This paper introduces a material technology relating to longer life bearings made of through-hardening steel.

Efforts to improve steel cleanliness led to the development of a more reliable through-hardening steel (SAE 52100). As the fruit, we have succeeded in mass-producing extremely purified steel (EP steel) in collaboration with a steel maker.

Among the rolling contact fatigue (RCF) life tests under clean lubrication conditions, only one bearing had accidentally failed substantially earlier than all the rest. This earlier failure was at about 1/20 of the L₁₀ life, so that it introduced a large deviation into the Weibull distribution. It was not due to surface originated flaking from a small dent. This extremely earlier failure was believed to be subsurface originated flaking from a large non-metallic inclusion. Under clean lubrication conditions, the large non-metallic inclusions exert a more harmful impact on RCF life than small indentations. Therefore, original evaluation methods of non-metallic inclusions for practical bearing applications, especially from the view point of particle size distribution, have been developed.

Since Japanese bearing manufactures have moved forward with overseas local production due to the recent economic situation, it has been necessary to define the lower limits of permissible bearing steels. Therefore, the permissible limits about cleanliness for bearing steels were studied. Consequently, successful local procurement of almost all bearing steels has been achieved.

By changing the chemical composition of SAE 52100, a new low cost through-hardening steel was developed. This steel was a non-soaking steel which was produced by controlling the primary large carbides and the heat-treatment condition of spheroidized annealing. The performance of bearings made of this steel is equivalent to that of the conventional bearing steel.