Rolling elements

Abstract

The pitting resistance of a gear is increased by hardening its tooth flanks through application of carburizing / quenching, bright hardening and induction hardening to a steel material capable of providing significantly improved softening resistance in tempering at a low temperature of 300 to 350° C. To this end, the steel material prepared so as to satisfy the relationship described by: 5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)) is carburized such that the carbon concentration of its carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the steel material is subjected to quenching and tempering at 300° C. or less subsequently to the carburization process, or alternatively the steel material is once cooled after the carburization process and then subjected to treatments of re-heating hardening and tempering at 300° C. or less so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.

Description

CROSS REFERENCE TO RELATED APPLICATION [0001] This application is a Divisional application of U.S. application Ser. No. 10 / 641,362, filed on Aug. 13, 2003.TECHNICAL FIELD [0002] The present invention relates to rolling elements produced by carburizing and quenching, bright hardening or induction hardening. More particularly, the invention relates to gears and rolling elements such as bearings, races and rollers, the gears being made from steel which provides significantly improved resistance to softening caused in low-temperature tempering at 300 to 350° C. and having high pitting resistance in the tooth flanks hardened by carburizing / quenching, bright hardening or induction hardening. BACKGROUND ART [0003] Up to now, gears produced by applying carburizing / quenching, carburizing / carbonitriding / quenching to SCr-based, SCM-based or SNCM-based low carbon steel have been commonly employed in the reducers of construction machines and earth-moving machines, since high contact fatigue stre...

AI Technical Summary

Benefits of technology

[0016] It has also been understood from the comparison between the 300° C.-temper hardness of the carburization-hardened layer of the SCM420 steel which has undergone carburizing / quenching and the 300° C.-temper hardness of the eutectoid carbon steel which has undergone quenching that since virtually no improvement in temper softening resistance can be attained by additions of Cr and Mo, a new alloy design intended for increasing temper softening resistance during low-temperature tempering at about 300° C. is necessary in order to achieve pitting resistance equal to or more than that of the carburized, quenched gears by bright hardening or induction hardening. Also, dispersion of fine cementite particles or the like in the martensitic phase has proved effective as seen from the cases of eutectoid carbon steel (0.82 wt % C) and SUJ2 which were improved in rolling contact fatigue strength.

[0069] Mo is a useful element as it improves the hardenability of steel and restrains temper brittleness, and therefore it is desirable for the invention to add Mo in an amount of 0.35 wt % or less which is at the same level as that of ordinary case-hardened steels.

Problems solved by technology

Construction machines and earth-moving machines often stride obstacles such as rocks and structures during travelling and drill the obstacles while making a turn, and therefore, the gears of the reducer used for running and turning such machines receive impulsive load.

This is a serious problem of damage to carburized quenched gears.

Bright-hardened or induction-hardened gears have higher toughness than carburized quenched gears, but are more likely to cause pitting or scuffing when they are used under high interface pressure such as noted above.

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