Taizo Makino

Influence of the Inclusion Shape on the Rolling Contact Fatigue Life of Carburized Steels

It has been well known that the flaking failure in rolling contact fatigue (RCF) originates from nonmetallic inclusions in steels, and their apparent size is one of the important factors affecting RCF life. However, the influence of inclusion shape on the RCF life has not been fully clarified. In this study, attention was paid to the influence of the inclusion shape on the RCF life. This was evaluated by using carburized JIS-SCM420 (SAE4320) steels that contained two different shapes of MnS—stringer type and spheroidized type—as inclusions. Sectional observations were made to investigate the relation between the occurrence of shear crack in the subsurface and the shape of MnS. It was found that the RCF life was well correlated with the length of MnS projected to the load axis, and the initiation of shear crack in subsurface was accelerated as the length of MnS increased.

The Effect of Slip Conditions on the Contact Fatigue Strength for Pitting of a Carburized Steel

The objective of the present study is to clarify the effect of slip conditions, such as slip ratio, outer surface velocity, and lubricant oil temperature on the contact fatigue strength for pitting (pitting strength) of carburized steels. Rolling contact fatigue (RCF) tests were conducted under oil lubrication on 26mm-diameter specimens made of a carburized steel with various combinations of slip ratio, outer surface velocity, and lubricant oil temperatures. Relationships among those three parameters were also evaluated. The pitting strength decreased with the increase of slip ratio, with the decrease of outer surface velocity, and with the decrease of lubrication oil temperature. It was found that the slip ratio, the outer surface velocity, and the lubricant oil temperature influenced the friction coefficient and the contact surface temperature. The friction coefficient increased when slip ratio increased. The friction coefficient was considered to be the dominant factor which directly affected the pitting strength.