less than 58 in HRC, the surface hardness is insufficient also in the case of the sliding part. The part will then be susceptible to surface indentations due to friction.
Further with the rolling or sliding part of the present invention, it is desired that the spheroidal carbide be up to 5 &mgr;m in mean particle size. If the mean particle size of the spheroidal carbide is over 5 &mgr;m, the amount of spheroidal carbide in excess of 5 &mgr;m in particle size will be about 80% of the whole amount of spheroidal carbide, consequently permitting stress concentration on the carbide portion exceeding 5 &mgr;m in particle size, and the part is likely to develop a break in this portion. Accordingly, the spheroidal carbide should be up to 5 &mgr;m, preferably up to 3 &mgr;m, in mean particle size. When the amount of spheroidal carbide up to 5 &mgr;m in particle size is less than 70%, the carbide in excess of 5 &mgr;m in particle size will be in an amount of at least 30% of the whole amount and will contain particles with a maximum size of 10 &mgr;m as the case may be, with the result that the portion of carbide exceeding 5 &mgr;m in size is subjected to stress concentration to develop a break. Accordingly, it i
Jacobson & Holman PLLC
King Roy
Koyo Seiko Co. Ltd.
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