Abstract
Fatigue tests were carried out up to 108 cycles for a series of 1800 MPa-class spring steels in order to investigate the relation between inclusions and fatigue properties. The fatigue tests were carried out both for billets and bars rolled from the billets and, in case of billets, the tests were both for RD and TD directions.
Al2O3rich inclusions were undeformable and not lengthened in rolling. On the other hand, SiO2 rich inclusions were divided into small pieces and the effective sizes for the axial direction were little reduced in rolling although the inclusions were deformable. In turn the difference of fatigue strength between billets and bars was not so large in most cases. The TD direction fatigue tests showed fish-eye fractures with the origins of large and lengthened MnS inclusions and the fatigue strengths were about a half of those in the RD direction.
The fatigue data plotted on a modified S-N diagram, in which stress amplitude were normalized by fatigue limits estimated by Murakami's equation, were divide into two groups. One consisted of large and lengthened MnS inclusions and large artificial notched defects. The other consisted of small defects such as Al2O3 and TiN inclusions and matrix cracks. The result showed that the effects of the lengthened MnS inclusions were almost equal to artificial notches and apparently different from relatively small defects. Moreover, the small defect group was also divided into Al2O3 inclusion group and others, suggesting the effect of properties of defects on the fatigue strengths.
