Naoki Matsui

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.

Cutting Mechanism of Sulfrized Free-Machining Steel

Leaded free-machining steel is used in production industry. However the use of the leaded free-machining steel is limited from an environmental problem. So sulfurized free-machining steel, not include lead, has been researched widely. In order to improve the machining performance of the sulfurized free-machining steel, it is necessary to find out the behavior of inclusion. The experiment was carried out to find out the mechanism of the surfurized inclusion on the machinability, using some kinds of materials which were changed the size of the inclusion. In orthogonal cutting, the deformation behaviors of the sulfurized inclusion in the work materials were observed in the vicinity of tool face. Cutting forces were also measured and finished surfaces were observed. Besides, low speed orthogonal cutting in Scanning Electron Microscope (SEM) which was mounted a small cutting device was carried out in order to observe the deformation behavior more microscopically. As a result, it is clear that some inclusions creates voids around them and some broke to several pieces depending on their conditions. Moreover, in order to investigate the formation and growth of BUE, a quick stop experiment was done during turning. As a result, it is clear that the larger inclusions can reduce the formation of BUE.