Spring steels subjected to torsional cyclic loading may fail even in the very high cycle region. In order to improve the very high cycle torsional fatigue strength of spring steels, it is necessary to clarify the governing factors of the very high cycle torsional fatigue strength. In this study, ultrasonic torsional fatigue tests were conducted for spring steel to investigate the fatigue crack initiation and growth processes with periodical observations of specimen surface using an optical microscope and with an observation of a cross section of a shear-mode fatigue crack. The results revealed that most of the shear-mode fatigue cracks originated in the early stage of the fatigue life at axially elongated MnS inclusions, the crack growth rates of the shear-mode fatigue cracks at surface were smaller than 10-10 m/cycles, and most of the fatigue life was consumed for the shear-mode fatigue crack growth.
