Effect of Microstructure on Fatigue Crack Propagation Behavior in Ultrafine-Grained Steel
Hidehiko KIMURA, Yoshiaki AKINIWA, Keisuke TANAKA, Jun KONDO and Tadashi ISHIKAWA
Abstract:Steel plates with surface layers having ultrafine-grained microstructure, called SUF steel, exhibit excellent crack arrestability against brittle crack propagation by forming shear-lips at the surface layers. Although SUF steels have been proven to possess superior fatigue properties as well, the mechanism of the improvement in fatigue properties is not fully understood. In this study, smooth specimens and single-edge-notched specimens were prepared from the ultrafine-grained surface layer and the medium-grained mid-thickness region in the LT and LS directions. The specimens were fatigued under cyclic axial tension compression at room temperature. The effects of the microstructures on the fatigue limits and crack propagation behavior were investigated.@The crack propagation rate, when compared at the same stress intensity range, was the lowest in the ultrafine-grained LS specimen. The threshold value of the stress intensity range in the ultrafine-grained LS specimen was higher than that in the medium-grained LS specimen, indicating that the ultrafine-grained specimen exhibited not only higher fatigue limits but also superior crack arrestability against long fatigue cracks. The higher crack arrestabiliy in the ultrafine-grained specimen is attributed to bifurcation of fatigue cracks. Key Words:Ultrafine-grained steel, Fatigue, Crack propagation, Microstructure, Grain size, Rolling direction