In order to discuss the influence of lamellar structure and MnS inclusion on fatigue fracture mechanism, the fatigue crack propagation tests were carried out along different orientations of a hot rolled SB49 steel plate with or without homogenized heat treatment.
The as-received material showed directionality in fatigue crack propagation due to the effect of lamellar structure, while the homogenizing treatment made crack growth isotropic both macroscopically and microscopically. Fractographic examination revealed that the ordinary striation mechanism was predominant when the crack plane was normal to the lamellar structure as well as for any orientation of the homogenized structure. For the case where crack plane was parallel to the lamellar structure, however, sub-cracks initiated at the edge of MnS inclusions locating ahead of the main crack and each propagated independently by striation mechanism to join with the main crack. The increase in fatigue crack propagation rate was explained by the interactive effect of MnS inclusions and lamellar structure.