The effect of inclusion geometry according to forging ratio and metal flow direction on very high-cycle fatigue properties of steel bars

The objectives of the present paper are to clarify the effect of inclusion geometry according to the forging ratio and the metal flow direction on very high-cycle fatigue properties of steel bars and to propose an evaluation method for characteristic inclusion geometries observed at fracture origin. Specimens were made of steel plates rolled to three levels of thickness from steel bars of low alloy steel. Then the specimens were cut in both parallel and perpendicular directions to the metal flow of the steel plates. Fatigue tests were conducted in a long life regime by using an ultrasonic fatigue testing machine. The fatigue limit for 109 cycles decreased with the increase of the thickness of steel plate, i.e. a decrease of the forging ratio. The fatigue limit of specimens cut parallel to metal flow was higher than that of specimens cut perpendicularly to metal flow. Fish-eye fractures appeared on the fracture surface of most specimens. Especially, characteristic inclusions, which are clusters of oxides, sulfides and their combination, were mainly observed at the origin of fish-eye fractures in specimens cut perpendicularly from thick plates.