Fatigue behaviour of low carbon steel plates with fine grained ferrite in surface layers

Low carbon steel plates with fine grained ferrite in the surface layers produced industrially by a special thermomechanical controlled process (TMCP) have been tested to investigate the effect of ferrite grain refinement on the fatigue behaviour of steel plates. Fatigue fractographs were observed and analysed using a scanning electron microscope (SEM). In plates rolled by the special TMCP (designated special plates), the ferrite grain size approached 11μm in the surface layers and reached 14 μm on average, while in usually rolled plates (designated usual plates), the grain size was 26 μm in the surface layers and 26.4 μm on average. For a similar stress condition, the fatigue lifetime of the special plates was more than 3.5 times as long as that of the usual plates; the first stage of fatigue crack propagation could be prolonged, and secondary fatigue cracks were smaller than in the usually hot rolled plates. With a lifetime 37% more than that of the usual plates, the special plates could sustain a stress range 40 MPa more than that of the usual plates. Therefore, the fatigue properties were obviously improved for the special plates with fine grains in the surface layers.