Fatigue crack growth test and in-situ SEM observation of ferritic/pearlitic and ferritic/bainitic steel plates were performed. Study was conducted to investigate the effect of microstructure on fatigue crack initiation and propagation behavior. The ferritic/pearlitic steel shows slightly higher FCG rate at region II and ΔK_<th> compared to the ferritic/bainitic steel. In-situ observation of the crack tip position at the ΔK_<th> showed that the crack was arrested in ferrite grains slightly below the pearlite phase band for the ferritic/pearlitic steel. In case of the ferritic/bainitic steel, the crack was arrested at the bainite phase of the sheaves oriented closely perpendicular to the crack growth direction. In the ferritic/pearlitic steel, crack generally passed through the ferrite grains and followed the interface of ferrite/pearlite when the crack faced the pearlite phases. The fracture path in the ferritic/bainitic steel was observed to: (1) follow the interface of bainite and ferrite (2) follow the bainite sheaves (3) pass through the bainite sheaves.
