Abstract
The inclusion content of recent high strength clean steels has been remarkably reduced by the progress of steel making process. Nevertheless, it is known that inclusions still cause fatigue fracture and decrease in fatigue strength. Thus, the quantitative correlation between fatigue strength and inclusion content or size must be made clear.
In order to investigate the critical lower bound of inclusion size which influences the fatigue limit, a special steel with extremely low inclusion content, the electron beam remelted super clean bearing steel (EB-CHR), was prepared.
The inclusion rating based on the statistics of extreme was applied for the quantitative evaluation of inclusion size in EB-CHR with decreasing size of inclusion, detrimental effect of inhomogeneities which have larger size than nonmetallic inclusions was revealed in place of inclusions.
The obtained results are summarized as follows ;
(1) The size of inclusions in EB-CHR measured by the inclusion rating based on statistics of extreme is extremely small in comparison with commercial clean bearing steels.
(2) The fatigue limit of EB-CHR is extremely high in comparison with commercial bearing steels.
This is due to the reduction of inclusion size by electron beam remelted process.
(3) The fatigue fracture origins of EB-CHR are seldom at inclusions but mostly at a small inhomogeneity which is presumed to be a local imperfectly heat-treated structure, bainite with lower hardness in the martensite matrix. This is because, based on the rating by statistics of extreme, the size of inhomogeneity (bainite) has relatively larger size than nonmetallic inclusion.
