Effects of Inclusion and ODA Sizes on Gigacycle Fatigue Properties of High-strength Steels

Abstract

Gigacycle fatigue tests up to 10⁹ cycles were conducted for SCM440 and SUJ2 steels. Five types of each steel were prepared with different melting processes. The gigacycle fatigue properties were divided into two groups: One group, consisting of SCM440 and double-melted SUJ2 steels, revealed higher fatigue strengths than the other group consisting of single-melted SUJ2 steels. The origins of fish-eye fracture were mostly an small Al₂O₃ in the high fatigue strength group and entirely a large (Cr, Fe)₃C in the low fatigue strength group. ODA sizes normalized with inclusion sizes at the fracture origin showed dependency on the inclusion sizes. The normalized ODA sizes increased according to the inclusion size decrease when the inclusion sizes were below about 10 μm, although the normalized ODA sizes were constant when above that size. Relating to this dependency, fatigue limits at 10⁹ cycles were independent of the inclusion sizes when the inclusion sizes were enough small. The inclusion sizes, below which fatigue limit lost dependency on the inclusion sizes, were expected to exist between 10 and 20 μm. On the other hands, fatigue limits depended on inclusion sizes when the inclusions were large enough to exceed that size, as well as on ODA sizes. The ODA and large inclusions were similar to small cracks in which ΔK_th depends on the crack size to the 1/3th power. This meant that the fatigue limits depended on the ODA and large inclusion sizes to the-1/6th power.