Using the Ando equation derived by treating as a nonlinear problem for the plastic behavior of fatigue fracture and the Tange equation derived from the El Haddad equation, the fatigue limit and threshold stress intensity factor were determined and the crack length dependence was evaluated. The threshold stress intensity factors were almost identical for the two equations although the basic idea and form of the equations are quite different. The ratio of stress intensity factor/threshold stress intensity factor using the modified Goodman fatigue limit is same for each crack length regardless of the stress ratio. The Tange equation (using α = 1) and the Ando equation were conservatively evaluated due to the decrease in the ratio of stress intensity factor/threshold stress intensity factor as the stress ratio increased at fatigue limit maximum value.

In this study, the crack length dependence of the threshold stress intensity factor (△K_th) for semi-elliptical cracks in the finite plate subjected to bending stress was evaluated. (△K_th) of ultra high strength steel (UHS steel) increased sharply at short crack depth and length. Mild steel (M steel) showed a tendency to gently increase at short crack depth and length, compared to ultra high strength steel. That is, different characteristics were shown depending on the material strength. (△K_th) of UHS steel and M steel was constant regardless of the width and crack aspect ratio (As). Regardless of the steel grade and specimen size, (△K_th) of As=0.3 for the crack depth was slightly larger than 1.0. On the other hand, that of As=1.0 for the crack length was much larger than 0.3.

This study investigated the mechanism of cementite formation in JIS-SCM440 low-alloy steel subjected to high-temperature nitriding (NH) at 913 K, followed by quenching and aging. Specimens with and without prior quenching and tempering (QT) treatment were analyzed to understand the influence of the initial microstructures on cementite precipitation. Comprehensive characterization using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) revealed that NH treatment promoted significant nitrogen diffusion into the steel matrix, resulting in the formation of a nitrogen compound layer and a hardened nitrogen martensite layer beneath the compound layer. In the NH specimens following QT treatment (QTNH), accelerated carbon diffusion to the surface led to cementite precipitation around the surface pores owing to the dissolution of fine carbides. Cementite enhanced the surface hardness of the porous regions compared to that of the NH specimens.